R. MooneyTime (sec) Power (mW) Time (sec) Power (mW) Time (sec) Power (mW)0 4.27 1800 4.21 3600 4.2330 4.26 1830 4.22 3630 4.2260 4.25 1860 4.23 3700 4.2390 4.24 1890 4.24 3730 4.24120 4.23 1920 4.25 3800 4.25150 4.23 1950 4.26 3830 4.26180 4.22 1980 4.27 3900 4.27210 4.22 2010 4.28 3930 4.28240 4.21 20
1. SJCC Laser 103
Laser Performance Evaluation and
Validation Manual
by
Randall Mooney
May 2013
2. ii
Written as partial fulfillment of course requirements
Table of Contents Page
Introduction…………..……………………………………………………..…….. 1
A. CW Lasers
1. Power…………………………………………………………………………… 4
2. Power Stability (Nova II/StarLab) ………………………………………………8
3. Wavelength …………..………………………………………………………... 14
4. Beam Width and Ellipticity …………………………………………………….. 18
5. M2
……………………………………………………………………………...... 22
6. Beam Divergence……………………………………………………………...... 30
7. Gaussian Fit…………………………………………………………………....... 32
8. Polarization Ratio and Extinction Angle……………………………………....... 36
9. Peak-to-Peak Noise……………………………….………………………….......40
10. RMS Noise…………..…………………………..……………………………...43
11. Irradiance (Nova II/StarLab) …………………………………………………...46
B. Pulsed Lasers
12. Average Power, Peak Power, Duty Cycle ………………………………….... 51
13. Energy Per Pulse (EPP)………………………………………………………... 58
14. Pulse Duration/FWHM …………..………………………………………….....60
15. Pulse Repetition Rate (PRR/PRF)……………………………………………... 65
16. Fluence ( Nova II/StarLab) …………..……………………………...………....69
3. iii
Table of Contents Page
C. Appendices
Best Laser Lab and Manufacturing Practices
Appendix C-1: Best Laser Safety Practices........................................................... 74
Appendix C-2: Best Laser Beam X-Y Alignment Practices................................ 76
Appendix C-3: Best Optics Microscopic Inspections Practices............................ 81
Appendix C-4: Best Optics Cleaning Practices..................................................... 82
Appendix C-5: Best ESD Avoidance Practices..................................................... 86
Appendix C-6: Measuring the Focal Length of a Positive Lens............................. 87
Appendix C-7: Measuring Slope Efficiency........................................................... 90
Appendix C-8: Laser Pulse Image & Data Capture (Agilent & Tektronix) ........ 92
Appendix C-10: Getting Started with LASCAD ................................................. 97
Appendix C-11: Unstable Cavity Analysis with LASCAD ................................. 101
Appendix C-12: Troubleshooting 35 Common Laser Problems........................... 112
Appendix C-13: DVM Bandwidth Analysis.......................................................... 124
Appendix C-14: Oscilloscope Bandwidth Analysis................................................ 128
Appendix C-15: Agilent Application Note 5990-9923EN:
Minimum Required Sample Rate for a 1-GHz Bandwidth Oscilloscope................. 136
4. 1
CONTINUOUS WAVE LASER SYSTEMS
Continuous wave laser systems are just that; continuous sources of coherent, monochromatic,
highly directional and highly intense (bright) light. Continuous wave systems were the first type
of laser systems developed. Today, half a century later, they continue to be a mainstay of the
industry.
Continuous wave laser systems consist of an optical resonator, a gain medium placed in the
resonator cavity and a pump source to provide energy to the gain medium. When the proper
conditions are met, the system will begin to emit laser radiation.
in this section of the manual we will examine performance parameters that characterize
continuous wave (CW) laser systems, such Power output, Power Stability, Wavelength, Beam
Width/Ellipticity, M2
, Beam Divergence, Gaussian Fit, Polarization Ration and Extinction
Angle, Peak-to-Peak noise, RMS noise and Irradiance.
5. 2
1. Power
Objective:
We will measure the power output of a test laser.
Theory:
Power (work per unit time) is one of the key performance characteristics of a laser system.
Power level determines the rate at which the laser beam deposits energy on the target. Proper
control of output power levels is a basic element of any laser system.
Method:
Set up a laser system and directly measure output power levels.
Equipment:
• HeNe laser for test
• optical breadboard and hardware to mount laser to breadboard
• hand tools for assembly and setup as needed (include a ruler)
• laser power meter (appropriate sensor for wavelength and power level)
• safety eyewear
Setup:
a. Setup your equipment as outlined in Figure 1 below. The beam block can be something
as ordinary as a piece of paper for a low power HeNe beam.
Figure 1
b. Note the test laser wavelength (633nm in this case) and other label information as
necessary, such as manufacturer, model number, serial number, etc. Do the same for the
power meter. If the measurement is of critical importance you may want to check for a
valid calibration sticker on the power meter & sensor.
6. 3
c. Take the time to make sure that the sensor on the power meter is appropriate for the
wavelengths and power levels expected. An experienced person I know has suggested
having a kit of neutral density (ND) filters and holders available. Start slow and work
your way up. It can save you the embarrassment of a cooked sensor. I write this based on
personal experience.
d. Verify that the wavelength and optical density (OD) of the laser eyewear being used are
appropriate for the situation.
Procedure:
a. Put on safety eyewear. Block the laser beam, this can be as simple as holding a piece of
paper in front of the laser for a low power HeNe beam. Turn on the laser and give it a few
minutes to warm up and stabilize per posted procedures.
b. Place the sensor head of the power meter in beam path. Adjust it both horizontally and
vertically so the laser beam is centered on the head. Adjust the sensor head so that the
focal plane of the sensor is normal to the direction of propagation of the beam.
c. Block beam and allow reading from sensor to stabilize. This value is due to ambient light
in the room. Note the value as PAMBIENT.
d. Remove the beam block so the beam strikes the sensor. Allow the reading on the power
meter to stabilize. Read the power level from the meter and record it as PBEAM.
e. Block the beam again and allow the sensor to stabilize. Note the ambient light power
level. It should be the same as before the beam power was measured. A significant
difference is a sign that something is amiss.
f. Repeat the above procedure 3 times, noting PAMBIENT and PBEAM each time.
g. Turn off laser, leave blower running if needed so you don’t cook the tube. Let the system
cool down. Remove eyewear when safe.
Calculations:
The data in Table 1 was recorded from a Melles Griot HeNe laser operating at 633nm. Model
number is 05-LLR-851, serial number is S1936. The power meter and sensor used were a
Coherent Fieldmaster model FM, serial number MJ95, with associated sensor Coherent mode
CM-2, serial number ML92. Three sets of readings were taken and averaged to smooth the
data.
PBEAM (mW) PAMBIENT (µW)
1 4.27 13
2 4.38 15
3 4.21 14
Table 1
7. 4
Since PBEAM - PAMBIENT = PLASER
PBEAM (mW) PAMBIENT (µW) PLASER (mW)
1 4.27 13 4.257
2 4.38 15 4.365
3 4.21 14 4.196
Table 2
We average the three readings to smooth observational error, yielding:
PAVG = (PLASER1 + PLASER2 + PLASER3)/3
PAVG = (4.257 + 4.365 + 4.196)/3 = 4.273 mW.
Illustrations:
Since power is inversely proportional to wavelength, the meter must be set properly to give an
accurate reading. In this case λ = 633nm so we can see the meter is set correctly.
8. 5
Here we can directly observe laser power being measured. Note that the beam is incident normal
to the sensor head for accurate results. Power in milliwatts is shown on the meter screen.
9. 6
2. Power Stability
Objective:
Determine stability of the output power of a CW laser over time.
Theory:
Laser systems exhibit transient effects at power up before settling into long-term steady state
operation. Power stability is a key factor in many applications, so the ability to characterize and
quantify it is important.
Method:
Set up a laser system and monitor power undisturbed for a period of time while recording data at
fixed intervals.
Equipment:
• HeNe laser for test
• optical breadboard and hardware to mount laser to breadboard
• hand tools for assembly and setup as needed (include a ruler)
• laser power meter (appropriate sensor for wavelength and power level)
• safety eyewear
Setup:
a. Setup as shown, see Figure 1 below. The laser and power meter should be firmly
installed on a breadboard if you want to get any valid readings.
Figure 1
10. 7
Power meter reading during stability observation.
Procedure:
a. Put on safety eyewear.
b. Place the sensor head of the power meter in beam path. Adjust it both horizontally and
vertically so the laser beam will be centered on the head. Adjust the sensor head radially
so that the focal plane of the sensor is normal to the beam path.
c. Turn on the laser.
d. Record the power reading at time T = 0 sec. as PBEAM.
e. Wait 30 seconds.
f. Note the reading on the power meter and record as PBEAM.
g. Repeat step e & f for the next hour.
h. Turn off laser. Remove eyewear when safe.
11. 8
Data & Results:
The data in Table 1 were recorded from a Melles Griot HeNe laser operating at 633nm.
Model number is 05-LLR-851, serial number is S1936. The power meter and sensor used
were a Coherent Fieldmaster model FM, serial number MJ95, with associated sensor
Coherent mode CM-2, serial number ML92.
A graph of time-dependent power output for the test laser is shown below in Chart 1.
HeNe Power Stability
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
7.50
8.00
00:0001:3003:0004:3006:0007:3009:0010:3012:0013:3015:0016:3018:0019:3021:0022:3024:0025:3027:0028:3030:0032:3034:0035:3038:0039:3041:0043:3045:0046:3048:0049:3051:0052:3054:0055:3058:00
Elapsed Time
Power(mW)
Chart 1
Data for Chart 1 is contained in Table 1, shown on the following page.
12. 9
Table 1
Power Stability Worksheet
Date: 12/14/2011
Laser: Melles Griot HeNe (633nm)
Model: 05-LLR-851
Serial #: S1936
rated as Class lIIb device
Power meter: Coherent Fieldmaster + sensor
Laser cooled to ambient. Room is quite cool, approximately 60° - 65°F
Unit powered on at T = 0, power logged at 30 sec intervals for 30 min
Spot check for stability at 45 & 60 minutes
Elapsed Time
(min:sec)
Power (mW)
00:00 7.54
00:30 7.28
01:00 6.88
01:30 6.47
02:00 5.92
02:30 5.36
03:00 4.95
03:30 4.77
04:00 4.74
04:30 4.71
05:00 4.68
05:30 4.65
06:00 4.61
06:30 4.60
07:00 4.57
07:30 4.56
08:00 4.55
08:30 4.53
09:00 4.48
09:30 4.47
10:00 4.45
10:30 4.43
11:00 4.42
11:30 4.40
12:00 4.38
12:30 4.38
13:00 4.37
13:30 4.36
15. 12
3. Wavelength
Objective:
Determine the wavelength of a laser system.
Theory:
When passed through a diffraction grating, light will be deflected at an angle that is dependent
on the wavelength λ, in a relationship defined by l sin θ = m λ.
We note from Figure 2, below, that:
tan θ = α / l, thus θ = tan-1
(α / l)
and that the diffraction grating groove slit separation d is given by
d = 1 / ρ
where ρ is the groove slit density and can usually be found inscribed on the grating.
We know the properties of thin-slit diffraction gratings are such that l sin θ = m λ holds true
for far field effects where l is much greater than the slit separation d. λ is the wavelength of the
incident beam and m is an integer greater than 0, known as the order of the diffraction. We will
examine the case where m = 1.
Figure 2
Equipment:
• argon laser
• diffraction grating and mount
• hand tools for assembly and setup as needed (include a ruler and caculator)
• safety eyewear
16. 13
• spectrometer (if available)
Setup: (diffraction grating)
a. Set up equipment as outlined in Figure 2 above. Leave a reasonable amount of distance
between the laser and the mounted diffraction grating. In our case it was roughly a foot. It
is best if both the laser and diffraction grating are firmly mounted to an optical
breadboard.
Procedure: (diffraction grating)
a. Put on safety eyewear.
b. Turn on the blower for the argon laser FIRST so you do not cook the tube.
c. Wait 15-20 seconds to make sure the blower is running properly and up to speed. Using
the control box, turn on the laser. Make sure the control box is set to ‘power’ mode (as
opposed to ‘current’) mode.
d. Wait 30 seconds. THIS LASER DOES NOT COME ON RIGHT AWAY. Be calm.
Pump time for this system is long enough for an impatient student to wonder why things
are not working right (WTF??, there should be a big bright spot right over there but it is
not showing up, hmm…..) who will then walk around in front of the laser TO LOOK AT
IT.
Hypothetically, this would happen just as the system hits threshold and begins to lase.
This is a worst case scenario that one hopes would not occur in the real world. Still, I am
just saying, hey, you know, it could happen. Plan accordingly…..
e. When the laser does come on, go to the screen and record the location of the beam. Turn
off the laser. Put the diffraction grating in the beam path. Note that the beam path deflects
from its original course. Approach the screen and record the location of the beam. The
distance between the two points is α.
f. Turn off the laser FIRST and leave the blower on so you do not cook the tube.
g. Remove eyewear when safe.
g. Wait at least 30 seconds with the blower running to let the tube cool off.
h. Measure the distance from the diffraction grating to the target screen and record its value.
This is l, which together with α will let us calculate the angle θ.
i. Turn off the blower.
17. 14
Procedure: (spectrometer)
a. Arrange the bench such that the laser is pointed directly at the thermopile power meter
sensor as shown below. Clamp the fiber input of the spectrometer to a vertical post so it
will have a stable mount. Take care to handle the fiber properly and not to damage the
optical surface at the tip of the fiber.
b. Point the fiber input of the spectrometer at the thermopile sensor. DO NOT POINT THE
LASER DIRECTLY INTO THE FIBER INPUT OF THE SPECTROMETER. The areal
intensity is too high for the sensor to tolerate, it will be damaged or destroyed almost
instantly. This is not an exaggeration. I speak from bitter personal experience.
c. Note: The fiber input should be pointed at the thermopile sensor at about 30 to 45
degrees off the optic axis. Both the laser and the spectrometer fiber input are pointed at
the thermopile sensor. You will be sampling the back-scattered laser radiation from the
sensor. The direct laser radiation is far too intense and will damage the spectrometer
CCD sensor.
d. Start with the fiber input about 6 – 8 inches away from the thermopile sensor. At this
point, if you are doing this right, when you turn the laser the screen display on the
spectrometer control computer will show only background noise with no detectable
signal.
Picture
1:
Laser
pointed
at
thermopile
sensor
head
18. 15
e. Put on safety eyewear.
f. Turn on the blower for the argon laser FIRST so you do not cook the tube.
g. Wait 15-20 seconds to make sure the blower is running properly and up to speed. Using
the control box, turn on the laser. Make sure the control box is set to ‘power’ mode (as
opposed to ‘current’) mode.
h. Wait 30 seconds. THIS LASER DOES NOT COME ON RIGHT AWAY. Be calm.
i. When the laser does come on, SLOWLY advance the fiber pickup closer to the
thermopile sensor. As the pickup moves toward the sensor head, watch the control
computer closely. You will see the primary wavelength of the laser start to spike above
the background levels. Most DPSS lasers have a fundamental wavelength of 1064nm,
and that is what we can see here, a pronounced spike at 1064nm. See below:
Since we are only trying to determine only the fundamental wavelength of the laser and
not its intensity (at least not with this instrument) we can now safely record the results in
our laboratory notebooks and power down the system.
If you are using the argon laser, turn off the laser FIRST and leave the blower on so you
do not cook the tube.
g. Remove eyewear when safe.
h. Wait at least 30 seconds with the blower running to let the tube cool off. Then turn off
the blower.
Picture
2:
1064nm
spike
detected
as
fundamental
19. 16
4. Beam Width and Ellipticity
Objective:
Measure the width of the beam in the X and Y axis at the 1 / e2
cutoff point and compare
measured results to the geometric ideal.
Theory:
For an ideal Gaussian beam, a transverse section through the beam would give a perfectly
circular profile. If this were the case, Dx would be exactly equal to Dy and Dx / Dy would be equal
to 1. In the real world beams often exhibit a slight degree of imperfection and this is expressed as
the ratio Dx / Dy, or ellipticity.
Method:
Characterize a laser beam through the use of a CCD camera and beam profiling software.
Equipment:
We will use an experimental setup to make a series of related measurements. The equipment
layout will be detailed below. Safety eyewear and hand tools as well as all the necessary optical
equipment and hardware to assemble the layout will be required
Additionally, we will be making extensive use of a CCD camera based beam profiling system
and its control software, known as BeamScan.
20. 17
Setup:
Set up the bench or breadboard as shown in the figure of the alignment setup below.
Photograph of a breadboard set up to match schematic in alignment figure
21. 18
Procedure:
a. Set up breadboard to match schematic. Perform X-Y alignment procedure as outlined in
Appendix B. Make sure proper safety procedures are observed.
b. Once near field and far field alignment has been achieved, replace the retro-reflective
mirror mounted on the slide on the optical rail with the sensor head of the BeamScan
beam profiler.
c. Adjust the locking knob on the slide so that it can move smoothly back and forth but
there is a slight amount of drag. There should be no play in the mechanism.
d. Pull the slide forward on the rail so the sensor head is in the near field position..
e. IF the beam is centered properly on the scan head, the BeamScan software will display a
screen like Screen 1 in the Illustrations section below.
f. Slowly move the slide on the optical rail towards the far field position. The beam should
stay centered on the sensor head as you slide it toward the rear (far field) position. The
width numbers shown in the Ap1 X (Dx) and Ap2 Y (Dy) are the beam width as
measured at the 1/e2
cutoff point in the X and Y dimensions. Continue to move the slide
toward the far field position. Depending on the focal length of the focusing lense, at some
point for Dx and Dy will reach a minimum value as the narrowest part of the beam, kown
as the beam waist, is approached. After passing through the beam waist the values for Dx
and Dy will begin to increase; continue to slowly move the slide back to the far field
position while watching to make sure aligment is maintained (no beam clipping on edge
of mirror, etc).
g. If the untreated beam width is the parameter to be characterized, do not install the focusing
lense. You will not see the the beam ‘pinch’ down to a narrow beam waist. However, it is
still necessary to check that aligment is maintained along the rail by checking it at both
ends, near and far field.
g. Note the values for Dx and Dy as we move toward the far field position. Assuming the
system is in proper alignment, the ratio of Dx / Dy should not change as the slide is moved
along the rail. If this is true, then the system is properly aligned and the sensor head is
placed normal to the beam.
h. If we get good readings from both near and far field positions then we have achieved the
goal of putting our measurement system together properly. Gently bring the slide to the
front of the rail at the near field position again. Assuming success, the values for Dx and
Dy will be quite close to their original readings when we started this trip.
Data & Results:
As seen in Screen 1 in Illustrations section below, Dx is measured at approximately 1594
µm while Dy is shown to be 1622 µm. Ellipticity is given by the ratio of Dx / Dy so 1594
µm / 1622 µm = 0.982. This is in close agreement with results for ellipticity as obtained
on the BeamScan profiler.
23. 20
5. M2
:
Objective:
Manually determine the value of M2
for a benchtop laser system
Theory:
If a beam of light is collimated is passed through a positive lens then it will come to its smallest
diameter at a point determined by the focal length of the lens. The M2
parameter is a
dimensionless ratio that contains important information about beam geometry in relation to the
parameters of size (beam width), focusability and beam divergence. The reference figure below
shows the key elements of beam geometry we need to measure in order to calculate M2
. For a
perfect beam, M2
is equal to 1
From our work in class, we know that for Gaussian beams
€
M2
= π D0( )
2
/ 2(2ZR )λ
24. 21
where
€
D0 = diameter of the beam at the beam waist
€
DZR = 2
€
D0
€
2ZR = ZFAR − ZNEAR , given that
€
ZFAR = Z0 + ZR ( far field distance ) and
€
ZNEAR = Z0 − ZR ( near
field distance ).
€
λ = the operating wavelength of the laser we are evaluating.
Method:
We will use a BeamSscan beam profiler to examine the geometry of a laser beam as illustrated
in the reference figure above. The profiler will help use determing the values of D0 and ZR ; once
these values are known accurately then M2
can be calculated.
Equipment:
We will use the same equipment that we used in the previous chapter; if we are lucky, it is still
sitting undisturbed on the bench from our earlier work measuring ellipticity, saving considerable
time and effort.
Safety eyewear appropriate for the system under test is always required.
Setup:
Set up the bench or breadboard as shown in the alignment reference figure below.
25. 22
Usually we work with a 12” optical rail. So that values for
€
ZR work out to be something
practical, it is suggested that a convex lens with a focal length of approximately 150mm to
180mm be used. Install it right at the near field end of the rail.
Procedure:
a. Set up the bench as we did in Section 4 to measure beam ellipticity. If we are
extremely lucky, the bench will still be set up.
b. X-Y alignment must be verified by checking alignment at near field and far field
ranges and that it is maintained along the Rayleigh range between the two points.
To check this, adjust the friction knob on the slide on the rail such that the slide
can move smoothly but has zero backlash. Slide the rail to the near field end of
the rail and get alignment with M1. Slide it to the far field end of the rail and get
alignment with M2. The system must be in alignment at near field and far field
and all points in between or measurements will not be valid.
c. Once alignment has been achieved, we can begin to take readings. Replace the
alignment mirror on the slide with the sensor head of the BeamScan beam
profiler. Take a look at the real time display and make sure that the beam is well
centered throughout the range of travel, showing no clipping or trimming by
edges of optics, mirrors, apertures or other obstructions
Figure 1
d. The schematic representation shown in Figure 1 above is a good representation of
what is going on in the small distance around the beam waist. We used a lens with
a focal length of 185mm ( a little more than 7 inches ) and installed it right off the
26. 23
near field end of the rail. This meant that for us the small region of space depicted
in Figure 1 was located near the middle of the rail, somewhere near the 6” mark.
The reader might note that all the calculations here work much more easily in
metric. We found converting English/metric so cumbersome and error-prone that
we unscrewed the rail, flipped it around so the metric hash marks could be used,
screwed it back down and re-aligned the whole system. Spare yourself some
frustration. Go metric with optical measurement, you will save yourself a lot of
grief.
e. Once you have the sensor head in roughly the right spot, let it sit there for a bit
while watching the Profile display on the BeamScan. You should see something
like the picture below:
Slowly and gently start to pull the slide toward the near field end of the rail.
Watch the BeamScan display. Go slow. The machine has noticeable conversion
time and it is easy to outrun it. If the numbers in Ap1 X and Ap2 Y (DX and DY
respectively) are getting bigger then the beam waist D0 is on the far side of the
sensor head from you. You are operating in the left half of the sketch in Figure 1.
If the Ap1 X and Ap2 Y numbers are getting smaller then the beam waist in
Initial view after first placing the sensor head on the rail
27. 24
between you and the sensor head. You are operating in the right half of the sketch
in Figure 1.
f. Locate the beam waist D0 . Float the sensor head slowly and gently back and forth
along the rail. It should be possible to localize the region shown in Figure 1 fairly
quickly. Watch the DX and DY values on the BeamScan display. When you think
you are close, stop and let the readings settle out. Move the sensor head
incrementally, either forward or backward. Stop and let the readings settle out. If
they are getting bigger you are going in the wrong direction. Start moving the
slide in the other direction. If DX and DY values are getting smaller, keep going in
the direction you are headed until a minimum value is reached and the next
movement of the head results in an increase in DX and DY values.
Iterate to limit of human resolution or patience, whichever comes first. Stop and
let the readings settle out to make sure you are really at a minimum. When you
are as close as you can get, stop and let the readings settle out. That is it. You
have found the beam waist D0.
You should see a screen that looks like this:
Screen shot with sensor head located at beam waist D0
28. 25
Record the values of DX and DY. Do a quick reality check, calculate the ellipticity
by finding the value of the ration DX / DY . The result should compare closely
with the value shown in the ellipticity field on the BeamScan display. In fact, it
should be the same. If it is not, something is wrong. Stop and figure out why. If
you feel good about your data, record your location on the rail, noting is as Z0.
g. Find the far field Rayleigh range
€
ZFAR . Gently knudge the slide away from Z0
towards the far field end of the rail. The values for DX and DY should start to
increase. When the beam diameter D increases to
€
D0 × 2, stop. Let readings
settle. You have located
€
ZFAR . Record your location on the rail.
h. Find the near field Rayleigh range
€
ZNEAR . Start sliding the sensor head forward
on the rail. The values for DX and DY should start to go down. Move past the
position Z0 where the beam reaches its minimum diameter. The values for DX and
DY should start to increase. Keep moving slowly toward the near field end of the
rail. When the values shown for DX and DY are equal to
€
D0 × 2 , stop. Let
readings settle out. Record DX and DY, and record your location on the rail noting
it as
€
ZNEAR .
You should be looking at a screen that looks like this:
Screen shot of BeamScan display while located at
€
ZNEAR with beam diameter
D =
€
D0 × 2
29. 26
Stop and let readings settle. Note the value of DX and DY to make a record that
they are equal to
€
D0 × 2. Note your location on the rail. Record it as
€
ZNEAR .
Congratulations!! You have now measured all the information necessary in order
to determine M2
.
i. Shut down laser. Remove eyewear. Clean up optical bench and put away
hardware per requirements. You have a nice lined up set of mirrors there, you
should leave them there for someone to use if security concerns permit it.
j. Go out and have a beer with your friends. You have successfully completed an
M2
data collection process. Take a moment to relax and enjoy life.
Calculations & Results:
Using our data just collected we can calculate the value of M2.
. We know that
M 2
= π D0( )
2
/ 2(2ZR )λ
where
€
D0 = diameter at the beam waist
€
DZR = 2
€
D0
€
2ZR = ZFAR − ZNEAR
€
λ =
the operating wavelength of the laser we are evaluating
We measured M2
on one of the Hughes HeNe lasers we use in the lab. It was a learning
experience for me. Those little machines put out a high quality ‘pretty’ beam. Our data
was convincing.
30. 27
With numbers:
π = 3.14159 (approx)
€
D0 = 75µm
€
ZNEAR = 9.7cm on the optical rail
€
ZFAR = 11cm on the optical rail
so
€
2ZR = ZFAR − ZNEAR = 11cm – 9.7cm = 1.3 cm = 13mm
while
€
λ = 633nm
yielding
M2
= 3.14159 (75 x 10-6
m)2
/ 2(13 x 10-3
m)(633 x 10-9
m)
€
≈ 1.08! That is so cool!
M2
results with a value of less than 1.00 should be viewed with suspicion.
31. 28
6. Beam Divergence:
Objective:
Determine the angle of divergence of a laser beam.
Theory:
From our work in the previous section, we know that
€
M2
= π D0( )
2
/ 2(2ZR )λ
Examining the reference diagram of Figure 1 below, it can be seen that
Figure 1
€
tanΘ/2 = DF / 2 f for half angle divergence. This means that
€
Θ/2 = tan−1
DF / 2 f( ). From the
previous chapter, we know that DF is the diameter of the beam waist, commonly noted as D0 .
Substituting and consolidating, we now have
€
tanΘ = D0 / f or
€
Θ = tan−1
D0 / f( ).
We know the value D0 from our work in the previous chapter. This means we have all the
information necessary to calculate the value of the angle of divergence
€
Θ.
Method:
Finding the angle of divergence
€
Θ is a purely mathematical exercise once M2
has been
determined.
32. 29
Calculations & Results:
As before,
Θ = tan−1
D0 / f( )
where
f is the focal length of the convex lens used to determine M2
and
D0 is the size of the beam at the beam waist at the focal point of the lens. The value of D0 is
known to us from our previous determination of M2
, unless you really want to go and measure it
again.
To determine the angle of divergence we take our equation
Θ = tan−1
D0 / f( )
and insert our data. For us, D0 was measured at about 75 µm. We used lens with a focal length f
of 185mm for our M2
work so the value of the distance 2ZR (for calculating M2
, earlier) would
be large enough value that a human could measure it reasonably well.
€
Θ = tan−1
75 ×10−6
m ÷185 ×10−3
m( ) = 4.05 ×10−4
= .405 mR (milliradians) which converts to
roughly 0.0232 degree of arc. That is a small angle.
33. 30
7. Gaussian Fit:
Objective:
Compare the profile of our beam to that of an ideal Gaussian beam.
Theory:
For a laser operating in
€
TEM00 mode, it can be shown (but not by me) that the energy intensity
profile across a transverse section of the beam will have a Gaussian (normal) distribution.
Deviation from the ideal Gaussian profile indicates the presence of higher order transverse
modes or some other beam aberration.
Method:
We will use the capabilities of the BeamScan beam profiler to measure the beam and establish a
Gaussian curve fit.
Equipment:
• An optical bench equipped and set up as outlined for M2
in the preceding chapter
• BeamScan or other beam profiler (DataRay, BeamGage), associated hardware &
software
• HeNe laser for test
Setup:
If not already blessed with an existing setup, set up your bench as we did for the M2
procedure.
Align the system and prepare it for use by the the BeamScan beam profiler.
34. 31
Procedure:
a. Get the beam centered on the BeanScan sensor head, make sure you are getting good
readings. You should see a screen that looks like:
This is the view at beam waist D0
Note that you already have a good amount of information about Gaussian curve fit in the
fields located on either side of the GFit button in the upper left quadrant on the screen. On
the left of the button is the fit of the beam sliced in the X plane, on the right of the button is
the fit of the beam sliced in the Y plane.
However, there is yet more information to be had.
b. Select the ‘XYMeas’ button in the line of control buttons across the top of the screen. You
will move to a screen that looks like the picture on the next page.
35. 32
This is the view of the XYMeas screen.
Note that the Gaussian fit button has been selected. The BeamScan software will perform
a Gaussian curve fit and display the result.
Results:
In this case the result was not ideal. From the screen shot above it is clear that the beam (in this
view shown head on) is not symmetrical around the geometric center of the circle. What this
means in the real world is that the sensor head is probably misaligned and is not perfectly
normally incident to the beam.
However, it turns out to be useful for learning purposes. The BeamScan software runs the
Gaussian curve fit and shows the results for the X and Y plane in the windows labeled Aperture
1 and Aperture 2 above, respectively. The actual curve is plotted in white, with the Gaussian
ideal overlayed on it in yellow. It may not be possible to see it in this picture, but when you are
in front of the screen it is quite apparent.
Additionally, if one looks at the data readout below the graphical displays it is possible to see a
line labeled ‘Fit’. This is the variance of the actual beam profile from the Gaussian ideal beam
profile.
36. 33
These numbers (for Aperture 1 (X axis) the fit is 0.036, for Aperture 2 (Y axis) the fit is 0.019
may look small but I don’t think they are that good. Even with my limited experience, on nicely
set up test stations I have seen fit numbers on the order of 0.005.
37. 34
8. Polarization Ratio and Extinction Angle
Objective:
Determine the polarization ratio (if any) and extinction angle of a laser beam.
Theory:
Even though laser light is coherent, it is not necessarily polarized. For a beam to be
polarized all of its transverse oscillations have to be in the same plane. This principle is
shown by the illustration drawn from Wikipedia, below.
Method:
Examine the power transmission through two sheets of linear polarizing film. The ratio of
of power transmitted is proportional the the sine of the angle between the two sheets of
film, such that
€
PT = P0 × sin
€
Θ. When the polarizing films are normal to each other,
€
PT
drops to zero, since sin (90°) = 0.
Equipment:
HeNe (or Argon) laser on adjustable platform with beam-block, power meter, linear
polarizer film and tape, protractor mount & associated hardware and tools, breadboard or
adjustable rail,and, of course, alignment laser safety eyewear.
Most of the HeNe lasers used in our lab at SJCC are not polarized. If you are using a
HeNe laser you will need to "create" a polarized beam by using an extra piece of linear
polarization film and placing it in front of the emitted laser beam.
38. 35
However, as far as I know all the argon laser used in our lab are polarized.
The argon laser is constructed in such a way that the polarization process carried out
externally with the HeNe beam is accomplished internally in the argon laser..
Setup:
Set up the optical breadboard or bench as shown in the schematic below.
Procedure:
a. Put on safety eyewear first.
b. Get the linear film on the protractor and line it up at 0 degrees.
c. Power up that laser and let it settle into steady state operation.
d. Note the reading on the power meter display and record it.
e. Advance the protractor by 10° and take another power reading.
f. Repeat step e and f until the full circumference of the circle (360°) has been tested.
Record your results at each step.
g. Power down the laser.
h. Remove safety eyewear.
39. 38
9. Peak-to-Peak Noise
Objective:
Determine the Peak-to-Peak Noise value for the optical power output of a laser.
Theory:
The electrical voltage output produced by a photodiode sensor is proportional to the power of the
optical power input. By measuring the voltage of the sensor output we can assign a value to the
power output of the laser VPOWER. By measuring the Peak-to-Peak variance in the voltage output
of the sensor we can assign a value to the noise VP-P. The ratio of VP-P / VPOWER is the Peak-to-
Peak Noise of the laser.
Method:
We will use the measurement tools built into modern digital oscilloscopes to determine the Peak-
to-Peak noise of a laser.
Equipment:
• Laser for test
• Digital oscilloscope, we used a Tektronix TDS 210, with scope probes
• Photodiode based optical sensor
Setup:
a. Set up your equipment as shown in Figure 1 below.
Figure 1
40. 36
Results:
The procedure was performed as described above.
The results support the theoretical predictions.
€
PT does drop (close) to zero when the two
polarizing films are at 90° to each other. The minima and maxima are separated by 180°, as
expected. They are offset at right angles, also an expected result.
Raw data is attached on the next page in Table 1 for reference.
42. 39
Procedure:
a. Have the test laser warmed up and stabilized before beginning. Follow all standard
safety procedures, eyewear, etc.
b. Attach the scope probe BNC connector to the CH1 input. Hook it to the calibration
output, in our case located to the lower right of the screen. Make sure you see the 5V
square wave calibaration signal.
c. When you are satisfied the oscilloscope is providing reliable readings, remove the
scope probe.
d. Attach the sensor output to the CH1 input on the scope. Make sure CH1 is set for DC
coupling or this procedure will not work.
e. Close the shutter on the laser. Use the CH1 position control knob to put the display line
near the bottom of the screen. This will be the zero point for the VPOWER measurement.
f. Open the shutter on the laser. The output voltage of the sensor will jump. Use the
measurement cursors on the scope or simply count the divisions on the scope to
determine the sensor output voltage. Again, this will not work if CH1 is not set for DC
coupling. Record this as VPOWER .
g. Use the Volt/Div knob on CH1 to scale the display so we can examine the VPOWER
signal more closely, as shown below in the screen shot.
Screen shot as signal is being scaled for VP-P measurement
43. 40
h. As we zoom in on the CH1 signal we can begin to see there is a small active signal laid
on top of the static DC signal. This is the noise signal. Scale the picture so you have a
good view of it and then use the measurement cursors to determine its value. I snapped
this picture while we were still fiddling around getting the cursors set up so unfortunately
they are not visible. Record the peak-to-trough value as VP-P.
Calculations & Results:
For VPOWER we recorded a value of 276mV, as can be seen in the lower right of the screen shot
above. For VP-P we recorded a value of 2.1mV.
Since Peak-to-Peak Noise is the ratio of VP-P / VPOWER , we set up the ratio with our measured
values, yielding
Peak-to-Peak Noise = VP-P / VPOWER
with numbers
Peak-to-Peak Noise = 2.1mV / 276mV = 0.0076 or 0.76%.
44. 41
10. RMS Noise
Objective:
Determine the RMS Noise value for the optical power output of a laser.
Theory:
The electrical voltage output produced by a photodiode sensor is proportional to the power of the
optical power input. By measuring the voltage of the sensor output we can assign a value to the
power output of the laser VPOWER. By measuring the Peak-to-Peak variance in the voltage output
of the sensor we can assign a value to the noise VP-P. The RMS value (VRMS) of a time-varying
signal is a way of comparing the energy in an AC signal the the energy in a DC signal. The ratio
of VRMS / VPOWER is the RMS Noise of the laser.
Method:
We will use the measurement tools built into modern digital oscilloscopes to determine the RMS
Noiseß of a laser.
Equipment:
• HeNe laser for test
• Digital oscilloscope, we used a Tektronix TDS 210, with scope probes
• Photodiode based optical sensor
Setup:
a. Set up your equipment as shown in Figure 1 below.
Figure 1
45. 42
Procedure:
a. Have the test laser warmed up and stabilized before beginning. Follow all standard
safety procedures, eyewear, etc.
b. Attach the scope probe BNC connector to the CH1 input. Hook it to the calibration
output, in our case located to the lower right of the screen. Make sure you see the 5V
square wave calibaration signal.
c. When you are satisfied the oscilloscope is providing reliable readings, remove the
scope probe.
d. Attach the sensor output to the CH1 input on the scope. Make sure CH1 is set for DC
coupling or this procedure will not work.
e. Close the shutter on the laser. Use the CH1 position control knob to put the display line
near the bottom of the screen. This will be the zero point for the VPOWER measurement.
f. Open the shutter on the laser. The output voltage of the sensor will jump. Use the
measurement cursors on the scope or simply count the divisions on the scope to
determine the sensor output voltage. Again, this will not work if CH1 is not set for DC
coupling. Record this as VPOWER .
g. Use the Volt/Div knob on CH1 to scale the display so we can examine the VPOWER
signal more closely, as shown below in the screen shot.
Screen shot as signal is being scaled for VP-P measurement
46. 43
h. As we zoom in on the CH1 signal we can begin to see there is a small active signal laid
on top of the static DC signal. This is the noise signal. Scale the picture so you have a
good view of it and then use the measurement cursors to determine its value. It is possible
to let the scope calculate RMS value for you if you can figure out how to configure the
measurement cursors properly. I snapped this picture while we were still fiddling around
getting the cursors set up so they are not visible. If you can get the cursors configured to
read RMS values, you can measure VRMS directly. If you can’t, record the peak-to-trough
value as VP-P.
Calculations & Results:
For VPOWER we recorded a value of 276mV, as can be seen in the lower right of the screen shot
above. For VP-P we recorded a value of 2.1mV.
RMS Noise is the ratio of VRMS / VPOWER . However, we only know the value of VP-P, since we
were not able to configure the measurement cursors in a reasonable amount of time. We decided
to calculate the value of VRMS from VP-P.
We can calculate an approximate value for VRMS by noting that in the special case of a sinusoidal
signal, VRMS = VP-P /
€
2 . This is usually sufficient for a first order estimation.
Accordingly,
RMS Noise = VRMS / VPOWER
substituting
RMS Noise = (VP-P /
€
2 ) / VPOWER
with numbers
RMS Noise = (2.1mV /
€
2 ) / 276mV = (2.1mV / 1.414) / 276mV = 0.0053 or 0.53%.
47. 44
11. Irradiance
Objective:
Determine the Irradiance value of a given optical power output of a laser. The sensor is located at
a fixed distance from the laser.
Theory:
Irradiance is defined at the Power per Unit Area of a given laser beam. Once we know the
average power output of the beam and its geometric properties, we can calculate the area of the
beam cross section, such that:
Irradiance = Pavg / Abeam
Alternatively, modern test equipment can directly measure the power of an incident beam. By
making assumptions regarding beam geometry that are true for Gaussian beams (generally the
case in our scenarios) the test equipment, in this case an Ophir Optronics Nova II Power/Energy
meters mated with an appropriately rated thermopile sensor head can calculate and directly
supply Irradiance values to the investigator.
Method:
We will use the Ophir Optronics Nova II Power/Energy meter to determine the Irradiance value
as a function of diode pump current for a Spectra Physics V70 Series Q-Switched 1064nm laser.
Equipment:
• Laser for test
• Ophir Optronics Nova II Power/Energy Meter.
• Appropriately rated thermopile sensor for use with Nova II.
• Ophir StarLab applications software, which operates the instrument, collects the raw
data, calculates Irradiance and presents it to us in a nice graphical user interface.
48. 45
Setup:
a. Set up your equipment as shown in Figure 1 below.
b. Picture of our actual setup:
Here
you
can
see
the
laptop
running
Starlab,
the
Nova
II
meter
and
the
remote
console
for
the
V70
laser.
The
T20
series
power
supply
is
in
the
background.
The
V70
laser
head
is
the
gray
rectangle
in
the
middle
of
the
picture,
the
thermopile
sensor
head
the
black
cube
just
to
the
right
of
center.
Thermopi
le Sensor
Head
Nova II
Power
Meter
V70 Series
1064nm Laser
Beam
Block
49. 46
Procedure:
a. Put on your safety goggles and make sure GATE is set to ‘OFF’ on the remote
console. Power on the laser and allow it to warm up so that the laser beam can
stabilize.
b. Install the power detector head so that the incoming beam is normally incident. The
Nova II has a feature that will subtract out the baseline noise caused by ambient light.
Turn this function ON. Note; some power meters do not have this function. If so, an
option to minimize ambient light is to use a tube extension that encloses the beam and
blocks ambient light from reaching the sensor.
c. Using the remote console, bring the laser up slowly to a low power level, several
hundred milliwatts is generally a good starting point. Adjust the power meter range to
the expected value. Reposition the detector so that the beam is centered and strikes
the face of the detector at normal incidence.
d. Adjust the laser power to the required level and fine tune the beam alignment as
necessary to get an accurate measurement.
e. Find the power density via Ophir Nova II power meter with Starlab. Refer to Figure 2
f. Switch the view from normal power reading to Irradiance reading by clicking on
“Density” under the Functions drop down menu on the Starlab software. This will
give you the Irradiance reading. Refer to Figure 3
g. Irradiance=Power/Area
h. Record your irradiance reading in W/cm2
i. If enough data has been collected, power down the laser as noted in the laser
shutdown lab procedure.
50. 47
Calculations & Results:
The following data was recorded using the Nova II power/energy meter and StarLab application
software.
51. 48
Irradiance as a function of pump current:
Discussion:
It is easy to see that irradiance (Irr = Pavg/A) varies directly as a function of pump current. Since
beam cross-sectional area remains constant, only the laser power can vary; since laser power
varies directly as a function of pump current, these results make sense.
52. 49
PULSE MODE LASER SYSTEMS
As the name indicates, pulse mode lasers do not emit a continuous stream of coherent
monochromatic radiation like CW lasers. Instead they use a variety of techniques to store
additional energy in the active medium, releasing the energy at periodic intervals in short
bursts at much higher energy levels than would be possible if the laser were operated in
continuous wave (CW) mode. This course focused on the use of DPSS systems, and so in our
case the active medium is typically Nd:YAG, Nd:YALF or Nd:YVO4 (vanadate)
There are three basic techniques used to create pulse mode laser systems. They are:
• Cavity Dumping – an old technique, rarely if ever used in modern systems. This
technique will not be addressed further in this manual.
• Mode Locking – Mode Locking is an advanced technique in which several fundamental
longitudinal modes of the laser are held in phase with each other. This creates both
constructive and destructive interference as the beat oscillation occurs between the
frequencies F1 (frequency being equal to c / λ ) and FN , where N is a relatively small
integer indicating the fundamental mode. This technique produces ultra short
(femtosecond range) pulses of laser radiation. We will not address it further in this entry-
level manual except to note that it exists at the leading edge of short-pulse generation
technology.
• Q-Switching – we will focus on Q-Switched laser systems in this manual. There are two
main types of Q-Switch technology. They are:
1. Electro-Optical Q-Switching – this technique relies on polarized light and an
electro-optical switch called a Pockel cell. A Pockel cell contains an optical
crystal that exhibits a piezoelectric effect such that the polarization angle of the
cell is dependent on the voltage applied to it. During the pump phase of the Q-
Switch laser the Pockel cell is activated and reflects the laser light reaching it.
Periodically the optical crystal in the Pockel cell is turned OFF by removing the
voltage creating the piezoelectric effect. This has the effect of flopping the
polarization angle of the Q-Switch crystal by 90°, which means that instead of an
opaque high reflective (HR) surface it becomes a transparent one and a laser pulse
is released.
It is critical to note that Electro-Optical (EO) Q-Switching REQUIRES the laser
light in the resonator to be polarized in order to operate. It can use polarized light
if that is what the gain medium happens to produce, but if the crystal does not
create polarized light on its own, a separate polarizing filter will be needed inside
the resonator cavity for the Q-Switch to operate properly. An EO Q-Switch needs
two parts to operate properly; a polarizing filter (or already polarized light
produced by the crystal) and a Pockel cell.
53. 50
2. Acousto-Optical (AO) Q-Switching – this technique relies on nonlinear optical
effects created by RF frequency longitudinal waves (like sound, as opposed to
transverse waves, like light) that are induced to occur in specific crystal types.
When the RF signal is stopped the non-linear optical effects cease, the Q-Switch
crystal becomes transparent (turns off) and a laser pulse is released from the
resonator cavity.
It is important to note that Acouso-Optical Q-Switch does not require the laser
light in the resonator to be polarized in order to operate. It can use polarized light
if that is what the gain medium happens to produce, but it is not required that the
light be polarized. Accordingly, an AO Q-Switch needs only one part to function
as intended, the RF-excited piezoelectric crystal.
Q-Switched laser systems have several additional figures of merit to help describe their
performance. These parameters are not used in continuous wave systems as they are by
definition only useful in the pulse world. The parameters include Energy Per Pulse (EPP), Pulse
Duration (FWHM), Pulse Repetition Rate (PRR), Duty Cycle, Average Power, Peak Power.
We will explore characterization of these parameters below.
54. 51
12. Average Power, Peak Power, Duty Cycle
Objective:
Determine the average power output of a pulsed laser system through observation. With this
value plus knowledge of Pulse Repetition Rate (PRR) and the Pulse Duration (FWHM) of the
system all other figures of merit that characterize a pulsed laser system can be calculated.
Theory:
Given a pulse train as described below
Figure
1:
Idealized
Pulse
Train
and making the assumption that the amount of energy E in each pulse is constant over time, we
find that the rate of flow of energy in each individual pulse is given by
PPEAK = Epp x FWHM
which defines the term Peak Power.
We also find that average power output of the system is given by
PAVG = Epp x PRR
and that Duty Cycle is given by PAVG / PPEAK or
Duty Cycle = FWHM x PRR
where:
Epp is the Energy Per Pulse (in Joules)
PRR is the Pulse Repetition Rate (in Hz).
FWHM is the Pulse Duration (in seconds), measured at Full Width Half Maximum.
55. 52
Method:
Direct observation and measurement, basic algebra.
Equipment:
Laser, Nova II Power/Energy Meter, thermopile sensor of appropriate power rating, brain,
pencil, paper, appropriate safety equipment.
Setup:
Our experimental setup is shown in the picture below:
Overall view of experimental setup for power readings
56. 53
Close-up of laser and thermopile sensor
Close-up of Nova II meter and the control box for the laser
57. 54
Start up screen of Ophir StarLab application software
StarLab screen during power measurement. The milliwatt reading is the instantaneous laser
power output. The line chart below and to the right is the laser power output over time
58. 55
Procedure:
Set up the system as shown in the photographs above. Observe all relevant safety precautions.
Bring up the laser and allow it to stabilize for approximately 10 minutes. For a given pump
current and PRR, note the power output of the laser system. When done recording data,
carefully power down the system.
Computations & Results:
We know that for our system, a Spectra Physics V70 Series laser driven by a T20 Series power
supply
Pulse duration (FWHM) ≈ 100 nsec (Spectra-Physics published performance data)
PRR = 50 KHz - set on laser control box, then checked by observation & measurement.
For illustration, assume an average power of 2.5W from the V70 laser. This gives us
Epp = PAVG / PRR or
Epp = 2.5W / 50 KHz = 2.5 / 5 x 104
= 0.5 x 10-4
= 5 x 10-5
J = 50 µJ.
Using the values we already have for FWHM, PRR and Epp we calculate
PPEAK = Epp / FWHM
PAVG = Epp x PRR
Duty Cycle = FWHM x PRR
giving
PPEAK = Epp / FWHM = (5 x 10-5
J) / (1 x 10-7
sec) = 5 x 102
watts = 500 W
(checking) PAVG = Epp x PRR = (5 x 10-5
J) x (5 x 104
/sec) = 25 x 10-1
= 2.5J/sec = 2.5 W. ==>
Pass reality check – very good!!
Accordingly
Duty Cycle = FWHM x PRR = (1 x 10-7
sec) x (5 x 104
/ sec) = 5 x 10-3
= 0.005 = 0.5%
59. 56
13. Energy per Pulse (Epp)
Objective:
Determine the Energy Per Pulse (Epp) for a laser system
Theory:
The energy per pulse of a laser is given by the product of the laser’s peak power (PMAX)
multiplied by the pulse duration (FWHM), so
Epp = PMAX x FWHM
Alternatively, Epp can also be calculated if the Average Power (PAVG) and the Pulse Repetition
Rate (PRR) are known, since
PAVG = Epp x PRR
Solving for Epp gives
Epp = PAVG / PRR
Method:
Determine average power from the system per previous procedure using Ophir Nova II
Power/Energy meter. Use this observed information in addition to the previously known value
for Pulse Repetition Rate (PRR) to calculate Epp
Equipment:
Same as used for FWHM and PRR.
Setup:
Same as used for FWHM and PRR.
Procedure:
Same as used for FWHM and PRR.
Computations & Results:
Epp = PAVG / PRR or
Epp = 2.5W / 50 KHz = 2.5 / 5 x 104
= 0.5 x 10-4
= 5 x 10-5
J = 50 µJ.
61. 58
14. Pulse Duration ( FWHM)
Objective:
Determine the pulse duration (or pulse width, also known as FWHM) of a typical Q-Switched
laser system.
Theory:
The pulse duration or pulse width of a laser is typically defined as Full Width Half Maximum,
the time from the beginning of the pulse until its power level drops to half of the maximum
value. Figure 1 below is a good illustration.
Figure
2:
Pulse
Profile
showing
FWHM
tR is the rise time of the pulse, FWHM baseline is marked at time T when pulse power has
reached 90% of maximum value. The ΔT from T90 to THALF-MAX is the pulse width or FWHM.
Method:
We will measure the pulse width using a photodiode sensor and an oscilloscope.
Equipment:
Tektronix TDS 3012 oscilloscope
Photodiode sensor
Continuum Mini-Lite II laser system
Appropriate safety eyewear
62. 59
Setup:
Setup is straightforward.
Figure
3:
Setup
for
Pulse
System
Measurements
A photograph of our actual bench setup is below.
Picture
1:
Setup
for
Pulse
Characterization
63. 60
Procedure:
Procedure was straightforward; put on eyewear, power up the system and let it stabilize for
several minutes, then take our data. Most of the effort involved getting the oscilloscope set up
properly so we could take the right measurements.
Once we got that behind us, we started measuring FWHM and took data.
Results:
We were able to measure the pulse width. A screen shot of our oscilloscope trace is below.
Picture
2:
Screen
shot
for
FWHM
We were able to make measurements for FWHM for the Continuum MiniLite II laser, but some
practical difficulties interfered. We had trouble scaling the scope display such that we could both
capture the rising edge of the pulse and see the FWHM point on the trailing edge of the pulse.
Our results, averaged over several readings to smooth observational error, yielded a value of
FWHM = 19 nanoseconds
However, when we compared our experimental results with the Minilite II specifications, we
found we were way off. The Minilite II specification sheet has been included for convenience as
Appendix F of this manual.
64. 61
Discussion:
The specifications for our configuration and wavelength (MLII operating @ 532nm, 10-15 Hz)
showed that we should have measured a pulse width of approximately 3-5 nanoseconds.
We were able to accept our results knowing that several problems existed with our experimental
setup.
We knew we had a problem with the test leads we were using to attach our oscilloscope to the
sensor. The leads we were using were not matched to the scope; also, the quality of the coax
connectors on the test leads was quite poor.
Either of these factors could have introduced a significant impedance mismatch into the
measurement circuit which would have had the effect of broadening the signal duration.
Evidence for this can be seen in Picture 2 above, where the sinusoidal ‘ringing’ waveform is
imposed on the exponential decay waveform of the pulse. This ‘ringing’ is characteristic of
scope measurements that have excess capacitative loading.
Additionally, we didn’t figure this out till later, but I do not believe our oscilloscope was fast
enough to accurately characterize the short-duration events that happen at the beginning of a Q-
Switch laser pulse. Our scope sampled at 1 GHz To accurately characterize events in the
nanosecond range like we were attempting would seem to require a scope that would sample
considerably faster, in the range of 5+ GHz
Taken together, I believe that these factors could account for the inaccuracies in our FWHM
measurements. We obtained a good characteristic waveform (overlayed with capacitive loading)
but it was stretched out in time. Probe capacitance and sampling error can account for a great
deal of our observational errors.
Further notes on FWHM measurement – added 12/12
FWHM is a key pulse laser system parameter. It is important to be able to characterize and
quantify it in order to specify the functionality and behavior of the system.
However, I believe we have technical limitations with our current test equipment that hinder our
ability to directly measure FWHM with accuracy.
For now, I would use OEM published FWHM specifications as a basis for calculations, rather
than any empirically determined value obtained in our lab, as there is a good chance it is likely to
be incorrect by an order of magnitude. Or more.
65. 62
Notes and discussion on this issue have been moved to Appendix C-14, Oscilloscope
Bandwidth Analysis. – added 5/13 - ROM
66. 63
15. Pulse Repetition Rate (PRR)
Objective:
Determine the Pulse Repetition Rate (PRR) of a typical Q-Switched laser system.
Theory:
A Q-Switch laser emits pulses of laser light at discrete interval. Knowing the frequency of the
pulse train is a key performance parameter of such systems. The graphic below illustrates the
pulse repetition rate as it relates to the pulse train.
Figure
4:
Pulse
Repetition
Rate
(PRR)
Note that the PRR is the reciprocal of the pulse separation period, T. Measuring T lets us
calculate PRR = 1/T
Method:
As before, we will measure the time between pulses using an oscilloscope.
Equipment:
Tektronix TDS 3012 oscilloscope
Photodiode sensor
Continuum Mini-Lite II laser system
Appropriate safety eyewear
67. 64
Setup:
Setup is straightforward.
Figure
5:
Setup
for
Pulse
System
Measurements
A photograph of our actual bench setup is below.
Picture
3:
Setup
for
Pulse
Characterization
Procedure:
68. 65
Procedure was straightforward; put on eyewear, power up the system and let it stabilize for
several minutes, then take our data. Most of the effort involved getting the oscilloscope set up
properly so we could take the right measurements.
Once
we
got
that
behind
us,
we
started
measuring
PRR
and
took
data.
Results:
We were able to measure the period T, and thus readily calculate PRR (1/T) . In fact, the scope
did all the hard work for us. A screen shot of our PRR measurement is below.
l
Picture
4:
Pulse
train
for
PRR
The Minilite II can operate in the range of 10-15 Hz. We measured T for each PRR setting on the
control panel and obtained the following results:
PRR Setting (Hz) T (msec) PRR-calculated (1/T)
10 100 10.00
11 91.2 10.96
12 83.2 12.01
13 77.0 12.89
14 72.0 13.89
15 66.6 15.06
Our results are in excellent agreement with the product specifications.
70. 67
16. Fluence
Objective:
Determine the fluence of a pulsed laser beam from a laser sytem using an Ophir Nova II
Power/Energy meter and StarLab application software.
Theory:
We note that:
Fluence = Energy per Pulse (Epp)/Area
Area = Average Power (Pavg)/Irradiance (Irr)
EPP (Energy Per Pulse) = Pavg / PRR (Pulse Repetition Rate)
Method:
We will measure the average power and irradiance of the beam using the Nova II. Then we will
use that information to calculate the fluence value for the beam.
Equipment:
V70 series laser and associated power supply, necessary mounting hardware and interconnect
cables, Ophir Nova II power meter, thermopile sensor head with appropriate power rating, laptop
computer with StarLab software, oscilloscope and appropriate safety eyewear.
71. 68
Procedure:
Set up the equipment as shown in the picture below; for safety reasons make sure the computer
and system operator are located behind the plane of the aperture opening in the laser head.
1. Collect power and irradiance data as described in Chapter 11 in the CW section of the
manual.
2. Determine the Pulse Repetition Rate (PRR) of the system. Record the value for PRR as set
on the control box. Check that the value is accurate by measuring the period (T) between
pulses. For this series of experiments all data was taken at a PRR of 50 KHz, equating to a
period (T) of 20 µsec.
Data & Calculations:
The data from the Irradiance section of this manual is used to find the area of the laser beam.
Irradiance = Power/Area so Area = Power/Irradiance
1. Use the power and irradiance measurements in the Chapter 11of the CW section to find the
area of the laser beam in mm2
.
2. The data for EPP (Energy Per Pulse) will be needed to calculate the fluence.
3. Calculate the fluence of your laser beam using the following formula:
Fluence = Energy per Pulse/Area
The results obtained are shown in Table 1 on the following page.
74. 71
Results & Discussion:
As with other system parameters such as average power and irradiance, fluence was found to
vary directly as a function of pump current.
75. 72
Appendices
Appendix C-1: Laser Safety: Best Practices
Thou shall:
1. Always wear protective eyewear for safety reasons. Blindness is bad.
2. Verify safety eyewear to make sure it is appropriate for the wavelength being used.
3. Verify safety eyewear Optical Density (OD) to make sure it is appropriate for the power
level being used.
4. Post visible laser radiation warning signs and labels as needed.
5. Always orient laser so that it does not point at doors or windows where people may
inadvertently wander into the beam path.
6. Initially bring up the laser system at the lowest possible power.
7. High power laser systems should be kept in a secured area with controlled access.
8. Install safety interlocks on entrance doors to areas where high power lasers are located.
9. Remove all jewelry (shiny or not) before working on laser system to prevent accidental
exposure to beam.
10. Always use a beam dump or screen at the end of the beam path. No loose beams in the
lab
Thou shall NOT:
1. EVER, EVER, EVER look directly into a laser beam.
2. Remove safety eyewear while in posted laser operating zone.
3. Use eyewear that is not appropriate for the wavelength or power level being used.
4. Wear shiny jewelry (or any jewelry) when working on the laser system.
5. Point a laser beam at another person, even a low intensity source such as a laser pointer.
6. Leave a laser system running unattended.
7. Bring up a laser system without knowing the entire beam path FIRST.
8. Position a chair such that one’s face is at the same height as the laser beam.
9. Attempt to defeat safety interlocks and/or switches. They are there for a reason.
10. Work on a laser system operating at high power. Always use the lowest possible power
for alignment and adjustment.
77. 74
Appendix C-2: X-Y Beam Alignment: Best Practices
The bench layout shown in Figure 1 below is a good general guide for an equipment
configuration that will allow for proper near and far field alignment so beam characterization
measurements such as Gaussian Fit, M2
, beam width/ellipticity and beam profiling can be
performed accurately.
Figure 1
Thou shall:
1. Measure the height of the laser beam and the mirrors and targets to start them out in the
same horizontal plane BEFORE beginning the alignment procedure.
2. Try to keep as close to the breadboard datum as possible; it is hard to get a good
alignment if everything is wobbling around way up in the air..
3. Use the breadboard; try to line things up using the breadboard grid so they are as
perpendicular as possible.
4. Mount lasers and optics firmly, so tolerances are minimized. However, do not over-
tighten mounting screws.
5. Set all X-Y adjustments to the middle of their adjustment range at start of alignment.
6. Normalize M1 & M2 to 45° (by eye, using breadboard holes for reference) for starting
point.
7. Center the beam on M1, M2 and target reflector.
8. Inspect optics before use.
9. Start with target at midpoint of rail at beginning of procedure. Get coarse alignment. Next
move to near field, align, then to far field and align.
HeNe laser
78. 75
10. Understand what each mirror does; M1 for near-field adjustments, M2 for far-field
adjustments.
Thou shall NOT:
1. Mount optical hardware without using washers under Allen cap screws so as to avoid
marring the anodize surface on hardware.
2. Allow the beam to clip the edge of the mirrors.
3. Call a system aligned if a beam is off center on optics.
4. Perform optics alignment with beam at high power.
5. Perform procedure without beam block at end of beam path.
6. Move the laser itself to try to achieve alignment.
7. Lose track of thy beam, that it may not wander across the laboratory and smite the
innocent.
8. Achieve alignment by putting one mirror at max adjustment in one direction and the
second mirror at max adjusment in the other direction. Mirrors should be near midpoint
of adjustment ranges at end of alignment procedure.
9. Remove safety eyewear at any time when the beam is active.
10. Reflect return beam directly back into alignment laser output window. This can cause
instability problems with the alignment laser.
On the following pages are several photographs for reference and instruction.
79. 76
Note off-center beam on M1 and target mirror on rail. Target mirror is set at far field
position. This system is NOT in alignment.
Note retro-reflected beam back to source laser. Target mirror is set at near field position. This
system IS in alignment.
80. 77
This picture shows the fully retro-reflected beam more clearly. Note the bright spot from the
return beam on the face of the source laser
A close picture of the return beam on the bezel of the alignment laser.
81. 78
Thou shall:
1. Always wear protective eyewear for safety reasons. Blindness is bad.
2. Verify safety eyewear to make sure it is appropriate for the wavelength being used.
3. Verify safety eyewear Optical Density (OD) to make sure it is appropriate for the power
level being used.
4. Post visible laser radiation warning signs and labels as needed.
5. Always orient laser so that it does not point at doors or windows where people may
inadvertently wander into the beam path.
6. Initially bring up the laser system at the lowest possible power.
7. High power laser systems should be kept in a secured area with controlled access.
8. Install safety interlocks on entrance doors to areas where high power lasers are located.
9. Remove all jewelry (shiny or not) before working on laser system to prevent accidental
exposure to beam.
10. Always use a beam dump or screen at the end of the beam path. No loose beams in the
lab
Thou shall NOT:
1. EVER, EVER, EVER look directly into a laser beam.
2. Remove safety eyewear while in posted laser operating zone.
3. Use eyewear that is not appropriate for the wavelength or power level being used.
4. Wear shiny jewelry (or any jewelry) when working on the laser system.
5. Point a laser beam at another person, even a low intensity source such as a laser pointer.
6. Leave a laser system running unattended.
7. Bring up a laser system without knowing the entire beam path FIRST.
8. Position a chair such that one’s face is at the same height as the laser beam.
9. Attempt to defeat safety interlocks and/or switches. They are there for a reason.
10. Work on a laser system operating at high power. Always use the lowest possible power
for alignment and adjustment.
82. 79
Appendix C-3: Optics Microscopic Inspection: Best Practices
Thou shall:
1. Set the seat at the right height for a proper sitting or standing posture.
2. Set the two eyepieces that you can see through both eyes (in stereo).
3. Try not to focus too fast which leads to under or over zooming, which wastes time.
4. Always set the coarse focus FIRST before trying to set the fine focus.
5. Try not to work on the microscope for an extended period of time. Switch to a
different task to avoid stress.
6. Adjust the magnification by rotating the objective lenses, starting with the lowest
power objective lens first.
7. Reduce eye strain. Exercise the eyes by looking up, down, and from side-to-side. Go
outside if possible. Look at the horizon or distant objects to make the eyes focus far
away. This will help in order to relieve eye strain.
Thou shall NOT:
1. Over stress the eyes by improper image focusing.
2. Spend too much time working on microscope without exercise the
eyes.
3. Use microscope with one eyes, and close the other.
4. Crush parts and objectives by not paying attention and getting too close to inspection
microscope stage.
83. 80
Appendix C-4: Optics Cleaning: Best Practices
Thou shall:
1. Hold optics by the side, not on the surface.
2. Inspect the optics first before cleaning.
3. Always wear gloves to prevent skin oils from getting on optics.
4. Prepare a clean work surface before opening the sealed bag containing the optics.
5. Try to blow air first to remove the debris or particles if there is no
fingerprint or oils on optic surface.
6. Use only a source of clean, dry, non-reactive gas to blow dust of optics. Compressed CO2
or N2 are good candidates.
7. Clean with a gentle firm pressure, using one motion and going in only one direction.
8. Never scrub back and forth on the optic surface.
9. Never re-use the lens cleaning tissue. They are single use only.
10. Always inspect the optics after every clean.
Thou shall NOT:
1. Handle precious optics with greasy, ungloved hands.
2. Touch the coated surface (or any optically active surface) of the optics
3. Use aggressive cleaning methods unless less invasive methods have been tried first.
4. Use your own breath to blow dust off of optics. You will just make things worse.
5. Re-use the lense cleaning tissues. Tissues are cheap. Optics are not. Act accordingly.
6. Clean optics unless inspection determines that it is necessary.
7. Work with optics directly over a hard unprotected work surface.
8. Dispose of cleaning tissues soaked with methanol or acetone (or other cleaning agents) in
the regular trash. They must go in approved Hazardous Material disposal cans.
84. 81
Best practices document prepared by Candace Gilletter, Laser 103 2007 is attached below for
reference.
87. 84
Appendix C-5: ESD Avoidance: Best Practices
Thou shall:
1. Wear ground straps while handling or working with ESD sensitive devices.
2. Discharge by touching ground before pickup up ESD sensitive devices.
3. Make sure the work station is tied to earth ground.
4. Keep ESD sensitive devices away from strong electrical and magnetic fields.
5. Make sure the carts used to move parts between work stations are grounded.
6. Always use ESD bags for storage.
7. Move slowly and avoid rubbing parts together so as to not build up electric charge.
8. Routinely test grounding strap.
Thou shall NOT:
1. Work with ESD sensitive devices without proper grounding.
2. Touch ESD sensitive parts unnecessarily.
3. Forget to test grounding strap regularly.
4. Let the workstation float with respect to earth ground.
5. Expose ESD sensitive device to strong electric or magnetic fields.
6. Move any faster than needed or rub parts together to minimize charge buildup.
7. Store ESD sensitive part in regular plastic bags. They must be stored in ESD bags.
88. 85
Appendix C-6: Measuring the Focal Length of Positive Lens
Objective: Calculate the focal length of a positive lens.
Theory: The focal length, f, of a lens is given by the Lensmaker’s Equation
1/ f =1/ d0 +1/ d1
Figure 6: Image of a positive lens
where
do is the object distance from the principal plane of the lens
and
di is the image distance from the principal plane of the lens.
We note that when d0 goes to infinity that the term 1/ d0 goes to 0, removing it from the
equation, leaving only
1/f =1/di which is the same as f = di . The value of di is the same as the focal length of the
lens.
89. 86
We will do an empirical test in order to do a quick reality check and see f we are getting valid
results from our calculations.
As shown in Figure 2 below, if the light source (the object) is located effectively at infinity,
such that d0 >> d1., then the bundle of light rays can be considered to be traveling in parallel;
thus they will all pass through the focal point of the lens. We will set up a lens on a rail, shine
a light on it from a fair distance away. The focal point will be where the minimum spot size
is obtained. The distance from there to the principal plane of the lens is the focal length, so
we measure it.
Figure
7:
Imaging
light
rays
from
infinity
Materials:
Light source, positive lens, a screen, and a ruler. Optical rail, 2 rail carrier, lens mount and
screen holder
Setup:
Bolt the optical rail to the benchtop. Position the light source at some distance from the end
of the rail, at least 5x the anticipated focal length of the lens. Mount the lens on a rail carrier.
Mount the rail carrier holding the lens on the rail. Lock it in place with the thumbscrew on
the base. Mount the screen on the other rail carrier and mount it on the rail, on the opposite
side of the lens from the light source. Use the ruler to make sure everything is co-planar.
90. 87
Procedure:
A. Turn on the light source and direct it through the lens. Move the screen along the rail as
needed until you determine the location where minimum spot size is obtained. Lock
down the screen rail carrier and measure the distance between the screen and the lens.
This is the focal length of the lens.
B. Calculate the Value of the Focal Length Using the Lensmaker’s Equation
C. Find the Focal Length Using Light Rays from Infinity
a. To obtain rays from a source at infinity you can use sunlight or rays from an
overhead lamp. A flashlight will do.
b. Attach a screen to a rail carrier and move the same positive lens ( used in A,
above) back and forth along the rail until you locate the focus point where the
minimum spot size is obtained.
c. Measure the distance from the screen to the lens.
D. Compare the results from parts A and B. Since the same lens was examined in part A and
in part B we expect the focal length to be the same. However, due to experimental and
human error these results may slightly differ.
a. Calculate the focal length difference ∆f= |fA-fB|
b. Calculate the average fave= fA+fB/2
c. % Difference= [∆f/ fave]x100 %
Data:
Measure
5
times
&
average
readings
to
smooth
noise
introduced
by
poorly
focusing
middle-‐aged
eyes.
D0
(cm)
D1
(cm)
7.5
13.5
8.0
14.0
7.5
14.0
808
11.5
11.8
8.7
8.72
12.4
giving
do = 8.72cm
di = 12.34cm
1/f=1/do +1 /dif = (do x di) / (do + di) = (8.72cm x 12.34cm) / (8.72cm + 12.34cm)
f = 107.6cm / 21.01 cm = 5.12 cm
f = 5.12 cm
91. 88
Appendix C-7: Threshold & Slope Efficiency
Using the techniques outlined earlier in the manual, set up a laser and power meter. Let
everthing come up to temperature and stabilize. Determine threshold level, then plot laser
power against pump current across the full range of pump current values the system will
support. Run a regression through the linear portion of the curve using a least squares curve
fit. The slope of this line is the slope efficiency of the laser.
Data & Results:
Figure 8: Power Curve for V70 Series 1064nm Laser
92. 89
Table 2: Optical Power as function of Pump Current
Pump
Current
(A)
Optical
Power
(W)
0
0.00
1
0.00
2
0.00
3
0.00
4
0.00
5
0.00
6
0.00
7
0.00
8
0.00
9
0.00
10
0.00
11
0.00
12
0.65
13
1.40
14
2.41
15
2.88
17
2.94
19
3.10
Data is charted above
93. 90
Appendix C-8: Laser Pulse Image and Data Capture (Agilent & Tektronix)
Objective:
Step through the process of acquiring a waveform on the oscilloscope and then capturing it to a
data file on disk. We will use Excel to plot the numeric data obtained from the data capture.
Theory:
Not much theory on this one.
Equipment:
Pulsed laser Tektronix Oscilloscope Photodetector
Setup:
Set up as shown in the picture below
94. 91
Connect the photodetector probe to Channel 1 on the TDS3000 oscilloscope. Turn the scope on
to initialize it. Do not point the photodetector directly at the beams. You will damage the detector
if you do. Follow the instruction below to capture data.
Procedure:
1) For the Tektronix oscilloscope:
Turn on the laser; set up the oscilloscope until the signal is clear and you have a stable
trigger.
Figure
9:
Waveform
on
Tektronix
oscilloscope
screen
You can save the waveform as an image file in .BMP format, or you can save the raw sampled
data as a .csv file. The steps below outline each method in turn:
95. 92
A. To save the screen as a .BMP file
a. Insert the diskett into the floppy drive.
b. Press the “Utility” button System Config”“Hardcopy” ”Format”“BMP”
c. Scroll to this selection using (”-More-“)
d. Press “Port” “File”; highlight the filename to use (choose the first one in the list to
create a new file).
e. Press the “Menu Off” button to clear the filename menu
f. Press the “Hardcopy” button (button on the lower left with a printer icon on it). The
scope will save the screen to the selected file on diskette in .BMP format. Note that the
“Hardcopy” button will save the current screen to disk NOT the waveform image you
actually want to capture. This is frustrating as well as an effective time-waster.
g. Press the “Menu Off” button in order to go back to the waveform screen, otherwise it
will capture the filename menu screen instead.
B. To save the screen as a .CSV file
a. Insert the diskett into the floppy drive.
b. Press the “Save/Recall” butto “Save waveform ch ” “To file”
“Spreadsheet File Format”; scroll to filename
c. Press “Save ch to save the selected fil
C. For both file formats, after data is captured and written to the floppy, take the floppy diskette
down to the computer lab. Move that file onto a USB stick so you can read it with a modern
computer. Use Excel to import the .csv file and plot the data. See below
2) For the Agilent oscilloscope:
Setup:
Setup as above, only use the Agilent scope .
On the Agilent scope data is stored is by printing the waveform into a file on the floppy disc.
To do that, use the printer configuration menu to set up the printer interface and printer format
type:
1. To Store Data
a. Press the Utility Key.
b. Press the Print Config soft key to display the print configuration menu.
c. Press the Print soft key to to select the printer interface.
Parallel prints to the printer connected to the parallel port on the back of theoscilloscope.
Disk sends a print file to the built-in floppy disk. The print file will be named
PRINT_nn.xxx, where xxx is the format of the output (BMP, TIF, or CSV). If you print to
the floppy disk again, the number nn will automatically increment (starting at 00) each
time you save a new file to floppy disk. Up to 100 files can be saved on each floppy disk if
space permits.
96. 93
d. Press the Format key to select the print format.
2. To Recall Data
The Floppy menu allows you to load or delete files from the floppy disk. To retrieve files
previously stored in the floppy disk drive:
a. Press the File: soft key or turn the Entry knob to select a file on the floppy disk.
b. Press the Load File softkey.
Files that can be loaded into the oscilloscope from the floppy drive:
1) QFILE_nn.SCP setup files
2) QFILE_nn.TRC trace files, and
3) other user-defined setup or trace files that were created using the Save/Recall key on
the front panel of the oscilloscope.
Illustrations:
Figure 10: Waveform Capture (.BMP file)
97. 94
Figure 11: Data Capture & Plot of .CSV File for Waveform in Figure 2
Conclusion:
The ability to store/retrieve the waveform both as a screen shot (.BMP file) and as discrete data
points in the .CSV file is powerful. It gives the investigator the ability to fully preserve complex
waveforms (.BMP) while enabling offline analysis with tools such as Excel.
98. 95
Appendix C-10: Getting Started with LASCAD
Objective:
This LASCAD software shows how a resonator can be configured using the GUI, and how to
experiment with different cavity parameters. This manual helps you get started.
Installation:
To install the software:
1. Down load a demo copy of LASCAD 3.6.4 from this site :
2. https://www.lascad.com/lascad_download.php
3. Run lascad_demo_3_6_4.exe manually.
Starting the Program:
1. After starting LASCAD the dialog "Select Working Directory" appears asking you to
define aworking directory which will contain all files and directories created during a
LASCAD session.
2. After clicking "OK" the main LASCAD window appears.
3. Click "File/New Project" in the menu bar of this window or the leftmost icon button in the
tool bar or simply press the RETURN key.
A dialog appears, with the options "Standing Wave Resonator", "Ring Resonator" and
"External Beam".
4. To get started the first time, choose the "Standing Wave Resonator", and leave the number
of face elements and the wavelength [µm] unchanged.
5. After closing this dialog with "OK" or the RETURN key two additional windows appear.
a . The upper one (Mode Plot window) shows the Gaussian mode shape for a simple
standing wave resonator with 2 mirrors, visualized by 5 lines. The upper line is
showing the shape of the spot size along the axis, the other lines are provided to give
the picture a nicer appearance.
b. The lower window ("Parameter Field”window) shows a parameter field with the
parameter values used in the computation. If you enter a new value into one of the
number boxes of this window and click afterwards the apply button or use the
RETURN key, a new Gaussian mode computation is carried through.
Details of these and the other LASCAD windows are described in the following. Many of
the windows can be scaled by the use of the maximize and minimize buttons or by dragging
their borders with the mouse. The windows are described sequentially in an order as a new
99. 96
user of LASCAD might open them. If a LASCAD project file in *.lcd format already exists,
you can start the LASCAD program by double clicking on this file in the LASCAD
Explorer.
Main Menu of LASCAD
1. File menu
a. New Projec opens the Window "New Project”.
b. Open Project reads all parameters including thermal lens parameters of a previous
project from a file to which they have been saved before.
c. Save Project and Save Project As saves all parameter values with explanations in
ASCII format to a file. You can edit and print this file with a text editor, but be
careful in changing values, and never clear lines.
d. Exit closes LASCAD.
2. Print Menu
a. Mode Plot opens the printer dialog and prints the Gaussian mode shape shown in
the mode plot window.
b. Parameter Field prints the parameter field shown in the window "Parameter Field"
to a printer. To get it on a printer page it may be advisable to reduce the horizontal
extension of the window in advance or to print it in landscape format.
c. Parabolic Fit prints the graphs of the window: "Parabolic fit of Temperature and
Deformation
3. Print to File
a. Mode Plot generates a bitmap file of the mode plot diagrams.
b. Optical Element Parameters generates a file containing a list of the optical
parameters shown in the window
c. Parameter Field - The elements are listed with increasing number together with
their position along the optical axis, element type, curvature or focal length, and
the parameters of the medium between the elements.
d. Spot Sizes along Resonator axis generates a file where the spot sizes of the
Gaussian mode inside the cavity are listed in small steps with increasing z-
coordinate. The list contains the spot sizes of the wave traveling from left to right
as well as of the wave traveling from right to left, also y-plane spot sizes are listed.
e. Spot Sizes along External Beam generates a file where the spot sizes of the
external Gaussian beam are listed in small steps with increasing z-coordinate.
f. Spot Sizes at Element Positions" generates a file where the spot sizes are listed at
the element positions together with the element type.
g. Intensity at Right Mirror generates a bitmap file of the intensity distribution at the
right mirror as computed by the BPM code.
100. 97
h. Phase at Right Mirror generates a bitmap file of the phase distribution at the right
mirror as computed by the BPM code.
i. Eigenmode generates a bitmap file of the eigenmode profile shown in the
window "Eigenmodes".
4. Copy to Clipboard copies bitmaps to the clipboard.
5. View
a. Gaussian Mode Plot reopens the Gaussian mode plot window.
b. Parameter Field reopens the window "Parameter Field”.
c. Mode Profile"opens the window “Mode Profile"
d. Stability Diagram opens a window showing the stability diagram
e. Stability Criterion opens a window showing values for resonator stability
criterions
f. Crystal, Pump Beam, and Material Parameters opens or reopens a window to enter
the corresponding parameters.
g. 2D Data Profiles and Parabolic Fit opens or reopens a window to start the
parabolic fit computation.
h. Input for External Beam opens or reopens the window: "Entrance Plane Beam
Parameters" to enter the beam parameters of an external beam at the starting plane.
This button can only be used if the "External Beam" option of the "New Project”
window was checked or if a project with this option was opened.
i. Pump Profile opens or reopens the window showing the Pump beam profile.
j. Curvature of Phase Front opens the window: "Curvature of Phase Front", which
shows a plot of the phase front curvature.
6. FEA
a. Parameter Input & Start of FEA Code opens a window to select a cavity
configuration, and to enter crystal dimensions, pump configuration, and material
parameters, and to start the FEA code.
b. 2D Data Profiles and Parabolic Fit opens a window to start the parabolic fit
computation using the file "FEA.Out" in the working directory which contains the
results of the FEA carried through previously together with all crystal, pump and
material parameters used with this computation,
c. 3D Visualizer opens a window to show the FEA results.
7. CW Laser Power
Opens a window to compute the laser power output for CW operation.
8. Dynamic Multimode Analysis
Opens a window to activate code for analysis of multimode and Q-switch operation
101. 98
9. BPM
a.Run BPM" opens a window to enter parameters for the BPM computation and to
start the latter one.
b.Show Beam Radius and Power Output opens the window
c.Beam Radius and Laser Power versus Cavity iteration shows how beam radius
and laser power output develop with increasing number of cavity iterations.
d. Show Beam Quality opens the window "Beam Quality versus Cavity iteration".
It shows, how the beam quality parameters Mx2 and My2 develop with
increasing number of cavity iterations.
e. Show Beam Profile" opens the window "Intensity and Phase at Right End
Mirror".
f. Show Frequency Spectrum opens the window "Spectrum of Eigenfrequencies".
Show Eigenmodes opens the window "Eigenmodes".
Laser Power opens a window to compute the laser power output.
10. Tool Bar (from left to right):
a.Button: opens the New Project Window (same function as corresponding menu
item).
b. Button: saves all parameters of the project to a file (same function as
corresponding menu item).
c.Button: opens a previous project file (same function as corresponding menu item).
d. Button: prints the mode shape shown in the mode plot window to a printer
102. 99
Appendix C-11: Unstable Cavity Analysis with LASCAD
Objective:
Analyzing the Stability of an Unstable Lasers Cavity Using LASCAD
Start LASCAD
After starting LASCAD the dialog "Select Working Directory" appears asking you to define a
working directory which will contain all files and directories created during a LASCAD session.
After clicking "OK" the main LASCAD window appears.
Setting:
Click "File/New Project" in the menu bar of this window or the leftmost icon button in the tool
bar or simply press the ENTER key. A dialog appears, with the options "Standing Wave
103. 100
Resonator", "Ring Resonator” and "External Beam”
To get started the first time, choose the "Standing Wave Resonator", and leave the number of
face elements and the wavelength [µm] unchanged. After closing this dialog with "OK" or the
ENTER key two additional windows appear.
The upper window (Mode Plot window) shows the Gaussian mode shape for a simple standing
wave resonator with 2 mirrors, visualized by 5 lines. The upper line is showing the shape of the
spot size along the axis, the other lines are provided to give the picture a nicer appearance.
104. 101
The lower window ("Parameter Field” window) shows a parameter field with the parameter
values used in the computation. If you enter a new value into one of the number boxes of this
window and click afterwards the apply button or use the ENTER key, a new Gaussian mode
computation is carried through.
Click FEA/Parameter Input & Start of FEA Code in the menu bar of the main LASCAD window
to open the window "Crystal, Pump Beam, and Material Parameter”.
105. 102
Click on Show Pump Profile to bring up the following window
106. 103
Boundaries & Material Parameter setting:
Doping & Mat. And FEA Option setting:
Click on “Apply & Run FEA”. After the FEA run completes, close the FEA Options window.
107. 104
Data profile & Parabolic Fit
On "New Project" window click: "FEA"- "2D Data Profiles & Parabolic Fit"
"Open Fit Window"
108. 105
3D Visualizer
On “New Project” window click: “FEA” then “3D Visualizer” Click File – Open DataFile then
select file “Temperature.dat”
Inserting the crystal into the mode plot
Press the ALT key and click into the mode plot, such as between element 1 and 2, to insert the
crystal between these two face elements. A yellow element appears in the mode plot
symbolizingthe thermally lensing crystal. The face elements 1 and 2 have been converted into the
left and right end face of the crystal, respectively; and their distance has been adjusted to the
length of the crystal. To compute the mode shape, ABCD matrices for all FEA subsections and
the deformed end faces of the crystal have been built by the use of the computed parabolic
coefficients, and have been combined with the matrices of the end mirrors of the cavity.
109. 106
Analyzing the stability of a laser cavity
Select the menu item, View→Stability Diagram in the mode plot window to open the window
Stability Diagram, as shown in the figure below
Click Plot g1*x g2* to show the stability of the actual resonator configuration.
110. 107
Showing transverse Gaussian mode profiles and overlap with pump profile
Select the menu item Show Additional Items→Transverse Gaussian Mode Profile in the Mode
Plot window to open the window Gaussian Mode Profile, shown below
Gaussian Mode Profile
111. 108
Computing the laser power output
Select the menu item, Laser Power CW in the main LASCAD window to open the window Laser
Power Output, below
113. 110
Appendix C-12: Troubleshooting 35 Common Laser Problems
1) Astigmatism
a. How to detect:
Astigmatism is a condition in which the apparent focal points of the two axes do not
coincide. It limits the ability to focus the laser beam to a small spot size and complicates
focusing the output beam to a sharp well defined point.
b. Cause:
• Thermal lensing due to non uniform heating in the laser rod
• Unmatched cylindrical lenses
• Elliptical beams give rise to astigmatism when focused
c. Solution:
• Resolve thermal lensing issue.
• Replace with match cylindrical lenses
• Cylindrical lenses are used to correct for astigmatism
2) Wrong beam shape
a. How to detect:
Beam is highly astigmatic
b. Cause:
• Beam is clipping by aperture
• Defective lenses
• Incorrect collimator distance
c. Solution
• Verify no clipping beam by aperture
• Replace defective lenses
• Adjust collimator distance
3) Large beam divergence
a. How to detect:
beam size x/y diameter is extremely large while the waist location is small
b. Cause:
• Thermal lensing effects
• Incorrect collimator location setting.
c. Solution
• Resolve thermal lensing issue by checking beam size and beam divergence
• Readjust collimator distance
