The realignment of a mirror in resonator requires a closed loop feed back control system. The system utilise PSD, Laser Diode, stepper motor, digital encoder, data acquisition card and motor controller card, motor driver for controlling the sequence of operation. In aligned position of mirror, beam from diode laser fall on to centre of the mirror which reflects the beam back to the centre of the PSD. The central position of spot is taken as reference position. When the mirror is misaligned, the position of light beam on PSD shifts from its reference position. The output signal from PSD is then compared with the reference position. These data are processed in data acquisition card in to find the misalignment of mirror. The necessary command is then generated through controller to move mirror in correct direction through stepper motor for realignment of mirror. The no of step counted with the help of digital encoder.

1. INVERTED ACTIVE MEDIUM
IN ABSENCE OF FEEDBACK MIRRORS THE EXCITED MOLE-
CULES DECAY BY SPONTANEOUS EMISSION OR BY COLLI-
SIONAL NON-RADIATIVE TRANSITIONS.
IF FEEDBACK IS WEAK THE EXCITED MOLECULES CAN
DECAY BY STIMULATING EMISSION OR BY COLLISIONAL
PROCESSES.
IMPORTANT
TOTAL REFLECTOR PARTIAL REFLECTOR
INVERTED ACTIVE MEDIUM
IN PRESENCE OF FEEDBACK MIRRORS THE EXCITED MOLE-
CULES DECAY DOMINANTLY BY STIMULATING EMISSION IF
INTRACAVITY INTENSITY IS EQUAL TO SATURATION INTENSITY
go Is = Maximum power that can be extracted.
EXTRACTING LASER FROM INVERTED MEDIUM

2. Optical Resonator Classification
Where gi = 1-L/Ri (i= 1,2)
L is resonator length
R is ROC of the mirror
On the basis of geometry they can be divided in to two categories
If ray retrace its path after each round Stable resonator
0 ≤ g1g2 ≤1
If ray change its path after each round Un-Stable resonator
-1≤g1g2≥1

3. Stability diagram
Stability Condition :-
0g1g2 1
Where gi = 1-L/Ri (i= 1,2)
L is resonator length
R is ROC of the mirror
(1,1)
PLANE PARALLEL RESONATOR
(-1,-1)
HEMISPHERICAL(0,1)
HEMISPHERICAL(1,0)
1-d/R
1-d/R
POSITIVE BRANCH UNSTABLE
RESONATOR
NEGATIVE BRANCH UNSTABLE RESONATOR
1
2
All the stable resonator which fall on blue line are
must be confocal
All the resonator along green line are
hemispherical

4. Types of Stable Resonator
Plane-Parallel
L
R1= R2=
Confocal
Concave-Convex
Hemispherical
R2=R1=L
R2=- (R1-L)R1L
R1=L/2 R2=L/2

5. Unstable Resonator
P
2
P1
M2
M1
Unstable resonator in general are non periodic focusing system
Are characterized by a geometrical magnification parameter,
Having characteristic magnifying geometrical eigen waves according to a purely geometric
or paraxial analysis

6. ( ) ( )
XYYXX
YXYX
L
x
IIII
IIII 2
2121
2112
=
+++
+−+
Electrode X2
Focused laser spot falling on
1st
quadrant 0f duo-lateral PSD
( ) ( )
YYYXX
YXYX
L
y
IIII
IIII 2
2121
1122
=
+++
+−+
LX
Y1
Y2
X1 X2
X
Y
LY
Ly X2
X1
Y2
Y1
Lx
Focused laser spot falling on
1st
quadrant 0f pincushion PSD
Output Ix2
Output Ix1
XB
XA Incident light
Electrode X1
Common
electrode
Resistance length Lx
Sectional view of PSD
P layer
I layer
N layer

7. Technical specifications
Material Silicon
Geometry Square
Active Area 12X12 mm2
Sensitivity 0.6 A/W @ 920nm
Rise time 3 µs
Dark current 1 nA
Resistance length 14 mm
Inter electrode resistance 10 KΩ
Position resolution 1 µm
Spectral response 320 nm -1060 nm
Terminal capacitance 300 pF
Quadrant
Detector
(S1880)

8. ``
A
B
C
D
A'
B'
C'
D'
A
B
C
D
A'
B'
C'
D'
`
A
B
C
D
A'
B'
C'
D'
`
A
B
C
D
A'
B'
C'
D'
15°
`
A
B
C
D
A'
B'
C'
D'
30°
45°
60°
Current is applied to the A and A’
windings so the A winding is north
Current is applied to the B and B’
windings so the B winding is north
Current is applied to the C and C’
windings so the C winding is north
Current is applied to the D and D’
windings so the D winding is north
Current is applied to the A and A’
windings so the A’ winding is north
Rotor
Stator
A A’
B B’
C C’
D D’
V

9. N NN SS
`
Permanent magnet S M Variable-reluctance S M Hybrid Stepper Motor
°




 −
= 360
rs
rs
NN
NN
ψ
ψ= step angle in degree
Ns=Number of teeth on stator core
Nr=Number of teeth on rotor core
6
)/( ss
N
ψ
=
N= motor speed in RPM
ψ= step angle in degree
s/s= number of steps per second

10. Stepper motor parameters
Speed 800 rpm
Dimension 60X60 mm
Encoder shaft diameter 8 mm
Step angle full 0.720
Maximum torque 2 N
Shaft diameter 8 mm
Parameter for digital encoder
Configuration Hollow (no attachment)
Shaft diameter 8 mm
Power supply 5 V DC
Resolution 40 to 1024 impulse /turn

11. Digital optical encoder is a device that converts motion into a sequence of
digital pulses. By counting a single bit or by decoding a set of bits, the pulses
can be converted to relative or absolute position measurements.
Types: Linear and rotary configurations but the most common type is rotary
Rotary encoders come in two configurations
Absolute encoder where a unique digital word corresponds to each rotational
position of the shaft.
Incremental encoder, which produces digital pulses as the shaft rotates,
allowing measurement of relative position of shaft
Rotary encoders are composed of a
glass or plastic code disk with
photographically organized tracks.
Digital pulses are produced when
radial lines in each track interrupt
the beam between a photoemitter-
detector pair.
Code disk
Shaft
Tracks
IR emitters
Phototransistors

12. Specifications
Selectable step angle
F (full step) position : 0.72 degree per step
H (half step) position : 0.36 degree per step
Current down
With auto current cut back feature when the
motor is at stand still (idle) to reduce motor
heat buildup when not running.
Input pulse type selector switch is given
Output current adjustment
Potted at max current value for the motor
Power supply : 24V DC

13.  PCI-bus mastering for data transfer
 16-channel single-ended or 8 differential A/D
inputs
 12-bit A/D conversion with up to 100kHz
sampling rate
 Programmable gain for each input channel
On board samples first input first output (FIFO)
buffer
 2-channel analog to digital output (PCI-1710)
Motor controller card
ৡ Number of axis : 1-4
ৡMaximum rate of steps : 3 MHz
ৡ Memory for application : 1000 lines x 80 character
ৡ Multitasking of application program : 8 simultaneously
ৡ Power requirements : ± 12 VDC (20 mA)

14. Y -tive
DCBA
DCBA
Y
DCBA
CBDA
X
+++
+−+
=
+++
+−+
=
)()(
)()(
Start
Stop
Drive the motor along
+ Y axis
Yes
No Y + tive
)()( DCBA +>+
Drive the motor
along – Y axis
Stop
No
X + tive
)()( CBDA +>+
No
Yes
Drive the motor
along –X axisX -tive
)()( DACB +>+
Yes
Drive the motor
along + X axis
No
Yes
Yes
Y -tive
)()( BADC +>+
Stop
Stop
B A
C D +X-X
+Y
-Y

15. Alignment accuracy 20 arc sec
Mirror misalignment angle 1/3˚
Time of alignment 10 sec
Diode laser beam diameter 2 mm
Diode laser beam divergence 0.7 mrad
Data acquisition
card
Motor
controller
Stepper motor
drivers
Digital
encoder
Biasing circuits
& amplifier
Stepper motor
Schematic of alignment of single mirror
Active Area of position sensing detector (PSD) 12X12 mm2

16. Resetting of reference
Manual misalignment
Powering

17. along x axis with focusing
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1 1774 3547 5320 7093 8866 10639 12412
Distance from centre (a.u)
photosignal(a.u)
Movement of spot in along X and Diagonally X
Diode laser spot on PSD scanned from left to right and diagonally
Diagonal
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
0 1 2 3 4 5
Distance (a.u)
Photocurrent(a.u)