2. point. The assembly has a cartridge heater to provide heat and an RTD (Resistance Temperature Detector) to monitor
temperature. Total power budget for the entire laser system is 3000W at End Of Life (EOL).
The diode drive circuitry has many fault detections to prevent misuse or damage to the sensitive diode arrays. The
diode drive FET (Field Effect Transistor) temperatures, diode array temperatures, drive inputs, and actual drive currents
are monitored to aid in protecting the diode arrays and prevent optical damage. The laser transmitter has a T=0 (time
zero when pulse leaves the LTU) circuit comprised of a photodiode with integrated pre-amplifier manufactured by
Advanced Photonix, Inc. The output of the photodetector in the laser routes to the LEU, where additional circuitry
buffers the fast T=0 digital signal to the host controller. A small Ohpir power meter head measures a portion of the total
optical power and a circuit on the control card converts this to a DC level which the host controller uses to determine
laser system health.
1.2 Mechanical Packaging
Proper thermal control and structural design are necessary to meet the full military range of temperature, vibration, and
shock environments specified for the system. Thermal control of the LTU involves inclusion of all temperature-critical
components in the system cooling loop including the laser pumpheads, Faraday rotator, IR beam dump, and SHG
assembly. The diode arrays within the pumpheads are mounted on high-efficiency heat exchangers to achieve the level
of heat removal required. The custom-designed chiller for the pod has an allowable setpoint range of ±2°C so the
design of the LTU is such that optical performance can be maintained over such a range. To save volume, the majority
of the floor of the LEU is liquid-cooled to act as a coldplate surface for the FET’s, converters, and other heat-generating
components.
Figure 1
ALMDS Laser Assembly
With regard to LTU structural design, system requirements on boresight wander demand tight control over laser
pointing stability for the complete operational range of ambient temperatures and vibration inputs. The optical bench
assembly is mounted to the LTU housing floor with proprietary kinematic mounts to allow for thermal expansion of the
bench without out-of-plane deflections. Structural design is also responsible for assuring that bench deflections are
acceptable with respect to optical alignment within the cavity and that resonant coupling between critical structures is
164 Proc. of SPIE Vol. 4968
3. minimized. Modal finite element analysis was performed on all critical structures within the LTU and LEU to optimize
the respective designs. All critical components achieved a minimum of half-octave separation from adjacent structures
and avoided the frequencies of the sine vibration inputs (see Sec. 2.1).
Figure 1 shows the ALMDS LTU/LEU assembly. The LTU weighs 67 pounds and has dimensions of 11” wide by 7”
tall by 27” long. The LEU weighs 37 pounds with dimensions of 13” wide by 4” tall by 23.5” long. This yields a
system volume of approximately 3300 cubic inches.
1.3 Optical
The basic laser design is a MOPA with single passes of the amplifier heads. The oscillator is a 2 mm diameter,
Nd:YAG rod pumped by 27 q-cw GaAs diode bars in a stable resonator. The Nd:YAG rod and diode heatsink dissipate
their heat into the 25°C coolant which is supplied by the system. The oscillator cavity length is physically 490mm long.
A low output coupler reflectivity along with good single pass gain allows for generation of moderate duration q-switch
pulses. At 350 Hz , <12 nsec pulses at 1064nm were achieved which resulted in 10-11 nsec pulses at 532nm as seen in
Figure 2 below.
A flat output coupler provides for a stable cavity and very good pointing stability. Zero-order waveplates are used to
throughout to assure the best possible performance and minimal change over temperature deviations. BBO was the
electro-optic q-switch material chosen for this laser due to its high damage threshold and minimal piezo-electric ringing,
as well as a lower index then LiNbO3 (Lithium Niobate).
Figure 2
Typical Pulsewidth at 532 nm
Proc. of SPIE Vol. 4968 165
4. A pair of matched optical wedges allows for fine alignment of the cavity with excellent lock down capability. This type
of rugged alignment technique has been widely used for years in typical LTD/RF’s (Laser Target Designator/Range
Finders). After the oscillator section is a 27 bar, pre-amplifier head, identical to the oscillator head, to increase the pulse
energy from the oscillator. Next, a Faraday isolator provides optical separation from the power amplifier. A telescope
sizes the beam for the power amplifier leg employing a 5mm rod head separated by a 90° rotator from a final 6.35 mm
rod head. The power amplifier heads are each pumped by 120 laser diode bars. Figure 3 shows the power output from
one of these heads in a short cavity test.
A half waveplate sets the input polarization to the proper orientation for type II doubling in KTP. The temperature of
the KTP is elevated well above ambient to reduce “gray-tracking” and provide closer thermal control. A final output
telescope provides the required final divergence. Total q-switched energy is approximately 280mj at 1064nm which
converted to 131 mj at 532nm via the type II KTP crystal. The beam quality at 532nm has an M2
of less than 8.
The diode arrays are run at a very conservative level to assure long life and high reliability. Total wall plug efficiency is
1.75% at 532nm. While this number does not reflect state of the art laser efficiency, it must be viewed in the light of
system reliability. It is not acceptable to have short-lived diode arrays or unacceptable MTBF’s (Mean Time Between
Failures) in a military application.
ALMDS 5mm Diode Pump Head Short Cavity Data (35%R, 5M HR, 25°C, 350Hz, 250mm CL)
y = 5.216x - 131.96
R2
= 0.9999
0
20
40
60
80
100
120
140
160
180
40 45 50 55
Diode Peak Current (Amps)
WattsOut
Series1
Linear (Series1)
Figure 3
166 Proc. of SPIE Vol. 4968
5. 2) Environmental Testing
2.1 Vibration and shock
Figure 4 shows the operational vibration spectrum over which the laser is tested while running. It consists of both a
random vibration spectrum from 4 to 2000 Hz and four discrete sine vibration spikes to simulate the helicopter
environment per Navy requirements and MIL-STD-810. The duration of vibration is 30 minutes per axis for all three
axes. The overall level is 2.75 Grms with a maximum static equivalent load of 8.4G. The laser system is also subjected
to a total of 18 shocks (three per direction), 8G in the vertical and longitudinal axes and 3G lateral. The shock pulse is
a half-sine with 6-9ms duration. Testing is ongoing at Systems and Electronics, Inc. in St. Louis, MO.
0.000100
0.001000
0.010000
0.100000
1.000000
1 10 100 1000 10000
FREQUENCY (HZ)
PSD(G^2/HZ)
Figure 4
Operational Vibration Spectrum
2.2 MIL-STD-704E
The complete laser system has undergone MIL-STD-704E power input testing. Testing was performed at Elite
Electronic Engineering in Chicago, IL. The laser system passed most steady-state and transient power input test
conditions required. All normal voltage transients conditions were passed except the 140Vrms, 60ms test condition.
Higher amplitude and longer duration transients were passed . No system faults occurred and laser output power did not
drop by >80% for more than two successive pulses. The laser system survived the abnormal voltage transients,
requiring only a logic reset to clear fault conditions and resume normal operation. The crest factor measurement test did
cause performance anomalies and is being reviewed.
Troubleshooting is ongoing to determine the circuits that are susceptible to the 140Vrms, 60ms transient. Additional
shielding and filtering should correct this anomaly.
2.3 MIL-STD-461E
The complete laser system has undergone MIL-STD-461E EMC testing, also at Elite Electronic Engineering. The laser
system is required to pass conducted emissions and susceptibility tests. The system has not yet passed conducted
emissions for the CE101/102 frequency bands from 800Hz to <3MHz. Low frequency noise is predominately
harmonics of the 400Hz input line frequency. These harmonics also account for the high crest factor measurement
reported above.
Proc. of SPIE Vol. 4968 167
6. Most conducted susceptibility tests were passed. The laser system was susceptible to sinusoidal bulk cable injection
(CS114) in the frequency band between 1.1MHz and 1.6MHz. Laser power dropped to approximately 14watts during
this portion of the test. Preliminary troubleshooting indicates the SHG heater controller is the cause. SHG oven
temperature rises causing a detuning of the SHG crystal. The laser system passed the impulse excitation and damped
sinusoidal transient bulk cable injection tests (CS115 and CS116).
Improved line filtering has been implemented in the LTU to reduce conducted emissions and susceptibilities. Reducing
the conducted emissions will also reduce the crest factor measurement and the CS114 susceptibility.
Retests for both MIL-STD-704E and MIL-STD-461E are planned for January 2003.
2.4 Temperature storage and operation
The laser system has been stored at a cold temperature extreme of –40°C as well as a hot extreme of 66°C, representing
a 106°C total temperature variation. The cold storage stage of testing consists of a dwell for 4 hours at –40°C followed
by a 6 hour rise in temperature to –20°C. After 4 hours of stabilization at this temperature, the laser system is operated
for 1 hour. The normal operating procedure for this laser subsystem consists of a five minute stabilization period after
power is applied before proceeding to the firing state. The laser displayed nominal power with consistent beam
pointing.
The hot storage stage of testing consists of a dwell for 4 hours at 66°C. Once the system stabilized for 4 hours, it was
cooled to the maximum operational temperature of 55°C. Again, the laser was stored here for 4 hours to achieve
thermal stabilization. The laser was then operated at that temperature after a 5 minute power-up stabilization period.
The laser displayed full power with consistent beam pointing once a steady state was reached. Average power,
pulsewidth and pulse jitter were recorded during the test period. The units performed nominally and all specifications
were met.
Acknowledgements
The laser system is presently in system integration and field trials are expected to begin in mid summer. Thanks to
Areté Associates, Northrop Grumman and the U.S. Navy/Coastal Sea Systems (CSS) for permission to publish this
work.
168 Proc. of SPIE Vol. 4968
