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Specific holographic design includes:

The configuration is a MOPA arrangement. Oscillator is normally operated at various amounts above threshold and uses an aperture for transverse mode selection. Axial mode selection is accomplished by use of resonant reflector and dye Qswitch. Resonant reflector will reduce to 8 modes or less and dye Qswitch (saturable absorber) will reduce further to 1 or 2 modes. Power amplifiers increase output to approximate 250 to 350 millijoules depending on pulse quantity. Changing Oscillator power and adjusting dye concentrations allowed 1 to 4 pulses and therefore gave a wider range of possible output.

 rwmopa5.gif Diagram of RW-MOPA ver 5.

 Image6.gif Oscilloscope of output.

rwmopa5.jpg Picture of ver 5 laser head. 250-350 mj holographic.

pw65.jpg Power supply used in version 5 and 6.  Version 5 was 4300 joules. Version 6 was increased to 7kj and then 8kj.

 dye3.htm Oscilloscope output of RW-MOPA 5 with dye cell adjusted for multi-pulse.

Q switch pulse from oscillator. The flashlamp pulse is approximately 700-800 usecs long. The Q switch pulse generally would occur at the 50 percent trailing gain point but in this case it occurred near the 30 percent trailing edge. The circuit was critically damped at A= 0.8 using 1180v at 933 ufd and 58 uh . Delivering 650 joules to an EG&G FX-42C 7mm flashlamp at the oscillator. Explosion limit was at 41 percent. Delivery of pump is 280 electrical joules/cm3 to a 6.3x76mm rod in a half-elliptical cavity. The resonator length is 77cm. HR is 3m spherical concave and OC is a planar 2 plate resonant reflector type from tank OC. Beam waist Wo radius = 0.538 mm. Aperture set at 1.2mm at HR due to the 16% increase in mode size. Fresnel number is 0.67 at aperture. The resonator G is 0.75

The HR forms part of 8mm dye cell with a 6.3mm dual AR coated window. I decide on DOTCI in ethanol as a q switch formula. DOTCI was used to prevent mode locking. DOTCI absorption peak is 690nm and importantly has a relaxation time of 1.7 nanosecs. Cryptocyanine in ethanol had a absorption peak at 710nm and a relaxation time of 17 picoseconds. Crypto dye is the dye of choice for passive mode locking with q switching. The reason not to mode lock (create multiple picosecond pulses in a q switch envelope is the main purpose here is to reduce the number of longitudinal modes. An example, the broadband frequency gains like TI-sapphire lasers convert it's large frequency bandwidth domain into femto-second short pulses in the time domain due to mode locking. Likewise a fast dye is likely to mode lock and not provide the slow pulse build time that reduces frequencies that oscillate from the gain differences created by the resonant reflector. That slow build time or larger number of cavity round trips allows the gain differences between axial modes to increase and therefore will help produce a more dominate single mode or two.

Adjustment of dye solution allows for a large single pulse or two shorter pulses. Adjustment of dye solution also affects location on the trailing edge of the pump cycle. By delaying the giant pulse and allowing more round trips for the noise signal to build, further improvement on axial mode selection occurs. Instead of reducing pump (electrical input) and therefore gain to obtain this, dye concentration can be increased. Power amplifiers were pumped to 380 electrical joule/cm3 with total of 30cm or 12" single pass length. Optics from CVI laser. Dye from Exciton. Inc. Series injection triggers and flashlamps from Perkin Elmer of former EG&G. Ultrastable kinematic 1" optic mount threaded model KS1-T using 1" SM1L10 lens tubes from Thorlabs Inc. yag Inc. machined all enclosures and optical benches. Optical bench is 1"X 38"X 6" 6061 aluminum plate milled for various component heights and rubber shock mounted on 1/2 x 10 x38 bottom plate. Rod, OC and cavity salvaged from tank surplus market. Cavities milled to accept 7mm flashtubes instead of 4mm. 5000 joule power supply and timing circuits engineered by yag Inc.


Image7.gif Dye Q switch pulse output.

At 10ns sampling rate you can see that the pulse is 50 nanoseconds long. A single 15-17mjoule TEM00 mode pulse was produced from this oscillator. Output is as follows from amp 1, 2, 3, and 4. 38mj, 80mj, 140mj, 246mj. Oscillator exit beam was 1.2mm and at the end of amp chain was 1.8mm.

Laser had 1 nanosec rise time photometer and bnc connector for cavity pulse monitoring by digital storage oscilloscope. Needed when adjusting DOTCI dye/ethanol ratios for single or double pulse. Also built in two thermistors and a digital thermometer show cavity and bench/etalon temp. Laser has a remote hand held fire switch which TTL logical level fires timing circuits for flashlamp triggers. System uses series injection transformers and scr firing switches. Ports are used for an external HENE laser for optic alignment of the laser system.

I found that when adjusting the resonator with a fixed position aperture over the HR, I would align the oscillator but then when I had to reduce the aperture, I had to adjust the OC by tilting it until I got good power level from the oscillator at the small setting. The fixed position aperture was due to the barrel mount design of it and the aperture also had the dual purpose of shielding the dye from the flashlamp light of the tank cavities. A interesting note I found that during alignment I found I had three reflections at the OC. One in the center and two reflections that hit above and below the main one and appeared to originate from the AR 694vcoat window over the HR that formed the dye cell. This HR/window dye cell is the most efficient method but later I decided to use a separate dye cell in future versions.