Jason Machan

Joint high power solid state laser program advancements at Northrop Grumman

Northrop Grumman Corporation has made significant progress in the development of compact, high power, continuous operation solid state lasers for military applications during the past six years. The Joint High Power Solid State Laser (JHPSSL) program is nearing completion of its third phase; its key objective is to demonstrate a 100kW solid state laser with excellent beam quality. Northrops unique scalable architecture coherently combines modular 15kW lasers to produce power levels of 100kW and beyond with excellent beam quality and run times. This paper describes the JHPSSL program history, Northrops high power solid state laser architecture and our demonstrated results.

Active tracker laser (ATLAS)

A high brightness diode-pumped, Nd-YAG solid state laser has been designed, fabricated, and tested. This phase conjugated master oscillator/power amplifier (MOPA) device produces 20-ns Q-switched pulses at 2500 Hz at an average power of 690 W and a beam quality of 1.1/spl times/DL when the pump diodes are operated at 27.5% duty cycle. With an external KTP doubler, this device has produced 175 W of green average power at a beam quality of 1.5 /spl times/ DL and a conversion efficiency of 45% over continuous operating times as long as one hour. This 1.06 /spl mu/m result is believed to be the highest average power brightness achieved, and the 532-nm performance is both the highest average green power and the highest average brightness ever reported.

Scalable Coherent Combining of Kilowatt Fiber Amplifiers Into a 2.4-kW Beam

We demonstrate coherent beam combining of multiple Yb-doped non polarization-maintaining fiber amplifiers operating at individual power levels above 1 kW. A 2.4-kW, M2 = 1.2 output beam was generated by combining three amplifiers with a diffractive optical element (DOE). A single low-power sample of the combined output beam provided error signals for active phase and polarization locking of all three fibers. The beam combining efficiency of 80% was nearly constant with amplifier power, indicating the absence of significant thermal or nonlinear effects. Modeling anchored by precise measurements of the beam characteristics indicates potential for this architecture to scale to higher fiber counts, higher combining efficiency, and higher power.

Active Tracker Laser (ATLAS)

A phase conjugated Nd:YAG amplifier has demonstrated 690 W average power and 1.1 ×DL beam quality. The frequency doubled output was 175 W at 1.5 ×DL and 45% conversion efficiency. This is the highest average green power and highest average brightness reported.

One Joule Per Pulse, 100 Watt, Diode-Pumped, Near Diffraction Limited, Phase Conjugated, Nd:YAG Master Oscillator Power Amplifier

We have assembled and tested a diode-pumped, phase conjugated Nd:YAG master oscillator power amplifier (PC MOPA) operating at an average power of 100 Watts. 1 J per pulse has been extracted at a repetition rate of 100 Hz with a beam quality (BQ) of 1.1 x diffraction limited (D.L.). This combination of average power and beam quality makes this the brightest short pulse solid-state laser reported to date. The optical efficiency of 22% and the overall efficiency of 9.4% also represent record performance for high energy short pulse lasers. Excellent spatial uniformity and a pulse length of 7 ns make this laser ideal for frequency doubling and parametric conversion.

Diode-pumped Nd:YAG laser for precision laser machining

Results are presented on a high power, diode‐pumped, pulsed Nd:YAG laser for precision laser machining. The laser is an unstable resonator with a graded reflectivity outcoupler, generating a beam with excellent beam quality. The gain medium is a single zig‐zag slab, pumped symmetrically by diode arrays. The use of diode arrays minimizes the thermal loading on the slab, and the zig‐zag path averages thermal distortions in the zig‐zag dimension. Measurements of beam divergence as a function of diode duty‐cycle will be presented. Available pulse formats will also be discussed. To date, the laser has produced 720 W at 20% diode duty‐cycle with a stable cavity and 550 W at 20% duty cycle with an unstable cavity in close agreement with model predictions. The beam divergence has been measured to be 1.7 times diffraction‐limited at 20% duty cycle. The laser has been operated with pulse lengths from 20 μs to 1 ms and is being used to obtain laser processing data, with some results shown.

An experimental analysis of a Nd:YAG laser cutting process for machining silicon nitride

The ceramic silicon nitride was machined with a short pulsed Nd:YAG laser. The cuts made were crack free and exhibited promising characteristics for precision laser machining of such materials. The cut kerf geometry was analysed and modelled as a function of energy density. This model proved fairly accurate for lower level energy densities, but the amount of material removed increased with energy density up to a maximum point and then began to decrease. This work shows that laser machining silicon nitride can be accomplished with no crack formation and that the kerf characteristics can be modelled using an energy density formulation.

Modeling high-brightness kW solid state lasers

Several kW-class solid-state lasers at TRW are described with an emphasis on the performance modeling used to aid development of high brightness operation. Comparisons of results and analysis are presented for key aspects of high power, diode pumped, Nd:YAG lasers and amplifiers that use zigzag slab configurations to minimize thermal effects. Devices described include multi-kW power oscillators suitable for high power machining, welding, and material processing; and phase conjugated master oscillator/power amplifiers (MOPAs) which provide short pulse, high brightness beams for active tracking, photolithography, or remote sensing. Laboratory measurements are in good agreement with predictions of diode pump profile and absorption efficiency; slab extraction efficiency and thermal load; and slab OPD.

High-average-power industrial laser for precision machining

High brightness, high average power, diode pumped Nd:YAG solid state lasers (DPSSL) are being developed by TRW as part of the Precision Laser Machining Technology Reinvestment Program. The use of diode pump arrays in place of flashlamps, and zig-zag slab geometry, allow lasers to be scaled to power beyond the current generation of lamp-pumped rod lasers while providing excellent beam quality. The efficiency is 3 - 4 times better using diode arrays in place of flashlamps resulting in less waste heat in the laser medium and reduced optical aberrations. The corresponding beam quality provides more than an order of magnitude increase in the average intensity available at the workpiece, thus enabling new machining capabilities.

5.4 kW diode-pumped, 2.4x diffraction-limited Nd:YAG laser for material processing

A 5.4 kW diode-pumped laser has been demonstrated with a beam quality of 2.4X the diffraction limit. This laser has achieved breakthrough capabilities in laser welding, producing high-quality single-pass welds in 65-mm steel and 50-mm titanium and aluminum.