Hiroshi Komine

Coherently coupled high-power fiber arrays

A four-element fiber array has demonstrated 470 watts of coherently phased, linearly polarized light energy in a single far-field spot. Each element consists of a single-mode fiber-amplifier chain. Phase control of each element is achieved with a Lithium-Niobate phase modulator. A master laser provides a linearly polarized, narrow linewidth signal that is split into five channels. Four channels are individually amplified using polarization maintaining fiber power amplifiers. The fifth channel is used as a reference arm. It is frequency shifted and then combined interferometrically with a portion of each channels signal. Detectors sense the heterodyne modulation signal, and an electronics circuit measures the relative phase for each channel. Compensating adjustments are then made to each channels phase modulator. This effort represents the results of a multi-year effort to achieve high power from a single element fiber amplifier and to understand the important issues involved in coherently combining many individual elements to obtain sufficient optical power for directed energy weapons. Northrop Grumman Corporation and the High Energy Laser Joint Technology Office jointly sponsored this work.

Brightness-Scaling Potential of Actively Phase-Locked Solid-State Laser Arrays

Recent progress in developing phased arrays of high-brightness solid-state lasers is summarized. We address the prospects for continued brightness-scaling via a model that extrapolates measured results to large numbers of array elements and provides a quantitative illustration of the features of coherent beam combination. This demonstrates that with present-day technology, both slab and fiber lasers have the capability to scale to unprecedented brightness levels.

Beam cleanup and low-distortion amplification in efficient high-gain hydrogen Raman amplifiers

We report the results of a theoretical and experimental investigation of two important properties of Raman converters. The first property is Raman beam cleanup, which refers to the generation of high-quality Stokes beams in a Raman amplifier pumped by laser beams of poor spatial quality. The second is nearly distortion-free amplification of aberrated Stokes beams in a Raman amplifier pumped by good-quality laser beams. A parallel beam geometry was used with collinear pump and Stokes beams. In both cases, excellent energy-conversion efficiencies into the first Stokes order, of the order of 60%, were demonstrated at high amplifier gains of 100 to 1000. The spatial characteristics of the amplified beams were found to be in good agreement with model predictions.

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.

Efficient H2 Raman conversion of long‐pulse XeF laser radiation into the blue‐green region

Efficient Raman conversion of microsecond pulse XeF laser radiation into the blue‐green region via the second Stokes shift in hydrogen has been demonstrated using a Raman oscillator‐amplifier scheme. Strong depletion of the pump and the first Stokes radiation accompanied by a dominant second Stokes output was observed for the first time.

Multi-kW near-diffraction-limited single-frequency Nd:YAG laser

Northrop Grumman is developing a laser architecture that can scale to >100 kW with a near-term goal of a 25 kW demonstration. The near-term 25 kW design is based on two chains of four slab amplifiers that produce average power of 12.5 kW each. Adaptive optics sense the output wavefront and piston relative to a reference, then adjust the phase of the master oscillator input to each chain to keep the wavefronts of each chain uniform and in phase. To reach the goal of 12.5 kW per chain, Northrop has demonstrated power scaling of individual amplifiers by extracting 4.5 kW form a single amplifier using a multimode resonator. This is well above the minimum needed to achieve 12.5 kW from a four-amplifier chain.

High Power Conversion to Mid-IR Using KTP and ZGP OPOs

A high average power, OPO based system has been developed for the purpose of generating output in the 2 - 5 µm mid-IR band. The system uses a cw diode array - pumped, Nd:YAG master oscillator power amplifier (MOPA) as the pump source and two tandem OPOs for wavelength conversion to the mid-IR. A Type II degenerate KTP OPO was used to convert the pump beam to 2.13 µm and a Type I near degenerate zinc germanium phosphite (ZGP) OPO was used to generate broadband radiation in the 3.7 - 4.1 and 4.4 - 4.8 µm wavelength range.

Noncritically phase matched mid-infrared generation in AgGaSe/sub 2/

A tunable mid-infrared optical parametric oscillator has been demonstrated using a noncritically phase matched AgGaSe/sub 2/ crystal with pump wavelength tuning. Average power experiments have generated several hundred milliwatts of 2.63 and 3.71 /spl mu/m radiation using a high-repetition-rate, multiwatt 1.54 /spl mu/m pump source. Interferometric measurements of the AgGaSe/sub 2/ crystal during operation indicated only minor thermal lensing. >

Average-power scaling for ultraviolet-pumped β-barium borate and lithium triborate optical parametric oscillators

Pulsed optical parametric oscillators for generating visible and near-IR output were investigated to determine the prospects of average-power scaling. The devices used β-barium borate and lithium triborate crystals. Experimental results show that the combination of a low-energy master oscillator and a higher-energy slave oscillator can be configured for devices of medium average power.

Efficient injection locking of an e‐beam‐excited XeF laser

Narrowband ultraviolet radiation from a frequency‐doubled dye laser has been used to control the linewidth and polarization of a long pulse length, electron‐beam‐excited XeF laser. Linewidths of 0.004 nm have been achieved in the 353‐nm band of the XeF B→X laser transition. Over 90% of the energy in the free‐running laser pulse has been extracted in the narrowband injection‐locked pulse. Ratios of injection signal power to output laser power on the order of 1:5000 are adequate for efficient injection‐locking to occur.