William A. Neuman

Design and operation of a 150 W near diffraction-limited laser amplifier with SBS wavefront correction

The design and operation of a Nd:glass regenerative amplifier using a stimulated Brillouin scattering (SBS) phase conjugate mirror is presented. The system can be operated at 25-30 J per pulse with a pulse width of 14 ns and a pulse repetition frequency (PRF) of 6 Hz. This results in an average output power of >150 W with a peak power of 2 GW. The experimentally measured divergence of the amplifier output is 1.25/spl times/ the diffraction limit and it can be frequency doubled with >80% efficiency. The detailed considerations required for this specific amplifier design are discussed as well as how these considerations apply to the design of high average power, high beam quality laser systems in general. >

Effect of Cavity Design on Optical Parametric Oscillator Performance

The effect of resonator cavity design on parametric oscillator performance is investigated theoretically. Certain unstable resonators produce superior energy conversion and beam quality than traditional resonators.

Multi-wavelength injection seeded mid-infrared optical parametric oscillator for DIAL

We have constructed and fielded a multi-wavelength injection seeded mid-IR OPO source for DIAL. This OPO system was built for ground based remote sensing measurements of species with both broad (300 cm-1) and narrow absorption bandwidth (0.07 cm-1 FWHM). The OPO utilizes a single frequency tunable diode laser for the injection seeded signal wavelength in the region from 6400 to 6700 cm-1 and an angle phase-matched 5 cm LiNbO3 crystal to provide large tuning excursions on a slow time scale. The pump was a diode pumped Nd:YAG MOPA (9398 cm-1) running at 180 Hz. This pump source was repeatedly injection seeded with a different wavelength on each of three sequential shots forming a set of three pulses having wavelength separations on the order of 0.4 cm-1 at a three color set repetition rate of 60 Hz. This combination of OPO signal and pump source produced a set of three time staggered idler wavelengths separated by 0.4 cm-1 with the center wavelength tunable from 2700 to 3000 cm-1. This OPO system was used in field test experiments to detect the release of chemicals from a standoff distance of 3.3 Km. We present key OPO design criteria, performance data, and numerical simulations that agree with our observation of pump induced spectral impurities in the OPO output.

Unique high-power fiber laser technologies

Continuous wave (CW) fiber laser systems with output powers in excess of 500 W with good beam quality have now been demonstrated, as have high energy, short pulse, fiber laser systems with output energies in excess of 1 mJ. Fiber laser systems are attractive for many applications because they offer the promise of high efficiency, compact, robust systems. We have investigated fiber lasers for a number of applications requiring high average power and/or pulse energy with good beam quality at a variety of wavelengths. This has led to the development of a number of custom and unique fiber lasers. These include a short pulse, large bandwidth Yb fiber laser for use as a front end for petawatt class laser systems and a narrow bandwidth 0.938 μm output Nd fiber laser in the > 10 W power range.

Energy scaling of SBS pulse compression

A two-cell stimulated Brillouin scattering (SBS) pulse compressor design is presented that can be scaled to large laser pulse energies and a numerical model has been developed which accurately predicts the performance of this pulse compressor system over a wide range of operating parameters. The compression of a 2.5 J input pulse from a width of 15.8 ns to 1.7 ns is demonstrated with 80% energy efficiency.

Midwave infrared DIAL noise phenomenology

LLNL has utilized optical parametric oscillator technology to develop and field a rapidly-tunable mid-wave IR DIAL system. The system can be tuned at up to 1 KHz over the 3.3- 3.8 micron spectral region, where hydrogen-bond stretching modes provide spectroscopic signatures for a wide variety of chemicals. We have fielded the DIAL system on the LLNL site on range, turbulence, and receiver aperture size. In this paper we describe the interplay of turbulence and speckle to produce the observed nose fluctuations at short range.

Fidelity fluctuation in SBS phase conjugation at high input energies

The shot-to-shot phase fidelity of an SBS phase conjugator operated many times above threshold has been found to be very sensitive to the slope of the leading edge of the input pulse. For a pulse with a rising edge that is short relative to the acoustic lifetime of the SBS medium, strong random fluctuations in the fidelity of the wavefront reversal are observed. However, by tailoring the leading edge of the pulse relative to the acoustic response time of the medium, good phase reproduction can be achieved. No increase in shot-to-shot fidelity fluctuation was observed using a carbon tetrachloride SBS cell at input energies up to 100X threshold, resulting in reflectivities of 90%. Conclusions are made about the source of the observed random fluctuations which are supported both by experimental measurements and numerical modeling.

OPO performance with an aberrated input pump beam

The performance of an optical parametric oscillator (OPO) with non-ideal input pump fields is investigated numerically. The analysis consists of a beam propagation calculation based on Fourier methods including walk-off in the non-linear crystal coupled with the three-wave interaction in the crystal. The code is time dependent enabling analysis of laser pulses. The pump beam aberrations are described by Zernike polynomials. The OPO investigated is a LiNbO3 crystal in a flat-flat resonator. The LiNbO3 crystal is cut to produce a 1.5 micrometers signal and 3.6 micrometers idler from a 1.06 micrometers input pump field. The results show that the type of aberration is significant when predicting the output performance of the OPO and not simply the beam quality or M2 angular divergence of the pump beam. While thresholds for input pump beams with M2 equals 2 only increase on the order of 10% over unaberrated beams, the divergence of the output fields can be much worse than the pump beam divergence. The output beam divergence is also a function of the input pump energy. Aberrated pump fields can also lead to angular displacements between the generated signal and idler fields.