Beam combining of laser arrays with high efficiency and good beam quality for power and radiance (brightness) scaling is a long-standing problem in laser technology. Recently, significant progress has been made using wavelength (spectral) techniques and coherent (phased array) techniques, which has led to the demonstration of beam combining of a large semiconductor diode laser array (100 array elements) with near-diffraction-limited output (M/sup 2//spl sim/1.3) at significant power (35 W). This paper provides an overview of progress in beam combining and highlights some of the tradeoffs among beam-combining techniques.

Laser beam combining for high-power, high-radiance sources

The fractional thermal loading, the ratio of heat generated to absorbed energy, at a 6.5 at.% doped Yb:YAG crystal diode pumped at 0.943 mu m has been measured to be >

Heat generation in Nd:YAG and Yb:YAG

Fiber lasers are well suited to scaling to high average power using beam-combining techniques. For coherent combining, optical phase-noise characterization of a ytterbium fiber amplifier is required to perform a critical evaluation of various approaches to coherent combining. For wavelength beam combining, we demonstrate good beam quality from the combination of three fiber amplifiers, and we discuss system scaling and design trades between laser linewidth, beam width, grating dispersion, and beam quality.

Beam combining of ytterbium fiber amplifiers (Invited)

Phase conjugation by stimulated Brillouin scattering represents a fundamentally promising approach for achieving power scaling of solid-state lasers. Following a summary of the power scaling problem and an overview of phase conjugation concepts, a review is presented of the application of phase conjugation to solid-state lasers. The author describes power scaling using demonstrated techniques for compensating static and thermally induced aberrations and depolarizations, as well as energy scaling by coherent coupling of multiple-gain media. Applications to diode-pumped lasers are discussed, as is a novel approach for power scaling of diode lasers themselves. Future research directions are indicated regarding conjugation fidelity at increasingly higher energies or with short-pulse and/or broadband lasers. >

A review of phase-conjugate solid-state lasers

We report to our knowledge the highest to date quasi-CW output power, 600 W and pulse energy, >1 J, for an InGaAs diode-pumped Yb:YAG laser. In separate preliminary results, we have also obtained 225 W of average output power under true CW diode pumping. This performance was obtained using a laser head designed to be part of a master oscillator power amplifier (MOPA) operating at 3 kW. We summarize why the diode-pumped Yb:YAG crystal laser is ideal for scaling to high average powers and the different approaches being pursued. We also report our latest results for side-pumped rod devices.

Low-heat high-power scaling using InGaAs-diode-pumped Yb:YAG lasers

We have experimentally demonstrated a laser energy-scaling technique, based on a phase-conjugate oscillator/amplifier scheme that permits the coupling of several parallel amplifiers to form a single coherent output beam. This, is achieved despite dramatic differences in the individual amplifier energies and optical path lengths. When many amplifiers are coupled together, energy can be scaled significantly beyond the limit imposed by the maximum available volume of a single active medium. Although the present demonstration utilized Nd:YAG, this technique is quite general and can be applied to a variety of laser media.

Coherent coupling of laser gain media using phase conjugation.

We observe shot-to-shot fluctuations in the far-field fidelity of stimulated Brillouin scattering phase-conjugate mirrors in the saturated reflectivity regime under various experimental conditions. The fluctuations, which are not seen in the reflectivity, reflect a partial breakdown in the discrimination mechanism that selectively amplifies the conjugate mode above the stimulated Brillouin scattering noise seed.

Stimulated Brillouin scattering phase-conjugation fidelity fluctuations

The authors have experimentally measured the energy stored and the heat generated in flashlamp-pumped Cr:Nd:GSGG for three Cr/sup 3+/ concentrations in the range of 1-2*10/sup 20/ ions/cm/sup 3/. It has been found that the energy storage efficiency in these samples is 1.7 times greater than that of the Nd:YAG sample, and the normalized heating parameter chi , defined as the heat deposited per unit of stored energy, is 2.5 under the specified pumping conditions, with no evident dependence on the Cr/sup 3+/ concentration. It has been been found that the measured chi value for the sample of Nd:YAG is 2.9 for the same pumping conditions. These observed chi values exceed expected values by factors of approximately 1.1 and 2 for the Cr:Nd:GSGG and Nd:YAG samples, respectively. The thermal focal length in the two materials was measured showing that the lensing is shorter in GSGG by a factor of 2.6 for the same available output power, or a factor of 4.5 for the same input pump power. The expected thermal lensing was determined using measured heat loads with no adjustable parameters, achieving satisfactory agreement with measured lensing values. >

Energy storage and heating measurements in flashlamp-pumped Cr:Nd:GSGG and Nd:YAG

We report the first demonstration to our knowledge of atmospheric compensation at extended ranges by stimulated Brillouin-scattering phase conjugation. With a field-of-view aperture product of 175 mm mrad, we studied horizontal ranges up to 6 km at a 2.5-m height above ground, with the refractive-index structure parameter Cn2 as large as 10−13 m−2/3. Compensation was achieved at all ranges, consistent with the turbulence-resolution capability of the 10-cm system aperture. The system was driven by frequency-doubled Nd:YAG lasers operating with ~10-ns pulses that were Raman shifted in gaseous hydrogen to the 683-nm operating wavelength. Raman amplification overcame as much as 85 dB of range and internal losses.

Real-time atmospheric compensation by stimulated Brillouin-scattering phase conjugation

Laser beam speckle effects are mitigated by frequency chirping a single-mode laser beam, reflecting it off a speckling surface, and then integrating the reflected beam over at least one chirp period. The speckle reduction is comparable to the use of a multi-longitudinal mode beam having a bandwidth similar to the chirped frequency excursion, while leaving the beam compatible with nonlinear optical processes that are not compatible with multi-longitudinal mode beams. Where the nonlinear process is stimulated Brillouin scattering (SBS) phase conjugation, the beam can be chirped at a rate up to the order of ##EQU1## where ΔνB is the gain bandwidth and L is the effective SBS interaction length of the phase conjugator, and C is the speed of light. Other nonlinear processes with which the beam may be used include harmonic generation and Raman-based beam cleanup or wavelength conversion.

David A. Rockwell

Papers

Coherent coupling of laser gain media using phase conjugation.

Stimulated Brillouin scattering phase-conjugation fidelity fluctuations

Energy storage and heating measurements in flashlamp-pumped Cr:Nd:GSGG and Nd:YAG

Real-time atmospheric compensation by stimulated Brillouin-scattering phase conjugation

Speckle resistant method and apparatus with chirped laser beam