Tso Yee Fan

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

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.

Room-temperature diode-pumped Yb:YAG laser

We have developed an efficient room-temperature ytterbium-doped YAG laser operating at 1.03 microm pumped by an InGaAs strained-layer diode laser operating at 968 nm. The threshold was 234 mW and 23 mW of output power was obtained for an absorbed pump power of 345 mW. This laser offers a number of advantages over AlGaAs pumped Nd:YAG lasers, such as broader absorption features, longer fluorescent lifetime, and lower thermal loading of the gain medium.

Effect of radiation trapping on fluorescence lifetime and emission cross section measurements in solid-state laser media.

Radiation trapping causes significant lengthening of the measured fluorescence lifetime in optically thin solid-state laser gain media, which leads to underestimates of the stimulated emission cross section by as much as 30%. A measurement technique is demonstrated that greatly reduces radiation trapping in Yb:YAG, a prototypical quasi-three-level laser medium. The radiative lifetime of the (2)F(5/2) manifold was measured to be 0.951 ms at 300 K, which is more than 10% lower than any previous measurement. This more-accurate radiative lifetime gives a 1.03-microm peak effective stimulated emission cross section of 2.3 x 10(-20) cm(2). Our measurement technique is expected to be most relevant for three-level and quasi-three-level laser media.

Cryogenic Yb

Cryogenically cooled solid-state lasers promise a revolution in power scalability while maintaining a good beam quality because of significant improvements in efficiency and thermo-optic properties. This is particularly true for Yb lasers because of their relatively low quantum defect and relatively broadband absorption even at cryogenic temperatures. Thermo-optic properties of host materials, including thermal conductivity, thermal expansion, and refractive index at low temperature, are reviewed and data presented for YAG (ceramic and single crystal), GGG, GdVO4, and Y2O3. Spectroscopic properties of Yb:YAG and Yb:LiYF4 (YLF) including absorption cross sections, emission cross sections, and fluorescence lifetimes at cryogenic temperatures are characterized. Recent experiments have pushed the power from an end-pumped cryogenically cooled Yb:YAG laser to 455-W continuous-wave output power from 640-W incident pump power at an of M2 1.4.

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Thermo-optic distortions often limit the beam quality and power scaling of high-average-power lasers. Cryogenically cooled Yb:YAG is used to efficiently generate 165 W of near-diffraction-limited beam from a power oscillator with negligible thermo-optic effects. End pumped with 215 W of incident pump power from two diode modules, the laser has an optical-optical efficiency of 76%, a slope efficiency of 85%, and an M2 value of 1.02.

-Doped Solid-State Lasers

A novel technique for scaling end-pumped lasers is demonstrated using three diode-laser arrays to pump a single Nd:YAG laser. Diffraction-limited output with a 58% slope efficiency was obtained. Simple arguments indicate that this method should allow at least an order of magnitude increase in pump power, compared with that from a single high-power diode array, to be focused into the same fixed-mode volume of the pumped laser. Such a scheme may be particularly useful with low-gain, quasi-three-level, or tunable lasers.

165-W cryogenically cooled Yb:YAG laser.

Five 500 W fiber amplifiers were coherently combined using a diffractive optical element combiner, generating a 1.93 kW beam whose M(2)=1.1 beam quality exceeded that of the inputs. Combining efficiency near 90% at low powers degraded to 79% at full power owing to thermal expansion of the fiber tip array.

Scalable, end-pumped, diode-laser-pumped laser

Aperture guiding, as opposed to thermal waveguiding, is shown to be the dominant mechanism for cavity mode stabilization in a quasi-three-level Yb:YAG laser in a flat-flat resonator. Excellent agreement is obtained between the experimental and modeled spot sizes in this laser when aperture guiding is included in the model. The models show that the spot size is insensitive to the exact form of the dependence of the transmission of the aperture on the transverse dimension. This finding is expected to affect other quasi-three-level lasers.

Diffractive coherent combining of a 2.5 kW fiber laser array into a 1.9 kW Gaussian beam

We demonstrate amplification of picosecond laser pulses to 40?mJ at a 2?kHz pulse repetition frequency (PRF) from a two-stage cryogenic chirped-pulse Yb:YAG amplifier, composed of a regenerative amplifier (RGA) and a two-pass booster amplifier. The RGA produces 8.2mJ of energy at 2kHz PRF and 13.2mJ at 1kHz PRF with excellent energy stability (approximately 0.3% rms) and beam quality (M(2)<1.1). Pulse stretching and compression are achieved by using a chirped fiber Bragg grating and a multilayer dielectric grating pair, respectively. Compressed 15?ps pulses from the RGA are obtained with a throughput efficiency of approximately 80% (approximately 6.5 mJ for 2kHz). The booster amplifier further amplifies the pulses to 40mJ at 2kHz PRF, and approximately 32 mJ, approximately 15 ps pulses are expected after compression. The amplifier chain seeded from a femtosecond Yb-fiber laser enables the optical self-synchronization between signal and pump in optical parametric chirped-pulse amplifier applications.

Aperture guiding in quasi-three-level lasers

Thermooptic effects often limit the power and beam quality of bulk-solid-state lasers. Cryogenically cooled (/spl sim/100 K) Yb:YAG lasers have been previously demonstrated to have relatively low thermooptic effects and high efficiency due to improved material properties at low temperatures. In this work, >300-W average power with M/sup 2//spl sim/1.2 and 64% optical-optical efficiency has been demonstrated from an end-pumped-rod geometry power oscillator. To our knowledge, this is the highest average power to date from a cryogenically cooled Yb:YAG laser.