Juan R. Ochoa

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

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.

165-W cryogenically cooled Yb:YAG laser.

A 21-/spl mu/m Ho:YAG laser end pumped by 1.9-/spl mu/m diode lasers has generated nearly 0.7-W CW output power. Laser operation was maintained even with Ho:YAG heat sink temperatures in excess of 60/spl deg/C. >

300-W cryogenically cooled Yb:YAG laser

Microchip lasers have produced single-frequency, gain-switched pulses with a FWHM as short as 760 psec for Nd:YAG (Nd(x)Y(3-x)Al(5)O(12)) and 80 psec for LiNdP(4)O(12) (LNP).

Ho:YAG laser pumped by 1.9-/spl mu/m diode lasers

Midinfrared InAs‐based and GaSb‐based semiconductor lasers with wavelengths from 3.3 to 4 μm have been used in a grating‐tuned external cavity configuration. At 80 K, a tuning range up to ∼8% of the center wavelength has been obtained. Power of 0.2 W peak, 20 mW average has been demonstrated for multimode operation with ∼1–2 nm linewidth.

Gain-switched pulsed operation of microchip lasers

This work describes a cryogenic, electro-optically <i>Q</i>-switched Yb:YAG laser that generates 114-W average TEM₀₀ power with 47% optical-to-optical efficiency. Pulse repetition frequency is 5 kHz, pulse duration is 16 ns full-width at half-maximum, and <i>M</i> ² is less than 1.05.

Broad wavelength tunability of grating‐coupled external cavity midinfrared semiconductor lasers

We are investigating the utility of UV light emitting diodes (LEDs) as the excitation source for fluorescence-based biological agent detection. These LEDs may enable the development of small and low-cost biological agent detectors. We have designed, and are currently fabricating, a test bed using UV LEDs for detecting biological agent aerosols. Using an experimental apparatus, we have measured the elastic scattering and fluorescence signals from single bacterial spores illuminated by low-power 408-nm laser diode radiation.

100-W

A cryogenic Yb amplifier using two laser materials, Gd3Sc2Al3O12 and Y3Al5O12 (YAG), has been used to obtain 70 W average power at 5 kHz pulse repetition frequency; the output was compressed to 1.6 ps, compared with an input compressible to 1.4 ps. The gain broadening obtained by combining two media enables shorter pulses than using Yb:YAG alone but retains the power-scaling advantages of cryogenic Yb:YAG.

Q

A number of double heterostructure and quantum well lasers with wavelengths approximately 3.1, 3.2, 3.4, 3.85 - 4.1, and 4.5 micrometers have been realized in InAsSb/GaSb and HgCdTe/CdZnTe material systems. Peak powers at the few W level and average power at the few hundred mW-level were obtained from optically pumped broad-area lasers at >= 80 K. Threshold, efficiency, internal loss, and gain saturation studies are reported. A compact laser package was built, using a high-power diode array for pumping and a Stirling pump for cooling. Its performance with a 4-micrometers laser is described.