R. L. Aggarwal

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.

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.

^{3+}

Thermo-optic materials properties of laser host materials have been measured to enable solid-state laser performance modeling. The thermo-optic properties include thermal diffusivity (β), specific heat at constant pressure (Cp), thermal conductivity (κ), coefficient of thermal expansion (α), thermal coefficient of the optical path length (γ) equal to (dO∕dT)∕L, and thermal coefficient of refractive index (dn∕dT) at 1064nm; O denotes the optical path length, which is equal to the product of the refractive index (n) and sample length (L). Thermal diffusivity and specific heat were measured using laser-flash method. Thermal conductivity was deduced using measured values of β, Cp, and the density (ρ). Thermal expansion was measured using a Michelson laser interferometer. Thermal coefficient of the optical path length was measured at 1064nm, using interference between light reflected from the front and rear facets of the sample. Thermal coefficient of the refractive index was determined, using the measured val...

-Doped Solid-State Lasers

Wavelength beam combining of five ytterbium fiber lasers is demonstrated in a master-oscillator power-amplifier configuration at combined powers up to 6 W. The combined beam profile has an M2 value of 1.14, which is equal to that of an individual fiber. Beam steering in one dimension over 140 resolvable spots is also demonstrated.

Measurement of thermo-optic properties of Y3Al5O12, Lu3Al5O12, YAIO3, LiYF4, LiLuF4, BaY2F8, KGd(WO4)2, and KY(WO4)2 laser crystals in the 80–300K temperature range

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.

Wavelength beam combining of ytterbium fiber lasers.

The vertical-gradient-freeze technique has been used to grow laser-quality Ti:Al/sub 2/O/sub 3/ single crystals. Ti/sup 3+/-Ti/sup 4+/ pairs have been shown to be responsible for the residual infrared absorption. Room-temperature oscillator and amplifier experiments are reviewed. >

165-W cryogenically cooled Yb:YAG laser.

Avalanche photodiodes have been demonstrated utilizing GaN grown by hydride vapor-phase epitaxy. Spatially uniform gain regions were achieved in devices fabricated on low-defect-density GaN layers that exhibit no microplasma behavior. A uniform multiplication gain up to 10 has been measured in the 320–360 nm wavelength range. The external quantum efficiency at unity gain is measured to be 35%. The electric field in the avalanche region has been determined from high-voltage C–V measurements to be ∼1.6 MV/cm at the onset of the multiplication gain. Electric fields as high as 4 MV/cm have been measured in these devices. Response times are found to be less than 5 μs, limited by the measurement system.

Crystal growth, spectroscopy, and laser characteristics of Ti:Al/sub 2/O/sub 3/

Data for as-grown and partially oxidized samples of Ti:Al/sub 2/O/sub 3/ grown by the vertical-gradient-freeze technique show that the residual infrared absorption in these samples is largely due to Ti/sup 3+/-Ti/sup 4+/ pairs. In agreement with this pair model, the residual absorption in as-grown samples has been substantially decreased by annealing in a reducing atmosphere. Data for an as-grown crystal grown by the heat exchanger method indicate the presence of a second mechanism for residual absorption that may set a lower limit on the ratio of this absorption to the Ti/sup +3/ absorption used to pump laser emission. >

GaN avalanche photodiodes grown by hydride vapor-phase epitaxy

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.

Residual infrared absorption in as-grown and annealed crystals of Ti:Al/sub 2/O/sub 3/

Pulses of 193 nm radiation from an ArF laser with energies exceeding 0.5 J/cm2 have been shown to modify 40–60 nm thick layers of {100} and {110} oriented diamond surfaces. These layers exhibit highly anisotropic electrical and optical properties which have principal in‐plane axes along the 〈110〉 directions. The minimum resistance is (4–10)×10−4 Ω cm, and minimum in the optical transmittance and maximum in the reflectance occur when the electric field vector of the incident polarized light is aligned along the low resistance direction. Transmission electron microscopy indicates that the modified layer primarily consists of unidentified graphite‐like carbon phases embedded in diamond. The first‐order electron diffraction spots correspond to lattice spacings of 0.123, 0.305, and 0.334 nm. The modified layer is stable at 1800 °C, forms ohmic contacts to type IIb diamond, and supports epitaxial diamond growth.