Mark A. Emanuel

115-W Tm:YAG diode-pumped solid-state laser

A compact diode-pumped Tm:YAG laser capable of generating greater than 100 W of CW power at 2 /spl mu/m has been demonstrated. A scalable diode end-pumping architecture is used in which 805-nm radiation, coupled to the wing of the Tm/sup 3+3/H/sub 6/-/sup 3/H/sub 4/ absorption feature, is delivered to the end of the laser rod via a lens duct. To facilitate thermal management, undoped YAG end caps are diffusion bonded to the central doped portion of the laser rod. For 2% and 4% Tm-doped rods of the same length, the lower doping level results in higher power, indicating that cross relaxation is still efficient while offering lower thermal stress and reduced absorption at the laser wavelength. Output powers for various output coupler reflectivities are compared to the predictions of a quasi-three-level model. Thermal lensing, cavity stability, and stress-induced birefringence measurements are described. The beam quality was analyzed with the 2% Tm-doped rod and a flat output coupler, yielding M/sup 2/ values of 14-23.

High-power dual-rod Yb:YAG laser

We describe a diode-pumped Yb:YAG laser that produces 1080 W of power cw with 27.5% optical optical efficiency and 532 W Q-switched with M2=2.2 and 17% optical–optical efficiency. The laser uses two composite Yb:YAG rods separated by a 90° quartz rotator for bifocusing compensation. A microlensed diode array end pumps each rod, using a hollow lens duct for pump delivery. By changing resonator parameters we can adjust the fundamental mode size and the output beam quality. Using a flattened Gaussian intensity profile to calculate the mode-fill efficiency and clipping losses, we compare experimental data with modeled output power versus beam quality.

Modular microchannel cooled heatsinks for high average power laser diode arrays

Detailed performance results for an efficient and low thermal impedance laser diode array heatsink are presented. High duty factor or CW operation of fully filled laser diode arrays is made possible at high average power. Low thermal impedance is achieved using a liquid coolant and laminar flow through microchannels. The microchannels are fabricated in silicon using an anisotropic chemical etching process. A modular rack-and-stack architecture is adopted for the heatsink design, allowing arbitrarily large two-dimensional arrays to be fabricated and easily maintained. The excellent thermal control of the microchannel cooled heatsinks is ideally suited to pump array requirements for high average power crystalline lasers. >

Anomalous temperature dependence of threshold for thin quantum well AlGaAs diode lasers

Thermally induced threshold wavelength shifts of 50 nm have been observed in short cavity length diode lasers fabricated from thin quantum well AlGaAs. Analysis suggests that the high‐energy radiation is generated by transitions between the n=2 level in the conduction band and the n=2 heavy hole level in the valence band. The threshold characteristic temperature (T0) of the laser material is found to be a strong function of cavity length.

High-average-power 1-/spl mu/m performance and frequency conversion of a diode-end-pumped Yb:YAG laser

Using a diode-end-pumped technology, a Yb:YAG laser capable of delivering up to 434 W of CW power has been demonstrated. The system incorporates a unique composite rod design which allows for high-average-power operation while simultaneously suppressing parasitic oscillations. Modeling and experimental data to support the quenching of parasitics are discussed. Beam quality measurements for CW operation with several cavity configurations are presented. In particular, beam quality measurements at 340-W CW yielded a beam quality factor of M/sup 2/=21. Predictions of a quasi-three-level model are compared with the experimental data for several output coupler reflectivities. An observed dependence of the cavity mode fill as a function of output coupler reflectivity is discussed. Employing a single acoustooptical switch, the system was Q-switched at 10 kHz and generated output powers up to 280 W with a measured beam quality of M/sup 2/=6.8 at 212 W, With an external dual-KTP crystal configuration, the Q-switched output was frequency converted to 515 nm and produced up to 76 W at 10 kHz in a 30-ns pulse length.

New ultrafast switching mechanism in semiconductor heterostructures

A new switching mechanism in a two‐terminal semiconductor heterolayer structure is proposed which capitalizes on nonlinear electron temperature effects in adjacent heterolayers. The estimated switching speed of an optimized heterostructure hot electron diode should be extremely fast, perhaps as fast as 200 fs. Data are presented on prototype devices which show the expected negative differential resistance and indicate that the basic physical model is correct.

Analysis of an intracavity-doubled diode-pumped Q-switched Nd:YAG laser producing more than 100 W of power at 0.532 µm

A diode-pumped Nd:YAG laser was frequency doubled to 0.532 microm with an intracavity KTiOPO(4) crystal in a V-cavity arrangement, achieving an output power of 140 W. Acousto-optic Q switching was employed at repetition rates of 10-30 kHz, and the beam quality was assessed at M(2) approximately 50. It was deduced on the basis of our model that the strength of the nonlinear frequency conversion is the main parameter determining the pulse width.

Impurity‐induced disorder‐delineated optical waveguides in GaAs‐AlGaAs superlattices

Layer intermixing in GaAs‐AlGaAs superlattices achieved by Zn thermal diffusion has been used to fabricate buried channel optical waveguides. Linear waveguides with small, abrupt bends were made and light was seen to be transmitted through these for bend angles up to 7.5 degrees corresponding to an effective index of refraction difference of 0.9% for a 50% average Al composition of the layers bounding the superlattice. For radiation between the band gap of bulk GaAs (870 nm) and the effective band gap of a 10‐nm GaAs quantum well (840 nm), the 3‐mm‐long waveguides appeared to be semitransparent suggesting a possible use in planar, buried channel all‐semiconductor integrated optoelectronics.

Diode-pumped ytterbium-doped Sr/sub 5/(PO/sub 4/)/sub 3/F laser performance

The performance of the first diode-pumped Yb/sup 3+/-doped Sr/sub 5/(PO/sub 4/)/sub 3/F (Yb:S-FAP) solid-state laser is discussed. An InGaAs diode array has been fabricated that has suitable specifications for pumping a 3/spl times/3/spl times/30 mm Yb:S-FAP rod. The saturation fluence for diode pumping was deduced to be 5.5 J/cm/sup 2/ for the particular 2.8 kW peak power diode array utilized in our studies. This is 2.5/spl times/ higher than the intrinsic 2.2 J/cm/sup 2/ saturation fluence as is attributed to the 6.5 nm bandwidth of our diode pump array. The small signal gain is consistent with the previously measured emission cross section of 6.0/spl times/10/sup -20/ cm/sup 2/, obtained from a narrowband-laser pumped gain experiment. Up to 1.7 J/cm/sup 3/ of stored energy density was achieved in a 6/spl times/6/spl times/44 mm Yb:S-FAP amplifier rod. In a free running configuration, diode-pumped slope efficiencies up to 43% (laser output energy/absorbed pump energy) were observed with output energies up to /spl sim/0.5 J per 1 ms pulse. When the rod was mounted in a copper block for cooling, 13 W of average power was produced with power supply limited operation at 70 Hz with 500 /spl mu/s pulses.

High-power InAlGaAs-GaAs laser diode emitting near 731 nm

High-power, reliable operation of an InAlGaAs-based QW laser diode structure emitting near 731 nm and having a strained InAlGaAs active region is described. Threshold currents for coated 100 /spl mu/m/spl times/1000 /spl mu/m devices are 281 mA, and a peak power conversion efficiency of 41% is measured. Internal losses are measured to be 1.2 cm/sup -1/. A system for fiber-coupling two-dimensional continuous-wave (CW) arrays of these devices is demonstrated.