Richard W. Solarz

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. >

Properties of Cr:LiSrAlF 6 crystals for laser operation

We have performed several physical and optical measurements on the Cr:LiSAF (LiSrAlF(6)) laser material that are relevant to its laser performance, including thermal and mechanical properties, water durabilities, and Auger upconversion constants. The expansion coefficient, Youngs modulus, fracture toughness, thermal conductivity, and heat capacity are all used to determine an overall thermomechanical figure of merit for the crystal. An investigation of the water durability suggests that the cooling solution should be maintained at pH = 7 to ameliorate problems associated with water dissolution. The Auger constant was found to become much more significant at higher Cr doping, in which excited-state migration leads to a substantial increase in the upconversion rate. We propose a design for a 50-W Cr:LiSAF laser system that is based on a detailed knowledge of all the relevant material parameters.

Demonstration of high-performance silicon microchannel heat exchangers for laser diode array cooling

A heat exchanger package has been demonstrated for semiconductor laser arrays using silicon microstructures with water as the coolant. A thermal impedance of 0.04 °C cm2/W has been achieved for a single linear bar. This design makes use of efficient, edge‐emitting laser diode arrays in a rack and stack architecture combined with a high‐performance silicon microchannel structure to allow cw operation. The architecture can be scaled to large areas and we project a thermal impedance of 0.09 °C cm2/W for close‐packed two‐dimensional arrays on this device.

First ionization potentials of lanthanides by laser spectroscopy

Photoionization and Rydberg spectra of ten lanthanides have been studied using stepwise laser excitation and ionization methods. These spectra were obtained from several different laser populated excited states in each case. Accurate ionization limits were derived from observed photoionization thresholds. Except for praseodymium, the observation of one or more long Rydberg progressions allowed more accurate limits to be determined. The Rydberg convergence values in eV are: Ce, 5.5387(4); Nd, 5.5250(6); Sm, 5.6437(6); Eu, 5.6704(3); Gd, 6.1502(6); Tb, 5.8639(6); Dy, 5.9390(6); Ho, 6.0216(6), and Er 6.1077(10). For praseodymium a threshold value of 5.464(+12-2) was obtained. When plotted against N, the lanthanide ionization limits normalized to correspond to ionization from the lowest level of fNs2 to the lowest level of fNs form two straight lines connected at the half-filled shell.

Radiative lifetimes, absorption cross sections, and the observation of new high-lying odd levels of 238 U using multistep laser photoionization*

We report the observation of over 100 new high-lying odd levels in atomic uranium between 32 600 and 34 200 cm−1 using tunable laser techniques. These augment the 32 odd levels previously determined in this energy regime by conventional atomic spectroscopy. The method used in these studies, multistep photoionization under pulsed dye laser excitation, was also employed to make J assignments, measure radiative lifetimes, and to obtain absorption cross sections of transitions involving these levels.

High average powers diode pumped slab laser

The authors have developed and tested stackable microchannel cooled laser bar diode pump packages suitable for direct pumping of slab lasers at high duty factor. A stack of 41 diode packages gives a pump array of 13.5 cm/sup 2/ and produces a peak power of 4000 W and an average power of 1000 W for an average irradiance of 75 W/cm/sup 2/. A high average power, total internal reflection face pumped Nd:YAG laser using 80 diode packages has been constructed. Preliminary testing of the slab laser using a 20% subset of diode packages arranged to pump a 4 mm*4 mm*80 mm volume of the slab has been completed. Seventy watts average power is obtained at 2.5 kHz pulse repetition rates and 100 mu s pulse widths. >

Scalable diode-end-pumping technology applied to a 100-MJ Q-switched Nd 3+ :YLF laser oscillator

A compact diode-end-pumped Nd3+:YLF laser oscillator capable of delivering 100 mJ of energy in a 4-ns pulse is demonstrated. A scalable pump architecture is used in which the output from a cylindrical-microlens conditioned-diode array is delivered to the laser rod via a lens duct. As a pump technology, this architecture may permit new applications for diode lasers that were previously not possible.

Optical absorption and stimulated emission of neodymium in yttrium orthosilicate

A spectroscopic investigation of the biaxial crystal yttrium orthosilicate doped with Nd/sup 3+/(Nd/sup 3+/:Y/sub 2/SiO/sub 5/) has been performed. Spectrally and orientationally resolved emission cross sections necessary for the evaluation of laser performance on the Nd/sup 3+/ /sup 4/F/sub 3/2/-/sup 4/I/sub 9/2/ and /sup 4/F/sub 3/2-//sup 4/I/sub 11/2/ transitions have been determined. The Judd-Ofelt theory has been applied to measured values of optical absorption line strengths to obtain the orientation averaged intensity parameters: Omega /sub 2/-3.34*10/sup -20/ cm/sup 2/, Omega /sub 4/=4.35*10/sup -20/ cm/sup 2/, and Omega /sub 6/=5.60*10/sup -20/ cm/sup 2/. These Judd-Ofelt intensity parameter values are significantly different from those previously reported by A.M. Tkachuk et al. Using these intensity parameters, the Nd/sup 3+/ /sup 4/F/sub 2/ metastable state lifetime is predicted to be 225 mu s. Measured low Nd concentration /sup 4/F/sub 3/2/ lifetimes of 214 mu s indicate a high radiative quantum efficiency. Because of the Stark level splitting of the Nd/sup 3+/ /sup 4/F/sub 3/2/ and /sup 4/I/sub 9/2/ manifolds, laser operation at twice one of the Cs atomic resonance filter acceptance wavelengths is possible. >

Passively Q-switched transverse-diode-pumped Nd 3+ :YLF laser oscillator

The design and performance of a diode-pumped Nd3+:YLF laser oscillator is described. A simple transverse-pump geometry in which a lensing duct efficiently couples the two-dimensional diode-pump array radiation to the YLF rod is employed. Using a color-center LiF crystal as a passive Q switch, we have produced burst-mode pulse trains that have a total energy of 115 mJ at a 30-Hz pulse-repetition frequency. The Q-switched pulse-train energy is 71% of the optimized free-lasing pulse energy, which is 163 mJ. Using an unstable cavity with a graded-reflectivity output coupler, we have generated Q-switched pulses that have 12-ns duration and near-diffraction-limited spatial profiles.

Microchannel heatsinks for high average power laser diode arrays

Detailed performance results and fabrication techniques for an efficient and low thermal impedance laser diode array heatsink are presented. High duty factor or even CW operation of fully filled laser diode arrays is enabled 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 a photolithographic pattern definition procedure followed by anisotropic chemical etching. 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 because of the stringent temperature demands that result from coupling the diode light to several nanometers wide absorption features characteristic of lasing ions in crystals.