Barry L. Freitas

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. >

The mercury project : A high average power, gas-cooled laser for inertial fusion energy development

Abstract Hundred-joule, kilowatt-class lasers based on diode-pumped solid-state technologies, are being developed worldwide for laser-plasma interactions and as prototypes for fusion energy drivers. The goal of the Mercury Laser Project is to develop key technologies within an architectural framework that demonstrates basic building blocks for scaling to larger multi-kilojoule systems for inertial fusion energy (IFE) applications. Mercury has requirements that include: scalability to IFE beamlines, 10 Hz repetition rate, high efficiency, and 109 shot reliability. The Mercury laser has operated continuously for several hours at 55 J and 10 Hz with fourteen 4 × 6 cm2 ytterbium doped strontium fluoroapatite amplifier slabs pumped by eight 100 kW diode arrays. A portion of the output 1047 nm was converted to 523 nm at 160 W average power with 73 % conversion efficiency using yttrium calcium oxy-borate (YCOB).

High-average-power femto-petawatt laser pumped by the Mercury laser facility

The Mercury laser system is a diode-pumped solid-state laser that has demonstrated over 60 J at a repetition rate of 10 Hz (600 W) of near-infrared light (1047 nm). Using a yttrium calcium oxyborate frequency converter, we have demonstrated 31.7 J/pulse at 10 Hz of second harmonic generation. The frequency converted Mercury laser system will pump a high-average-power Ti:sapphire chirped pulse amplifier system that will produce a compressed peak power > 1 PW and peak irradiance > 1023W/cm2.

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.

Silicon monolithic microchannel-cooled laser diode array

A monolithic microchannel-cooled laser diode array is demonstrated that allows multiple diode-bar mounting with negligible thermal cross talk. The heat sink comprises two main components: a wet-etched Si layer that is anodically bonded to a machined glass block. The continuous wave (cw) thermal resistance of the 10 bar diode array is 0.032 °C/W, which matches the performance of discrete microchannel-cooled arrays. Up to 1.5 kW/cm2 is achieved cw at an emission wavelength of ∼808 nm. Collimation of a diode array using a monolithic lens frame produced a 7.5 mrad divergence angle by a single active alignment. This diode array offers high average power/brightness in a simple, rugged, scalable architecture that is suitable for large two-dimensional areas.

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.

Applications of microlens-conditioned laser diode arrays

The ability to condition the radiance of laser diodes using shaped-fiber cylindrical-microlens technology has dramatically increased the number of applications that can be practically engaged by diode laser arrays. Lawrence Livermore National Laboratory (LLNL) has actively pursued optical efficiency and engineering improvements in this technology in an effort to supply large radiance-conditioned laser diode array sources for its own internal programs. This effort has centered on the development of a modular integrated laser diode packaging technology with the goal of enabling the simple and flexible construction of high average power, high density, two-dimensional arrays with integrated cylindrical microlenses. Within LLNL, the principal applications of microlens-conditioned laser diode arrays are as high intensity pump sources for diode pumped solid state lasers (DPSSLs). A simple end-pumping architecture has been developed and demonstrated that allows the radiation from microlens- conditioned, two-dimensional diode array apertures to be efficiently delivered to the end of rod lasers. This architecture enables the generation of pump bemas that are scalable in absolute power with intensities approaching 100 kW/cm2. To date, pump powers as high as 2.5 kW have been delivered to 3 mm diameter laser rods. Such high power levels are critical for pumping solid state lasers in which the terminal laser level is a Stark level lying in the ground state manifold. Previously, such systems have often required operation of the solid state gain medium at low temperature to freeze out the terminal laser Stark level population, so as to minimize losses resulting from reabsorption of the laser radiation. The necessity of low temperature operation has rendered such systems impractical for many applications. Our recently developed high intensity pump sources overcome this difficulty by effectively pumping to much higher inversion levels, allowing efficient operation at or near room temperature. Because the end-pumping technology is scalable in absolute power, the number of rare-earth ions and transitions that can be effectively accessed for use in practical DPSSL systems has grown tremendously. Unique laser systems for applications in fields such as medicine and remote sensing can now be simply realized. We have also been involved in programs to evaluate the use of direct diodes for material processing applications. Here, diode radiation from an extended two-dimensional microlens-conditioned array is focused and delivered directly onto a work piece. Systems based on this concept can be utilized in the heat treating and hardening of metals. Another application of microlens-conditioned laser diode arrays is in the direct coupling of their radiation to optical fibers. Direct diode-to-fiber coupling has recently been demonstrated for a medical application in which 22 W of cw 690 nm radiation was delivered from a microlens-conditioned stack of AlGaInP laser diode bars through a 1 mm core fused silica fiber. This approach used a simple and inexpensive 1 cm focal length lens to direct the microlens-conditioned radiation from the diode stack into the optical fiber.

ND:GLASS LASER DESIGN FOR LASER ICF FISSION ENERGY (LIFE)

Abstract We have developed preliminary conceptual laser system designs for the Laser ICF (Inertial Confinement Fusion) Fission Energy (LIFE) application. Our approach leverages experience in high-energy Nd: glass laser technology developed for the National Ignition Facility (NIF)1, along with high-energy-class diode-pumped solid-state laser (HEC-DPSSL) technology developed for the DOE’s High Average Power Laser (HAPL) Program and embodied LLNL’s Mercury laser system.2 We present laser system designs suitable for both indirect-drive, hot spot ignition and indirect-drive, fast ignition targets. Main amplifiers for both systems use laser-diode-pumped Nd:glass slabs oriented at Brewster’s angle, as in NIF, but the slabs are much thinner to allow for cooling by high-velocity helium gas as in the Mercury laser system. We also describe a plan to mass-produce pump-diode lasers to bring diode costs down to the order of