We have assembled and tested a diode-pumped, phase conjugated Nd:YAG master oscillator power amplifier (PC MOPA) operating at an average power of 100 Watts. One joule per pulse has been extracted at a repetition rate of 100 Hz with a beam quality (BQ) of 1.1 x diffraction limited (D.L.). This combination of average power and beam quality makes this the brightest short pulse solid-state laser reported to date. The optical efficiency of 22% and the overall efficiency of 9.4% also represent record performance for high energy short pulse lasers. Excellent spatial uniformity and a pulse length of 7 ns make this laser ideal for frequency doubling and parametric conversion.
High average power Nd:YAG lasers are increasingly interesting for industrial applications such as drilling and machining. Diode pumping of this solid state medium offers longer services intervals, reduced thermal optical distortions, higher system efficiency and more compact packaging than lamp pumping. The zigzag slab geometry is well suited for applications where the average power exceeds a few hundred watts and a good beam quality is desired, particularly if the laser pumping level is to be varied. We present the status of our latest upgrade to our (originally 300 watt1) diode pumped slab laser. In what follows we first describe the diode pump source. We then discuss the zigzag slab laser design and its present performance.
To achieve the design goal of 50 watts cw in a single-frequency, fundamental mode we have chosen a slab laser design. The slab geometry with a zig-zag optical path eliminates stress birefringence and thermal focusing to first order in a uniformly pumped, ideal slab. However, at the thermal loads we are considering, higher order effects become significant. We have developed a computer program to analyze the full three dimensional behavior of the thermally loaded slab and have used this code in an attempt to minimize the wavefront distortion of a beam as it traverses the slab. The resonator design is also critical in achieving single- frequency, fundamental mode operation with high extraction efficiency. Because of the rectangular geometry and size of the slab laser, we have chosen to build a stable-unstable resonator. Single frequency operation is obtained by injection locking a ring cavity. Mode selection is achieved in the wide transverse direction of the slab by using an unstable cavity with a super-Gaussian mirror. An unstable resonator supports large mode volumes and has large discrimination against higher order transverse modes. In addition, a super-Gaussian mirror profile provides efficient extraction in a high quality mode. Our resonator design should lead to efficient operation in a near TEM00 mode with a slope efficiency approaching those of slab lasers with multimode output.
Diode-pumped solid-state lasers with average power output of up to 200 W and peak power to 100 MW have been developed for military and commercial applications. This paper discusses engineering issues related to scaling solid-state lasers to the kW level while maintaining high beam quality, high nonlinear conversion efficiency, and avoiding optical damage.
One way of achieving high-power diode-end pumped lasers is to angularly multiplex several diodes on each end of the laser rod. We have successfully multiplexed four 15 W diode arrays on each end of a 6.3 mm diameter X 7.5 mm long Nd:YAG rod to produce an approximately equals 2 mm diameter pump spot. Higher laser power was achieved by adding a second laser rod pumped at both ends. The addition of the second rod facilitates thermally induced birefringence compensation by introducing a quartz polarization rotator between the rods. In addition, it was necessary to add an aspheric lens to compensate the thermal aberration induced at these high, nonuniform pump powers. With this arrangement, > 90 W has been extracted multimode, and > 60 W in a near-diffraction-limited beam.
Current results for diode pumped solid state lasers are driven by advances in diode laser technology. Diode arrays in the 10 - 20 watt range are now available in useful formats which allow coupling into side- and end-pumped laser configurations. Side-pumped designs have traditionally produced higher output power at the expense of mode quality. End-pumped devices, on the other hand, have shown high mode quality but have been limited in their output power or pulse energy. Results are given which demonstrate new end-pumped laser coupling and cavity configurations which allow both high mode quality and increased power output.
The influence of thermal lensing and mode matching on the oscillating mode in an end pumped Nd:YAG laser is investigated. Thermal lensing was measured by probing the output beam divergence of a dynamically stable resonator. The magnitude of lensing expected due to thermal dispersion was confirmed in a distinct range of adequate mode matching. Measurements on arrangements which typically exhibit higher order ring mode oscillation were performed, and compared to a refined model. The refined model is based on a modified numerical calculation of the eigenstate of the electrical field in the active resonator. The active medium is described by a general complex aperture transmission function. This function defines the transverse mode discrimination in the active resonator. Finally the conditions for fundamental mode oscillation without the use of aperture stops in the active resonator are estimated.
We report the scaling considerations, design details, and experimental results for an efficient, high-power, diode-pumped laser. This Nd:YAG laser produces a cw output power of 60 W in a near-diffraction-limited beam (i.e., M2 < 1.3). In multimode operation, the laser produces an output power of 92 W. The optical-to-optical efficiency is 26% for TEM00 operation and 44% for multimode operation (based on the diode output power). The near- diffraction-limited average power in Q-switched operation is 40 W at a repetition rate 10 kHz. Polarized output powers of 40 W and extinction ratios of 40:1 have also been obtained for cw TEM00 operation.
Compared to other Nd3+ doped materials such as YAG:Nd3+, YAP:Nd3+, and YLF:Nd3+, crystal LaxNd1-xMgAl11O19 (LNA) has relatively much higher Nd concentration, long upper state lifetime, and large absorption bandwidths. LNA crystals have been pumped by laser diode arrays emitting around 800 nm. High-quality LNA crystals for this purpose have been grown.
Solid-state diode pump lasers are placing new design criteria for intracavity electro-optics. Potassium titanyl phosphate (KTP) material properties and limitations for Q-switching and modelocking diode pumped systems are discussed. Specific applications include low jitter synchronization to an electronic reference (phase modulation) and high average power Q- switching and regenerative pulse amplification (amplitude modulation).
A cw-pumped Nd:YAG oscillator followed by a cw-pumped, multipass, travelling wave amplifier using a face-pumped, total internal reflection (TIR) Nd:YAG slab as the gain element, has produced a TEM00, diffraction limited beam with up to 150 W of average power. The oscillator has been operated in the Q-switched, modelocked, and Q- switched/modelocked output formats. When the oscillator is Q-switched at a rate of 12 kHz, the system produced over 90 W of average power; with a Q-switched/modelocked oscillator at a Q-switch rate of 10 kHz the system produced over 80 W (50 W at 3 kHz). The output of this laser system has been used to generate the 532 nm second harmonic in lithium triborate (LBO), and to synchronously pump a degenerate OPO which utilized six potassium titanyl phosphate (KTP) crystals as nonlinear converters.
Elevated temperature operation of pump arrays holds the possibility for utilizing passive cooling of lasers, even in environments with high ambient temperatures. Traditionally, such arrays have used active cooling to maintain junction temperatures near 30 degree(s)C. Although diode junctions operating at elevated temperatures would still require thermostating to maintain the required output wavelength, simpler thermal control systems could be employed. System level weight and power advantages can be realized through the use of such diode arrays. In this paper, elevated temperature operation of laser diode arrays at up to 90 degree(s)C is reported. These arrays were operated at conditions that would yield up to 1300 W/cm2 at the face of a pumping array. In order to address a prime concern in elevated temperature operation, an initial lifetest of these devices is reported. The devices operated for over 350 million shots without catastrophic failure and exhibited a degradation rate of about 4.4% in output power at constant drive current per 100 million shots.
Recent advances in diode pump sources include higher efficiency, higher power, longer lifetimes, wider wavelength coverage, and higher operating temperatures. In addition, advances in diode array packaging have provided for higher average power capability and an optical format that is more convenient for a variety of pump applications. We discuss advances in laser performance as well as advances in packaging technology with an emphasis on pumping of high average power solid state lasers.
Simple thermal management of large multi-bar laser diode arrays can be accomplished by directly mounting edge emitting laser material directly into a grooved substrate, thereby greatly reducing the part count and substantially reducing the thermal path. The laser mounts can be `Flat' for pumping `Slabs' or `Curved' for pumping `Rods' or as a base for fiberoptic pump applications. There are three basic concepts for fabrication of laser diode arrays using the bars in grooves assembly methodology. These are best described as: monolithic, composite, and doped.
The development of separate confinement, heterostructure, semiconductor lasers diodes continues to be facilitated by the use of models. These models allow one to examine how the performance of a laser diode depends on epistructure growth and device processing parameters. In this way one can determine the optical and thermal constraints on a semiconductor laser's performance. For pulsed operation at low duty factor this is generally sufficient to design a laser diode system. Although the models are a good predictor of the initial performance of laser diode bars operated either in cw or a high duty factor mode, the performance degrades during the first 100 hours of operation. In our investigation of the design of cw operating, AlGaAs laser diode bars we have found that degradation mechanisms along with thermal properties provide design constraints. The problem which we have encountered is that a laser diode bar generally contains many independently emitting laser diodes. Each laser diode bar therefore has an unscreened population of laser diodes. In an unscreened population of laser diodes there are generally some which exhibit `infant mortality,' i.e., they cease lasing in the first 50 to 100 hours of on time. We have found that the `infant mortalities' among the laser diodes can significantly lower the yield of bars by reducing the number of emitting laser diodes on the bars below acceptable limits. The sources of this initial, rapid degradation of laser diode bars are discussed.
Detailed performance results for an efficient and low thermal impedance laser diode array heatsink are presented. High duty factor and even cw operation of fully filled laser diode arrays at high stacking densities are 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 an anisotropic chemical etching process. A modular rack-and- stack architecture is adopted for heatsink design, allowing arbitrarily large two-dimensional arrays to be fabricated and easily maintained. The excellent thermal control of the microchannel heatsinks is ideally suited to pump array requirements for high average power crystalline lasers because of the stringent temperature demands that are required to efficiently couple diode light to several-nanometer-wide absorption features characteristic of lasing ions in crystals.
We have investigated diode pumped thulium and holmium YAG lasers. Comparisons of laser performance for several different Tm/Ho concentrations and the results of a prototype Q- switched laser are reported.
The development of a high brightness diode laser pump source and a scalable end-pumped resonator design are key issues in achieving high average power diode-pumped solid-state lasers without sacrificing efficiency and beam quality. The above is particularly true for the low gain, quasi-three level 2 micrometers solid-state laser systems. Spatial multiplexing of multiple diode lasers into a single optical fiber creates a high power, versatile pump source which is compatible with end-pumping. A diode laser pump module was built that produced 15 W of 785 nm light out of a 400 micrometers core diameter multimode fiber within numerical aperture of 0.25. A periodically imaged resonator (PIR) incorporates multiple end-pumping ports for high efficiency average power scaling, yet can be made very resistant to the adverse thermal effects of end-pumping. Fifteen watts of continuous wave, fundamental mode, 2 micrometers radiation were obtained from a 4% Tm:YAG PIR end-pumped with 46.5 W of fiber- coupled 785 nm diode laser emission.
This work presents a procedure to design an aspherical corrective mirror to remove the effects of thermally induced optical aberrations in end-pumped solid-state lasers. The design is based on solving the inverse problem of bending of a thin plate of variable thickness, i.e., given the plate deflection profile a thickness profile must be calculated by solving the differential equation for bending. The advantage of this type of aberration correction is the fact that it can be scaled to different pump powers during operation while still matching the aspherical profile in question. Guidelines for fabrication of the mirror are also presented.