Many laser applications now require sources providing high average powers in a high-quality diffraction-limited beam either for continuous wave (CW) operation or for the nanosecond and even picosecond pulsed regimes. These applications include high-precision micromachining, semiconductor processing, laser displays, and visible or UV harmonic generation. Diode-pumped solid-state lasers (DPSSLs) most often represent the best solution in terms of wall-plug efficiency, simplicity, reliability, compactness, and ease of use; however, there is always a tradeoff between output power and beam quality.
End-pumped DPSSLs based on neodymium-doped orthovanadate (Nd:YVO4) crystals have become an attractive solution for most commercial lasers delivering from a few watts to several tens of watts in a diffraction-limited beam. The key advantages of Nd:YVO4 over other popular gain media such as Nd:YAG reside in its high gain, high diode-pump-light absorption, and constantly polarized output. These features allow the realization of high-efficiency oscillators or amplifiers, high repetition-rate Q-switched oscillators providing short nanosecond pulses, and quasi-CW high-repetition-rate mode-locked lasers. However, their output power is limited by strong thermal and lensing effects in the vanadate crystal. We present a novel pumping technique that retains Nd:YVO4's advantages while extending its output power capabilities.
Vanadate is often pumped by fiber-coupled diodes emitting around its absorption peak at 808nm (see Figure 1). Recently, it has also been pumped at 880nm, which provides the added advantage of a reduced quantum defect. At these wavelengths, howevewr, the pump light polarized along the c-axis of the crystal is strongly absorbed, while the portion polarized along the a-axis has an absorption coefficient about four times smaller. This leads to a very strong thermal load in the first millimeters of the crystal, resulting in a bulging of the input face of the crystal, severe local heating, and thus a strong aberrated thermal lens that effectively limits the output power.
Figure 1. Nd:YVO4 absorption spectrum for a 1% doping concentration. Red trace: c-axis polarization. Blue trace: a-axis polarization
Pumping at 888nm, in contrast, provides low and isotropic absorption, with equal coefficients for a and c-polarized light. This leads to a much smoother absorption with a reduced front-face heat load and temperature, and polarization-insensitive absorption, so that the uncontrolled pump polarization state emerging from the fiber does not affect laser performance. Long crystals with reliable doping levels can now be used in conjunction with high-brightness fiber-coupled diodes to achieve a large and uniform pump volume.1,2
We implemented this technique in the resonator illustrated in Figure 2. A 30mm-long a-cut YVO4 crystal doped at 0.5% with Nd is end-pumped by a 400μm fiber-coupled diode with a numerical aperture of 0.22 focused on a 1350μm-diameter spot. The non-absorbed pump light is retro-reflected by a lens/mirror combination (L, M3) that provides a long and smooth pump volume. The resonator is formed between a highly reflective (HR) end-mirror and a 40% transmission output coupler (OC), folded by convex pump mirrors M1 and M2 for thermal lens compensation.
Figure 2. TEM00 cavity setup with double-pass pump absorption. HR: highly reflective mirror. Mx: mirrors. L: lens. OC: output coupler. N.A.: numerical aperture.
The system was optimized to provide 60W of output power when pumped with a total of 108W, corresponding to an optical efficiency of 55%. The output beam was round and diffraction-limited with M2=1.05. The output of this oscillator was further amplified in a single pass through an identical pump/crystal configuration, achieving 58W of extracted power and a 53% optical efficiency, for a total output power of 117W in an M2=1.05 beam (see Figure 3). Based on this simple high-power oscillator, we realized various systems covering a wide range of applications. First, intracavity-doubling a two-crystal resonator provided 62W of low-noise green light at 532nm.3 Next, cavity-dumping a Q-switched oscillator provided up to 47W with a constant 6ns-pulse at repetition rates up to 100kHz,4 which avoids the pulse-lengthening usually associated with the increasing repetition rate in Q-switched oscillators, as illustrated in Figure 4. Finally, we realized a mode-locked master-oscillator power-amplifier (MOPA), providing 111W of power in a 110MHz, 33ps pulse train.5 This output was effectively frequency-doubled to 87W of green light and frequency-tripled to 35W of 355nm UV light.
Figure 3. TEM00 beam (M2=1.05) and corresponding vertical profile of the output of the mode-locked master-oscillator power-amplifier.
Figure 4. Pulse duration vs. repetition rate during Q-switched (blue squares) and cavity-dumped Q-switched (red dots) operation.
Pumping Nd:YVO4 at 888nm allows the fabrication of a wide range of laser sources in CW, or in the nanosecond or picosecond pulsed regimes, up to powers in the 100W range in a diffraction-limited beam. Our approach benefits from the simplicity of end-pumped vanadate technology, and avoids the drawbacks and limitations of more recent high-power technologies such as disks or fibers.
Louis McDonagh, Richard Wallenstein
Department of Physics
Technical University of Kaiserslautern
Louis McDonagh graduated in optical engineering from the Ecole Supérieure d'Optique in Orsay, France. His current research as a graduate student is focused on the development of high-power, high-beam-quality solid-state lasers.