New Gluing Technique Rejuvenates Abandoned Lasers
Tiny Q-switched microchip laser achieves many kilowatts of peak power.
During the late 1990s, scientists investigated Q-switched microchip lasers that were fabricated by pressing a thin slab of laser material — usually, but not always, Nd:YAG — between an output coupler and a semiconductor saturable absorber mirror (SESAM). The promising technique ultimately was abandoned, however, because the unavoidable air gaps between the components introduced etalon effects that undermined stable laser operation.
Figure 1. The thin resonator consisting of Nd:YVO4, glue and a SESAM was only 200 μm thick, restricting the laser to oscillating in a single longitudinal mode. A dichroic mirror placed between the pump-focusing lenses separated the laser’s 1.06-μm output from the incoming pump light. f = focal length.
Recently, a collaboration of scientists in Europe has resurrected these microchip lasers, eliminating the air gaps between components by gluing them together. They believe that the versatile and robust lasers may find applications in lidar, range finding and micromachining.
The scientists, associated with the Friedrich Schiller University of Jena, Germany, with Batop GmbH in Weimar, Germany, and with RefleKron Ltd. of Tampere, Finland, borrowed the gluing technique from the integrated circuit industry, where spin-on-glass glues are used for overcoat passivation of the circuits. The glue functioned as the peanut butter in a sandwich consisting of a thin slice of Nd:YVO4, the glue and the SESAM. The 3 × 3-mm slice of Nd:YVO4 was only 200 μm thick and was doped with 3 percent neodymium so that sufficient absorption of pump light could occur in a single pass. The scientists pressed the components together tightly during the gluing process, so that the glue layer was less than a few hundred nanometers thick.
Figure 2. The microchip laser is mounted on an aluminum heat sink.
They then pumped the sandwich with up to 1 W of 808-nm light from a fiber-coupled diode laser (Figure 1). A coating on the front surface of the Nd:YVO4 transmitted 10 percent of the Nd:YVO4 wavelength (1.06 μm), providing the laser’s output coupling. The spacing between longitudinal modes of the very short (200 μm) resonator was ~1.5 nm, significantly greater than the Nd:YVO4 gain bandwidth of 0.8 nm. As a result, only a single longitudinal mode oscillated. The laser was restricted also by resonator and pump geometry to a single transverse mode.
The scientists characterized two lasers fabricated this way, using two slightly different SESAMs. Both SESAMs were grown by molecular-beam epitaxy on n-GaAs substrates, and their absorption regions both consisted of InGaAs/GaAs quantum wells. Twenty-four λ/4 AlAs/GaAs layers provided a reflectivity of greater than 99 percent when the absorption was saturated. A fluence of ~500 μJ/cm2 saturated the absorption, and the scientists measured the recovery time of both SESAMs at 320 ps. The two mirrors differed in modulation contrast, the first having a contrast of ~20.5 percent, and the second, an 11 percent contrast.
The first laser, the one whose SESAM had the greater modulation depth, generated 50-ps full width half maximum (FWHM) pulses containing 1 μJ with a 40-kHz repetition rate. The scientists believe that the 20-kW peak power is the highest reported from such a laser. The intracavity intensity of more than 20 GW/cm2 did not damage the glue, and the investigators expect that the gluing technique can be refined to withstand significantly higher power.
Although the first laser operated stably for many hours, the second one was more reliable, and the scientists characterized it more fully. It produced 110-ps FWHM pulses with a peak power of 6 kW at a 166-kHz repetition frequency. It reached threshold with 200 mW of pump power and operated with 14 percent slope efficiency. The transfer function (output versus input power) was flat all the way up to 1 W of pump power, with no rolloff at higher power. The output beam’s M2 was 1.5, and the peak-to-peak amplitude fluctuation was ~3 percent. The pulse-to-pulse temporal jitter decreased from 3 μs at low repetition frequency (~20 kHz) to ~100 ns at a high frequency (150 kHz).
Optics Letters, Aug. 1, 2007, pp. 2115-2117.
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