- Cavity-Dumped Laser with Pulses Adjustable from 170 to 900 ns
Disk laser generates up to 100-W second-harmonic output at 515 nm.
High-power visible lasers with a high repetition rate are essential to many materials-processing applications. Scientists in Germany recently demonstrated a green thin-disk laser that generates up to 100 W of average power at frequencies as high as 100 kHz. The laser’s suitability for a variety of tasks is enhanced by its design, which allows its pulse duration to be adjusted from 170 to 900 ns.
Figure 1. The thin-film polarizer normally reflected the incident intracavity power, but when the Pockels cell rotated its polarization, the intracavity power was dumped out through the thin-film polarizer (HR = highly reflective, HT = highly transmittive). Image reprinted with permission of Optics Letters.
The scientists, from the University of Stuttgart and from Technologiegesellschaft für Strahlwerkzeuge, both in Stuttgart, and from Jenoptik Laser, Optik, Systeme in Jena, designed an internally doubled Yb:YAG thin-disk laser that was cavity-dumped with a beta-barium-borate Pockels cell (Figure 1). They placed the lithium-borate doubling crystal nearly against the high-reflecting rear mirror of the laser, so the intracavity harmonic power generated in both directions was combined into a single output beam that emerged through a dichroic folding mirror.
Figure 2. Dumping the infrared power out of the resonator also terminates the green pulse. All data shown in this article were taken at 50 kHz (TFP = thin-film polarizer, SH = second harmonic).
They adjusted the laser’s pulse duration by waiting until the green pulse was an appropriate length and then dumping the fundamental power out of the resonator, terminating everything, including the green pulse. Because the time between pulses was much shorter than the spontaneous decay time of the population inversion (~20 μs vs. ~1 ms), nearly all the population inversion left behind was still available to the next pulse. This led to some unusual intracavity dynamics.
The folded-cavity laser used a 9-percent-atomic-doped Yb:YAG thin disk as its gain medium. The back surface of the disk was highly reflective at both the laser and pump (940-nm) wavelengths, and the disk was soldered onto a heat sink. Up to 520 W of pump radiation reflected through the 180-μm-thick disk 12 times, resulting in a pump-power density of 4.1 kW/cm2. Despite six antireflection-coated intracavity surfaces, an etalon, four folding mirrors and two end mirrors, the resonator had an unusually low round-trip loss of only 0.5 percent (excluding output coupling). The ultrathin disk introduced only 0.1 percent depolarization loss, even at the highest pump powers.
Figure 3. The longer the Pockels cell waits before dumping the pulse, the less population inversion is left behind for the next pulse. The next pulse, seeing less gain, takes longer to build up. Thus, increasing the delay time increases the pulse duration, as shown here from ~170 ns to ~400 ns. Even longer delay times, as long as 8 μs, result in pulses as long as 900 ns.
The intracavity (infrared) pulse and the second-harmonic (green) output built up simultaneously after the Pockels cell switched the cavity to its high-Q condition (Figure 2). When the green pulse was about half as long as desired, the scientists triggered the Pockels cell, dumping the intracavity power in several hundred nanoseconds.
Figure 4. The average green power and the average infrared power dumped from the cavity vary with Pockels cell delay. The “amplification period” here is the delay between the time the Pockels cell initially switches the cavity to a high-Q condition and the time it switches it back to a low-Q condition, dumping the pulse (SH = second harmonic).
But the sooner the pulse was dumped, the more energy was left behind in the population inversion. That meant that the next pulse began with more population inversion, and thus saw more gain. More gain means that shorter pulses build up faster (Figure 3). Thus, when the Pockels cell dumped the intracavity power after only 2 μs, a short (~170-ns) pulse emerged from the output mirror relatively quickly. On the other hand, when the Pockels cell waited 3.5 μs before dumping the pulse, a longer (~400-ns) pulse emerged after a longer buildup time. Not shown in Figure 3 are the 900-ns pulses that resulted from an 8-μs delay in dumping the pulse.
The longer the infrared power passes through the doubling crystal, the more infrared energy is converted to green. Thus, the average second-harmonic power increased as the Pockels cell delay increased (Figure 3). But the more energy converted to the second harmonic, the less fundamental energy there is to dump out of the cavity. So the average dumped power decreased with the delay, and, as previously explained, the pulse duration increased.
Figure 5. The scientists maintained a 300-ns pulse duration, independent of pump power, by adjusting the Pockels cell delay. They obtained a maximum second-harmonic average power of slightly more than 100 W.
To demonstrate the versatility of their laser, the scientists adjusted the Pockels cell delay so the pulse duration remained a constant 300 ns, independent of pump power. As expected, the green output power increased with pump power (Figure 5). As the pump power increased, the Pockels cell delay necessary to maintain a 300-ns pulse decreased because more pump power meant more gain, and more gain meant faster buildup of the intracavity pulse.
By measuring the energy of every pulse during a 500-μs period, the scientists determined that the standard deviation of pulse energy was less than 0.5 percent and that the maximum peak-to-valley deviation was less than 1.5 percent. Although they had not designed the cavity for a high beam quality — the measured M2 value was ~5 — they believe that a diffraction-limited beam could be obtained from a carefully designed resonator without altering the laser’s other parameters. They also believe that further refinements could lead to similar lasers producing green output in excess of a kilowatt.
Optics Letters, May 1, 2007, pp. 1123-1125.
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