New Saturable Absorber Material Q-Switches Vanadate Laser

Breck Hitz

Passive Q-switching has found broad acceptance with diode-pumped solid-state lasers because saturable-absorber Q-switches are rugged and simple to operate, resulting in compact and robust lasers. But there are trade-offs among the different saturable-absorber materials used to fabricate the Q-switches. The most common is probably Cr⁴⁺:YAG, but its absorption cross section is less than ideal. InGaAs/GaAs quantum wells, fabricated into semiconductor saturable-absorber mirrors (SESAMs), also find frequent use in solid-state lasers, but the inherent lattice mismatch between these two materials results in lower modulation depth than is desirable.

Recently, investigators at the National Chiao Tung University in Hsinchu, Taiwan, demonstrated, for what they believe is the first time, AlGaInAs quantum wells fabricated into a saturable-absorber Q-switch. Experimenting with their new Q-switch in a diode-pumped Nd:YVO₄ laser, they have already achieved results comparable to those obtainable with Cr⁴⁺:YAG Q-switches.

Q-switching with AlGaInAs

The researchers grew AlGaInAs quantum wells via metallorganic chemical-vapor deposition on Fe-doped InP substrates, which are nearly transparent at the neodymium wavelength of 1.06 μm. Each Q-switch consisted of 30 pairs of quantum wells, separated from each other by the 1.06-μm laser wavelength. When placed in the laser resonator, these lossy quantum wells tended to extinguish any longitudinal modes whose standing waves had significant amplitude overlapping the wells.

Figure 1. Placed near the center of the resonator, the AlGaInAs quantum wells discriminated against any longitudinal mode whose standing wave had significant amplitude at the quantum wells. As a result, only modes with minima at the quantum wells oscillated, reducing the probability of damage to the Q-switch. Images reprinted with permission of Optics Letters. (HR = high reflectivity, HT = high transmission).

By placing the Q-switch in the middle of the resonator, where the standing waves have minimal overlap with each other, the investigators assured that only modes with minima at the quantum wells oscillated (Figure 1). Thus, the probability of damage to the quantum wells was minimized, but there was still sufficient interaction between the oscillating modes and the quantum wells to Q-switch the laser.

Even though the investigators applied antireflection coatings to the high-index Q-switch, its surfaces still reflected about 5 percent of the incident 1.06-μm radiation. The scientists aligned the Q-switch carefully so that the reflected radiation was not ejected from the resonator. The resulting etalon effect introduced further longitudinal-mode selectivity into the laser, but these effects apparently had little deleterious effect on the output.

Figure 2. The maximum output from the laser was 4.4 W when the Q-switch was removed and 3.5 W when the Q-switch was present and Q-switching the laser.

When operated with the Q-switch removed from the cavity, the laser produced approximately 4.4 W of continuous-wave output. With the Q-switch present, it produced 3.5-W average power (Figure 2). At the 3.5-W level, the Q-switched pulses had 900-ps duration and a peak power greater than 36 kW, at a 110-kHz repetition rate.

The investigators measured the normal (nonsaturated) insertion loss of the Q-switch to be ∼30 percent, and they found that loss decreased to ~2 percent when the absorption became saturated. From numerical simulations, they calculated that the saturating flux was in the range of 1 mJ/cm² and that the relaxation time for the Q-switch to return to its high-loss state after saturating was on the order of 100 ns.

Optics Letters, June 1, 2007, pp, 1480-1482.