Fiber Laser Q-Switched by Stimulated Brillouin Scattering

Breck Hitz

Fiber lasers increasingly are becoming attractive for commercial applications because of their compact and rugged packaging, efficient operation with passive cooling and high beam quality. Recently, a novel approach to passive Q-switching has boosted the peak power of the systems an order of magnitude above that of conventionally Q-switched fiber lasers.

In the technique, stimulated Brillouin scattering, which is considered detrimental in most optical fiber, generates a narrow seed pulse that travels backward through the population inversion and quickly gains amplitude. The width of the seed pulse depends on the interaction between the acoustic wave and the fiber core -- typically a nanosecond for a 6-µm-diameter core.

Another way of understanding the effect is to view the scattering as a feedback mechanism that dramatically increases the resonator's Q during the nanosecond lifetime of the acoustic wave. The stimulated Brillouin scattering thus serves as a passive Q-switch inside the fiber laser resonator.

This effect had been observed previously, but now researchers at Nankai University in Tianjin, China, have stabilized the effect and have set a record for peak power by combining it with a pulsed pump source. They used 20 m of ytterbium-doped double-clad fiber featuring an inner cladding with a D-shaped cross section to suppress doughnut modes that do not couple energy to the core.

Figure 1. Researchers have employed stimulated Brillouin scattering to passively Q-switch a diode-pumped Yb-doped double-clad fiber laser. The laser produces a peak power of 105 kW, an order of magnitude greater than conventionally Q-switched fiber lasers.

They end-pumped the fiber laser with a diode laser capable of both continuous and pulsed operation (Figure 1). Under continuous pumping, the fiber laser reached threshold at approximately 570 mW and began random pulsing at about 800 mW. The pulses, generated by Brillouin backscattering, were very short -- less than 10 ns -- but were sporadic and unstable.

The researchers stabilized the pulses by switching the pump diode to pulsed mode. As they increased the average pump power from the laser threshold to approximately 5 W at a repetition frequency of approximately 8 kHz, they observed stable Q-switched pulses with a duration of 5 ns and an energy of 0.5 mJ. The 105-kW peak power of these pulses is an order of magnitude greater than had been previously observed from Q-switched fiber lasers, and the output beam was measured to have an M² of 2.5.

Figure 2. Introducing an external-cavity grating to the setup narrowed the bandwidth of the laser to 0.04 nm and enabled tunability over a range of 1080 to 1140 nm.

By slightly varying the resonator configuration, they reduced the bandwidth of the laser to 0.04 nm and tuned it over a range of 1080 to 1140 nm (Figure 2). The external-cavity grating and high-reflectivity mirror were in the Littman configuration, with the first-order diffracted beam directed toward the mirror and the zeroth-order beam as the laser output. This setup produced a peak power of 28.6 kW.