Vibrating Microsphere Q-Switches Fiber Laser

Breck Hitz

Although the continuous-wave outputs of fiber lasers have already found numerous real-world applications, scientists and engineers have more recently begun exploring potential applications of these lasers’ pulsed-power outputs.

Bulk acousto-optic Q-switches are the most widely used technique of pulsing a laser, but with a fiber laser, such a bulk device forfeits the advantage of all-fiber construction. Accordingly, many researchers are investigating all-fiber approaches to Q-switching. Recently, scientists at the University of Arizona in Tucson demonstrated a novel technique based on replacing one of the resonator’s end reflectors with a vibrating microsphere.

Six meters of erbium-doped fiber provided the gain medium of the scientists’ laser, whose total resonator length was roughly 10 m (Figure 1). A fiber Bragg grating reflected light from one end of the resonator, and a 60-μm-diameter glass microsphere, placed within 2 μm of a tapered section of the fiber, reflected light from the other end. Light resonant with the microsphere — that is, light whose wavelengths fit an integral number of times around the sphere’s circumference — was evanescently coupled from the fiber into the microsphere. A high level of power circulated within the microsphere, and part of that power was coupled back into the fiber, providing the feedback necessary for laser oscillation.

Figure 1. A microsphere provided feedback from one end of the fiber laser. By vibrating it, the scientists modulated the resonator feedback and Q-switched the laser. Reprinted with permission of Optics Letters.

The scientists previously showed that the narrow resonance of such a microsphere (Q~10⁸) could force single-frequency oscillation, even in a long fiber laser with closely spaced longitudinal modes. But now their purpose was different: By vibrating the microsphere back and forth as indicated by the arrow in Figure 1, they modulated the resonator feedback and Q-switched the laser.

They obtained the best results when they vibrated the microsphere at the resonant frequency of the sphere and its piezoelectric transducer, approximately 940 Hz. The pulse train of 160-ns pulses contained 15 mW of average power, with ~102 W of peak power in the individual pulses (Figure 2). At an off-resonance frequency, where the amplitude of the sphere’s vibration was significantly less than the 100-μm, on-resonance amplitude, the scientists observed much longer pulses, ~7 μs. Both on- and off-resonance, they observed laser threshold at 3 mW of pump power, which they believe is the lowest value reported for a Q-switched fiber laser.

Figure 2. When the microsphere vibrated at its resonant frequency, the laser produced a stable pulse train containing up to 15 mW of average power (left). The individual pulses were 160 ns wide, with a peak power of ~102 W (right). The inset shows the 60-μm sphere vibrating next to the tapered fiber.

Besides the erbium laser line at 1553 nm, the laser generated multiple lines around 1670 nm, separated from the laser line by ~120 nm, which corresponds to the first Stokes shift in glass (Figure 3). The scientists believe that the stimulated Raman gain occurs not in the fiber itself, but in the tiny microsphere where the power density is significantly higher.

Figure 3. The laser’s spectrum shows multiple Raman lines in addition to the laser line at 1553 nm. These lines apparently result from stimulated Raman gain in the microsphere, not in the fiber itself. The inset shows the low end of the laser’s power transfer function. In the laboratory, the laser pumping went as high as 120 mW (with a correspondingly linear increase in average output up to 15 mW), but the scale here is chosen to make the threshold visible.

Higher repetition frequencies than the ~1 kHz rate observed in these experiments are required for many applications. The scientists believe that the rate of their laser could be increased to tens of kilohertz by optimizing the microsphere’s piezoelectric mounting structure. The inherently lightweight nature of the tiny sphere makes it well suited for high-frequency mechanical vibration and, potentially, for integration into microelectromechanical systems.

Optics Letters, Dec. 15, 2006, pp. 3568-3570.