Researchers Russell Wilcox and Donald Browning of the Lawrence Livermore National Laboratory (LLNL, Livermore, CA) have fabricated a Q-switched, tunable diode-pumped fiber ring laser based on ytterbium-doped silica (Yb:silica) as the gain medium. With piezoelectrically stretched tuned fiber gratings, the device shows single-longitudinal-mode operation at 1053 nm. A feedback control circuit adjusts cavity length, also by piezo fiber stretching, for stable, operation. The laser is seen by the developers as a tunable source for applications in which a short pulse will be chopped from a long, Q-switched pulse by electro-optic modulators. This pulse could then be amplified in neodymium-doped phosphate glass, as in the proposed National Ignition Facility at LLNL, to apply the proper temporal variation of power to a target for laser fusion (see Laser Focus World, Nov. 1996, p. 107).
The Yb:silica fiber is pumped with a 980-nm diode laser. The gain in Yb:silica is distributed over a 90-nm range, which makes it suitable for operation at many wavelengths. Pumping at 980 nm, however, allows use of the stable diode pump lasers made for erbium-doped fiber amplifiers. In order to take advantage of the wideband gain medium and still operate in a single-cavity mode, the laser incorporates a circulator that causes unidirectional operation and thus allows use of a fiber grating for reflection. The grating—with a bandwidth of 0.02 nm—determines the coarse laser tuning; piezoelectrically stretching the grating tunes it to the desired wavelength. A single mode of the cavity is selected by a piezo-tuned fiber-grating Fabry-Perot etalon of 64-MHz bandwidth.
The laser is Q-switched by a bulk acousto-optic device at a repetition rate of 1 kHz. Loss is controlled to allow the oscillator to lase near its threshold for 500 µs before completely turning off the Q-switch, creating a pulse. Such prelasing stabilizes the single mode because the Q-switched pulse builds from the prelase level. To circumvent modehopping during long-term operation, cavity length is controlled by feedback of an error signal, de rived from the output optical signal, to another piezoelectric actuator that stretches the cavity tuner fiber.
Because of the long length of the high-loss cavity, the Q-switched pulse is roughly 200 ns long. The researchers are looking to gate the central part of this pulse by an electro-optic modulator to yield a 30-ns square pulse to be used for further amplification and modulation. Wilcox described this laser at the LEOS Annual Meeting last month in Boston, MA.