Researchers from the Laser Optics Group of Imperial College, (London, England), and the Optoelectronics Research Centre, University of Southampton (Southampton, England), have demonstrated a continuous wave (CW) diode-pumped Nd:YVO4 laser with an adaptive gain-grating resonator. The system produces more than 7 W output in a TEM00 single longitudinal mode, with an M2 of 1.3 and 1.1 in the x and y axes, respectively.
Standard diode-pumped solid-state (DPSS) systems typically exhibit thermally induced problems at high pump powers, including phase distortions and depolarization. This has a detrimental effect on beam quality, which affects many applications. One solution to this problem has been to use adaptive nonlinear optical techniques—such as a dynamic gain grating—to correct the laser beam inside the cavity and thereby improve the output beam quality. A gain grating is produced by gain saturation in an amplifying medium in the laser cavity. The interference of a pair of intersecting coherent beams creates a volume hologram, through saturation of the gain, and this encodes the distortions in the laser loop. The hologram is then used as a grating that diffracts light going round the loop of the laser cavity. The system is self-adaptive because it is the beams in the cavity itself that are used to create the hologram, and so correct the distortions even when they change.
Although they have been used successfully in pulsed lasers, an efficient system based on gain gratings requires high small-signal gain, which makes CW implementation of this technique more difficult to achieve. The UK researchers have developed a technique using four-wave mixing in a side-pumped CW amplifier that provides enough gain for an adaptive resonator.
The new oscillator can run in both injected and self-starting modes. In either case, at the heart of the system is a cooled Nd:YVO4 slab, transversely pumped by a 25-W CW diode bar at 808 nm. An external CW single-longitudinal-mode diode-pumped Nd:YVO4 laser was used as an injection source for the injected version of the system. The beam is injected in the forward direction, passed through the amplifier, and fed back to self-intersect in the amplifier. The interference pattern produced by the crossing of the beams produces the volume gain grating, which is encoded with any phase aberrations experienced from the round trip of the system. Bragg-matched diffraction of light from the gain grating forms a closed-loop resonator in which light can oscillate in the backward direction with respect to the injection beam, when the loop gain is above threshold. The efficiency of operation can be enhanced by the inclusion in the loop of a nonreciprocal transmission element (NRTE)—a leaky optical diode, with different forward-to-backward transmission, that maximizes the modulation of the gain grating and ensures unidirectional (backward) oscillation. In the self-starting version of the system, the injected laser beam is replaced with an output coupler.
The researchers have seen output power of as much as 7.2 W in a good transverse mode. The system is also capable of running on a single longitudinal mode. "We verified the self-adaptive nature of the resonator further by inserting a severe phase aberration into the loop," said Researcher Matthew Trew of Imperial College. "Little adverse effect was observed on either the high-output quality or the power level." Scaling to higher powers looks promising.