Laser development for gravitational wave detection

There is a recurrent need for high power, frequency stable, lasers in applications as diverse as laser radar, remote sensing, gravitational wave interferometry and non-linear optics. This need is often satisfied by using a low power, frequency stable laser followed by a chain of amplifiers, an architecture made complicated by the need to limit the gain-length product of the amplifiers to prevent parasitic oscillations and amplified spontaneous emmission (ASE). Furthermore, the power extraction efficiency from an amplifier is usually significantly less than that available from a laser resonator.

A preferred approach, therefore, is to injection lock a high power (slave) laser to a lower power, frequency stable master laser. With this approach, efficient coupling of the lasing mode to the gain medium can be accomplished in the usual way by using an optimized resonator, the resonator of the slave laser discriminates against parasitic oscillations and ASE, and the frequency control is maintained by keeping the master laser tuned, to within the locking range, to the master laser. The width of the locking range is proportional to the square root of the the ratio of the injected power to the slave laser output power. It thus becomes increasingly difficult to maintain lock for high power gains. For this reason, it is desirable to develop a medium power master for injection locking a high power slave laser.

We are presently working on three projects in this area: (a) development of a medium power (5-10 W) injection locked, diode pumped Nd:YAG laser, (b) development of a higher power injection locked, diode pumped, unstable resonator solid-state laser which lases at 1064 nm, and (c) development of an injection seeded, Q-switched, quasi-cw diode pumped Nd:YAG laser. The slave laser in the medium power project uses a diode-pumped slab architecture, developed by Richards et al at DSTO, which has already been characterized to have excellent slope efficiency and good beam quality when configured as a standing wave laser. This gain medium will be incorporated in a ring slave laser and injection locked using a super stable NPRO. It is unclear whether injection-locking a stable resonator slave laser can be scaled-up to produce a laser that has an output power of 100 W diffraction-limited TEMoo. An alternative approach is to use an unstable resonator slave laser. Such resonators are also useful in Q-switched lasers which need to produce pulses that have good beam quality at high repetition rates.