External-Resonator Raman Laser Emits 1.3 W

Breck Hitz

In the last 20 years, a host of wavelengths have become available from conventional lasers, and manufacturers are offering systems that can be tuned to virtually any wavelength from the short ultraviolet to the long infrared.

In general, these systems feature an Nd:YAG laser operating at 1.06 µm and employ frequency-doubling crystals and optical parametric oscillators to reach the desired wavelength. Ongoing research, however, indicates that another nonlinear process, stimulated Raman scattering, may enable improved tunable sources.

Researchers in Australia from Macquarie University in Sydney and the Department of Defence in Salisbury have generated 1.32 W at the 1197-nm first-Stokes wavelength in a Ba(NO₃)₂ crystal pumped with 2.88 W of 1064-nm radiation from an Nd:YAG laser.

Researchers in Australia have generated 1.32 W of 1197-nm radiation from an external-resonator Raman laser. To demonstrate the versatility of the approach, they frequency-doubled the output in LBO to produce 270 mW of 599-nm light.

Raman lasers have been placed inside the pump laser to take advantage of increased pump power, but although lower pump powers are available to an external-resonator configuration, thermal distortions in the Raman crystal do not degrade the pump laser.

In the setup, the diode-pumped slab Nd:YAG laser produced up to 5 W in the TEM₀₀ mode when pumped with 20 W at 808 nm. An electro-optic modulator Q-switched the laser at rates of up to 10 kHz, and the measured M² of the output beam from the 50 percent coupler was 1.1 in both planes.

A pair of cylindrical lenses focused the asymmetric YAG beam into the external Raman oscillator. An optical isolator consisting of a polarizer and quarter-wave plate protected the YAG laser from backreflection. The input mirror of the Raman oscillator was highly transmissive to the 1064-nm pump radiation and highly reflective to the first-Stokes wavelength. The output mirror was 70 percent reflective to the 1197-nm radiation but reflected most of the 1064-nm radiation for a second pass through the Ba(NO₃)₂ crystal.

The quality of the beam emerging from the Raman oscillator decreased with increasing power. At a relatively low, 630-mW Stokes power, the measured M² was 1.4. As the power increased to 1.05 W, the M² increased to 2.5 and, at the maximum Stokes power of 1.32 W, to 3.

To illustrate the versatility of stimulated Raman scattering for producing new wavelengths, the group also frequency-doubled the output of the Raman oscillator in a lithium triborate crystal to produce 270 mW of 599-nm light.