SOLID-STATE LASERS: Dual-cavity resonator tunes pulsed lasers

Oct. 1, 1996
Researchers have demonstrated a low-threshold single-frequency resonator for gain-switched Cr4+:forsterite and Ti:sapphire lasers excited by harmonics from a Q-switched Nd:YAG pump laser.

Researchers at the University of Otago (Dunedin, New Zealand), in collaboration with colleagues at Manchester University (Manchester, England), have demonstrated a low-threshold single-frequency resonator for gain-switched Cr4+:forsterite and Ti:sapphire lasers excited by harmonics from a Q-switched Nd:YAG pump laser. The high-dopant-level Cr:forsterite laser has single-longitudinal-mode pulsed output with broadband tunability in the 1150-1350-nm range. Bandwidth is less than 150 MHz for nearly transform-limited pulses. The resonator also could be used with other solid-state lasers, such as Cr:YAG, and with optical parametric oscillators.

Many tunable solid-state lasers feature low gain or high passive loss, hence the requirement for a resonator combining low threshold with high mode discrimination--both characteristics of the dual-cavity configuration developed by the Otago group. The master cavity is a Littrow oscillator, incorporating a four-prism expander and Littrow grating. The etalon is designed to isolate a single-cavity mode, but the Littrow cavity operating bandwidth exceeds the etalon-free spectral range. A dichroic filter simultaneously retroreflects the undepleted pump beam and defines a low-finesse slave resonator. Cavity optics and prism coatings are optimized so the interplay of resonator modes with the dispersive elements produces a region of stable, single-frequency operation.

Threshold pump fluence energy for the coupled resonator was 0.7 J/cm2 for Ti:sapphire and 0.3 J/cm2 for Cr:forsterite. For both materials, the single-frequency output was tunable across a 100-nm operating range, limited only by the optics. Measured linewidths, less than 100 MHz in Cr:forsterite and less than 300 MHz in Ti:sapphire, were near the instrumentation resolution, indicating the pulses were single-mode, with close to transform-limited bandwidth.

The researchers are now investigating wavelength scanning and optimum output coupling for the resonator cavity.

About the Author

Rick DeMeis | Associate Editor, Technology

Rick DeMeis was Associate Editor, Technology for Laser Focus World from March 1995 through March 1997.

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