SOLID-STATE LASERS: Near-IR and white light result from one crystal

The heart of a newly developed superbroadband color-center laser that can generate light over both near-infrared and white-light spectral bands is a single lithium fluoride (LiF:F₂+**) crystalline active element. According to Neil Jenkins and Sergey Mirov of the Laser & Photonics Research Center at the University of Alabama at Birmingham (Birmingham, AL), with alexandrite laser pumping, the LiF:F₂+** crystal should provide tunability of 800-1300 nm at room temperature.

The researchers believe the 500-nm emission spectrum band that they observed from LiF:F₂+** is one of the largest for any active medium emitting in the near-IR. Phase-matching this output into standard nonlinear crystals, such as lithium iodide (LiIo₃), will provide second-harmonic generation into the visible spectral range (400-650 nm). Collimation of this radiation will make it possible to produce a true white-light laser from one solid-state active medium.

To obtain the second-harmonic generation of superbroadband radiation with just one nonlinear crystal, the laser designers had to satisfy the phase-matching conditions for all oscillating wavelengths by fitting the angular-wavelength distribution of superbroadband near-IR output to the angular dependence of phase-matching in nonlinear crystal.

According to Jenkins and Mirov, another design goal of the laser was to maintain simultaneous lasing operation in an optically active gain medium at different wavelengths without mode competition. The key to providing this function lies within the unique structure of the laser's external grating cavity. In essence, the superbroadband laser system creates its own microcavities, each lasing at a different wavelength within the fluorescence band of the gain medium. The laser operates in a pulsed mode, and different wavelengths appear at spatially different positions at the output of the laser cavity.

Unlike conventional tunable lasers that can switch between different lasing wavelengths within a given wavelength band, the superbroadband laser simultaneously emits at multiple wavelengths. It can thus provide either superbroadband spectral output (a combination of hundreds of independent lasing channels of the laser) or a preassigned multiline spectral composition.

Preliminary experiments produced LiF:F₂+** superbroadband lasing under 633-nm excitation from the Raman-shifted second harmonic of a Q-switched Nd:YAG laser. The result was ultrabroadband oscillation within an 850-1050-nm spectral range. The output was then phase-matched into a LiIo₃ nonlinear crystal for second-harmonic generation into the visible spectral range of 425-525 nm (see photo).

Based on spectroscopic analysis, Jenkins and Mirov expect alexandrite laser excitation to produce the full range of superbroadband laser radiation possible with the LiF:F₂+** crystal. Realization of this scheme will be a subject of their next publication. They presented the preliminary results on the superbroadband laser at the Advanced Solid State Lasers meeting this month in Davos, Switzerland.