Single-frequency operation of a Cr:YAG laser from 1332-nm to 1554-nm
D. Welford and M.A. Jaspan
Q-Peak, Inc., 135 South Road, Bedford, MA 01730
Phone (781)-275-9535, Fax (781)-275-9726, E-mail welford@qpeak.com
Abstract: Single-frequency cw operation of a Cr:YAG laser has been demonstrated for the first time. An output power of 680 mW was generated at 1457 nm with single-frequency operation obtained from 1332 nm to 1554 nm.
©1999 Optical Society of America
OCIS Codes: (140.3570) and (140.3600)
1. Introduction
We describe the first demonstration, to our knowledge, of single-frequency operation from a Cr:YAG laser. The wavelength range of operation, from 1332 nm to 1554 nm, with output powers as high as 680 mW, provides a unique narrow linewidth source. Potential applications include, but are not limited to, injection seeding of optical parametric oscillators for remote sensing applications such as DIAL, near-infrared spectroscopy, and optical telecommunications instrumentation.
2. Experimental Results
Our laser oscillator design was a unidirectional ring laser, a scheme employed in commercial single-frequency dye lasers or Ti:Al2O3 lasers to eliminate the effects of spatial hole-burning, thereby maximizing single-frequency output. Figure 1 shows a schematic layout of the oscillator, which consisted of four mirrors in an X-configuration and Brewster-angled intracavity elements, except for the intracavity etalon, to minimize loss.
Fig. 1. Optical schematic of the Cr:YAG ring-resonator.
The two curved mirrors on either side of the Cr:YAG crystal act as collimation and focusing optics and provide a 50-
m m diameter beam waist in the laser crystal. The pump laser was a TEM00 mode, cw Nd:YLF laser focused with a plano-convex lens to match the beam waist of the TEM00 mode in the Cr:YAG crystal, thereby ensuring TEM00-mode operation of the laser.Wavelength tuning was obtained with a single-plate, Brewster-angled, quartz birefringent filter element. A solid CaF2 etalon 9.5-mm thick was added to suppress neighboring longitudinal modes and ensure single-mode operation. Although the etalon was uncoated and near normal incidence, it is essentially lossless for resonance transmission peaks.
Unidirectional operation was obtained by the action of an optical diode that used a non-reciprocal polarization rotation in the Cr:YAG laser material, due to the Faraday effect, and a non-planar resonator geometry to provide the reciprocal polarization compensation. A Nd/Fe ring magnet placed on top of the Cr:YAG crystal mount provided sufficient field strength to obtain between 0.1
° and 0.2° of polarization rotation. The use of the laser crystal for the Faraday element is ideal given the small emission cross section of Cr:YAG and the need to minimize intracavity losses.
Fig. 2. Scanning Fabry-Perot interferometer output showing Cr;YAG laser single-frequency operation. The lower trace is a scan over approximately 2 free-spectral-ranges and shows two transmission peaks separated by one free-spectral range or 2 GHz. The upper trace is a 100x expansion of a single longitudinal mode with an instrument limited FWHM of 3.6 MHz.
Fig. 3. Single-frequency Cr:YAG tuning curves.
A scanning Fabry-Perot interferometer (Burleigh, Model SA-Plus-200) was used to monitor and verify single-frequency operation of the Cr:YAG laser (see Fig. 2). The upper trace of Fig. 2 shows a single longitudinal mode with an instrument limited FWHM of 3.6 MHz. The Schawlow-Townes linewidth for the laser mode should be < 1kHz, but we expect resonator fluctuations to broaden the effective laser linewidth to a value approaching 1 MHz over a 1 second time interval.
The laser tuned smoothly over the 1332-nm to 1554-nm range with unstable behavior or power drops only in regions of water vapor absorption. An engineered product will require a sealed and purged enclosure to eliminate this effect. The long-wavelength tuning limit appears to be due to increasing total output coupling loss with significant contribution, 0.3% compared to 0.1% at the output mirror, from the three high-reflectivity mirrors. Improved high-reflectors with <0.05% transmission should enable single-frequency operation of the Cr:YAG laser to be extended to 1600 nm.
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