Interest in the development of tunable solid-state lasers has increased significantly in recent years. The potential for storing energy in the population inversion and the long, maintenance-free operating lifetimes of solid-state lasers make the current generation of laser- and lamp-pumped, transition-metal-doped, insulating crystal lasers superior to dye lasers for many scientific and technical applications including laser spectroscopy and remote sensing. Unfortunately, widespread commercial use of these lasers will require significant reductions in complexity, size, and cost.
These lasers are characterized by a broad gain spectrum (which, in some cases, is several hundred nanometers wide) and typically have an intra-cavity tuning element which can be used to tune the output wavelength over a portion of this curve. Earlier devices (e.g., ones using Co:MgF.sub.2 or Ti:Al.sub.2 O.sub.3) used a mechanical Lyot Filter which was inserted inside a laser cavity and rotated to shift the output wavelength of the laser. See Lovold et al., IEEE J.Q.E., QE-21(3) page 202 (March, 1985). Galvanometer controlled etalons or Lyot Filters incorporating electro-optic crystals have also been used for this purpose. See Otsuka et al., Optics Communications, 63(1), p. 37 (July 1987). One disadvantage of using a diffraction grating is that it cannot be tuned electronically.
Tuning elements, like the mechanically-tuned Lyot Filter and galvano-driven etalon, in which an optical component is physically translated are comparatively slow and bulky. The electro-optic Lyot Filter is faster and smaller but requires a voltage of several hundred volts to tune it. For these reasons, the tuning techniques described in the prior art are not well suited to compact, diode-pumped tunable laser systems, such as Cr:LiSAF.
A liquid crystal Lyot Filter is described by Shin-Tson Wu, Applied Optics, 28,48, (1989): J. R. Andrews, IEEE Photonics Technology Letters, 2, 334 (1990) and in U.S. Pat. No. 4,394,069 to W. I. Kaye entitled "Liquid Crystal Tuned Birefringent Filter." As with any conventional birefringent tuner, light that is transmitted through the polarizer passes through the variable waveplate, is reflected by the mirror and again transmitted through the waveplate and polarizer. The return beam is attenuated if the retardation of the variable birefringent plate is not an integral number of half-waves. The liquid crystal waveplate will only act like a half- or full-waveplate for a band of wavelengths. Wavelengths lying outside this band are attenuated by the polarizer as they return through it.
Unfortunately, the maximum single-pass transmission of the polarizer/waveplate combination is between 90% and 98%, making it useless as an intra-cavity tuning element in a low-gain, diode-pumped system. It is useful, however, in high-gain systems and compound liquid crystal birefringent filters have been used as an intra-cavity tuning element in external cavity laser diodes (see "Electronically tunable single-mode external-cavity diode laser", J. R. Andrews, Optics Letters, 16, 732-734 (1991)). The development of AlGaInP semiconductor lasers with output wavelengths near 670 nm has led to the demonstration of CW, diode-pumped lasers in Alexandrite or (Cr.sup.3+ : BeAl.sub.2 O.sub.4) (See R. Scheps, et al., Appl. Phys. Lett. 56, 2288 (1990)), in Cr.sup.+3 :LiSrAlF.sub.6 or (Cr:LiSAF) (See G. J. Dixon, et al., Digest of Sixth Interdisciplinary Laser Science Conference, (American Physical Society, New York, 1990), paper B3-1; Q. Zhang et al., Digest of Conference on Lasers and Electro-Optics (Optical Society of America, Washington, D.C., 1991), paper CTHR6; and R. Scheps, et al., Opt. Lett. 16, 820 (1991)) and in Cr.sup.3+ :LiCaAlF.sub.6 or (Cr:LiCAF) (See R. Scheps, IEEE J. Quantum Electron., 27, 1968 (1991)). Because of a broad, intense absorption near 670 nm and an advantageous combination of emission cross section and lifetime, diode-pumped operation of 10%-doped Cr:LiSAF lasers with threshold powers as low as 3 mW have been demonstrated. With dye-laser excitation, a slope efficiency of 41% was measured for broadband operation near 850 nm. High-power, quasi-CW diode-pumped operation from a laser incorporating lower-doped Cr:LiSAF has also been reported (See R. Scheps, et al., Digest of the Advanced Solid State Laser Conference (Optical Society of America, Washington, D.C., 1991), p. 291).
Although the cross-section lifetime product (i.e., product of stimulated emission cross section and lifetime) of Cr:LiSAF is higher than that of other tunable chromium-doped materials, it is approximately 50 times smaller than that of Nd:YAG (and many other rare-earth doped hosts). As a result, increasing the intra-cavity losses of a diode-pumped laser through the addition of intra-cavity tuning elements significantly increases its threshold.
It would be most advantageous if a relatively simple, low cost, compact apparatus and method were available which would allow the use of materials like Cr:LiSAF. It would be especially advantageous if such a laser could be efficiently tuned over a range of frequencies.