Stimulated emission at the approximate wave length of 4.5 microns, 2.25 microns and 1.6 microns is obtained from pumping on an excited energy level. The phenomenon of photon avalanching is utilized to create a population inversion within three lower level states that have a substantially equal energy spacing to provide resonance between neighboring ions. This resonance feeds the photon avalanching process when pump having sufficient power in excess of the critical pump power irradiates a LaCl.sub.3 :Pr.sup.+3 crystal at an excited energy level to maintain a population inversion in an optical cavity.

Ce.sup.3+ -doped LiSrAlF.sub.6 crystals are pumped by ultraviolet light which is polarized along the c axis of the crystals to effectively energize the laser system. In one embodiment, the polarized fourth harmonic light output from a conventional Nd:YAG laser operating at 266 nm is arranged to pump Ce:LiSrAlF.sub.6 with the pump light polarized along the c axis of the crystal. The Ce:LiSrAlF.sub.6 crystal may be placed in a laser cavity for generating tunable coherent ultraviolet radiation in the range of 280-320 nm. Additionally, Ce-doped crystals possessing the LiSrAlF.sub.6 type of chemical formula, e.g. Ce-doped LiCaAlF.sub.6 and LiSrGaF.sub.6, can be used. Alternative pump sources include an ultraviolet-capable krypton or argon laser, or ultraviolet emitting flashlamps. The polarization of the pump light will impact operation. The laser system will operate efficiently when light in the 280-320 nm gain region is injected or recirculated in the system such that the beam is also polarized along the c axis of the crystal. The Ce:LiSrAlF.sub.6 laser system can be configured to generate ultrashort pulses, and it may be used to pump other devices, such as an optical parametric oscillator.

A method is provided to control the lasing wavelength of a laser material without changing or adjusting the mechanical components of a laser device. The rate at which the laser material is pumped with the pumping energy is controlled so that lasing occurs at one or more lasing wavelengths based on the rate. The lasing wavelengths are determined by transition lifetimes and/or energy transfer rates.

A laser catheter for insertion in a body passage and for treatment of a relatively inaccessible location with laser radiation in a preselected first wavelength range, typically in the mid-infrared band, that is outside the transmission passband of silica optical fibers. The laser catheter includes an elongated flexible tube, an optical fiber for carrying optical pumping laser radiation in a second wavelength range through the flexible tube, and a laser attached to the flexible tube at or near the distal end thereof and responsive to optical pumping laser radiation in the second wavelength range for generating output laser radiation in the first wavelength range. The laser can comprise a cylindrical laser crystal attached to the distal end of the flexible tube and having laser mirrors on opposite end faces thereof. The laser crystal can be a suitable host material doped with a rare earth ion selected to produce the desired output wavelength. A preferred laser crystal is erbium-doped YAG, which produces an output at 2.94 micrometers. The optical pumping radiation is supplied from an external pump laser, which is preferably an alexandrite laser operating in the wavelength range of 0.7 to 0.8 micrometer.

Laser isotope separation is accomplished using at least two photoionization pathways of an isotope simultaneously, where each pathway comprises two or more transition steps. This separation method has been applied to the selective photoionization of erbium isotopes, particularly for the enrichment of .sup.167 Er. The hyperfine structure of .sup.167 Er was used to find two three-step photoionization pathways having a common upper energy level.