1. Technical Field
This invention relates to diode-pumped lasers and more specifically to configurations of resonator cavities for solid-state lasers.
2. Discussion of the Prior Art
Laser diode light emissions are commonly dedicated to "pumping" gain materials to achieve lasing in larger host cavities which in turn output continuous waves or pulses of light. Laser diode side-pumping complicates controlling transverse modes of lasing in solid-state gain materials such as 1% neodymium-doped yttrium aluminum garnet (Nd:YAG) or yttrium lithium fluoride (Nd:YLF), but end-pumping facilitates controlling transverse modes. Laser diode light pumped into a solid-state gain material is converted along the light's "absorption length" (typically four to five mm) most efficiently within an axially cylindrical TEM.sub.00 mode volume through the gain material. Solid-state laser gain "G" is inversely proportional to the sum of the cross-sectional areas of the pumping beam A.sub.p and of the cavity mode A.sub.c : ##EQU1##
Higher power laser diodes have light-emitting junctions or "slits" which are dimensioned non-symmetrically, typically one micron high vertically and many times as long horizontally. Laser diodes pump light in beams which diverge non-symmetrically, typically ten degrees horizontally and fifty degrees vertically full-width at half-maximum intensity ("FWHM"). Any laser diode pump light beams which diverge more widely than ten degrees beyond their "waists" spread, through several millimeters, to objectionably broad spots. Fortunately, a laser diode beam initially only one micron high can afford to have its short vertical height increased by magnification five times in a trade-off to achieve having its excessive vertical divergence decreased by collimation from fifty degrees inward to ten degrees, to be suitable for imaging axially into a TEM.sub.00 mode volume.
Non-symmetrically divergent elliptical beams having their collimations improved by vertical magnification fortuitously tend to become rounder. Minimally elliptical beams (i.e., those with short, say.ltoreq.several hundred micron, lengths not dominating their heights) undergoing vertical magnification become fairly circular. Diode pump beam-rounding distributes light energy more uniformly, and thereby reduces thermal focusing and gain-guiding effects in pumped mode volumes of solid-state laser cavities, which in consequence reduces distortions in output beams.
Laser output beams are preferably circular for applications in general. The transverse shape of a laser output beam corresponds to that of the lasing mode beam inside the cavity at the out-coupling mirror. This predisposes solid-state laser designers to specify cavities for resonating in cross-sectionally circular modes. To match circular mode areas, pumping beams are preferably also circular in cross-section.
Non-symmetrically divergent light beams may be re-shaped for example by passage through a cylindrical lens or a prism, or by reflections against a pair of concave mirrors oriented off-axis as described in U.S. patent application Ser. No. 185,466, filed Apr. 25, 1988 and assigned to the assignee of this application. Optical cavities have also used prisms for different purposes, for example tilted steeply in "An Improved Line Narrowing Technique for a Dye Laser Excited by a Nitrogen Laser," Optical Communications vol. #4 pg. 187 (1971).
Laser output beams have been made more powerful conventionally by cavities being pumped with laser diodes in turn made more powerful through their junctions being lengthened proportionately. Laser diode powers and operational lifetimes also have been further increased by arranging multiple arrays side-by-side to form long "bars". For example, thirty discrete one hundred micron-long diode arrays are aligned on 333 micron centers in ten-watt bar model SDL 3490-S from the Spectra Diode Labs. Such a diode-array bar has a slit with a very high aspect ratio of one micron high by one centimeter long.
Unfortunately however, laser-diode radiations are not diffraction-limited, i.e. longer diode pump beams do not have narrower divergences. An optical system which images a laser diode pump beam by horizontally reducing its length simultaneously increases its divergence so that before the pumping light can be entirely absorbed, the beam expands beyond the diameter of the fundamental (TEM.sub.00) mode area in a solid-state gain medium. This results either in lasing in higher transverse modes or, in cavities restricted to the fundamental mode (for example by an aperture), less efficient lasing.
Despite thermal effects and gain-guiding considerations, thin elliptical laser diode beams can be used to pump solid-state lasing gain cavities. U.S. Pat. No. 4,653,056 by Baer describes multi-strip arrays of diodes which emit light compiled into beams possessing too much spacial structure and having poor focusing qualities (col. 2 lines 52-61). This disadvantage may be addressed by configuring a cavity to expand its lasing mode diameter to match the long diameter of the focused image of the pump beam, which allows utilizing longer diode bars with higher powers. However, Baer considers only circular modes.
Increases in diode pump power accompanied by proportional increases in diode length dictate equal increases in diameters of circular modes in end-pumped solid-state lasers. Since solid-state laser gain G is inversely proportional to the cross-sectional area A.sub.c of the cavity mode volume, diode pump power-increases accompanied by length-increases are traded off against gain decreases. This poses dilemmas in making longer diode arrays or array-bars to be compatible for end-pumping conventional solid-state laser cavities.
Laser diodes in discrete arrays arranged either serially along a bar or individually can be used to pump light into ends of optical fibers, which can in turn either separately couple the light into multiple-bounce point cavities (see Optics Letters, Vol. 13, No. 4, pg. 306, April 1988), or be bundled together to couple the light into single- or multiple-end cavities.
U.S. Pat. No. 4,785,459 by Baer describes multiple laser diode arrays each two hundred microns long centered one mm apart along a bar. The bar must be precisely and meticulously aligned to couple the light from the diodes into respective bounce points also one m apart along a multi-bounce solid-state gain element of Nd:YAG.
There remains, therefore, a need for a laser cavity configuration to facilitate converting longer laser diode pump light beams efficiently and conveniently into TEM.sub.00 lasing output energy.