A multi-layer graded reflectivity mirror (GRM) with high effective reflectivity is suitable for use in large aperture laser systems with relatively low gain. The GRMs are manufactured with multiple dielectric layers with a thickness profile that eliminates the interference fringes, while providing a reflectivity which tapers smoothly from a peak to zero. The mirror is formed on a substrate having a first surface and a second surface opposite the first. The substrate consists of a material which has low absorption at a given wavelength .lambda.. A first dielectric layer is formed on the first surface of the substrate which has an index of refraction n.sub.1 and having an optical thickness profile with a maximum optical thickness of .lambda./4 at a center which essentially continuously decreases away from the center to a minimum optical thickness of Z at a perimeter P. A second dielectric layer is formed on the first dielectric layer having an index of refraction n.sub.2 and a thickness profile which is similar to the thickness profile of the first dielectric layer. Additional dielectric layers can be added as necessary to achieve the reflectivity magnitude desired. The thickness Z is the thickness at which the reflectivity profile reaches zero at the first interference fringe.

A Q-switched pulsed laser resonant cavity having an output coupling mirror with a radially variable reflectivity function across its surface and a magnification level that combine to provide a very high discrimination against transverse modes of oscillation higher than a desired fundamental mode. This resonant cavity is especially adapted for use with injection seeding to limit the longitudinal modes of oscillation to one mode. The cavity arrangement develops a high powered light pulsed beam at its output that also has a uniform intensity profile.

In a laser apparatus which includes a laser medium exhibiting thermally induced optical distortions, and flash lamp mounted near the laser medium to provide laser pumping radiation, an absorbing filter is mounted between the flash lamp and the laser medium for absorbing radiation in the output spectrum of the lamp which exhibits a low fluorescence efficiency in the laser medium. By absorbing radiation with the low fluorescence efficiency in the laser medium, heat which develops in the laser medium can be limited to control the thermally induced optical distortions.

A stereolithographic (STL) apparatus for forming structures such as semiconductor die packages which uses a laser beam focused to a generally annular "spot" having an annulus of enhanced laser power surrounding a central "hole" of reduced (or no) laser power. The structures are formed of a stack of light-polymerized photopolymer layers. Scanning of a beam having power concentrated in the annulus enables simultaneous production of a self-supporting structure having at least semisolid, smooth lateral outer polymer walls and an upper hardened polymer skin extending over liquid polymer still lying between the walls. The structure may be subjected to heat or broad source light of suitable wavelength after removal from the STL apparatus to accelerate complete polymerization of the structure to a solid state.

A stable resonator for solid-state lasers which exhibit a thermally induced lensing effect, includes a laser rod, a rear mirror and a semi-reflecting output mirror. The invention is characterized in that the rear mirror has an extremely asymmetrical configuration, allowing the laser rod to move totally or almost totally toward the side of the output mirror. The laser rod is curved in a convex manner on one end in order to achieve a refractive effect, and a convex rear mirror is provided. As a result of the extreme asymmetry, the resonator has a beam quality as a function of the pump power with a comparably flat maximum even at relatively short resonator lengths in contrast to the state of the art. The effects of the thermal lens have practically no influence on processing results. Starting pulse behavior lies below the detection limit.