A laser oscillator/amplifier system in which thermally induced birefringence in the gain media of the oscillator is compensated by thermally induced birefringence in the gain media of the amplifier. The system produces a quality output beam efficiently and with high power density in the preferred polarization. The oscillator cavity is characterized by a high reflector at one end and an output coupler at the second end with an oscillator gain medium exhibiting thermally induced birefringence in between. An amplifier exhibiting thermally induced birefringence which matches the thermally induced birefringence in the oscillator gain medium is mounted to receive the output of the oscillator cavity. A ninety degree rotator is mounted between the oscillator gain medium and the amplifier so that compensation for thermally induced birefringence in the oscillator gain medium is compensated by the matching thermally induced birefringence in the amplifier. The ninety degree rotator may be mounted inside the oscillator cavity, that is, between the oscillator gain medium and the output coupler; or it may be mounted outside of the oscillator cavity, that is, between the output coupler and the amplifier.

The present invention features a slab laser apparatus having a stable/unstable resonator. The slab laser comprises a slab-shaped medium for producing a beam of coherent light. The slab-shaped medium is physically defined by an entrance end face; a rear, exiting end face; and two oppositely disposed and substantially parallel side faces. The respective end faces of the medium are disposed along an axis defining a "p" plane; the side faces are disposed along an axis defining an plane. A light beam entering at the entrance end face of the slab-shaped medium along the "p" plane axis reflects back and forth within the slab-shaped medium via total internal reflection, exiting at the rear end face. A stable optical cavity is defined for the "p" plane, comprising a hemispherical, high reflector curvature having an finite radius of curvature, and a flat, partially reflecting, outcoupler. An unstable optical resonator is defined for the plane, which operates in a low order transverse mode. The resonator in the plane consists of a high reflector mirror, and a negative radius of curvature, partially reflective mirror. The partially-reflecting outcoupler is a substantially cylindrical optical component onto which has been coated a variable reflectivity mirror (VRM) profile. The transmission of this mirror surface varies in the transverse direction. The optical component is flat in the "p" plane and cylindrical in the "s" plane. The VRM coating is disposed on the cylindrical face. The outside surface may be coated with an anti-reflective coating.

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 tunable laser system having a wide tunable range, and narrow line widths, achieves relatively high output powers. The tunable laser system includes a master optical parametric oscillator which generates a seed beam, and a power optical parametric oscillator which is responsive to the seed beam to generate a narrow line width, high power output beam. The master OPO and power OPO comprise gain media consisting of BBO, tunable over a range from about 400 nanometers to more than 2000 nanometers. The master OPO includes line narrowing elements, such as a tunable grating, which limits the line width of the output beam to less than one wave number (centimeter.sup.-1). Pump energy is supplied to the master OPO and power OPO using a Nd:YAG laser with a harmonic generator, so that the second, third, or fourth harmonics of the primary 1064 nanometer line of YAG can be used to pump the BBO crystals. The power OPO may be an unstable resonator.

A laser 10 comprises a pump source 11, a resonator 13 including an etalon of gain material 15, and means 12 to focus the pump source 11 output onto one end 14 of the resonator 13 as a spot having a diameter in the range 50-150 .mu.m. The resonator 13 includes substantially parallel end faces 14,19 having a relative taper better than 0.25 milliradians, and the end faces 14,19 are substantially free of scratches greater than 10 .mu.m in size and pits greater than 5 .mu.m in size. A thermally induced microlens 22 is formed at the surface 14 of the gain material 15.