A stable multi-fold telescopic resonator is applied to a slab laser to provide a high power laser beam of a high optical quality. The current invention could be applied to both waveguide and free-space slab lasers with either gas or solid-state active media. A reduced sensitivity to misalignment is achieved through the use of two short-radii concave folding mirrors forming a nearly-confocal or confocal optical resonator. In order to obtain a nearly-symmetrical output beam profile from the large thickness slab lasing medium, the resonator includes a slit diaphragm(s), positioned between the slab and resonator mirrors and aimed to eliminate higher order laser modes in the direction transverse to the plane of the slab. Furthermore, a three mirror multi-fold telescopic resonator is provided by substituting one of the resonator mirrors with a surface of one of the folding mirrors. Also, a four-mirror multi-fold telescopic resonator with the on-axis confocal or near-confocal folding mirror alignment is provided to cover a very large area slab lasing media.

A folded waveguide CO.sub.2 laser includes a plurality of waveguides arranged in a zigzag pattern with ends thereof overlapping. The laser includes a resonator having an axis extending through the plurality of waveguides. At least a portion of at least one of the waveguides has a uniform minimum width selected cooperative with the height of the waveguide and the laser wavelength such that the resonator can oscillate in only a single mode. At least a portion of one of the waveguides is tapered such that its width increases in one direction along the resonator axis. Tapering one or more of the waveguides provides that the total waveguide area and potential power output of the laser is greater than that of a zigzag arrangement of waveguides having the same total length waveguides each having a uniform width equal to the minimum width of the waveguide in the tapered waveguide arrangement.

An optical amplifier arrangement, including an amplifying medium, which exhibits an approximately rectangular cross section with a long edge and a short edge, as well as at least two highly reflecting mirrors, between which the amplifying medium is disposed, whereby the long or the short edge of the cross section is along the X axis or the Y axis; the Z axis is the optical axis; and the X, Y and Z axis form a rectangular coordinate system. The mirrors are designed and arranged in such a manner that one beam, which is to be amplified and beamed in by an oscillator, passes repeatedly through the amplifying medium in the XZ plane and is amplified; and the size of the beam to be amplified in the X direction becomes larger after each passage.

A gas laser comprises a pair of elongated electrodes arranged to define a discharge region between two opposing surfaces of said elongated electrodes, wherein the discharge region defines a longitudinal axis, a wide axis and a narrow axis. The gas laser further includes a lasing gas disposed in said discharge region and an excitation means for energizing the electrodes to excite the lasing gas. A first mirror is arranged in front of a first end of the pair of elongated electrodes, wherein the first mirror is spaced apart from the first end along the longitudinal axis by a first distance, and a second mirror is arranged in front of a second end of the pair of elongated electrodes. Moreover, the two opposing electrode surfaces define an electrode curvature, respectively, that is adapted such that a wave front of the a fundamental transverse radiation mode with respect to the narrow axis substantially coincides with a mirror curvature of the first mirror at the first distance.