A method of initializing the position of an optical shaft encoder without having to precisely index the encoder with respect to its shaft. The optical encoder includes an optical shutter mounted for rotation with the shaft and light emitters and light detectors associated with the optical shutter for respectively illuminating and detecting a predetermined pattern of code markings formed on the optical shutter. An electrical circuit, such as a suitably programmed microprocessor, is responsive to the status of the light emitters and light detectors for determining the angular position of the optical shutter and shaft relative to the light emitters and detectors from the detected predetermined pattern. The optical shutter is mounted to the shaft in some arbitrary position and the code pattern associated with this position is detected by the electrical circuit. This position is then used as the "zero" or index position and all other angular positions of the optical shutter and shaft are thereafter indexed to this initial position. This eliminates the need to align the optical shutter with some indexing mark on the shaft or with a shaft driven pointer, such as utilized in the dial-type register display commonly used in gas, water or electric meters.

Pulses from a first laser source (10) at a predetermined high repetition rate are passed through an isolator (38), a mode-matched with an amplifier and then temporally multiplexed to a predetermined high repetition rate. The high repetition rate pulses (63) are then injected into a second source (70) where they are amplified to derive a stream (79) of high power pulses at the predetermined high repetition rate and then selectively released.

Lenses (30', 32') are orthogonally tilted with respect to one another in a telescope to provide imaging for an amplifier-phase conjugate mirror (PCM)(10) in a laser oscillator/amplifier (12). This mutually orthogonal tilting avoids the problem of air breakdown which occurs when the laser energy is otherwise focused at a single point and thus avoids the need to use a vacuum cell to suppress the sparking at the telescope's focus. An adjustment of the tilting of one lens (32') with respect to the other lens (30') also avoids astigmatism.

A multiple-pass laser amplifier that uses optical focusing between subsequent passes through a single gain medium so that a reproducibly stable beam size is achieved within the gain region. A confocal resonator or White Cell resonator is provided, including two or three curvilinearly shaped mirrors facing each other along a resonator axis and an optical gain medium positioned on the resonator axis between the mirrors (confocal resonator) or adjacent to one of the mirrors (White Cell). In a first embodiment, two mirrors, which may include adjacent lenses, are configured so that a light beam passing through the gain medium and incident on the first mirror is reflected by that mirror toward the second mirror in a direction approximately parallel to the resonator axis. A light beam translator, such as an optical flat of transparent material, is positioned to translate this light beam by a controllable amount toward or away from the resonator axis for each pass of the light beam through the translator. The optical gain medium may be solid-state, liquid or gaseous medium and may be pumped longitudinally or transversely. In a second embodiment, first and second mirrors face a third mirror in a White Cell configuration, and the optical gain medium is positioned at or adjacent to one of the mirrors. Defocusing means and optical gain medium cooling means are optionally provided with either embodiment, to controllably defocus the light beam, to cool the optical gain medium and to suppress thermal lensing in the gain medium.

A multiple-pass laser amplifier that uses optical focusing between subsequent passes through a single gain medium so that a reproducibly stable beam size is achieved within the gain region. A resonator or a White Cell cavity is provided, including two or more mirrors (planar or curvilinearly shaped) facing each other along a resonator axis and an optical gain medium positioned on a resonator axis between the mirrors or adjacent to one of the mirrors. In a first embodiment, two curvilinear mirrors, which may include adjacent lenses, are configured so that a light beam passing through the gain medium and incident on the first mirror is reflected by that mirror toward the second mirror in a direction approximately parallel to the resonator axis. A light beam translator, such as an optical flat of transparent material, is positioned to translate this light beam by a controllable amount toward or away from the resonator axis for each pass of the light beam through the translator. A second embodiment uses two curvilinear mirrors and one planar mirror, with a gain medium positioned in the optical path between each curvilinear mirror and the planar mirror. A third embodiment uses two curvilinear mirrors and two planar mirrors, with a gain medium positioned adjacent to a planar mirror. A fourth embodiment uses a curvilinear mirror and three planar mirrors, with a gain medium positioned adjacent to a planar mirror. A fourth embodiment uses four planar mirrors and a focusing lens system, with a gain medium positioned between the four mirrors. A fifth embodiment uses first and second planar mirrors, a focusing lens system and a third mirror that may be planar or curvilinear, with a gain medium positioned adjacent to the third mirror. A sixth embodiment uses two planar mirrors and a curvilinear mirror and a fourth mirror that may be planar or curvilinear, with a gain medium positioned adjacent to the fourth mirror. In a seventh embodiment, first and second mirrors face a third mirror, all curvilinear, in a White Cell configuration, and a gain medium is positioned adjacent to one of the mirrors.