Abstract: In a laser emitter-receiver system, a tunable diode laser provides a beam of coherent light having a selected frequency. An infrared detector spaced from the source detects the light transmitted along the beam path between the source and the detector and produces a corresponding output in response thereto. A heating laser provides radiation to the diode laser to heat, and thereby to change the frequency of, the latter laser. The radiation from the heating laser is varied periodically to modulate the intensity of the radiation incident on the diode laser thereby to frequency-modulate the diode laser output over a selected tuning range. A controller responsive to the detector output regulates the current to the diode laser to minimize selected frequency components of the modulated light beam from that laser so that the power output of the diode laser varies minimally when that laser is tuned over the selected tuning range.

Abstract: A gas replenishment system for a gas laser utilizes a gas metering device between a gas reservoir and the gas laser tube which operates at a pressure below the gas reservoir pressure. The metering device incorporates a body of gas entrapping material. The tube pressure is detected and when low an electrically controlled valve is energized for a predetermined time which allows gas to flow from the reservoir, into the gas entrapping material and from there to the tube. If the tube pressure is not restored to its proper level, the system recycles for another equivalent period of time.

Abstract: Monochromatic light from a laser gain generator passes through a first aperture to a beam expander which expands the beam width and reduces the angular divergence of the beam. The light from the beam expander has a curved wavefront because of (1) light diffraction in the aperture and (2) deviations from planar surfaces in the optical components in the light path. The light is then reflected by a first mirror to a grating which has a plurality of steps in a stepped configuration. The grating is adjustable curved so that the steps in the grating are parallel to the curved wavefront and so that light from only a particular band of wavelengths is reflected back to the first mirror. The light then passes back through the beam expander, which compresses the beam width, to the discharge chamber for further amplification. The light then passes through a second aperture to a partially reflecting mirror. A portion of the light is reflected by the partial mirror back to the discharge chamber for further amplification.

Abstract: A 2.7Sg(m)m laser comprises a single mode fluoro-zirconate optical fibre doped with 0.086 mole % Er.sup.3+. C-W operation of the usually self-terminating lasing transition .sup.4 I.sub.11/2 to .sup.4 I.sub.13/2 is achieved by applying excitation energy to a suitable wavelength and intensity to elevate ions from the lower lasing level to maintain a population inversion between the upper and lower lasing levels during lasing. The laser may be pumped at a single wavelength of about 785 nm by a semiconductor laser.

Abstract: An NC laser device is provided which is a combination of a gas laser oscillator having a cooling mechanism for forcibly cooling a laser gas by a blower and a cooling unit, and a numerical control device. The NC laser device includes a gas temperature measuring device for measuring a gas temperature at an inlet of the blower, a casing temperature measuring device for measuring a temperature of a casing of the blower, and an abnormality detecting device, etc. The abnormality detecting device detects an abnormal state of the cooling mechanism, and stops the operation of the gas laser oscillator, when the inlet gas temperature or the casing temperature becomes higher than the respective reference value. Accordingly, an abnormal state of the cooling mechanism can be detected at an early stage, and thus secondary damage to the blower can be prevented.

Abstract: An apparatus for controlling an output from an excimer laser device wherein at least two etalons each having a large diameter are arranged between a rear mirror and a laser chamber for the excimer laser device is disclosed. With the apparatus, the operative range of laser light outputted from the excimer laser device can be narrowed by executing superposition control for a wavelength of laser light which transmits through each etalon, while maintaining the number of spatial lateral modes. The apparatus is preferably employable as a light source particularly for a unit for projecting and exposing figures in a reduced size. Superposition control with the use of a superposition control unit, excitation intensity control for laser gas with the use of an excitation intensity control unit and partial gas replacement control for laser gas with the use of a gas control unit are executed in a predetermined timing relationship, respectively.

Abstract: Disclosed is a laser apparatus for effecting laser oscillation by exciting a laser medium by discharge between an anode and a cathode opposedly arranged to each other in a discharge tube, comprising means disposed in the discharge tube to support both the electrodes movably along the axial direction of the discharge tube so as to prevent warp of the electrodes on laser oscillation and keep the parallelism therebetween with high accuracy. Also disclosed is a laser apparatus comprising main discharge means for effecting laser oscillation by exciting a laser medium by generating discharge, and preionization means for generating ionization previously to the discharge by the main discharge means so as to stabilize laser oscillation under a high gas pressure and a high current density.

Abstract: An external cavity laser assembly in which the laser chip is within a hermetically sealed package and the external cavity is outside the hermetic package. A collimating lens in a screw threaded mounting is disposed outside the hermetic package, the screw thread permitting focussing. The cavity is turned by rotation and translation of a grating, the position of which is electrically controlled via piezoelectric stacks. Thermal expansion of the stacks is compensated by mounting the grating on a rod of suitable material (e.g., ceramic) which also serves to reduce the cavity length while allowing maximum piezoextension. In a second embodiment the assembly is a tunable laser amplifier, with a grating being driven in a similar manner to provide the tuning.

Abstract: A gas laser housing having a housing and a metal cap attached thereto, includes a mirror mount attached to the metal cap. The mirror mount is provided with an annular region of reduced diameter affixed to the metal cap to absorb mechanical stresses and thereby prevents stress crack corrosion at the circumference of the housing tube.

Abstract: A laser device includes an optical resonator comprising a laser medium and two totally reflecting mirrors disposed on respective sides of the laser medium. The laser device also has a wavelength selection element disposed between one of the totally reflecting mirrors and the laser medium. A partially reflecting mirror is disposed between the wavelength selection element and the laser medium. Any distortion and deterioration of the wavelength selection device are greatly reduced.

Abstract: An electronics controller for controlling the power supply of a laser operating in a pulse mode is disclosed. The power supply is of the type having a resonant DC converter, charging a capacitive storage means with an inverter stage driving the laser discharge tube. The current supplied to the laser discharge tube is monitored. In addition, the voltage of the capacitive storage means is also measured. Set points for the current, the voltage, and on/off are provided to the electronics controller as the comparison points for the controller. Based upon the voltage and current detected, and the set points provided, the controller controls the on/off of the converter and its frequency, and the on/off of the inverter.

Abstract: An actively stabilized laser is disclosed which includes a temperature tunable etalon in the laser cavity for single frequency operation. The operation of the laser is actively monitored to detect variations induced by changes in the length of the resonator. These changes tend to cause the frequency of the selected longitudinal mode to shift so that it is no longer aligned with the peak of the transmission curve of the etalon. When this misalignment occurs, operation deteriorates. By monitoring laser performance, the misalignment can be detected and the temperature of the etalon can be changed to realign its transmission peak to the selected mode. In the preferred embodiments, either the laser output power or the current supplied to the laser is monitored. This approach substantially reduces mode hopping.

Abstract: A laser system is disclosed wherein a solid state gain medium (30) is longitudinally pumped with the beam (40) from another laser (42). The laser is configured with an optically stable resonator while at the same time matching the shape of the pump beam to the shape of the cavity beam in the gain medium. Optimum matching is achieved by taking into consideration thermal lens effects in the gain medium resulting from absorption of the pump beam.

Abstract: The present invention proposes a dielectric waveguide formed on a substrate of ultra-low thermal expansion glass which is assembled with a commercially available diode laser to create a temperature stabilized laser. The waveguide comprises multiple dielectric films which have equal and opposite temperature induced changes in refractive index with respect to each other into which is formed a Bragg grating, the grooves of which are sufficiently shallow to allow penetration of light into the waveguide of 1 mm to 1 cm. This provides a signal which is both narrowband and frequency stable so that the optical signal can be guaranteed to remain in a given narrow frequency band. The dielectric layers are deposited using ion assisted deposition (IAD) to provide uniform, high density films with reduced index-temperature coefficients and increased density, resulting in a waveguide with near-zero temperature variations in refractive index.

Abstract: A birefringent filter ("BRF") unit, for use in the cavity of a tunable pulsed laser generating ultrashort pulses, including means for suppressing satellite pulses resulting from surface reflections. In one preferred embodiment, the inventive BRF has substantially parallel front and back surfaces, and is cut so that its optical axis is not parallel to its front surface. For example, the inventive BRF may be a quartz crystal having a thickness of 3 mm, with its optical axis oriented at an angle substantially equal to 50 degrees from the plane of its front surface. In a second preferred embodiment, the inventive BRF includes a relatively thick, non-birefringent component coupled with index matching material to a relatively thin birefringent component. In a third preferred embodiment, the inventive BRF includes a pair of thick birefringent components designed so that the ordinary ray of the first component becomes the extraordinary ray of the second component.

Abstract: A semiconductor laser controller converts the light output of the driven laser to a photovoltaic current proportional to that light output and separates the current into high and low frequency components. A negative feedback loop controls the forward current of the laser such that the sum of these high and low frequency components of the photovoltaic current equal a reference signal current defining the light output of the laser. A control unit controls the reference current such that a voltage proportional to the low frequency component of the photovoltaic current equals a reference voltage corresponding to the reference current.

Abstract: In a laser device for producing a laser beam along a first predetermined direction by exciting a slab-shaped laser medium by an excitation member, a reciprocating motion is carried out along a second predetermined direction transverse to the first predetermined direction and is divided into a plurality of cycles which have different amplitudes. Both edge zones of the slab-shaped laser medium are excited and heated by the excitation member during a selected one of the cycles while they are not excited during the remaining cycles. Thus, heat accumulated in the edge zones is diffused and dissipated by intermittently heating the edge zones. The slab-shaped laser medium may be reciprocated relative to the excitation member. Alternatively, the excitation member may be reciprocated relative to the slab-shaped laser medium.

Abstract: A servo alignment method and apparatus for maintaining laser beam alignment. An operating parameter of the laser (such as output beam power) is measured, and error signals are generated from the measured operating parameter signal for controlling the laser beam alignment. In one class of embodiments, modulation signals are supplied to an intra-cavity component of a laser system to modulate the beam path. The beam path may be modulated by reciprocating a laser resonator mirror about a mirror axis or two substantially orthogonal mirror axes. In the latter case, the mirror may be vibrated about the axes either alternately (first about one axis and then about the other) or simultaneously.

Abstract: The stability of the frequency and/or the efficiency of a laser ultimately depends on the constancy of the resonator length. To ensure this demand the resonator elements are mounted on holders supported by longitudinal stabilizer bars, with the holders and bars being made of ceramic material.

Abstract: An echo cancelling circuit for use with a laser whose generated light is connected to an optical fiber by a connector. The connector reflects part of the laser light to a detector associated with the laser. The echo cancelling circuit attenuates the laser generated light by a predetermined factor. The cancelling circuit then detects the attenuated laser light and develops a signal whose amplitude and phase is such as to substantially cancel the signal representative of the light reflected from the connector.

Abstract: The frequency conversion element for producing light of higher harmonic wave from the fundamental wave laser oscillator is provided with strain gauges 4, 4 or piezoelectric elements 4', 4' or thermo-couples 6, 6, output signals whereof are led to a signal processor and to a comparator, thereby to issue a control signal to decrease power of the input laser light, to prevent thermal breakdown of the frequency conversion element.

Abstract: A semiconductor laser drive device modulates optical output of a semiconductor laser in accordance with an externally provided modulation signal comprises a compensation circuit for calculating a compensation coefficient and for generating a compensation modulation signal, a threshold value current generation circuit for generating and a threshold value current to the compensation modulation signal and sending the added current as a drive current, and a current drive circuit for driving the semiconductor laser based on the drive current sent from the threshold value current generation circuit. In accordance with the semiconductor laser, the optical output error caused by the temperature change of the semiconductor laser is compensated.

Abstract: A method and apparatus which corrects the amplutide variation in the output in an electro-optic modulated laser system (10). Birefringence within the electro-optic crystal (25) causes elliptical polarization of the linearly polarized laser light (22) produced by the laser gain medium (16), in turn causing a loss in the laser output (27). The loss owing to the elliptical polarization is corrected by either applying a D.C. bias from voltage source (30) directly to the electro-optic crystal (25) or by inserting a quarter-wave plate (32) within the laser's resonant cavity.

Abstract: This invention describes a method and apparatus that can be used in optic quality monitoring of a gas laser. The method involves measuring the temperature of at least one of the optics in a gas laser, then subtracting the temperature of the laser enclosure from the optic temperature to obtain a temperature differential, and finally dividing the differential by a value proportional to the output power of the laser. The final value, termed "figure of merit", gives a quantitative measure of the condition of the optic. As a result appropriate cleaning and replacement decisions can be made for one or both of the optics in a gas laser.

Abstract: In order to compensate for the thermally induced lens in the active element of an optical stage of a laser, the optical stage is provided with a telescope. The telescope comprises first and second lens arrangements. The first lens arrangement is located so that thermally induced lens is located at one of its focal planes. The position of the second lens arrangement is adjusted so as to compensate for variations in the power of flashlamps which pump the active element. In one embodiment, the laser has an oscillator stage and an amplifier stage. The telescope is placed after the oscillator stage and comprises two convex lenses.

Abstract: A laser diode impulse circuit has a normally forward biased step recovery diode coupled to the output of a current pulse generating circuit. An inductor in parallel with a series combination of a pulse shaping network and a laser diode is coupled to the output of the step recovery diode. When the step recovery diode becomes reverse biased in response to a current pulse from the current pulse generating circuit, energy is stored in the inductor. The stored energy is then transferred from the inductor to the laser diode via the pulse shaping network to produce an optical pulse having a desired shape.

Abstract: A laser diode control circuit maintains a desired average output power of the laser diode by detecting the optical output, amplifying the detected output, and controlling a bias current of the laser diode accordingly. A pilot tone is combined with the detected output and amplified therewith, and the amplified level of the tone is detected and used to control a modulation current for the laser diode. In one described arrangement the laser diode is controlled to have a high extinction ratio with the bias current at the knee of the diode's characteristic curve, and in another arrangement a lower extinction ratio is provided with the bias current above the knee.

Abstract: Novel designs for an efficient coupling mechanism for the conversion of typical profile laser beams into Jo profile beams are presented. Five specific embodiments are provided which employ non-linear optical mechanisms in the conversion process.

Abstract: A laser apparatus includes an optical resonator having a laser medium and generating a laser light. A wavelength selection element selects a given wavelength of the laser light. The selected wavelength resides within a gain bandwidth of the laser medium. A sealed container houses the wavelength selection element. The sealed container may be filled with a gas. A density of the gas within the sealed container may be changed to vary the selected wavelength.

Abstract: A mutually pumped phase conjugator (MPPC) is used to remove spatial distortions from a high power laser beam. The high power beam with its "dirty" spatial profile is directed into one side of the MPPC. A spatially clean beam at the same nominal frequency as the high power beam is output from a continuous wave (CW) laser and directed into the opposite side of the MPPC. As a result of mutually pumped phase conjugation, the MPPC returns a phase conjugate beam that retains the clean spatial profile of the CW beam but acquires the temporal characteristics of the high power beam. A Faraday isolation system may be used to separate the phase conjugate beam from the incident CW beam to provide an output beam having a clean spatial profile. If desired, the high power beam can be transformed into an image bearing output beam simply by modifying the incident CW beam with a transparency or spatial light modulator.

Abstract: A monolithically integrated structure comprises a laser diode, a photodetector diode and a reference diode which are monolithically integrated on a common substrate. The photodetector diode is optically coupled to the laser diode. The reference diode is substantially identical to the photodetector diode and optically decoupled from the laser diode. When substantially equal reverse biases are applied to the photodetector diode and the reference diode, the reference diode conducts a leakage current which can be used to substantially cancel the leakage current of the photodetector diode. The monolithically integrated structure may also include a modulator diode which is monolithically integrated on the common substrate and optically coupled to the laser diode. The monolithically integrated structure is useful in optical communications systems.

Abstract: In case that the light beam emitted from a light source mounted on a base element is focused to a spot light on a data storage media to write the data, when the light beam reflected from the data storage media to return to the light source is reflected from the base element and focused again on the data storage media, it becomes difficult to precisely read/write the data on the storage media. In order to resolve the above problem, a light beam treating means is provided in the base element to prevent the re-incident light beams from being reflected toward the data storage media and focused thereon.

Abstract: A method is disclosed of adjusting the light-emission of a light-emitting diode to a predetermined output level for a given driving current. The light-emitting diode is irradiated with a laser beam in a manner to burn tiny holes in the light-emitting surface to such an extent that said predetermined output level is obtained. The invention furthermore comprises a method of obtaining a substantially uniform light output of an array of light-emitting diodes.

Abstract: A tunable solid state laser having a titanium:sapphire grain medium and a birefringent filter mounted within a resonant cavity is provided. The resonant cavity is longitudinally pumped. The cavity is enclosed in a casing which has a small window admitting the pump beam into the resonant cavity, and a small window with an anti-reflection coating allowing the laser output to exit the casing. The casing is maintained at a positive pressure of nitrogen gas during operation of the laser to purge the resonant cavity of air and water vapor.

Abstract: An excimer laser including a discharge chamber containing a mixture of halogen, rare gas and buffer gas uses two gas sources to replenish the halogen in the discharge chamber while maintaining the concentrations of all the gases at the optimum levels. One of the gas sources, source A, contains a mixture of the rare gas and the buffer gas in optimum concentrations. The other gas source, source B, contains both the rare gas and the buffer gas, in optimum relative concentrations, and also the halogen, in a concentration which is greater than optimum. As the laser operates and the gain of the laser decreases due to halogen depletion, gas from source B is injected into the discharge chamber to raise the halogen concentration. Gas is then released from the chamber to reduce the chamber pressure to the original level.

Abstract: According to present invention, the central wavelength of the central wave of the output laser beam, and the sideband wave power or central wave power of the output laser beam are detected, and the wavelength selective characteristics of wavelength selective elements disposed between a laser chamber and a rear mirror are controlled such that the detected central wavelength falls within a desired allowable range and that the detected power becomes minimum or maximum.

Abstract: A low-pressure mercury resonance radiation source includes a discharge lamp containing mercury vapor and producing broad band radiation directly emitted from the mercury vapor through excitation of same by electric discharge, and a resonance cell containing mercury vapor and disposed adjacent the discharge lamp for absorbing the broad band radiation and reemitting narrow band mercury resonance radiation. The pressure of the mercury vapor within the resonance cell is controlled to a first predetermined value by controlling the temperature of a first mercury sump arranged within the resonance cell, and the pressure of the mercury vapor within the discharge lamp is controlled to a second predetermined value, preferably by controlling the temperature of a second mercury sump arranged in a side tube communicating with the discharge lamp.

Abstract: A high-frequency discharge pumped laser device for producing a laser oscillation through application of a high-frequency voltage to a discharge tube, which includes a plurality of tube segments constituting the discharge tube, means for passing a laser gas axially through each of the tube segments, high-frequency power supplies for supplying a high-frequency voltage to the tube segments to generate a laser beam output, and a partition wall made of an electroconductive material and arranged between adjacent tube segments, the potential of the partition wall being maintained at a constant level. As the potential of the partition wall is maintained at a constant level, a mutual current caused by a stray capacity between the tube segments is eliminated, and thus fluctuation of the laser beam output is reduced.

Abstract: A gas laser device for effecting a laser oscillation while circulating a laser gas is disclosed, the gas laser device comprising a laser gun supply unit (5), a gas supply valve (6) provided at the outlet of the laser gas supply unit, an exhaust unit (9) for exhausting the laser gas in a discharge tube, a pressure sensor (7), a gas cylinder replacement check switch (5a), a control unit (10) and the like, wherein the control unit (10) exhausts the air after closing the gas supply valve, introduces a laser gas by opening the gas supply valve after the laser oscillator has been brought to a vacuum state, and effects a vacuum exhausting of the laser oscillator after closing the gas supply valve, whereby air in a pipe is exhausted from the oscillator when a gas cylinder is replaced to eliminate such disadvantages as a reduced laser output, and an unstable laser output, and the like.

Abstract: A metal vapor laser discharge tube configuration for preventing accumulation of the condensed metal form interfering with the generated laser beam. A heater arrangement is positioned on the tube in an area corresponding to the area in which the metal vapor condenses within the tube. Energization of the heater when required melts the condensate thus extending the laser tube lifetime.

Abstract: 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.

Abstract: The liquid crystal is written with a laser beam. The laser beam generated by a laser source is applied through an optical control unit such as galvanometer mirrors to the liquid crystal. The intensity of the laser beam applied to the liquid crystal is controlled by a laser beam intensity varying unit. A writing control unit controls the mechanical displacement caused by the optical axis control unit and controls the laser beam intensity varying unit. The laser beam energy applied to the liquid crystal can be maintained constant, thus enabling a constant written line width even during acceleration and deceleration of the optical axis control unit.

Abstract: Misalignment of a laser beam due to bending of a plasma tube for an internal resonator ion laser, or another structural element supporting beam guiding optics, is controlled by varying the distribution of heat flow out of the plasma tube or structural element to compensate for any misalignment in response to position of the laser beam. A detector is mounted along the optical path of the laser beam, and generates a position signal indicating drift of the laser beam from a preferred position. A cooling system, thermally connected to the structural material of the plasma tube or the structural element and connected to the detector, conducts heat out of the structural material in a controlled distribution in response to the position signal, so that misalignment of the optics due to thermal bending is minimized.

Abstract: A control apparatus for a pumped rod-type laser includes an arc lamp disposed to illuminate the lasing medium, such as a NdYAG crystalline rod. The apparatus includes a full wave rectifier to power the arc lamp, and a MOSFET switching circuit to turn on and off the arc lamp power at controlled times during each half cycle of the power waveform so that the laser medium is pumped and optically discharged once during each half cycle of the power supply. The laser power output is measured by a photodetector during each half cycle, and the photodetector output is integrated and compared with a manually set, variable laser output power level. When the actual laser power reaches the preset power level, the comparator initiates turning off the MOSFET switching circuit power for that respective half cycle of the power waveform. The full wave rectified power supply also permits the use of 110 VAC utility power.

Abstract: Sensitive components of a laser such as a resonator mirror, a decoupling mirror or the like, are protected against an initial laser beam surge caused by the ignition of a laser with an arc discharge lamp, by a beam interrupting switch having two switch sections each carrying a filter or shutter switchable into a laser beam path in response to an arc discharge lamp ignition pulse. For a beam interruption the filters or shutters are located directly in front of and directly behind of a lasing element as viewed in the beam path direction. The switch sections are operated by a shifting mechanism responsive to the ignition pulse to bring the switch sections into the beam path and to a control signal to bring the switch sections out of the beam path or into a beam releasing position.

Abstract: Squeezed states of light having significant degrees of squeezing are achieved with modest values of atomic density and intracavity electromagnetic (EM) field by a device and method for coupling the EM field within the optical cavity. An oscillatory exchange of excitation then occurs between the atomic polarization and the EM field within the cavity.

Abstract: A modified laser tube including a second pinch off tube or port for flowing argon and krypton gas through the laser for processing to remove contaminants to improve performance of new and used tubes such as ALC 60X argon ion lasers. Lasing discharge is maintained during processing to release contaminants for removal by plasma scrubbing of the laser bore. A magnetic field is applied and rotated in a plane perpendicular to the laser during discharge to enhance contamination release during plasma scrubbing. Preliminary cathode activation, control of gas flow rates and tube power as well as alternating of argon and krypton gas controls the contaminant release rate to prevent recontamination of the laser by released contaminants before removal.

Abstract: The laser diode modulator disclosed herein employs a series of field-effect transistors provided with an input circuit utilizing a succession of inductive elements arranged to form a lumped element transmission line of essentially constant characteristic impedance. The drain or output terminals of the transistors drive an output circuit employing a series of inductive elements which, together with shunt capacitors, form a singly terminated filter providing a graduated and decreasing characteristic impedance. The laser diode is connected as the termination of the filter.

Abstract: A solid state laser including a block of Nd:YVO.sub.2, Nd:YSAG, or Nd:YAB laser material having a cavity-defining mirror at one of its end surfaces is pumped by a laser diode pumping source having narrow bandwidth and is caused to lase in a single longitudinal mode by controlling the relationship between the pump power, the bandwidth of the lasing transition within the laser material, and the absorption depth of the pump radiation within the laser material.

Abstract: A high average power pockels cell is disclosed which reduces the effect of thermally induced strains in high average power laser technology. The pockels cell includes an elongated, substantially rectangular crystalline structure formed from a KDP-type material to eliminate shear strains. The X- and Y-axes are oriented substantially perpendicular to the edges of the crystal cross-section and to the C-axis direction of propagation to eliminate shear strains.