Raytheon Company

Clarke E Harris

Wayne H Keene

Albert V Jelalian

Charles M Sonnenschein

Arthur A Chabot

Clifford E Morrow

Christopher R Miller

William A Richardson

Donald A Kawachi

Edward Barsack

Francis J Cusack, Jr

Richard E Seavey

An interferometer for transmitting a beam of electromagnetic energy toward a target, portions of the transmitted beam reflected by the target being directed onto a detecting surface of a detector element with a local oscillator beam to provide a beat frequency signal representative of, for example, target range. The interferometer comprises a first lens for focusing the target-reflected return beam onto substantially a point on a detecting surface. The interferometer additionally comprises a second lens, for example, for focusing the local oscillator beam through a focal point onto the first lens. The first and second lenses are disposed to align such focal point of the second lens with a focal point of the first lens to thereby direct the local oscillator beam onto the detecting surface of the detector element with a substantially uniform intensity distribution thereon. With such arrangement, the detector element may be implemented as a photoconductive, rather than a photovoltaic, detector with only a minimal net increase in the noise equivalent power (NEP) thereof, such as 5 dB, resulting in similar slight reduction in the signal-to-noise ratio of the interferometer. Since such photoconductive detector element may be cooled by thermoelectric stacks, the requirement for liquid nitrogen coolant is eliminated, thereby significantly reducing the size and weight of the interferometer system.

Optical system including interferometer for enhancing the performance of

A CO.sub.2 waveguide laser frequency modulation (FM) system which applies the Fourier series components of a triangular drive waveform selectively to the tuning PZT at one end of the waveguide and the FM PZT at the other end to achieve an improvement over the waveform than can be achieved by applying the fundamental and its harmonics to the FM PZT alone. The system applies the fundamental component to the tuning PZT and the odd harmonics to the FM PZT. The frequency-separated drive signals are applied in proper phase to the PZT's at each end of the waveguide via waveform synthesizer, frequency splitter and amplifiers. The waveform synthesizer includes a ROM containing digitized values of segments of the waveform amplitudes and a digital-to-analog converter for converting the digital ROM data to an analog signal.

Apparatus and method for frequency modulating a waveguide laser

A scanning laser radar system utilizing a frequency modulated (fm)-continuous wave (cw) beam to coherently detect echo signal returns from an object in a predetermined region. The frequency of the beam is repeatedly changed as a function of time to produce a symmetrical triangular-shaped frequency modulated waveform. The echo signals received are frequency shifted, as a function of range and Doppler shift, from the signal being transmitted at that time. The frequency shift is detected by homodyning a portion of the instantaneous transmitted signal with the received echo signal to produce an output signal having a frequency which is substantially constant during a portion of the frequency modulation period. The constant frequency portion of the output signal is related to the range and Doppler speed of the target. Target information such as range, Doppler speed, intensity and angle information are derived by processing the output signal. The signal to noise ratio of the output signal is improved by using a frequency modulation period greater than the scanning time across an arc of a size equal to a predetermined multiple of the beamwidth because with such relationship the time duration of the constant amplitude portion of the output signal is increased. Further, with such arrangement, because of the relatively long modulation period, the ambiguity in detection of the range to the target is reduced. The system is particularly useful in terrain following or obstacle avoidance applications.

FM-CW laser radar system

Ringing caused by acoustic resonances in an electro-optical crystal modulator and commencing at the trailing edge of a pulse may be suppressed by the combination of a second electro-optical crystal and a polarizer. The polarization of the ringing passing through the second crystal is rotated 90.degree. by applying an appropriate voltage to the second crystal at the trailing edge of the pulse. The polarizer, properly oriented, then blocks the ringing. The crystals may comprise such materials as Cadmium Telluride or Gallium Arsenide.

Optical modulator with ringing suppression

A laser heterodyne system is disclosed wherein received reflections of a beam of coherent light are amplified in a laser. The amplified received reflections are heterodyned with the light produced by the laser and the resulting beat frequency is detected using a photodetector. A controller, responsive to the beat frequency, controls, in one case, the frequency of a transmitting laser, and hence the frequency of the received light, and, in another case, the frequency of the light produced by the amplifying laser so that, in either case, the difference between the frequency of the received light and the frequency of the light produced by the amplifying laser is adjusted to maximize the amplification of the received light by the amlifying laser prior to detection by the photodetector.

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Wayland, MA, United States of America

Clarke E Harris