Hughes Aircraft Company

United States of America as Represented by the Secretary of the Air Force

David M Henderson

Richard L Abrams

Maurice J Halmos

Robert L Duvall, Iii

Douglas A Pinnow

Ronald R Stephens

A laser radar system employing a linear FM chirp laser followed by post detection pulse compression by a surface acoustic wave (SAW) device. The system includes an FM chirp modulator that provides the needed tuning range and linearity for pulse compression. This modulation is accomplished by using a high-pressure CO.sub.2 laser with an intracavity electro-optic modulator of CdTe. The frequency of the transmitter laser is accurately controlled through the application of high linearity, high frequency, sawtooth voltages to the two plates of the CdTe crystal, with the voltage waveform applied to one plate being out of phase with that applied to the other crystal plate. With the performance the system offers, absolute radar ranges can be measured to 0.22 m and velocities to 0.37 m/sec.

Linear FM chirp laser

A laser radar system employing a linear FM chirp laser followed by post detection pulse compression by a surface acoustic wave (SAW) device. The heart of the invention is the FM chirp modulator that provides the needed tuning range and linearity for pulse compression. This modulation is accomplished by using a high pressure CO.sub.2 laser with an intracavity electric-optic modulator of CdTe. The frequency of the laser is controlled through the voltage applied to the crystal. With the performance the system offers, absolute radar ranges can be measured to 0.22 m and velocities to 0.37m/sec.

A chirp laser system includes an intracavity electro-optical crystal modulated at a relatively high frequency, for example 250 kilohertz, to sweep the frequency of the transmitter laser above and below its nominal center frequency. One of the mirrors of the laser transmitter is adjustable to vary the length of the cavity and to shift the frequency of the transmitter laser. When the frequency of the transmitter laser is shifted, the output amplitude of the laser changes. With the center frequency of the laser at a maximum gain point, the amplitude of the output signals during positive and negative frequency excursions of the transmitter laser will be substantially symmetrical. However, if thermal or other effects cause the frequency of the laser to shift away from the maximum gain point, the output during positive and negative excursions will be asymmetrical. A small part of the output from the transmitter laser is sampled, and the amplitude during positive and negative frequency shift excursions is compared, using a square wave derived from the chirp modulating driver. Where the detected signals for the positive and negative frequency excursions are substantially equal, no change in the position of the mirror is accomplished; however, when the excursions result in a difference in the amplitude of the detected signals, the position of the piezoelectrically adjustable mirror is shifted to restore the nominal center frequency of the laser to its maximum amplitude point on the gain curve.

Chirp laser stabilization system

An imaging apparatus which permits the use of a transverse electrode electro-optic tunable filter (TEOTF), despite its obscured aperture, to produce a high quality image. The imaging apparatus includes a suitable conventional anamorphic input optical system in combination with a stacked plurality of TEOTFs in which the platelet of each TEOTF is made of an electro-optic material of a known index of refraction (such as CdS of an index of .about.2.4), and is clad between its surface and its electrodes with a material having a lower index of refraction than the platelet material (such as SiO.sub.2 of an index of .about.1.5).

Imaging apparatus for transverse electrode electro-optic tunable filter

Apparatus for encoding a laser beam with information indicative of position in the beam. A linearly polarized laser beam passes through a birefringent wedge which encodes a continuously varying polarization across one dimension thereof. The polarization-encoded laser beam is applied to an electro-optic modulator which induces a second harmonic component of an applied modulation signal to appear in the beam due to optical biasing by the polarization encoding. On beam center, only the fundamental appears, while going away from beam center the second harmonic appears, having a varying magnitude and phase. A polarizer transmits a linearly polarized component of the encoded beam. To encode a second dimension of the beam orthogonal to the first, a second wedge and modulator are provided. To provide discrimination between the two, the second modulator operates at a different fundamental modulation frequency. The doubly-encoded beam is applied to a polarizer which transmits a linearly polarized component of the laser beam. A receiver detects and separates the encoded signals by frequency, and the fundamental signals are frequency-doubled and compared with the second harmonic signals. A signal is developed which is a function of the magnitude and phase of the second harmonic of each encoded dimension. These signals are indicative of the receiver position in the beam, and may be used as error signals, as in a missile guidance system or the like.

Spatial coding of laser beams by optically biasing electro-optic

Apparatus for coding a laser beam to provide guidance signals, or the like. In the transmitter, the laser beam traverses two modulators to provide two-dimensional encoding. In the receiver, the detected signals are amplified, frequency-discriminated, and mixed to form guidance signals. The modulator is a modified Stark cell wherein four split longitudinal planar electrodes allow for beam encloding as a function of two frequencies by the process of molecular absorption. Bias and modulating signals applied to the electrodes interact with the molecular absorbing gas and with the beam to encode position information as a function of the specific frequencies. The amplitude of the first encoding frequency varies from a maximum at one edge of the beam, decreasing to a minimum at the opposite edge, and vice versa for the second encoding frequency. The amplitudes of both are equal on beam center. A second embodiment utilizes a pair of internal bias electrodes and four external electrodes to apply bias and modulating signals. The modulator wall is thin for penetration of the modulating signals so as to interact with the beam. A third embodiment utilizes four semicircular disc electrodes transparent to laser light affixed to the cell endcaps to provide both bias and modulation. A gas separates electrodes affixed to the same endcap. The endcaps are oriented such that the gaps are mutually perpendicular to allow for orthogonal encoding of the beam.

Spatial encoding of a laser beam by means of a Stark cell modulator

A wideband optical ring modulator is disclosed. A ring resonator containing a synchronous travelling wave electro-optic modulator is disposed external to a laser cavity. The invention provides a compact lightweight optical modulator which combines the power enhancement capabilities of a resonant cavity with the wideband capabilities of a travelling wave coupling modulator. The present invention is especially useful for, although not limited to, ultra-wideband infrared laser communication systems utilizing PCM techniques.

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Playa Del Rey, CA, United States of America

David M Henderson