Terry A. Dorschner

Optical phased array technology

Optical phased arrays represent an enabling new technology that makes possible simple affordable, lightweight, optical sensors offering very precise stabilization, random-access pointing programmable multiple simultaneous beams, a dynamic focus/defocus capability, and moderate to excellent optical power handling capability. These new arrays steer or otherwise operate on an already formed beam. A phase profile is imposed on an optical beam as it is either transmitted through or reflected from the phase shifter array. The imposed phase profile steers, focuses, fans out, or corrects phase aberrations on the beam. The array of optical phase shifters is realized through lithographic patterning of an electrical addressing network on the superstrate of a liquid crystal waveplate. Refractive index changes sufficiently large to realize full-wave differential phase shifts can be effected using low (<10 V) voltages applied to the liquid crystal phase plate electrodes. High efficiency large-angle steering with phased arrays requires phase shifter spacing on the order of a wavelength or less; consequently addressing issues make 1-D optical arrays much more practical than 2-D arrays. Orthogonal oriented 1-D phased arrays are used to deflect a beam in both dimensions. Optical phased arrays with apertures on the order of 4 cm by 4 cm have been fabricated for steering green, red, 1.06 /spl mu/m, and 10.6 /spl mu/m radiation. System concepts that include a passive acquisition sensor as well as a laser radar are presented.

High-efficiency liquid-crystal optical phased-array beam steering.

Efficient, electrically tunable, agile, inertialess, near-diffraction-limited one-dimensional optical beam steering is demonstrated at the infrared wavelength of 10.6 microm with a liquid-crystal phased array.

Applications look at the use of liquid crystal writable gratings for steering passive radiation

Liquid crystal writable grating technology is being developed for beam steering in laser radar systems. We consider the ability of writable gratings to steer broad-spectral-band radiation for use in passive sensors. We find that there is potential for these devices in microscan systems because there is little or no dispersion for the small scan angles required in microscanning. The dispersion that is present is less than the resolution of the sensor considered here. For large angle steering we find that dispersion correction or a narrowing of the spectral bandwidth is required. The degradation in sensitivity resulting from narrowing the spectral bandwidth is considered. We find that a high-quantum-efficiency step-stare sensor with a two-dimensional focal plane array responsive over a narrow spectral width can achieve the same sensitivity as current linear scanning sensors while being able to steer the field of view (FOV) over a larger field of regard with no moving parts. Approaches for dispersion correction and postdetection correction are discussed. A promising approach for steering a narrow FOV with broad spectral content and good resolution is described.

Laser gyro at quantum limit

We show that a certain fundamental limit applies to the accuracy of all optical rotation sensors which use laser light as a probe. We derive this fundamental rotation-rate uncertainty from the Heisenberg uncertainty relations and Glaubers minimum uncertainty states. The same relationship is obtained from a spontaneous-emission noise formulation. We present experimental data on a (nondithered) four-frequency ring laser gyroscope for which this limit is attained.

The Multioscillator Ring Laser Gyroscope

Abstract The multioscillator (or four-frequency) ring laser gyroscope is discussed from both a theoretical and a practical point of view. Fundamentals of device operation are presented, important nonideal behaviors (error sources) are discussed and analyzed from first principles, typical multioscillator gyroscopes are described, and samples of representative data from developmental instruments in our laboratories are reviewed. A key to the development of practical multioscillator instruments has been the introduction of nonplanar ring resonators. The theoretical formalisms (geometric and wave optic) necessary for understanding the properties of nonplanar ring resonators, and nonplanar gyroscopes, are derived and discussed in detail. Much of the material presented is new.

Spatially resolved phase imaging of a programmable liquid-crystal grating

Phase imaging is used to compare near-field measurements with the corresponding far-field intensity distribution. A liquid-crystal device serves as a phase object that can be programmed as a variable grating. Real-time phase visualization then provides an avenue for direct optimization of complex phase gratings.

Electrically tunable liquid-crystal wave plate in the infrared

Phase retardance of a liquid-crystal-based, electrically tunable wave plate as a function of voltage and incident light intensity at 10.6 microm is measured using the Stokes-MacCullaugh ellipsometry technique. At intensities of up to 900 W/cm(2), device performance is found to be driven by thermal effects and not optically induced reorientation effects.

An optical phased array for lasers

High-performance, optical phased arrays for electronic control of laser beams have recently been demonstrated. The optical aperture is lithographically fabricated to form a unity-fill-factor array of liquid-crystal-based optical phase shifters. The liquid-crystal phase shifters can be fabricated smaller than a wavelength of light, with spacings equally small, although this is not required in these devices for small angle steering applications. Computer control of the individual phase shifters establishes a phase profile across the optical aperture that steers or otherwise controls an incident optical beam. Prototypes of such optical phased arrays have been used to demonstrate electronically programmable optical beam steering, lensing, fanout, and conditioning. The devices offer high pointing accuracy and resolution in a small size, owing to the much shorter wavelength of light as compared to microwaves. This inertialess technology makes available to optical systems many of the performance capabilities and the functional versatility long afforded microwave systems by microwave phased array antennas. Operation of such optical phased arrays has been successfully demonstrated from the green (doubled Nd:YAG lasers) to the long-wave infrared (carbon dioxide lasers). The capability represents a major technological advance and is expected to enable numerous new optical systems. This paper focuses on a comparison of the new optical phased array technology with that of conventional microwave phased arrays. Operating principles are briefly reviewed from dual perspectives: namely, those of the microwave and optical regimes. A brief discussion of system applications is included.

Nonplanar Rings For Laser Gyroscopes

Conventional ring laser gyroscopes (RLGs) are planar; the light beams are constrained to lie in a single plane. This need not be the case. We have found that nonplanar ring resonators offer certain advantages for the multioscillator (four frequency) RLGs developed by Raytheon. An overview is given of those properties of nonplanar rings which are pertinent to RLGs, including the inertial rotation sensitivity (the scale factor and the direction of the sensitive axis), the mode polarization, and the frequency spectrum. Design equations for a class of simple nonplanar rings are given and applications to multioscillator RLGs are discussed.

Nonmechanical beam steering for active and passive sensors

Liquid crystal writable grating technology is being developed for beam steering in laser radar systems. To date, steering of 10.6 micrometers , 1.06 micrometers , and 0.53 micrometers wavelengths has been demonstrated. Preliminary results are described. In this paper we also consider the ability of writable gratings to steer broad spectral band radiation for use in passive sensors. We find that there is potential for these devices in microscan systems because there is little or no dispersion for the small scan angles required in microscanning. The dispersion that is present is less than the resolution of the sensor considered here. For large angle steering we find that dispersion correction or a narrowing of the spectral bandwidth is required. Approaches for dispersion correction and post-detection compensation are discussed.