Paul F. McManamon

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.

A Review of Phased Array Steering for Narrow-Band Electrooptical Systems

Nonmechanical steering of optical beams will enable revolutionary systems with random access pointing, similar to microwave radar phased arrays. An early approach was birefringent liquid crystals writing a sawtooth phase profile in one polarization, using 2pi resets. Liquid crystals were used because of high birefringence. Fringing fields associated with voltage control required to implement the 2pi resets have limited the efficiency and steering angle of this beam steering approach. Because of steering angle limitations, this conventional liquid crystal steering approach is usually combined with a large angle step-steering approach. Volume holograms, birefringent prisms or sawtooth-profile birefringent phase gratings, and circular-type polarization gratings are the large angle step steering approaches that will be reviewed in this paper. Alternate steering approaches to the combined liquid crystal and step-steering approach exist. Microelectromechanical system mirrors, lenslet arrays, electrowetting, and a variable birefringent grating approach will be reviewed and compared against the conventional liquid crystal and step-steering approaches. Step-steering approaches can also be combined with these approaches. Multiple nonmechanical steering approaches are developing that will allow high-efficiency steering, excellent steering accuracy, and wide fields of view.

Review of ladar: a historic, yet emerging, sensor technology with rich phenomenology

Ladar is becoming more prominent due to the maturation of its component technologies, especially lasers. There are many forms of ladar. There is simple two-dimensional (2-D) ladar, similar to a passive electro-optic sensor, but with controlled illumination and the ability to see at night even at short wavelengths. There is three-dimensional (3-D) ladar, with angle/angle/range information. 3-D images are very powerful because shape is an invariant. 3-D images can be easily rotated to various perspectives. You can add gray scale or color, just like passive, or 2-D ladar, imaging. You can add precise velocity measurement, including vibrations. Ladar generates orders of magnitude higher frequency change then microwave radar for velocity measurement, because frequency change is proportional to one over the wavelength. Orders of magnitude higher frequency change means you can measure a given velocity orders of magnitude quicker, in many cases making an accurate measurement possible. Polarization can be used. With an active sensor you control both the illumination and the reception, so you can pattern the illumination. Also, because ladar can use narrow band illumination it is easier to easier to coherently combine sub-aperture images to obtain the higher resolution of an 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.

Modeling and Design of an Optimized Liquid-Crystal Optical Phased Array

In this paper, the physics that determines the performance limits of a diffractive optical element based on a liquid-crystal (LC) optical phased array (OPA) is investigated by numerical modeling. The influence of the fringing electric fields, the LC material properties, and the voltage optimization process is discussed. General design issues related to the LC OPA configuration, the diffraction angle, and the diffraction efficiency are discussed. A design for a wide-angle LC OPA is proposed for high-efficiency laser beam steering. This work provides fundamental understanding for a light beam deflected by a diffractive liquid-crystal device.

Liquid Crystal Optical Phase Plate with a Variable In-Plane Gradient

We propose a nematic liquid crystal (LC) optical phase plate, with a large continuous in-plane gradient that is variable, and its application to a beam steering device with high efficiency. The device is a vertically aligned, continuous phase, optical phased array (V-COPA) that uses a negative dielectric anisotropy LC material. High steering efficiency of over 95% is demonstrated by modeling the LC director field and its effect on transmitted light. The period of the V-COPA grating can be varied by adjusting an applied voltage profile, which allows for continuous angular control of the diffraction angle.

An overview of optical phased array technology and status

Optical Phased Array technology will revolutionize optical systems. Within the last year or two we have had a demonstration of nonmechanical beam steering over a 45 degree x 45 degree field of regard. This demonstration had a response time of 6 - 40 millisecond, and a beam size of about a half of a centimeter. It had an efficiency of 15 -20 %. We are now working to increase beam size and efficiency, and to decrease speed. This demonstration was for narrow band radiation. We are also working on methods of decreasing dispersion so we can steer broadband radiation. We have approaches for all of the above areas. Another area of work is to move into the MWIR and LWIR regions. Currently we are working mostly in the visible and NIR. In the long run we would like to combine optical phased array technology with microwave phased arrays, so it would be possible to steer both 15 Ghz , or about 1 cm wavelength, radiation and 200 THz, or about 1.5 um, radiation.

Agile Nonmechanical Beam Steering

The day is coming when engineers will be able to replace mechanical complex gimbal and steering mirror assemblies in electro-optical systems with thin, conformal devices that either have no moving parts or only make use of micro-motion.

Wavefront correction with photon echoes

Abstract If a first pulse with accumulated phase ϕ( x , y , z 0 ) and a plane wave second pulse generate a photon echo, the echo has phase -ϕ( x , y , z 0 ) and is wavefront corrected when returned along the path of the first pulse.

Broadband optical phased-array beam steering

An array of phase retarders can be used as an optical phased array OPA to steer light McManamon et al., Proc. IEEE 842, 268- 298 1996. The introduction of resets enables steering to larger angles without requiring an optical path difference OPD greater than one wavelength. These resets, however, are correct only at the design wave- length. The beam steerer is therefore very dispersive. It has been shown theoretically that resets of an integer multiple of the wavelength will make the beam steerer less dispersive McManamon and Watson, Proc. SPIE 4369, 140-148 2001. We offer the first experimental proof that resets of n are less dispersive than resets of a single . We also show experimentally that the dispersion associated with fixed period resets does vary, but only within a fixed limit. Last, we show the equivalent of power shifting from one order to the next as larger resets move from being divisible by one integer times the nondesign wavelength toward being divisible by the next integer times the nondesign wavelength.