Optical Phased Array Technology
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`PAUL F. MCMANAMON, SENIOR MEMBER, IEEE, TERRY A. DORSCHNER, MEMBER, tEEE,
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`DAVID L. CORK'UM, LARRY J. FRIEDMAN, MEMBER, IEEE,DOUGLAS S. HOBBS,
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`'
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`MICHAEL HOLZ, SERGEY LIBERMAN, HUY Q. NGUYEN, DANIEL P. RESLER,
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`RICHARD C. SHARP, MEMBER, IEEE, AND EDWARD A. WATSON
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`Optical phased arrays represent an enabling new technology
`that makes possible simple, affordable, lightweight, optical sensors
`ofl‘ering very precise stabilization, random-access pointing, pro—
`grammable multiple simultaneous beams, a dynamicfocus/defocus
`capability, and moderate to excellent optical power handling ca-
`pability. These new arrays steer or otherwise operate. on an
`already firrmed beam. as compared to modern microwave phased
`arrays which both generate a beam and direct it in a specific
`direction. 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 ofa liquid crystal
`waveplate. Refractive index changes sufificiently large to realize
`full-wave difi‘erential phase shifts can be eflected using low (<10
`V) voltages applied to the liquid crystal phase plate electrodes.
`High ejj‘iciency large-angle steering with phased arrays requires
`phase shifter spacing on the order of a wavelength or less; con-
`sequently addressing issues make I-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
`fabricatedfor steering green, red, 1.06 am, and 10.6 pm radiation.
`Steering efficiencies of about 60% at 4° and 85% at about 2°
`have been achieved to date with switching times as short as
`a few milliseconds in the visible. Fluences of several hundred
`W/cm2 have been demonstrated at 10.6 pm with nonoptimally
`engineered devices. Higher fluences can be handled at shorter
`wavelengths. Larger apertures are feasible, as is operation at
`other wavelengths and significantly faster switching times. System
`concepts that include a passive acquisition sensor as well as a
`laser radar are presented.
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`Manuscript received June 30. 1995; revised November 14, 1995. This
`work was supported in part by Raytheon intomal funds. and in part by
`the Air Force Wright Laboratory at Wright Patterson AFB, Dayton. OH.P.
`F. McManamon and E. A. Watson are with Wright Laboratory, Wright
`Patterson AFB, OH 45433 USA.
`D. L. Corkum is with Texas Instruments, Attlcborough, MA 02703
`USA.
`.
`T. A. Dorschncr, L. J. Friedman. D. S. Hobbs, M. Holz, D. P. Reslcr, and
`R. C. Sharp arc with Raytheon Company, Electronic Systems, Lexington,
`MA 02173 USA.
`H. Q. Nguyen is with Kopin Corp, Tnunton, MA 02173 USA.
`S. Libemtan is with SemiTcst. Billcrica, MA 0182l USA.
`Publisher ltcm identifier S 00l8-9219[96)01390-4.
`
`I.
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`INTRODUCTION
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`Currently optical sensor systems. including laser radar,
`are often limited in performance and cost by mechanical
`beam directing and stabilization mechanisms. The requisite
`pointing and stabilization usually requires precise, rapid,
`mechanical motion, and is often associated with substan-
`tial masses. Submicroradian steering precisions are often
`desired, but are usually impractical for available, afford-
`able, mechanical beam directing systems. Most mechanical
`systems do not facilitate rapid random pointing. Further—
`more, the rapid steering of a large aperture optical sensor
`often requires a prohibitive amount of power. Despite the
`considerable accumulated manufacturing experience in this
`field, mechanical beam steering for optical sensors remains
`complex, precise, and expensive.
`to
`Optical phased arrays appear to have the potential
`overcome many of the limitations of mechanical beam
`steering. Liquid-crystal-based phased arrays require very
`little prime power, even for large apertures, thereby opening 4
`up application areas such as missile intcrceptors, satellite
`communications, and portable sensors of all types. Phased
`arrays are inherently random-access devices, a distinct
`advantage when regions of interest are distributed widely
`across a sensor field of regard (FOR). Unlike mechanical
`systems,
`liquid crystal devices are generally insensitive
`to accelerations, and their costs can drop rapidly with
`volume production, as is the general case for the electronic
`devices they resemble. Flat panel displays, fabricated using
`technologies that are similar to those required for liquid-
`crystal optical phased arrays, are now inexpensive enough
`to be in every notebook computer.
`In the related microwave radar arena, phased arrays are
`rapidly displacing conventional horn antennas. The clear
`benefits of random-access, rapid beam pointing with no
`moving parts have made phased arrays the technology of
`choice, despite their high cost. Fortunately, cost trades for
`optical radars using optical phased aways promise to be
`more favorable since the optical arrays are monolithically
`fabricated with no discrete elements, consist of an array
`of phase shifters rather than individual
`transmit/receive
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`PROCEEDINGS OF THE [BEE VOL. 84, NO. 2. FEBRUARY 1996
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`0018—9219/96$05.00 © 1996 IEEE
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`FNC 1011
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`FNC 1011
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`modules, and are designed to use low—cost addressing
`electronics. The optical phased arrays discussed here are
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`passive arrays, consisting soley of phase shifters, and are
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`operated as space—fed arrays, meaning that an already
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`formed beam is fed to the array of phase shifters, which
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`then effects steering of that beam. This contrasts to an active
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`phased array in which individual transmit modules form a
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`beam as it exits a large array of transmitters.
`There are many application areas that can benefit from
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`the performance/cost benefits made possible by optical
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`phased arrays. Inexpensive, reliable laser radar for target
`detection, wind profiling, and gas cloud identification are
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`examples of high interest. Laser communication, whether
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`effected with directed beams in free space or by switching
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`of guided beams within fiber links, is another application
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`area. Defense against infrared guided missiles benefits from
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`directed laser energy, and is another potential optical phased
`array application area. Later in the paper issues associ—
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`ated with steering broadband optical energy are addressed.
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`Passive infrared sensors for imaging or point detection
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`applications can also benefit from phased array optical beam
`steering, but to a more limited degree at this time; however,
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`future applications are expected to expand as techniques for
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`reducing the influence of dispersion are developed.
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`Optical beam steering by means of phased elements is
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`a rich area heavily researched by prior workers. As early
`as 1971, Meyer [1] had developed a l—D optical phased
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`array using bulk,
`lithium tantalate phase shifters. The
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`array comprised 46 phase shifters on one—half millimeter
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`spacings. The number of addressable beam positions, beam
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`widths, scan angles, and beam spacings all were shown to
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`agree with theory as developed for microwave phased array
`antennas. Shortly thereafter Ninomiya [2] demonstrated a
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`1—D array of lithium niobate electrooptic prism deflectors.
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`The resolving power of the array was shown to be N
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`times that of a single prism, where N is number of arrayed
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`prisms. The array successfully demonstrated 50 resolvable
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`spots with 600 V applied. Both discrete and continuous
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`steering were demonstrated. The power required was noted
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`to be similar to that for acousto-optic deflectors. Although
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`these early phased arrays clearly demonstrated the concept,
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`they were neither developed for high performance nor
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`were intended for practical application. Large phase shifter
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`spacings of hundreds of wavelengths were unavoidable,
`given the state of the technology, and precluded achieving
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`efficient large angle beam steering. The small aperture fill
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`factors also guaranteed large insertion losses. However,
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`many of the key advantages of the phased array approach to
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`beam steering were well appreciated by these early workers.
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`Ninomiya pointed out that a phased array offers random
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`access, that the resolving power of a phased array is high,
`that the steering angle is very accurate, and that there is no
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`shift of optical frequency as with acousto—optic deflectors.
`Beam steering of visible light has recently been reported
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`using a liquid crystal
`television panel as an elementary
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`phased array [3]. Although liquid crystal displays are usu-
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`ally configured to effect intensity modulation, when the po—
`larizers are removed the accompanying phase shift becomes
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`observable. The display pixels are programmed to effect
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`a discrete blazed-grating phase ramp across the aperture.
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`However, the relatively large pixel spacing (several hundred
`the nonunity array fill
`factor, and the limited
`waves),
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`available phase modulation depth (1.3a) have severely
`limited the achievable steering efficiency and angle (<0.l°).
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`Other workers in the field have attempted to develop
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`higher performance optical phased arrays by greatly re-
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`ducing the phase shifter spacings. Vasey et al.
`[4] have
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`developed an integrated optics approach comprising a l-
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`D phased array based on a linear array of closely spaced
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`AlGaAs waveguides, the relative phases of which can be
`electrically adjusted using the electrooptic effect
`in the
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`waveguiding material itself. Beams are coupled into the
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`guided structure and launched into free space using grating
`couplers. Continuous steering is achieved by electrically
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`imposing a linear phase ramp of adjustable slope across
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`the aperture. Addressing is accomplished via a fine/coarse
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`architecture, somewhat similar to the approach discussed
`in Section IV. Continuous steering of a 900 nm beam over
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`a 2127.5 mrad field has been reported. Element spacings
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`are orders of magnitude less than those in earlier bulk
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`demonstrations, but remain multiple (1344) wavelengths.
`Consequently, maximum steering angles are limited and
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`efficiencies are low due to the large number of radiated
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`diffraction orders (so-called grating lobes). Although the
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`electrooptic effect used in this approach is inherently fast
`(ns), achieving the steering angles and efficiency levels
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`required for laser radar is expected to be difficult, as is
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`scaling to required aperture sizes and obtaining steering in
`two-dimensions.
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`Another approach reported recently [5] uses a thin, 2—D
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`array of liquid crystal phase shifting elements configured to
`operate as coherent rnicroprisms with a relatively high fill
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`factor. The individual elements are multiple wavelengths
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`in extent and spacing, but are constructed to produce a
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`linear phase ramp of adjustable slope across each element
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`thereby simulating a discrete blazed grating with
`face,
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`programmable blaze angle. The current device steers in
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`one dimension only, although in principal
`two devices
`could be cascaded to steer in both dimensions. Unlike
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`most preceding optical arrays, this device was specifically
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`designed for laser radar application and is,
`in principle,
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`capable of high efficiency at large angles (20°) and of being
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`fabricated with large apertures. However,
`to date, only
`small apertures (2 mm square), moderate steering angles
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`(5°), and low efficiencies (1%—9%) have been achieved
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`owing to fabrication difficulties inherent to the approach.
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`One of the difficulties is that this approach requires using
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`only the linear portion of the liquid crystal versus voltage
`curve, resulting in a limited use of available birefringence.
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`A more classic approach to optical phased arrays has been
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`under development by the authors. This work has resulted in
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`development of a true optical phased array with l‘-D phase
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`shifter spacings smaller than a single free-space wave—
`length, 100% aperture fill factors over significant apertures,
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`and performance approaching theoretical predictions for
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`small to moderate angles. The unity fill factor, small phase
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`MCMANAMON et al.: OPTICAL PHASED ARRAY TECHNOLOGY
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`shifter spacings. and careful fabrication techniques used
`result in very low sidelobe levels. These are the first optical
`phased arrays which are capable of redirecting a single input
`beam into essentially a single. diffraction limited, output
`beam with negligible sidelobes. Using this approach, high
`performance optical phased array based steering of carbon
`dioxide laser beams (10.6 um) was first demonstrated in
`1989, with demonstrations of NszAG steering (1.06 #111)
`following soon thereafter [6].
`[7]. That work will soon
`appear in the open. reviewed. literature [8]. [9].
`These new optical phased arrays are direct functional
`analogs of the well known microwave phased array anten-
`nas [10] that make possible the agile, inertialess steering of
`microwave beams. The underlying fundamental concepts
`are identical
`to those for a microwave array. However,
`due to the orders-of-magnitude difference in wavelengths
`between the microwave and optical worlds, these new opti-
`cal phased arrays have been implemented quite differently.
`The current optical devices are 1-D, space-fed, passive,
`phase-only, apertures. This differs from modern microwave
`phased arrays with which a beam is usually both formed
`and steered in two dimensions by a 2—D array of active
`elements. The field intensity across the aperture of an active
`microwave array is generally tapered at the edges in order
`to achieve low sidelobe levels. This is not an option with
`a passive, phase-only array. However, being space-fed, if
`the input optical beam is Gaussian in spatial profile, as is
`the usual case, additional tapering is not needed. The 1-D
`phase-only array steers an optical beam in one dimension
`only. Unlike modern microwave arrays, and most other
`optical phased arrays to date, these new arrays are designed
`to be easily cascaded. This allows simple mounting of
`orthogonal l-D arrays to steer the beam in two dimensions.
`Microwave arrays are built using discrete phase shifters,
`as have been most early optical phased arrays. However,
`since a vast number of phase shifters is needed to realize a
`high performance optical array, distributed liquid crystal
`phase shifters have been implemented, as described by
`Huignard er al. [11]; however, it has proven essential to
`implement additional innovative addressing means to avoid
`the otherwise impractical numbers of interconnects.
`The organization of this paper is as follows. In Section II
`liquid crystal optical phased array technology is summa-
`rized.
`In Section III we briefly discuss alternative can-
`didates to optical phased arrays for eliminating complex
`and expensive mechanical motion from laser radar optical
`systems. A more detailed description is presented in section
`IV. Section V summarizes performance levels achieved
`and predicted performance potential. Section VI considers
`the pointing of an acquisition sensor, often a passive in-
`frared (IR) sensor. Section VII discusses laser radar system
`concepts that incorporate target acquisition and tracking
`capabilities. Section VIII contains conclusions.
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`II. OVERVIEW OF LIQUID CRYSTAL OPTICAL
`PHASED ARRAY CONCEPTS
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`A prism inserted into the aperture of an optical system
`introduces a linear gradient of optical path delay (OPD)
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`270
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`across the aperture which tilts the phase front and thereby
`steers the optical beam. For a given wavelength a phase
`shift of 21' (corresponding to an OPD of one wavelength)
`can be subtracted periodically from the phase front without
`influencing the far—field pattern produced by the phase front
`[12]. The “folded” phase profile represents a blazed grating.
`The phase ramp, or its equivalent modulo—21r sawtooth
`phase profile, further can be approximated by a series of
`discrete phase steps, as long as the steps are small.
`Fig.
`1
`illustrates the use of nematic liquid crystal cells
`as phase shifters. With no applied fields, the liquid crystal
`molecules align with an average orientation parallel
`to
`the substrates. according to the liquid crystal alignment
`layer applied at the substrate interface. Application of a
`relatively low voltage, on the order of l—lO V, reorients
`the liquid crystal molecules and changes the effective index
`of refraction as seen by light polarized along the direction
`of quiescent molecular orientation. The maximum phase
`shift available is proportional to the thickness of the liquid
`crystal layer. The case of a Zn phase retarder is illustrated.
`The switching speed of a nematic liquid crystal phase
`shifter is generally inversely proportional to the square of
`the thickness of the nematic liquid crystal layer [13]. For
`steering angle/aperture size combinations that require phase
`resets, the minimum thickness of the liquid'crystal layer
`to produce efficient steering requires a liquid crystal layer
`sufficiently thick to produce a full wavelength of CPD and
`allow modulo 27r operation. Only a combination of very
`small angles, or very small aperture size, allows practical
`beam steering without the use of resets. The liquid crystal
`layer thickness, t, for a 27r phase shift is: given by
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`t2 A/An
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`(1)
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`where An = (72,, - no) is the birefringence of the material
`and A is the free space wavelength. As an, example, the ne-
`matic liquid crystal E7 has a birefringence of approximately
`0.2 in the visible and near infrared spectrum. It requires a 5
`pm layer thickness to achieve a relative phase delay of 27r
`radians at a l um wavelength. If a reflective—mode design is
`used, allowing two passes through the liquid crystal layer,
`a full wave OPD is created using only half that thickness,
`or 2.5 um.
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`The diffraction efficiency, 7], of a grating. with a stair-
`step blaze designed to maximize energy in the. first order
`is given by [23]:
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`77 : (sm(@)
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`7W
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`where q is the number of steps in the blaze profile. From
`(2) it can be seen that an eight—step approximation gives a
`theoretical efficiency of approximately 95%. Fig. 2 shows
`a step approximation to the wavefront deflected by a prism,
`including the 27r phase resets. Note that a 27r phase reset has
`that value only for the design wavelength. Fig. 3 shows the
`deviation from a straightiine in the unfolded phase profile
`when a wavelength other than the design wavelength is
`used. This variation in phase reset va]ues causes dispersion
`
`PROCEEDINGS OF THE IEFE, VOL. 84, NO. 2. FEBRUARY 1996
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`Polarization
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`Optleal Beam
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`Fig. 1. Nematic liquid crystal phase shifters. The liquid crystal molecules are birefringent. Light
`polarized along the long axis of the molecule will experience a different index of refraction than light
`polarized along the short axis of the crystal. The molecules will rotate when a voltage is applied,
`producing an effective index change for light polarized perpendicular to the long axis of the crystal.
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`Unfolded Phase
`Profile
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`
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`Undleturbed
`Phase Front
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`Indlvlduel
`Phase snlfters
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`Flg. 2. Optical phased array agile beam steering. The optical
`phase delay introduced by a prism in an aperture can be approx-
`imated by a series of stair-step ramp phase delays. When a ramp
`has an optical path difference equal to or largcr than the design
`wavelength one design wavelength of optical path difference is
`subtracted from the ramp. At the design wavelength, the phased
`array effectively reproduces the steering caused by a prism.
`
`[14], which will be discussed further in Section VI. As
`shown in Fig. 3, the unfolded OPD is in error by (A —— Ad)
`after each reset, and the unfolded phase is in error by
`21r(A — A4)//\ after each reset. where A is the actual
`wavelength and Ad is the design wavelength.
`Practical factors can cause the measured efficiency of
`an actual phased array beam steerer to deviate from the
`theoretical value given by (2). One such factor, evident in
`liquid crystal phased arrays currently being developed, is a
`spatial “flyback” in the molecular orientation of the liquid
`crystals which results from the minimum spatial extent
`required to change from the orientation for a phase shift
`of 27r to that for a phase shift of zero. The actual flyback
`transition is a complex function of device design and liquid
`crystal visco-mechanical properties. Fig. 4 depicts phase
`versus position for a simple flyback model. As a result of
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`MCMANAMON £101.: OPTICAL PHASED ARRAY TECHNOLOGY
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`“Sign
`Steered
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`Wavelength Wavelength
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`
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`Unfolded
`Phase Profile
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`Fig. 3. Unfolded phase profile. This figure shows the influence
`on the unfolded phase profile of operation at a wavelength other
`than the design wavelength.
`
`flyback, only a portion of the grating imposes the correct
`phase distribution to steer a beam in the design direction.
`That portion of the grating over which flyback occurs can
`be thought of as steering the beam in a different direction.
`The resulting diffraction efficiency 77 into the desired grating
`order can be approximated by [15]
`
`2
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`n=(1—ATF)
`
`(3)
`
`where A F is the width of the flyback region and A is the
`period of the programmed grating. The energy that is not
`directed into the desired grating order is distributed among
`numerous other grating orders, causing a loss in efficiency
`for the primary order. The overall steering efficiency is
`given by the product of (2) and (3). Depending on the
`grating period (which affects both the number of steps in
`the blaze profile as well as the relative size of the flyback),
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`Fig. 4. Flyback, When one design wavelength of optical path
`difference is subtracted it requires finite spatial extent. This region
`is refcn-ed to as the flybnck region, The steering efficiency into a
`given order is influenced by the relative size of the flyback region
`with respect to the grating period.
`
`either (2) or (3) may dominate the overall steering efficiency
`of an optical phased array beam steerer.
`For a normally incident input beam the steered angle is
`given by [16]
`
`A
`(4)
`sin 0 = 13
`where A0 is the design wavelength for the beam steerer,
`A = qd is the period of the staircase ramp, q is the number
`of phase shifters between resets, and d is the center-to-
`center spacing between phase shifters. which is assumed
`to equal
`the width of the phase shifter as well. Large
`steering angles correspond to high spatial frequencies (small
`periods) and vice versa. From (3) and (4) it can be seen
`that the steering efficiency decreases monotonically with
`steering angle, for fixed flyback.
`Two-dimensional beam steering can be achieved using
`two orthogonally oriented l-D liquid crystal phase gratings.
`In addition, any optical distortion that
`is separable in
`Cartesian coordinates can be fully compensated, modulo
`2n. Spherical aberrations can be fully compensated with a
`crossed grating system. For a full adaptive optics capability,
`a third layer, with a 2-D array of phase shifters, would be
`required. This would add the ability to clean up an arbitrar-
`ily aberrated beam and adapt for atmospheric turbulence.
`Such a liquid crystal adaptive optics layer has recently
`been discussed [17]. The spacing of elements on such an
`adaptive optics layer would be orders of magnitude courser
`than the spacing required for large angle beam steering.
`Current adaptive optics mirror systems have on the order
`of 50—400 elements correcting for turbulence while using
`apertures up to a few meters [18]. However, the adaptive
`optic element is usually used prior to final beam expansion
`and is much smaller in aperture. Pixelated phase shifters
`of about
`1 mm square would probably suffice for most
`' applications and could be readily fabricated with the current
`technology. Thus liquid crystal phase shifter technology
`could replace the current piezoelectrically driven adaptive
`optic components, resulting in a single three-layer compo~
`nent that both deflects and phase compensates a beam. If
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`272
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`operated modulo 21r, such adaptive optic elements would
`be dispersion limited to narrow band applications.
`
`111. BEAM STEERING APPROACHES USING
`LIMITED MECHANICAL MOTION
`
`the only approach to
`An optical phased array is not
`realizing rapid beam steering without
`the use of con-
`ventional mechanical systems. Some of the more viable
`alternate options are briefly reviewed here. All of the
`options discussed here potentially allow the redirection
`of the field—of—view of an optical sensor without the use
`of complex, costly, mechanical mechanisms. Unlike the
`optical phased array, most of these alternate options do
`not eliminate mechanical motion, but instead minimize the
`degree of mechanical motion required. To date, nonegof
`these alternate approaches have demonstrated the scope of
`performance characteristics desired for laser radar and most
`other optical sensors.
`One such option is the use of cascaded microlens arrays
`[19], [20] an example of which is shown in Fig. 5. Each
`microlens array consists of a (generally) close packed,
`periodic array of miniature lenses which can be fabricated
`in either diffractive or refractive forms. Beam steering is
`effected by translating one microlens array with respect
`to the other. The concept can be understood by first
`considering a single microlens pair from a set of aligned
`afocal arrays. A collimated input beam is focused to the
`back focal point of the first microlens, which is also the
`front focal point of the second microlens, resulting in
`an unsteered, collimated output beam. However,
`if the
`second microlens is offset, then the back focal point of the
`first microlens appears as an off—axis point to the second
`microlens. The point remains in the front focal plane of the
`second microlens, so the second microlens still recollimates
`the light, but the beam 'is redirected to a nonzero field
`angle. A paraxial ray trace shows that the tangent of this
`field angle is equal to the amount of ioffset divided by
`the focal length of the second microlens. Maximum useful
`steering occurs with an offset equal
`to the radius of a
`microlens. It may be noted that it does not matter if the
`second microlens has a positive or negative focal
`length
`so long as the condition of overlapping focal planes is
`met. If the individual microlenses of the arrays are aligned,
`periodically spaced, and designed to fill the aperture, the
`output beam replicates the input beam. If the offset is small,
`the steered beam approximates a simple redirection of the
`input beam.
`However. if the offset is large, significant fractions of the
`input beam are coupled into other grating modes. This can
`be appreciated by noting that phased arrays and microlens
`arrays both approximate blazed gratings [2 l ]. If the periodic
`quadratic phase profiles of two offset microlens arrays
`are superimposed,
`the result is a (generally asymmetric)
`triangular waveform, which approximates a blazed grating.
`If the composite phase profile were a sawtooth, the approx-
`imation would be exact. Motion of the lenses alters the
`
`slope(s) of the phase profile, thereby changing the blaze
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`PROCEEDINGS OF THE IEEE. VOL. 84. NO. 2. FEBRUARY 1996
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`profile of the equivalent grating, and shifting the light to
`different grating orders. To the extent that the composite
`phase profile approximates a true sawtooth, light is steered
`to a single direction. However, the offset of two lens arrays
`inherently causes each input lenslet to illuminate adjacent
`output lenslets, resulting in the multiple-slope profile of
`the triangular wave, and steering to multiple directions. To
`mitigate this effect, designs using a third microlens array
`as a field lens have been put fonh, but demonstrations have
`not yet been reported.
`The agile steering of a beam using the microlens array
`concept requires the agile motion of one microlens array
`with respect to the other. Microlens arrays inherently have
`small focal lengths (typically on the order of a few lenslet
`diameters, usually a millimeter or less); consequently, the
`amount of mechanical offset required to achieve a desired
`steering angle can also be quite small. Compared to steering
`via a displaced bulk lens having the same aperture as
`the microlens array, the reduction of motion required to
`steer to a given angle is proportional to the ratio of the
`individual microlens diameter to the array diameter. Due
`to its essentially planar structure, a microlens array can be
`made much lighter than a bulk lens of equivalent aperture.
`The combination of low mass and small motion allows agile
`positioning (and agile beam steering) to be accomplished
`with more simplified mechanical drivers than would be
`required for macroscopic lenses. The microlens arrays can
`be designed to effect substantial steering with mechanical
`motions that can be achieved with piezoelectric transducers.
`However, small errors in mechanical positioning are ampli-
`fied by the same optical leverage that makes possible the
`reduction in mechanical motion. This means the amount of
`
`energy at the desired steering angle will be influenced by
`a small amount of mechanical motion. Thus fine angular
`beam steering with this approach generally requires very
`precise motion.
`Microlens arrays can be programmed onto the liquid-
`crystal based optical phased arrays reported here, thereby
`making possible an electronic translation of one lens array
`with respect
`to the other, and complete elimination of
`all mechanical motion. Since only one microlens array
`must move to achieve beam steering, only a single array
`would have to be programmable. Owing to the precise
`displacement control available with an optical phased array,
`this option may be preferable to piezoelectrically driven
`motion for applications requiring precision pointing.
`Flexure beam micromirror technology is another ap-
`proach with large numbers of small apertures arranged in
`regular arrays [22]. The individual apertures are lithographi-
`cally fabricated mirror “pixels” on hinges with micromotion
`effected by an electrostatic field. The field attracts the-ele-
`ment and moves it rapidly. on the order of a microsecond.
`This can create a piston phase shift for the individual
`aperture. These devices have demonstrated 2n phase shifts
`at 633 nm wavelength with a 60—75% fill factor. Much of
`the same phased array theory discussed later in this paper
`applies to these array structures. although the physical im-
`plementation is significantly different. The implementation
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`MCMANAMON et aL: OPTICAL PHASED ARRAY TECHNOLOGY
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`
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`Fig. 5. Decemened microlens army beam steering. The figure
`shows two decentered microlens arrays, and their influence in
`steering an incoming beam. If one array is moved with respect
`to the other array it causes the beam to s