外文文献Novel Laser Beam Steering Techniques（新型激光束转向

Invited Paper

Novel Laser Beam Steering Techniques

Hans Dieter Tholl

Dept. of Optronics & Laser Techniques

Diehl BGT Defence

PO Box 10 11 55, 88641 überlingen, Germany

ABSTRACT

The paper summarizes laser beam steering techniques for power beaming, sensing, and communication applications. Principles and characteristics of novel mechanical, micro-mechanical and non-mechanical techniques are compiled. Micro-lens based coarse beam steering in combination with liquid crystal or electro-optical phase control for fine steering is presented in more detail. This review addresses beam steering devices which modulate the phase distribution across a laser beam and excludes intra-cavity beam steering, beam steering based on combining tuneable lasers with dispersive optical elements, active optical phased arrays, and optical waveguides.

Keywords: Laser beam steering, optical phased arrays, decentered micro-lenses, spatial light modulators

1. INTRODUCTION

The integration of laser power beaming, laser-assisted sensing, and laser communication subsystems into autonomous vehicles, airborne and space platforms demands new techniques to steer a laser beam. The new techniques should promote the realization of beam steering devices with large optical apertures which are conformally integrated into the mechanical structure of the platform. The wish list of requirements comprise well-known properties: compact, lightweight, low power, agile, multi-spectral, large field of regard.

The angular spread of a laser beam, especially for long range applications, is inherently small because of the high antenna gain of apertures at optical wavelengths. Consequently, the direction of propagation of a laser beam is generally controlled in two steps: (1) A turret with gimballed optical elements points the field-of-view of a transmitting/receiving telescope into the required direction and compensates for platform motions with moderate accuracy and speed. (2) A beam steering device steers the laser beam within the field-of-view of the telescope in order to acquire and track a target.

The subject matter of this review are novel laser beam steering techniques. Beam steering devices are capable of

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pointing a laser beam randomly within a wide field-of-regard,

stepping the beam in small increments from one angular position to the next, dwelling in each position for the required time on target.

In contrast, scanning devices move the beam axis continuously and switching devices are only able to address predefined directions. Reviews of current technologies for steering, scanning, and switching of laser beams are found in references [1,2,3,4].

Correspondence. Email: hans.tholl@diehl-bgt-defence.de; Phone: +49 7551 89 4224

Technologies for Optical Countermeasures III, edited by David H. Titterton, Proc. of SPIE Vol. 6397, 639708, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.689900

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In general, beam steering is accomplished by imposing a linear phase retardation profile across the aperture of the laser beam. The slope of the corresponding wavefront ramp determines the steering angle: large steering angles correspond to large slopes and vice versa. Large wavefront slopes in combination with large apertures require large optical path differences (OPD) across the aperture which have to be realized by the beam steering device.

Large wavefront slopes may be generated directly by macro-optical elements such as rotating (Risley) prisms and mirrors or decentered lenses. Compared to gimballed mirrors these steering devices are relative compact, possess low moments of inertia and do not rotate the optical axis. Recently, these macro-optical approaches gained renewed popularity.

The way for compact, lightweight, low power beam steering devices is smoothed by micro-optics technology. Single micro-optical elements such as electro-optic prisms, dual-axis scanning micro-mirrors, or micro-lenses attached to micro-actuators imitate the steering mechanism of their macro-optical counterparts. Single, small aperture micro-opto- electro-mechanical systems (MOEMS) are mounted near the focal plane of macro-optical systems and provide rapid pointing of the laser beam. These configurations combine the benefits of macro-optical beam steering devices with the high bandwidth of MEMS and are candidates for beam steering applications at low optical power levels.

In order to build large apertures with micro-optical elements, they have to be arranged in rectangular two-dimensional arrays. Promising techniques are one-dimensional arrays of electro-optic prisms or two-dimensional arrays of micro- mirrors and decentered micro-lenses. At visible and infrared wavelengths the array pitch is larger than the wavelength and the arrangement acts like a diffraction grating. Suppression of undesired diffraction orders is accomplished by actively blazing the grating structure in an appropriate way.

Micro-optical actively blazed gratings are a rudimentary form of phased arrays. A phased array is a periodic arrangement of subapertures each radiating its own pattern into space. The interference of the individual radiation patterns simulate a large coherent aperture in the far field. This review addresses only so called passive phased arrays which modulate the phase distribution across an impinging laser beam. For this purpose the phase piston of each subaperture is varied, thus creating a programmable diffractive optical element across the device aperture.

There are many more beam steering techniques described in the literature: intra-cavity beam steering, beam steering based on combining tuneable lasers with dispersive optical elements (e. g. photonic crystals), active optical phased arrays, and steering techniques associated with optical waveguides. These techniques are excluded from this review.

2. PARAMETER SPACE OF BEAM STEERING DEVICES

Functional requirements for laser beam steering devices cover the following topics:

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maximum steering angle,

beam divergence/imaging capability, aperture/vignetting,

spectral range and dispersion, throughput,

control of the steering angle.

The quantitative parameters associated with each function depend strongly on the operational requirements. In general, two classes of steering devices can be distinguished: (1) Power beaming (e.g. directional optical countermeasures, transfer of power to remote devices) and free space laser communication applications require the laser beam to pass only once through the beam steering device. (2) Active sensing techniques such as laser radar transmit (Tx) the laser beam and receive (Rx) a signal through the beam steering device. Table 1 gives nominal values for functional parameters associated with the specific applications directional infrared countermeasures (DIRCM), imaging laser radar (ladar) and deep space laser communications as stated in references [6,9,10]. These examples run the gamut of system level

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parameters such as maximum steering angle, aperture diameter, beam divergence, and pointing accuracy. The parameters which characterize a beam steering device independently of its location within the optical system are spectral range, time constant, angular dynamic range, and etendue.

Table 1. Compilation of nominal beam steering parameters for different applications.

Parameter Maximum steering angle Aperture diameter Beam divergence (Tx) Instantaneous FOV(1) (Rx) Pointing accuracy Spectral range Time constant (2) Angular dynamic range (3) Etendue (4) (1) FOV: Field-of-View(2)

DIRCM [6] 45 deg 50 mm (Tx) 1 mrad - 100 μrad 2 to 5 μm Imaging Ladar [9] 5.4 deg 75 mm (Rx) 10 mrad 333 μrad 30 μrad 0.532 μm Deep Space Lasercom [10] 0.6 deg 300 mm (Tx) 6.3 μrad - 1 μrad 1.064 μm 1 ms 1 ms 0.7 ms 38 dB 42 dB 43 dB 78 mm*rad 28 mm*rad 10 mm*rad Time required to step from one angular position to the next

(3)

10 log(2*[max steering angle]/[pointing accuracy]) (4)

2*[max steering angle]*[aperture diameter]

The etendue of the beam steering device (BSD) restricts its location within the optical system. The large etendues required for the DIRCM system demands the BSD to be placed in the exit pupil of the transmitting telescope. Moderate etendues give the opportunity to mount the BSD in the exit pupil or the entrance pupil of a beam expanding telescope depending on the technologies available. It is also possible to split the steering capability between a coarse steering element situated in the exit pupil and a fine steering element in the entrance pupil. For imaging ladar applications the division in coarse/fine beam steering is preferable if the fine beam steerer also functions as a fan out diffractive optical element (DOE). The DOE creates an array of laser spots which illuminate the footprints of the receiving FPA pixels [9]. Small etendues in combination with large apertures as for deep space lasercom require the BSD to be mounted in the entrance pupil of the telescope which expands the laser beam and reduces the steering angle.

The applications compiled in table 1 serve as a guide through the following sections although a particular beam steering technique is not unique to an application.

3. BEAM STEERING WITH MACRO-OPTICAL COMPONENTS

In a recent series of papers the application of rotating prisms and decentered lenses to wide angle beam steering for infrared countermeasures applications was reported [5,6,7]. The research was focused on macro-optical coarse beam steering devices based on rotating prisms and decentered lenses.

Macro-optical devices enable achromatic designs, avoid blind spots within the field-of-view and concentrate the steered energy into a single beam. Employing prisms and decentered lenses to deviate the chief ray of a ray bundle are standard techniques in the design of visual instruments. The design challenge of this well-known approach is the search for the right combination of opto-mechanical parameters and materials to ensure wide-angle achromatic steering in the infrared spectral range between 2-5 μm.

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3.1 Risley prism beam steering device [5,6]

Principle of operation. Risley prisms are a pair of achromatic prisms cascaded along the optical axis. The rotation of the prisms in the same or the opposite directions with equal or unequal angular velocities generates a variety of scan patterns which fill a conical field-of-regard continuously. The prism configuration should be optically reciprocal in order to ensure precise beam steering along the optical axis for all wavelengths of interest. Optical reciprocity is a symmetry property: in the reference position the prism configuration remains invariant after reflections at an internal plane perpendicular to the optical axis.

Maximum steering angle. According to reference [6] a maximum steering angle of 45 deg is attainable with proper control of the dispersion.

Beam divergence. All beam steering devices which do not change the direction of the optical axis exhibit a reduction of the effective beam diameter projected perpendicular to the steering direction. Additionally, a device dependent beam compression may occur. The prism beam steerer compresses the laser beam in such a way that a circular input beam leaves the device with an elliptical shape. The compression preserves the beam’s phase space volume (etendue) and the beam power but reduces the peak irradiance in the far field because of an increase in the beam divergence along the direction of compression. This effect ultimately limits the maximum steering angle for a given upper bound of the beam divergence.

Spectral range. Risely prisms work throughout the optical spectral range (VIS to VLWIR). The operational optical bandwidth is limited by the material dispersion. Achromatism to the first order is achieved by using achromatic prism doublets. Among a wide range of material alternatives the combination LiF/ZnS leads to small secondary dispersion of 1.78 mrad within the spectral range 2-5 μm at a maximum steering angle of 45 deg [6].

Throughput. Large clear apertures and apex angles of several degrees generate long optical path lengths within the prisms which has an impact on the device transmittance due to absorption and scattering in the prism material. With proper anti-reflection coatings multiple-interference effects between the prisms are reduced and a transmittance in the order of 75-80% seems to be achievable [5].

Comments. Steering a laser beam rapidly and randomly through a wide angular range requires control over the direction of rotation, the instantaneous angular position, and the angular velocities of the prism pairs. The azimuth and elevation steering angles are complicated continuous functions of the prism rotation angles and the wavelength. For smooth steering trajectories no singularities, e.g. prism flipping, are encountered [6]. The implementation of prism drives for scanning the line of sight of passive and ladar sensors is established [9,11]. However, the realization of the control loops for random step and stare mode is not an easy and straight forward task. In a recent publication a Risley beam steering device with a maximum steering angle of 60 degrees, an aperture of 100 mm, a wavelength range of 2-5 μm, and an aiming repeatability of better than 50 μrad was announced [12].

3.2 Decenterd lens beam steering device [5,7]

Principle of operation. Ideally, a beam steering device is an afocal optical system which transforms a plane input wavefront into a plane output wavefront. Besides prisms, lens telescopes of the Kepler or the Galileo type are candidates for macro-optical beam steering devices. The telescope comprises two lenses which are separated by the sum of their focal lengths. Steering of the chief ray and the associated ray bundle is accomplished by a lateral displacement of the exit lens with respect to the input lens.

Maximum steering angle. The maximum steering angle depends on the focal length and the distortion of the exit lens and on the maximum lateral displacement which is acceptable. In practice, the lateral displacement is limited to half the diameter of the aperture of the exit lens due to vignetting of the ray bundles. This leads to a maximum steering angle of roughly 25 degrees.

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Beam divergence. The compression of the laser beam depends on the ratio of the focal lengths of the two lenses. For the Galileo type the absolute value of this ratio is always smaller than one. For the Kepler type a focal length ratio of one is possible and preferable if the beam steering device should operated in a combined transmit/receive mode. The lateral displacement of the two lens apertures relative to each other reduces the clear aperture and leads to vignetting and to an asymmetric increase in beam divergence. This effect is controlled by the introduction of a field lens. Furthermore, the beam divergence is strongly affected by optical aberrations of the lens system. The transmitted beam should only illuminate the central portion of each lens in order to stay within the divergence requirement. The received wavefront may illuminate the full aperture and suffer a higher degree of aberration than the transmitted beam.

Vignetting. Vignetting due to the lateral displacement of lenses is reduced substantially for the Kepler configuration by the introduction of a field lens in the focal plane common to both lenses. Positive and negative field lenses are possible. The positive field lens is rigidly connected to the exit lens and both are displaced together. This facilities the driving mechanism but introduces an internal focus near the field lens. For high power applications this is undesirable. A negative field lens needs an extra drive which moves the field lens and the exit lens in opposing directions in a nearly 1:2 relationship [7]. In this way, an internal focus is avoided.

Spectral range. As for prisms there is no limitation on the spectral range. Ideally, each lens of the beam steerer has to be an achromat. In reference 7 the material combination Ge/AMTIR-1 was chosen to minimize the chromatic aberrations of a Fraunhofer doublet (positive first, negative second component) over the spectral range 2-5 μm. The authors designed a Kepler telescope with a negative field lens which steers a laser beam up to 22.5 deg and a secondary dispersion of 0.65 mrad over the spectral range 2-5 μm.

Throughput. The achromatic beam steering device of reference [7] comprises 6 external and 3 internal interfaces and rougly 40 millimeters of material thickness. As for the prism beam steerer the throughput should be in the order of 75- 80%. The encircled energy within the specified divergence of 1 mrad depends on the wavelength and the steering angle. At 2 μm the encircled energy remains above 95 % for all steering angles; at 5 μm the encircled energy varies from 98% on axis to 63% at 22.5 degrees.

Comments. In order to steer a laser beam the lateral displacement of two lens groups must be controlled. Fortunately, the relationship between the displacements of the lens groups is constant. For each wavelength, the azimuth and elevation steering angles are almost linear functions of the displacements. The required maximum displacement is equal to the aperture radius of the exit lens which is approximately 35 mm. The overall dimensions are 180 mm length and a height of 135 mm at maximum lens displacement. Decentering macro-optic lenses for beam steering is a possible but, because of the complexity involved, not a practical approach compared to Risley prisms. This is in contrast to the micro-optics world where micro-optical elements are arranged in a regular array. Electro-optic prism arrays are capable of one- dimensional beam steering with small steering angles. Decentered micro-lens arrays including field lenses are an option for steering laser beams up to angles of 25 degrees in two dimensions.

3.3 Beam steering with macro-optical mirrors

Transmissive optical elements are the first choice for compact optical systems with large fields-of-view. The drawback of this approach is the wavelength dependence of the optical functions due to the refraction at the interfaces between materials of different refractive indices. Reflective optical designs offer independence on the wavelength. Both approaches which were discussed in the preceding paragraphs can be realized with mirrors. A Risley type beam steering device for mm-waves based on rotating mirrors is discussed in reference [8].

4. BEAM STEERING WITH MICRO-OPTO-ELECTRO-MECHANICAL SYSTEMS (MOEMS)

Ladars find applications in targeting, missile guidance, terrain mapping and surveillance, or robotic navigation to name only a few. Short range applications of ladars (several 10 m) will rely on a flash illumination of the field-of-view and a reception of the scattered light by snapshot focal plane arrays. Intermediate and long range imaging ladars must sequentially illuminate a portion of the field-of-view because of limited laser power. These systems need a beam

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steering device to step through the field-of-view and to dwell on a specific portion in order to accumulate several laser pulses reflected from the scene.

The next generation intermediate range (several 100 m) 3-D imaging ladars will operate in a time-of-flight mode and integrate lasers, optical, electrical, and mechanical devices into micro-opto-electro-mechanical-systems (MOEMS) [4]. Currently, the development of MOEMS technologies is driven by fiber-optics communication with the focus on optical switches and wavelength multiplexers/demultiplexers. Most of the MOEMS switching devices are either digital (e.g. DLP technology introduced by Texas Instruments) or scan in an open loop mode without sensing the actual beam direction. According to the definition given in the introduction, these devices do not steer a laser beam (they are unable to point randomly or dwell on a particular direction). Nevertheless, scanning micro-mirrors are shortly reviewed because they pave the way for micro-optical phased arrays.

Two-axis scanning micro-mirrors

Principle of operation: Gimballed micro-mirrors reflect a laser beam in the same way as the large-scale counterparts. MOEMS offer advantages with respect to mass, volume, and electrical power consumption. The design challenge of these micro-systems lies in the fact that the macro-forces do not scale linearly with size. New design approaches to mount, drive, and control the tilt of the micro-mirrors are necessary. The micro-mirrors are fabricated on silicon wafers and are then bonded to another chip which contains the electrode structure to electro-statically drive the mirror motion. Mirror diameters are in the order of several millimeters. The first generation of these devices flipped back and forth between two positions, the newer versions are capable of performing a controlled continuous scanning in two dimensions [4].

Maximum tilt angles of up to (±)15 degrees are reported in the literature [13]. Depending on the optical layout this gives a maximum scan angle of (±)30 degrees which may be magnified optically with a negative lens.

Beam divergence: The diffraction limited beam divergence is limited by the diameter of the micro-mirror. For a wavelength of 1μm the divergence of a laser beam reflected off a mirror of 6 mm is about 330 μrad. With large scanning angles the beam is compressed in one direction.

Spectral range: The reflection law is independent of wavelength. Dispersion is introduced through the packaging of the mirrors which are usually sealed behind windows. These windows limit the spectral range and introduce chromatic aberrations.

Throughput is limited by the reflective coating of the mirrors (aluminium for the visible, gold for the IR spectral range), by reflections at the window interfaces, and by diffraction effects at the edges of the micro-mirrors. The throughput should be in the order of 85%.

Comments. To provide real steering capability the gimballed micro-mirror must be tilted in a step-and-stare fashion. Currently, this mode of operation is not on the research agenda because the areas of application of mirror devices are either switching or scanning of laser beams. Another interesting approach which is more relevant to beam steering is the flexure beam micromechanical spatial light modulator [21]. In this device the micro-mirrors are suspended with four hinges which results in a piston-like motion. This mode of operation brings about phase modulation of the reflected laser beam which may be relevant for fine beam steering with phased arrays.

5. BLAZED GRATING BEAM STEERING

Blazed grating beam steering utilizes an array of micro-optical elements (micro-telescopes, micro-mirrors, micro- prisms) with a fixed pitch. Each micro-optical element samples the incoming laser beam and radiates a beamlet into space. The beamlets interfere coherently and form a diffraction pattern which comprises several main lobes (grating lobes) surrounded by side-lobes. The directions of propagation of the main lobes are governed by the grating equation. The periods of the micro-optical grating are actively blazed in order to steer a laser beam. The most promising two-

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dimensional blazed grating beam steering approach is based on decentered micro-lens arrays. Other techniques such as tilting micro-mirror or (electro-optical) prism arrays are too limited with respect to deflection angles.

Decentred micro-lens arrays

Principle of operation. An array of micro-telescopes comprises a two-dimensional regular arrangement of telescopes of the Kepler or Galileo type. The lens trains which form the micro-telecopes are distributed among two planar substrates which hold mircro-lens arrays on their surfaces. Micro-lens arrays were realized as refractive and diffractive elements in glass for the visible spectral range and in silicon and other materials with high format (up to 512 x 512) and high fill- factors for mid IR applications [14,15,16,17]. Active blazing of the telescope array is realized by translating one mirco- lens substrate laterally with respect to the other. This is similar to the operation of the macroscopic arrangement. The difference lies in the fact that the amount of wavefront aberration scales with the size of the aperture and the field size. Therefore, the micro-optical realization of the principle of decentered lenses requires fewer optical surfaces than the macro-optical counterpart.

Maximum steering angle. The maximum lateral displacement of one half of the array pitch restricts the maximum steering angle to roughly 25 degrees. The maximum steering angle depends on the telescope type and the refractive index of the lens material (see table 2) [14]. The maximum value can only be reached with acceptable performance with a Kepler telescope and a field lens array (see Fig. 1).

Beam divergence. The far field of the decentred micro-lens beam steering device is composed of main lobes (called grating lobes) and sidelobes determined by the grating structure. The angular width of the grating lobes depends on the size of and the coherence length across the array aperture. In silicon, arrays with diameters of up to 6 inches should be possible. A major factor which determines the beam divergence is the spatial coherence across the array. Variations of the geometric-optical parameters due to imperfections of the fabrication process reduce the spatial coherence of the beamlets [18,19] and broadens the grating lobes. The non-uniformity of the optical parameters of the arrays should be well below 3% in order to attain good performance with respect to beam width, steering angle and diffraction efficiency. Beam compression is an issue for the Galileo and the simple Kepler type because of the different focal lengths involved and the vignetting induced by the lateral displacement.

Vignetting. Vignetting is introduced by the lateral displacement of the micro-lenses and depends on the type of micro- telescope. The performance of the Galileo and the simple Kepler type is strongly influenced by vignetting. Introduction of a positive field lens in the Kepler telescope eliminates vignetting at the cost of a reduced laser damage threshold. Fig. 2 illustrates the gain in uniformity of the Strehl ratio across the addressable grating lobes. Introduction of a negative field lens is not suitable because of the increased complexity of the driving mechanism.

Spectral range. Beam steering with decentered micro-lens arrays works over the entire optical waveband (UV to VLWIR, see table 2). Dispersion is caused by the materials involved and is induced by the grating structure of the arrays.

Table 2. Comparison of materials for decentered micro-lens arrays (adapted from ref. [14])

Property Refractive index Max. steering angle (deg) Best choice waveband (μm) Ge 4.0 28 6.5 - 13 Si 3.4 25 1.5 – 6.5 GaAs 3.3 24 13 – 17 ZnSe 2.4 18 0.65 – 1.5 ZnS 2.3 17 0.46 – 1.5

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Galileo micro-telescope array

Kepler micro-telescope array with field lens array

Figure 1. Ray tracing through decentered micro-lens arrays with the same pitch and maximum steering angle.

1

0,8

Strehl Ratio Galileo Telescope

0,6

0,4

0,2

0

Galileo Telescope Kepler Telescope with Field Lens Kepler Telescope

0

2

4

6

8

10

Diffraction Order

Figure 2. Calculated Strehl ratio of the addressed grating lobe (diffraction order) for a decentered micro-lens array beam steerer with 3% non-uniformity of the focal length across each lens surface. Both telescope arrays have identical pitch and maximum steering angle.

Throughput. Several loss mechanisms reduce the intensity in the addressed diffraction order for the Kepler type arrangement with field lens: Fresnel losses at the AR coated interfaces, losses due to non-ideal aperture ratios (the ratio of the width of the clear aperture to the grating pitch), reduced diffraction efficiency due to insufficient blazing, and extinction in the lens material. For lenses fabricated in silicon the energy balance predicts a minimum throughput of 65% of the incident power.

Comments. Micro-lens-arrays of large formats are well suited for agile steering with demonstrated high rates (up to kHz) and moderate accuracy (typical about 0.3% of micro-lens aperture). Piezoelectric transducers are the appropriate choice for driving the lenslets arrangements. Fig. 3 shows the second generation prototype of a broadband decentered micro-lens image steering device for the spectral range 3-5 μm [20].

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Figure 3. Prototype imaging steering device for the spectral range 3-5μm.

Blazed grating beam steerers exhibit two disadvantages: (i) Due to the grating nature only discrete angular positions (the

grating lobes) can be addressed. (ii) The non-uniformity of the optical parameters across the array leads to a reduction of the spatial coherence between the interfering beamlets and an increase in the beam divergence. These disadvantages can be resolved. (i) The angular positions between the grating lobes are accessible with a fine beam steering device in front of the blazed grating. The fine beam steerer imposes a local phase ramp across each grating period and steers the laser beam between adjacent grating lobes for a fixed blaze angle [14]. Alternatively, the blazed grating beam steerer is combined with a phased array which modulates the incident wavefront. In lowest order, the phase piston of each grating period is varied. In this way, a global phase ramp across the grating is approximated by a phase staircase. The blaze angle of the grating must be adjusted to the slope of the global phase ramp to retain maximum diffraction efficiency because each grating period only exhibits a variation in phase piston. (ii) The non-uniformity in optical thickness due to fabrication and alignment errors can be compensated and the spatial coherence across the grating aperture may be improved with sufficient dynamic range of the phase pistons (see Fig. 4). In addition, if the number of pixels of the phased array is sufficiently large, several pixels may cover one period of the blazed grating and higher wavefront errors (tip/tilt, defocus) may be corrected. In the VIS and NIR spectral range liquid crystal phased arrays are available for adaptive correction of the blazed grating beam steerer. Phased arrays based on micro-mirrors or electro-optical ceramics which cover a broader spectral range are under development [21,28].

6. PHASED ARRAY BEAM STEERING

Free-space optical communications between ground, airborne, and satellite platforms attracts increasing attention due to the evolution towards rugged lasers and compact optical systems. In this context non-mechanical laser beam steering provides technical means to realize cost-effective communication links. The system level functions of the beam steering device comprise (i) coarse steering for pointing the laser beam and active tracking of the receiver/retro-reflector, and (ii) fine steering to compensate for line-of-sight fluctuations due to small movements of the receiver, atmospheric turbulence and platform vibrations.

Optical phased arrays (OPAs) emerged as an attractive approach for these tasks. OPAs impose a phase delay ramp across a laser beam by controlling the spatial variation of the refractive index or the geometrical ray path across the device aperture. Currently, three techniques are under investigation: (i) spatial modulation of the refractive index of thin liquid crystal (LC) films and (ii) electro-optics ceramics (EOC) and (iii) spatially resolved alteration of the geometrical path with arrays of micro-mechanical (MEMS) mirrors. LC technology is mature and commercially available. EO ceramics and MEMS phased arrays with piston-like motion of the micro-mirrors are in their infancy [21,28].

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Blazed Wavefront Output

Blazed Wavefront Output

Blazed Grating Beam Steerer Blazed Grating Beam Steerer

Plane Wave Input

Phase Piston

Phased Array Plane Wave Input

(a)

(b)

Blazed Wavefront Output

Blazed Grating Beam Steerer

Phase Piston + Fine Steering Ramp Phased Array

Figure 4. Illustration of the compensation of array non-uniformity

and interpolation between grating lobes using a phased array in front of the blazed grating beam steerer. One pixel of the phased array corresponds to one period of the grating. (a) Phased array is absent. (b) Correction of phase piston error. (c) Correction of phase piston error and interpolation between grating lobes.

If several pixels of the phased array cover one grating period the wavefront curvature can also be corrected.

Plane Wave Input

(c)

Liquid crystal spatial light modulators (LC SLM)

LC SLMs have been evaluated for laser beam steering and shaping since more than a decade [22]. Gradually, SLMs suitable for high-quality phase modulation of up to 2π at VIS and NIR wavelengths, with a large number of individually addressable resolution cells in a two-dimensional array, and with acceptable frame rates appear on the market [23].

Principle of operation. The detailed structure of a LC SLM depends on the type of liquid crystal (nematic, ferroelectric, or polymer dispersed), the addressing scheme (electrical or optical), and the mode of operation (transmissive or reflective). Most LC SLM which are available commercially consist of nematic LC. Schematically, the nematic LC layer is sandwiched between plane substrates which are coated with electrodes. The LC molecules are pre-aligned parallel to the electrodes. A voltage applied between the electrodes generates an electrical field which is mainly perpendicular to the electrodes and which controls the orientation of the LC directors in the layer: the LC directors line up with the electric field lines. From an optical point of view this arrangement is birefringent and the change in optical path length induced by the reorientation of the LC directors depends on the polarisation of the incident light beam. Most LC SLM operate in a reflective mode. In comparison with the transmissive mode the layer thickness is reduced and the modulation bandwidth is increased for a required depth of optical phase modulation. The reflective mode of operation also eases the addressing of the resolution cells of the LC layer.

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An electrically addressed LC SLM (EA-SLM) comprise a patterned electrode (line pattern for 1D, pixelated for 2D beam steering) with highly reflective pads which are bonded to a silicion backplane and a transparent electrode which is antireflection coated. The gaps which define the pattern on the backplane electrode distort the electric field distribution and give rise to fringing fields. The fringing fields separate the EA-SLM in optically active and inactive regions. Due to fabrication limitations a gap of at least 0.5 μm is required in order to prevent dielectric break down between electrodes. Together with the requirement of having a high aperture ratio this results in a minimum width of a resolution cell of 1.5 μm [24]. For VIS and NIR wavelength the pixel structure acts like a diffraction grating causing diffraction into several orders in the far field independently of the imposed phase profile [25].

The disturbing diffraction effect of the electrode structure is prevented by the use of optically addressed LC SLM (OA- SLM). Instead of a patterned backplane electrode a reflective OA-SLM comprises a large-area photoconductor, an absorbing light-blocking layer, and a high reflective dielectric mirror. In operation, the photoconductor is electrically biased relative to the transparent electrode and irradiated with patterned light. The irradiance distribution determines the local electric field which controls the orientation of the LC director. The OA-SLM exhibits reduced diffraction artefacts because of the absence of a pixelated electrode structure and is scaleable to large apertures [26].

Maximum steering angle. Commercially available LC SLM have a size up to 20 mm and 8 bit phase dynamic range [23]. Depending on the wavelength (0.5 to 1.5 μm) this results in maximum steering angles between 25 to 75 μrad with tenth of nanoradians resolution. These small steering angles are ideal for ultra fine beam steering. Larger steering angles are realized through the implementation of diffractive optical elements (DOE), i.e. diffraction gratings. The phase profile of a DOE is limited to a phase range between 0 and 2 π radians. As a rule of thumb, high diffraction efficiency (>95%) in the first diffraction order is attained for a minimum of 8 phase steps per grating period with the appropriate blazing of the unit cell. This technique results in a maximum steering angle between 6 and 18 mrad for a pixel pitch of 8 to 40 μm and wavelengths between 0,5 and 1,5 μm. Extension of the steering angle is possible with macro-optical components known from fish-eye and projection lens designs, with decentered micro-lens blazed gratings (as explained above), or with multiple exposure volume (Bragg) gratings [27].

Beam divergence. In general, the far field energy distribution of a DOE comprises the desired first diffraction order, unwanted grating lobes, and diffraction artefacts due to the underlying pixelation and quantization of the phase profile [25]. The beam divergence is determined by the diameter of the clear aperture of the LC SLM and the exit pupil of the optical system. Compression effects are not an issue.

Vignetting. The pixelation of the phase profile due to the addressing of individual resolution cells divides the SLM into active and inactive regions. Aperture ratios of 60 - 70% are reported [25,26] regardless of the addressing scheme.

Spectral range. The liquid crystal currently in use work best for the VIS and NIR spectral range. Some LC material exhibit small transmission windows in the 3-5 μm range [28]. Ultra fine beam steering without diffractive structures suffer the normal material dispersion. Larger steering angles which require the implementation of DOE are subject to structural dispersion. Methods to cope with this type of dispersion are currently under investigation [29].

Throughput. With 8 phase levels the maximum diffraction efficiency (energy in the first diffraction order divided by the energy in all diffraction orders) is 95%. The net optical reflectivity of an OA-SLM was measured to be 93% [26]. Multiplying the diffraction efficiency, the net reflectivity, and the aperture ratio of 70% (1D grating) gives a total maximum throughput of roughly 60%.

Comments. OPAs are programmable diffractive optical elements based on an array of phase shifting pixels. Thus, different optical functions in addition to beam steering such as fanouts and focusing may be realized. OPAs based on LC are a very promising approach for laser beam steering backed up by applications with high market volumes such as micro-displays and projection devices. Challenges to be resolved are: reduction of the response time, reduction of the polarization dependence, reduction of the pixel size below the application wavelength, extension of the spectral range up to 5 μm. New liquid crystal composites such as polymer dispersed liquid crystal materials (PDLC) can overcome part of these problems [28]. A PDLC comprises a polymer matrix with embedded LC domains (droplets). It is possible to form domains of few tens of nanometers, the so-called nano-droplet regime (Fig. 5). Each individual droplet of LC can

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be considered as an uniaxial birefringent medium whose optical axis is controlled by an externally applied electric field. A laser beam propagating through a LC droplet experiences no phase shift if the optical axis is orientated in such a way that the ordinary index of the droplet matches the index of the polymer matrix. Otherwise, the PDLC film imposes a phase shift onto the laser beam due to the refractive index mismatch between the LC droplets and the polymer matrix. The orientation of the LC droplets and the resulting phase shift is controlled by driving the PDLC film either by an array of electrodes or by analogue optical addressing with a photoconductive layer.

Nano-droplet Nano-droplet liquid crystal liquid crystal

Phoconductor Phoconductor

Figure 5. Structure of an optically addressed PDLC device (left) and photographs of the structure of a PDLC nano-droplet composite

film (graphic and photographs provided by Thales Research and Technology [28]).

7. PROGRAMS

Novel laser beam steering techniques for achieving significant reduction in size, weight, power, and cost over conventional gimballed mirror systems are topics of several research projects worldwide. Two programs established in the military research arena are ATLAS in Europe and STAB in the US.

7.1 Advanced Techniques for Laser Beam Steering (ATLAS)

The European Defence Agency (EDA) is funding a program to develop component technologies for advanced laser beam steering. The program addresses innovative concepts for non-mechanical beam steering and beam shaping for DIRCM, active imaging, designation, tracking and ranging systems in a multi-target context. The following techniques are investigated:

? ? ? ?

electro-optics ceramics and polymer-dispersed liquid crystal spatial light modulators for fine beam steering and wavefront correction,

decentered micro-lens arrays for extended field of view beam steering,

optically addressed liquid crystal spatial light modulators for laser beam shaping, intra-cavity beam steering.

The consortium comprises the European defence corporations Thales Optronique SA (consortium leader), Thales Research and Technology, Galileo Avionica, and Diehl BGT Defence.

7.2 Steered Agile Beams (STAB)

The STAB program is funded by the US Defense Advanced Research Projects Agency (DARPA) with the objective “to develop and demonstrate novel chip-scale laser beam steering technologies for military applications” [30]. The STAB program comprises several projects which focus on optical MEMS, diffractive and micro-optics, liquid and photonic crystals technologies for free space laser communications and electro-optical countermeasures applications. The program team consists of major US defence corporations and universities such as Raytheon, BAE Systems, Hughes

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Research Lab, Rockwell Scientific Company, Honeywell, University of California at Berkley, Kent University, UCLA, UCSD, and USC.

8. CONCLUSION

Table 3 serves as a guide to the performance of the most promising beam steering techniques reviewed in this paper. The numbers form a coarse parameter grid to ease the association of beam steering technologies and applications. For broadband wide-angle beam steering Risley prisms are the first choice although they do not comply with the requirements of low weight and power. Blazed gratings offer a broadband solution with moderate angular coverage, but they need additional optical devices to close the angular gaps between grating lobes. Optical phased arrays are a promising technique for small angle beam steering in the lasercom wavebands (LC) and beyond (MEMS, EOC) if coarse beam steering is provided by other means, for example by blazed gratings. Such a combined coarse/fine beam steering device may exhibits large apertures with a throughput of roughly 40% with an update rate in the order of kHz.

Table 3. Guide to the performance of different beam steering techniques.

Property Max. steering angle Angular coverage Aperture diameter Spectral range Update rate Etendue Throughput (1) ultra fine steering mode(2)

Risley prisms 60 deg continuous < 100 mm 0.4 – 12 μm 500 Hz 105 mm*rad 75 % Blazed grating 25 deg discrete < 150 mm 0.4 – 12 μm 1000 Hz 65 mm*rad 65 % Optical phased arrays 75 μrad(1) / 1.5 deg(2) quasi-continuous < 20 mm 0.4 – 1.5 μm (LC) 0.4 – 5 μm (MEMS/EOC)(3) 50 Hz 0.0015 / 0.5 mm*rad 60 % programmable diffractive optical element (3)

LC: liquid crystal, MEMS: micro-electro-mechanical systems, EOC: electro-optical ceramics

ACKNOWLEDGEMENTS

The author takes this opportunity to acknowledge several people who support the ATLAS program. At the very beginning, Anne-Marie Bouchardy (TOSA), David Titterton (DSTL), and Giorgio Leonardi (GA) initiated the formation of an industry consortium. ATLAS is funded by the French, German, and Italian MODs and supervised by a Management Group comprising Julie Poupard (DGA), Gerhard Traeger (BWB/WTD 81), and Giuseppe Licciardello (IT MOD DGAT). The industry consortium is managed by Nathalie Gerbelot-Barrillon (TOSA).

Brigitte Loiseaux, Patrick Feneyrou, Jér?me Bourderionnet (TRT), and Matthias Rungenhagen (DBD) supplied valuable information and viewgraphs for this paper concerning PDLC, EO ceramics and decentered micro-lens arrays. Their contribution is gratefully acknowledged.

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中文翻译：

特邀论文

新型激光束转向技术

汉斯·戴特tholl 光电子与激光技术系 迪尔BGT防务

邮政信箱101155，88641ü贝尔林根，德国

文摘

本文总结了大功率激光光束转向技术和通信应用。编制原则和新型机械，微型机械和非机械的技术特点。基于微透镜的粗波束或细转向液晶的电光相位控制相结合，提出更多的细节。这项检讨涉及调节通过激光束的相位分布的光束转向装置和排除腔内的波束，波束在色散光学元件，光有源相控阵，光波导相结合的可调谐激光器的基础上。

关键词：激光束转向，光学相控阵，偏心微型透镜，空间光调制器

1引言

激光功率的整合,激光辅助传感和激光通信子系统到自主车辆、航空和太空平台需要新的技术来引导雷射光束。新技术应该促进实现大光孔径的光束转向装置融入的机械结构平台。愿询价单包括著名的特性:结构紧凑、重量轻、低功耗、灵活、多光谱、大的方面领域。

角传播的激光束，特别是远距离应用，本质上是由于光波长的高孔径天线增益小。因此，激光束的传播方向一般控制在两个步骤：（1）与万向光学元件的炮塔指向发射/接收到所需的方向望远镜的视场和平台运动补偿中度精度和速度。 （2）光束转向装置，转向在望远镜的视场的激光束，以获取和跟踪目标。

本次审查的标的物是一种新的激光束转向技术。光束转向装置能够 ?指向激光束，在广泛的领域方面的随机， ?步进梁从一个角度位置到下一个小增量，

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?居住在每个位置上的目标所需的时间。

相反，在扫描装置移动光轴不断开关设备是唯一能够解决预定的方向。评论目前的技术指导，扫描，激光束的切换在引用[1,2,3,4]。

在一般情况下，波束完成整个激光束的孔径施加一个线性相位延迟配置文件。相应的波前斜坡的斜率决定转向角：大转向角对应大斜坡，反之亦然。结合大光圈大波前的斜坡需要大光路的光圈上的差异（OPD），其中由光束转向装置实现。

大波前的斜坡可直接生成宏观光学元件，如旋转（里斯利）棱镜和反射镜或偏心镜片。这些转向装置是相对紧凑的万向镜子相比，具有低惯量矩和不旋转光轴。最近，这些宏光学方法获得了新的流行。

结构紧凑，重量轻，低功耗波束设备的方式进行平滑微光学技术。单一的微型光学元素，如电光棱镜，双轴扫描微镜，或连接到微执行器的微镜片模仿转向宏观光机制。宏观光学系统焦平面附近安装单，小孔径的微型光机电系统（MOEMS的），并提供快速的激光束的指向。这些配置结合起来，高带宽的MEMS宏光束转向装置的好处是在低光功率水平波束应用的候选。 为了建立与微光学元件的大光圈，他们被安排在矩形二维阵列。有前途的技术，是电光棱镜或二维微镜和偏心微透镜阵列的一维数组。在可见光和红外波段的阵列间距大于波长的安排，像一个衍射光栅的作用。抑制中衍射命令了积极炽热的光栅结构以适当的方式。

积极开辟微型光学光栅的相控阵的基本形式。相控阵是定期安排一个子孔径每一个进入太空的辐射其自己的模式。干涉个人的辐射模式模拟在远场的连贯的大光圈。这种审查涉及所谓的被动相控阵调节整个撞击的激光束的相位分布。为了这个目的，每个子孔径相活塞是多种多样的，从而创造一个可编程的跨设备的光圈衍射光学元件。

有更多的波束技术在文学描述：腔内波束，波束在色散光学元件（如光子晶体），主动光学相控阵，光波导转向技术相结合的可调谐激光器的基础上。这些技术被排除在这次审查。 2光束转向装置的参数空间

激光束转向装置的功能要求包括以下主题： ?最大转向角，

?光束发散/成像能力， ?光圈/暗角，

?光谱范围和分散， ?吞吐量， ?控制转向角

每个功能相关的定量参数，很大程度上取决于业务需求。在一般情况下，可以区分两类转向装置（1）电源整经（如定向光的对策，将权力移交给远程设备）和自由空间激光通信的应用程序需要通过激光束光束转向装置只有一次。 （2）有源传感技术，如激光雷达激光束发射（Tx）和接收（Rx）信号，通过波束装置。表1给出了具体应用定向红外对抗（DIRCM系统），激光成像雷达（激光雷达）和深空激光通信中引用相关的功能参数的标称值[6,9,10]。这些例子运行的系统级域参数，如最大转向角，孔径，光束发散角，指向精度。表征光束转向装置，独立的光学系统内的位置参数的光谱范围，时间常数，棱角分明的动态范围，并扩展光束。

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表1。标称波束参数为不同的应用程序的编译。 参数 定向红外对抗[6] 激光成像雷达 深空激光通信 [9] [10] 最大转向角 45 deg 5.4 deg 0.6 deg 光圈直径 50 mm (Tx) 75 mm (Rx) 300 mm (Tx) 光束发散角（TX） 1 mrad 10 mrad 6.3 μrad 瞬时视场（1）（RX） - 333 μrad - 指向精度 100 μrad 30 μrad 1 μrad 光谱范围 2 to 5 μm 0.532 μm 1.064 μm 时间常数（2） 角动态范围（3） 1 ms 0.7 ms 1 ms 光束扩展（4） 42 dB 38 dB 43 dB （1）视野：视场 （2）所需的时间从一个角度位置到下一个步骤 （3）10日志（2 *[最大转向角] / [定位精度]） （4）2*[最大转向角] *[孔径]

光束转向装置（BSD）光束扩展限制其在光学系统中的位置。所需的的大光束扩展要求的DIRCM系统的BSD被放置在发射望远镜的出瞳。温和光束扩展给有利的环境的BSD安装在出瞳光束扩展技术取决于望远镜的入口瞳孔。这也是可能的分裂粗转向位于出瞳元素和精细入口瞳孔转向元素之间的转向能力。为成像激光雷达应用中的粗/细的光束转向分工是可取的，如果精细光束辅助镜也作为一个风扇出衍射光学元件（DOE）的功能。能源部建立一个激光点的数组，它照亮接收焦平面像素[9]的足迹。深空激光通信的需要结合大光圈小光束扩展的BSD安装在入瞳的望远镜，它扩大了激光束，并减少转向角。

表1中编译应用程序通过以下各节的指南作为一个特定的波束技术虽然不是唯一的应用程序。 3光束转向宏观光学元件

在最近的一系列文件，应用红外对抗应用旋转棱镜和偏心镜片广角波束报道[5,6,7]。这项研究是侧重于宏观光学粗光束旋转棱镜和偏心镜头转向装置。

宏观光学器件使消色差设计，避免盲点领域内的视图，并集中到一个单一的光束转向能源。用人棱镜和偏心镜片偏离射线束的主要射线是在可视化工具的设计技术标准。这个著名的设计挑战是光电力学参数和材料，以确保在2-5微米之间的红外光谱范围内的消色差转向广角正确组合搜索。

3.1里斯利棱镜光束转向装置[5,6]

工作原理：里斯利棱镜是一对沿光轴级联的消色差棱镜。在相同的角速度相等或者不相等的棱镜或相反的方向旋转，产生多种扫描模式，填补一个圆锥形的方面领域不断。棱镜的配置应该是光学互惠，以确保所有感兴趣的波长沿光轴精确的波束。光学互惠是一个对称属性：在基准位置后，在内部的平面垂直于光轴的反射棱镜的配置仍然不变。

最大转向角：根据参考[6]的最大转向角45度，是实现与适当的分散控制。

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光束发散。所有光束转向装置不改变光轴的方向，表现出有效的光束直径减少预计垂直转向方向。此外，设备依赖光束压缩可能会发生。棱镜光束辅助镜压缩在这样一种方式，一个圆形的输入光束留下一个椭圆形的设备的激光束。压缩保存梁的相空间体积（光束扩展）和激光功率，但减少了在远场的高峰，因为在沿压缩方向的光束发散角增加辐射。这种效应最终限制了光束发散角的约束给定上最大转向角。

光谱范围： risely棱镜光的光谱范围（可见光VLWIR）整个工作。业务的光纤带宽是有限的材料色散。无色第一顺序是通过使用消色差棱镜双峰。其中的物质替代品的广泛结合LIF /硫化锌导致小型二次分散1.78 mrad的2-5微米的光谱范围内的最大转向角度在450[6]。 吞吐量：清澈的大光圈和顶点的角度几度产生长期内设备透过率的影响，由于吸收和散射棱镜材料的棱镜光学路径长度。适当的防反射涂层的多棱镜之间的干扰影响降低，透光率在75-80％的情况似乎是可以实现的[5]。

评论：快速随机转向激光束，通过广泛的角度范围，需要控制旋转方向，瞬时角位置，和棱镜对角速度。复杂的方位角和仰角转向角度棱镜旋转角度和波长的连续函数。对于平稳的转向轨迹无奇，如棱镜翻转，遇到[6]。棱镜驱动器的扫描线被动和激光雷达传感器的视线的实施，建立[9,11]。然而，随机步骤和盯模式控制回路的实现不是一个简单的直线前进的任务。在最近出版的1的里斯利光束转向设备最大转向角60度，口径为100毫米，2-5微米的波长范围内，针对重复性优于50微弧度公布[12]。

3.2 Decenterd镜头的光束转向装置[5,7]

工作原理：理想的情况下，光束转向装置是一个无焦光学系统转换成一个平面输入波前平面输出波前。除了棱镜，镜头的开普勒或伽利略式望远镜是宏观光束转向装置的候选人。该望远镜由两个分离的镜头焦段的总和。完成由出口镜头侧向位移与输入镜头转向行政射线和相关的射线束。

最大转向角：最大转向角度取决于退出镜头的焦距和失真，这是可以接受的最大侧向位移。在实践中，侧向位移是由于暗角的射线束，退出镜头的光圈直径的一半。这导致了大约25度的最大转向角。

光束发散：激光束的压缩取决于两个镜头的焦段比。伽利略类型的，这个比例的绝对值总是比一个小。对于开普勒类型的焦距比是可能的，最好的光束转向装置应在联合发送/接收模式运作。彼此相对的两个镜头光圈的侧向位移，减少了明确的光圈和暗角和光束发散角的不对称增加。这种效果是通过引进一个场镜头控制。此外，强烈的光束发散透镜系统的光学像差的影响。传输的光束应该只照亮了每个镜头的中央部分，以保持在分歧的要求。接收到的波前可以照亮全口径和遭受传输光束的像差高于程度。

暗角：减少侧向位移的镜头暗角由于为开普勒配置大幅引进一个现场镜头中常见的两个镜头焦平面。正面和负面的现场镜头是可能的。阳性场镜头是硬性退出镜头，并连接到都流离失所一起。这个设施的驱动机制，但内部重点介绍了近场镜头。对于高功率的应用，这是不可取的。负场镜头需要一个额外的驱动器移动相反方向在近场镜头和退出镜头1:2的关系[7]。这样，避免内部的焦点。

光谱范围。棱镜光谱范围没有限制。理想的情况下，每个镜头的光束辅助镜是一个消色：透镜。在参考文献7的材料组合Ge/AMTIR-1选择在2-5微米光谱范围的弗劳恩霍夫双峰（积极的，消极的第二部分），以尽量减少色差。 1开普勒望远镜的设计与负场镜头转向激光束可达22.5度和0.65 mrad的多光谱范围2-5微米的二次分散。

吞吐量：无色梁文献[7]转向装置包括6个外部和3个内部接口和rougly材料厚度40毫米。为棱镜辅助镜的吞吐量应该在75% - 80％。围内指定1 mrad的发散能源取决于波长和转向角度。在2微米的包围能源仍高于95％，为所有的转向角度;在5微米的包围能量从98％上

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轴22.5度至63％不等。

评论：为了引导激光束的两个镜头组的横向位移，必须加以控制。幸运的是，镜头组的位移之间的关系是恒定的。每个波长的方位角和仰角转向角位移几乎线性函数。所需的最大位移是等于退出镜头，这是约35毫米的孔径半径。整体尺寸为长180毫米和135毫米的高度最大的镜头位移。偏心光束转向宏观光学镜片是可能的，但由于所涉及的复杂性，而不是一个实用的方法相比，里斯利棱镜。这是被安排在一个普通的数组微光学元件的微型光学世界的对比。电光棱镜阵列是一维小角度转向波束。偏心微透镜阵列，包括场镜头转向激光束可达25度角在两个方面的选项。

3.3波束与宏观光学镜

透射光学元件的大领域的看法紧凑型光学系统的第一选择。这种方法的缺点是由于在不同折射率的材料之间的界面的折射光功能的波长依赖。反射光学设计提供了独立的波长上。在前款讨论的两个方法可以实现用镜子。一的里斯利型梁毫米波基于旋转的镜子转向装置进行了讨论，在文献[8]。

4波束与光电微机电系统（MOEMS的）

ladars发现目标，导弹制导，地形测绘和监视，或命名只有少数的机器人导航应用。 ladars（数10米）的短距离应用将依靠闪光灯照明的视场和接待的由快照焦平面阵列的散射光。 ，中级远距离成像ladars，必须按顺序照亮的，因为有限的激光功率视场的一部分。这些系统需要加强通过现场视图和住在一个特定的部分，以积累几个激光脉冲从现场反映的光束转向装置。

下一代中程（几个100米）的3-D的成像ladars将在一段时间的飞行模式操作，并融入激光器，光学，电学，机械设备，微型光机电系统（MOEMS的）[4]。目前，微光机电系统技术的发展是由光纤光学通信与光开关和波长复用器/解复用器的焦点。开关设备的微光机电系统，大多是数字（如由德州仪器推出的DLP技术），或无传感的实际光束方向在开环模式下的扫描。据介绍给出的定义，这些设备没有引导激光束（他们无法指出随机或停留在一个特定的方向）。然而，扫描微反射镜是在短期内检讨，因为他们铺平了微光学相控阵的方式。 两轴扫描微镜

工作原理：万向微反射镜反射激光束在同样的方式作为大型同行。微光机电系统提供与质量，体积和耗电量方面的优势。这些微系统设计的挑战在于，在事实上，宏力不缩放大小线性。新的设计方法，安装，驱动器和控制微镜倾斜是必要的。微镜制作在硅片上，然后粘贴到另一个芯片，其中包含了电极结构电静态驱动镜议案。镜直径几毫米为了这些设备的第一代两个位置之间来回翻转，较新的版本是在两个层面进行控制的连续扫描能力[4]。

最大倾斜角度可达（±）15度，在文献[13]报道。根据光学的布局，这给了可能用负透镜的光学放大（±）30度，最大扫描角。

光束发散衍射极限的光束发散的微反射镜的直径是有限的。为1微米波长的激光束的发散反映了6毫米的镜子，是约330微弧度。具有大扫描角度的光束被压缩在一个方向。

光谱范围:反射法是独立的波长。介绍了弥散通过包装的镜子,通常是密封窗户后面。这些窗户限制光谱范围及介绍色差。

吞吐量是有限的,由反射涂料镜(铝为可见,金红外光谱覆盖范围),反射在窗口界面,通过衍射效应在微反射镜的边缘。吞吐量应在85%。

评论:提供真正的转向能力万向微镜必须在一步步盯时尚倾斜。目前，这种运作模式的研

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究议程上，不因为镜装置的应用领域是开关或激光束扫描。另一个有趣的方法，这是有关波束的弯光束微机械的空间光调制器[21]。在这个装置的微反射镜暂停有四个铰链，像活塞运动的结果。这种运作模式带来了相位调制反射的激光束，这可能是有关精细相控阵波束。 5。闪耀光栅波束

闪耀光栅的光束转向利用微光学元件阵列的固定摊位（微型望远镜，微反射镜，微棱镜）。每个微光学元件样品传入的激光束，并辐射到空间小波束。的相干干扰波束形成的衍射图，其中包括几个主要的叶栅瓣，旁瓣包围。主瓣的传播方向是由光栅方程。微光栅期间积极开辟以引导激光束。最有前途的二维闪耀光栅波束的方法是基于偏心微透镜阵列。其他技术，如倾斜微反射镜（光电）棱镜阵列是太有限了偏转角。 decentred微透镜阵列

工作原理:一个微型望远镜阵列包括两维规则排列的开普勒或伽利略式望远镜。形成微telecopes镜头列车分布在两个平面的基板上，保持其表面mircro透镜阵列。微透镜阵列，实现了可见光谱范围内的玻璃折射和衍射光学元件和高格式（512×512）和高填充因子为中红外应用[14,15,16硅和其他材料， 17]。积极炽烈的望远镜阵列实现横向翻译与其他方面的一个mirco镜头基板。这是类似的宏观安排的运作。不同之处在于一个事实，即光圈大小和字段大小像差尺度。因此，偏心镜片的原则，微光学的实现需要比宏观光学对应较少的光学表面。 最大转向角:阵列间距的二分之一的最大侧向位移最大转向角约25度的限制。最大转向角度取决于望远镜的类型和镜片材料的折射率（见表2）[14]。只能达到最大值，与开普勒望远镜和场透镜阵列（见图1）与可接受的性能。

光束发散:远场偏心微透镜的光束转向装置由主瓣（称为栅瓣）和光栅结构所决定的旁瓣。栅瓣角宽度取决于整个阵列孔径的大小和相干长度。在硅，直径6英寸的阵列应该是可能的。这决定了光束发散的一个主要因素是整个阵列的空间相干性。由于在制造过程中的缺陷的几何光学参数的变化减少的beamlets的空间相干性[18,19]和拓宽了栅瓣。非均匀阵列的光学参数应该远低于3％，以实现波束宽度性能良好，转向角和衍射效率。束压缩为伽利略的问题，因为不同焦距长度和侧向位移引起的暗角简单的开普勒型。

暗角:暗角介绍了微透镜的横向位移和依赖型微型望远镜。伽利略的性能和简单的开普勒型的强烈影响暗角。在开普勒望远镜的积极场透镜的引入消除成本降低激光损伤阈值的暗角。图2说明了在整个寻址栅瓣Strehl比均匀的增益。引入一个负场镜头是不适合的，因为日益复杂的动力机制。

光谱范围:梁偏心微透镜阵列的指导工作在整个光波段（紫外到VLWIR，见表2）。色散是由所涉及的材料和光栅结构的阵列诱导。

表2。偏心微透镜阵列材料的比较（摘自文献[14]） 特性 Ge Si GaAs ZnSe ZnS 折射率 4.0 3.4 3.3 2.4 2.3 最大转向角（度） 28 25 24 18 17 最好的选择波段(μm) 6.5 - 13 1.5 – 6.5 13 – 17 0.65 – 1.5 0.46 – 1.5

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伽利略微型望远镜阵列

开普勒微场透镜阵列望远镜阵列

图1。射线追踪，通过偏心微透镜阵列具有相同的间距和最大转向角。

伽利略望远镜

开普勒望远镜 衍射

图2。处理光栅叶的(衍射的顺序)为一个偏心微透镜阵列与光束辅助镜计算斯特列尔比3％的非均匀性，在每个镜片表面的焦距。两个望远镜阵列具有相同的间距和最大转向角。 吞吐量:解决开普勒型安排与现场镜头衍射顺序减少一些损失机制的力度：在镀膜接口，

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造成的损失非理想的光圈比率（孔径的宽度比光栅间距菲涅尔损失），减少由于衍射效率不足炽烈，在镜片材料的灭绝。对于在硅制造的镜片的能量平衡预测的最低吞吐量65％的入射功率。

评论:非常适合与敏捷的转向表现出高利率（千赫）和中等精度（典型的约0.3％的微镜头光圈）大格式的微透镜阵列。压电换能器是驾驶lenslets安排适当的选择。图3显示了一个宽带的第二代样机偏心微透镜图像转向装置的光谱范围3-5微米[20]。

图3。原型成像转向装置为3-5μm光谱覆盖范围。 闪耀光栅光束控制的表现出两个缺点：

（一）由于光栅的性质，只有离散角位置（栅瓣）可以解决。

（二）跨阵列的光学参数的非均匀性，减少之间的干扰光束增加光束发散的空间相干性。这些缺点都可以解决。 （I）之间的栅瓣角位置是用细光束在前面闪耀光栅转向装置访问。精细光束辅助镜规定，在每个光栅周期当地相斜坡和指导之间为一个固定的闪耀角[14]相邻的栅瓣的激光束。另外，闪耀光栅光束辅助镜相结合，相控阵调制事件眼波。在最低阶，光栅周期的每个阶段活塞是多种多样的。这样一来，整个光栅的全局相近似相楼梯坡道。光栅的闪耀角必须调整坡道保留最大衍射效率的全局相斜坡，因为每个光栅周期只表现在一个变化阶段活塞。 （三）光学厚度非均匀性可以补偿由于制造和对准误差和整个光栅孔径的空间相干性，可提高阶段活塞有足够的动态范围（见图4）。此外，如果相控阵像素数足够大，可以覆盖几个像素闪耀光栅和更高的的眼波错误（头/倾斜，离焦）期间可能得到纠正。在可见光和近红外光谱范围内的液晶相控阵闪耀光栅光束辅助镜的自适应校正。基于微反射镜或电光陶瓷，它涵盖了更广泛的光谱范围的相控阵下的发展[21,28]。 6。相控阵波束

自由空间光通信地面，空中和卫星平台之间，吸引了越来越多的关注，由于对崎岖的激光器和紧凑的光学系统的演变。在此背景下，非机械式激光束转向提供技术手段来实现成本效益的通讯联系。波束装置的系统级功能，包括（i）为指向的激光束和接收器/反射器主动跟踪粗转向，及（ii）精细转向以补偿线的视线波动，因小变动接收机，大气湍流和平台的振动。 光学相控阵（OPAS）成为有吸引力的办法为这些任务。 OPAS对整个激光束，通过控制空间变化的折射率或几何射线路径，跨设备的光圈相延迟坡道。目前，三种技术正在调查：（一）超薄液晶（LC）薄膜的折射率和空间调制（二）电光陶瓷（EOC）及（三）变更阵列的几何路径空间分辨微机械（MEMS）的镜子。立法技术是成熟和商用。电光陶瓷和MEMS相控阵活塞般的微反射镜的议案是在他们处于起步阶段[21,28]。

曝晒波输出 曝晒波输出

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平面波输入 平面波输入 (a) (b)

曝晒波输出

平面波输入 （c）

图4。插图阵列非均匀性和使用相控阵在前面闪耀光栅光束辅助镜叶之间光栅插值补偿。相控阵的一个像素对应一个光栅周期。 （一）相控阵是缺席的。 （二）修正阶段活塞错误。

（三）纠正错误的阶段活塞和栅瓣之间的插值。

如果几个像素的相控阵盖一个光栅周期波前曲率也可以得到纠正平面波输入 液晶空间光调节器(LC应)

LC SLMs评价激光光束转向和塑造超过十年以来[22]。渐渐地,SLMs适合高质量的相位调制的2π对世界和近红外波段,拥有了一大批的独立可寻址的分辨率细胞在一个二维数组,满意的帧速率的上市(23)。

工作原理。一个LC可持续土地管理的详细结构依赖型液晶（液晶，铁电，或聚合物分散），解决方案（电气或光学），和运作模式（透射或反射）。大多数立法会的SLM是商业，包括向列型液晶。示意图，向列液晶层夹着涂电极的基板之间的平面。液晶分子预电极平行排列。电极之间施加电压产生电场，这主要是垂直的电极和控制立法会董事层的方向：在立法会董事线与电场线。从光学的角度来看，这种安排是双折射和调整立法会董事诱导光学路径长度的变化取决于入射光束的偏振。大多数立法会可持续的土地管理工作在反射模式。在传输模式的比较层的厚度减少，增加所需的光学相位调制深度和调制带宽。的反光操作模式，也简化了寻址的液晶层的决议细胞。

一个电子信用证应解决(EA-SLM)组成图案的电极(线模式1 D,2 D失常波束控制)与高度反光垫是粘结在silicion底板以及一个透明电极是抗反射涂层。差距定义模式扭曲了底板上电

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极电场分布和引起边缘领域。边缘领域分离EA-SLM在光学活性和不活跃的地区。由于制造限制一个缺口至少需要0.5μm为了防止介质分解电极间。在一起的要求有高孔径比这导致的最小宽度为1.5μm[24]。为能见度和近红外波段的像素结构就像一个衍射光栅衍射造成分成几个独立的订单的远场征收相剖面[25]。

干扰衍射效应的电极结构是预防使用时应解决的信用证(OA -应)。而不是一个有图案的底板电极反射OA-SLM大面积光电导体组成,一个吸引人的光堵层,和高介电镜子反射。在操作过程中, 光电导体偏见是电相对于透明电极、图案灯下。但同时决定了局域电场分布控制的方向LC导演。展览的文物OA-SLM减少衍射由于缺乏一个像素化电极结构和可用于大孔[26]。

最大转角:商业上可用信用证应大小是20毫米和8位相动态范围(23)。根据波长(0.5 - 1.5μm)这一结果在最大转向角度25 ~ 75之间μrad与nanoradians决议。这些小转向角度来说是很理想的超细波束控制。大转向角度实现通过实施的衍射光学元件(DOE),即衍射光栅。相位类不局限于某一阶段是介于0和2π弧度。就想拇指规则一样,高衍射效率(> 95%)在第一衍射订单达到了至少8阶段步骤/光栅周期与相应的炽热的单元。这种技术结果最大转角6到18 mrad为一个像素8到40μm音高和波长介于0、5、1、5μm。扩展的转角可能有macro-optical部件已知得像是透过鱼眼镜头设计和投影, 偏心微型镜头曝晒光栅(解释以上),或与多重曝光量(布拉格)光栅[27]。

光束发散:一般而言,远场能量分布的一种不包括预期的第一个衍射秩序,不必要的光栅叶和衍射文物由于潜在的像素及量化相剖面[25]。光束发散角由直径的清晰的孔径和出口的信用证应由光学系统的学生。压缩效果并不是一个问题。

装饰图案。像素的相位剖面由于解决个人解决细胞分应积极的和消极的地区。孔径比报道60 - 70%[25,26]无论向方案演说。

光谱覆盖范围:液晶目前正在使用的最适合能见度和近红外光谱覆盖范围。一些LC材料展览小传递窗在3 - 5μm范围[28]。超细波束控制无衍射结构受正常的材料色散。大转向角度需要实施不受结构色散。方法来应付这种类型的色散正在调查[29]。

吞吐量:最大衍射效率（第一衍射能量除以能源在所有衍射订单顺序），与8相的水平是95％。是衡量一个OA - SLM净光学反射是93％[26]。乘以衍射效率，净反射率，70％的孔径比（一维光栅），提供了大约60％的最大总吞吐量。

评论:OPAS是一个相移像素阵列为基础的可编程衍射光学元件。因此，除了如扇出和聚焦光束转向不同的光学功能可以实现。基于对立法会的OPAS是一个非常有前途的激光束转向高市场容量的应用，如微显示器和投影设备备份的方法。要解决的挑战是：响应时间减少，减少的偏振依赖性，减少应用波长，光谱范围扩展到5微米以下的像素大小。新液晶复合材料，如聚合物分散液晶材料（牙周膜）可以克服这些问题的一部分[28]。一个PDLC的包括聚合物基体与嵌入式LC域（水滴）。这是可能形成几十纳米，所谓的纳米液滴的政权（图5）域。每个LC单个液滴被视为一个单轴双折射介质，其光轴是由外加电场控制。激光束通过一个LC飞沫传播体验无相移，如果光轴定位是在这样一种方式，的液滴普通指数相匹配的聚合物基体的指数。否则，PDLC膜征收到的激光束的相移，由于液晶液滴和聚合物基体之间的折射率不匹配。液晶液滴的方向，以及由此产生的相移控制驾驶PDLC膜电极阵列或模拟光学与光导层的寻址

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图5。结构的光学解决PDLC的设备（左）和结构的PDLC显示器的纳米复合薄膜液滴（泰雷兹研究与技术[28]所提供的图形和照片）的照片。

7方案

新型的激光光束转向技术达到显著减少尺寸、重量、电力、和成本比常规反射镜系统的命题给出几个研究世界项目。两个项目建立在军事研究领域是阿特拉斯在欧洲和美国的刺。 7.1高级的技术来激光转向(阿特拉斯)

欧洲防务局（EDA）正在资助一个项目，开发先进的激光束转向组件技术。该方案涉及非机械光束转向和塑造DIRCM系统，主动成像，标识，跟踪和测距系统在一个多目标的情况下，梁的创新概念。研究以下技术：

?电光陶瓷和聚合物分散液晶空间光调制器的细光束转向波前校正， ?偏心微透镜阵列的角度波束扩展字段， ?光寻址液晶激光束的空间光调制器的塑造， ?腔内波束。

该财团由欧洲防务公司Thales公司Optronique SA（财团领袖），泰勒斯 研究和技术，伽利略Avionica，迪尔BGT防务。 7.2掌舵的灵活的光束(刺)

刺的计划是由美国国防部高级研究计划局（DARPA）资助的目标是“用于军事用途的新型芯片级的激光束转向技术开发和示范”[30]。之刺方案包括光学MEMS，衍射微光学，液体和光子晶体的自由空间激光通信和电光学对策应用技术集中的几个项目。项目团队由美国主要国防公司和大学，如雷声公司，BAE系统公司，休斯研究实验室，罗克韦尔科技公司，霍尼韦尔公司，美国加州大学伯克利分校，肯特大学，加州大学洛杉矶分校，加州大学圣迭戈分校，南加州大学。 8结论

表3作为本文综述最有前途的波束技术的性能指南。数字形成一个粗参数网格缓解波束控制技术和应用的关联。宽带宽角波束里斯利棱镜的第一选择，尽管他们不符合低体重和力量的要求。闪耀光栅提供一个适度的角覆盖的宽带解决方案，但他们需要额外的光学器件之间的栅瓣关闭角的差距。光学相控阵是一个小角度转向束激光通信波段（LC）及以上（MEMS，平机会）的有前途的技术，例如，如果通过其他方式提供粗波束闪耀光栅。这种联合粗/细的光束

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转向装置可能会表现出与的与千赫秩序的更新率大约40％的吞吐量大光圈。

表3。引导到不同的波束技术的性能。 特性 里斯利棱镜 闪耀光栅 光学相控阵列 最大转向角 60 deg 25 deg 75 μrad(1) / 1.5 deg(2) 覆盖角度 continuous discrete quasi-continuous 孔径 < 100 mm < 150 mm < 20 mm 光谱范围 0.4 – 12 μm 0.4 – 12 μm 0.4 – 1.5 μm (LC) 0.4 – 5 μm 更新率 500 Hz 1000 Hz 50 Hz 光束扩展 105 mm*rad 65 mm*rad 0.0015 / 0.5 mm*rad 吞吐量 75 % 65 % 60 % （1 ）超细转向模式 （2）可编程衍射光学元件

（3）立法会：液晶，微机电系统：微机电系统，平机会：电光陶瓷

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鸣谢

笔者借此机会感谢那些支持的ATLAS方案的几个人。在开始的时候，安妮 - 玛丽·Bouchardy（TOSA），大卫Titterton（DSTL），乔治·莱奥纳尔迪（GA）发起的产业联盟的形成。 Atlas是由法国，德国和意大利的外挂和监督管理，包括集团朱莉Poupard（DGA的），格哈德，Traeger（翼身融合/ WTD的81），和Giuseppe Licciardello（当日DGAT）。行业协会管理由纳塔莉Gerbelot Barrillon（TOSA的）。 碧姬Loiseaux，帕特里克Feneyrou，，杰罗姆Bourderionnet（泰爱泰党），：马蒂亚斯Rungenhagen（DBD）提供有价值的信息和视图本文关于PDLC的，电光陶瓷，偏心微透镜阵列。他们的贡献表示感谢。

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