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1、Invited PaperNovel Laser Beam Steering TechniquesHans Dieter ThollDept. of Optronics & Laser TechniquesDiehl BGT DefencePO Box 10 11 55, 88641 berlingen, GermanyABSTRACTThe paper summarizes laser beam steering techniques for power beaming, sensing, and communication applications. Principles and char
2、acteristics 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 mod
3、ulate 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-len
4、ses, spatial light modulators1. INTRODUCTIONThe 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 bea
5、m 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 be
6、am, 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
7、 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 revie
8、w are novel laser beam steering techniques. Beam steering devices are capable ofpointing 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 dev
9、ices 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.tholldiehl-bgt-defence.de; Phone: +49 7551 89 42
10、24Technologies for Optical Countermeasures III, edited by David H. Titterton,Proc. of SPIE Vol. 6397, 639708, (2006) 0277-786X/06/$15 doi: 10.1117/12.689900In general, beam steering is accomplished by imposing a linear phase retardation profile across the aperture of the laser beam. The slope of the
11、 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 dev
12、ice.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, th
13、ese 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 i
14、mitate 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
15、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
16、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 b
17、y 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 sim
18、ulate 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
19、 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 associa
20、ted with optical waveguides. These techniques are excluded from this review.2. PARAMETER SPACE OF BEAM STEERING DEVICESFunctional requirements for laser beam steering devices cover the following topics:maximum steering angle,beam divergence/imaging capability,aperture/vignetting,spectral range and d
21、ispersion,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 pow
22、er 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 no
23、minal 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 levelparameters such as maximum steeri
24、ng 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 pa
25、rameters for different applications.ParameterDIRCM 6Imaging Ladar 9Deep Space Lasercom 10Maximum steering angle45 deg5.4 deg0.6 degAperture diameter50 mm (Tx)75 mm (Rx)300 mm (Tx)Beam divergence (Tx) Instantaneous FOV(1) (Rx)1 mrad-10 mrad333 rad6.3 rad-Pointing accuracy100 rad30 rad1 radSpectral ra
26、ngeTime constant (2)Angular dynamic range (3)Etendue (4)2 to 5 m1 ms42 dB78 mm*rad0.532 m0.7 ms38 dB28 mm*rad1.064 m1 ms43 dB10 mm*rad(1) FOV: Field-of-View(2) 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*apertu
27、re diameterThe 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 exi
28、t 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
29、 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 fo
30、r 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 a
31、n application.3. BEAM STEERING WITH MACRO-OPTICAL COMPONENTSIn 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 d
32、evices 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 techni
33、ques 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 m.3.1 Risley prism beam steering device 5
34、,6Principle 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.
35、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 pla
36、ne 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 eff
37、ective 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 bea
38、ms 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
39、. 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
40、 to small secondary dispersion of1.78 mrad within the spectral range 2-5 m 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 absorpti
41、on 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 co
42、ntrol 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,
43、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
44、 Risley beam steering device with a maximum steering angle of 60 degrees, an aperture of 100 mm, a wavelength range of 2-5 m, and an aiming repeatability of better than 50 rad was announced 12.3.2 Decenterd lens beam steering device 5,7Principle of operation. Ideally, a beam steering device is an af
45、ocal 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 lengt
46、hs. 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 displace
47、ment 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.Beam divergence. The compression of the laser beam depends on the ratio
48、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
