1、Proc. of SPIE Vol. 6397 639708-1 Invited Paper Novel Laser Beam Steering Techniques Hans Dieter Tholl Dept. of Optronics & Laser Techniques Diehl BGT Defence PO Box 10 11 55, 88641 berlingen, Germany ABSTRACT The paper summarizes laser beam steering techniques for power beaming, sensing, and communi
2、cation 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 a
3、ddresses 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, op
4、tical 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 techn
5、iques 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
6、 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 gimball
7、ed 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 tra
8、ck a target. The subject matter of this review are novel laser beam steering techniques. Beam steering devices are capable of 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 r
9、equired 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: han
10、s.tholldiehl-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 Proc. of SPIE Vol. 6397 639708-2 In general, beam steering is accomplished by imposing a li
11、near 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 di
12、fferences (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 c
13、ompact, 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
14、 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
15、 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
16、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
17、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 radia
18、ting 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 pist
19、on 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 elemen
20、ts (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: maxi
21、mum 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 device
22、s 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 trans
23、mit (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
24、 in references 6,9,10. These examples run the gamut of system level Proc. of SPIE Vol. 6397 639708-3 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
25、 optical system are spectral range, time constant, angular dynamic range, and etendue. Table 1. Compilation of nominal beam steering parameters for different applications. Parameter DIRCM 6 Imaging Ladar 9 Deep Space Lasercom 10 Maximum steering angle 45 deg 5.4 deg 0.6 deg Aperture diameter 50 mm (
26、Tx) 75 mm (Rx) 300 mm (Tx) Beam divergence (Tx) Instantaneous FOV(1) (Rx) 1 mrad - 10 mrad 333 rad 6.3 rad - Pointing accuracy 100 rad 30 rad 1 rad Spectral range Time constant (2) Angular dynamic range (3) Etendue (4) 2 to 5 m 1 ms 42 dB 78 mm*rad 0.532 m 0.7 ms 38 dB 28 mm*rad 1.064 m 1 ms 43 dB 1
27、0 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*aperture diameter The etendue of the beam steering device (BSD) restricts its location within the optical system. The large eten
28、dues 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
29、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 opt
30、ical 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 a
31、nd 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 p
32、risms 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 sp
33、ots 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 m.
