外文翻譯--新型激光束轉向技術

1、<p>　　Invited Paper</p><p>　　Novel Laser Beam Steering Techniques</p><p>　　Hans Dieter Tholl</p><p>　　Dept. of Optronics & Laser Techniques</p><p>　　Diehl BGT

2、Defence</p><p>　　PO Box 10 11 55, 88641 Überlingen, Germany</p><p><b>　　ABSTRACT</b></p><p>　　The paper summarizes laser beam steering techniques for power beaming,

3、 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

4、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-c</p>

5、<p>　　Keywords: Laser beam steering, optical phased arrays, decentered micro-lenses, spatial light modulators</p><p>　　1. INTRODUCTION</p><p>　　The integration of laser power beaming, las

6、er-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 steer

7、ing 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, agil

8、e, multi-spectral</p><p>　　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,

9、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 a

10、nd compensates for platform motions with moderate accuracy and speed. (2) A beam steering device ste</p><p>　　The subject matter of this review are novel laser beam steering techniques. Beam steering devices

11、 are capable of</p><p>　　?pointing a laser beam randomly within a wide field-of-regard,</p><p>　　?stepping the beam in small increments from one angular position to the next,</p><p&

12、gt;　　?dwelling in each position for the required time on target.</p><p>　　In contrast, scanning devices move the beam axis continuously and switching devices are only able to address predefi

13、ned directions. Reviews of current technologies for steering, scanning, and switching of laser beams are found in references [1,2,3,4].</p><p>　　Correspondence. Email: hans.tholl@diehl-bgt-defence.de; Phone:

14、 +49 7551 89 4224</p><p>　　Technologies for Optical Countermeasures III, edited by David H. Titterton,</p><p>　　Proc. of SPIE Vol. 6397, 639708, (2006) · 0277-786X/06/$15 · doi: 10.111

15、7/12.689900</p><p>　　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 t

16、he 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 t

17、o be realized by the beam steering device.</p><p>　　Large wavefront slopes may be generated directly by macro-optical elements such as rotating (Risley) prisms and mirrors or decentered lenses. Compared to g

18、imballed 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.</p><p>　　T

19、he 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 attac

20、hed 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 a

21、nd provide rapid pointing of the laser beam. These configur</p><p>　　In order to build large apertures with micro-optical elements, they have to be arranged in rectangular two-dimensional arrays. Promising t

22、echniques 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

23、the arrangement acts like a diffraction grating. Suppression of undesired diffraction orders is accomplished by actively blazing the grating s</p><p>　　Micro-optical actively blazed gratings are a rudi

24、mentary 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

25、 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

26、, thu</p><p>　　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. p

27、hotonic crystals), active optical phased arrays, and steering techniques associated with optical waveguides. These techniques are excluded from this review.</p><p>　　2. PARAMETER SPACE OF BEAM STEERING DEVIC

28、ES</p><p>　　Functional requirements for laser beam steering devices cover the following topics:</p><p>　　?maximum steering angle,</p><p>　　?beam divergence/imaging capability,<

29、/p><p>　　?aperture/vignetting,</p><p>　　?spectral range and dispersion,</p><p>　　?throughput,</p><p>　　?control of the steering angle.</p><p>　　The quanti

30、tative 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 countermeasur

31、es, 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 transm

32、it (Tx) the laser beam and receive (Rx) a signal t</p><p>　　parameters such as maximum steering angle, aperture diameter, beam divergence, and pointing accuracy. The parameters which characteriz

33、e a beam steering device independently of its location within the optical system are spectral range, time constant, angular dynamic range, and etendue.</p><p>　　Table 1. Compilation of nominal beam steering

34、parameters for different applications.</p><p>　　(2) Time required to step from one angular position to the next</p><p>　　(3) 10 log(2*[max steering angle]/[pointing accuracy])</p><p&g

35、t;　　(4) 2*[max steering angle]*[aperture diameter]</p><p>　　The etendue of the beam steering device (BSD) restricts its location within the optical system. The large etendues required for the DIRCM system de

36、mands 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 t

37、echnologies available. It is also possible to split the steering capability between a coarse steering element situated in the exit pu</p><p>　　The applications compiled in table 1 serve as a guide through th

38、e following sections although a particular beam steering technique is not unique to an application.</p><p>　　3. BEAM STEERING WITH MACRO-OPTICAL COMPONENTS</p><p>　　In a recent series of papers

39、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

40、based on rotating prisms and decentered lenses.</p><p>　　Macro-optical devices enable achromatic designs, avoid blind spots within the field-of-view and concentrate the steered energy into a single beam. Emp

41、loying 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 combi

42、nation of opto-mechanical parameters and materials to ensure wide-angle achromatic steering in the infrared spectral range between</p><p>　　3.1 Risley prism beam steering device [5,6]</p><p>　　P

43、rinciple 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

44、 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 i

45、nterest. Optical reciprocity is a symmetry property: in the</p><p>　　Maximum steering angle. According to reference [6] a maximum steering angle of 45 deg is attainable with proper control of the dispersion.

46、</p><p>　　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

47、. 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 preserv

48、es the beam’s phase space volume (etendue) and the beam power but reduces the peak </p><p>　　Spectral range. Risely prisms work throughout the optical spectral range (VIS to VLWIR). The operational optical b

49、andwidth 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

50、dispersion of</p><p>　　1.78 mrad within the spectral range 2-5 µm at a maximum steering angle of 45 deg [6].</p><p>　　Throughput. Large clear apertures and apex angles of several degrees ge

51、nerate 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 effect

52、s between the prisms are reduced and a transmittance in the order of 75-80% seems to be achievable [5].</p><p>　　Comments. Steering a laser beam rapidly and randomly through a wide angular range requires con

53、trol 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

54、 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</p><p>　　3.2 Decente

55、rd lens beam steering device [5,7]</p><p>　　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

56、 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

57、 chief ray and the associated ray bundle is accomplished by a lateral displacement of the exit lens with respect to t</p><p>　　Maximum steering angle. The maximum steering angle depends on the focal length a

58、nd 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

59、the ray bundles. This leads to a maximum steering angle of roughly 25 degrees.</p><p>　　Beam divergence. The compression of the laser beam depends on the ratio of the focal lengths of the two lenses. For the

60、 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/rec

61、eive 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 incr</p><p>　　Vignetting. Vignetting due to the la

62、teral 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 positi

63、ve 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 undesir

64、able. A negative field l</p><p>　　1:2 relationship [7]. In this way, an internal focus is avoided.</p><p>　　Spectral range. As for prisms there is no limitation on the spectral range. Ideally, e

65、ach 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)

66、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 spect</p><p>

67、　　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

68、 the order of 75-</p><p>　　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 s

69、teering angles; at 5 µm the encircled energy varies from 98% on axis to 63% at 22.5 degrees.</p><p>　　Comments. In order to steer a laser beam the lateral displacement of two lens groups must be control

70、led. 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 m

71、aximum 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 d</p><p>　　3.3 Beam steering

72、 with macro-optical mirrors</p><p>　　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 th

73、e 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

74、preceding paragraphs can be realized with mirrors. A Risley type beam steering device for mm-waves based on ro</p><p>　　4. BEAM STEERING WITH MICRO-OPTO-ELECTRO-MECHANICAL SYSTEMS (MOEMS)</p><p>

75、;　　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

76、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

77、power. These systems need a beam</p><p>　　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.</p>

78、<p>　　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-mechan

79、ical-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 switchin

80、g devices are either digital (e.g. DLP technology introduced by Texas In</p><p>　　Two-axis scanning micro-mirrors</p><p>　　Principle of operation: Gimballed micro-mirrors reflect a laser beam in

81、 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

82、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 c</p>

83、<p>　　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

84、 a negative lens.</p><p>　　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

85、 mirror of 6 mm is about 330 µrad. With large scanning angles the beam is compressed in one direction.</p><p>　　Spectral range: The reflection law is independent of wavelength. Dispersion is introduced

86、through the packaging of the mirrors which are usually sealed behind windows. These windows limit the spectral range and introduce chromatic aberrations.</p><p>　　Throughput is limited by the reflective coat

87、ing 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

88、 of 85%.</p><p>　　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

89、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

90、 [21]. In this device the micro-mirrors are suspended with four hinges which results in a p</p><p>　　5. BLAZED GRATING BEAM STEERING</p><p>　　Blazed grating beam steering utilizes an array of mi

91、cro-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 an

92、d 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-opt

93、ical gra</p><p>　　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 w

94、ith respect to deflection angles.</p><p>　　Decentred micro-lens arrays</p><p>　　Principle of operation. An array of micro-telescopes comprises a two-dimensional regular arrangement of telescopes

95、 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 an

96、d 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</p><p>　　Maximum steering angle. The max

97、imum 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

98、(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).</p><p>　　Beam divergence. The far field of the decentred mic

99、ro-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 acro

100、ss 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-op