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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