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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