外文文献Novel Laser Beam Steering Techniques（新型激光束转向(3)

An electrically addressed LC SLM (EA-SLM) comprise a patterned electrode (line pattern for 1D, pixelated for 2D beam steering) with highly reflective pads which are bonded to a silicion backplane and a transparent electrode which is antireflection coated. The gaps which define the pattern on the backplane electrode distort the electric field distribution and give rise to fringing fields. The fringing fields separate the EA-SLM in optically active and inactive regions. Due to fabrication limitations a gap of at least 0.5 μm is required in order to prevent dielectric break down between electrodes. Together with the requirement of having a high aperture ratio this results in a minimum width of a resolution cell of 1.5 μm [24]. For VIS and NIR wavelength the pixel structure acts like a diffraction grating causing diffraction into several orders in the far field independently of the imposed phase profile [25].

The disturbing diffraction effect of the electrode structure is prevented by the use of optically addressed LC SLM (OA- SLM). Instead of a patterned backplane electrode a reflective OA-SLM comprises a large-area photoconductor, an absorbing light-blocking layer, and a high reflective dielectric mirror. In operation, the photoconductor is electrically biased relative to the transparent electrode and irradiated with patterned light. The irradiance distribution determines the local electric field which controls the orientation of the LC director. The OA-SLM exhibits reduced diffraction artefacts because of the absence of a pixelated electrode structure and is scaleable to large apertures [26].

Maximum steering angle. Commercially available LC SLM have a size up to 20 mm and 8 bit phase dynamic range [23]. Depending on the wavelength (0.5 to 1.5 μm) this results in maximum steering angles between 25 to 75 μrad with tenth of nanoradians resolution. These small steering angles are ideal for ultra fine beam steering. Larger steering angles are realized through the implementation of diffractive optical elements (DOE), i.e. diffraction gratings. The phase profile of a DOE is limited to a phase range between 0 and 2 π radians. As a rule of thumb, high diffraction efficiency (>95%) in the first diffraction order is attained for a minimum of 8 phase steps per grating period with the appropriate blazing of the unit cell. This technique results in a maximum steering angle between 6 and 18 mrad for a pixel pitch of 8 to 40 μm and wavelengths between 0,5 and 1,5 μm. Extension of the steering angle is possible with macro-optical components known from fish-eye and projection lens designs, with decentered micro-lens blazed gratings (as explained above), or with multiple exposure volume (Bragg) gratings [27].

Beam divergence. In general, the far field energy distribution of a DOE comprises the desired first diffraction order, unwanted grating lobes, and diffraction artefacts due to the underlying pixelation and quantization of the phase profile [25]. The beam divergence is determined by the diameter of the clear aperture of the LC SLM and the exit pupil of the optical system. Compression effects are not an issue.

Vignetting. The pixelation of the phase profile due to the addressing of individual resolution cells divides the SLM into active and inactive regions. Aperture ratios of 60 - 70% are reported [25,26] regardless of the addressing scheme.

Spectral range. The liquid crystal currently in use work best for the VIS and NIR spectral range. Some LC material exhibit small transmission windows in the 3-5 μm range [28]. Ultra fine beam steering without diffractive structures suffer the normal material dispersion. Larger steering angles which require the implementation of DOE are subject to structural dispersion. Methods to cope with this type of dispersion are currently under investigation [29].

Throughput. With 8 phase levels the maximum diffraction efficiency (energy in the first diffraction order divided by the energy in all diffraction orders) is 95%. The net optical reflectivity of an OA-SLM was measured to be 93% [26]. Multiplying the diffraction efficiency, the net reflectivity, and the aperture ratio of 70% (1D grating) gives a total maximum throughput of roughly 60%.

Comments. OPAs are programmable diffractive optical elements based on an array of phase shifting pixels. Thus, different optical functions in addition to beam steering such as fanouts and focusing may be realized. OPAs based on LC are a very promising approach for laser beam steering backed up by applications with high market volumes such as micro-displays and projection devices. Challenges to be resolved are: reduction of the response time, reduction of the polarization dependence, reduction of the pixel size below the application wavelength, extension of the spectral range up to 5 μm. New liquid crystal composites such as polymer dispersed liquid crystal materials (PDLC) can overcome part of these problems [28]. A PDLC comprises a polymer matrix with embedded LC domains (droplets). It is possible to form domains of few tens of nanometers, the so-called nano-droplet regime (Fig. 5). Each individual droplet of LC can

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be considered as an uniaxial birefringent medium whose optical axis is controlled by an externally applied electric field. A laser beam propagating through a LC droplet experiences no phase shift if the optical axis is orientated in such a way that the ordinary index of the droplet matches the index of the polymer matrix. Otherwise, the PDLC film imposes a phase shift onto the laser beam due to the refractive index mismatch between the LC droplets and the polymer matrix. The orientation of the LC droplets and the resulting phase shift is controlled by driving the PDLC film either by an array of electrodes or by analogue optical addressing with a photoconductive layer.

Nano-droplet Nano-droplet liquid crystal liquid crystal

Phoconductor Phoconductor

Figure 5. Structure of an optically addressed PDLC device (left) and photographs of the structure of a PDLC nano-droplet composite

film (graphic and photographs provided by Thales Research and Technology [28]).

7. PROGRAMS

Novel laser beam steering techniques for achieving significant reduction in size, weight, power, and cost over conventional gimballed mirror systems are topics of several research projects worldwide. Two programs established in the military research arena are ATLAS in Europe and STAB in the US.

7.1 Advanced Techniques for Laser Beam Steering (ATLAS)

The European Defence Agency (EDA) is funding a program to develop component technologies for advanced laser beam steering. The program addresses innovative concepts for non-mechanical beam steering and beam shaping for DIRCM, active imaging, designation, tracking and ranging systems in a multi-target context. The following techniques are investigated:

? ? ? ?

electro-optics ceramics and polymer-dispersed liquid crystal spatial light modulators for fine beam steering and wavefront correction,

decentered micro-lens arrays for extended field of view beam steering,

optically addressed liquid crystal spatial light modulators for laser beam shaping, intra-cavity beam steering.

The consortium comprises the European defence corporations Thales Optronique SA (consortium leader), Thales Research and Technology, Galileo Avionica, and Diehl BGT Defence.

7.2 Steered Agile Beams (STAB)

The STAB program is funded by the US Defense Advanced Research Projects Agency (DARPA) with the objective “to develop and demonstrate novel chip-scale laser beam steering technologies for military applications” [30]. The STAB program comprises several projects which focus on optical MEMS, diffractive and micro-optics, liquid and photonic crystals technologies for free space laser communications and electro-optical countermeasures applications. The program team consists of major US defence corporations and universities such as Raytheon, BAE Systems, Hughes

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Research Lab, Rockwell Scientific Company, Honeywell, University of California at Berkley, Kent University, UCLA, UCSD, and USC.

8. CONCLUSION

Table 3 serves as a guide to the performance of the most promising beam steering techniques reviewed in this paper. The numbers form a coarse parameter grid to ease the association of beam steering technologies and applications. For broadband wide-angle beam steering Risley prisms are the first choice although they do not comply with the requirements of low weight and power. Blazed gratings offer a broadband solution with moderate angular coverage, but they need additional optical devices to close the angular gaps between grating lobes. Optical phased arrays are a promising technique for small angle beam steering in the lasercom wavebands (LC) and beyond (MEMS, EOC) if coarse beam steering is provided by other means, for example by blazed gratings. Such a combined coarse/fine beam steering device may exhibits large apertures with a throughput of roughly 40% with an update rate in the order of kHz.

Table 3. Guide to the performance of different beam steering techniques.

Property Max. steering angle Angular coverage Aperture diameter Spectral range Update rate Etendue Throughput (1) ultra fine steering mode(2)

Risley prisms 60 deg continuous < 100 mm 0.4 – 12 μm 500 Hz 105 mm*rad 75 % Blazed grating 25 deg discrete < 150 mm 0.4 – 12 μm 1000 Hz 65 mm*rad 65 % Optical phased arrays 75 μrad(1) / 1.5 deg(2) quasi-continuous < 20 mm 0.4 – 1.5 μm (LC) 0.4 – 5 μm (MEMS/EOC)(3) 50 Hz 0.0015 / 0.5 mm*rad 60 % programmable diffractive optical element (3)

LC: liquid crystal, MEMS: micro-electro-mechanical systems, EOC: electro-optical ceramics

ACKNOWLEDGEMENTS

The author takes this opportunity to acknowledge several people who support the ATLAS program. At the very beginning, Anne-Marie Bouchardy (TOSA), David Titterton (DSTL), and Giorgio Leonardi (GA) initiated the formation of an industry consortium. ATLAS is funded by the French, German, and Italian MODs and supervised by a Management Group comprising Julie Poupard (DGA), Gerhard Traeger (BWB/WTD 81), and Giuseppe Licciardello (IT MOD DGAT). The industry consortium is managed by Nathalie Gerbelot-Barrillon (TOSA).

Brigitte Loiseaux, Patrick Feneyrou, Jér?me Bourderionnet (TRT), and Matthias Rungenhagen (DBD) supplied valuable information and viewgraphs for this paper concerning PDLC, EO ceramics and decentered micro-lens arrays. Their contribution is gratefully acknowledged.

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中文翻译：

特邀论文

新型激光束转向技术

汉斯·戴特tholl 光电子与激光技术系 迪尔BGT防务

邮政信箱101155，88641ü贝尔林根，德国

文摘

本文总结了大功率激光光束转向技术和通信应用。编制原则和新型机械，微型机械和非机械的技术特点。基于微透镜的粗波束或细转向液晶的电光相位控制相结合，提出更多的细节。这项检讨涉及调节通过激光束的相位分布的光束转向装置和排除腔内的波束，波束在色散光学元件，光有源相控阵，光波导相结合的可调谐激光器的基础上。

关键词：激光束转向，光学相控阵，偏心微型透镜，空间光调制器

1引言

激光功率的整合,激光辅助传感和激光通信子系统到自主车辆、航空和太空平台需要新的技术来引导雷射光束。新技术应该促进实现大光孔径的光束转向装置融入的机械结构平台。愿询价单包括著名的特性:结构紧凑、重量轻、低功耗、灵活、多光谱、大的方面领域。

角传播的激光束，特别是远距离应用，本质上是由于光波长的高孔径天线增益小。因此，激光束的传播方向一般控制在两个步骤：（1）与万向光学元件的炮塔指向发射/接收到所需的方向望远镜的视场和平台运动补偿中度精度和速度。 （2）光束转向装置，转向在望远镜的视场的激光束，以获取和跟踪目标。

本次审查的标的物是一种新的激光束转向技术。光束转向装置能够 ?指向激光束，在广泛的领域方面的随机， ?步进梁从一个角度位置到下一个小增量，

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