【正文】 ing 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 (. directional optical countermeasures, transfer of power to remote devices) and free space laser munication 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 munications as stated in references [6,9,10]. These examples run the gamut of system levelparameters 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.ParameterDIRCM [6]Imaging Ladar [9]Deep Space Laser [10]Maximum steering angle45 deg deg degAperture diameter50 mm (Tx)75 mm (Rx)300 mm (Tx)Beam divergence (Tx) Instantaneous FOV(1) (Rx)1 mrad10 mrad333 181。rad 181。radPointing accuracy100 181。rad30 181。rad1 181。radSpectral rangeTime constant (2)Angular dynamic range (3)Etendue (4)2 to 5 181。m1 ms42 dB78 mm*rad 181。m ms38 dB28 mm*rad 181。m1 ms43 dB10 mm*rad(1) FOV: FieldofView(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 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 bination with large apertures as for deep space laser 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 piled 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 MACROOPTICAL COMPONENTSIn 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 macrooptical coarse beam steering devices based on rotating prisms and decentered lenses.Macrooptical devices enable achromatic designs, avoid blind spots within the fieldofview 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 wellknown approach is the search for the right bination of optomechanical parameters and materials to ensure wideangle achromatic steering in the infrared spectral range between 25 181。m. 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 fieldofregard 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 pression may occur. The prism beam steerer presses the laser beam in such a way that a circular input beam leaves the device with an elliptical shape. The pression 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 pression. 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 bination LiF/ZnS leads to small secondary