【正文】 optical grating are actively blazed in order to steer a laser beam. The most promising twodimensional blazed grating beam steering approach is based on decentered microlens arrays. Other techniques such as tilting micromirror or (electrooptical) prism arrays are too limited with respect to deflection angles.Decentred microlens arraysPrinciple of operation. An array of microtelescopes prises a twodimensional regular arrangement of telescopes of the Kepler or Galileo type. The lens trains which form the microtelecopes are distributed among two planar substrates which hold mircrolens arrays on their surfaces. Microlens 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 microoptical realization of the principle of decentered lenses requires fewer optical surfaces than the macrooptical 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 microlens beam steering device is posed 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 geometricoptical parameters due to imperfections of the fabrication process reduce the spatial coherence of the beamlets [18,19] and broadens the grating lobes. The nonuniformity 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 pression 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 microlenses 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 plexity of the driving mechanism.Spectral range. Beam steering with decentered microlens 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 microlens arrays (adapted from ref. [14])PropertyGeSiGaAsZnSeZnSRefractive indexMax. steering angle (deg)2825241817Best choice waveband (181。m the encircled energy varies from 98% on axis to 63% at 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 macrooptic lenses for beam steering is a possible but, because of the plexity involved, not a practical approach pared to Risley prisms. This is in contrast to the microoptics world where microoptical elements are arranged in a regular array. Electrooptic prism arrays are capable of one dimensional beam steering with small steering angles. Decentered microlens arrays including field lenses are an option for steering laser beams up to angles of 25 degrees in two dimensions. Beam steering with macrooptical mirrorsTransmissive optical elements are the first choice for pact optical systems with large fieldsofview. 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 mmwaves based on rotating mirrors is discussed in reference [8].4. BEAM STEERING WITH MICROOPTOELECTROMECHANICAL 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 fieldofview 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 fieldofview because of limited laser power. These systems need a be