This invention relates to the steering of beams of electromagnetic radiation, such as light beams, by relative translation of lens arrays in combination with phase shifters.
Coherent beams of electromagnetic radiation are scanned for use in communication systems, radar, weapons, welding, supermarket label checking, and optical disc reading and writing. Very often, the transmitted beams are made up from a combination of plural individual beams.
The scanning function may be provided by gimballed, mechanically moveable mirrors, lenses or reflectors. However, the mass of such structures may impede the ability to scan in a random fashion, although repetitive scanning at high speeds may be possible. An article entitled "Binary micro optics: an application to beam steering", by Goltsos et al., published by Lincoln Laboratory in connection with the SPIE: OE LASE 89, 1052 (January 89) describes the relative translation of a pair of microlens arrays for beam steering. As described in the article, beam steering is accomplished by relative translation of a pair of microlens arrays cascaded in the path of an array of light beams. The translation of the microlens arrays is in a direction lateral to the beam direction, and the magnitude of the motion which is required for scanning is less than the diameter of the individual lens of the array.
FIG. 1a illustrates a portion of a cascade of two microlens arrays. In FIG. 1a, a scanner designated generally as 10 includes a first microlens array 12 which includes individual lenses 14, 16, 18 and 20. Adjacent light beams illustrated as 22, 24, 26 and 28 fill the apertures of lenses 14, 16, 18 and 20, respectively. Lenses 14-20 cause the light beams to converge toward focal points (not illustrated). A second microlens array 32 includes diverging or defocussing lenses 34, 36, 38 and 40. Microlens array 32 is capable of translation relative to microlens array 12 in a direction of arrows 41. When the lenses of the microlens arrays 12 and 32 are registered, i.e., when the corresponding lenses are coaxial as illustrated in FIG. 1a, the output light beams, illustrated as 42, 44, 46 and 48, propagate parallel to the direction of propagation of incoming light beams 22, 24, 26 and 28, respectively.
FIG. 1b illustrates as plots 52, 54, 56 and 58 the phase of the wave fronts associated with light beams 42, 44, 46 and 48, respectively, as a function of distance from an arbitrary reference point relative to the lens arrays. The spaces between plots 52, 54, 56 and 58 represent regions in which the light beams have a small amplitude. In FIG. 1b, plots 52, 54, 56 and 58 are, in effect, portions or continuations of the same straight dash-line 51 having the same phase. Other plots could be made at other distances from the lens arrays, with the phases increasing gradually with increasing distance from the lens arrays, and with the phases recurring if reduced by subtraction of multiples of 2.pi..
As illustrated in FIG. 1a, the output apertures of the lenses of array 32 are not filled. If the output apertures were filled, plots 52, 54, 56 and 58 of FIG. 1b would run together to create a continuous phase front representing a coherent beam of light, the direction of propagation of which is normal to the phase front.
FIG. 1c illustrates a portion of scanner 10 of FIG. 1a, with lenses 36 and 38 of lens array 32 translated vertically upward (in the direction of arrow I) relative to corresponding lenses 16 and 18 of lens array 12, and with the input light beams 24 and 26 illustrated as not completely filling the input aperture to enable the beam paths to be clearly depicted. As illustrated, output beams 44 and 46 propagate in a direction different from that of the incoming beams, i.e. the beams have been scanned. FIG. 1d illustrates the phase of the wave fronts of beams 44 and 46. As illustrated in FIG. 1b, phase fronts 64 and 66 exhibit a slope, the normal to which defines the direction of propagation of the beam. As also illustrated in FIG. 1d, there is an offset, which is illustrated between arrows 50, which represents the offset between the phases of adjacent continuations of beams 44 and 46 of FIG. 1c. If this phase offset is zero or zero plus a multiple of 2.pi., the beams are in-phase for the illustrated direction of propagation, and a beam maximum occurs. In general, however, the phase offset will vary with the scanning direction, with the result that for some scanning directions the individual beams will be mutually out-of-phase with another beam, resulting in destructive interference. This in turn results in a far-field scanned radiation pattern which contains grating lobes or angles at which the radiated energy is high, and other angles at which the radiated energy is low. The result of translating a lens array in one direction is to gradually reduce the amplitude of one grating lobe, while the adjacent grating lobe becomes larger. The Goltsos et al. article suggests the use of a scanning mirror at the system input for fine or vernier beam steering. Such a scanning mirror has the disadvantages of a mechanical system referred to above, and in addition, causes the beams to enter the lenses of the lens array at an angle, which reduces the efficiency of the lens. This may be particularly important when two lens arrays are involved, because the entry at an angle occurs in both lens arrays, so the losses are cascaded. It is desirable to scan in a manner which allows the beam(s) to be directed at any angle, and not just at angles at which grating lobes occur.
FIG. 2a illustrates a lens array similar to that of FIG. 1, with the lenses of the two arrays registered, and FIG. 2b illustrates the same arrangement with one of the arrays laterally offset by translation in the direction of arrow I. In FIG. 2, elements corresponding to those of FIG. 1 are designated by the same reference numerals. In FIG. 2a, circular input light beams 24 and 26 are centered on axes 6 and 8, respectively, and fill the apertures of converging lenses 16 and 18, pursuant to the Goltsos et al. suggestion. Lenses 16 and 18 focus the light to form converging beam portions 74 and 76, respectively, which come to a focus at a focus plane 99. From focus plane 99, diverging beam portions 84 and 86 propagate toward the input apertures of converging lenses 234 and 236, respectively. As illustrated, the spacings are such that beam portions 84 and 86 do not fill the apertures of lenses 234 and 236. Lenses 234 and 236 collimate the beams to produce parallel output beams 44 and 46, respectively, which are centered on axes 4 and 6, respectively.
FIG. 2b illustrates the result of moving lens array 32 of FIG. 1a downward, in the direction of arrow I. As illustrated, light beams 84 and 86 intercept lenses 234 and 236 in a region in which the lens curvature causes output beams 44 and 46 to be deflected or scanned downward.
FIG. 3a is identical in subject matter to FIG. 1c, and is included as a reference for comparison with FIG. 3b. In FIG. 3b, array 32, which includes diverging lenses 36 and 38, has been moved or translated upward in the direction of arrow I, thereby causing exit beams 44 and 46 to be deflected downward.
By comparison of FIGS. 2b and 3b, it is apparent that deflection of output beams in a given direction in accordance with the Goltsos et al. arrangement requires that the output lens array be moved in the direction of the desired deflection in the case of converging lens array, and in a direction opposite to the desired scanning direction for a diverging lens array.
In FIG. 4, an arrangement similar to that of FIG. 2 has had its output lens array 32 translated upward by an amount .DELTA. in an attempt to increase the scan angle. As illustrated, translation .DELTA. is sufficient to cause diverging light beam portion 84 to illuminate portions of both lenses 234 and 236. This may be viewed as a form of overfilling of the aperture of lens 234. As illustrated, output beam 244 is deflected or scanned upward by lens 234. That portion of beam 84 falling onto lens 236, however, is deflected downward. When overfilling of the aperture occurs in this manner, the far-field peak beam amplitude decreases, because the effective aperture decreases. Put another way, translation of the moving lens array in the direction of the arrow in any of FIGS. 2, 3 or 4 may result in a secondary portion of each beam (246) being directed away from the main scanned beam (244). The energy which goes into the secondary beam is not available for the main beam, and the secondary beam amounts to a scanning sidelobe which may not be desired. The undesirable effect of overfilling also occurs with the arrangement of FIG. 3. It would be advantageous to be able to translate the lenses to achieve additional scanning, with less loss of peak amplitude.