WO2006020888A2 - Afocal beam steering system corrected for excess diffraction due to phase error from microelectromechanical mirror offsets - Google Patents
Afocal beam steering system corrected for excess diffraction due to phase error from microelectromechanical mirror offsets Download PDFInfo
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- WO2006020888A2 WO2006020888A2 PCT/US2005/028777 US2005028777W WO2006020888A2 WO 2006020888 A2 WO2006020888 A2 WO 2006020888A2 US 2005028777 W US2005028777 W US 2005028777W WO 2006020888 A2 WO2006020888 A2 WO 2006020888A2
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- Prior art keywords
- array
- phase
- mirrors
- mirror
- interposing
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
- G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
- G02B26/0816—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements
- G02B26/0833—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
- G02B26/06—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the phase of light
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/42—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect
- G02B27/46—Systems using spatial filters
Definitions
- This invention relates to systems and procedures in which, a microelectromechanical system (MEIlS) mirror array is used to steer substantially coherent imaging or projection beams.
- MIlS microelectromechanical system
- mirrors in a MEMS array are small elements, closely ju_ ⁇ taposed and independently manipulated — usually under microprocessor control.
- Some preferred embodiments of the present invention address the diffraction-limited resolution in a remote-sensor optical system with an optical astis 11 (Fig. 1) , and with a collimated beam 12 passing through an afocal lens 13 (having a magnification ratio of Z to 1) to form a magnified or minified beam 14 that reaches a MEMS scan-mirror array 15.
- the array in turn produces from that beam, 14 a deflected beam 16 which nest reaches a reimaging lens 17.
- This lens in turn forms from the deflected beam a focused beam 18, at an image plane 19 spaced from the reimager by that element's focal length f.
- the MEMS mirrors in the array 15 are necessarily set to be nominally planar, as a group — that is, all substantially aligned with a common base plane 38 of the array (or a common plane 38 of the mirror pivots) .
- planar condition all the light 12, 14 that is coherent initially — before reflection by the MEMS scan-mirror array — is again coherent at each point 16, 18, 19 after the reflection.
- light that is all in phase initially is also all in phase later.
- the MEMS mirrors in this condition they be ⁇ have, for purposes of diffraction analysis, very much as if they were a single mirror having the overall size of the array. Accordingly the resulting diffraction-limited spot size ⁇ , 19 after passage of the beam 16, 18 through the reimaging lens 17, is inversely proportional to the size of that effective "single" mirror, which is to say the size of the array.
- the diffraction-controlling dimension NB is tx ⁇ ice the linear dimension D of each individual mirror.
- the diffraction limit is twice as fine a ⁇ (i. e. is half the size of) the spot sise which corresponds to that dimension D of each individual mirror.
- This condition may be regarded as characterising sensed beams that are addressing field sources which are on axis (e. ⁇ . , normal) with respect to the MEMS mirror array — or more generally whenever the individual mirror surfaces as a group are aligned with their common base plane. It will shortly be seen that a lil ⁇ e condition applies to projected beams that are addressing field transmission targets, provided only that the beam outside the system is on axis and the array in its aligned, groupwise-planar condition.
- a coherent-beam (most typically laser) projection system (Pig. 3), in which a collimated projection beam 21 is deflected by a MEMS scan-mirror array 22 to direct plural individual beams 23 toward an afocal lens 24 (again with Z-to-1 magnification) .
- the deflected beams 23 are on-axis (i. e. , parallel to the optical axis 26) — and thereby producing, at the lens 24, an on-axis projected beam 25.
- the beam divergence a that is controlled by the overall dimension of the mirror array 22, provided that the mirrors are in fact groupwis ⁇ planar to yield an on-axis beam 25.
- the divergence is controlled by the product ND
- the particular phenomenon of interest is the coarser diffraction limit corresponding to the dimension D of one in- dividual MEMS mirror, when the system is modified (simply by rotation of the MEMS array) to operate with a phase difference 2 ⁇ between the two deflected subheams 123 (Fig. 4) entering the afocal lens 24, and with the projected beam 125 off-axis;
- the number N of individual mirrors is typically at least ten and sometimes on the order of a hundred.
- the described diffraction-limit-degrading effect is significant only if phase mismatch between adjacent mirrors departs from an integral number of wavelengths by roughly a tenth of one wavelength or more.
- diffraction in a sensor system is controlled by the individual mirror dimension.
- the phase difference increases beyond about 90% of a wave, hoi ⁇ ever, then once again the diffraction i ⁇ controlled by the overall array dimension — until again the difference exceeds 110% of a wave.
- the introductory discussion here is not rigorous but may be regarded as a first-order approximation.
- the mirror scan angle ⁇ causes the diffraction limit to be either ⁇ /D or ⁇ /ND multiplied by some theoretical form-factor; and what is of interest is the basic phenomenon — particularly the dominant effect of N — rather than that form-factor.
- a projection system illustrated in Fig. 3 pro ⁇ jecting parallel to the system axis — has a beam divergence a, in- versely proportional to the MEMS array size.
- the identical optical system suffers a phase difference 2 ⁇ , between adjacent MEMS rows, proportional to the MEMS scan angle ⁇ — and the beam divergence is then inversely proportio ⁇ nal to the individual MEMS mirror size D (Fig. 4) .
- phase differ ⁇ ence can be maintained such that it is an integral number of wave ⁇ lengths ⁇
- the system when operating at a single wavelength ⁇ or over a narrow band about that wavelength can obtain imaging performance determined by the dimension of the entire scan-mirror array, not the individual mirrors. This provides a significant improvement over what would be possible in terms of the beam diver ⁇ gence of a projection system — in the cases being considered, again, factors of ten to one hundred.
- MEMS scan-mirror array develops from the phase error introduced by the arrangement of the mirrors. This phase error very undesirably forces the diffraction limit to scale with the area of each individual mirror, rather than the total area of the mirrors in the array.
- the prior art although providing powerful and very sophisticated imaging and sensing capabilities — has left some refinements to foe desired in the area of ideally fine- focused images and optical projection.
- aspects of the invention operate by holding the phase dif ⁇ ference at substantially an integral number of wavelengths ⁇ , at least within the tenth-of-a-wavelength threshold mentioned earlier. If this condition is met, then the system when operating at a single wavelength ⁇ or over a narrow band about that wavelength can achieve imaging performance determined by the dimension of the entire scan mirror array, rather than only the individual mirrors.
- Fig. 1 is a conceptual optical diagram of an afocal MEMS beam- steering system (AMBS) with a MEMS mirror array in a planar orientation, used in an imaging or sensing mode;
- AMBS afocal MEMS beam- steering system
- Fig. 2 is a like diagram of the Fig. 1 AMBS system but introducing phase error due to imaging of an off-axis field location;
- Fig. 3 is a diagram of an AMBS system in planar orientation, analogous to the Fig. 1 system, but used in a projection application rather than imaging or sensing;
- Fig. € is a lilze diagram for the Fig. 3 projection system but with phase error as in Fig. 2;
- Fig. 5 is a one-dimensional slice of a theoretical single- wavelength diffraction pattern from four rectangular mirrors with s random relative phase;
- OTF optical transfer function
- Fig. 8 is a like OTF graph for an optical system with cubic phase mask p 90ix 3 with
- ⁇ 1 for misfocus parameter ⁇ 0, ⁇ 2 /2, ⁇ 2 ;
- Fig. 10 is an imaging/sensor-system diagram like Fig. 2 but for 0 a system with wavefront control achieved through M ⁇ MS-mirror piston modulation as a function of mirror angle;
- Fig. 11 is a like diagram but for an analogous projection system as in Fig. 4;
- Fig. 12 is an imaging/sensor-system diagram like Figs. 2 and 10 5 but provided with a nonlinear optic, e. ⁇ . an addressable in-path refractive index/thickness wavefront correction;
- Fig. 13 is a like diagram but for a projection system as in Figs. 4 and 11j and
- Fig. 14 is a diagram generally like Figs. 2, 4, and 10 through 13, but for a tandem dual-array assembly of MEBSS mirrors that per- s forms phase-error autocancellation — good for both imaging and projection.
- an afocal MEMS-arry beam steering device is subject to an apparent diffraction limit.
- the limit is defined by the size of the individual mirror elements in the MEMS array.
- the s limit can be overcome by proper application of hardware design or a combination of hardware and signal-processing software.
- NPME nonlinear phase modification element
- NPME correct choice of NPME enables a mixing that can be unmixed while simultaneously aligning the phase outputs from the individual mirrors.
- the resulting process provides an image that is diffraction 0 limited at the scale of the entire mirror array, not at the scale of individual mirrors.
- postprocessing can 5 circumvent the diffraction limit for incoming beams only — i. e. for imaging or sensing, not projection. Examination of a linear array of mirrors with displacements in the normal direction reveals more than one feature that can be exploited in a control loop or in a postprocessing step. With an array of four mirrors, for instance, every eighth zero in the diffraction pattern (Figs. 5 and ⁇ ) is independent of the path differences resulting from the mirror offset positions.
- Wavefront coding is a relatively new optical tech- nique, in which a specially-designed phase mask is added to a stan ⁇ dard optical system to compensate for misfocus.
- E. R. Dowski and W. T. Cathey have described the general procedure in "Extended Depth of Field through Wavefront Coding," 34 Applied Optics 11, at 1859-66 (April 1995) . All images are "blurred" by the phase mask, regardless of whether the original optical system is in focus or not; therefore the resulting image requires postprocessing. Because it is possible to form phase masks that make the overall OTF,
- a single filter can be used to obtain clear images for a large range of misfocus (Figs. 7 through 9).
- the same deconvolution of the point-spread function can occur with no physical optical device.
- the element and filter together remove phase mismatch at the MEMS array while increasing the depth of field.
- this first form of the invention may represent the best mode of practice. The reason is that at least some variants of this form of the invention are almost exclusively computational, requiring relatively little or nothing in the way of hardware.
- Postprocessing for this form of the invention is not limited to fixing one wavelength or extremely narrow waveband at a time.
- Rath ⁇ er this system is capable of deconvolving wavelength intervals amounting to more than ⁇ 10% of the nominal wavelength — or 0.5 to 1 ⁇ m, and this is better than at least some of the optomechanical approaches discussed below.
- This form of the invention controls the wavefront by maintain- ing an integral number of M wavelengths between rays from immediately adjacent mirrors, as the mirrors 215, 22 (Figs. 10 and 11) are rotated in ⁇ to address different field locations. This is accom ⁇ plished simply by driving each mirror in z., the so-called "piston" direction 31, 32 (Figs. 10 and 11), normal to the plane of pivots of the array or backing plane 38, 39. (The dimension z. is to be dis ⁇ tinguished from the magnification Z of the afocal lens 13, 124.)
- the reimage ⁇ beam 218 at the image plane has a spot size 219 that is only 1/N times the size 119 (Fig. 2) .
- the microprocessor which drives the mirrors is programmed to satisfy the integral-wavelengths condition at all scan angles ⁇ . Unlike the postprocessing technique introduced above, this method serves well for not only a sensor system (Fig. 10) but also a pro ⁇ jection system (Fig. 11) .
- the diffraction-limited focal spot size ⁇ for a reimaged sensor beam 218 at the image plane 219, or beam divergence angle ⁇ for a projected beam 225 is determined once again by the MEMS array size ND and not the individual mirror size D.
- this form of the invention can be implemented by a programmer.
- this piston-com ⁇ pensation aspect is particularly straightforward, and therefore may represent the best mode of practice — especially for projection systems, in which the deconvolution method and other mainly computa ⁇ tional postprocessing approaches appear to be unworkable.
- the piston facet of the invention is particularly appealing — not only for its simplicity and ease of implementation, but also for the property that it is fully broadband.
- This form of the invention too controls the wavefront to main ⁇ tain an integral number of M wavelengths between pathlengths at adjacent mirrors.
- this phase relationship is produced by insertion of controllably variable delays, in the form of respective nonlinear optical elements 33, 34 (Figs. 12 and 13), into the optical path.
- such an optic either may vary the refractive index n for a constant thickness d of transmissive material, or may vary the thickness d for a material of constant index n — or both.
- a nonrefractive element is equally appropriate.
- a Bragg cell can be used.
- the system is programmed for automatic servocontrol, using known materials of variable index or thickness — or other delay-inducing physical characteristic — and varying at least one of those parameters in a suitable dependence on the scan angle ⁇ .
- ⁇ diffraction-limited performance
- a sensor Fig. 12
- a for a projection system Fig. 13
- Phase-equalized subbeams 316 produce, in the sensor case, a reimaged sensing beam 318 with fine spot 319 (Fig. 12) — and in the projector case, an external projected beam 425 with fine divergence (Pig. 13) .
- the overall phase difference is forced to:
- FIG. 14 Another layout for controlling the relative phase of collimated light reflecting from the MEMS scan mirror array in an afocal config ⁇ uration is a tandem dual system with two opposing arrays 22, 622 (Fig. 14) .
- the o lower array rotates by the same amount and in a compensating sense.
- the net optical path difference (OPD) between these two collimated beams 625a, 625b at the output plane 36 is nominally zero. This is true for either an AMBS projection or sensor con ⁇ figuration.
- diffraction-limited performance (focal blur 5 size ⁇ for a sensor, and divergence a for a projection system) is determined by the MEMS array size ND and not the individual mirror size D. Diffraction-limited performance for nonzero scan angles ⁇ is thus restored to equal the performance of the favorable zero-angle configurations (Figs. 1 and 3) discussed earlier.
- a "compensating sense" of rotation is actually the same absolute sense. That is, when the mirrors in the upper array 22 rotate clockwise, those in the loiter array 622 also rotate clockwise.
- the limits of operation can be critical: as rotation rises to encompass relatively larger angles, a ray initially reflected from e. ⁇ . the upper array to the lower array may fall off the specific lower-array mirror which that ray struck initially.
- rays from a specific upper-array mirror can be divided, and divided differently, between plural or multiple lower-array mirrors as the rotation angle changes. Careful programming must take into account all these simple geometrical effects, to produce a functional device.
- Such offset between adjacent mirrors can arise during manufac ⁇ ture of the array, or thereafter due to thermal or other influences.
- This kind of imperfection can be corrected by piston-dimension (j2, Figs. 10 and 11) movement of the individual mirrors that are in- volved.
- the piston movement is simply added in to the basic rota ⁇ tional movements of those mirrors.
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Priority Applications (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
EP05788879A EP1810069A2 (en) | 2004-08-11 | 2005-08-10 | Afocal beam steering system corrected for excess diffraction due to phase error from microelectromechanical mirror offsets |
AU2005272650A AU2005272650A1 (en) | 2004-08-11 | 2005-08-10 | Afocal beam steering system corrected for excess diffraction due to phase error from microelectromechanical mirror offsets |
US11/705,809 US7791786B2 (en) | 2005-08-10 | 2007-02-12 | Afocal beam steering system corrected for excess diffraction due to phase error from microelectromechenical mirror offsets |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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US60101504P | 2004-08-11 | 2004-08-11 | |
US60/601,015 | 2004-08-11 |
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WO2006020888A2 true WO2006020888A2 (en) | 2006-02-23 |
WO2006020888A3 WO2006020888A3 (en) | 2006-06-08 |
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Application Number | Title | Priority Date | Filing Date |
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PCT/US2005/028777 WO2006020888A2 (en) | 2004-08-11 | 2005-08-10 | Afocal beam steering system corrected for excess diffraction due to phase error from microelectromechanical mirror offsets |
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EP (1) | EP1810069A2 (en) |
AU (1) | AU2005272650A1 (en) |
WO (1) | WO2006020888A2 (en) |
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
CN114967118A (en) * | 2022-04-20 | 2022-08-30 | 清华大学深圳国际研究生院 | Optical parameter control method and device for orthogonal reflector array |
Families Citing this family (1)
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CN109270515B (en) * | 2018-11-29 | 2020-06-16 | 北京理工大学 | Variable scanning area coaxial receiving and transmitting scanning laser radar |
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2005
- 2005-08-10 WO PCT/US2005/028777 patent/WO2006020888A2/en active Application Filing
- 2005-08-10 EP EP05788879A patent/EP1810069A2/en not_active Withdrawn
- 2005-08-10 AU AU2005272650A patent/AU2005272650A1/en not_active Abandoned
Non-Patent Citations (4)
Title |
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BOYD J G IV ET AL: "Fast-response Variable Focusing Micromirror Array Lens" PROCEEDINGS OF THE SPIE, vol. 5055, 2003, pages 278-286, XP002990703 ISSN: 0277-786X * |
DOWSKI E R ET AL: "EXTENDED DEPTH OF FIELD THROUGH WAVE-FRONT CODING" APPLIED OPTICS, OSA, OPTICAL SOCIETY OF AMERICA, WASHINGTON, DC, US, vol. 34, no. 11, 10 April 1995 (1995-04-10), pages 1859-1866, XP000497512 ISSN: 0003-6935 cited in the application * |
KRISHNAMOORTHY U ET AL: "Dual-mode micromirrors for optical phased array applications" SENSORS AND ACTUATORS A, vol. 97-98, 1 April 2002 (2002-04-01), pages 21-26, XP004361577 ISSN: 0924-4247 * |
LI K ET AL: "COHERENT MICROMIRROR ARRAYS" OPTICS LETTERS, OSA, OPTICAL SOCIETY OF AMERICA, WASHINGTON, DC, US, vol. 27, no. 5, 1 March 2002 (2002-03-01), pages 366-368, XP001117266 ISSN: 0146-9592 * |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
CN114967118A (en) * | 2022-04-20 | 2022-08-30 | 清华大学深圳国际研究生院 | Optical parameter control method and device for orthogonal reflector array |
CN114967118B (en) * | 2022-04-20 | 2024-02-09 | 清华大学深圳国际研究生院 | Method and device for controlling optical parameters of orthogonal reflector array |
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AU2005272650A1 (en) | 2006-02-23 |
EP1810069A2 (en) | 2007-07-25 |
WO2006020888A3 (en) | 2006-06-08 |
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