Classifications

• • 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
  • G02B26/10—Scanning systems
  • G02B26/106—Scanning systems having diffraction gratings as scanning elements, e.g. holographic scanners

Definitions

• the present invention relates to optics, systems, and optical elements generally, and more particularly to scanners utilizing diffraction gratings (including diffraction gratings produced by holographic techniques) for deflecting a light beam.
• the invention may utilize a laser source, with light beam deflection by means of a periodically moving element, such as a transmissive or a reflective diffraction grating.
• Background Art The prior art includes the use of both reflective and refractive diffractive gratings which are rotated to scan a light beam. In general, the principal ray of the scanned beam in such arrangements describes a cone. The intersection of this conical scan with an image plane is, of course, a curve, and various schemes have been utilized to correct for the curvature so as to yield a straight line scan.
• U.S. Patent No. 4,094,576 is illustrative of one such technique.
• a reflective hologram scanner is combined with a cylindrical lens, post-scanning, to produce a straight-line scan.
• U.S. Patent No. 3,972,582 while it does not appear to be directed to a diffraction-grating scanner, but rather to a rotating mirror-type scanner, utilizes a fixed holographically created plate to correct for facet-to-facet errors in parallelism of the rotating polygonal mirror.
• U.S. Patent No. 3,984,171 is also directed to a rotating or oscillating mirror type scanner, rather than a scan generated by diffraction.
• the optical arrangement disclosed in this patent is, in some sense, related to the disclosure of the present invention.
• U.S. Patent No. 3,984,171 describes the use of a spherical compensator which has the effect of straightening, or linearizing, the curved scan inherent in that scheme.
• the inventor suggests that in some arrangements the spherical reflector may, in fact, become a compound toric surface.
• U.S. Patent No. 4,176,907 while also directed to a scanner utilizing a rotating polygonal mirror, shows another technique for linearizing the resultant scan.
• a prism is incorporated which, along with a condensing lens, has the effect of producing a linear scan from the inherently conical scanning beam produced by the rotating polygonal reflector.
• U.S. Patent No. 3,953,105 describes an arrangement in which the scan is effected by means of a rotating reflective holographic disc. Curvature correction is accomplished by means of a convex spherical reflector, which serves to redirect the rays to be perpendicular to a recording medium.
• the hologram used for scanning must be generated using the same convex spherical reflector.
• the field, in this arrangement is cylindrical, whereas a planar or substantially planar field would be more desirable.
• U.S. Patent No. 3,951,509 discloses another scanning apparatus utilizing a rotating mirror. In this scheme, a fixed hologram is utilized to convert the resultant conical scan into a linear scan at the image plane. Disclosure of the Invention The present invention is directed to an improved apparatus for scanning a light beam by means of a rotating, multifaceted diffraction grating.
• An object of the present invention is to provide an improved, economical, simple apparatus for scanning a light beam.
• Another object of the present invention is to provide an apparatus comprising anamorphic imaging for correcting the inherent curved scan characteristic of rotating diffraction grating scanning arrangements.
• a further object of the present invention is to provide a diffraction grating scanner with improved resolution and length of scan.
• a further object of the present invention is to provide an apparatus wherein focus, in addition to curvature correction, is accomplished by means of anamorphic imaging.
• a further object of the present invention is to provide a diffraction grating scanning apparatus in which field curvature correction is accomplished by means of toroidal, or crossed cylindrical lenses and/or reflectors.
• a further object of the present invention is to provide an improved diffraction grating scanner with a high duty factor.
• a further object of the invention is to provide an improved diffraction grating scanner that is relative insensitive to vibration of the rotating disc bearing the diffraction gratings.
• a further object of the present invention is to provide an improved diffraction grating scanning apparatus whose performance is relatively unaffected by changes in the wavelength of the light source.
• a further object of the present invention is to provide an improved method of fabrication of a holofacet scanner disc, in which the holograms are created at a first wavelength, and playback is, or may be, accomplished at a different wavelength, for example, an infrared wavelength.
• a further object of the present invention is to provide an improved diffraction grating scanning apparatus which operates satisfactorily over a range of laser wavelengths.
• a further object of the present invention is to provide an improved diffraction grating scanner in which mechanical and optical alignment tolerances are relatively uncritical.
• a further object of the present invention is to provide an improved diffraction grating scanning apparatus incorporating an injection laser diode, thereby obviating the requirement for complex and expensive acousto-optical or electro-optical light modulators.
• a further object of the present invention is to provide an improved laser diffraction grating scanning apparatus with relatively short warm-up time by virtue of its incorporation of a semiconductor injection laser as light source.
• Figure 1 is a top view of the preferred embodiment of a diffraction grating scanning apparatus according to the present invention.
• Figure 2 is a front view of the apparatus of Figure 1.
• Figures 3, 4 and 5 show one facet of the diffraction grating scanner as it rotates through the incident beam, illustrating the high duty factor obtainable with the present invention.
• Figure 6 is a front view of a simplified version of the apparatus according to the present invention, illustrating the two important features: first, varying deflection angles, associated with different laser wavelengths, although they are deflected through different angles by the diffraction grating, converges to the same point at the same image plane. Second, vibration or wobble of the diffraction grating scanning disc, which also produces a varying deflection of the incident light beam, is also compensated by the anamorphic imaging system so as to bring the light beam to the same point in the image plane.
• Figure 7 is a top view of one embodiment of the invention which incorporates a cylindrical reflector of circular or elliptical cross-section.
• this embodiment there is no correction for field curvature; in other words, the image is created on a curved surface, rather than a plane.
• Figure 8 shows an improved version of the apparatus according to the present invention, in which correction is introduced for the curvature of the image field.
• the reflector rather than being cylindrical, is now generally toroidal, that is, the cross-section in each of two orthogonal planes is circular, with different radii of curvature.
• the image surface can be made substantially planar, as shown in Figure 8.
• Figure 9 illustrates another embodiment of the anamorphic imaging arrangement of the present invention.
• a cylindrical mirror in combination with a cylindrical lens, arranged such that the axes of the lens and mirror are at right angles. This arrangement achieves substantially the same result as the toroidal reflector previously described.
• Figure 10 illustrates another, somewhat different, embodiment of the anamorphic imaging scheme.
• a cylindrical lens again having its axis perpendicular to the axis of the cylindrical mirror, although placement of the cylindrical lens is between the planar reflector and the image plane.
• Figure 11 shows an embodiment which utilizes two cylindrical reflectors having their axes at right angles. Again, substantially the same effect is realized as was provided by the toroidal reflector of Figure 8.
• Figure 12 illustrates the anamorphic imaging arrangement utilizing a single cylindrical lens. In the arrangement of Figure 12, there is no correction for field curvature.
• Figure 13 illustrates an embodiment with two cylindrical lenses having their axes orthogonal. This arrangement provides correction for field curvature as well as for the inherent arcuate scan.
• Figure 14 shows another embodiment comprising two cylindrical lenses. In this arrangement one of the crossed cylindrical lens is located adjacent the image plane. Best Mode for Carrying Out the Invention
• Figure 1 is a top view of an apparatus according to the present invention
• Figure 2 is a front elevational view of the same apparatus.
• light source 10 which may typically be an injection laser diode, but may also be any source of reasonably collimated, relatively monochromatic light.
• Light from light source 10 is collected by collecting optics 11, which produce a shaped, collimated beam of light 12.
• Shaped, collimated light beam 12 is applied to first cylindrical lens 13, having its axis parallel to the desired scan.
• First, cylindrical lens 13, as can be seen from Figures 1 and 2, has no focusing effect in one plane (as is apparent in Figure 1), but does have a focusing effect in the view of Figure 2.
• first cylindrical lens 13 serves to concentrate the energy light source 10 into a focused line segment 30 of light at scanning disc 14.
• cylindrical lens 15 is shown, in Figures 1 and 2, intermediate first cylindrical lens 13 and scanning disc 14.
• Second cylindrical lens 15, in conjunction with the converging power of toroidal reflector 20, serves to bring the light beam to a focus at individual points along the scan in the direction parallel to scan line 25.
• Scanning disc 14 typically bears a number of individual diffraction patterns, or facets 16, each of which may be constructed by holographic techniques. Of course, it is also possible to construct such diffraction patterns mechanically, i.e., by rulling, or even by computer generation.
• a typical facet 16 is shown in Figures 1 and 2. Scanning disc 14 is driven by, for example, motor 17, as is well-known to the prior art, to produce a scanned beam of light 18.
• central ray 19 of scanned beam of light 18 describes a portion of a cone as scanning disc 14 rotates.
• toroidal reflector 20 is characterized by different radii of curvature in the two planes illustrated in Figures 1 and 2. Hence, the converging power of toroidal reflector 20 is different in these two planes. Indeed, its function in these two planes is quite different. In the plane visible in Figure 1, the convering power of toroidal reflector 20 is established so as to provide a focus, and to correct for the curved image surface which would otherwise result. The converging power of toroidal reflector 20 in the plane visible in Figure 2 serves to correct for the arcuate scan by reimaging object point 24, visible in Figure 2, onto image point 23.
• Figure 1 shows the same scanning system, illustrating the imaging components perpendicular to scan line 25. Again, one is looking at light source 10 whose output is collected and collimated by collecting optics 11 to produce shaped, collimate light beam 12. The axis of second cylindrical lens 15 is perpendicular to scan line 25. Shaped, collimated light beam 12 is focused onto the image plane 26 by second cylindrical lens 15, in conjunction with the first component of curvature of toroidal reflector 20, i.e., the component apparent in Figure 1. This focusing action, together with the focusing action parallel to scan line 25, illustrated in Figure 2, results in a well-focused spot on the image plane 26 and, more specifically, along scan line 25.
• the second curvature component of toroidal reflector 20 i.e., the curvature component shown in cross section in Figure 2 serves to produce a straight-line scan at image plane 26. Even though scanned beam of light 18 describes a conical locus during scanning, this second curvature component of toroidal reflector 20 serves to correct for the conical scan, bringing the scanned beam to a straight-line sweep at image plane 26.
• the first curvature component of toroidal reflector 20, as shown in cross-section in Figure 1 is chosen (in conjunction with second cylindrical lens 15) so as to produce a focus condition and a substantially flat field at image plane 26.
• the second curvature component of toroidal reflector 20, as shown in crosssection in Figure 2 is chosen to provide a straight-line scan at image plane 26.
• Figure 7 which is a view analogous to Figure
• this spot growth may become the limitation in certain applications of the scanner, in that it establishes the maximum number of resolvable spots .along scan line 25. To some extent, it is possible to "trade off" the length of scan line 25 and spot density. So, to increase the scan length or the number of resolvable spots per unit length beyond the depth of focus compensation of the sag, the toroidal system of Figures 1 and 2 may be used, or a crossed cylindrical imaging system might also be utilized.
• Figure 8 shows how the sag is reduced by introducing the first component of curvature of toroidal reflector 20, as already described in connection with Figure 1.
• This first component of curvature folds the beam back into the center of the scan, thereby reducing the sag of scanned beam of light 18.
• This arrangement allows increasing the length of scan line 25, or an increase in resolution, because the depth of focus is less critical. In other words, since the sag is less, the length of scan line 25 can be increased until the sag approaches that which would have occurred with a cylindrical reflector in place of toroidal reflector 20.
• FIGs 3, 4 and 5 show a front view of scanning disc 14, which bears n facets 16.
• Each facet 16 scans a line (visible in Figures 1 and 2) at the image plane 26, also visible in Figures 1 and 2, for each revolution of scanning disc 14. There are, therefore, n scans per revolution of scanning disc 14.
• Figures 3-5 show successive positions of scanning disc 14 as it rotates, and serve to illustrate the high duty factor achieved by this system.
• Figure 3 shows the beginning
• Figure 4 shows the intermediate
• Figure 5 shows the end of scanned conditions relative to focused line segment 30, which represents the profile of the light beam incident on facet 16.
• the usable duty factor of each facet i.e., the fraction of the facet that is used in the scan, is determined by the amount of time required for focused line segment 30 to traverse the spaces 29 between facets 16, i.e., that amount of time required for scanning disc 14 to rotate between positions such that focused line segment 30 is entirely contained within one facet 16 and when focus line 30 is entirely contained within the succeeding facet 16.
• the diffraction gratings which constitute facets 16 of scanning disc 14 can be fabricated by any of several different methods including holographic means, as are well known to the art, and techniques of scribing or mechanically ruling contact printing or embossing, or other methods known to the art.
• each facet 16 bears a plurality of parallel grating lines as is illustrated in Figures 3-5, and functions as a diffraction grating.
• the diffraction grating is holographically recorded, it is possible to produce the requisite hologram at a shorter wavelength than that used for playback, since most high diffraction efficiency holographic materials are only sensitive at shorter wavelengths.
• playback at infrared wavelengths such as are characteristic of solid state injection lasers, can be accomplished without any aberrations resulting from the fact that recording of the hologram and playback are accomplished at different wavelengths.
• the only effect is the differing diffraction angles, which can be taken into account in producing the hologram in the first instance.
• wavelengths of a semiconductor injection laser diode are, in general, very different from the well-defined wavelengths of a gas laser, e.g., HeNe at 633nm. This effect requires that the system be substantially insensitive to wavelength, if it is to function properly with a semiconductor injection laser as a light source 10. Also, the wavelength of a semiconductor injection laser tends to vary as a function of temperature, thereby providing an additional reason to minimize wavelength sensitivity if high resolution scanning is to be achieved.
• Figure 6 shows how a change in wavelength, which causes a change in diffraction angle, is corrected by the second component of curvature of reflector 33 (that component of curvature which corresponds to the previously-described second component of curvature in Figure 2).
• the characteristic of this second component of curvature, as is apparent from Figure 6, is to image, at scan line 25 on image plane 26, any ray emanating from object point 24.
• first central ray 34 might correspond to a ray at a certain wavelength.
• first central ray 34 is imaged at scan line 25.
• Second central ray 35 which is diffracted through a greater thangle than was first central ray 34, corresponds to a longer wavelength than that of first central ray 34.
• the curvature of reflector 33 serves to direct second central ray 35 onto scan line 25, in the same manner as occurred for first central ray 34. In this way, it can be seen that if a new laser of longer wavelength were substituted in the system, or if the laser warms up so as to change wavelength, the performance of the system is not adversely affected.
• first central ray 34 and second central ray 35 emanate from the very same object point 24 at facet 16 of scanning disc 14, and they are therefore imaged to the same scan line 25 on the image plane 26.
• Figure 6 also illustrates another important attribute of the present invention.
• the imaging characteristics of reflector 33 correct for cross4ine jitter caused by mechanical deflections, e.g., vibration, wobble, etc., in the scanning disc 14 as it rotates.
• first central ray 34 and second central ray 35 might represent variations in the angle of deflection resulting from mechanical instability of scanning disc 14.
• all rays emanating from object point 24 are imaged onto scan line 25, in accordance with well-known principles of geometric optics.
• Figures 9 through 14 illustrate various possible modifications or alternate embodiments of the invention.
• the arrangements shown in Figures 9 through 14 are not intended to be exhaustive; rather, they are intended to demonstrate just a few of the many different embodiments employing the principles of the present invention.
• Figure 9 shows that the anamorphic correction can be realized by making reflector 33 a cylindrical reflector, and including a cylindrical lens in front of reflector 33.
• the axis of cylindrical lens 36 must be at right angles to the axis of cylindrical mirror 33.
• different converging powers are realized in the planes parallel to, and perpendicular to, the scan, as taught by the present invention.
• the arrangement is similar to that of Figure 9, except cylindrical lens 36 is positioned intermediate plane reflector 22 and the image plane 26.
• FIG 11 shows yet another embodiment of the invention, in which a second cylindrical reflector 37 replaces the plane reflector 22 of, for example, Figure 10.
• second cylindrical reflector 37 in the embodiment of Figure 11 provides the same benefits as does the first component of curvature of toroidal reflector 20 in Figure 1.
• the two cylindrical reflectors together provide the same optical characteristics as does the toroidal reflector 20 in the embodiment of Figures 1 and 2. It is important to note, as a practical consideration, that a pair of cylindrical reflectors may be less expensive and more readily procured than, for example, a single toroidal reflector.
• FIG 12 illustrates another embodiment of the invention, similar in its properties to that of Figure 7, except that third cylindrical lens 38 serves the same function, in the embodiment of
• Figure 12 as does the cylindrical mirror 39 in the embodiment of Figure 7.
• third cylindrical lens 38 serves to image object point 24 onto scan line 25 at image plane 26; however, the embodiment of Figure 12 does not include any correction for the purpose of reducing sag in the image field, as we discussed in connection with Figure 8.
• Figure 13 adds, to the embodiment of Figure 12, the cylindrical lens 36, having its axis orthogonal to that of third cylindrical lens 38.
• the embodiment of Figure 13 in addition to providing the benefits already discussed in connection with Figure 12, also minimizes the sag of the image field.
• cylindrical lens 36 and third cylindrical lens 38 can be "collapsed" into a single toroidal lens, providing the very same performance and benefits.
• the radii of curvature of the two cross-sections of such a toroidal lens are appropriately selected to (a) image object point 24 onto scan line 25 on image plane 26; and (b) to minimize the "sag" of the locus points constituting scan line 25, as already discussed in connection with Figure 8.
• cylindrical lens 36 has been moved, from its position proximate image plane 26.
• This arrangement serves to demonstrate that the positioning of the two cylindrdical lenses 36 and 38 can be modified to accommodate other design considerations.
• the particular radii of curvature of the cylindrical lenses must depend upon their placement in the optical path.
• Plane mirror 22 serves merely to fold the beam, and can be located at any point within the optical path. Of course, additional folding mirrors may also be included.
• conjugate ratio associated with correcting imaging elements 33, 36 and 38 may take on any appropriate value suitable to the desired image size and other design considerations.
• the invention has been discussed in terms of an image plane 26, in fact the image may be made to fall upon any suitable surface.
• the invention may be used in connection with a cylindrical photoconductive rotating drum having its axis parallel to scan line 25, as is well-known to the prior art, thus producing a two dimensional scan.
• Thare are various other techniques known to the prior art for achieving a two-dimensional scan from the inherently one-dimensional scan discussed in connection with this invention.
• the image plane 26 can be made to translate in a direction perpendicular to scan line 25 to achieve this to-dimensional scan. It is also possible, of course, to include means for optically scanning the beam in a direction orthogonal to scan line 25 onto a fixed image plane 26.
• anamorphic imaging and correcting that is the substance of this invention can be realized by a pair of cylindrical el ements, either lenses, reflectors, or a lens-reflector combination.
• the axes of the cylindrical dements must, of course, be perpendicular.
• second cylindrical lens 15 could, in fact, be a spherical lens. Since this is a relatively slow lens, and located near the focus of first cylindrical lens 13, it would only be necessary to make a slight adjustment in the power of first cylindrical lens 13 to compensate for the converging power, in view of Figure 2 of a spherical lens substituted for second cylindrical lens 15. It will be understood by those skilled in the art that many modifications and variations of the present invention may be made without departing from the spirit and scope thereof.

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• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Mechanical Optical Scanning Systems (AREA)

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• 1982
  • 1982-02-22 EP EP19820901080 patent/EP0078269A4/de not_active Withdrawn
  • 1982-02-22 JP JP50109182A patent/JPS58500262A/ja active Pending
  • 1982-02-22 WO PCT/US1982/000225 patent/WO1982002955A1/en not_active Application Discontinuation

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