LASER SCANNER WITH POST-SCANNING BEAM SHAPING MEANS

Abstract: In a laser scanner of the type having post-scanning beam shaping employing prisms between a hologon scanner and an f- THETA lens, the prisms are replaced with a planar beam expander such as a diffraction grating, thereby enabling the f- THETA lens to be positioned closer to the hologon, and therefore made smaller and hence less costly.

Description: LASER SCANNER WITH POST-SCANNING BEAM SHAPING MEANS Technical Field This invention relates to laser scanning apparatus for producing large format high quality images, and more particularly to such apparatus having means for shaping the scanning spot produced by such apparatus subsequent to the scanning means. Background Art U.S. patent no. 4,707,055 issued November 17, 1987 to Stark, and U.S. patent no. 4,904,034 issued February 27, 1990 to Narayan, Roddy, Stark and Thompson (which are incorporated herein by reference) describe scanning apparatus for providing a substantially straight line scan of a laser spot having a selected shape. The scanning apparatus shown schematically in Fig. 2 includes a laser beam light source 10, such as a laser diode. Means (not shown) are provided for modulating the output of the laser diode 10 in accordance with information contained in a stream of electronic signals. As such means are well known in the art, no further description is provided herein. The beam 12 from the laser diode 10 is collimated by collimating optics 14, and is incident on a stationary diffraction grating 16, which directs the beam 12 to a rotating holographic beam scanner disc 18, referred to in the art as a hologon spinner. The hologon spinner disc 18 comprises a plurality of holographically produced diffraction grating facets 20 as shown in Fig. 3. As the disc is rotated by a high speed motor 22, the diffraction grating facets 20 cause the beam to scan in a direction perpendicular to the plane of the drawing in Fig. 2. The scanned beam 12' then passes through a pair of beam shaping prisms 2 and 26 which serve as beam expanders for expanding the cross section of the beam in the cross scan direction. The shaped beam 12" then passes through an f-Θ lens 28 which focuses the scanning beam onto a target such as a photosensitive element 34 attached to a rotating drum 30. The beam is scanned by hologon spinner 18 in a direction parallel to the axis 32 of the rotating drum 30 to provide a line scan on the photosensitive element 34. A page scan of the element 34 is provided by rotation of drum 30 about axis 32. For optimum exposure of the photosensitive element 34, it is desirable that the spot produced by the laser beam be narrower in the line scan direction than in the page scan direction. For large format scanning apparatus, it is also desirable that the useful duty cycle of the hologon be as large as possible, and for a high resolution scanner (i.e. a small spot at the photosensitive element 34) it is desirable that the beam at the hologon spinner 18 be large. These requirements dictate that the beam 12 at the hologon spinner 18 be narrower in the page scan direction, as shown in Fig. 3. The prisms 24 and 26 are employed to generate the desired beam shape at the photosensitive element 34. In the scanning apparatus described above, the required size of the f-Θ lens 28, which needs to be large to accommodate the large format scan, varies directly with its distance from the hologon scanner 18, and the cost of the f-Θ lens increases approximately with the square of the distance to the hologon. Therefore, it can be seen that the presence of the beam shaping prisms 24 and 26 exact a cost in the size of the f-Θ lens required by the scanner. Summary of the Invention It is therefore the object of the present invention to provide an improved hologon laser scanner of the type having means for post-scanning shaping of the scanning spot that avoids the shortcoming noted above. The object is achieved according to the present invention by providing a planar beam expander means between the f-Θ lens and the hologon scanner, whereby the f-Θ lens can be moved closer to the 5 hologon scanner, and therefore be made smaller and hence less expensive. In a preferred embodiment of the invention, either one or both of the beam expander prisms is replaced by a diffraction grating. In an alternative embodiment, the planar beam expander is a 0 gradient index optical plate having an index of refraction that continuously increases from one surface to the other. Brief Description of the Drawings Figure 1 is a schematic diagram of laser scanning apparatus accordingly to the present invention; Figure 2 is a schematic diagram of laser scanning apparatus according to the prior art; Figure 3 illustrates a radial hologon employed in the apparatus of Figures 1 and 2; Figure 4 is a schematic diagram illustrating a diffraction grating beam expander employed in one embodiment of the present invention; Figure 5 is a schematic diagram of a gradient index optical plate beam expander employed in an alternative embodiment of the present invention; Figure 6 is a schematic diagram useful in describing the scanner shown in Figure 1; Figure 7A and B are diagrams useful in describing the orientation of the diffraction grating beam expander with respect to the hologon spinner; and Figure BA and B are diagrams useful in describing the incident ray and diffraction ray at the diffraction grating, relating to the signs of their respective angles. Modes of Carrying out the Invention Referring to Figure 1, scanning apparatus according to the present invention is shown. Elements corresponding the the elements in the prior art scanner shown in Figure 2 are similarly numbered. In the apparatus shown in Figure 1, the beam expanding prisms 24 and 26 of the prior art have been replaced by a single planar beam expander 36, thereby enabling the f-Θ lens 28 to be moved closer to hologon spinner 18, and hence made smaller, realizing considerable cost savings in the f-Θ lens. Only a single planar beam expander 36, oriented parallel to the hologon spinner 18, is employed in the example shown in Figure 1 for achieving the maximum space savings and hence maximum size reduction in the f-Θ lens 28. As a result, the expanded beam 12" is no longer parallel with the output beam 12, as in the apparatus shown in Figure 1. There are some advantages to having the input and output beams parallel as is discussed in the prior art. These advantages may be achieved albeit at the increased cost of the f-Θ lens by providing a second prism or a second planar beam expander. However, since the maximum cost savings in the f-Θ lens is achieved by employing a single planar beam expander, the configuration shown in Figure 1 employing a single planar beam expander is presently preferred. The planar beam expander 36 preferably comprises a diffraction grating 36' employed with a large incidence angle θ. and a small diffraction angle θ, as shown in Figure 4. Alternatively, the planar beam expander 36 comprises a sheet of gradient index material 36" such as doped glass (as shown in Fig. 5), wherein the index of refraction varies continuously from one surface 42 to the opposite surface 44. With the index gradient in the direction of arrow A as shown in Figure 5, parallel rays 46 entering the sheet of gradient index glass 36" will be continuously refracted as they traverse the glass sheet, and will exit parallel as shown by rays 48. A procedure for designing the diffraction grating beam expander 36' shown in Figure 4 will now be described. Design Procedure The initial data needed to begin the diffraction grating design process are: λ: spectral wavelength of the laser light source 10 θ: angle of incidence of the input beam 12 on the spinner 18 G: grating factor for the spinner 18 φ : rotation angle of the spinner corresponding to the position where bow is to be nulled by the output grating. f- i anamorphic beam magnification factor for the output grating. The grating factor G is related to diffraction order number (m), wavelength (λ) and groove frequency (f ) by the expression: G - mλf g (1) The direction cosines of the diffracted beam 12 leaving the hologon spinner 18 are calculated next. These direction cosines are: cosα ■ CB - DA (2) cosfl - - E (3) cosγ ■ AB + CD (4) where A«[cos 2 θ-G 2 +2G sinθcosφ] 172 (5) B«[cos 2 θ-G 2 +2G εinθ] 1 2 (6) C»εinθ-Gcosφ (7) D-sinθ-G (8) E«Gsinφ (9) In general, the beam incidence angle (θ..) on the diffraction grating 36' can be either a negative or a positive value. The sign identifies the orientation of the output grating with respect to the hologon spinner 18, as shown in Figures 7A and 7B. Figure 7A illustrates the case where the angle θ. is negative, and Figure 7B illustrates the case where θ, is positive. As a special case, we begin by setting the value for the beam incidence angle θ. on the diffraction grating 36* at mid-scan equal to θ. This special condition causes the grating to be parallel with the spinner, giving the most compact scanner configuration. θ^ θ (10) The diffraction grating requires a specific grating factor that causes the bow to be nulled when the spinner rotation angle is φ . This grating factor is calculated from the quadratic formula: G 1 «[-b± b 2 -4ac]/2a (11) where c«cosα[cos(2θ, )cosα-sin(2θ 1 )cosγ] (14) The required grating factor is calculated from eg. (11). In general, this quadratic equation will give two solutions for the grating factor. The good solution will have RQ ■ SP in eq. (15) below, causing cosα.-O, and this is the condition for zero bow. The bad solution will have RQ « -SP and should be discarded. -7- Also, the calculated grating factors from eg. (11) can be negative. These are acceptable solutions. The negative sign only means that the grating order number (m) used is -1 instead of +1. The direction cosines of the beam leaving the diffraction grating 36 when the spinner rotation is φ are: cosα.-RQ-SP (15) 10 15 20 25 The diffraction angle (θ.) can either have the same sign as the incidence angle (θ.) or the opposite sign. This condition identifies the orientation of the diffracted beam with respect to the incident beam as shown in Figures 8A and 8B. Fig. 8A 30 illustrates the case where the angles θ, and θ 1 have opposite signs, and Fig. 8B illustrates the case where they have the same signs. The anamorphic magnification M. of the beam caused by the diffraction grating is: 35 χ - cosθ 1 /cosθ 1 (25) The magnification calculated above is compared with the specified value of magnification. If adjustment is needed, the above process is repeated using new values for θ and θ, . In general, reducing the beam incidence angle θ on the spinner while maintaining the grating parallel to the spinner (θ, - θ) will reduce the magnification. The scan trajectory is calculated from the equations: where θ c is the beam deflection angle in the scan direction and θ β is the beam deflection angle in the orthogonal bow direction. The trajectory is generated by using a sequence of values for spinner rotation φ in eq. (2-9). The sets of direction cosines thus calculated are used as input to eq. (15-22). Finally, eq. (26-27) are used to generate beam deflections corresponding to the selected spinner rotation values. Working Example As an example, a single diffraction grating planar beam expander for the holographic scanner shown in Figure 1, having the following characteristics was designed: a. The spectral wavelength (λ) of the laser light source is 830 nanometers. b. The beam incidence angle (θ) on the spinner is 63.82 degrees. c. The spinner operates at the Bragg angle where the incidence angle and diffraction angle of the beam are equal. To meet this condition, the grating factor for the spinner is G - 2 εinθ - 1.794824859. d. The diffraction grating nulls the bow at the point where the spinner rotation angle (φ ) is 5.90 degrees. Using eq. (2-9), the direction cosines of the diffracted beam leaving the spinner are calculated: A - 0.421410260 B - 0.441192624 C - 0.887904913 D - 0.897412429 E - 0.184494600 cosα ■ -0.013558293 COSβ - -0.184494600 cosγ « 0.982740004 The mid-scan beam incidence angle, θ, , on the diffraction grating is made equal to the beam incidence angle, θ, on the spinner. This causes the grating to be parallel with the spinner, θ, » θ » 63.82 degrees From eq. (11-14) the required grating factor for the diffraction grating is calculated: a - 0.034038258 b - 0.042077678 c - 0.010438746 Solving the quadratic eq. (11), we have two values for the grating factor: G 1 + - -0.343570312 G j^ - - -0.892617466 Then, solving eq. (15-22) for both of the above grating factors, we find that G.+ is the good solution because it yields the desired bow null condition (cosα. ■ 0) . For the good grating factor of -0.343570312: P - 0.818328532 Q - 0.832621710 Cθεβ χ - -0.184494600 Cθsγ 1 - 0.982833527 From eq. (23) the groove frequency for the output grating is 413.940139 grooves/mm. The diffraction angle of the beam leaving the diffraction grating (at mid-scan) is calculated from eq. (24). θ. - -33.630999 degrees Finally, the beam magnification is calculated from eq. (25) M. - 1.887207 The scan trajectory iε found by firεt calculating sets of direction cosineε for the scanning beam at the spinner output using eq. (2-9) for a sequence of spinner rotation angles φ. Then, uεing these direction cosines in eq. (15-22), corresponding εetε of direction coεineε are generated for the scanning beam leaving the diffraction grating. Finally, the beam angular deflections are calculated from eq. (26-27). The resulting trajectory iε given in the following table: Hologon Rotation Scan Angle Bow Angle (Degrees) (Degrees) (Degrees) 0.00 0.0000 0.00000 1.00 - 1.7950 0.00011 2.00 - 3.5913 0.00040 3.00 - 5.3900 0.00075 4.00 - 7.1923 0.00098 5.00 - 8.9997 0.00081 6.00 -10.8134 -0.00014 6.50 -11.7230 -0.00105 The above table shows that bow increases to a maximum value of 0.00098 degrees at a spinner rotation angle of 4.00 degrees and then decreases to zero at the specified spinner rotation angle of 5.90 degrees. Further rotation of the spinner to 6.50 degrees causes the bow to again increase to a value of -0.00105 degrees, which is approximately equal in magnitude to the maximum bow. This point of balanced bow is usually chosen as the full scan position. Thus, for this scanner example, rotating the spinner ±6.50 degrees scanε the beam from itε mid-scan position (0 degrees) to its full-scan position (±11.7230 degrees) and, over this scan, the maximum angular bow is 0.00105 degrees. The configuration for this example scanner is shown in Figure 6. Industrial Applicability and Advantages A laser scanner according to the present invention is useful for making large format high-quality images, such as graphic arts quality images and is advantageous in that the f-Θ lens may be placed closer to the hologon scanner thereby employing a smaller and hence lower cost f-Θ lens realizing manufacturing economy in the laser scanner.

Claims: We Claim; 1. Scanning apparatus for scanning a radiation spot along a line at a target, comprising: a) a source of monochromatic radiation; b) a hologon mounted for rotation about an axis to scan a beam of radiation incident thereon; c) means for forming radiation from the source into a collimated beam and directing the beam onto the hologon at a predetermined angle; d) a target station for receiving the εcanned beam; e) lenε meanε between the target station and the hologon for focusing the beam to a spot at the target station; and f) planar beam expanding means between the lens means and the hologon for modifying the crosε εectional shape of the beam. 2. The scanning apparatus claimed in claim 1, wherein the planar beam expanding means introduces bow in the scan line, and the hologon is a radial hologon selected to produce a compensating bow in the scan line. 3. The scanning apparatus claimed in claim 1, wherein the planar beam expanding meanε comprises a diffraction grating. 4. The scanning apparatus claimed in claim 1, wherein the planar beam expanding means is a sheet of gradient index glass having a continuously varying gradient from the input to the output surface.

Inventors: NARAYAN BADHRI, RODDY JAMES E, STARK RICHARD A, NARAYAN, BADHRI, RODDY, JAMES, E., STARK, RICHARD, A.
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1991
Main classification: G02B27
IPC codes: G02B26, G02B27, H04N1