GB2293460A

GB2293460A - Scanning optical system

Info

Publication number: GB2293460A
Application number: GB9519159A
Authority: GB
Inventors: Ronald J Straayer
Original assignee: Gerber Systems Corp
Current assignee: Gerber Systems Corp
Priority date: 1994-09-23
Filing date: 1995-09-19
Publication date: 1996-03-27
Anticipated expiration: 2015-09-19
Also published as: GB9519159D0; GB2293460B; JP2740481B2; JPH08194183A; DE19535283A1; DE19535283C2; FR2725100A1

Description

1 2293460

DUAL SCAN SPINNER FOR USE IN SCANNER SYSTEMS TECHNICAL FIELD

The present invention relates to scanners and imagers in general and, more particularly, to scanners in having a dual scan spinner for an enhanced operational efficiency.

CROSS REFERENCE TO RELATED APPLICATIONS

Some of the subject matter herein is disclosed and claimed in the following U.S. patents, all of which are incorporated herein by reference.

U.S. Patent No. 5,291,392 entitled "Method And Apparatus For Enhancing The Accuracy Of Scanner Systems"; U.S. Patent No. 3,555,254, entitled "Error Correcting System And Method For Use With Plotters, Machine Tools And The Like"; U.S. Patent No. 4,851,656 entitled "Method And Apparatus For Enhancing Optical Photoplotter Accuracy".

BACKGROUND OF THE INVENTION

Raster scan photoplotters or imagers having both planar and internal drum design are known in the art. These devices are used in the fabrication of printed circuit boards. Conversely, scanners which read data from a substrate have similar geometries. Planar photoplotters such as disclosed and claimed in U.S. Patent No. 4,851,656 have a planar surface for receiving a substrate. An optical exposure head is located on a movable gantry apparatus and is rastered above the substrate during exposure. Internal drum photoplotters are characterized by a substantially cylindrical surface portion which receives the substrate. The exposure beam emanates from an optical exposure head and is scanned across the substrate by a rotating spinner. The optical exposure head is indexed along the longitudinal axis of the cylinder to complete the substrate exposure. Internal drum raster photoplotters of the 2 type disclosed in U.S. Patent No. 5,291,392 have inherent advantages over planar type scanners, including simplicity of design and lower costs.

An exemplary internal drum laser raster imager, the Crescent 42 manufactured by Gerber Scientific, Inc. of South Windsor, CT, has an internal drum that utilizes a 180' curved surface to receive the substrate. It also has a spinner centered on a longitudinal drum axis. With this configuration, one rotation of the spinner with its nominal 450 scan mirror produces one scan line; yielding a duty cycle of about 50%. As the raster image processing or "RIPing" technology of transferring data and thereafter interpreting it progresses, so does the desire to image faster. However, there are diffici3lties in advancing the imaging speed of internal drum imagers. The spinner itself is limited to a speed in the range of 20,000 to 24,000 RPM by the air bearing/motor technology and mirror deformation considerations. Another avenue of inquiry involves the use of multiple beams. However, a multiple beam approach is highly difficult to implement due to the internal drum scanning geometry which produces an undesirable rotation in the image plane of multiple beams so that they no longer lie in a plane with respect to the motion axes. Solving this problem requires the addition of a costly and complicated rotating prism assembly which must be synchronIzed to the spinner.

A further, related issue is the desire to increase the temporal efficiency of the scanner or imager. As noted, prior art systems are limited to a maximum 50% duty cycle. Internal drum imagers can be manufactured with higher angular utilization (i.e. 270') with higher duty cycle but they add complexity for material handling. A limited duty cycle is undesirable from two respects. First, the lower the duty cycle, the faster the video electronics must be for an equivalent scan rate. Secondly, for systems such as computer-toplate and direct imaging of printed circuit boards, there can be an exposure

3 limitation. A higher duty cycle improves the system's ability to expose the substrate media.

Earlier efforts to improve the overall throughput of imaging or scanner systems include the device disclosed in U.S. Patent 5,187,606 to Kondo et al. The '606 device shows a scanning optical apparatus that has a light source for emitting a light beam and a deflector, such as a rotating polygonal mirror, with a plurality of mirror surfaces for deflecting the light beam. Each mirror surface of the polygonal mirror has a pair of reflecting surfaces inclined toward the center axis of rotation of the polygonal mirror and orthogonal to each other. There is a fixed reflecting mirror arranged in an opposed relationship with one of the pair of reflecting surfaces so that the light beam deflected by the reflector is reflected, to be returned to the deflector again. The '606 system is used to increase the scanning angle of the laser beam to twice the width as compared to that of conventional polygonal mirrors, thereby increasing the speed of scan without increasing the rotational speed of the polygonal mirror. U.S. Patent 4,445,126 to Tsukada discloses an image forming apparatus in which recording medium is scanned with a plurality of light beams. The '126 apparatus includes a beam generator for generating a plurality of light beams and pre;enting them simultaneously to a facet of rotating polygonal mirror. The purpose of the '126 apparatus is to generate a plurality of scan lines at a given time during operation.

An image recording device which relies on multiple beams is disclosed in U.S. Patents 4,506,275 and 4,517,608 to Maeda et al. The Maeda et al device includes a recording unit for duplicating and recording halftone images on photosensitive material. The recording unit comprises an acoustooptic light modulating element including a plurality of ultrasonic wave exciting portions disposed side by side on a single acousto-optic medium. The ultrasonic - portions independently modulate an incident light beam into a wave excitint, 4 plurality of modulated light beams in response to image signals from a photoelectrical scanning means. There is a scaled down optical system which then reduces the diameter of the plurality of modulated light beams at a plurality of light transfer elements to transfer the light beams from the scaled down optical system to a focusing lens to be projected onto a film in a recording cylinder. The system as set forth in the Maeda et al patents relies on a fixed scanning head. The substrate is located on external surface of the rotating drum.

A multiple beam optical modulation system is disclosed in'U.S.

Patent 5,251,057. The'057 system is used in a raster output scanner that employs one original beam and a facet of a rotating polygon to generate two consecutive scan lines. The original beam is first separated into two beams in a beam splitter. The resultant beams are polarized ninety degrees apart, and directed to a modulator. The beams are a sufficient distant apart so that the acousto-optic (alo) modulator can modulate each beam with a minimum of crosstalk. The output beams are put brought together to within one scan line separation by a beam recombination device, which is a reversed beam splitter.

The beams can be brought together to close proximity without optical interference because the beams are polarized ninety degrees apart.

None of the systems disclosed by the prior art offer a doubling of scan system efficiency nor is there found a system which achieves any improvement in throughput without extensive and cumbersome modifications to system optics and electronics. It would be advantageous to have a system for use with internal drum type scanners or photoplotters which provides two scans for each rotation of the system spinner. The present invention is drawn toward such a system.

SUMMARY OF INVENTION

An object of the present invention is to provide an optical spinner for use with a photoplotter or scanner that provides two scan lines for each rotation and/or is to provide a spinner of the foregoing type that allows for approximately one hundred percent duty cycle operation and/or to provide a system of the foregoing type in which the system throughput approximately doubles for a given spinner rotation speed and/or other improvements generally.

According to one aspect of the present invention, a scanning optical system includes an optical source for generating a circularly polarized optical beam. There is a curved platen to receive a substrate and a modulator for providing optical modulation to the circularly polarized optical beam in response to received modulator control signals. A raster scanner is responsive to advancement control signals and advances, relative to the substrate, the circularly polarized optical beam across the substrate in a first direction forming a scan line. The raster scanner also advances the circularly polarized optical b;arn relative to the substrate in a second direction substantially perpendicular to the first direction displacing one scan line from another. There is also an apparatus for switching the optical beam polarization between first and second directions in response to polarization control signals. An encoder generates signals indicative of the position of the circularly polarized optical beam along a current scan line. A controller receives the encoder signals and generates the advancement signals and the modulator control signals. The controller further provides the optical beam polarization switching signals in dependence on the encoder signals such that the optical beam circular polarization is switched after the completion of the current scan line. The b 6 scanning optical system also includes a spinner that receives the optical beam from the polarization switching apparatus. The spinner has a quarter wave plate that receives the circularly polarized optical beam and provides a linearly polarized scan beam. A polarization sensitive beamsplitter reflects the linearly polarized scan beam at an internal beamsplitter surface if the linearly polarized scan beam is polarized in a first linear direction. A quarter wave plate receives the linearly polarized scan beam from the polarization sensitive beamsplitter if the linearly polarized scan beam is polarized in a second linear direction orthogonal to the first linear direction. The quarter wave plate rotates the second direction polarized scan beam by ninety degrees as it transits the same.

There is a retroreflector for retuming the ninety degree rotated second direction polarized scan beam through the quarter wave plate to the polarization sensitive beamsplitter.

According to another aspect of the present invention, a spinner for use in a scanning optical system that has an apparatus for generating a circularly polarized optical beam, an apparatus for switching the optical beam polarization between first and second directions, a curved platen for receiving a substrate and a raster scanner responsive to control signals for advancing, relative io the substrate, the circularly polarized optical beam across the substrate in a first direction forming a scan line and an encoder for generating signals indicative of the position of the circularly polarized optical beam along the current scan line. The spinner includes a quarter wave plate for receiving the circularly polarized optical beam and provides therefrom a linearly polarized scan beam. There is a polarization sensitive beamsplitter reflecting the linearly polarized scan beam at an internal beamsphtter surface if the linearly polarized scan beam is polarized in a first linear direction. A quarter wave plate receives the linearly polarized scan beam from the polarization sensitive beamsplitter if the linearly polarized scan beam is polarized in a second linear direction orthogonal to the first linear direction. The quarter wave plate rotates the second direction polarized scan beam by ninety degrees. A retroreflector receives and returns the ninety degree rotated second direction polarized scan beam through the quarter wave plate to the polarization sensitive beamsplitter.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 is a simplified schematic illustration of a portion of an internal drum raster imager system including a spinner provided in accordance with the present invention.

Fig. 2 is a diagrammatic illustration of a portion of the system of Mg. 1 showing a relationship between the scanned beam and the spinner.

Fig. 3 is a simplified schematic illustration showing an initial portion of a scan of an optical beam across the internal drum surface by a prior art photoplotter.

Fig. 4 is a simplified schematic illustration showing a final portion of the scan of Fig. 3.

Fig. 5 is a diagrammatic illustration showing the effective duty cycle of the photoplotter of Fig. 2.

Fig. 6 is a simplified schematic illustration of a spinner provided in accordance with the present invention receiving a counterclockwise, circular polarized light beam.

Fig. 7 is a simplified schematic illustration of a spinner provided in accordance with the present invention receiving a clockwise, circular polarized light beam.

8 DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to both Figs. 1 and 2, there is shown in simplified schematic form a portion of an internal drum raster photoplotter 10 having an internal drum 12 with a surface 14 that comprises a portion of a cylinder. The internal drum is carefully fabricated and must maintain the cylindricity of the drum surface with great accuracy regardless of variations in environmental parameters such as temperature. To that end the internal drum is a substantial structure preferably of cast aluminum with a series of reinforcing ribs (not shown) spaced along an outside perimeter.

The drum surface is adapted to receive a substrate and includes a plurality of holes 16 which communicate with a plurality of internal channels 18 through which a vacuum is generated by conventional apparatus not shown in the drawing. The vacuum is used to hold a substrate 21 in place during the exposure process. Alternative methods can be equivalently used to hold the substrate in place, including electrostatic and mechanical retention techniques.

The photoplotter also includes a rail 20 that has a carriage mounted raster scanner 22 for scanning an optical beam 24 about the substrate surface in response to command signals received from controller 26 in a mar;ner detailed hereinafter. The raster scanner includes a linear encoder 28 for generating signals indicative of the position of the raster scanner as it moves along the rail. Also included is a fast scan apparatus 30, preferably comprised of a motor 32 and a spinner 34, for receiving the optical beam at a mirror surface 35 from an optical beam source, such as laser 36, and for exposing a series of scan lines 38 on the substrate by rotating the spinner about a spin a-)ds 40, typically at 12,000 rpm. A rotary encoder 42 is included for generating signals indicative of the angular position of the mirror surface during a scan. The optical beam is provided along the spin axis to be received at a central point on the mirror surface.

9 Fig. 3 is a simplified schematic illustration of a portion of a prior art photoplotter 44. Shown in Fig. 3 is a first portion of a rotary drum substrate surface 46 which receives a beam of light 48 reflected from a mirror surface of spinner 50. The spinner 50 is rotated about a rotational axis 52 and advances the beam from right to left in the figure. The spinner mirror surface is oriented at 45 degrees along the central axis of the internal drum which also corresponds to the optical aids along which the exposure beam traverses before presentation to the substrate surface. The mirror surface is oriented to the optical axis of a laser beam and presents the beam directly to the surface. A full rotation of the spinner will yield a laser beam presented to the entire internal drum surface; both the section containing the substrate and the remainder thereof.

There is an initial spinner position 54 before which the beam would otherwise be presented above the internal raster drum substrate surface 46 and therefore not to the substrate. Fig. 4 shows a second spinner position 56 subsequent to the initial position shown in Fig. 3 in which the beam is almost completely advanced across substrate. The rotational extent of these two positions is displayed diagrammatically with respect to Fig. 5 by curve 58.

Beyond spinner position 56, the spinner must rotate around to its initial position shown in Fig. 3 before the controller can again present the modulated exposure beam for creating a scan line. In many scanners, the internal drum surface wl-iich receives the substrate extends only 165 degrees, much less than the practical upper bound of 180 degrees. As a result, the duty cycle of prior art systems is even less than 50%.

Figs. 6 and 7 are simplified schematic drawings showing a spinner provided according to the present invention. The spinner allows for two scan lines for each rotation. In the present invention, the speed of the spinner is substantially the sal-ne as in known systems. Fundamental to the present design is the concept of polarization switching of the incident laser beam. In Fig. 6, a collimated beam of light 62 which feeds this scanner is circularly polarized in a clockwise rotation. A first quarter wave plate 64 positioned on a first spinner surface 66 to receive the beam. A linearly polarized, first scan beam 68 is created through the interaction of the quarter wave plate and transits a polarization sensitive beamsplitter (PSBS) 70 with an "S" orientation. This first scan beam is received and reflected by a internal surface 72 of the PSBS such that the reflected beam e-Nits the spinner to follow the direction of scanner rotation. The internal surface is polarization sensitive such that incident light of select polarizations will be transmitted while other polarizations (e.g., "S" orientation) will be reflected. The PSBS surface reflects nearly 100% of the linearly polarized light. There is also a lens 74 which focuses the first scan beam before presentation to the substrate.

The first scan beam, therefore, is generated in a manner similar to that done in prior art systems and constitutes the initial beam generated by the present system. For the second scan, an input (feed) beam to the scanner is polarization switched by 180" to circular/counterclockwise, as represented by beam 76 in Fig. 7. The first quarter wave plate now creates a linearly polarized second can beam 77 in a'T" orientation perpendicular to the S beam. The light of the second scan beam propagates through the polarization sensitive beam splitter past the internal surface with nearly 100% efficiency. Following the PSBS, the polarization of the light is further rotated by 90 degrees by quarter wave plate 78 and reflected back by retroreflector 79. On return, the - retroreflected beam is again polarization rotated by an additional 90 degrees by 0 the quarter wave plate to be polarized in the "S" orientation, as was the first scan beam. The PSBS internal surface reflects the now S polarized second scan beam which transits a focusing lens 80 and presents the same to the substrate.

11 The present system takes advantage of the above spinner by including an acousto-optical device 81 which receives switching signals from the controller to change the polarization of the input beam between clockwise and counterclockwise polarizations. Since the preferred encoder generates a once per revolution signal, the controller now enables presentation of the modulated beam at two predetermined times during each revolution of the spinner, as opposed to once per revolution. Similar changes to the other components and system parameters are accomplished as well, including a doubling of the advancement speed in the slow scan direction.

Similarly, although the invention has been shown and described with respect to a preferred embodiment thereof, it would be understood by those skilled in the art that other various changes omissions and additions thereto may be made without departing from the spirit and scope of the present invention.

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Claims

We Claim

1. A scanning optical system (10) receiving a circularly polarized optical beam (62), the system having a curved platen for receiving a substrate (21), a raster scanning means (22) responsive to advancement control signals and including a spinner (60) for advancing, relative to said substrate (21), said optical beam (62) across a substrate surface (14) in a first direction forming a scan line (38) and for cooperatively advancing, relative to said substrate surface, said optical beam in a second direction substantially perpendicular to said first direction, a modulator means for providing modulation to the optical beam in response to received modulator control signals, a controller (26) receiving encoder signals, for generating said advancement control signals and said modulator control signals, said scanning optical system characterized by a means for switching (81) said optical beam polarization between first and second directions in response to polarization control signals, an encoder means (28) for generating said encoder signals indicative of the angular position of said spinner about a scan axis (40) and consequently indicative of the optical beam along a current scan line, said controlle further providing said optical beam polarization switching signals in dependence on said encoder signals such that said optical beam polarization is switched between said first and second direction after the completion of the current scan line, said spinner receiving said optical beam from said polarization switching means and including a quarter wave plate (64) for receiving said optical beam and providing a linearly polarized scan beam, a polarization sensitive bearnsplitter (70) reflecting said linearly polarized scan beam at internal beamsplitter surface (72) if said linearly polarized scan beam is polarized in a fixst linear direction, a quarter wave plate (78) receiving said hnearly polarized scan beam from said polarization sensitive beamsplitter if 13 said linearly polarized scan beam is polarized in a second linear direction orthogonal to said first linear direction, said quarter wave plate (78) rotating said second direction polarized scan beam by ninety degrees, and a retroreflector means (79) for returning said ninety degree rotated second direction polazized scan beam through said quarter wave plate (78) to said polarization sensitive bearnsplitter to reflect said retroreflected ninety degree rotated second direction polarized scan beam off of a second internal bearnsplitter surface and in a direction opposite to the direction said linearly polarized scan beam travels from said first internal bearnsplitter surface, thereby generating two scan beams per bearnsplitter revolution about said scan w6s.

2. The scanning optical system of claim 1 wherein said platen is further characterized by an internal drum (12) of a raster photoplotter.

3. The scanning optical system of claim 1 ftirther characterized by first and second lenses (74, 80) positioned in opposed relation to one another to receive a respective one of said first or second direction polarized scan beams e3dting said polarization sensitive beamsplitter.

4. The scanning optical system of claim 1 further characterized by an optical source (36) for generating said circularly polarized optical beam (24).

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5. The scanning optical system of claim 4 further characterized by a means for cooperatively advancing relative to said substrate said optical beam in a second direction substantially perpendicular to said first direction and a second encoder means for generating signals indicative of a current position of said raster scanning means along a drum longitudinal a-Nis.

6. The scanning optical system of claim 1 further characterized by a second encoder means for generating signals indicative of a current position of said raster scanning means along a longitudinal a3ds.

7. A scanning optical system comprising an optical source for generating a polarized optical beam.

8. A scanning optical system substantially as described herein with reference to the drawings.

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