WO1985005750A1

WO1985005750A1 - Apparatus for producing clock signals for scanner

Info

Publication number: WO1985005750A1
Application number: PCT/US1984/001732
Authority: WO
Inventors: David Kessler
Original assignee: Eastman Kodak Company
Priority date: 1984-05-29
Filing date: 1984-10-29
Publication date: 1985-12-19
Also published as: US4616132A; EP0185662B1; DE3481030D1; EP0185662A1; JPH0614661B2; JPS61502300A

Abstract

Apparatus for producing clock signals which are used to precisely control the modulation of a first light beam as it is line scanned by a moving mirror. The apparatus employs a second beam of light which is reflected from the moving mirror and is intensity modulated by a grating. A paraboloid mirror recives such intensity modulated second beam and reflects a collimated beam. A lens focuses such collimated beam to form a stationary image on the surface of a detector which produces the clock signals.

Description

APPARATUS FOR PRODUCING CLOCK SIGNALS FOR SCANNER Technical Field

This invention relates to apparatus for providing clock signals which precisely control the modulation of a laser beam as it is line scanned. Background Art

Optical printing systems use output scanner systems wherein the intensity of a laser light beam focused on a moving two-dimensional photosensitive surface is modulated as the beam is line scanned relative to such moving surface to provide a two- dimensional output image. In one common output scanner system, a deflector, such as a rotating polygon mirror, line scans a beam of laser light. The intensity of -such scanned light beam is modu¬ lated by an acoustooptic cell. Precise synchroniza¬ tion clock signals are necessary to represent the position of the laser beam as it is line scanned. The timing of the modulation of the laser beam is controlled by these clock signals. The clock sig¬ nals control the flow of information from an elec¬ tronic data buffer to the modulator. One common technique used to provide clock signals is a grating clock. In this technique, a second unmodulated beam is also reflected off the rotating mirror surface and scans a grating that intensity modulates the second light beam. A mirror projects the intensity modulated second light beam onto the surface of a detector which provides the synchronization clock signals as the line scan progresses. This system offers a number of advantages in that the clock sig¬ nals produced are representative of the instantane¬ ous beam position. U.S. Patent 3,835,249 discloses such a system. One problem with this system is that the light image formed on the photodetector surface will move or wobble as the second light beam is line scanned. Since the photosensitivity of the detector surface may change from position to position, the modulated moving light image may interlace noise into the clock signals.

The object of this invention is to provide relatively noiseless clock signals for use in syn¬ chronizing the modulation of a first light beam as it is line scanned. Disclosure of the Invention

This object is accomplished by an apparatus which uses a laser light beam intensity modulated by a grating and a paraboloid mirror which receives such intensity modulated beam and projects it as a collimated beam.. Lens means focuses the collimated beam, as a stationary image, onto the surface of the detector. The detector produces relatively noiseless clock signals.

In order for the grating to sharply modu- lated the light beam, the light beam is shaped so as to have its waist at the surface of the grating. As the light beam is scanned, the instantaneous posi¬ tions of the light beam waists locus describe a curve. The grating is curved so that its curvature matches such curve.

Brief Description of the Drawing

The drawing shows a simplified top plan view depicting an output polygon scanner and an apparatus for producing synchronization clock signals. Ways of Carrying Out the Invention

As shown in the drawing, a first laser light beam 10 from laser 43 which is modulated by an acoustooptical modulator 14 and shaped by lens 44, is reflected off a mirrored facet of a rotating polygon 15 through optical element 25 and line scan¬ ned onto a photosensitive member M. The photosen¬ sitive member M is moved in a direction perpendicu- lar to the line scan by means not shown to provide a two-dimensional image. Clock signals are needed to control th operation of modulator 14 in accordance with the position of the beam 10 in the line scan. A second unmodulated laser beam 12 from laser 42, or split off laser 43, is focused by lens 24 on a curved grating 16 after being reflected off a mirrored facet of the polygon 15. To reduce the tolerance for facet power variations and other deformations in the facets, the second light beam 12 is generally in coincidence with the first light beam 10 at a polygon facet but at a different inter¬ section angle than the first light beam. Lens 24 forms a beam waist of beam 12 at the grating 16. The grating 16 is located along the path of the second light beam 12 between the polygon 15 and a paraboloid mirror 18. Grating 16 is curved so that its curvature matches the locus of the scanned waist positions. As beam 12 is scanned, light passes through the grating. The grating thereby intensity modulates the scanned beam 12. The scanned beam 12 is then reflected off the paraboloid mirror 18 which has its focal point at or in the vicinity of the intersection of beam 12 and a polygon facet. The paraboloid mirror 18 receives the beam at an off axis position, e.g. a position spaced from the posi¬ tion where the paraboloid axis would engage the paraboloid surface. Because the beam 12 passes in the vicinity of the focal point of the paraboloid and is received by the paraboloid at an off-axis position, the paraboloid mirror collimates the scanned beam 12 which is then collected and focused by a lens 20 onto the surface of detector 22. The detector surface and the facet are coj_^ugate to pro¬ vide a stationary image on the detector surface thereby eliminating beam wobbling on the detector surface. The detector 22 produces relatively noise¬ less clock signals in response to the modulation of the stationary image and inputs them to an amplifier 32. Clock signals from amplifier 32 gates data from the storage buffer 34 to a power amplifier 38. It will be understood that the storage buffer 34 in¬ cludes a conventional digital-to-analog converter (not shown) . The storage buffer receives digital data representative of the information to be record- ed. on the member .M from an information source not shown. The amplifier 38 provides driving analog signals to the acoustooptical modulator 14 in the usual manner. The acoustooptic modulator creates a diffraction grating that causes a first-order beam to be diffracted out of the original beam. Beam 10 is actually the first-order beam. Beam 10 is information-wise modulated in accordance with the data delivered from the buffer 34. Industrial Applicability Apparatus for providing clock signals for synchronizing the modulation of a light beam as it is line scanned across an image zone is useful as a product, for example, in an output laser printer which makes prints of photographic negatives and in electrostatic copiers and duplicators. The apparatus has the advantage of producing precise, relatively noiseless clock signals.

Claims

Clai s:

1. Apparatus for providing relatively noiseless clock signals for synchronizing the modu¬ lation of a first light beam with its position as it is line scanned by a moving reflective surface, in¬ cluding means for projecting a second light beam off said moving reflective surface as it line scans the first light beam, an optical grating positioned to intercept said scanned second light beam for in- tensity modulating such light beam, characterized by: a paraboloid mirror disposed so that it receives such intensity modulated second light beam and pro¬ jects a collimated light beam; first lens means which receive such collimated light beam and focuses it at a plane to form a stationary image; and a detector having a surface disposed to receive such stationary image and which in response to the mod¬ ulation of such stationary image, produces the clock signals.

2. In apparatus which includes a moving reflective surface that line scans an information- wise modulated first laser light beam across a photosensitive surface, means for projecting a second laser light beam onto said reflective surface generally in coincidence with said first light beam but at a different intersection angle than said first light beam, a grating located in the path of said second light beam which intensity modulates said second light beam, characterized by: a parabo- loid mirror disposed to have its focal point in the vicinity of the reflective surface and receive said intensity modulated second light beam at an off-axis position so as to collimate said second light beam; lens means for collecting the modulated collimated second light beam and forming modulated stationary

OMPI light image which is conjugated with the position of the reflective surface; and means including a detector which receives the modulated stationary light image for producing said clock signals.

3. The invention as set forth in Claim 2, wherein said grating has a curved surface and said projecting means includes second lens means which forms a beam waist of said second light beam_^on said curved grating surface while said second light beam is scanned.

4. Apparatus comprising a grating which intensity modulates a laser light beam, a paraboloid mirror which receives such intensity modulated light beam and projects it as a collimated beam, and lens means which focuses the collimated beam as a sta¬ tionary image onto the surface of a detector which produces clock signals.