Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2002-02-06
2004-09-28
Tran, Huan (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
Reexamination Certificate
active
06798438
ABSTRACT:
Priority to German Application No. 101 11 871.6, filed Mar. 13, 2001 and hereby incorporated by reference herein, is claimed.
BACKGROUND INFORMATION
The present invention is directed to a device for recording images on a printing form, including an array of light sources and imaging optics for generating (n×m) imaging spots on the printing form, n>1 and m>=1 being natural numbers.
The time it takes to record an image on printing forms, whether it is in a printing-form imaging unit or in a print unit, is substantially determined by the number of available imaging channels. For that reason, typical image-recording devices have a plurality of light sources, at least one light source being assigned to an imaging channel. By way of an image-recording channel, an imaging spot is projected onto the surface of a printing form, enabling printing dots to be placed on the printing form. Many documents in the technical literature deal with the multi-beam image-recording of printing forms.
U.S. Pat. No. 5,291,329, for example, describes a device for recording images, where a plurality of laser light sources configured in a two-dimensional array are imaged using an optical system onto a recording surface to generate a plurality of imaging spots. The laser light sources are typically configured on a spherical-shell sector such that the main beam of any one laser passes through a focus of a first lens of the optical system.
To reduce the pitch of imaging spots, U.S. Pat. No. 5,995,475 discusses a two-dimensional array of individual multimode laser diodes having separate collimating lenses. The array is imaged at a large reduction ratio onto a recording medium. In this instance, the imaging optics includes anamorphic, optical elements to form the beam of the divergent laser light.
What these image-recording devices have in common is that they use edge-emitting diode lasers to generate the laser radiation. A disadvantageous property of edge-emitting diode lasers is, however, that they exhibit laser radiation having a large angle of divergence and a pronounced astigmatism, making for a complex focusing of the laser radiation and usually requiring an expensive optical lens system. Moreover, edge-emitting diode lasers must first be broken out of a wafer and mounted, prior to performing a functional test. These manufacturing steps are, in part, very expensive; they reduce manufacturing efficiency, and, as a result, increase the price of the lasers.
While conventional semiconductor lasers are edge emitters, thus the light is propagated perpendicularly to the surface of the p-n junction, and the light emerges perpendicularly from the cleavage faces of the chip, so-called surface-emitting laser diodes (VCSEL laser diodes, vertical-cavity surface-emitting lasers) are known, which emit light perpendicularly to the wafer surface. The cavity resonator axis is disposed in parallel to the surface of the p-n junction. In the context of this description and of the device according to the present invention, the concept of a VCSEL light source may embrace all diode lasers, whose beam direction is at right angles to the active zone. These may be, in particular, surface emitters, whose resonator cavity length is short in comparison to the thickness of the active zone, surface emitters, whose resonator cavities are monolithically lengthened, or surface emitters, which have an external or a coupled resonator cavity (also referred to as NECSELs). Furthermore, a VCSEL light source may be a diode laser, whose resonator cavity is situated essentially in parallel to the active zone and is provided with a diffracting or reflecting structure which couples out the laser radiation at right angles to the active zone.
Generally, for VCSEL light sources, it holds that the active length of the resonator can be very short, typically only a few micrometers, and that highly reflecting resonator cavity mirrors are needed to obtain small threshold currents. Surface-emitting laser diodes, VCSEL light sources, have numerous interesting properties. By employing an extremely short resonator cavity, often of a length of less than ten micrometers, one achieves a large longitudinal mode spacing, which promotes single-mode emission above the laser threshold. By using a rotationally symmetric resonator cavity of, typically, six to eight micrometers diameter, one obtains a circular near field and—due to the relatively large diameter—a small beam divergence. Moreover, the geometry of the laser permits a simple monolithic integration of two-dimensional VCSEL laser-diode arrays. Finally, it is possible to test the lasers directly on the wafer following manufacturing.
The typical layer structure of a surface-emitting laser is known to one skilled in the art and can be taken from the relevant literature (see, for example, K. J. Ebeling “Integrierte Optoelektronik” [Integrated Opto-electronics], Springer Publishers, Berlin, 1992). EP Pat. 090 5 835 A1, for example, describes a two-dimensional array of VCSEL light sources which are individually addressable or controllable.
Typical VCSEL light sources have, however, only a modest output power. To increase the attainable output power and to restrict a laser to oscillate in its fundamental mode, U.S. Pat. No. 5,838,715 describes a special resonator cavity form for a VCSEL layer structure. However, the drawback of a procedure of this kind is, inter alia, the expensive manufacturing.
In this connection, it is worth mentioning that it is also known from the literature that light from a plurality of emitter diodes is combined to generate an intensive light beam using imaging optics. For example, U.S. Pat. 5,793,783 describes how light from a plurality of light sources or subarrays of light sources in one array is converged into an overlapping focus.
In addition, it is established from the literature, for example, from U.S. Pat. No. 5,477,259, that an array of light sources can be combined from individual modules of subarrays. Typically, this constitutes a row, thus, one-dimensionally configured laser diodes, which are fixed side-by-side on a mounting element, resulting in a two-dimensional array of light sources.
There are usually two classes of image-recording methods for recording images on a printing form using a one- or two-dimensional array of imaging spots. The first class is based on a dense arrangement of imaging spots of the image-recording light sources. In other words: The spacing among the image-recording points corresponds to the spacing among the printing dots to be set. The two-dimensional printing form to be recorded on is then covered in a translatory movement in the two linearly independent directions defining the surface. The second class of image-recording methods is distinguished by the spacing among the imaging spots being greater than the spacing among the adjacent printing dots to be set. Therefore, a complete recording of images on the two-dimensional printing form requires that the imaging spots of one specific imaging step come to rest between imaging spots of an imaging step that preceded in time. Methods of this kind are also called interleaving methods. U.S. Pat. No. 4,900,130 is cited here as an example of such an interleaving method. This document discusses both a one-dimensional as well as a two-dimensional method for interleaving raster scan lines for a one- and/or two-dimensional array of light sources, whose imaging spots are situated on a recording medium at a larger spatial interval than adjacent printing dots.
SUMMARY OF THE INVENTION
An object of the present invention is, therefore, to create a device for recording images on a printing form which is able to record using a multiplicity of image-recording channels and whose light exhibits advantageous beam properties, and/or whose focal points are able to be generated using simple optics. It is additionally or alternatively intended to achieve a long system service life and to render possible inexpensive repairs, in the event of partial failures.
The present invention provides a device for
Beier Bernard
Vosseler Bernd
Davidson Davidson & Kappel LLC
Heidelberger Druckmaschinen AG
Tran Huan
LandOfFree
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