AU756499B2 - Diode-pumped imaging system - Google Patents

Description

-1- DIODE-PUMPED IMAGING SYSTEM BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to digital printing apparatus and methods, and more particularly to a system for imaging of recording media such as lithographic printing members.

Description of the Related Art 0Imaging devices that utilize laser power sources frequently deliver the output of the laser to its destination using an optical fiber arrangement. This frees the designer from the need to physically locate the lasers directly adjacent the recording medium. For example, U.S. Pat. Nos. 5,351,617 and 5,385,092 (the entire disclosures of which are hereby incorporated by reference) disclose the use of lasers *ooo to impress images onto lithographic printing-plate constructions. As described in these patents, laser output can be generated remotely and brought to the recording blank by means of optical fibers and focusing lens assemblies.

It is important, when focusing radiation onto many types of recording medium, to maintain satisfactory depth-of-focus--that is, a tolerable deviation from perfect focus on the recording surface. Adequate depth-of-focus is important to construction and use of the imaging apparatus; the smaller the working depth-of-focus, the greater will be the need for fine mechanical adjustments and vulnerability to performance degradation due to the alignment shifts that can accompany normal use. Depth-offocus is maximized by keeping output beam divergence to a minimum.

Optical efforts to reduce beam divergence also diminish power density, since a lens cannot alter the brightness of the -2radiation it corrects; a lens can only change the optical path. Thus, optical correction presents an inherent tradeoff between depth-of-focus and power loss. U.S. Pat. No.

5,822,345 discloses an approach that utilizes the divergent output of a semiconductor or diode laser to optically pump a laser crystal, which itself emits laser radiation with substantially less beam divergence but comparable power density; the laser crystal converts divergent incoming radiation into a single-mode output with higher brightness. The output of the laser crystal is focused onto the surface of a recording medium to perform the imaging function.

The arrangements described in the '345 patent employ a separate crystal for 10 each diode pumping source. This is ordinarily necessary due to the nature of laser crystals and their operation. In the absence of optical excitation, resonant cavities S. formed from these optical-gain crystals are flat-flat monoliths; when optical power is delivered to an end face of such a crystal, however, this and the opposed face bowan effect called bulk thermal lensing. To obtain a single transverse mode of operation (preferably the lowest-order, fundamental TEMoo 00 mode), with the output divergence as close as possible to that of a diffraction-limited source, the crystal must be implemented in a design that accounts for bulk thermal lensing.

This phenomenon makes it even more difficult to obtain multiple, independent S. outputs from a single laser crystal. Even if the energy of each pumping source is confined to a discrete region on one of the crystal faces, the thermal lensing action required for lasing in one region will ordinarily affect the other regions, resulting in mutual interference. This condition is known as "thermal crosstalk." Accordingly, the current state of the art prescribes the use of a separate crystal for each laser channel, resulting not only in added cost for the crystals and their mounts, but also for separate focusing assemblies.

In addition, the configurations described in the '617 and '092 patents (and, somewhat more pertinently, in U.S. Pat. No. 5,764,274) contemplate permanent affixation of the diode laser packages to the optical fiber. This is due to the need for efficient coupling of the laser energy into the end face of the fiber. Components are therefore permanently joined so that the alignment therebetween remains undisturbed during operation. Should a diode fail, not only the diode but the entire optical-fiber assembly must be replaced. Such a requirement is of little consequence in the arrangements described in the '274 patent, since the the fiber is coupled to a -3focusing assembly using an SMA connector or the like, which is conveniently removed and replaced. In arrangements having fiber outputs that are less accessible or which require more involved mounting operations, however, permanent diode affixation at the input side of the optical fiber can prove decidedly disadvantageous.

DESCRIPTION OF THE INVENTION Summary of the Invention According to a first aspect of the present invention there is provided a laser diode package facilitating removable coupling of an optical fiber extension to the diode, the package comprising: a. a laser diode; b. a microlens associated with the laser diode; c. a connecting optical fiber having first and second ends, the first end S. 15 receiving radiation exiting the microlens and the second end emerging from the package; and d. means for aligning an end of the optical fiber extension with the second end of the connecting optical fiber and maintaining contact therebetween.

20 The aligning means may comprise a spring for biasing the end of the optical fiber extension towards the second end of the connecting optical fibre. Optionally, the aligning means may further comprise a mechanical member that urges the optical fiber extension end to contact the second end of the connecting optical fiber.

The second end of the connecting optical fibre or the optical fiber extension end may optionally have an index-matching fluid coating.

Thus this invention provides for removable affixation of the pumping laser diodes at the input ends of the fibers. Pumping laser output is coupled into a fiber whose other end face is butted against the anterior face of the crystal. This avoids the need for permanent mounting of the fibers that conduct the pumping energy to the crystal.

-4- BRIEF DESCRIPTION OF THE DRAWINGS The foregoing discussion will be understood more readily from the following detailed description of the invention, when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a plan schematic illustration of the basic components of the invention in a representative implementation; 10 FIG. 2 is an isometric view of a crystal adapted to receive four separate inputs .o.o•i without substantial crosstalk; FIG. 3 is a sectional view of a first structure for removably coupling an optical fiber to a laser pumping diode, with the fiber partially inserted into the structure; and FIG. 4 is a sectional view of a second structure for removably coupling an optical fiber to a laser pumping diode, with the fiber removed from the structure.

*Detailed description of the Preferred Embodiments Refer first to FIG. 1, which schematically illustrates the basic components of the invention, recording medium 50, such as a lithographic plate blank or other graphic-arts construction, is affixed to a support during the imaging process. In the depicted implementation, that support is a cylinder 52 around which recording medium 50 is wrapped, and which rotates as indicated by the arrow. If desired, cylinder 52 may be straightforwardly incorporated into the design of a conventional lithographic press, serving as the plate cylinder of the press. Cylinder 52 is supported in a frame and rotated by a standard electric motor or other conventional means. The angular position of cylinder 52 is monitored by a shaft encoder associated with a detector 55. The optical components of the invention, described hereinbelow, may be mounted in a writing head for movement on a lead screw and guide bar assembly that traverses recording medium 50 as cylinder 52 rotates. Axial movement of the writing head results from rotation of a stepper motor, which turns the lead screw a indexes the writing head after each pass over cylinder 52.

Imaging radiation, which strikes recording medium 50 so as to effect an imagewise scan, originates with a series of pumping laser diodes 60, four of which are representatively designated D 1

D

2

D

3

D

4 The optic components discussed below concentrate laser output onto recording medium 50 as small features, resulting in high effective power densities. A controller 65 operates a series of laser drivers collectively indicated at 67 (and described more fully below) to produce imaging bursts when the outputs of the lasers 60 are directed at appropriate points opposite recording medium Controller 65 receives data from two sources. The angular position of cylinder 10 52 with respect to the laser output is constantly monitored by detector 55, which .:o.oi provides signals indicative of that position to controller 65. In addition, an image data source a computer) 70 also provides data signals to controller 65. The image data define points on recording medium 50 where image spots are to be written.

Controller 65, therefore, correlates the instantaneous relative positions of the focused outputs of lasers 60 and recording medium 50 (as reported by detector 55) with the image data to actuate the appropriate laser drivers at the appropriate times during scan of recording medium 50. The driver and control circuitry required to implement this scheme is well-known in the scanner and plotter art; suitable designs are described in the '092 patent and in U.S. Pat. No. 5,174,205, both commonly owned with the present application and hereby incorporated by reference.

The output of each of the lasers 60 is conducted, by means of an optical fiber 721, 722, 723, 724, to an alignment bench 75 that has a series of parallel grooves 77 for receiving the fibers. Bench 75, which may be fabricated from materials such as metal or silicon, is aligned with a laser crystal to direct the outputs of lasers 60 at appropriate points on the anterior face 80f of laser crystal 80. Because of the construction of laser crystal 80 as described below, each laser 60 stimulates a separate output from laser crystal 80 without substantial thermal crosstalk.

It is the emissions of crystal 80 that actually reach the recording medium 50. A first lenslet array 82 concentrates the outputs of lasers D 1

-D

4 onto crystal 80, and a second lenslet array 84 concentrates the outputs from crystal 80 onto a focusing lens The latter lens, in turn, demagnifies the incident beams in order to concentrate them further and draw them closer together on the surface of recording medium The relationship between the initial pitch or spacing P between beams from crystal -6and their final spacing on recording medium 50 is given by Pf where Pf is the final spacing and D is the demagnification ratio of lens 85. For example, the grooves 77 of bench 75 may be spaced 400 pIm apart, which also determines the pitch P. If the emagnification ratio of lens 85 is 4:1, then the spots will he spaced 100 pm apart on the surface of recording medium Given the characteristics of currently available laser crystals, four pumping sources per crystal is a preferred configuration. Different configuratons are of course possible, however. Most commercial imaging applications will require more than four 10 simultaneously actuable laser beams. One may therefore employ a writing head o: having multiple crystals (each receiving, for example, four pumping inputs) focused through the same or separate optical components 82, 85 and all advanced by the same lead screw. The use of a series of multiply pumped laser crystals is also favored in order to minimize imaging artifacts, as described below.

A variety of laser crystals can serve in the present invention so long as they S-lase efficiently at the desired imaging wavelength and produce a collimated output.

Preferred crystals are doped with a rare earth element, generally neodymium (Nd), and include Nd:YVO 4 Nd:YLF and Nd:YAG crystals. It should be understood, however, that advantageous results may be obtainable with other laser crystals.

With reference to FIG. 2, laser crystal 80 is modified in order to receive energy from multiple pumping sources and to provide, in response thereto, discrete outputs without substantial thermal crosstalk. Crystal 80 has a series of parallel longitudinal grooves 100 and transverse grooves 101 cut into end face 80f. Grooves 100, 101 may be, for example, 2-10 pm deep and spaced 100 pm apart. (Typically, crystal is 0.5-2.0 mm thick, with a polarization vector Vp oriented as shown.) A pair of metal strips 1021, 1022 extend across face 80f of crystal 80 parallel to grooves 101; a complementary pair of metal strips 1023, 1024 extend across the posterior face of crystal 80. Metals strips 102 may be, for example, gold, 0.8 pIm in height and 0.005 pm thick, and may be applied by vacuum deposition or other suitable means. Their purpose is to thermally couple the contacted regions of crystal to a heat sinking arrangement (such as that disclosed in copending application Ser. No. 08/966,492, filed Nov. 7, 1997, the entire disclosure of which is hereby incorporated by reference).

Grooves 100, 101 define a series of four bounded regions. The outputs of the pumping lasers are desirably directed at the centerpoints 105 of these regions. In response, crystal 80 will produce four separate outputs without substantial thermal crosstalk.

The grouped structure of the laser diodes is advantageously employed to minimize imaging artifacts. These tend to occur at the boundaries between zones imaged by adjacent imaging devices, and reflect slight imperfections in inter-device spacings. The visual effect of these imperfections can be reduced or eliminated by exploiting the inter-device spacing within each array and the spacing between arrays to permit indexing by different amounts. Variable indexing disrupts the periodicity of .,..imaging errors, making them less noticeable.

Suppose, for example, that the array shown in FIG. 1 is one of several arrays in a single writing head, that the pitch in P in each array is 400 pm, and that the 15 demagnification ratio of lens 85 is 4:1 to produce spots spaced 100 pm apart on the A. surface of recording medium 50. Suppose, further, that the desired dot resolution the spacing between adjacent dots on recording medium 50) is 20 pm. Each time cylinder 52 rotates, each of the four diodes 60 produces a column or "swath" of image spots. After a rotation, the array is indexed by 20 pm (the resolution or spotpitch distance), and after the array, has been indexed four times.(so that four columns spaced 20 pm apart have been applied), the entire zone spanned by the array has been imaged. The writing head is then indexed by 300 pm the distance representing the width of the imaging zone. Since the spacing between arrays ordinarily is substantially larger than the zone width, each array will be indexed through multiple zone widths throughout the course of a scan. Because of this variable indexing at both the resolution and zone-width distances), imaging errors will generally be less noticeable as compared with, for example, a system in which the devices are indexed only by the resolution distance.

FIG. 3 illustrates a first mounting structure facilitating removable coupling of any of laser diodes D 1

-D

4 to its respective fiber 721 -724 (see FIG. The structure, indicated generally at 150, guides the output of a laser diode 155 into the end face of an optical fiber without the need for permanent affixation thereto. Mounting structure 150 includes a housing 158 having an interior cavity for receiving the diode package -8- 155, which is permanently affixed therein. Housing 158 contains suitable openings, not shown, that facilitate electrical connection to diode 155.

Diode 155 has an emission slit 160 through which laser radiation is emitted.

Radiation disperses as it exits slit 160, diverging at the slit edges. Generally the dispersion (expressed as a "numerical aperture," or NA) along the short or "fast" axis is of primary concern; this dispersion is reduced using a divergence-reduction lens 165. A preferred configuration is a cylindrical lens; however, other optical arrangements, such as lenses having hemispherical cross-sections or which correct both fast and slow axes, can also be used to advantage.

10 Lens 165 may be bonded directly to diode 155 at slit 160. In front of lens 165 is a sapphire window 168, which is carried at the end of a tubular cartridge 170, forming the end face thereof. Cartridge 170 is received within the interior cavity is of housing 158, and is preferably bonded therein such that the exterior face of window 168 contacts (and may be bonded to) the flat face of cylindrical lens 165. Cartridge 170 and housing 158 are preferably metal.

Cartridge 170 includes a threaded stem 175 for receiving a fiber-optic cable 180 terminating in an SMA (or similar, ST or FC) connector package 182, which includes a threaded collar 184 that is free to rotate. Cable 180 emerges within collar 184 and protrudes beyond the collar, terminating in an end face 180f. (The optical fiber resides within cable 180 and is indicate.by the dashed line.) The length of stem S'175 is chosen such that, with collar 184 fully threaded thereover, the end face 180f of cable 180 makes contact with the interior face of sapphire window 168. Accordingly, if diode 155 fails, its removal need not disturb the optical cabling assembly. Instead, this is simply removed by detaching connector 182, and the diode structure replaced.

FIG. 4 illustrates a second mounting structure facilitating removable coupling of any of laser diodes D 1

-D

4 to its respective fiber 721 -724 (see FIG. Once again, the illustrated structure, indicated generally at 200, guides the output of a laser diode 155 into the end face of an optical fiber without the need for permanent affixation thereto. Mounting structure 200 includes a housing 210 having an interior cavity for receiving the diode package 155, which is permanently affixed therein. Housing 200 contains suitable openings, not shown, that facilitate electrical connection to diode 155.

-9- The emission slit 160 of diode 155 is again directed through a divergencereduction lens 165, which may be a cylindrical lens. Lens 165 is bonded to a length 215 of optical fiber, which exits housing 210 through a ceramic sleeve 218 encased within housing 210. Projecting from housing 210 and concentric with sleeve 218 is a tubular stem 220 having one or more guide slots or channels 222 therein. The fiberoptic cable 180 terminates in a connector 225 having a rimmed or flanged end 227 whose diameter approximately matches the interior diameter of stem 220 (so as to permit connector 225 to be conveniently received within stem 220). A pin 230 projects radially from flange 227 and fits within guide slot 222 as connector 225 10 travels axially within stem 220. The optical fiber carried within cable 180 emerges 0*oo** from connector 225 through a ceramic sleeve 235, which is encased within connector o 225.

The depth of guide slot 222 is chosen such that, before pin 230 reaches the terminus of the slot, the end face of ceramic sleeve 235 makes mechanical contact with the end face of sleeve 218, thereby aligning optical fiber 215 with the optical fiber carried within cable 180. One or both end faces may be coated with an indexmatching fluid a cis-trans mixture of decahydronaphthalene) to ensure proper light transmission through the junction.

In order to ensure maintenance of mechanical contact between the end faces 201 of sleeves 218, 235 notwithstanding the vibrational rigors of a commercial printing.

environment, connector 225 may be provided with a spring 237, one end of which butts against flange 227. The other end of spring 237 is engaged by a mechanical member (not shown) that is urged toward the mounting structure 200. The resulting axial force transmitted to flange 227, the magnitude of which is determined by the spring constant of spring 237, maintains contact between the end faces of sleeves 218, 235. The spring constant of spring 237 is chosen so as to ensure reliable contact without damage to sleeves 218, 235 or, more likely, skew or shifting of the end faces.

It will therefore be seen that we have developed new and useful approaches to the design and operation of multiple-beam, diode-pumped laser systems applicable to a variety of digital-imaging environments. The terms and expressions employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

4

Claims (6)

1. A laser diode package facilitating removable coupling of an optical fiber extension to the diode, the package comprising: e. a laser diode; f. a microlens associated with the laser diode; g. a connecting optical fiber having first and second ends, the first end receiving radiation exiting the microlens and the second end emerging from the package; and h. means for aligning an end of the optical fiber extension with the second 10 end of the connecting optical fiber and maintaining contact therebetween.

2. The package of claim 1 wherein the aligning means comprises a spring for biasing the end of the optical fiber extension towards the second end of the connecting optical fiber.

3. The package of claim 2 wherein the aligning means further comprises a S 15 mechanical member that urges the optical fiber extension end to contact the second end of the connecting optical fiber.

4. The package of any one of claims 1 to 3 wherein the second end of the connecting optical fiber has an index-matching fluid coating.

The package of any one of claims 1 to 3 wherein the optical fiber extension end has an index-matching fluid coating.

6. A laser diode package substantially as described herein in the detailed description with reference to the drawings. DATED THIS TWENTY-NINTH. DAY OF JANUARY 2002. PRESSTEK, INC. BY PIZZEYS PATENT AND TRADE MARK ATTORNEYS