Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Utility Patent
1999-03-31
2001-01-02
Le, N. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S239000
Utility Patent
active
06169565
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to laser printers utilizing total internal reflecting (TIR) electro-optic spatial light modulators, and in particular to a system with a multi-mode laser diode array source, illumination optics, imaging optics, and a TIR spatial light modulator optimized to work with a partially coherent laser source.
BACKGROUND OF THE INVENTION
Laser printers, particularly flying spot printers, are used in a wide variety of applications, from office printing, to medical printing, to bar code printing. These systems typically print with a single lower power beam, onto fairly sensitive media. Other systems, such as those used in the graphic arts industry, often are configured as multispot printers to attain sufficient productivity. As many of the graphic arts media are rather insensitive to light exposure, each of these incident beams must provide a high light level in a small spot at the printing plane. The printer is typically configured like a “lathe”, where the page scan is obtained by rotating a drum which holds the media, and line scan, by translating the multiple laser beams in a direction parallel to the axis of rotation of the drum.
There are several approaches to solving the graphic arts printer design problem. In one approach, each of the laser sources is separately coupled to optical fibers, which are then mounted to form a linear array of sources. Each of these channels can then be independently modulated. Such systems are described in U.S. Pat. Nos. 4,900,130 and 5,351,617. Another approach is to utilize a monolithic array of laser sources and then image the elements of the laser array directly onto the light sensitive media to produce multiple spots. Power to each element of the laser array is individually modulated to obtain pixel densities. Such a system, described by U.S. Pat. No. 4,804,975 is potentially of lower cost and higher efficiency as compared to systems which couple the lasers to optical fibers. However, these systems are significantly disadvantaged as the failure of even one lasing element or emitter of the laser diode array source will appear as an artifact in the printed image, requiring replacement of the entire laser array source.
One approach to improving a printing system using a monolithic diode array source is to split each lasing element or emitter into an array of subarray sources, such as described in U.S. Pat. No. 5,619,245. Each writing element is assembled from the combined light of all the lasing elements or emitters of a given subarray, and each of the subarrays are directly and individually modulated to provide the image data output. This approach desensitizes the system to the failure of the lasing elements or emitters within a subarray.
Another approach to improving a system with a monolithic diode array source is to combine the light from each lasing element or emitter to flood illuminate a linear spatial light modulator array. The pixel elements of the modulator break up the light into image elements, and each pixel of the modulator is subsequently imaged onto the media plane to form the desired array of printing spots. Printing systems employing this approach are described in U.S. Pat. Nos. 4,786,918, 5,517,359 and 5,521,748. These systems improve upon the prior art designs by providing indirect light modulation means, so that the laser diode array operates at full power, and serves only as a light source. Also, as the light from the emitters overlaps in illuminating the modulator, the resulting redundancy desensitizes the system to the failure or poor behavior of any of the lasing elements or emitters within the array.
The performance of such systems, in which a linear spatial light modulator array is flood illuminated, is highly dependent on both the design of the illumination system and the design and operation of the modulator array. Optimally, the illumination system should provide highly uniform illumination with minimal loss of brightness. In U.S. Pat. No. 4,786,918, the Gaussian beams from many single mode lasers are combined in the far field to create a broad and generally slowly varying illumination profile, but one which still falls off in a generally Gaussian manner. The array of single mode lasers is carefully structured so that the beams from the individual laser sources are mutually incoherent, and therefore they can be superimposed without interference. Such a structure may provide the effect required of a relatively incoherent source that may be used in conjunction with a spatial light modulator. However, great care needs to be taken to guarantee that the source does not exhibit any phase locking, or coherence effects. Additionally, the modulator will require extremely uniform illumination in order to avoid streaking in the images. While this may be achievable within the constraints shown in U.S. Pat. No. 4,786,918, the care, detail and effort required may render the system expensive and difficult to maintain in a manufacturing environment.
U.S. Pat. No. 5,517,359 provides for a printing system with a laser diode array consisting of multimode emitters, each of which typically has a rather non-uniform near field profile. A mirror system, included in the illumination optics, partially improves the light uniformity by substantially removing the macro-nonuniformities in the light profile. Another method, as described in U.S. patent application Ser. No. 08/757,889, filed Nov. 27, 1996, and assigned to the same assignee as the present invention, also describes a printing system with a laser diode array including multimode emitters, but with an illumination system utilizing a fly's eye integrator. With the fly's eye integrator, both the micro and macro light non-uniformity can be substantially improved.
Given that the illumination optics efficiently provides a uniform illumination of the linear spatial light modulator, the overall system performance is highly dependent on the design and operation of the spatial light modulator array. Generally, candidate technologies for a spatial light modulator to be used in a laser printer for graphic arts should be highly transmissive with a high optical fill factor, have high thresholds for optical damage and altered behavior under exposure to high optical energy densities, and provide sufficiently high modulation contrast at high data rates. There are both electromechanical and electro-optical modulator technologies which meet these various criteria for use in a laser thermal printer.
Specifically, the TIR modulator, as described in U.S. Pat. Nos. 4,281,904 and 4,376,568, which is of the electro-optic variety, has many traits which lends itself to use in a laser thermal printer. These devices are transmissive modulators used with schlieren optics, produced from Lithium Niobate (LiNbO3) or Lithium Tantalate (LiTaO3), which can be designed for a high optical fill factor. Lithium Niobate is highly transmissive in the near infrared, and it has a high threshold to optical damage. Furthermore, the TIR modulator, as described by U.S. Pat. No. 4,376,568, is a device which modulates the light by imposing a grating structure on it when an electric field is applied. As a result the light is diffracted, and the modulated light is separated from the unmodulated light by spatial filtering at a fourier plane later in the optical system. As the TIR modulator uses schleiren phase modulation, as opposed to directly absorbing or blocking the light, the thermal load on the modulator is greatly reduced.
The TIR modulator as described in U.S. Pat. Nos. 4,281,904 and 4,376,568 performs admirably when illuminated by light from a highly coherent source. However, the high power laser sources needed for laser thermal printing applications are at best partially coherent.
SUMMARY OF THE INVENTION
An object of the present invention is to provide for a laser thermal printing system where both the optical system and the TIR modulator are designed to optimally work together when the light used therein is partially coherent.
The present invention relates to a printi
Kurtz Andrew F.
Ramanujan Sujatha
Blish Nelson Adrian
Eastman Kodak Company
Le N.
Nguyen Lamson D.
Novais David A.
LandOfFree
Laser printer utilizing a spatial light modulator does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Laser printer utilizing a spatial light modulator, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Laser printer utilizing a spatial light modulator will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2519696