Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Reexamination Certificate
2001-11-06
2003-12-16
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
C359S626000
Reexamination Certificate
active
06665121
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical imaging heads that produce a plurality of light spots on light sensitive medium such as photographic film or a printing plate. The optical head incorporates an array of laser diodes (LDA) as a light source, a Micro-Scramblers Array (MSA) as a beam-shaping element, a Spatial Light Modulator (SLM) for spatial modulation of the light emitted from the LDA in accordance with data representing the image to be obtained, and an imaging lens for imaging the SLM on the light sensitive medium.
BACKGROUND OF THE INVENTION
Multi-channel optical imaging systems are well known in the art and are currently being used in a variety of applications, including imaging of different printing media. Such systems often utilize a LDA and a SLM as a light source. In a typical offset platesetter for example, the LDA is used as a light source for illuminating the SLM, which, in turn, spatially modulates the light in accordance with the pattern to be imaged. The SLM on the other hand, is imaged by an imaging lens on the photosensitive medium, such as an offset printing plate, and thus the desired pattern is achieved.
A LDA can be an array of ordered individual laser emitters, a laser-diode bar, or a laser-diode stack. For simplicity, all these options will be referred to as LDA hereinbelow. A laser diode bar (linear array of laser diode emitters) will be used as an example in the explanations below (
FIG. 1
a
). It will, however, be appreciated by any person skilled in the art, that the embodiments described may be easily scaled up in Y direction to a two-dimensional array such as a diode laser bars stack. All the conclusions drawn for a linear array will be valid for a two-dimensional array. In many applications, such as imaging of thermal offset printing plates, the individual emitters of the LDA are expected to deliver a significant amount of light power, namely, from several hundred milliwatts to several watts. In such cases, multimode laser diode emitters are used.
FIG. 1
a
illustrates a conventional art LDA
310
. The individual emitters
311
are usually with length
between 50 and 150 &mgr;m and height
of approximately 1 &mgr;m. The pitch
of the emitters
311
can be of several hundred micrometers. All references to X, Y and Z directions hereinbelow refer to the coordinate system illustrated in
FIG. 1
b
. The emission properties of the emitters
311
are different in the directions X (parallel to the LDA length) and Y (perpendicular to the LDA length). The beam divergence in the Y direction is usually between 50 and 90 degrees full angle, while the beam divergence in X direction is usually well below 20 degrees full angle. It is commonly accepted for the Y-axis to be termed “fast axis” and the X-axis to be termed “slow axis”. Thus, the orientation of the LDA
310
in an optical system is defined in the XYZ coordinate system by the fast and slow axes. The Z-axis is considered an optical axis of the optical system.
An important part of optical imaging heads utilizing LDA and SLM is the illumination system of the SLM. There are a number of prior art works tackling the issue of illuminating an SLM by LDA, for example U.S. Pat. No. 5,900,981 to Oren et al. and U.S. Pat. No. 5,333,077 to Legar et al.
FIGS. 2
a
and
2
b
schematically illustrate an optical system
20
for illumination of SLM
16
.
FIG. 2
a
schematically illustrates the propagation of the beams in the X-Z plane (the plane of the slow axis) and
FIG. 2
b
schematically illustrates the propagation of the beams in the Y-Z plane (the plane of the fast axis). The main idea in this, as in most illumination systems, is to image each individual emitter
11
on the SLM
16
by overlapping their images, so that each point on the SLM
16
receives light from each emitter
11
. This method of illumination provides a high degree of redundancy, namely, partial or total loss of emitting abilities of one or more emitters
11
of LDA
10
will result in decreased illumination power, but will hardly affect the power distribution over SLM
16
.
An important characteristic of LDA-SLM imaging systems is the uniformity of the SLM illumination. The illumination method of
FIGS. 2
a
and
2
b
provides illumination which, in each point of the SLM
16
, is a sum of the contributions of the individual emitters
11
of the LDA
10
. However, as was already mentioned, a wide range of contemporary applications requires high-power LDAs of multimode emitters. The near field spatial power distribution of a multimode laser emitter along the slow axis is highly non-uniform and variable over time, depending on parameters such as current through the diode, junction temperature, etc. In a laser diode bar there is a tendency to similarity in the near field power distribution between the individual emitters.
FIGS. 3
a
and
3
b
illustrate the spatial power distribution along the slow axis of two 150 &mgr;m long emitter members of a commercially available laser diode bar and clearly indicate this similarity. The illumination pattern of the SLM
16
is an overlap of the images of the individual emitters
11
. Typical power distribution of such illumination pattern along the slow axis direction is illustrated in
FIG. 3
c
. It can be seen that the distribution approximately resembles the power distribution of the individual LDA members and therefore is highly non-uniform.
The non-uniformity of the SLM illumination leads to uneven power distribution among the writing beams, causing undesired image density irregularities. The non-uniformity of the SLM illumination can be compensated by using a SLM with calibration capabilities for each channel. In this case, all channels are equalized to the least illuminated one. The result of such calibration is an even power distribution among the writing channels, at the expense of energy loss. The amount of energy lost is illustrated in
FIG. 3
c
by the hatched area and in some cases can reach several tens of percents.
Another method of obtaining a uniform illumination of the SLM is disclosed in U.S. Pat. No. 6,137,631 to Moulin. The inventor suggests using a glass blade as a mixing means for obtaining substantially uniform illumination. The proposed embodiments include a focusing lens for delivering the light emitted by each emitter of the LDA to the entrance aperture of a single (common to all emitters) mixing glass blade, which should have a significant length (It is well known to any person skilled in the art that good mixing capabilities can be obtained when the length of the blade is at least ten times greater than its aperture diagonal). These features increase the complexity and the cost of the system. The main disadvantage, however, is the significant loss of brightness in the slow axis direction (by factor
/
,
FIG. 1
a
), making the proposed embodiments low energy efficient.
SUMMARY OF INVENTION
The present invention provides a multi-beam, LDA-SLM based optical imaging head with a high degree of illumination uniformity of the SLM.
The present invention additionally provides a high energy-efficient LDA-SLM based, multiple laser-beam recording apparatus, free of image density irregularities due to non-uniform illumination of the SLM.
In a first embodiment, the present invention comprises an optical head. The optical head comprises an array of laser diodes (LDA) where each member of the LDA having a fast axis direction and a slow axis direction of beam divergence, a spatial light modulator (SLM), an imaging lens; and a Micro-Scramblers array (MSA) where each member of the MSA has exit apertures and each member of the MSA is associated with a member of the LDA for scrambling the light only in the direction of the slow axis of the member of the LDA.
In another embodiment, the present invention comprises an external-drum electro-optical plotter comprising an optical head. The optical head comprises an array of laser diodes (LDA) where each member of the LDA having a fast axis direction and a slow axis direction of beam divergence, a spatial light modulator (SL
Cork Russell
Gelbart Daniel
Pilossof Nissim
Creo Il. Ltd.
Hanig Richard
Love Jane M.
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