Illumination System and Method for Spatial Modulators

The present invention is directed to a system and a method particularly for illuminating spatial modulators. In particular, it concerns an optical system for directing radiant energy produced by a laser source, such as a high power laser array, to a spatial modulator for imaging heat or light sensitive media.

It concerns more particularly systems in which a spatial modulator is used in connection with a laser source to produce a column of individually controllable spots as described in U.S. Pat. No. 4,746,942. This column of spots is moved transversally to the motion of the media to produce successive adjacent bands as shown in FIG. 3 of said patent. In order to avoid any noticeable discontinuity between successive bands it is not only necessary that the relative displacement of the bands and the media corresponds exactly to the height of a column of spots but also that all the spots be substantially identical in form and in intensity.

In the axis corresponding to the height of the column of optical gates, the illumination of the modulator should preferably be uniform and the impinging light beam must show very little divergence so that substantially all the light crossing the modulator enters the pupil of the projection optics. The other axis, width axis, demands a highly collimated beam to produce a column of spots measuring 10 microns or less in width. This necessitates the use of a laser source having a resolution of the order of one micron in this width axis.

To expose highly sensitive media such as silver halides surfaces, electrophotographic plates or films, it is possible to use low-power laser diodes. In this case, the gaussian intensity profile can be transformed by optical means as described in U.S. Pat. No. 4,746,942.

Temperature sensitive polymers (offset plates or any heat sensitive media) demand exposure to stronger luminous power. This can be obtained by the use of high power laser diode(s) to illuminate the spatial modulator. The emission profile location of the diode in the direction of the height (also named "fast axis") is of the order of a micron and has a gaussian emission profile. This must match the width of the column of spots.

In the other direction, particularly high power diodes are available with different types of emitters and configurations. With one-emitter diodes the width of the emitted beams can measure from one to several hundred microns. The most powerful diodes have a multiplicity of emitters located along a one-centimeter bar. They can be located serially or in regularly spaced groups. The angular divergence in this axis is small but the angular emission profiles are irregular and vary from a diode to another. Thus, the distribution of the radiant energy is not uniform.

U.S. Pat. No. 5,517,359 is directed to a complex apparatus for coupling a broad emitting area laser diode made of multiple emitters operating in parallel to a linear light valve. The apparatus images each one of the emitters onto the linear light valve, superimposing them in order to increase the immunity to defects occurring in any individual emitter.

It is the object of the present invention to provide an improved system and an improved method for illuminating a spatial modulator.

This object is achieved with an illumination system and an illumination method according to the respective claims.

The means utilized with low power laser diodes are not applicable with high power lasers because their distribution profiles are not defined. It could be assumed that with laser diodes having a plurality of emitters the superposition of the luminous energy of different diodes (as can be obtained by a simple lens in front of the diode as shown in FIG. 1), could produce substantial uniform light distribution. However, for a given diode, emitters basically have the same characteristics and their superposition shows the same profile as an individual emitter and is consequently non-uniform. In addition, an important part of the energy is lost on the edges.

This invention makes it possible to produce a substantially uniform energy distribution with a well defined width, a very small loss and an optimal collimation.

The invention has the advantage that it provides a system and a method for obtaining a substantially uniform light distribution on the total height of at least one modulator associated with any type of power laser diode. The system preferably includes various lenses associated with an optical mixer preferably in the form of a glass blade to equalize by multiple internal reflections the light energy of the laser emitters. The glass blade is preferably made of boronsilicate-glass and has in a particularly preferred embodiment a width of approximately 2.5 mm and a length of approximately 75 mm.

A preferred laser source emits the radiant energy in the infrared spectrum with a wavelength of approximately 800 nm to approximately 850 nm, preferably with a wavelength of approximately 808 nm. The preferred laser source is further a high power diode having a plurality of emitters arranged in a linear array.

A preferred focusing means comprises a collimator with a cylindrical lens for collimating the radiant energy in the vertical dimension and an array of cylindrical lenses substantially aligned with each emitter of the high power diode for collimating the radiant energy in the horizontal dimension. The cylindrical lens for collimating the radiant energy in the vertical dimension is preferably an asperic lens. Alternatively, an array of lenses could be used for collimating the radiant energy in both horizontal and vertical dimensions.

In a furthermore preferred embodiment there is provided a pair of additional cylindrical lenses between the focal point and the input of the mixing means to form an image of the radiant energy source at the input of the mixing means and to obtain substantially the same divergence for the radiant energy being emitted from the laser source.

Other objects, features and advantages of the invention will be apparent from the following detailed description and drawings which show by way of illustration, and not limitation, preferred embodiments of the invention.

In the drawings preferred embodiments of the present invention are exemplified. Same reference numerals in the figures represent identical or similar components.

FIG. 1 represents a laser having a plurality of emitters.

FIG. 2 represents the energy distribution from the laser of FIG. 1.

FIG. 3 is a schematic representation of the new illumination system in a first plane parallel to the vertical axis of the system.

FIG. 3a represents the same system in a second plane parallel to the horizontal axis of the system.

FIG. 4 represents a portion of the path of the light rays through the optical system, with the path the rays would follow if not subjected to multiple reflection inside the mixing means.

FIG. 4a represents the path of the light rays to the entrance of the blade.

FIGS. 4b and 4c represent the light distribution curve at the entrance of the blade.

FIG. 5 illustrates a further improved system of FIG. 3 in a plane parallel to the horizontal axis of the system.

FIG. 5a shows the light rays entering a mixing means in the improved system with the path the rays would follow if not subjected to multiple reflection inside the mixing means.

FIG. 5b represents schematically the power distribution at the output of the mixing means.

FIG. 6 represents along the fast axis, the path of light rays emerging from a power laser diode with multiple lenslets.

FIG. 6a represents along the slow axis the same arrangement as in FIG. 6.

FIG. 1 shows a laser 1 with several emitters, and FIG. 2 the respective energy distribution of the laser in the direction of the height.

FIGS. 3 and 3a represent the illumination system as per the present invention in the case of a laser diode 1 with several scattered emitters (reference numeral 1' in FIG. 3a). The fast axis shown in FIG. 3 is collimated by a cylindrical lens 2 which is preferably aspherical. Typical fast axis rays after collimation by lens 2 are represented by rays (8).Such a laser diode 1 and the cylindrical lens 2 are available, e.g. from the company Jenoptik Laseroptik GmbH in Germany, having the brand name JOLD-32.

For diodes having spaced groups of emitters or large section emitters, such as for example groups or emitters 150 microns wide, spaced by 500 microns, an array of cylindrical microlenses 12 (FIG. 4) spaced according to the emitter spacing, can be located between lenses 2 and 3 of FIG. 6. This array permits the reduction of the divergence of the bundle at the entry to the modulator. Such an array of lenses 12 is available, e.g., from the company Limo, Germany.

In the other axis (as shown in FIG. 3a) the cylindrical lens 3 is located after the collimation lens 2. The light rays emerging from each emitter 1' merge at the focal point 9 of the lens. Typical slow axis rays after collimation by lens 2 are represented by rays (8').

The bundle of rays produced by the different emitters enters into a mixing means 4 the width of which is substantially equal to the width of the ray bundle at this point 9. The mixing means 4 is most preferably a glass blade 4, which is preferably made of boronsilicate-glass. Upon entry of the light rays into the input or input end of the glass blade 4 they are subjected to multiple reflections inside the glass blade 4 so that the distribution of the radiant energy is uniform. In the shown embodiment the different luminous intensities are mixed at the output or exit end of glass blade 4. The image of this output is formed on modulator 7 by cylindrical lenses 5 and 6. Lens 6 is more specifically used to collimate the light bundle impinging the modulator 7. It will be clear to the person skilled in the art that the glass blade 4 can be replaced by mirrors or can have other appropriate forms which may be standard forms.

FIG. 4a represents the path of the light rays to the entrance of the mixing means and FIGS. 4b and 4c the light distribution at the entrance.

In FIG. 3 where the general configuration is shown, the width of the bundle at point 9 is not exactly defined and the divergence of rays emerging from the edges of the diode 1 is more important that those from the center (see angles (b and (c in FIG. 4). Because the divergence of the bundle at the output of the blade corresponds to the maximum divergence at its input with a blade 4 having parallel sides, this arrangement does not necessarily lead to the optimum collimation of the rays on the modulator as well as maximum power efficiency. This arrangement can be further improved as shown in FIGS. 5, 5a, 6 and 6a where a pair of additional cylindrical lenses 11 is located between the focal point 9 of lens 3 and the input 13 of blade 4. This pair of lenses is designed and located to form an image of the emitters at the input 13 of the glass blade 4 and also to obtain the same divergence for the rays emerging from the edges of the diode as from those emerging from its center. In this arrangement the width of the glass blade 4 corresponds to the width of the image of the emitters and all the power of the diode collected by lenses 2 and 3 entering the glass blade 4 emerges at its end and on its image at the modulator. The divergence of the bundle at the output is minimized thus allowing as good collimation as possible and the luminous power is substantially uniformly spread on the width of the image at the output of the blade as shown in FIG. 5b. Lenses 3, 5, 6 and 11 are commercially available, e.g. from the company Opto Sigma, USA.

The modulator 7 can be electro-optical, acousto-optical or electromechanical (deformable mirrors). A preferred modulator is a TIR-modulator, as for example described in U.S. Pat. No. 4,281,904. As also described in this reference with respect to its FIGS. 6(a) to 6(e), it is possible to introduce various phase microscopy configurations for converting the phase modulated by the TIR-modulator into corresponding light intensity variations. In particular, an embodiment is preferred for the present invention with a multiple point laser source which makes use of a dark field illumination, as is for example described in the above-mentioned reference with respect to its FIG. 6(e).