Micromachined reflective light valve

[0002] The invention herein disclosed is related to co-owned, U.S. patent application Ser. No. 09/072,753, now U.S. Pat. No. 6,147,789.

[0003] The invention relates to the modulation of light beams and particularly to the modulation of light beams employing reflective light valves with micromachined deformable mirrors.

[0004] In the prior art, micromachined ribbons have been used for various types of optical systems. A primary use for such ribbons is a phased-array mirror, wherein many ribbons act in unison as one large mirror. The deformable ribbons adjust the surface of the mirror so as to change the shape of the wave front. The manner in which phased-array mirrors act in unison to change the shape of the wave front represents a different concept than light valves in which individual ribbons are addressable, so as to permit modulation of individual “sub-beams” of light.

[0005] Micromachined ribbons have also been used in interferometric light valves, the operation of which depend on the wave nature of light. Such an interferometric scheme is disclosed in U.S. Pat. No. 5,508,840, wherein each light valve comprises two ribbons and the modulation technique involves deforming the ribbons in a manner that causes the light to interfere constructively or destructively. Depending on the deformation of the two ribbons, the constructive and destructive interference cause differences in the light intensity at a given location. A second type of interferometric light valve employing micromachined ribbons is known in the art as a grating light valve. In a grating light valve, a group of ribbons form a diffraction grating which causes diffraction spots to occur in various locations. Once again, the difference in the light intensity between constructively and destructively interfering light waves form high and low intensity diffraction spots which allow for modulation of data. In general, interferometric light valves require more than one micromachined optical element per channel, and can operate only at a narrow range of wavelength since the motion of the ribbon has to correspond to a known fraction (typically one quarter or one half) of a wavelength.

[0006] An additional drawback of the diffraction grating light valves is low optical efficiency. The zero order beam is not suitable for digital recording because the contrast ratio is poor (i.e. the difference between the bright spots caused by constructive interference and the dark spots caused by destructive interference is insufficient to record the digital data faithfully). The alternative, the first order diffraction spot, contains less than 50% of an incident beam's light energy, representing a low optical efficiency. In order to reduce the loss of light incurred by using the first order beam, a technique known as “blazing” has been developed. Blazing incorporates multiple ribbons aligned in a “staircase” fashion. Although blazing increases the optical efficiency of the first order spot, it effectively recreates the aforementioned disadvantage of requiring a plurality of optical elements for each individual light valve.

[0007] The related U.S. patent application Ser. No. 09/072,753, employs a micromachined ribbon which reflects incident light. On the selective application of an electrostatic field, the ribbon deforms into a cylindrical shape focusing the reflected light through a collection slit. In contrast to the interferometric devices mentioned above, the invention of application Ser. No. 09/072,753 uses the geometric properties of light rays as a basis for modulation. When the ribbon is flat (i.e. in the absence of an electrostatic field), the reflected light is scattered broadly and only a small fraction makes it through the collection slit. In contrast, when the ribbon forms a cylindrical mirror, the reflected light converges on the slit and a relatively high intensity spot is formed.

[0008] The application Ser. No. 09/072,753, has several shortcomings, the main one being that the performance is dependent on the shape of the deformed ribbon. When the ribbon is perfectly cylindrical, the focal point can be accurately predicted and the collection slit can be appropriately positioned. However, in the working device, the deforming ribbons do not form perfect cylindrical surfaces and, consequently, a portion of the incident light energy is lost. Additionally, the application Ser. No. 09/072,753 process uses the entire surface of the ribbon to reflect light. As a result, the device is sensitive to small variations and processing imperfections on the reflective surface of the ribbon. Small imperfections can cause light to reflect in random directions so that some of the reflected light does not reach the collection slit and is lost. Furthermore, because of the micromachine processing, an array of ribbons must have small gaps between each pair of ribbons to permit the etching of the cavity behind the ribbons. Naturally, these gaps must be of a certain width to properly etch the cavity. When light incident on the ribbon array impinges on one of the gaps, then that portion of the incident light is not reflected and does not contribute to the modulation process. These disadvantages all represent losses of optical efficiency and reduced contrast ratios.

[0009] It is an object of the present invention to form a light valve based on reflective deformable micromachined ribbons requiring only one ribbon per device.

[0010] Another object of the present invention is to create a light valve which has an improved contrast and optical efficiency over prior-art micromachined light valves.

[0011] A further object of the present invention is to create a light valve which uses only a fraction of the surface of the micromachined ribbon while maintaining high optical efficiency, such that the device is relatively insensitive to the shape of the deformed ribbon or minor processing defects in the profile of the reflective surface.

[0012] FIG. 1 depicts a preferred embodiment of the present invention. An array of light valves is used to record data onto a recording surface.

[0013] FIG. 2 shows a schematic diagram of an individual light valve.

[0014] FIG. 3 depicts an implementation of the light valve array using a single cylindrical lens.

[0015] FIG. 4 depicts an implementation of the light valve array employing an array of lenslets with each lenslet corresponding to one light valve.

[0016] FIG. 5ashows a prior art ribbon assembly and

[0017] FIG. 5bshows a ribbon assembly having shaped ribbons according to a preferred embodiment of the invention.

[0018] Referring to FIG. 1, laser diode 1 emits a beam 15 which is collimated in both horizontal and vertical directions by lenses 2 and 3. Lenses 2 and 3 can be spherical, cylindrical or any combination; typically, lens 2 is a strong cylindrical lens used to collimate the axis with the larger divergence and lens 3 is a weaker cylindrical lens. If the polarization of the laser is low, then an additional polarizer 4 may be introduced. The linearly polarized beam then passes through the polarizing beam splitter 5 and the quarter-wave plate 6 (where the light becomes circularly polarized) before impinging on the light valve array 8. The light valve array 8 comprises a lenslet array 7 and an array of deformable micromachined ribbons 16. Each of the ribbons 16 can be moved by electrical signals 9 such that the properties of the reflected light are changed. Upon reflection, the beam passes back through the quarter-wave plate 6, causing it to become linearly polarized again, but on an axis 90 degrees from the incident beam polarization. Thus, when the reflected light returns to the polarizing beam splitter 5, it is reflected rather than transmitted. The reflected beam then passes through slit 10 into imaging lens 11 which focuses the light in order to form an image 12 of the data 9 on recording surface 13.

[0019] Referring now to FIG. 2, an individual light valve is depicted. Incident collimated beam 15 travels through the polarizing beam splitter 5, the quarter-wave plate 6 and the lenslet 7. The incident light 15 will only be retro-reflected (i.e. reflected as a collimated beam) when the reflective surface of ribbon 16 is placed at the focal point of lenslet 7. Since the ribbon 16 is electrically moveable, the distance between the reflective surface and lenslet 7 can be adjusted by changing the electrical signal 14 applied between the ribbon 16 and a fixed electrode 17. Retro-reflection is achieved at position 16′, where the incident light 15 is focused perfectly on the reflective surface. While this example used a collimated beam, any type and shape of beam can be used as long as this shape is changed by the motion of mirror 16, and this change in shape can be used to change the beam intensity.

[0020] The exact shape of the ribbon 16 is of relatively little importance, since only a small point on the ribbon 16 is illuminated when it is deformed to position 16′ (i.e. at the focal point of lenslet 7). When retro-reflected in this manner, the reflected beam 15 has relatively little divergence, allowing it to pass easily through slit 10 after being reflected by the polarizing beam splitter 5. The reason beam 15 is reflected on the return and not on the incoming path is that its polarization was changed by passing twice through quarter-waveplate 6. When the ribbon 16 is not in position 16′, the reflected beam 15 does not stay collimated and diverges rapidly, becoming the wide beam 15′. Because of this divergence, most of the reflected beam 15′ is blocked by slit 10 and only a relatively small percentage of the light makes it to recording surface 13. The arrangement of prism 5, waveplate 6, lens 7 and reflective surface 16, is well known in optics and is sometimes referred to as an optical isolator, since it also prevents the reflected light from going back into the laser. While this example uses a slit to convert change in collimation (or convergence) into intensity change, there are many other well known methods for achieving this conversion. One of the best known is Schlieren imaging, in which all light reflected from one state of the mirrors is blocked by an opaque object, while any change in collimation allows the light to by-pass the opaque object.

[0021] FIGS. 3 and 4 depict different implementations of the lens 7. FIG. 3 depicts a configuration with a single cylindrical lens 7 which directs light towards the light valve 8, whereas FIG. 4 depicts an implementation with an array of aspherical lenslets 7, each of which corresponds to one of the ribbons 16. It should be understood that throughout this disclosure, the principal lens 7 of the light valve may be either a single lens of an array of lenslets and any discussion of one or the other is meant to apply to both implementations. It is important to note that while each ribbon 16 modulates an individual spot 12 on the recording medium 13, all sub-beams forming these spots are part of a single beam passing through a single output lens 11.

[0022] The present invention has several advantages over the invention of the related application Ser. No. 09/072,753 For reasons discussed below, the present invention performs best when lens 7 has a high numerical aperture (NA). The present invention is relatively insensitive to imperfections in the reflective surface of ribbon 16, particularly when a high NA lens 7 is used. Even if there are minor imperfections in the ribbon surface, the reflected light may still be collected by lens 7 and directed onto the recording surface 13. Furthermore, the present invention focuses the light on the ribbon 16, thereby using only a small portion of its reflective surface. As such, the present invention is relatively insensitive to the shape of the deformed ribbon 16.

[0023] Referring now to FIGS. 5a5b,an additional advantage involves the shape of the ribbons 16. As discussed above, micromaching processes require small gaps 19 between each ribbon 16 in order to etch the cavity (not shown) behind the ribbons 16. These gaps 19 are shown in the prior art ribbon array of FIG. 5a.In the prior art invention, these gaps 19 represent a source of optical inefficiency, because no light is reflected from them. In the preferred embodiment of the present invention (FIG. 5b), the ribbons 16 can be shaped so as to minimize the gaps 19 in the reflection zones 20 and maximize the gaps 19 in the non-reflecting zones 21. In this manner, the fabrication process is made easier, because larger gaps 19 are available, yet high optical efficiency is maintained.

[0024] Referring back to FIG. 1, the preferred embodiment employs a lenslet array 7, wherein each individual lenslet has a high numerical aperture (NA). High NA lenses are preferable for a number of reasons. As mentioned previously, a high NA lens is less sensitive to minor imperfections in the reflective surface of the ribbon 16. In addition, the depth of focus of an optical system is proportional to 1(NA)². Thus, the focusing properties of an optical system with a high NA lens will vary more substantially when the position of the reflective ribbon 16 is changed.

[0025] The preferred embodiment also employs a lenslet array 7 with a relatively small focal length. A small focal length is preferable because it makes the physical dimensions of the light valve as small as possible, a significant advantage because each array may have from hundreds to thousands of ribbons.

[0026] For high power laser sources, an additional advantage is provided by merely changing the system focus without actually focusing the beam on the ribbon. Such a process requires the ribbons to change from an out of focus state (at no deformation) to a different focus state (after deformation), the second focus state being not exactly perfect focus. This design feature would prevent the high power laser source from destroying the structural or reflective surface-properties of the ribbons by excessive power density, while maintaining a relatively collimated reflected beam. An additional collimating lens may be added in the path of the reflected beam to achieve better collimation.

[0027] The preferred embodiment was tested in a system wherein the light valve ribbons were 40 microns wide by 300 microns long on a 50 micron pitch (i.e. the gaps between ribbons were approximately 10 microns). The ribbons were made of 2000 Å thick silicon nitride covered with 500 Å of gold on top of 250 Å of chrome. The airgap was about 1 micron. The lenslet array was a commercial product made of silica with square lenslets having focal lengths of 100 microns and a 50 micron pitch. One supplier of such arrays is Mems Optical Inc. (Huntsville, Ala.). The system response time (to go from 0% to 90% deflection) was measured at about 10 μs for a drive voltage of 30 V. The contrast ratio was about 10:1.

[0028] It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present invention in any way. Those skilled in the art will appreciate that various modifications can be made to the embodiments discussed above without departing from the spirit of the present invention.