Optical: systems and elements – Light control by opaque element or medium movable in or... – Electro-mechanical
Reexamination Certificate
1999-06-11
2001-07-31
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Light control by opaque element or medium movable in or...
Electro-mechanical
C359S291000, C359S295000, C359S900000, C347S135000, C347S255000
Reexamination Certificate
active
06268948
ABSTRACT:
FIELD OF THE INVENTION
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.
BACKGROUND OF THE INVENTION
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.
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.
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.
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.
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.
SUMMARY OF THE INVENTION
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.
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.
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.
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Raji Krishnamoorthy Mali, et al., “Development of Microelectromechanical Deformable Mirrors for Phase Modulation of Light”, Optical Eng., vol. 36, No. 2, pp. 542-548, Feb. 1977.
Creo Products Inc.
Jr. John Juba
Nguyen Thong
Oyen Wiggs Green & Mutala
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