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Cascaded self-induced holography
| Details |
Inventors: Lin, Chih-Hsien Jason; Lo, Yu-Hwa;
Assignee: Cornell Research Foundation, Inc. (Ithaca, NY)
Primary Examiner: Henry; Jon W.
Assistant Examiner:
Attorney, Agent or Firm: Jones, Tullar & Cooper
Cascaded diffraction grated pairs are employed to form submicron gratings and other patterns in various types of devices using photolithographic, UV exposure and other techniques. Cascaded grating pairs are formed in masks which are positioned between a source of radiation and the device or substrate in which a grating is to be formed. The interaction of the diffracted beams generated by each diffraction grating in a cascade grating pair results in the device grating having a period which is substantially smaller than that of either of the gratings in the cascaded grating pair. As a result, gratings with substantially smaller periods than were previously obtainable can now be achieved using conventional fabrication techniques. To eliminate sensitivity to background exposure in photolithographic fabrication applications, a mask is employed in which first and second cascaded grating pairs are separated from one another by an opaque region that acts as a spatial filter. The mask is positioned above the target device or substrate at a distance which causes the diffracted radiation beams from each of the cascaded grating pairs to interfere with one another on the device in the area where the diffraction grating is to be formed. |
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DETAILED DESCRIPTION To address the foregoing problems, the present invention provides a method and apparatus for fabricating submicron periodic gratings and other fine patterns in optical devices at a resolution well in excess of that which is achievable with conventional fabrication techniques.
The heart of the invention resides in the use of a technique known as cascaded, self-induced holography in which multiple diffraction gratings are used in an optical mask in such a manner that optical gratings or patterns having grating spacings (periods) substantially smaller than the periods in the mask gratings are obtained.
More particularly, the present invention employs an optical mask having at least two diffraction gratings which are cascaded in series, one beneath or in front of the other, between a radiation source and a device in or on which a grating is to be formed.
The diffraction patterns generated by each of the mask gratings interact with one another so that the resulting diffraction grating formed on or in the device has a period a number of times (e.
g.
4) smaller than the periods in either of the mask's diffraction gratings.
In its simplest form, the first and second cascaded diffraction gratings are disposed directly above or adjacent the surface of the optical device, such as an optical fiber array, in which an optical grating is to be formed.
A monochromatic beam of light is passed through the cascaded gratings onto the surface of the device, thereby forming the optical grating therein with a period up to four times smaller than that of the period in the cascaded gratings.
The period of the device grating can be reduced even further by employing multiple pairs of the cascaded grating pairs in series.
In one embodiment of the present invention, two pairs of cascaded optical gratings are disposed on either side of a central opaque region which acts as a spatial filter element, and minimizes sensitivity to background or noise exposure.
The resulting mask is positioned between a substrate in which the optical grating is to be formed and a source of polarized monochromatic light
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Optical Systems 
