Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2002-12-23
2004-07-27
Dunn, Drew A. (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S291000, C359S223100, C359S566000, C385S024000, C398S081000
Reexamination Certificate
active
06768589
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to actuatable diffraction gratings and more particularly, to actuatable diffraction gratings providing reduced polarization dependent loss.
BACKGROUND OF THE INVENTION
An important characteristic of desired for optical telecommunications components is that they have low polarization-dependent loss (PDL). PDL is defined as the degree to which an optical device attenuates an input signal as a function of polarization, commonly expressed in terms of a logarithm the ratio of the diffraction efficiency of transverse electric (TE) polarized light and transverse magnetic (TM) polarized light. Conventional diffraction gratings (i.e., one-dimensional gratings having a single elongate dimension) are intrinsically polarization-dependent devices. Accordingly, MEMS-based (i.e., made using microelectromechanical system manufacturing techniques) conventional diffraction gratings have suffered from significant PDL.
Modifications to MEMS-based diffraction gratings have been made to reduce PDL. One such modification involves using wider elements, either for the entire grating or by making selected grating elements wider. Such modifications provide reduced PDL at a specific actuation state of the MEMS grating, but do not provide significant reduction of PDL over all actuation depths. Additionally, the use of wider grating elements results in larger devices, and smaller diffraction angles. Both of these drawbacks have led to increased package size for devices employing gratings that have been so modified.
An example of a diffraction grating design having equal diffraction efficiency for TE-polarized light and TM-polarized light (i.e., a PDL of zero) is a bi-grating. A bi-grating is a two-dimensional grating which diffracts light in two orthogonal planes. The grating described in U.S. Pat. No. 6,188,519 B1, to Johnson, issued Feb. 13, 2001 is an example of a MEMS-based bi-grating for use in maskless lithography and high resolution printing. The device described in Johnson requires an actuatable membrane and fixed islands, which are difficult to fabricate. Furthermore, when the device described in Johnson is actuated, the membrane is not flat, resulting in a reduction in diffraction efficiency.
SUMMARY OF THE INVENTION
Some aspects of the present invention apply a recognition that in an actuatable, one-dimensional diffraction grating, the PDL contribution caused by two grating elements is a function of the relative displacement of the two grating elements (i.e., the gap size; gap size is defined as separation in the z-direction, as illustrated in
FIG. 2
a
); and a further recognition that PDL as a function of displacement has regimes of positive and negative PDL (see FIG.
1
). Each grating element of a diffraction grating has a reflective surface that is either integrated with the grating element (i.e., the grating elements is made of a reflective material) or has a reflective surface otherwise disposed on the grating element. The size of a gap is measured between the tops of the reflective surfaces of the relevant grating elements.
Accordingly, by processing light of a channel (e.g., a signal having a single wavelength of light) with a diffraction grating configured such that one or more regions of the grating correspond to a positive PDL (i.e., the relative displacement of at least two grating elements within the grating corresponds to greater throughput efficiency for TE-polarized light than TM-polarized light) and one or more regions correspond to a negative PDL (i.e., the relative displacement of at least two elements within the grating correspond to greater throughput efficiency for TM-polarized light than TE-polarized light L), the overall PDL of the light processed by the grating can be reduced relative to a conventional diffraction grating in which all regions of the diffraction grating correspond to a PDL of the same sign (i.e., all positive or all negative).
One aspect of the present invention is directed to a diffraction grating optical processor having one or more groups of grating elements, each group including three or more grating elements that function together to process light of a channel in a manner that provides reduced PDL relative to a conventional diffraction grating.
The grating elements processing light of a single channel are referred to herein as a “pixel.” It is to be understood that it is the aggregate effect of the positioning of all the grating elements of a pixel (i.e., the relative displacements of all grating elements relative to one another) that determines the amount of PDL present in a signal processed by a pixel. However, it is instructive and convenient to ascribe a PDL to individual pairs of grating elements comprising a pixel (e.g., a reference grating element, typically a non-actuatable grating element, and an operational grating element, typically an actuatable grating element); the sum of the PDLs ascribed to the pairs of grating elements are indicative of the PDL for the entire pixel. When referring to the PDL contribution of a pair of grating elements of a pixel (as determined by the gap therebetween), the arrangement will be said “to correspond to a PDL.”
Embodiments of actuatable gratings as taught herein may have pixels including a plurality of adjacent actuatable grating elements. Accordingly, gaps (in the z-direction) may be defined between one or more operational grating elements of a pixel and a non-adjacent reference grating element. The gaps in such devices may correspond to positive and negative PDLs, so as to reduce the overall PDL of a pixel (e.g., see FIG.
8
).
The present invention also includes, but is not limited to, embodiments of actuatable diffraction gratings that provide reduced PDL over all actuation depths. The phrase “all actuation depths” means including actuation distances equal to at least one quarter of a processed wavelength of light, such that a selected amount of diffraction achieved by a given pixel of the diffraction grating may range from zero diffraction (i.e., substantially all of the light reflecting from the pixel remains in the zeroth order) to complete diffraction (i.e., substantially all of the light reflecting from the pixel is diffracted out of the zeroth order).
A first aspect of the invention is directed to an optical processor characterized by an axis extending in a direction, the optical processor comprising: (a) a pixel to process light having a wavelength &lgr;, comprising (1) a first grating element having a reflective surface, at least a portion of the reflective surface being disposed normal to a direction of the axis, (2) a second grating element, parallel to the first grating element and having a reflective surface, at least a portion of the reflective surface being disposed normal to the direction of the axis, and (3) a third grating element, parallel to the first grating element and having a reflective surface, at least a portion of the reflective surface being disposed normal to the direction of the axis; and (b) a controller operable to displace at least the reflective surface of the second grating element relative the reflective surface of the first grating element, a displacement of the reflective surface of the second grating element forming a first gap in the direction of the axis relative the reflective surface of the first grating element, the first gap corresponding to a PDL of a sign, and the reflective surface of the third grating element forming a second gap, relative one of the reflective surface of the first grating element and the reflective surface of the second grating element, in the direction of the axis and corresponding to a PDL of the opposite sign.
In some embodiments, in a direction perpendicular to the axis, the second grating element is located intermediate the first grating element and the third grating element. In some embodiments, the first grating element and the third grating element are non-actuatable grating elements. The reflective surface of the first grating element and reflective surface of the
Deutsch Erik R.
Smith Malcolm C.
Boutsikaris Leo
Dunn Drew A.
Lowrie Lando & Anastasi, LLP
Polychromix Corporation
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