Patterned diffractive light modulator ribbon for PDL reduction

Details Patterned diffractive light modulator ribbon for PDL reduction Patterned diffractive light modulator ribbon for PDL reduction

2003-02-28

2004-10-19

Schwartz, Jordan M. (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave directional modulation

C359S563000

Reexamination Certificate

active

06806997

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an apparatus for reducing the effects of Polarization Dependent Losses (PDL). More particularly, this invention relates to a patterned diffractive light modulator ribbon for PDL reduction.
BACKGROUND OF THE INVENTION
Designers and inventors have sought to develop a light modulator which can operate alone or together with other modulators. Such modulators should provide high operating speeds (KHz frame rates), a high contrast ratio or modulation depth, have optical flatness, be compatible with VLSI processing techniques, be easy to handle and be relatively low in cost. Two such related systems are found in U.S. Pat. Nos. 5,311,360 and 5,841,579 which are hereby incorporated by reference.
According to the teachings of the '360 and '579 patents, a diffractive light modulator is formed of a multiple mirrored-ribbon structure. An example of such a diffractive light modulator
10
is shown in FIG.
1
. The diffractive light modulator
10
comprises elongated elements
12
suspended by first and second posts,
14
and
16
, above a substrate
20
. The substrate
20
comprises a conductor
18
. In operation, the diffractive light modulator
10
operates to produce modulated light selected from a reflection mode and a diffraction mode.
FIGS. 2 and 3
illustrate a cross-section of the diffractive light modulator
10
in a reflection mode and a diffraction mode, respectively. The elongated elements
12
comprise a conducting and reflecting surface
22
and a resilient material
24
. The substrate
20
comprises the conductor
18
.
FIG. 2
depicts the diffractive light modulator
10
in the reflection mode. In the reflection mode, the conducting and reflecting surfaces
22
of the elongated elements
12
form a plane so that incident light I reflects from the elongated elements
12
to produce reflected light R.
FIG. 3
depicts the diffractive light modulator
10
in the diffraction mode. In the diffraction mode, an electrical bias causes alternate ones of the elongated elements
12
to move toward the substrate
20
. The electrical bias is applied between the reflecting and conducting surfaces
22
of the alternate ones of the elongated elements
12
and the conductor
18
. The electrical bias results in a height difference between the alternate ones of the elongated elements
12
and non-biased ones of the elongated elements
12
. A height difference of a quarter wavelength &lgr;/4 of the incident light I produces maximum diffracted light including plus one and minus one diffraction orders, D
+1
and D
−1
.
FIGS. 2 and 3
depict the diffractive light modulator
10
in the reflection and diffraction modes, respectively. For a deflection of the alternate ones of the elongated elements
12
of less than a quarter wavelength &lgr;/4, the incident light I both reflects and diffracts producing the reflected light R and the diffracted light including the plus one and minus one diffraction orders, D
+1
and D
−1
. In other words, by deflecting the alternate ones of the elongated elements
12
less the quarter wavelength &lgr;/4, the diffractive light modulator
10
produces a variable reflectivity.
Unfortunately, when arbitrarily polarized light impinges on a linear one-dimensional (1D) diffractive light modulator, each polarization state interacts with the diffractive light modulator differently. Such a scenario is illustrated in
FIG. 4
in which an incident light
40
impinges upon a diffractive light modulator
50
comprising a series of reflective ribbons placed in parallel. The incident light
40
includes a polarization state P and a polarization state S. Light polarized parallel to the ribbons (polarization state P) interacts with the diffractive light modulator
50
differently than light polarized perpendicular to the ribbons (polarization state S). Polarization states S and P each “see” different environments at the diffractive light modulator. This can lead to Polarization Dependent Losses (PDL) in which one polarization state is attenuated more than the other.
PDL can be defined as a function of the width of the ribbons and the width of the gaps between the ribbons. When the ribbon width is sufficiently large, then PDL becomes a weak function of the ribbon width. When the gap width is sufficiently small, then PDL becomes a weak function of the gap width. Theoretically, with no gaps and a single wide ribbon, there is no PDL in the center of the ribbon. However, light impinging the edge of the ribbon still experiences PDL. This is known as the edge effect or edge polarizability. Light with a polarization state perpendicular to the edge “sees” the edge differently than light polarized parallel to the edge, leading to polarization dependent loss. The polarization state of the impinging light can not be guaranteed. As a result, the edge effect is very difficult to overcome. What is needed is a light modulator that minimizes PDL due to the edge effect.
Since the polarization state of the incident light at the edge of the ribbon impacts PDL, efficiency of a diffractive light modulator can be expressed as a function of the polarization state of the incident light. In general, light includes polarization states TM and TE which are perpendicular to each other. Since the polarization state at any given time and place can not be guaranteed, the orientation of the polarization states TM and TE relative to the ribbon edges can not be predetermined. As such, a polarization state can be parallel to the ribbon edge, perpendicular to the ribbon edge, or somewhere in between. What is needed is a diffractive light modulator with an output response that is as independent of the polarization state as possible.
What is also needed is a grating system that normalizes edge effect PDL across input polarization states.
SUMMARY OF THE INVENTION
Embodiments of the present invention include a modulator for modulating an incident beam of light. The modulator includes a plurality of elements, each element including a first end, a second end, a first linear side, a second linear side, and a light reflective planar surface with the light reflective planar surfaces of the plurality of elements lying in one or more parallel planes, wherein the plurality of elements are arranged parallel to each other and further wherein the light reflective planar surface of each of the plurality of elements includes a first non-linear side and a second non-linear side. The modulator also includes a support structure coupled to each end of the plurality of elements to maintain a position of each element relative to each other and to enable movement of selective ones of the plurality of elements in a direction normal to the one or more parallel planes of the plurality of elements, and between a first modulator configuration wherein the plurality of elements act to reflect the incident beam of light as a plane mirror, and a second modulator configuration wherein the plurality of elements act to diffract the incident beam of light.
The modulator according to embodiments of the present invention wherein the first non-linear side and the second non-linear side each include one or more projections, wherein each projection is perpendicular to each of the linear side of the element and the one or more projections do not extend beyond the first linear side and the second linear side. The modulator also embodying each projection on the first non-linear side repeated according to a constant period, and each projection on the second non-linear side is repeated according to a constant period, wherein the shape of each projection is the same.
The modulator according to embodiments of the present invention also includes the projections on the first non-linear side being symmetric in relation to the projections on the second non-linear side and wherein the non-linear sides of adjacent elements are symmetrical. The modulator of the present invention is also a diffractive MEMS device and the selective ones of the elements are alternating elements and are moved by

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