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.
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 xcex/4 of the incident light I produces maximum diffracted light including plus one and minus one diffraction orders, D+1 and Dxe2x88x921.
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 xcex/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 Dxe2x88x921. In other words, by deflecting the alternate ones of the elongated elements 12 less the quarter wavelength xcex/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 xe2x80x9cseexe2x80x9d 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 xe2x80x9cseesxe2x80x9d 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.
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 applying an electrostatic force.
According to other aspects of the embodiments, a modulator for modulating an incident beam of light including a plurality of elements, each element including a first end, a second end, a first linear side, a second linear side, and a non-continuous light reflective planar surface with the non-continuous light reflective planar surfaces of the plurality of elements lying in one or more parallel planes, wherein the elements are arranged parallel to each other and further wherein the non-continuous 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 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 sides of the element. The modulator also embodying the one or more projections not extending beyond the first linear side and the second linear side.
The modulator of the present invention also includes each projection on the first non-linear side being repeated according to a constant period, and each projection on the second non-linear side being repeated according to a constant period and wherein the shape of each projection is the same and the projections on the first non-linear side are symmetric in relation to the projections on the second non-linear side and 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 applying an electrostatic force. The modulator of the present invention also embodies the non-continuous portion of each of the light reflective planar surfaces being non-reflective.
The present invention is also a modulator for modulating an incident beam of light including means for supporting each of a 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, wherein each element includes 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, and the elements are arranged parallel to each other and further wherein the light reflective planar surfaces of each of the plurality of elements includes a first non-linear side and a second nonlinear side. The modulator of the present invention also includes means for moving selective ones of the plurality of elements 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.