Sub-wavelength efficient polarization filter (SWEP filter)

Details Sub-wavelength efficient polarization filter (SWEP filter) Sub-wavelength efficient polarization filter (SWEP filter)

2001-05-04

2003-12-09

Stafira, Michael P. (Department: 2877)

Optical waveguides

With optical coupler

Input/output coupler

C385S011000

Reexamination Certificate

active

06661952

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to a method and apparatus for separation of light having more than one polarization.
BACKGROUND
Polarization sensitive materials have been used for many years, in a variety of applications. Polarization sensitive materials refer to the selective transmission or reflection of light from a material based on the polarization state of the light. A common method of producing polarized light is through the use of dichroic polarizing materials. The most common dichroic polarizing material is manufactured by assembling a plurality of long, parallel polymer chains. This type of polarizing filter nearly completely absorbs light which has its electric field vector parallel to the length of the polymer chains, while only partially absorbing light that has its electric field vector perpendicular to the length of the polymer chains. Thus, a polarizing filter formed from an oriented polymer chain material absorbs a significant amount of incident light from an unpolarized light source, making them low efficiency polarizing filters. Accordingly, these materials are generally unusable for high optical power applications.
Other transmission polarizing techniques take advantage of the fact that at a certain angle of incidence (i.e. Brewster's angle), only one polarization state (TE mode) is partially reflected while the other state (TM mode) is completely transmitted. Polarizing beamsplitters are in this class of polarizers. These polarizing filters are difficult to fabricate as they require light to strike a surface at a specific angle of incidence. Moreover, these filters are only moderately efficient.
High efficiency, frequency selective, reflective, optical filters (mirrors) are typically constructed from quarter wave stacks of high and low refractive index dielectric materials. Such stacks are polarization insensitive at normal incidence and become increasingly polarization sensitive at higher incident angles.
In order to select, or filter, one polarization state from an optical beam having both polarization states, current optical systems generally includes a bulky polarizing beam splitter positioned in the beam path. Beam splitters add considerable bulk and are known to introduce significant optical aberrations in optical signals when they are used.
SUMMARY
A polarization filter includes a first sub-wavelength resonant grating structure (SWS) for receiving incident light and a second SWS. The SWS are disposed relative to one another such that a portion of incident light applied to the first SWS passes through the second SWS. The combination of two SWS being optically connected produce a strong, polarization dependent, resonance. As a result, incident light is either strongly reflected or transmitted, depending on the polarization state of the incident light.
The polarization sensitive resonance of the filter substantially reflects a first polarization component of incident light while substantially transmitting a second polarization component of incident light, the polarization components being orthogonal to one another. This resonance condition can hold over a broad band of wavelengths. In one embodiment, the first and second SWS are spaced apart a distance being less than one half an optical wavelength. However, in an alternate embodiment, the first and second SWS are spaced apart a distance greater than one half an optical wavelength, thus creating a high Q narrowband filter. For most efficient operation, incident light can be directed substantially normal to the surface of the filter.
Each SWS can be formed from respective pluralities of sub-wavelength features, the plurality of features being disposed in respective dielectric waveguide materials. The features preferably have a higher refractive index as compared to the respective waveguide materials. The plurality of features can be embedded in the waveguides.
Each SWS can be formed from features having a length dimension and a shorter width dimension, the features disposed in respective waveguide materials. The SWEP filter can reflect substantially all incident light over a range of wavelengths if the electric field vector of the incident light is parallel to the length dimension of the features. If the electric field vector of the incident light is perpendicular to the feature lengths, substantially all incident light will substantially be transmitted over the same bandwidth of wavelengths. If unpolarized light is normally incident on the SWEP filter, then approximately half the light is reflected with one polarization and approximately half the light is transmitted with the other orthogonal polarization component.
The filter can be configured to have a pixelated surface, the filter having a plurality of areas defining a plurality of pixels. Pixels can each be adapted to produce a configurable resonant reflective response.
The resonant reflective response of the filter can also be modulated by including a structure for modulation. The structure for modulation can include at least one electro-optic layer, the electro-optic layer disposed in optical contact with at least one SWS. The electro-optic layer can modulate light reflected by the filter over a narrow band portion of a reflective bandwidth of the filter.
Incident light gets separated into two orthogonal polarization states without any substantial absorption by the filter. Accordingly, it is possible to efficiently convert the polarization of one separated beams using a polarization converter. After conversion, a structure for combining beams can be used to recombine the two beams into a single beam, the single beam having approximately the same energy as the incident beam. The combined beam can generate high visibility interference fringes from incident unpolarized light.
A method for forming a polarization filter includes the steps of forming a first sub-wavelength resonant grating structure (SWS) and a second SWS. The SWS are each formed by providing a waveguide and forming the SWS by disposing a plurality of sub-wavelength features in the respective waveguides. The features are positioned with substantially equal spacing, the features formed from a material having a refractive index greater than the waveguide refractive index. The first and second SWS are disposed relative to one another such that a portion of incident light applied to the first SWS passes through the second SWS. The two SWS can share a single unitary waveguide material. Preferably, the features are embedded in the waveguides.
In one embodiment of the method, the waveguide can be provided by selecting at least one mold, the mold having a pattern defining a plurality of features, filling the mold with a liquid, the liquid adapted to produce a solid material following hardening, and hardening the liquid to form the waveguide. The SWS can be formed by filling the plurality of features with a feature material. A method such as chemical vapor deposition (CVD) may be used for this purpose. The liquid is preferably sol gel. Most preferably, the sol gel is a silica gel, the silica gel adapted to form silica after the hardening step. In this embodiment, the method can include the step of disposing the SWS relative to one another such that a portion of incident light applied to the first SWS passes through the second SWS, and annealing after the disposing step, the annealing to create an integrated filter structure. The method can include the step of interposing an optically transparent material between the first and second SWS before the annealing step.
A method for separating orthogonal polarizations of light includes the steps of providing a filter including a first sub-wavelength resonant grating structure (SWS) for receiving incident light, the incident light having orthogonal polarization components, and a second SWS. The SWS are disposed relative to one another such that a portion of incident light applied to the first SWS passes through the second SWS. The interaction of the two SWS results in the filter substantially reflecting one of the orthogon

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