Double sided wire grid polarizer

Details Double sided wire grid polarizer Double sided wire grid polarizer

2001-10-15

2004-03-30

Chang, Audrey (Department: 2872)

Optical: systems and elements

Polarization without modulation

Polarization variation over surface of the medium

C359S483010, C359S900000

Reexamination Certificate

active

06714350

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to wire grid polarizers and their use in a modulation optical system. The present invention relates in particular to double sided wire grid polarizers and beamsplitters for the visible spectrum, and the use of these double sided wire grid polarizers within a modulation optical system.
BACKGROUND OF THE INVENTION
The use of an array of parallel conducting wires to polarize radio waves dates back more than 110 years. Wire grids, generally in the form of an array of thin parallel conductors supported by a transparent substrate, have also been used as polarizers for the infrared portion of the electromagnetic spectrum.
The key factor that determines the performance of a wire grid polarizer is the relationship between the center-to-center spacing, sometimes referred to as period or pitch, of the parallel grid elements and the wavelength of the incident light. If the grid spacing or period is long compared to the wavelength, the grid functions as a diffraction grating, rather than as a polarizer, and diffracts both polarizations, not necessarily with equal efficiency, according to well-known principles. However, when the grid spacing (p) is much shorter than the wavelength, the grid functions as a polarizer that reflects electromagnetic radiation polarized parallel (“s” polarization) to the grid, and transmits radiation of the orthogonal polarization (“p” polarization). The transition region, where the grid period is in the range of roughly one-half of the wavelength to twice the wavelength, is characterized by abrupt changes in the transmission and reflection characteristics of the grid. In particular, an abrupt increase in reflectivity, and corresponding decrease in transmission, for light polarized orthogonal to the grid elements will occur at one or more specific wavelengths at any given angle of incidence. These effects were first reported by Wood in 1902, and are often referred to as “Wood's Anomalies.” Subsequently, in 1907, Rayleigh analyzed Wood's data and had the insight that the anomalies occur at combinations of wavelength and angle where a higher diffraction order emerges. Rayleigh developed following equation to predict the location of the anomalies, which are also commonly referred to in the literature as “Rayleigh Resonances.”
&lgr;=&egr;(
n
+/−sin &thgr;)/
k
  (1)
wherein epsilon (&egr;) is the grating period; n is the refractive index of the medium surrounding the grating; k is an integer corresponding to the order of the diffracted term that is emerging; and lambda and theta are the wavelength and incidence angel (both measured in air) where the resonance occurs.
For gratings formed on one side of a dielectric substrate, n in the above equation may be equal to either 1, or to the refractive index of the substrate material. Note that the longest wavelength at which a resonance occurs is given by the following formula:
&lgr;=&egr;(
n
+sin &thgr;)  (2)
where n is set to be the refractive index of the substrate.
The effect of the angular dependence is to shift the transmission region to larger wavelengths as the angle increases. This is important when the polarizer is intended for use as a polarizing beamsplitter or polarizing turning mirror.
In general, a wire grid polarizer will reflect light with its electric field vector parallel (“s” polarization) to the wires of the grid, and transmit light with its electric field vector perpendicular (“p” polarization) to the wires of the grid, but the plane of incidence may or may not be perpendicular to the wires of the grid as discussed here. Ideally, the wire grid polarizer will function as a perfect mirror for one polarization of light, such as the S polarized light, and will be perfectly transparent for the other polarization, such as the P polarized light. In practice, however, even the most reflective metals used as mirrors absorb some fraction of the incident light and reflect only 90% to 95%, and plain glass does not transmit 100% of the incident light due to surface reflections. The performance of wire grid polarizers, and indeed other polarization devices, is mostly characterized by the contrast ratio, or extinction ratio, as measured over the range of wavelengths and incidence angles of interest. For a wire grid polarizer or polarization beamsplitter, the contrast ratios for the transmitted beam (Tp/Ts) and the reflected beam (Rs/Rp) may both be of interest.
Historically, wire grid polarizers were developed for use in the infrared, but were unavailable for visible wavelengths. Primarily, this is because processing technologies were incapable of producing small enough sub-wavelength structures for effective operation in the visible spectrum. Nominally, the grid spacing or pitch (p) should be less than ~&lgr;/5 for effective operation (for p~0.10-0.13 &mgr;m for visible wavelengths), while even finer pitch structures (p~&lgr;/10 for example) can provide further improvements to device contrast. However, with recent advances in processing technologies, including 0.13 &mgr;m extreme UV photolithography and interference lithography, visible wavelength wire grid structures have become feasible. Although there are several examples of visible wavelength wire grid polarizers devices known in the art, these devices do not provide the very high extinction ratios (>1,000:1) across broadband visible spectra needed for demanding applications, such as for digital cinema projection.
An interesting wire grid polarizer is described by Garvin et al. in U.S. Pat. No. 4,289,381, in which two or more wire grids residing on one side of a single substrate are separated by a thin dielectric interlayer. Each of the wire grids are deposited separately, and the wires are thick enough (100-1000 nm) to function as a polarizer without significant light leakage through the metal wires. As the dielectric interlayer is thick enough to avoid resonance, the wire grids effectively multiply, such that while any single wire grid may only provide 500:1 polarization contrast, in combination a pair or grids may theoretically provide 250,000:1. This device is described relative to usage in the infrared spectrum (2-100 &mgr;m), although presumably the concepts are extendable to visible wavelengths. However, as this device employs two or more wire grids in a series, the additional contrast ratio is exchanged for reduced transmission efficiency and angular acceptance. Furthermore, the device is not designed for high quality extinction for the reflected beam, which places some limits on its value as a polarization beamsplitter.
A wire grid polarization beamsplitter for the visible wavelength range is described by Hegg et al. in U.S. Pat. No. 5,383,053, in which the metal wires (with pitch p<<&lgr; and ~150 nm features) are deposited on top of metal grid lines, each of which are deposited onto glass or plastic substrate. While this device is designed to cover much of the visible spectrum (0.45-0.65 &mgr;m), the anticipated polarization performance is rather modest, delivering an overall contrast ratio of only 6.3:1.
Tamada et al., in U.S. Pat. No. 5,748,368, describes a wire grid polarizer for the near infrared spectrum (0.8-0.95 &mgr;m) in which the structure of the wires is shaped in order to enhance performance. In this case, operation in the near infrared spectrum is achieved with a wire structure with a long grid spacing (&lgr;/2<p<&lgr;) rather than the nominal small grid spacing (p~&lgr;/5) by exploiting one of the resonances in the transition region between the wire grid polarizer and the diffraction grating. The wires, each ~140 nm thick, are deposited on a glass substrate in an assembly with wedge plates. In particular, the device uses a combination of trapezoidal wire shaping, index matching between the substrate and a wedge plate, and incidence angle adjustment to tune the device operation to hit a resonance band. While this device provides reasonable extinction of ~35:1, which would be useful for many applications

Affiliated with

Also associated with

Rating

Double sided wire grid polarizer does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Double sided wire grid polarizer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Double sided wire grid polarizer will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3217116