Polarization beam splitter

Details Polarization beam splitter Polarization beam splitter

2002-11-26

2004-09-14

Juba, Jr., John (Department: 2872)

Optical: systems and elements

Polarization without modulation

Polarizarion by dichroism

C359S485050, C359S583000, C359S588000

Reexamination Certificate

active

06791750

ABSTRACT:

This application is based on Japanese Patent Application No. 2002-280188 filed on Sep. 26, 2002, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polarization beam splitter that separates P- and S-polarized light.
2. Description of the Prior Art
Polarization beam splitters that separate polarized light components that are polarized on mutually perpendicular polarization planes are used in optical systems of image display apparatuses and optical disk apparatuses. A polarization beam splitter is provided with a dielectric multilayer film having two types of dielectric with different refractive indices alternately laid on top of one another so that each layer has an optical film thickness equal to ¼ of the wavelength of light to be separated. Of the light obliquely incident on this dielectric multilayer film, P-polarized light is transmitted therethrough and S-polarized light is reflected therefrom. This makes possible the separation of the two differently polarized light components.
For efficient separation of P- and S-polarized light, it is advisable to make the angle of incidence at which light is incident on the dielectric multilayer film as close to the Brewster angle as possible. Moreover, it is preferable that the refractive index of the dielectric multilayer film and the angle at which the light travels therethrough fulfill formula (0) below.
sin
2
&phgr;=NH
2
·NL
2
/[NE
2
·(
NH
2
+NL
2
)]  (0)
where NE represents the refractive index of the medium that is located contiguous with the dielectric multilayer film and from which light enters the dielectric multilayer film, NH represents the refractive index of the high-refractive-index layers of the dielectric multilayer film, NL represents the refractive index of the low-refractive-index layers of the dielectric multilayer film, and &phgr; represents the angle of the light relative to a normal to the dielectric multilayer film.
Moreover, it is necessary to protect the dielectric multilayer film and to package it into an easy-to-handle optical device.
In consideration of these requirements, a polarization beam splitter is generally composed of a dielectric multilayer film sandwiched between two transparent media, and the dielectric multilayer film is designed to be used at an angle of incidence of 45° and to fulfill formula (0). Moreover, to minimize the deflection of light at the interface between the transparent media and air, and to make easy the handling of separated light by making P- and S-polarized light components travel in mutually perpendicular directions after separation, the two transparent media are each formed into the shape of a prism of which the section has the shape of a right-angled isosceles triangle, with the dielectric multilayer film sandwiched between the hypotenuse surfaces of those prisms.
A conventional polarization beam splitter built as described above exhibits, when light is incident on the dielectric multilayer film at an angle of incidence of 45°, high transmissivity for P-polarized light and high reflectivity for S-polarized light over a wide wavelength range about a reference wavelength, and thus it separates the two differently polarized light components very effectively. However, a conventional polarization beam splitter exhibits high dependency on angle of incidence; that is, when the angle of incidence at which light is incident on the dielectric multilayer film deviates even slightly from the design value of 45°, the polarization beam splitter separates P- and S-polarized light less effectively, exhibiting particularly lower transmissivity for P-polarized light. That is, unless light is incident at 45°, the wavelength range in which differently polarized light components are separated effectively is extremely narrow.
Some optical systems employing a polarization beam splitter handle only light of a single wavelength. However, many such optical systems handle light spreading over a certain range of wavelengths. For example, optical systems for use in image display apparatuses handle light spreading all over the range of wavelengths of visible light. To miniaturize such optical systems, it is preferable to use as few polarization beam splitters as possible, and to make a convergent or divergent beam of light incident on a polarization beam splitter.
However, because of the above-mentioned dependency on angle of incidence, it is impossible to effectively separate light spreading over a wide wavelength range when the light is in the form of a convergent or divergent beam. Even when the principal ray of a convergent or divergent beam of light is made incident on a dielectric multilayer film at an angle of incidence of 45° as designed, the rays other than the principal ray are incident at angles deviated from 45°. Thus, only the portion of the beam quite near its principal ray is separated effectively into P- and S-polarized light, and the other portion, particularly the peripheral portion, of the beam is separated markedly less effectively.
FIGS. 25
to
27
show the relationship between the wavelength of light and transmissivity in a conventional polarization beam splitter. In these figures, thick lines represent the transmissivity for S-polarized light, and thin lines represent the transmissivity for P-polarized light.
FIG. 25
deals with the ray that makes an angle of 45° with a normal to the dielectric multilayer film before entering the prism (i.e., in the layer of air). This ray is incident on the dielectric multilayer film at an angle of incidence of 45°.
FIG. 26
deals with the ray that makes an angle of 34.7° with a normal to the dielectric multilayer film before entering the prism.
FIG. 27
deals with the ray that makes an angle of 55.3° with a normal to the dielectric multilayer film before entering the prism. These three rays correspond to the principal ray and the two outermost rays of a convergent or divergent beam of light of which the f-number as observed in the layer of air is 2.8 and of which the principal ray makes an angle of 45° with the dielectric multilayer film. The refractive index Nd of the prism is 1.84.
As
FIG. 25
clearly shows, with the principal ray, which is incident on the dielectric multilayer film at an angle of incidence of 45°, it is possible to separate P- and S-polarized light effectively over a wide wavelength range of from about 440 nm to about 640 nm. However, as
FIG. 26
shows, with the outermost ray that is incident on the dielectric multilayer film at the smallest angle of incidence, effective polarization separation is possible only in wavelength ranges of from about 490 nm to about 540 nm and from about 600 nm to about 670 nm. Moreover, as
FIG. 27
shows, with the outermost ray that is incident on the dielectric multilayer film at the largest angle of incidence, effective polarization separation is possible only in wavelength ranges of from about 380 nm to about 430 nm and from about 490 nm to about 600 nm.
Thus, the wavelength range in which the whole beam of light having an f-number of 2.8 is effectively separated into P- and S-polarized light is extremely narrow, namely from 490 nm to 540 nm. The greater the f-number of the beam of light, the wider the wavelength range in which effective polarization separation is possible, but increasing the f-number poses a demanding requirement on the beam of light to be subjected to polarization separation. This makes it difficult to miniaturize optical systems incorporating polarization beam splitters.
As one way to overcome this inconvenience, there have conventionally been proposed (for example, in Japanese Patent Applications Laid-Open Nos. H7-281024, H11-211916, and 2001-350024) polarization beam splitters in which the dielectric multilayer film is composed of two multilayer portions, of which the first is designed to fulfill formula (0) at the angle (&phgr;
1
) of a first ray with respect to a first reference wavelength and of which the second is designed t

Affiliated with

Also associated with

Rating

Polarization beam splitter does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Polarization beam splitter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polarization beam splitter will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3209248