Birefringence diffraction grating type polarizer

A birefringence diffraction grating type polarizer is composed of an optically anisotropic sheet crystal substrate. The optically anisotropic sheet crystal substrate is provided with periodical ion-exchanged regions on the main plane thereof, thereby providing an optical diffraction grating. The ion-exchanged regions are covered with dielectric layers, respectively, and further diffused in the vicinity of the surface thereof with metal. Utilizing this structure, it a linearly polarized incident light does not become an elliptically polarized light.

FIELD OF THE INVENTION 
This invention relates to a birefringence polarizer, and more particularly 
to a grating type polarizer based on different diffraction efficiencies 
depending on the direction of polarization. 
BACKGROUND OF THE INVENTION 
A polarizer element, particularly a polarizing beam splitter, is an element 
in which a specific polarized light is obtained at different angles of 
propagation which depend on the direction of polarization of two 
orthogonally polarized components of an incident light. 
Conventional polarizing beam splitters, such as the Glan-Thompson prism or 
Rochon prism, include an element using a crystal with enhanced 
birefringence in which a light path is split due to the difference between 
refraction angles or total reflection angles of two orthogonally polarized 
components of light at the reflection plane of the crystal. In addition, 
the conventional polarizing beam splitter includes an element using a 
total reflection prism which consists of an isotropic optical medium such 
as glass and provided with a multilayer dielectric film formed on the 
reflection plane thereof whereby light is totally reflected, or otherwise 
transmitted, in accordance with refractive indices of polarized 
components. 
These conventional elements, however, have a disadvantage that they are 
large in size, low in production efficiency and expensive. 
The reflection boundary surface in these conventional elements is slanted 
by at least 45 degrees relative to the light axis. Therefore, such an 
element has to take the form of a cube having a side no less than 
.sqroot.2 times a diameter of the light beam to be transmitted. 
Specifically, the side of the cube is as long as 8 to 10 mm where the 
conventional polarizer element is applied to an optical disc recording or 
reproducing apparatus. 
Another type of a conventional polarizer element is disclosed in "National 
conference record, 1982, Optical & Radio Wave Electronics, the Institute 
of Electronics & Communication Engineers of Japan, Part 2". The 
conventional polarizer element consists of a birefringent tapered plate of 
Rutile (TiO.sub.2) having a tapered angle of 4 degrees. When parallel 
light beam is incident to the birefringent tapered plate on one side 
thereof, the parallel light beam is subject to different refractions 
between an extraordinary ray component and an ordinary ray component, so 
that the two components are split on the other side of the birefringent 
tapered plate with a split angle of approximately 1 degree. 
The birefringent tapered plate, however, is associated with a disadvantage 
that a fabricating process is complicated, because the cutting of a 
tapered configuration is difficult on a mass-production basis, and the 
polishing of a tapered surfaces is also difficult to be carried out. There 
is a further disadvantage that Rutile is expensive. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide a birefringence 
diffraction grating type polarizer, the size of which is small. 
It is a further object of the invention to provide a birefringence 
diffraction grating type polarizer which is of a low material cost. 
It is a still further object of the invention to provide a birefringence 
diffraction grating type polarizer which is fabricated by a simple 
process. 
It is a yet still further object of the invention to provide a 
birefringence diffraction grating type polarizer in which it is avoided 
that a linearly polarized light becomes an elliptically polarized light, 
thereby providing a high extinction ratio. 
According to the invention, a birefringence diffraction grating type 
polarizer comprises, 
an optically anisotropic sheet crystal substrate provided with periodical 
ion-exchanged regions on the principal plane thereof to form an optical 
diffraction grating, 
dielectric layers each provided on each of the ion-exchanged regions, and 
a metal diffusion layer in which metal is diffused in the vicinity of the 
surface of the crystal substrate at least within the regions where the 
ion-exchange is effected on the crystal substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before explaining a birefringence polarizer in an embodiment according to 
the invention, the principle and the feature of the invention will be 
briefly explained. 
A birefringence diffraction grating type polarizer according to the 
invention is composed of an optically anisotropic crystal substrate which 
is provided, on the principal plane parallel to its optical axis, with a 
series of ion-exchanged regions, each of which is provided with a 
dielectric layer deposited thereon, whereby a light path is split due to 
the difference in diffraction efficiencies. 
The refractive index within ion-exchanged regions provided periodically on 
the principal plane of, for example, a lithium niobate crystal substrate, 
is changed due to the ion-exchange as much as +0.10 for the ordinary ray 
and -0.04 for the extraordinary ray at the wavelength of 1.3 .mu.m. 
Therefore, if the decrease in the refractive index for the ordinary ray 
within the ion-exchanged region is compensated by increasing the thickness 
of the dielectric layer on the ion-exchanged region, both the diffraction 
efficiency of first or higher order for the ordinary ray and the zero 
order diffraction efficiency for the extraordinary ray can be made null, 
so that a polarizer is formed. 
It is found, however, that distortion of the crystal lattice is sometimes 
caused by the ion-exchange on the substrate. As a result, linearly 
polarized incident light becomes elliptically polarized light, thereby 
resulting in degradation of the extinction ratio. For this reason, a high 
extinction ratio is not obtained, even if polarizers of this type are 
arranged in tandem, or where an optical isolator is composed in the 
combination of the polarizer with a Faraday rotor. 
The polarizer according to the invention is prevented from the distortion 
of the crystal lattice caused by the ion-exchange by means of a metal 
diffused layer near the surface of the crystal, consequently, it is 
avoided that a linearly polarized incident light becomes an elliptical 
polarized light. That is, the crystal lattice is subject to distortion in 
an opposite direction to that in the ion-exchange by the diffusion of 
metal thereinto, where an orientation of the crystal substrate and a kind 
of the metal are appropriately selected. Accordingly, the distortion of 
the crystal lattice caused by the metal diffusion and that of the crystal 
lattice caused by the ion-exchange cancel each other to avoid the 
distortion of the crystal lattice. Therefore, a birefringence diffraction 
grating type polarizer having a high extinction ratio and a minimized 
insertion loss is obtained. 
A birefringence diffraction grating type polarizer in an embodiment 
according to the invention will be explained referring to FIGS. 1 and 2. A 
crystal substrate 1 having optical anisotropy is, in this embodiment, a Y 
cut crystal sheet of lithium niobate (LiNbO3). A series of 
proton-exchanged regions 3 are provided periodically on the substrate 1. 
Each of these proton-exchanged regions 3 is provided with a dielectric 
film 4 of a required thickness deposited thereon to form an optical 
diffraction grating. A metal diffusion layer 2 is formed covering the 
upper surface of the substrate 1. 
As shown in FIG. 2, an incident light 10 of circular polarization is split 
to a zero order diffraction light 11 polarized in the x-axis, a plus first 
order diffraction light 12 and a minus first order diffraction light 13, 
such that the latter two lights are polarized orthogonally to the 
polarization of the zero order light 11. 
The intensity of the zero order diffraction light from the diffraction 
grating as shown in FIG. 1 and FIG. 2 is given by the equation 
EQU cos.sup.2 {.pi.[.DELTA.nT.sub.p +(n.sub.d -1)T.sub.d ]/.lambda.} 
where .lambda. represents a wavelength of light, .DELTA.n represents the 
change in the refractive index by the protonexchange, T.sub.p represents a 
depth of the proton-exchanged regions 3, n.sub.d represents a refraction 
index of the dielectric film 4. If a wavelength of light is 1.3 .mu.m, the 
change in the refractive index due to the protonexchange is about +0.10 
for the ordinary ray and about -0.04 for the extraordinary ray. 
If a film of niobium oxide (Nb.sub.2 O.sub.5) having a refractive index of 
approximately 2.2 is used for the dielectric film 4, the proton- exchanged 
region 3 of about 4.6 .mu.m in depth and the niobium oxide film of 160 nm 
in thickness cause the zero-order diffraction intensity for the 
extraordinary ray to be zero and the zero-order diffraction intensity for 
the ordinary ray to be 1, so that the element functions as a polarizer. 
The polarizer according to the invention is fabricated by 
(1) depositing a metal film on a crystal substrate 1 having an optical 
anisotropy such as lithium niobate etc. by means of sputtering, electron 
beam evaporation or the like, 
(2) providing a metal diffusion layer 2 having the approximately same depth 
as that of proton-exchanged regions 3 by heating in an electric furnace, 
and where the metal is titanium, diffusing titanium thermally by heating 
the substrate at about 1000.degree. C. or higher for several hours, for 
example at 1050.degree. C. for 8 hours, to form titanium diffusion layer 2 
in accordance with a deposited titanium film of a 300 .ANG. thickness, 
(3) forming a mask in the form of a grating over the substrate 1 by a 
conventional lithographic technology or the like, 
(4) immersing the crystal substrate with the mask in an acid at the 
temperature of 200.degree. C. or higher for several hours, for example, in 
benzoic acid at 249.degree. C. for four and half hours, to form 
proton-exchanged regions 3, and 
(5) forming a dielectric film 4 on each of the proton-exchanged regions 3. 
In forming the dielectric film 4, a sputtering process in which niobium 
oxide (Nb.sub.2 O.sub.5) is used for a target, or a reactive sputtering 
process in which Nb is used for a target in an atmosphere of O.sub.2 is 
carried out to provide a grown film, and an ordinary lithographic 
technology is utilized to provide a predetermined pattern, where niobium 
oxide (Nb.sub.2 O.sub.5) is used for the dielectric film 4. If required, 
an anti-reflection film is provided on both the dielectric film 4 and a 
region in which the dielectric film 4 is not provided. The anti-reflection 
film is provided in the form of an SiO.sub.2 film having a thickness of 
approximately 2200 .ANG. and a refractive index of approximately 1.5 by an 
ordinary sputtering process, where the anti-reflection film faces, for 
instance, the air. 
In a polarizer according to the invention, a diffraction angle is inversely 
proportional to a pitch of a diffraction grating. Therefore, the pitch is 
determined, such that the diffraction angle is more than a predetermined 
separation angle. For instance, a first order diffraction angle is 
0.74.degree., where the pitch is 100 .mu.m at a light wavelength of 1.3 
.mu.m, and the first order diffraction angle is 7.5.degree., where the 
pitch is 10 .mu.m at the same wavelength. 
Where a Y plate of lithium niobate is diffused with titanium, crystal 
lattice is subject to distortion in an opposite direction to that in the 
ion-exchange as described before. Therefore, the distortion of the crystal 
lattice caused by a metal diffusion and that of the crystal lattice caused 
by the ion-exchange cancel each other, thereby eliminating the distortion 
of the crystal lattice. Accordingly, either an ordinary or extraordinary 
ray split in a polarizer according to the invention is does not become 
elliptically polarized. This provides a polarizer having a very high 
extinction ratio, where polarizers of the invention are in a tandem 
arrangement, and an isolator having a high extinction ratio, where the 
isolator is provided in the combination of the polarizer with a Faraday 
rotor. 
The diffusion of metal such as titanium may be restricted to 
proton-exchanged regions, though titanium is diffused on the whole surface 
of the substrate in the described embodiment. Copper, nickel, vanadium, 
magnesium or the like may be used in place of titanium in the metal 
diffusion layer 2. 
According to the invention, a polarizer in a thin form can be obtained at a 
low cost, since it can be fabricated from a thin lithium niobate sheet 
crystal in quantities by batch process, and the polarizer provides a high 
extinction ratio and a low insertion loss. 
Although the invention has been described with respect to specific 
embodiment for complete and clear disclosure, the appended claims are not 
to thus limited but are to be construed as embodying all modification and 
alternative constructions that may occur to one skilled in the art which 
fairly fall within the basic teaching herein set forth.