JPH02205802A - Polarizing element - Google Patents

Abstract

PURPOSE:To obtain the small-sized, thin, and low-price polarizing element which has superior mass-productivity by arranging 1st areas which have specific one- dimensional periodic structure and 2nd areas which have surface oxidized layers alternately in a lattice shape. CONSTITUTION:The 1st areas 1 which have surface irregular type one- dimensional periodic structure which has a pitch less than a half of the wavelength of a light wave and the 2nd areas 2 which have the surface oxidized layers are arranged on a substrate 3 alternately in a lattice shape. Namely, diffraction gratings are used to obtain 0-th order diffraction efficiency, i.e. transmissivity which is 100% to specific polarized light and 0% to polarized light perpendicular to said specific polarized light, thereby constituting the polarizing element which has a polarization beam splitter function. Consequently, birefringent crystal is not required, the element is composed of a material which is easily obtained at low cost like silicon, and many elements can easily be manufactured at the same time by photolithography while assembly is not required, so that cost is reducible. Further, the substrate is only thick enough to give strength to the element, so the thin, lightweight element can be constituted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a polarizing element used as a polarizing beam splitter to configure isolators and optical circulators for optical fiber communications, optical heads for optical disks, and the like.

[Conventional technology]

Conventionally, Glan-Thompson prisms and Rochon prisms have been used as polarizing beam splitters for optical isolators and optical circulators.

These prisms are made by gluing two triangular prisms of birefringent crystals such as quartz or calcite with different crystal axes. Regarding these prisms, see Kunio Yoshiwara, “
No. 213 of “Physical Optics” (Kyoritsu Shuppan, published in 1966)
It is explained in detail on pages 1 to 216.

[Problem to be solved by the invention]

The conventional polarizing elements mentioned above use birefringent crystals, so the materials are expensive, and such polarizing elements require a process of polishing all four sides and then bonding them, which increases the number of manufacturing steps. The problem was that it was expensive and not suitable for mass production. Furthermore, since it has a structure of two triangular prisms glued together, it is large, which is an obstacle to miniaturizing optical isolators and optical circulators.

An object of the present invention is to solve the above problems and to achieve a compact design.

The object of the present invention is to provide a polarizing element that is thin, inexpensive, and excellent in mass production.

[Means to solve the problem]

The polarizing element of the present invention includes, on a substrate, a first region having a one-dimensional periodic structure with a surface unevenness having a pitch smaller than 172 wavelengths of light;
The second region having a surface oxidized layer is alternately arranged in a grid pattern.

[Effect]

The working principle of the present invention is as follows. In the present invention, by using a diffraction grating, the O-th order diffraction efficiency, that is, the transmittance is 100% for a specific polarized light and 0% for polarized light orthogonal to this, thereby achieving a polarizing beam splitter function. It constitutes a polarizing element.

The diffraction efficiency of the O-order diffracted light of a phase grating with a rectangular cross section is ηo=
(1+cosr) (1) is given. Here, T is the phase difference that light receives at the grating portion, and is expressed as γ=2πt·Δn/λ (2). In equation (2), L is the thickness of the grating, Δn is the difference in refractive index between the two media forming the phase grating, and λ is the wavelength of light.

In order to obtain the change in the zero-order diffraction efficiency as described above, the phase difference γ of the grating must be Tl1=o due to the polarization.

It is necessary to change T□=π (where I+ and earth represent polarization parallel and perpendicular to the grating grooves, respectively), and (
In equation 2), Δn must change depending on the polarization, and usually requires a birefringent material.

The present invention utilizes a dense periodic structure in order to obtain birefringence without using expensive birefringent crystals.

A phase grating with a pitch smaller than 1/2 of the wavelength of light does not generate diffracted light and exhibits birefringence.

Let n be the effective refractive index in the direction parallel to the grooves of the surface uneven grating, and n be the effective refractive index in the direction perpendicular to the grooves, then n, = (nt"q+nz"(1-q))"" (3
)”A −((1/nl”) q + (1/nz”) (1q) )−”” (4)
Here, n is the refractive index of the groove, n2 is the refractive index of the land, and q is the ratio of the width of the groove to the grating pitch. This birefringence is used to change the diffraction efficiency depending on the polarization.

FIG. 2 shows a grating type polarizing element using a dense grating. A dense lattice is used in the first region 1 and the second region 1 has a flat surface.
A lattice is formed on the substrate 3 in which areas 2 of the lattice are alternately arranged.

If the traveling direction of light is the direction of arrow 4, the substrate refractive index is n
2, the first polarization in the direction parallel to the grating grooves is
The refractive index difference between the region and the second region is (Δfi),
-fi, -fill, (Δn) for polarized light in the direction perpendicular to the grating grooves.

=nz, -n. However, with only this change in Δn, even if the grating thickness t is selected, T in equation (2) cannot satisfy both of γm=0=rh''π for both polarized lights.

Therefore, the present invention adopts the structure shown in FIG. 1 in which the surface refractive index of the second region of the grating shown in FIG. 2 is changed for phase adjustment. In FIG. 1, the depth of the uneven groove in the first region 1 is Ll.
, the thickness of the surface layer having a different refractive index n in the second region 2 is t2, then the phase difference of the bihead region for each polarized light is:
It is given by (6). In equations (5) and (6), γ,
=0. There exists t+, Lx that satisfies both conditions r, = π.

The above is the principle of the present invention. In order to make the groove depth t and the surface layer thickness t2 shallow, that is, to easily manufacture them, n,
It is necessary that l−n be large, and for this purpose it is a good idea to use a material with a large refractive index n2.

Therefore, in the present invention, for example, silicon (
The surface layer is made of an oxide layer, for example, Si obtained by thermally oxidizing silicon.

[Example] Next, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a partial perspective view showing the basic configuration of an embodiment of the present invention. As mentioned above, since it is preferable for the material of the substrate 3 to have a large refractive index, silicon (Si) crystal was used for long-wave optical communication with an optical wavelength of 1.3 μm. At this wavelength, silicon is transparent and has a refractive index of n,=3.5. If the groove width ratio to the pitch of the dense lattice is q = 0.5, (3),
From formula (4), n1I=2.5739゜n=1.3
It becomes 598. Since the pitch of the dense lattice should be smaller than λ/2, it was set to 0.6 μm.

Further, the period of the grating structure consisting of the first region 1 and the second region 2 was set to 50 am in this example, since the design condition was that the desired zero-order diffraction light and other-order diffraction light could be separated.

The above n z =3-5, n 3 =1.446 +
I'l m ” 2.5739.

Solving equations (5) and (6) using n = 1.3598, t, -〇, 535 μm, tz = 0.241 μm
Therefore, t+ = 0.54 μm, tz = 0.
A 21 μm grid was fabricated.

First, a portion of the substrate 3 that will become the first region 1 is masked, and the second region 2 is thermally oxidized to a thickness tz = 0.21μ.
A SiO□ layer of m was formed. Next, after removing the masking, the resist in the first region is patterned by holographic interference, and the substrate 3 is etched by reactive ion etching.
．． A dense lattice of 6 μm was formed.

According to the polarizing element of this example, the extinction ratio between polarized light parallel to the grating grooves and polarized light perpendicular to the grating grooves was 20 dB.

Note that in this example, the first region and the second region of the grating polarizing element are
The periodic direction of the dense lattice in the first region is perpendicular to the boundary surface of the region, but the periodic direction of the dense lattice in the first region is It is clear from the explanation in the operation section above that any angle with respect to the boundary surface may be used.

〔Effect of the invention〕

The polarizing element of the present invention does not require a birefringent crystal, is made of an easily and inexpensively available material such as silicon, and can be easily manufactured in large numbers at the same time using photolithography. It is extremely inexpensive as no assembly is required.

In addition, in the polarizing element of the present invention, the refractive index of the surface layer in both the first region and the second region is smaller than the refractive index of the substrate, and acts as an antireflection film. Despite using a high refractive index material, there is also the effect of low reflection loss even without coating.

Further, the polarizing element of the present invention is essentially a thin film element,
Since the substrate only needs to have a thickness sufficient to provide the strength of the element, it is possible to construct an extremely thin and lightweight element of approximately 100 μm.

[Brief explanation of the drawing]

FIG. 1 is a partial perspective view of an embodiment of the invention, and FIG. 2 is a partial perspective view of a grid for explaining the invention in detail. 1...First area 2...Second area 3...Substrate 4...Arrow indicating the direction of travel of light Figure 1 Agent: Yoshi Iwasa, patent attorney Happy Figure 2

Claims (1)

[Claims] (1) On a substrate, a first region having a one-dimensional periodic structure of a surface unevenness type with a pitch smaller than 1/2 of the wavelength of light and a second region having a surface oxide layer are alternately arranged in a lattice. A polarizing element characterized by being arranged in a shape.