JPH0677089B2 - High birefringence single mode glass optical waveguide - Google Patents

Description

TECHNICAL FIELD The present invention relates to a single-mode glass optical waveguide required for the construction of a waveguide type optical component, and more particularly to a high birefringence single-mode glass optical waveguide. It is a thing.

[Conventional technology and problems]

Single-mode glass optical waveguides manufactured on flat substrates, especially silica-based glass single-mode optical waveguides, have their core cross-sectional dimensions set to about 5 to 10 μm in accordance with commonly used single-mode optical fibers. Therefore, it is expected as a practical means for realizing a waveguide-type optical component having excellent compatibility with an optical fiber.

High-quality silica glass single-mode optical waveguide with low light propagation loss is a glass film deposition technology by flame hydrolysis reaction of glass forming raw material gas such as SiCl ₄ , TiCl ₄ on quartz glass substrate or crystalline silicon substrate. It is known that it can be produced by a combination of the above and a glass film processing technology by reactive ion etching (Masao Kawauchi: “Microfabrication of silica-based optical waveguide” Journal of Japan Society of Applied Physics, Micro Optics Research Group, 1986, 4 / vol.No.2, pp.33-38).

The silica glass single mode optical waveguide fabricated on the silica glass substrate exhibits weak birefringence due to the weak tensile stress received from the silica glass substrate. Usually, the birefringence value B is of the order of 10 ^-5 . The silica glass single mode optical waveguide fabricated on the silicon substrate is subjected to compressive stress due to the difference in coefficient of thermal expansion from the silicon substrate, and exhibits relatively strong birefringence. Birefringence value B in this case is of the order of ^10-4, the maximum 4 × 10 about ^-4.

The birefringence of the optical waveguide is one of the important factors that control the performance such as the polarization characteristics of the waveguide type optical component, and it is desirable to select various birefringence values according to the application purpose of the optical waveguide. However, it has been difficult to realize a large birefringence value exceeding 5 × 10 ⁻⁴ with a conventional silica glass single mode optical waveguide on a silica glass substrate or a silicon substrate.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a high birefringence single mode glass optical waveguide and an applied optical component thereof, which solve the above-mentioned conventional problems.

[Means for solving problems]

The main feature of the present invention is to use a substrate having a coefficient of thermal expansion α of more than 5 × 10 ⁻⁶ / ° C. as a substrate for a silica-based glass single mode optical waveguide. A strong compressive stress acts on the optical waveguide formed on such a substrate under high temperature conditions at room temperature. The optical waveguide exhibits a high birefringence exceeding 5 × 10 ⁻⁴ in response to a large compressive stress.
By the way, the coefficient of thermal expansion of conventionally used quartz glass substrate is α 0.5 × 10 ^-6 / ° C, and that of silicon substrate is α 4
× 10 ^-6 / ℃, silica glass optical waveguide (α1
(× 10 ⁻⁶ / ° C.), the difference in thermal expansion coefficient is small, and strong stress-induced birefringence cannot be generated.

Examples of the substrate satisfying the above requirements of the present invention include zirconium oxide (ZrO ₂ ), that is, a zirconia substrate. Zirconium oxide has a large coefficient of thermal expansion of α10.6 × 10 ^-6 / ° C and excellent heat resistance, and as described later, it is used as a high-quality, highly birefringent single-mode glass optical waveguide. Are better.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1 FIG. 1 is a diagram for explaining a first example of the present invention,
It is a section structure explanatory view of a silica glass single mode optical waveguide (embedding type). In the figure, reference numeral 1 is a zirconium oxide substrate, 2 is a silica-based glass core portion, and 3 is a silica-based glass clad layer surrounding the core portion 2 so as to be embedded therein. A small amount of TiO ₂ is added to the core portion 2 as a refractive index control dopant, and the relative refractive index difference Δ between the core and the cladding is adjusted to 0.25%. The cross-sectional dimension of the core portion 2 is 8 μm, and the thickness of the cladding layer 3 is about 50 μm. The zirconium oxide substrate 1 used is No. Z-201, manufactured by Kyocera Corporation,
It has a thickness of 2 mm and its upper surface is pre-polished to be flat.

FIG. 2 is an explanatory view of an example of a manufacturing process of the single mode optical waveguide shown in FIG. First, (a) a lower clad porous glass layer made of glass fine particles on a zirconium oxide substrate 1 by a flame hydrolysis reaction using SiCl ₄ as a source gas.
21 is deposited, and TiCl ₄ is further added as the next source gas to deposit the core porous glass layer 22. (B) The substrate 1 with the porous glass layer is heated to a high temperature (about 1300 ° C.) in an electric furnace to defoam and become transparent vitrified, and the lower clad glass layer 21a.
And the core layer 22a. At this time, the porous glass layer shrinks in the thickness direction until it becomes about 1/5 to 1/10 in size.
(C) The unnecessary portion of the core layer 22a is removed by reactive ion etching to form the ridge-shaped core portion 2. (D)
The upper clad porous glass layer 23 is again deposited by the flame hydrolysis reaction method so as to cover the core portion 2. (E) Finally, in the electric furnace, the porous glass layer 23 is heated again to a high temperature to become a transparent glass, and the clad layer 3 is combined with the lower clad layer 21a. In the above manufacturing process, to the purpose PCl _3, BCl ₃ or the like of the dopant softening temperature reduction of the glass particles in the raw material gas previously added trace is advisable in order to facilitate vitrification.

The birefringence value B of the single-mode optical waveguide manufactured by the above process was measured by a photoelastic optical method to find that B9 ×
It was 10 ⁻⁴ , which was much larger than that of the conventional optical waveguide formed on the quartz glass substrate or the silicon substrate, and the effect of the zirconium oxide substrate was confirmed. Estimating from the above B value, it is considered that a large compressive stress of about 30 kg / mm ² is acting on the optical waveguide of the present example, but no cracks are generated in the optical waveguide core portion and the clad layer. there were. This is quartz glass, which has a tensile strength of 5 kg / mm ²
However, the compressive strength is as strong as about 100 kg / mm ² , which is presumed to have sufficient resistance to the above compressive stress.

The optical propagation loss of the produced optical waveguide was measured and found to be 0.1 dB / cm or less, which was low enough to form various waveguide type optical components.

By the way, the so-called beat length (the period Lp in which the polarization state changes periodically with propagation), which is an important parameter in discussing the polarization characteristics of the optical waveguide, is defined by the birefringence value B and the used light wavelength λ. , Lp = λ / B, the optical waveguide of this embodiment has a wavelength λ
= 1.3 μm, it has a short beat length of about Lp to 1.4 mm. This means that by using the optical waveguide of the present invention, a polarization control element can be constructed with a short element length on the order of mm.

Example 2 FIG. 3 is a structural explanatory view of a waveguide type optical wave plate as an application example of the high birefringence single mode optical waveguide of the present invention.
(A) is a plan view and (b) is a cross-sectional view taken along the line segment AA '. In the figure, reference numeral 31 is a cladding layer 3 on one side of the core portion 2.
The groove is provided along the core portion 2. Since a part of the stress acting on the optical waveguide from the zirconium oxide substrate 1 is released by the groove 31, the left-right symmetry about the core portion 2 is broken in FIG.
It is possible to incline a and 32b from the direction perpendicular and horizontal to the substrate 1 when there is no stress relief groove 31 by an angle θ. FIG. 4 shows the principal axis angle θ as a function of the groove distance S when the core portion 2 is arranged at the center position of the clad layer 3 having a thickness of 50 μm. FIG. 4 was derived by stress distribution calculation by the so-called finite element method. FIG. ⁴ also shows the normalized birefringence value B / B _{0 based on} the birefringence values B _{0 to} 9 × 10 ⁻⁴ when the groove 31 is not provided.

Referring to FIG. 4, a waveguide type 1/2 optical wave plate was designed and manufactured as follows. First, the groove distance S is set so that the main axis angle θ is 22.5.
S = 34 μm so as to obtain the desired frequency. At this time, B / B ₀ ≈ 0.
6, and it is possible to obtain birefringence with a tilted principal axis angle of B≈5.4 × 10 ⁻⁴ . The length l of the groove 31 is such that B · l = 1/2 · λ, that is, l = 1.2 at the used wavelength λ = 1.3 μm.
It was set to be mm. The width of the groove 31 can be selected relatively arbitrarily and is set to 200 μm in this embodiment. Here, the stress release groove 31 was formed by removing a part of the cladding layer 3 by reactive ion etching.

When linearly polarized light (TM wave) perpendicular to the substrate surface was made incident on the core 2, it was confirmed that the polarization plane rotates 2θ, that is, 45 °, after passing through the stress relief groove 31 forming portion, that is, the half-wave plate portion. Was done. As described above, according to the present invention, it is possible to impart the wave plate function to a desired place of the optical waveguide with a required length as short as about 1 mm.

In the above examples, TiO ₂ was used as the dopant for controlling the refractive index of the core portion, but it is needless to say that GeO ₂ which is often used when manufacturing an optical fiber can be used.

As described above, the zirconium oxide substrates used in the two examples were manufactured by Kyocera Corporation, but are not particularly limited to those manufactured by Kyocera Corporation. It is important to use a high-purity substrate as much as possible because a low-purity substrate often contains impurities such as alkali metals, which causes crystallization of the optical waveguide fabricated on the substrate. .

Further, as a ceramic substrate material having a large coefficient of thermal expansion, in addition to zirconium oxide described above, alumina Al
₂ O ₃ (α8 × 10 ⁻⁶ / ° C.), etc., but according to the results of intensive investigations by the present inventors, on an alumina substrate, the silica glass waveguide causes an obstacle to crystallization during production. Note that no product that could be used practically was obtained. However, it should be pointed out that crystallization can be suppressed by using a composite substrate in which a zirconium oxide thin film is coated on an alumina substrate. It goes without saying that a high birefringence optical waveguide on a composite substrate having such a large coefficient of thermal expansion is also included in the scope of the present invention. In addition to the above-mentioned oxides, carbides, nitrides, refractory metals, and the like have potential as substrate materials. In addition to the large thermal expansion coefficient, heat resistance, mechanical properties (Young's modulus, etc.), It is necessary to select in consideration of the presence or absence of crystallization of quartz glass.

〔The invention's effect〕

As described above, according to the present invention, it is possible to provide a single mode optical waveguide having high birefringence, which is unprecedented. Therefore, for coherent optical communication or optical sensor in which polarization characteristics play an important role. It is extremely effective for compactly constructing a waveguide type optical component, for example, a wave plate, a polarizer, a polarization separation element and the like.

[Brief description of drawings]

FIG. 1 is a sectional structural view of an optical waveguide shown as an embodiment of the present invention, FIGS. 2 (a) to 2 (e) are manufacturing process diagrams of the optical waveguide of the present invention, and FIG. FIG. 4 is a structural explanatory view of another embodiment, and FIG. 4 is a birefringence value prediction diagram by the finite element method. DESCRIPTION OF SYMBOLS 1 ... Zirconium oxide substrate, 2 ... Core part, 3 ... Clad layer, 21 ... Lower clad porous layer, 22 ... Core porous layer, 23
... upper clad porous layer, 21a ... lower clad layer, 22a ... core layer, 31 ... stress release groove, 32a, 32b ... birefringent principal axis direction.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kaname Jinguji Temple 162 Shirahane, Shirahoji, Tokai-mura, Naka-gun, Ibaraki Prefecture, Nippon Steel Telephone Corporation, Ibaraki Telecommunications Research Institute (56) Reference JP-A-59-208509 (JP, A)

Claims (1)

[Claims] 1. A silica-based glass clad layer is formed on a flat zirconium oxide substrate with a silica-based glass core portion having a refractive index larger than that of the clad layer being located inside, and a silica-based glass formed on the substrate. A high birefringence single mode glass optical waveguide, wherein the single mode optical waveguide is subjected to compressive stress from the substrate.