JPH11352349A - Waveguide type optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a waveguide type optical element wide in application range and sufficiently high in performance as a TM mode filter while being manufacturable at a low cost. SOLUTION: This optical element is provided with a substrate 1 provided with anisotropy in a refraction index, the optical waveguide 2 of a three- dimensional type embedded and formed on one surface of the substrate 1 and a constructive double refraction film 3 formed on the surface of the substrate 1 covering the waveguide 2. The structural double refraction film 3 is a laminated body for which a high refraction index layer which is a thin film provided with an isotropic refraction index larger than the refraction index of the optical waveguide 2 and a low refraction index layer which is the thin film provided with the isotropic refraction index smaller than the refraction index of the optical waveguide 2 are alternately laminated. Since the structural double refraction film 3 for imparting a function as the TM mode filter to the optical waveguide 2 can be formed by normal sputtering without using an epitaxial growth method, the sufficiently high performance as the TM mode filter is obtained while being manufacturable at a low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of a waveguide type optical element, and more particularly, to the technical field of a waveguide polarizer having a function of a TM mode filter.

[0002]

2. Description of the Related Art A conventional waveguide polarizer (TM mode filter) is disclosed in a document by Ichikawa et al. (Preliminary Lectures of the 44th Lecture Meeting on Related Physics, 30a-NE-6 (1997)). There are things. In this waveguide polarizer, a two-dimensional titanium diffusion waveguide is formed on a lithium niobate crystal (Z-cut: LiNbO ₃ ) substrate, and an anisotropic crystal is further formed thereon to form a three-dimensional titanium diffusion waveguide. As a waveguide, TM
This is a mode filter.

[0003]

However, the above-described waveguide polarizer according to the prior art has a disadvantage because an epitaxial crystal growth process is required to form an anisotropic crystal. That is, since the control of the processing conditions becomes strict and the number of steps is increased, and the manufacturing cost is considerably increased, there is an inconvenience that the cost is higher than when a thin film forming method such as sputtering is adopted. Not only that, there are also disadvantages in that the places where elements can be formed and the shape of the waveguide are greatly restricted for the same reason, and applicable cases are limited.

Accordingly, an object of the present invention is to provide a waveguide type optical element which can be manufactured at low cost, has a wide range of application, and has sufficiently high performance as a TM mode filter.

[0005]

According to the present invention, there is provided a waveguide type optical device comprising: a substrate having anisotropic refractive index; a three-dimensional optical waveguide buried on one surface of the substrate; And a structural birefringent film formed on the surface of the substrate so as to cover the optical waveguide. A feature of the waveguide type optical element of the present invention is that the structural birefringent film is a thin film made of a material having an isotropic refractive index larger than the refractive index of the material forming the optical waveguide. A layer and a low-refractive-index layer, which is a thin film made of a material having an isotropic refractive index smaller than the refractive index of the material forming the optical waveguide, are alternately laminated. .

That is, the waveguide type optical element according to the present invention comprises a substrate having an anisotropic refractive index, for example, a substrate having an anisotropic refractive index for TM mode light is lower than that for TE mode light. And a structural birefringent film in which high-refractive-index layers and low-refractive-index layers are alternately stacked in contact with an optical waveguide formed on the substrate. Therefore, while the light wave propagates in the optical waveguide, the light in the TE mode in which the oscillating electric field surface of the propagating light is parallel to the substrate surface enters the structural birefringent film and departs from the optical waveguide. On the other hand, TM mode light in which the oscillating electric field surface of the propagating light is perpendicular to the substrate surface is confined in the optical waveguide without entering the structural birefringent film, and propagates in the optical waveguide with almost no attenuation. Therefore, the waveguide type optical element of the present invention can exhibit high performance as a TM mode filter that removes TE mode light and transmits only TM mode light.

Further, since the high refractive index layer and the low refractive index layer forming the laminated body do not need to be epitaxy crystals, when alternately laminating the high refractive index layer and the low refractive index layer, epitaxial growth is performed. No need to do. Therefore, a relatively inexpensive thin film forming method such as sputtering can be adopted in forming the laminate, and the cost is reduced as compared with the conventional technology. In addition, for the same reason, the restrictions on the location where the element can be formed and the shape of the waveguide are small, and the applicable cases are wider than in the related art.

Therefore, according to the waveguide type optical element of the present invention, there is an effect that it can be manufactured at low cost, has a wide application range, and can obtain a sufficiently high performance as a TM mode filter.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As a substrate having anisotropic refractive index, the Z-axis (a horizontal bar is originally attached to Z, but it is impossible to use it in the application, simply substitute for Z) is used. Lithium niobate crystal (Z-cut: LiNb
O ₃ / Originally, a horizontal bar is attached on Z, but it is a character that cannot be used in the application, so it is simply referred to as Z in this specification.) However, since an appropriate anisotropy can be obtained, it can be adopted in terms of cost The best of the best materials. On the other hand, as a three-dimensional optical waveguide formed by being buried on one surface of the same substrate, it is technically established and is the most common and has good waveguide characteristics. It is desirable to employ a titanium diffused waveguide.

Since the substrate has an anisotropic refractive index as described above, there are two types of refractive index of the lithium niobate crystal (Z-cut: LiNbO ₃ ) forming the substrate. . That is, for a light beam of wavelength λ = 830 nm,
The value of the refractive index no in the ordinary ray direction in which the electric field oscillation plane is perpendicular to the Z axis is 2.25, and the value of the refractive index ne in the extraordinary ray direction in which the electric field oscillation plane is parallel to the Z axis is 2.25. Eighteen.

Therefore, the material of the high refractive index layer and the material of the low refractive index layer which form the structural birefringent film formed on the surface of the substrate over the optical waveguide are formed of a light having a wavelength λ = 830 nm. On the other hand, the following conditions must be satisfied. That is, the refractive index in the direction perpendicular to the entire film surface of the structural birefringent film is nfn, and the refractive index in the in-plane direction of the film surface is nfn.
Assuming that fp, the refractive index n1 of the high refractive index layer has a magnitude relation (no
<Nfp <n1), and the refractive index n2 of the low-refractive-index layer must satisfy the magnitude relationship (n2 <nfn <ne). As a light-transmitting substance that satisfies this condition, there are several candidates as described below. Here, the term “translucent substance” means a substance where k = 0 substantially when the complex refractive index is n + ik.

That is, TiO ₂ (n 1 ≒ 2.3) and CeO ₂ (n
1 ≒ 2.3), ZnS (n1 ≒ 2.3), ZnSe (n
1 ≒ 2.4). On the other hand, as a material candidate for forming the low refractive index layer, SiO ₂ (n ₂ 21.4
5), MgF ₂ (n2 ≒ 1.37), Na ₃ AlF ₆ (n
2 ≒ 1.37) and CaF ₂ (n2 ≒ 1.37). The values in parentheses above indicate the value of the refractive index of the film obtained by the ordinary sputtering method.

If a candidate having excellent water resistance (not deteriorated by water and not corroded) and scratch resistance (mechanical scratch resistance) is selected from these candidates, the high refractive index layer is made of titanium dioxide TiO _2. And the low refractive index layer is silicon dioxide S
It is most preferably made of iO _2. Therefore, in the waveguide type optical element described in the section of “Means for Solving the Problems”, the substrate is made of lithium niobate crystal (Z-cut: LiNbO ₃ ) cut with the Z axis as a normal line. The high refractive index layer is preferably made of titanium dioxide TiO ₂ , and the low refractive index layer is preferably made of silicon dioxide SiO ₂ .

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the waveguide type optical device according to the present invention will be described below with reference to the drawings. Embodiment 1 (Structure and Operation and Effect of Embodiment 1) As shown in FIG. 1, a waveguide type optical element as Embodiment 1 of the present invention
It comprises an optical waveguide 2 formed on a substrate 1 and a structural birefringent film 3 formed so as to cover both of them.

The substrate 1 is a substrate made of lithium niobate crystal (Z-cut: LiNbO ₃ ) cut with the Z axis as a normal line and having anisotropic refractive index. That is, the substrate 1 has anisotropy such that the refractive index nn for TM mode light is lower than the refractive index np for TE mode light (nn <np). Here, the arrows in FIG. 1 showing the TM mode light and the TE mode light indicate the vibration directions of the electric field of the light beam in the mode.

The optical waveguide 2 is a titanium diffusion waveguide, and is a three-dimensional optical waveguide buried and formed on one surface of the substrate 1. The structural birefringent film 3 is again shown in FIG.
As shown in FIG. 2, a structural birefringent film formed on the surface of the substrate 1 so as to cover the optical waveguide 2, and as shown in FIG.
The high refractive index layers 31 and the low refractive index layers 32 each having a very small thickness are alternately laminated. That is, the structural birefringent film 3 forms the optical waveguide 2 and the high refractive index layer 31 which is a thin film made of a material having an isotropic refractive index larger than the refractive index of the material forming the optical waveguide 2. A low-refractive-index layer 32, which is a thin film made of a material having an isotropic refractive index smaller than that of the material, is alternately laminated. Here, the high refractive index layer 31 is made of titanium dioxide TiO.
₂ and the low refractive index layer 32 is made of silicon dioxide SiO ₂ .

The refractive index n1 of the high refractive index layer 31 of this embodiment formed by a manufacturing method described later has reached 2.43, and the wavelength λ of the transparent titanium dioxide TiO ₂ film is λ = 830.
The value for nm is exceptionally high, which cannot be considered with common sense. That is, in the present embodiment, the structural birefringent film 3
Of these, a very high value is obtained for the refractive index n1 of the high refractive index layer 31, so that it is considered that extremely good performance is obtained as a TM mode filter.

That is, titanium dioxide TiO_TwoConsists of
The refractive index n1 of each high refractive index layer 31 is about 2.43,
Has a thickness t1 of about 20 nm. On the other hand, silicon dioxide S
iO _TwoHas a refractive index of about 1.45.
And the film thickness t2 is about 4 nm. therefore,
The wavelength λ of the light incident on the waveguide type optical element of this embodiment is 830n.
m, the height at which the structural birefringent film 3 is formed is high.
Respective film thicknesses t1 and t2 of the refractive index layer 31 and the low refractive index layer 32
Is sufficiently thin with respect to the wavelength λ of the incident light (t1, t2 ≪
λ). On the other hand, the spread of the film surface of the structural birefringent film 3 depends on the wavelength.
It has a scale size large enough for λ.

The structural birefringent film 3 has a thickness of 20 nm.
The high refractive index layer 31 and the low refractive index layer 32 having a thickness of 4 nm are respectively laminated over 21 layers, and the total thickness of the structural birefringent film 3 is about 0.5 μm. This level is appropriate as the total thickness of the structural birefringent film 3.
The reason is that if the total thickness of the structural birefringent film 3 is too thin, the insertion loss increases, which is inconvenient. On the other hand, if the total thickness is too thick, separation occurs or the polarization extinction ratio deteriorates. This is inconvenient.

Further, a thickness 2 from the top of the structural birefringent film 3
A 00 nm aluminum film is formed, and functions as a light absorbing film. Here, the high refractive index layer 3
The ratio of the refractive index between 1 and the low refractive index layer 32 is defined as η≡n1 / n2, and the ratio between the film thicknesses of both layers is defined as δ≡t1 / t2. Then, the refractive index nfn in the direction perpendicular to the film surface of the entire structural birefringent film 3 and the refractive index n in the in-plane direction of the film surface
fp is uniquely determined by the following Equations 1 and 2, respectively.

[0021]

## EQU1 ## nfn ² = n 1 ² (1 + δ) / (δ + η ² )

[0022]

## EQU2 ## nfp ² = n ^{2 2} (δη ² +1) / (1+
δ) When calculated by the above equations (1) and (2), the refractive index nfn in the direction perpendicular to the film surface of the entire structural birefringent film 3 is 2.
It is about 1, and the refractive index nfp in the in-plane direction of the film surface is about 2.3. The refractive index of the lithium niobate crystal (Z-cut: LiNbO ₃ ) forming the substrate 1 is no = 2.25 in the ordinary ray direction (the electric field vibration plane is perpendicular to the Z axis), and the extraordinary ray direction ( Ne = 2.18 when the electric field vibration plane is parallel to the Z axis)
It is. Here, the conditions for the structural birefringent film 3 to act as a TM mode filter on the substrate 1 are nfn <ne for TM mode light and nfn for TE mode light.
Although fp <no, the above numerical values satisfy these conditions. Therefore, the light in the TE mode enters the structural birefringent film 3 and departs from the optical waveguide 2, but the light in the TM mode propagates through the optical waveguide 2 without entering the structural birefringent film 3. The waveguide type optical element of the embodiment functions as a TM mode filter.

Further, as described above, the refractive indices of the high refractive index layer 31 and the low refractive index layer 32 forming the structural birefringent film 3 are n 1 ≒ 2.43 and n 2 ≒ 1.45, respectively. The following magnitude relation for acting as a TM mode filter is satisfied. That is, assuming that the refractive index in the direction perpendicular to the entire film surface of the structural birefringent film 3 is nfn and the refractive index in the in-plane direction of the film surface is nfp, the high refractive index layer 3
1 satisfies the magnitude relation (no <nfn <n1), and the refractive index n2 of the low refractive index layer 32 has the magnitude relation (n2 <n).
fp <ne). Here, the material forming the high refractive index layer 31 is TiO ₂ (n1 ≒ 2.43), and the material forming the low refractive index layer 32 is SiO ₂ (n2 ≒ 1.4).
5).

On the surface of the structural birefringent film 3, an aluminum film having a thickness of 200 nm is formed as an absorbing film. (Manufacturing Method of Embodiment 1) The manufacturing method of the waveguide type optical element of this embodiment includes a substrate forming step, a waveguide forming step, a structural birefringent film forming step, and a light absorbing film forming step. Have.

In the substrate forming step, a single crystal of lithium niobate crystal was cut with the Z axis as a normal line, and a surface perpendicular to the Z axis was mirror-polished to form a substrate 1 made of Z-cut: LiNbO ₃ . In the waveguide forming step, the mirror-polished Z-cu
t: On the surface of the substrate 1 of LiNbO ₃ , a titanium film having a desired waveguide shape (in this embodiment, linear) is formed with a thickness of about 50 nm. Thereafter, in an atmosphere in which oxygen bubbled with hot water at 60 ° C. is flowed at a flow rate of 100 ml / min, the temperature of the substrate 1 is raised to 1025 ° C. and maintained for 6 hours, heat treatment is performed to diffuse titanium, and titanium diffusion is performed. An optical waveguide 2 as a waveguide was formed.

In the step of forming a structural birefringent film, first, a rectangular region for forming a TM mode filter was left around the above-mentioned waveguide and its peripheral portion, and the peripheral portion was masked with a thin stainless steel plate. Thereafter, titanium dioxide TiO ₂
The substrate 1 is placed in an RF magnetron sputtering apparatus in which a target and a silicon dioxide SiO ₂ target are placed, and the temperature of the substrate 1 is raised to 350 ° C. and maintained. The atmosphere sealed in the RF magnetron sputtering apparatus is a sputtering gas of 90% argon and 10% oxygen, and the gas pressure is set to 0.67 Pa (= 5 × 10 ⁻³ Torr). The sputtering power is about 200 W per target of about φ51 mm (φ2 inch) for titanium dioxide TiO ₂ and about φ51 for silicon dioxide SiO ₂ .
The power was set to about 100 W per mm (φ2 inch) target.

Then, first, a 4 nm thick silicon dioxide SiO ₂ film is formed as the low refractive index layer 32, and then a 20 nm thick titanium dioxide TiO ₂ is formed as the high refractive index layer 31.
_Two films were formed and these were repeated. As a result, the thickness 4
low-refractive-index layer 32 made of silicon dioxide SiO ₂
And a high-refractive-index layer 31 made of a titanium dioxide TiO ₂ film having a thickness of 20 nm were alternately formed 21 times, and a total of 42 layers were formed. After completion of the film formation, the temperature of the substrate 1 was lowered to room temperature, and the step of forming a structural birefringent film was completed. The layer structure formed in this step is [substrate / ｛SiO ₂ (4 nm) /
TiO ₂ (20 nm)｝ × 21 / surface], and the thickness of the formed 21 pairs of the high refractive index layer 31 and the low refractive index layer 32 is about 0.5 μm.

When the refractive index of the thus formed structural birefringent film 3 was measured with a laser beam having a wavelength λ = 850 nm, nfn = 2.13 for TM mode light and TE mode light. For light, nfp = 2.30.
In the last light absorbing film forming step, the thickness of the structural birefringent film 3 is set to 2
A 00 nm aluminum film was formed. At this time, the atmosphere was 100% argon gas and the pressure was 0.40 Pa (= 3 ×
10 ⁻³ Torr) and the sputtering power is about φ51 m
The power was set to about 100 W per m (φ2 inch) target.

As described above, the structural birefringent film 3 having the light absorbing film formed on the surface was formed so as to cover the optical waveguide 2 formed on the substrate 1. (Evaluation of Characteristics of Example 1) In order to measure the performance of the waveguide type optical device of the present example manufactured as described above, a portion of the optical waveguide 2 covered with the structural birefringent film 3 was measured. The polarization extinction ratio and the excess loss for TE mode light were measured. The width of the optical waveguide 2 of the waveguide type optical device of this embodiment is 5 μm.
In the optical waveguide 2, the length of the TM mode filter covered with the structural birefringent film 3 was 20 mm. In the above measurement, a single mode semiconductor laser having a wavelength λ = 850 nm is used as a light source, a polarization maintaining fiber is used to introduce a laser beam into the optical waveguide 2, and a single mode fiber is used to derive an output from the optical waveguide 2. Using.

When measured by such a measurement arrangement, the polarization extinction ratio was about 30 dB, and the excess loss for TE mode light was about 1.5 dB. As described above in detail, according to the waveguide type optical element of the present embodiment, a TM mode filter having a wide range of application and sufficiently high performance can be provided while being inexpensive (because epitaxial growth is not required). I found that I could do it.

(Various Modifications of Embodiment 1) In this embodiment, the material forming the substrate 1 and the optical waveguide 2 is such that the refractive index nn of the optical waveguide 2 with respect to the TM mode light is TE.
If the material is lower than the refractive index np for the mode light,
It is possible to adopt a material other than the first embodiment. For example, as the substrate 1, a lithium niobate crystal (Z-cu
t: LiNbO ₃ ) instead of LiTaO ₃ , ZnO,
Modifications using KTa _x Nb _1-x O ₃ or the like are possible. Further, in order to form the optical waveguide 2, a modified embodiment using a proton exchange waveguide, an outer diffusion waveguide, or the like instead of the titanium diffusion waveguide is possible.

Further, as the material for forming the high refractive index layer 31 and the material for forming the low refractive index layer 32 forming the structural birefringent film 3, the candidates listed in the section of “Embodiments of the Invention” can be used. An appropriate one can be selected and adopted from among them. Alternatively, instead of forming a metal film such as an aluminum film as the light absorbing film, a light absorbing inorganic film such as silicon or carbon may be formed.

[0033]

As described above in detail, according to the waveguide type optical element of the present invention, it can be manufactured at low cost, has a wide application range, and has a sufficiently high performance as a TM mode filter. There is.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a configuration of a waveguide type optical element as a first embodiment.

FIG. 2 is a perspective view schematically showing a configuration of a structural birefringent film of Example 1.

[Explanation of symbols]

1: Substrate (lithium niobate crystal (Z-cut: LiNb
O3)) 2: optical waveguide (titanium diffusion waveguide) 3: structural birefringent film (TM mode filter) 31: high refractive index layer (titanium dioxide TiO ₂ ) 32: low refractive index layer (silicon dioxide SiO ₂ )

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yasuhiko Takeda 41-cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central R & D Laboratories Co., Ltd. 41 at Yokomichi, Toyota Central Research Laboratory, Inc.

Claims (2)

[Claims] A substrate having anisotropic refractive index; a three-dimensional optical waveguide buried in one surface of the substrate; and a three-dimensional optical waveguide formed on the surface of the substrate so as to cover the optical waveguide. And a formed structural birefringent film, wherein the structural birefringent film is made of a material having an isotropic refractive index larger than the refractive index of the material forming the optical waveguide. A high refractive index layer, which is a thin film, and a low refractive index layer, which is a thin film made of a material having an isotropic refractive index smaller than the refractive index of the material forming the optical waveguide, are alternately laminated. A waveguide type optical element, which is a laminate. 2. The substrate is made of lithium niobate crystal (Z-cut: LiNbO ₃ ) cut with the Z axis as a normal line, the high refractive index layer is made of titanium dioxide TiO ₂ , and the low refractive index is made of rate layer is made of silicon dioxide SiO _2, waveguide type optical device according to claim 1, wherein.