JPH055809A - Optical waveguide type isolator - Google Patents

Abstract

PURPOSE:To eliminate the need for utilizing a magneto-optical effect and to provide the isolator which is smaller in size and has an excellent matching property by constituting optical waveguides formed by a proton exchange method into the optical path of an optical waveguide formed by a titanium diffusion method. CONSTITUTION:A lithium niobate crystal is used as a substrate and the proton exchange waveguide 2 and the titanium diffused waveguides 1, 3 are formed by the different production processes in the optical waveguide formed as a high-refractive index region on the substrate. The boundary in the juncture between the optical waveguides is so set that the boundary attains an angle thetasatisfying a total reflection condition with an abnormal optical refractive index and the other boundary is so set that the boundary is perpendicular to the progressing direction of the optical waveguides. The difference in losses by a difference in the progressing direction of the waveguides is increases by utilizing the total reflection of the light waves generated in the case of the progression from the high-refractive index region to the low-refractive index region in such a manner.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide type isolator used in fields such as optical communication and optical measurement.

[0002]

2. Description of the Related Art In the fields of optical communication and optical measurement using an optical fiber as a transmission path, it is important to block reflected light returning to a light source in order to realize stable operation of the light source.

According to an optical isolator that has already been put into practical use in order to solve such a problem, a YIG (yttrium iron garnet) thin film having a magneto-optical effect is used as a Faraday rotator, and the Faraday rotator is used as a polarizer and By combining with the analyzer, non-reciprocity utilizing the Faraday effect is realized. The optical isolator is constructed as a so-called bulk type element.

On the other hand, although in a research stage, an optical waveguide type isolator is proposed in which a YIG thin film is formed as an optical waveguide like the bulk type element and the magneto-optical effect is utilized. Then, basic evaluations of the prototyped optical waveguide type isolator have been conducted (R. Wolfe et.
al.: Appl. Phys. Lett. 57 , 960, 1990).

[0005]

However, in such a conventional optical isolator, first, in the case of a bulk type isolator, since it is a bulk type element, the optical axis must be adjusted and the spatial coupling of light waves must be performed. As a result, an additional optical component such as a coupling lens is required, so that the device has to be complicated and large. Further, since a magnetic field is used, a magnet is indispensable for its structure. Therefore, when an optical fiber is used as a transmission line, the coupling configuration between the optical fiber and the optical isolator is indispensable, but especially when a bulk type optical isolator is used, the insertion loss caused by the optical isolator itself is accompanied by such coupling. There is a problem such as an increase in power consumption and instability of binding. On the other hand, in the case of the optical waveguide type isolator, the transmission loss of the optical waveguide becomes large, and even if the transmission loss can be reduced, it is necessary to use a polarizer, an analyzer, a magnet, etc. for the bulk type element. It is similar to the case of.

As described above, since the conventional optical isolator utilizes the magneto-optical effect in any case, it is indispensable to apply a magnetic field from the outside. Also, for example, when used in combination with another waveguide type optical functional element such as a waveguide type optical switch or an optical modulator, it is necessary to connect with a waveguide type element that requires a minute optical axis adjustment. It becomes large. Furthermore, in recent years, a so-called one-chip device has been formed to enable the optical isolator to be a waveguide, downsized, and integrated with other devices. Realization of an isolator is desired.

In view of such circumstances, the present invention does not need to use the magneto-optical effect, realizes miniaturization and excellent coupling, and stable coupling with an optical fiber. It is an object of the present invention to provide an optical waveguide type isolator that enables integration of the above.

[0008]

The optical waveguide type isolator according to the present invention is formed by using a lithium niobate crystal (LiNbO ₃ ) which is a ferroelectric substance as a substrate and a high refractive index region on this LiNbO ₃ crystal substrate. The optical waveguide has a structure including the optical waveguide formed by the proton exchange method in the optical path of the optical waveguide formed by the titanium (Ti) diffusion method.

Further, in the optical waveguide type isolator according to the present invention, the proton exchange waveguide and the titanium diffusion waveguide are formed by different manufacturing methods, and the boundary surface at one of the boundaries between the optical waveguides is abnormal light. The angle is set so as to satisfy the total reflection condition with respect to the refractive index, and the boundary surface at the other boundary is set to be perpendicular to the traveling direction of the optical waveguide.

[0010]

According to the present invention, the optical waveguide 2 is formed by the proton exchange method in the optical path of the Ti diffusion waveguide (optical waveguide 1 and optical waveguide 3) formed on the LiNbO ₃ crystal substrate.
In this way, when the proton exchange waveguide is formed on the LiNbO ₃ crystal substrate, assuming that the refractive index changes with respect to the extraordinary light refractive index n _e and the ordinary light refractive index n _o are Δn _e and Δn _o , respectively, in the proton exchange region, Δn _e = 0.13, and Δn
It is already known that _o = -0.04. Therefore, light waves having a field component corresponding to n _o in such a proton exchange region can not be guided. Further, it is known that Δn _e and Δn _o are almost equal to each other in the Ti diffusion region and are about 0.01 or less. Therefore, the magnitude relation of the refractive index changes in the optical waveguides 1, 2 and 3 shown in FIG. 1 can be expressed by the following equations (1) and (2). 0 <Δn _e1 = Δn _e3 <Δn _e2 (1) Δn _o2 <0 <Δn _o1 = Δn _o3 (2)

Here, in FIG. 1, it is assumed that the incident signal light traveling from the left side is a light wave having a linearly polarized light corresponding to the extraordinary light refractive index n _e , and this light wave is set to the extraordinary light mode. Further, the light wave corresponding to the ordinary light refractive index n _o having an electric field component orthogonal to the extraordinary light mode is defined as the ordinary light mode. And
In FIG. 1, the incident extraordinary light mode passes through one boundary surface inclined by an angle θ with respect to the traveling direction of the light wave. In this case, since the travel is from the low refractive index region to the high refractive index region, total reflection occurs. Does not occur. The other boundary surface on the right side of the boundary surface is perpendicular to the traveling direction of the light wave.

Next, in the case of the reflected return light traveling from the right side in FIG. 1, considering the extraordinary light mode as the reflected return light, the extraordinary light mode is perpendicular to the traveling direction of the light wave. To the proton exchange region (optical waveguide 2). When the light travels from the optical waveguide 2 to the optical waveguide 1, it does not travel to the optical waveguide 1 as guided light because it is totally reflected at one boundary surface inclined by the angle θ and is emitted to the outside of the optical waveguide. This is because, as described above, the angle θ of the boundary surface is set so as to satisfy the total reflection condition with respect to the extraordinary light mode traveling from the proton exchange region to the Ti diffusion region. is there. Therefore, by utilizing the total reflection of the light wave that occurs when traveling from the high refractive index region to the low refractive index region,
The difference in loss can be increased by the difference in the traveling direction of the light wave.

Even if the polarized light of the incident light is linearly polarized light, a light wave having polarized light orthogonal to the polarized light may be incident as reflected return light. Since this ordinary mode cannot guide the proton exchange region (that is, Δn _o <0), it becomes a radiation mode in this case, and the progress from the optical waveguide 2 to the optical waveguide 1 is blocked.
In such a case, the extinction ratio of the ordinary light mode to the extraordinary light mode, which is the pass mode, is a function of the length of the proton exchange region. For example, if the length of the proton exchange region is 1.5 mm or more, 50 dB or more. It is known that the extinction ratio can be obtained (PGSuchoski et al .: Opt. Let
t. 13 , 172, 1988).

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an optical waveguide type isolator according to the present invention will be described below with reference to FIG. 2 and FIG. First, in the case of producing the optical waveguide type isolator of the present invention,
A thickness of 1.0 in the thickness direction, longitudinal direction and width direction of the a-axis direction, b-axis direction and c-axis direction of the LiNbO ₃ crystal, respectively.
mm, length 25 mm and width 10 mm LiNbO ₃ crystal substrate 1
One main surface of 1 is mirror-polished, and linear optical waveguide patterns 12 and 12 ′ having a width of 5 μm are formed by photolithography. After depositing Ti with a thickness of about 45 nm on the optical waveguide patterns 12 and 12 ′, patterning is performed by a lift-off method, and then thermal diffusion is performed at a temperature of 1000 ° C. for 5 hours. 3 (see FIG. 1) was produced. Furthermore, after forming the Ti diffusion waveguide in this way, the optical waveguide pattern 13 is formed in the same manner as in the case of this Ti diffusion waveguide, and the optical waveguide pattern 13 is formed in benzoic acid.
Proton exchange was carried out at a temperature of ° C for 10 minutes, whereby an optical waveguide 2 (see Fig. 1) was produced. The length of the proton exchange waveguide thus formed is 2 mm in order to obtain sufficient extinction for the ordinary mode.

A boundary surface 14 between the optical waveguides 1 and 2
The inclination angle θ of is the following formula (3), where Δn _e = 0.13 is the change in the refractive index of the extraordinary light by the proton exchange method, and 2.175 (wavelength 0.83 μm) is the refractive index of the Ti diffusion waveguide region. When calculated according to the formula of the total reflection angle θ _c represented by, θ _c = 19.3 °. θ _c = cos ⁻¹ (n _e1 / n _e2 ) (3) where n _e1 (= 2.175) is the refractive index of the Ti diffusion waveguide region, and n _e2 (= 2.305) is the proton exchange waveguide. The refractive index of the area. Therefore, if θ _c <19.3 °, the total reflection condition is always satisfied. However,
With respect to the light wave traveling in the forward direction (see FIG. 2), the reflection loss increases as the angle θ becomes smaller. Therefore, it is necessary to set the angle θ as large as possible while satisfying the total reflection condition. is there. Therefore, in the actually manufactured optical waveguide type isolator, θ = 18 °. The calculation result of the reflection loss when θ = 18 ° is 0.08.
It becomes dB, and it can be seen that the reflection loss is sufficiently small.

The optical waveguide type isolator according to the present invention is constructed as described above. Next, an example of the evaluation result of the optical waveguide type isolator manufactured as described above will be described. TE for forward and backward incidence
The (extraordinary light) mode insertion loss was evaluated. The TM (ordinary light) mode insertion loss was also evaluated in the opposite direction. In evaluating the insertion loss, a semiconductor laser having a wavelength of 0.83 μm is used as a light source, and the light is incident on the optical waveguide from the light source via a polarization-maintaining optical fiber. The light emitted from the optical waveguide was condensed by a 20 × objective lens, and the amount of output light was measured by a silicon photodiode. The extinction ratio at the exit end of the polarization-maintaining optical fiber on the incident side was 15 dB.

First, the TE (extraordinary light) mode was incident in the forward direction, and the insertion loss in that case was measured. Here, in order to compare with the optical waveguide type isolator of the present invention, as shown in FIG.
When the insertion loss was measured for the linear waveguide 15 formed by the Ti diffusion method that does not include a proton exchange region formed at a distance of 0 μm, an excessive loss of 0.8 dB was observed.

Next, TE (extraordinary light) is applied to the backward incidence.
Mode and TM (normal light) mode were incident, and the insertion loss in this case was measured. Then, the linear waveguide 1 formed by the Ti diffusion method not including the proton exchange region
Compared to the insertion loss in 5, the excess loss is
It was 30 dB and 50 dB for the (extraordinary light) mode and the TM (ordinary light) mode, respectively. From the above measurement results, it is clear that an optical waveguide type isolator having an isolation of 30 dB or more was realized.

In the above-mentioned embodiment, the substrate in which the thickness direction of the LiNbO ₃ crystal substrate is the a-axis direction is used. However, even when the thickness direction is the b-axis direction or the c-axis direction, it is completely different from the above. An optical waveguide type isolator can be manufactured by the same method. Further, only the extraordinary light mode is used as the pass mode, but since the isolator is originally installed at a position very close to the light source, incident light having a specific linearly polarized light is used.
Further, in the case of integrating the optical switch or the optical modulator and the optical waveguide type isolator of the present invention on the same LiNbO ₃ crystal substrate, it is almost the case that the extraordinary optical mode is used to realize a low operating voltage. .. Therefore, also in this point, it does not matter that the optical waveguide type isolator of the present invention sets only the extraordinary optical mode to the pass mode. Further, in coupling with the optical fiber, the connection loss due to the mode mismatch between the optical waveguide and the optical fiber was 0.5 dB, and it could be confirmed that it was a sufficiently low level. Since the optical waveguide type isolator according to the present invention uses the LiNbO ₃ crystal as a substrate as described above, when an optical functional element such as an optical switch or an optical modulator produced by using the LiNbO ₃ crystal substrate is produced at the same time. Only the process of proton exchange is required. Therefore, it becomes extremely easy to integrate various types of optical functional elements on the same substrate.

[0020]

As described above, according to the present invention, it is not necessary to use an external magnetic field, and it is possible to integrate the optical waveguide with other waveguide type optical functional elements. A practical effect can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a basic configuration of an optical waveguide type isolator according to the present invention.

FIG. 2 is a perspective view showing a configuration example of an optical waveguide formed on a LiNbO ₃ crystal substrate in an embodiment of the optical waveguide type isolator according to the present invention.

[Explanation of symbols]

 1 Ti diffusion waveguide 2 Proton exchange waveguide 3 Ti diffusion waveguide

Claims (1)

Claim: What is claimed is: 1. An optical waveguide formed as a high refractive index region in a portion including a surface of a lithium niobate crystal substrate by a proton exchange method in an optical path of the optical waveguide formed by a titanium diffusion method. An optical waveguide type isolator including the formed optical waveguide. 2. A proton exchange waveguide and a titanium diffusion waveguide are formed by different manufacturing methods, and at one boundary at the junction between the optical waveguides, the boundary surface has a total reflection condition for the extraordinary refractive index. 2. The optical waveguide type according to claim 1, wherein the angle is set to satisfy an angle, and the boundary surface at the other boundary is set to be perpendicular to the traveling direction of the optical waveguide. Isolators.