JPH04293004A - Optical waveguide with isolator function - Google Patents

Abstract

PURPOSE:To obtain the optical waveguide which has a function for preventing reflecting light without using any nonreciprocal material such as a polarizer and a rotary polarizer. CONSTITUTION:A 2nd optical waveguides 12 which is small in equivalent refractive index and has an input port at one end has the other end coupled in a Y branch shape with a 1st optical waveguide 11 which is large in equivalent refractive index and has an output port at one end at a fine angle theta to the other end side of the 1st optical waveguide 11, and an optical transition area where the core areas of both the waveguides 11 and 12 contact each other is formed at the coupling part.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide that is used in optical transmission systems and optical integrated circuits and has an isolator function for suppressing reflected return light.

0002

2. Description of the Related Art An "isolator" has been known as a typical device having a function of controlling reflected return light in optical transmission systems and the like.

[0003] An isolator has a polarizer and an optical rotator as its constituent elements, and its operating principle is that when light passes through the optical rotator, the polarization direction always rotates, for example, by 45 degrees to the right or left depending on the direction of passage. It is said that

[0004] This operating principle will be explained in more detail. For example, the angle of the polarizer is set in the direction in which linearly polarized incident light passes, and the light that has passed through the polarizer is made to pass through an optical rotator with a clockwise rotation of 45°. If there is a reflection from behind the optical rotator and this reflected light passes through the optical rotator again, its polarization direction will be rotated 45 degrees to the left, so the polarization direction of the reflected light will be rotated 90 degrees with respect to the incident light. It turns out. The reflected light whose polarization direction has been rotated by 90 degrees cannot pass through the polarizer at the entrance. Therefore, reflected light is blocked.

[0005]

[Problems to be Solved by the Invention] The above-mentioned conventional isolator requires an optical polarizer in its construction, and in order to realize the above-mentioned operation, it is necessary to apply a magnetic field to a non-reciprocal material having an optical rotation function. becomes essential. therefore,
Conventional isolators have large element dimensions, making it difficult to downsize them. Furthermore, it is difficult to form a waveguide using a non-reciprocal material, so it is not compatible with waveguide devices. Furthermore, because non-reciprocal materials are different from typical semiconductor materials, monolithic integration of isolators and laser diodes has long been desired but has not been realized.

[0006] In view of these circumstances, the present invention provides a polarizer,
The object of the present invention is to provide a novel optical waveguide having a function of blocking reflected light, that is, an isolator function, without using a non-reciprocal material such as an optical rotator.

[0007]

[Means for Solving the Problems] An optical waveguide having an isolator function according to the present invention that achieves the above object is composed of a striped optical waveguide capable of propagating only a single mode, and has a first end serving as an output port. The other end of the second optical waveguide, which has a relatively smaller equivalent refractive index than the first optical waveguide and whose one end serves as an input port, is connected to the optical waveguide other than the first optical waveguide. A light transition region is formed in which the core regions of the first and second optical waveguides are connected to each other in a Y-shaped branching manner from a direction inclined at a slight angle to the end side, and are in contact with each other at the joint portion. shall be.

[0008]

[Operation] When light enters from the input port, that is, one end of the second optical waveguide, the area where the second core region of the second optical waveguide and the first core region of the first optical waveguide are in contact with each other. Under the influence of the equivalent refractive index distribution, light moves from the second core region to the first core region, which has a relatively high equivalent refractive index, and is output from one end of the first optical waveguide, which is the output port. be done. On the other hand, even if the reflected light of the light output in this way enters from the output port, the reflected light is guided to the core region with a large equivalent refractive index in the light transition region and propagates, so that the first optical waveguide The light is emitted from the other end of the first optical waveguide without transferring from the core region of the second optical waveguide to the core region of the second optical waveguide. Therefore, such an optical waveguide functions as an isolator.

[0009]

EXAMPLES The present invention will be explained below based on examples.

FIG. 1 conceptually shows the basic configuration of an optical waveguide according to an embodiment. In the figure, 11 is a first striped optical waveguide with a large equivalent refractive index (hereinafter referred to as an optical waveguide), 12 is a second striped optical waveguide with a small equivalent refractive index (hereinafter referred to as an optical waveguide); The second optical waveguide 12 is coupled to the first optical waveguide 11 at a small angle θ in a Y-shaped branching manner. Furthermore, in this joint portion, an optical transfer region 13 is formed in which the side surfaces of the core regions of both optical waveguides 11 and 12 are in contact with each other, and in this optical transfer region, it is as if one striped optical waveguide is formed. It seems like it is. Note that equivalent refractive index distributions in the AA cross section and the BB cross section of such an optical waveguide are shown in FIGS. 1(B) and 1(C).

Here, the coupling ends of the second optical waveguide 12 include port A on the reflection side, port B on the side of the first optical waveguide 11 forming a small angle θ with the second optical waveguide 12, etc. Let the end be port C. The light incident from port A is influenced by the equivalent refractive index distribution shown in FIG. It moves to the wave path 11, propagates, and reaches port C. Conversely, the light incident from port C encounters the core region of the second optical waveguide 12 with a small equivalent refractive index in the light transition region 13, but the light is guided by the core region with a large equivalent refractive index and proceeds. The light passes through the light transition region 13 and reaches port B.

That is, in the optical waveguide shown in FIG. 1, light that enters from port A exits from port C, but for example, when reflected light enters from port C, it does not reach port A, which is the input end, but exits from port B. Since it will be emitted,
If port A is used as an input end and port C is used as an output end, it will work as an isolator. The operating principle of the isolator function of the optical waveguide of the present invention is completely different from that of conventional isolators, so there is no need for polarizers or optical rotators, and there is no need to use non-reciprocal materials.

In the optical waveguide of the present invention, the small angle θ may be any angle such that the light from the second optical waveguide 12 smoothly transfers to the first optical waveguide in the optical transition region 13, and is usually 1, Up to 2°. This is because if the angle is larger than this, the loss increases, and the maximum angle is about 10°. Moreover, the refractive index difference Δn between the first optical waveguide 11 and the second optical waveguide 12
In the optical transition section 13, the light from port A is transferred from the second optical waveguide 2 to the first optical waveguide 1 without significant loss, but the light from port C is transferred from the first optical waveguide 11 to the second optical waveguide 1. What is necessary is to set it so that it does not migrate to the optical waveguide 12. In reality, it is necessary to determine the angle by taking into account the minute angle θ, the length of the light transition region 13, etc., but normally it is sufficient to set it to Δn. Furthermore, the length of the light transition region 13 in the longitudinal direction is, in principle, better as it is longer, and it is necessary to set it in consideration of the dimensions of the optical waveguide itself, the above-mentioned refractive index difference Δn, minute angle θ, etc.
Usually, 200 μm or more is required.

The optical waveguide of the present invention will be explained in more detail below with reference to specific manufacturing examples.

In FIG. 2, in order to realize a striped optical waveguide with an equivalent refractive index having a relative relationship between sizes, a metal film having a relatively large and small film thickness is loaded in a stripe shape on the surface of a dielectric crystal. , shows an optical waveguide that uses a method of thermal diffusion.

In FIG. 2, 20 is a substrate made of lithium niobate single crystal, and 21 and 22 are striped light guides formed in a Y-shaped bonded state by thermally diffusing titanium metal. It is a wave path, and its width is approximately 4 microns. Here, the refractive index difference Δn between the first optical waveguide 21 and the substrate 20 is about 2×10 −3 , and the refractive index difference Δn between the second optical waveguide 22 and the substrate 20 is 1×1
It is about 0-3. That is, the equivalent refractive index of the second optical waveguide 22 is smaller than that of the first optical waveguide 21. Such optical waveguides 21 and 22 can be easily created by simultaneously subjecting diffusion sources formed in stripes of titanium metal with different thicknesses to high temperature treatment. In addition,
The small angle θ formed by the optical waveguides 21 and 22 is 0.5°,
The total length of the element is approximately 10 mm.

FIG. 2B shows a refractive index distribution in a cross-sectional view (cross-section taken along the line C--C) of the light transfer region 23. The refractive index distribution in this cross section is divided into four regions, where both sides a and d are cladding regions made of the lithium niobate substrate 21 outside the core region, region b is the core region of the optical waveguide 22 with a small equivalent refractive index, and region c is the core region of the optical waveguide 21 having a large equivalent refractive index.

Generally, in an optical waveguide in which a core portion made of a material with a high refractive index is surrounded by a cladding made of a material with a low refractive index, light has a characteristic that light propagates through the core region with a high refractive index. Therefore, the optical waveguide shown in FIG. 2 has an isolator function similar to that shown in FIG. The light will not return to A but will be emitted from port B.

Laser light with a wavelength of 1.5 μm is transmitted through the optical waveguide 12.
When the light enters from the port A of the optical waveguide 22, the light passes through the optical transition region 23 and then exits from the port C of the optical waveguide 22 without suffering a large loss. On the other hand, when laser light of the same wavelength enters from port C, it propagates through the core region of the optical waveguide 21 with a large equivalent refractive index in the light transition region 23, and finally exits from port B, and the light exits from port A. I didn't. When the output optical power from port B and port A was accurately measured, the ratio thereof was PB /PA = 20 dB or more. That is,
The isolation ratio of the optical waveguide shown in FIG. 2 was 20 dB or more. This value did not change significantly even if the polarization direction of the laser beam was changed. This was a problem in that conventional isolators required a polarizer and the polarization direction of usable laser light was regulated, but in the optical waveguide with an isolator function according to the present invention, the polarization direction This shows that the characteristic of operating regardless of the situation arises. Furthermore, even when the wavelength of the laser beam was changed to 1.50 μm, there was no significant change in the isolator characteristics. This indicates that the isolator function waveguide according to the present invention is characterized by a small dependence on the operating wavelength.

In the embodiments described above, titanium is thermally diffused into a lithium niobate single crystal to form a striped optical waveguide with an equivalent refractive index having a relative relationship in size. An isolator can be similarly formed by controlling the type of dopant used in the core material of glass fiber or the like.

Next, another embodiment will be described with reference to FIG. In this embodiment, a striped optical waveguide with an equivalent refractive index having a relative size relationship is realized by giving the core region a relatively large or small dimension.

As shown in FIG. 3(A), in this optical waveguide, a first optical waveguide 31 having a large equivalent refractive index and a second optical waveguide 32 having a small equivalent refractive index are arranged at a small angle as in FIG. They are coupled in a Y-shape at an angle of θ, and a light transfer region 33 is formed at the coupled portion. That is, in this optical waveguide, an InGaAsP core 35 having a bandgap wavelength of 1.3 μm and a rib portion is formed on an InP substrate 34, and this is covered with an InP cladding layer 36, and the height of the core 35 is The rib portion 35a with a large height forms a first optical waveguide 31, and the rib portion 35b with a small height forms a second optical waveguide 32. Note that such a waveguide structure is conventionally known as a rib structure.

In this embodiment, the thickness Tc of the core 35 is set to 50
The height h1 of the larger rib portion 35a was 500 angstrom, the height h2 of the smaller rib portion 35b was 250 angstrom, and the width of each rib portion 35a, 35b was 5 μm.

In the optical waveguide of this example, the wavelength is 1.5μ.
The waveguide characteristics were investigated using a laser beam of m. Optical waveguide 3
When light enters from port A of 2, the light enters optical waveguide 3.
It was emitted from port C of 1. On the other hand, the light incident from port C of the optical waveguide 31 is emitted from port A, and the output optical power ratio at port B and port A at this time is PB /PA
was 20 dB or more. Furthermore, as a result of observing the near-field pattern, it was confirmed that propagation occurred in a single mode. Furthermore, the same result was obtained when light with a wavelength of 1.50 μm was used.

From the above results, it was confirmed that the optical waveguide of this example had a good isolation function. In this example, the heights of the core regions of the two striped optical waveguides, that is, the heights of the ribs, were varied to give a difference in the equivalent refractive index; A difference can be given to the equivalent refractive index, and an optical waveguide having an isolation function can also be formed.

Next, another embodiment will be described with reference to FIG. In this example, a striped optical waveguide having equivalent refractive indexes having a magnitude relationship is realized by making one of the core regions having a superlattice structure mixed crystal.

FIG. 4(A) is a cross-sectional view in the light transfer region, FIG.
(B) is a cross-sectional view in another area. In the figure, 41 is a first optical waveguide, and 42 is a second optical waveguide, both of which are shown in FIG.
Similarly, the light transfer region 43 is formed by coupling in a Y-shape, and a description of this structure will be omitted.

Here, the optical waveguides 41 and 42 are composed of rib portions 45a and 45b processed into a superlattice 45 made of InGaAs/InP formed on an InP substrate 44, but the first optical waveguide 41 has been mixed crystallized and has a different refractive index from the original. That is, a dielectric thin film 46 made of Si3N4 is loaded on the upper surface of the rib portion 35a, and when it is rapidly heated at an appropriate temperature in this state, the superlattice structure of the rib portion 35a is destroyed and mixed crystal formation progresses. A mixed crystal region 47 is formed and serves as the first optical waveguide 41.

The refractive index of the mixed crystal region is different from that of the original superlattice, and the magnitude relationship depends on the polarization direction of light. That is, T.E.
For mode light, the refractive index of the mixed crystal region is smaller than that of the original superlattice, and the first optical waveguide 41 has a smaller equivalent refractive index than the second optical waveguide 42. on the other hand,
For TM mode light, the above relationship is reversed, and the first optical waveguide 41 is larger than the second optical waveguide 42. In any case, since there is a size relationship between the optical waveguides 41 and 42, the optical waveguides have the same isolator function as in the embodiment described above.

[0030] In the embodiments described above, the embodiments are mainly related to waveguides using a semiconductor as a substrate. Also, by using the same principle as the present invention, a high-performance isolator function can be easily realized.

[0031]

[Effects of the Invention] As explained above, according to the present invention, it is possible to realize an isolator function only with a semiconductor without using a polarizer or a rotator, so monolithic integration with a semiconductor laser is facilitated. In addition, it is expected that it will be used as a key device in optical integrated circuits. Also,
It also becomes possible to incorporate it into integrated circuits made of materials other than semiconductors.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram showing the configuration of an optical waveguide according to an example.

FIG. 2 is a conceptual diagram showing the configuration of an optical waveguide according to another embodiment.

FIG. 3 is a conceptual diagram showing the configuration of an optical waveguide according to another embodiment.

FIG. 4 is a conceptual diagram showing the configuration of an optical waveguide according to another embodiment.

[Explanation of symbols]

11, 21, 31, 41 first optical waveguide 12, 22
, 32, 42 Second optical waveguide 13, 23, 33, 4
3 Optical transition region 20 Lithium niobate single crystal 34 InP substrate 35 InGaAs core 36 InP cladding layer 44 InP substrate 45 InGaAs/InP superlattice 46 Dielectric thin film (Si3N4)

Claims (1)

[Claims] Claim 1: A first optical waveguide consisting of a striped optical waveguide capable of propagating only a single mode, one end of which is an output port, has an equivalent refractive index relative to that of the first optical waveguide. The other end of the second optical waveguide, which is small in size and has one end serving as an input port, is coupled to the other end of the first optical waveguide in a Y-shaped branching direction from a direction inclined at a slight angle,
An optical waveguide having an isolator function, characterized in that an optical transition region is formed in which the core regions of the first and second optical waveguides are in contact with each other at the coupling portion.