JPH1152158A - Waveguide type optical circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a waveguide type optical circuit capable of efficiently removing a higher-order mode even if an impurity layer having a high refractive index is formed at an interface between a core and a clad due to a manufacturing deviation caused in a manufacturing process. I do. SOLUTION: In a waveguide type optical circuit 13 in which a silica-based glass is laminated on a silicon substrate to form an optical waveguide 8 composed of a core and a clad, a part of the optical waveguide 8 has a width larger than that of the optical waveguide 8. A narrow mode filter 22 is provided. As a result, when an element such as an optical fiber, an LD, or a PD is mounted on the waveguide type optical circuit 13, an optical axis shift occurs,
Even if a higher-order mode occurs, it can be removed by the mode filter 22.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a waveguide type optical circuit, and more particularly to a waveguide type optical circuit provided with a mode filter.

[0002]

2. Description of the Related Art Generally, when an optical fiber, a photodiode (PD), a laser diode (LD), or the like is mounted on a waveguide type optical circuit, an optical axis shift or the like causes the inside of the waveguide type optical circuit to fail. A higher-order mode occurs.

[0003] When a higher-order mode exists in a waveguide type optical circuit, the fundamental mode and the higher-order mode interfere with each other at the Y branch portion of the waveguide type optical circuit, and the ratio of light branching becomes unstable.
There is a problem that the light receiving sensitivity of D and the LD light output vary greatly. In addition, since the polarization state changes when a higher-order mode exists, there is a problem that polarization-dependent loss (PDL) increases.

For this reason, conventionally, a higher-order mode has been removed by forming a bent portion in an optical waveguide.

[0005]

However, a manufacturing deviation occurs in the device manufacturing process, and this manufacturing deviation causes minute deviations in the refractive index of each film such as the core and the clad. An impurity layer having a high refractive index is formed at the interface of the clad.

For this reason, in the conventional waveguide type optical circuit, a higher-order mode is confined in the core by this impurity layer. Therefore, when the impurity layer is formed due to manufacturing deviation, a bent portion is formed in the optical waveguide. Could not eliminate higher order modes.

An object of the present invention is to provide a waveguide type optical circuit capable of efficiently removing a higher-order mode even if an impurity layer having a high refractive index is formed at an interface between a core and a clad due to a manufacturing deviation caused in a manufacturing process. Is to provide.

[0008]

According to a first aspect of the present invention, there is provided a waveguide type optical circuit in which a silica-based glass is laminated on a silicon substrate to form an optical waveguide comprising a core and a clad. , A mode filter is provided in a part of the optical waveguide.

According to a second aspect of the present invention, the optical waveguide comprises an E1
A single mode in which only one mode propagates, or E21,
The higher-order modes E12 and E22 are multimodes that can propagate.

According to a third aspect of the present invention, the optical waveguide width of the mode filter is formed to be narrower than the optical waveguide width of the connection portion of the optical fiber.

According to a fourth aspect of the present invention, the mode filter is bent at a predetermined bending radius.

According to a fifth aspect of the present invention, the bending radius of the mode filter is set to 10 mm or less.

According to a sixth aspect of the present invention, the optical waveguide is tapered from the width of the optical waveguide other than the mode filter to the width of the optical waveguide of the mode filter.

That is, the gist of the present invention is to increase the core width of the coupling portion between the optical waveguide and the optical fiber and to reduce the core width of the optical waveguide in the mode filter portion, thereby enhancing the ability to remove higher-order modes. It is in.

According to the above configuration, in the portion where the core width of the waveguide is narrowed, the cutoff wavelength shifts to the shorter wavelength side, so that the higher-order mode is greatly attenuated. Therefore, the ability to remove higher-order modes can be enhanced.

[0016]

Next, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 3 shows an example of a surface mount type bidirectional transmission module using the present invention.
1 shows a μm bidirectional transmission module.

In the figure, the solid arrow A indicates the transmission direction of 1.3 μm light, and the broken arrow V indicates the transmission direction of 1.55 μm light.

As shown in FIG. 3, 1.3 μm / 1.55
In the μm bidirectional transmission module, a bidirectional transmission optical waveguide type module element (waveguide type optical circuit) 13 having both sides of an optical waveguide 8 branched is formed on a silicon substrate, and the waveguide type optical circuit is formed. An LD (1) that outputs 1.3 μm light into the optical waveguide 8 and a PD (6) that receives 1.3 μm light from inside the optical waveguide 8 are provided on one side (right side in the drawing) of the optical waveguide 8.
And a monitor for monitoring the backward light emission of the LD (1) to control the light output of the LD (1) to be stable.
A PD (m-PD) 5 is provided, and single-mode optical fibers (SMFs) 4 and 7 are connected to the other side (left side in the drawing) of the waveguide type optical circuit 13, respectively, and are mainly configured. .

The bidirectional transmission waveguide type module element (waveguide type optical circuit) 13 is provided with a multilayer filter 3 for multiplexing / demultiplexing light at one branch and at the other branch. A Y-branch portion 2 for branching light at a predetermined ratio is formed. Further, the optical waveguide 8 between the multilayer filter 3 and the Y branch 2 has a bent portion 16 for removing a higher-order mode.
Are formed.

Element 13 for Bidirectional Transmission Waveguide Module
The end of the LD is connected to an optical fiber or element.
LD port 18 for entering light from (1), PD port 19 for emitting light from optical waveguide 8 to PD (6), 1.5 port for connecting to optical fiber 7 (wavelength 1) .55 μm optical signal output port) 1
5 and COM port 1 for connecting to optical fiber 4
The optical fiber 4 is connected to the optical fibers 4 and 7 using a resin or the like.

The vicinity of the LD port 18 will be described in detail. As shown in FIG. 4, a bidirectional transmission waveguide type module element (waveguide type optical circuit) 13 is formed on a first silicon substrate 9. A buffer layer 10 and a second buffer layer 11 are sequentially stacked, and an optical waveguide (core) 8 and an over clad 12 are sequentially stacked on the buffer layers 10 and 11.

The first buffer layer 10 has a large difference between the refractive index (3.5166) of the silicon substrate 9 and the refractive index (1.4646) of the optical waveguide (core) 8 so that light is transmitted to the silicon substrate 9. It is provided in order to prevent easy absorption, thereby reducing the absorption loss. Further, the second buffer layer 11 is made of LD (1) or PD
The optical waveguide 8 is formed to have an appropriate thickness so as to match the height of the optical waveguide 8 with (6).

The optical waveguide 8 of the bidirectional transmission waveguide module element (waveguide optical circuit) 13 has a relative refractive index difference of 0.45%, and the diameter of the optical waveguide is 7.0.
2 μm × 7.2 μm (mode field diameter 4.08 μm
m). As a result, a mode field mismatch loss with a general SMF (4, 8) having a mode field diameter of 4.75 μm is about 0.1 dB. On the other hand, the waveguide type optical circuit 1 having a relative refractive index difference of 0.45%
In No. 3, if the core diameter is 7.2 μm, the cutoff wavelength is 1.46 μm as shown in FIG. That is,
In an optical waveguide having a core diameter of 7.2 μm, the wavelength is 1.46.
In the case of μm or more, single mode operation is performed, and other modes do not propagate. Therefore, when the wavelength of the signal light propagating in the optical waveguide is 1.3 μm, the fundamental mode E1
In addition to the first mode, E21, E12, E
Twenty-two modes propagate and become a multi-mode.
It is not easy to remove this higher-order mode only by the bent portion 16 of the optical waveguide.

It should be noted that here, JEGoell "A Circular Harm
onic conputer analysis of rectangular dielectric w
aveguides "Bell Syst.Tech, J, 48, pp2133-2160

The modes E11, E21, E12 and E22 will be described in detail with reference to FIGS.

FIGS. 6 to 9 show the electric field distribution of each mode. The solid arrow E indicates the direction of the electric field, and the broken arrow H indicates the direction of the magnetic field.

As shown in FIG. 6, the E11 mode 40 is
The electromagnetic distributions in the X and Y directions are even symmetric modes (modes in which the field distribution changes by a cosine function) in both directions, and the light power distribution is concentrated at the center.

As shown in FIG. 7, the E21 mode 42 is
The electromagnetic distribution in the X direction is an odd symmetric mode (mode in which the field distribution changes by a sine function), the electromagnetic distribution in the Y direction is an even symmetric mode, and the optical power distribution is separated into two in the X direction. .

As shown in FIG. 8, the E12 mode 44 is
The electromagnetic distribution in the X direction is an even symmetric mode, the electromagnetic distribution in the Y direction is an odd symmetric mode, and the optical power distribution is distributed in the Y direction in two separate ways.

As shown in FIG. 9, the E22 mode 46 is
The electromagnetic distributions in the X direction and the Y direction are odd symmetric modes in both directions, and the optical power distribution is distributed in the X direction and the Y direction, and is distributed in four.

In the present invention, in order to eliminate these higher modes (E21, E12, E22 modes),
In addition to the bent portion 16 of the optical waveguide, a mode filter is provided in the waveguide type optical circuit 13 of the element for the bidirectional transmission waveguide type module.

FIG. 1 is an enlarged view of a waveguide type optical circuit 13 provided with this mode filter.

As shown in FIG. 1, the waveguide type optical circuit 13
Are the modes between the COM port 14 and the multilayer filter 3, between the multilayer filter 3 and the Y branch 2, between the Y branch 2 and the LD port 18, and between the Y branch 2 and the PD port 19, respectively. A filter unit 20 is provided.

Specifically, the Y branch 2 and the LD port 18
A mode filter 22a between the multilayer filter 3 and the Y-branch 2;
A mode filter 22c is provided between the Y-branch unit 2 and the PD port 19, and a mode filter 22c is provided between the Y-branch unit 2 and the PD port 19.

FIG. 2 is an enlarged view of the mode filter section 20.

As shown in FIG. 2, the mode filter 20
Is composed of a mode filter 22 formed narrower than the core width of the optical waveguide 8 and a tapered portion 23 tapered from the optical waveguide 8 other than the mode filter 22 to the mode filter 22. H is the optical waveguide 8
The same 7.2μm and a core width W _1, the width W ₂ of the mode filter 22 is a 6.2 .mu.m, and the spread angle α of the tapered portion 23 is formed in 0.5 °. That is, the mode filter 22 is formed so that the cutoff wavelength becomes 1.31 μm, and the tapered portion 23 is formed so that the mode field mismatch loss becomes 0.1 dB or less.

Mode filter 2 having a narrow core width
2 should be as long as possible,
In the optical circuit configuration of FIG. 1, the mode filter 22b and the mode filter 22c may be a single connected mode filter.

In the mode filter section 20, the mode filter 22 is bent with a bending radius of 10 mm or less.

Next, the operation will be described.

The 1.3 μm light emitted from the LD 1 passes through the multi-layer filter 3 through the mode filter 22a, the Y branch 2 and the mode filter 22b in this order as indicated by the solid arrow A, and passes through the mode filter 22c. After that, the light is emitted from the optical fiber 4.

On the other hand, 1.3 .mu.m light for sound incident from the optical fiber 4 passes through the multilayer filter 3 through the mode filter 22c, passes through the mode filter 22b, the Y branch 2, the mode filter 22d, and passes through the PD 6 for reception. Is received.

The 1.55 μm light for video incident from the optical fiber 4 passes through the mode filter 22c, is reflected by the multilayer filter 3 as shown by the dashed arrow V, and is reflected by the other optical fiber. 7 is emitted.

Since the light that has passed through the mode filter section 20 has its cutoff wavelength shifted to the shorter wavelength side (1.31 μm), the higher-order mode is removed and the light becomes a single mode.

As a result, even if a manufacturing deviation occurs in the element manufacturing process and an impurity having a high refractive index is formed at the interface between the core and the clad, the transmission mode is converted to a single mode. not exist.

The 1.5 port 15 and the multilayer filter 3
Since only 1.55 μm light for video is propagated between, the mode filter is unnecessary.

The optical waveguide 8 has a different refractive index (birefringence) between the X direction and the Y direction shown in FIG. 3, but in the case of a single mode waveguide, even if the birefringence is present, the Y branch portion is formed. Since the ratio of light branching in 2 is stable irrespective of the polarization direction, the light receiving sensitivity of the PD (6) and the light output of the LD (1) are also stable, and the polarization dependent loss (PDL) is reduced.

As described above, by providing the mode filter unit 20 of the present invention, higher-order modes can be effectively removed, and an element resistant to process deviation can be manufactured.

In this embodiment, the element formed by laminating quartz glass on a silicon substrate has been described. However, the same applies to an element formed by laminating quartz glass on a glass substrate. Effective and adaptable.

In the present embodiment, LD (1), P
Although the waveguide type optical circuit of the element on which the D (6) element is mounted has been described, the present invention can be similarly applied to a waveguide type optical circuit of a passive element (such as a splitter).

[0051]

In summary, according to the present invention, each film such as a core and a clad has a slight refractive index deviation due to a manufacturing deviation generated in an element manufacturing process, and an impurity having a high refractive index is present at an interface between the core and the clad. Is formed,
It is possible to realize an optical circuit having a high order mode removal capability. For this reason, variation in the light receiving sensitivity and LD light output of the PD can be reduced, and PDL can also be reduced. Therefore, an element that is resistant to process deviation can be manufactured.

[Brief description of the drawings]

FIG. 1 is a diagram showing an element for a bidirectional transmission waveguide type module (waveguide type optical circuit) according to the present invention.

FIG. 2 is a partially enlarged view of a mode filter unit in FIG.

FIG. 3 is a schematic view showing a surface mount type bidirectional transmission module.

FIG. 4 is a sectional view of a device for a bidirectional transmission waveguide module.

FIG. 5 is a diagram showing cutoff wavelength characteristics with respect to a core width of a waveguide having a refractive index of 0.45%.

FIG. 6 is a diagram showing an electromagnetic distribution of an E11 mode.

FIG. 7 is a diagram showing an electromagnetic distribution of an E21 mode.

FIG. 8 is a diagram showing an electromagnetic distribution of an E12 mode.

FIG. 9 is a diagram showing an electromagnetic distribution of an E22 mode.

[Explanation of symbols]

Reference Signs List 8 optical waveguide (core) 13 element for bidirectional transmission optical waveguide module (waveguide optical circuit) 20 mode filter section 22 mode filter 23 taper section

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Naoto Uezuka 5-1-1, Hidaka-cho, Hitachi City, Ibaraki Prefecture Inside the Opto-System Research Laboratory, Hitachi Cable, Ltd. (72) Inventor Yasuyuki Inoue Nishishinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Co., Ltd. 3-chome 19-2 Nippon Telegraph and Telephone Co., Ltd. (72) Inventor Norio Takato 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Co., Ltd. (72) Inventor Tatsunori Kanaya Totsuka-ku, Yokohama 216 Totsukacho In the Information and Communication Division, Hitachi, Ltd.

Claims (6)

[Claims] 1. A waveguide type optical circuit in which a silica-based glass is laminated on a silicon substrate to form an optical waveguide comprising a core and a clad, wherein a mode filter is provided in a part of the optical waveguide. Waveguide type optical circuit. 2. The waveguide type optical circuit according to claim 1, wherein the optical waveguide is a single mode in which only the E11 mode propagates or a multimode in which higher-order modes of E21, E12, and E22 can propagate. 3. The waveguide type optical circuit according to claim 1, wherein an optical waveguide width of the mode filter is formed to be smaller than an optical waveguide width of a connection portion of the optical fiber. 4. The waveguide type optical circuit according to claim 1, wherein the mode filter is bent at a predetermined bending radius. 5. The waveguide type optical circuit according to claim 4, wherein a bending radius of the mode filter is set to 10 mm or less. 6. The waveguide type optical circuit according to claim 3, wherein the optical waveguide is tapered from the width of the optical waveguide other than the mode filter to the width of the optical waveguide of the mode filter.