JPH10186153A - Optical waveguide directional coupler and method of manufacturing the same - Google Patents

Abstract

(57) [Problem] To provide a directional coupler in which a branching ratio of an optical waveguide type directional coupler is adjusted to a predetermined value, and a method of manufacturing the same. SOLUTION: In the optical waveguide type directional coupler, a trench 6 whose depth reaches the core 3 or is shallower is provided immediately above a coupling portion 5 where two waveguides 12 are close to each other,
A predetermined branching ratio is provided by filling the trench 6 with a first filler.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type directional coupler used in optical communication, and more particularly to a directional coupler having a branching ratio accurately adjusted and a method of manufacturing the same.

[0002]

2. Description of the Related Art In an optical waveguide type directional coupler, two optical waveguides are brought close to each other by a few μm and branched from one waveguide to another waveguide by optical coupling by evanescent light from a core.

[0003] However, in the conventional flame deposition (FHD) method or plasma chemical deposition (CVD) method, a few μm
Cannot be precisely controlled to the order of sub-μm, and the branching ratio inevitably varies within a certain range. Here, the branch ratio of the directional coupler is defined as a value obtained by dividing the output of the branch waveguide by the sum of the optical outputs of the branched waveguide and the branch waveguide.

For example, in a Mach-Zehnder type optical switch using a directional coupler having a branching ratio of 50%, exactly 5
If the branching ratio is 0%, the extinction ratio of the switch should be extremely high. However, in practice, the branching ratio has a variation of about ± 5%, and therefore, the extinction ratio of the switch is 20 dB.
Limited to a degree. Therefore, at present, if an optical switch having a high extinction ratio is to be manufactured, there is no other way than to manufacture an optical switch having a directional coupler having a small variation in the branching ratio from among them.

Further, in the method of selecting a directional coupler having a small branching ratio by chance, a loop-back type arrayed waveguide grating using a large number of 2 × 2 optical switches and a large number of directional couplers are used. It is difficult to manufacture a large-scale optical waveguide circuit such as a transversal optical filter or a lattice optical filter (for example, Suzuki Ogita et al., NTT R & D 43, No. 11, pages 1299 to 1301).

In particular, in a wavelength division multiplexing (WDM) system in optical fiber communication, a signal is extracted from a fiber,
A so-called ADD-DROP circuit which is put into a fiber is required. An optical integrated circuit in which a large number of optical components are integrated on one silicon wafer, such as a loop-back type arrayed waveguide grating circuit using a large number of 2 × 2 optical switches as one candidate of the ADD-DROP circuit. Is used.

The 2 × 2 optical switch used in this circuit has a structure in which an arm waveguide is sandwiched between directional couplers having a branching ratio of 50%, and the extinction ratio of this optical switch is ADD-D.
The characteristics of the entire circuit such as the wavelength selectivity of the ROP circuit are determined. For this reason, it is necessary to adjust the branch ratio of the two directional couplers to 50% as accurately as possible. However, in the directional coupler manufactured by the conventional technology, the variation of the branch ratio is 50% ±.
It is as large as about 5%.

For this reason, the extinction ratio of the optical switch using this is as low as about 20 dB. As a result, ADD-D using a large number of these optical switches with a low extinction ratio is used.
The performance of the ROP circuit deteriorates as a whole, and there is a problem in practical use.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a directional coupler in which the branching ratio of an optical waveguide type directional coupler is adjusted to a predetermined value, and a method of manufacturing the same. The most important feature of the present invention is that a trench is formed immediately above a coupling portion of an optical waveguide type directional coupler, and a filler having an arbitrary refractive index is filled therein to obtain a desired branching ratio.

[0010]

An optical waveguide type directional coupler according to a first aspect of the present invention that achieves the above object has an optical waveguide type directional coupler in which two optical waveguides are brought close to each other on a substrate. The optical waveguide has a trench just above the junction where the two optical waveguides are adjacent to each other, and has a trench reaching the core or shallower than the core, and filling the trench with the first filling material to form a predetermined branch. It is characterized by giving a ratio.

According to a second aspect of the present invention to achieve the above object, a method of adjusting a branching ratio of an optical waveguide type directional coupler according to the first aspect of the present invention is the optical waveguide type directional coupler according to the first aspect. Is set to a predetermined refractive index.

According to a third aspect of the present invention, there is provided a method of manufacturing an optical waveguide type directional coupler according to the first aspect of the present invention, comprising the steps of: Forming a directional coupler having two optical waveguides in close proximity to each other, followed by forming a trench whose depth reaches the core or is shallower immediately above the coupling portion of the directional coupler, and Filling the trench with a second filler having a known index of refraction and removable by cleaning, subsequently measuring the branching ratio of the directional coupler and removing the second filler, It is characterized in that the trench is filled with a first filler that gives a predetermined branching ratio.

According to a fourth aspect of the present invention, a method for determining a refractive index of a filler of an optical waveguide type directional coupler according to the present invention is provided by using a measurement result of a branching ratio according to the third aspect. A predetermined refractive index of the filler is determined.

[Operation] The directional coupler is formed by bringing two optical waveguides close to each other in parallel. As a result, the light is gradually branched from the branched waveguide to the branch waveguide.
This branching ratio is determined by the waveguide spacing, the parallel distance (coupling length),
It is determined by the waveguide structure. The reason why the branching ratio varies during manufacture is mainly because it is not possible to manufacture a waveguide interval of several μm uniformly over a fixed coupling length.

In the present invention, the deviation of the branching ratio from the design value caused by the variation of the waveguide spacing is reduced by forming a minute trench immediately above the coupling portion and embedding a medium having an appropriate refractive index therein. Is to re-adjust. By changing the refractive index of the filling material embedded in the trench, the coupling state of the directional coupler changes, and the branching ratio can be changed. Therefore, by utilizing this effect, it is possible to make the branching ratio of the directional coupler close to the design value, and it is possible to manufacture the directional coupler with the adjusted branching ratio, which is the object of the present invention. Become.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. 1 is a perspective view of a trench type directional coupler according to a first embodiment of the present invention, and FIG. Show. In the figure, 1 is a silicon substrate, 2 is a lower clad, 3 is a core, 4 is an upper clad, 5 is a coupling portion, 6 is a trench,
7 is a filler, 8, 9 are input ports, 10 and 11 are output ports, and 12 is a bent waveguide.

The fabricated waveguide is capable of single mode transmission at a wavelength of 1.55 μm, and has a core diameter of 6.5 μm × 6.
5 μm, refractive index of cladding 1.444, refractive index of core 1.455, gap of coupling part 3 μm, coupling length 600 μ
m. The trench 6 has a width of 10 μm, a length of 600 μm, and a depth of 30 μm (corresponding to the thickness of the upper clad), with the center line of the two waveguides at the coupling portion as the center.

The basic operation of the trench directional coupler according to the present embodiment is the same as that of a normal directional coupler, and the communication light (wavelength is 1.55 μm) input to the input port 8 or the input port 9 is input. Output port 10 at a fixed branching ratio.
And 11 are output.

Here, when light enters the input port 8,
The branching ratio is defined by the following equation. Branching ratio (%) = (output of output port 10) / (output of output port 10 + output of output port 11) × 100 Generally, since the gap interval of the coupling portion varies at the time of manufacturing, the directivity manufactured by the conventional method is used. It is known that the branch ratio of the coupler varies in the range of 45 to 55% even when the design value is 50%.

The trench type directional coupler shown in the present embodiment has a structure in which a trench 6 is formed immediately above a coupling portion. By embedding a resin having an arbitrary refractive index into the trench 6, a space between the waveguides is obtained. It is possible to change the coupling state and lead to different branching ratios.

Therefore, if the characteristics are used, the branching ratio that varies during manufacturing can be readjusted to a desired branching ratio. Since the refractive index changes depending on the temperature, the following adjustment of the branching ratio was performed in a place where the temperature was adjusted to 25 ° C. Here, the value of the refractive index is a value at a temperature of 25 ° C.

1 cut out from one silicon wafer
Silicon chips having different refractive indices were put into the chips of the trench type directional couplers, and the results were examined.
It turned out to be 9.7%. Therefore, after curing, 1.
An epoxy resin having a refractive index of 447 was added and cured. To cure this resin, a mercury xenon lamp (2
(00 W) for 5 minutes and then heat-treated at 80 ° C. for 3 hours.
When the branching ratio of the directional coupler was measured after curing, it was 4
It was 9.7%.

Similarly, the branching ratio of the remaining 19 trench-type directional couplers manufactured on one silicon wafer was adjusted, and as a result, the branching ratio was 49.5% to 50.50.
It could be readjusted between 5%. The refractive index of the epoxy resin used for the adjustment was in the range of 1.438 to 1.448 divided into 21 every 0.0005, and selected from these.

In the directional coupler having no trench 6 manufactured under the same manufacturing conditions, the branch ratio varied from 45% to 55%. Thus, according to the present embodiment,
The branching ratio of the trench-type directional coupler can be suppressed to 50% ± 0.5%, and the variation is improved by 10 times compared to the branching ratio of the conventional directional coupler without a trench of 50% ± 5%. Was done.

When the division number of the refractive index of the silicone oil and the epoxy resin used for the adjustment is 41, the variation is 50%
It was even smaller at ± 0.25%. Similarly, the degree of variation could be further reduced by increasing the number of divisions of the refractive index.

[Embodiment 2] In this embodiment, a diallylethene photochromic material (hereinafter referred to as epoxy resin) is used as the resin to be embedded in the trench 6 instead of increasing the number of divisions of the refractive index of the silicone oil and the epoxy resin used for adjustment. PC
(Abbreviated as material) was used. Others are the same as in the first embodiment.

By adding the PC material whose refractive index can be adjusted, the branching ratio can be adjusted with high accuracy without increasing the number of divisions of the refractive index. Here, the refractive index after curing is 0.0 in the range of 1.438 to 1.448.
Twenty-one kinds of resins having each 005 were prepared. The PC molecules in this resin are in a thermally stable state of a ring-closed state and a ring-opened state, and can have different refractive indexes according to the ratio of the number of molecules in each state in the epoxy resin.

It is known that these two molecular states are controlled by visible light or ultraviolet light and do not change at a high temperature of 80 ° C. for several years. The change width of the refractive index of an epoxy resin to which 10% of a PC material is added is about 0.00.
05, and the refractive index can be finely adjusted. Since the refractive index changes depending on the temperature, the following adjustment of the branching ratio was performed in a place where the temperature was adjusted to 25 ° C. Here, the value of the refractive index is a value at a temperature of 25 ° C.

A silicon oil having a refractive index of 1.4455 was put into the trench 6 of the trench type directional coupler taken out of one silicon wafer, and the branching ratio was measured.
The branching ratio was 49.9%. Next, the refractive index is 1.445.
An epoxy resin containing a PC material to be No. 5 was put into the trench 6 and cured.

After curing, the branching ratio of the directional coupler was measured.
However, it was 40.9%. The branching ratio of the directional coupler
In the measured state, ultraviolet rays (wavelength 356 nm, intensity 1 mW /
cm ^Two) For 3 minutes, the branching ratio becomes large.
And 50.1%. So this time, halogen run
Irradiate a pump (150W) for 10 seconds to return to 50.03%
I was able to. In this way, the branching ratio approaches 50%
After performing the operation several times, the black shielding table that does not transmit light
To the top of the trench 6 so that the branching ratio does not change
Caught.

The remaining 19 directional couplers cut out from one silicon wafer could all be adjusted to about 50% by the same operation. The adjusted branching ratio ranged from 49.95% to 50.05%.
Thus, according to the present embodiment, first, roughly
After the material-containing epoxy resin is put into the trench 6 to adjust the branching ratio, the branching ratio can be readjusted to 50% ± 0.05% on the spot while observing the branching ratio.

As a result, the variation is 1% as compared with the branching ratio of 50% ± 5% of the conventional directional coupler without trench.
Improved by a factor of 00. The material having a variable refractive index used in the present embodiment is not limited to the epoxy resin containing the PC material at all, and other materials having a variable refractive index can be used. In addition to a reversible material whose refractive index can be increased or decreased, a reversible material that can be changed only in one direction can be used. Examples of the latter include photodegradable organic compounds such as organic polysilanes and dyes.

Embodiment 3 FIG. 3 shows a method of manufacturing a trench type directional coupler according to a third embodiment of the present invention. In the figure, 1 is a silicon substrate, 2 is a lower clad, 3 is a core, 4 is an upper clad, 6 is a trench, 7 is a filler, and 13 is silicon oil.

First, as shown in FIG. 3A, a lower clad 2 (refractive index: 1.444, wavelength: 1.55 μm) is formed on a silicon substrate to a thickness of 30 μm by a flame deposition method and heat treatment.

Next, a core layer (refractive index: 1.455, wavelength: 1.55 μm) having a slightly increased refractive index was formed to a thickness of 6 μm. Subsequently, the core portion of the directional coupler was processed by combining the straight waveguide and the bent waveguide 12 by photolithography and reactive ion etching. The core width was 7 μm.

In order to produce a directional coupler having a branching ratio of 50%, the coupling portion was set to a gap of 3 μm and a coupling length of 600 μm. Further, the upper clad was formed to a thickness of 36 μm by a flame deposition method. Thereafter, as shown in FIG. 3B, a trench 6 was formed in the directional coupler on the wafer. Processing is 10μm width, 600μm length, 30μ depth right above the core
m trench 6 was formed by a method combining a photoresist method and reactive ion etching.

Although the trench processing step shown in FIG. 3B can be performed after the wafer is cut into chips or as it is in the wafer state, here, it is cut into chips and executed. The cutting was performed after covering with a protective seal so that the shavings did not enter the trench 6 when cutting out the chip.

The refractive index is from 1.438 to 1.44.
Up to 8, 21 types of silicone oils 13 having different refractive indexes every 0.0005 were prepared. The value of the refractive index is a value at 25 ° C.
C. and carried out. As shown in FIG. 3D, a silicon oil 13 having an arbitrary refractive index was put in the trench 6 of the directional coupler, and the branching ratio was measured.

In order that the branch ratio approaches 50%,
50% branch ratio while changing several types of silicone oil
The value of the refractive index close to was determined. After that, the silicon oil in the trench was cleanly cleaned by ultrasonic cleaning with acetone, and then epoxy resin was put therein and cured.

The refractive index of these epoxy resins after curing was previously determined. An epoxy resin having a refractive index closest to the refractive index obtained in FIG. Here, an ultraviolet-curable epoxy resin was used for ease of operation. The epoxy resin was cured with ultraviolet rays (mercury xenon lamp, 200 W) for 5 minutes, and then heat-treated at 80 ° C. for 3 hours. When an epoxy resin containing a PC material is used, the trench 6 is formed as a thermosetting type.
The resin was cured at 80 ° C. for 1 day.

The resin to be put into the trench 6 is not limited to the above-mentioned epoxy resin, and any resin such as an epoxy resin containing an organic silicate, a silicone resin, or sol-gel glass may be used as long as the refractive index after formation is known. No problem. As described above, the processes of FIGS. 3B to 3D can be executed following the conventional manufacturing process shown in FIG. 3A, so that an optical integrated circuit having excellent characteristics can be produced with the same manufacturing equipment as the conventional one. The point that it can be manufactured well is superior to the conventional one.

[0042]

As described above in detail with reference to the embodiments, the trench type directional coupler of the present invention has a branch ratio of 50% as compared with the conventional directional coupler without trench.
Because it can be adjusted with high accuracy of ± 0.05%, it can be used as an individual component in a light branching circuit for optical communication that requires a high-precision branching ratio. Further, if the trench type directional coupler of the present invention is used, 50% ±
Since the branching ratio of all directional couplers can be adjusted to 0.05%, the extinction ratio of the optical switch using the directional coupler is remarkably improved to 60 dB.
An optical integrated circuit having excellent practicability can be realized by using it for a DD-DROP circuit. Furthermore, this trench-type directional coupler can be compared with the reactive ion etching process for forming trenches, the branch ratio adjustment process, and the resin curing process without changing the conventional quartz-based optical integrated circuit manufacturing process. It can be manufactured by adding a simple process. Therefore, there is an advantage that an optical integrated circuit or the like having excellent performance can be manufactured with high yield.

[Brief description of the drawings]

FIG. 1 is a perspective view of a trench directional coupler according to a first embodiment of the present invention.

FIG. 2 is a structural diagram of the trench directional coupler shown in FIG.

FIG. 3 is a process diagram of a method of manufacturing a trench-type directional coupler according to a third embodiment of the present invention, and illustrates the manufacturing process in the order of (A) to (D).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Silicon substrate 2 Lower clad 3 Core 4 Upper clad 5 Coupling part 6 Trench 7 Filler 8,9 Input port 10,11 Output port 12 Bend waveguide 13 Silicon oil

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Mitsutoshi Hoshino 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Japan Telegraph and Telephone Corporation (72) Inventor Ken Sukegawa 3-19, Nishishinjuku, Shinjuku-ku, Tokyo 2 Nippon Telegraph and Telephone Corporation (72) Inventor Kenji Yokoyama 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (4)

[Claims] 1. An optical waveguide type directional coupler in which two optical waveguides are brought close to each other on a substrate, and a depth of the directional coupler reaches a core just above a coupling portion where the two optical waveguides come close to each other or more than that. An optical waveguide type directional coupler having a shallow trench and providing a predetermined branching ratio by filling the trench with a first filler. 2. An optical waveguide type directional coupler according to claim 1, wherein a refractive index of said first filler is set to a predetermined refractive index. 3. A method for manufacturing an optical waveguide type directional coupler according to claim 1, wherein a directional coupler in which two optical waveguides are brought close to each other is formed on a substrate. Immediately above the junction of the sexual coupler, a trench is formed having a depth reaching the core or shallower than it, followed by a second filling material having a known refractive index and removable by cleaning. Filling, subsequently measuring the branching ratio of the directional coupler, removing the second filler, and then filling the trench with a first filler that provides a predetermined branching ratio. A method for manufacturing a waveguide directional coupler. 4. A method for manufacturing an optical waveguide type directional coupler, wherein a predetermined refractive index of the first filler is determined using the measurement result of the branching ratio according to claim 3.