JPH0561076A - Waveguide type optical branching element - Google Patents

Abstract

PURPOSE:To obtain a waveguide type optical branching element whose loss is small and having the small wavelength dependency of a coupling ratio in a wide wavelength area, and also whose polarization dependency is mitigated. CONSTITUTION:Two directional couplers 2a, 2b are installed on the way of two optical waveguides 1a 1b arranged on a substrate 1, and the length difference DELTAL of an optical path between the optical waveguides 1a, 1b in between the directional couplers 2a, 2b is arranged, and also, a tapered optical waveguide 1c is provided on some parts of one optical waveguide, then, the wavelength and the polarization dependency of the directional couplers 2a, 2b are counterbalanced with the wavelength and the polarization dependency of the tapered optical waveguide 1c, so that the optical branching element whose loss is small and having the small wavelength dependency in a desired wavelength area can be formed as a whole.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical branching device suitable for use in the field of optical communication, and more specifically, it shows both the wavelength dependence and the polarization dependence of the power coupling rate. The present invention relates to a relaxed waveguide type optical branching device.

[0002]

2. Description of the Related Art An optical branching element is an optical component that is indispensable for distributing optical information, monitoring and testing optical fiber transmission lines, and branches such as 50% branching, 20% branching, and several% branching depending on the purpose. A branching element with a ratio (coupling rate) is required. further,
As it is convenient to use for actual optical fiber communication,
There is a demand for an optical branching device whose branching ratio hardly changes even when the wavelength and polarization state change.

Optical branching devices can be roughly classified into 1) bulk type, 2) fiber type, and 3) waveguide type, depending on their forms.

The bulk type is a microlens or prism,
Although it is configured by combining an interference film filter and the like, it is possible to provide a branch element with little wavelength and polarization dependence.
It takes a long time to assemble and adjust, leaving problems in terms of long-term reliability, price, and size.

The fiber type is formed by polishing, fusing, and stretching steps using the optical fiber itself as a constituent material, and it is possible to realize a type in which wavelength dependency is reduced and polarization dependency is hardly present. However, the manufacturing process thereof is poor in controllability, so that it has poor reproducibility and is not suitable for mass production.

On the other hand, the waveguide type has an advantage that it can be mass-produced on a flat substrate in a batch by a photolithography process, and is excellent in reproducibility and small-sized integration possibility. Further, regarding a waveguide type optical branching element having a small wavelength dependence, for example, see K. Jinguji et al. : "M
ach-Zhender interferomete
rtype optical wave guide c
ouplerwithwavelength-flat
tened coupling ratio ", Ele
ctron. Lett. , Vol. 26, pp. 132
6-1327, 1990.

FIG. 11 is a plan view showing an example of the configuration of a conventional wide-wavelength operation waveguide type optical branching device. In the figure, optical waveguides 1a and 1b are arranged on a silicon substrate 1, and the optical waveguides 1a and 1b are close to each other at two locations to form directional couplers 2a and 2b. One end 3a of the optical waveguide 1a is used as an input port, and the other ends 3b and 4b of the optical waveguides 1a and 1b are
These are the main output port and the sub output port, respectively. Two
The optical path length difference between the optical waveguides 1a and 1b between the directional couplers 2a and 2b is set to a minute amount ΔL. In the Mach-Zehnder interferometer circuit set in this way, the phase difference due to the optical path length difference ΔL is provided between the two directional couplers.

[0008]

[Equation 1] θ = 2π · n · ΔL / λ (1) (where, n = refractive index of optical waveguide, λ = wavelength) exists, and the entire Mach-Zehnder optical interferometer type optical branching device of FIG. 11 is present. The power coupling rate η is given by the following equation.

[0009]

[Equation 2] η = sin ² φ ₁ cos ² φ ₂ + cos ² φ ₁ sin ² φ ₂ + 2cos θ sin φ ₁ sin φ ₂ cos φ ₁ cos φ ₂ (2) where φ ₁ and φ ₂ are two directions This is a variable having a relationship of η ₁ = η ₂ and η ₁ = sin ² φ ₁ and η ₂ = sin ² φ ₂ with the coupling ratios η ₁ and η _{2 of the directional} couplers 2a and 2b. It depends on the coupling length, wavelength, polarization, etc.

The wavelength dependence of the two directional couplers 2a, 2b is set by appropriately setting the structural parameters such as the minute optical path length difference ΔL and the optical waveguide spacing of the two directional couplers 2a, 2b and the length of the coupling portion. And the wavelength dependence of the phase difference θ are canceled out in a desired wavelength range, and a waveguide type optical branching device having a desired coupling rate with small wavelength dependence can be realized.

[0011]

However, in this branch element, since the coupling rate of the directional coupler itself has a dependency, the power coupling rate has a slight polarization dependency.

FIG. 12 shows an example of the wavelength and polarization dependence of the coupling rate of the wide-wavelength operation waveguide type optical branching device shown in FIG. 11. In this example, the wavelength is 1.25 to 1.65 μm. , The wavelength dependence is extremely small, but especially on the short wavelength side, T
The difference in coupling rate between M and TE2 modes is about 2.5%. FIG. 13 further shows the polarization angle dependence of the loss of the main output port and the sub output port at the wavelength of 1.31 μm of this element, but the loss variation due to the polarization state of the sub output port is large. Such a drawback has been a serious problem when used for optical fiber communication.

Therefore, the object of the present invention is to solve the above-mentioned drawbacks, and the wavelength dependence of the coupling rate is very small in a wide wavelength range, and the polarization dependence thereof is, for example, 20% ±.
An object is to provide a waveguide-type optical branching element with a low loss that is reduced to 0.5%.

[0014]

In order to achieve such an object, the present invention provides a substrate, first and second optical waveguides arranged on the substrate, and first and second optical waveguides. A first directional coupler and a second directional coupler in which the optical waveguides are arranged close to each other at two locations, and one end of the first optical waveguide is used as an input port, and the first and second optical waveguides are provided. In the waveguide type optical branching device having the other end of each of the waveguides as the main output, the first and second directional couplers of the first and second optical waveguides are connected to each other. While providing the optical path length difference in the portion, the tapered optical waveguide is provided in at least one of the portions,
The wavelength and polarization dependences of the first and second directional couplers are alleviated by the wavelength and polarization dependence of the tapered optical waveguide.

Here, the optical waveguide may be a silica-based optical waveguide having a core portion size substantially equal to the core diameter of the single mode optical fiber.

[0016]

In the present invention, the directional coupler 2 is composed of two optical waveguides provided with a controlled difference in length by a minute amount ΔL.
In a Mach-Zehnder optical interferometer circuit-type optical branching device formed by connecting a plurality of optical waveguides, at least one of two optical waveguides between two directional couplers has a tapered optical waveguide. By using a waveguide, a different phase difference is given to the part connecting these two directional couplers depending on the polarization, and not only the wavelength dependence of the directional coupler but also the polarization dependence is relaxed. , Is very different from the conventional Mach-Zehnder interferometer circuit type optical branching element, and relaxes not only the wavelength dependence of the coupling ratio of the directional coupler itself but also the polarization dependence, and the entire wavelength range of the element as desired. Thus, it is possible to provide an optical branching element with low wavelength dependence and low loss.

[0017]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a plan view showing a basic configuration example of a waveguide type optical branching device of the present invention, in which optical waveguides 1a and 1b are arranged on a flat substrate 1 and optical waveguides 1a and 1b are The directional couplers 2a and 2b are formed close to each other at two locations. One end 3a of the optical waveguide 1a serves as an input port, and the other ends 3b and 4b of the optical waveguides 1a and 1b serve as a main output port and a sub output port, respectively. Of the optical waveguide 1a,
A part between the directional couplers 2a and 2b is a tapered wide optical waveguide 1c. Two directional couplers 2
The optical path length difference between the optical waveguides 1a and 1b between a and 2b is a small amount Δ.
It is set to L. In the Mach-Zehnder optical interferometer circuit set in this way, two directional couplers 2a, 2
between b

[0019]

## EQU3 ## There is a phase difference of Θ = θ + θ '(3). The first term θ in the equation (3) is a phase difference generated by the optical path difference ΔL and is given by the equation (2),
The term θ ′ is a phase difference caused by a part of the waveguide connecting the two directional couplers being tapered. Although θ has wavelength dependence but hardly depends on polarization, θ ′ has both wavelength dependence and polarization dependence.

The power coupling ratio η of the Mach-Zehnder interferometer type optical branching element shown in FIG. 1 is calculated by the following equation, as in the case of the conventional Mach-Zehnder interferometer type optical branching element shown in FIG. Given.

[0021]

[Equation 4] η = sin ² φ ₁ cos ² φ ₂ + cos ² φ ₁ sin ² φ ₂ + 2cos θ sin φ ₁ sin φ ₂ cos φ ₁ cos φ ₂ (4) where φ ₁ and φ ₂ are two directions Coupling ratio η of the sex coupler
₁ and η ₂ and η ₁ = sin ² φ ₁ and η ₂ = sin ² φ ₂ are variables that have a relationship such as the optical waveguide spacing, the coupling length, the wavelength, and the polarization of the coupling portions of the directional couplers 2a and 2b. It depends on the waves. In order to facilitate understanding of the operation of the present invention, the case where two directional couplers are the same (φ ₁ = φ ₂ = φ) will be described. In this case, the equation (4) can be rewritten as follows.

[0022]

Η = sin (2φ) · (1 + cos Θ) / 2 (5) In the present invention, the sin (2φ) term in the equation (5), that is, the wavelength dependence and polarization dependence of the directional coupler, (1 + cos
Θ) / 2 wavelength dependence and polarization dependence, ie,
The principle is that the wavelength dependence of the phase difference θ and the wavelength dependence and the polarization dependence of the phase difference θ ′ in the equation (3) are used to cancel at the same time. Of course, the wavelength dependence and the polarization dependence of the sin (2φ) term, the wavelength dependence and the polarization dependence of the (1 + cosΘ) / 2 term cancel each other out in a desired wavelength range, and the wavelength dependence and the polarization dependence In order to realize a desired coupling rate with a small dependency, the formula (4) or (5) is examined to determine the wavelength dependency of the coupling rate of the directional couplers 2a and 2b, the ΔL value, and the tapered waveguide shape. It is necessary to set it appropriately.

The design guideline for alleviating the polarization dependence will be described below. In a directional coupler using a silica glass optical waveguide formed on a silicon substrate, the coupling ratio (sin ² φ) is
The TM mode is larger than the TE mode. In the configuration of FIG. 1, in order to obtain a branching element having a small wavelength dependence in a certain wavelength range, the branching element is constituted by a directional coupler whose coupling rate increases monotonically in the wavelength range, and (5) The 1 + cos Θ) / 2 term needs to decrease monotonically. Therefore, if the phase difference term is set to be smaller in the TE mode than in the TM mode, that is, the phase difference θ ′ due to the tapered waveguide. However, if it is set to be larger in the TM mode, the wavelength dependence of the entire optical branching element is less and the polarization dependence is alleviated.

The present invention will be described in detail below with reference to examples. In the following examples, a silica-based single-mode optical waveguide formed on a silicon substrate is used as an optical waveguide. This is because the silica-based single-mode optical waveguide has good connectivity with a single-mode optical fiber. This is because it is possible to provide an excellent and practical waveguide-type optical branching element, and the present invention is not limited to a silica-based optical waveguide.

Example 1 FIG. 2, FIG. 3, FIG. 4 and FIG. 5 are wavelength ranges of 1.25 .mu.m to 1.7 as a first example of the waveguide type optical branching device of the present invention.
2 is a plan view and an enlarged cross-sectional view showing a cross-section taken along line segments AA ′, BB ′ and CC ′ in FIG. 2, respectively, showing the configuration of a branch element designed to have a coupling rate of 20% ± 2% in μm Is.

Here, 1 is a silicon substrate, and 1a and 1b are silica-based optical waveguides formed on the silicon substrate 1 with a silica-based glass material. The optical waveguides 1a and 1b are close to each other at two locations to form directional couplers 2a and 2b. The optical waveguide 1a forms a wide tapered optical waveguide 1c between the directional couplers 2a and 2b. Optical waveguide 1
a and 1b are S having a cross-sectional dimension of about 8 μm × 8 μm embedded in the SiO ₂ -based glass clad layer 5 having a thickness of about 50 μm.
consists iO ₂ -GeO ₂ based glass core unit, the Mach-Zehnder optical interferometer circuit is constituted by the combination of the arc pattern of the linear pattern and the curvature radius of 50 mm.

Such a quartz optical waveguide can be formed by a known combination of a glass film deposition technique utilizing a flame hydrolysis reaction of silicon tetrachloride or titanium tetrachloride and a fine processing technique by reactive ion etching.

The coupling portion of the directional couplers 2a and 2b keeps the two optical waveguides 1a and 1b at a distance of 5 μm, and one of them is arranged in parallel over a distance of 0.13 mm and the other of 0.52 mm. Configured by The tapered optical waveguide 1c has a total length of 4 mm and a length of 0.7 at both ends.
A tapered portion having a width of 5 mm and linearly changing from 8 μm to 9.5 μm was provided, and a central portion between the tapered portions at both ends had a rectangular shape with a width of 9.5 μm and a length of 2.5 mm. The distance between the input ports 3a and 4a and the distance between the output ports 3b and 4b were both designed to be 0.250 mm. The waveguide lengths of the portions connecting the two directional couplers 2a and 2b are L and L + ΔL, and the (n · ΔL) value is set to 1.595 μm. Since the refractive index n of the quartz optical waveguide is about 1.45, ΔL is set to 1.1 μm. ΔL was accurately set at the photomask pattern stage by using the slight difference in length between the curved waveguide and the straight waveguide between the two directional couplers 2a and 2b in FIG.

FIG. 6 is a diagram showing the wavelength and polarization dependence of the coupling rate of the optical branching element of this embodiment. Wavelength 1.25
The wavelength dependence is extremely small in the range of 1.65 μm, and the difference in coupling rate between the TM and TE modes is 1% or less in the above wavelength range.

FIG. 7 further shows that the wavelength of this device is 1.31 μm.
The polarization dependence of the loss of the main output port and the sub output port at m is shown, and it can be seen that the loss variation due to the polarization state of the sub output port is as small as ± 0.08 dB.

FIG. 8 is a diagram for explaining the polarization characteristics of the coupling rate of the optical branching device of this embodiment. Curves (a), (a '),
(B) and (b ') are component directional couplers 2.
The coupling rate characteristic with respect to TM and TE modes of a and 2b itself is shown. Curves (c) and (c ') are wavelengths of the phase difference generated in the tapered optical waveguide 1c between the two optical waveguides 1a and 1b connecting the two directional couplers 2a and 2b for the TM and TE modes, respectively. The dependence is shown, and the curve (d) shows the wavelength dependence of the phase difference in the case of the conventional example without the tapered optical waveguide as a reference. The coupling ratios of the directional couplers 2a and 2b are both larger for the TM mode, and inevitably when the phase difference having no polarization dependence shown in the curve (d) is given as in the conventional example. As a whole, the polarization dependency of the coupling rate occurs in the entire optical branching element. In the case of the present embodiment, as shown by the curves (c) and (c '), the polarization dependence of the phase difference is generated in the tapered waveguide 1c, which causes the directional couplers 2a and 2b. It is possible to cancel the polarization dependence of the above, and as a result, it is possible to reduce the polarization dependence of the directional couplers 2a and 2b.

The dimensions of the optical branching element of this embodiment will be described below. The length was 20 mm and the width was 2.5 mm, which was about the same as the conventional optical branching element. This is because only a part of the waveguide connecting the two directional couplers in the conventional optical branching element with a straight line is replaced with a tapered shape. The loss value of the optical branching element of this embodiment is about 0.2 dB, which is about the same as that of the conventional optical branching element, but this is excessive because the taper angle of the tapered waveguide portion is designed to be small. This is because no radiation loss occurs.

Embodiment 2 FIG. 9 shows a wavelength range of 1.
It is a top view of the optical branching element which has a coupling rate of 50% +/- 5% in 25 micrometers-1.6 micrometers. The outline is similar to that of the first embodiment, but in this embodiment, the tapered optical waveguide 1c is used.
Optical waveguide 1 for connecting two directional couplers by detouring
The difference is that it is provided in the middle of b. Of course, also in this case, the same arrangement as in the first embodiment can be adopted.

The waveguide spacing of the coupling portions of the directional couplers 2a and 2b was set to 5 μm, and the coupling portion lengths were set to 0.72 mm and 1.28 mm, respectively, as compared with the strong coupling of the first embodiment.
The structure of the tapered optical waveguide 1c is also different from that of the first embodiment and has a total length of 4 mm, and both ends are provided with tapered portions whose width decreases linearly from 8 μm to 7.5 μm at a length of 0.75 mm,
And the width between the taper parts at both ends is 7.5μ.
The shape was an elongated rectangular shape having a length of m and a length of 2.5 mm. Further, ΔL was set to 1.225 μm. The element length of this example was 25 mm. In the present embodiment, the linear tapered waveguide is used, but the tapered portion may be provided in the curved portion as long as the phase difference effectively generated is the same.

FIG. 10 is a diagram showing the wavelength and polarization dependence of the coupling rate of the optical branching element of this embodiment. Wavelength 1.25
It has a nearly constant coupling rate of 50 ± 5% in the range of 1.60 μm, and the difference in the coupling rate between the TM and TE modes is 1.5% or less in the above wavelength range, and the conventional wide wavelength range operation. The polarization dependence of the waveguide-type optical branching element was ± 3.5% at maximum, which is significantly reduced compared to the maximum.

In the above embodiments, the structural parameters of the coupling portion of the directional coupler have been described. However, since the directional coupler is an optical circuit element having extremely sensitive structure, the manufacturer has a habit of each manufacturing process. The parameters can be changed in consideration of the above. In short, for example, in Example 1,
The directional couplers, which are the constituent elements of the Mach-Zehnder optical interferometer circuit, are respectively represented by curves (a), (a '), and
It may be designed and manufactured so as to exhibit wavelength and polarization characteristics close to those of (b) and (b ').

Furthermore, in the above embodiment, the tapered waveguide 1c is provided on one of the two optical waveguides connecting the two directional couplers, but a tapered portion may be formed on both of them. Absent. The point is that the tapered optical waveguide may be arranged so that the polarization dependence of the phase difference between the two waveguides relaxes the polarization dependence of the directional coupler.

In each of the above-mentioned embodiments, the most important region of 1.3 μm in the optical fiber communication application field.
Although an example showing a flat coupling rate and a wavelength characteristic in the wavelength range including 1.55 μm has been shown, the present invention is not necessarily limited to such a wide wavelength range operation type optical branching element, and the wavelength dependence is conventionally. It should be pointed out that it is possible to design and manufacture an optical branching device that has almost the same polarization dependence as that of the directional coupler.

Further, in the above embodiments, the optical branching element was constituted by the silica-based optical waveguide on the silicon substrate, but in the present invention, the substrate is not limited to silicon.
It is also possible to change to a quartz glass substrate. Alternatively, as described above, the present invention is not limited to these silica-based optical waveguides, and can be applied to other waveguide material systems such as multi-component glass waveguide systems and lithium niobate waveguide systems. Is added.

[0040]

As described above, according to the present invention, two directional couplers are connected to form a Mach-Zehnder optical interferometer circuit, and an optical waveguide for connecting these two directional couplers. By providing a controlled slight difference ΔL to the difference in length, and providing a taper waveguide in the middle of the optical waveguide to operate the entire Mach-Zehnder optical interferometer circuit as an optical branching element, By reducing not only the wavelength dependence of the coupling rate of the coupler itself but also the polarization dependence, it is possible to provide an optical branching element with a small wavelength dependence and a low loss in a desired wavelength region as a whole element.

Such a waveguide type optical branching device is expected to have a wide range of uses for distribution, monitoring, and tapping of optical signals spread over a wide wavelength range. On the contrary, it is expected to be used as an optical merging element for merging two signal lights.

Furthermore, by arranging the branching elements according to the present invention in multiple stages on a flat substrate, it is easy to expand to 4-branch elements or 8-branch elements. Alternatively, it is also possible to form the optical branching elements in an array on the same substrate and connect the optical branching element to an optical fiber array having a pitch of 250 μm for use.

Since the device of the present invention can be mass-produced on a flat substrate in a large amount, cost reduction can be expected, and the optical branching device and its applied device of the present invention are expected to greatly contribute to the spread of optical communication systems.

[Brief description of drawings]

FIG. 1 is a plan view showing a configuration of an embodiment of a waveguide type optical branching device of the present invention.

FIG. 2 is a plan view showing the configuration of the first embodiment of the waveguide type optical branching device of the present invention.

3 is a cross-sectional view taken along the line AA ′ of FIG.

FIG. 4 is a sectional view taken along line BB ′ of FIG.

5 is a sectional view taken along the line CC ′ of FIG.

FIG. 6 is a characteristic diagram of wavelength and polarization dependence of the coupling rate of the optical branching device of the first embodiment.

FIG. 7 is a characteristic diagram of polarization angle dependence of port loss of the optical branching element of the first example.

FIG. 8 is a characteristic diagram of wavelength and polarization dependence of the coupling rate of the optical branching device of the first example.

FIG. 9 is a plan view showing the configuration of a second embodiment of the waveguide type optical branching device of the present invention.

FIG. 10 is a characteristic diagram of wavelength and polarization dependence of the coupling rate of the optical branching device of the second embodiment.

FIG. 11 is a plan view showing a configuration example of a conventional waveguide type optical branching element.

FIG. 12 is a characteristic diagram of wavelength and polarization dependence of the coupling rate of a conventional waveguide type optical branching element.

FIG. 13 is a characteristic diagram of polarization angle dependence of port loss of a conventional waveguide type optical branching element.

[Explanation of symbols]

 1 substrate 1a, 1b optical waveguide 1c tapered optical waveguide 2a, 2b directional coupler 3a, 4a input port 3b, 4b output port 5 clad layer

Claims (2)

[Claims] 1. A substrate, first and second optical waveguides arranged on the substrate, and first and second optical waveguides formed by bringing the first and second optical waveguides close to each other at two locations. A directional coupler, wherein one end of the first optical waveguide serves as an input port, and the other end of each of the first and second optical waveguides serves as a main output. In the optical branching element, an optical path length difference is provided in a portion connecting the first and second directional couplers of the first and second optical waveguides, and a part of at least one of the portions. A tapered optical waveguide is provided in the optical waveguide, and the wavelength and polarization dependences of the first and second directional couplers are relaxed by the wavelength and polarization dependence of the tapered optical waveguide. Guided-type optical branching device. 2. The waveguide type optical branching device according to claim 1, wherein the optical waveguide is a silica-based optical waveguide having a core portion size substantially equal to a core diameter of a single mode optical fiber.