JPH06102424A - Waveguide type directional coupler - Google Patents

Abstract

PURPOSE:To provide the waveguide type directional coupler which is improved in the yield of productivity and obtains an optional branch ratio and also has small wavelength dependency over a wide wavelength band. CONSTITUTION:The waveguide type directional coupler is constituted by forming two cores 12 and 13 in a clad 11 closely to each other. The core 12 is thin at an intermediate point between the incidence and projection end and thick at the incidence end and projection end, which are tapered symmetrically about the intermediate point; and the core 13 is close to the core 12 at an intermediate point and the incidence end and projection end are formed in a symmetrically curved shape about the intermediate point.

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 in which two cores are formed close to each other in a clad.

[0002]

2. Description of the Related Art Optical fiber communication requires various optical parts in addition to an optical fiber, a light receiving element, a light emitting element and the like. There is a directional coupler in the optical component, which is formed by, for example, forming two waveguides (cores) close to each other in a clad,
The light propagating in one core is branched into the other core.

By the way, it is well known that if the propagation constants of the two cores of the coupling portion of the directional coupler are different, the branching ratio of light changes gently with respect to the wavelength. For this reason, conventionally, two linear waveguides having different core widths are arranged in parallel in close proximity to each other to form a coupling portion, and in order to facilitate connection with an optical fiber, 2
It has been attempted to gradually increase the interval between the cores of a book to obtain a constant branching ratio independent of wavelength (Japanese Patent Laid-Open No. 2-287408).

FIG. 3A is a plan view of a conventional waveguide type directional coupler, and FIG. 3B is a sectional view taken along the line I-I.

As shown in FIGS. 1A and 1B, the waveguide type directional coupler has two straight lines having the same height and different widths in a clad 2 provided on a substrate 1. Shaped cores 3, 4
In parallel with each other near the midpoint, and for coupling with the optical fiber, the thicker core (upper side of the figure) 3 is formed in a curved shape, and the thinner core (lower side of the figure) 4 is tapered. It was formed in.

[0006]

However, in the conventional waveguide type directional coupler, the branching ratio greatly changes only when the distance between the two cores 3 and 4 in the coupling portion changes by about 0.2 μm. Therefore, in manufacturing, the tolerance is small and the yield is low (the substrate is glass, the core is silica waveguide,
When manufacturing a directional coupler using a photolithography technique, it is difficult to manufacture the core width and the core interval according to design values, and it is unavoidable that an error of 0.1 μm or more occurs. . There is also a problem that the wavelength band in which a constant branching ratio is obtained is narrow.

Therefore, an object of the present invention is to solve the above problems, to improve the yield, to obtain an arbitrary branching ratio, and to reduce the wavelength dependence over a wide wavelength band. To provide.

[0008]

In order to achieve the above object, the present invention provides a waveguide type directional coupler in which two cores are formed in a clad so as to be close to each other. Has a thin intermediate point and a thick incident end and an outgoing end, and the incident end and the outgoing end are formed in a symmetrical tapered shape across the intermediate point, and the other core is close to one core and the intermediate point. At the same time, the entrance end and the exit end are formed in a symmetrical curved shape across the intermediate point.

[0009]

According to the above structure, one core is formed in a curved shape and the other core is formed in a tapered shape, whereby a phase mismatch state is caused in the optical coupling and the maximum coupling rate is 100.
% Or less, and the change in the coupling rate with respect to the wavelength becomes gradual. That is, the wavelength independence becomes stronger, and since the straight parallel portion is eliminated, it is less likely to be affected by a mask shift, an etching defect, or the like in the case of manufacturing using the photolithography technique, and the yield can be improved.

[0010]

An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 (a) is a plan view of an embodiment of the waveguide type directional coupler of the present invention, and FIG. 1 (b) is its II-II.
It is a line sectional view.

As shown in FIGS. 1A and 1B, the waveguide type directional coupler includes a clad 1 provided on a substrate 10.
Two cores 12 and 13 are provided in one, and one core (lower side in the figure) 12 has a taper such that the core width becomes the smallest at the midpoint between the incident end Pin and the outgoing end Pc. The other core (upper side in the figure) 13 is formed in a linear shape including a portion, and is formed in a curved shape so as to come close to the one core 12 at an intermediate point. The curved portion and the tapered portion of the cores 12 and 13 which are adjacent to each other form a coupling portion of the directional coupler.

The cores 12 and 13 are embedded in a clad 11 having a refractive index lower than that of the cores 12 and 13.

The substrate 10 is made of quartz glass (SiO ₂ ), the cladding 11 is made of SiO ₂ —B ₂ O ₅ —P ₂ O ₃ , and the cores 12 and 13 are made of SiO ₂ —TiO ₂ .

In manufacturing this waveguide type directional coupler, the widths and heights of the cores 12 and 13, the intervals between the cores 12 and 13, the taper angles of the cores 12 and 13, and the curvatures of the cores 12 and 13. By setting the refractive indices of the cores 12 and 13, the cladding 11 and the substrate 10 to appropriate values, the light Pin incident on the incident end 13a can be broadened from the emission ends 13b and 12a of the core 13. It is possible to output with a desired branching ratio Pt: Pc over the wavelength band.

For example, the height of the cores 12 and 13 is about 8 μm,
The minimum width of the core 12 is about 3.8 μm, and the widths of the entrance ends 13a and the exit ends 12a, 13b of the cores 12 and 13 are about 8 μm.
Length of tapered portion of core 12 (8 μm core width is 3.8 μm
m is about 1600 μm, the curvature of the core 13 is about 50 mm, the core interval between the cores 12 and 13 is about 3.0 μm, and the refractive index of the cores 12 and 13 is 1. A high-performance wideband directional coupler could be realized by using quartz glass having a refractive index of 4624, a cladding 11 of 1.458, and a substrate 10 of 1.458.

FIG. 2 is a diagram showing the evaluation result of the wavelength independence of the branching ratio of the waveguide type directional coupler.

In the figure, the horizontal axis represents wavelength and the vertical axis represents loss. The curve L ₁ is the wavelength characteristic of the branched light by the waveguide type directional coupler of the present application, the curve L ₂ is the wavelength characteristic of the transmitted light by the waveguide type directional coupler of the present application, and the curve L ₃ is the conventional waveguide type direction. The wavelength characteristic of the branched light by the directional coupler, the curve L ₄ shows the wavelength characteristic of the transmitted light by the conventional waveguide type directional coupler.

From the figure, the wavelength is about 1.25 μm to 1.75.
Within the band of up to μm, the change of the loss of the branched light is between about 7 dB and 7.6 dB (conventionally about 6.9 dB to about 9.3 dB).
B, especially in the long wavelength region), and the change in loss of transmitted light is between approximately 0.9 dB and 1.2 dB (conventionally between 0.5 dB and 1.2 dB), It can be seen that the transmitted light and the branched light hardly depend on the wavelength.

Next, the operation of the embodiment will be described.

Since one core 13 is formed in a curved shape and the other core 12 is formed in a tapered shape to eliminate the linear parallel portion, the wavelength independence becomes strong, and the photolithography technique is used to form a cladding. When the core pattern is manufactured, it is unlikely to be affected by the mask shift of the photomask, etching defects, and the like, and the yield can be improved.

Here, in the directional coupler composed of two cores having the same propagation constant, the phases of light are in a matched state and 0
~ 100% binding can occur. In this case, the coupling rate changes sinusoidally with respect to the wavelength, and it is difficult to keep the coupling rate constant in a certain wavelength band.

Therefore, the propagation constants of the two cores of the directional coupler are made different. Then, in the optical coupling, the phase mismatching state occurs, the maximum coupling rate becomes 100% or less, and the variation of the coupling rate with respect to the wavelength becomes gentle. By utilizing this, it is possible to realize a wavelength-independent coupler having a constant coupling rate in a certain wavelength band.

The conventional wavelength-independent coupler has a structure in which a thick core and a thin core are arranged in parallel (the propagation constant is varied depending on the thickness of the core). In the present application, one of the cores is tapered and the other is curved (curvature ∞ =
Including the case of a straight line), the mixing efficiency of the phase was strengthened to create a stronger phase mismatch state, and the wavelength band was widened.

As described above, according to the present embodiment, in the waveguide type directional coupler in which the two cores 12 and 13 are formed in the clad 11 in close proximity to each other, one core 12
Of the entrance and exit ends are thin, the entrance end and the exit end are thick, and the entrance end and the exit end are formed in a symmetrical tapered shape across the middle point. Since the entrance end and the exit end are formed in a symmetrical curved shape across the middle point, the yield of productivity can be improved and an arbitrary branching ratio can be obtained.
Moreover, it is possible to provide a waveguide type directional coupler having a small wavelength dependence over a wide wavelength band.

[0026]

In summary, according to the present invention, the following excellent effects are exhibited.

(1) Wavelength can be made independent over a wide band.

(2) The yield of productivity is improved.

[Brief description of drawings]

FIG. 1A is a plan view of an embodiment of a waveguide type directional coupler of the present invention, and FIG. 1B is a sectional view taken along line II-II thereof.

2 is a diagram showing an evaluation result of wavelength independence of a branching ratio of the waveguide type directional coupler shown in FIG.

FIG. 3A is a plan view of a conventional waveguide type directional coupler, and FIG. 3B is a sectional view taken along line I-I thereof.

[Explanation of symbols]

 1, 10 Substrate 3, 4, 12, 13 Core 2, 11 Clad

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Hideaki Arai Inventor Hideaki Arai 5-1-1 Hidaka-cho, Hitachi City, Ibaraki Hitachi Cable Company, Ltd., Optoro System Laboratories (72) Inventor Naoto Uezuka Hidaka, Hitachi City, Ibaraki Prefecture 5-1-1, Machi, Hitachi Cable, Ltd. Optro System Research Center

Claims (1)

[Claims] 1. A waveguide type directional coupler in which two cores are formed close to each other in a clad, and one of the cores has a thin midpoint between the input and output ends and a thick input and output ends.
In addition, the entrance end and the exit end are formed in a symmetrical tapered shape across the midpoint, and the other core is placed close to the one core and the midpoint, and the entrance end and the exit end are placed across the midpoint. A waveguide-type directional coupler, wherein and are formed in symmetrical curved shapes.