JPH0736050B2 - Method of coupling semiconductor light emitting device and optical waveguide - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coupling method for a semiconductor light emitting device and an optical waveguide, and more particularly to a coupling method suitable for suppressing reflected light from returning to the semiconductor light emitting device.

[Background of the Invention]

Conventionally, in an optical communication device using a semiconductor laser, it is known that C / N and distortion greatly change when the reflected light from the optical fiber end face, connector end face, optical multiplexer / demultiplexer end face, etc. returns to the semiconductor laser. ing. Therefore, as measures against this, a method of obliquely polishing the end face so that reflected light does not return to the semiconductor laser (FIG. 1A), a method of offsetting the semiconductor laser and the coupling optical system lens, and a reflecting surface Is used with respect to the optical axis, and good characteristics have been obtained. However, in the case of an optical integrated circuit, which is attracting attention as a future optical device, a semiconductor laser, a light receiving element, an optical waveguide, an optical multiplexing / demultiplexing unit,
It is composed of an optical module in which optical devices such as an optical modulator and an optical switch are integrated, and an optical signal is transmitted in the optical fiber by connecting the optical fiber to the optical module (for example, Japanese Patent Laid-Open No. -There is a 49288 bulletin.).
In this case, it was found that the reflected light of the semiconductor laser generated in the optical module returns to the semiconductor laser and causes C / N and distortion deterioration. In this case, the above-mentioned problems occurred even though the end faces of the optical fibers were obliquely polished (angle 8 °). That is, in a system in which the emitted light of a semiconductor laser is coupled to an optical fiber via an optical waveguide (including optical devices such as the above optical multiplexer / demultiplexer in addition to simple optical waveguides), it is important to take measures against reflected light. I got caught.

[Object of the Invention]

An object of the present invention is to provide a coupling method of a semiconductor light emitting device and an optical waveguide, which prevents reflected light from returning to the semiconductor laser in an optical integrated circuit that couples the emitted light of the semiconductor laser to the optical fiber through the optical waveguide. is there.

[Outline of Invention]

According to the present invention, a semiconductor laser and an optical waveguide for propagating emitted light of the laser are provided on a substrate, and any one of the width, thickness, and refractive index of the optical waveguide on the laser light incident side, or two or more of them are partially formed. It is configured to have a tapered shape. That is, since the emitted light of the semiconductor laser has a width or thickness on the incident side of the optical waveguide, or a refractive index that is partially increased in the taper direction in the propagation direction of the incident light, it is coupled and propagates. Propagation is maintained in a steady state in the part where the shape is constant, but with respect to the reflected light from the end face of the optical fiber, the light is concentrated in the part where the width or thickness or the refractive index is tapered. Leakage, and it becomes difficult to return to the semiconductor laser. As the optical waveguide, a buried type, a diffusion type, a loading type, a ridge type, a raised type and the like can be applied.

In the present invention, it will be described that reflected light can be suppressed.
In the above optical waveguide, for example, when the width of the optical waveguide is tapered partially and continuously toward the laser light incident side, the optical waveguide propagates in a tapering decreasing direction. The optical signal with the wavelength λ ₀ gradually loses the confinement of light in the optical waveguide and radiates out at a desired angle from the tapered portion, and finally the width of the optical waveguide and the refractive index of the optical waveguide and the cladding portion. When the normalized frequency determined by the difference becomes a small value that does not satisfy the propagation condition, the above λ ₀
The optical signal is cut off and does not propagate. The present invention is an application of this principle, and one or more of the width, thickness, and refractive index of the above-mentioned optical waveguide is partially tapered toward the laser light incident side up to the very vicinity having the cut-off characteristic. The reflected light is leaked radially. In the case of the optical waveguide, the reflected light leaked in the radial direction does not return to the inside of the optical waveguide again because the cladding is thick enough to the air or near infinity. I can't come. In addition, conventionally,
As a method for efficiently coupling the emitted light of a semiconductor laser to a single mode optical fiber, as shown in FIG. 1 (b),
There is a method of tapering the tip of the optical fiber and making the tip spherical, but in this case, the reflected light of the semiconductor laser leaks little by little in the tapered portion. However, the leaked light enters the cladding portion, propagates in the cladding portion as a cladding mode while being reflected at the interface between the cladding portion and air, and returns to the semiconductor laser, so that there is no effect as in the present invention. . Further, in the case of an optical fiber, there is no cut-off characteristic, and there is also little field of exudation to the cladding.

Further, it is known that the input / output ends of the optical waveguide are tapered optical waveguides as shown by 9 in FIGS. 2 and 3 to reduce the coupling loss at the input / output portion (Japanese Patent Laid-Open No. 58-118610). ). This is a taper shape continuously over a long distance in order to reduce the coupling loss due to the difference in the core shapes of the optical waveguide and the optical fiber as much as possible. This is different from the configuration in which a semiconductor laser and an optical waveguide are provided and a partially tapered shape is provided to leak reflected light, and the effect is also different.

Example of Invention

FIG. 4 shows an embodiment of the coupling system of the semiconductor laser and the optical waveguide of the present invention. The figure (a) is a top view, (b) and (c) are front views. Semiconductor substrate 1 (eg, n-type
A semiconductor laser 2 and an optical waveguide 3 are monolithically formed on a GaAs substrate. In the figure (a), 5 is the optical waveguide 3.
Is a portion in which the width of the is tapered. (C) is a portion where the thickness is also tapered. That is, the width or the thickness of the optical waveguide 3 is increased partially and continuously in the propagation direction of the emitted light from the emitted light end side of the semiconductor laser.

FIG. 5 shows an enlarged view of the tapered portion 5 in FIG. In the figure, 2 is a semiconductor laser that oscillates in a single mode at a wavelength λ ₁ (for example, 1.55 μm). The width w ₂ of the optical waveguide 3 is a width (about 8 μm) set so that the optical signal of the wavelength λ ₁ can be transmitted in a single mode. w ₁ is the wavelength λ ₁ (1.55μ
Set a value that is slightly larger than the width (5.25 μm) at which the light mode and disconnection in (m) occur. When such a size structure is set, the emitted light of the semiconductor laser 2 propagates in the optical waveguide 3, but the reflected light returning to the semiconductor laser 2 side is converted into light by the tapered portion 5. It exudes from 3 and leaks to the outside, and almost never returns to the semiconductor laser 2. This amount of returning light depends on w ₁ . That is, the smaller w ₁ is, the smaller the amount of light confined in the optical waveguide 3 is, and the larger the amount of light seeping out of the optical waveguide 3 is. The length L of the tapered portion 5 depends on the spread angle of the reflected light. The shorter L is,
The spread angle of the reflected light becomes large. The same effect can be obtained by changing the thickness and the refractive index of the optical waveguide 3 instead of changing the width of the optical waveguide 3.

FIG. 6 shows an embodiment in which the emitted lights of the arrayed semiconductor lasers 2a and 2b are combined and transmitted. 2a and 2b may be semiconductor lasers having the same wavelength or different waveforms.

As the optical waveguide of the present invention, those shown in FIGS. 7 and 8 can be applied. FIG. 7 shows the embedded type, and FIG. 8 shows the ridge type. In these figures, (a) is a top view, (b) is a side view,
(C) is a front view.

In FIGS. 4 to 8, when the tip portion of the optical waveguide on the semiconductor laser side is obliquely inclined (angle θ is about several degrees) as shown in FIG. 9A, reflection from the optical waveguide side to the semiconductor laser side is performed. It is possible to prevent light from returning to the semiconductor laser. Further, in order to increase the coupling efficiency of the light emitted from the semiconductor laser to the optical waveguide, the tip of the optical waveguide may be formed into a curved surface shape (preferably a parabolic surface shape) as shown in FIG. 9 (b). . Further, as shown in FIG. 9 (c), if the tip end is doped with a doping material that is equal to or lower than the refractive index of the semiconductor laser but higher than the refractive index of the optical waveguide by ion implantation or the like. Therefore, higher coupling efficiency characteristics can be obtained. Further, in the embodiment of the present invention, only the semiconductor laser and the optical waveguide are formed on the semiconductor substrate, but the present invention is not limited to this. That is, an optical device such as a light receiving element, an optical multiplexer / demultiplexer, an optical switch, an optical modulator, and a lens may be formed. Further, by forming an antireflection film on the tip of the optical waveguide, it is possible to reduce the reflected light from the tip. Also, the optical waveguide 3
In order to prevent the reflected light at the connection portion of the optical fiber 4 and the optical fiber 4 from returning to the semiconductor laser, it is more effective to provide the end face of the optical waveguide of the connection portion with an inclination. Further, although the semiconductor laser is used as the semiconductor light emitting element in the above-mentioned embodiment, a light emitting diode may be used.

〔The invention's effect〕

According to the present invention, since reflected light hardly returns to the semiconductor laser, it is possible to realize a stable optical transmission system with high C / N and low distortion.

[Brief description of drawings]

1 to 3 are views for explaining the prior art, and FIG.
FIG. 5 is a diagram showing an embodiment of the present invention, and FIGS. 5 to 9 are diagrams showing another embodiment of the present invention. 1 ... Semiconductor substrate, 2,2a, 2b ... Semiconductor laser, 3,3a, 3b
…… Optical waveguide core, 5, 5a, 5b …… Optical waveguide taper, 7 …… High refractive index material.

Claims (5)

[Claims] 1. A semiconductor light emitting element, an optical waveguide for propagating the emitted light of the light emitting element are provided on a substrate, and at least one of the width, thickness and refractive index of the optical waveguide on the incident side of the emitted light is set to A method of coupling a semiconductor light emitting device and an optical waveguide, which is characterized in that it is formed by partially tapering in the propagation direction. 2. The coupling method for a semiconductor light emitting device and an optical waveguide according to claim 1, wherein the tip of the optical waveguide on the outgoing light end side of the semiconductor light emitting device is formed in a curved shape. 3. The curved end portion of the optical waveguide on the outgoing light end side of the semiconductor light emitting device according to claim 2,
A method of coupling a semiconductor light emitting device and an optical waveguide, wherein a doping material of a material having a refractive index equal to or lower than that of the semiconductor light emitting device and higher than that of the optical waveguide is doped. 4. The method for coupling a semiconductor light emitting device and an optical waveguide according to claim 1, wherein the tip end of the optical waveguide on the side of the emitted light of the semiconductor light emitting device has an inclined structure. 5. A semiconductor light emitting device and an optical waveguide according to claim 1, wherein the optical waveguide has any one of a buried type, a diffused type, a ridge type and a raised type structure. Method of combining.