JPH05114762A - Optical coupling device - Google Patents

Abstract

(57) [Abstract] [Purpose] Optically couples two different optical functional elements, especially an optical functional element integrating a plurality of devices with low loss. A first optical waveguide layer 208 formed on a semiconductor substrate 201 and a second optical waveguide layer 208 formed on the first waveguide layer 208.
From the optical waveguide layer 209 to the spot size conversion optical waveguide 20
2 is formed, and the thickness of the second optical waveguide layer 209 is gradually changed along the light propagation direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low loss optical coupling device for converting the spot diameter of a light wave propagating through an optical waveguide.

[0002]

2. Description of the Related Art Optical communication systems are required to have high performance, multifunction, and downsizing such as optical transceivers and optical switching devices. For this reason, the integration of semiconductor optical devices used in these optical communication devices is progressing. For example, a semiconductor monolithic optical integrated device in which a plurality of light active elements such as light emitting elements and optical switches are formed on the same semiconductor substrate, or a hybrid optical integrated device in which optical devices in which optical functional elements are formed on different substrates are optically coupled Development is in progress. In these optical semiconductor devices, 2
It is necessary to optically couple the two optical functional elements or the optical functional element and the optical fiber with low loss.

When optically coupling a semiconductor laser diode (LD) and a single-mode fiber, the end face of the LD element and the fiber are directly butt-coupled (butt joint).
Then, since the optical waveguide light wave spot sizes are different from each other, the coupling loss of the direct butting portion becomes a problem.
Usually, the LD light wave spot size (mode radius: W) is about 1 μm, and the fiber spot size is about 5 μm.
m, the coupling loss in this case is about 10 dB. Therefore, a method of converting the spot size with a lens to reduce the coupling loss is generally used.

FIG. 5 shows a conventional configuration example in which an optical functional element having a plurality of laser diodes (LDs) and an array fiber are optically coupled by a single lens. In FIG. 5, 504 is a semiconductor substrate, 505 is an LD active region (optical waveguide portion), 516 is a lens, 506 is a fiber,
507 is a V for fixing the fiber 506 at regular intervals.
-It is a groove array.

[0005]

However, in such a structure, as the integration scale of the LD increases, the coupling loss increases due to the influence of the aberration of the lens and the like, so that the LD can be integrated on one semiconductor substrate. However, it was difficult to realize a high performance and highly integrated optical device.

Therefore, the present invention has been made in order to solve the above-mentioned conventional problems, and an object thereof is to provide a function between two different optical functional elements described above, particularly between optical functional elements in which a plurality of devices are integrated. An object is to provide an optical coupling device capable of optical coupling with low loss.

[0007]

In order to achieve such an object, the present invention comprises a waveguide consisting of at least two optical waveguide layers, and at least one of the waveguide layers has a tapered thickness or refractive index. It is formed into a shape.

[0008]

In the present invention, the light wave spot size is converted into a low loss by forming the waveguide layer with a tapered thickness or refractive index.

[0009]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows an embodiment of the present invention, which is an array L.
It is a figure which shows the structure at the time of inserting the optical coupling device by this invention between a D element and a fiber, and taking optical coupling with low loss. 2 and 3 are views for explaining a device structure and an operation principle when a semiconductor substrate is used as an embodiment of the optical coupling device of the present invention. 1A is a plan view seen from above, and FIG. 1B is a sectional view. In the figure, 101 is a semiconductor substrate of an optical coupling device according to the present invention, 102 is a spot size conversion waveguide, 103 is an antireflection film, 104 is a semiconductor laser substrate, 105 is an LD active layer (optical waveguide portion), and 106 is a single layer. One mode optical fiber, 1
07 is a V-groove array. Since the light wave spot size of the LD is converted into the spot size of the fiber by the optical waveguide of the optical coupling device, it is optically coupled to the fiber 106 with low loss.

FIG. 2 (a) is a plan view of the optical coupling device according to the present invention as seen from above, and FIG. 2 (b) is A- of FIG. 2 (a).
2A is a cross-sectional view taken along the line A ′, FIG. 2C is a cross-sectional view taken along the line BB ′ of FIG. 2A, and FIG. 2D is an enlarged cross-sectional view of a C portion of FIG. 2B. In the figure, 201 is a semiconductor substrate made of InP, which serves as a clad portion of the optical waveguide. 208 is I
A first waveguiding layer (pseudo clad layer) made of nGaAsP, InAlAs, or the like, and 209 is an InGaAs well layer 21.
2 and a second waveguide layer (pseudo core layer) 210 composed of a multiple quantum well layer including InP barrier layer 214 and InP barrier layer 214.
Is a clad layer. Reference numeral 211 is incident light, and 212 is outgoing light. The width w ₁ and the thickness t _{1 of} the first waveguide layer 208
_Are w _i1 and t _i1 at the light incident part and w _o1 and t _o1 at the light emitting part, respectively. In addition, the second waveguide layer 20
The width w ₂ and the thickness t ₂ of 9 are respectively w _i2 and
t _i2 , and w _o2 and t _o2 at the light emitting portion, respectively.
Further, the magnitudes of the refractive indexes of the first waveguide layer 208 and the second waveguide layer 209 are n ₁ and n ₂ , respectively.

In such a configuration, the light wave spot size in the optical waveguide depends on the dimensions w and t of each waveguide layer and the refractive index n. For example, in the configuration of FIG. 2, the calculation result by the slab waveguide model of the light wave spot size (radius of the fundamental mode) W in the depth direction of the optical waveguide is shown in FIG. Here, the wavelength λ = 1.55 μm
And the thickness of the first waveguide layer 208 is t _i1 = t _o0 = 3 μm
Constant, its refractive index n ₁ = 3.186 (normalized refractive index difference Δn ₁ = (n ₁ −n _c ) / n _c = 0.
5%, n _c : refractive index of cladding layer = 3.166),
The normalized refractive index difference Δn ₂ of the second waveguide layer is used as a parameter. As can be seen from FIG. 2, when t ₂ = 0, the spot size of the optical waveguide having only the first waveguide layer (w = ˜2 μm)
However, as t ₂ increases, the light spot size W tends to decrease after reaching a maximum value.
It approaches the spot size of the waveguide with only the second waveguide layer (t ₁ = 0).

From the above, for example, when the LD and the optical fiber are optically coupled, in FIG. 2, the width w _{i2 is set} so that the spot size on the light incident side is about the same as the spot size of the LD. , The thickness t _i2 and the refractive index n ₂ may be set. On the light emitting side, a width w _o1 giving a spot size similar to the spot size of the fiber and a thickness t
_o1, the refractive index n ₁ and a width w _o2, the thickness t _o2 (≦ t _i2), may be a refractive index n _o2 (≦ n _i2). At this time, the length l of the region where the waveguide layer is tapered may be set to a size that radiation loss associated with mode size conversion is sufficiently small (l = several hundred μm to several mm). Further, as shown in FIG. 2, for higher order mode conversion loss due to mode size conversion, for example, w _o1 > w _i1 (∼0), w _i2 > w _o2 (∼).
If the structure of (0) has an optical waveguide structure in which higher-order modes are cut off, the loss can be reduced.

The thickness t ₁ of the first waveguide layer 208 or the second
As a method of forming the thickness t ₂ of the waveguide layer 209 in a tapered shape, for example, when the waveguide layer is formed on the semiconductor substrate 201 by epitaxial growth, the growth rate depends on the substrate temperature (reference document: IEEE, for example). Journal
of Quantum Electronics, vo
l. QE-27, no. 3, pp687, 1991) or the shape and size of the selective growth mask made of an insulator such as SiO ₂ (Reference: Japanese Patent Application No.
170452). It can also be formed by a wet etching or dry etching technique. Further, the widths w ₁ and w ₂ can also be formed in a tapered shape by using an etching technique.

The magnitudes n ₁ and n ₂ of the refractive indexes of the first waveguide layer 208 and the second waveguide layer 209 are adjusted by controlling the composition of InGaAsP or selecting an appropriate semiconductor material such as InAlAs. it can. Also, FIG.
Well layer as a multi-quantum well structure like the second waveguide layer of
The thickness of the barrier layers can be adjusted by an appropriate combination. That is, this refractive index n ₂ is equal to the thickness d of the well layer 213.
Depending on _w and the thickness d _b of the barrier layer 214, for TE mode light, for example, n ₂ = [(n _w ² d _w + n _b ² d _b ) / (d _w + d _b )] ^{1 /} It is known to be given in a relationship of ² . Where n
_w and n _b are the refractive indices of the well layer and the barrier layer, and the wavelength λ =
For light in the 1.55 μm band, n _w = 3.6, n _b =
The size is about 3.17. Well layer 213 thickness d _w
Is configured so that the effective band gap energy of the quantum well structure is sufficiently larger than the photon energy, the absorption loss due to the second waveguide layer 209 can be reduced. For example, λ =
For light in the 1.55 μm band, d _w may be ˜0.5 nm or less.

FIG. 4 is a cross-sectional view showing another embodiment of the optical coupling device according to the present invention, in which a third waveguide layer 415 is formed between the cladding layer 410 and the second waveguide layer 409. Is. In this case, the thickness t ₃ of the third waveguide layer 415,
The total thickness (t ₁ + t ₃ ) of the first waveguide layer 408 and the third waveguide layer 415 is shown by setting the refractive index n ₃ to be t _i3 = t _i1 , t _o3 = t _o1 , and n ₃ = t ₁ . First Waveguide Layer 2 of Example 2
By setting the thickness to be approximately the same as the thickness t _{1 of} 08, the spot size can be similarly converted and a light wave mode shape having good symmetry in the depth direction can be realized.

In the above embodiment, InP is used.
The case where the waveguide layer for spot size conversion is formed on the substrate has been described, but a GaAs semiconductor substrate is used for the AlG.
Similar effects can be obtained by using a semiconductor material such as aAs or a dielectric material for the substrate and the waveguide layer.

Although FIG. 1 shows a configuration example of optical coupling by a butt joint, a low lens is inserted between the LD element 104 and the optical coupling device 101 or between the optical coupling device 101 and the fiber 106. It is also possible to obtain lossy optical coupling.

Since the optical coupling device according to the present invention can use a semiconductor material, the present coupling device is monolithically integrated at the light input / output end of an optical functional element such as an LD element or an optical switch using the semiconductor material. In this case, a high-density optical integrated device can be realized.

[0019]

As described above, according to the present invention,
It is composed of a waveguide consisting of at least two optical waveguide layers, and the light wave spot size is gradually converted by changing the thickness or the refractive index of at least one of the optical waveguide layers, so that it is different. It is possible to optically couple the two optical functional devices with low loss. Also,
Since it can be configured in a small size, an extremely excellent effect such as realization of a highly integrated optical device can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration in which an optical coupling device according to the present invention is inserted between an array LD element and a fiber.

FIG. 2 is a diagram showing a configuration according to an embodiment of an optical coupling device according to the present invention.

FIG. 3 is a diagram illustrating an operation principle of the optical coupling device shown in FIG.

FIG. 4 is a diagram showing the configuration of another embodiment of the optical coupling device according to the present invention.

FIG. 5 is a diagram showing a configuration of a conventional optical coupling device.

[Explanation of symbols]

 101 semiconductor substrate 102 spot size conversion waveguide 103 antireflection film 104 semiconductor laser substrate 105 LD active layer 106 optical fiber 107 V-groove array 201 semiconductor substrate 202 spot size conversion waveguide 208 first waveguide layer 209 second waveguide layer 210 clad layer 211 incident light 212 emitted light 213 quantum well layer 214 barrier layer 401 semiconductor substrate 402 spot size conversion waveguide 408 first waveguide layer 409 second waveguide layer 410 cladding layer 415 third waveguide layer

Claims (2)

[Claims] 1. A semiconductor substrate, a first optical waveguide layer formed on the semiconductor substrate, and a second optical waveguide layer formed on the first optical waveguide layer. In an optical functional device having at least one optical waveguide, at least the thickness or the refractive index of the first optical waveguide layer or the second optical waveguide layer is gradually changed along the light propagation direction. Optical coupling device. 2. The method according to claim 1, wherein at least the first
2. An optical coupling device characterized in that said optical waveguide layer or said second optical waveguide layer has a multiple quantum well structure.