JP2873857B2 - Integrated optical coupler - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention has an optical waveguide layer structure configured so that the electromagnetic field distribution of propagating light is asymmetric with respect to a main waveguide layer, The present invention relates to an integrated optical coupler in which a slit is formed to perform wavefront division of guided light.

2. Description of the Related Art A device having a branching / coupling portion inside a semiconductor optical amplifier plays an important role as a bus node of an optical communication system, such as coupling of a transmission / reception portion and compensation of coupling loss by an optical amplifier. The branching / coupling of such a device requires branching / coupling of guided light, especially channel guided light, and a directional coupler, a Y-shaped branching device, or the like has been conventionally used.

However, the directional coupler has a strong wavelength dependence of the branching efficiency, and cannot be applied to wavelength multiplexed optical transmission. Further, the Y-type branching device has a problem that the device length is relatively long (on the order of mm).

On the other hand, as another branching / coupling device, a so-called wavefront splitting type integrated coupler in which slit processing is performed in the waveguide layer direction has been proposed (for example, J. Saltzman et al.
al .: “Cross coupled cavity semiconductor laser” App
l. Phys. Lett. 52 , 10, pp. 767-769 (March, 1988)).

FIG. 10 shows a cross section of a conventional optical coupler having a slit formed in an optical amplifier including an active layer. In the figure, 101 is a GaAs substrate, 102 is a lower cladding layer, 103 is an active layer, 104 is an upper cladding layer, and 105 is a cap layer. These layers 102 to 105 are formed on the substrate 101 by a molecular beam epitaxy (MBE) method or the like. Also, 106 and 107 are Au
The electrode 108a is a non-reflective (A
R) Coating, 109 is a wavefront splitting coupler which is a slit in the layer direction, and 110 is a field distribution of guided light.

In this example, the position of the tip of the slit 109 is
The electromagnetic field distribution 110 is designed so as to be substantially at the center, and the branching ratio is designed to be about 1: 1. Also, in order to increase the confinement coefficient of the optical amplifier, the active layer 103
Is set at the center of the guided light field distribution 110.

This slit 109 is usually processed by reactive ion etching (RIE) or reactive ion beam etching (RIB).
E), since it is performed by a focused ion beam (FIB) or the like, damage to the active layer 103 is not small. Therefore, the conventional integrated optical coupler in which the vicinity of the tip of the slit 109 is located near the active layer 103 has a problem that the active layer 103 is deteriorated due to the damage caused by the slit processing and the durability of the device is remarkably reduced.

Therefore, an object of the present invention is to provide an optical waveguide layer structure in which the center of the field distribution of guided light is displaced from the main waveguide layer in view of the above problem, and to guide the slit in the layer direction to improve the durability of the device. It is an object of the present invention to provide an integrated optical coupler which is stopped up to the top of the wave layer.

[Means for Solving the Problems] An integrated optical coupler according to the present invention that achieves the above object includes at least a first clad layer, an optical waveguide layer, and a second clad layer sequentially laminated on a semiconductor substrate. A slit is formed so as to cross a part of the photoelectric field distribution of the guided light propagating through the optical waveguide layer from the second clad layer side, and a part of the guided light is formed by the slit. Is divided into wavefronts, and the center of the optical magnetic field distribution of the guided light is the second from the center of the optical waveguide layer.
The structure of the laminate is asymmetric with respect to the optical waveguide layer so as to be shifted toward the cladding layer side of the optical waveguide layer, and the end of the slit does not reach the optical waveguide layer, and stops before the optical waveguide layer. It is characterized by comprising.

 More specifically, the following can be performed.

The refractive index of the first cladding layer and the refractive index of the second cladding layer may be different from each other so that the configuration of the laminate is asymmetric with respect to the optical waveguide layer.

Further, an electrode for injecting a current into the optical waveguide layer may be provided, and the optical waveguide layer may function as an active layer for amplifying the guided light. In this case, a first and a second SCH layer are provided between the optical waveguide and the first and second cladding layers, respectively, in order to make the configuration of the laminated body asymmetric with respect to the optical waveguide layer. At least one of the refractive index and the layer thickness of the first and second SCH layers may be different from each other. Further, the first and second SCH layers have a refractive index distribution type structure in which the refractive index changes in the layer thickness direction, and the refractive index distributions of the first and second SCH layers are asymmetric with each other. You may do so.

In this way, the main waveguide layer of the optical waveguide layer and the field distribution center or the center of gravity of the guided light are appropriately shifted. Formed without damaging the layer.

Embodiment FIG. 1 and FIG. 2 show a configuration of a first embodiment of the present invention.
In the figure showing an integrated optical coupler, 1 is an n-type GaAs substrate, 2 is an n-type Al _x Ga _{1 -x} As cladding layer (Al mixed crystal ratio x = 0.
3) 3 is a GaAs active layer, 4 is a P-type Al _x Ga _{1 -x} As cladding layer (x = 0.2), 5 is a P ⁺ GaAs cap layer, and 6 and 7 are formed on the cap layer 5, respectively. Au electrode formed on the back surface of the substrate 1 and the Au electrode, 8a and 8b are AR coating, 9 is
A coupler section slit by FIB, 10 is an electromagnetic field distribution of guided light, and 11 is a channel waveguide section which is a ridge type channel waveguide. The slit 9 is formed at a crossing portion of the channel waveguide portion 11 at a 45 ° angle to the propagation direction of the guided light in the layer direction, and branches and couples the guided light.

The light wave 12 incident from the outside is branched and coupled by the integrated coupler having such a configuration, is optically amplified and multiplexed, and is output as the light wave 13. Optical amplification is performed by applying a forward bias between the electrodes 6 and 7 with a magnitude slightly smaller than the threshold value.

The position of the active layer 3 in the optical waveguide layer is below (from the center of the substrate 1) the center of the electromagnetic field distribution of the guided light as shown in FIG.
The active layer 3 can be set at the front end in the layer direction of the slit coupler by shifting the position of the slit coupler to the active layer 3, eliminating the need to cut the slit 9 into the active layer 3. Thereby, a highly durable integrated coupler can be configured.

As means for shifting the position of the active layer 3 and the center of the field distribution,
In this embodiment, the AlGaAs on both sides of the double hetero (DH) structure is used.
The refractive index of the cladding layers 2 and 4 is made asymmetric with respect to the active layer 3. That is, as shown in FIG. 3, the Al mixed crystal ratio (x = 0.3) of the n-type Al _x Ga _{1 -x} As cladding layer 2 on the substrate 1 side is changed to the p-type Al _x Ga _{1 -x} As cladding Layer 4 A
l By increasing the mixed crystal ratio (x = 0.2),
A refractive index is given to both cladding layers 2 and 4, and a high refractive index p-type
The electromagnetic field distribution is shifted to the AlGaAs cladding 4 side.

In this way, in order for the branching ratio of the slit 9 to be approximately 1: 1, the leading end position of the slit 9 may be at the position shown by the broken line in FIG. 3 (c), and the active layer 3 may be damaged by the slit processing. Disappears. Of course, the form of the electromagnetic field distribution and the position of the tip of the slit 9 may be appropriately changed according to the branching ratio of the slit 9 or the like.

FIG. 4 is for comparison with FIG. 3 relating to the present embodiment.
A conventional case in which both clad layers have the same refractive index is shown.
From the comparison between FIG. 3 and FIG. 4, it can be seen that the center (more precisely, the center of gravity) of the photoelectric field distribution can be displaced from the active layer 3, which is the main waveguide layer, by adopting the asymmetric cladding structure.

FIG. 5 is a schematic top view of a device according to a second embodiment in which the integrated optical coupler according to the present invention is applied to an integrated bus node including a transmission / reception unit. In the figure, reference numeral 20 denotes an n-type GaAs substrate,
21 and 22 are optical amplifiers for injecting current into the active layer, 24 is a distributed reflection (DBR) laser transmitter, 26 is a receiver, 23a and 23b are A
The R coating, and 25 is an integrated slit coupler as shown in FIG.

In this configuration, the light wave 27 entering the bus node from the left bus is amplified by the optical amplifier 21, and a part of the signal is reflected by the integrated coupler 25 and received by the light detector 26. On the other hand, the remaining signal passes through the coupler 25 as it is, and the optical amplifier unit compensates for the loss caused by the branch.
The light is amplified at 22 and output as a light wave 28. If you want to send a signal from this bus node,
An optical signal is output from 4, branched and coupled by the coupler 25, and superimposed on the output lightwave 28.

In order to discriminate between the own signal and the signal from the other, a device for performing wavelength multiplexing and demultiplexing / detecting by the photodetection unit 26 is necessary. This is directly related to the present invention or is not shown in FIG. Omitted for simplicity.

Regarding the fabrication of the branch coupler, the waveguide is processed into a ridge shape by the RIBE method after a normal photolithography process, and an electrode is formed through an insulating film deposition process and a ridge upper window opening process. The formation of the coupler 25 was performed at an accelerating voltage of 40 kV by FIB (Ga ⁺ ion beam). The slit 25 was formed by performing time control so that the processing depth was set at the center of the field distribution of the guided light as described above. The machining depth at this time is about 2
μm.

In the first embodiment, the waveguide structure is a step-like DH structure, and the main waveguide layer and the center of the field distribution of the guided light are shifted, but another example will be described below.

FIG. 6 shows a SCH structure (Separ structure) in which a waveguide structure and an active layer are separated.
3 shows a third embodiment having an ate-confinement-heterostructure). In the figure, 31 is an n-type GaSa substrate, 32 is an n-type
Al _x Ga _1-x As (x = 0.3) cladding layer, 33 is an n-type Al _x Ga _1-x A
s (x = 0.1) SCH layer, 34 is a multiple quantum well active layer (well is G
aAs, barrier consists of Al _0.1 Ga _0.9 As), 35 is p-type Al _x Ga
_1-x As (x = 0.1) SCH layer, 36 is p-type Al _x Ga _1-x As (x = 0.
3) a cladding layer, 37 is a p ⁺ GaAs cap layer, 38 and 39 are an Au electrode formed on the cap layer 37 and an Au electrode formed on the back surface of the substrate 31, respectively, 40 is a slit coupler, 41a, 41b
Is an AR coating.

The SCH structure has a structure in which the Al mixed crystal ratio, the refractive index, and the field distribution of the guided light are shown in FIGS. 7 (a), (b) and (c), and the quantum well structure is used as an active layer. Often used in laser structures. In the layer structure of the ordinary laser structure, the SCH layers on both sides of the active layer are set symmetrically. In this embodiment, the thicknesses of both SCH layers 33 and 35 are asymmetric (the refractive index is the same), and The field distribution center of the wave light is shifted from the position of the active layer 34. That is, the thickness of the SCH layer 35 on the p-type side is made larger than that of the other side 33 so that the guided light distribution is increased on the air side (the
To the opposite side).

Accordingly, the tip position of the slit coupler 40 is stopped at the position shown by the broken line in FIG. 7 (c).
Of course, also in the SCH layer, the thickness and the refractive index of both SCH layers 33 and 35 are appropriately set depending on the case, and the shift between the center of the field distribution of the guided light and the active layer corresponding to the desired branching ratio. Whatever may be desired. Other points are the same as the first embodiment.

FIG. 8 shows the Al mixed crystal ratio and the like of the SCH layer used in the fourth embodiment. In the fourth embodiment, the Al mixed crystal ratio of both SCH layers is asymmetrical (thickness is the same), the variation of the refractive index is asymmetrical as shown in FIG. It is drawn to the air side as shown in FIG. In this manner, the position of the active layer is shifted from the center of the field distribution, and the position of the tip of the slit coupler can be stopped at the position indicated by the broken line in FIG. 8 (c). The mixed crystal ratio of both SCH layers is p-type AlGaA
The s layer has x = 0.08 and the n-type AlGaAs layer has x = 0.12.

FIG. 9 shows the Al mixed crystal ratio and the like of the fifth embodiment in which the SCH layer has a GRIN (graded-index) structure. In the case of the GRIN structure, parameters such as the mixed crystal ratio x and the film thickness of the SCH layer in the third and fourth embodiments can be given by an arbitrary function, and a desired guided light distribution can be obtained. Become. Therefore, the offset between the active layer and the field distribution center due to the asymmetric structure can be set relatively freely.

 The other points are the same as the above embodiment.

By the way, in the above embodiment, the center of the field distribution of the guided light is shifted from the main waveguide layer in order to eliminate the need to cut the tip position of the slit coupler into the main waveguide layer such as the active layer. The principle of the present invention can also be used when shifting the distribution center and the main waveguide layer is required from other viewpoints.

For example, when it is necessary to shift the field distribution center of the guided light from the main waveguide layer, for example, when the guided light is emitted from the optical waveguide layer using a prism coupler or the like, the principle of the present invention is applied to this waveguide. What is necessary is just to apply to a layer structure.

[Effects of the Invention] As described above, according to the present invention, in an integrated optical coupler having an optical waveguide layer structure including a main waveguide layer such as an active layer, the center of the field distribution of guided light is located at the position of the main waveguide layer. By using an asymmetric waveguide structure that deviates from the above, for example, in forming a layer-directional wavefront splitting type split coupler, a branch coupler can be formed without processing damage to the active layer and the like, and is used in a semiconductor optical amplifier and the like. An integrated coupler with excellent durability can be realized as a branch coupler.

In addition, if necessary, displacement between the center of the field distribution of the guided light and the main waveguide layer of the optical waveguide layer can be easily performed.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a device according to a first embodiment of the present invention, FIG. 2 is a top view of the device of the first embodiment, FIG.
(B) and (c) are graphs showing the asymmetry of the cladding and the distribution of the photoelectric magnetic field for explaining the principle of the present invention in accordance with the first embodiment. FIGS. 4 (a), (b) and (c) are conventional graphs. FIG. 5 is a graph showing the symmetry of the cladding and the distribution of the photoelectric field of the example, FIG. 5 is a top view of the device of the second embodiment in which the device of the first embodiment is applied to an integrated optical bus node having a transmitting / receiving function, and FIG. FIG. 7 (a), (b), sectional views of the device of the third embodiment.
(C) is a graph showing the SCH structure and the photoelectric field distribution of the third embodiment, and FIGS. 8 (a), (b) and (c) are SCs of the fourth embodiment.
Graphs showing the H structure and field distribution, FIG. 9 (a), (b),
(C) is a graph showing the GRIN-SCH structure and field distribution of the fifth embodiment, and FIG. 10 is a cross-sectional view of a conventional example. 1, 31, 20 ... substrate, 2, 4, 32, 36 ... clad layer,
3, 34 ... active layer, 5, 37 ... cap layer, 6, 7, 3
8, 39 ... electrodes, 8a, 8b, 23a, 23b, 41a, 41b, ... AR
Coating, 9, 25, 40 ... Wavefront splitting coupler, 11
…… Ridge type channel waveguide, 21, 22 …… Optical amplifier, 24
…… DBR laser transmitter, 26 …… transmitter, 33, 35 …… SCH layer

Claims (5)

(57) [Claims] 1. A laminated body including at least a first clad layer, an optical waveguide layer and a second clad layer which are sequentially laminated on a semiconductor substrate, and the light guide is formed from the side of the second clad layer. A slit is formed so as to cross a part of the optical magnetic field distribution of the guided light propagating through the waveguide layer, and in the integrated optical coupler for dividing a part of the guided light by the wavefront by the slit, the slit of the photoelectric field distribution of the guided light is The configuration of the laminate is asymmetric with respect to the optical waveguide layer so that the center is shifted from the center of the optical waveguide layer toward the second cladding layer, and the tip of the slit does not reach the optical waveguide layer. An integrated optical coupler, wherein the integrated optical coupler is configured to stop before the optical waveguide layer. 2. The integrated optical coupler according to claim 1, wherein a refractive index of said first cladding layer and a refractive index of said second cladding layer are different from each other. 3. The integrated optical coupler according to claim 1, further comprising an electrode for injecting a current into the optical waveguide layer, wherein the optical waveguide layer functions as an active layer for amplifying the guided light. 4. The semiconductor device according to claim 1, further comprising a first SCH layer and a second SCH layer between said optical waveguide and said first and second cladding layers.
4. The integrated optical coupler according to claim 3, wherein at least one of the refractive index and the thickness of the second SCH layer is different from each other. 5. The first and second SCH layers have a refractive index distribution type structure in which a refractive index changes in a layer thickness direction, and the refractive index distributions of the first and second SCH layers are different from each other. 5. The integrated optical coupler according to claim 4, wherein the optical coupler is asymmetric.