JP2006251088A - Semiconductor optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem associated with a conventional n-i-n structure optical modulator in which centering control of an optical fiber and a lens is not conducted with simplicity, because light incident on an optical waveguide core layer is only inefficiently converted into a light absorption current and a sufficient light absorption current is not obtained, and also it has been difficult to make the optical waveguide core layer emit light with current injection. <P>SOLUTION: In the semiconductor optical modulator, by arranging a p-type semiconductor region extending to an n-type semiconductor cladding layer and a semi-insulating cladding layer in the vicinity of input and output terminals of an input-output waveguide, it becomes possible to withdraw a hole generated with light absorption and to make the optical waveguide core layer 13 emit light with current injection from the p-type semiconductor region 15-2. Adjustment of optical coupling between the optical modulator and the optical fiber or the lens is conducted with simplicity. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a structure of a semiconductor optical modulator used in an optical communication system or an optical information processing system.

A waveguide-type optical control device such as an optical modulator is one of key elements of a high-speed optical communication system and an optical information processing system. Optical modulators are roughly classified into those using a dielectric such as LiNbO ₃ (LN) and those using an InP or GaAs semiconductor.

LiNbO ₃ optical modulators made of dielectric materials are widely used at present. This optical modulator has features such as a small optical propagation loss and a small optical coupling loss due to a small difference in spot size from the optical fiber.

  There are the following two types of typical optical modulators made of semiconductors. That is, an electroabsorption optical modulator (EA modulator) that utilizes the effect that the absorption edge shifts to the long wavelength side by applying an electric field, and an electro-optic effect (Pockels effect) in which the refractive index changes by applying the electric field. This is an electro-optic modulator (EO modulator) using an effect).

  In particular, electro-optic modulators (EO modulators) have been actively researched. For example, lumped-constant type modulators having a p-i-n structure (Non-Patent Document 1), and Schottky electrodes. Is a traveling wave electrode type modulator (Non-patent Document 2). Further, an n-i-n structure semiconductor Mach-Zehnder modulator (Patent Document 1) aimed at further lowering the voltage, reducing the size, and increasing the speed has been studied.

  In order for these optical modulators to actually function in a high-speed optical communication system, it is necessary to couple the chip of the optical modulator and the optical fiber without loss and to connect to an external optical circuit. In other words, it is important to optimally adjust the positions of the optical fiber and the chip and optically couple them so as to minimize the coupling loss.

For example, in a method of optically coupling an optical fiber and a LiNbO ₃ modulator, light output from the optical fiber is input to the waveguide of the modulator and coupled, and output light from the end surface opposite to the coupling end is coupled. There is a method of adjusting the position while observing with an infrared vidicon camera or the like. Even when optically coupling the opposite end face and the output optical fiber, after fixing the optical fiber of the end face that was adjusted first with epoxy resin, the light emitted from the modulator is reflected from the output optical fiber side to the light intensity. By adjusting the position while monitoring, it can be performed relatively easily.

On the other hand, when optically coupling an optical fiber and an optical modulator made of a semiconductor material, it is necessary to adopt a method different from that of the above-described LiNbO ₃ modulator. This is because there is a large difference in spot size. That is, while the spot size of light from the optical fiber is about 8 μm, the spot size of light input / output from the semiconductor optical modulator is about 2 μm, which is a quarter of the size of the optical fiber. Therefore, when optically coupling an optical fiber and an optical modulator, it is necessary to reduce the optical coupling loss by arranging a lens between the optical fiber and the optical modulator and adjusting the spot size. is there. In addition, due to the difference in spot size described above, it is necessary to adjust the position of the optical fiber with an accuracy of about 0.1 μm. For this reason, adjustment by observation with an infrared vidicon camera is difficult as in the case of the LiNbO ₃ modulator using the dielectric described above.

  FIG. 6 is a conceptual diagram in which adjustment is performed by coupling an optical modulator and an optical fiber via a lens. FIG. 6A shows a state in which the positions of the optical fiber 6 and the optical modulator 7 are adjusted using two lenses 8a and 8b. As shown in FIG. 6B, the number of lenses 8a may be one. Although not shown in FIG. 6, coupling to the optical fiber is performed on the output side of the optical modulator 7 similarly to the input side. When performing the above coupling, it is necessary to optimally adjust the position of the lens and the optical fiber so that the coupling loss between the optical modulator and the optical fiber is minimized.

  There are the following two specific methods for optically coupling an optical fiber and a modulator made of a semiconductor material to optimally adjust the coupling. One is a method of monitoring a light absorption current that flows when light incident from an optical fiber is absorbed by the optical waveguide layer. While monitoring this light absorption current, the coupling between the optical modulator and the optical fiber can be adjusted optimally. The other is a method in which the semiconductor optical waveguide layer is caused to emit light and the light coupled to the optical fiber is converted into an amount of current by a light intensity meter. In the case of this method, a monitoring mechanism using an electric signal of the light intensity meter is required. While monitoring this electrical signal, the coupling between the optical modulator and the optical fiber can be adjusted optimally.

  An electroabsorption optical modulator (EA modulator) has a light absorption current that flows when light is incident. Therefore, the light absorption current can be monitored, or the current can be flowed into the modulator to cause the light absorption layer to emit light. The electric signal of the light intensity meter can be monitored. Therefore, it is possible to relatively easily optically couple with the optical fiber and adjust the position of the optical fiber and the lens.

  On the other hand, in an electro-optic modulator (EO modulator), for example, an optical modulator having a pin structure, an electrode is disposed in the vicinity of the light input / output waveguide in the same manner as the electroabsorption optical modulator. Thus, the light absorption current can be measured from this electrode, or the light reception current of the light emitted by the injection current from this electrode can be measured. Based on these currents, the position of the optical fiber or lens can be adjusted.

International Publication No. 2004/081638 Pamphlet C. Rolland et al., “10 Gbit / s, 1.56 μm multiquantum well InP / InGaAsP Mach-Zehnder optical modulator,” Electron. Lett., Vol. 29, no. 5, pp. 471-472, 1993. R. Spickermann et al., “GaAs / AlGaAs electro-optic modulator with bandwidth> 40 GHz,” Electron. Lett., Vol. 31, no. 11, pp. 915-916, 1995.

  However, when the optical fiber and the modulator having the Schottky electrode structure (Non-Patent Document 2) or the optical modulator having the n-i-n structure disclosed in Patent Document 1 are to be coupled, the following problems are encountered. There was a point.

  FIG. 5 is a cross-sectional view of a waveguide of an optical modulator having an nin structure. An n-type InP clad layer 42, an optical waveguide core layer 43, a semi-insulating clad layer 44, and an n-type InP clad layer 45 are sequentially laminated on the substrate. An electrode 46 and an electrode 47 are connected to the n-type InP cladding layer 45 and the n-type InP cladding layer 42, respectively.

  Holes generated by light absorption in the optical waveguide core layer 43 of the present optical modulator accumulate in the semi-insulating clad layer 44 that is a barrier layer. The accumulated holes 48-2 are recombined with the electrons injected into the n-type InP cladding layer 45 in contact with the semi-insulating clad layer 44 with the semi-insulating clad layer 44 interposed therebetween. Since the light absorption current flows only by this recombination, the amount of light absorption current is very small. For this reason, the light absorption current is insufficient as a monitor current for optically coupling with the optical fiber. Further, since the two electrodes 46 and 47 are both on the n-type semiconductor layer, there is a problem that the optical waveguide core layer 43 cannot emit light by current injection. This problem is further explained in detail using a band diagram.

  FIG. 4 is a diagram showing a waveguide band diagram of a conventional optical modulator having an n-i-n structure. Bands corresponding to the respective layers shown in FIG. 5 are assigned the same numbers as in FIG. Fe atoms doped in the semi-insulating clad layer 44 act as acceptors at deep saddle positions. This raises the energy of the valence band and acts as a potential barrier for electrons. By this potential barrier, electron injection from the n-type InP clad layer 45 is suppressed, and a voltage is applied to the optical waveguide core layer 43 in a state where the generation of leakage current is small, thereby modulating the optical phase based on the electro-optic effect. be able to.

  On the other hand, electrons 49-1 generated by light absorption in the optical waveguide core layer 43 can be extracted from the n-type InP cladding layer 42 to the electrode 47. However, the hole 48-1 generated by light absorption can only be taken out from the electrode 46 as a light absorption current if it is electron-hole recombined at the interface between the semi-insulating clad layer 44 and the n-type InP clad layer 45. . For this reason, the efficiency with which light incident on the optical waveguide core layer 43 from the optical fiber is converted into a light absorption current is low, and a light absorption current signal sufficient to adjust the position of the optical fiber or lens can be obtained. could not.

  Further, since both the electrodes 46 and 47 are the same n-type semiconductor, it is impossible to adjust the position using a method of emitting light from the optical waveguide core layer 43 by current injection.

  The object of the present invention is to solve the problem that measurement is difficult due to insufficient light absorption current, and the problem that it is difficult to cause the optical waveguide core layer to emit light by current injection. An object is to provide an element structure of an optical modulator capable of adjusting the position of a lens.

  In order to achieve such an object, the present invention provides a first n-type clad layer, a semiconductor optical waveguide layer, a semi-insulating clad layer, In a semiconductor optical modulator including a semiconductor optical modulation waveguide having a layer structure in which two n-type cladding layers are sequentially stacked, an input waveguide having the layer structure, and an output waveguide having the layer structure, In the vicinity of the input end of the input waveguide for inputting incident light or the output end of the output waveguide for outputting outgoing light, the second n-type cladding layer in a certain length section in the light traveling direction. A p-type semiconductor region, all or a part of which has p-type conductivity, and an electrode formed on the p-type semiconductor region are provided.

  The invention according to claim 2 is a layer structure in which a first n-type cladding layer, a semiconductor optical waveguide layer, a semi-insulating cladding layer, and a second n-type cladding layer are sequentially laminated on a substrate. In a semiconductor optical modulator including a semiconductor optical modulation waveguide having a layer structure, an input waveguide having the layer structure, and an output waveguide having the layer structure, an input end or an output of the input waveguide for inputting incident light The semi-insulating type in contact with the second n-type cladding layer and the second n-type cladding layer in the vicinity of the output end of the output waveguide that outputs incident light and in a certain length section in the light traveling direction A p-type semiconductor region in which a part of the cladding layer is a p-type conductive region, and an electrode formed on the p-type semiconductor region are provided.

  The invention according to claim 3 is characterized in that the semiconductor optical modulation waveguide constitutes a Mach-Zehnder interferometer.

  Unlike the prior art, holes generated by light absorption are extracted from the electrode through the p-type semiconductor region, and a sufficient amount of light absorption current can be detected. In addition, it is possible to cause the optical waveguide core layer to emit light by injecting current into the optical waveguide core layer.

  Further, unlike a conventional optical waveguide having a pin structure, a semi-insulating clad layer exists between the optical waveguide core layer and the p-type semiconductor region, so that there is little light absorption loss due to the p-type semiconductor region, and regrowth or ion The manufacturing accuracy error due to the implantation can be sufficiently wide.

  In the optical modulator having the structure of the present invention, a p-type semiconductor region is introduced between the semi-insulating clad layer and the electrode in the vicinity of the input / output waveguide of the waveguide of the optical modulator. For this reason, it becomes possible to detect the light absorption current through the semi-insulating clad layer or to emit light from the optical waveguide core layer.

  The position of the lens and the optical fiber can be easily adjusted by monitoring the light absorption current that flows when the light incident from the optical fiber is absorbed by the optical waveguide core layer. In addition, by making the semiconductor optical waveguide core layer emit light and converting this light coupled to the optical fiber into a current amount with a light intensity meter, it becomes possible to use a monitoring mechanism based on an electric signal, and a semiconductor optical modulator can be easily used. And the position of the lens and the optical fiber can be adjusted.

  In the optical modulator according to the present invention, a part of the n-type cladding layer has p-type conductivity in the vicinity of the input end of the input / output waveguide for inputting light or in the vicinity of the output end of the output waveguide for outputting light. It has a p-type semiconductor region. For this reason, even if the semi-insulating clad layer is held, holes generated by light absorption can be extracted from the electrode through this p-type semiconductor region, and a sufficient amount of light absorption current can be detected. . The optical waveguide core layer can also emit light by current injection into the optical waveguide core layer. By converting this light coupled to the optical fiber into a current amount with a light intensity meter, a monitoring mechanism using an electric signal can be used.

  Therefore, it is possible to easily adjust the position of the semiconductor optical modulator, the lens and the optical fiber, which is the object of the present invention. Hereinafter, it demonstrates in detail based on an Example.

  FIG. 1 is a diagram showing a structure in the vicinity of an input / output waveguide of a waveguide used in a semiconductor optical modulator according to the present invention. This waveguide has a layer structure in which an n-type InP cladding layer 12, an optical waveguide core layer 13, a semi-insulating cladding layer 14, and an n-type InP cladding layer 15-1 are sequentially stacked on a semi-insulating InP substrate 11. This is a semiconductor light modulation waveguide. Semi-insulating with the n-type InP cladding layer 15-1 in contact with the semi-insulating clad layer 14 in a section having a certain length away from the input end face or the output end face for inputting / outputting light in the inner direction of the optical modulator. A part of the type cladding layer 14 is a p-type semiconductor region 15-2 having p-type conductivity, and an electrode 16 is provided on the p-type semiconductor region 15-2. The electrode 16 is in contact only with the p-type semiconductor region 15-2.

  In this p-type semiconductor region 15-2, for example, after growing layers from the n-type InP clad layer 12 to the n-type InP clad layer 15-1, a portion corresponding to the p-type semiconductor region 15-2 is etched. It can be formed by removing and regrowing p-type InP. It can also be formed by introducing a Be acceptor into a part of the n-type InP cladding layer 15-1 by an ion implantation method. At this time, the thickness of the semi-insulating clad layer 14 is relatively thick, about 0.5 to 1.0 μm. For this reason, the light absorption loss in the p-type semiconductor region 15-2 that occurs when the p-type semiconductor region 15-2 approaches the optical waveguide core layer 13 can be sufficiently suppressed. Therefore, a low-loss optical modulator waveguide can be formed with sufficiently moderate manufacturing error accuracy.

  In this embodiment, the p-type semiconductor region 15-2 penetrates into the n-type InP clad layer 15-1 and the semi-insulating clad layer 14, but the present invention is not limited to this. It may be up to a depth just in contact with the layer 14. Further, it may be a case where it does not reach the semi-insulating clad layer 14.

  In the present embodiment, the p-type semiconductor region 15-2 is located at a position that enters the inner direction of the optical modulator by a certain distance from the input end face or the output end face, but is not limited thereto. That is, the p-type semiconductor region 15-2 may be located on the input end face or the output end face and exposed at each end face. Therefore, it may be in the vicinity of the input / output waveguide end of the semiconductor optical modulator in which the effect of the present invention provided with the p-type semiconductor region described above is exhibited. One p-type semiconductor region sufficiently exhibits the effect of the present invention, but two or more p-type semiconductor regions may be provided. The specific length in the light traveling direction of the p-type semiconductor region can be set to, for example, 50 μm or more.

  When the optical fiber is optically coupled to the optical modulator waveguide, the electrode 16 and the electrodes 17a and 17b are in a state where light is incident in a direction perpendicular to the cross section of the mesa structure shown in FIG. Adjustment is performed by detecting the light absorption current flowing between them.

  FIG. 2 is a diagram showing a band diagram of an optical modulator waveguide according to the configuration of the present invention. The same number is assigned to the band corresponding to each layer shown in FIG. Fe atoms doped in the semi-insulating clad layer 14 act as acceptors at deep saddle positions and raise the energy of the valence band, but do not function as potential barriers against holes. For this reason, movement of holes is performed between the p-type semiconductor region 15-2 and the light absorption current can be detected with high efficiency.

  In addition, when current is injected into the optical waveguide core layer 13, electrons injected from the electrode 17 are injected into the optical waveguide core layer 13 through the n-type InP cladding layer 12, and holes injected from the electrode 16 are p-type. Since the light is injected into the optical waveguide core layer 13 through the semiconductor region 15-2 and the semi-insulating clad layer 14, the optical waveguide core layer 13 can emit light by electron-hole recombination.

  For this reason, the position of the lens and the optical fiber can be easily adjusted by monitoring the light absorption current that flows after the light incident from the optical fiber is absorbed by the optical waveguide core layer. Alternatively, the position of the lens and the optical fiber can be simply adjusted by causing the semiconductor optical waveguide layer to emit light, converting the light coupled to the optical fiber into a current amount with a light intensity meter, and using a monitoring mechanism based on an electrical signal. be able to.

  FIG. 3 is an overview of a Mach-Zehnder modulator which is an example of a semiconductor optical modulator according to the present invention. The Mach-Zehnder modulator is composed of two optical waveguides having an nin structure. Part of the two optical waveguides constitute phase modulation waveguides 4a and 4b, respectively. The phase modulation waveguides 4a and 4b are respectively connected to the optical multiplexer / demultiplexers 3a and 3b at two locations. The optical multiplexer / demultiplexer 3a is further connected to the waveguide 1 having a p-type semiconductor region, and the other optical multiplexer / demultiplexer 3b is further connected to the waveguide 2 having a p-type semiconductor region. Input light is input to the waveguide 1 having the p-type semiconductor region, and output light is output from the waveguide 2 having the p-type semiconductor region. For the waveguides 1 and 2 having the p-type semiconductor region, the nin structure unique to the present invention described in the first embodiment is used. Electrodes 6a and 6b are provided and correspond to the electrode 16 in the first embodiment.

  The Mach-Zehnder interferometer configured as described above enables light intensity modulation. That is, the input light incident on the waveguide 1 having the p-type semiconductor region is divided into two phase modulation waveguides 4a and 4b by the optical multiplexer / demultiplexer 3a. After phase modulation is performed by each of the phase modulation waveguides 4a and 4b, they are synthesized again by the optical multiplexer / demultiplexer 3b. The signal lights phase-modulated by the phase modulation waveguides 4a and 4b are interfered and synthesized by the optical multiplexer / demultiplexer 3b, and output light whose intensity is modulated is output from the waveguide 2 having the p-type semiconductor region. Perform the action. In order to perform optical modulation at high speed, a structure of a coplanar waveguide 5 is adopted as an electrode to which a high-frequency electric field as a modulation signal is applied.

  As described in the first embodiment, in the vicinity of the input / output waveguide of the Mach-Zehnder optical modulator of the present embodiment, an n-i-n structure having a p-type semiconductor region reaching the semi-insulating clad layer is employed. Yes. The feature of the Mach-Zehnder optical modulator is that the waveguides 1 and 2 having the p-type semiconductor region are provided. The electrode 16 described in the first embodiment corresponds to the electrodes 6a and 6b in FIG. 2 in the second embodiment. As described with reference to FIG. 2 in the first embodiment, the presence of the p-type semiconductor region allows the holes generated by the light absorption to be extracted, or conversely, an electric current is injected into the p-type semiconductor region to Can also emit light.

  For this reason, the position of the lens and the optical fiber can be easily adjusted by monitoring the light absorption current that flows after the light incident from the optical fiber is absorbed by the optical waveguide core layer. Alternatively, the position of the lens and the optical fiber can be easily adjusted by causing the semiconductor optical waveguide core layer to emit light, converting the light coupled to the optical fiber into a current amount with a light intensity meter, and using a monitoring mechanism based on an electrical signal. It can be carried out.

It is a figure which shows the waveguide structure of the optical modulator of Example 1 and Example 2 of this invention. It is a figure which shows the band diagram of the optical waveguide of Example 1 and Example 2 of this invention. It is a figure which shows the structure of the optical modulator of Example 2 of this invention. It is a figure which shows the waveguide band diagram in the semiconductor electro-optic modulator of the conventional nin structure. It is waveguide sectional drawing of the conventional optical modulator of an nin structure. It is a conceptual diagram which adjusts by combining an optical modulator and an optical fiber through a lens.

Explanation of symbols

1, 2 Waveguide having p-type semiconductor region 3a, 3b Optical multiplexer / demultiplexer 4a, 4b Phase modulation waveguide 5 High frequency electric field application line (coplanar waveguide)
6 Optical fiber 7 Optical modulator 8a, 8b Lens 11 Semi-insulating InP substrate 12, 15-1, 42, 45 n-type InP cladding layer 13, 43 Optical waveguide core layer 14, 44 Semi-insulating cladding layer (semi-insulating Fe layer) Doped InP cladding layer)
15-2 p-type semiconductor region (p-type region provided in n-type InP cladding layer)
6a, 6b, 16, 17, 17a, 17b, 46, 47 Electrode 26-1, 48-1 Hole generated by light absorption 26-2 A hole that passes through the semi-insulating clad layer and moves to the p-type semiconductor region 27-1, 49-1 Electrons generated by light absorption 27-2 Electrons present in the n-type InP cladding layer 12 48-2 Holes accumulated in the semi-insulating cladding layer 44 49-2 n-type InP cladding layer Electrons present in 42

Claims (3)

A semiconductor optical modulation waveguide having a layer structure in which a first n-type cladding layer, a semiconductor optical waveguide layer, a semi-insulating cladding layer, and a second n-type cladding layer are sequentially stacked on a substrate; In a semiconductor optical modulator including an input waveguide having the layer structure and an output waveguide having the layer structure,
In the vicinity of the input end of the input waveguide for inputting incident light or the output end of the output waveguide for outputting outgoing light, the second n-type cladding layer in a certain length section in the light traveling direction. A p-type semiconductor region in which all or part of the region has p-type conductivity;
An electrode formed on the p-type semiconductor region;
A semiconductor optical modulator comprising: A semiconductor optical modulation waveguide having a layer structure in which a first n-type cladding layer, a semiconductor optical waveguide layer, a semi-insulating cladding layer, and a second n-type cladding layer are sequentially stacked on a substrate; In a semiconductor optical modulator including an input waveguide having the layer structure and an output waveguide having the layer structure,
The second n-type cladding layer in the vicinity of the input end of the input waveguide for inputting incident light or the output end of the output waveguide for outputting outgoing light, and having a certain length section in the light traveling direction, and A p-type semiconductor region in which a part of the semi-insulating clad layer in contact with the second n-type clad layer is a region having p-type conductivity;
An electrode formed on the p-type semiconductor region;
A semiconductor optical modulator comprising: The semiconductor optical modulator according to claim 1, wherein the semiconductor optical modulation waveguide constitutes a Mach-Zehnder interferometer.

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