Semiconductor optical device

A semiconductor optical device includes a first optical waveguide including first, second, and third sections; a second optical waveguide including fourth, fifth, and sixth sections; an input optical coupler; and an output optical coupler. The first and second optical waveguides and the input and output optical couplers each include a first cladding layer composed of an n-type semiconductor and a core layer. The second and fifth sections each include an intermediate semiconductor layer on the core layer, and a second cladding layer composed of an n-type semiconductor. The first, third, fourth, and sixth sections and the input and output optical couplers each further include a third cladding layer on the core layer. At least one of the third cladding layers includes a first cladding section on the core layer and a second cladding section on the first cladding section. The second cladding section is composed of a semi-insulating semiconductor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor optical devices.

2. Description of the Related Art

Patent Document 1 (Japanese Unexamined Patent Application Publication No. 62-183406) describes a waveguide-type optical interferometer. This waveguide-type optical interferometer includes a substrate, two optical guides composed of glass or a plastic formed on the substrate, two optical couplers that connect the optical waveguides to each other at different positions, and phase shifters disposed in the optical waveguides between the optical couplers. Each phase shifter includes a heater disposed on the optical waveguide. The optical path length of the optical waveguide is changed by controlling the temperature of the optical waveguide by heating the heater of the phase shifter.

In recent years, optical modulators that modulate light in response to electric signals from outside have become one of the essential components in configuring optical fiber communication systems and optical information processing systems. In particular, a Mach-Zehnder interferometer type optical modulator that uses a waveguide-type optical interferometer described in Patent Document 1 enables high-speed modulation of 40 Gbps or higher. Since Mach-Zehnder interferometer type optical modulators have a low wavelength chirp under high-speed modulation, Mach-Zehnder interferometer type optical modulators can be used for future ultra high-speed, high-capacity optical communication systems. In particular, Mach-Zehnder interferometer type optical modulators composed of semiconductors are small in size, have low power consumption, and can be monolithically integrated with other semiconductor optical devices such as a laser diode through to achieve wider versatility.

One example of a known semiconductor Mach-Zehnder interferometer type optical modulator is an npin-structured modulator. An npin structured Mach-Zehnder interferometer type optical modulator includes a waveguide structure formed by sandwiching a core layer (i layer) composed of an undoped semiconductor with two cladding layers composed of an n-type semiconductor and then inserting a thin p-type semiconductor layer between the core layer and one of the cladding layers.FIGS. 13,14A, and14B are diagrams showing one example of an npin-structured Mach-Zehnder interferometer type optical modulator.

As shown inFIG. 13, a Mach-Zehnder interferometer type optical modulator100includes two optical waveguides110and120, an input optical coupler130, an output optical coupler140, and upper electrodes150and160. These components are formed on an n-type semiconductor substrate101(refer toFIGS. 14A and 14B). The optical waveguide110includes a waveguiding section211, a phase shifting section212, and a waveguiding section213, aligned in that order in the waveguiding direction. The optical waveguide120includes a waveguiding section121, a phase shifting section122, and a waveguiding section123also aligned in that order in the waveguiding direction. The phase shifting sections212and122are sections in which the upper electrodes150and160are respectively disposed and to which a signal voltage is applied. Each of the optical waveguides110and120has one end connected to the input optical coupler130and the other end connected to the output optical coupler140.

Referring now toFIGS. 14A and 14B, the Mach-Zehnder interferometer type optical modulator100includes an n-type lower cladding layer103, core layers104aand104b, p-type semiconductor layers105aand105b, and n-type upper cladding layers106aand106b. The core layer104ais interposed between the n-type lower cladding layer103and the n-type upper cladding layer106a. The p-type semiconductor layer105ais interposed between the core layer104aand the n-type upper cladding layer106a. The core layer104bis interposed between the n-type lower cladding layer103and the n-type upper cladding layer106b. The p-type semiconductor layer105bis interposed between the core layer104band the n-type upper cladding layer106b. The upper electrode150is disposed on the n-type upper cladding layer106a. The upper electrode160is disposed on the n-type upper cladding layer106b. A lower electrode170is formed on the back of the n-type semiconductor substrate101.

A part of the n-type lower cladding layer103, the core layer104a, the p-type semiconductor layer105a, and the n-type upper cladding layer106aform a mesa structure107a. The mesa structure107aconstitutes the optical waveguide110. Similarly, another part of the n-type lower cladding layer103, the core layer104b, the p-type semiconductor layer105b, and the n-type upper cladding layer106bform another mesa structure107b. The mesa structure107bconstitutes the optical waveguide120. Side surfaces of the mesa structures107aand107bare buried by, for example, a polyimide resin108.

According to the Mach-Zehnder interferometer type optical modulator100, the refractive indices of the core layers104aand104bcan be changed by applying a reverse bias voltage between the lower electrode170and the upper electrodes150and160. As a result, the phase of the light guided in the core layers104aand104bcan be shifted.

SUMMARY OF THE INVENTION

According to this npin-structured Mach-Zehnder interferometer type optical modulator100, the n-type upper cladding layers106aand106bare continuously formed over the optical waveguides110and120, the input optical coupler130, and the output optical coupler140. It has been found that current leakage occurs between the optical waveguides for the Mach-Zehnder interferometer type optical modulator having an n-type upper cladding layer formed over the optical waveguides110and120. It has also been found that this is due to a relatively small electric resistance of the n-type semiconductor layer constituting the n-type upper cladding layer. For example, when a reverse bias voltage is applied between the upper electrode150and the lower electrode170, a leakage current flows from the n-type upper cladding layer106aof the phase shifting section212of the optical waveguide110to the other optical waveguide120via the n-type upper cladding layer106aof the waveguiding section211and the n-type upper cladding layer of the input optical coupler130. When the leakage current flows, electrical cross-talk occurs between the optical waveguides. The cross-talk degrades the characteristics of the Mach-Zehnder interferometer type optical modulator100.

An aspect of the present invention provides a semiconductor optical device that includes a first optical waveguide, a second optical waveguide, an input optical coupler, and an output optical coupler. The first optical waveguide includes a first section, a second section, and a third section aligned in that order in a waveguiding direction. The second optical waveguide includes a fourth section, a fifth section, and a sixth section aligned in that order in the waveguiding direction. The input optical coupler is connected to one end of each of the first optical waveguide and the second optical waveguide. The output optical coupler is connected to the other end of each of the first optical waveguide and the second optical waveguide. The first, second, and third sections of the first optical waveguide, the fourth, fifth, and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler each include a first cladding layer composed of an n-type semiconductor and a core layer disposed on the first cladding layer. The core layer is composed of an undoped semiconductor. The second section of the first optical waveguide and the fifth section of the second optical waveguide each include an intermediate semiconductor layer disposed on the core layer and a second cladding layer disposed on the intermediate semiconductor layer. The second cladding layer is composed of an n-type semiconductor. The first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler each further include a third cladding layer on the core layer. At least one of the third cladding layers included in the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler includes a first cladding section disposed on the core layer and a second cladding section disposed on the first cladding section. The second cladding section is composed of a semi-insulating semiconductor.

In this semiconductor optical device, at least one of the third cladding layers included in the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler includes a section composed of a semi-insulating semiconductor. A third cladding layer including a section composed of a semi-insulating semiconductor exhibits a relatively high electric resistance. Accordingly, the leakage current that flows from one optical waveguide to the other when a reverse bias voltage is applied to the second section of the first optical waveguide and the fifth section of the second optical waveguide is reduced. As a result, electrical cross-talk can be suppressed and degradation of device characteristics can be suppressed.

The input optical coupler and the output optical coupler of the semiconductor optical device may be multimode interference (MMI) couplers or Y-branch optical couplers. When the input optical coupler and the output optical coupler are MMI couplers or Y-branch optical couplers, the third cladding layers of these optical couplers are physically connected to the third cladding layers of the first and third sections of the first optical waveguide and the fourth and sixth sections of the second optical waveguide. When at least one of the third cladding layers of the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input coupler, and the output coupler is configured to include a section composed of a semi-insulating semiconductor, the leakage current flowing from one optical waveguide to the other can be reduced even in the case where the MMI couplers or Y-branch optical couplers are used as the input optical coupler or the output optical coupler.

The intermediate semiconductor layers of the second section of the first optical waveguide and the fifth section of the second optical waveguide may be composed of a p-type semiconductor or a semi-insulating (SI) semiconductor. In this semiconductor optical device, the second section of the first optical waveguide and the fifth section of the second optical waveguide have an n-p-i-n structure or n-SI-i-n structure including an intermediate semiconductor layer composed of a p-type semiconductor or a semi-insulating semiconductor. According to this semiconductor optical device, at least one of the third cladding layers included in the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler includes a section composed of a semi-insulating semiconductor. Therefore, the leakage current flowing from one optical waveguide to the other can be reduced even if the second section of the first optical waveguide and the fifth section of the second optical waveguide have an n-p-i-n structure or n-SI-i-n structure.

The third cladding layers of the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler of this semiconductor optical device may each include the first cladding section and the second cladding section, and the first cladding section may be composed of a semi-insulating semiconductor. In such a case, since each of the third cladding layers of the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler includes a section composed of a semi-insulating semiconductor, the electric resistance of the third cladding layer can be further increased. As a result, the leakage current can be reduced.

According to this semiconductor optical device, the third cladding layers of the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler may each include the first cladding section and the second cladding section, and the first cladding section may be composed of an undoped semiconductor or an n-type semiconductor. Since the second cladding section of each of the third cladding layers of the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler is composed of a semi-insulating semiconductor, the leakage current can be reduced. Since the first cladding section in which the guided light is intensely distributed is composed of an undoped semiconductor or n-type semiconductor having a relatively small optical absorption, the optical loss can be reduced.

Some of the third cladding layers of the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler of the semiconductor optical device may each include a plurality of semiconductor sections aligned in the waveguiding direction. Some of the semiconductor sections may be composed of a semi-insulating semiconductor. The other semiconductor sections may be composed of an undoped semiconductor or an n-type semiconductor. The other third cladding layers of the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler may be composed of an undoped semiconductor or an n-type semiconductor. According to this configuration, some sections of some of the third cladding layers of the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler are composed of a semi-insulating semiconductor while the remaining sections are composed of an undoped semiconductor or n-type semiconductor. As a result, the leakage current can be reduced and the optical loss of guided light can be further reduced.

Some of the third cladding layers of the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler of the semiconductor optical device may each include a plurality of semiconductor sections aligned in the waveguiding direction. Some of the semiconductor sections may each include the first cladding section and the second cladding section and the first cladding section may be composed of an undoped semiconductor or an n-type semiconductor. The other semiconductor sections may be composed of an undoped semiconductor or an n-type semiconductor. The other third cladding layers of the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler may be composed of an undoped semiconductor or an n-type semiconductor. According to this configuration, only the second cladding sections of some sections of the some of the third cladding layers of the first and third sections of the first optical waveguide, the fourth and sixth sections of the second optical waveguide, the input optical coupler, and the output optical coupler are composed of a semi-insulating semiconductor while the first cladding sections in which the guided light is intensely distributed are composed of an undoped semiconductor or n-type semiconductor having a relatively small optical absorption. Thus, the leakage current can be reduced and the optical loss of the guided light can be reduced.

The first and second optical waveguides of the semiconductor optical device may each be configured to have a mesa structure including the core layer. The side surfaces of the mesa structure may be buried by a resin layer. The resin layer may be composed of a polyimide resin or a BCB resin. The semiconductor optical device may further include a first electrode disposed on the second section of the first optical waveguide and a second electrode disposed on the fifth section of the second optical waveguide.

According to the semiconductor optical device, the semi-insulating semiconductor may be a group III-V compound semiconductor doped with one transition metal element selected from Fe, Ti, Cr, and Co, and the group III-V compound semiconductor may be one of InP, GaInAsP, AlGaInAs, and AlInAs. As a result, sections containing a semi-insulating semiconductor can be easily formed by a crystal growth method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the semiconductor optical device of the present invention will now be described in detail with reference to the attached drawings. In the description of the drawings, the same components are given the same reference numerals and the description therefor is omitted to avoid redundancy.

First Embodiment

A Mach-Zehnder interferometer type optical modulator is described below as an embodiment of the semiconductor optical device according to the present invention. Referring toFIG. 1, a Mach-Zehnder interferometer type optical modulator1A of this embodiment includes two optical waveguides10and20, an input optical coupler30, an output optical coupler40, and two upper electrodes50and60. The optical waveguides10and20, the input optical coupler30, and the output optical coupler40are disposed on a main surface2aof an n-type semiconductor substrate2as shown inFIG. 2. An n-type InP substrate can be used as the n-type semiconductor substrate2, for example. The Mach-Zehnder interferometer type optical modulator1A further includes a lower electrode70disposed on a rear surface2bof the n-type semiconductor substrate2(refer toFIGS. 2 to 4).

The optical waveguide10is a first optical waveguide according to this embodiment and the optical waveguide20is a second optical waveguide according to this embodiment. The optical waveguides10and20extend between the input optical coupler30and the output optical coupler40and each have one end connected to the input optical coupler30and the other end connected to the output optical coupler40. The optical waveguides10and20are provided in parallel with each other in an extending direction. The optical waveguides10and20may have the same optical length. However, the optical waveguides10and20may have different optical lengths.

The optical waveguide10includes a waveguiding section11, a phase shifting section12, and a waveguiding section13. The waveguiding section11, the phase shifting section12, and the waveguiding section13are aligned in that order in the waveguiding direction (the direction in which the optical waveguide10extends). The waveguiding section11is a first section according to this embodiment. The phase shifting section12is a second section according to this embodiment. The waveguiding section13is a third section according to this embodiment.

The optical waveguide20includes a waveguiding section21, a phase shifting section22, and a waveguiding section23. The waveguiding section21, the phase shifting section22, and the waveguiding section23are aligned in that order in the waveguiding direction (the direction in which the optical waveguide20extends). The waveguiding section21is a fourth section according to this embodiment. The phase shifting section22is a fifth section according to this embodiment. The waveguiding section23is a sixth section according to this embodiment.

The input optical coupler30branches incoming light L1coming into the Mach-Zehnder interferometer type optical modulator1A from outside to the optical waveguide10and the optical waveguide20. The output optical coupler40combines the light that has propagated through the optical waveguides10and20. The input optical coupler30and the output optical coupler40are each constituted by, for example, a multimode interference (MMI) coupler.

The upper electrode50is disposed on the phase shifting section12, and the upper electrode60is disposed on the phase shifting section22.

The phase shifting sections12and22will now be described with reference toFIG. 2. The phase shifting section12includes a lower cladding layer4a, a core layer5a, an intermediate semiconductor layer6a, and an upper cladding layer7a. The lower cladding layer4a, the core layer5a, the intermediate semiconductor layer6a, and the upper cladding layer7aform a mesa structure8a. The phase shifting section22includes a lower cladding layer4b, a core layer5b, an intermediate semiconductor layer6b, and an upper cladding layer7b. The lower cladding layer4b, the core layer5b, the intermediate semiconductor layer6b, and the upper cladding layer7bform a mesa structure8b. Both side surfaces of the mesa structures8aand8bare buried in a resin layer9composed of benzocyclobutene (BCB) resin or a polyimide resin.

A lower cladding layer4is disposed on the main surface2aof the n-type semiconductor substrate2. The lower cladding layers4aand4bare integrated with the lower cladding layer4. The lower cladding layers4,4a, and4bare a first cladding layer of this embodiment. The lower cladding layers4,4a, and4bare composed of an n-type semiconductor. The lower cladding layers4,4a, and4bmay be composed of a semiconductor such as InP, GaInAsP, AlGaInAs, or AlInAs. The core layer5ais disposed on the lower cladding layer4aand the core layer5bis disposed on the lower cladding layer4b. The core layers5aand5bare composed of an undoped semiconductor. Here, the “undoped semiconductor” refers to a semiconductor to which an impurity element is not intentionally added. For example, an undoped semiconductor can be formed by not adding an impurity element during formation of the semiconductor layer by crystal growth. The impurity concentration in the undoped semiconductor may be, for example, not more than 1×1016cm−3. The core layers5aand5bmay be composed of a semiconductor such as GaInAsP, AlGaInAs, AlInAs, or GaInAs. The core layers5aand5bmay each be a single layer (bulk layer) or may have a quantum well structure constituted by alternately stacked well layers and barrier layers.

The intermediate semiconductor layer6ais disposed on the core layer5a, and the intermediate semiconductor layer6bis disposed on the core layer5b. The intermediate semiconductor layers6aand6bare composed of a p-type semiconductor. The intermediate semiconductor layers6aand6bmay be composed of a semiconductor such as InP, GaInAsP, AlGaInAs, AlInAs, or GaInAs. The upper cladding layer7ais disposed on the intermediate semiconductor layer6a, and the upper cladding layer7bis disposed on the intermediate semiconductor layer6b. The upper cladding layers7aand7bare second cladding layers of this embodiment. The upper cladding layer7ais in contact with the upper electrode50, and the upper cladding layer7bis in contact with the upper electrode60. The upper cladding layers7aand7bare composed of an n-type semiconductor.

The waveguiding sections11and21will now be described with reference toFIG. 3. The waveguiding section11includes a lower cladding layer14a, a core layer15a, an intermediate semiconductor layer16a, and an upper cladding layer17a. The lower cladding layer14a, the core layer15a, the intermediate semiconductor layer16a, and the upper cladding layer17aform a mesa structure18a. The waveguiding section21includes a lower cladding layer14b, a core layer15b, an intermediate semiconductor layer16b, and an upper cladding layer17b. The lower cladding layer14b, the core layer15b, the intermediate semiconductor layer16b, and the upper cladding layer17bform a mesa structure18b. Both side surfaces of the mesa structures18aand18bare buried by the resin layer9.

A lower cladding layer14is disposed on the main surface2aof the n-type semiconductor substrate2. The lower cladding layers14aand14bare integrated with the lower cladding layer14. The lower cladding layers14,14a, and14bare first cladding layers of this embodiment. The lower cladding layer14amay be integrated with the lower cladding layer4a. The lower cladding layer14bmay be integrated with the lower cladding layer4b. The lower cladding layers14,14a, and14bare composed of an n-type semiconductor. The core layer15ais disposed on the lower cladding layer14a. The core layer15bis disposed on the lower cladding layer14b. The core layer15amay be integrated with the core layer5a. The core layer15bmay be integrated with the core layer5b. The core layers15aand15bare composed of an undoped semiconductor. The core layers15aand15bmay each be a single layer (bulk layer) or may have a quantum well structure constituted by alternately stacked well layers and barrier layers.

The intermediate semiconductor layer16ais disposed on the core layer15a, and the intermediate semiconductor layer16bis disposed on the core layer15b. The intermediate semiconductor layer16amay be integrated with the intermediate semiconductor layer6a. The intermediate semiconductor layer16bmay be integrated with the intermediate semiconductor layer6b. The intermediate semiconductor layers16aand16bare composed of a p-type semiconductor.

The upper cladding layer17ais disposed on the intermediate semiconductor layer16aon the core layer15ain this embodiment. The upper cladding layer17ais a third cladding layer of this embodiment. The upper cladding layer17aincludes a first cladding section29aand a second cladding section31a. The first cladding section29ais disposed on the intermediate semiconductor layer16aon the core layer15ain this embodiment. The second cladding section31ais disposed on the first cladding section29a. The first cladding section29aand the second cladding section31aare composed of a semi-insulating semiconductor. In other words, the upper cladding layer17ais composed of a semi-insulating semiconductor.

The semi-insulating semiconductor preferably has a high resistivity, e.g., 105Ωcm or higher. Such a semi-insulating semiconductor can be obtained by doping a transition metal element in a group III-V compound semiconductor. The transition metal element can form, for example, an electron deep trapping level in a band gap of the group III-V compound semiconductor. The semi-insulating semiconductor can be easily prepared by adding a transition metal element during crystal growth of the group III-V compound semiconductor. Alternatively, the semi-insulating semiconductor can be obtained by growing group III-V compound semiconductor crystals and then diffusing a transition metal element into the crystals through ion implantation or thermal diffusion. Preferred examples of the transition metal element include Fe, Ti, Cr, and Co. Examples of the group III-V compound semiconductor include InP, GaInAsP, AlGaInAs, and AlInAs.

The upper cladding layer17bis disposed on the intermediate semiconductor layer16bon the core layer15bin this embodiment. The upper cladding layer17bis a third cladding layer of this embodiment. The upper cladding layer17bincludes a first cladding section29band a second cladding section31b. The first cladding section29bis disposed on the intermediate semiconductor layer16bon the core layer15bin this embodiment. The second cladding section31bis disposed on the first cladding section29b. The first cladding section29band the second cladding section31bare composed of a semi-insulating semiconductor. In other words, the upper cladding layer17bis composed of a semi-insulating semiconductor. The waveguiding sections13and23have the same structures as the waveguiding sections11and21, respectively.

The input optical coupler30will now be described with reference toFIG. 4. The input optical coupler30includes a lower cladding layer24a, a core layer25a, an intermediate semiconductor layer26a, and an upper cladding layer27a. The lower cladding layer24a, the core layer25a, the intermediate semiconductor layer26a, and the upper cladding layer27aform a mesa structure28a. Both side surfaces of the mesa structure28aare buried by the resin layer9.

A lower cladding layer24is disposed on the main surface2aof the n-type semiconductor substrate2. The lower cladding layer24ais integrated with the lower cladding layer24. The lower cladding layers24and24aare a first cladding layer of this embodiment. The lower cladding layer24amay be integrated with the lower cladding layers14aand14b. The lower cladding layers24and24aare composed of an n-type semiconductor. The core layer25ais disposed on the lower cladding layer24a. The core layer25amay be integrated with the core layers15aand15b. The core layer25ais composed of an undoped semiconductor. The core layer25amay be a single layer (bulk layer) or may have a quantum well structure constituted by alternately stacked well layers and barrier layers.

The intermediate semiconductor layer26ais disposed on the core layer25a. The intermediate semiconductor layer26amay be integrated with the intermediate semiconductor layers16aand16b. The intermediate semiconductor layer26ais composed of a p-type semiconductor. The upper cladding layer27ais formed on the intermediate semiconductor layer26aon the core layer25ain this embodiment. The upper cladding layer27ais a third cladding layer of this embodiment. The upper cladding layer27amay be integrated with the upper cladding layers17aand17b. The upper cladding layer27aincludes a first cladding section39aand a second cladding section41a. The first cladding section39ais disposed on the intermediate semiconductor layer26aon the core layer25ain this embodiment. The second cladding section41ais disposed on the first cladding section39a. The first cladding section39aand the second cladding section41aare composed of a semi-insulating semiconductor. In other words, the upper cladding layer27ais composed of a semi-insulating semiconductor. The output optical coupler40has the same structure as the input optical coupler30.

Next, operation of the Mach-Zehnder interferometer type optical modulator1A is described. Incoming light L1(refer toFIG. 1) from outside the Mach-Zehnder interferometer type optical modulator1A enters the core layer25aof the input optical coupler30. The incoming light L1is branched to the waveguiding section11of the optical waveguide10and the waveguiding section21of the optical waveguide20. Then a branched light beam reaches the output optical coupler40via the phase shifting section12and the waveguiding section13of the optical waveguide10and another light beam reaches the output optical coupler40via the phase shifting section22and the waveguiding section23of the optical waveguide20. These beams are optically coupled in the core layer25aof the output optical coupler40and form outgoing light L2emitted to outside the Mach-Zehnder interferometer type optical modulator1A.

A reverse bias voltage is applied between the lower electrode70and one or both of the upper electrodes50and60to generate an electrical field in one or both of the core layer5aof the phase shifting section12of the optical waveguide10and the core layer5bof the phase shifting section22of the optical waveguide20. As a result, the refractive index of one or both of the core layer5aand the core layer5bcan be changed due to the electro-optic effect or the quantum confined Stark effect (QCSE). Consequently, a phase difference is generated between the light propagating in the optical waveguide10and the light propagating in the optical waveguide20. Interference caused by the phase difference between light occurs in the output optical coupler40and intensity-modulated outgoing light L2is generated.

In the Mach-Zehnder interferometer type optical modulator1A of this embodiment, the upper cladding layer17aof the waveguiding section11of the optical waveguide10, the upper cladding layer of the waveguiding section13of the optical waveguide10, the upper cladding layer17bof the waveguiding section21of the optical waveguide20, the upper cladding layer of the waveguiding section23of the optical waveguide20, the upper cladding layer27aof the input optical coupler30, and the upper cladding layer of the output optical coupler40are each composed of a semi-insulating semiconductor. Thus, these upper cladding layers have an electrical resistance greater than that of the n-type semiconductor layers. Accordingly, when a reverse bias voltage is applied to the phase shifting sections12and22by using the upper electrodes50and60and the lower electrode70, the electric current is suppressed from flowing from one optical waveguide to another through the upper cladding layers (reduction of leakage current). Therefore, electrical cross-talk can be suppressed, and degradation of optical modulation characteristics (device characteristics) can be suppressed. In this embodiment, surfaces of the upper cladding layer17aof the waveguiding section11of the optical waveguide10, the upper cladding layer7aof the waveguiding section12, and the upper cladding layer of the waveguiding section13are flat. A groove or a similar structure for electrically isolating these sections is not needed at the borders between the sections of the upper cladding layers. Similarly, surfaces of the upper cladding layer17bof the waveguiding section21, the upper cladding layer7bof the waveguiding section22, and the upper cladding layer of the waveguiding section23of the optical waveguide20are flat. Thus, propagation loss and scattering that occur during propagation of light in the optical waveguides10and20can be suppressed.

Next, an example of a method for making the Mach-Zehnder interferometer type optical modulator1A is described with reference toFIGS. 5A to 5C,6A to6C, and7. As shown inFIG. 5A, an n-type semiconductor substrate2composed of n-type InP is prepared. A lower cladding layer34composed of an n-type semiconductor, a core layer35composed of an undoped semiconductor, an intermediate semiconductor layer36composed of a p-type semiconductor, and an upper cladding layer37composed of an n-type semiconductor are sequentially grown on a main surface2aof the semiconductor substrate2. Each layer is grown by a crystal growth method such as metal-organic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE). An n-type dopant such as Si or Se is added in growing the layers composed of n-type semiconductors, and a p-type dopant such as Zn is added in growing the layers composed of p-type semiconductors. A mask38is formed on the upper cladding layer37. The mask38is formed over the region where the upper cladding layers7aand7bof the phase shifting sections12and22are to be formed. A dielectric material such as SiO2or SiN is used as the material for the mask38.

Next, as shown inFIG. 5B, the upper cladding layer37is etched using the mask38. Portions of the upper cladding layer37not covered with the mask38are etched away and part of the surface of the intermediate semiconductor layer36is exposed. A new upper cladding layer37ais formed as a result of etching the upper cladding layer37. The method for etching is, for example, dry etching.

Next, as shown inFIG. 5C, an upper cladding layer47is formed on the surface of the intermediate semiconductor layer36by using the mask38as a selective growth mask. Growth is preferably conducted so that the level of the surface of the upper cladding layer47is substantially the same as the level of the surface of the upper cladding layer37composed of an n-type semiconductor. When the surface of the upper cladding layer47and the surface of the upper cladding layer37are at substantially the same level, a flat surface that has no level gap at the border between the upper cladding layers47and37can be obtained. The upper cladding layer47is composed of a semi-insulating semiconductor. An impurity such as Fe or Ti, is added during the growth of the upper cladding layer47. These impurities can form a deep level in the band gap of a group III-V compound semiconductor.

After the mask38is removed, as shown inFIG. 6A, a mask48is formed on the surfaces of the upper cladding layers37aand47. The mask48defines the outlines of the optical waveguides10and20, the input optical coupler30, and the output optical coupler40. In the mask48, a portion48athat defines the phase shifting sections12and22is located on the surface of the upper cladding layer37a, and portions48bthat define the waveguiding sections11,13,21, and23, the input optical coupler30, and the output optical coupler40are located on the surface of the upper cladding layer47. A dielectric material such as SiO2or SiN may be used as the material for the mask48.

The upper cladding layers37aand47, the intermediate cladding layer36, the core layer35, and the lower cladding layer34are etched by using the mask48. The etching depth is set so that part of the lower cladding layer34remains unetched. It is necessary to sufficiently confine the guided light in the mesa structure by increasing the difference in refractive index between the mesa structure formed by etching and the peripheral regions thereof. Accordingly, etching is continued at least until part of the lower cladding layer34is reached. In order to intensify optical confinement in the mesa structure, etching may be continued until part of the n-type semiconductor substrate2is reached. As a result of etching, as shown inFIG. 6B, the optical waveguides10and20, the input optical coupler30, and the output optical coupler40are formed.

As shown inFIG. 6C, a resin layer M composed of BCB, polyimide, or the like, is formed on the main surface2aof the n-type semiconductor substrate2to cover the entire top surfaces of the optical waveguides10and20, surfaces of the input optical coupler30other than an input end surface30a, and surfaces of the output optical coupler40other than an output end surface40a.

Then the resin layer M is partly removed by, for example, dry etching to expose top surfaces of the upper cladding layers of the optical waveguides10and20, the input optical coupler30, and the output optical coupler40. As a result, the resin layer9is formed. Then as shown inFIG. 7, the upper electrodes50and60are respectively formed on the phase shifting sections12and22. The rear surface of the n-type semiconductor substrate2is polished to reduce the thickness to a certain level (e.g., about 100 μm) and the lower electrode70is formed on the rear surface2bof the polished semiconductor substrate2. The upper electrodes50and60and the lower electrode70can be formed by, for example, an evaporation method or a sputtering method. As a result, the Mach-Zehnder interferometer type optical modulator1A is made.

Second Embodiment

Another Mach-Zehnder interferometer type optical modulator is described below as a second embodiment of the semiconductor optical device according to the present invention. Referring toFIG. 8, a Mach-Zehnder interferometer type optical modulator1B of this embodiment includes two optical waveguides10B and20B, an input optical coupler30B, an output optical coupler40B, and two upper electrodes50and60. The optical waveguides10B and20B, the input optical coupler30B, and the output optical coupler40B are formed on a main surface2aof an n-type semiconductor substrate2as shown inFIGS. 9A and 9B. The Mach-Zehnder interferometer type optical modulator1B further includes a lower electrode70formed on a rear surface2bof the n-type semiconductor substrate2(refer toFIGS. 9A and 9B).

The optical waveguide10B is a first optical waveguide according to this embodiment and the optical waveguide20B is a second optical waveguide according to this embodiment. The optical waveguides10B and20B extend between the input optical coupler30B and the output optical coupler40B and each have one end connected to the input optical coupler30B and the other end connected to the output optical coupler40B. The optical waveguides10B and20B are provided in parallel with each other in an extending direction.

The optical waveguide10B includes a waveguiding section11B, a phase shifting section12B, and a waveguiding section13B. The waveguiding section11B, the phase shifting section12B, and the waveguiding section13B are aligned in that order in the waveguiding direction (the direction in which the optical waveguide10B extends). The waveguiding section11B is a first section according to this embodiment. The phase shifting section12B is a second section according to this embodiment. The waveguiding section13B is a third section according to this embodiment.

The optical waveguide20B includes a waveguiding section21B, a phase shifting section22B, and a waveguiding section23B. The waveguiding section21B, the phase shifting section22B, and the waveguiding section23B are aligned in that order in the waveguiding direction (the direction in which the optical waveguide20B extends). The waveguiding section21B is a fourth section according to this embodiment. The phase shifting section22B is a fifth section according to this embodiment. The waveguiding section23B is a sixth section according to this embodiment.

The input optical coupler30B branches incoming light L1coming into the Mach-Zehnder interferometer type optical modulator1B from outside to the optical waveguide10B and the optical waveguide20B. The output optical coupler40B combines the light that has propagated through the optical waveguides10B and20B. The input optical coupler30B and the output optical coupler40B are each constituted by, for example, a MMI coupler.

The upper electrode50is formed on the phase shifting section12B, and the upper electrode60is formed on the phase shifting section22B.

The waveguiding sections11band21B and the input optical coupler30B will now be described with reference toFIGS. 9A and 9B. Note that the phase shifting section12B has the same structure as the phase shifting section12of the optical waveguide10shown inFIG. 2, and the phase shifting section22B has the same structure as the phase shifting section22of the optical waveguide20shown inFIG. 2.

Referring toFIG. 9A, the waveguiding section11B differs from the waveguiding section11of the optical waveguide10shown inFIG. 3in that an upper cladding layer57ais provided instead of the upper cladding layer17a. The waveguiding section21B differs from the waveguiding section21of the optical waveguide20shown inFIG. 3in that an upper cladding layer57bis provided instead of the upper cladding layer17b. The upper cladding layers57aand57bare third cladding layers of this embodiment.

The upper cladding layer57aincludes a first cladding section49aand a second cladding section51a. The first cladding section49ais disposed on the intermediate semiconductor layer16aon the core layer15ain this embodiment. The second cladding section51ais disposed on the first cladding section49a. The first cladding section49ais composed of an undoped semiconductor and the second cladding section51ais composed of a semi-insulating semiconductor. In other words, an undoped semiconductor constitutes a portion of the upper cladding layer57a, the portion being located on the core layer15a-side and including an interface S1with the intermediate semiconductor layer16a. And a semi-insulating semiconductor constitutes another portion of the upper cladding layer57a, the another portion including a surface S2opposite the interface S1of the upper cladding layer57a.

The upper cladding layer57bincludes a first cladding section49band a second cladding section51b. The first cladding section49bis disposed on the intermediate semiconductor layer16bon the core layer15bin this embodiment. The second cladding section51bis disposed on the first cladding section49b. The first cladding section49bis composed of an undoped semiconductor and the second cladding section51bis composed of a semi-insulating semiconductor. In other words, an undoped semiconductor constitutes a portion of the upper cladding layer57b, the portion being located on the core layer15b-side and including an interface S3with the intermediate semiconductor layer16b. And a semi-insulating semiconductor constitutes another portion of the upper cladding layer57b, the another portion including a surface S4opposite the interface S3of the upper cladding layer57b. The waveguiding sections13B and23B have the same structures as the waveguiding sections11B and21B, respectively.

Referring toFIG. 9B, the input optical coupler30B differs from the input optical coupler30shown inFIG. 4in that an upper cladding layer67ais provided instead of the upper cladding layer27a. The upper cladding layer67ais a third cladding layer of this embodiment. The upper cladding layer67aincludes a first cladding section59aand a second cladding section61a. The first cladding section59ais disposed on the intermediate semiconductor layer26aon the core layer25ain this embodiment. The second cladding section61ais disposed on the first cladding section59a. The first cladding section59ais composed of an undoped semiconductor and the second cladding section61ais composed of a semi-insulating semiconductor. In other words, an undoped semiconductor constitutes a portion of the upper cladding layer67a, the portion being located on the core layer25a-side and including an interface S5with the intermediate semiconductor layer26a. And a semi-insulating semiconductor constitutes another portion of the upper cladding layer67a, the another portion including a surface S6opposite the interface S5of the upper cladding layer67a. The first cladding sections59a,49a, and49bmay be integrated with each other. The second cladding sections61a,51a, and51bmay be integrated with each other. The output optical coupler40B has the same structure as the input optical coupler30B.

Operation of the Mach-Zehnder interferometer type optical modulator1B is the same as the Mach-Zehnder interferometer type optical modulator1A.

In the Mach-Zehnder interferometer type optical modulator1B of this embodiment, the second cladding section51aof the upper cladding layer57aof the waveguiding section11B, the second cladding section of the upper cladding layer of the waveguiding section13b, the second cladding section51bof the upper cladding layer57bof the waveguiding section21B, the second cladding section of the upper cladding layer of the waveguiding section23B, the second cladding section61aof the upper cladding layer67aof the input optical coupler30B, and the second cladding section of the upper cladding layer of the output optical coupler40B are each composed of a semi-insulating semiconductor. Thus, the leakage current between the optical waveguides10B and20B can be reduced. In the Mach-Zehnder interferometer type optical modulator1B of this embodiment, the first cladding section49aof the upper cladding layer57aof the waveguiding section11B, the first cladding section of the upper cladding layer of the waveguiding section13b, the first cladding section49bof the upper cladding layer57bof the waveguiding section21B, the first cladding section of the upper cladding layer of the waveguiding section23B, the first cladding section59aof the upper cladding layer67aof the input optical coupler30B, and the first cladding section of the upper cladding layer of the output optical coupler40B are each composed of an undoped semiconductor. Optical absorption of the undoped semiconductor is smaller than that of the semi-insulating semiconductor doped with a transition metal such as Fe or Ti. Thus, according to the Mach-Zehnder interferometer type optical modulator1B of this embodiment, absorption loss of the light propagating in the optical waveguides10B and20B can be reduced compared to the structure in that first and second cladding sections of the upper cladding layers of the waveguiding sections11B,13B,21B, and23B, the input optical coupler30B, and the output optical coupler40B are all composed of a semi-insulating semiconductor.

In this embodiment, the sections composed of an undoped semiconductor may be composed of an n-type semiconductor. As with the undoped semiconductor, an n-type semiconductor also has a small optical absorption compared to the semi-insulating semiconductor. Thus, absorption loss of the light propagating in the optical waveguides10B and20B can be reduced compared to the structure in which first and second cladding sections of the upper cladding layers of the waveguiding sections11B,13B,21B, and23B, the input optical coupler30B, and the output optical coupler40B are all composed of a semi-insulating semiconductor.

Third Embodiment

Another Mach-Zehnder interferometer type optical modulator is described below as a third embodiment of the semiconductor optical device according to the present invention. Referring toFIG. 10, a Mach-Zehnder interferometer type optical modulator1C of this embodiment includes two optical waveguides10C and20C, an input optical coupler30C, an output optical coupler40C, and two upper electrodes50and60. The optical waveguides10C and20C, the input optical coupler30C, and the output optical coupler40C are formed on a main surface2aof an n-type semiconductor substrate2as shown inFIGS. 11A and 11B. The Mach-Zehnder interferometer type optical modulator1C further includes a lower electrode70formed on a rear surface2bof the n-type semiconductor substrate2(refer toFIGS. 11A and 11B).

The optical waveguide10C is a first optical waveguide according to this embodiment and the optical waveguide20C is a second optical waveguide according to this embodiment. The optical waveguides10C and20C extend between the input optical coupler30C and the output optical coupler40C and each have one end connected to the input optical coupler30C and the other end connected to the output optical coupler40C. The optical waveguides10C and20C are provided in parallel with each other in an extending direction.

The optical waveguide10C includes a waveguiding section11C, a phase shifting section12C, and a waveguiding section13C. The waveguiding section11C, the phase shifting section12C, and the waveguiding section13C are aligned in that order in the waveguiding direction (the direction in which the optical waveguide10C extends). The waveguiding section11C is a first section according to this embodiment. The phase shifting section12C is a second section according to this embodiment. The waveguiding section13C is a third section according to this embodiment.

The optical waveguide20C includes a waveguiding section21C, a phase shifting section22C, and a waveguiding section23C. The waveguiding section21C, the phase shifting section22C, and the waveguiding section23C are aligned in that order in the waveguiding direction (the direction in which the optical waveguide20C extends). The waveguiding section21C is a fourth section according to this embodiment. The phase shifting section22C is a fifth section according to this embodiment. The waveguiding section23C is a sixth section according to this embodiment.

The input optical coupler30C branches incoming light L1coming into the Mach-Zehnder interferometer type optical modulator1C from outside to the optical waveguide10C and the optical waveguide20C. The output optical coupler40C combines the light that has propagated through the optical waveguides10C and20C. The input optical coupler30C and the output optical coupler40C are each constituted by, for example, a MMI coupler.

The upper electrode50is formed on the phase shifting section12C, and the upper electrode60is formed on the phase shifting section22C.

The waveguiding sections11C and21C will now be described. Note that the phase shifting section12C has the same structure as the phase shifting section12of the optical waveguide10shown inFIG. 2, and the phase shifting section22C has the same structure as the phase shifting section22of the optical waveguide20shown inFIG. 2.

The waveguiding section11C includes a first portion111, a second portion112, and a third portion113. The first portion111, the second portion112, and the third portion113are aligned in that order in the waveguiding direction. Since the waveguiding section11C includes a plurality of (three) portions sequentially aligned in the waveguiding direction, the upper cladding layer19(refer toFIGS. 11A and 11B) of the waveguiding section11C also includes a plurality of semiconductor sections sequentially aligned in the waveguiding direction. In this embodiment, the upper cladding layer19includes a first semiconductor section corresponding to the first portion111, a second semiconductor section92corresponding to the second portion112, and a third semiconductor section93corresponding to the third portion113. Note that the upper cladding layer19is a third cladding layer of this embodiment.

Referring toFIG. 11A, the second portion112differs from the waveguiding section11of the optical waveguide10shown inFIG. 3in that the second semiconductor section92is provided instead of the upper cladding layer17a. The second semiconductor section92includes a first cladding section69aand a second cladding section71a. The first cladding section69ais disposed on the intermediate semiconductor layer16aon the core layer15ain this embodiment. The second cladding section71ais disposed on the first cladding section69a. The first cladding section69aand the second cladding section71aare composed of a semi-insulating semiconductor. In other words, the second semiconductor section92is composed of a semi-insulating semiconductor.

Referring toFIG. 11B, the third portion113differs from the waveguiding section11of the optical waveguide10shown inFIG. 3in that the third semiconductor section93is provided instead of the upper cladding layer17a. The third semiconductor section93includes a first cladding section79aand a second cladding section81a. The first cladding section79ais disposed on the intermediate semiconductor layer16aon the core layer15ain this embodiment. The second cladding section81ais disposed on the first cladding section79a. The first cladding section79aand the second cladding section81aare composed of an undoped semiconductor. In other words, the third semiconductor section93is composed of an undoped semiconductor. The first portion111has the same structure as the third portion113.

The waveguiding section21C differs from the waveguiding section21of the optical waveguide20shown inFIG. 3in that an upper cladding layer77bis provided instead of the upper cladding layer17b. The upper cladding layer77bis a third cladding layer of this embodiment. The upper cladding layer77bincludes a first cladding section69band a second cladding section71b. The first cladding section69bis disposed on the intermediate semiconductor layer16bon the core layer15bin this embodiment. The second cladding section71bis disposed on the first cladding section69b. The first cladding section69band the second cladding section71bare composed of an undoped semiconductor. In other words, the upper cladding layer77bis composed of an undoped semiconductor. The waveguiding section23C has the same structure as the waveguiding section21C.

The waveguiding section13C differs from the waveguiding section11shown inFIG. 3in that an upper cladding layer composed of an undoped semiconductor is provided instead of the upper cladding layer17a. The upper cladding layer is a third cladding layer of this embodiment.

The input optical coupler30C differs from the input optical coupler30shown inFIG. 4in that an upper cladding layer composed of an undoped semiconductor is provided instead of the upper cladding layer27acomposed of a semi-insulating semiconductor. The upper cladding layer is a third cladding layer of this embodiment. The output optical coupler40C has the same structure as the input optical coupler30C.

Operation of the Mach-Zehnder interferometer type optical modulator1C is the same as the Mach-Zehnder interferometer type optical modulator1A.

Among the upper cladding layer19of the waveguiding section11C, the upper cladding layer of the waveguiding section13C, the upper cladding layer77bof the waveguiding section21C, the upper cladding layer of the waveguiding section23C, the upper cladding layer of the input optical coupler30C, and the upper cladding layer of the output optical coupler40C of the Mach-Zehnder interferometer type optical modulator1C, the upper cladding layer19of the waveguiding section11C is constituted by a first semiconductor section, a second semiconductor section92, and a third semiconductor section93. Among these semiconductor sections, only the second semiconductor section92is composed of a semi-insulating semiconductor. Other semiconductor sections and the upper cladding layers other than the second semiconductor section92of the upper cladding layer19are composed of an undoped semiconductor having a lower optical absorption than the semi-insulating semiconductor. Thus, the leakage current can be reduced, and optical absorption loss of the guided light is reduced compared to the structure in which the upper cladding layers of the waveguiding regions11C,13C,21C, and23C, the input optical coupler30C, and the output optical coupler40C are all composed of a semi-insulating semiconductor.

The second semiconductor section92can have the structure shown inFIG. 12. According to this structure, the second semiconductor section92includes a first cladding section82aand a second cladding section83a. The first cladding section82ais disposed on the intermediate semiconductor layer16aon the core layer15ain this embodiment. The second cladding section83ais disposed on the first cladding section82a. The first cladding section82ais composed of an undoped semiconductor and the second cladding section83ais composed of a semi-insulating semiconductor. In other words, an undoped semiconductor constitutes a portion of the second semiconductor section92shown inFIG. 12, the portion being located on the core layer15a-side and including an interface S7with the intermediate semiconductor layer16a. And a semi-insulating semiconductor constitutes another portion of the second semiconductor section92, the another portion including a surface S8opposite the interface S7. In this case, the propagation loss of the guided light can be reduced more reliably.

According to this embodiment, the upper cladding layer19of the waveguiding section11C is constituted by a plurality (three) semiconductor sections and, of these sections, the second semiconductor section92includes a section composed of a semi-insulating semiconductor. However, the arrangement is not limited to this. Among the upper cladding layer19of the waveguiding section11C, the upper cladding layer of the waveguiding section13C, the upper cladding layer77bof the waveguiding section21C, the upper cladding layer of the waveguiding section23C, the upper cladding layer of the input optical coupler30C, and the upper cladding layer of the output optical coupler40C, some of the upper cladding layers may be configured to include a plurality of semiconductor sections sequentially aligned in the waveguiding direction, and some of the plurality of semiconductor sections may be composed of a semi-insulating semiconductor.

In this embodiment, the layers and sections composed of an undoped semiconductor may be composed of an n-type semiconductor. As with the undoped semiconductor, an n-type semiconductor also has a small optical absorption compared to the semi-insulating semiconductor. Thus, optical absorption loss of the guided light is reduced compared to the structure in which the upper cladding layers of the waveguiding regions11C,13C,21C, and23C, the input optical coupler30C, and the output optical coupler40C are all composed of a semi-insulating semiconductor.

The intermediate semiconductor layers of the Mach-Zehnder interferometer type optical modulators1A,1B, and1C of the first to third embodiments described above may be composed of a semiconductor other than a p-type semiconductor. For example, the intermediate semiconductor layers of the phase shifting section may be composed of a semi-insulating semiconductor. The intermediate semiconductor layers of the waveguiding sections, input optical couplers, and output optical couplers may be composed of a semi-insulating semiconductor, an n-type semiconductor, or an undoped semiconductor. In such a case, optical absorption loss and the like caused by a p-type semiconductor can be reduced.

Although the core layers of the waveguiding sections, the input optical couplers, and the output optical couplers of the Mach-Zehnder interferometer type optical modulators1A,1B, and1C of the first to third embodiments are composed of an undoped semiconductor, the material is not limited to this. For example, the core layer of at least part of these sections may be composed of a semi-insulating semiconductor. Aforementioned semiconductor materials doped with transition metal such as Fe or Ti may be used as the semi-insulating semiconductor. In such a case, since the core layers exhibit high resistivity in addition to the upper cladding layers, electrical cross-talk between the optical waveguides of the Mach-Zehnder interferometer type optical modulators can be effectively reduced.

According to the Mach-Zehnder interferometer type optical modulators1A,1B, and1C of the first to third embodiments, at least some of the upper cladding layers of the waveguiding sections and the upper cladding layers of the input optical coupler and the output optical coupler may be configured to include a first cladding section on the core layer, and a second cladding section composed of a semi-insulating semiconductor and disposed on the first cladding section.

Although MMI-couplers are used as the input optical couplers and the output optical couplers of the Mach-Zehnder interferometer type optical modulators1A,1B, and1C of the first to third embodiments, Y-branch optical waveguides and the like may be used instead.

In the aforementioned embodiments, group III-V compound semiconductors doped with transition metal elements are described as an example of the semi-insulating semiconductor. However, the semi-insulating semiconductor is not limited to these. The semi-insulating semiconductor may be a semiconductor having resistivity increased by proton injection.

In the Mach-Zehnder interferometer type optical modulators1A,1B, and1C of the first to third embodiments, the width of the mesa structures8aand8bof the phase shifting sections is preferably substantially the same as that of the mesa structures18aand18bof the waveguiding sections. The core layers5aand5bof the phase shifting sections preferably have substantially the same thickness and refractive index as those of the core layers15aand15bof the waveguiding sections. The n-type semiconductor, the p-type semiconductor, the undoped semiconductor, and the semi-insulating semiconductor preferably have substantially the same refractive index. In this case, the effective refractive index of the phase shifting sections can be made substantially the same as that of the waveguiding sections. Thus, scattering of the guided light at the interfaces between the phase shifting sections and the waveguiding sections can be suppressed. As a result, deterioration of the device characteristics can be avoided.

Although the optical waveguides of the Mach-Zehnder interferometer type optical modulators1A,1B, and1C of the first to third embodiments each have a high mesa waveguide structure, the structure is not limited to this. For example, a ridge structure or a buried heterostructure (BH structure) may be employed.

Although Mach-Zehnder interferometer type optical modulators are described here as an example of the semiconductor optical device of the present invention, the structure of the present invention can be applied to any device that includes a plurality of optical waveguides, such as semiconductor laser array devices and integrated optical switches. In such a case also, electrical cross-talk between the waveguides and degradation of element characteristics can be suppressed as in these embodiments.

Although the principle of the present invention has been described heretofore through preferred embodiments, persons skilled in the art should recognize that alterations and modifications of details may be made without departing from the principle. All modifications and alterations which come within the scope of the claims and the spirit of the present invention are covered and protected.