Phase adjuster and semiconductor optical integrated element

Abstract

A phase adjuster (500) according to the present disclosure comprises: a first optical waveguide part (10a) having a striped ridge structure including a lower cladding layer (101), an optical waveguide core layer (102), and an upper cladding layer (103) that are laminated on the surface of a semiconductor substrate (100); a phase adjustment part (20) provided in contact with the first optical waveguide part (10a) in a stripe direction and having a ridge structure; a second optical waveguide part (10b) provided in contact with the phase adjustment part (20) in the stripe direction and having a ridge structure; an upper phase adjustment electrode (211) provided at the top of the ridge structure of the phase adjustment part (20) and extending along the stripe direction; a lower phase adjustment electrode (210) electrically connected to the lower cladding layer (101); a first extraction electrode (220) electrically connected to the upper phase adjustment electrode (211); and a second extraction electrode (221) electrically connected to the upper phase adjustment electrode (211).

Description

Phase adjuster and semiconductor optical integrated device

[0001] The present disclosure relates to a phase adjuster and a semiconductor optical integrated device.

[0002] As a semiconductor optical integrated device for optical communications, a Mach-Zehnder type optical interferometer using an optical demultiplexer, a phase modulator, a phase adjuster, and an optical multiplexer has been disclosed (for example, Patent Documents 1 to 3).

[0003] Patent documents 1 to 3 disclose phase adjusters that use a method of applying a reverse bias voltage to a diode provided in the optical waveguide to change the phase of signal light propagating through the optical waveguide, a method of applying a forward bias voltage to a diode provided in the optical waveguide to pass a current, or a method of passing a current through a heater electrode provided around the optical waveguide to change the temperature of the optical waveguide, etc.

[0004] JP 2012-163876 A JP 2016-133664 A JP 2019-191252 A

[0005] When adjusting the phase in a phase shifter by applying a reverse bias, the phase shift and optical loss increase sharply with the applied voltage. Therefore, in the bias region where a sufficient phase shift is obtained, the optical loss of the optical signal propagating through the optical waveguide increases sharply, making it difficult to control.

[0006] The method of adjusting the phase using a heater has the advantage of being easy to control, since the amount of phase change is roughly proportional to the applied power. However, it has issues such as the difficulty of adjusting the optical loss and the need to apply a relatively large amount of power to the heater electrode.

[0007] The technique of applying a forward bias voltage to pass a current exhibits characteristics intermediate between the two techniques described above and is relatively easy to control, but it has the problem that it is difficult to obtain both the desired phase shift amount and the desired optical loss amount in a compact phase shifter.

[0008] Furthermore, when multiple phase adjusters using the techniques described in Patent Documents 1 to 3 are combined in series, the controllability of the amount of phase change and the amount of optical loss can be improved, but there is a problem that the element size becomes large.

[0009] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a compact phase adjuster that is capable of adjusting the amount of loss and the amount of phase change of an optical signal propagating through an optical waveguide, and a semiconductor optical integrated device in which such a phase adjuster is integrated on a semiconductor substrate.

[0010] A phase adjuster according to the present disclosure includes: a first optical waveguide portion having a stripe-shaped ridge structure including a lower cladding layer, an optical waveguide core layer, and an upper cladding layer stacked on a surface of a semiconductor substrate; a phase adjuster portion having the ridge structure and provided in contact with the first optical waveguide portion in the stripe direction; a second optical waveguide portion having the ridge structure and provided in contact with the phase adjuster portion in the stripe direction; an upper phase adjuster electrode provided on a top of the ridge structure of the phase adjuster portion and extending along the stripe direction; a lower phase adjuster electrode electrically connected to the lower cladding layer; a first extraction electrode electrically connected to the upper phase adjuster electrode; and a second extraction electrode electrically connected to the upper phase adjuster electrode.

[0011] The semiconductor optical integrated device according to the present disclosure includes an optical demultiplexer that branches incident signal light into a plurality of optical waveguides; a plurality of optical modulators, one end of which is connected to each of the plurality of optical waveguides; the above-mentioned phase adjusters, one end of which is connected to the other ends of the plurality of optical modulators via the optical waveguides; and an optical multiplexer / demultiplexer, connected to the other ends of the plurality of phase adjusters via the optical waveguides.

[0012] The phase adjuster according to the present disclosure has the advantage that it is possible to adjust the amount of loss and the amount of phase change of an optical signal propagating through an optical waveguide, and that a compact phase adjuster can be obtained.

[0013] The semiconductor optical integrated device according to the present disclosure has the advantage of being able to adjust the amount of loss and the amount of phase change of an optical signal propagating through an optical waveguide, and of being able to obtain a semiconductor optical integrated device in which a miniaturized phase adjuster is integrated on a semiconductor substrate.

[0014] 1 is a cross-sectional view of a phase adjuster according to a first embodiment in a direction parallel to the propagation direction of signal light; FIG. 2 is a top view of the phase adjuster according to the first embodiment; FIG. 3 is a cross-sectional view of an optical waveguide portion of the phase adjuster according to the first embodiment in a direction perpendicular to the propagation direction of signal light; FIG. 4 is a cross-sectional view of a phase adjuster portion of the phase adjuster according to the first embodiment in a direction perpendicular to the propagation direction of signal light; FIG. 5 is a cross-sectional view of a phase adjuster according to a second embodiment in a direction parallel to the propagation direction of signal light; FIG. 6 is a cross-sectional view of a phase adjuster according to a third embodiment in a direction parallel to the propagation direction of signal light; FIG. 7 is a top view of a phase adjuster according to the third embodiment; FIG. 8 is a top view of a semiconductor optical integrated device according to a fourth embodiment; FIG. 9 is a cross-sectional view of an optical modulator of the semiconductor optical integrated device according to the fourth embodiment in a direction parallel to the propagation direction of signal light; FIG. 10 is a cross-sectional view of an optical modulator of the semiconductor optical integrated device according to the fourth embodiment in a direction perpendicular to the propagation direction of signal light;

[0015] 1 is a cross-sectional view of a phase adjuster 500, which is an example of a phase adjuster according to the first embodiment, taken along a direction parallel to the propagation direction of signal light. FIG. 2 is a top view of the phase adjuster 500, which is an example of a phase adjuster according to the first embodiment.

[0016] <Configuration of Phase Adjuster According to First Embodiment> The phase adjuster 500 according to the first embodiment includes, on a semiconductor substrate 100, a first optical waveguide portion 10 a, a phase adjustment portion 20 provided in contact with the first optical waveguide portion 10 a in the stripe direction, and a second optical waveguide portion 10 b provided in contact with the phase adjustment portion 20 in the stripe direction.

[0017] 3 is a cross-sectional view of the first optical waveguide portion 10 a and the second optical waveguide portion 10 b of the phase adjuster 500 according to the first embodiment, taken along a direction perpendicular to the propagation direction of the signal light. The first optical waveguide portion 10 a and the second optical waveguide portion 10 b have a striped ridge structure including a lower cladding layer 101, an optical waveguide core layer 102, and an upper cladding layer 103, which are sequentially stacked on the surface of the semiconductor substrate 100.

[0018] The refractive index of the material constituting the optical waveguide core layer 102 is greater than the refractive index of the material constituting the lower cladding layer 101 and the refractive index of the material constituting the upper cladding layer 103. Due to the difference in refractive index between the optical waveguide core layer 102 and the lower cladding layer 101 and the upper cladding layer 103, the signal light input to the phase adjuster 500 can be confined in the vertical direction within the optical waveguide core layer 102 and propagated to the light output side.

[0019] Furthermore, as described above, the first optical waveguide portion 10 a and the second optical waveguide portion 10 b have a striped ridge structure in the direction horizontal to the surface of the semiconductor substrate 100. By forming the protective insulating film 300, which has a refractive index smaller than that of the optical waveguide core layer 102, so as to cover this ridge structure, the input signal light can be confined in the optical waveguide core layer 102 in the direction horizontal to the surface of the semiconductor substrate 100 due to the refractive index difference, and can propagate to the light output side.

[0020] 4 shows a cross-sectional view of the phase adjustment unit 20 in a direction perpendicular to the propagation direction of the signal light. The phase adjustment unit 20 has a striped ridge structure including a lower cladding layer 201, an optical waveguide core layer 202, and an upper cladding layer 203, which are sequentially stacked on the surface of the semiconductor substrate 100.

[0021] 1 and 2 , an upper phase adjustment electrode 211 electrically connected to the upper clad layer 203 is formed on the surface of the upper clad layer 203 of the phase adjustment unit 20. A lower phase adjustment electrode 210 is formed on the back surface of the semiconductor substrate 100. The lower phase adjustment electrode 210 is electrically connected to the lower clad layer 201. Note that in FIG. 3 , the lower phase adjustment electrode 210 is electrically connected to the lower clad layer 201 via the semiconductor substrate 100, but the lower phase adjustment electrode 210 may be formed on the lower clad layer 201, for example, and may be arranged so that the lower phase adjustment electrode 210 is directly and electrically connected to the lower clad layer 201. Note that both of the above-mentioned connection modes of the lower phase adjustment electrode 210 are expressed as the lower phase adjustment electrode 210 being electrically connected to the lower clad layer 201. In FIG. 2, the optical waveguide core layers 102 of the first optical waveguide portion 10a and the second optical waveguide portion 10b are not actually visible from the top surface side, but for convenience of explanation, the internal positions are shown as optical waveguide core layers 102a.

[0022] The lower cladding layer 101 of the first optical waveguide portion 10 a and the second optical waveguide portion 10 b and the lower cladding layer 201 of the phase adjustment portion 20 may be a single continuous semiconductor layer as a crystal growth layer, or may be different semiconductor layers. The optical waveguide core layer 102 and upper cladding layer 103 of the first optical waveguide portion 10 a and the second optical waveguide portion 10 b, and the optical waveguide core layer 202 and upper cladding layer 203 of the phase adjustment portion 20 may also be a single continuous semiconductor layer as a crystal growth layer, or may be different semiconductor layers. Furthermore, the ridge structure may be a single continuous ridge structure common to the first optical waveguide portion 10 a, the phase adjustment portion 20, and the second optical waveguide portion 10 b, or may be different optical waveguide core layer structures.

[0023] The refractive index of the optical waveguide core layer 202 is greater than the refractive index of the lower cladding layer 201 and the refractive index of the upper cladding layer 203. Due to the difference in refractive index between the optical waveguide core layer 202 and the refractive index of the lower cladding layer 201 and the upper cladding layer 203, the input signal light can be confined in the vertical direction within the optical waveguide core layer 202 and propagated to the optical output side.

[0024] Furthermore, as described above, the phase adjustment unit 20 has a striped ridge structure in the direction horizontal to the surface of the semiconductor substrate 100. By forming the protective insulating film 300, which has a refractive index smaller than that of the optical waveguide core layer 202, so as to cover this ridge structure, the input signal light can be confined in the optical waveguide core layer 202 in the direction horizontal to the surface of the semiconductor substrate 100 due to the refractive index difference, and can propagate to the light output side.

[0025] In the phase adjustment section 20, an upper phase adjustment electrode 211 provided on the surface of the upper cladding layer 203 exposed at the top of the ridge structure extends along the direction in which the signal light propagates, i.e., the stripe direction. In the phase adjustment section 20, a first extraction electrode 220 is electrically connected to one end of the upper phase adjustment electrode 211, i.e., the side where the signal light is input, and a second extraction electrode 221 is connected to the other end of the upper phase adjustment electrode 211, i.e., the side where the signal light is output. Note that the first extraction electrode 220 and the second extraction electrode 221 are formed on the protective insulating film 300 except for the portion connected to the upper phase adjustment electrode 211.

[0026] A potential difference is applied between the first extraction electrode 220 and the second extraction electrode 221 by a power supply provided outside the phase adjuster 500. In response to this potential difference, a current flows inside the upper phase adjustment electrode 211 along the stripe direction in which the upper phase adjustment electrode 211 extends.

[0027] Since the upper phase adjustment electrode 211 has electrical resistance in the extending stripe direction, Joule heat is generated when a current flows through the upper phase adjustment electrode 211. In other words, the upper phase adjustment electrode 211 functions as a heater. When the Joule heat generated from the upper phase adjustment electrode 211 is thermally conducted to the optical waveguide core layer 202 via the upper cladding layer 203, the refractive index of the optical waveguide core layer 202 changes due to the thermo-optic effect. In other words, the phase of the signal light propagating through the optical waveguide core layer 202 of the phase adjustment unit 20 can be changed using the thermo-optic effect.

[0028] Furthermore, when a potential difference is applied between the first and second extraction electrodes 220 and 221, a potential difference is also applied between the first and second extraction electrodes 220 and 221 and the lower phase adjustment electrode 210 by a power supply provided outside the phase adjuster 500. In response to this potential difference, a current flows through the lower cladding layer 201, the optical waveguide core layer 202, and the upper cladding layer 203.

[0029] When the amount of carriers present in the optical waveguide core layer 202 changes due to the flow of such current, the carrier plasma effect changes the refractive index and the amount of optical loss of the optical waveguide core layer 202. In other words, by adjusting the amount of current passing through the lower cladding layer 201, the optical waveguide core layer 202, and the upper cladding layer 203, it is possible to change the phase and the amount of optical loss of the signal light propagating through the optical waveguide core layer 202 of the phase adjustment unit 20.

[0030] Therefore, according to the phase adjuster 500 of the first embodiment, it is possible to simultaneously obtain the phase adjustment function by the heater using electrical resistance in the phase adjustment unit 20 and the phase and optical loss adjustment function by the current passing through the optical waveguide core layer 202, thereby making it possible to realize a miniaturized phase adjuster, and ultimately to miniaturize the semiconductor optical integrated device that integrates such a phase adjuster.

[0031] The above has described the basic semiconductor layer structure of the phase adjuster 500 according to the first embodiment. Furthermore, in order to reduce the contact resistance between the upper phase adjust electrode 211 and the semiconductor layer, a contact layer made of a low-resistance semiconductor may be formed between the upper phase adjust electrode 211 and the upper cladding layer 203. Also, a commonly known structure may be added, such as covering the sidewalls of the ridge shape with a semiconductor layer having a lower refractive index than the optical waveguide core layer 202 and then forming the protective insulating film 300.

[0032] <Method of Manufacturing Phase Adjuster According to First Embodiment> Next, the details of the method of manufacturing the phase adjuster 500 according to the first embodiment will be described below. On the surface of an n-type InP (indium phosphide) substrate, which is an example of a semiconductor substrate 100, an n-type InP layer having a thickness of 2.0 μm is grown by epitaxial crystal growth, such as metal organic chemical vapor deposition (MOCVD), to form the lower cladding layer 101 and the lower cladding layer 201. Subsequently, an alloy semiconductor layer having an appropriate composition selected from Al, In, Ga, As, and P according to the wavelength of the signal light, such as AlInGaAs or InGaAsP, to form the optical waveguide core layer 102 and the optical waveguide core layer 202, is grown by crystal growth. Furthermore, a p-type InP layer having a thickness of 3.0 μm is sequentially grown by crystal growth to form the upper cladding layer 103 and the upper cladding layer 203.

[0033] The composition and film thickness of the optical waveguide core layer 102 and the optical waveguide core layer 202 may be appropriately selected depending on the wavelength of the input optical signal. For example, when the wavelength of the optical signal is 1.55 μm, which is commonly used in optical communications, an example of an alloy semiconductor that constitutes the optical waveguide core layer 102 and the optical waveguide core layer 202 is an InGaAsP alloy semiconductor with a layer thickness of 400 nm, which has an emission peak wavelength of 1.2 μm as measured by photoluminescence.

[0034] Alternatively, the optical waveguide core layer 102 and the optical waveguide core layer 202 may have a structure called a multiple quantum well structure, in which layers each made of alloy semiconductors with different compositions and having a layer thickness of about 10 nm are alternately stacked multiple times. In this case, due to the so-called quantum confined Stark effect, a steep change in refractive index and optical loss can be obtained in the optical waveguide core layer 202 when a bias is applied.

[0035] Furthermore, in order to confine light efficiently in the optical waveguide core layer, a laminated structure called an SCH (Separate Confinement Heterostructure) layer, which is made of an alloy semiconductor and has a refractive index higher than those of the lower cladding layer 101, the lower cladding layer 201 and the upper cladding layer 103, the upper cladding layer 203, and a refractive index lower than those of the optical waveguide core layer 102, the optical waveguide core layer 202, may be added above and below the optical waveguide core layer 102, the optical waveguide core layer 202.

[0036] Furthermore, different optical waveguide core layer structures may be employed between the first optical waveguide portion 10 a and the second optical waveguide portion 10 b and the phase adjustment portion 20. For example, after forming the optical waveguide core layer 202 of the phase adjustment portion 20, the optical waveguide core layer may be selectively removed from the regions that will become the first optical waveguide portion 10 a and the second optical waveguide portion 10 b by wet etching using a chemical solution or dry etching using plasma of a reactive gas, and an optical waveguide core layer having a different layer structure may be crystal-grown again by MOCVD in the removed regions.

[0037] Next, in order to process the first optical waveguide portion 10a, the second optical waveguide portion 10b, and the phase adjustment portion 20 into a ridge shape, a SiO2 film patterned into stripes having a width of 1.0 μm to 3.0 μm is formed as a protective mask using photolithography and etching techniques to form the desired optical waveguide shape on the surfaces of the upper clad layer 103 and the upper clad layer 203. The areas not covered with the SiO2 film are etched using dry etching or wet etching until the lower clad layer 101 and the lower clad layer 201 are exposed.

[0038] Next, in order to protect both side surfaces of the ridge-shaped first and second optical waveguide portions 10 a and 10 b and the phase adjustment portion 20, as well as the surface of the semiconductor region other than the ridge structure, from contamination, moisture, and the like, a SiO film having a thickness of, for example, 500 nm is deposited as a protective insulating film 300 using a plasma CVD method or the like.

[0039] Next, for the phase adjustment section 20, the upper part of the ridge structure, i.e., the SiO2 film at the top, is selectively removed, leaving the upper cladding layer 203 exposed on the surface, and a conductive film that will become the upper phase adjustment electrode 211 is then formed. As described above, the upper phase adjustment electrode 211 must have a certain resistance value to function as a heater. Therefore, examples of materials that can be used to form the upper phase adjustment electrode 211 include metals such as platinum and titanium, alloys such as nichrome, metal nitrides such as tantalum nitride, and metal oxides such as indium tin oxide. The upper phase adjustment electrode 211 is made of a thin film with a thickness of approximately 200 nm.

[0040] Next, a first extraction electrode 220 is formed in contact with a portion near one end of the upper phase adjustment electrode 211, which is the input side of the optical signal in the phase adjustment unit 20, and a second extraction electrode 221 is formed in contact with a portion near the other end of the upper phase adjustment electrode 211, which is the output side of the optical signal. Methods for forming the first extraction electrode 220 and the second extraction electrode 221 include vapor deposition, sputtering, and plating.

[0041] A low-resistance metal such as gold is used as the material for forming the first extraction electrode 220 and the second extraction electrode 221. An example of the material for forming the first extraction electrode 220 and the second extraction electrode 221 is gold with a film thickness of 3 μm.

[0042] Here, it is preferable to set the resistance R1 (first resistance) between both ends of the upper phase adjustment electrode 211, i.e., the resistance R1 between the first extraction electrode 220 and the second extraction electrode 221, to be approximately the same as the resistance R2 (second resistance) of the semiconductor layer when a forward voltage is applied between the upper phase adjustment electrode 211 and the lower phase adjustment electrode 210. The reason for this is that if the resistance of one electrode is too low, a sufficient current will not flow between the other electrode, and it will be impossible to simultaneously obtain both a refractive index change due to the thermo-optic effect and a refractive index change and optical loss change due to the carrier plasma effect.

[0043] Generally, to shift the phase of an optical signal by half a wavelength in a phase shifter using an InP-based semiconductor material, an applied power of about 20 mW is required for the carrier plasma effect due to current injection, and about 50 mW is required for the thermo-optic effect using a heater. Therefore, in order to ensure that the currents required to produce each effect flow, the ratio of the resistance R1 between both ends of the upper phase adjustment electrode 211 to the resistance R2 of the semiconductor layer between the upper phase adjustment electrode 211 and the lower phase adjustment electrode 210, i.e., R1 / R2, should be set to 100 or less, preferably 10 or less, and even more preferably about 1.

[0044] <Effects of First Embodiment> As described above, according to the phase adjuster of the first embodiment, an upper phase adjustment electrode having one end connected to the first extraction electrode and the other end connected to the second extraction electrode is provided in the phase adjustment section of the phase adjuster, thereby achieving the effect of being able to adjust the amount of loss and the amount of phase change of an optical signal propagating through an optical waveguide and obtaining a compact phase adjuster.

[0045] 5 is a cross-sectional view of a phase adjuster according to the second embodiment taken along a direction parallel to the propagation direction of the signal light. The phase adjuster 600 according to the second embodiment differs from the phase adjuster 500 according to the first embodiment in that, as shown in FIG. 5 , an insulating film 310 is formed in a part of the region between the upper phase adjuster electrode 211 and the upper clad layer 203 so that the amount of current flowing in the upper phase adjuster electrode 211 does not become extremely small compared to the amount of current flowing between the upper phase adjuster electrode 211 and the lower phase adjuster electrode 210.

[0046] In general semiconductor devices, in order to reduce the contact resistance at the interface between the electrode and the semiconductor layer, the semiconductor layer that forms the contact surface is doped with a high concentration of impurities or is a semiconductor layer with high carrier mobility, thereby providing a low resistance. On the other hand, the upper phase adjustment electrode 211 uses a thin film metal or the like with a high resistance value as its constituent material in order to generate Joule heat sufficient to change the refractive index of the optical waveguide core layer by the thermo-optic effect.

[0047] Here, if the resistance value of the semiconductor layer near the interface between the upper phase adjustment electrode 211 and the semiconductor layer becomes sufficiently smaller than the resistance value R1 between both ends of the upper phase adjustment electrode 211, the current flowing between the first extraction electrode 220 and the second extraction electrode 221 will not flow through the upper phase adjustment electrode 211, which has a constant resistance value R1, but will flow through the semiconductor layer, which has a low resistance, and there is a risk that sufficient Joule heat will not be generated to heat the optical waveguide core layer 202.

[0048] Therefore, in the phase adjuster 600 according to the second embodiment, an insulating film 310 is formed in a partial region between the upper phase adjust electrode 211 and the upper cladding layer 203. In the region where the upper phase adjust electrode 211 is formed on the insulating film 310, current flows only within the upper phase adjust electrode 211 and does not flow to the semiconductor layer side, so that Joule heat can be reliably generated according to the resistance value R1 between both ends of the upper phase adjust electrode 211. Therefore, in the phase adjuster 600 according to the second embodiment, both a refractive index change due to the thermo-optic effect and a refractive index change and a change in the amount of optical loss due to the carrier plasma effect can be obtained simultaneously.

[0049] <Effects of Second Embodiment> As described above, according to the phase adjuster of the second embodiment, an insulating film is formed in a partial region between the upper phase adjustment electrode and the upper cladding layer, and therefore, it is possible to simultaneously obtain both a refractive index change due to the thermo-optic effect and a refractive index change and a change in the amount of optical loss due to the carrier plasma effect. This has the effect of making it possible to easily adjust the amount of loss and the amount of phase change of an optical signal propagating through an optical waveguide, and also to obtain a compact phase adjuster.

[0050] Embodiment 3 Fig. 6 is a cross-sectional view of a phase adjuster according to embodiment 3 in a direction parallel to the propagation direction of the signal light. Fig. 7 is a top view of the phase adjuster according to embodiment 3.

[0051] 6 and 7, in the phase adjuster 700 according to the third embodiment, the upper phase adjust electrode 211 is divided into at least two or more electrodes along the stripe direction, and two lead electrodes are formed on each of the divided upper phase adjust electrodes, one of which is connected to an equipotential. The following description will be given of an example in which the upper phase adjust electrode 211 is divided into two.

[0052] The upper phase adjustment electrode 211 of the phase adjuster 700 is composed of a first upper phase adjustment electrode 211a and a second upper phase adjustment electrode 211b, which are divided into two parts along the stripe direction. One end of the first upper phase adjustment electrode 211a is connected to one end of the first upper phase adjustment electrode 211a, and one end of the second upper phase adjustment electrode 221 is connected to the other end of the first upper phase adjustment electrode 211a. Furthermore, the other end of the first upper phase adjustment electrode 220 is connected to one end of the second upper phase adjustment electrode 211b, and the other end of the second upper phase adjustment electrode 211b is connected to the other end of the second upper phase adjustment electrode 221. In the above description, "one end" refers to the input side of an optical signal, and "the other end" refers to the output side of an optical signal.

[0053] In the phase adjuster of the present disclosure, both the current flowing in the upper phase adjust electrode 211 and the current flowing between the upper phase adjust electrode and the lower phase adjust electrode 210 must flow sufficiently. Therefore, in the third embodiment, the phase adjuster 20 is divided into a plurality of parts, and each of the divided phase adjusters 20 is electrically connected in parallel to an external power supply. With this configuration, the current flowing in the upper phase adjust electrode 211 and the current flowing between the upper phase adjust electrode 211 and the lower phase adjust electrode 210 can obtain the current amount necessary to change the phase in the optical waveguide core layer 202. Therefore, the phase of the optical signal and the amount of optical loss can be easily adjusted without increasing the size and complexity of the power supply configuration required for the phase adjuster.

[0054] <Effects of Third Embodiment> As described above, according to the phase adjuster of the third embodiment, the upper phase adjustment electrode is divided into at least two or more electrodes along the stripe direction, and therefore the amount of current required to change the phase in the optical waveguide core layer can be obtained, thereby achieving the effect of easily adjusting the amount of loss and the amount of phase change of the optical signal propagating through the optical waveguide and obtaining a compact phase adjuster.

[0055] Fourth Embodiment Fig. 8 is a top view of a semiconductor optical integrated device 1000 according to a fourth embodiment, Fig. 9 is a cross-sectional view of the semiconductor optical integrated device 1000 according to the fourth embodiment in a direction parallel to the propagation direction of signal light in the optical modulator, and Fig. 10 is a cross-sectional view of the semiconductor optical integrated device 1000 according to the fourth embodiment in a direction perpendicular to the propagation direction of signal light in the optical modulator. Note that a Mach-Zehnder modulator is given as an example of the semiconductor optical integrated device 1000.

[0056] 8 , a Mach-Zehnder modulator, which is an example of a semiconductor optical integrated device 1000 according to the fourth embodiment, includes: an optical demultiplexer 30 that branches input signal light into two optical waveguides 10, each provided on a substrate 100a; an optical modulator 40 that is provided in each of the two optical waveguides 10 branched by the optical demultiplexer 30 and generates a modulated optical signal by applying a high-speed modulated electrical signal; and an optical multiplexer / demultiplexer 50 to which a phase adjuster according to any one of the first to third embodiments is connected via the optical waveguides 10 to each of the optical modulators 40, and which receives the optical signals from the two optical waveguides 10 as input and branches them into two more optical waveguides to output optical signals. Note that the substrate 100a may include a semiconductor substrate 100 and each semiconductor layer formed on the semiconductor substrate 10.

[0057] One end of an output from an optical multiplexer / demultiplexer 50 of a Mach-Zehnder modulator, which is an example of a semiconductor optical integrated device 1000 according to the fourth embodiment, is connected to an optical output unit 60 that outputs an optical signal to the outside via an optical waveguide, and the other end is connected to an optical monitor 70. That is, of the two outputs from the optical multiplexer / demultiplexer 50, namely a first output and a second output, the first output is connected to the optical output unit 60 via an optical waveguide, and the second output is connected to the optical monitor 70 via an optical waveguide.

[0058] The optical demultiplexer 30 can use, for example, a one-input, two-output type or two-input, two-output type multi-mode interference (MMI) optical waveguide.

[0059] 9 shows a cross-sectional view of the optical modulator 40 in a direction parallel to the propagation direction of the signal light, and FIG. 10 shows a cross-sectional view of the optical modulator 40 in a direction perpendicular to the propagation direction of the signal light. The optical modulator 40 is composed of semiconductor layers, such as a lower cladding layer 401 made of n-type InP, an optical waveguide core layer 402 made of an i-type alloy semiconductor layer such as AlInGaAs or InGaAsP, or a quantum well structure formed by stacking these layers, and an upper cladding layer 403 made of p-type InP, a lower phase modulation electrode 410 electrically connected to the lower cladding layer 401, and an upper phase modulation electrode 411 electrically connected to the upper cladding layer 403. The optical modulator 40 can modulate the phase of the signal light propagating through the optical waveguide core layer 402 by applying a high-speed modulated voltage signal to the upper phase modulation electrode 411.

[0060] A commonly used technique is to provide a differential signal by pairing the upper phase modulation electrodes 411 formed on each of the two branched optical waveguides 10. This technique makes it possible to generate an efficiently modulated optical signal. The optical multiplexer / demultiplexer 50 can be, for example, a two-input, two-output MMI.

[0061] In the Mach-Zehnder modulator, the optical modulator 40 imparts a phase difference between the signal lights branched and output to the two optical waveguides 10 by the optical demultiplexer 30, and an optical signal is output to the optical output unit 60 or the optical monitor 70 according to this phase difference at the output side of the optical multiplexer / demultiplexer 50. Therefore, an optical signal whose optical intensity and phase have been modulated according to the phase difference imparted by the optical modulator 40 can be output from the optical output unit 60 to the outside of the Mach-Zehnder modulator.

[0062] Here, in order to improve the signal-to-noise ratio of the output optical signal, it is necessary to accurately control the phase difference between the optical signals from the two optical waveguides 10 input to the optical multiplexer / demultiplexer 50, and furthermore, to match the optical signal intensities. Therefore, by adjusting the phase and the amount of optical loss using the phase adjustment unit of the phase adjuster mounted on the semiconductor optical integrated device 1000 according to the fourth embodiment, it is possible to accurately match the phase and optical signal intensity to desired values. Therefore, the semiconductor optical integrated device 1000 according to the fourth embodiment can realize a Mach-Zehnder modulator that can be miniaturized and has a high signal-to-noise ratio of the optical signal.

[0063] A photodiode for detecting the intensity of signal light propagating through the optical waveguide is an example of the optical monitor 70. The photodiode constituting the optical monitor 70 may be a photodiode integrated in the optical waveguide on a semiconductor substrate, or may be a photodiode installed at a position where it receives signal light output from the end of the optical waveguide to the outside of the Mach-Zehnder modulator.

[0064] The procedure for improving the signal-to-noise ratio of a Mach-Zehnder modulator using the optical monitor 70 will be described below. Signal light input to the Mach-Zehnder modulator is branched by the optical branching filter 30 and propagates through two optical waveguides 10. By applying a current flowing along the upper phase adjustment electrode 211 of the phase adjustment unit 20 of the phase adjuster connected to each of the two optical waveguides 10 and a current flowing between the upper phase adjustment electrode 211 and the lower phase adjustment electrode 210, the phase and intensity of the light propagating through each optical waveguide 10 can be changed.

[0065] In a Mach-Zehnder optical waveguide, the signal light output from the optical multiplexer / demultiplexer 50 changes depending on the phase difference between the signal lights propagating through the two optical waveguides 10. Therefore, by measuring the intensity of the optical signal using the optical monitor 70, the phase difference between the two optical waveguides can be monitored.

[0066] In the semiconductor optical integrated device 1000 according to the fourth embodiment, when the current value to the phase adjustment unit 20 is adjusted so that the optical signal intensity detected by the optical monitor 70 is maximized, the optical output to the other optical output end of the optical multiplexer / demultiplexer 50, i.e., the optical output at the optical output unit 60, becomes minimum due to the optical characteristics of the two-input / two-output MMI. In the ideal case, that is, when the optical intensities of the two optical waveguides 10 are perfectly matched and the phase difference is a half wavelength, the optical output from the optical output unit 60 becomes zero.

[0067] As explained above, by applying a high-speed modulated voltage signal to the upper phase modulation electrode 411 of the optical modulator 40 with the state in which the current value of the phase adjustment unit 20 is adjusted as the initial state, an optical output modulated at high speed according to the phase difference between the two optical waveguides can be obtained from the optical output unit 60. If the optical output in the initial state can be made as close to zero as possible, the ratio with the optical output in the on state becomes extremely large, and therefore a small Mach-Zehnder modulator with a high signal-to-noise ratio can be realized.

[0068] <Effects of Fourth Embodiment> As described above, according to the semiconductor optical integrated device of the fourth embodiment, by applying a high-speed modulated voltage signal to the upper phase modulation electrode of the optical modulator, an optical output modulated at high speed in accordance with the phase difference between the two optical waveguides can be obtained from the optical output unit, thereby achieving the effect of obtaining a semiconductor optical integrated device that can adjust the phase of an optical signal and the amount of optical loss without increasing the size and complexity of the power supply configuration required for the phase adjuster.

[0069] Although the present disclosure describes various exemplary embodiments and examples, the various features, aspects, and functions described in one or more embodiments are not limited to application to a particular embodiment, but may be applied to the embodiments alone or in various combinations.

[0070] Therefore, countless variations not illustrated are conceivable within the scope of the technology of the present disclosure, including, for example, cases where at least one component is modified, added, or omitted, and even cases where at least one component is extracted and combined with a component of another embodiment.

[0071] 10 Optical waveguide, 10a First optical waveguide section, 10b Second optical waveguide section, 20 Phase adjustment section, 30 Optical demultiplexer, 40 Optical modulator, 50 Optical multiplexer / demultiplexer, 60 Optical output section, 70 Optical monitor, 100 Semiconductor substrate, 100a Substrate, 101, 201, 401 Lower clad layer, 102, 102a, 202, 402 Optical waveguide core layer, 103, 203, 403 Upper clad layer, 210 Lower phase adjustment electrode, 211 Upper phase adjustment electrode, 211a First upper phase adjustment electrode, 211b Second upper phase adjustment electrode, 220 First extraction electrode, 221 Second extraction electrode, 300 Protective insulating film, 410 Lower phase modulation electrode, 411 Upper phase modulation electrode, 500, 600, 700 Phase adjuster, 1000 Semiconductor optical integrated device

Claims

1. A phase adjuster comprising: a first optical waveguide section having a stripe-shaped ridge structure including a lower cladding layer, an optical waveguide core layer, and an upper cladding layer stacked on a surface of a semiconductor substrate; a phase adjustment section provided in contact with the first optical waveguide section in the stripe direction and having the ridge structure; a second optical waveguide section provided in contact with the phase adjustment section in the stripe direction and having the ridge structure; an upper phase adjustment electrode provided on the top of the ridge structure of the phase adjustment section and extending along the stripe direction; a lower phase adjustment electrode electrically connected to the lower cladding layer; a first extraction electrode electrically connected to the upper phase adjustment electrode; and a second extraction electrode electrically connected to the upper phase adjustment electrode.

2. The phase shifter according to claim 1, wherein the first and second extraction electrodes are provided on a protective insulating film.

3. A phase adjuster according to claim 2, wherein the first lead electrode is connected to one end of the upper phase adjustment electrode, and the second lead electrode is connected to the other end of the upper phase adjustment electrode.

4. The phase adjuster according to claim 3, wherein a portion of said upper phase adjustment electrode is provided on the top of said ridge structure via said protective insulating film.

5. The phase adjuster according to claim 1 or 2, characterized in that the upper phase adjustment electrode comprises a first upper phase adjustment electrode and a second upper phase adjustment electrode divided into two along the stripe direction, one end of the first upper phase adjustment electrode is connected to one end of the first upper phase adjustment electrode, one end of the second extraction electrode is connected to the other end of the first upper phase adjustment electrode, the other end of the second extraction electrode is connected to one end of the second upper phase adjustment electrode, and the other end of the second extraction electrode is connected to the other end of the second upper phase adjustment electrode.

6. A phase adjuster as claimed in any one of claims 1 to 4, characterized in that the ratio of the first resistance value between the first extraction electrode and the second extraction electrode to the second resistance value between the upper phase adjustment electrode and the lower phase adjustment electrode is 100 or less.

7. A phase shifter according to any one of claims 1 to 6, characterized in that the lower cladding layer and the upper cladding layer are made of semiconductor layers made of InP, and the optical waveguide core layer is made of a semiconductor layer containing a mixed crystal semiconductor made of either AlInGaAs or InGaAsP.

8. A semiconductor optical integrated device comprising: an optical splitter that splits incident signal light into a plurality of optical waveguides; a plurality of optical modulators, one end of which is connected to each of the plurality of optical waveguides; a phase adjuster according to any one of claims 1 to 7, one end of which is connected to the other end of each of the plurality of optical modulators via the optical waveguide; and an optical multiplexer / demultiplexer connected to the other ends of the plurality of phase adjusters via the optical waveguide.

9. The semiconductor optical integrated device according to claim 8, wherein of the two outputs branched from the optical multiplexer / demultiplexer, a first output is connected to an optical output section via the optical waveguide, and a second output is connected to an optical monitor via the optical waveguide.

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