JP2017003669A - Semiconductor Mach-Zehnder optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor Mach-Zehnder optical modulator that can reduce propagation loss of a microwave, achieve impedance matching and velocity matching, simultaneously, and can further achieve higher speed and lower driving voltage.SOLUTION: A semiconductor Mach-Zehnder optical modulator comprises a Mach-Zehnder interferometer which is formed with first and second optical waveguide arms including an optical waveguide formed by sequentially laminating on a semiconductor substrate a first semiconductor clad layer, a non-doped semiconductor core layer, and a second semiconductor clad layer. Travelling-wave electrodes including ends to each of which a modulation electric signal is inputted are provided along the first and second optical waveguide arms, and comprise a ground electrode consisting of upper and lower stages. The lower ground electrode has a protruding part closer and in contact with the first and second optical waveguides than the upper ground electrode.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a semiconductor Mach-Zehnder optical modulator that modulates an optical signal with an electric signal.

  In order to respond to the increasing demand for communication traffic, researches to increase the multiplicity of optical signals are being actively conducted. As a specific method for increasing the optical signal multiplicity, quaternary phase modulation method (QPSK) that doubles the transmission capacity by assigning two values (multiplicity 2) to one symbol, or four values (multiple) for one symbol. There are known multilevel optical modulation schemes such as a 16-value quadrature amplitude modulation scheme (16QAM) and a 16-value amplitude phase modulation scheme (16APSK), in which the transmission capacity is quadrupled by assigning the severity 4). A method of further doubling the transmission capacity by optical polarization multiplexing is also known.

  Usually, when performing such multilevel optical modulation, an I / Q modulator is used as the optical modulator. The I / Q modulator is also called an orthogonal modulator, and is an optical modulator that can independently generate orthogonal optical electric field components (I channel and Q channel), and is a Mach-Zehnder (MZ) optical modulator. It has a special configuration in which are connected in parallel.

As a typical MZ optical modulator, an LN modulator using LiNbO ₃ (LN) is widely used. This operates using an electro-optic effect in which the refractive index of the medium changes according to the electric field applied to the LN. However, the LN modulator has a relatively long element length due to the physical constant of the material. In recent years, miniaturization and low drive voltage of optical transmitter modules have become issues, and research on semiconductor MZ optical modulators that are compact and capable of low drive voltages has been actively pursued.

  As a structure of the semiconductor MZ optical modulator, a pin-type InP / InGaAsP optical modulator in which a voltage is effectively applied to the core portion of the waveguide while confining light using a hetero pin junction is generally used. It is. (For example, Non-Patent Document 1)

  A general semiconductor MZ optical modulator is as shown in FIG. The electrodes used in the semiconductor MZ optical modulator shown in FIG. 1 will be described in detail.

  In the top view of the semiconductor MZ optical modulator shown in FIG. 1A, the modulated light input from the left end to the optical waveguide 2 provided on the InP substrate 1 is divided into two upper and lower structures by the optical demultiplexer 3. The optical waveguide arms 6 and 7 are respectively demultiplexed and modulated by two modulated electric signals applied to the traveling wave type electrode 4 provided along the arm, and after being multiplexed by the optical multiplexer 5, Is output as modulated light from the optical waveguide.

  FIG. 1B is a cross-sectional view of the optical waveguide at the central portion b of one arm portion 7 of the semiconductor MZ optical modulator of FIG. The optical waveguide of the arm portion 7 provided on the substrate 1 is generally composed of three layers: a core layer 7a for confining and transmitting an optical signal, and upper and lower cladding layers 7b and 7c provided above and below the core layer 7a. .

  The traveling wave electrode 4 to which a modulated electric signal is applied is also provided in the optical waveguide. The traveling electrode 4 is provided with a signal electrode 4a provided on the upper clad layer 7b of the optical waveguide arm 7 and a lower portion extending on the substrate 1. The ground electrode 4b is provided on both sides of the clad layer 7c at a predetermined distance from the arm portion optical waveguide.

  It is known that a traveling wave type electrode structure is useful for realizing a high-speed Mach-Zehnder optical modulator (Non-Patent Document 2). As can be seen from the top view of FIG. The modulated electric signal input from the modulated electric signal source 8 to the left end of the traveling wave type electrode structure propagates as a traveling wave to the right side on the signal electrode 4a and travels through the core layer 7a of the lower optical waveguide arm 7. It acts on the modulated light and is terminated with a matching resistor 9 (for example, 50Ω) on the right side.

  In such a traveling wave electrode structure, in order to realize a wide band, three points are important: impedance matching, speed matching between light and electricity in an optical modulator, and reduction of microwave propagation loss.

  A general electrical transmission line model is represented as an equivalent circuit as shown in FIG. 2, and in this circuit model, the impedance Z_0 and the propagation constant γ are represented by the following equations (1) and (2).

  R, G, L, and C represent the resistance, conductance, inductance, and capacitance per unit length, respectively, and when ωL >> R, ωC >> G,

At this time, the velocity of electricity v and the effective refractive index n are

It can be expressed as. This model can also be applied to traveling wave electrodes. That is, this indicates that the impedance and the speed of electricity can be adjusted by qualitatively controlling the inductance component of the optical modulator.

  As specific impedance matching conditions, it is desirable that the impedance be close to 50Ω, which is the impedance of the external electric circuit. If it deviates from 50Ω, electrical reflection occurs, and the voltage cannot be applied efficiently.

The frequency band Δf due to the difference in speed between light and electricity is expressed as follows using the light velocity c, the group velocity v _{0 of} light propagating through the optical waveguide, and the electrode length l.

From this equation, it can be seen that the maximum frequency band can be obtained when the light group velocity v ₀ and the electric velocity v coincide. However, since this equation is an approximate equation when there is no propagation loss and the impedance matching is the same, it is actually greatly influenced by the propagation loss and the impedance matching.

  As a method for reducing the propagation loss, an n-type having a lower electrical resistivity than that of the p-type is used as both upper and lower InP cladding layers, and a thin p-type semiconductor layer (p-type) is used as a barrier layer for suppressing electron current. An npin-type semiconductor optical modulator having a barrier layer interposed therebetween has been proposed. (For example, Patent Document 1)

  FIG. 3 is a detailed cross-sectional view of such a semiconductor optical modulator having an npin structure (see Non-Patent Document 3). Also in this structure, the n-type InP lower cladding layer 7c is formed on the semi-insulating InP substrate 1, and a part thereof forms a lower layer of the optical waveguide 7 as a mesa structure in FIG. ).

  A core layer 7a composed of a non-doped MQW (Multi-Quantum Well) optical core layer and an InP layer is formed on the lower cladding layer 7c, and transmits an optical signal.

  An n-type InP upper clad layer 7b is formed on the thin p-type semiconductor layer (p-type barrier layer 7d) as a barrier layer as described above to form an npin type structure.

  In FIG. 3, as in FIG. 1B, the signal electrode 4a provided on the upper clad layer 7b having such an npin structure and the arm portion 7 on the lower clad layer 7c extending on the substrate 1 are provided. A ground electrode 4b provided at a predetermined distance d from the side wall of the optical waveguide constitutes a traveling wave electrode 4, and a modulated electric signal is applied to modulate an optical signal propagating through the optical waveguide body of the arm portion 7.

  In the space between the both side walls of the optical waveguide of the arm portion 7 and the ground electrodes 4b on both sides, the dielectric layer 11 can be disposed as shown in the figure, or there can be no air.

  In such a conventional semiconductor optical modulator having an npin type structure, the distance d between the ground electrode 4b and the optical waveguide side wall is limited from the viewpoint of impedance matching and speed matching, and is about 10 μm to 20 μm. Therefore, there is a problem that the propagation loss of the traveling wave type electrode becomes large in a region where the modulation frequency is high.

  As described above, impedance matching, velocity matching, and propagation loss are important in determining the performance of an MZ optical modulator with a traveling wave electrode structure, but each generally has a trade-off relationship. Therefore, how to satisfy these is a design issue.

Japanese Patent No. 4047785

C. Rolland et al, “10 Gbit / s, 1.56 μm multiquantum well InP / InGaAsP Mach-Zehnder optical modulator,” Electron. Lett., Vol. 29, no. 5, pp. 471-472, 1993 R. G. Walker, “High-Speed III-V Semiconductor Intensity Modulators” IEEE J. Quantum Electron., Vol. 27, no. 3, pp. 654-667, 1991 N. Kikuchi et al, “80-Gb / s lowdriving-voltage InP DQPSK modulator with an n-p-i-n structure,” IEEE Photon. Technol. Lett. 21 (12), 787-789 (2009) R. Lewen et al, “Segmented Transmission-Line Electroabsorption Modulators,” IEEE J. Lightwave Technol., Vol. 22, no. 1, pp. 172-179, 2004

With the structure of the conventional semiconductor Mach-Zehnder optical modulator as described above, it is difficult to simultaneously realize reduction in microwave propagation loss, impedance matching, and speed matching in order to realize high speed and low driving voltage. It was.
Therefore, by combining a structure that can reduce the propagation loss of microwaves and a mechanism that adjusts the impedance and the speed of electricity, optical modulation with a broadband or low driving voltage is possible compared to conventional Mach-Zehnder optical modulators using traveling-wave electrodes. The purpose is to provide a vessel.

  In order to solve the above problems, a semiconductor Mach-Zehnder optical modulator of the present invention is formed by sequentially laminating a first semiconductor cladding layer, a non-doped semiconductor core layer, and a second semiconductor cladding layer on a semiconductor substrate. In a semiconductor Mach-Zehnder optical modulator in which a Mach-Zehnder interferometer is formed by first and second optical waveguide arms each having an optical waveguide, a modulated electric signal is provided at one end along each of the first and second optical waveguide arms. An input traveling wave electrode is provided, and the ground electrode of the traveling wave electrode is composed of two upper and lower stages, and the lower ground electrode is closer to the first and second optical waveguides than the upper ground electrode. It has the overhang | projection part which is carrying out.

  In the above optical modulator, one of the first semiconductor clad layer and the second semiconductor clad layer is an n-type semiconductor and the other is a p-type semiconductor. A Mach-Zehnder optical modulator.

  In the optical modulator, both the first semiconductor cladding layer and the second semiconductor cladding layer are n-type semiconductors, and the non-doped semiconductor core layer and the first semiconductor cladding layer, or the first semiconductor cladding layer, A semiconductor Mach-Zehnder optical modulator, characterized in that a p-type third semiconductor cladding layer is inserted between at least one of the two semiconductor cladding layers.

  Further, in the above optical modulator, the traveling wave type electrode has a signal electrode provided on an optical waveguide, and the signal electrode is loaded with a capacitance or an inductance. A Mach-Zehnder optical modulator.

  In the above optical modulator, a semiconductor Mach-Zehnder optical modulator characterized in that at least a part of the non-doped semiconductor core layer has a multiple quantum well layer structure.

  As described above, according to the present invention, a conventional Mach-Zehnder optical modulator using a traveling wave electrode is combined by combining a structure capable of reducing the propagation loss of microwaves and a mechanism for adjusting the impedance and the speed of electricity. As compared with the above, it is possible to provide an optical modulator having a wide band or a low driving voltage.

It is the top view (a) of the conventional semiconductor MZ light modulator, and sectional drawing (b) of the one side arm part. It is a general electrical transmission line model diagram. FIG. 6 is a cross-sectional view of a conventional semiconductor MZ optical modulator having an npin type structure. It is sectional drawing of the npin structure at the time of applying the electrode structure of Embodiment 1 of this invention. It is a figure explaining the outline of the manufacturing process of the electrode structure of this invention. It is a figure which shows the frequency characteristic of the propagation loss of a microwave in the electrode structure of this invention. It is a figure which shows the capacity | capacitance loading type traveling wave electrode structure of Embodiment 2 of this invention. It is a figure which shows the inductor loading type traveling wave type | mold electrode structure of Embodiment 2 of this invention.

Embodiments of the present invention will be described below with reference to the drawings.
(First embodiment)
Hereinafter, the optical modulator according to the first embodiment of the present invention will be described.

  FIG. 4 shows an example of a cross-sectional view of the waveguide portion of the semiconductor Mach-Zehnder optical modulator according to the first embodiment of the present invention. Differently, the ground electrode 4b has two upper and lower stages, and the lower stage has a thin ground electrode projecting portion 4c projecting to the optical waveguide side along the lower cladding layer 7c. It has a different structure. Of course, the upper ground electrode 4b and the lower ground electrode extension 4c may be configured as a two-layer integrated electrode structure formed of metal.

  By adopting a ground electrode structure with two upper and lower stages, only the lower thin ground electrode projecting portion 4c (about 1 μm) can approach the optical waveguide, and the frequency characteristics of the propagation loss as a traveling wave type electrode can be obtained. Can be improved.

  If the upper and lower two-stage structure is not used, the ground electrode and the signal electrode may be too close to each other, causing a short circuit. In addition, as shown in the schematic manufacturing process in FIG. 5, this electrode structure is formed by forming an optical waveguide 7 on the substrate 1 and then performing exposure and development using a lower resist to deposit a metal by vapor deposition. A two-layer resist can be used for manufacturing.

  FIG. 6 shows the calculation result of the frequency characteristics of the propagation loss obtained by the electromagnetic field simulation in the electrode structure of the present invention shown in FIG. The size of the signal electrode and the distance d from the upper ground electrode 4b to the waveguide are fixed, and the distance d 'from the lower ground electrode overhanging portion 4c to the waveguide is set to 0 μm, 1 μm, 2 μm, 5 μm, 10 μm ( The characteristics in five cases calculated as equivalent to the conventional structure are shown.

  From these results, it can be seen that the distance to the waveguide can be reduced by providing the overhanging portion on the ground electrode as compared with the conventional structure, and the propagation loss is greatly reduced at a frequency of 50 GHz or higher. This result shows that, for example, assuming that the electrode length of the phase modulation unit is 3 mm, a modulation band approximately 1.5 times that of the conventional structure can be realized by using this structure.

  As described above, it is effective to reduce the propagation loss by providing the projecting portion and reducing the distance between the ground electrode and the waveguide. Specifically, it is shown that the propagation loss in the lower cladding layer can be reduced by making the ground electrode close.

(Second Embodiment)
When the above-described ground electrode structure according to the first embodiment of the present invention is applied, the propagation loss of the modulated electric signal microwave in the traveling wave type electrode can be reduced to half or less of the conventional structure.

  However, in the electrode structure according to the first embodiment of the present invention in which the signal electrode 4a constituting the traveling wave electrode is provided on the optical waveguide 7, the impedance is reduced by about 10Ω compared to the conventional structure, and the microwave is effective. Since the refractive index is reduced by about 0.5 (high speed direction), impedance matching and speed matching, which are satisfied in the conventional structure, are greatly shifted.

  If this is not done, the E / E band (Electrical bandwidth, which does not interact with light in the electrical band (S21)) increases due to the effect of greatly reducing propagation loss, but impedance mismatch and speed mismatch. Due to the effect of matching, the E / O band (Electro-Optic band with, this 3 dB band corresponds to the modulation band of the optical modulator) does not increase.

  Therefore, in the second embodiment of the present invention, in the optical modulator of the first embodiment of the present invention shown in FIG. 4, the signal electrode 4a has a T-shape as shown in FIG. 7 (see Non-Patent Document 2). A capacitance-loaded traveling wave electrode structure for loading the capacitance formed by the mold-extending electrode 4d, or an inductance for loading the inductance formed by the U-shaped meander electrode 4e as shown in FIG. 8 (see Non-Patent Document 4) A loaded traveling wave electrode structure is applied.

  7 and 8, each partial view (a) is a top view of an optical modulator to which each loaded traveling-wave electrode structure is applied, and each partial view (b) and (c) is each arm. 4 is a cross-sectional view of a loaded traveling wave electrode structure at two locations b and c of a waveguide 7. FIG.

  In these top views, the specific shapes of the loading electrode portions 4d and 4e are not limited to the T-shape and U-shape shown in the drawing, and any arbitrary shape exhibiting capacitive or inductive properties in the transmission line. As described above, the shape is possible and the dielectric layer 11 is not essential in the cross-sectional view.

  Since these loaded lines can adjust the impedance and speed according to the electrode structure, the shifted impedance and speed can be compensated by first bringing the ground electrode closer to the waveguide. Thereby, it is possible to satisfy both impedance matching and velocity matching while reducing propagation loss, and it is possible to widen the bandwidth.

  If the electrode length is constant, the effect of broadening the band can be obtained as described above. On the other hand, if the electrode length is increased, the effect of widening the band can be reduced by reducing the driving voltage. It is possible to realize an optical modulator with a low driving voltage in a modulation band equivalent to that of the prior art.

  Now, a general method for 50Ω matching has been described, but conversely, impedance matching should be achieved by making the input / output terminals have a lower impedance than 50Ω such as 40Ω or 35Ω in accordance with the impedance of the optical modulator. Is also possible.

  In addition, it is n-type contacts such as nin-type, npin-type (Fig. 4), nnip-type, nip-type, and n-type that greatly improve propagation loss by bringing the ground electrode closer to the waveguide. A semiconductor layer structure having a relatively high ratio is most effective, and a semiconductor having a p-type ratio such as a pin type is less effective.

  This is because, in general, the pin type has a higher electrical resistivity ratio than the n-type, compared to the nin-type, npin-type, nipn-type, and nip-type, and the metal contact resistance is also higher than that of the n-type. This is because the p-type has a large loss by about an order of magnitude. The effect of improving the propagation loss by bringing the ground electrode closer to the waveguide is the same, but the loss due to the p-type is large, so the effect is difficult to see.

  In the optical modulator according to this embodiment, an n-InP layer, a lower cladding layer made of InP, a non-doped semiconductor core layer, and an upper cladding layer made of InP are sequentially stacked on an SI-InP substrate. The semiconductor core layer functions as an optical waveguide layer, for example, using a material system such as InGaAsP or InGaAlAs, and is composed of a single-component quaternary mixed crystal bulk layer or multiple quantum well layer, or a multiple quantum well layer. It is also possible to use a structure having an optical confinement layer above and below the band gap that is larger than the multiple quantum well layer and smaller than the upper and lower cladding layers.

  The band gap wavelengths of the quaternary mixed crystal bulk layer and the multiple quantum well layer are set so that the electro-optic effect acts effectively and the light absorption does not become a problem at the light wavelength used.

  The present invention is not limited to InP-based materials, and for example, a material system that matches a GaAs substrate may be used.

  As described above, unlike conventional pin types, microwave propagation loss is reduced in structures with a high proportion of n-types with low electrical resistivity, such as nin type, npin type, and nipn type, which have lower loss. Therefore, the propagation loss can be greatly reduced by making the ground electrode in a two-stage structure and bringing the lower ground electrode close to the vicinity of the optical waveguide. However, with this structure alone, impedance matching and velocity matching will shift as propagation loss is reduced, so combining with a traveling wave electrode that can adjust impedance and velocity by loading capacitance or inductance. Thus, a semiconductor Mach-Zehnder optical modulator with a wide band or a low driving voltage is realized.

DESCRIPTION OF SYMBOLS 1 Substrate 2 Optical waveguide 3 Optical demultiplexer 4 Traveling wave type electrode 4a Signal electrode 4b Ground electrode 4c Ground electrode projecting part 4d T-shaped projecting electrode 4e U-shaped meander electrode 5 Optical multiplexer 6, 7 Optical waveguide arm 7a Core layer 7b Upper clad layer 7c Lower clad layer 7d Barrier layer 8 Modulated electric signal source 9 Matching resistor 11 Dielectric layer

Claims (5)

A Mach-Zehnder interferometer is composed of first and second optical waveguide arms each having an optical waveguide formed by sequentially laminating a first semiconductor cladding layer, a non-doped semiconductor core layer, and a second semiconductor cladding layer on a semiconductor substrate. In the formed semiconductor Mach-Zehnder optical modulator,
A traveling wave electrode to which a modulated electric signal is input is provided at one end of each of the first and second optical waveguide arms, and the ground electrode of the traveling wave electrode is composed of two upper and lower stages, A semiconductor Mach-Zehnder optical modulator, wherein the ground electrode has an overhanging portion that is closer to the first and second optical waveguides than the upper ground electrode.   2. The semiconductor Mach-Zehnder light modulation according to claim 1, wherein one of the first semiconductor clad layer and the second semiconductor clad layer is an n-type semiconductor and the other is a p-type semiconductor. vessel.   Both the first semiconductor clad layer and the second semiconductor clad layer are n-type semiconductors, and at least one of the non-doped semiconductor core layer and the first semiconductor clad layer, or the second semiconductor clad layer. 2. The semiconductor Mach-Zehnder optical modulator according to claim 1, wherein a p-type third semiconductor clad layer is inserted between the first and second semiconductor clad layers. The traveling wave electrode has a signal electrode provided on an optical waveguide,
4. The semiconductor Mach-Zehnder optical modulator according to claim 1, wherein a capacitance or an inductance is loaded on the signal electrode.   5. The semiconductor Mach-Zehnder optical modulator according to claim 1, wherein at least a part of the non-doped semiconductor core layer has a multiple quantum well layer structure.