JP2015129906A - Semiconductor mach-zehnder modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a travelling wave type semiconductor Mach-Zehnder modulator which has a wide band or is driven at a low voltage.SOLUTION: The Mach-Zehnder interferometer is formed with a first and second optical guide 203 and 204, and a first and second travelling wave type electrodes are formed on the first and second optical guide 203 and 204. The first and second travelling wave type electrodes respectively include a first regions 205 and 207 with a first element length having a first inductance and a first capacitance, and a second regions 206 and 208 with a second element length having a second inductance and a second capacitance. The second inductance is larger than the first inductance and the second capacitance is smaller than the first capacitance. The first element length and the second element length are sufficiently smaller than the operation frequency.

Description

  The present invention relates to a semiconductor Mach-Zehnder modulator that converts an electrical signal into an optical signal.

  The development of wavelength division multiplexing (WDM) technology is progressing to meet the increasing demand for communication traffic, and at present, the bandwidth that can be transmitted by the fiber is depleted, and the improvement of the frequency utilization efficiency is a problem. Increasing the transmission rate per wavelength is useful for improving frequency utilization efficiency and further increasing the capacity of a communication system.

  In recent years, therefore, research on increasing the signal multiplicity has been actively conducted. As a specific method for increasing signal multiplicity, quaternary phase modulation method (QPSK) that doubles the transmission capacity by assigning binary (multiplicity 2) to one symbol, or four values (multiplicity) for one symbol. There are known multi-level modulation schemes such as a 16-value quadrature amplitude modulation scheme (16QAM) and a 16-value amplitude phase modulation scheme (16APSK) that quadruple the transmission capacity by assigning 4). A method of doubling the transmission capacity by polarization multiplexing is also known.

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

  As a typical MZ modulator, an LN modulator using LiNbO3 (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 miniaturization and low drive voltage of optical modulators are indispensable issues. In order to meet these demands, research on a semiconductor MZ modulator that is small and can be driven at a low driving voltage has been actively conducted.

  FIG. 1 shows a semiconductor MZ modulator having a conventional structure (see, for example, Non-Patent Document 1), (a) is a top view, and (b) is a cross-sectional view. As shown in FIG. 1A, the semiconductor MZ modulator includes an input waveguide 101, an optical demultiplexer 102 that demultiplexes light guided through the input waveguide 101, and an optical demultiplexer. Waveguides 103 and 104 through which the light demultiplexed by the optical unit 102 is respectively guided, an optical multiplexer 107 that combines the light guided through the two waveguides 103 and 104, and an optical multiplexer 107 And an output waveguide 108 that outputs the combined light. The semiconductor MZ modulator shown in FIG. 1 is connected to a high-frequency signal source 109 and traveling-wave electrodes 105 and 106 for applying a voltage to light guided through the waveguides 103 and 104. And electrodes 110 and 111 for applying a bias voltage to the waveguides 103 and 104, respectively. FIG. 1B is a cross-sectional view of two waveguides 103 and 104 between the optical demultiplexer 102 and the optical multiplexer 107. As shown in FIG. 1B, the semiconductor MZ modulator includes a semiconductor substrate 112, an n-type semiconductor layer 113, a lower cladding layer 114, a core layer 115, and an upper cladding layer 116. , 106 are formed on the upper cladding layer 116.

  Here, the configuration and operation principle of the general semiconductor MZ modulator shown in FIG. 1 will be described. The input light first enters from the input waveguide 101, is branched into two by the optical demultiplexer 102, and is guided to two arms (waveguide 103 and waveguide 104). When a voltage is applied from the electrodes 105 and 106, a refractive index change occurs due to an electro-optic effect in the semiconductor core layer 115, and as a result, the phase of light guided through the waveguide 103 and the waveguide 104 changes. Due to the phase difference of the light propagating through the two arms (waveguides), the output light intensity combined and output by the optical multiplexer 107 changes. This is the operating principle of the MZ modulator.

  In the structure shown in FIG. 1, the electrodes 105 and 106 form a coplanar strip line, and the electric signal from the high-frequency signal source 109 is propagated in the opposite phase. Is a so-called push-pull type configuration, and this configuration is called a single-end drive type MZ modulator.

  2 shows another conventional semiconductor MZ modulator (see, for example, Non-Patent Document 2 or Non-Patent Document 3). FIG. 2A is a top view and FIG. 2B is a cross-sectional view. The semiconductor MZ modulator of FIG. 2 is also called a differential drive type MZ modulator. Similar to the semiconductor MZ modulator of FIG. 1, the semiconductor MZ modulator of FIG. 2 includes an input waveguide 101 and an optical demultiplexer 102 that demultiplexes light guided through the input waveguide 101 into two. The optical waveguides 103 and 104 through which the light demultiplexed by the optical demultiplexer 102 is guided respectively, and the optical multiplexer 107 that multiplexes the light guided through the two waveguides 103 and 104, respectively, And an output waveguide 108 that outputs the light combined by the wave filter 107. Further, the semiconductor MZ modulator shown in FIG. 2 is connected to high-frequency signal sources 121 and 122, traveling wave electrodes 105 and 106 for applying a voltage to light guided through the waveguides 103 and 104, and traveling waves And ground electrodes 123, 124, and 125 that form differential drive type transmission lines together with the mold electrodes 105 and 106. FIG. 2B is a cross-sectional view of the waveguide 104 portion of the two waveguides 103 and 104 between the optical demultiplexer 102 and the optical multiplexer 107. As shown in FIG. 2B, the semiconductor MZ modulator includes a semiconductor substrate 112, a lower cladding layer 114, a core layer 115, and an upper cladding layer 116, and the traveling wave electrode 106 is disposed on the upper cladding layer 116. Is formed. The cross section of the waveguide 103 is the same as that shown in FIG.

  The configuration and operation principle of the general semiconductor MZ modulator (differential drive type MZ modulator) shown in FIG. 2 will be described. As in the general semiconductor MZ modulator (coplanar stripline modulator) shown in FIG. 1, the input light first enters from the input waveguide 101 and is branched into two by the optical demultiplexer 102, and two arm waveguides. Guided to (waveguide 103 and waveguide 104). In the differential drive type MZ modulator shown in FIG. 2, unlike the coplanar stripline type modulator shown in FIG. 1, the semiconductor core layer (waveguide 103 and The electro-optic effect in the waveguide 104) causes refractive index changes that are independent of each other, resulting in a change in the phase of the light. Due to the phase difference between the two arms, the output light intensity combined and output by the optical multiplexer changes. This is the operating principle of the differential drive type MZ modulator of FIG. The electrode structure shown in FIG. 2 is a differential drive type electrode structure formed of traveling wave electrodes 105 and 106 and ground electrodes 123, 124 and 125.

  In the semiconductor MZ modulator shown in FIGS. 1 and 2, as described above, a voltage is applied to the light propagating through the core layer 115 by the electric signal propagating through the electrodes 105 and 106, thereby acting as an optical modulator. At this time, the refractive index change per unit length is

It is expressed. a is a coefficient of the first-order electro-optic effect (Pockels effect), and b is a coefficient of the second-order electro-optic effect.

Next, electrodes used in the semiconductor MZ modulator shown in FIG. 2 will be described in detail. It is known that a traveling wave electrode structure is useful for realizing a high-speed Mach-Zehnder modulator (for example, see Non-Patent Document 3). In the traveling wave type electrode structure, impedance matching and speed matching between light and electricity in the optical modulator are important. The electrical transmission line model is expressed as shown in FIG. 3, and in such a circuit model, the impedance Z ₀ and the propagation constant γ are expressed by the following equations.

  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 speed difference 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.

  As described above, impedance matching and speed matching are important in determining the performance of an MZ modulator having a traveling wave electrode structure.

K. Tsuzuki et al. “0.3 Vpp single-drive push-pull InP Mach-Zehnder modulator module for 43-Gbit / s systems” Optical Fiber Communication Conference, 2006 and the 2006 National Fiber Optic Engineers Conference. OFC 2006 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

  With the demand for higher capacity and lower power consumption in communication systems, it is essential to increase the speed and drive voltage of semiconductor Mach-Zehnder modulators. To that end, it is important to satisfy impedance matching and speed matching. is there. However, in the conventional structure, it is difficult to adjust the two in a balanced manner. For example, when priority is given to speed matching, the impedance tends to be too small.

  The present invention has been made in view of such problems, and its object is to provide a mechanism for adjusting the impedance and the speed of electricity, compared to a conventional Mach-Zehnder modulator using traveling wave electrodes. Another object is to provide a broadband or low drive voltage modulator.

  In order to achieve such an object, according to a first aspect of the present invention, a first conductive semiconductor cladding layer, a non-doped semiconductor core layer, and a second conductive semiconductor cladding layer are sequentially stacked on a semiconductor substrate. A Mach-Zehnder interferometer is formed by the first optical waveguide and the second optical waveguide each having the optical waveguide formed in the first, and the first optical waveguide is formed on the first optical waveguide and the second optical waveguide, respectively. A semiconductor Mach-Zehnder modulator including a traveling wave electrode and a second traveling wave electrode.

  In one embodiment, each of the first traveling wave type electrode and the second traveling wave type electrode of the semiconductor Mach-Zehnder modulator includes a first region formed with a first inductance and a first element length, A second region formed with a second inductance and a second element length, and the first region and the second region are electrically connected. The inductance of the second region is larger than the inductance of the first region, and the element lengths of the first region and the second region are sufficiently smaller than the operating frequency. Alternatively, each of the first traveling wave electrode and the second traveling wave electrode of the semiconductor Mach-Zehnder modulator has a first region having a first inductance, a first element length, and a first capacitance. The second region is formed with a second inductance, a second element length, and a second capacitance, and the capacitance of the second region is smaller than the capacitance of the first region.

  In one embodiment, the first traveling wave electrode and the second traveling wave electrode of the semiconductor Mach-Zehnder modulator form a coplanar strip line. Alternatively, the semiconductor Mach-Zehnder modulator further includes a ground electrode that sandwiches the first traveling wave type electrode and the second traveling wave type electrode, respectively, and is different from the first traveling wave type electrode and the second traveling wave type electrode. A motion signal is input.

In one embodiment, in the semiconductor Mach-Zehnder modulator, the first traveling wave type electrode and the second traveling wave type electrode each have a plurality of sets of the first region and the second region. The first regions of the first and second traveling wave electrodes are formed on the first and second optical waveguides, respectively.
In one embodiment, at least one of a portion corresponding to the second region on the first traveling wave electrode side in the ground electrode and a portion corresponding to the second region on the second traveling wave electrode side in the ground electrode. The two shapes change in response to the shape of the corresponding second region, and sandwich the second region of the first traveling wave electrode and the second traveling wave electrode with the second region. The distance between the ground electrode, or the average distance between the second region of the first traveling wave electrode and the second traveling wave electrode and the ground electrode sandwiching the second region is The distance between the first region of the traveling wave type electrode and the second traveling wave type electrode and the ground electrode sandwiching the first region is substantially the same.

  In one embodiment, the second conductive semiconductor cladding layer of the first optical waveguide and the second optical waveguide in a region between two adjacent first regions is a non-doped semiconductor cladding layer.

  In one embodiment, the second region portions of the first traveling wave electrode and the second traveling wave electrode are respectively on the first optical waveguide and the second optical waveguide, or on the side of the first optical waveguide and It is formed on a dielectric, an insulating semiconductor, a semi-insulating semiconductor, or an air layer formed beside the second optical waveguide. The second region portions of the first traveling wave electrode and the second traveling wave electrode are respectively positioned higher than the first region portions of the first traveling wave electrode and the second traveling wave electrode. It is formed.

  In one embodiment, one of the first conductive semiconductor cladding layer and the second conductive semiconductor cladding layer is an n-type semiconductor and the other is a p-type semiconductor.

  In one embodiment, both the first conductive semiconductor cladding layer and the second conductive semiconductor cladding layer are n-type semiconductors. A p-type third conductive semiconductor clad layer is provided between at least one of the first conductive semiconductor clad layer and the non-doped semiconductor core layer and between the non-doped semiconductor core layer and the second conductive semiconductor clad layer. Has been inserted.

  In one embodiment, at least a portion of the non-doped semiconductor core layer has a multiple quantum well layer structure.

  As described above, according to the present invention, by providing a mechanism for adjusting impedance matching and speed matching, compared with a conventional single-end drive or differential drive traveling-wave semiconductor Mach-Zehnder modulator, wideband or low drive. A voltage modulator can be realized.

It is a figure explaining the conventional semiconductor Mach-Zehnder optical modulator, (a) is a top view, (b) is sectional drawing. It is a figure explaining the conventional semiconductor Mach-Zehnder optical modulator, (a) is a top view, (b) is sectional drawing. It is a figure which shows the equivalent circuit model of a transmission line model. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining the semiconductor Mach-Zehnder optical modulator based on one Embodiment of this invention, (a) is a top view, (b) thru | or (e) are sectional drawings. It is a figure which shows the equivalent circuit model of the transmission line model after inductor loading. It is a figure explaining the semiconductor Mach-Zehnder optical modulator based on one Embodiment of this invention. It is a figure explaining the semiconductor Mach-Zehnder optical modulator based on one Embodiment of this invention. It is a figure explaining sectional drawing of the advancing direction of the light of the semiconductor Mach-Zehnder optical modulator which concerns on one Embodiment of this invention. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining the semiconductor Mach-Zehnder optical modulator based on one Embodiment of this invention, (a) is a top view, (b) And (c) is sectional drawing. It is a figure explaining the semiconductor Mach-Zehnder optical modulator based on one Embodiment of this invention. It is a figure explaining the semiconductor Mach-Zehnder optical modulator based on one Embodiment of this invention. It is a figure explaining the semiconductor Mach-Zehnder optical modulator based on one Embodiment of this invention. It is a figure explaining the semiconductor Mach-Zehnder optical modulator based on one Embodiment of this invention. It is a figure explaining the semiconductor Mach-Zehnder optical modulator based on one Embodiment of this invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or similar reference numerals indicate the same or similar elements. Therefore, repeated description of the same or similar elements is omitted.
(First embodiment)
The optical modulator according to the present embodiment is provided with an inductor adjustment unit in an electrode configuration as a mechanism for adjusting impedance matching and speed matching based on a conventional traveling wave type semiconductor Mach-Zehnder modulator. A voltage modulator is realized.

  FIG. 4 shows the configuration of the optical modulator of this embodiment. 4A is a top view, and FIGS. 4B, 4C, 4D, and 4E are cross-sectional views. As shown in FIG. 4A, the optical modulator of this embodiment includes an input waveguide 201, an optical demultiplexer 202 that demultiplexes light guided through the input waveguide 201, and an optical Waveguides 203 and 204 through which the light demultiplexed by the demultiplexer 202 is guided, an optical multiplexer 209 that multiplexes the light guided through the two waveguides 203 and 204, and an optical multiplexer 209, respectively. And an output waveguide 210 that outputs the light combined at. 4 includes a high-frequency signal source 211, traveling-wave electrodes (electrodes 205 and 206, and 207 and 208) connected to the high-frequency signal source 211, and waveguides 203 and 204 with a bias voltage. , And electrodes 212 and 213.

  The traveling wave type electrode connected to the high frequency signal source 211 forms a coplanar strip line, and adjusts the linear portions 205 and 207 in the light propagation direction on the waveguides 203 and 204, respectively, and impedance matching and velocity matching. Inductor adjustment units (also referred to as impedance / speed adjustment units) 206 and 208 are provided. By applying a voltage to the straight line portions 205 and 207, an electric field is generated in each of the waveguides 203 and 204, resulting in a change in refractive index, resulting in a change in light phase. The straight line portion 205 and the inductor adjustment unit 206 are electrically connected. Similarly, the straight line portion 207 and the inductor adjustment unit 208 are electrically connected.

  FIG. 4B is a cross-sectional view of the straight portions 205 and 207 of the traveling wave electrode. 4C to 4D are cross-sectional views of the inductor adjusting units 206 and 208. FIG. FIG. 4E is a cross-sectional view of a modified example of the inductor adjustment units 206 and 208 shown in FIG. 4B to 4E, the optical modulator includes an n-type semiconductor layer 215, a lower cladding layer 216, a core layer 217, and an upper portion, which are sequentially formed on the semiconductor substrate 214. A cladding layer 218 or 220. 4B to 4D, the optical modulator includes a dielectric or semiconductor 219 formed on a semiconductor substrate 214. In FIG. 4E, an air layer is used as an alternative to the dielectric or semiconductor 219 in FIG.

  As shown in FIG. 4B, the straight line portions 205 and 207 of the traveling wave electrode are formed on the cladding layer 218 at a position where the center of the straight line portion coincides with the center of the core layer 217. Further, as shown in FIGS. 4C to 4E, the inductor adjustment units 206 and 208 are arranged such that the center of the inductor adjustment unit does not coincide with the center of the core layer 217 (a part or all of the inductor adjustment unit is At a position that does not overlap with the core layer 217), it is formed on the cladding layer 220, the dielectric or semiconductor 219, or the air layer.

  The operation principle of the optical modulator of this embodiment will be described. The signal light first enters from the input waveguide 201, is branched into two by the optical demultiplexer 202, is guided to the waveguides 203 and 204, and a voltage is applied by the linear portions 205 and 207 of the traveling wave electrode. In the semiconductor core layer 217, a change in refractive index occurs due to the electro-optic effect, and as a result, the phase of light changes (FIG. 4B). Due to the phase difference between the two arms (waveguides), the output light intensity combined and output by the optical multiplexer 209 changes. This is the operating principle of the MZ modulator.

  Next, the effect of traveling wave type electrodes on impedance matching and speed matching by introducing the inductor adjusting mechanisms 206 and 208 will be described. For example, in the case of widening the band, it can be said that not only impedance matching and velocity matching but also the band is proportional to the loss, so it is necessary to reduce the propagation loss in the traveling wave type electrode. It is necessary to take it widely. For example, if the distance between two core layers is 20 μm, the thickness of the core layer is 1 μm, the width is 2 μm, and the widths of the electrodes 206 and 208 are 20 μm, the electrode width should be 100 μm to reduce the propagation loss at that time to about 2/3 It is necessary to do it above. However, when the electrode width is set to 100 μm or more, the impedance is 40Ω or less along with it, and the band is deteriorated because it is largely deviated from 50Ω. Further, at this time, in terms of speed matching, a situation occurs in which the speed of electricity is about 3/2 times faster than the speed of light, and the band is degraded due to speed mismatch. For this reason, it has been impossible to obtain the advantage of increased bandwidth due to a reduction in propagation loss with the conventional structure.

  Here, the impedance and speed of the traveling wave electrode can be expressed as follows.

In other words, as described above, when the electrodes of the modulation part (straight line parts 205, 207) are determined, the inductance L and the capacitance C are determined, and a situation of low impedance and high speed is born. If the inductance Lload can be loaded by the inductor adjusting units 206 and 208,

It can be expressed as. That is, impedance matching and speed matching can be achieved by loading a desired inductance Lload when the impedance is too low and the speed is too high. As a result, the propagation loss can be reduced, impedance matching, and speed matching can all be improved, so that the bandwidth can be increased. Further, by increasing the electrode length, the band can be narrowed, but it is also possible to operate at a low driving voltage.

  Compared to the modulator composed of only the straight-line portions 205 and 207 of the traveling wave electrode, the modulator loaded with the inductors 206 and 208 as described above is required to realize impedance matching and velocity matching. It is necessary to have a larger inductance. For this purpose, the inductance portions 206 and 208 (impedance / speed adjusting portion) to be loaded must have a larger inductance than the straight portions 205 and 207.

  The inductance Lload can be greatly changed by adjusting the number, size, and configuration of the inductor adjustment portions 206 and 208. FIG. 4 shows a loop type inductor as an example. Here, the loop type inductor has a structure in which the traveling type electrodes are arranged in a loop shape (a ring shape including a half ring shape) like the inductor adjustment portions 206 and 208 in FIG. Thereby, an inductor larger than the straight portions 205 and 207 of the progressive electrode can be realized. The inductor adjusting portions 206 and 208 are not formed on the waveguide constituted by 216, 217, and 218 but on the dielectric or semiconductor 219 arranged on the side thereof or in the air (on the air layer). For example, when a larger inductance is required, a larger inductance can be loaded by simply increasing the number of the inductor adjusting portions 206 and 208 per unit length. Although only one loop is shown in the inductor adjustment portions 206 and 208 in FIG. 4, a larger inductance is loaded by creating a plurality of loop structures in this portion or increasing the distance and size of the loop. be able to.

  In order to load a larger inductance, traveling wave type electrodes such as a spiral type formed in a spiral shape on the same plane as shown in FIG. 6 or a meander type consisting of a plurality of folded portions as shown in FIG. Other inductor configurations can be used.

  On the other hand, if a small inductance is required, the number of inductor loads may be reduced or the size may be reduced, contrary to the above.

  In the configuration of the present embodiment, in order to eliminate the interaction between the inductor adjustment portions 206 and 208, the inductor adjustment portion 206 is directed upward, the inductor adjustment portion 208 is directed downward, and they are separated from each other. The design may be made in consideration of the interaction (mutual inductance) by bringing the adjustment portion 206 downward and the inductor adjustment portion 208 upward.

  Note that the lengths of the linear portions 205 and 207 of the traveling wave electrode and the lengths of the inductor adjustment portions 206 and 208 are sufficiently smaller than the wavelength of the high-frequency signal so that they can be regarded as lumped constants.

  Although omitted in the above for the sake of simplification, the capacitor is actually loaded simultaneously with the loading of the inductor. Considering an equivalent circuit model as shown in FIG. 5 in order to express the effect of loading the inductor more accurately, the impedance and speed of the traveling wave electrode can be expressed as follows.

  That is, it can be seen that the situation where the impedance is too low and the speed is too high can be realized by adjusting the inductance Lload and the capacitance Cload. In the case of the above example (when the impedance is 40Ω or less and the speed of electricity is about 3/2 times faster than the speed of light), it is necessary to effectively load the inductance component Lload from the formula. This indicates that it is effective to reduce the capacitance component.

  In order to significantly reduce the capacitance of the impedance / velocity adjustment unit (206, 208) compared to the modulation unit, the impedance / velocity adjustment unit is formed at a position higher than the straight portions 205, 207 (for example, with a thickness of It is effective to fabricate on a certain dielectric medium) and on a low dielectric medium such as BCB or polyimide or on air. This is because capacitance is expressed by dielectric constant × cross section / dielectric medium height. Since the change that occurs in the inductance is the direction in which the inductance, which is a desired direction, increases, it can be said that it is effective to increase the dielectric medium. Thus, by selecting the type of dielectric medium and the height of the dielectric medium, it is possible to adjust the loading amounts of the inductance and the capacitance to desired values. For example, the position of the impedance / speed adjusting unit (206, 208) can be formed to be about 10 μm higher than the position of the straight line portion (205, 207), but how much inductance and capacitance are loaded. It is desirable to change depending on the situation. Further, if the inductance per unit length to be loaded is increased and the capacitance per unit length is controlled to be small, the length of the adjustment unit can be shortened, leading to a reduction in loss.

  Note that there is generally a dielectric medium such as BCB on the side (side) of the waveguide portion. However, in order to effectively load inductance (reduce capacitance), the side of the waveguide is air. It is desirable that

Next, description will be made with reference to the cross-sectional view of FIG.
In the optical modulator according to this embodiment, an n-InP layer 215, a lower cladding layer 216 made of InP, and a non-doped semiconductor core layer 217 are sequentially stacked on an SI-InP substrate 214. The upper clad layer 218 is formed of a conductive clad layer (for example, a p-InP layer) in the cross-sectional view of FIG. 4B, that is, below the straight portions 205 and 207 of the traveling wave electrode. The semiconductor core layer 217 functions as an optical waveguide layer. For example, a material system such as InGaAsP or InGaAlAs is used, and the semiconductor core layer 217 is configured by 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 having a band gap higher and lower than that of the multiple quantum well layer and a lower band gap than the upper and lower cladding layers.

  In the cross-sectional view of FIG. 4D, that is, a portion where the straight-line portions 205 and 207 of the traveling wave electrode are not present (portions where the inductor adjustment portions (impedance / speed adjustment portions) 206 and 208 are formed). A non-doped cladding layer (eg, non-doped InP) is used. This is because the conductive cladding layer 218 reduces light loss and provides electrical isolation. Between the cross-sectional views (b) and (d) is as shown in the cross-sectional view (c), and the upper cladding layer 220 is also a non-doped cladding layer.

  It should be noted that the dielectric or the material of the semiconductor 219 greatly affects the impedance, and an insulator having a low dielectric constant is desirable in order to make the inductor adjusting portions 206 and 208 work effectively. An example of the material is non-doped InP. As another material, a material having a low dielectric constant such as BCB (benzocyclobutene) is more desirable. In addition, polyimide, epoxy, and various polymers can be used. A semi-insulating semiconductor such as GaAs may be used.

  Further, air having a low dielectric constant and excellent insulation can be used as the dielectric or semiconductor 219. That is, a structure in which the inductor adjusting portions 206 and 208 are floated in the air (the dielectric or the semiconductor 219 is eliminated) as shown in the sectional view (e) may be used.

  FIG. 8 is a diagram for explaining a cross section in the light traveling direction of the optical modulator according to the present embodiment. As shown in the figure, below the straight portions 205 and 207 of the traveling waveform electrode, the upper cladding layer 218 is formed of a conductive cladding layer (for example, a p-InP layer), and is a portion of the traveling waveform electrode that is not the straight portions 205 and 207. In the portion where the inductor adjusting portions 206 and 208 are formed, the upper cladding layer 220 is a non-doped cladding layer (for example, non-doped InP).

  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.

  In FIG. 4, each of the two arms (waveguides) of the MZ modulator has three linear portions 205 and 207 of traveling waveform electrodes, and two inductor adjustment portions 206 and 208 (impedance / speed adjustment portions). Although these are arranged one by one, these numbers are not limited to the above.

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

  One of the upper cladding layer and the lower cladding layer in FIG. 4 may be an n-type semiconductor and the other may be a p-type semiconductor. On the other hand, both the upper clad layer and the lower clad layer are n-type semiconductors, and a third p-type clad layer is inserted between the upper clad layer and the core layer or between the lower clad layer and the core layer. It can also be.

  As described above, according to the present embodiment, it is possible to reduce the propagation loss of the transmission line of the optical modulator, improve the impedance matching and the speed matching between electricity and light, and to widen the bandwidth. Further, by adjusting the electrode length, it is possible to realize a low driving voltage operation or a wider band operation.

  For example, by increasing the electrode length, a low driving voltage operation is possible although a narrow band occurs. On the other hand, by shortening the electrode length, a high driving voltage can be obtained, but a wide band can be obtained.

(Second embodiment)
The optical modulator according to the present embodiment is provided with a mechanism for adjusting impedance matching and speed matching based on a conventional differentially driven traveling wave type semiconductor Mach-Zehnder modulator (FIG. 2) in the electrode configuration, thereby providing a wider band than the conventional one. Alternatively, a modulator with a low driving voltage is realized.

  FIG. 9 shows the configuration of the optical modulator of this embodiment. FIG. 9A is a top view, and FIGS. 9B and 9C are cross-sectional views. As shown in FIG. 9A, the optical modulator of this embodiment includes an input waveguide 201, an optical demultiplexer 202 that demultiplexes the light guided through the input waveguide 201 into two, Waveguides 203 and 204 through which the light demultiplexed by the demultiplexer 202 is guided, an optical multiplexer 209 that multiplexes the light guided through the two waveguides 203 and 204, and an optical multiplexer 209, respectively. And an output waveguide 210 that outputs the light combined at. 9 includes traveling wave electrodes (electrodes 205 and 206, and 207 and 208) connected to the high-frequency signal sources 221 and 222, and ground electrodes 223, 224, and 225, respectively. A traveling wave electrode (signal electrode) formed from the electrodes 205 and 206 is sandwiched between ground electrodes 223 and 225. Similarly, a traveling wave electrode (signal electrode) formed from the electrodes 207 and 208 is sandwiched between the ground electrodes 224 and 225. The other end of each traveling wave type electrode is connected to a 50Ω termination resistor.

  Two traveling wave electrodes connected to the high-frequency signal sources 221 and 222 respectively form a differential drive type transmission line, and linear portions 205 and 207 in the light propagation direction on the waveguides 203 and 204, respectively. Impedance / speed adjusting units (inductor adjusting units) 206 and 208 for adjusting impedance matching and speed matching are provided. The straight portions 205 and 207 apply a voltage to the light propagating through the waveguides 203 and 204, respectively. The straight line portion 205 and the impedance / speed adjusting unit 206 are electrically connected. Similarly, the straight line portion 207 and the impedance / speed adjusting unit 208 are electrically connected.

  In the single-ended drive type optical modulator of FIG. 4, a single-ended signal from a single signal source is connected to two traveling wave electrodes. Driving is performed so as to interact by applying voltages of mutually opposite phases between traveling wave electrodes (push-pull driving). Due to the interaction, the distance between the traveling wave electrodes is usually about 20 to 40 μm. On the other hand, in the optical modulator of this embodiment, one signal source is connected to each traveling wave electrode, and the two traveling wave electrodes are controlled independently. Push-pull driving is performed by inputting mutually opposite-phase voltages (differential signals) between the traveling wave electrodes. The interval between the traveling wave electrodes is about 100 μm, which is wider than the interval between the traveling wave electrodes in the single-end drive type optical modulator.

  FIG. 9B is a cross-sectional view of the straight portions 205 and 207 of the traveling wave electrode. FIG. 9C is a cross-sectional view of the impedance / speed adjusting units 206 and 208. 9B and 9C, the optical modulator includes a lower cladding layer 216, a core layer 217, and an upper cladding layer 218 or 220 that are sequentially formed on the semiconductor substrate 214. Prepare. Further, as shown in FIGS. 4B and 4C, the optical modulator includes a dielectric or semiconductor 219 formed on the semiconductor substrate 214. The dielectric layer 219 can be a low dielectric medium such as BCB or polyimide, or an air layer. The height of the dielectric or semiconductor 219 is higher than the height of the core layer 217, and the position of the impedance / speed adjusting unit (206, 208) is about 10 μm higher than the position of the straight line portion (205, 207). ing.

  As shown in FIG. 9B, the straight line portions 205 and 207 of the traveling wave electrode are formed on the cladding layer 218 at a position where the center of the straight line portion coincides with the center of the core layer 217. Further, as shown in FIG. 9C, the impedance / speed adjusting units 206 and 208 are located at positions where the center of the impedance / speed adjusting unit does not coincide with the center of the core layer 217 (a part of the impedance / speed adjusting unit or The dielectric layer is formed on the dielectric or semiconductor 219 or on the air layer at a position where it does not overlap with the core layer 217. As described with reference to the model of FIG. 4 in the above embodiment, by forming the impedance / speed adjusting units 206 and 208 at a position higher than the height of the core layer 217, the capacitance is reduced and the inductance is effectively reduced. Can be loaded.

  Also, as shown in FIGS. 10 and 11, in order to load a larger inductance, the structure of the traveling wave type electrode impedance / velocity adjusting portion (306, 308, 406, 408) is formed in a spiral shape. It is also possible to adopt another inductor configuration such as a meander type composed of a plurality of folded portions.

  Also in this embodiment, as described with reference to FIG. 8, one of the upper cladding layer and the lower cladding layer may be an n-type semiconductor and the other may be a p-type semiconductor. On the other hand, both the upper clad layer and the lower clad layer are n-type semiconductors, and a third p-type clad layer is inserted between the upper clad layer and the core layer or between the lower clad layer and the core layer. It can also be. Various materials may be used for various materials as in the above embodiment.

(Third embodiment)
Similarly to the optical modulator according to the above-described embodiment, the optical modulator according to this embodiment is provided with a mechanism for adjusting impedance matching and speed matching in the electrode configuration, so that a modulator having a wider band or a lower driving voltage than the conventional one can be obtained. It is realized. The optical modulators shown in FIGS. 12 to 14 correspond to the optical modulators shown in FIGS. Unlike the ground electrodes 223, 224, and 225 in the optical modulators of FIGS. 9 to 11, in the optical modulators of FIGS. 12 to 14, the shape of the ground electrode is changed to correspond to the shape of the inductor adjustment portion of the traveling wave electrode. ing. FIGS. 12 to 14 are examples in which the distance between the “linear portion of the traveling wave electrode” and the ground and the distance between the “inductor adjustment portion of the traveling wave electrode” and the ground are substantially constant.

  In the optical modulator shown in FIG. 12, the traveling wave type electrodes (205 and 206, and 207 and 208) have the same shape as the traveling wave type electrode of the optical modulator shown in FIG. 233, 234 and 235) are different from the shape of the ground electrode (223, 224 and 235) of the optical modulator shown in FIG. The shape of the ground electrode on the side of the traveling wave type electrode sandwiching the traveling wave type electrode responds to the shape of the traveling wave type electrode and has irregularities in the direction perpendicular to the light guiding direction of the waveguide (203, 204). ing. The distance between the linear portion 205 (207) of the traveling wave electrode and the ground electrode 233 (234) is equal to the distance between the linear portion 205 (207) of the traveling wave electrode and the ground electrode 235, and the impedance of the traveling wave electrode The distance between the speed adjusting unit 206 (208) and the ground electrode 233 (234) is equal to the distance between the impedance / speed adjusting unit 206 (208) of the traveling wave electrode and the ground electrode 235. In the optical modulator of FIG. 12, the distance between the traveling wave electrode impedance / velocity adjusting unit 206 (208) and the ground electrode 233 (234) is the same as that of the traveling wave electrode linear portion 205 (207) and the ground electrode. It is also equal to the distance to 233 (234). However, as will be described below, the distance between the straight portion 205 (207) of the traveling wave electrode and the ground electrode 233 (234), the impedance / speed adjusting unit 206 (208) of the traveling wave electrode, and the ground electrode 233 The distance from (234) may be adjusted individually. Similarly, the distance between the straight line portion 205 (207) of the traveling wave electrode and the ground electrode 235 and the distance between the impedance / speed adjusting unit 206 (208) of the traveling wave electrode and the ground electrode 235 are individually adjusted. May be.

By adjusting the distance between the straight part of the traveling wave electrode and the ground electrode that sandwiches it, and the distance between the impedance / velocity adjustment unit (inductor adjustment unit) of the traveling wave electrode and the ground electrode that sandwiches this, impedance and speed Can be adjusted. For example, the impedance can be increased by increasing the distance between the inductor adjustment portion of the traveling wave electrode and the ground electrode sandwiching the inductor adjustment portion, and the impedance can be decreased by decreasing the distance.
Similarly, the distance between the straight line portion 205 (207) of the traveling wave electrode and the ground electrode 235 and the distance between the impedance / velocity adjusting unit 206 (208) of the traveling wave electrode and the ground electrode 235 may be individually adjusted. Good. That is, the optical modulator of FIG. 12 is an example in which the distance from the ground electrode is the same regardless of the position perpendicular to the traveling direction of the light of the traveling wave electrode impedance / velocity adjusting unit 206 (208). Although it is not necessary to make the distances equal in any (all) locations, the distances in the individual locations perpendicular to the light traveling direction of the traveling wave electrode impedance / velocity adjusting unit 206 (208) are not equal. Alternatively, the distances may be equal on average (impedance / speed adjustment unit 206 (208) is located between the two ground electrodes on average). Similarly, the distance between the linear portion 205 (207) of the traveling wave electrode and the ground electrode 233 (234) and the distance between the linear portion 205 (207) of the traveling wave electrode and the ground electrode 235 are all the locations. It is not necessary to make the distances equal.

  In the optical modulator shown in FIG. 13, the traveling wave type electrodes (205 and 306, and 207 and 308) have the same shape as the traveling wave type electrode of the optical modulator shown in FIG. 323, 324, and 325) are different from the shape of the ground electrodes (223, 224, and 225) of the optical modulator shown in FIG. The impedance / velocity adjustment unit (306, 308) of the traveling wave electrode is a spiral type, and the distance from the ground electrode 323 (324) is individually adjusted to load a larger inductance. The same applies to the ground electrode 325. Similar to the optical modulator shown in FIG. 12, in the optical modulator shown in FIG. 13, the shape on the traveling wave electrode side of the ground electrode sandwiching the traveling wave electrode responds to the spiral shape portion of the traveling wave electrode. In addition, there are irregularities in the direction perpendicular to the light guiding direction of the light in the waveguides (203, 204). The average distance between the linear portion 205 (207) of the traveling wave electrode and the ground electrode 323 (324) is equal to the average distance between the linear portion 205 (207) of the traveling wave electrode and the ground electrode 325. The average distance between the impedance / velocity adjusting unit 306 (308) of the traveling wave electrode and the ground electrode 323 (324) is the average of the impedance / velocity adjusting unit 306 (308) of the traveling wave electrode and the ground electrode 325. Equal to the distance. It should be noted that the average distance between the impedance / velocity adjusting unit 306 (308) of the traveling wave electrode and the ground electrode 323 (324) is the distance between the straight portion 205 (207) of the traveling wave electrode and the ground electrode 323 (324). May be equal to the distance. The same applies to the ground electrode 325.

  In the optical modulator shown in FIG. 14, the traveling wave electrodes (205 and 406 and 207 and 408) have the same shape as the traveling wave electrode of the optical modulator shown in FIG. The shape of the electrodes (423, 424 and 425) is different from the shape of the ground electrodes (223, 224 and 225) of the optical modulator shown in FIG. The impedance / velocity adjusting unit (406, 408) of the traveling wave type electrode is a meander type, and a larger inductance is loaded by individually adjusting the distance from the ground electrode 423 (424). The same applies to the ground electrode 425. Similar to the optical modulator shown in FIG. 12, in the optical modulator shown in FIG. 14, the shape of the traveling wave electrode side of the ground electrode sandwiching the traveling wave electrode responds to the meandering portion of the traveling wave electrode. In addition, there are irregularities in the direction perpendicular to the light guiding direction of the light in the waveguides (203, 204). The distance (or average distance) between the linear portion 205 (207) of the traveling wave electrode and the ground electrode 423 (424) is the distance between the linear portion 205 (207) of the traveling wave electrode and the ground electrode 425 (or Equal to the average distance). The distance (or average distance) between the traveling wave electrode impedance / velocity adjusting unit 306 (308) and the ground electrode 323 (324) is equal to the traveling wave electrode impedance / velocity adjusting unit 306 (308) and the ground electrode. Equal to the distance (or average distance) from 325. The distance (or average distance) between the impedance / velocity adjustment unit 406 (408) of the traveling wave electrode and the ground electrode 423 (424) is equal to the straight portion 205 (207) of the traveling wave electrode and the ground electrode 423. It may be equal to the distance to (424). The same applies to the ground electrode 425.

DESCRIPTION OF SYMBOLS 101 Input optical waveguide 102 Optical demultiplexer 103, 104 Optical waveguide 105, 106 Traveling wave type electrode 107 Optical multiplexer 108 Output optical waveguide 109, 121, 122 High frequency signal source 110, 111 Bias electrode 112 Semiconductor substrate 113 n-InP Layer 114 Lower cladding layer 115 Core layer 116 Upper cladding layer 123, 124, 125 Ground electrode 201 Input optical waveguide 202 Optical demultiplexer 203, 204 Optical waveguide 205, 207 Electrodes 206, 208, 306, 308, 406 of the voltage application unit , 408 Inductor adjustment portion electrode 209 Optical multiplexer 210 Output optical waveguide 211, 221, 222 High frequency signal source 212, 213 Bias electrode 214 Semiconductor substrate 215 n-InP layer 216 Lower cladding layer 217 Core layer 218 Upper cladding layer 219 Air Invite Electrical body or semiconductor 223, 224, 225, 233, 234, 235 Ground electrode 323, 324, 325, 423, 424, 425 Ground electrode

Claims (13)

First and second optical waveguides having an optical waveguide formed by sequentially laminating a first conductive semiconductor clad layer, a non-doped semiconductor core layer, and a second conductive semiconductor clad layer on a semiconductor substrate A semiconductor Mach-Zehnder modulator comprising a first traveling wave electrode and a second traveling wave electrode formed on the first optical waveguide and the second optical waveguide, respectively, wherein a Mach-Zehnder interferometer is formed by the waveguide. Because
Each of the first traveling wave electrode and the second traveling wave electrode is
A first region formed with a first inductance and a first element length;
A second region formed with a second inductance and a second element length, the second region electrically connected to the first region, and
The semiconductor Mach-Zehnder modulator characterized in that the inductance of the second region is larger than the inductance of the first region, and the element lengths of the first region and the second region are sufficiently smaller than the operating frequency. .   2. The semiconductor Mach-Zehnder modulator according to claim 1, wherein the first traveling wave electrode and the second traveling wave electrode form a coplanar strip line. A ground electrode sandwiching the first traveling wave electrode and the second traveling wave electrode;
2. The semiconductor Mach-Zehnder modulator according to claim 1, wherein differential signals are input to the first traveling wave electrode and the second traveling wave electrode. At least a portion corresponding to the second region on the first traveling wave electrode side of the ground electrode and a portion corresponding to the second region on the second traveling wave electrode side of the ground electrode One shape has changed in response to the shape of the corresponding second region,
The distance between the second region of the first traveling wave electrode and the second traveling wave electrode and the ground electrode sandwiching the second region, or the first traveling wave electrode And the average distance between the second region of the second traveling wave electrode and the ground electrode sandwiching the second region is
The distance between the first region of the first traveling wave electrode and the second traveling wave electrode and the ground electrode sandwiching the first region is substantially the same. The semiconductor Mach-Zehnder modulator according to claim 3. The first region of each of the first traveling wave electrode and the second traveling wave electrode is formed with the first inductance, the first element length, and a first capacitance;
The second region of each of the first traveling wave electrode and the second traveling wave electrode is formed with the second inductance, the second element length, and a second capacitance,
5. The semiconductor Mach-Zehnder modulator according to claim 1, wherein a capacitance of the second region is smaller than a capacitance of the first region.   6. The first traveling wave electrode and the second traveling wave electrode each include a plurality of sets of the first region and the second region, respectively. The semiconductor Mach-Zehnder modulator described. The first region of the first traveling wave electrode is formed on the first optical waveguide;
7. The semiconductor Mach-Zehnder modulator according to claim 1, wherein the first region of the second traveling wave electrode is formed on the second optical waveguide.   The second conductive semiconductor clad layer of the first optical waveguide and the second optical waveguide in a region between two adjacent first regions is a non-doped semiconductor clad layer. The semiconductor Mach-Zehnder modulator according to claim 6 or 7. The second region portion of the first traveling-wave electrode is a dielectric, an insulating semiconductor, a semi-insulating semiconductor formed on the first optical waveguide or beside the first optical waveguide, or Formed on the air layer,
The second region of the second traveling wave electrode is a dielectric formed on the second optical waveguide or beside the second optical waveguide, an insulating semiconductor, a semi-insulating semiconductor, or 9. The semiconductor Mach-Zehnder modulator according to any one of 1 to 8, which is formed on an air layer.   The second region portion of the first traveling wave electrode and the second traveling wave electrode is the first region portion of the first traveling wave electrode and the second traveling wave electrode. 10. The semiconductor Mach-Zehnder modulator according to 9, wherein the semiconductor Mach-Zehnder modulator is formed at a higher position.   One of the first conductive semiconductor clad layer and the second conductive semiconductor clad layer is an n-type semiconductor, and the other is a p-type semiconductor. The semiconductor Mach-Zehnder modulator described.   Both the first conductive semiconductor clad layer and the second conductive semiconductor clad layer are n-type semiconductors, between the non-doped semiconductor core layer and the first conductive semiconductor clad layer, and the non-doped semiconductor. The p-type third conductive semiconductor clad layer is inserted in at least one of the core layer and the second conductive semiconductor clad layer. The semiconductor Mach-Zehnder modulator described.   13. The semiconductor Mach-Zehnder modulator according to claim 1, wherein at least a part of the non-doped semiconductor core layer has a multiple quantum well layer structure.

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