JP2018128506A - Light modulator - Google Patents

Abstract

An optical modulator having excellent high frequency characteristics and good waveform quality is provided. Two RF electrodes to which a pair of differential electrical signals are applied, two first conductive semiconductor layers to which the two RF electrodes are connected, and the two first conductive semiconductors An optical modulator comprising: a second conductive semiconductor layer formed between the layers and connected to a fixed potential; and two optical waveguides formed at a pn junction boundary between the first and second conductive semiconductor layers. The optical modulator may further include a plurality of conductive regions that are spaced apart along the RF electrode and capacitively coupled to the RF electrode. [Selection] Figure 6

Description

  The present invention relates to an optical modulator used in an optical communication system and an optical information processing system, and more particularly to a structure for providing an optical modulator having excellent frequency characteristics when performing an optical modulation operation with a high-speed electrical signal. .

  A Mach-Zehnder (MZ) type optical modulator splits incident light into two optical waveguides with an intensity of 1: 1, propagates the branched light for a certain length, combines the light after phase modulation. Has a basic structure to output. By changing the phase of the two lights relative to each other by the modulated electric signal applied to the phase modulation unit provided in each of the two branched optical waveguides, the interference condition of the light is changed, The intensity and phase of the output light can be modulated.

As a material constituting the optical waveguide, a dielectric such as LiNbO ₃ or a semiconductor such as InP, GaAs, or Si is used, and a modulated electric signal is input to an electrode disposed in the vicinity of these optical waveguides to apply a voltage to the optical waveguide. By applying this, the phase of light propagating through the optical waveguide is changed. As the principle of changing the phase of light, LiNbO ₃ mainly uses the Pockels effect, InP and GaAs mainly use the Pockels effect and the quantum confined stark effect (QCSE), and Si mainly uses the carrier plasma effect. It is done.

  In order to perform optical communication with high speed and low power consumption, an optical modulator having a high modulation speed and a low driving voltage is required. In order to perform optical modulation with a voltage amplitude of several volts at a high speed of 10 Gbps or higher, the high-speed modulated electric signal and the speed of light propagating in the optical waveguide are matched, and the interaction is performed while propagating. A wave electrode is required. An optical modulator using a traveling wave electrode having an electrode length of several millimeters to several tens of millimeters has been put into practical use. (For example, Non-Patent Document 1)

  A traveling wave electrode optical modulator requires an electrode structure and an optical waveguide structure with low loss and low reflection so that the electrical signal propagating through the electrode and the light propagating through the waveguide can interact with each other without reducing the intensity. It is done.

Further, the MZ type optical modulator includes a Si optical modulator in which an optical waveguide is made of Si. The Si optical modulator is composed of an SOI (Silicon on Insulator) substrate in which a Si thin film is pasted on an oxide film (BOX) layer obtained by thermally oxidizing the surface of the Si substrate. After processing into a thin wire, a dopant is implanted so as to become a p-type / n-type semiconductor, and SiO _{2 serving} as a light cladding layer is deposited and electrodes are formed. At this time, it is necessary to design and process the optical waveguide so as to reduce the optical loss. The p-type / n-type doping and the electrode fabrication suppress the generation of the optical loss and reflect the high-speed electric signal. In addition, it is necessary to design and process to minimize loss.

(Cross-section structure of optical waveguide)
FIG. 1 shows a cross-sectional structure diagram of an optical waveguide which is the basis of a conventional Si optical modulator. In FIG. 1, light is assumed to propagate in the direction perpendicular to the paper surface. The optical waveguide of this Si optical modulator is composed of the Si layer 2 sandwiched between the upper and lower SiO ₂ cladding layers 1 and 3, and the Si thin wire for confining the light in the center of FIG. It has a cross-sectional structure called a waveguide.

An optical waveguide 7 that confines light propagating in the direction perpendicular to the paper surface by using the difference in refractive index with the surrounding SiO ₂ cladding layers 1 and 3 using the thick Si layer 201 at the center of the Si layer 2 in FIG. Configure.

  In order to realize the light modulation function, a high-concentration p-type semiconductor layer 211 and a high-concentration n-type semiconductor layer 214 are provided in the slab regions 202 on both sides of the optical waveguide 7. A pn junction structure is formed by the p-type semiconductor layer 212 and the medium-concentration n-type semiconductor layer 213, and a modulated electric signal and a bias are applied from the left and right ends of FIG. The pn junction structure formed by the medium-concentration p-type semiconductor layer 212 and the medium-concentration n-type semiconductor layer 213 may be a pin structure having an i-type (intrinsic) semiconductor that is not doped therebetween.

  Although not shown in FIG. 1, metal electrodes connected to the high-concentration semiconductor layers 211 and 214 at both ends are provided, and a reverse bias electric field (in FIG. To the left) is applied to change the carrier density inside the optical waveguide core 201 (carrier plasma effect), and the optical phase can be modulated by changing the refractive index of the optical waveguide.

  Since the waveguide dimension depends on the refractive index of the core / cladding material, it cannot be uniquely determined. However, the rib-type silicon waveguide structure including the optical waveguide core portion 201 and the slab regions 202 on both sides as shown in FIG. As an example, the waveguide core width is 400 to 600 (nm) × height 150 to 300 (nm) × slab thickness 50 to 200 (nm) × length number (mm).

(Single electrode structure MZ type Si optical modulator)
FIG. 2 is a plan view of a Si optical modulator constituting an MZ type modulator having a structure called a conventional single electrode, and FIG. 3 is a sectional view taken along the line AA ′ of FIG. (For example, see Non-Patent Document 3)

  In the plan view of FIG. 2, the light input from the left side is branched into two optical waveguides 7 a and 7 b, between the two upper and lower RF electrodes 5 a and 5 b arranged in parallel as traveling wave electrodes and the central DC electrode 6. And phase modulation by a modulated electric signal (RF signal, differential electric signal) applied as a pair of differential signals to the RF electrodes 5a and 5b. Thereafter, the two phase-modulated optical signals are combined and interference-coupled, and light is output from the right end to constitute a single electrode structure MZ type Si optical modulator.

  In the cross-sectional structure diagram of FIG. 3 (corresponding to the AA ′ portion of FIG. 2), the Si layer 2 has a basic structure in which two optical waveguides having the same cross-sectional structure as FIG. There are two high-frequency lines (RF electrodes 5a and 5b) for inputting a pair of differential modulation electric signals (RF signals) on both sides, and a DC electrode 6 for applying a common bias voltage between them. Is provided.

  Two optical waveguides 7a and 7b are provided between the two RF electrodes 5a and 5b with the DC electrode 6 interposed therebetween, and a pn junction structure is formed symmetrically in the optical waveguides 7a and 7b. Yes. The RF electrodes 5a and 5b are respectively connected to the high-concentration p-type semiconductor layer 211 of the Si layer 2 via one or a plurality of vias (connection electrodes). The same applies to the following, but vias (connection electrodes) are not shown in the plan view of FIG. 2 for the sake of simplicity.

  Similarly, the DC electrode 6 is connected to the central high-concentration n-type semiconductor layer 214. By applying a positive voltage to the DC electrode 6 with respect to the RF electrodes 5a and 5b, the left and right optical waveguides 7a and 7b are applied. A reverse bias can be applied to the pn junction.

  In the Si optical modulator having such a single electrode structure, the RF electrodes 5a and 5b and the DC electrode 6 are electrically independent by a reverse bias of a pn junction, and positive bias voltage application to the RF electrode is possible. No longer needed. This eliminates the need for a bias tee for applying a bias voltage superimposed on the RF electrode and a capacitor for a DC block installed between the driver IC and the RF electrode. Has merit.

  In this example, the RF electrodes 5a and 5b are connected to the p-type semiconductor layer, and the DC electrode 6 is connected to the n-type semiconductor layer. However, the conductivity type of the semiconductor layer is changed and the RF electrode is changed to the n-type semiconductor layer. The DC electrode may be connected to the p-type semiconductor layer.

  At this time, as a bias voltage applied to the DC electrode 6, a negative voltage can be applied to the RF electrode, whereby the pn junction can be reverse-biased.

  The same applies to the following, but without losing generality, when the p-type semiconductor layer or the n-type semiconductor layer is the first conductive semiconductor layer, the corresponding n-type semiconductor layer or p-type semiconductor layer is the second conductive type. It can be called a type semiconductor layer.

  According to such a notation, the MZ type optical modulator having the single electrode structure shown in FIGS. 2 and 3 includes two RF electrodes to which a pair of differential electrical signals are applied, and the two RF electrodes. Two first conductive semiconductor layers to which electrodes are connected, a second conductive semiconductor layer formed between the two first conductive semiconductor layers and connected to a fixed potential, and the first and second conductive layers It can be said that this is an optical modulator comprising two optical waveguides formed at the pn junction boundary of the type semiconductor layer.

In the conventional MZ type optical modulator having the single electrode structure shown in FIGS. 2 to 3, the RF electrodes 5a and 5b acting as traveling wave electrodes are provided on the SiO ₂ cladding layer 3 of the optical modulator substrate serving as a dielectric layer. In addition, a structure called a coplanar stripline (CPS line) is formed.

  FIG. 4 conceptually shows how a high-frequency electrical signal propagates in such a CPS line. In the CPS line, a pair of differential RF electric signals V + and V− are input to the two high-frequency electrodes 5a and 5b and propagated. A high-frequency signal propagating as a traveling wave through a CPS line is positively or negatively applied to a pair of differential lines due to Coulomb interaction when a dense portion of charges moves like a wave in a high-frequency propagation line electromagnetically. It can be understood by a model in which a dense portion of opposite charges is induced and moves in the same manner as a high-frequency signal. The CPS line is one of balanced lines in which positive and negative charge amounts are balanced by two electrodes that form a pair of differential lines.

Po Dong, Long Chen, and Young-kai Chen, "High-speed low-voltage single-drive push-pull silicon Mach-Zehnder modulators", OPTICS EXPRESS Vol.20, No.6, pp.6163-6169, 12 March 2012. R. Ding et al., "Design and characterization of a 30-GHz bandwidth low-power silicon traveling-wave modulator", Optics. Communication, vol. 321, pp. 124-133, 7 February 2014. David Patel, Samir Ghosh, Mathieu Chagnon, Alireza Samani, Venkat Veerasubramanian, Mohamed Osman, and David V. Plant, "Design, analysis, and transmission system performance of a 41 GHz silicon photonic modulator," OPTICS EXPRESS Vol.23, 14263- 142871, 22 May 2015

  In order to perform large-capacity optical communication, an optical modulator capable of modulating light at high speed is required. In order to perform high-speed optical modulation, frequency characteristics that can operate over a wide frequency band from several hundred kHz to several tens GHz are required. In particular, in order to increase the signal baud rate (baud rate: modulation rate) and perform larger capacity communication, the optical modulation band (EO band), which is the signal frequency characteristic of the optical signal with respect to the modulated electric signal, is widened. is important.

  There are mainly two factors that limit the EO band of the MZ type optical modulator. One is a mismatch between optical and electrical signal propagation speeds, and the other is a propagation loss of high-frequency signals.

  In the MZ type optical modulator, the phase of the optical signal propagating through the two optical waveguides is modulated by a pair of high-frequency electric signals propagating through the two traveling wave type RF electrodes provided on the optical waveguide. . For this reason, when the light signal propagation speed is different from the electrical signal propagation speed, a phase shift occurs gradually between the light and the electricity propagating through the modulator. This phase shift reduces the conversion efficiency from an electrical signal to an optical signal. In particular, in the high frequency region, the modulation efficiency drops significantly due to the phase shift. Therefore, it is necessary to make the electrical signal speed of the RF electrode and the optical signal speed of the optical waveguide sufficiently match.

  The MZ type optical modulator can be regarded as a kind of the above-described CPS transmission line from the viewpoint of propagation of a high-frequency electric signal. In an ideal transmission line, a high-frequency electric signal can be propagated without loss, but in an actual line, loss occurs due to resistance or scattering. Even in the MZ type optical modulator, the signal amplitude is reduced while the modulated electric signal propagates through the RF electrode, so that the EO band is deteriorated.

(Distributed constant circuit model of Si optical modulator)
FIG. 5 shows a distributed constant circuit model of the Si optical modulator. Based on this model, the electric velocity and propagation loss of the MZ type Si optical modulator will be described. Here, C _pn and R _pn of the transmission line element in FIG. 5 are the capacitance and resistance per unit length of the pn junction, R _tl is the resistance per unit length of the transmission line, L _tl is the inductance per unit length of the line, C _tl is a capacitance per unit length of the line other than the pn junction.

  This model represents a single-phase driving mode, but even in the case of differential driving, a similar model can be considered by extracting effective parameters for differential signals. The voltage amplitude of the differential signal is expressed as the amplitude on the signal line in the figure.

  When this model is used, the attenuation of the radio frequency (RF) electric signal of the Si optical modulator is expressed by the following formula (1) (Non-patent Document 2).

However,

It is.

Here, α on the left side of Equation (1) is RF signal attenuation (propagation loss, unit is Np / m), f is signal frequency, Z _{dev in} Equation (2) is characteristic impedance, and f _{rc in} Equation (3) is RC band, v _p in equation (4) is the propagation speed of the electrical signal.

In Equation (1), the first term on the right side represents the loss due to the resistance R _tl of the RF electrode, and the second term represents the loss due to the resistance R _{pn in the} pn doping region. R _{tl in} the first term is R _tl ˜√f due to the skin effect of the RF electrode, and is proportional to the square root of f, while the second term is proportional to f ² . Therefore, the second term determines the propagation loss of the electric signal in the high frequency region and becomes a factor that limits the EO band. To reduce this effect, the second term by utilizing the fact that depend on Z _dev, it is effective to reduce the Z _dev.

As a design for reducing Z _dev with a conventional CPS single electrode type Si optical modulator, it is considered to increase the effective C _tl by increasing the width of the RF electrode or reducing the distance between the two RF electrodes. It is done. However, when such a design is adopted, the effective L _tl decreases at the same time. For this reason, there is a problem that the electric speed v _p represented by the equation (4) increases, and the speed matching between light and electricity cannot be achieved.

Although speed matching can be achieved with a structure such as a capacitively loaded modulator (Non-patent Document 3), it is difficult to set Z _dev low due to a design restriction that the electrode area is small. It was.

  In order to achieve such an object, the present invention is characterized by having the following configuration.

(Structure 1 of the invention)
Between two RF electrodes to which a pair of differential electrical signals are applied, two first conductive semiconductor layers to which the two RF electrodes are connected, and between the two first conductive semiconductor layers An optical modulator comprising: a second conductive semiconductor layer formed and connected to a fixed potential; and two optical waveguides formed at a pn junction boundary of the first and second conductive semiconductor layers,
An optical modulator, further comprising a plurality of conductive regions that are spaced apart from each other along the RF electrode and capacitively coupled to the RF electrode.

(Configuration 2)
An optical modulator according to Configuration 1 of the invention,
The plurality of conductive regions are formed in a lower layer or an upper layer of the RF electrode,
At least one of the plurality of conductive regions is a conductive region having both a region where the RF electrode is disposed in an upper layer or a lower layer and a region where the RF electrode is not disposed in an upper layer or a lower layer, Light modulator.

(Structure 3 of the invention)
An optical modulator according to Configuration 1 or 2 of the invention,
The plurality of conductive regions are formed in a region sandwiched between the two RF electrodes including a portion that is multilayered with the two RF electrodes.

(Configuration 4)
An optical modulator according to Configuration 1 or 2 of the invention,
The plurality of conductive regions are formed toward an outside of a region sandwiched between the two RF electrodes.

(Structure 5 of the invention)
An optical modulator according to any one of configurations 1 to 4 of the invention,
The light modulator according to claim 1, wherein the plurality of conductive regions are a plurality of elongated electrodes extending in a direction perpendicular to the RF electrode.

(Structure 6 of the invention)
The optical modulator according to any one of configurations 1 to 5 of the invention,
The light modulator, wherein the plurality of conductive regions are electrically independent and floating.

(Configuration 7)
The optical modulator according to any one of configurations 1 to 5 of the invention,
An optical modulator, wherein at least a part of the plurality of conductive regions is connected to a specific potential.

(Configuration 8)
The optical modulator according to any one of configurations 1 to 7 of the invention,
An optical modulator characterized in that an arrangement interval of the plurality of conductive regions and a length in a high-frequency signal propagation direction are each ½ or less of a high-frequency signal operating wavelength.

In the optical modulator according to the present invention, the RF electrode and a plurality of conductive regions spaced apart along the RF electrode are capacitively coupled. Due to the effect of this capacitive coupling, the present modulator can increase the capacitance C _{tl of the} transmission line while _achieving optoelectrical speed matching, so that the characteristic impedance Z _dev can be reduced. Thereby, it is possible to reduce the propagation loss of the high frequency signal in the high frequency region. Therefore, it is possible to provide an optical modulator having excellent high frequency characteristics and good waveform quality.

It is sectional drawing of the optical waveguide of the conventional Si optical modulator. It is a top view of the Si optical modulator which comprises the conventional Mach-Zehnder (MZ) type Si optical modulator of a single electrode structure. FIG. 3 is a cross-sectional structure diagram of the conventional MZ type Si optical modulator of FIG. 2. It is the schematic diagram which showed notionally the mode of the propagation of the high frequency electric signal in a CPS line. It is explanatory drawing of the distributed constant circuit model of Si optical modulator. It is a top view which shows the structure of the MZ type | mold optical modulator by Example 1 of this invention. It is sectional drawing which shows the structure of the MZ type | mold optical modulator by Example 1 of this invention. It is the calculation result which shows the frequency characteristic of (a) attenuation | damping and (b) speed of the MZ type | mold optical modulator by Example 1 of this invention compared with a prior art example. It is the calculation result which shows the frequency characteristic of (a) characteristic impedance and (b) capacity | capacitance of the MZ type | mold optical modulator by Example 1 of this invention compared with a prior art example. It is the calculation result which shows the frequency characteristic of (a) inductance and (b) EO band of the MZ type | mold optical modulator by Example 1 of this invention compared with a prior art example. FIG. 6 is a calculation result showing frequency characteristics of (a) band limitation due to speed mismatch and (b) band limitation due to propagation loss in the MZ type optical modulator according to the first embodiment of the present invention as compared with the conventional example. FIG. It is a top view which shows the structure of the MZ type | mold optical modulator by Example 2 of this invention. It is sectional drawing which shows the structure of the MZ type | mold optical modulator by Example 2 of this invention. It is a top view which shows the structure of the MZ type | mold optical modulator by Example 3 of this invention. It is sectional drawing which shows the structure of the MZ type | mold optical modulator by Example 3 of this invention.

Example 1
Hereinafter, the form of the optical modulator of the present invention will be described in detail using preferred examples.

  FIG. 6 is a plan view showing the configuration of the MZ type optical modulator according to the first embodiment of the present invention.

  As shown in FIG. 6, the optical modulator according to the first embodiment of the present invention is disposed on a Si optical modulator having a single electrode structure similar to the conventional one along the RF electrode, and is capacitive with the RF electrode. A plurality of conductive regions 9a and 9b that function as floating electrodes are provided. Although various shapes can be applied as the conductive region, in the following, for the sake of simplicity, the conductive region is perpendicular to the RF electrode toward the outside of the region sandwiched between the two RF electrodes. A configuration in which a plurality of elongated linear electrodes 9a and 9b extending apart from each other along the RF electrode is illustrated.

  In the optical modulator of the first embodiment shown in FIG. 6, the plurality of orthogonal electrodes 9a and 9b extend from the lower part of the RF electrodes 5a and 5b toward the outside of the optical modulator, respectively, and the signal propagation direction ( It is an elongated linear conductive region disposed perpendicular to the longitudinal direction of the RF electrode), and a plurality of orthogonal electrodes are illustrated as having an equal length and an equal interval.

  As shown in the cross-sectional view of Example 1 in FIG. 7, the orthogonal electrodes 9 a and 9 b which are conductive regions are formed on the electrode layer at the same level as the DC electrode 6 by the same process. In the first embodiment, the orthogonal electrodes 9a and 9b, which are conductive regions, are formed at the same level as the DC electrode 6. However, if the capacitive coupling with the RF electrode can be formed, the orthogonal electrodes 9a and 9b may be formed at different levels. Good.

  For example, even if an insulating layer such as a dielectric is formed above the RF electrodes 5a and 5b and the orthogonal electrodes 9a and 9b are disposed thereon, the same effect as in the first embodiment can be obtained. However, in a general silicon photonics process, since the uppermost electrode is the thickest electrode, a signal electrode is usually formed on the uppermost layer. Therefore, in the first embodiment, the orthogonal electrodes 9a and 9b are exemplified by a configuration in which the orthogonal electrodes 9a and 9b are formed in a lower layer than the electrode layer of the RF electrode.

  In addition, the plurality of conductive regions (orthogonal electrodes) are not necessarily all formed in the lower layer or the upper layer of the RF electrode, and at least one of the plurality of conductive regions has the RF electrode as the upper layer. Alternatively, capacitive coupling can be formed as long as it is a conductive region having both a region disposed in a multilayer in the lower layer and a region in which the RF electrode is not disposed in the upper layer or the lower layer.

  In the first embodiment, the plurality of conductive regions constituting the orthogonal electrodes 9a and 9b are floating electrodes that are independent of each other in terms of DC and are not connected to other portions in terms of direct current. Further, the length, width, density, etc. of each linear electrode constituting the orthogonal electrode can be freely designed within a range that can be produced, for example, it can be designed with a length of 200 μm, an interval of 30 μm, and an electrode width of 2 μm. .

  Here, the length (electrode width) and electrode interval in the signal propagation direction of the plurality of conductive regions constituting the orthogonal electrodes 9a and 9b are set to be ½ or less of the operating wavelength of the RF electrode of the high-frequency signal. Designed. This is a design for preventing resonance in a plurality of conductive regions constituting the RF electrode and the orthogonal electrode.

The propagation speed of the electromagnetic wave in vacuum is about 3 × 10 ⁸ m / s, and the effective refractive index of the high-frequency signal propagating through the RF electrode of the Si modulator is about n _eff = 3, the wavelength of the high-frequency signal of 10 GHz is The wavelength of the high-frequency signal of 10 mm and 40 GHz is about 2.5 mm. Therefore, if the length in the signal propagation direction (electrode width) and the electrode interval of each orthogonal electrode is ½ the wavelength of the RF electrode of the high frequency signal, which is 5 mm or less, up to 10 GHz. If it is 25 mm or less, an optical modulator that does not resonate up to a signal of 40 GHz can be realized.

  Here, the expression of the effect of widening the EO band in the present invention will be described. FIG. 7 shows a cross-sectional view taken along the line BB ′ of the first embodiment shown in FIG. The propagation of the electric signal is considered with the charge model shown in FIG.

  In the cross-sectional view of the first embodiment shown in FIG. 7, it is assumed that a negative charge is applied to the RF electrode 5a and a positive charge is applied to the 5b. At this time, the potential generated by the positive charge of the RF electrode 5b on the right side of FIG. 7 is considered. Considering the distance from the RF electrode 5a, the potential at the right end and the left end of the orthogonal electrode 9b (potential A and potential B) is potential A> potential B, and there is a potential difference inside the orthogonal electrode 9b. Here, since the orthogonal electrode 9b is a conductor, a charge is induced on the surface so as to cancel the internal potential difference. This state can be considered as a state in which a capacitor is formed in a multilayer portion where the RF electrode 5b and the orthogonal electrode 9b overlap each other and are capacitively coupled.

Similarly, the RF electrode 5a and the orthogonal electrode 9a on the left in FIG. 7 also form a capacitor. As described above, the AC electrical coupling between the RF electrode and the orthogonal electrode is increased, thereby increasing the capacitance C _tl of the transmission line in the circuit model of FIG.

Further, since the orthogonal electrodes 9a and 9b are divided in the signal traveling direction (RF electrode longitudinal direction) of the RF electrode, almost no return current induced by the signal potential flows. For this reason, the value of the inductance _Ltl in the circuit model of FIG. 5 hardly changes with or without the orthogonal electrodes 9a and 9b.

Therefore, it is possible to increase only C _tl due to the presence of the orthogonal electrodes 9a and 9b, and Z _dev represented by the equation (2) is reduced. As described above, since the propagation loss in the high frequency region is reduced by reducing Z _dev , the EO band can be designed wider when there is an orthogonal electrode.

Further, by adjusting the L _tl of the modulator according to the design of the electrode widths and electrode intervals of the RF electrodes 5a and 5b, the propagation speed of the electric signal (v _{p in} Expression (4)) can also be adjusted. Orthogonal electrodes 9a, by the effect of 9b, since the C _tl increases, L _tl can design the direction of reducing. Since this adjustment is an adjustment in the direction of reducing Z _dev from the equation (2), the propagation loss does not increase by this adjustment.

  In other words, according to the present invention, it is possible to design to match the photoelectric velocity while expanding the EO band.

  Here, in FIG. 8 to FIG. 11, when there are orthogonal electrodes 9a and 9b of the MZ type optical modulator according to the present invention (With Cross Electrode, solid line) and when there are no orthogonal electrodes of the conventional MZ type optical modulator ( A design example of “Without Cross Electrode” (dotted line) will be described by comparing the calculation results of the characteristics of various optical modulators by the electromagnetic field simulator HFSS.

In the calculation, two orthogonal single electrode Si optical modulators of the same design are driven differentially, one with orthogonal electrodes 9a and 9b and the other without orthogonal electrodes 9a and 9b. In addition, the group refractive index of the optical signal was 3.8 (optical signal speed 0.79 × 10 ⁸ m / s), and the modulator had a 3 mm modulator. The orthogonal electrode was a floating electrode.

  As shown in the capacity of FIG. 9B and the inductance of FIG. 10A, in the present invention, by adding the orthogonal electrodes 9a and 9b, only the effective capacity is increased without reducing the effective inductance. You can see that As a result, the characteristic impedance can be reduced as shown in FIG. 9A, and the attenuation in the high frequency region (> 13 GHz) can be reduced as shown in FIG. 8A. In FIG. 8A, the attenuation in the low frequency region (<13 GHz) is slightly increased, which is due to the increase in the first term of Equation (1).

  As described above, the EO band is determined by the band limitation due to the mismatch of the photoelectric velocity and the band limitation due to the propagation loss. As shown in FIG. 11B, the EO band limitation is relaxed by reducing the propagation loss. It is possible. In the present design, the orthogonal electrodes 9a and 9b are attached so that the speed can be substantially matched. Therefore, the bandwidth limitation due to the speed mismatch can be relaxed (FIG. 11 (a)).

  This effect is due to the fact that if there is a conductive region in the vicinity of the RF electrodes 5a and 5b, capacitive coupling between the RF electrodes 5a and 5b and the conductive region is realized by a potential difference in the conductor. Therefore, the shape of the orthogonal electrodes 9a, 9b does not need to be a linear shape or a rectangle, and is a taper type that becomes wider or narrower toward the outside of the modulator, a bent shape, a wave shape, The same effect can be obtained even in the case of a shape extending in a direction inclined from a direction perpendicular to the RF electrodes 5a and 5b.

  In addition, although the present Example discussed RF electrode 5a, 5b of simple shape, these also do not need to be a rectangle and depending on the convenience of design, deformed electrodes such as a capacitive loading type as described in Non-Patent Document 3 It may be.

(Example 2)
FIG. 12 is a plan view showing the configuration of the MZ type optical modulator according to the second embodiment of the present invention. In the second embodiment, with respect to the features described in the first embodiment, portions having the same function are denoted by the same reference numerals, and redundant description is omitted.

  As shown in FIG. 12, the optical modulator according to the second embodiment of the present invention includes a Si optical modulator having a single electrode structure similar to the conventional one, orthogonal electrodes 9a and 9b having a structure similar to that of the first embodiment, and a plurality of Are formed of bus electrodes 10a and 10b that are electrically connected to each other at the end portion away from the RF electrode.

  The plurality of conductive regions constituting the orthogonal electrodes 9a and 9b described in the first embodiment in FIG. 6 are independent floating electrodes, but in this second embodiment, as shown in the cross-sectional view in FIG. The orthogonal electrodes 9a and 9b are connected to a bias potential via bus electrodes 10a and 10b formed on the upper ends of the orthogonal electrodes 9a and 9b on the side away from the RF electrode. Here, the bias potential is a specific potential applied to the DC electrode 6 for bias.

  FIG. 13 is a cross-sectional view taken along the line CC ′ of FIG. The bus electrodes 10a and 10b are fabricated on the same electrode layer as the RF electrodes 5a and 5b by the same process, and are connected to the orthogonal electrodes 9a and 9b by short vias (connection electrodes) at the end on the side away from the RF electrode. The In the plan view of FIG. 12, vias are not shown for simplicity of illustration.

  In the second embodiment, the bus electrodes 10a and 10b are realized using the electrode layer at the same level as the RF electrodes 5a and 5b. However, the bus electrodes 10a and 10b may be formed using another conductive layer. The bus electrode is connected to a plurality of conductive regions constituting the orthogonal electrode through vias. The bus electrode need not be rectangular, and may be divided. When dividing, all the bus electrodes need to be connected to the bias potential.

In the floating orthogonal electrodes 9a and 9b of the first embodiment, the amount of charge induced in the orthogonal electrodes is proportional to the potential difference between the right end and the left end of the orthogonal electrodes 9a and 9b. Therefore, in order to increase _Ctl , it is necessary to lengthen the orthogonal electrode. However, there is a problem that the footprint (planar size) of the modulator becomes large at this time.

In the second embodiment, the plurality of conductive regions constituting the orthogonal electrode are connected to the bias potential by the bus electrodes 10a and 10b. In this case, the potential difference between the right end and the left end in the orthogonal electrode is equal to the absolute value of the signal potential, and a potential difference larger than that of the first embodiment can be provided. Since more electric charges can be induced by this potential difference, in Example 2, even when the orthogonal electrode length is short, a sufficient _Ctl increasing effect can be obtained.

  In the second embodiment, the example in which all the orthogonal electrodes 9a and 9b on the respective sides are connected by the bus electrodes 10a and 10b has been described. However, it is not necessary to connect all the orthogonal electrodes, and as necessary. Only at least a part of the orthogonal electrodes (conductive regions) may be connected to the bus electrode. The number of bus electrodes is not necessarily two, and the same effect can be obtained even if the orthogonal electrodes 9a and 9b are connected by a plurality of wirings. Further, in the second embodiment, all the orthogonal electrodes are connected to the bias potential. However, if necessary, even if at least a part of the plurality of conductive regions (orthogonal electrodes) are connected to any specific potential, Similar effects can be obtained.

(Example 3)
The third embodiment of the present invention shown in the plan view of FIG. 14 and the DD ′ cross-sectional view of FIG. It is. The difference from the second embodiment is that in the third embodiment, the DC electrode 6 for bias driving located between the two RF electrodes is partially extended as it is in a branch shape perpendicularly to the signal propagation direction at several positions (widening). ) And used as a plurality of orthogonal electrodes 9a, 9b. For this reason, in the plan view of FIG. 14, a plurality of orthogonal electrodes 9a are disposed in a region between the two RF electrodes 5a and 5b, including a portion that overlaps the two RF electrodes and becomes a multilayer. 9b are formed.

  In FIGS. 14 and 15 of the third embodiment, the orthogonal electrodes 9a and 9b are described as two electrodes in accordance with the other embodiments, but the electrodes are configured by extending (widening) the DC electrode 6 as it is. Therefore, they are electrically at the same potential, and may be configured as an integrated electrode including the DC electrode 6.

  In the first and second embodiments, there is a merit that the operation is stable because the orthogonal electrode is not disposed in the vicinity of the optical waveguide. On the other hand, the orthogonal electrode extends toward the outside of the two RF electrodes. The overall plane size (footprint) may increase. In the third embodiment, since the orthogonal electrode is formed between the two RF electrodes and does not spread outside, it is advantageous for downsizing.

  As shown in the cross-sectional view of Example 3 in FIG. 15, even the orthogonal electrodes 9a and 9b having such a structure are spaced apart from each other along the RF electrodes 5a and 5b, and capacitively connected to the RF electrode. It can be said that there are a plurality of conductive regions to be combined.

Therefore, as in the first and second embodiments, the transmission line capacitance C _tl can be increased in the third embodiment, Z _dev can be reduced, and the propagation loss in the high frequency region can be reduced. Can be designed widely.

  As described above, in the optical modulator according to the present invention, the optical modulation efficiency in the high frequency region can be increased. For this reason, it is possible to improve an adverse effect such as deterioration of waveform quality during high-speed modulation due to deterioration of frequency response characteristics of the optical modulator, and to provide an optical modulator having excellent high frequency characteristics and good waveform quality. Is possible.

1,3 SiO2 cladding layer 2 Si layer 201 Optical waveguide core portion 202 Slab region 211 High concentration p-type semiconductor layer 212 Medium concentration p type semiconductor layer 213 Medium concentration n type semiconductor layer 214 High concentration n type semiconductor layer 5a, 5b RF electrode 6 DC electrodes 7, 7a, 7b Optical waveguides 9a, 9b Orthogonal electrodes (conductive regions)
10a, 10b bus electrode

Claims (8)

Between two RF electrodes to which a pair of differential electrical signals are applied, two first conductive semiconductor layers to which the two RF electrodes are connected, and between the two first conductive semiconductor layers An optical modulator comprising: a second conductive semiconductor layer formed and connected to a fixed potential; and two optical waveguides formed at a pn junction boundary of the first and second conductive semiconductor layers,
An optical modulator, further comprising a plurality of conductive regions that are spaced apart from each other along the RF electrode and capacitively coupled to the RF electrode. The optical modulator according to claim 1, comprising:
The plurality of conductive regions are formed in a lower layer or an upper layer of the RF electrode,
At least one of the plurality of conductive regions is a conductive region having both a region where the RF electrode is disposed in an upper layer or a lower layer and a region where the RF electrode is not disposed in an upper layer or a lower layer, Light modulator. The optical modulator according to claim 1 or 2,
The plurality of conductive regions are formed in a region sandwiched between the two RF electrodes including a portion that is multilayered with the two RF electrodes. The optical modulator according to claim 1 or 2,
The plurality of conductive regions are formed toward an outside of a region sandwiched between the two RF electrodes. The optical modulator according to any one of claims 1 to 4,
The light modulator according to claim 1, wherein the plurality of conductive regions are a plurality of elongated electrodes extending in a direction perpendicular to the RF electrode. An optical modulator according to any one of claims 1 to 5,
The light modulator, wherein the plurality of conductive regions are electrically independent and floating. An optical modulator according to any one of claims 1 to 5,
An optical modulator, wherein at least a part of the plurality of conductive regions is connected to a specific potential. The optical modulator according to any one of claims 1 to 7,
An optical modulator characterized in that an arrangement interval of the plurality of conductive regions and a length in a high-frequency signal propagation direction are each ½ or less of a high-frequency signal operating wavelength.

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