JP2017111338A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulator that compensates for degradation of optical signal characteristic attributable to a fabrication error.SOLUTION: The present invention is an optical modulator comprising: an optical coupler for branching input light into two, and formed on a substrate; first and second arm waveguides for guiding branched lights from the optical coupler, and formed on the substrate and having a pn junction along the waveguide direction of light; and an optical coupler for combining light from the first arm waveguide and light from the second arm waveguide, and formed on the substrate. The light input side of light of the first and second arm waveguides is a first phase modulation region, and the light output side of light of the first and second arm waveguides is a second phase modulation region, a p-doped region and an n-doped region of the first arm waveguide in the first phase modulation region and the first arm waveguide in the second phase modulation region being at inverse positions, the p-doped region and the n-doped region of the second arm waveguide in the first phase modulation region and the second arm waveguide in the second phase modulation region being at inverse positions.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an optical modulator for modulating an optical signal with a high-frequency electrical signal, and more particularly to an optical modulator in which optical waveform distortion caused by a manufacturing error is compensated.

  With the rapid increase in communication demand, optical communication technology for increasing the capacity of communication networks has been energetically studied. A conventional optical modulation format used in optical communication is mainly an amplitude shift keying (ASK) method in which one channel of a high-frequency electric signal is assigned to one channel of light. However, the ASK method can only give a 1-bit signal to a certain frequency band. Therefore, in recent years, modulation methods such as a quadrature phase shift keying (QPSK) method and a quadrature amplitude modulation (QAM) method that can give a signal of 1 bit or more to a certain frequency band have been actively researched. It has been developed and put to practical use. In order to generate a QPSK signal in the QPSK system and to generate a QAM signal in the QAM system, normally, the real axis and the imaginary axis when light is complexly expressed are individually amplitude-modulated using a Mach-Zehnder modulator. An IQ modulator is used (see, for example, Non-Patent Document 1). A QPSK signal is generated by inputting a 1-bit signal to one Mach-Zehnder modulator, and a QAM signal is generated by inputting a 2-bit or more signal to one Mach-Zehnder modulator. Using such multi-level modulation, an increase in communication capacity is achieved.

JP 2014-6389 A

S. Kanazawa, et al., "112-Gb / s InP DP-QPSK modulator integrated with a silica-PLC polarization multiplexing circuit", OFC 2012, PDP5A.9, (2012)

  FIG. 1 is a diagram showing a configuration of a Mach-Zehnder modulator 100 that is a conventional optical modulator. FIG. 1A shows a top view of the Mach-Zehnder modulator 100, and FIG. Sectional drawing in -A 'is shown. The Mach-Zehnder modulator 100 of FIG. 1 includes one of an input optical port 102, a 1 × 2 optical coupler 103 connected to the input optical port 102, and a 1 × 2 optical coupler 103 provided on a substrate 101. A first arm waveguide 104 connected to the output and a second arm waveguide 105 connected to the other output of the 1 × 2 optical coupler 103 are provided. The Mach-Zehnder modulator 100 includes a 2 × 1 optical coupler 106 in which the first arm waveguide 104 is connected to one input and the other input of the second arm waveguide 105, and a 2 × 1 optical coupler 106. And an output optical port 107 connected to the output.

  The Mach-Zehnder modulator 100 includes a phase modulation region configured by a Mach-Zehnder interferometer. In the phase modulation region, the first arm waveguide 104 is a rib-shaped optical waveguide formed on the substrate center side by the n-doped region 104-1 and on the substrate edge side by the p-doped region 104-2. The center of the light traveling direction is a pn junction between the n-doped region 104-1 and the p-doped region 104-2. The second arm waveguide 105 is a rib-shaped optical waveguide in which the substrate center side is formed by the n-doped region 105-1 and the substrate edge side is formed by the p-doped region 105-2, and the waveguide center is n. It becomes a pn junction between doped region 105-1 and p-doped region 105-2. A first arm electrode 108 is formed on the p-doped region 104-2, a second arm electrode 109 is formed on the p-doped region 105-2, and the n-doped regions 104-1 and 105- are formed. On 1, a bias electrode 110 is formed.

  FIG. 1B is a cross-sectional view of the phase modulation portion of the Mach-Zehnder modulator 100. As shown in FIG. 1B, the phase modulation region of the Mach-Zehnder modulator 100 is a clad layer formed on the substrate 101. 112, a core layer 111 formed on the cladding layer 112, and a cladding layer 113 formed on the core layer 111. The core layer 111 forms two rib-shaped arm waveguides 304 and 305, and is composed of n-doped regions 104-1 and 105-1 and p-doped regions 104-2 and 105-2.

  When a differential high-frequency signal is input to the first arm electrode 108 and the second arm electrode 109, the potential difference between the first arm electrode 108, the second arm electrode 109, and the bias electrode 110 is changed by the high-frequency signal. To do. This potential difference changes the size of the depletion layer region of the pn junction, so that the so-called carrier plasma effect changes the refractive index of the waveguide to cause a light phase change, and the Mach-Zehnder modulator 100 is pushed-pull. To work.

  In the conventional Mach-Zehnder modulator, since the interval between symbol points on the complex plane (constellation map) of the QAM signal is narrowed, waveform degradation due to the asymmetry of the Mach-Zehnder modulator and imperfections such as chirp becomes the bit error rate. There is a problem that the influence is large (see Patent Document 1). For example, since the position accuracy of the pn junction is determined by the alignment accuracy of the photomask during the manufacturing process, the pn junction position may deviate from about several tens nm to several hundreds nm with respect to the design value. The optical waveguide may become asymmetric.

  FIG. 2 is a diagram illustrating a case where the pn junction positions of the arm waveguides 104 and 105 are symmetric and asymmetric. Here, FIG. 2A is a sectional view in the light traveling direction of the arm waveguides 104 and 105 when the pn junction positions of the arm waveguides 104 and 105 are symmetric, and FIG. 2B is a diagram showing the pn junction position. FIG. 2C is a diagram showing the relationship between the amount of phase change of light with respect to the voltage applied to the pn junction in the case of 2 (a), and FIG. 2C is the case of quaternary amplitude phase modulation when the pn junction position is in FIG. It is a figure showing the constellation map which shows a signal characteristic. 2D is a cross-sectional view of the light propagation directions of the arm waveguides 104 and 105 when the pn junction positions of the arm waveguides 104 and 105 are asymmetric, and FIG. The figure which shows the relationship of the light phase change amount with respect to the voltage applied to a pn junction in the case of (d), FIG.2 (f) is a signal at the time of quaternary amplitude phase modulation in case a pn junction position is FIG.2 (d). It is a figure showing the constellation map which shows a characteristic.

  When the pn junction position is shown in FIG. 2A, the voltage dependency characteristics of the phase change amount are the same between the first arm and the second arm (FIG. 2B), and quaternary amplitude phase modulation is performed. It can be seen that the constellations at the time are arranged in a straight line at equal intervals, and a signal with good characteristics can be generated (FIG. 2C).

  On the other hand, for example, when the n-doped region is shifted downward on the cross-sectional view due to a manufacturing error in the manufacturing process, the pn junction position is as shown in FIG. In such a case, the first arm and the second arm have a difference in the voltage-dependent characteristics of the phase change amount ((FIG. 2 (e)), and the constellation during quaternary amplitude phase modulation has a bow shape. It can be seen that the signal characteristics are distorted and deteriorated (FIG. 2 (f)).

  As described above, the conventional Mach-Zehnder modulator has a problem that the modulation signal characteristic is deteriorated when the asymmetry of the pn junction position occurs between the arms of the Mach-Zehnder modulator due to a manufacturing error during the manufacturing process. .

  The present invention has been made in view of the above-described prior art, and an object thereof is to provide an optical modulator in which signal characteristic deterioration due to a manufacturing error is compensated.

  In order to achieve such an object, according to a first aspect of the present invention, there is provided an optical coupler formed on a substrate for branching input light into two, and formed on the substrate along a light guiding direction. A first arm waveguide for guiding one of the branched lights from the optical coupler, and a pn junction formed along the light guiding direction, A second arm waveguide that guides the other of the branched light from the optical coupler; and a light that is formed on the substrate and that combines the light from the first arm waveguide and the light from the second arm waveguide. An optical modulator including a wave coupler, wherein light input sides of the first and second arm waveguides are first phase modulation regions, and the first and second arm guides are provided. The light output side of the waveguide is a second phase modulation region, and the first arc in the first phase modulation region is the second phase modulation region. The waveguide and the first arm waveguide in the second phase modulation region are in positions where the p-doped region and the n-doped region are opposite to each other, and the second arm in the first phase modulation region In the waveguide and the second arm waveguide in the second phase modulation region, the p-doped region and the n-doped region are in opposite positions.

  According to a second aspect of the present invention, there is provided the optical modulator according to the first aspect, wherein the first phase modulation region is one of the first phase modulation region and the first phase modulation region. And the second arm waveguide has a p-doped region on the substrate edge side, and the first and second phase modulation regions in the other one of the first phase modulation region and the first phase modulation region. The second arm waveguide is characterized in that an edge side of the substrate is an n-doped region.

  According to a third aspect of the present invention, there is provided the optical modulator according to the second aspect, wherein the phase modulation region having the first and second arm waveguides wherein the edge side of the substrate is a p-doped region. , One bias electrode formed between the first arm waveguide and the second arm waveguide, the substrate edge side of the first arm waveguide, and the second arm waveguide A phase modulation region having the first and the second arm waveguides, wherein the edge side of the substrate is an n-doped region. An arm electrode for applying two RF signals formed between the arm waveguide and the second arm waveguide; the substrate edge side of the first arm waveguide; and the second arm waveguide. A bias electrode formed on the substrate edge side. The features.

  According to a fourth aspect of the present invention, there is provided the optical modulator according to the third aspect, wherein the edge side of the substrate is a p-doped region and includes the first and second arm waveguides. The bias electrode is connected to the bias electrode in the phase modulation region having the first and second arm waveguides in which the edge side of the substrate is an n-doped region by a bridge electrode.

  According to a fifth aspect of the present invention, there is provided the optical modulator according to any one of the first to fourth aspects, wherein the length of the first phase modulation region is the length of the first phase modulation region. And 30 to 50% of the total of the length of the second phase modulation region.

  A sixth aspect of the present invention is the optical modulator according to the third or fourth aspect, wherein the bias electrode has the same length as a total length of the first and second phase modulation regions. It is characterized by having.

  According to a seventh aspect of the present invention, there is provided the optical modulator according to any one of the first to sixth aspects, wherein the substrate is a single substrate.

  The present invention has an effect of compensating for deterioration in optical signal characteristics caused by manufacturing errors in an optical modulator.

It is a figure which shows the structure of the Mach-Zehnder modulator which is the conventional optical modulator, (a) is a top view of a Mach-Zehnder modulator, (b) is sectional drawing in AA 'of a Mach-Zehnder modulator. FIG. 2 is a diagram for explaining a case where pn junction positions of two arm waveguides of the Mach-Zehnder modulator of FIG. 1 are symmetric and asymmetric. (A) is a sectional view of the light propagation direction of the arm waveguide when the pn junction positions of the two arm waveguides are symmetric, and (b) is a voltage applied to the pn junction when the pn junction position is (a). FIG. 6C is a diagram showing a constellation map showing signal characteristics at the time of quaternary amplitude phase modulation when the pn junction position is (a), and FIG. Sectional drawing of the light advancing direction of an arm waveguide when the pn junction position of two arm waveguides is asymmetrical, (e) is the phase of the light with respect to the voltage applied to a pn junction when a pn junction position is (d). The figure which shows the relationship of a variation | change_quantity, (f) is a figure showing the constellation map which shows the signal characteristic at the time of quaternary amplitude phase modulation in case a pn junction position is (d). 1A and 1B are diagrams illustrating a configuration of a Mach-Zehnder modulator that is an optical modulator according to a first embodiment of the present invention, in which FIG. 1A is a top view of the Mach-Zehnder modulator, and FIG. (C) is sectional drawing in BB ', (d) is sectional drawing in CC'. FIGS. 3A and 3B are diagrams for comparing the conventional optical modulator and the optical modulator according to the first embodiment, in which FIG. 3A is a constellation map of the conventional Mach-Zehnder modulator in FIG. 1, and FIG. 2 is a constellation map of the Mach-Zehnder modulator. It is a top view which shows the Mach-Zehnder modulator which concerns on the Example of the 1st Embodiment of this invention. FIG. 6 is a diagram for comparing a conventional optical modulator and the optical modulator according to the present embodiment, in which (a) is a constellation map of a conventional Mach-Zehnder modulator, and (b) is a constellation map of the Mach-Zehnder modulator. It is. It is a top view which shows the structure of the Mach-Zehnder modulator which is an optical modulator which concerns on the 2nd Embodiment of this invention. It is a figure which shows the ratio of the front half area length with respect to the full length of a Mach-Zehnder modulator, and EVM in the 2nd Embodiment of this invention, (a) is a mode that EVM changes with the ratio of the front half area length with respect to the phase modulation area full length. (B) shows the relationship between the 3 dB band normalized by the symbol rate and the optimum point of the ratio of the first half region length to the total length. FIG. 5 is a diagram illustrating a configuration of a Mach-Zehnder modulator that is an optical modulator according to a third embodiment of the present invention, where (a) is a top view of the Mach-Zehnder modulator, and (b) is an AA ′ of the Mach-Zehnder modulator. (C) is sectional drawing in BB '.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the form used in the following description is an example, and there is no intention to exclude implementation of the present invention in other forms.

[First Embodiment]
FIG. 3 is a diagram showing a configuration of a Mach-Zehnder modulator 300 that is an optical modulator according to the first embodiment of the present invention. FIG. 3A is a top view of the Mach-Zehnder modulator 300, and FIG. ) Is a cross-sectional view taken along the line AA ′ of the Mach-Zehnder modulator 300, FIG. 3C is a cross-sectional view taken along the line BB ′, and FIG. 3D is a cross-sectional view taken along the line CC ′.

  The Mach-Zehnder modulator 300 of FIG. 3 includes one of an input optical port 302, a 1 × 2 optical coupler 303 connected to the input optical port 302, and a 1 × 2 optical coupler 303 provided on a substrate 301. A first arm waveguide 304 connected to the output and a second arm waveguide 305 connected to the other output of the 1 × 2 optical coupler 303 are provided. The Mach-Zehnder modulator 300 includes a 2 × 1 optical coupler 306 in which the first arm waveguide 304 is connected to one input and the other input of the second arm waveguide 305, and a 2 × 1 optical coupler 306. And an output optical port 307 connected to the output. Further, the Mach-Zehnder modulator 300 includes two phase modulation regions constituted by a Mach-Zehnder interferometer. The light incident side is a first phase modulation region 351 and the light emission side is a second phase modulation region. 352.

  In the first phase modulation region 351, the first arm waveguide 304 is a rib-shaped optical waveguide in which the substrate center side is formed by the n-doped region 304-1 and the substrate edge side is formed by the p-doped region 304-2. The center of the waveguide is the pn junction between the n-doped region 304-1 and the p-doped region 304-2. The second arm waveguide 305 is a rib-shaped optical waveguide in which the substrate center side is formed by the n-doped region 305-1 and the substrate edge side is formed by the p-doped region 305-2, and the waveguide center is n. It becomes a pn junction between doped region 305-1 and p-doped region 305-2. Further, a first arm electrode 308 is formed on the p-doped region 304-2, a second arm electrode 309 is formed on the p-doped region 305-2, and the n-doped regions 304-1 and 305- are formed. A first bias electrode 310 is formed on 1.

  In the second phase modulation region 352, the first arm waveguide 304 is a rib-shaped optical waveguide formed on the substrate edge side by the n-doped region 304-3 and on the substrate center side by the p-doped region 304-2. The waveguide center is the pn junction between the n-doped region 304-3 and the p-doped region 304-2. The second arm waveguide 305 is a rib-shaped optical waveguide in which the substrate edge side is formed by the n-doped region 305-3 and the substrate edge center side is formed by the p-doped region 305-2. It becomes a pn junction between n-doped region 305-3 and p-doped region 305-2. A first arm electrode 308 is formed on the p-doped region 304-2, a second arm electrode 309 is formed on the p-doped region 305-2, and a first arm electrode 308 is formed on the n-doped region 304-3. The second bias electrode 314 is formed, and the third bias electrode 315 is formed on the n-doped region 305-3.

  The first bias electrode 310, the second bias electrode 314, and the third bias electrode 315 are connected by the bridge electrode 316 between the first phase modulation region 351 and the second phase modulation region 352. .

  FIG. 3B is a cross-sectional view of the AA ′ portion in the first phase modulation region 351 of the Mach-Zehnder modulator 300, and as shown in FIG. The phase modulation region 351 includes a clad layer 312 formed on the substrate 301, a core layer 311 formed on the clad layer 312, and a clad layer 313 formed on the core layer 311. The core layer 311 forms two rib-shaped arm waveguides 304 and 305. The n-doped regions 304-1 and 305-1 and the p-doped formed outside the n-doped regions 304-1 and 305-1. It consists of regions 304-2 and 305-2.

  FIG. 3C is a cross-sectional view of a BB ′ portion that is a region between the first phase modulation region 351 and the second phase modulation region 352 of the Mach-Zehnder modulator 300. As shown in FIG. 3C, a region between the first phase modulation region 351 and the second phase modulation region 352 of the Mach-Zehnder modulator 300 includes a clad layer 312 formed on the substrate 301, A core layer 311 formed on the cladding layer 312 and a cladding layer 313 formed on the core layer 311 are configured. The first arm waveguide 304 in the region between the first phase modulation region 351 and the second phase modulation region 352 includes a pn junction between the n-doped region 304-1 and the p-doped region 304-2, The n-doped region 304-3 and the p-doped region 304-2 are connected to the pn junction. The second arm waveguide 305 includes a pn junction between the n-doped region 305-1 and the p-doped region 305-2, and a pn junction between the n-doped region 304-1 and the p-doped region 304-2. Is connected. The bridge electrode 316 is formed in the width direction of the substrate 301 in a region between the first phase modulation region 351 and the second phase modulation region 352 of the Mach-Zehnder modulator 300.

  FIG. 3D is a cross-sectional view of a CC ′ portion in the second phase modulation region 352 of the Mach-Zehnder modulator 300. As shown in FIG. The phase modulation region 352 includes a cladding layer 312 formed on the substrate 301, a core layer 311 formed on the cladding layer 312, and a cladding layer 313 formed on the core layer 311. The core layer 311 forms two rib-shaped arm waveguides 304 and 305, and p-doped regions 304-2 and 305-2 and n-doped formed outside the p-doped regions 304-2 and 305-2. It consists of areas 304-3 and 305-3.

  The Mach-Zehnder modulator 300 shown in FIG. 3 includes a p-doped region (304-2 and 304-2) outside the Mach-Zehnder interferometer including the first arm waveguide 304 and the second arm waveguide 305 in the first phase modulation region 351. 305-2) is provided with n-doped regions (304-1 and 305-1) inside. On the other hand, in the second phase modulation region 352, n-doped regions (304-3 and 305-3) are provided outside the Mach-Zehnder interferometer, and p-doped regions (304-2 and 305-2) are provided inside. . By adopting such a positional relationship between the p-doped region and the n-doped region, the first phase modulation region 351 and the second phase modulation region 352 cancel each other in the pn junction position caused by a manufacturing error. it can.

  For example, when the n-doped regions (304-1 and 305-1) are shifted to the lower side in FIG. 3A, the pn junction position of the first phase modulation region 351 of the first arm waveguide 304 is p. The doped region 304-2 is shifted so as to increase, but the pn junction position of the second phase modulation region 352 of the first arm waveguide 304 is shifted so that the n-doped region 304-3 is increased. Similarly, the pn junction position of the first phase modulation region 351 of the second arm waveguide 305 is shifted so that the n-doped region 305-1 becomes larger, but the second phase modulation of the second arm waveguide 305 is increased. The pn junction position of the region 352 is shifted so that the p-doped region 305-2 becomes larger. As described above, by reversing the shift of the pn junction position between the first phase modulation region 351 and the second phase modulation region 352, the phase change amount of the first arm waveguide 304 and the second phase modulation region 352 can be reduced. The phase change amount of the arm waveguide 305 is kept symmetrical.

Below, it demonstrates using numerical formula. When a deviation of the PN junction position due to a manufacturing error occurs, the first phase modulation region of the first arm waveguide 304, the second phase modulation region of the first arm waveguide 304, and the second arm guide Response characteristics of the phase change amount with respect to the voltage in each of the first phase modulation region of the waveguide 305 and the second phase modulation region of the second arm waveguide 305 are φ _u1 (V), φ _u2 (V), φ _{Assuming that l1} (V) and φ _l2 (V), the output electric fields E _u (V) of the first arm waveguide 304 and the second arm waveguide 305 of the Mach-Zehnder modulator 300 according to the first embodiment of the present invention. ) And E _l (V) are

It can be expressed. From the positional relationship between the p-doped region and the n-doped region in FIG.

It is. here

Then, Equation 1 and Equation 2 are

It becomes. From Equation 6 and Equation 7, the output electric fields of the upper arm waveguide 304 and the lower arm waveguide 305 are symmetrical, and the asymmetry between arms of the phase change amount with respect to the voltage due to the deviation of the pn junction position can be compensated by the manufacturing error. I understand that.

  FIG. 4 is a diagram for comparing the conventional optical modulator and the optical modulator according to the first embodiment, and FIG. 4A shows a constellation map of the conventional Mach-Zehnder modulator 100 of FIG. FIG. 4B shows a constellation map of the Mach-Zehnder modulator 300 of FIG. Both of the two constellation maps are the characteristics of the optical modulation signal as a result of performing quaternary amplitude phase modulation. The conventional Mach-Zehnder modulator 100 of FIG. 4A is a case where the pn junction position is shifted by 60 nm from the design value in the direction of the n-doped region due to manufacturing errors. In the conventional Mach-Zehnder modulator 100, four symbol points are distorted in a bow shape due to the effect of asymmetry between arms, and the lower the value, the better the error vector amplitude (EVM) indicating that the signal characteristics are better. It was 5%. On the other hand, in the constellation map of the Mach-Zehnder modulator 300 according to the first embodiment, the four symbol points are linearly arranged in an almost ideal state, and the EVM is 2.1%, which is compared with the conventional optical modulator. It can be seen that improvement has been seen.

[Example]
FIG. 5 is a top view showing a Mach-Zehnder modulator 500 according to an example of the first embodiment of the present invention. The Mach-Zehnder modulator 500 of FIG. 5 is mounted on a ceramic package 501. Light input / output is performed by the input optical connector 502 and the output optical connector 503. Also, the RF signal input to the arm electrode 521 is input by the RF connector 504-1, the RF signal input to the arm electrode 523 is input by the RF connector 504-2, and the RF signal input to the arm electrode 524 is input by the RF connector 504-. 3, the RF signal is input to the arm electrode 525 through the RF connector 504-4. Further, the voltages applied to the phase adjusting arm electrodes 521, 522, 524, and 525 and the bias electrodes 523, 526, and 527 to 530 of the Mach-Zehnder interferometer were connected so as to be input from the DC pin 505. Continuous wave light (continuous wave light: CW light) input from the input optical connector 502 passes through the optical fiber 506 and is input to the modulator chip 508 by the fiber block 507. The input CW light is modulated in the modulator chip 508 and output from the output optical connector 503 as an optical modulation signal.

  The modulator chip 508 is composed of an optical waveguide and an electrode formed on a silicon substrate. For the optical waveguide, rib-type silicon was used as a core and quartz glass was used as a cladding. The height of the core cross section was 250 μm, the width was 600 μm, and the thickness of the slab part was 100 μm. The p-doped region was formed by implanting boron, and the n-doped region was formed by implanting phosphorus. On the modulator chip 508, two Mach-Zehnder modulators 300 according to the first embodiment of the present invention are mounted in parallel, and the input light is branched by a 1 × 2 optical coupler, and each of the two Mach-Zehnder modulators is split. The outputs from the two Mach-Zehnder modulators are combined by a 2 × 1 optical coupler and output. Phase adjustment heaters 509-1 and 509-2 are provided on one arm of each of the two Mach-Zehnder modulators, and the modulator chip 508 has a function as a so-called IQ modulator. Electrical connection between the electrode on the modulator chip 508 and the package 501 was performed by wire bonding. The end of the arm electrode 521 is connected to a chip resistor 510-1 having a resistance value of 50Ω, the end of the arm electrode 522 is connected to a chip resistor 510-2 having a resistance value of 50Ω, and the end of the arm electrode 524 is connected to a resistor having a resistance value of 50Ω. The chip resistor 510-3 is connected, and the end of the arm electrode 525 is connected to the chip resistor 510-4 having a resistance value of 50Ω. The arm electrodes 521, 522, 524, and 525 were connected to the chip resistors 510-1 to 510-4 by wire bonding, and aluminum was used as an electrode material for the arm electrode and the bias electrode. Chip resistors 510-1 to 510-4 are mounted on a termination substrate 511 mounted on the package 501.

  FIG. 6 is a diagram for comparing the conventional optical modulator and the optical modulator according to the present embodiment. FIG. 6A shows a constellation map of the conventional Mach-Zehnder modulator 100, and FIG. ) Shows a constellation map of the Mach-Zehnder modulator 500 of FIG. Both of the two constellation maps are the results when a 16QAM signal is generated. In the conventional Mach-Zehnder modulator 100, the pn junction position is shifted by 60 nm from the design value in the direction of the n-doped region due to a manufacturing error. In the constellation map of the conventional Mach-Zehnder modulator 100 shown in FIG. 6A, the arrangement of the symbol points is distorted due to the production error of the pn junction position, and the EVM is 6.3%. On the other hand, in the constellation map of the output optical signal of the Mach-Zehnder modulator 500 of FIG. 5 in FIG. 6B, the symbol points are aligned in a straight line, and EVM is 2.8%, which is the influence of the production error of the pn junction position. It can be seen that the signal quality deterioration due to can be compensated. When the Mach-Zehnder modulator 500 is driven, each Mach-Zehnder modulator applies a voltage to the phase adjustment heater so that light is in a phase state where light is transmitted through a minimum, and the phase of the output light of the two Mach-Zehnder modulators is about 90 degrees. A phase difference was given. The amplitude of the input RF signal was 3 Vppd, and the pattern was a 9-stage pseudo random pulse signal.

  In the Mach-Zehnder modulator 300 of FIG. 3, the positions of the p-doped region and the n-doped region are uniquely fixed. However, the Mach-Zehnder modulator according to the present invention is not limited to this example. May be reversed.

  In the Mach-Zehnder modulator 300 of FIG. 3, the positional relationship between the first phase modulation region 351 and the second phase modulation region 352 is the first phase modulation region 351 on the side close to the input optical port. The Mach-Zehnder modulator according to the invention is not limited to this example, and may have a configuration in which the positional relationship between the first phase modulation region 351 and the second phase modulation region 352 described above is reversed.

  In the Mach-Zehnder modulator 300 of FIG. 3, a 1 × 2 multimode interferometer (Multi Mode Interferometer, MMI) is used as the 1 × 2 optical coupler 303 or the 2 × 1 optical coupler 306. However, the optical modulator according to the present invention is The present invention is not limited to this example, and may be a Y-branch waveguide or a 2 × 2 MMI.

  In the Mach-Zehnder modulator 500 of FIG. 5, a 50Ω chip resistor is used as the termination resistance of the RF electrode. However, the optical modulator according to the present invention is not limited to this example, for example, a 25Ω chip resistor. Alternatively, a resistive element manufactured on a 50Ω substrate may be used.

  In the Mach-Zehnder modulator 500 of FIG. 5, an optical modulator in which the Mach-Zehnder modulator is integrated on one substrate is used. However, the optical modulator according to the present invention is not limited to this example. Only the waveguide portion may be integrated, and the functional element such as a 1 × 2 optical coupler may be formed of separate components.

  In the Mach-Zehnder modulator 500 of FIG. 5, the material of the Mach-Zehnder modulator is silicon as the core and the clad is quartz-based glass. However, the optical modulator according to the present invention is not limited to this example. The compound semiconductor material may be an organic material such as a polymer.

  In the Mach-Zehnder modulator 500 of FIG. 5, aluminum is used as the electrode material. However, the optical modulator according to the present invention is not limited to this example, and may be another metal such as gold or copper. Absent.

  In the Mach-Zehnder modulator 300 of FIG. 3, the first arm waveguide 304 and the second arm waveguide 305 are bent between the first phase modulation region 351 and the second phase modulation region 352, and the p-doped region and n Although the positional relationship of the doped region is reversed, the Mach-Zehnder modulator according to the present invention is not limited to this example. For example, the positional relationship between the p-doped region and the n-doped region may be reversed by bending the arm electrode. I do not care.

  In the Mach-Zehnder modulator 500 shown in FIG. 5, phosphorus and boron are used as impurities when forming the doped region. However, the optical modulator according to the present invention is not limited to this example. For example, antimony or arsenic is used. It does not matter.

  In the Mach-Zehnder modulator 500 of FIG. 5, ceramic is used as the package material. However, the optical modulator according to the present invention is not limited to this example, and may be a metal material such as copper tungsten or plastic. It may be a resin such as

[Second Embodiment]
FIG. 7 is a top view showing a configuration of a Mach-Zehnder modulator 700 which is an optical modulator according to the second embodiment of the present invention. In the Mach-Zehnder modulator 700 of FIG. 7, the length of the first phase modulation region near the input optical port is 30 which is the sum of the length of the first phase modulation region 751 and the length of the second phase modulation region 752. It occupies a percentage of 50% to 50%.

  In the following description, the sum of the length of the first phase modulation region 751 and the length of the second phase modulation region 752 is simply expressed as the total length, and the length of the modulation region closer to the input optical port is the first half region length. It expresses. The arm electrode to which the RF signal is applied has a loss depending on the relative permittivity and conductivity of the substrate material. In order to cancel the influence of the manufacturing error strictly in the first phase modulation region and the second phase modulation region, the integral value of the RF amplitude in each region needs to be equal, so that it is close to the input. The length of the phase modulation area on the side should be shorter than that on the side far from the input.

  FIG. 8 is a diagram showing the relationship between the ratio of the first half area length to the total length of the Mach-Zehnder modulator and the EVM in the second embodiment of the present invention, and FIG. 8A shows the ratio of the first half area length to the total length of the phase modulation area. FIG. 8B shows the relationship between the 3 dB band normalized by the symbol rate and the optimum point of the ratio of the first half area length to the total length. In FIG. 8A, plots are made when the 3 dB band normalized by the symbol rate is 0.1, 0.2, 0.6, and 1. When the 3 dB band normalized by the symbol rate is less than 0.1, the 3 dB band is extremely small with respect to the signal band, so that it is practically impossible to use. In FIG. 8A, each curve has a minimum value when the ratio of the first half area length to the total length is a specific value, and the ratio (optimum point) of the first half area length to the total length where the EVM has the minimum value is Different for each 3 dB band normalized by the symbol rate. In FIG. 8B, it can be seen that the optimum point of the ratio of the first half region length to the total length varies depending on the 3 dB band, but is within the range of 0.3 to 0.5 in the realistic 3 dB band range. Even with such a configuration, the effect of the optical modulator according to the present invention is exhibited.

[Third Embodiment]
FIG. 9 is a diagram showing a configuration of a Mach-Zehnder modulator 900 which is an optical modulator according to the third embodiment of the present invention. FIG. 9A is a top view of the Mach-Zehnder modulator 900, and FIG. ) Is a cross-sectional view taken along line AA ′ of the Mach-Zehnder modulator 900, and FIG. 9C is a cross-sectional view taken along line BB ′. In the Mach-Zehnder modulator 900 of FIG. 9, the first bias electrode 910, the second bias electrode 914, and the third bias electrode 915 extend over the entire length of the first phase modulation region 951 and the second phase modulation region 952. Is formed. Here, as shown in the AA ′ cross-sectional view of the first phase modulation region 951 in FIG. 9B, the second bias electrode 914 is not connected to the core layer 911, and The bias electrode 915 is also not connected to the core layer 911. On the other hand, the first bias electrode 910 is not connected to the core layer 911 as shown in the BB ′ sectional view of the second phase modulation region 952 in FIG.

  The arm electrode to which the RF signal is applied has a characteristic impedance determined by the structural relationship with the surrounding electrodes. Therefore, by unifying the electrode structure in the first phase modulation region and the second phase modulation region, The mismatch of characteristic impedance between the modulation regions is suppressed to a small level, and good modulation characteristics can be obtained. Even with such a configuration, the effect of the optical modulator according to the present invention is exhibited.

101, 301, 701, 901 Substrate 102, 302, 702, 902 Input optical port 103, 303, 703, 903 1 × 2 optical coupler 104, 105, 304, 305, 704, 705, 904, 905 Arm waveguide 104 -1, 105-1, 304-1, 305-1, 304-3, 305-3, 904-1, 905-1, 904-3, 905-3 n-doped regions 104-2, 105-2, 304 -2, 305-2, 904-2, 905-2 p-doped regions 106, 306, 706, 906 2 × 1 optical couplers 107, 307, 707, 907 Output optical ports 108, 109, 308, 309, 521, 522, 524, 525, 708, 709, 908, 909 Arm electrodes 110, 310, 314, 315, 523, 526, 527-530 , 710, 714, 715, 910, 914, 915 Bias electrode 111, 311, 711, 911 Core layer 112, 113, 312, 313, 712, 713, 912, 913 Clad layer 316, 716, 916-1 to 916 5 Bridge electrodes 351, 751, 951 First phase modulation area 352, 752, 952 Second phase modulation area 501 Package 502 Input optical connector 503 Output optical connector 504-1 to 504-4 RF connector 505 DC pin 506 Optical fiber 507 Fiber block 508 Modulator chips 509-1 and 509-2 Phase adjusting heaters 510-1 to 510-4 Chip resistor 511 Termination substrate

Claims (7)

An optical coupler formed on the substrate for splitting the input light into two branches;
A first arm waveguide formed on the substrate, having a pn junction along a light guiding direction, and guiding one of the branched lights from the optical coupler;
A second arm waveguide formed on the substrate, having a pn junction along a light guiding direction, and guiding the other of the branched light from the optical coupler;
An optical modulator comprising: an optical coupler formed on the substrate and configured to multiplex light from the first arm waveguide and light from the second arm waveguide;
The light input sides of the first and second arm waveguides are first phase modulation regions,
The light output side of the first and second arm waveguides is a second phase modulation region,
In the first arm waveguide in the first phase modulation region and the first arm waveguide in the second phase modulation region, the p-doped region and the n-doped region are in opposite positions,
The second arm waveguide in the first phase modulation region and the second arm waveguide in the second phase modulation region have p-doped regions and n-doped regions in opposite positions. An optical modulator characterized by.   The first and second arm waveguides in any one of the first phase modulation region and the first phase modulation region have a p-doped region on the substrate edge side, and The first and second arm waveguides in the other phase modulation region of the first phase modulation region and the first phase modulation region have an n-doped region on the edge side of the substrate. Item 4. The optical modulator according to Item 1. The phase modulation region having the first and second arm waveguides in which the edge side of the substrate is a p-doped region is formed between the first arm waveguide and the second arm waveguide. 1 bias electrode, and an arm electrode for applying an RF signal formed on the substrate edge side of the first arm waveguide and the substrate edge side of the second arm waveguide,
The phase modulation region having the first and second arm waveguides whose edge side of the substrate is an n-doped region is formed between the first arm waveguide and the second arm waveguide. An arm electrode for applying two RF signals; and a bias electrode formed on the substrate edge side of the first arm waveguide and on the substrate edge side of the second arm waveguide. The optical modulator according to claim 2.   The bias electrode in the phase modulation region having the first and second arm waveguides where the edge side of the substrate is a p-doped region, and the first and second regions where the edge side of the substrate is an n-doped region The optical modulator according to claim 3, wherein the bias electrode in the phase modulation region having an arm waveguide is connected by a bridge electrode.   The length of the first phase modulation region is 30 to 50% of the total of the length of the first phase modulation region and the length of the second phase modulation region. 5. The optical modulator according to any one of 4 above.   5. The optical modulator according to claim 3, wherein the bias electrode has a length equal to a total length of the first and second phase modulation regions.   The optical modulator according to claim 1, wherein the substrate is a single substrate.