JP2011043575A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To compactly achieve an integrated optical modulator having a means for accurately regulating an intensity ratio of an optical signal output from each of a plurality of optical modulation circuits and being coupled to an output port. <P>SOLUTION: The optical modulator includes optical branch circuits (21-1 to 7) for branching the optical signal input to an input port into eight signals; the optical modulation circuits (24-1 to 8); and optical coupling circuits (26-1 to 7) for coupling the optical signal to be output to the output port. The optical modulator further includes MZI circuits (30-1 to 8), provided between the optical branch circuit and the optical modulation circuit, or between the optical modulation circuit and the optical coupling circuit and being capable of regulating an attenuation amount of intensity of light for regulating the optical intensity output from the optical modulation circuit. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical modulator applicable to an optical communication system.

  In recent years, in order to improve the band utilization efficiency in an optical communication system, studies have been actively conducted to introduce a multi-level modulation method used in the field of wireless communication into optical communication.

  As one of the multi-level modulation schemes, there is a quadrature amplitude modulation (QAM) scheme in which signal points are arranged in a lattice pattern on a signal space diagram. Non-Patent Document 1 reports optical transmission using 64-value QAM modulation (64QAM). Since the symbol rate is 1 Gbaud with respect to the data rate of 6 Gb / s, the signal spectrum width is as small as 2 GHz, and high spectrum utilization efficiency (3 bits / s / Hz) is realized. Non-Patent Document 2 reports 112 Gb / s × 10 channel WDM transmission using 16-level QAM modulation (16QAM) and orthogonal polarization multiplexing. Since the symbol rate is 14 Gbaud due to 16QAM and polarization multiplexing, narrow channel spacing transmission with a wavelength channel spacing of only 25 GHz has been successful.

  As described above, a large number of optical transmission experiments using multilevel modulation have been reported in recent years. Generally, in order to generate a multilevel optical modulation signal, a relatively complex optical modulator is required.

FIG. 1 is a diagram showing an optical modulator (conventional example 1) using a nested Mach-Zehnder Interferometer (MZI) configuration used in many examples including Non-Patent Documents 1 and 2. is there. As shown in FIG. 1, in the optical modulator of Conventional Example 1, MZI modulation circuits 1 and 2 having high-frequency signal electrodes 12-1 to 12-4 are embedded in each arm of a large MZI circuit (nested). The output optical signals from the respective MZI modulation circuits are combined and output by the optical coupling circuit 16 after the relative phase is adjusted by the phase shifters 14-1 and 14-2. Yes. Each MZI modulation circuit is push-pull driven, and the output signals of both are coupled to the output port with an intensity ratio of 1: 1 and a phase difference of 90 degrees, so that the output signal of the MZI modulation circuit 1 is the I phase and the MZI modulation circuit 2 An output signal having a Q-phase output signal can be obtained. If each MZI modulation circuit is push-pull driven with a binary drive electric signal, a quadrature phase-shift keying (QPSK) signal can be obtained. Can drive 16QAM signals (Non-Patent Document 2) by driving 4 values, and 64QAM signals (Non-Patent Document 3) by driving 8 values. It is often called a modulator or an optical IQ modulator. Further, it can also be used as a single side-band (SSB) modulator. In this embodiment, a modulator is formed on a Z-cut LiNbO ₃ (LN) substrate. As is well known, an LN crystal has a Pockels effect, which is a kind of electro-optic (EO) effect, and is capable of high-speed refractive index modulation by applying an electric field. There is a sufficient track record of practical application.

FIG. 2 is a diagram showing a more complicated optical modulator (conventional example 2) proposed in Non-Patent Documents 3, 4 and 5. As shown in FIG. 2, the optical modulator of Conventional Example 2 has a configuration in which two nested MZI modulation circuits shown in Conventional Example 1 are integrated in parallel. When each nest type MZI modulation circuit is driven by two systems of binary electrical signals to output a QPSK signal and combined with an intensity ratio of 4: 1 (electric field amplitude ratio of 2: 1) and a phase difference of zero or 90 degrees, A 16QAM signal is obtained by vector synthesis on a signal space diagram as shown in FIG. The configuration as in Conventional Example 2 shown in FIG. 2 is advantageous for high-speed driving because a 16QAM signal can be generated by driving only with a binary electric signal and there is no need to use a multi-level electric signal. As discussed in detail in Non-Patent Document 4, if the MZI modulation circuit is binary push-pull driven with a voltage amplitude that gives the phase difference between arms + π to −π, the optical output caused by the drive electric signal noise can be reduced. There is also an advantage that fluctuations can be minimized and interference between symbols can be suppressed. As shown in Non-Patent Document 6, this configuration is further expanded and N nested MZI modulation circuits are connected in parallel and each of them is QPSK driven, and their outputs are zero relative phase or 90 degrees, intensity ratio 4 ^{N- 1} : 4 ^N-2 : ...: 4: 1 (electric field amplitude ratio 2 ^N-1 : 2 ^N-2 : ...: 2: 1), 4 ^N QAM signal can be generated It is.

  As described above, an optical modulator used for multilevel modulation needs to have a function of combining a plurality of optical signals with a predetermined intensity ratio and relative phase inside the modulator. It is desirable that the intensity ratio and the relative phase are accurately adjusted. However, in the conventional technique, although a phase shifter is provided as a relative phase adjusting means, an intensity ratio adjusting means is not provided. In the single nest type optical modulator shown in FIG. 1, since the relative intensity is usually set to 1: 1 as described above, for example, the optical axis (the light incident on the input port when viewed as a 1 × 2 coupler) is used. If a Y-shaped coupler having a line-symmetric structure with respect to the traveling direction) is used, a desired intensity ratio can be obtained with relatively high accuracy, and the necessity of intensity ratio adjusting means is not necessarily high. However, when the intensity ratio deviation due to a manufacturing error is large, or when used in an asymmetric intensity ratio (4: 1 or the like) as described later, it is desirable that an intensity ratio adjusting means is provided. In addition, in a more complicated configuration as shown in FIG. 2, it is indispensable to accurately adjust to an asymmetric intensity ratio such as 4: 1. In Non-Patent Document 3, the output optical signal intensity is attenuated by 75% by driving one nested MZI modulation circuit with a voltage amplitude of ½ of the other, and an intensity ratio of 4: 1 is obtained. Since it is necessary to adjust the intensity ratio of the drive electric signal, accurate adjustment is difficult, and the MZI modulation circuit is driven between the phase differences between arms + π / 2 to −π / 2. The effect of suppressing intersymbol interference as shown is not obtained. In order to enable high-precision adjustment and obtain a good output signal, it is desirable to use intensity ratio adjusting means provided in the optical circuit of the modulator instead of attenuation of the drive voltage amplitude.

  FIG. 4A shows a configuration of an optical modulator (conventional example 3) provided with an optical branching circuit 20 that can adjust the branching ratio, which is proposed in Patent Document 1, as a means for solving such a problem. FIG. FIG. 4B is a diagram showing a detailed configuration of the optical branch circuit 20 in which the branch ratio can be adjusted. Although Patent Document 1 assumes use as an SSB modulator, the circuit configuration is basically the same nested MZI modulation circuit as in Conventional Example 1. The major feature of the optical modulator of Conventional Example 3 is that a total of three substrates, one LN substrate 52 and two quartz-based planar lightwave circuit (PLC) substrates 50 and 54 as shown in FIG. It is comprised on the multichip board | substrate which joined end surfaces of each directly. Quartz PLC does not have the EO effect like LN, but the propagation loss is smaller than LN, and the optical path length is precisely adjusted using the thermo-optic (TO) effect and the refractive index change of quartz glass due to ultraviolet irradiation. Therefore, it is optimal as a platform for manufacturing passive optical circuits such as filters, optical branching, and optical coupling with high accuracy, low loss, and compactness. As shown in FIG. 4, the optical modulator of Conventional Example 3 has a configuration utilizing the features of both the LN and PLC substrate platforms, and the arm portion of the MZI modulation circuit (signal electrode 24- 1 to 24-4) is formed on the LN substrate 52, and all other circuit portions are formed on the PLC substrates 50 and 54. By making each of the optical branching portions on the PLC substrate (first substrate) 50 on the input side into MZI circuits 20-1 to 20-3 capable of adjusting the branching ratio, four LNs loaded with signal electrodes respectively. Means are provided for accurately adjusting the intensity ratio of the optical signal coupled from the waveguide (light modulation means) to the output port. As shown in FIG. 4B, as the MZI circuit 20 capable of adjusting the branching ratio, a 2 × 2 MZI circuit in which a TO phase shifter 204 in which a thin film heater is loaded on one waveguide arm is used. .

Japanese Patent No. 3800594

Jumpei Hongo, Keisuke Kasai, Masato Yoshida, and Masataka Nakazawa, “1-Gsymbol / s 64-QAM Coherent Optical Transmission Over 150 km,” IEEE Photon. Technol. Lett., Vol. 19, no. 9, pp. 638- 640, May. 2007. Peter J. Winzer and Alan H. Gnauck, “112-Gb / s Polarization-Multiplexed 16-QAM on a 25-GHz WDM Grid,” Proc. Of ECOC2008, paper Th.3.E.5, 2008. Elza Ip and Joseph M. Kahn, “Carrier Synchronization for 3- and 4-bit-per-Symbol Optical Transmission,” J. Lightwave Technol. Vol. 23, No. 12, pp. 4110-4124, 2005. N. Kikuchi, “Intersymbol Interference (ISI) SuppressionTechnique for Optical Binary and Multilevel Signal Generation,” J. Lightwave Technol., Vol. 25, No. 8, pp. 2060-2068, 2007. T. Sakamoto, A. Chiba, and T. Kawanishi, “50-Gb / s 16 QAM by a quad-parallel Mach-Zehnder modulator,” Proc. Of ECOC2007, PDS 2.8, 2007. T. Sakamoto, A. Chiba, and T. Kawanishi, “High-bit-rate optical QAM,” Proc. OFC2009, OWG5, 2009. K. Jinguji, N. Takato, A. Sugita, and M. Kawachi, “Mach-Zehnder interferometer type optical waveguide coupler with wavelength-flattened coupling ratio,” Electron. Lett., Vol. 26, No. 17, pp. 1326 -1327, 1990.

  However, the intensity ratio adjusting means as in Conventional Example 3 has the following problems. In general, the size of the light propagation direction (longitudinal direction) of an MZI circuit capable of adjusting the branching ratio is several mm to 1 cm larger than that of a single directional coupler or Y-shaped branching circuit in which the branching ratio cannot be adjusted. . This is because the MZI circuit is originally composed of two couplers (each of which is a directional coupler and a Y-shaped branch circuit) and two arms, and each arm has a development section for sufficiently increasing the mutual distance. In addition, in order to be able to adjust the branching ratio, it is necessary to provide the arm with a portion for adjusting the optical path length (a TO phase shifter or an ultraviolet irradiation portion). If such large MZI circuits are connected in series as in Conventional Example 3, the circuit size increases.

  This effect increases as the degree of circuit integration increases. For example, an optical modulator having a configuration in which the idea of the conventional example 3 is simply applied to the multi-level optical modulator configuration as in the conventional example 2 can be considered.

  FIG. 6A is a diagram illustrating a configuration of the optical modulator having the above configuration (Reference Example 1). FIG. 6B is a diagram illustrating a configuration of a branch circuit (an MZI circuit capable of adjusting a branch ratio) included in the optical modulator of Reference Example 1. As shown in FIG. 6A, the reference example 1 adopts a PLC-LN-PLC three-substrate configuration as in the conventional example 3, and the nested MZI modulation circuit shown in the conventional example 3 (the branching ratio can be adjusted). Two MZI circuits 20-1 to 20-6) are connected in parallel. Further, the branch circuit 20-7 for branching light from the input port to each of the nested MZI modulation circuits 1 and 2 is also an MZI circuit capable of adjusting the branch ratio, and the TO phase shifter 22- 3 and 22-6 are arranged. In other words, the optical modulator of Reference Example 1 has a configuration in which the optical modulator of Conventional Example 3 is simply further scaled up by one stage. With such a configuration, the intensity ratio of the optical signal coupled to the output port from the eight LN waveguides (optical modulation means) loaded with the signal electrodes 24-1 to 24-8 is accurately adjusted. be able to. Therefore, a 16-QAM signal can be generated by binary driving as in Conventional Example 2, and a multi-level optical modulator capable of accurately adjusting the intensity ratio of the internal optical signal can be realized. However, MZI circuits that can adjust the branching ratio are connected in cascade in three stages, and the size in the longitudinal direction is about several centimeters compared to the case where all of these are configured with Y-shaped branches or the like (the branching ratio cannot be adjusted). growing. This is a non-negligible size increase considering that the longitudinal substrate size of a typical LN optical modulator is 10 cm or less.

  The present invention has been made in view of such problems, and an object thereof is to accurately adjust the intensity ratio of an optical signal output from each modulation circuit and coupled to an output port, including a plurality of optical modulation circuits. It is to realize an integrated optical modulator having means for compactly.

  In order to achieve such an object, the present invention provides an optical modulator having a main input port and a main output port, wherein n is an integer of 2 or more, and the main input Optical branching means for branching an optical signal input to a port to n output ports, n optical modulation means, m is an integer equal to n or an integer less than n, and m light intensity adjustments And optical coupling means for combining optical signals input to n input ports and outputting the combined optical signals to the main output port, wherein i is an integer of 1 to n, and the optical modulation means is an arbitrary i Is arranged in an optical path connecting the i-th output port of the optical branching means and the i-th input port of the optical coupling means, j is an integer of 1 to m, and the light intensity adjusting means is , For any j, the optical branching hand In the optical path connecting the j-th output port and the j-th optical modulation means, or in the optical path connecting the j-th optical modulation means and the j-th input port of the optical coupling means. Features.

  According to a second aspect of the present invention, each of the light intensity adjusting means according to the first aspect includes two arm optical waveguides and two optical couplers connecting them, and the optical path length between the arm optical waveguides. It is a Mach-Zehnder circuit capable of adjusting the light intensity of output light by adjusting the difference.

  According to a third aspect of the present invention, the Mach-Zehnder circuit according to the second aspect can arbitrarily adjust the optical intensity and optical phase of the output light by independently adjusting the optical path lengths of the two arm optical waveguides. It is a Mach-Zehnder circuit that can be adjusted to the above.

  According to a fourth aspect of the present invention, the optical modulator according to the second or third aspect further includes a monitor output port, and the Mach-Zehnder circuit is connected to the main output port and the monitor output port. It has an output port.

  According to a fifth aspect of the present invention, each of the optical modulation means according to the first aspect is a linear waveguide loaded with a high-frequency signal electrode, a single Mach-Zehnder interferometer circuit, or a nested Mach-Zehnder interferometer circuit. It is characterized by.

  According to a sixth aspect of the present invention, the optical modulator according to the first aspect comprises a planar lightwave circuit formed on three substrates whose end faces are directly bonded to each other, and the first and third substrates. The planar lightwave circuit formed on the substrate is made of silica glass, and the planar lightwave circuit formed on the second substrate is made of a multi-component oxide, compound semiconductor, or polymer whose refractive index or light absorption property changes when an electric field is applied. Each of the light modulation means includes a high-speed phase shifter formed on the second substrate, the light intensity adjusting means is formed on the first or third substrate, and the light branching means is the It is formed on a first substrate, and the optical coupling means is formed on the third substrate.

  As described above, according to the present invention, n (n is an integer satisfying n ≧ 2) light modulations by m (integers satisfying n ≧ m ≧ n−1). The intensity ratio of the optical signal output from the means and coupled to the main output port can be accurately adjusted, and the j-th (j is an integer between 1 and m) light intensity adjusting means , Disposed in the optical path connecting the jth output port of the optical branching means and the jth optical modulation means, or in the optical path connecting the jth optical modulation means and the jth input port of the optical coupling means. Therefore, the light intensity adjusting means are all connected in parallel, and an increase in circuit size due to the series connection of the intensity ratio adjusting means can be avoided, and a compact integrated optical modulator can be configured.

It is a figure showing the circuit structure of the nest type MZI modulator which is a 1st prior art example. It is a figure showing the circuit structure of the optical 16QAM modulator which is a 2nd prior art example. It is a figure explaining the principle which produces | generates a 16QAM modulation signal in a 2nd prior art example, a 1st reference example, and the 1st, 2nd Example of this invention. (A) is a figure showing the circuit structure of the optical SSB modulator which is a 3rd prior art example, (b) is an adjustable MZI circuit which comprises the branch part of the optical SSB modulator of a 3rd prior art example. It is a figure showing a structure. It is a perspective view which shows the structure of 3 board | substrate structure used by the 3rd prior art example and the Example of this invention. (A) is a figure showing the circuit structure of the integrated modulator which is a reference example, (b) shows the structure of the MZI circuit which can adjust the branching ratio which comprises the branch part of the integrated modulator of a reference example. FIG. (A) is a figure showing the circuit structure of the integrated modulator which is a 1st Example of this invention, (b) is output from each modulation circuit in the integrated modulator of a 1st Example, and is output port It is a figure showing the structure of the adjustable MZI circuit which comprises the means to adjust the intensity ratio of the optical signal couple | bonded with. (A) is a figure showing the circuit structure of the integrated modulator which is a 2nd Example of this invention, (b) is output from each modulation circuit in the integrated modulator of a 2nd Example, and is output port It is a figure showing the structure of the adjustable MZI circuit which comprises the means to adjust the intensity ratio of the optical signal couple | bonded with. It is a figure explaining the principle which produces | generates 4APSK-BPSK modulation signal in the 1st and 2nd Example of this invention. (A) is a figure showing the circuit structure of the optical 64QAM modulator which is a 3rd Example of this invention, (b) is output from each modulation circuit in the integrated modulator of a 3rd Example, and is output port It is a figure showing the structure of the adjustable MZI circuit which comprises the means to adjust the intensity ratio of the optical signal couple | bonded with. It is a figure explaining the principle of 64QAM signal generation in the 3rd example of the present invention.

Hereinafter, embodiments of an optical modulator according to the present invention will be described in detail with reference to the drawings. In the following embodiments of the present invention, LN is used as a material constituting the high-speed phase shifter of the optical modulator. However, the present invention is not limited to this, and multi-element oxides such as KTa _1-x Nb _x O ₃ and K _1-y Li _y Ta _1-x Nb _x O ₃ which also have the Pockels effect instead of LN. EO effects such as crystal, electro-absorption (EA) effect and quantum confined Stark effect (Quantum Confined Stark Effect: QCSE) that can modulate the refractive index or absorption coefficient of GaAs or InP compound semiconductors, chromophores, etc. It is also possible to use a polymer having In the examples shown below, the crystal axis direction of LiNbO ₃ is Z-cut, that is, a substrate whose crystal rotation axis is perpendicular to the substrate surface is used, but the axis direction is X-cut, that is, the six-rotation axis is A substrate parallel to the substrate surface and perpendicular to the waveguide of the phase shifter portion may be used. However, since the electric field application direction is different when X cut is used, the position of the phase shifter electrode (signal electrode) in the drawings referred to in each embodiment is different.

In the embodiments described below, the optical modulator is constituted by a planar lightwave circuit formed on three substrates whose end faces are directly bonded to each other. The first substrate and the third substrate are PLCs in which an optical waveguide made of glass mainly composed of SiO ₂ is formed on a silicon substrate. The optical waveguide is an embedded type in which the core is rectangular and embedded in the clad, and the relative refractive index difference between the core and the clad is 1.5%. The second substrate is an LN substrate, on which an optical waveguide is formed. The effects of the present invention are not limited to the above-mentioned substrate types, and as described above, other multi-element oxides, compound semiconductors, EO polymers, etc. can be used as the second substrate. As the third substrate, a quartz substrate on a quartz substrate, a substrate of a low-loss waveguide made of another material such as a silicon waveguide, a polymer waveguide, or the like can be used. Quartz-based optical waveguides do not have the EO effect of LN, but can form various low-loss and small passive optical circuits such as branches, couplers, bends, and filters, and are sufficient as optical splitters and optical multiplexers / demultiplexers Has a practical introduction record. Actually, the Ti diffusion LN waveguide has a propagation loss of about 0.3 dB / cm and a minimum bending radius of about several cm, whereas the quartz-based waveguide has a propagation loss of about 0.01 dB / cm or less, and non- When the refractive index difference is 1.5%, the minimum bending radius is 1.5 mm, which is superior to the LN waveguide. The basic concept of the three-substrate configuration is to make use of the characteristics of each waveguide material by combining different substrates. In other words, an LN waveguide is used for the phase shifter portion of a modulator that requires a high-speed response, and branching, multiplexing, and phase adjustment of a modulator that does not require high-speed response but is required to be small and have low loss. As for the responsible portion, a quartz PLC is used to realize a high-performance optical modulator as a whole. FIG. 5 is a perspective view showing the connection between the substrates. An LN substrate is sandwiched between two quartz PLC substrates, and each substrate is end-face connected using an ultraviolet curable adhesive. An input optical fiber and an output optical fiber are connected to the first and third substrates, respectively. The Ti diffusion LN waveguide on the second substrate has a spot size of about 3.6 μm, and the end size of the first and third quartz-based waveguides uses a spot size conversion waveguide to change the spot size to LN. Matching with the waveguide, the coupling loss is 0.2 dB on average and the loss variation is within 0.1 dB, so that very good optical coupling is obtained. Further, this end face connection method is technically equivalent to a method of connecting an optical waveguide circuit substrate and an optical fiber array, which have a sufficient track record including reliability, and therefore has sufficient reliability. In all of the following embodiments, only the straight waveguide loaded with signal electrodes is arranged on the LN substrate, and all other circuit elements such as couplers are arranged on the PLC. Is not limited to such a configuration. For example, an MZI modulation circuit comprising two Y-shaped couplers and two arm waveguides connecting the two Y-shaped couplers is arranged in an array on the LN substrate, and other circuit portions are formed on the PLC. Is possible.

  Furthermore, in the embodiment shown below, a PLC circuit having a TO phase shifter on an arm is used as an adjustable MZI circuit, but an MZI circuit not having a TO phase shifter is used, and adjustment is performed by ultraviolet irradiation. Is also possible.

(First embodiment)
FIG. 7A is a diagram showing a configuration of an integrated optical modulator according to the first embodiment of the present invention. The optical modulator shown in FIG. 7A includes a quartz PLC substrate 50, an LN substrate 52, and a quartz PLC substrate 54.

  On the quartz PLC substrate 50, 1 × 2 couplers 21-1 to 21-7 and adjustable MZI circuits 30-1 to 30-8 are arranged.

  The 1 × 2 couplers 21-1 to 21-7 are connected in series in three stages, and constitute an optical branch circuit (optical branch means) having one input port and eight output ports. Further, the first stage 1 × 2 coupler 21-1 and the second stage 1 × 2 couplers 21-2 to 21-3 and the second stage 1 × 2 couplers 21-2 to 21-3 and the third stage Between the 1 × 2 couplers 21-4 to 21-7 of the eye, TO phase shifters 22-1 to 22-6 for phase adjustment are arranged. Adjustable MZI circuits (light intensity adjusting means) 30-1 to 30-8 are arranged in parallel on the optical path connecting the output port of the optical branching circuit and the light modulating means described later.

  2 × 1 couplers 26-1 to 26-7 are arranged on the quartz PLC substrate 54. The 2 × 1 couplers 26-1 to 26-7 are connected in series in three stages, and constitute an optical coupling circuit (optical coupling means) having eight input ports and one output port. The optical modulation means described later is connected to each input port of the optical coupling circuit.

  On the LN substrate 52, eight straight waveguides are arranged. The eight straight waveguides are loaded with signal electrodes 24-1 to 24-8, respectively. The straight waveguide loaded with the signal electrode constitutes an optical modulation means together with the LN substrate. That is, the optical modulator of this embodiment has eight optical modulation means.

  FIG. 7B is a diagram illustrating a configuration of the MZI circuit 30 that can adjust the attenuation of light intensity. The adjustable MZI circuit 30 shown in FIG. 7B connects the 1 × 2 coupler 208, the 2 × 1 coupler 210, the output port of the 1 × 2 coupler 208, and the input port of the 2 × 1 coupler 210. This is a 1 × 1 MZI circuit including a TO phase shifter 204 provided on one of the arm waveguides. In the adjustable MZI circuit 30, the input port of the 1 × 2 coupler 208 is connected to the output port of the optical branch circuit on the quartz PLC substrate 50, and the output port of the 2 × 1 coupler 210 is optically modulated on the LN substrate 52. And a light intensity adjusting means for adjusting the light intensity output from the light modulating means. That is, the optical modulator of the present embodiment has eight light intensity adjusting means, and using these, the intensity ratio adjusting means for adjusting the intensity ratio of the light output from the eight light modulating means, respectively. Constitute.

  1 × 2 coupler (21-2, 21-4, 21-5), TO phase shifter (22-1 and 22-2), adjustable MZI circuit (30-1 to 30-4), light modulation means ( Signal electrodes 24-1 to 24-4) and 2 × 1 couplers (26-1 to 26-3) constitute a nested MZI modulation circuit 1, and 1 × 2 couplers (21-3, 21-6, 21-6). 21-7), TO phase shifters (22-4, 22-5), adjustable MZI circuits (30-5 to 30-8), light modulation means (including signal electrodes 24-5 to 24-8) and The 2 × 1 couplers (26-4 to 26-6) constitute a nested MZI modulation circuit 2.

  The optical modulator of the present embodiment, like the first reference example, has a configuration in which two nested MZI modulation circuits are integrated in parallel and includes an MZI circuit capable of adjusting intensity attenuation as intensity ratio adjusting means. However, the circuit size is reduced by devising the arrangement.

  In this embodiment, all of the optical branching circuit and the optical coupling circuit (the branching ratio cannot be adjusted) are 1 × 2 Y-shaped couplers (21-1 to 21-7) and 2 × 1 Y-shaped couplers (26− 1 to 26-7), 8 output ports of 1 × 8 optical branching means by 1 × 2 coupler connected in series in three stages of input port side PLC 50, and 8 each loaded with signal electrodes An MZI circuit (light intensity adjusting means) that can be adjusted is disposed in the optical path connecting the waveguides (light modulating means) on the LN substrate. That is, all adjustable MZI circuits are arranged in parallel. By adopting such an arrangement, the circuit size can be reduced while enabling accurate adjustment of the intensity ratio of the optical signal coupled from each optical modulation means to the output port, as in Reference Example 1. This is because, in Reference Example 1, three stages of MZI circuits that can be adjusted in the longitudinal direction are arranged in series in the PLC board on the input port side, whereas in this embodiment, an equivalent function is provided in the PLC board on the output port side. This is realized by three stages of Y-shaped couplers arranged in the longitudinal direction and one stage of adjustable MZI. Since the longitudinal size of the Y-shaped coupler is generally about several hundred μm to 1 mm except for the developed portion, the adjustable longitudinal direction size of the MZI circuit is generally about 4 mm to 1 cm except for the developed portion. According to the configuration of the embodiment, the size in the longitudinal direction can be reduced by about several mm to several cm with respect to the reference example 1.

(Second embodiment)
FIG. 8A is a diagram showing a configuration of an integrated optical modulator according to the second embodiment of the present invention.
The optical modulator shown in FIG. 8A includes a quartz PLC substrate 50, an LN substrate 52, and a quartz PLC substrate 54.

  On the quartz PLC substrate 50, 1 × 2 couplers 21-1 to 21-7 are arranged. The 1 × 2 couplers 21-1 to 21-7 are connected in series in three stages, of which the first stage 1 × 2 coupler 21-1 and the second stage 1 × 2 couplers 21-2 and 21-3 are connected. Constitute an optical branching circuit (optical branching means) having one input port and four output ports.

  2 × 1 couplers 26-1 to 26-7 are arranged on the quartz PLC substrate 54. The 2 × 1 couplers 26-1 to 26-7 are connected in series in three stages, of which the second stage 2 × 1 couplers 26-5 and 26-6 and the third stage 2 × 1 coupler 26-7 Constitutes an optical coupling circuit (optical coupling means) having four input ports and one output port. Between the output ports of the first-stage 2 × 1 couplers 26-1 to 26-4 and the input ports of the second-stage couplers 26-5 and 26-6 (input ports of the optical coupling circuit) will be described later. MZI circuits 40-1 to 40-4 capable of adjusting the amount of attenuation of light are arranged in parallel.

  On the LN substrate 52, eight straight waveguides are arranged. The eight straight waveguides are loaded with signal electrodes 24-1 to 24-8, respectively. Two adjacent straight waveguides out of the eight linear waveguides loaded with signal electrodes constitute an optical modulation means together with the 1 × 2 coupler 21 and the 2 × 1 coupler 26 at both ends thereof. Specifically, the 1 × 2 coupler 21-4, the linear waveguide (including the signal electrodes 24-1 and 24-2), and the 2 × 1 coupler (26-1) constitute the MZI modulation circuit 1, and 1 × The 2 coupler 21-5, the straight waveguide (including the signal electrodes 24-3 and 24-4) and the 2 × 1 coupler (26-2) constitute the MZI modulation circuit 2. Similarly, the 1 × 2 coupler 21-6, the linear waveguide (including the signal electrodes 24-5 and 24-6) and the 2 × 1 coupler (26-3) constitute the MZI modulation circuit 3, and the 1 × 2 coupler. 21-7, the straight waveguide (including the signal electrodes 24-7 and 24-8) and the 2 × 1 coupler (26-4) constitute the MZI modulation circuit 4. The four MZI modulation circuits (light modulation means) are connected to the input port of the optical coupling circuit. Adjustable MZI circuits (light intensity adjusting means) 30-1 to 30-8 are arranged on the optical path connecting the output port of the optical coupling circuit and the light modulating means, respectively.

  FIG. 8B is a diagram illustrating a configuration of the adjustable MZI circuit 40. The adjustable MZI circuit 40 shown in FIG. 8B connects the 2 × 2 3 dB coupler 202, the 2 × 2 3 dB coupler 206, the output port of the 2 × 2 coupler 202, and the input port of the 2 × 2 coupler 206. This is a 2 × 2 MZI circuit including TO phase shifters 204-1 and 204-2 provided in two arm waveguides respectively. In the adjustable MZI circuit 40, one of the input ports of the 2 × 2 coupler 202 is connected to the output port of the 2 × 1 coupler 26 in the first stage, and one of the output ports of the 2 × 2 coupler 206 is connected to the second stage. Connected to the input port of the 2 × 1 coupler 26. The other one of the output ports of the 2 × 2 coupler 206 of the adjustable MZI circuit 40 is used as a monitor port. In other words, the optical modulator of this embodiment has four light intensity adjusting means, and these are used to constitute intensity ratio adjusting means for adjusting the intensity ratio of light output from each light modulating means.

  The configuration of the optical modulator of the present embodiment is an integrated optical modulator in which two nested MZI modulation circuits, which are the same as those of the first embodiment, are integrated in parallel, but further improvements are added in the following four points.

  The first point is that an adjustable MZI circuit is provided in the PLC on the output port side instead of the input port side. The adjustment of light intensity is essentially the same regardless of whether it is performed on the input port side or the output port side, but the latter is advantageous in terms of circuit layout when a monitor light extraction function described later is added.

  The second point is that the MZI circuit capable of adjusting the amount of light attenuation is not immediately after the waveguide on the LN substrate, but is composed of two waveguides on the LN substrate and two Y-shaped couplers on the PLC. The point is that it is arranged immediately after the MZI modulation circuit (the light modulation means in this embodiment). As described above, if the binary push-pull drive is performed with a voltage amplitude that gives the phase difference between arms + π to −π, the MZI modulation circuit minimizes the fluctuation of the optical output caused by the drive electric signal noise, and between the symbols. Although interference can be suppressed, this effect is obtained when the ratio of the intensity of the optical signal coupled from each arm to the output port in the MZI modulation circuit is 1: 1. It is relatively easy to accurately realize the optical signal intensity ratio of 1: 1, and the optical axis (the traveling direction of light incident on the input port when viewed as a 1 × 2 coupler) as in this embodiment. Two Y-shaped couplers having a line-symmetric structure are used, or two 2 × bidirectional couplers having a coupling rate of about 3 dB and two multimode interference (MMI) couplers are used as an MZI modulation circuit. The input port and the output port of the MZI modulation circuit may be arranged in a cross arrangement (an arrangement in which the input port and the output port are different from each other with respect to the straight line connecting the centers of the couplers on the input side and the output side). The necessity for light intensity adjustment is not necessarily high. Therefore, in this embodiment, an adjustable MZI circuit 40 is provided outside the MZI modulation circuit. Thus, the total number of MZI circuits that can be adjusted is eight in the first embodiment, but is reduced to four in this embodiment. Since the number of stages of the Y branch and MZI circuit connected in series is the same as that of the first embodiment, there is no change in that the circuit can be configured compactly.

The third point is that in the adjustable MZI circuit 40, the TO phase shifter is arranged on both arms instead of one arm. As a result, the function of adjusting the optical phase in addition to the light intensity can be combined with one adjustable MZI circuit 40, so that it is not necessary to provide a phase shifter outside the MZI circuit, and the circuit size can be further reduced. It becomes. Since the longitudinal size of the TO phase shifter is generally several millimeters or less, the circuit size reduction effect with respect to Conventional Example 1 is about several millimeters or less. The reason why the MZI circuit 40 with the two-arm TO phase shifter shown in FIG. 8B can serve both the light intensity and phase adjustment functions will be described below. In an MZI circuit comprising two 2 × 2 3 dB couplers 202 and 206 and two arms connecting them, the optical phase delay amounts given by the TO phase shifters 204-1 and 204-2 of each arm are respectively φ ₁ , Φ ₂ . When light of intensity 1 is input, the output optical electric field in the cross arrangement is

であるから、出力光強度、光位相はそれぞれ

Therefore, the output light intensity and optical phase are

It becomes. That is, the output light intensity is a function of Δφ, and _the output light phase is a function of φ _avg . If the TO phase shifters of both arms are used, Δφ _and φ _avg can be controlled independently of each other, so that the output light intensity and phase can be arbitrarily set. If φ ₁ = 0 and φ ₂ = 0 when the applied electric power to the TO phase shifter is zero, φ ₁ ≧ 0 and φ ₂ ≧ 0 because of heater control.

  The fourth point is that, of the two ports on the output side of the adjustable MZI circuit 40, one port (cross side) is used as the main output port and the other one port (through side) is used as the monitor output port. The monitor output port is directly pulled out to the end of the PLC chip, and the monitor light can be taken out by connecting an optical fiber here. Thereby, the optical output of each MZI modulation circuit can be individually monitored, and the DC bias and the voltage amplitude of the drive signal can be adjusted. That is, in the present embodiment, the adjustable MZI circuit 40 also serves as three roles of light intensity adjustment, optical phase adjustment, and monitor tap. As a means for extracting the monitor light to the outside of the circuit, in addition to the above, for example, a method of extracting the monitor light vertically above the PLC substrate 54 using a 45 degree mirror is conceivable.

  As described above, the present embodiment realizes an integrated optical modulator that has a sufficient number of intensity ratio adjusting means, is compact, and includes means for monitoring each optical modulation circuit.

  The modulator shown in the present embodiment can not only generate a 16QAM modulation signal on the same driving principle as the conventional example 2 shown in FIG. 3, but also, for example, optical signals coupled from each MZI modulation circuit to the output port as shown in FIG. The signal intensity ratio is 16: 4: 1: 0 (electric field intensity ratio 4: 2: 1: 0), and the relative phases are all zero, whereby four-level intensity modulation (4-ASK) and 2 A 4ASK-BPSK modulation signal combined with binary phase-shift keying (BPSK) can also be generated. Of course, the same driving is possible with the configuration of the first embodiment.

(Third embodiment)
FIG. 10A is a diagram showing a configuration of an integrated optical modulator which is a third embodiment of the present invention. The optical modulator shown in FIG. 10A includes a quartz PLC substrate 50, an LN substrate 52, and a quartz PLC substrate 54.

  On the quartz PLC substrate 50, 1 × 2 couplers 21-1 to 21-11 are arranged. The input port of the 1 × 2 coupler 21-2 is connected to one of the two output ports of the 1 × 2 coupler 21-1, and the input port of the 1 × 2 coupler 21-8 is connected to the other output port. Is connected. Input ports of 1 × 2 couplers 21-3 and 21-9 are connected to two output ports of the 1 × 2 coupler 21-8, respectively. Further, the two output ports of the 1 × 2 couplers 21-2, 21-3, and 21-9 include 1 × 2 couplers 21-4 and 21-5, 21-6, 21-7, and 21-10. Input ports 21-11 are connected to each other. The 1 × 2 couplers 21-1 and 21-8 constitute an optical branching circuit (optical branching unit) having one input port and three output ports.

  On the LN substrate 52, 12 linear waveguides are arranged. The 12 linear waveguides are loaded with signal electrodes 24-1 to 24-12, respectively.

  On the quartz PLC substrate 54, 2 × 1 couplers 26-1 to 26-11 and adjustable MZI circuits 40-1 to 40-3 are arranged. The input ports of the 2 × 1 couplers 26-1 to 26-6 are connected to 12 straight waveguides on the LN substrate 52, respectively. The output ports of 2 × 1 couplers 26-1 and 26-2 are connected to the input port of 2 × 1 coupler 26-7, and the output ports of 2 × 1 couplers 26-3 and 26-4 are 2 × 1 coupler 26−. The output ports of the 2 × 1 couplers 26-5 and 26-6 are connected to the input port of the 2 × 1 coupler 26-9. The output ports of the 2 × 1 couplers 26-8 and 26-9 are connected to the input ports of the 2 × 1 coupler 26-10, and the output ports of the 2 × 1 couplers 26-7 and 26-10 are connected to the 2 × 1 coupler 26−. 11 input ports. The 2 × 1 couplers 26-10 and 26-11 constitute an optical coupling circuit (optical coupling means) having three input ports and one output port. Between the output port of the 2 × 1 couplers 26-7 to 26-9 and the input port of the optical coupling circuit, MZI circuits 40-1 to 40-3 capable of adjusting the attenuation of light are arranged in parallel. Yes.

  FIG. 10B is a diagram showing the configuration of the adjustable MZI circuit 40. As shown in FIG. Since the configuration is the same as that of the adjustable MZI circuit 40 shown in FIG. In this embodiment, one of the input ports of the 2 × 2 coupler 202 constituting the adjustable MZI circuits 40-1 to 40-3 is connected to the output port of the 2 × 1 couplers 26-7 to 26-9. One of the output ports of the × 2 coupler 206 is connected to the input port of the optical coupling circuit.

  Of the 12 linear waveguides loaded with signal electrodes on the LN substrate 52, the adjacent four waveguides are optically modulated together with the 1 × 2 coupler 21 and the 2 × 1 coupler 26 connected in two stages at both ends thereof. Configure the means. Specifically, 1 × 2 couplers 21-2, 21-4, and 21-5, optical modulation means (including signal electrodes 24-1 to 24-4), 2 × 1 couplers 26-1, 26-2, and 26-7 constitutes a nested MZI modulation circuit 1 and includes 1 × 2 couplers 21-3, 21-6 and 21-7, optical modulation means (including signal electrodes 24-5 to 24-8) and 2 × 1. The couplers 26-3, 26-4, and 26-8 constitute a nested MZI modulation circuit 2, and 1 × 2 couplers 21-9, 21-10, and 21-11, optical modulation means (signal electrodes 24-9 to 24). -12) and 2 × 1 couplers 26-5, 26-6, and 26-9 constitute a nested MZI modulation circuit 3.

  The configuration of the optical modulator of the present embodiment is a configuration in which the integration scale is further increased and three nested MZI modulation circuits are connected in parallel. With such a configuration, as shown in FIG. 10, a 64QAM signal can be generated by driving only with a binary electric signal. For this purpose, each nest type MZI modulation circuit (optical modulation means in this embodiment) is coupled to an output port. It is necessary to accurately adjust the optical signal intensity ratio to be 16: 4: 1 (electric field amplitude ratio 4: 2: 1). In this embodiment, the optical branch circuit (1 × 3) and the optical coupling circuit (3 × 1) are configured by 1 × 2 and 2 × 1 Y-shaped couplers, and each of the nested MZI modulation circuits and the optical coupling circuit An adjustable MZI circuit is arranged in each of the optical paths connecting the three input ports. The adjustable MZI circuit has a structure in which both arms are equipped with a TO phase shifter and a main output port and a monitor output port, as in the second embodiment, and has three functions: light intensity adjustment, optical phase adjustment, and monitor tap. It is a circuit that doubles as.

  Unlike the second embodiment, the MZI circuit capable of adjusting the amount of light attenuation is arranged not immediately after the MZI modulation circuit (total of six in this embodiment) but immediately after the nested MZI modulation circuit (total of three). No means for adjusting the intensity ratio of the output signal light from each MZI modulation circuit is provided in the nested MZI modulation circuit for the following reason. The modulator of this embodiment is specialized for the purpose of generating a 64QAM modulated signal based on the principle shown in FIG. 10, and each nested MZI modulation circuit is designed on the assumption that it is QPSK driven. In this case, since the output signal light from each MZI modulation circuit in the nested MZI modulation circuit is always coupled at an intensity ratio of 1: 1, the necessity of adjusting the light intensity ratio at this portion is not necessarily high for the reasons described above. Therefore, in this embodiment, an MZI circuit that can be adjusted from the viewpoint of configuring the circuit more simply is arranged at the subsequent stage of the nested MZI modulation circuit.

  As another approach for constructing an optical modulator equivalent to the present embodiment, the adjustable MZI circuit is not arranged immediately after the nested MZI modulation circuit, but the idea of Reference Example 1 is applied to apply 3 × 1 light. A method is also conceivable in which the 2 × 1 coupler portion of the coupling circuit is an adjustable MZI circuit. However, in this method, the adjustable MZI circuit has a configuration in which two stages are connected in series in the longitudinal direction, so that the size in the longitudinal direction of several millimeters to several centimeters increases as compared with the arrangement of FIG. Furthermore, in order to individually monitor the output of each nested MZI modulation circuit, a tap circuit must be newly provided between each nested MZI modulation circuit and each input port of the 3 × 1 optical coupling circuit. Therefore, the circuit size is further increased. Therefore, instead of using an adjustable MZI circuit connected in series to the optical branching unit or coupling unit as in Reference Example 1, it is used between the modulation circuit and the optical branching unit or coupling unit as in the present invention. It can be seen that the circuit can be configured more compactly by arranging the adjustable MZI circuit in parallel.

  In each of the embodiments described above, the case where the number of light modulating means is the same as the number of light intensity adjusting means (adjustable MZI circuits) has been described. However, the number of adjustable MZI circuits is reduced by one. The light intensity attenuation amount of the adjustable MZI circuit may be set on the basis of the output intensity of the light modulation means to which the adjustable MZI circuit is not connected.

DESCRIPTION OF SYMBOLS 10 Optical branch circuit 12 Signal electrode 14 Phase shifter 16 Optical coupling circuit 21,208 1 * 2 coupler 20, 30, 40 Adjustable MZI circuit 22,204 TO phase shifter 24 Signal electrode 26,210 2 * 1 coupler 50,54 Quartz-based PLC substrate 52 LiNbO ₃ substrate 202,206 2 × 2 3 dB coupler

Claims (6)

An optical modulator having a main input port and a main output port,
an optical branching means for branching an optical signal input to the main input port to n output ports, wherein n is an integer of 2 or more;
n light modulation means;
m is an integer equal to n or an integer less than n, and m light intensity adjusting means;
optical coupling means for coupling optical signals input to the n input ports and outputting the combined optical signals to the main output port;
i is an integer not less than 1 and not more than n, and the optical modulation means is arranged in an optical path connecting the i-th output port of the optical branching means and the i-th input port of the optical coupling means for an arbitrary i. And
j is an integer not smaller than 1 and not larger than m, and the light intensity adjusting means is, for an arbitrary j, in the optical path connecting the j-th output port of the optical branching means and the j-th light modulating means or the j-th An optical modulator, wherein the optical modulator is disposed in an optical path connecting the optical modulation means and the jth input port of the optical coupling means.   Each of the light intensity adjusting means comprises two arm optical waveguides and two optical couplers connecting them, and the light intensity of the output light can be adjusted by adjusting the optical path length difference between the arm optical waveguides. The optical modulator according to claim 1, wherein the optical modulator is a Mach-Zehnder circuit.   The Mach-Zehnder circuit is a Mach-Zehnder circuit capable of arbitrarily adjusting the light intensity and optical phase of output light by independently adjusting the optical path lengths of the two arm optical waveguides. The optical modulator according to 2. A monitor output port;
4. The optical modulator according to claim 2, wherein the Mach-Zehnder circuit has two output ports connected to the main output port and the monitor output port. 5.   2. The optical modulator according to claim 1, wherein each of the optical modulation means is a linear waveguide loaded with a high-frequency signal electrode, a single Mach-Zehnder interferometer circuit, or a nested Mach-Zehnder interferometer circuit. The optical modulator is an optical modulator composed of a planar lightwave circuit formed on three substrates whose end faces are directly bonded to each other,
The planar lightwave circuit formed on the first and third substrates is made of silica glass,
The planar lightwave circuit formed on the second substrate is composed of a multi-component oxide, a compound semiconductor, or a polymer whose refractive index or light absorption property changes when an electric field is applied,
Each of the light modulation means includes a high-speed phase shifter formed on the second substrate,
The light intensity adjusting means is formed on the first or third substrate, the light branching means is formed on the first substrate, and the optical coupling means is formed on the third substrate. The optical modulator according to claim 1.

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