JP2014006389A - Optical modulation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulation circuit in which non-linearity of an output optical signal is inhibited.SOLUTION: An optical modulation circuit comprises: an optical input port configured to input an optical signal; a 1×2 optical coupler that divides the input optical signal at a light intensity ratio of 1:1; optical modulation means connected to each output of the 1×2 optical coupler; and a 2×1 optical coupler that couples the outputs from the optical modulation means and outputs the coupled outputs to an optical output port. In the optical modulation circuit, if N is an integer of 2 or greater, the optical modulation means includes 1×N optical dividing means and N×1 optical multiplex means. Also, optical phase modulation means is provided for modulating an optical phase for a plurality of optical paths. 2N optical paths include at least one or more optical phase modulation means.

Description

  The present invention relates to an optical modulation circuit for generating an optical signal.

  In large-capacity optical transmission systems, multi-level modulation such as QAM (Quadrature Amplitude Modulation) and orthogonal frequency multiplexing such as OFDM (Orthogonal Frequency Division Multiplexing) are used to improve the efficiency of optical frequency use in the frequency band of transmitted optical signals. Application of modulation has been actively studied.

  FIG. 1 shows an example of a conventional multilevel optical modulation circuit. As one of multi-level optical modulation circuits, a push-pull drive type modulator that applies a multi-level voltage signal to two arm waveguides of a Mach-Zehnder interference circuit in opposite phases is known. The input signal is branched into the optical waveguide 102 and the optical waveguide 103 by the optical branch circuit 101. For the optical branch circuit 101, a directional coupler, an MMI (Multimode Interference), a Y branch 1 × 2 coupler, or the like can be used. A first light modulation electrode 104 is formed on the optical waveguide 102, and a second light modulation electrode 105 is formed on the optical waveguide 103.

When the driving voltage V is an applied voltage _Vπ that changes the phase of an optical signal propagating through the optical waveguide by a half wavelength, φ = (π / _Vπ ) · V, and the first light modulation electrode 104 and the second light modulation electrode 104 Phase modulation of + φ / 2 and −φ / 2 is applied to each of the light modulation electrodes 105. Thereafter, the modulated signals are combined by an optical coupling circuit 106 to become an output signal. At this time, the electric field E of the output optical signal is represented by cos (φ / 2).

  FIG. 2 shows a response curve of the output optical signal with respect to the drive voltage. The modulation output in the multilevel optical modulation circuit of FIG. 1 has a nonlinear response curve (sine wave) with respect to the drive voltage. When a driving voltage that is a multi-valued electrical signal is applied, the signal level of the multi-valued output optical signal deviates from an ideal signal level. In other words, the output optical signal does not have ideal equidistant signal levels obtained when the response curve is a linear triangular wave.

Shogo Yamanaka, Takayuki Kobayashi, Akihide Sano, Hiroji Masuda, Eiji Yoshida, Yutaka Miyamoto, Tadao Nakagawa, Munehiko Nagatani, and Hideyuki Nosaka, "11 x 171 Gb / s PDM 16-QAM Transmission over 1440 km with a Spectral Efficiency of 6.4 b / s / Hz using High-Speed DAC, "Proc. ECOC 2010, paper We. 8. C. 1 (2010)

FIG. 3 shows optical loss in a conventional multilevel optical modulation circuit. In order to suppress the signal level deviation described above, a region that can be regarded as a linear response curve is used. However, when the amplitude of the drive voltage is reduced from 2V _{π at} which the optical loss is minimized, there is a problem that the optical loss occurs as shown in FIG.

  The present invention has been made in view of such problems, and an object thereof is to provide an optical modulation circuit in which nonlinearity of an output optical signal is suppressed.

  In order to achieve such an object, an embodiment of the present invention includes an optical input port for inputting an optical signal, a 1 × 2 optical coupler for branching the input optical signal at an optical intensity ratio of 1: 1, In an optical modulation circuit comprising: an optical modulation means connected to the output of each of the 1 × 2 optical couplers; and a 2 × 1 optical coupler that combines the outputs of the optical modulation means and outputs them to an optical output port. When N is an integer greater than or equal to 2, the optical modulation means is a first 1 × N optical branching unit comprising at least one optical coupler optically connected to one output of the 1 × 2 optical coupler. And a first 1 × N optical branching means comprising at least one optical coupler optically connected to the other output of the 1 × 2 optical coupler, and the first 1 × N optical branching means A first optical coupler comprising at least one optical coupler optically connected to the output of N × 1 optical multiplexing means, and second N × 1 optical multiplexing means composed of at least one optical coupler optically connected to the output of the second 1 × N optical branching means, At least one between the 1 × 2 optical coupler and the first and second 1 × N optical branching means, or between the first and second N × 1 optical multiplexing means and the 2 × 1 optical coupler. , Optical phase modulation means for performing optical phase modulation on a plurality of optical paths, and at least one or more lights in 2N optical paths between the optical input port and the optical output port A phase modulation means is provided.

  As described above, according to the present invention, by increasing the parallel number N of optical paths for performing optical phase modulation, an ideal signal level shift can be reduced and nonlinearity can be suppressed. In addition, by applying modulation for each of the plurality of optical paths and performing modulation, the applied voltage can be reduced or the electrode length can be reduced.

It is a figure which shows an example of the conventional multilevel light modulation circuit. It is a figure which shows the response curve of the output optical signal with respect to the drive voltage in the conventional multi-value optical modulation circuit. It is a figure which shows the optical loss in the conventional multilevel light modulation circuit. It is a figure for demonstrating the fundamental view of this invention. It is a figure which shows the response curve obtained by the light modulation circuit of FIG. It is a figure for demonstrating the method to evaluate the shift | offset | difference of the signal level of an output optical signal, and an ideal signal level. It is a figure which shows the relationship between the parallel number N and the evaluation value D of a light modulation part. It is a figure which shows the structure of the optical modulation circuit which concerns on Example 1 of this invention. FIG. 3 is a diagram illustrating an equivalent circuit of the light modulation circuit according to the first embodiment. It is a figure which shows the structure of the optical modulation circuit which concerns on Example 2 of this invention. FIG. 6 is a diagram illustrating an equivalent circuit of an optical modulation circuit according to a second embodiment. It is a figure which shows the structure of the optical modulation circuit which concerns on Example 3 of this invention. FIG. 10 is a diagram illustrating a response curve obtained by the light modulation circuit according to the third embodiment. It is a figure which shows the structure of the optical modulation circuit which concerns on Example 4 of this invention. It is a figure which shows the structure of the optical modulation circuit which concerns on Example 5 of this invention. FIG. 10 is a diagram illustrating a response curve obtained by the light modulation circuit according to the fifth embodiment. It is a figure which shows the structure of the optical modulation circuit which concerns on Example 6 of this invention. It is a figure which shows the structure of the optical modulation circuit based on Example 7 of this invention. It is a figure which shows the structure of the optical modulation circuit which concerns on Example 8 of this invention. It is a figure which shows the structure of the vector modulator which concerns on Example 9 of this invention. It is a figure which shows an example of the conventional vector modulator. It is a figure which shows the output signal diagram at the time of the 4-value drive of a vector modulator.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Basic concept of the present invention)
The basic concept of the present invention will be described with reference to FIG. An optical modulation circuit according to an embodiment of the present invention includes an optical input port 401 that inputs an optical signal, and a first 1 × 2 optical coupler 402 that branches the input optical signal at an optical intensity ratio of 1: 1. Prepare. First and second optical modulation means are connected to the outputs of the first 1 × 2 optical coupler 402.

  The first light modulation means includes a second 1 × 2 optical coupler 403 optically connected to one output of the first 1 × 2 optical coupler 402, and two of the second 1 × 2 optical coupler 403. And a first 2 × 1 optical coupler 405 optically connected to the output waveguide.

  The second optical modulation means includes a third 1 × 2 optical coupler 404 optically connected to the other output of the first 1 × 2 optical coupler 402, and two of the third 1 × 2 optical coupler 404. And a second 2 × 1 optical coupler 406 optically connected to the output waveguide.

  The outputs of the first 2 × 1 optical coupler 405 and the second 2 × 1 optical coupler 406 are optically connected to the input of the third 2 × 1 optical coupler 407, and the third 2 × 1 optical coupler 407 is connected. Are optically connected to an optical output port 408.

  In the waveguide between the first 1 × 2 optical coupler 402 and the second 1 × 2 optical coupler 403, a first light modulation electrode 409 is formed as optical phase modulation means, and the first 1 × 2 In the waveguide between the optical coupler 402 and the third 1 × 2 optical coupler 404, a second light modulation electrode 410 is formed as an optical phase modulation means.

  A third light modulation electrode 411 is formed in the waveguide between one output of the second 1 × 2 optical coupler 403 and the first 2 × 1 optical coupler 405, and the second 1 × 2 light is formed. A fourth light modulation electrode 412 is formed in the waveguide between the other output of the coupler 403 and the first 2 × 1 optical coupler 405. A fifth light modulation electrode 413 is formed in the waveguide between one output of the third 1 × 2 optical coupler 404 and the second 2 × 1 optical coupler 406, and the fifth 1 × 2 light is formed. A sixth light modulation electrode 414 is formed in the waveguide between the other output of the coupler 404 and the second 2 × 1 optical coupler 406.

  The light intensity branching ratio of the output port connected to the third light modulation electrode 411 and the output port connected to the fourth light modulation electrode 412 of the second 1 × 2 optical coupler 403 is 1 : 9. The light intensity coupling ratio of the input port connected to the third light modulation electrode 411 and the input port connected to the fourth light modulation electrode 412 of the first 2 × 1 optical coupler 405 is also 1: Nine. The light intensity branching ratio of the output port connected to the fifth light modulation electrode 413 and the output port connected to the sixth light modulation electrode 414 of the third 1 × 2 optical coupler 404 is 1 : 9. The light intensity coupling ratio of the input port connected to the fifth light modulation electrode 413 and the input port connected to the sixth light modulation electrode 414 of the first 2 × 1 optical coupler 406 is also 1: Nine.

  In this way, the first and second optical modulation means include 1 × 2 optical branching means and 2 × 1 optical multiplexing means. Further, the first and second optical modulation means include 1 × N optical branching means and N × 1 optical multiplexing means so as to include N (N is an integer of 2 or more) optical paths connected in parallel. Can be included.

Here, the modulation degree of the first light modulation electrode 409 is + φ1, the modulation degree of the second light modulation electrode 410 is −φ1, the modulation degree of the third light modulation electrode 411 is + φ2, and the fourth light modulation electrode 412. When the modulation degree is + φ3, the modulation degree of the fifth light modulation electrode 413 is −φ2, and the modulation degree of the sixth light modulation electrode 414 is −φ3,
φ1 + φ2 = 3 × φ / 2 and φ1 + φ3 = φ / 2
To satisfy.

  The modulation degree and the light intensity ratio of this light modulation circuit are based on the equation (1) which is the Fourier expansion of a triangular wave.

By adding the second-order Fourier component in this way, nonlinearity of the response curve with respect to the drive voltage can be suppressed. For example, in the optical modulation circuit of FIG. 4, the light intensity branching ratio in the second 1 × 2 optical coupler 403 and the third 1 × 2 optical coupler 404 is 1: 9, and the first 2 × 1 The light intensity coupling ratio in the optical coupler 405 and the second 2 × 1 optical coupler 406 is 1: 9. Therefore, the total intensity ratio of the optical signal given ± (φ ₁ + φ ₂ ) and the optical signal given ± (φ ₁ + φ ₃ ) is 1:81. On the other hand, since the electric field amplitude ratio of the optical signal is the square root of the intensity ratio, the total ratio is 1: 9, which satisfies Expression (1).

  FIG. 5 shows a response curve obtained by the light modulation circuit of FIG. It can be seen that the linearity of the response curve has been expanded and the linearity has been improved.

In the optical modulation circuit of this embodiment, only the second order component has been studied and described. Similarly, by adding a higher order Fourier component to the first order component, the nonlinearity of the response curve with respect to the drive voltage can be reduced. Can be suppressed. When generalized, the light modulation circuit according to the present embodiment has N (N is an integer of 2 or more) optical paths connected in parallel. n is an integer between 1 and N, and the n-th optical path changes the electric field E ₀ of the optical signal branched into each path by the optical branch circuit with respect to the drive voltage V as follows.

Then, the optical signals modulated in the respective paths are coupled by the optical coupling circuit. Here, an optical branch circuit is designed so that the optical signal is branched at a light intensity ratio of 1: 1 / (2n−1) ² in the first and nth optical paths. The optical coupling circuit is designed so that the optical signals from the nth optical modulation unit are coupled at a light intensity ratio of 1: 1 / (2n−1) ² . As can be seen from the equation (2), the response period ratio with respect to the input voltage of the phase modulation given in the first and nth optical paths is 1: 1 / (2n−1).

  In the present embodiment, a modulation electrode is formed on each of a plurality of optical paths, and modulation is performed, compared with a configuration in which modulation is performed on one optical path by one modulation electrode. The applied voltage can be reduced or the electrode length can be reduced.

In the present embodiment, any one of X-cut LN (LiNbO ₃ ), Z-cut LN, and an oxide crystal, semiconductor, or polymer having an electro-optic effect can be used as a substrate material. Furthermore, it should be noted that the same effect can be obtained regardless of whether the light modulation electrode of the Mach-Zehnder interference circuit is a single drive driven by a single-end signal or a dual drive driven by a differential signal.

  A method for evaluating the difference between the signal level of the output optical signal and the ideal signal level will be described with reference to FIG. The evaluation value of deviation is defined as follows.

Here, M is the modulation level, E _k is the actual output optical signal level, E _{k, ideal} is the ideal signal level, and E _MAX is the maximum signal level. E _MAX is normalized here to 1.00.

  FIG. 7 shows N (N is an integer of 2 or more) parallel-connected optics included in the first and second light modulation means when the modulation multilevel number M = 4, 8, 16 is changed. The change of the evaluation value D with respect to the parallel number N of the static path is shown. Basically, the evaluation value D decreases as the parallel number N increases. That is, increasing the parallel number N reduces the deviation and suppresses the non-linearity.

  However, when the modulation multi-level number is small, it does not necessarily decrease monotonously. For example, when M = 4, D (N = 3) = 1.1% against D (N = 2) = 0.8%. That is, N = 2 is optimal for quaternary modulation. For 8-level modulation and 16-level modulation, it is necessary to select a modulation multi-level number in consideration of a trade-off between a decrease in deviation due to an increase in the parallel number N and a complicated configuration of the optical modulation circuit. There is.

  Examples will be described below.

  FIG. 8 shows the configuration of the optical modulation circuit according to the first embodiment of the present invention. The optical modulation circuit according to the first embodiment includes an optical input port 801 that inputs an optical signal, and a first 1 × 2 optical coupler 802 that branches the input optical signal at an optical intensity ratio of 1: 1. First and second optical modulation means are connected to the outputs of the first 1 × 2 optical coupler 802.

  The first optical modulation means includes a second 1 × 2 optical coupler 803 optically connected to one output of the first 1 × 2 optical coupler 802 and a second 1 × 2 optical coupler 803. And a first 2 × 1 optical coupler 805 optically connected to the output waveguide.

  The second light modulation means includes a third 1 × 2 optical coupler 804 optically connected to the other output of the first 1 × 2 optical coupler 802, and two of the third 1 × 2 optical coupler 804. And a second 2 × 1 optical coupler 806 optically connected to the output waveguide.

  The outputs of the first 2 × 1 optical coupler 805 and the second 2 × 1 optical coupler 806 are optically connected to the input of the third 2 × 1 optical coupler 807, and the third 2 × 1 optical coupler 807 is connected. Are optically connected to an optical output port 808.

  In the waveguide between the first 1 × 2 optical coupler 802 and the second 1 × 2 optical coupler 803, a first light modulation electrode 809 is formed as an optical phase modulation means, and the first 1 × 2 optical coupler is formed. In the waveguide between the optical coupler 802 and the third 1 × 2 optical coupler 804, a second light modulation electrode 810 is formed as an optical phase modulation means.

  In the waveguide between one output of the second 1 × 2 optical coupler 803 and the first 2 × 1 optical coupler 805, a third light modulation electrode 811 is formed as optical phase modulation means, and the third In the waveguide between one output of the 1 × 2 optical coupler 804 and the second 2 × 1 optical coupler 806, a fourth light modulation electrode 812 is formed as an optical phase modulation means.

  The lengths of the first light modulation electrode 809, the second light modulation electrode 810, the third light modulation electrode 811 and the fourth light modulation electrode 812 are equal, and x is a real number larger than 1, and the applied voltage is V, −V, (x−1) × V, − (x−1) × V. The first light modulation electrode 809 and the second light modulation electrode 810 perform optical phase modulation on a plurality of optical paths.

  FIG. 9 illustrates an equivalent circuit of the light modulation circuit according to the first embodiment. An optical modulation circuit having the same modulation degree and light intensity ratio as the optical modulation circuit of the first embodiment is configured to modulate one optical path by one modulation electrode. The maximum applied voltage in this light modulation circuit is xV. On the other hand, in the optical modulation circuit according to Example 1, the maximum applied voltage is (x−1) × V, which indicates that the applied voltage is reduced.

  FIG. 10 shows a configuration of an optical modulation circuit according to the second embodiment of the present invention. 6 shows a modification of the optical modulation circuit of the first embodiment. The optical modulation circuit includes an optical input port 1001 that inputs an optical signal, and a first 1 × 2 optical coupler 1002 that branches the input optical signal at an optical intensity ratio of 1: 1. First and second optical modulation means are connected to the outputs of the first 1 × 2 optical coupler 1002.

  The first optical modulation means includes a second 1 × 2 optical coupler 1003 optically connected to one output of the first 1 × 2 optical coupler 1002 and a second 1 × 2 optical coupler 1003. And a first 2 × 1 optical coupler 1005 optically connected to the output waveguide.

  The second light modulating means includes a third 1 × 2 optical coupler 1004 optically connected to the other output of the first 1 × 2 optical coupler 1002 and two of the third 1 × 2 optical coupler 1004. And a second 2 × 1 optical coupler 1006 optically connected to the output waveguide.

  The outputs of the first 2 × 1 optical coupler 1005 and the second 2 × 1 optical coupler 1006 are optically connected to the input of the third 2 × 1 optical coupler 1007, and the third 2 × 1 optical coupler 1007. Are optically connected to an optical output port 1008.

  In the waveguide between the first 1 × 2 optical coupler 1002 and the second 1 × 2 optical coupler 1003, a first light modulation electrode 1009 is formed as an optical phase modulation means, and the first 1 × 2 optical coupler 1003 is formed. In the waveguide between the optical coupler 1002 and the third 1 × 2 optical coupler 1004, a second light modulation electrode 1010 is formed as an optical phase modulation means.

  In the waveguide between one output of the second 1 × 2 optical coupler 1003 and the first 2 × 1 optical coupler 1005, a third light modulation electrode 1011 is formed as an optical phase modulation means, and the third In the waveguide between one output of the 1 × 2 optical coupler 1004 and the second 2 × 1 optical coupler 1006, a fourth light modulation electrode 1012 is formed as an optical phase modulation means.

  When y is a real number larger than 1, the electrode lengths of the first light modulation electrode 1009, the second light modulation electrode 1010, the third light modulation electrode 1011 and the fourth light modulation electrode 1012 are L, −L, (Y-1) * L,-(y-1) * L. The magnitude of the voltage applied to each electrode is equal, and the sign of the applied voltage is the same for the first light modulation electrode 1009 and the third light modulation electrode 1011, and the second light modulation electrode 1010 and the fourth light modulation electrode 1012 have the same sign, and are inverted between the former and the latter. In addition, the first light modulation electrode 1009 and the second light modulation electrode 1010 perform optical phase modulation on a plurality of optical paths.

  FIG. 11 illustrates an equivalent circuit of the light modulation circuit according to the second embodiment. The optical modulation circuit has the same modulation degree and light intensity ratio as the optical modulation circuit of the second embodiment, and is configured to modulate one optical path by one modulation electrode. The longest electrode length in this light modulation circuit is yL. In contrast, in the light modulation circuit according to Example 2, the longest electrode length is (y−1) × L, and it can be seen that reduction of the electrode length is realized.

  FIG. 12 shows the configuration of an optical modulation circuit according to Embodiment 3 of the present invention. The optical modulation circuit includes an optical input port that inputs an optical signal, and a first 1 × 2 optical coupler 1204 that branches the input optical signal at an optical intensity ratio of 1: 1. One output of the first 1 × 2 optical coupler 1204 is optically connected to a second 1 × 2 optical coupler 1205 having a light intensity branching ratio of 9: 1. A third 1 × 2 optical coupler 1206 having a light intensity branching ratio of 9: 1 is optically connected to the other output.

  The two output waveguides of the second 1 × 2 optical coupler 1205 are optically connected to the two inputs of the first 2 × 1 optical coupler 1207 having a light intensity coupling ratio of 9: 1. The two output waveguides of the 1 × 2 optical coupler 1206 are optically connected to the two inputs of the second 2 × 1 optical coupler 1208 having a light intensity coupling ratio of 9: 1. The outputs of the first 2 × 1 optical coupler 1207 and the second 2 × 1 optical coupler 1208 are optically connected to the input of a third 2 × 1 optical coupler 1209 that couples at a light intensity coupling ratio of 1: 1. The output of the third 2 × 1 optical coupler 1209 is optically connected to the optical output port.

  A modulation electrode 1201 is formed between the output of the first 1 × 2 optical coupler 1204 and the second 1 × 2 optical coupler 1205 and the third 1 × 2 optical coupler 1206, and the second 1 × 2 optical A modulation electrode 1202 is formed in a waveguide (first optical modulation path indicated by a broken line) having a branching ratio of 9 between the coupler 1205 and the third 1 × 2 optical coupler 1206, and the second 1 × 2 optical coupler 1205 and A modulation electrode 1203 is formed on a waveguide having a branch ratio of 1 (second optical modulation path indicated by a one-dot chain line) of the third 1 × 2 optical coupler 1206. The modulation electrodes 1201, 1202, and 1203 are push-pull type modulation electrodes. In addition, phase adjustment units 1221 and 1222 are provided in the first light modulation path to make the phase difference between the two light modulation units zero.

  The modulation electrodes 1201 and 1203 have the same length, and the multi-value electric signal is amplified by the electric amplifiers 1231 and 1232 to drive the modulation electrodes 1201 and 1203 terminated by the resistors 1235 and 1236. The modulation electrode 1202 is one third of this length, the voltage applied to the modulation electrode 1201 is inverted by the inverter 1234, amplified by the electric amplifier 1233, and driven to the modulation electrode 1202 terminated by the resistor 1237. To do.

Response E ₁ of the first optical modulation path comprising the waveguide branch ratio of 9, a response E ₂ of the second optical modulation path comprising the waveguide branch ratio 1, is designed to satisfy the following.

The response of the entire optical modulation circuit according to the third embodiment is E = 0.9E ₁ + 0.1E ₂ .

FIG. 13 shows a response curve obtained by the light modulation circuit according to the third embodiment. Equally spaced drive voltage, when driven in the four-level amplitude 2V _[pi, ideal output optical signal level -1.00, -0.33, + 0.33, to + 1.00, in the conventional single MZM, While -1.00, -0.50, +0.50, and +1.00 are obtained, in Example 3, -1.00, -0.35, +0.35, and +1.00 are obtained, and the linearity of the response is obtained. The signal level is improved and a signal level closer to the ideal is obtained.

FIG. 14 shows a configuration of an optical modulation circuit according to Embodiment 4 of the present invention. In the light modulation circuit according to the third embodiment, the modulation electrode 1202 is omitted. The length of the modulation electrode 1401 corresponding to the modulation electrode 1201 of the third embodiment is ½ the length of the modulation electrode 1402 corresponding to the modulation electrode 1203 of the third embodiment. As a result, a simple modulation circuit can be realized, and the drive system becomes simpler. Also in Example 4, the response of the entire optical modulation circuit is E = 0.9E ₁ + 0.1E ₂ , and the linearity of the response is improved as compared with the conventional optical modulation circuit.

  FIG. 15 is a diagram illustrating a configuration of an optical modulation circuit according to the fifth embodiment of the present invention. The optical modulation circuit includes an optical input port for inputting an optical signal, and a first 1 × 2 optical coupler 1506 that branches the input optical signal at an optical intensity ratio of 1: 1. One output of the first 1 × 2 optical coupler 1506 is optically connected to a second 1 × 2 optical coupler 1508 having a light intensity branching ratio of 225: 34. A third 1 × 2 optical coupler 1509 having a light intensity branching ratio of 225: 34 is optically connected to the other output.

  One output of the second 1 × 2 optical coupler 1508 is optically connected to the first 2 × 1 optical coupler 1510 having a light intensity coupling ratio of 225: 34. A fourth 1 × 2 optical coupler 1512 having a light intensity branching ratio of 25: 9 is optically connected to the other output. One output of the third 1 × 2 optical coupler 1509 is optically connected to a second 2 × 1 optical coupler 1511 having a light intensity coupling ratio of 225: 34. A fifth 1 × 2 optical coupler 1513 having a light intensity branching ratio of 25: 9 is optically connected to the other output.

  The two output waveguides of the fourth 1 × 2 optical coupler 1512 are optically connected to the two inputs of the fourth 2 × 1 optical coupler 1514 having a light intensity coupling ratio of 25: 9, and the fifth The two output waveguides of the 1 × 2 optical coupler 1513 are optically connected to the two inputs of the fifth 2 × 1 optical coupler 1515 having a light intensity coupling ratio of 25: 9. The output of the fourth 2 × 1 optical coupler 1514 is optically connected to the input of the first 2 × 1 optical coupler 1510, and the output of the fifth 2 × 1 optical coupler 1515 is the second 2 × 1 optical coupler 1510. Optically connected to the input of the optical coupler 1511.

  The outputs of the first 2 × 1 optical coupler 1510 and the second 2 × 1 optical coupler 1511 are optically connected to the input of the third 2 × 1 optical coupler 1507, and the third 2 × 1 optical coupler 1507 is connected. Is optically connected to the optical output port.

  A modulation electrode 1501 is formed between the output of the first 1 × 2 optical coupler 1506 and the second 1 × 2 optical coupler 1508 and the third 1 × 2 optical coupler 1509, and the second 1 × 2 optical coupler 1509 is formed. A modulation electrode 1504 is formed in the waveguide (the first optical modulation path indicated by a one-dot chain line) having a branching ratio 225 of the coupler 1508 and the third 1 × 2 optical coupler 1509, and the second 1 × 2 optical coupler 1508 is formed. In addition, a modulation electrode 1502 is formed in the waveguide having a branch ratio of 34 (second optical modulation path indicated by a broken line) of the third 1 × 2 optical coupler 1509. A modulation electrode 1505 is formed in a waveguide (third optical modulation path indicated by a two-dot chain line) having a branching ratio of 25 of the fourth 1 × 2 optical coupler 1512 and the fifth 1 × 2 optical coupler 1513. A modulation electrode 1503 is formed in the waveguide having a branch ratio of 9 (second optical modulation path indicated by a broken line) of the fourth 1 × 2 optical coupler 1512 and the fifth 1 × 2 optical coupler 1513.

  In addition, phase adjustment units 1521 and 1522 and phase adjustment units 1523 and 1524 for making the phase difference between the two light modulation units zero are provided in the first light modulation path and the third light modulation path.

  The ratio of the lengths of the modulation electrode 1501, the modulation electrode 1502, the modulation electrode 1503, the modulation electrode 1504, and the fifth modulation electrode is 5: 5: 5: 2: 1. The multi-value electric signal is amplified by the electric amplifiers 1531, 1532, and 1533, and the modulation electrodes 1501, 1502, and 1503 terminated by the resistors 1537, 1538, and 1539 are driven. The voltage applied to the modulation electrode 1504 and the modulation electrode 1505 is inverted by the inverter 1536 with the voltage applied to the modulation electrode 1501 and amplified by the electric amplifiers 1534 and 1535.

  The optical intensity response to the applied voltage of the first, second, and third optical paths is designed to satisfy the following.

The light intensity ratio of the light passing through the first, second, and third optical paths is designed to be 225: 25: 9 (= 1: 1/9: 1/25). The response of the entire optical modulation circuit according to the fourth embodiment is E = (225E ₁ + 25E ₂ + 9E ₃ ) / 259.

  FIG. 16 shows a response curve obtained by the optical modulation circuit according to the fifth embodiment. It can be seen that a more linear response is obtained than in Example 3 and Example 4.

  FIG. 17 shows the configuration of an optical modulation circuit according to Embodiment 6 of the present invention. In the light modulation circuit according to the fifth embodiment, the modulation electrode 1504.1505 is omitted. The lengths of the modulation electrodes 1701, 1702, and 1703 corresponding to the modulation electrodes 1501, 1502, and 1503 of Example 5 are set to 1: 2: 2. As a result, a simple modulation circuit can be realized, and the drive system becomes simpler. Also in Example 6, the response of the entire light modulation circuit is designed to satisfy the following.

The response of the entire optical modulation circuit according to the sixth embodiment is E = (225E ₁ + 25E ₂ + 9E ₃ ) / 259, and the same response as in the fifth embodiment is obtained.

  FIG. 18 shows a configuration of an optical modulation circuit according to the seventh embodiment of the present invention. In the optical modulation circuit according to the fourth embodiment illustrated in FIG. 14, the modulation electrodes 1801 and 1802 corresponding to the modulation electrodes 1401 and 1402 of the fourth embodiment are connected in cascade to form a drive system (electric amplifier 1831 and resistor). 1832) can be integrated into one system. Note that the modulation electrodes 1802 and 1801 may be cascaded in this order.

  FIG. 19 shows a configuration of an optical modulation circuit according to the eighth embodiment of the present invention. In the optical modulation circuit according to the sixth embodiment illustrated in FIG. 17, the modulation electrodes 1901, 1902, and 1903 corresponding to the modulation electrodes 1701, 1702, and 1703 of the sixth embodiment are connected in cascade, so that the drive system (electrical The amplifier 1931 and the resistor 1932) can be integrated into one system. A configuration in which the modulation electrodes 1903, 1902, and 1901 are cascade-connected in this order may be employed.

  FIG. 20 shows the configuration of the vector modulator according to the ninth embodiment of the present invention. The vector modulator includes an optical input port for inputting an optical signal, and a 1 × 2 optical coupler 2003 that branches the input optical signal at an optical intensity ratio of 1: 1. Optical modulation circuits 2001 and 2002 according to the seventh embodiment illustrated in FIG. 18 are optically connected to outputs of the 1 × 2 optical coupler 2003.

  The optical modulation circuit 2001 modulates with an I-channel multilevel electrical signal, and the optical modulation circuit 2002 modulates with a Q-channel multilevel electrical signal. The output of the light modulation circuit 2001 and the output of the light modulation circuit 2002 via the π / 2 phase difference providing circuit 2004 are optically connected to the input of the 2 × 1 optical coupler 2005.

  FIG. 21 shows an example of a conventional vector modulator. For comparison, a vector modulator in which the multilevel optical modulation circuit shown in FIG. 1 is arranged in parallel is shown.

FIG. 22 shows an output signal diagram at the time of four-value driving of the vector modulator. FIG. 22 is an output signal diagram when driven by quaternary I and Q channel signals (amplitude 2 _Vπ ), where ♦ indicates an ideal signal point, and Δ indicates a signal of the conventional vector modulator shown in FIG. Points and ◯ are signal points of the vector modulator of the ninth embodiment shown in FIG. It can be seen that the vector modulator of the ninth embodiment has less deviation of the signal point from the ideal point than the conventional vector modulator.

DESCRIPTION OF SYMBOLS 101 Optical branch circuit 102,103 Optical waveguide 104,105,409-414,809-812,1009-1012,1201-1203,1401,1402,1501-1505,1701-1703,1801,1802,1901-1903 Optical modulation Electrodes 401, 801, 1001 Optical input ports 402-404, 802-804, 1002-1004, 1204-1206, 1506, 1508, 1509, 1512, 1513, 2003 1 × 2 optical couplers 405-407, 805-807, 1005 ˜1007, 1207 to 1209, 1507, 1510, 1511, 1514, 1515, 2005 2 × 1 optical couplers 408, 808, 1008 optical output ports 1221, 1222, 1521-1524 phase adjustment units 1231-12 3,1531～1525,1831,1931,2011,2012 electric amplifiers 1234,1536 inverter 1235～1237,1537～1541,1832,1932,2013,2015 resistor 2001 and 2002 optical modulator 2004 [pi / 2 phase difference providing circuit

Claims (4)

An optical input port for inputting an optical signal, a 1 × 2 optical coupler for branching the input optical signal at an optical intensity ratio of 1: 1, and an optical modulation means connected to the output of each of the 1 × 2 optical couplers An optical modulation circuit including a 2 × 1 optical coupler that combines outputs of the optical modulation means and outputs the combined output to an optical output port;
When N is an integer of 2 or more, the light modulation means is
First 1 × N optical branching means comprising at least one optical coupler optically connected to one output of the 1 × 2 optical coupler;
Second 1 × N optical branching means comprising at least one optical coupler optically connected to the other output of the 1 × 2 optical coupler;
First N × 1 optical multiplexing means composed of at least one optical coupler optically connected to the output of the first 1 × N optical branching means;
Second N × 1 optical multiplexing means composed of at least one optical coupler optically connected to the output of the second 1 × N optical branching means,
At least one between the 1 × 2 optical coupler and the first and second 1 × N optical branching means, or between the first and second N × 1 optical multiplexing means and the 2 × 1 optical coupler. And optical phase modulation means for performing optical phase modulation on a plurality of optical paths,
An optical modulation circuit comprising at least one optical phase modulation means in 2N optical paths between the optical input port and the optical output port. When n is an integer of 2 to N,
The light intensity ratio of the first optical path and the nth optical path of the 1 × N optical branching means is 1: 1 / (2n−1) ² ,
In the first optical path and the nth optical path, the response period ratio with respect to the input voltage of the phase modulation given to the optical phase modulation means is 1: 1 / (2n−1). The light modulation circuit according to claim 1.   3. The optical modulation circuit according to claim 1, wherein the optical modulation electrodes of the plurality of optical phase modulation means are connected in cascade. An optical input port for inputting an optical signal;
A 1 × 2 optical coupler that branches an input optical signal at a light intensity ratio of 1: 1;
The optical modulation circuit according to claim 1, 2 or 3 connected to an output of each of the 1 × 2 optical couplers;
2. A vector modulator comprising: a 2 × 1 optical coupler that combines outputs of the optical modulation circuits and outputs the combined outputs to an optical output port.

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