JP6265850B2 - Optical multilevel signal modulation apparatus and optical multilevel signal modulation method - Google Patents

Description

  The present invention relates to a Mach-Zehnder optical amplitude modulator for generating a Star QAM optical signal, an optical multilevel signal modulator using the same, and an optical multilevel signal modulation method thereof.

  For the purpose of improving frequency utilization efficiency, a multi-level modulation scheme that arranges a signal number greater than 2 on the amplitude direction, phase direction, or IQ plane of light and transmits a signal exceeding 1 bit in one symbol, Used in telecommunications, wireless communications, and optical communications. Among them, a modulation scheme in which phase modulation and amplitude modulation are combined and signal points are arranged on a plurality of concentric circles on the constellation is called a Star QAM modulation scheme.

  FIG. 1 shows an example of a Star QAM type signal that is modulated in binary in the amplitude direction. From the left, constellations on the IQ plane of signals modulated in BPSK, QPSK, and 8-PSK in the phase direction are not limited to this example, and are modulated at several levels in the phase direction. Also good.

  In the optical access system, it is considered to use the Star QAM type modulation method for the coexistence of an ONU that can receive only OOK and an ONU that can receive a multilevel modulation signal in the same PON branch ( For example, see Patent Document 1.) Also in wireless communication, a system that eliminates the need to exchange channel information by adopting the Star QAM type modulation scheme has been studied (for example, see Non-Patent Document 1).

  The structure of the modulator for baseband modulating light with the Star QAM modulation method is a method using only an IQ modulator, a method combining an IQ modulator and an amplitude modulator, and combining a plurality of phase modulators and amplitude modulators. There are methods. Examples of generating Star 8-QAM signals in which binary signal points are arranged in the amplitude direction and quaternary signal points are arranged in the phase direction are shown in FIGS. 2 to 5, respectively.

  As shown in FIG. 2, the IQ modulator 51 is a modulator that applies amplitude modulation to the orthogonal I-axis and Q-axis, respectively. The signal needs to be a quaternary signal. However, in general, an electrical multilevel signal is more susceptible to the effects of band limitation and amplifier nonlinearity than a binary electrical signal, and generating a multilevel electrical signal with good characteristics is a binary electrical signal with the same characteristics. Is more difficult to generate.

  FIG. 3 shows a configuration in which an amplitude modulation unit is combined in a subsequent stage of the IQ modulation unit. In this configuration, first, a QPSK signal is generated in the IQ modulation unit, and further, amplitude modulation is performed in the subsequent amplitude modulation unit, thereby generating a Star 8-QAM signal. Here, all electrical signals input to each port are binary signals. In addition to the example of the Star 8-QAM signal, the number of levels of the input electric signal can be reduced by increasing the number of modulation units that are generally combined.

  In the configuration shown in FIG. 4, the amplitude modulation unit is arranged in the subsequent stage of the two phase modulation units. A QPSK signal is generated using two phase modulation units, and a Star 8-QAM signal is generated by applying amplitude modulation in the subsequent amplitude modulation unit. Even in this configuration, all electric signals input to the modulation units are binary signals.

  In addition, as shown in FIG. 5, it can also be set as the structure which reduced the phase modulation part one by making the electric signal input into a phase modulation part into a multi-value signal.

  In addition, the number of levels of the electrical signal to be generated and the complexity of the configuration of the modulator are trade-offs. In addition to those shown in FIG. It is possible to generate a Star QAM signal in various configurations.

  In the present application, a description will be given of a configuration in which the binary amplitude modulation unit is separated from other modulation units at a clear boundary, as in the configurations of FIGS. At this time, if it is a Star QAM type signal modulated in binary in the amplitude direction, the lower level in binary amplitude modulation corresponds to the inner circle of the Star QAM signal, and the upper level corresponds to the outer circle. The level difference of the binary amplitude modulation becomes the level difference between the two circles on the constellation of the Star QAM signal.

  FIG. 6 is a diagram illustrating an example of the configuration and principle of a Mach-Zehnder (MZ) type amplitude modulator 10 which is an example of an amplitude modulation signal generation unit. The MZ type amplitude modulator 10 includes a substrate 15 having an electro-optic effect, such as lithium niobate, and a waveguide 11 having such a shape that the input light is separated into an optical power branching ratio of 1: 1 and is multiplexed at a subsequent stage at 1: 1. And an electrode 12 disposed so as to sandwich the waveguide branched into two.

  Since the substrate 15 has an electro-optic effect, the refractive index of each of the branched waveguides 11, that is, the optical path length, can be changed by applying a voltage having a positive arrow direction to the electrode 12. Therefore, by controlling the voltage applied to the electrode 12, the phase difference of the light passing through each waveguide 11 can be controlled. If the phase difference of light from each waveguide is 0 at the multiplexing unit 14, the optical power is added as it is, and if the phase difference of light is π, the optical power cancels each other and becomes zero.

  FIG. 7 is a modulation curve of a general MZ type amplitude modulator. The horizontal axis is the voltage applied to the electrodes, the vertical axis is the optical output power, and the curve is a function curve of cos square. When a voltage is applied so that the phase difference of light becomes zero with the MZ type amplitude modulator of FIG. 6, the light output power takes a maximum point on the modulation curve of FIG. 7, and the voltage is set so that the phase difference of light becomes π. Is applied, the light output power takes the minimum point on the modulation curve of FIG. In general, when a binary voltage is input to an MZ type amplitude modulator for the purpose of generating a light amplitude binary signal, a voltage corresponding to the minimum point to the maximum point of the modulation curve is applied to the electrodes.

  This is because when modulation is performed using the maximum and minimum points of the modulation curve, compared to the case where modulation is performed using other parts, the fluctuation of the signal level due to noise of the input voltage or insufficient bandwidth, This is because fluctuations in the level of the output optical power can be reduced, and an amplitude binary optical signal with good characteristics can be obtained. For example, as shown in FIG. 7, the fluctuations of the optical output power generated at the maximum point, the minimum point, and other parts of the modulation curve with respect to the fluctuation width ΔV of the input voltage of the same magnitude are a, b, c. Then, it is understood that c> a and c> b, and the fluctuation of the level of the output light power is small at the maximum point and the minimum point of the modulation curve.

  Further, the optical power at the local maximum point and the local minimum point in the modulation curve, and the ratio d / e between d and e is the extinction ratio of the MZ type amplitude modulator. When a voltage is applied so that the phase difference of light passing through the two waveguides of the MZ type amplitude modulator is π, e is zero and the extinction ratio is infinite if they completely cancel each other. E is not completely zero because, for example, the branching ratio of the demultiplexing part that divides the signal into upper and lower parts is not exactly 1: 1. However, in general, a modulator with a large extinction ratio can generate an optical amplitude binary signal with a large extinction ratio, and is an amplitude modulator with good characteristics. Therefore, the amplitude modulator fabrication technology basically aimed at a high extinction ratio. (For example, refer nonpatent literature 2).

JP 2012-222456 A

D. Liang, M.M. Song, S.M. X. Ng and L. Hanzo, “Turbo-coded and cooperative network coded non-coherent soft-decision star-QAM dispensing with channel ensemble”, in Global Telecomment. T.A. Kawanishi, T .; Sakamoto, M .; Tsuchiya, M .; Izutsu, S .; Mori, and K.M. Higuma, “70 dB extension-ratio LiNbO3 optical intensity modulator for two-tone lightwave generation”, Proc. OFC, Anaheim, CA, 2006, Paper OWC4. N. Iiyama, J .; Kani, J .; Terada, and N.A. Yoshimoto, “Feasibility study on a scheme for coexistence of DSP-based PON and 10-Gbps / λ PON using hierarchical star QAM format”. Lighttw. Technol. , Vol. 31, no. 18, pp. 3085-3092, Sep. 2013.

  When a Star QAM signal having a certain characteristic is generated using a configuration in which a binary amplitude modulation unit and another modulation unit are separated, the magnitude required as a level under the binary amplitude modulation is It is larger than the optical output power corresponding to the minimum point of the modulation curve of the amplitude modulator. As shown in FIG. 8, if the level under the amplitude modulation of the Star QAM signal is too small, the distance between adjacent signals of the phase modulation signal in the circle inside the Star QAM signal becomes too small. This is because separation becomes difficult. For this reason, when the binary modulation is performed while maintaining the level under the amplitude modulation of the Star QAM signal at a somewhat large value, the level under the binary voltage input to the amplitude modulation unit is the modulation curve of the modulator. As described with reference to FIG. 7, the fluctuation of the light output power increases with respect to the fluctuation of the voltage below the binary voltage. As shown in FIG. 9, the Star QAM signal of the light generated at that time has the signal point of the inner circle spread in the amplitude direction as compared with the signal point of the outer circle, and the transmission characteristics deteriorate. Become. As described above, in the Star QAM type modulation method, the optical power at the level below the optical amplitude binary signal is kept large to some extent while suppressing the fluctuation of the optical output power, and a Star QAM signal with good characteristics can be generated. There was a problem of difficulty.

  Therefore, in order to solve the above-described problem, the present invention can suppress the fluctuation of the optical output power while maintaining the optical power at the lowest level of the optical amplitude signal to some extent in the Star QAM type modulation system, and has a good characteristic of the Star QAM. It is an object of the present invention to provide a Mach-Zehnder type modulator, an optical multilevel modulation device, and an optical multilevel modulation method capable of generating a signal.

  In order to achieve the above object, the Mach-Zehnder type modulator according to the present invention intentionally sets the branching ratio of the optical power of the demultiplexing unit for separating the input light into two and the multiplexing unit for multiplexing the separated light. x: y (x ≠ y). In the Mach-Zehnder type modulator according to the present invention, x and y are designed so that the extinction ratio of the output amplitude modulated light is in the range of about 6 dB to 12 dB, and the optical multi-level modulation device for generating the optical Star QAM signal is provided. Used in the amplitude modulation section.

  Specifically, an optical multilevel modulation method according to the present invention is an optical multilevel modulation method that combines phase modulation and amplitude modulation and baseband modulates light using a Star QAM type modulation method, and is the most efficient on the constellation. The amplitude of the signal point existing in the inner concentric circle is set by the branching ratio of the input light of the Mach-Zehnder type amplitude modulator that performs the amplitude modulation.

  In the Mach-Zehnder optical modulator, by changing the ratio (X: Y) for splitting the input light into the two waveguides, even if the phase difference when combining them is π (the minimum point of the modulation curve) One light cannot cancel the other light, and the modulated light of the optical power corresponding to the branching ratio is output. For this reason, this optical multilevel modulation method can set the innermost signal point of the optical Star QAM signal to a desired light intensity by using the minimum point of the modulation curve by setting the branching ratio.

  Therefore, the present invention can suppress the fluctuation of the optical output power while maintaining the optical power of the lowest level of the optical amplitude signal to some extent in the Star QAM type modulation system, and can generate a Star QAM signal with good characteristics. An optical multilevel modulation method can be provided.

A Mach-Zehnder type amplitude modulator according to the present invention capable of realizing the above optical multilevel modulation method,
A substrate having an electro-optic effect;
A branching portion formed on the substrate for branching optical power at a design ratio of x: y (where x ≠ y), and a combined ratio of the branched input light with a design value of x: y. An optical waveguide having a multiplexing portion to be combined at
An electrode for changing a refractive index of each of the optical waveguides between the branching portion and the combining portion with an applied voltage;
A Mach-Zehnder type amplitude modulator comprising:
The light power of the minimum point of the modulation curve is set by x and y.

  At this time, the electric field intensity ratio is 1.66 ≦ y / x ≦ 3.00, or 1.66 ≦ x / y ≦ 3.00, or the light intensity ratio is 2.78 ≦ y / x ≦ 9.34, or 2.78 ≦ x / y ≦ 9.34 is preferable.

  This Mach-Zehnder type amplitude modulator can have an extinction ratio in the range of 6 to 12 dB, and can be mounted on an optical multilevel modulation device that generates a Star 8-QAM signal.

An optical multilevel modulation apparatus according to the present invention capable of realizing the optical multilevel modulation method is as follows:
A phase modulation signal generator for phase-modulating input light with an electrical signal;
An optical signal output from the phase modulation signal generation unit, an amplitude modulation signal generation unit that performs amplitude modulation with an electrical binary signal;
An optical multilevel modulation device comprising:
The amplitude modulation signal generation unit is the Mach-Zehnder type amplitude modulator.

An optical multilevel modulation device according to the present invention is
A phase modulation signal generator for phase-modulating input light with an electrical signal;
An optical signal output from the phase modulation signal generation unit, an amplitude modulation signal generation unit that performs amplitude modulation with an electrical binary signal;
An optical multilevel modulation device comprising:
The amplitude modulation signal generation unit includes a plurality of Mach-Zehnder type amplitude modulators connected in series, and at least one of the Mach-Zehnder type amplitude modulators is the Mach-Zehnder type amplitude modulator.
An optical multilevel modulation device that generates an optical Star QAM signal modulated with four or more values in the amplitude direction can be realized.

  The present invention is a Mach-Zehnder type capable of suppressing the fluctuation of the optical output power while maintaining the optical power of the lowest level of the optical amplitude signal to some extent in the Star QAM type modulation system, and generating a Star QAM signal with good characteristics. A modulator, an optical multilevel modulation device, and an optical multilevel modulation method can be provided.

It is a figure explaining the example of the Star QAM type signal modulated by binary in the amplitude direction. In the phase direction, modulation is performed by (a) BPSK, (b) QPSK, and (c) 8-PSK. It is a figure explaining the optical multi-value modulation apparatus which produces | generates a Star QAM signal. It is a figure explaining the optical multi-value modulation apparatus which produces | generates a Star QAM signal. It is a figure explaining the optical multi-value modulation apparatus which produces | generates a Star QAM signal. It is a figure explaining the optical multi-value modulation apparatus which produces | generates a Star QAM signal. It is a figure explaining an MZ type amplitude modulator. It is a figure explaining the modulation curve of an MZ type amplitude modulator. It is a figure explaining a subject. It is a figure explaining a subject. It is a figure explaining the MZ type | mold amplitude modulator based on this invention. It is a figure explaining the modulation curve of the MZ type amplitude modulator concerning the present invention. It is a figure explaining the conditions of the extinction ratio of a Star 8-QAM signal. It is a constellation of a Star 8-QAM signal output from the optical multilevel modulation device according to the present invention. It is a constellation of a Star 16-QAM signal output from the optical multilevel modulation device according to the present invention. It is a figure explaining the optical multi-value modulation apparatus which concerns on this invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

  The present invention is an optical multi-level modulation method that combines phase modulation and amplitude modulation, and baseband modulates light using the Star QAM type modulation method, and the amplitude of signal points existing in the innermost concentric circle on the constellation is It is set by the branching ratio of the input light of the MZ type amplitude modulator that performs the amplitude modulation. The present invention will be described with the configurations described in the following first to third embodiments.

(Embodiment 1)
3 to 5 are diagrams for explaining the configuration of the optical multilevel modulation apparatus according to this embodiment. This optical multilevel modulation apparatus includes a phase modulation signal generation unit 50 that phase-modulates input light with an electric signal, and an amplitude modulation signal generation that amplitude-modulates an optical signal output from the phase modulation signal generation unit 50 with an electric binary signal. The amplitude modulation signal generation unit 60 is an MZ-type amplitude modulator 20 to be described later.

  In the optical multilevel modulation apparatus of FIG. 3, the phase modulation signal generation unit 50 has the configuration of the IQ modulator 51. The IQ modulator 51 has two amplitude modulators (52a, 52b) and a phase modulator 53. The IQ modulator 51 amplitude-modulates the input light with the electric binary signal input to each of the two amplitude modulators (52a, 52b), and multiplexes by rotating the phase of one output by 90 degrees. The IQ modulator 51 modulates light in this way and outputs a QPSK signal.

  The optical multilevel modulation device of FIG. 4 has a configuration in which the phase modulation signal generation unit 50 connects two phase modulators (55a, 55b) in series. The phase modulation signal generation unit 50 amplitude-modulates the input light with the electric binary signal input to each of the two phase modulators (55a, 55b), and outputs a QPSK signal.

  In the optical multilevel modulation apparatus of FIG. 5, the phase modulation signal generation unit 50 has a single phase modulator 55. The phase modulation signal generation unit 50 amplitude-modulates the input light with the electrical multilevel signal input to the phase modulator 55 and outputs a QPSK signal.

  3 to 5 includes an amplitude modulation signal generation unit 60 subsequent to the phase modulation signal generation unit 50, and amplitude-modulates the QPSK signal output from the phase modulation signal generation unit 50 to generate a Star QAM signal. Is generated.

FIG. 10 is a diagram for explaining the amplitude modulator 20 of the optical multilevel modulation apparatus of FIGS. 3 to 5. The amplitude modulator 20 includes a substrate 15 having an electro-optic effect,
A branch unit 13 formed on the substrate 15 for splitting the input light with an optical power branching ratio with a design value of x: y (where x ≠ y), and the branched input light with a design value of X: Y. An optical waveguide 11 having a multiplexing section 14 for multiplexing at a ratio;
An electrode 12 that changes the refractive index of each optical waveguide 11 between the branching section 13 and the multiplexing section 14 by an applied voltage;
An MZ type amplitude modulator comprising:
The light power of the minimum point of the modulation curve is set by x and y.

  In the amplitude modulator 20, the demultiplexing unit 13 that separates the two waveguides and the multiplexing unit 14 that connects the two waveguides are set to x: y such that the branching ratio of the optical power is x ≠ y. Have been made. When a voltage is applied to such an MZ type amplitude modulator to control the phase difference of the signal passing through each waveguide, when the phase difference is zero, each waveguide is the same as when the branching ratio is 1: 1. However, when the phase difference is π, the light power does not become zero even if the light from the respective waveguides cancel each other.

  Therefore, as shown in FIG. 11, the modulation curve of such an MZ type amplitude modulator is such that the minimum value f of the output optical power is larger than e in FIG. 7, and the MZ type amplitude modulator of FIG. In comparison, the modulator has a smaller maximum extinction ratio. Here, the value of f is determined by the difference | x−y | of the branched optical power. Since the value f increases as the value increases, the desired value f can be designed by selecting x: y. .

  x and y are 1.66 ≦ y / x ≦ 3.00 or 1.66 ≦ x / y ≦ 3.00 in electric field intensity ratio, or 2.78 ≦ y / x ≦ 9.34 in light intensity ratio. Or 2.78 ≦ x / y ≦ 9.34 is preferably selected.

  For example, a Star 8-QAM signal may be used for the purpose of coexistence in the same PON branch of an ONU including a 10G-EPON or XG-PON and a digital coherent detector (for example, Patent Document 1). The condition of the extinction ratio of the Star 8-QAM signal at that time is shown in Non-Patent Document 3. The numerical calculation results in this document are shown in FIG. The horizontal axis is the extinction ratio of the Star 8-QAM signal, and the condition is that the light receiving sensitivity of all the signals in this figure is lower than the standard minimum light receiving sensitivity indicated by the dotted line. Therefore, it can be seen that the condition that the extinction ratio of the Star 8-QAM signal is in the range of about 6 to 12 dB is a condition that enables coexistence. For the same purpose, when the MZ type amplitude modulation unit of the present invention is used as a part of the Star 8-QAM signal generation unit, the extinction ratio is about 6 to 12 dB. Is designed so that the electric field intensity ratio is about 1.66 times to 3.00 times and the light intensity ratio is about 2.78 times to 9.34 times with respect to the smaller value. do it.

  When the MZ type amplitude modulator 20 of FIG. 10 is used to generate the Star QAM signal, the voltage applied to the MZ type amplitude modulator 20 is set to a magnitude from the minimum point to the maximum point of the modulation curve in FIG. Thus, a signal as shown in FIG. 13 can be obtained. This is because the MZ type amplitude modulation section of the present invention has a modulation curve with a large optical power at the minimum point compared to FIG. 7, so that the inner circle can be set larger than that of FIG. This is because the signal point on the inner circle does not spread in the amplitude direction.

(Embodiment 2)
In the present embodiment, the phase modulation signal generation unit 50 of the optical multilevel modulation apparatus of FIGS. 3 to 5 generates a phase modulation signal (such as BPSK or 8-PSK) other than QPSK, and serves as the amplitude modulation signal generation unit 60. The MZ type amplitude modulator 20 of FIG. 10 is used. The optical multilevel modulation apparatus of this embodiment generates a Star QAM type signal of the constellation shown in FIG. 1 (a) or FIG. 1 (c).

(Embodiment 3)
FIG. 15 is a diagram for explaining an optical multilevel modulation device according to the present embodiment. This optical multilevel modulation apparatus includes a phase modulation signal generation unit 50 that phase-modulates input light with an electric signal, and an amplitude modulation signal generation that amplitude-modulates an optical signal output from the phase modulation signal generation unit 50 with an electric binary signal. An optical multilevel modulation device comprising: a unit 60;
The amplitude modulation signal generation unit 60 includes a plurality of MZ type amplitude modulators connected in series, and at least one of the MZ type amplitude modulators is the MZ type amplitude modulator described with reference to FIG. .

  The amplitude modulation signal generator 60 of the present optical multilevel modulation device is configured to connect the MZ type amplitude modulator 10 of FIG. 6 and the MZ type amplitude modulator 20 of FIG. 10 in series. With this configuration, the amplitude modulation signal generation unit 60 modulates the BPSK, QPSK, or 8-PSK signal output from the phase modulation signal generation unit 50 with four or more values in the amplitude direction. For this reason, the present optical multilevel modulation apparatus can generate a Star 16-QAM signal modulated by a quaternary value in the amplitude direction and a quaternary value in the phase direction, which is represented by a constellation as shown in FIG.

  With this configuration, the present optical multi-level modulation device maintains a level of the innermost circle of the constellation as shown in FIG. A 16-QAM signal can be generated.

  Not limited to the configuration of FIG. 15, the amplitude modulation signal generation unit 60 of FIGS. 3 and 5 is configured by connecting the MZ type amplitude modulator 10 and the MZ type amplitude modulator 20 in series as in this embodiment. Can do.

[Appendix]
The following describes the optical multilevel modulation device of the present invention.
<Issues>
When generating a Star QAM type optical signal, generating a Star QAM signal having good characteristics with small fluctuation of the output optical power while keeping the optical power corresponding to the signal point on the innermost circle to a certain extent. However, it was difficult to use a conventional amplitude modulation unit aiming at a high extinction ratio.

<Solution>
In the MZ-type amplitude modulation unit, the branching ratio of the demultiplexing unit that branches the waveguide up and down and the multiplexing unit that connects the upper and lower waveguides is made to be x: y such that x ≠ y. Then, an amplitude modulation unit with a small extinction ratio is intentionally produced. This is used in an amplitude modulation section for generating a Star QAM type signal, and amplitude modulation is performed by applying a voltage from the minimum point to the maximum point of the modulation curve to the electrode. The extinction ratio at this time is about 6 to 12 dB, and x and y are about 1.66 to 3.00 times in terms of electric field intensity ratio and 2 in terms of light intensity ratio with respect to the smaller value. It is designed to have a value of about .78 times to about 9.34 times.

That is,
(A) The voltage of the modulation signal is adjusted so as to use the minimum point of the modulation curve of the MZ type amplitude modulator.
(B) The branching ratio of the MZ type amplitude modulator is not set to 1: 1.
With the above (A), the fluctuation of the output light power of the modulator can be reduced.
With the above (B), the minimum point of the modulation curve can be increased, and the signal point separation of the inner circle of the constellation of the Star QAM signal can be facilitated.

Specifically, the present invention provides:
(1):
In the MZ type amplitude modulator, a branching ratio in a demultiplexing unit that divides a waveguide through which input light passes vertically and a multiplexing unit that connects the upper and lower waveguides is set to x: y such that x ≠ y, and output An MZ type amplitude modulator manufactured so that the extinction ratio of light becomes small.
(2):
An optical multilevel signal generation unit that includes the MZ type amplitude modulation unit configured as described in (1) above and generates an optical Star QAM type signal in combination with the phase modulation signal generation unit.
(3):
In the MZ type amplitude modulation unit such as 1 in the optical multilevel signal generation unit as in (2) above, the branch x of the MZ type amplitude modulation unit so that the extinction ratio of the generated signal is about 6 to 12 dB: x and y of y are about 1.66 times to 3.00 times in electric field intensity ratio and about 2.78 times to 9.34 times in light intensity ratio with respect to the smaller value of the larger one. MZ type amplitude modulation unit designed to be a value.
(4):
An optical multilevel signal generation unit including the MZ type amplitude modulation unit as in (3) above.

10, 20: MZ type amplitude modulator 11: Waveguide 12: Electrode 13: Branching unit 14: Multiplexing unit 15: Substrate 50: Phase modulation signal generation unit 51: IQ modulators 52, 52a, 52b: Amplitude modulator 53 : Phase modulator 55, 55a, 55b: Phase modulator 60: Amplitude modulation signal generator

Claims (4)

A phase modulation signal generator for phase-modulating input light with an electrical signal;
An optical signal output from the phase modulation signal generation unit, an amplitude modulation signal generation unit that performs amplitude modulation with an electrical binary signal;
An optical multilevel modulation device comprising:
The amplitude modulation signal generator is
A substrate having an electro-optic effect;
A branching portion formed on the substrate for branching optical power at a design ratio of x: y (where x ≠ y), and a combined ratio of the branched input light with a design value of x: y. An optical waveguide having a multiplexing portion to be combined at
An electrode for changing a refractive index of each of the optical waveguides between the branching portion and the combining portion with an applied voltage;
With
An optical multilevel modulation device, characterized in that it is a Mach-Zehnder type amplitude modulator that sets the optical power at the minimum point of the modulation curve by x and y . A phase modulation signal generator for phase-modulating input light with an electrical signal;
An optical signal output from the phase modulation signal generation unit, an amplitude modulation signal generation unit that performs amplitude modulation with an electrical binary signal;
An optical multilevel modulation device comprising:
The amplitude modulation signal generation unit includes a plurality of Mach-Zehnder type amplitude modulators connected in series, and at least one of the Mach-Zehnder type amplitude modulators includes:
A substrate having an electro-optic effect;
A branching portion formed on the substrate for branching optical power at a design ratio of x: y (where x ≠ y), and a combined ratio of the branched input light with a design value of x: y. An optical waveguide having a multiplexing portion to be combined at
An electrode for changing a refractive index of each of the optical waveguides between the branching portion and the combining portion with an applied voltage;
With
An optical multilevel modulation device, characterized in that it is a Mach-Zehnder type amplitude modulator that sets the optical power at the minimum point of the modulation curve by x and y . The electric field intensity ratio is 1.66 ≦ y / x ≦ 3.00, or 1.66 ≦ x / y ≦ 3.00, or the light intensity ratio is 2.78 ≦ y / x ≦ 9.34, or 2.78. optical multilevel modulator according to claim 1 or 2, characterized in that a ≦ x / y ≦ 9.34. An optical multilevel modulation method that combines phase modulation and amplitude modulation, and baseband modulates light using a Star QAM modulation method,
By setting the branching ratio and the multiplexing ratio of the input light of the Mach-Zehnder type amplitude modulator that performs the amplitude modulation to x: y (where x ≠ y),
An optical multilevel modulation method characterized in that a signal point existing in the innermost concentric circle on the constellation has a desired light intensity .

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