JP4341776B2 - Optical modulation method and optical transmitter using the same - Google Patents

Description

  The present invention relates to an optical modulation method and an optical transmitter using the same, and more particularly to an optical modulation method and an optical transmitter that generate duobinary signal light by using a Mach-Zehnder optical modulator.

  In recent years, in the field of optical communication technology, with the increase in transmission information, a communication method capable of large capacity and long-distance transmission has been demanded, and in particular, excellent performance against waveform degradation due to wavelength dispersion of optical fibers. A duobinary method is known as an optical transmission method having the above.

  The duobinary method is a method in which a binary data signal has redundancy in the amplitude direction and is mapped to a ternary value, thereby narrowing the signal band (spectrum width) to ½ or less and transmitting it. The electrical signals “0”, “1”, and “2” mapped to the three values are modulated corresponding to on (phase zero), off, and on (phase π), respectively, in the optical signal. Since the modulated spectrum width is narrow, it is known that optical transmission by the duobinary method is less susceptible to waveform degradation due to chromatic dispersion in an optical fiber and has high dispersion resistance.

As a method for generating duobinary signal light, a method of applying a signal to a light modulator after converting a binary signal into a ternary signal using a low-pass filter or a code conversion circuit is disclosed in Patent Document 1 below. As described, there is a method in which duobinary light is generated by a Mach-Zehnder type optical modulator having two modulation electrodes while keeping a binary signal.
US Pat. No. 5,917,638

  A configuration example of a conventional example is shown in FIG. Also, the optical modulation state in FIG. 1 is shown in FIGS. 2 and 3, where (a) the relationship between the modulation curve of the optical modulator and the input data, (b) the optical output when the delay amount is 1 bit (bit). A waveform, (c) an optical output waveform when the delay amount is 0.8 bits, and (d) an optical spectrum waveform are shown.

  In FIG. 1, modulation signals V1 and V2 corresponding to a data signal 2 are applied to a Mach-Zehnder type optical modulator 1 to generate duobinary signal light. Data 20 that is a modulation signal is converted to EX-OR. The data is converted into an encoded data signal 11 through a differential encoder composed of a precoder composed of an exclusive OR circuit 21 and a 1-bit delay circuit 22 and a D-type flip-flop circuit 10.

  Next, the data signal 11 is divided into two signals by a divider 4 and applied to the two modulation electrodes of the optical modulator 1. However, one branched data signal is applied to the modulation electrode via the delay circuit 5. Each branched data signal is appropriately amplified through a microwave (RF) amplifier 6 and input to modulation electrodes as modulation signals V1 and V2.

  A semiconductor laser 3 is incident on the optical modulator 1 as stationary light, undergoes intensity modulation corresponding to the modulation signals V1 and V2, and generates duobinary signal light. The generated duobinary signal can suppress deterioration in reception sensitivity as compared with a duobinary modulation method using a low-pass filter. In addition, the spectral component has characteristics greatly different from NRZ. Specifically, as shown in FIG. 2, the bias point (operating point) of the Mach-Zehnder optical modulator is set at the bottom of the modulation curve indicated by the arrow in FIG. It is possible to obtain an optical spectrum in which such residual carriers are suppressed. By suppressing such carrier components, resistance against stimulated Brillouin scattering (SBS) is also provided.

  2A shows the modulation curve of the optical modulator, and the lower rectangular wave shows an input signal inputted to the optical modulator. 2B shows the optical output waveform (eye waveform) when the delay amount in the delay circuit 5 is 1 bit, and FIG. 2C shows the delay amount in the delay circuit 5 is 0.8 bits. The optical output waveform (eye waveform) in each case is shown.

  As the delay time of the delay circuit 5 shown in FIG. 1 is changed, the duty cycle of the pulse can be varied to obtain an RZ (return-to-zero) signal. The RZ (return to zero) transmission method is known to be superior in terms of high non-linear tolerance, reception sensitivity, and the like as compared with a general NRZ (non return to zero) transmission method.

However, compared to the NRZ system, the RZ system requires a pulse generation modulator in addition to a data modulator, and also requires a signal source and an amplifier for the pulse generation modulator. There was room for improvement in terms of score, space, and cost. For this reason, an RZD (RZ-Duobinary) signal that generates an RZ pulse using duobinary coding is reported in Non-Patent Document 1 below. This method is also called AMI-RZ (Alternate Mark Inversion RZ) (see Non-Patent Document 2 below).
Jianjun Yu, “Generation of modified duobinary RZ signals by using one single dual-arm LiNbO3 modulator”, IEEE Photonics technology letter, Vol.15, No.10, Oct. 2003 PJWinzer etal., “Return-to-zero modulation with electrically continuously tunable duty cycle using single NRZ modulator”, Electronics letter, vol.39, No.11,29th MAY 2003

In FIG. 3, by setting the bias point (operating point) of the Mach-Zehnder type optical modulator at the top of the modulation curve indicated by the arrow in FIG. 3A, light having a single wavelength as shown in FIG. A spectrum can be obtained. 3B and 3C show the RZ eye waveform generated by changing the delay time, as in FIG.
In the case of the configuration as shown in FIG. 1, compared to the conventional RZ pulse generation method, the data and pulse modulators can be integrated into one, so that the cost can be reduced and the configuration can be simplified. Become.

The duobinary system of FIG. 1 has a very simple structure because it is configured to apply only two different modulated signals (data signals) shifted by a predetermined time such as 1 bit to the Mach-Zehnder type optical modulator. This has the advantages of low cost and easy drive adjustment.
However, since the data signal 2 is split into two by the divider 4, a divider with little deterioration of the signal characteristics is required in order to improve the signal characteristics. In addition, when the signal is branched, the output amplitude of the signal is reduced to half, and there is a concern that the minimum operation amplitude for operating the electric amplifier 6 thereafter is insufficient.

The problem to be solved by the present invention is to solve the above-mentioned problems and prevent the deterioration of the modulation signal characteristics and amplify the modulation signal when generating duobinary signal light using a Mach-Zehnder type optical modulator. It is an object to provide an optical modulation method and an optical transmission device using the same, which can ensure a signal amplitude for sufficiently operating an electric amplifier.
It is another object of the present invention to provide an optical modulation method and an optical transmission apparatus using the same, which contribute to simplification of manufacturing and cost reduction by simplifying the circuit configuration related to the modulation signal.

In the invention according to claim 1, a Mach-Zehnder type optical waveguide having two branching waveguides and an independent modulation signal are applied on a substrate having an electro-optic effect, and an electric field corresponding to the modulation signal is applied to each branching waveguide. In an optical modulation method for generating duobinary signal light using a Mach-Zehnder type optical modulator having two modulation electrodes to be applied to the signal Q, the signal Q output from the encoder circuit and the inverted signal Q bar are shifted for a predetermined time. The two branch waveguides are applied to each modulation electrode, and the substrate region including one of the two action portions of the branch waveguide and the electric field generated by the modulation signal is inverted. The phase change Φ _A and Φ _B of the light is adjusted so as to be in phase.

In the invention according to claim 2, a Mach-Zehnder type optical waveguide having two branch waveguides and an independent modulation signal are applied to a substrate having an electro-optic effect, and an electric field corresponding to the modulation signal is branched into each branch. In an optical modulation method for generating duobinary signal light using a Mach-Zehnder type optical modulator having two modulation electrodes applied to a waveguide, the signal Q and the inverted signal Q bar output from the encoder circuit are shifted by a predetermined time. After that, in addition to applying to each modulation electrode and adjusting the relationship between the signal Q and the inverted signal Q bar between the encoder circuit and the Mach-Zehnder optical modulator, a logic inversion circuit is connected, It is characterized in that the phase changes Φ _A and Φ _B of the light in the two branch waveguides are adjusted to be in phase .

In the invention according to claim 3, in the optical modulation method according to claim 1 or 2, the predetermined time is adjusted so that the duobinary signal becomes an RZ signal, and the modulation operating point of the Mach-Zehnder optical modulator is set. It is characterized by being set to the bottom of the modulation curve .

According to a fourth aspect of the present invention , there is provided an optical transmission apparatus using the optical modulation method according to any one of the first to third aspects.

According to a fifth aspect of the present invention, in the optical transmission device according to the fourth aspect , the encoder circuit is composed of a precoder composed of a bit delay device and an EX-OR circuit, and a D-type flip-flop circuit. It is characterized by.

According to the first aspect of the present invention, the signal Q and the inverted signal Q bar output from the encoder circuit are applied to each modulation electrode, so that a conventional divider is not required, and the circuit configuration is simplified. It becomes possible to achieve easy manufacturing and cost reduction. In addition, since both the signal Q that is the output signal of the encoder circuit and the inverted signal Q bar are used, it is possible to sufficiently secure the signal amplitude necessary for the operation of the electric amplifier that amplifies the modulation signal of the signal.
Further, after the signal Q and the inverted signal Q bar output from the encoder circuit are shifted for a predetermined time and then applied to each modulation electrode, the phase changes Φ _A and Φ _B of the light in the two branch waveguides are in phase. Therefore, it is possible to generate duobinary signal light that suppresses the generation of residual carriers as will be described later, and it is possible to prevent deterioration of the modulation signal characteristics.
The “in-phase” state means that the phase change of the light wave propagating through each branch waveguide is shifted in the same direction by the modulation signal voltage applied to the branch waveguide.
In addition, since the substrate region including one of the two acting portions of the branching waveguide and the electric field generated by the modulation signal is polarized, the signal Q and the inverted signal Q bar, which are output signals from the encoder circuit, are inverted. Is simply applied to the modulation electrode of the Mach-Zehnder optical modulator, the phase change of the light wave propagating through each branch waveguide can be easily brought into the in-phase state, and the device itself using the optical modulation method of the present invention It becomes possible to further simplify the configuration.

According to the second aspect of the present invention, as in the first aspect, the signal Q and the inverted signal Q bar output from the encoder circuit are applied to each modulation electrode, so that a conventional divider is not required. Thus, it becomes possible to simplify the circuit configuration and to facilitate manufacturing and reduce costs. In addition, since both the signal Q that is the output signal of the encoder circuit and the inverted signal Q bar are used, it is possible to sufficiently secure the signal amplitude necessary for the operation of the electric amplifier that amplifies the modulation signal of the signal.
Further, after the signal Q and the inverted signal Q bar output from the encoder circuit are shifted for a predetermined time and then applied to each modulation electrode, the phase changes Φ _A and Φ _B of the light in the two branch waveguides are in phase. Therefore, it is possible to generate duobinary signal light that suppresses the generation of residual carriers as will be described later, and it is possible to prevent deterioration of the modulation signal characteristics.
In addition, a logic inversion circuit is connected between the encoder circuit and the Mach-Zehnder optical modulator, and the relationship between the signal Q and the inversion signal Q bar can be adjusted to obtain the above-described in-phase state. The present invention can be effectively applied to an optical modulator using an X-cut plate or the like that is difficult to form inversion.

By adjusting the predetermined time and the modulation operation point (bias point) of the delay circuit, the duobinary RZ signal can be easily obtained, and an appropriate signal can be obtained with an extremely simple circuit configuration. It becomes possible.

The invention according to claim 4 can provide an optical transmission device having the effect of the optical modulation method according to any one of claims 1 to 3.

  According to the fifth aspect of the present invention, the encoder circuit includes a precoder composed of a bit delay device and an EX-OR circuit, and a D-type flip-flop circuit. A differential encoder circuit can be configured, and the manufacturing cost can be further reduced.

Hereinafter, the present invention will be described in detail using preferred examples.
In the present invention, a Mach-Zehnder type optical waveguide having two branch waveguides and an independent modulation signal are applied on a substrate having an electro-optic effect, and an electric field corresponding to the modulation signal is applied to each branch waveguide. In an optical modulation method for generating duobinary signal light using a Mach-Zehnder optical modulator having a modulation electrode, a signal Q and an inverted signal Q bar output from an encoder circuit are applied to each modulation electrode and branched The relationship between the signal Q and the inverted signal Q bar at the action portion between the waveguide and the electric field by the modulation signal is adjusted so that the phases Φ _A and Φ _B of the two branch waveguides are in phase and shifted for a predetermined time. It is characterized by.

7 and 8 are schematic views showing an embodiment of an optical modulation method according to the present invention.
The Mach-Zehnder type optical modulator 1 forms an optical waveguide having a shape having two branch waveguides (Mach-Zehnder type) on a substrate having an electro-optic effect, and further applies an independent electric field to each branch waveguide. In addition, two modulation electrodes (not shown) to which independent modulation signals can be applied are formed.

  The substrate having the electro-optic effect is composed of, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), and a quartz-based material. Lithium niobate (LN) is preferably used because it is composed of a cut plate, a Y cut plate, and a Z cut plate, and is particularly easy to configure as an optical waveguide device and has high anisotropy. Further, as described below, in the present invention, it is more preferable to use a Z-cut plate as a preferred embodiment because it is necessary to form polarization inversion on a part of the substrate.

The optical waveguide on the substrate can be formed by diffusing Ti or the like on the substrate surface by a thermal diffusion method or a proton exchange method. The modulation electrode, the ground electrode surrounding the modulation electrode, and the like can be formed by forming a Ti / Au electrode pattern, a gold plating method, or the like. Furthermore, a buffer layer such as a dielectric SiO ₂ can be provided on the substrate surface after the formation of the optical waveguide, if necessary. The modulation electrode is formed for each branch optical waveguide, and independent modulation signals V1 and V2 are applied to the two modulation electrodes.

Next, an example in which duobinary signal light is generated without using a divider for broadband operation is shown in FIG. By applying the output signal Q (11) and the inverted signal Q bar (12) of the D-type flip-flop circuit 10 to the modulation electrode of the optical modulator 1 via the delay circuit 5 to one of the signals, duobinary Signal light can be generated.
With this configuration, since no frequency divider is used, the output amplitude of the signal is not reduced by half, and a minimum operating amplitude sufficient for operating the subsequent electric amplifier 6 can be secured.

However, in the light modulation method shown in FIG. 4, since the carrier light as shown in FIG. 6D remains in the optical spectrum component, the SBS resistance is inferior. In the present invention, suppression of such residual carrier components is also one of the main purposes.
For reference, FIGS. 5 and 6 show the case where the bias point (operating point) of the modulation curve is set to the bottom and the case where the bias point (operating point) is set to the top. In each figure, (a) the relationship between the modulation curve of the optical modulator and the input data, (b) the optical output waveform when the delay amount is 1 bit (bit), and (c) the delay amount is 0.8 bits. And (d) shows the optical spectrum waveform.

The cause of the residual carrier component is closely related to the direction of phase change of the light wave propagating through the branching waveguide.
FIG. 9 is a schematic cross-sectional view of an optical modulator for explaining a state in which a modulation signal is applied to each modulation electrode of the Mach-Zehnder optical modulator. Branching waveguides 32 and 33 are formed on the substrate, and modulation electrodes 30 and 31 are provided on the upper surface of the substrate.

In the optical modulator in FIG. 1, since the same modulation signal is applied to the two modulation electrodes with a predetermined time shift, if the delay of the predetermined time is ignored, as shown in FIG. signal voltage V _a to form _a, is V _B are applied to the branching waveguides 32 and 33, the branch waveguides [Phi _a phase change in the same direction of beam _propagation, and [Phi that _B shift to (phase state )

On the other hand, when the signal Q and the inverted signal Q bar, which are output signals of the D-type flip-flop circuit shown in FIG. 4, are used, as shown in FIG. 9B, the two modulation electrodes 30, 31 are used. signal voltages to form the reverse electric field V _a, and -V _B is applied to the branching waveguides 32 and 33, the phase change of the light waves propagating through the branch waveguide in opposite directions [Phi _a, -Φ _B shift (referred to as reverse phase state). In this case as well, a predetermined time shift by the delay circuit 5 is ignored.

Therefore, residual carriers can be effectively suppressed by setting the phase change of the light wave propagating through the branching waveguide to the in-phase state.
In the present invention, in order to obtain the above-described in-phase state, a logic inversion circuit is used in FIG. 7, and in FIG. 8, a polarization inversion region is formed in a part of the substrate of the optical modulator.
When a logic inversion circuit is used, there is an advantage that residual carriers can be effectively suppressed even when an X-cut plate or the like that is difficult to form polarization inversion is used as a substrate of an optical modulator. On the other hand, when using polarization inversion, electronic components such as logic inversion circuits can be reduced, and the circuit configuration can be simplified. In addition, it is possible to ensure stable modulation signal characteristics even at high frequency driving.

In the case of utilizing polarization reversal, as shown in FIG. 9 (d), the domain reversal region (the shaded portion in the figure) is applied to the action portion where the branch waveguide 33 and the electric field formed by the modulation electrode 31 interact. ), And even if signal voltages V _A and −V _B that form electric fields in opposite directions at the two modulation electrodes 30 and 31 are applied to the branch waveguides 32 and 33, they propagate through the branch waveguides. The phase change of the light wave is configured to shift Φ _A and Φ _{B in the} same direction. This phase change is the same change as the optical modulation method shown in FIG. 1, and as a result, it is possible to suppress the generation of residual carriers as in the optical spectrum shown in FIG.
For reference, even when the domain-inverted region is formed, FIG. 9C shows a state in which the phase change is reversed when an electric field in the same direction is applied. The light spectrum in this case is the same as that shown in FIG.

From the above, in one example of the present invention, as shown in FIG. 8, the domain-inverted region 15 is placed on the substrate region including one of the two working parts of the branching waveguide and the electric field by the modulation signal. At least forming. As a result, even when the modulation signals apply electric fields in opposite directions, it is possible to obtain the same effect as the case of the modulation signal applying an electric field in the same direction for light waves propagating through the branching waveguide. Become.
In order to perform polarization inversion, a substrate having an electro-optic effect is a substrate in which the direction of the crystal axis whose refractive index can be changed most efficiently by the electro-optic effect is a direction perpendicular to the substrate surface, so-called “Z It is preferable to use a “cut substrate”.

An example using the logic inversion circuit of FIG. 7 will be described more specifically.
As in FIG. 4, FIG. 7 applies duobinary signal light by applying modulation signals V 1 and V 2 corresponding to the data signal 2 to the Mach-Zehnder optical modulator 1, and the data that is the modulation signal 20 is a signal Q that is an encoded data signal through a differential encoder composed of a precoder including an EX-OR circuit (exclusive OR circuit) 21 and a 1-bit delay circuit 22, and a D-type flip-flop circuit 10. (11) and inverted signal Q bar (12).

  The output signal 2 of the precoder is input to the D-type flip-flop circuit 10 and, for example, at the time of rising from L to H, according to a clock signal (not shown) input to the flip-flop circuit, A signal indicating the state is output to Q (11), and an inverted signal 12 of the signal Q is output to the Q bar (12).

The signal Q11 is amplified via the electric amplifier 6 and applied to one of two modulation electrodes (not shown) formed in the Mach-Zehnder optical modulator.
On the other hand, the inverted signal Q bar 12 becomes a signal having the same polarity as the signal Q by the logic inverting circuit 7. Further, the signal is delayed by a predetermined time τ by the delay circuit 5 and then amplified by the electric amplifier 6 and applied to the other modulation electrode.
As a result, the signal applied to the modulation electrode can make the phase change of the light wave propagating through the branch waveguide in the same phase as shown in FIG.

  On the other hand, when using polarization reversal as shown in FIG. 8, the signal Q (11) is amplified via the electric amplifier 6 and is one of two modulation electrodes (not shown) formed in the Mach-Zehnder optical modulator. To be applied. On the other hand, the inverted signal Q bar (12) becomes a signal delayed by a predetermined time τ by the delay circuit 5, and then amplified by the electric amplifier 6 and applied to the other modulation electrode.

  A modulation signal related to the signal Q and the inverted signal Q bar is applied to the modulation electrode of the Mach-Zehnder optical modulator 1 in a state shifted by a predetermined time. However, since one of the acting parts is reversed in polarity, as a result, the phase changes in the same direction as shown in FIG. 9D, and a modulation signal that forms an electric field in the same direction as shown in FIG. 1 is applied. It will be in the same state as

  Stationary light from the semiconductor laser 3 is incident on the Mach-Zehnder optical modulator 1, and the incident light is subjected to light modulation similar to that in FIG. 1 by applying the above-described modulation signal. Duobinary signal light as shown in FIG. 3 is generated.

In the optical modulation method shown in FIG. 7 or FIG. 8, in order to obtain an RZ signal of duobinary signal light, a predetermined time τ is set within a range of 0 <τ <1 bit with respect to the antiphase signal by the signal Q and the inverted signal Q bar. This can be realized by shifting the time and setting the bias point (operating point) of the optical modulator at the bottom of the modulation curve. At this time, the optical output waveform of the duobinary signal light and the state of the optical spectrum are the same as those shown in FIGS. 2B, 2C, and 2D. In particular, as shown in FIG. 2D, residual carriers can be effectively suppressed.

  The shift amount of the predetermined time by the delay circuit 5 is not limited to the above-described 0 <τ <1 bit, and the modulation signal due to the difference in the length of the signal line from the D-type flip-flop circuit to each modulation electrode It is also possible to set in consideration of the phase difference during propagation. Further, when the delay time to be set in the delay circuit is known in advance, the shape (length, thickness, thickness, etc.) of the modulation electrode of the optical modulator is adjusted, and each modulation signal is branched optically. It is also possible to adjust the timing of starting the operation with the waveguide so as to shift by the delay time. As a result, the delay circuit can be reduced.

As described above, according to the present invention, when duobinary signal light is generated using a Mach-Zehnder optical modulator, the electric amplifier that sufficiently prevents the deterioration of the modulation signal characteristic and amplifies the modulation signal operates sufficiently. It is possible to provide an optical modulation method capable of ensuring a signal amplitude to be transmitted and an optical transmission device using the same.
In addition, it is possible to provide an optical modulation method and an optical transmission apparatus using the same that simplify the circuit configuration related to the modulation signal and contribute to facilitating manufacturing and cost reduction.

It is a figure which shows the conventional optical modulation method using a frequency divider. It is a figure which shows the state (at the time of bottom drive) of duobinary signal light in the light modulation method of FIG. It is a figure which shows the state (at the time of a top drive) of duobinary signal light in the light modulation method of FIG. It is a figure which shows the optical modulation method using the signal Q and inverted signal Q bar | burr of a D type flip-flop circuit. FIG. 5 is a diagram illustrating a state of duobinary signal light (at the time of bottom driving) in the light modulation method of FIG. 4. FIG. 5 is a diagram illustrating a state of duobinary signal light (at the time of top driving) in the light modulation method of FIG. 4. It is a figure which shows the Example of the optical modulation method based on this invention using a logic inversion circuit. It is a figure which shows the Example of the light modulation method based on this invention using polarization inversion. It is a figure which shows the phase change of the light wave which propagates the signal voltage and branching waveguide which are applied to a modulation | alteration electrode, and a situation change in case polarization inversion exists.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Mach-Zehnder type optical modulator 2 Encoded signal 3 Semiconductor laser 4 Frequency divider 5 Delay circuit 6 Electric amplifier 7 Logic inversion circuit 10 D type flip-flop circuit 11 Electric signal line of signal Q 12 Electric signal line of inversion signal Q bar 15 Polarization inversion region 20 Data signal 21 EX-OR circuit 22 1-bit delay device 30, 31 Modulation electrode 32, 33 Branched waveguide

Claims (5)

A Mach-Zehnder type optical waveguide having two branch waveguides on a substrate having an electro-optic effect, and two modulation electrodes to which an independent modulation signal is applied and an electric field corresponding to the modulation signal is applied to each branch waveguide In an optical modulation method for generating duobinary signal light using a Mach-Zehnder type optical modulator,
The signal Q and the inverted signal Q bar output from the encoder circuit are shifted by a predetermined time , and then applied to each modulation electrode , and one of the two action portions of the branch waveguide and the electric field generated by the modulation signal is applied . An optical modulation method characterized in that the phase change Φ _A and Φ _B of the light in the two branch waveguides is adjusted to be in phase by polarization inversion of the substrate region including the action part .   A Mach-Zehnder type optical waveguide having two branch waveguides on a substrate having an electro-optic effect, and two modulation electrodes to which an independent modulation signal is applied and an electric field corresponding to the modulation signal is applied to each branch waveguide In an optical modulation method for generating duobinary signal light using a Mach-Zehnder type optical modulator,
  The signal Q and the inverted signal Q bar output from the encoder circuit are shifted by a predetermined time and then applied to each modulation electrode, and between the encoder circuit and the Mach-Zehnder optical modulator, the signal Q and the inverted signal Q bar are In order to adjust the relationship, the phase change Φ of the light in the two branch waveguides by connecting a logic inversion circuit _A And Φ _B Is adjusted so as to be in phase with each other. 3. The optical modulation method according to claim 1, wherein the predetermined time is adjusted so that the duobinary signal becomes an RZ signal, and the modulation operating point of the Mach-Zehnder optical modulator is set at the bottom of the modulation curve. A light modulation method. An optical transmission apparatus using the optical modulation method according to claim 1. 5. The optical transmission apparatus according to claim 4 , wherein the encoder circuit includes a precoder including a bit delay unit and an EX-OR circuit, and a D-type flip-flop circuit. .

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