JP3306573B2 - Optical transmission device and optical transmission system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a long-distance, large-capacity optical transmission system used for backbone transmission.

[0002]

2. Description of the Related Art In an optical communication system, a light intensity modulation-direct detection method (hereinafter referred to as an "IM-DD method") is the simplest optical transmission method. Light intensity modulation includes a direct modulation method for directly driving a semiconductor laser and an external modulation method using a light intensity modulator. In the direct modulation method, an excessive spectrum spread occurs due to the occurrence of a frequency modulation component, that is, chirping due to a change in light intensity, and transmission characteristics deteriorate due to chromatic dispersion of an optical fiber serving as a transmission path. On the other hand, various forms of light intensity modulators are used in the external modulation method, but the Mach-Zehnder interferometer type (hereinafter, referred to as “MZ type”) can suppress chirping in principle, and therefore has an ultra-high speed and a long length. It is widely used as a light intensity modulator suitable for distance transmission.

FIG. 13 shows a system configuration of a conventional optical transmission system using an MZ type optical intensity modulator. In the figure, M
The Z-type light intensity modulator 70 is configured to perform light intensity modulation by giving a phase difference to the light branched into the two optical waveguides 72 a and 72 b by the coupler 71 and joining the lights by the coupler 73. As shown in FIG. 14, the MZ type light intensity modulator 70
The transmittance changes periodically due to a phase difference corresponding to the voltage applied to the electrodes. In the conventional binary IM-DD system, a binary signal is modulated in correspondence with an adjacent point A having a maximum transmittance and a point B having a minimum transmittance. In this configuration, the binary data signal is branched into two, one of which is input to the inverting circuit 75 to generate two data signals whose logics are inverted with respect to each other, and a voltage proportional thereto is applied to each of the optical waveguides 72a and 72b. Electrode 74
a, 74b. As a result, the applied voltage performs a push-pull operation, and the chirping can be completely removed (F. Koyama and K. Iga, IEEE J. Lightwav
e Technol., vol.6, No.1, pp.87-93, 1988).

The output light of the semiconductor laser 76 is subjected to light intensity modulation in accordance with the two data signals whose logics are inverted, and the light intensity modulated signal is transmitted to the optical fiber transmission line 77. The light intensity modulated light output from the optical fiber transmission line 77 is directly detected by an optical detection circuit 78, and the data signal is demodulated by identifying the detected signal by an identifier 79.

As shown in FIG. 15, the spectrum of a binary light intensity modulated signal has a large spectral component at the carrier frequency and spreads up to twice the bit rate around the carrier frequency. In the example shown in FIG. 15, the bit rate is 5 Gbit / s, and the horizontal axis is one scale.
It is 1.2 GHz, and the vertical axis is 5 dB per division.

[0006] The carrier frequency component degrades the transmission characteristics due to the nonlinearity of the optical fiber, and particularly restricts the optical fiber input power due to stimulated Brillouin scattering (T. Su
gie, IEEE J. Lightwave Technol., vol.9, pp.1145-115
5, 1991), and further increases crosstalk due to four-wave mixing in an optical wavelength division multiplexing transmission system (N. Shib.
ata et al., IEEE J. Quantum Electron., vol.QE-23,
pp.1205-1210, 1987). In addition, the spread of the signal spectrum causes deterioration in reception sensitivity due to chromatic dispersion of an optical fiber that occurs when performing long-distance large-capacity transmission, and causes a reduction in frequency use efficiency due to crosstalk between channels in an optical wavelength division multiplexing transmission system. In long-distance transmission of a high-speed WDM signal, even if a dispersion-shifted fiber is used, the influence of chromatic dispersion cannot be ignored depending on the channel due to the dispersion slope. These are transmission distance, transmission speed,
This is a major factor that limits the transmission capacity, and must be solved when expanding the optical network.

In order to extend the transmission distance limited by chromatic dispersion, an optical transmission system for transmitting ternary light intensity using a duobinary signal has been proposed as means for suppressing the spectrum spread of the light intensity modulation signal. (X. Gu and LC Blank, Electron. Lett. Vol. 29, N
o.25, pp.2209-2211, 1993).

FIG. 16 shows a system configuration of a conventional optical transmission system using a duobinary signal. In the figure, 2
The value data signal is converted by the code conversion circuit 80 into a ternary duobinary signal. The code conversion circuit 80 is a 1-bit delay (T) 81 that differentially codes a binary data signal.
And an exclusive OR circuit (EXOR) 82 and a low-pass filter 87 for generating a duo-binary signal from the intermediate sequence. The low pass filter for duobinary signal generation also functions as a band limiting filter. The light intensity modulator 85 modulates the optical carrier output from the semiconductor laser 86 with the duobinary signal, and sends out a light intensity modulation signal that changes with ternary light intensity to the optical fiber transmission line 77. The light intensity modulation signal output from the optical fiber transmission line 77 is directly detected by an optical detection circuit 78. Since this detection signal indicates a ternary value, it can be discriminated by the two discriminators 79a and 79b, and a binary data signal can be demodulated by inputting the discriminated two signals to the EXOR circuit 79c.

In such an optical transmission system using a duobinary signal, since the spectrum of the light intensity modulation signal is narrow,
The characteristic is that deterioration of transmission characteristics due to chromatic dispersion of the optical fiber is small. Already used MZ type light intensity modulator
Its effectiveness has been confirmed in transmission experiments at Gbit / s and 100 km (X. Gu and LC Blank, Electron. Lett. Vo
l.29, No.25, pp.2209-2211, 1993).

[0010]

However, in an optical transmission system in which the light intensity is set to three values using a duobinary signal,
Reducing the distance between signal points due to ternarization causes reception sensitivity degradation of about 3 dB (X. Gu and LC Blank, Electron.
Lett. Vol.29, No.25, pp.2209-2211, 1993). In addition, a complicated receiving circuit is required to demodulate a binary data signal from a ternary baseband signal directly detected on the receiving side. Furthermore, like a binary light intensity modulation signal, it has a large spectral component in the carrier frequency, which limits the input power of the optical fiber due to stimulated Brillouin scattering and increases crosstalk due to four-wave mixing in an optical wavelength division multiplexing transmission system. It becomes a factor.

According to the present invention, in a configuration in which light intensity modulation is performed by a duobinary signal, a carrier frequency component in a signal spectrum is suppressed without deteriorating reception sensitivity and complicating a reception circuit, and a signal band is reduced to a conventional binary value. It is an object of the present invention to provide an optical transmission device and an optical transmission system that can achieve a large capacity, a high speed, and a long distance by halving the optical intensity modulation signal.

[0012]

An optical transmitting apparatus according to the present invention comprises:
Code conversion means for converting a binary data signal into a duobinary signal, and light intensity modulation for minimizing the light intensity with respect to the median value of the duobinary signal and inverting the phase with respect to the other binary values with the same light intensity And a light modulation means for performing the following. An optical transmission system according to the present invention includes the optical transmission device, an optical transmission line that transmits an optical signal output from the optical transmission device, and an optical receiving unit that receives the optical signal.

The light intensity modulating means preferably performs light intensity modulation with the light intensity with respect to the median of the duobinary signal being zero. The light intensity modulating means includes: a modulator driving signal generating means for generating two modulator driving signals having the same amplitude and inverted phases from each other from the duobinary signal; a light source for generating an optical carrier; An MZ that applies a drive signal to two electrodes, minimizes the output light intensity when the modulator drive signal is at the center value, and performs light intensity modulation in which the phases are inverted with the same light intensity when the other two values are used. A light intensity modulator.

The light modulating means includes: a light source for generating an optical carrier; a light intensity modulator for modulating output light on and off using an inverted binary data signal; and an output using a duobinary signal. An optical phase modulator that modulates the phase of light at 0-π / 2-π. At this time, the code conversion means is composed of a branch circuit for splitting the binary data signal into two, an inversion circuit for inverting one signal, and a code conversion circuit for converting the other signal into a duobinary signal. It is preferable to insert a low-pass filter into the input section of the light intensity modulation means to limit the band of the modulated light.

[0015] The receiving means includes an optical detection circuit for directly detecting the optical modulation signal, an identifier for identifying the detected binary signal,
An inverted circuit for logically inverting the identified binary signal. As means for demodulating the digital data of the transmission light output from the optical transmitter, not only the DD method but also a coherent detection method in which local oscillation light is superimposed and received can be applied.

In the optical transmitter and the optical transmission system according to the present invention, a binary data signal is converted into a duobinary signal. At this time, for example, “0” of the data signal corresponds to “0” and “2” of the duobinary signal, and “1” corresponds to “1”. Here, the duobinary signal “0”,
Light intensity “1”, “0”, “1” for “1” and “2”
And the optical phases of the duobinary signals “0” and “2” are inverted with respect to each other to perform light intensity modulation. That is, by modulating the light intensity with respect to the central value of the duobinary signal to the minimum (ideally 0) and generating the light intensity modulation signals in which the phases are inverted with respect to the other two values with the same light intensity. The spectrum of the obtained optical signal is obtained by moving the spectrum of the duobinary signal in the baseband to the optical frequency band as it is. Therefore, in the optical modulation signal spectrum, the carrier frequency component is suppressed, and the signal band is half that of the conventional binary optical intensity modulation signal.

Further, since the intensity of the light intensity modulation signal in the present invention has only two values, "ON" and "OFF", a binary direct detection receiver can be used as it is. However, since the intensity of the optical modulation signal is “off” or “on” with respect to “1” or “0” of the binary data signal, it is necessary to perform logical inversion in the receiver.

In addition, the distance between signal points of the light intensity modulated signal in the method of the present invention is equal to that of the binary light intensity modulated signal of the conventional method, and therefore, in principle, compared with the conventional binary IM-DD method. No deterioration in reception sensitivity occurs. In addition, by inserting a low-pass filter into the input section of the light modulation means, the band of the light modulation signal is limited, so that the dispersion resistance can be improved.

[0019]

FIG. 1A shows a first embodiment of the system configuration of the optical transmission system of the present invention. This embodiment shows a configuration using a dual-drive MZ type light intensity modulator that can independently change the delay of light propagating through two optical waveguides by applying a voltage to two electrodes.

In the figure, a binary digital data signal is converted by a code conversion circuit 80 into a duobinary signal. The code conversion circuit 80 is the same as the conventional configuration, and includes 1-bit delay units (T) 81 and 83 and an exclusive OR circuit (EXO).
R) 82 and an adder 84. The ternary (0, 1, 2) duobinary signal is split into two, one of which is input to an inverting circuit 11 and has two DC components having the same amplitude and inverted phases with respect to each other and having no DC component (-).
1, 0, 1) is generated, and a voltage proportional thereto is applied to the electrodes 74a, 74b of the MZ type light intensity modulator 70, respectively.

The output light of the semiconductor laser 76 is light-intensity-modulated in accordance with the two duo-binary signals whose phases are inverted from each other.
7 is sent. The light intensity modulated light output from the optical fiber transmission line 77 is directly detected by the optical detection circuit 78, the detected signal is identified by the discriminator 79, and the data signal is demodulated by logical inversion by the inversion circuit 12.

A feature of the present invention is that when light intensity modulation is performed with a ternary (0, 1, 2) duobinary signal, a bias voltage is set so that the output light intensity becomes minimum with respect to the median value (1). Is to adjust. Ideally the median (1)
In this case, it is desirable that the output light intensity be 0. Also,
The voltages applied to the two electrodes 74a and 74b are set so that their absolute values are equal and equal to or less than the voltage required to change the phase of light by π, that is, a half-wavelength voltage. Ideally, the peak-to-peak voltage is desirably a half-wave voltage. At this time, as shown in FIG. 2, three signal points of the duobinary signal correspond to a point A having a maximum transmittance, a point B having a minimum transmittance, and a point C having a maximum transmittance. At points A and C, the intensity of the light intensity modulation signal is maximum, but the phases are inverted.

In order to make these adjustments, amplitude adjustment circuits 76a and 76b for adjusting the amplitude of the complementary duobinary signal are provided in each branch, and one branch has an appropriate bias voltage applied to the electrode 74a. Is provided for the bias adjustment circuit 76c. Electrodes 74a and 74b
Are grounded via resistors 78a and 78b, respectively.

FIG. 1B shows a modification of FIG. 1A. The characteristic of FIG. 1B is that a low-pass filter (LPF) 75a
The purpose of the present invention is to limit the band of the baseband signal for driving the optical modulator by removing the higher harmonic component by inserting 75b. This filter may be inserted only one before the signal branch.
The passband of these filters can be arbitrarily designed from infinity to zero. When the band of the signal for driving the optical modulator is restricted, the transmission distance can be extended as shown in FIG. 1C. Here, the horizontal axis indicates the transmission distance (km), the vertical axis indicates the eye opening degradation (dB), and the white point curve indicates the bandwidth B = 0.5.
(3dB bandwidth normalized by transmission rate, Gaussian LP
F), the black point curve shows the case without band limitation. It can be seen that the white point curve (B = 0.5) that limits the bandwidth has a longer transmittable distance than the black point curve. As shown in FIG. 16, when the code converter 80 that generates a duobinary signal has a low-pass filter, the low-pass filter 87 of the code converter includes a low-pass filter 75 a that limits the band of the duobinary signal. , 75b. FIG.
The use of a code converter having a low-pass filter is equivalent to the case where an ideal rectangular filter that cuts off at half the bit rate is used as the low-pass filter in FIG. 1B.

FIG. 1D shows a modification 70b of the Mach-Zehnder type light intensity modulator, in which one electrode is divided into 74a-
1 and 74a-2, and the DC bias voltage is applied to the specific electrode 74a-2. In this case, the bias adjustment circuit 7
6c is omitted.

FIG. 3 shows the relationship between the ternary (0, 1, 2) duobinary signal and the light intensity and light phase of the light intensity modulation signal in the present invention. The light intensity is a binary value of “1 (on)” and “0 (off)”.
(ON), the optical phase takes two values of “0” and “π”. Since the duobinary signal does not cause a code transition between “0” and “2”, the optical phase does not directly change between “0” and “π” while the light intensity is constant. .

The power spectral density of the duobinary signal (-1, 0, 1) has no DC component as shown in FIG. 4, and most of the power is concentrated in a band half the bit rate. Further, if the phase inversion relationship of the light intensity modulation signal in the present invention is regarded as positive and negative amplitudes, the light intensity modulation signal can be represented by multiplication of a duobinary signal and an optical carrier. Therefore, the spectrum of the optical intensity modulation signal in the present invention is obtained by moving this spectrum as it is to the optical frequency band, the carrier is suppressed, and the power is concentrated in the bit rate band centered on the carrier frequency.

The light intensity modulation signal obtained in this way is directly detected and logically inverted by the inverting circuit 12, whereby
A binary data signal can be demodulated. here,
Binary data signal { _ak }, differentially encoded intermediate sequence { _bk }, duobinary signal { _dk }, inverted duobinary signal

(Equation 1) , Light intensity modulation signal {e _k }, detection signal {| e _k |
² ｝, demodulated data signal

(Equation 2) Table 1 shows an example.

[Table 1]

The light intensity modulation signals "1" and "-"
"1" indicates phase inversion, and the detected signals are both 1. Here, the signal flow from the encoding to the decoding in the 2 ⁴ -1 pseudo-random bit sequence is shown, but the same applies to a general signal sequence.

The results of experiments using the above-described configuration will be described. In the experiment, the bit rate was set to 5 Gbit / s. FIG. 5A shows a duobinary signal waveform for light intensity modulation.
(B) shows a received signal waveform. FIG. 6 shows the duobinary signal spectrum. It can be seen that most of the power is concentrated in the half band of the bit rate. FIG. 7 shows a light intensity modulation signal spectrum in the present invention. It can be seen that the carrier frequency component is suppressed and the band is half that of the conventional binary light intensity modulation signal (FIG. 15). FIG. 8 shows the method of the present invention and the conventional method (binary IM-DD method).
2 shows the bit error rate characteristics of FIG. There is almost no difference between them,
It can be seen that the reception sensitivity does not deteriorate even when the method of the present invention is used.

FIG. 9A shows a second embodiment of the system configuration of the optical transmission system of the present invention. The feature of this embodiment resides in that two duobinary signals whose phases are inverted from each other are generated from a binary data signal using a demultiplexer.

In the figure, a binary data signal is converted to a 1-bit delay (T) 81 and an exclusive OR circuit (EXOR).
The differential sequence is differentially encoded at 82, and the intermediate sequence is input to a demultiplexer (DEMUX) 21. The demultiplexer 21 demultiplexes the input intermediate sequence in bit units,
Output together with the inverted data. Demultiplexer 2
By adding the two-channel data output from 1 by the adder 84a and adding the inverted data of the two channels by the adder 84b, the two signals having the same amplitude and inverted phases are obtained.
Two duobinary signals are generated. A voltage proportional to the two duobinary signals is applied to the MZ type light intensity modulator 7.
0 is applied to the electrodes 74a and 74b. Other configurations are the same as in the first embodiment. FIG. 9B shows a modification of the second embodiment of the present invention. This is to improve dispersion resistance by inserting a low-pass filter as in the first embodiment.

FIG. 10 shows a process of generating a duobinary signal using a demultiplexer. Is a binary data signal, is an intermediate sequence, and is a demultiplexer 2.
The output of channel 1 (ch.1) and channel 2 (ch.2) of 1 indicates a duobinary signal.

FIG. 11A shows a third embodiment of the system configuration of the optical transmission system of the present invention. The feature of this embodiment is that the light intensity modulator and the optical phase modulator are connected in cascade, the light intensity modulator is driven by an inverted signal of the binary data signal, and the duobinary generated from the binary data signal is used. It is where the optical phase modulator is driven by the signal. The light intensity modulator can perform an on-off operation, and the optical phase modulator can perform a 0-π phase modulation operation.

In the figure, a binary data signal is input to a code conversion circuit 80 and an inversion circuit 75. The output light of the semiconductor laser 76 is input to the light intensity modulator 31 and is light intensity modulated by the inverted data signal via the inverting circuit 75. The light intensity modulated light is further transmitted to the optical phase modulator 32.
And is optically phase-modulated by the duobinary signal converted from the binary data signal by the code conversion circuit 80. The light intensity modulation signal modulated by the light intensity modulation 31 and the optical phase modulator 32 is transmitted to the optical fiber transmission line 77. Note that the code conversion circuit 80 has the same configuration as the conventional one, and includes 1-bit delay units (T) 81 and 83, an exclusive OR circuit (EXOR) 82, and an adder 84. The configuration on the receiving side is the same as in the first embodiment. FIG. 11B shows a modification of the third embodiment of the present invention. This is to improve the dispersion resistance by inserting a low-pass filter as in the first and second embodiments.

FIG. 12 shows drive signals for the light intensity modulator 31 and the optical phase modulator 32. Optical phase modulator 32
Binary signals "0", "1", "2"
It can be seen that the light intensities “1”, “0”, and “1” correspond to, and the optical phases of the duobinary signals “0” and “2” are mutually inverted. Since the optical signal thus modulated is the same as the optical intensity modulated signal of the first embodiment, the data signal can be demodulated by performing direct detection and logical inversion similarly to the first embodiment. .

[0037]

As described above, in the optical transmission system of the present invention, the carrier frequency component in the signal spectrum can be suppressed without deteriorating the receiving sensitivity and complicating the receiving circuit. Therefore, it is possible to alleviate the restriction on the input power of the optical fiber due to stimulated Brillouin scattering, and to reduce crosstalk due to four-wave mixing in the optical wavelength division multiplexing transmission system. Further, since the signal band can be reduced to half that of the conventional binary light intensity modulation signal, the influence of the optical fiber chromatic dispersion can be reduced, and the frequency utilization efficiency in the optical wavelength division multiplexing transmission system can be improved. That is, in the optical transmission system of the present invention, it is possible to further increase the capacity, increase the speed, and extend the distance, which were limited by the wavelength dispersion and nonlinearity of the optical fiber in the conventional system.

[Brief description of the drawings]

FIG. 1A is a block diagram showing a first embodiment of a system configuration of an optical transmission system according to the present invention.

FIG. 1B is a modification of the first embodiment of the present invention.

FIG. 1C is a graph showing wavelength dispersion resistance characteristics of a light intensity modulation signal according to the present invention.

FIG. 1D is a modification of the light intensity modulator of FIG. 1A.

FIG. 2 is a diagram for explaining the operation of the MZ light intensity modulator in the first embodiment.

FIG. 3 is a view for explaining a relationship between a duobinary signal in the first embodiment and a light intensity modulation signal in the present invention.

FIG. 4 is a diagram showing a power spectrum density of a duobinary signal.

FIG. 5 is a diagram showing a duobinary signal waveform and a received signal waveform.

FIG. 6 is a diagram showing a duobinary signal spectrum.

FIG. 7 is a diagram showing a light intensity modulation signal spectrum according to the present invention.

FIG. 8 is a diagram showing code error rate characteristics of the present invention system and a conventional system (binary IM-DD system).

FIG. 9A is a block diagram showing a second embodiment of the system configuration of the optical transmission system of the present invention.

FIG. 9B is a modification of the second embodiment of the present invention.

FIG. 10 is a diagram showing a process of generating a duobinary signal using a demultiplexer.

FIG. 11A is a third system configuration of the optical transmission system according to the present invention;
FIG. 2 is a block diagram showing an embodiment.

FIG. 11B is a modification of the third embodiment of the present invention.

FIG. 12 is a diagram showing drive signals for an optical intensity modulator and an optical phase modulator in a third embodiment.

FIG. 13 is a diagram showing a system configuration of a conventional optical transmission system using an MZ type optical intensity modulator.

FIG. 14 is a diagram for explaining the operation of an MZ light intensity modulator in a conventional optical transmission system.

FIG. 15 is a diagram showing a light intensity modulation signal spectrum in a conventional binary optical transmission system.

FIG. 16 is a diagram showing a system configuration of a conventional optical transmission system using a duobinary signal.

[Explanation of symbols]

 11, 12, 75 Inverting circuit 21 Demultiplexer (DEMUX) 31 Optical intensity modulator 32 Optical phase modulator 70 MZ type optical intensity modulator 71, 73 Coupler 72 Optical waveguide 74 Electrode 75a, 75b Low-pass filter 76 Semiconductor laser 76a , 76b Amplitude adjustment circuit 76c Bias adjustment circuit 77 Optical fiber transmission line 78 Optical detection circuit 78a, 78b Resistance 79 Discriminator 80 Code conversion circuit 81, 83 1-bit delay (T) 82 Exclusive OR circuit (EXOR) 84 Addition Device 85 laser modulator 86 semiconductor laser 87 low-pass filter for duobinary signal generation

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Seiji Norimatsu 1-6-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-1-223837 (JP, A) JP-A-Hei 5-14286 (JP, A) Kazushige Yonenaga, Shigeru Kuwano, New optical duobinary transmission system without degradation of receiving sensitivity, Proc. Communication Association, August 15, 1995, Communication 2, B-773, 440 (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04L 25/497

Claims (7)

(57) [Claims] An input terminal for inputting a binary data signal, code conversion means coupled to the input terminal for converting the binary data signal into a duobinary signal, and an output coupled to the code conversion means; An optical modulator for applying intensity modulation to an optical carrier by the duobinary signal, wherein the intensity of the modulated light with respect to the median value of the duobinary signal is minimum, and the intensity of the modulated light with respect to the other two values of the duobinary signal. And the output terminal coupled to the output of the optical modulation means and transmitting the modulated light to the optical fiber. An optical transmission device, comprising: 2. The light modulating means gives a light intensity of 0 as a minimum output with respect to a median of a duobinary signal.
The optical transmission device according to claim 1. 3. An optical modulator comprising: a light source for providing an optical carrier; and a modulation for providing a pair of complementary modulator drive signals having the same amplitude but opposite phases according to the value of the duobinary signal. Modulator driving signal generating means, and two modulator driving signal input terminals for inputting the pair of complementary modulator driving signals, and the phase of the light split into two for the two input signals. A Mach-Zehnder interferometer type light intensity modulator capable of performing light intensity modulation by changing each of them, wherein the modulator modulates the output light of the light source and outputs the light when the modulator drive signal has a median value. 2. The modulation method according to claim 1, wherein the output light is maximum when the light is minimum and the modulator drive signal is another binary value, and the phases of the output light for these binary values are opposite to each other. Optical transmitter. 4. A code conversion circuit for converting a binary data signal into two, a code conversion circuit for converting one binary data signal into a duobinary signal, and the other binary conversion signal. A light source for providing an optical carrier, a light intensity modulator for modulating the optical carrier according to an output of the inverting circuit, and an optical modulator for inverting the data signal. An optical phase modulator that modulates the output light of the light intensity modulator, wherein the light intensity for the median of the duobinary signal is the minimum, the light intensity for the other two values is the maximum, 2. The optical transmission apparatus according to claim 1, wherein the operation is performed such that the phases with respect to the maximum binary value are opposite to each other. 5. The optical modulator according to claim 1, wherein a low-pass filter is provided between an output of said code converter and an input of said optical modulator to limit a band of an input signal to said optical modulator. The optical transmission device according to claim 1. 6. An optical transmitter according to claim 1, an optical receiver for providing a demodulated binary signal, an optical transmitter, and an optical transmission line that couples the optical receiver. An optical transmission system comprising: 7. An optical receiving apparatus comprising: an optical detection circuit for directly detecting received light; an identifier coupled to an output of the optical detection circuit to identify each value of a binary signal; and an output of the identifier. 7. The optical transmission system according to claim 6, further comprising a logic inversion circuit for inverting the sign, and an output terminal coupled to an output of the inversion circuit to provide a demodulated binary signal.