JP2003177362A - Device and method for optical modulation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device and method for optical modulation which can suppress deterioration to XPM by outputting a light signal of RZ code as an optical transmit signal and eliminates the need for a phase modulator to have a wide frequency band. <P>SOLUTION: The optical modulating device 100 is equipped with a light source 101, the phase modulator 102, a Mach-Zehnder interferometer 103, an optical band-pass filter 104, an NRZ/NRZI converter 105, and a driving circuit 106. The phase modulator 102 drives light 1 with a driving signal 6 of NRZI code to generate a light signal 7 of NRZI code and inputs it to the Mach- Zehnder interferometer 103. The Mach-Zehnder interferometer 103 sets an operation point so that when the light signal 7 of NRZI code does not vary, the light signal 11 is not outputted. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical modulator and an optical modulation method.

[0002]

2. Description of the Related Art At present, in a high-speed and high-density wavelength division multiplexing optical transmission system, an optical fiber amplifier is installed in a transmission device or an intermediate repeater in order to compensate optical fiber loss and realize long-distance transmission. It is necessary to increase the optical power input to the optical fiber in order to increase the distance between the intermediate repeaters while appropriately maintaining the signal-to-noise ratio on the receiving side. However, when the optical power input to the optical fiber is increased, the nonlinear optical effect of the optical fiber greatly affects the transmission characteristics. Especially in high-density wavelength division multiplexing transmission, XPM
(Cross Phase Modulation) effect becomes a problem. XPM has different group velocities due to dispersion when optical signals with different wavelengths propagate in the optical fiber, but they simultaneously interact in the optical fiber for a short time to cause mutual interaction and deteriorate the waveform. It is the effect of ending.

Research has been conducted on the deterioration of the waveform due to the XPM. For example, NRZ (Non Ret)
urn to Zero) RZ (Return
It has been reported that the to zero) code has less deterioration with respect to XPM (see Non-Patent Document 1).
As shown in this report, it is known that the RZ code is suitable for performing long-distance transmission using an optical fiber amplifier in a high-speed / high-density wavelength division multiplexing optical transmission system.

Therefore, the NRZ / RZ converter NRZ / RZ-converts the data signal and the clock signal into an output signal of RZ code, and the drive circuit uses the output signal of the RZ code as a drive signal to drive the light intensity modulator. An optical modulator that is driven is adopted (see Patent Document 1). In this light modulator, the light intensity modulator RZ converts the light input from the light source into RZ.
It is modulated by a drive signal of a code and modulated into an optical signal of an RZ code. Since such a conventional optical modulator can output an optical signal of RZ code, it is effective as a measure against XPM.

[0005]

[Non-Patent Document 1] M Suyama, et al.
Name, "Optical Fiber Communication 1996 Technical Document PD2
6 ", February 29, 1996, p. 1-5

[Patent Document 1] Japanese Unexamined Patent Publication No. 2001-147411

[0006]

However, in the conventional optical modulator, the light intensity modulator, the NRZ / RZ converter, and the drive circuit operate with the RZ code, and thus it is necessary that they have a wide frequency band. . Further, such a broadband light intensity modulator requires a higher modulation voltage than the narrow band light intensity modulator in order to operate. Furthermore, a high-speed drive circuit that drives the broadband light intensity modulator is also required, and its output amplitude value must also be larger than that of the drive circuit that drives the narrow-band light intensity modulator. Met. That is, a high voltage output drive circuit was required. However, in a high-speed drive circuit, the size of the transistor used is small and the breakdown voltage tends to be low.
Therefore, it is technically difficult to construct a high-speed and high-voltage output drive circuit.

Therefore, according to the present invention, the RZ is used as an optical transmission signal.
An object of the present invention is to provide an optical modulation device and an optical modulation method that can output a coded optical signal and suppress deterioration with respect to XPM and that does not require a phase modulator to have a wide frequency band.

[0008]

An optical modulator according to the present invention is a phase modulator that drives light by a drive signal of an NRZI code to perform phase modulation, and a phase modulator that is input from the phase modulator. An intensity modulator for intensity-modulating an optical signal of NRZI code, and the operating point of the intensity modulator is NRZ.
It is characterized in that the output of the intensity-modulated optical signal by the intensity modulator is set to the extinction state when the optical signal of the I code does not change. Here, NRZI (Non Retur)
The “n to Zero Inverted” code is a code generated by the feedback shift register represented by the generator polynomial X + 1. The RZ code is a code in which a pulse shorter than the bit time length of the code is sent and the pulse is not sent for the rest and the state returns to 0.

According to the above-described optical modulator of the present invention, the phase modulator drives the light by the drive signal of the NRZI code to generate the optical signal of the NRZI code.
It is input to the intensity modulator. When the intensity modulator does not output the intensity-modulated optical signal when the input phase-modulated NRZI coded optical signal does not change, the intensity modulator outputs an optical signal of the RZ-coded optical signal. Become a signal. Therefore, the intensity modulator can output an RZ code optical signal as an optical signal to be output to the optical transmission path, and can suppress deterioration with respect to XPM.

Further, since the phase modulator is driven by a drive signal of NRZI code, the frequency band required for the phase modulator is about half of the data signal bit rate according to the concept of Nyquist frequency (Nyquist theorem). Good. Therefore, the phase modulator does not need to have a wide frequency band and does not require a high modulation voltage. Further, as the phase modulator does not need to have a wide frequency band, a high speed, high voltage output drive circuit is not required as a drive circuit for driving the phase modulator. Therefore, it is difficult to form a high-speed and high-voltage output drive circuit because of the breakdown voltage.

As the intensity modulator, the phase-modulated optical signal input from the phase modulator is divided into a first optical signal and a second optical signal, and the first optical signal and the second optical signal are separated. A signal for synthesizing and outputting the signal is used to adjust the phase difference between the first optical signal and the second optical signal to set the operating point of the intensity modulator to NRZ.
It is preferable to set the output of the optical signal intensity-modulated by the intensity modulator to be in the extinction state when the optical signal of the I code does not change. According to this, the operating point of the intensity modulator can be easily set by adjusting the phase difference.

Further, the adjustment of the phase difference can be realized, for example, by providing the intensity modulator with a phase adjusting section for adjusting the phase difference. Further, the optical modulator may be provided with a temperature control unit that adjusts the phase difference by controlling the temperature of the intensity modulator. In that case, the intensity modulator has a first output port that outputs an intensity-modulated optical signal and a second output port that outputs an optical signal complementary to the optical signal output to the first output port. It is preferable that the temperature control unit generates a temperature control signal based on the optical signal output from the second output port, and controls the temperature of the intensity modulator by the generated temperature control signal.

Further, the optical modulator may be provided with a frequency controller for adjusting the phase difference by controlling the frequency of the light input to the phase modulator. The frequency control unit, for example,
By controlling the temperature of the light source of light, the frequency of light can be controlled. In that case, the intensity modulator includes a first output port for outputting the intensity-modulated optical signal and the first output port.
And a second output port that outputs an optical signal that is complementary to the optical signal that is output to the output port of the frequency control unit, and the frequency control unit controls the temperature based on the optical signal output from the second output port. It is preferable to generate a control signal and control the temperature of the light source by the generated temperature control signal.

As the intensity modulator, a first output port for outputting an intensity-modulated optical signal and a second output for outputting an optical signal complementary to the optical signal output to the first output port. A port having a port is used so that the optical intensity of the optical signal output from the second output port is maximized.
The phase difference between the first optical signal and the second optical signal can also be adjusted by setting the operating point of the intensity modulator.

Further, the optical modulator comprises an NRZ / NRZI converter for converting the data signal of the NRZ code into the signal of the NRZI code when the data signal which is the basis of the drive signal is the signal of the NRZ code. It is preferable. According to this, even when the NRZ code data signal is input to the optical modulator, the NRZI code signal can be input to the drive circuit that generates the drive signal.

Further, it is preferable that the optical modulator comprises an optical bandpass filter for narrowing the spectrum of the intensity-modulated optical signal output from the intensity modulator. According to this, the optical modulator can make the wavelength of the optical signal compact. Therefore, the optical modulator can avoid the influence of dispersion of the optical fiber. The optical bandpass filter can pass both modulation spectral sidebands of the intensity modulated optical signal. In this case, the optical modulator can perform double sideband (Double Side Band, hereinafter referred to as "DSB") modulation. As a result, demodulation becomes easier.

The optical bandpass filter may pass one modulation spectrum sideband of the intensity-modulated optical signal more than the other modulation spectrum sideband. In this case, the optical modulator is a single sideband system (hereinafter referred to as “SS”).
It is possible to perform modulation of "B system". SSB modulation uses only half the bandwidth of DSB modulation. Therefore, the optical modulator can achieve a transmission capacity that is approximately double.

Further, it is preferable that the intensity modulator intensity-modulates a plurality of phase-modulated NRZI code optical signals.
According to this, the optical modulator can intensity-modulate a plurality of optical signals with one intensity modulator. Therefore, the optical modulator can be downsized and reduced in cost.

Further, in the optical modulation method according to the present invention, the phase modulator drives the light by the drive signal of the NRZI code to perform the phase modulation, and the intensity modulator performs the phase modulation input from the phase modulator. The optical signal of the NRZI code is intensity-modulated to
It is characterized in that the output of the intensity-modulated optical signal is put into the extinction state when the optical signal of the I code does not change.

The intensity modulator divides the phase-modulated optical signal input from the phase modulator into a first optical signal and a second optical signal, and the first optical signal and the second optical signal. It is preferable that the signal is intensity-modulated by combining and outputting the signal, and the output of the optical signal is put into the extinction state by adjusting the phase difference between the first optical signal and the second optical signal.

The intensity modulator has a phase adjusting section for adjusting the phase difference, and the phase adjusting section can adjust the phase difference. Also, the phase difference may be adjusted by controlling the temperature of the intensity modulator. Further, the phase difference may be adjusted by controlling the frequency of the light input to the phase modulator. The intensity modulator outputs the intensity-modulated optical signal to the first output port, and outputs the optical signal complementary to the optical signal output to the first output port to the second output port, The phase difference between the first optical signal and the second optical signal can also be adjusted by maximizing the light intensity of the optical signal output from the second output port.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

[First Embodiment] FIG. 1 is a block diagram showing the arrangement of an optical modulator 100 according to the first embodiment. The optical modulator 100 includes a light source 101, a phase modulator 102, a Mach-Zehnder interferometer 103, an optical bandpass filter 104, and an NRZ / NRZI converter 10.
5 and a drive circuit 106.

The light source 101 inputs the output light 1 into the phase modulator 102. As the light source 101, for example, a semiconductor laser that oscillates in a single longitudinal mode can be used. The phase modulator 102 converts the light 1 input from the light source 101 into N
It is driven by a drive signal 6 of RZI code to perform phase modulation.
The phase modulator 102 is the phase modulator 1 from the drive circuit 106.
It is driven by the drive signal 6 input to 02 to perform phase modulation. The phase modulator 102 obtains N obtained by performing phase modulation.
An optical signal 7 of RZI code is transmitted to a Mach-Zehnder interferometer 10
Enter in 3.

The Mach-Zehnder interferometer 103 is an intensity modulator that intensity-modulates the phase-modulated NRZI coded optical signal 7 input from the phase modulator 102. The Mach-Zehnder interferometer 103 divides the optical signal 7 input from the phase modulator 102 into a first optical signal and a second optical signal, and synthesizes the first optical signal and the second optical signal. Output.
FIG. 2 is a diagram showing a configuration of the Mach-Zehnder interferometer 103. The Mach-Zehnder interferometer 103 includes a directional coupler 103a, a directional coupler 103b, and an optical waveguide 10.
3c, an optical waveguide 103d, and a phase adjusting unit 103f.

The directional coupler 103a splits the optical signal 7 input to the Mach-Zehnder interferometer 103 into two optical waveguides 103c and 103d. That is, the directional coupler 103a transmits the optical signal 7 to the optical waveguide 103c as the first signal.
And the second optical signal traveling to the optical waveguide 103d. Of the two optical waveguides 103c and 103d,
One optical waveguide 103d is longer than the other optical waveguide 103c by ΔL103e.

The directional coupler 103b synthesizes the first optical signal and the second optical signal branched into the two optical waveguides 103c and 103d, and outputs the optical signal 11. In the directional coupler 103b, the phase difference between the first optical signal and the second optical signal from the two optical waveguides 103c and 103d input to the directional coupler 103b is 2kπ + π / 2 (k is an integer). In the case of, the optical signal 11 is transmitted and output, and the phase difference is 2 kπ−
In the case of π / 2 (k is an integer), the optical signal 11 is blocked and the output of the directional coupler 103b is put into the extinction state.

From the light source 101 to the Mach-Zehnder interferometer 1
Let A ₀ be the intensity of the light 1 input to 03, and let A be the optical intensity of the optical signal 11 output from the Mach-Zehnder interferometer 103. Optical signal 1 output from Mach-Zehnder interferometer 103
The light intensity A of 1 can be expressed by the following equation (1).

[0029]

[Equation 1] In the equation (1), β = (2 × π × f × n _eff /
C ₀ ), f is the frequency of the light source 101, C ₀ is the speed of light,
n _eff is the equivalent refractive index of the optical waveguides 103c and 103d.

In the equation (1), the value of βΔL + Φ is the phase difference between the first optical signal and the second optical signal from the two optical waveguides 103c and 103d input to the directional coupler 103b. Is. As described above, ΔL is the amount by which the length of the one optical waveguide 103d is set longer than the length of the other optical waveguide 103c. Here, theoretically,
The phase difference between the first optical signal and the second optical signal from the two optical waveguides 103c and 103d is represented by βΔL. However, this ΔL can actually be set only on the order of several mm to several tens of mm, and in order to set the phase difference βΔL to a desired value, ΔL can be set in advance on the wavelength order (micron unit). Can not. That is, ΔL cannot be initialized in the wavelength order (micron unit).

Therefore, the value of Φ is adjusted so that the phase difference between the first optical signal and the second optical signal output from the two optical waveguides 103c and 103d becomes a desired value. as a result,
The phase difference is actually represented by βΔL + Φ as described above. Therefore, Φ is the two optical waveguides 103
A parameter indicating the residual phase component due to the fact that ΔL that sets the phase difference between the first optical signal and the second optical signal output from the c and 103d cannot be initialized in the wavelength order (micron unit). Is.

FIG. 3 shows the drive circuit 106 to the phase modulator 1.
The state in which the drive signal 6 input to the phase modulator 102 for driving the phase modulator 102 is not input to the phase modulator 102, that is,
6 is a graph showing the relationship between the light transmittance of the Mach-Zehnder interferometer 103 and the frequency of the light when the light 1 from the light source 101 is input to the Mach-Zehnder interferometer 103 as it is. FIG. 3 shows a Mach-Zehnder interferometer 103.
This is for explaining the bias state of. The vertical axis of the graph shown in FIG. 3 represents the light transmittance (A / A ₀ ) of the Mach-Zehnder interferometer 103 obtained based on the equation (1),
The horizontal axis represents the optical frequency of the light 1.

In FIG. 3, since the light 1 from the light source 101 is directly input to the Mach-Zehnder interferometer 103, the light intensity of the light input to the Mach-Zehnder interferometer 103 becomes A ₀ , and the light transmittance is A / It is represented by A ₀ . In FIG. 3, the interval between the values of the frequency of the light that maximizes the light transmittance is the free spectral space (hereinafter referred to as “FSR”) of the Mach-Zehnder interferometer 103. FSR is shown in (2) below.
It can be calculated from the relation of the equations and can be expressed by the equation (3).

[0034]

[Equation 2]

[Equation 3] The phase adjustment unit 103f adjusts the phase difference between the first optical signal and the second optical signal from the two optical waveguides 103c and 103d input to the directional coupler 103b. To adjust the phase difference, the operating point of the Mach-Zehnder interferometer 103 is N
When the optical signal 7 of the RZI code does not change, the output is set to be in the extinction state. As mentioned above,
The directional coupler 103b includes two optical waveguides 103c and 1c.
When the phase difference between the first optical signal and the second optical signal from 03d is 2kπ + π / 2 (k is an integer), the optical signal 11 is transmitted and output, and the phase difference is 2kπ−π / 2 (k Is an integer), the optical signal 11 is blocked, and the output of the directional coupler 103b is put into the extinction state. Therefore, the phase adjusting unit 103f adjusts the phase difference so that the phase difference becomes 2kπ−π / 2 (k is an integer) when the optical signal 7 of the NRZI code does not change.

As described above, the phase difference is βΔL + Φ
And ΔL is set in advance. Therefore, the phase adjustment unit 103f adjusts Φ to adjust the phase difference. For the phase adjusting unit 103f, for example, a heater or the like can be used and is provided in the middle of the optical waveguide 103d.
The heater, which is the phase adjustment unit 103f, adjusts the temperature of the optical waveguide 103d and expands or contracts the length of the optical waveguide 103d, thereby adjusting Φ and adjusting the phase difference.

The NRZ / NRZI converter 105 converts the data signal 2 of the NRZ code into the NRZI signal when the data signal 2 which is the basis of the drive signal 6 is the signal of the NRZ code.
Convert to a sign signal. That is, the NRZ / NRZI converter 105 uses the data signal 2 input to the optical modulator 100.
Is an NRZ code, the NRZ code data signal 2 is converted into an NRZI code output signal 5. Here, the NRZ code is a code of a system in which pulses are continuously output for the length of time and the voltage does not return to zero. The NRZ / NRZI converter 105 performs N conversion for code conversion.
The clock signal 3 input to the RZ / NRZI converter 105 is used.

FIG. 4 is a diagram showing the configuration of the NRZ / NRZI converter 105. NRZ / NRZI converter 105
Is an EX-OR (Exclusive-OR) circuit 10
5a and 1-bit delay circuit 105b. The 1-bit delay circuit 105b includes the NRZ / NRZI converter 105.
The output signal 5 output by is delayed by 1 bit. The 1-bit delay circuit 105b can be configured by, for example, a flip-flop circuit that uses the clock signal 3 as a timing signal. In the present embodiment, the 1-bit delay circuit 1
The initial value of 05b is 0. 1-bit delay circuit 105b
Is a 1-bit delay signal 4 obtained by delaying the output signal 5 output from the NRZ / NRZI converter 105 by 1 bit, EX-
It is input to the OR circuit 105a.

The EX-OR circuit 105a includes the optical modulator 1
Data signal 2 input to 00 and 1-bit delay circuit 1
05b calculates the exclusive OR of the output signal 5 output from the NRZ / NRZI converter 105 and the 1-bit delay signal 4 obtained by delaying the output signal 5 by 1 bit. The calculated result is NRZ / N
It becomes the output signal 5 of the RZI converter 105. The EX-OR circuit 105a outputs "0" as the operation result when the values of the data signal 2 and the 1-bit delay signal 4 are the same, and when they are different. As described above, the 1-bit delay circuit 1
The initial value of 05b is "0", so EX-OR
The circuit 105a initially calculates the exclusive OR of the initial value "0" and the data signal 2 (when the output signal 5 is not yet output).

FIG. 5 shows the data signal 2 input to the NRZ / NRZI converter 105 and the NRZ / NRZI converter 1.
It is a figure which shows the output signal 5 which 05 outputs. In the present embodiment, a bit string “010111001” is used as the data signal 2. This data signal 2 is NRZ / NR
When input to the ZI converter 105, as an output signal 5,
A bit string “01101110” of the NRZI code is obtained. The bit string of the data signal 2 input to the NRZ / NRZI converter 105 is x (n), NRZ / NRZI
The bit string of the output signal 5 output from the converter 105 is y
Assuming that (n), the calculation performed by the NRZ / NRZI converter 105 is expressed by equation (4).

[0040]

[Equation 4] The calculation of the equation (4) is performed by the calculation of mod2. mod
The operation of 2 means 0 + 0 = 0, 1 + 0 = 1, 0 + 1 = 1,
This is an operation according to 1 + 1 = 0.

Drive signal 6 output from drive circuit 106
Drives the phase modulator 102. The drive circuit 106 is
It is an electric circuit for amplifying the electric signal inputted from the NRZ / NRZI converter 105, that is, the output signal 5 of the NRZI code. The drive circuit 106 drives the phase modulator 102 with the drive signal 6 of the NRZI code obtained by amplifying the output signal 5. As described above, the phase modulator 102 driven by the drive signal 6 modulates the light 1 input from the light source 101 and outputs the optical signal 7. The optical signal 7 obtained by the modulation by the phase modulator 102 can be expressed by the following equation (5).

[0042]

[Equation 5] In the equation (5), V is the drive voltage of the phase modulator 102,
V _π is the half-wave voltage of the phase modulator 102, φ ₀ is the initial phase, and t is time. The drive circuit 106 does not necessarily have to output up to V _π .

As described above, the phase modulator 102 has the NRZ
It is driven by a drive signal 6 having an I code. The drive circuit 106 that drives the phase modulator 102 also amplifies the output signal 5 of the NRZI code, and the NRZ / NRZI converter 105 also outputs N.
The output signal 5 of the RZI code is output. Therefore, the total frequency band required by combining the phase modulator 102, the drive circuit 106, and the NRZ / NRZI converter 105 is determined by the concept of the Nyquist frequency (Nyquist's theorem) so that the bit rate of the data signal 2 is B (b / S), B / 2
(Hz) is sufficient. For example, 40 Gbit / sec
In order to obtain the optical signal 11 of the RZ code having the bit rate of
The frequency band required for the phase modulator 102 may be approximately 20 GHz.

Therefore, the phase modulator 102 does not need to have a wide frequency band and does not need a high modulation voltage. Further, as the phase modulator 102 does not need to have a wide frequency band, the drive circuit 106 for driving the phase modulator 102 does not need to have high speed and high voltage output. Therefore, it is difficult to form a high-speed and high-voltage output drive circuit because of the breakdown voltage.

Next, a method of modulating the light 1 input from the light source 101 by the drive circuit 106, the phase modulator 102, and the Mach-Zehnder interferometer 103 will be described with reference to FIG. FIG. 6 shows the drive signal 6 for driving the phase modulator 102 by the drive circuit 106, the phase state of the phase-modulated optical signal 7 output by the phase modulator 102, and the two optical signals inside the Mach-Zehnder interferometer 103. The first optical signal 8 and the second optical signal 9 immediately before the waveguides 103c and 103d input to the directional coupler 103b, that is, the optical waveguides 103c and 103.
d, the phase states of the first optical signal 8 and the second optical signal 9, and the phase difference 10 between the first optical signal and the second optical signal,
It is a figure explaining the relationship of the light intensity of the optical signal 11 which the Mach-Zehnder interferometer 103 outputs.

Incidentally, in the explanation using FIG. 6, the F of the Mach-Zehnder interferometer 103 is shown in order to simplify the explanation.
SR is set to the reciprocal 1 / T of the 1-bit time slot T of the data signal 2 input to the NRZ / NRZI converter 105. The waveform of the drive signal 6 that drives the phase modulator 102 is a triangular waveform. Further, the phase modulator 102
The difference between the time when the Mach-Zehnder interferometer 103 outputs the optical signal 11 and the time when the Mach-Zehnder interferometer 103 outputs the optical signal 7 is zero. That is, as shown in FIG. 6, the optical signal 7 and the optical signal 11 are output in the same time slot. In reality, the optical signal 7 is delayed by the time it takes to transmit the Mach-Zehnder interferometer 103, and as shown in FIG. The NRZ / NRZI converter 105
The bit string of the output signal 5 output by is “01” shown in FIG.
101110 ”is used.

The waveform of the drive signal 6 shown in FIG. 6 is a signal waveform for driving the phase modulator 102. The drive signal 6 is
It is an electrical signal of the NRZI code obtained by amplifying the output signal 5.
As described above, the waveform of the drive signal 6 is a triangular waveform, and it is assumed that the voltage changes like a ramp. That is,
It is assumed that the voltage waveform linearly rises from the code “0” to “1” and the voltage waveform linearly drops from the code “1” to “0”. Here, “0” is the voltage 0, and “1” is the half-wave voltage V _π of the phase modulator 102.

The waveform of the optical signal 7 shown in FIG. 6 is the waveform of the phase state of the phase-modulated optical signal 7 output from the phase modulator 102. The waveform of the optical signal 7 is obtained by driving the light 1 input from the light source 101 to the phase modulator 102 by the drive signal 6 shown in FIG. This waveform shows the phase state of the optical signal 7 represented by the above equation (5). Here, the initial phase φ _{0 in the} equation (5) is, for convenience of explanation,
Assume 0. The phase of the optical signal 7 is, as shown in FIG.
The phase changes from 0 to π or from π to 0 in a sinusoidal manner.

The waveforms of the first optical signal 8 and the second optical signal 9 shown in FIG. 6 are the first optical signal 8 and the second optical signal 9 output by the two optical waveguides 103c and 103d of the Mach-Zehnder interferometer 103, respectively. 2 is a waveform showing the phase state of the optical signal 9 of 2.
The first optical signal 8 and the second optical signal 9 are the phase modulator 102.
It is obtained by branching the optical signal 7 shown in FIG. As shown in FIG. 6, the second optical signal 9 output by the optical waveguide 103d is delayed by one time slot as compared with the first optical signal 8 output by the optical waveguide 103c. That is, the second optical signal 9 output by the optical waveguide 103d is delayed by one bit as compared with the optical signal 8 output by the optical waveguide 103c.

The phase state of the first optical signal 8 output from the optical waveguide 103c is the same as the phase state of the optical signal 7, and changes between the phases 0 and π. On the other hand, the optical waveguide 10
The phase state of the second optical signal 9 output by 3d changes between π / 2 and 3π / 2, as shown in FIG. This is because the phase adjusting unit 103f adjusts the value of Φ so that the phase of the second optical signal 9 output from the optical waveguide 103d changes between π / 2 and 3π / 2. .
In this way, the phase of the first optical signal 8 is the same as the phase state of the optical signal 7 so as to change between the phases 0 and π, and the phase of the second optical signal 9 is π / 2 to 3π. The value of Φ is adjusted so as to change between the second optical signal 8 and the second optical signal 8.
The phase difference of the optical signal 9 is adjusted. The phase of the first bit of the second optical signal 9 output from the optical waveguide 103d of the Mach-Zehnder interferometer 103 is assumed to be π / 2.

The optical signal phase difference 10 shown in FIG. 6 is the same as the first optical signal 8 output from the optical waveguide 103c and the optical waveguide 10.
3d is the phase difference from the second optical signal 9 output. That is, the directional coupler 10 of the Mach-Zehnder interferometer 103
3 is a waveform showing the phase difference between the first optical signal 8 and the second optical signal 9 from the two optical waveguides 103c and 103d input to 3b. As shown in FIG. 6, the phase difference 10 is −π.
It takes a value from + π / 2 to −3π / 2 centering on / 2. Further, as described above, the phase of the first optical signal 8 is
The second optical signal 9 is made to vary between the phases 0 and π.
Of the first optical signal 8 by delaying the second optical signal 9 by 1 bit as compared with the optical signal 8 by changing the phase of the first optical signal 8 from the phase of π / 2 to 3π / 2. And the phase difference 10 of the second optical signal 9 is the optical signal 7 of the NRZI code.
, The first optical signal 8 and the second optical signal 9
Is adjusted so that the phase difference of 10 becomes −π / 2.
That is, when the optical signal 7 of the NRZI code does not change, for example, when “1” continues continuously, the phase difference 10 is −π / 2.
Has been adjusted to be

The waveform of the optical signal 11 shown in FIG. 6 is a waveform showing the light intensity of the optical signal 11 output from the Mach-Zehnder interferometer 103. The light intensity of the optical signal 11 output by the Mach-Zehnder interferometer 103 can be derived from the equation (1).
As a result, when the phase difference 10 between the first optical signal 8 and the second optical signal 9 is −π / 2, the Mach-Zehnder interferometer 10
The optical signal 11 of 3 is cut off, and the output of the optical signal 11 becomes 0 (extinction state). Also, the phase difference 10 is + π / 2 or -3π
In the case of / 2, the Mach-Zehnder interferometer 103 transmits the optical signal 11, and the output of the optical signal 11 becomes 1 (output state). Therefore, the waveform of the optical signal 11 output from the Mach-Zehnder interferometer 103 becomes the waveform shown in FIG.

Here, the optical signal 7 of the NRZI code which the phase modulator 102 inputs to the Mach-Zehnder interferometer 103.
And the optical signal 1 output from the Mach-Zehnder interferometer 103
Compare with 1. When the optical signal 7 of the NRZI code does not change, for example, when “1” continues, the output of the optical signal 11 is “0”, which is in the extinction state. On the other hand, when the optical signal 7 changes, that is, "0" to "1"
, Or when changing from “1” to “0”, the optical signal 1
The output of 1 is "1", which is in the output state.

Thus, when the optical signal 7 of the NRZI code does not change, the phase difference 10 is adjusted so that the phase difference 10 between the first optical signal 8 and the second optical signal 9 becomes -π / 2. By doing so, the operating point of the Mach-Zehnder interferometer 103 can be set so that the output of the Mach-Zehnder interferometer 103 is in the extinction state when the optical signal 7 of the NRZI code does not change. In this way, the Mach-Zehnder interferometer 10
In No. 3, the operating point can be easily set by providing the phase adjusting unit 103f in the optical waveguide 103d and adjusting the phase difference 10.

The first bit of the optical signal 11 output from the Mach-Zehnder interferometer 103, that is, the head of the bit string of the optical signal 11 is "0", is that the optical waveguide 103d of the Mach-Zehnder interferometer 103. Is assumed to be π / 2 in phase of the second optical signal 9 output by the. However, the second optical signal 9 output from the optical waveguide 103d of the Mach-Zehnder interferometer 103 is delayed by 1 bit from the first optical signal 8 output from the optical waveguide 103c. Therefore, the first bit of the optical signal 9 may be 3π / 2 in addition to π / 2. In that case, the first bit of the optical signal 11 output from the Mach-Zehnder interferometer 103 is “1”.

Thus, the Mach-Zehnder interferometer 10
The operating point 3 is set so that the output of the Mach-Zehnder interferometer 103 is in the extinction state when the optical signal 7 of the NRZI code does not change. As a result, the Mach-Zehnder interferometer 1
As shown in FIG. 6, the waveform of the optical signal 11 output by 03 is RZ code, and the bit string is "0101100".
1 ”. This bit string is NRZ / N shown in FIG.
NRZ before code conversion by the RZI converter 105
It is the same as the bit string of the data signal 2 of the code. That is, according to the optical modulation device 100, an optical signal 11 in which only the code is the RZ code is obtained from the data signal 2 of the NRZ code input to the optical modulation device 100 without changing the bit string of the data signal 2. You can As a result, the optical modulator 100 can output the optical signal 11 of the RZ code, and thus can suppress the deterioration with respect to XPM.

Incidentally, in the explanation using FIG. 6, in order to simplify the explanation, the Mach-Zehnder interferometer 103 is
FSR is the reciprocal 1 / T of the 1-bit time slot T of the data signal 2 input to the NRZ / NRZI converter 105.
, And the second optical signal 9 is delayed by 1 bit as compared with the first optical signal 8. However, the delay of the second optical signal 9 from the first optical signal 8 is not limited to 1 bit as long as it is 1 bit or less. Mach-Zehnder interferometer 1
03 compares the second optical signal 9 with the first optical signal 8 and
When delaying by less than 1 bit, the interference noise of the output optical signal 11 can be made smaller than when delaying by 1 bit.

Next, the Mach-Zehnder interferometer 103 is
The optical signal 11 to be output is input to the optical bandpass filter 104. The optical bandpass filter 104 narrows the spectrum of the optical signal 11 input from the Mach-Zehnder interferometer 103. By inserting the optical bandpass filter 104 next to the Mach-Zehnder interferometer 103, the wavelength of the optical signal 11 can be made compact and the influence of dispersion of the optical fiber can be minimized. When the optical modulator 100 is used as an optical modulator of a wavelength division multiplexing system, the optical bandpass filter 104 can prevent the influence on the adjacent wavelength channels. As the optical bandpass filter 104, a dielectric multilayer film filter, a fiber Bragg grating type filter, a rotary grating type filter, a Fabry-Perot interference type filter or the like can be used.

When the optical modulator 100 is used as an optical modulator of a wavelength division multiplexing system, a multiplexer (hereinafter referred to as “MUX”) for wavelength multiplexing may be used as the optical bandpass filter 104. M for wavelength multiplexing
This is because the UX has a bandpass filter characteristic and can utilize it. According to this, the optical signal 11 output from the Mach-Zehnder interferometer 103 can be directly input to the MUX that performs wavelength multiplexing, and the optical modulator 100 does not need to be provided with a bandpass filter.

Next, a simulation performed using the optical modulator 100 will be described.

(Simulation 1) Light modulator 100
In addition, as the data signal 2, a 64-bit bit string of the NRZ code "10100100101011010000100.
1011011100100010101110101
A simulation for inputting "11110011011010010" is performed. The simulation parameter is that the bit rate of the data signal 2 of NRZ code is 40 Gb.
it / sec, the modulation factor of the phase modulator 102 is 50% (V
drive to π / 2), F of Mach-Zehnder interferometer 103
Set SR to 40 GHz. FIG. 7 is a graph showing the spectrum of the optical signal 11 output from the Mach-Zehnder interferometer 103, which is the simulation result.
The vertical axis of the graph shown in FIG. 7 indicates the output of the optical signal 11 (dB
m), the horizontal axis represents the optical frequency (GHz), and the center value 0 on the horizontal axis represents 193.4 THz.

As shown in FIG. 7, the spectrum of the optical signal 11 output from the Mach-Zehnder interferometer 103 and input to the bandpass filter 104 includes both modulation spectrum sidebands, that is, the upper sideband and the lower sideband. There is a band. Further, the optical carrier of the optical signal 11 is suppressed. As described above, according to the optical modulation device 100, it is possible to obtain the optical signal 11 of the RZ code in which the optical carrier is suppressed. A modulation method for obtaining an optical signal of an RZ code in which such an optical carrier is suppressed is CS-
RZ (Carrier Suppressed Ret)
urn to Zero) modulation method. According to the CS-RZ modulation method, even when the optical power input to the optical fiber is high, it is possible to obtain the effect that the deterioration of the reception sensitivity due to the nonlinear optical effect is small.

As a CS-RZ modulation type optical modulator, conventionally, there is an apparatus in which two Mach-Zehnder type intensity modulators are connected in series. In this conventional device, first, an optical signal of NRZ code is obtained by the Mach-Zehnder type intensity modulator in the preceding stage. Next, in the conventional device, the Mach-Zehnder type intensity modulator at the subsequent stage is driven by a clock signal having a frequency half the bit rate of the data signal. At this time, in the conventional device, the operating point of the Mach-Zehnder type intensity modulator is set so that the optical signal is not output when the driving voltage is 0. Then, in the conventional device, in the part where the light transmission characteristic of the Mach-Zehnder type intensity modulator has a quadratic function, by driving the Mach-Zehnder type intensity modulator in the subsequent stage, the optical carrier of the RZ code is suppressed. Get the signal. However, in this conventional device, it is necessary to exactly match the phases between the two Mach-Zehnder type intensity modulators, and since two Mach-Zehnder type intensity modulators are required, the configuration becomes complicated. On the other hand, according to the optical modulator 100, the phase modulator 102 and the Mach-Zehnder interferometer 1
The CS-RZ modulation method can be realized with a simple configuration in which the above-mentioned No. 03 and No. 03 are combined. Then, the optical modulator 100 can easily obtain the effect that the deterioration of the reception sensitivity due to the nonlinear optical effect is small even when the optical power input to the optical fiber is high.

(Simulation 2) Next, a simulation for converting the optical signal output from the optical bandpass filter 104 into an electrical signal and displaying an eye pattern by using a super-Gaussian type fourth-order filter as the bandpass filter 104. To do. The full width at half maximum of the super-Gaussian fourth-order filter is set to 80 GHz. In addition, the super Gauss type fourth-order filter is an optical bandpass filter 104.
The center frequency of is substantially matched with the oscillation wavelength of the light source 101, is substantially matched with the frequency of the light 1 output from the light source 101, and both modulation spectrum sidebands are passed. FIG. 8 is a graph showing an eye pattern which is the result of the simulation. The vertical axis of the graph shown in FIG. 8 is an arbitrary axis, and the horizontal axis is time (ps).

As shown in FIG. 8, according to the optical modulator 100, a normal eye pattern can be obtained even if the optical bandpass filter 104 narrows the spectrum, and the RZ pattern is almost RZ.
A coded electrical signal is obtained. In addition, the center frequency of the optical bandpass filter 104 is set to the light 1 output from the light source 101.
The optical modulator 100 is capable of performing DSB modulation by allowing the frequencies to be substantially equal to each other and allowing both modulation spectrum sidebands (upper side and lower side) to pass. The DSB method modulation has an advantage that demodulation becomes easy.

(Simulation 3) Next, NRZ / R
A drive circuit that operates with an RZ code using a Z converter, an optical modulation method using a conventional optical modulator that directly obtains an optical signal of an RZ code by an optical intensity modulator, an NRZ / NRZI converter 105, and a drive circuit 106. The frequency bands required for the optical modulation method by the optical modulator 100 according to the present embodiment for obtaining the RZ code optical signal 11 using the phase modulator 102 and the Mach-Zehnder interferometer 103 will be compared. Therefore, the relationship between the total modulation band (total frequency band) of the light intensity modulator of the conventional optical modulator and its drive circuit and the eye opening degree of the receiver, and the phase modulator 102 and its drive circuit 106 of the present embodiment. The relationship between the total modulation band (total frequency band) and the eye opening at the receiver is simulated.

FIG. 9 is a graph showing the relationship between the modulation band as a result of the simulation and the deterioration of the eye opening. In the graph shown in FIG. 9, the horizontal axis represents the normalized modulation band f / B obtained by dividing the total modulation band (3 dB lower frequency) f by the bit rate B of the data signal 2.
The vertical axis represents the eye opening degree at the receiver when f / B = 1 as a reference (0 dB), and represents the deterioration of the eye opening degree from the reference. The eye opening degree means the ratio of the difference between the maximum value and the minimum value of the output waveform and the maximum value of the eye opening of the eye pattern.

As shown in FIG. 9, in the optical modulation method using the optical modulator 100 according to this embodiment, the phase modulator 10 is used.
The total modulation band (3 dB lower frequency) of 2 and its driving circuit 106 may be approximately half the bit rate of the data signal 2. On the other hand, in the conventional optical modulation method using the conventional optical modulator, the total modulation band (3 dB lower frequency)
Requires a frequency approximately at the bit rate of the data signal 2.

According to the optical modulation device 100 and the optical modulation method according to the first embodiment of the present invention as described above, the phase modulator 102 drives the light 1 by the drive signal 6 of the NRZI code, so that the NRZI A coded optical signal 7 is generated,
It is input to the Mach-Zehnder interferometer 103. And
When the Mach-Zehnder interferometer 103 does not output the optical signal 11 when the input phase-modulated NRZI coded optical signal 7 does not change, the optical signal 11 output from the Mach-Zehnder interferometer 103 becomes RZ. It becomes the optical signal 11 of the code. Therefore, Mach-Zehnder interferometer 1
Reference numeral 03 can output the optical signal 11 of the RZ code as an optical signal to be output to the optical transmission line, and can suppress deterioration with respect to XPM.

Furthermore, since the phase modulator 102 is driven by the drive signal 6 of the NRZI code, the phase modulator 102
The frequency band required for the above may be about half of the data signal bit rate by the Nyquist frequency concept (Nyquist theorem). Therefore, the phase modulator 102 does not need to have a wide frequency band and does not need a high modulation voltage. Further, as the phase modulator 102 does not need to have a wide frequency band, the drive circuit 106 for driving the phase modulator 102 does not need to have high speed and high voltage output. Therefore, it is difficult to form a high-speed and high-voltage output drive circuit because of the breakdown voltage.

Further, as the intensity modulator, the phase modulator 102
The Mach-Zehnder interferometer 103 which divides the optical signal 7 input from the first optical signal 8 and the second optical signal 9 and combines the first optical signal 8 and the second optical signal 9 and outputs the combined signal. To use. Then, the phase adjusting unit 103f is provided in the optical waveguide 103d, the phase difference 10 between the first optical signal 8 and the second optical signal 9 is adjusted, and the operating point of the Mach-Zehnder interferometer 103 is changed to the NRZI code. The output of the Mach-Zehnder interferometer 103 is set to be in the extinction state when the optical signal 7 does not change. Therefore, the phase adjusting unit 1 is provided in the optical waveguide 103d.
By providing 03f and adjusting the phase difference 10,
The operating point of the Mach-Zehnder interferometer 103 can be easily set.

Further, the optical modulator 100 is NRZ / NRZ
Since the I-converter 105 is provided, the optical modulator 100 has NR
Even when the data signal 2 of Z code is input, the output signal 5 of NRZI code can be input to the drive circuit 106 that generates the drive signal 6.

[Second Embodiment] Next, a second embodiment of the present invention will be described.
The optical modulator 200 according to the embodiment will be described. FIG. 10 shows an optical modulation device 20 according to the second embodiment.
It is a block diagram which shows the structure of 0. Light modulator 200
Is a light source 201, a phase modulator 202, a Mach-Zehnder interferometer 203, an optical bandpass filter 204,
An NRZ / NRZI converter 205, a drive circuit 206,
An O / E converter 207 and a temperature control circuit 208 are provided. In FIG. 10, a light source 201, a phase modulator 202,
The optical bandpass filter 204, the NRZ / NRZI converter 205, and the drive circuit 206 are the optical modulator 10 shown in FIG.
The light source 101, the phase modulator 102, the optical bandpass filter 104, the NRZ / NRZI converter 105, and the drive circuit 106, which are substantially the same as the light source 0, are not described here.

FIG. 11 shows a Mach-Zehnder interferometer 203.
It is a figure which shows the structure of. Mach-Zehnder interferometer 203
Is a directional coupler 203a and a directional coupler 203b.
And an optical waveguide 203c, an optical waveguide 203d, a first output port 203f, and a second output port 203g. The directional coupler 203a and the optical waveguide 203c are shown in FIG.
Directional coupler 1 of Mach-Zehnder interferometer 103 shown in FIG.
03a and the optical waveguide 103c are substantially the same,
The description is omitted here.

The optical waveguide 203d is the other optical waveguide 20.
The length is set longer than 3c by ΔL203e. The directional coupler 203b has a first output port 203f.
And is connected to the second output port 203g, and inputs an optical signal to both. The directional coupler 203b includes a first optical signal 8 split into two optical waveguides 203c and 203d.
And the second optical signal 9 are combined, and the combined optical signal 11 is input to the first output port 203f. Also, the directional coupler 2
03b is an optical signal 1 input to the first output port 203f
The optical signal 12 having a complementary relationship with 1 is output to the second output port 203
Enter in g.

The first output port 203f outputs an optical signal 11 which is a combination of the first optical signal 8 and the second optical signal 9 split into the two optical waveguides 203c and 203d. The optical signal 11 is similar to the optical signal 11 output from the Mach-Zehnder interferometer 103 shown in FIG. 2, and its bit string is “0.
1011001 ". The second output port 203g is
The optical signal 12 which is complementary to the optical signal 11 output from the first output port 203f is output. The optical signal 12 is used by the temperature control circuit 208 to control the temperature of the Mach-Zehnder interferometer 203. The first output port 20
The optical signal 11 output from 3f and the second output port 203g
The power sum with the optical signal 12 output by is always constant.

The first output port 203f inputs the optical signal 11 to the optical bandpass filter 204. The second output port 203g inputs the optical signal 12 to the O / E converter 207. In this way, the Mach-Zehnder interferometer 203
By providing the first output port 203f and the second output port 203G, the optical signal 11 can be output from the first output port 203f, and the second output port 203f.
An optical signal 12 used for temperature control of the Mach-Zehnder interferometer 203 can be output from g. The O / E converter 207 receives the optical signal 1 input from the second output port.
2 is converted into an electric signal 13. O / E converter 207
Inputs the converted electric signal 13 to the temperature control circuit 208.

The temperature control circuit 208 uses the electric signal 13 to generate the temperature control signal 16. Temperature control circuit 20
8 indicates the temperature control signal 16 by the Mach-Zehnder interferometer 20.
By inputting in 3, the Mach-Zehnder interferometer 2
03 Control the temperature of the whole. The temperature control circuit 208 controls the temperature of the Mach-Zehnder interferometer 203, so that the above-mentioned Φ can be adjusted. As described above, the phase difference 10 between the first optical signal 8 and the second optical signal 9 from the two optical waveguides 203c and 203d is represented by βΔL + Φ, and ΔL is preset. Therefore, the temperature control circuit 208 is a temperature control unit that controls the temperature of the Mach-Zehnder interferometer 203 and adjusts Φ to adjust the phase difference 10.

The temperature control circuit 208 adjusts the phase difference 10 to adjust the operating point of the Mach-Zehnder interferometer 203 to NR.
When the ZI coded optical signal 7 does not change, the output of the optical signal 11 output from the first output port 203f is set to the extinction state. As described above, the directional coupler 203
In b, the phase difference between the first optical signal 8 and the second optical signal 9 from the two optical waveguides 203c and 203d is 2kπ + π / 2.
In the case of (k is an integer), the optical signal 11 is transmitted, the optical signal 11 is output to the first output port 203f, and the phase difference is 2kπ−π.
/ 2 (k is an integer), the optical signal 11 is blocked and the first
The output of the output port 203f is in the extinction state. Therefore, the temperature control circuit 208 has a phase difference of 2kπ−π / 2 (k is an integer) when the optical signal 7 of the NRZI code does not change.
Therefore, the temperature of the Mach-Zehnder interferometer 203 is controlled so that Φ is adjusted.

The temperature control circuit 208 controls the temperature of the Mach-Zehnder interferometer 203 to adjust Φ,
When the phase difference 10 is adjusted so as to be in the extinction state when the optical signal 7 of ZI code does not change, the second output port 203
The light intensity of the optical signal 12 output by g is maximized. Therefore, the temperature control circuit 208 controls the temperature of the Mach-Zehnder interferometer 203 so that the optical intensity of the optical signal 12 output from the second output port 203g is maximized, thereby changing the optical signal 7 of the NRZI code. When not doing so, the operating point of the Mach-Zehnder interferometer 203 can be set so that the phase difference 10 becomes −π / 2 and the light is extinguished.

FIG. 12 is a diagram showing the structure of the temperature control circuit 208. As shown in FIG. 12, the temperature control circuit 208
Is a phase comparator 208a and a dither signal generator 208b.
With. The dither signal generator 208b generates the low frequency dither signal 14. The dither signal generator 208b is
The generated low frequency dither signal 14 is supplied to the phase comparator 208a.
To enter. The dither signal generator 208b superimposes the generated low frequency dither signal 14 on the phase comparison output signal 15 output from the phase comparator 208a. Phase comparator 20
8a receives the converted electric signal 13 from the O / E converter 207 and receives the low frequency dither signal 14 from the dither signal generator 208b.

The phase comparator 208a receives the electric signal 13 and
Conversion to the low frequency dither signal 14 is performed, and the result of the conversion is output as the phase comparison output signal 15. Then, in the phase comparison output signal 15 output from the phase comparator 208a,
The dither signal generator 208b superimposes the low frequency dither signal 14 to generate the temperature control signal 16 of the Mach-Zehnder interferometer 203. As described above, the temperature control circuit 208 generates the temperature control signal 16 using the electric signal 13 obtained by converting the optical signal 12 by the O / E converter 207. Then, the temperature control circuit 208 outputs the temperature control signal 1
By inputting 6 to the Mach-Zehnder interferometer 203, the temperature of the Mach-Zehnder interferometer 203 can be controlled.

According to the optical modulation device 200 and the optical modulation method according to the second embodiment of the present invention, the first output port 203f of the Mach-Zehnder interferometer 203 has the phase-modulated NRZI code optical signal. By not outputting the optical signal 11 when the signal 7 does not change, the optical signal 11 output from the first output port 203f becomes the RZ code optical signal 11. Therefore, the Mach-Zehnder interferometer 203 can output the optical signal 11 of the RZ code as an optical transmission signal, and can suppress deterioration with respect to XPM. In addition, with a simple configuration in which the phase modulator 202 and the Mach-Zehnder interferometer 203 are combined, the CS-
The RZ modulation method can be realized, and the optical signal 11 of the RZ code in which the optical carrier is suppressed can be obtained. Further, since the phase modulator 202 is driven by the drive signal of the NRZI code, it does not need to have a wide frequency band, does not need a high modulation voltage, and needs a high-speed and high-voltage output as the drive circuit 206. Not taken

Further, the optical modulator 200 includes the temperature control circuit 2
No. 08 and adjusting the phase difference 10 make it easy to set the operating point of the Mach-Zehnder interferometer 203 to NRZI.
The first output port 203 when the optical signal 7 of the code does not change
The output of the optical signal 11 output by f can be set to be in the extinction state. The temperature control circuit 208 also generates a temperature control signal 16 using the electric signal 13 obtained by converting the optical signal 12 by the O / E converter 207. Then, the temperature control circuit 208 can easily control the temperature of the Mach-Zehnder interferometer 203 by inputting the temperature control signal 16 to the Mach-Zehnder interferometer 203. or,
The temperature control circuit 208 controls the temperature of the Mach-Zehnder interferometer 203 by controlling the temperature of the Mach-Zehnder interferometer 203 so that the optical intensity of the optical signal 12 output from the second output port 203g becomes maximum.
The phase difference 10 can be adjusted so that the extinction state occurs when the RZI coded optical signal 7 does not change.

The optical waveguide 203d of the Mach-Zehnder interferometer 203 shown in FIG. 11 does not have the phase adjusting section as shown in FIG. 2, and the temperature control circuit 208 controls the temperature of the entire Mach-Zehnder interferometer 203 to be controlled. I am trying. However, the phase adjusting unit may be provided inside the Mach-Zehnder interferometer 203, and the temperature control circuit 208 may control the phase adjusting unit. For example, similarly to FIG. 2, a heater may be provided in the optical waveguide 203d of the Mach-Zehnder interferometer 203 as a phase adjusting unit, and the temperature control circuit 208 may control the temperature of the heater.

In the optical modulator 200, the Mach-Zehnder interferometer 203 having the first output port 203f and the second output port 203g is provided, and the temperature control circuit 208 is provided.
Uses the optical signal 12 output from the second output port 203g for temperature control. However, the light modulation device 200 is
Using the Mach-Zehnder interferometer 203 without the second output port 203g, the temperature control circuit 208 uses the optical signal 12
The temperature may be controlled by a method not using. Further, the optical modulator 200 may control the temperature of the Mach-Zehnder interferometer 203 using a temperature control unit other than the temperature control circuit 208.

[Third Embodiment] Next, the third embodiment of the present invention will be described.
The optical modulator 300 according to the embodiment will be described. FIG. 13 shows an optical modulation device 30 according to the third embodiment.
It is a block diagram which shows the structure of 0. Light modulator 300
Is a light source 301, a phase modulator 302, a Mach-Zehnder interferometer 303, an optical bandpass filter 304,
An NRZ / NRZI converter 305, a drive circuit 306,
An O / E converter 307 and a temperature control circuit 308 are provided. In FIG. 13, a light source 301, a phase modulator 302,
Mach-Zehnder interferometer 303, optical bandpass filter 304, NRZ / NRZI converter 305, drive circuit 30
6. The O / E converter 307 is the optical modulator 2 shown in FIG.
00 light source 201, phase modulator 202, Mach-Zehnder interferometer 203, optical bandpass filter 204, NRZ
Since it is substantially the same as the / NRZI converter 205, the drive circuit 206, and the O / E converter 207, description thereof will be omitted here.

In the optical modulator 100 shown in FIG. 13, the second
The optical signal 12 output from the output port is used by the temperature control circuit 308 to control the temperature of the light source 301. Like the Mach-Zehnder interferometer 203 shown in FIG. 11, the Mach-Zehnder interferometer 303 includes the first output port and the second output port, so that the optical signal 11 can be output from the first output port. Therefore, the optical signal 12 used for controlling the temperature of the light source 301 can be output from the second output port.

The temperature control circuit 308 includes an O / E converter 3
The electrical signal 13 obtained by converting the optical signal 12 from 07 is input. The temperature control circuit 308 uses the electric signal 13 to generate a temperature control signal 17 for controlling the temperature of the light source 301. The temperature control circuit 308 sends the temperature control signal 17 to the light source 3
By inputting 01, the temperature of the light source 301 is controlled. By controlling the temperature of the light source 301 by the temperature control circuit 308, the frequency of the light 1 output from the light source 301 can be controlled, and by controlling the frequency of the light 1, the above-mentioned Φ can be adjusted. You can Therefore, the temperature control circuit 308 controls the frequency of the light 1 by controlling the temperature of the light source 301, and the phase difference 10
Is a frequency control unit that adjusts.

The temperature control circuit 308 has a phase difference of 1 so that the operating point of the Mach-Zehnder interferometer 303 is in the extinction state when the optical signal 7 of the NRZI code does not change.
Adjust 0. The temperature control circuit 308 controls the temperature of the light source 301 to control the frequency of the light 1 and adjust Φ so that the phase difference becomes −π / 2 when the optical signal 7 of the NRZI code does not change.

The temperature control circuit 308 controls the temperature of the light source 301 to control the frequency of the light 1, and when the optical signal 7 of the NRZI code does not change, the Mach-Zehnder interferometer 3
If the phase difference 10 is adjusted so that the first output port of port 03 will be in the extinction state, the optical signal 1 output from the second output port
The light intensity of 2 is maximum. Therefore, the temperature control circuit 30
An optical signal 7 of NRZI code 8 is provided by controlling the temperature of the light source 301 and controlling the frequency of the light 1 so that the optical intensity of the optical signal 12 output from the second output port is maximized.
Can be adjusted so that the phase difference 10 becomes −π / 2 when does not change.

The structure of temperature control circuit 308 is substantially the same as that of temperature control circuit 208 shown in FIG. The phase comparator converts the electric signal 13 and the low-frequency dither signal, and outputs the conversion result as a phase comparison output signal. Then, the dither signal generator superimposes the low-frequency dither signal on the phase comparison output signal output from the phase comparator to generate the temperature control signal 17 of the light source 301.

According to the optical modulator 300 and the optical modulation method according to the third embodiment, the first output port of the Mach-Zehnder interferometer 303 is phase-modulated NR.
By not outputting the optical signal 11 when the optical signal 7 of ZI code does not change, the optical signal 11 output from the first output port becomes the optical signal 11 of RZ code. Therefore, the Mach-Zehnder interferometer 303 can output the RZ-coded optical signal 11 as an optical transmission signal.
It is possible to suppress deterioration with respect to PM. In addition, the CS-RZ modulation method can be realized with a simple configuration in which the phase modulator 302 and the Mach-Zehnder interferometer 303 are combined, and the optical signal 11 of the RZ code in which the optical carrier is suppressed can be obtained. Further, since the phase modulator 302 is driven by the drive signal of the NRZI code, it does not need to have a wide frequency band, does not require a high modulation voltage, and the drive circuit 30
No high-speed, high-voltage output 6 is required.

In addition, the optical modulator 300 includes the temperature control circuit 3
08 and adjust the phase difference 10 to easily adjust the operating point of the Mach-Zehnder interferometer 303 to NRZI.
The output of the optical signal 11 output from the first output port can be set to the extinction state when the optical signal 7 of the code does not change. Further, the temperature control circuit 308 generates a temperature control signal 17 using the electric signal 13 obtained by converting the optical signal 12 by the O / E converter 307. Then, the temperature control circuit 308
The temperature control of the light source 301 can be performed by inputting the temperature control signal 17 to the light source 301. or,
The temperature control circuit 308 adjusts the temperature of the light source 301 so that the light intensity of the optical signal 12 output from the second output port is maximized, so that when the optical signal 7 of the NRZI code does not change, the extinction state is set. The phase difference 10 can be adjusted so that

The optical modulator 300 includes the temperature control circuit 3
The temperature of the light source 301 may be controlled by using a frequency control unit other than 08. In addition, the light modulation device 300 is
The frequency of the light 1 may be controlled by a method other than the method of controlling the temperature of the light source 301.

[Fourth Embodiment] Next, a fourth embodiment of the present invention will be described.
The optical modulator 400 according to the embodiment will be described. FIG. 14 is an optical modulation device 40 according to the fourth embodiment.
It is a block diagram which shows the structure of 0. Light modulator 400
Is a plurality of light sources 401a and 401b, a plurality of phase modulators 402a and 402b, and a Mach-Zehnder interferometer 40.
3, a comb filter 404, a plurality of NRZ / NR
ZI converters 405a and 405b and a plurality of drive circuits 40
6a, 406b and MUX 407. Incidentally, FIG.
4, a plurality of light sources 401a and 401b, a plurality of phase modulators 402a and 402b, and a plurality of NRZ / NRZIs.
Converters 405a and 405b and a plurality of drive circuits 406
Although only two each of a and 406b are illustrated, in actuality, any number may be provided as long as it is plural (two or more). In FIG. 14, a plurality of light sources 401a,
401b, a plurality of NRZ / NRZI converters 405a, 405
05b and the plurality of drive circuits 406a and 406b are respectively the light source 101 and the NRZ of the light modulation device 100 shown in FIG.
Since it is substantially the same as the / NRZI converter 105 and the drive circuit 106, the description thereof is omitted here.

The plurality of phase modulators 402a and 402b are
Similarly to the phase modulator 102 of the optical modulator 100 shown in FIG. 1, the light 1 is phase-modulated by the drive signal 6 of the NRZI code. The plurality of phase modulators 402a and 402b are
NRZI coded optical signal 7 obtained by phase modulation
Is input to the MUX 407. The MUX 407 has a plurality of Ns input from the plurality of phase modulators 402a and 402b.
The optical signal 7 of the RZI code is wavelength-multiplexed. MUX407
Is an optical signal 18 obtained by wavelength-multiplexing a plurality of optical signals 7.
To the Mach-Zehnder interferometer 403.

The Mach-Zehnder interferometer 403 is a MUX.
The plurality of optical signals 7 input from 407 are wavelength-multiplexed, and the plurality of optical signals 7 included in the optical signal 18 obtained are intensity-modulated. As shown in FIG. 3, the Mach-Zehnder interferometer 40
The light transmittance of No. 3 changes periodically with respect to the frequency of light. Therefore, the Mach-Zehnder interferometer 403 is shown in FIG.
Intensity modulation can be periodically performed in the valley portion of the waveform shown in. Therefore, one Mach-Zehnder interferometer 403 intensity-modulates each optical signal 7 included in the optical signal 18 obtained by wavelength-multiplexing a plurality of optical signals 7. That is, the frequency interval of the wavelength division multiplexed signal is set to be an integral multiple of the FSR of the Mach-Zehnder interferometer 403.

The Mach-Zehnder interferometer 403 is the Mach-Zehnder interferometer 103 of the optical modulator 100 shown in FIG.
Similarly, the intensity of each optical signal 7 is modulated, and the optical signals 11 of a plurality of RZ codes are output. Mach-Zehnder interferometer 4
Reference numeral 03 denotes an optical comb filter 404 that periodically passes a plurality of optical signals 11 obtained by intensity-modulating the signals having a plurality of wavelengths.
To enter.

However, the Mach-Zehnder interferometer 403 is
A plurality of optical signals 7 included in the wavelength-multiplexed optical signal 18
In order to perform intensity modulation in each period, the delay of the second optical signal 9 of the Mach-Zehnder interferometer 403 from the first optical signal 8 needs to be 1 bit or less. That is, the Mach-Zehnder interferometer 403 delays the second optical signal 9 by 1 bit or less as compared with the first optical signal 8. Specifically, the Mach-Zehnder interferometer 40
3 sets ΔL so as to satisfy the equation (3), and
Is set to the reciprocal 1 / T of the time slot T of the 1-bit time slot of the data signal 2 or the reciprocal 1 / T 'of the time slot T ′ that is less than the 1-bit time slot T. As a result, the Mach-Zehnder interferometer 103 can delay the second optical signal 9 by 1 bit or less as compared with the first optical signal 8.

According to the light modulation device 400 and the light modulation method according to the fourth embodiment, a plurality of light sources 401 are provided.
a, 401b and a plurality of phase modulators 402a, 402b
And a plurality of NRZ / NRZI converters 405a, 405b
, A plurality of drive circuits 406a and 406b, and MUX 40
7 is provided, it is possible to intensity-modulate a plurality of optical signals 7 wavelength-multiplexed by the MUX 407 with one Mach-Zehnder interferometer 403. Therefore, the light modulator 400 can be downsized and the cost can be reduced.

[Fifth Embodiment] FIGS. 1, 10 and 1
3, the optical modulators 100, 200, 300 shown in FIG.
In the optical spectrum of the optical signal 11 obtained by the optical modulation method using 400, the optical carrier frequency is suppressed as shown in FIG. Utilizing this, the optical bandpass filters 104, 204, 304 or the optical comb filter 4
The center frequency of 04 is the light source 101, 201, 301, 4
The optical band pass filters 104, 204, 304,
Alternatively, the optical modulation device 10 provided with the optical comb filter 404
It is set to 0, 200, 300, 400. As a result, the optical bandpass filters 104, 204, 304 or the optical comb filter 404 can extract one modulation spectral sideband by allowing it to pass more than the other modulation spectral sideband. For example, the optical bandpass filters 104, 204, 304 or the optical comb filter 40
4 makes it possible to extract the upper side band by allowing the upper side band to pass more than the lower side band.

According to the optical modulators 100, 200, 300, 400 and the optical modulation method according to the fifth embodiment, the SSB system modulation can be easily performed. Light modulator 1
00, 200, 300, and 400 are DSs when the SSB modulation method is used as the high-density wavelength division multiplexing modulation method.
Since only half the bandwidth is used as compared with the B modulation method, it is possible to achieve approximately twice the transmission capacity. Further, in the optical modulators 100, 200, 300, 400, the spectrum width of the optical signal is narrowed, so that the optical modulators are less susceptible to the dispersion of the optical fiber.

[Modification] The present invention is not limited to the above embodiment, and various modifications can be made. For example, the Mach-Zehnder interferometers 103, 203, 303, 403 are
A directional coupler is provided, but Y is used instead of the directional coupler.
A branch may be provided. When the Y branch is provided, in order to put the output of the Mach-Zehnder interferometer into the extinction state, the first optical signal 8 output from the two optical waveguides is used.
The phase difference 10 between the second optical signal 9 and the second optical signal 9 needs to be adjusted to π. As described above, the phase difference 10 for bringing the output of the Mach-Zehnder interferometer into the extinction state differs depending on the structure of the Mach-Zehnder interferometer.

The optical modulator may use a bulk optical system Mach-Zehnder interferometer as the Mach-Zehnder interferometer. The light modulator may use a modulator other than the Mach-Zehnder interferometer as the intensity modulator.
Further, for convenience of explanation, the phase modulators 102, 202, 30
Although the phase modulation degree of 2,402 is set to π, the optical signal 11 of the RZ code can be similarly obtained with any value as long as it does not exceed π. In addition, the optical modulator 10
0, 200, 300, 400 are provided with NRZ / NRZI converters 105, 205, 305, 405a, 405b, but the data signal which is the basis of the drive signal 6 input to the optical modulator is an NRZI code. For example, NRZ / NR
The ZI converter can be omitted.

[0106]

As described above, according to the present invention,
The optical signal of RZ code is output as the optical transmission signal, and XPM
It is possible to provide an optical modulation device and an optical modulation method that can suppress deterioration of the optical modulator and that does not require the phase modulator to have a wide frequency band.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an optical modulation device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a Mach-Zehnder interferometer according to the first embodiment of the present invention.

FIG. 3 is a graph showing a relationship between light transmittance and light frequency of the Mach-Zehnder interferometer according to the first embodiment of the present invention.

FIG. 4 is an NRZ / NR according to the first embodiment of the present invention.
It is a figure which shows the structure of a ZI converter.

FIG. 5 is a diagram showing a data signal and an output signal according to the first embodiment of the present invention.

FIG. 6 is a diagram illustrating a drive signal for the phase modulator according to the first embodiment of the present invention, an optical signal for the phase modulator, and a first optical signal,
It is a figure which shows the 2nd optical signal, the phase difference of a 1st optical signal and a 2nd optical signal, and the relationship of the optical signal of a Mach-Zehnder interferometer.

FIG. 7 is a graph showing a spectrum of an optical signal of the Mach-Zehnder interferometer according to the first embodiment of the present invention.

FIG. 8 is a graph showing an eye pattern according to the first embodiment of the present invention.

FIG. 9 is a graph showing the relationship between the modulation band and the deterioration of the eye opening degree according to the first embodiment of the present invention.

FIG. 10 is a block diagram showing a configuration of an optical modulation device according to a second embodiment of the present invention.

FIG. 11 is a diagram showing a configuration of a Mach-Zehnder interferometer according to a second embodiment of the present invention.

FIG. 12 is a diagram showing a configuration of a temperature control circuit according to a second embodiment of the present invention.

FIG. 13 is a block diagram showing a configuration of an optical modulation device according to a third embodiment of the present invention.

FIG. 14 is a block diagram showing a configuration of an optical modulation device according to a fourth embodiment of the present invention.

[Explanation of symbols]

100, 200, 300, 400 Light modulator 101, 201, 301, 401a, 401b Light source 102, 202, 302, 402a, 402b Phase modulator 103, 203, 303, 403 Mach-Zehnder interferometer 103a, 103b, 203a, 203b Directional couplers 103c, 103d, 203c, 203d Optical waveguide 103f Phase adjusters 104, 204, 304 Optical bandpass filters 105, 205, 305, 405a, 405b NRZ
/ NRZI converter 105a EX-OR circuit 105b 1-bit delay circuit 106, 206, 306, 406a, 406b Drive circuit 203f First output port 203g Second output port 207, 307 O / E converter 208, 308 Temperature control circuit 208a Phase comparator 208b Dither signal generator 404 Optical comb filter 407 MUX

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H04B 10/152 F term (reference) 2H079 AA02 AA06 AA12 BA01 BA03 CA05 EA05 FA01 FA02 FA03 GA01 HA14 KA07 5K102 AA01 AA15 AH22 AH31 KA09 KA39 MB04 MC28 MH02 MH12 MH22 PH02

Claims (20)

[Claims] 1. A phase modulator that drives light by a drive signal of an NRZI code to perform phase modulation, and an intensity modulator that intensity-modulates the phase-modulated optical signal of the NRZI code input from the phase modulator. An optical modulator comprising: an operating point of the intensity modulator, wherein an output of the intensity-modulated optical signal is in an extinction state when the optical signal of the NRZI code does not change. 2. The intensity modulator converts the phase-modulated optical signal input from the phase modulator into a first optical signal and a second optical signal.
Of the optical signal, the first optical signal and the second optical signal are combined and output, and the phase difference between the first optical signal and the second optical signal is adjusted to perform the operation. The optical modulator according to claim 1, wherein a point is set. 3. The optical modulator according to claim 2, wherein the intensity modulator includes a phase adjuster that adjusts the phase difference. 4. The optical modulator according to claim 2, further comprising a temperature controller that adjusts the phase difference by controlling the temperature of the intensity modulator. 5. The intensity modulator outputs a first output port for outputting the intensity-modulated optical signal and a second output port for outputting an optical signal complementary to the optical signal output to the first output port. The temperature control unit generates a temperature control signal based on the optical signal output from the second output port, and controls the temperature of the intensity modulator by the temperature control signal. The light modulation device according to claim 4. 6. The optical modulator according to claim 2, further comprising a frequency controller that adjusts the phase difference by controlling a frequency of light input to the phase modulator. 7. The light modulation device according to claim 6, wherein the frequency control unit controls the frequency of the light by controlling the temperature of the light source of the light. 8. The intensity modulator outputs a first output port for outputting the intensity-modulated optical signal and a second output port for outputting an optical signal complementary to the optical signal output to the first output port. And a frequency control unit that generates a temperature control signal based on the optical signal output from the second output port, and controls the temperature of the light source by the temperature control signal. The optical modulator according to claim 7. 9. The intensity modulator outputs a first output port for outputting the intensity-modulated optical signal and a second output port for outputting an optical signal complementary to the optical signal output to the first output port. 3. The optical modulator according to claim 2, wherein the operating point is set so that the optical intensity of the optical signal output from the second output port is maximized. 10. An NRZ / NR for converting a data signal of the NRZ code into a signal of the NRZI code when the data signal which is the basis of the drive signal is the signal of the NRZ code.
The optical modulator according to claim 1, further comprising a ZI converter. 11. The optical modulation device according to claim 1, further comprising an optical bandpass filter that narrows the spectrum of the intensity-modulated optical signal output from the intensity modulator. . 12. The optical modulator according to claim 11, wherein the optical bandpass filter passes both modulation spectrum sidebands of the intensity-modulated optical signal. 13. The light according to claim 11, wherein the optical bandpass filter passes one modulation spectrum sideband of the intensity-modulated optical signal more than the other modulation spectrum sideband. Modulator. 14. The optical modulator according to claim 1, wherein the intensity modulator intensity-modulates a plurality of the phase-modulated optical signals of the NRZI code. 15. A phase modulator drives light with a drive signal of an NRZI code to perform phase modulation, and an intensity modulator intensity-modulates a phase-modulated NRZI code optical signal input from the phase modulator. An optical modulation method is characterized in that when the optical signal of the NRZI code does not change, the output of the intensity-modulated optical signal is put into a quenching state. 16. The intensity modulator divides the phase-modulated optical signal input from the phase modulator into a first optical signal and a second optical signal, and separates the first optical signal and the second optical signal. Intensity modulation is performed by synthesizing and outputting the second optical signal, and the output of the optical signal is put into the extinction state by adjusting the phase difference between the first optical signal and the second optical signal. 16. The light modulation method according to claim 15, wherein the light modulation method is the same as that of the first embodiment. 17. The optical modulator according to claim 16, wherein the intensity modulator includes a phase adjusting unit that adjusts the phase difference, and the phase adjusting unit adjusts the phase difference. Method. 18. The optical modulation method according to claim 16, wherein the phase difference is adjusted by controlling the temperature of the intensity modulator. 19. The optical modulation method according to claim 16, wherein the phase difference is adjusted by controlling the frequency of light input to the phase modulator. 20. The intensity modulator outputs the intensity-modulated optical signal to a first output port, and outputs an optical signal complementary to the optical signal to be output to the first output port to a second output. The optical modulation method according to claim 16, wherein the optical intensity of the optical signal output to the port and output from the second output port is maximized.