JPH11331091A - Optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize the optical transmitter with small power consumption where it is prevented that a residual light is optically phase-modulated and becomes a cause to a code error when an extinct ratio of intensity modulation is limited. SOLUTION: In the transmitter, a light source 1 that generates an optical signal, a luminous intensity modulator 2 that applies luminous intensity modulation to the optical signal based on a luminous intensity modulator drive signal, and an optical phase modulator 11 that applies optical phase modulation to the optical signal that is luminous-intensity-modulated based on an optical phase modulator drive signal synchronously with the luminous intensity modulator drive signal are all connected in cascade, and the optical phase modulator drive signal is supplied to the optical phase modulator 11 via a switch 13 that is turned on/off by a switch control signal synchronously with the luminous intensity modulator drive signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system, and more particularly to an optical transmitter for modulating an optical signal according to an electric signal and transmitting the modulated signal in a long-distance optical amplification repeater transmission system using an optical amplification transmission technique. is there.

[0002]

2. Description of the Related Art In a long-distance optical amplification repeater transmission system, it is known that an optical S / N ratio is deteriorated or fluctuated due to polarization hole burning of an optical amplifier repeater or polarization dependent loss of a transmission line. Polarization scrambling is performed to improve this. In particular, in order to average the latter optical S / N ratio fluctuation (signal fading), it is necessary to scramble the polarization at least at a speed higher than the signal bit rate. In general, a lithium niobate optical phase modulator is used for the polarization scrambling.

[0003] This lithium niobate optical phase modulator is characterized in that phase modulation occurs simultaneously with polarization modulation, and it can be used for the purpose of compensating waveform deterioration due to dispersion of a transmission line. Is shown in
According to the discussion, it is discussed that driving a polarization scrambler with a data clock synchronized with the data modulation bits has a significant effect of improving the bit error rate in ultra-long distance optical amplification repeater transmission systems such as transoceanic. Have been. In other words, by setting the driving phase of the polarization scrambler to a predetermined relationship with the data, the eye can be further opened so that amplitude fluctuations at the receiving end caused by dispersion of the optical fiber and changes in the non-linear refractive index can be distinguished. Direction can be set.

FIG. 24 is a block diagram showing a conventional optical transmitter disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 8-111662. In the figure, 1 is a light source, 2 is a light intensity modulator, 3 is a polarization scrambler, 4 is a data signal input terminal, 5 is a clock signal input terminal, 6 is a discriminator, 7 is a light intensity modulator driving circuit, 8 Is a delay unit, 9 is a polarization scrambler driving circuit, 10
Is an optical transmitter output terminal.

Next, the operation will be described. The discriminator 6 generates a light intensity modulator driving signal from the data signal and the clock signal input from the data signal input terminal 4 and the clock signal input terminal 5, and the light intensity modulator driving circuit 7 amplifies the signal. Input to the light intensity modulator 2. On the other hand, the clock signal input from the clock signal input terminal 5 is branched and input to the delay unit 8 as a polarization scrambler driving signal to be delayed. The polarization scrambler driving signal delayed by the delay unit 8 is amplified by the polarization scrambler driving circuit 9 and drives the polarization scrambler 3. The optical signal output from the light source 1 is subjected to optical intensity modulation by the optical intensity modulator 2 according to the data signal by the optical intensity modulator drive signal,
The signal is input to the polarization scrambler 3, is polarization-scrambled to zero polarization degree, and is output from the optical transmitter output terminal 10.

Next, the operation of the polarization scrambler 3 will be described with reference to FIG. Here, FIG. 25 (a) shows a configuration of a lithium niobate optical phase modulator used as the polarization scrambler 3, and FIG.
(B) is a driving waveform of the polarization scrambler 3, and FIG.
Is a trajectory of the polarization state of the optical signal output from the polarization scrambler 3 on a Poincare sphere. In the Poincare sphere shown in FIG. 25C, L is left-handed circularly polarized light, R is right-handed circularly polarized light, H is horizontal polarized light, V is vertical polarized light, Q is 45-degree linearly polarized light, and P is -45 degrees. Each represents linearly polarized light.

[0007] Polarization scrambling is to make the degree of polarization zero. To make the degree of polarization zero, on the Poincare sphere, (A) the probability of existence in the L-side hemisphere within a certain time and R (B) The probability of being in the Q-side hemisphere within a certain time and the probability of being in the P-side hemisphere are equal. (C) The probability of being in the H-side hemisphere within a certain time and V It is necessary to satisfy the three conditions that the probabilities of existence in the lateral hemisphere are equal.

[0008] As shown in FIG. 25A, 45-degree linearly polarized light is input to the polarization scrambler 3. When the drive signal is zero, the polarization scrambler 3 is output as it is. When a drive signal is applied to the polarization scrambler 3, the phase modulation degrees for the two linearly polarized light components are different.
As shown in FIG. 5 (b), the polarization state changes on the Poincare sphere according to the driving signal in the order of Q → L → A → L → Q → R → B → R
→ Move to Q. When the optical signal intensity input from the optical intensity modulator 2 to the polarization scrambler 3 is constant, the polarization state draws a locus indicated by a thick line on the surface on the Poincare sphere. At this time, since the above three conditions (a) to (c) are satisfied, the degree of polarization can be made zero.

When the drive waveform shown in FIG. 25 (b) is applied to the polarization scrambler 3, the polarization is scrambled and, at the same time, the phase modulation proportional to the drive waveform in FIG. 25 (b) is applied. This is because the in-phase component of the phase modulation superimposed on the two linearly polarized light components becomes the residual phase modulation of the output light.

Next, the signal waveform of each part of the circuit shown in FIG. 24 will be described with reference to FIG. The optical signal output from the light source 1 is modulated by a light intensity modulator 2. FIG. 26A shows a waveform example of the output signal of the light intensity modulator. Generally, a digital signal is expressed as “1” when an optical signal exists within a prescribed time and “0” when it does not exist within a prescribed time. FIG. 26B shows a phase modulation waveform of the optical signal output from the polarization scrambler 3. As described above, when the lithium niobate optical phase modulator is used as the polarization scrambler 3, the phase modulation as shown in FIG. 26B remains at the same time as the polarization scrambler operation. At this time, the phase of the phase modulation can be adjusted by the delay unit 8.

Here, the phase modulation and the frequency modulation are essentially the same, and FIG. 26 (c) shows a frequency modulation waveform. The phase modulation shown in FIG.
This is equivalent to the frequency modulation shown in FIG. The time for an optical signal to propagate through an optical fiber depends on its frequency. Therefore, as shown in FIGS. 26D and 26E, the waveform after propagation through the optical fiber changes. FIG. 26D shows a waveform change state, and FIG. 26E shows a waveform after optical fiber transmission. As shown in FIG. 26 (d) and FIG. 26 (e), when the pulse is compressed, the eyes are opened, which is advantageous for identification. On the other hand, when the sign of the phase modulation is opposite, the pulse spreads and the eye opening deteriorates, which is disadvantageous for identification.

As described above, by synchronizing the polarization scrambler driving signal for driving the polarization scrambler 3 with the optical intensity modulator driving signal and optimizing the driving position, the eye opening ratio at the receiving end is improved. be able to. Also, by replacing the polarization scrambler 3 with an optical phase modulator, the effect of improving the eye opening at the receiving end can be obtained.

References that have a description related to such a conventional optical transmitter include, for example, Japanese Patent Application Laid-Open No. H4-1401.
0, JP-A-7-7475, JP-A-5-11
No. 0516 is also available.

[0014]

Since the conventional optical transmitter is constructed as described above, a large amplitude signal is generally required to drive the polarization scrambler 3 or the optical phase modulator. In addition, there is a problem that the power consumption of the polarization scrambler driving circuit 9 or the optical phase modulator driving circuit increases.

When the extinction ratio of the light intensity modulator 2 is finite, a slight optical signal remains even when the data signal is "0". At this time, the residual light is phase-modulated. If the data signal is "0" and a pulsed light is generated, a code error occurs because the receiving end discriminates this as "1" at the receiving end. There was also a problem that there was something.

When a return-to-zero signal is used as the driving signal of the optical intensity modulator, the driving phase of the polarization scrambler 3 is set so that the eye opening ratio is optimized. There was a problem that could not be done.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide an optical transmitter which consumes less power in a polarization scrambler driving circuit or an optical phase modulator driving circuit.

Another object of the present invention is to provide an optical transmitter capable of preventing the residual light from being phase-modulated and causing a code error even when the extinction ratio of the intensity modulation is finite. And

Another object of the present invention is to provide an optical transmitter capable of making the degree of polarization zero even under the condition that the polarization scrambler maximizes the eye opening in the return-to-zero signal. .

[0020]

SUMMARY OF THE INVENTION An optical transmitter according to the present invention comprises: a light intensity modulator for inputting a light intensity modulator drive signal and modulating the light intensity of an optical signal from a light source; Based on the synchronized optical phase modulator drive signal, an optical phase modulator that optically modulates the optical signal whose optical intensity has been modulated is cascaded, and the optical phase modulator drive signal to this optical phase modulator is The power is supplied via a switch which is turned on / off by a switch control signal synchronized with the intensity modulator drive signal.

In the optical transmitter according to the present invention, the optical intensity modulator drive signal generated by the discriminator from the data signal and the clock signal is branched, and the switch is used as a switch control signal to perform switch on / off control. It was done.

An optical transmitter according to the present invention has a first delay unit that inserts an optical amplifier between an optical intensity modulator and an optical phase modulator to delay an optical phase modulator drive signal, and a switch control signal. The path length is controlled by the amount of delay of a second delay device that delays the path length.

In the optical transmitter according to the present invention, as a switch control signal, a part of the optical signal input to the optical phase modulator from the optical intensity modulator and split by the optical splitter is electrically converted by the optical receiver. The signal is converted into a signal, and the signal is used as a switch control signal to control the on / off of the switch.

An optical transmitter according to the present invention converts a clock signal extracted by a clock extraction circuit from an output signal of a photodetector obtained by converting a part of an optical signal demultiplexed by an optical demultiplexer into an electric signal, and converts the clock signal into an optical phase. This is input to a switch as a modulator drive signal.

In the optical transmitter according to the present invention, the optical phase modulator drive signal generation circuit converts a part of the optical signal input from the optical intensity modulator demultiplexed by the optical demultiplexer to the optical phase modulator. Based on an electric signal converted by a light receiver, an optical phase modulator drive signal for driving the optical phase modulator is generated only when an optical signal is input to the optical phase modulator. It is.

In the optical transmitter according to the present invention, the discriminator is provided to the optical phase modulator based on the optical intensity modulator drive signal generated from the data signal and the clock signal by the optical phase modulator drive signal generation circuit. Only when an optical signal is input, an optical phase modulator drive signal for driving the optical phase modulator is generated.

The optical transmitter according to the present invention is output from the optical phase modulator driving signal inverting means by the optical phase modulator driving signal inverting means controlled by the switching control signal synchronized with the optical intensity modulator driving signal. The polarity of the optical phase modulator drive signal is inverted according to whether or not an optical signal is input to the optical phase modulator, and is input to the optical phase modulator.

In the optical transmitter according to the present invention, the optical phase modulator drive signal inverting means includes a branch circuit for branching the optical phase modulator drive signal into two, and an inverter circuit for inverting one polarity of the output signal of the branch circuit. And a switch for selecting one of the output signal of the inverter circuit and the other of the output signal of the branch circuit in accordance with the switching control signal branched from the light intensity modulator drive signal generated by the discriminator. It is.

In the optical transmitter according to the present invention, the optical phase modulator drive signal inverting means includes a branch circuit that branches the optical phase modulator drive signal into two, and an inverter circuit that inverts one polarity of the output signal of the branch circuit. And the other of the output signals of the branch circuit according to a switching control signal obtained by converting an optical signal transmitted from the optical intensity modulator demultiplexed by the optical demultiplexer to the optical phase modulator into an electric signal by the photodetector. And a switch for selecting one of the output signals of the inverter circuit.

An optical transmitter according to the present invention comprises an optical phase modulator by adding an optical intensity modulator drive signal generated by a discriminator based on a data signal and a clock signal to the clock signal by an adder. An optical phase modulator drive signal for driving the optical phase modulator is generated only when an optical signal is input to the optical phase modulator.

The optical transmitter according to the present invention further includes a return-to-zero conversion circuit, and converts the optical intensity modulator drive signal generated by the discriminator from the non-return-to-zero code to the return-to-zero code. -It is converted into a zero code and sent to a light intensity modulator.

The optical transmitter according to the present invention converts the non-return-to-zero code light intensity modulator drive signal generated by the discriminator into a return-to-zero code by a return-to-zero conversion circuit. The signal is converted and sent to the optical intensity modulator, and a signal obtained by multiplying the frequency of the clock signal by the frequency multiplier is used as an optical phase modulator drive signal for driving the optical phase modulator.

In the optical transmitter according to the present invention, the return-to-zero conversion circuit converts the optical intensity modulator drive signal generated by the discriminator into a return-to-zero code and sends the signal to the optical intensity modulator. The optical intensity modulator drive signal converted into the return-to-zero code and the clock signal are added by an adder to generate an optical phase modulator drive signal.

An optical transmitter according to the present invention is a frequency multiplier, which multiplies the frequency of a clock signal input from a clock signal input terminal and converts the frequency to a return-to-zero code and an optical intensity modulator drive signal. , And the frequency-multiplied clock signal are added by an adder to generate an optical phase modulator drive signal.

An optical transmitter according to the present invention uses a polarization scrambler as an optical phase modulator.

The optical transmitter according to the present invention comprises: a λ / 4 plate for rotating the polarization state of an optical signal by 90 degrees; and a Faraday rotator for rotating the polarization state of an input optical signal according to a drive signal. It is connected to the output side of the modulator.

The optical transmitter according to the present invention has two or more optical phase modulators.

In the optical transmitter according to the present invention, a polarization scrambler is used for at least one of two or more optical phase modulators prepared.

[0039]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below. Embodiment 1 FIG. FIG. 1 is a block diagram showing an optical transmitter according to Embodiment 1 of the present invention. In the figure, reference numeral 1 denotes a light source for generating an optical signal, and reference numeral 2 denotes an optical intensity modulator for optically modulating an optical signal output from the optical source 1 in accordance with a data signal. 4 is a data signal input terminal to which a data signal is input, and 5 is a clock signal input terminal to which a clock signal is input. 6 is a data signal input terminal 4
And a clock signal input from the clock signal input terminal 5 to generate an optical intensity modulator drive signal. 7 is this discriminator 6
This is a light intensity modulator drive circuit that amplifies the light intensity modulator drive signal output from and applies it to the light intensity modulator 2 to drive it. An optical transmitter output terminal 10 outputs an optical signal modulated by the optical transmitter to an optical fiber transmission line. These are the parts corresponding to those of the related art shown with the same reference numerals in FIG.

Reference numeral 8a branches the clock signal input from the clock signal input terminal 5 as an optical phase modulator drive signal for driving an optical phase modulator, which will be described later, and delays the signal to the optical phase modulator drive signal. Is a first delay device that provides the following. Reference numeral 8b denotes a second delay unit that branches a light intensity modulator drive signal output from the discriminator 6 as a switch control signal for controlling a switch described later, and delays the switch control signal. Reference numeral 11 denotes a light which is cascade-connected to the optical intensity modulator 2, performs optical phase modulation on the optical signal modulated by the optical intensity modulator 2, and outputs the obtained optical signal to the optical transmitter output terminal 10. It is a phase modulator.

Reference numeral 12 denotes an optical phase modulator driving circuit that amplifies the optical phase modulator driving signal branched from the clock signal and delayed by the first delay unit 8a and drives the optical phase modulator 11. . The switch 13 is turned on and off by the switch control signal delayed by the second delay unit 8b, and is delayed by the first delay unit 8a only when an optical signal is input to the optical phase modulator 11. A switch for inputting a clock signal as an optical phase modulator drive signal to the optical phase modulator drive circuit 12.

Here, a semiconductor laser diode can be used as the light source 1, and a lithium niobate Mach-Zehnder modulator, a semiconductor optical modulator, or the like can be used as the light intensity modulator 2. Needless to say, a modulator integrated semiconductor laser diode in which the light source 1 and the light intensity modulator 2 are integrated can be used. Further, the optical phase modulator 11
Generally, a lithium niobate modulator is used.
As the first delay unit 8a and the second delay unit 8b, a microwave band phase modulator, a phase shifter using a varactor diode, a delay unit having a variable path length, or the like can be used. As the light intensity modulator drive circuit 7 and the optical phase modulator drive circuit 12, high output amplifiers can be procured from the market.

Next, the operation will be described. The data signal input from the data signal input terminal 4 and the clock signal input from the clock signal input terminal 5 are input to the discriminator 6, and the discriminator 6 generates a light intensity modulator drive signal based on them. . This light intensity modulator drive signal is amplified by the light intensity modulator drive circuit 7 and
Is input to On the other hand, the clock signal input from the clock signal input terminal 5 is branched and input to the first delay unit 8a as an optical phase modulator drive signal. The optical phase modulator drive signal delayed by the first delay unit 8a is input to the switch 13. The optical phase modulator drive signal output from the switch 13 is sent to the optical phase modulator drive circuit 12, and is amplified by the optical phase modulator drive circuit 12 to drive the optical phase modulator 11.

The light intensity modulator drive signal generated by the discriminator 6 is also branched and input to the second delay 8b as a switch control signal. The on / off of the switch 13 is controlled by a switch control signal input through the second delay unit 8b. The amount of delay of the second delay unit 8b is determined by turning on the switch 13 only when an optical signal is input to the optical phase modulator 11 from the optical intensity modulator 2.
The optical phase modulator drive signal is set to be input to the optical phase modulator drive circuit 12.

The light signal output from the light source 1 is subjected to light intensity modulation according to the data signal by the light intensity modulator drive signal amplified by the light intensity modulator drive circuit 7. Generally, a digital signal is expressed as “1” when an optical signal exists within a prescribed time and “0” when it does not exist within a prescribed time. Here, FIG. 2 shows the signal waveform of each part of the optical transmitter configured as described above. FIG. 2A shows an example of a data signal sequence input from the data signal input terminal 4, FIG. 2B shows an example of a waveform of an optical signal modulated by the optical intensity modulator 2, and FIG. 2) shows a waveform example of the optical phase modulator drive signal input to the optical phase modulator 11 when the switch 13 is always turned on, and FIG. 2D shows the switch 13 turned on by the output of the second delay unit 8b. .Optical phase modulator 1 when turned off
FIG. 2 shows an example of the waveform of the optical phase modulator drive signal input to FIG.
2E shows an example of a waveform of the optical signal modulated by the optical phase modulator drive signal shown in FIG. 2C after transmission through the optical fiber, and FIG. 2F shows the optical phase shown in FIG. 2D. Each waveform example after the optical fiber transmission of the optical signal optically modulated by the modulator drive signal is shown.

When the data signal sequence shown in FIG. 2A is input from the data signal input terminal 4, the light intensity modulator 2
The waveform of the output signal shown in FIG. Since the extinction ratio of the light intensity modulator 2 is finite, a small amount of light remains even when the data signal is “0”. Here, when the switch 13 is always on, that is, when there is no switch 13, the optical phase modulator driving signal shown in FIG. 2C is input to the optical phase modulator 11. At this time, the phase of the phase modulation can be adjusted by the delay amount of the first delay unit 8a.

Here, the phase modulation and the frequency modulation are essentially the same. The fact that the propagation time of a light wave propagating in an optical fiber varies depending on the frequency (wavelength) is generally called dispersion, and the waveform of a frequency-modulated optical signal changes due to this dispersion. Accordingly, the waveform of the optical signal that has been optically phase-modulated by the optical phase modulator 11 to which the optical phase modulator drive signal shown in FIG. 2C has been input becomes the waveform shown in FIG. Change. As shown in the figure, even when the data signal is "0", a slight optical signal remains, and the residual optical signal may be pulsed through the dispersion of the optical fiber due to optical phase modulation. obtain. When such a phenomenon occurs, the receiving side may erroneously determine “0” of the data signal as “1”.

Therefore, when the data signal is "0", the switch 13 is turned off by the switch control signal branched from the light intensity modulator drive signal generated by the discriminator 6. Thus, the waveform of the optical phase modulator drive signal input to the optical phase modulator 11 is as shown in FIG. The optical signal modulated by the optical phase modulator 11 to which the optical phase modulator driving signal as shown in FIG. 2D is input has the waveform shown in FIG. The aforementioned problems are avoided. The timing for driving the switch 13 can be controlled by the amount of delay of the second delay unit 8b as needed.

Although the transmission waveform is disrupted by the dispersion of the optical fiber, it is also possible to compensate for the waveform collapse due to the dispersion of the optical fiber to some extent by applying optical phase modulation. To control the waveform at the receiving end, the depth and phase of the optical phase modulation may be controlled. As shown in FIG. 2 (f), when the optical phase modulation is applied so that the pulse returns to zero at the receiving end, there is an advantage that the aperture ratio of the receiving eye increases and the receiving sensitivity increases. is there.

The optical phase modulator drive circuit 12 uses a high-output amplifier as described above. Depending on the configuration of this amplifier, it is possible to reduce the power consumption when the input is off. For example, there are methods such as DC coupling input configuration, class B operation, and class C operation.

In the above description, the optical intensity modulator 2 and the optical phase modulator 11 are directly connected. However, the optical intensity modulator 2 and the optical phase modulator 11 may be connected between the optical intensity modulator 2 and the optical phase modulator 11 if necessary. Alternatively, an optical amplifier may be provided. FIG. 3 is a block diagram showing such an optical transmitter according to the first embodiment of the present invention, in which an optical amplifier 14 is disposed between the optical intensity modulator 2 and the optical phase modulator 11. It is. The basic operation is the same as that of the optical transmitter shown in FIG. 1, but this optical amplifier 14 makes it possible to set a high light intensity level and suppress the deterioration of the optical S / N ratio. Become. Here, when the optical amplifier 14 is provided, it is desirable to control the path length by using the first and second delay units 8a and 8b.

As described above, according to the first embodiment, the optical signal modulated by the optical intensity modulator is input to the optical phase modulator, and the optical phase modulator drives the optical phase modulator. Since the drive signal is applied to the optical phase modulator using a switch whose on / off is controlled by the optical intensity modulator drive signal synchronized with the optical phase modulator drive signal, the optical signal is applied to the optical phase modulator. When no signal is input, no phase modulation is applied, and when the extinction ratio of the light intensity modulation is finite, the optical signal remaining when the data signal is "0" is pulsed after fiber transmission, resulting in a code error. Can be prevented, and since the optical phase modulator is operated only when an optical signal is input to the optical phase modulator, consumption of the optical phase modulator driving circuit is reduced. Power can be saved Effect of Runado can be obtained.

Further, since the optical phase modulator drive signal is delayed using the first delay device and the light intensity modulator drive signal is delayed using the second delay device, the output from the phase modulator is delayed. The phase modulation phase of the optical signal and the on / off timing of the switch can be easily adjusted by the delay amounts of the first and second delay units. The optical intensity level can be set high by inserting an optical amplifier between the devices and controlling the path length by the delay amount of the first delay device and the second delay device. An effect is obtained such that the deterioration of the ratio can be suppressed.

Embodiment 2 FIG. 4 is a block diagram showing an optical transmitter according to Embodiment 2 of the present invention. In the figure, 1 is a light source, 2 is a light intensity modulator, 4 is a data signal input terminal, 5 is a clock signal input terminal, 6 is a discriminator, 7 is a light intensity modulator driving circuit, 8a is a first delay unit, 8b is the second
, An optical transmitter output terminal, 11 an optical phase modulator, 12 an optical phase modulator drive circuit, and 13 a switch, which are denoted by the same reference numerals in FIG. 1, the detailed description is omitted. Reference numeral 15 denotes an optical splitter that splits a part of the optical signal input from the optical intensity modulator 2 to the optical phase modulator 11. Reference numeral 16 denotes a photodiode serving as a light receiver for converting the optical signal split by the optical splitter 15 into a switch control signal based on an electric signal. Reference numeral 17 denotes an amplifier for amplifying the switch control signal photoelectrically converted by the photodiode 16 and inputting the amplified signal to the second delay unit 8b.

As described above, the optical transmitter according to the second embodiment does not branch the switch control signal for turning on / off the switch 13 from the optical intensity modulator drive signal generated by the discriminator 6. Embodiment 1 in that a part of an optical signal input to the optical phase modulator 11 is obtained by demultiplexing by an optical demultiplexer 15 and converting the demultiplexed signal into an electric signal by a photodiode 16. Is different from that of

Next, the operation will be described. The data signal input from the data signal input terminal 4 and the clock signal input from the clock signal input terminal 5 are input to the discriminator 6 to generate a light intensity modulator drive signal. This light intensity modulator drive signal is amplified by the light intensity modulator drive circuit 7 and input to the light intensity modulator 2. Further, the clock signal input from the clock signal input terminal 5 is branched as an optical phase modulator drive signal and input to the first delay unit 8a. The optical phase modulator drive signal delayed by the first delay unit 8a is input to the optical phase modulator drive circuit 12 via the switch 13, where it is amplified and drives the optical phase modulator 11. This operation is the same as in the first embodiment.

On the other hand, from the optical intensity modulator 2 to the optical phase modulator 1
A part of the optical signal sent to 1 is split by the optical splitter 15 and photoelectrically converted by the photodiode 16 to become a switch control signal based on an electrical signal. The switch control signal output from the photodiode 16 is amplified by the amplifier 17. The switch control signal amplified by the amplifier 17 is proportional to the intensity of the optical phase modulator drive signal input to the optical phase modulator 2. The switch 13 is turned on / off by a switch control signal output from the amplifier 17. That is, the optical phase modulator 1
The switch 13 is turned on only when an optical signal is input to the switch 1. As shown, by providing the second delay unit 8b between the amplifier 17 and the switch 13, control accuracy can be improved.

Further, by replacing the photodiode 16 with an avalanche photodiode, it may be possible to reduce the gain of the amplifier 17 or to omit the amplifier 17.

As described above, since the optical phase modulator drive signal is supplied to the optical phase modulator 11 only when an optical signal is input, the power consumption of the optical phase modulator drive circuit 12 is reduced. Can be expected. Further, when the data signal is "0", the slight residual optical signal is phase-modulated, so that the optical signal is pulsed through the dispersion of the optical fiber, thereby preventing a code error. .

In the optical transmitter having the configuration shown in FIG. 4, since the light intensity modulation component input to the optical phase modulator 11 is extracted from the optical signal input to the optical phase modulator 11, the light intensity modulation is performed. Even if the length of the optical fiber connecting the modulator 2 and the optical phase modulator 11 fluctuates, stable operation can be achieved.

The optical intensity modulator 2 and the optical phase modulator 1 are provided for the purpose of increasing the S / N ratio of the output light of the optical transmitter.
It is assumed that an optical amplifier is inserted between the two. The length of the optical fiber constituting the optical amplifier is considered to be several tens of meters or more, and the fiber length may fluctuate due to a change in environmental temperature. It is known that the propagation delay time of the optical amplifier becomes a maximum of several picoseconds due to a temperature change of 1 ° C. In such a case, the third embodiment described later which is not affected by the delay time fluctuation due to the temperature fluctuation. This is particularly effective for the optical transmitter.

Embodiment 3 FIG. 5 is a block diagram showing an optical transmitter according to Embodiment 3 of the present invention. The corresponding parts are denoted by the same reference numerals as in FIG. 4, and description thereof will be omitted. In the figure, a clock extraction circuit 18 extracts a clock signal from an output signal of a photodiode 16 amplified by an amplifier 17 and inputs the extracted clock signal as an optical phase modulator drive signal to a switch 13 via a first delay unit 8a. It is. Reference numeral 19 denotes a branch circuit that branches the output signal of the amplifier 16 into two, and inputs one to the clock extraction circuit 18 and the other to the second delay unit 8b as a switch control signal.

As described above, the difference between the third embodiment and the second embodiment is that an optical phase modulator drive signal is branched by the branch circuit 19 instead of the clock signal input from the clock signal input terminal 5. The point is that the clock signal extracted by the clock extraction circuit 18 from the output signal of the photodiode 16 is used.

Next, the operation will be described. An optical signal input from the optical intensity modulator 2 to the optical phase modulator 11 is split by the optical splitter 15, photoelectrically converted by the photodiode 16, and amplified by the amplifier 17. This amplifier 17
Is sent to the branch circuit 19 and branched, one of which is supplied to the second delay unit 8b and the other is supplied to the clock extraction circuit 18
Respectively. The signal input to the second delay unit 8b is delayed as a switch control signal to control the on / off of the switch 13. Further, a clock signal is extracted from the signal input to the clock extraction circuit 18. This clock signal is input to the first delay unit 8a as an optical phase modulator drive signal, and is delayed by an appropriate value and sent to the switch 13. As the clock extracting circuit 18, a non-linear clock extracting circuit using a frequency multiplier is generally used.

As described above, also in the third embodiment, the optical phase modulator drive signal is supplied to the optical phase modulator 11 only when an optical signal is input. Can be expected to have an effect of reducing power consumption. Further, when the data signal is "0", it is possible to prevent a slight residual optical signal from being pulse-modulated through dispersion of the optical fiber due to phase modulation, thereby causing a code error, Further, the optical phase modulator 1
Since the light intensity modulation component input to the optical phase modulator 11 is extracted from the optical signal input to 1, the light intensity modulator 2
Even if the length of the optical fiber connecting the optical fiber and the optical phase modulator 11 fluctuates, stable operation can be achieved.

As described above, according to the second and third embodiments, the switch for turning on / off the optical phase modulator drive signal is extracted from the optical signal input to the optical phase modulator. When the extinction ratio of the light intensity modulation is finite because the control is performed by the photoelectrically converted switch control signal, the optical signal remaining when the data signal is "0" is pulsed after fiber transmission and becomes a code error. Not only can reduce the power consumption of the optical phase modulator drive circuit, but also can be affected by variations in the propagation time due to the temperature of the optical fiber connecting the optical intensity modulator and the optical phase modulator. Effects such as disappearance can be obtained.

Embodiment 4 FIG. 6 is a block diagram showing an optical transmitter according to Embodiment 4 of the present invention. The corresponding parts are denoted by the same reference numerals as in FIG. 4 and description thereof is omitted. In the figure, reference numeral 20 denotes an optical phase modulator that removes high-frequency components from the output signal of the photodiode 16 amplified by the amplifier 17 and drives the optical phase modulator 2 only when an optical signal is input thereto. It is a low-pass filter as an optical phase modulator drive signal generation circuit that generates a drive signal. As described above, the fourth embodiment is different from the third embodiment in that the output signal of the amplifier 17 is directly connected to the optical phase modulator driving circuit 12 via the low-pass filter 20.

Next, the operation will be described. Here, FIG.
FIG. 4 is an operation explanatory diagram showing signal waveforms of various parts of the optical transmitter configured as described above. FIG. 7A shows an example of a data signal sequence input from the data signal input terminal 4, and FIG. 7B shows an example of a waveform of an optical signal modulated by the optical intensity modulator 2; () Shows a waveform example of the optical phase modulator drive signal output from the low-pass filter 20.

The signal photoelectrically converted by the photodiode 16 and output from the amplifier 17 is a light intensity modulator drive signal as shown in FIG. When this signal is input to the low-pass filter 20 and its high-frequency component is removed, FIG.
A signal as shown in (c) is obtained. The waveform of the output signal of the low-pass filter 20 is similar to the phase modulation waveform when the switch 13 shown in FIG. 2D is turned on and off. The signal from which the high-frequency component has been removed by the low-pass filter 20 is sent to the optical phase modulator drive circuit 12 as an optical phase modulator drive signal, where it is amplified and drives the optical phase modulator 11. This optical phase modulator driving circuit 1
2 is proportional to the intensity of the optical signal input from the optical intensity modulator 2 to the optical phase modulator 11.

Although the optical phase modulator drive signal shown in FIG. 7C has a waveform different from that shown in FIG. 2D when "1" is continuous, it is compared when it is "1". Therefore, an effect similar to that of the optical transmitter according to the third embodiment shown in FIG. 5 can be obtained.

If the output signal of the amplifier 17 is input to the low-pass filter 20 via the delay device, the phase of the optical phase modulator drive signal can be set more accurately.

As described above, according to the fourth embodiment, the output signal of the amplifier is directly input to the optical phase modulator driving circuit via the low-pass filter. Uses a band-limited optical intensity modulator drive signal, and the optical phase modulator operates only when an optical signal is input to the optical phase modulator, thus reducing the power consumption of the optical phase modulator drive circuit. When the extinction ratio of the light intensity modulation is finite, it is possible not only to prevent a phenomenon that an optical signal remaining at the time of data "0" is pulsed after fiber transmission and a code error occurs. As compared with the optical transmitter according to the third embodiment, it is possible to obtain an effect that a predetermined function can be realized with fewer components.

Embodiment 5 FIG. 8 is a block diagram showing an optical transmitter according to a fifth embodiment of the present invention. The corresponding parts are denoted by the same reference numerals as in FIG. 6, and description thereof will be omitted. In the figure, reference numeral 8c gives a delay to a signal branched from the optical intensity modulator drive signal generated based on the data signal and the clock signal, which is input from the data signal input terminal 4 and the clock signal input terminal 5 by the discriminator 6. It is a delay unit. The fifth embodiment is different from the fourth embodiment in that the output signal of the delay unit 8c instead of the output signal of the amplifier 17 is input to the low-pass filter 20.

Next, the operation will be described. The discriminator 6 generates a light intensity modulator drive signal for driving the light intensity modulator 2 based on the data signal input from the data signal input terminal 4 and the clock signal input from the clock signal input terminal 5. I do. This light intensity modulator drive signal is amplified by the light intensity modulator drive circuit 7 and input to the light intensity modulator 2. The light intensity modulator drive signal output from the discriminator 6 is branched and input to the delay unit 8c. The delay unit 8 c delays the optical intensity modulator drive signal and inputs the signal to the low-pass filter 20. The low-pass filter 20 includes the delay unit 8
An optical phase modulator drive signal is generated by removing the high frequency component of the output signal of c, and is sent to the optical phase modulator drive circuit 12. The optical phase modulator drive circuit 12 amplifies the optical phase modulator drive signal and drives the optical phase modulator 11. The signal output from the optical phase modulator driving circuit 12 is proportional to the intensity of the optical signal input from the optical intensity modulator 2 to the optical phase modulator 11.

Also in the fifth embodiment, although the waveform of the data signal when "1" is continuous is different from that shown in FIG. 2D, the intersymbol interference is relatively small when "1" is continuous.
An effect similar to that of the optical transmitter according to the first embodiment shown in FIG. 1 can be obtained.

As described above, according to the fifth embodiment, the output signal of the discriminator delayed by the delay unit is directly input to the optical phase modulator driving circuit via the low-pass filter. A band-limited optical intensity modulator drive signal is used as the optical phase modulator drive signal, and the optical phase modulator operates only when an optical signal is input to the optical phase modulator. The power consumption of the optical drive circuit can be saved, and when the extinction ratio of the light intensity modulation is finite, the phenomenon that an optical signal remaining at the time of data "0" is pulsed after fiber transmission and causes a code error does not occur. Not only is it possible to achieve, but also an effect is obtained such that a predetermined function can be realized with fewer parts as compared with the optical transmitter of the first embodiment.

Embodiment 6 FIG. FIG. 9 is a block diagram showing an optical transmitter according to Embodiment 6 of the present invention. The corresponding parts are denoted by the same reference numerals as in FIG. 1 and description thereof is omitted. In the figure, reference numeral 21 denotes a branch circuit which branches the clock signal input to the clock signal input terminal 5 and branches the optical phase modulator drive signal delayed by the first delay unit 8a into two, and 22 denotes a branch circuit. This is an inverter circuit that inverts one polarity of the signal branched by the branch circuit 21. 2
Reference numeral 3 designates a signal branched from the optical intensity modulator drive signal output from the discriminator 6 and a signal delayed by the second delay unit 8b as a switching control signal, which is controlled by the signal and has two lights having different polarities. A two-input one-output switch for selecting one of the phase modulator drive signals and inputting the selected signal to the optical phase modulator drive circuit 12. Reference numeral 24 denotes an optical phase modulator drive signal inverting means formed by the branch circuit 21, the inverter circuit 22, and the changeover switch 23 and inverting the polarity of the optical phase modulator drive signal.

Next, the operation will be described. The discriminator 6 generates a light intensity modulator drive signal for driving the light intensity modulator 2 based on the data signal input from the data signal input terminal 4 and the clock signal input from the clock signal input terminal 5. I do. This light intensity modulator drive signal is amplified by the light intensity modulator drive circuit 7 and input to the light intensity modulator 2. The light intensity modulator drive signal output from the discriminator 6 is branched and input to the second delay unit 8b to be delayed, and becomes a switching control signal for controlling the switch 23.

On the other hand, the optical phase modulator drive signal branched from the clock signal input from the clock signal input terminal 5 is input to the first delay unit 8a to be delayed.
The optical phase modulator drive signal delayed by the first delay unit 8a is input to the branch circuit 21 and is branched into two. One of the output signals of the branch circuit 21 is input to one input terminal of the changeover switch 23, and the other is input to the inverter circuit 22. The inverter circuit 22 inverts the polarity of the optical phase modulator drive signal from the branch circuit 21 and inputs the inverted signal to the other input terminal of the switch 23. The changeover switch 23 is switched by a changeover control signal from the second delay unit 8b, selects an optical phase modulator drive signal inputted from one of the input terminals, and outputs it from an output terminal. The optical phase modulator drive signal output from the changeover switch 23 is sent to the optical phase modulator drive circuit 12 where it is amplified and drives the optical phase modulator 11.

Here, the optical signal output from the light source 1 is subjected to light intensity modulation corresponding to the data signal in the light intensity modulator 2 by the light intensity modulator drive signal amplified by the light intensity modulator drive circuit 7. Done. The optical signal modulated by the light intensity modulator 2 is input to the optical phase modulator 11.
The optical phase modulator 11 outputs the polarity of the optical phase modulator drive signal output from the changeover switch 23 when light is present (when “1”) and when light is not present (when “0”). Is controlled to be inverted.

The branch circuit 21 and the inverter circuit 2
2, and the optical phase modulator driving signal inverting means 24 formed by the changeover switch 23 inverts the polarity of the output optical phase modulator driving signal by the switching control signal synchronized with the light intensity modulator driving signal as described above. Perform the operation of As a device that realizes the same function as the optical phase modulator drive signal inverting means 24 by itself, there is a microwave phase modulator or the like. Further, the inverter circuit 22 can be replaced with a suitable delay device.

Here, FIG. 10 is an operation explanatory diagram showing the signal waveform of each part of the optical transmitter configured as described above. FIG.
0 (a) shows an example of a data signal sequence input from the data signal input terminal 4, FIG. 10 (b) shows an example of a waveform of an optical signal modulated by the optical intensity modulator 2 and FIG. 10 (c). 10D is a waveform example of the optical phase modulator drive signal input to the optical phase modulator 11, and FIG. 10D is an optical signal of the optical signal modulated by the optical phase modulator drive signal shown in FIG. 7 shows a waveform example after fiber transmission.

From the data signal input terminal 4, FIG.
When the data signal sequence shown in FIG. 2 is input, the waveform of the optical signal output from the optical intensity modulator 2 is as shown in FIG. Since the extinction ratio of the light intensity modulator 2 is finite, a small amount of light remains even when the data signal is “0”. When an optical signal having the waveform shown in FIG. 10B is output from the optical intensity modulator 2, the optical phase modulator 11 outputs the signal shown in FIG.
An optical phase modulator drive signal having the waveform shown in FIG. The optical signal that has been optically phase-modulated by the optical phase modulator driving signal having the waveform shown in FIG. 10C by the optical phase modulator 11 has the waveform shown in FIG. It changes as shown in d).

Here, the optical signal slightly remaining when the data signal is “0” is optically phase-modulated in a direction opposite to that when the data signal is “1”, so that the dispersion of the optical fiber is reduced. It is not pulsed through. Therefore, the possibility that the receiving side erroneously determines “0” of the data signal as “1” is small.

As described above, according to the sixth embodiment, the case where an optical signal is input to an optical phase modulator using two types of optical phase modulator drive signals of a positive phase and a negative phase. Since the optical phase modulator is driven by the optical phase modulator drive signals having different polarities when the signal is not input, the data signal is “0” when the extinction ratio of the light intensity modulation is finite. Even if there is, it is possible to prevent a phenomenon that a residual optical signal is pulsed after transmission through a fiber and a code error occurs, and an effect of increasing an eye opening ratio can be obtained.

Although the above description has been given of the case where the present invention is applied to the optical transmitter of the first embodiment shown in FIG. 1, it is needless to say that the present invention is also applicable to the optical transmitter of the second embodiment shown in FIG. Nor. FIG. 11 is a block diagram showing such an optical transmitter. As a switching control signal for controlling the changeover switch 23 of the optical phase modulator drive signal inverting means 24, the light intensity modulator drive output from the discriminator 6 is provided. The optical signal transmitted from the optical intensity modulator 2 to the optical phase modulator 11 is split by the optical splitter 15, and is obtained by photoelectric conversion by the photodiode 16, not by splitting the signal. Using electrical signals. Therefore, the same effects as those of the first embodiment can be obtained, and a stable operation can be performed without being affected by the fluctuation of the propagation time due to the temperature of the optical fiber.

Embodiment 7 FIG. 12 is a block diagram showing an optical transmitter according to Embodiment 7 of the present invention. The corresponding parts are denoted by the same reference numerals as in FIG. 1 and description thereof is omitted. In the figure, reference numeral 25 denotes a low-pass filter that removes high-frequency components of a signal branched from the light intensity modulator drive signal output from the discriminator 6 and inputs the signal to the second delay unit 8b. Reference numeral 26 denotes a signal which is branched from the output signal of the low-pass filter 25 and the clock signal input to the clock signal input terminal 5, and
This is an adder that adds the signal delayed by the delay unit 8a to generate an optical phase modulator drive signal. As the adder 26, various analog adders and multiplexers can be used.

Next, the operation will be described. The data signal and the clock signal input from the data signal input terminal 4 and the clock signal input terminal 5 are input to the discriminator 6 to generate a light intensity modulator drive signal. This light intensity modulator drive signal is amplified by the light intensity modulator drive circuit 7 and input to the light intensity modulator 2. The light intensity modulator drive signal output from the discriminator 6 is branched and input to the low-pass filter 25. The signal from which the high-frequency component has been removed by the low-pass filter 25 is input to the second delay unit 8b to be delayed.

On the other hand, the signal branched from the clock signal input from the clock signal input terminal 5 is input to the first delay unit 8a to be delayed. First delay unit 8a
The output signal is supplied to one input terminal of the adder 26,
The signal output from the second delay unit 8b is input to the other input terminal of the adder 26. The level of the signal input to the two input terminals of the adder 26 is adjusted as needed. To adjust the signal level, an attenuator or an amplifier may be used.

FIG. 13 is an operation explanatory diagram showing signal waveforms at various parts of the optical transmitter configured as described above. FIG.
3 (a) shows an example of a data signal sequence input from the data signal input terminal 4, and FIG. 13 (b) shows an example of the waveform of an optical signal modulated by the optical intensity modulator 2 and FIG. 13 (c). 13D is a waveform example of a signal input from the second delay unit 8b to the adder 26, FIG. 13D is a waveform example of a signal input to the adder 26 from the second delay unit 8b, and FIG. 13 is a waveform example of the optical phase modulator drive signal output from the adder 26, FIG.
(F) shows an example of the waveform of the optical signal phase-modulated by the optical phase modulator drive signal shown in FIG.

From the data signal input terminal 4, FIG.
When the data signal series shown in FIG. 13 is input, the waveform of the optical signal output from the optical intensity modulator 2 is as shown in FIG. The signal branched from the light intensity modulator drive signal output from the discriminator 6 at that time is supplied to the low-pass filter 25.
13 is delayed by the second delay unit 8b.
A signal having the waveform shown in (c) is input to one of the input terminals of the adder 26. The signal branched from the clock signal becomes a signal having the waveform shown in FIG. 13D and is input to the other input terminal of the adder 26. Here,
It is assumed that the amplitude of the signal input from the second delay unit 8b is larger than the amplitude of the signal input from the first delay unit 8a.

The signal input to the adder 26 from the second delay unit 8b is upwardly convex when the data signal is "1" as shown in FIG. The signal input to the adder 26 from 8a is convex downward when the data signal is "1" as shown in FIG.
Accordingly, the waveform of the signal output from the adder 26 after adding them is as shown in FIG. This figure 1
The waveform shown in FIG. 3E is similar to the waveform shown in FIG. 10C, and the optical signal modulated by the optical phase modulator drive signal having the waveform shown in FIG. As shown in FIG. 13 (f), the waveform after transmission through the optical fiber has a small possibility that the receiving side erroneously determines “0” of the data signal as “1”.

Here, generally, it is easy to obtain the adder 26 having a wider operation band than the changeover switch 23 in the sixth embodiment shown in FIG. Therefore, the optical transmitter according to the seventh embodiment has an advantage that the realization is easier.

As described above, according to the seventh embodiment, as the optical phase modulator drive signal, the optical intensity modulator drive signal and the clock signal are added, and the optical signal is input to the optical phase modulator. A signal whose polarity is inverted between when the data signal is not input and when it is not input is used. Therefore, when the extinction ratio of the light intensity modulation is finite, even if the data signal is “0”, the signal remains. It is possible to prevent a phenomenon in which an optical signal is pulsed after being transmitted through a fiber and generate a code error, and it is possible to more easily realize an optical transmitter capable of increasing an eye opening ratio.

Embodiment 8 FIG. FIG. 14 is a block diagram showing an optical transmitter according to Embodiment 8 of the present invention. The corresponding parts are denoted by the same reference numerals as in FIG. 1 and description thereof is omitted. In the figure, reference numeral 27 denotes a return signal for converting the optical intensity modulator drive signal output from the discriminator 6 into a return-to-zero code.
Reference numeral 28 denotes an AND gate which constitutes the return-to-zero conversion circuit 27. As described above, in the eighth embodiment, the light intensity modulation drive signal converted from the non-return-to-zero code to the return-to-zero code by the return-to-zero conversion circuit 27 is used to perform the light intensity modulation. Embodiment 2 is different from Embodiment 1 in that the drive of the container 2 and the control of the switch 13 are performed.

Next, the operation will be described. The data signal and the clock signal input from the data signal input terminal 4 and the clock signal input terminal 5 are input to the discriminator 6 to generate a light intensity modulator drive signal. Here, the light intensity modulator drive signal output from the discriminator 6 is a non-return-to-zero code. The light intensity modulator drive signal based on the non-return-to-zero code is input to the return-to-zero conversion circuit 27. The AND gate 28 constituting the drive signal drives the clock signal input from the clock signal input terminal 5. And is converted to a return-to-zero code. The light intensity modulator drive signal based on the return-to-zero code is amplified by the light intensity modulator drive circuit 7 and input to the light intensity modulator 2.
Therefore, the optical intensity modulator 2 outputs an optical signal whose optical intensity has been modulated in a return-to-zero format to the optical phase modulator 11.

The optical intensity modulator drive signal converted into the return-to-zero code by the return-to-zero conversion circuit 27 is branched and input to the second delay unit 8b. The delay is given by the second delay unit 8b and sent to the switch 13 as a switch control signal. on the other hand,
The clock signal input from the clock signal input terminal 5 is input to the switch 13 as an optical phase modulator drive signal via the first delay unit 8a. The switch 13 is turned on / off by a switch control signal output from the second delay unit 8b.
The optical phase modulator driving signal output from the first delay unit 8a is input to the optical phase modulator driving circuit 12 only when the OFF is controlled and the optical signal is input to the optical phase modulator 11.

In the above description, an AND gate 28 is used as the return-to-zero conversion circuit 27. However, the return-to-zero conversion circuit 27 is not limited to this. , For example, using a mixer or the like. Also, return
As a method of obtaining a two-zero modulated optical signal, a second optical intensity modulator driven by a clock signal can be used.

As described above, according to the eighth embodiment, the output signal of the discriminator using the non-return-to-zero code is used as the light intensity modulator drive signal.
Since the signal converted to the zero code is used, the optical signal modulated by the optical intensity modulator in the return-to-zero format is input to the optical phase modulator, and the optical signal is input to the optical phase modulator. Since the optical phase modulator operates only when the data is "0", the power consumption of the optical phase modulator driving circuit can be saved, and when the extinction ratio of the light intensity modulation is finite, the light remaining when the data is "0". Not only is it possible to prevent the signal from being pulsed after a fiber transmission and causing a phenomenon in which a code error occurs, but the use of a return-to-zero modulated optical signal generally reduces intersymbol interference with adjacent pulses. There is an advantage that the phase modulation is used together with the return-to-zero modulated optical signal, and an effect such as an improvement in the eye opening ratio at the receiving end can be obtained.

In the above description, in the optical transmitter shown in the first embodiment, discriminator 6 has a non-return-to-
Although the case where the optical intensity modulator drive signal output with the zero code is converted to the return-to-zero code has been described, the present invention can be applied to the optical transmitters described in the second to seventh embodiments. There is an effect similar to that of the description applied to the optical transmitter according to the first embodiment.

Embodiment 9 FIG. FIG. 15 is a block diagram showing an optical transmitter according to a ninth embodiment of the present invention. The corresponding parts are denoted by the same reference numerals as in FIG.
In the drawing, reference numeral 8d denotes a delay unit for delaying a signal branched from the clock signal input from the clock signal input terminal 5, and 29 denotes a frequency multiplier for multiplying the frequency of the clock signal delayed by the delay unit 8d. Is input to the optical phase modulator drive circuit 12. As the frequency multiplier 29, for example, a doubler using a diode can be used. As described above, the ninth embodiment differs from the eighth embodiment in that a clock signal whose frequency is multiplied is used as an optical phase modulator drive signal.

Next, the operation will be described. The data signal and the clock signal input from the data signal input terminal 4 and the clock signal input terminal 5 are input to the discriminator 6 to generate a light intensity modulator drive signal. The light intensity modulator drive signal output from the discriminator 6 is a non-return-to-zero code. The light intensity modulator drive signal based on the non-return-to-zero code is input to the return-to-zero conversion circuit 27 and converted into a return-to-zero code. The light intensity modulator drive signal based on the return-to-zero code is amplified by the light intensity modulator drive circuit 7 and input to the light intensity modulator 2, from which the return-to-zero signal is output. An optical signal that has been optically modulated in the format is output to the optical phase modulator 11.

On the other hand, the clock signal input from the clock signal input terminal 5 is branched and input to the frequency multiplier 29 via the delay unit 8d. In the frequency multiplier 29, the frequency of the clock signal is multiplied and sent to the optical phase modulator drive circuit 12 as an optical phase modulator drive signal. The optical phase modulator drive circuit 12 amplifies the input optical phase modulator drive signal, sends it to the optical phase modulator 11, and drives it.

FIG. 16 is an operation explanatory diagram showing signal waveforms at various parts of the optical transmitter configured as described above. FIG.
6 (a) shows an example of a data signal sequence input from the data signal input terminal 4, and FIG. 16 (b) shows an example of a waveform of an optical signal modulated by the optical intensity modulator 2;
(C) shows the state of the optical phase modulator 1 from the optical phase modulator drive circuit 12.
16 shows an example of the waveform of the optical phase modulator drive signal output to 1 in FIG.
(D) shows an example of the waveform of the optical signal phase-modulated by the optical phase modulator drive signal shown in FIG.

From the data signal input terminal 4, FIG.
Is input, the waveform of the optical signal output from the optical intensity modulator 2 that performs optical intensity modulation in a return-to-zero format is as shown in FIG. On the other hand, the clock signal input from the clock signal input terminal 5 is sent to the frequency multiplier 29 via the delay unit 8d, and its frequency is multiplied. From this frequency multiplier 29, a sine wave having a cycle twice as long as the data sequence is input to the optical phase modulator drive circuit 12. The optical phase modulator drive circuit 12 amplifies the signal, sends it to the optical phase modulator 11 as an optical phase modulator drive signal shown in FIG. 16C, and drives it.

According to the principle described with reference to FIG. 26 for the conventional optical transmitter, the optical intensity modulator 2 shown in FIG.
16 is pulse-compressed, and after transmission through the optical fiber, as shown in FIG. 16 (d), has a waveform more advantageous for reception with an open eye. Here, the degree of the pulse compression is determined by the frequency shift amount of the frequency modulation caused by the optical phase modulation and the dispersion of the optical fiber. The frequency shift amount is proportional to the frequency of the optical phase modulation signal if the degree of phase modulation is the same. For this reason, if the frequency of the optical phase modulator drive signal for driving the optical phase modulator 11 is doubled, the amplitude of the optical phase modulator drive signal required to obtain the pulse compression effect can be halved, Power consumption can be reduced.

In order to make the frequency of the optical phase modulator drive signal twice the data modulation frequency, the duty of the light intensity-modulated pulse should be set to 1/2 or less. In order to make the frequency of the drive signal three times the data modulation frequency, it is desirable that the duty of the light intensity-modulated pulse be １ ／ or less.

Further, the light intensity modulator 2 is supplied to the optical phase modulator 11.
When the optical signal modulated by the optical intensity is not input,
Similarly to the eighth embodiment, the optical phase modulator driving circuit 12
Providing the switch 13 for turning off the signal input to the FF is more advantageous for improving transmission characteristics and reducing power consumption. In this case, as described in each of the above embodiments, the control of the switch 13 controls the output signal of the discriminator 6 or the optical signal input to the optical phase modulator 11 by the photodiode 1
6 and the like can be used.

As described above, according to the ninth embodiment, the optical signal modulated by the light intensity modulator with the return-to-zero code is input to the optical phase modulator, and the optical phase modulator As an optical phase modulator drive signal for driving
Since a signal obtained by multiplying the clock signal is used, it is possible to reduce the amplitude of the optical phase modulator driving signal for obtaining the required pulse compression effect, thereby saving power consumption of the optical phase modulator driving circuit. Is obtained.

Embodiment 10 FIG. FIG. 17 is a block diagram showing an optical transmitter according to the tenth embodiment of the present invention. The same reference numerals as in FIGS. 12 and 14 denote the same parts, and a description thereof will be omitted. Embodiment 10 and FIG.
The difference from the seventh embodiment shown in FIG.
5 is removed and the output signal of the discriminator 6 is converted into a return-to-zero code by the return-to-zero conversion circuit 27.

Next, the operation will be described. The light intensity modulator drive signal generated by the discriminator 6 is converted from a non-return-to-zero code to a return-to-zero code in a return-to-zero conversion circuit 27, and is converted to a light intensity modulator drive circuit 7. Sent. The light intensity modulator driving circuit 7 amplifies the signal and inputs the amplified signal to the light intensity modulator 2. Therefore, the optical signal sent from the optical intensity modulator 2 to the optical phase modulator 11 is optically intensity-modulated in a return-to-zero format.

On the other hand, the signal branched from the output signal of the return-to-zero conversion circuit 27 and the signal branched from the clock signal input to the clock signal input terminal 5 are supplied to the second delay unit 8b and the second delay unit 8b. The delay is given by one delay unit 8a, phase control is performed, and input to the adder 26. The signal level of the signal input to the adder 26 is optimized using an attenuator or an amplifier as needed. These two signals are added by the adder 26, and as a result, the optical phase modulator drive signal having the waveform shown in FIG. 13E is input from the adder 26 to the optical phase modulator drive circuit 12. Therefore, unlike the case of using a non-return-to-zero code, intersymbol interference does not occur when the data signal is continuously "1".

As described above, according to the tenth embodiment, the optical phase modulator is driven by the optical phase modulator drive signal obtained by adding the signal converted by return-to-zero conversion circuit 27 and the clock signal. Is non-return to two
As in the case where a zero code is used, intersymbol interference does not occur when "1" s of data signals are continuous, and an effect is obtained that transmission characteristics can be more effectively improved.

Embodiment 11 FIG. FIG. 18 is a block diagram showing an optical transmitter according to an eleventh embodiment of the present invention. The same reference numerals as in FIGS. 15 and 17 denote the same parts, and a description thereof will be omitted. The eleventh embodiment has the first
Instead of directly inputting the clock signal delayed by the delay unit 8a to the adder 26, the frequency signal is multiplied by the frequency multiplier 29 and then input to the adder 26.
This is different from the tenth embodiment shown in FIG.

Next, the operation will be described. The light intensity modulator drive circuit 7 amplifies the light intensity modulator drive signal converted by the return-to-zero conversion circuit 27 into a return-to-zero code and inputs the amplified signal to the light intensity modulator 2. Therefore, the optical signal sent from the optical intensity modulator 2 to the optical phase modulator 11 is optically intensity-modulated in a return-to-zero format. The output signal of the return-to-zero conversion circuit 27 is branched and sent to the second delay unit 8b. The output signal is delayed and the phase is adjusted, and then input to the adder 26. On the other hand, the clock signal input to the clock signal input terminal 5 is also branched, delayed by the first delay unit 8a, adjusted in phase, and then input to the frequency multiplier 29. After the frequency of the clock signal is multiplied by the frequency multiplier 29, the adder 2
6 is input. The signal level of each signal input to the adder 26 is optimized using an attenuator or an amplifier as needed. These two signals are added by the adder 26 and input to the optical phase modulator drive circuit 12 as an optical phase modulator drive signal, and are amplified by the optical phase modulator drive circuit 12 to drive the optical phase modulator 11.

As described above, according to the eleventh embodiment, when the frequency doubler is used as the frequency multiplier 29, for the same reason as in the ninth embodiment shown in FIG.
Even if the output amplitude of the optical phase modulator driving circuit 12 is halved,
Since the same frequency shift amount as in the tenth embodiment shown in FIG. 7 can be applied, the same transmission characteristics as in the tenth embodiment can be obtained, and the optical phase modulator driving circuit 12 Since the output level can be reduced, effects such as reduction in power consumption can be obtained.

Embodiment 12 FIG. FIG. 19 is a block diagram showing an optical transmitter according to a twelfth embodiment of the present invention. The corresponding parts are denoted by the same reference numerals as in FIG. 14 and description thereof is omitted. In the figure, 3 is a polarization scrambler for making the polarization state of the optical signal output from the optical transmitter output terminal 10 random on average, and 9 is a first delay unit 8a to which a delay is given. A polarization scrambler driving circuit which amplifies the obtained signal, inputs the amplified signal as a polarization scrambler driving signal to the polarization scrambler 3, and drives the signal. in this way,
In the twelfth embodiment, the polarization scrambler 3 is used as the optical phase modulator 11, and the optical phase modulator driving circuit 12 is replaced by the polarization scrambler driving circuit 9 in accordance with the polarization scrambler 3, and the optical phase modulator driving circuit is branched from the clock signal and delayed. The present embodiment is different from the eighth embodiment in that the used signal is used as a polarization scrambler drive signal.

Next, the operation will be described. The discriminator 6
From the data signal and the clock signal input from the data signal input terminal 4 and the clock signal input terminal 5, a light intensity modulator drive signal based on a non-return to zero code is generated. This light intensity modulator drive signal is return-to-
It is input to the zero conversion circuit 27 and converted from a non-return-to-zero code to a return-to-zero code. The light intensity modulator drive signal based on this return-to-zero code is amplified by the light intensity modulator drive circuit 7 and input to the light intensity modulator 2, and is returned from the light intensity modulator 2 in a return-to-zero format. The optical signal whose optical intensity has been modulated is output to the polarization scrambler 3. The signal branched from the return-to-zero type light intensity modulator drive signal output from the return-to-zero conversion circuit 27 is also input to the second delay unit 8b to be delayed. , Switch 13
As a switch control signal.

On the other hand, the signal branched from the clock signal input from the clock signal input terminal 5 is delayed through the first delay unit 8a, and is input to the switch 13 as a polarization scrambler driving signal. Switch 13 is the second
ON / OFF is controlled by a switch control signal output from the delay unit 8b, and the polarization output from the first delay unit 8a only when an optical signal is input to the polarization scrambler 3. The scrambler drive signal is input to the polarization scrambler drive circuit 9. The polarization scrambler 3 driven by the polarization scrambler drive signal from the polarization scrambler drive circuit 9 changes the polarization state of the optical signal output from the optical transmitter output terminal 10 into an averagely random state. And The polarization scrambler 3 can suppress the effect that the optical S / N ratio deteriorates or fluctuates due to polarization hole burning in the optical amplifier used in the transmission line or the polarization dependent loss of the transmission line.

Here, as the polarization scrambler 3, a lithium niobate optical phase modulator is generally used, which has a feature that the phase modulation occurs simultaneously with the polarization modulation, as described in FIG. It is. Therefore,
In the optical transmitter according to the twelfth embodiment shown in FIG. 19, the phenomenon that the receiving side mistakenly determines that the data signal is "1" when the data signal is "0" is performed in the embodiment shown in FIG. This can be avoided as in the case of mode 8, and the power consumption can be similarly reduced.

That is, in all of the above embodiments of the present invention, the optical phase modulator 11 can be replaced with the polarization scrambler 3. This is because the polarization scrambler 3 is a kind of optical phase modulator.

In the optical transmitter according to the twelfth embodiment shown in FIG. 19, there may be some problems in the degree of polarization that can be achieved. Hereinafter, the polarization scrambler operation in the optical transmitter in FIG. 19 will be described with reference to FIG. here,
FIG. 20A shows an example of the trajectory of the polarization state of the optical signal output from the polarization scrambler 3 when the optical intensity modulation is performed with the return-to-zero code and the optical signal is polarization scrambled. FIG. 20B shows a waveform example of the polarization scrambler driving signal applied to the polarization scrambler 3 at that time.
FIG. 20C shows an example of the waveform of the optical signal that is intensity-modulated with the return-to-zero code input from the optical intensity modulator 2 to the polarization scrambler 3 at that time.

Here, the polarization of the optical signal output when the optical signal modulated with the non-return-to-zero code or the optical signal not modulated with the optical intensity is input to the polarization scrambler 3. The state where the state draws a locus on a great circle on the Poincare sphere is as described with reference to FIG. FIG.
As shown in FIG. 0 (c), the optical signal input to the polarization scrambler 3 is optically intensity-modulated with a return-to-zero code, and the polarization scrambler 3 output from the polarization scrambler driving circuit 9 Assuming that the phase of the polarization scrambler driving signal for driving the optical signal is as shown in FIG. 20B, the trajectory of the polarization state of the optical signal output from the polarization scrambler 3 at that time is shown in FIG. a). As shown, the polarization state when the optical signal amplitude is weak is plotted inside the Poincare sphere.

As described with reference to FIG. 25, in order for the degree of polarization to become zero, on the Poincare sphere, (A) the probability of being present in the L-side hemisphere and the probability of being present in the R-side hemisphere within a certain time are equal. (B) The probability of being in the Q-side hemisphere and the probability of being in the P-side hemisphere within a certain time are equal. (C) The probability of being in the H-side hemisphere and the probability of being in the V-side hemisphere within a certain time are equal. The following three conditions must be satisfied.

In this case, as shown in FIG. 20A, the condition (A) cannot be satisfied because the state of polarization does not draw a locus on the R-side hemisphere. Thus, return-to-two
The optical transmitter according to the twelfth embodiment, which uses light intensity modulation with a zero code, may not be able to make the degree of polarization zero. The achievable degree of polarization depends on the phase relationship between the light intensity modulation waveform and the polarization scrambler drive waveform.

As described above, in the optical transmitter having the configuration shown in FIG. 19 according to the twelfth embodiment, the degree of polarization may not be made zero, but it is sufficiently smaller than the case where the polarization scrambler 3 is not used. In many cases, this is not a serious problem because the degree of polarization can be achieved.

As described above, according to the twelfth embodiment, since the optical phase modulator is substituted by the polarization scrambler, the degree of polarization of the output optical signal is smaller than when the polarization scrambler is not used. Can be sufficiently reduced.

In the above description, the polarization scrambler 3 is used as the optical phase modulator 11 of the eighth embodiment shown in FIG.
However, the polarization scrambler 3 may be used for the optical phase modulator 11 in the first to seventh embodiments and the ninth to eleventh embodiments. The same effects as in the case of application to the eighth embodiment are achieved.

Embodiment 13 FIG. FIG. 21 is a block diagram showing an optical transmitter according to a thirteenth embodiment of the present invention. The same reference numerals as in FIG. 19 denote the corresponding parts, and a description thereof will be omitted. In the figure, reference numeral 30 denotes a λ / 4 plate for rotating the polarization state of the optical signal output from the polarization scrambler 3 by 90 degrees. Reference numeral 31 denotes a Faraday rotator for rotating the polarization state of the optical signal input from the λ / 4 plate 30 in accordance with the drive signal. For example, a device using the magneto-optical effect can be used. A Faraday rotator driving circuit 32 drives the Faraday rotator 31.
It is configured using an amplifier. An oscillator 33 oscillates at a predetermined frequency and supplies an output to a Faraday rotator drive circuit.

As described above, in the thirteenth embodiment, between the polarization scrambler 3 and the optical transmitter output terminal 10, the λ / 4
In that a plate 30, a Faraday rotator 31, a Faraday rotator drive circuit 32, and an oscillator 33 are added,
This is different from the twelfth embodiment.

As described above, in the optical transmitter according to the twelfth embodiment shown in FIG. 19, when the return-to-zero code is used, the polarization scrambler driving signal shown in FIG. Depending on the phase relationship of the light intensity modulation waveform shown in c), the degree of polarization may not be zero. The thirteenth embodiment having the configuration shown in FIG. 21 shows a method for solving such a problem.

The operation of the optical transmitter shown in FIG. 21 will be described below with reference to FIG. Here, FIG. 22A shows an example of the trajectory of the polarization state of the optical signal after the light intensity modulation is performed with the return-to-zero code output from the polarization scrambler 3, and FIG. 3A illustrates an example of a trajectory of the polarization state of the optical signal in which the light intensity modulation is performed with the return-to-zero code output from the λ / 4 plate 30 and the state of the polarization rotation by the Faraday rotator 31.

The polarization state of the optical signal output from the polarization scrambler 3 is as shown in FIG. 22 (a), which corresponds to the polarization scrambler 3 in the twelfth embodiment shown in FIG. 20 (a). Is the same as the output of Optical intensity modulation is performed with the return-to-zero code output from the polarization scrambler 3, and the optical signal is input to the λ / 4 plate 30, and its polarization state is changed as shown in FIG. Rotate 90 degrees. In FIG. 22 (b), λ / λ is set so that the probability of existence in the L-side hemisphere and the probability of existence in the R-side hemisphere within a certain time are equal.
The main axis direction of the four plates 30 is set.

Here, the Faraday rotator 31 is oscillated by the oscillator 33 and is driven by the Faraday rotator driving circuit 3.
According to the drive signal amplified in 2, the polarization state rotates on the Poincare sphere in parallel with the equator. Accordingly, the polarization state of the optical signal output from the Faraday rotator 31 rotates as shown by the arrow in FIG. At this time, the above-mentioned (A), (B),
Even when the three conditions (C) are satisfied and the return-to-zero code is adopted, the degree of polarization is zero.

As described above, according to the thirteenth embodiment, since the optical signal output from the optical phase modulator is output through the λ / 4 plate and the Faraday rotator, the return-to-two signal is output. An effect is obtained that the degree of polarization can be made zero even for an optical signal that has been subjected to optical intensity modulation with a zero code.

In the above description, the case where the present invention is applied to the optical transmitter according to the twelfth embodiment is shown. However, in any of the first to eleventh embodiments other than the twelfth embodiment, the same applies to the optical transmitter. The output signal of the phase modulator (polarization scrambler 3) can be output through the λ / 4 plate and the Faraday rotator. In any case, the first embodiment is used.
The same effect as in the case of applying No. 2 is obtained.

Embodiment 14 FIG. FIG. 23 is a block diagram showing an optical transmitter according to a fourteenth embodiment of the present invention. The corresponding parts are denoted by the same reference numerals as in FIGS. 1 and 19, and description thereof is omitted. In the figure, 8e is the first shown in FIG.
A third delay device 8a corresponding to the delay device 8a, a fourth delay device 8f corresponding to the second delay device 8b shown in FIG. 19, and a third delay device 13a corresponding to the switch 13 shown in FIG. 1
The switch 13b is a second switch corresponding to the switch 13 shown in FIG. The difference between the fourteenth embodiment and the first embodiment is that the polarization scrambler 3, the polarization scrambler driving circuit 9, the third delay unit 8e, and the fourth delay unit 8
f and the second switch 13b.

Next, the operation will be described. Here, the operation until the optical phase modulator 11 is turned on / off by the first switch 13a and driven by the optical phase modulator drive signal amplified by the optical phase modulator drive circuit 12 and outputs an optical signal. Is the same as in the first embodiment. The polarization scrambler 3 to which the optical signal from the optical phase modulator 11 is input
Realizes the same pulse compression effect as that of the optical phase modulator 11 in addition to the effect of reducing the degree of polarization of the optical signal output from the optical transmitter output terminal 10.

The polarization scrambler 3 delays the signal branched from the clock signal by the third delay unit 8e, then turns it on and off by the second switch 13b, and turns it on and off. , And is driven by the polarization scrambler drive signal amplified by. The second switch 13b is turned on / off by branching off the optical intensity modulator drive signal generated by the discriminator 6 and turning on / off the first switch 13a by the switch control signal delayed by the second delay unit 8b. It is controlled by a switch control signal branched from the light intensity modulator drive signal generated by the discriminator 6 and delayed by the fourth delay unit 8f so as to be turned off.

As described above, according to the fourteenth embodiment, since a plurality of optical phase modulators are used, the waveform at the receiving end can be controlled more precisely. Since a polarization scrambler is used in one of the devices, effects such as a reduction in the degree of polarization of the output optical signal can be obtained.

[0141]

As described above, according to the present invention, the switch control in which the optical phase modulator drive signal to the optical phase modulator cascaded to the optical intensity modulator is synchronized with the light intensity modulator drive signal. Since the power is supplied via a switch that is turned on / off by a signal, the optical phase modulator operates only when an optical signal is input to the optical phase modulator. Since power consumption can be saved, and phase modulation is not performed when no optical signal is input to the optical phase modulator, when the extinction ratio of light intensity modulation is finite, the data signal There is an effect that an optical transmitter can be obtained which can prevent a phenomenon that an optical signal remaining at the time of “0” is pulsed after being transmitted through the fiber and a code error occurs.

According to the present invention, since the discriminator uses a signal obtained by branching the light intensity modulator drive signal generated from the data signal and the clock signal as the switch control signal, the light intensity modulator drive signal is used. The switch ON / OFF control is performed by the switch control signal synchronized with the optical phase modulator, and the optical phase modulator can be operated only when the optical signal is input to the optical phase modulator. Since the phase modulation is not applied when the signal is not input, there is an effect that power consumption of the optical phase modulator driving circuit can be saved and occurrence of a code error can be prevented.

According to the present invention, an optical amplifier is inserted between an optical intensity modulator and an optical phase modulator, and the path length of the optical amplifier is adjusted by a first delay unit for delaying an optical phase modulator drive signal. Since the control is performed by the delay amount of the second delay device that delays the switch control signal, the optical amplifier can set the light intensity level to be high, and the optical S
There is an effect that deterioration of the / N ratio can be suppressed.

According to the present invention, a part of the optical signal input from the optical intensity modulator to the optical phase modulator is demultiplexed by the optical demultiplexer, converted into an electric signal by the photodetector, and converted into an electric signal. Since it is configured to be used as a switch control signal for controlling on / off, the control of the switch for turning on / off the optical phase modulator drive signal is performed by controlling the light intensity modulated from the light intensity modulator to the optical phase modulator. Since it is performed using the signal extracted from the optical signal, the optical fiber connecting the optical intensity modulator and the optical phase modulator has the effect of operating stably without being affected by the fluctuation of the propagation time due to the temperature of the optical fiber. .

According to the present invention, the clock signal is extracted by the clock extraction circuit from the signal obtained by converting the optical signal demultiplexed by the optical demultiplexer into an electric signal by the photodetector, and the clock signal is extracted by the clock extraction circuit. Is input to the switch, so that there is an effect that the operation is stable without being affected by the fluctuation of the propagation time due to the temperature of the optical fiber connecting the optical intensity modulator and the optical phase modulator.

According to the present invention, a part of the optical signal input from the optical intensity modulator to the optical phase modulator is demultiplexed by the optical demultiplexer, converted into an electric signal by the photodetector, and An optical phase modulator drive signal for driving the optical phase modulator is generated by the optical drive signal generation circuit only when an optical signal is input to the optical phase modulator based on the output signal of the optical receiver. Therefore, there is an effect that it is possible to realize an optical transmitter which is not affected by the fluctuation of the propagation time due to the temperature of the optical fiber connecting the optical intensity modulator and the optical phase modulator with fewer components.

According to the present invention, only when an optical signal is input to the optical phase modulator from the optical intensity modulator drive signal generated by the discriminator based on the data signal and the clock signal, the optical phase modulation is performed. The optical phase modulator drive signal for driving the optical modulator is configured to be generated by the optical phase modulator drive signal generation circuit, so the optical fiber connecting the optical intensity modulator and the optical phase modulator with fewer components This has the effect of realizing an optical transmitter which is not affected by the fluctuation of the propagation time due to the temperature.

According to the present invention, the optical phase modulator drive signal inverting means is controlled by the switching control signal synchronized with the light intensity modulator drive signal, and the control is performed according to whether an optical signal is input to the optical phase modulator. Since the polarity of the optical phase modulator driving signal input to the optical phase modulator by the optical phase modulator driving signal inverting means is configured to be inverted, when the extinction ratio of the light intensity modulation is finite, the data It is possible to prevent a phenomenon that an optical signal remaining when the signal is "0" is pulsed after being transmitted through the fiber and a code error occurs, thereby increasing an eye opening ratio.

According to the present invention, the optical phase modulator drive signal inverting means includes a branch circuit for branching the optical phase modulator drive signal into two, an inverter circuit for inverting one of the polarities, and an output signal of the branch circuit. Since the other and one of the output signals of the inverter circuit are configured by a changeover switch that is selected according to a changeover control signal branched from the light intensity modulator drive signal generated by the discriminator, code error prevention and There is an effect that an optical transmitter capable of improving the eye opening ratio can be easily realized.

According to the present invention, the optical phase modulator drive signal inverting means includes a branch circuit for branching the optical phase modulator drive signal into two, an inverter circuit for inverting one of the polarities, and an output signal for the branch circuit. The other of the output signal of the inverter circuit and one of the output signals of the inverter circuit, the optical signal sent from the optical intensity modulator demultiplexed by the optical demultiplexer to the optical phase modulator into a switching control signal that is converted to an electrical signal by the optical receiver It operates stably because it is configured with a changeover switch that selects according to
There is an effect that an optical transmitter capable of preventing a code error and improving an eye opening ratio can be easily realized.

According to the present invention, the adder adds the optical intensity modulator drive signal generated by the discriminator based on the data signal and the clock signal to the clock signal, and the optical signal is input to the optical phase modulator. Only when the optical phase modulator is driven, the optical phase modulator drive signal for driving the optical phase modulator is generated. When the polarity of the optical phase modulator drive signal is inverted and the extinction ratio of the light intensity modulation is finite, the optical signal remaining when the data signal is "0" is pulsed after fiber transmission and causes a code error. Thus, there is an effect that the eye opening ratio can be increased.

According to the present invention, the drive signal of the light intensity modulator based on the non-return-to-zero code generated by the discriminator is converted into the return-to-zero code by the return-to-zero conversion circuit. Since the signal is sent to the optical intensity modulator, the phase modulation is not applied when no optical signal is input to the optical phase modulator, and the data signal is transmitted when the extinction ratio of the optical intensity modulation is finite. There is an effect that it is possible to prevent a phenomenon that an optical signal remaining at the time of “0” is pulsed after fiber transmission and causes a code error to occur.

According to the present invention, as the optical phase modulator drive signal for driving the optical phase modulator to which the optical signal modulated with the return-to-zero code is input, the frequency multiplier converts the clock signal. Since the configuration is such that the frequency-multiplied signal is used, the amplitude of the optical phase modulator drive signal can be reduced by increasing its frequency,
There is an effect that power consumption of the optical phase modulator driving circuit can be saved.

According to the present invention, the light intensity modulator drive signal generated by the discriminator is converted into a return-to-zero code and sent to the light intensity modulator, and the converted light intensity modulator drive signal and Since the optical phase modulator is driven by the optical phase modulator drive signal obtained by adding the clock signal, the code is output when the data signal is "1" continuous as in the case of using the non-return-to-zero code. There is an effect that the transmission characteristics can be more effectively improved without the occurrence of interference between them.

According to the present invention, a frequency multiplier is further provided to multiply the frequency of the clock signal, and the frequency is multiplied by the optical intensity modulator drive signal converted to the return-to-zero code. Since the optical phase modulator is configured to be driven by a signal, the output level of the optical phase modulator driving circuit can be reduced, and the power consumption can be reduced.

According to the present invention, since the polarization scrambler is used as the optical phase modulator, there is an effect that the degree of polarization of the output optical signal can be sufficiently reduced.

According to the present invention, since the optical signal is output from the optical phase modulator via the λ / 4 plate and the Faraday rotator, the light intensity modulated by the return-to-zero code is output. There is an effect that the degree of polarization can be made zero for a signal.

According to the present invention, since two or more optical phase modulators are provided, there is an effect that the waveform can be more precisely controlled at the receiving end.

According to the present invention, since the polarization scrambler is used as at least one of the two or more optical phase modulators, the degree of polarization of the output optical signal is adjusted by the polarization scrambler. There is an effect that can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration example of an optical transmitter according to a first embodiment of the present invention.

FIG. 2 is an operation explanatory diagram showing signal waveforms of respective units of the optical transmitter according to the first embodiment of the present invention.

FIG. 3 is a block diagram showing another configuration example of the optical transmitter according to the first embodiment of the present invention.

FIG. 4 is a block diagram illustrating a configuration example of an optical transmitter according to a second embodiment of the present invention.

FIG. 5 is a block diagram illustrating a configuration example of an optical transmitter according to a third embodiment of the present invention.

FIG. 6 is a block diagram illustrating a configuration example of an optical transmitter according to a fourth embodiment of the present invention.

FIG. 7 is an operation explanatory diagram showing signal waveforms of respective parts of the optical transmitter according to Embodiment 4 of the present invention.

FIG. 8 is a block diagram showing a configuration example of an optical transmitter according to a fifth embodiment of the present invention.

FIG. 9 is a block diagram illustrating a configuration example of an optical transmitter according to a sixth embodiment of the present invention.

FIG. 10 is an operation explanatory diagram showing signal waveforms of respective parts of the optical transmitter according to Embodiment 6 of the present invention.

FIG. 11 is a block diagram showing another configuration example of the optical transmitter according to the sixth embodiment of the present invention.

FIG. 12 is a block diagram illustrating a configuration example of an optical transmitter according to a seventh embodiment of the present invention.

FIG. 13 is an operation explanatory diagram showing signal waveforms at various parts of the optical transmitter according to Embodiment 7 of the present invention.

FIG. 14 is a block diagram showing a configuration example of an optical transmitter according to an eighth embodiment of the present invention.

FIG. 15 is a block diagram showing a configuration example of an optical transmitter according to a ninth embodiment of the present invention.

FIG. 16 is an operation explanatory diagram showing signal waveforms at various parts of the optical transmitter according to Embodiment 9 of the present invention.

FIG. 17 is a block diagram illustrating a configuration example of an optical transmitter according to a tenth embodiment of the present invention.

FIG. 18 is a block diagram illustrating a configuration example of an optical transmitter according to an eleventh embodiment of the present invention.

FIG. 19 is a block diagram illustrating a configuration example of an optical transmitter according to a twelfth embodiment of the present invention.

FIG. 20 is an operation explanatory diagram illustrating an operation of an optical transmitter according to Embodiment 12 of the present invention.

FIG. 21 is a block diagram illustrating a configuration example of an optical transmitter according to Embodiment 13 of the present invention.

FIG. 22 is an operation explanatory diagram illustrating an operation of an optical transmitter according to Embodiment 13 of the present invention.

FIG. 23 is a block diagram illustrating a configuration example of an optical transmitter according to Embodiment 14 of the present invention.

FIG. 24 is a block diagram illustrating a configuration example of a conventional optical transmitter.

FIG. 25 is an operation explanatory diagram for explaining an operation of a polarization scrambler of a conventional optical transmitter.

FIG. 26 is an operation explanatory diagram showing signal waveforms of various parts of the conventional optical transmitter.

[Explanation of symbols]

1 light source, 2 light intensity modulator, 3 polarization scrambler,
4 data signal input terminal, 5 clock signal input terminal,
6 discriminator, 8a first delay, 8b second delay, 8c, 8d delay, 8e third delay, 8f
A fourth delay unit, 11 optical phase modulator, 13 switch,
13a first switch, 13b second switch, 1
4 Optical amplifier, 15 Optical demultiplexer, 16 Photodiode (receiver), 18 Clock extraction circuit, 20 Low-pass filter (Optical phase modulator drive signal generation circuit), 21
Branch circuit, 22 inverter circuit, 23 changeover switch, 24 optical phase modulator drive signal inverting means, 26 adder, 27 return-to-zero conversion circuit, 29 frequency multiplier, 30 λ / 4 plate, 31 Faraday rotator, 32 Faraday rotator drive circuit.

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁶ Identification symbol FI H04B 10/00 (72) Inventor Hitoshi Matsushita 2-3-2 Marunouchi, Chiyoda-ku, Tokyo Mitsubishi Electric Corporation

Claims (19)

[Claims] A light source that generates an optical signal; a light intensity modulator that modulates an optical signal output from the light source based on a light intensity modulator driving signal; and a cascade connection to the light intensity modulator. An optical phase modulator that performs optical phase modulation on an optical signal that has been subjected to optical intensity modulation based on an optical phase modulator drive signal synchronized with the optical intensity modulator drive signal; and the optical phase modulator drive signal Is input, ON / OFF is controlled by a switch control signal synchronized with the optical intensity modulator drive signal, and only when an optical signal is input to the optical phase modulator, the optical phase modulator drive signal is An optical transmitter, comprising: a switch for inputting to the optical phase modulator. 2. An identification for generating an optical intensity modulator drive signal for driving an optical intensity modulator based on a data signal input from a data signal input terminal and a clock signal input from a clock signal input terminal. 2. The optical transmitter according to claim 1, further comprising: a branching device that branches the optical intensity modulator drive signal generated by the discriminator, and uses the branched signal as a switch control signal for controlling on / off of a switch. . 3. A first delay unit which inserts an optical amplifier between an optical intensity modulator and an optical phase modulator, applies a delay to an optical phase modulator drive signal obtained by splitting the clock signal, and inputs the delayed signal to a switch. And a second delay unit that delays the signal obtained by splitting the optical intensity modulator drive signal and sends the signal to a switch as a switch control signal. The first delay unit and the second delay unit 3. The optical transmitter according to claim 2, wherein the path length is controlled by the delay amount. 4. An optical demultiplexer for demultiplexing a part of an optical signal input from an optical intensity modulator to an optical phase modulator, and converting the optical signal demultiplexed by the optical demultiplexer into an electric signal. The optical transmitter according to claim 1, further comprising: a light receiving device that uses an output signal of the light receiving device as a switch control signal for controlling on / off of a switch. 5. A clock extraction circuit for extracting a clock signal from an output signal of a photodetector, wherein the clock signal extracted by the clock extraction circuit is input to a switch as an optical phase modulator drive signal. Item 5. The optical transmitter according to item 4. 6. A light source that generates an optical signal, a light intensity modulator that modulates an optical signal output from the light source based on a light intensity modulator drive signal, and a cascade connection to the light intensity modulator An optical phase modulator that performs optical phase modulation on an optical signal that has been subjected to optical intensity modulation based on an optical phase modulator drive signal synchronized with the optical intensity modulator drive signal; An optical demultiplexer that demultiplexes a part of the optical signal input to the modulator, a photodetector that converts the optical signal demultiplexed by the optical demultiplexer into an electric signal, and an output signal of the photodetector. An optical phase modulator drive signal generation circuit that generates an optical phase modulator drive signal for driving the optical phase modulator only when an optical signal is input to the optical phase modulator. Optical transmitter. 7. A light source that generates an optical signal, a light intensity modulator that modulates an optical signal output from the light source based on a light intensity modulator drive signal, and a cascade connection to the light intensity modulator An optical phase modulator that performs optical phase modulation on an optical signal that has been subjected to optical intensity modulation based on an optical phase modulator drive signal synchronized with the optical intensity modulator drive signal; A discriminator that generates a light intensity modulator drive signal for driving the light intensity modulator based on the data signal and a clock signal input from a clock signal input terminal; and a light intensity modulator generated by the discriminator. An optical phase modulator drive signal generation circuit that generates an optical phase modulator drive signal for driving the optical phase modulator only when an optical signal is input to the optical phase modulator based on the drive signal, Optical transmission with Vessel. 8. A light source that generates an optical signal, a light intensity modulator that modulates an optical signal output from the light source based on a light intensity modulator drive signal, and a cascade connection to the light intensity modulator An optical phase modulator that performs optical phase modulation on an optical signal that has been subjected to optical intensity modulation based on an optical phase modulator drive signal synchronized with the optical intensity modulator drive signal; and the optical phase modulator drive signal The input is controlled by a switching control signal synchronized with the light intensity modulator drive signal, and depending on whether an optical signal is input to the optical phase modulator, the polarity of the optical phase modulator drive signal An optical phase modulator drive signal inverting means for inverting the optical phase modulator and inputting the inverted signal to the optical phase modulator. 9. An optical phase modulator driving signal inverting means, a branch circuit for branching the optical phase modulator driving signal into two, an inverter circuit for inverting one polarity of an output signal of the branch circuit, and a generation of a discriminator. A switching switch that selects one of the output signal of the branch circuit and one of the output signals of the inverter circuit by a switching control signal branched from the light intensity modulator drive signal. Item 10. The optical transmitter according to Item 8. 10. An optical phase modulator drive signal inverting means, a branch circuit for branching the optical phase modulator drive signal into two, an inverter circuit for inverting one polarity of an output signal of the branch circuit, and a light intensity modulator. An optical signal sent to the optical phase modulator from the other of the output signal of the branch circuit, by a switching control signal that is demultiplexed by the optical demultiplexer and converted to an electrical signal by the photodetector,
9. The switching circuit according to claim 8, wherein said switching circuit selects one of output signals of said inverter circuit.
An optical transmitter as described. 11. A light source that generates an optical signal, a light intensity modulator that modulates an optical signal output from the light source based on a light intensity modulator drive signal, and a cascade connection to the light intensity modulator. An optical phase modulator that performs optical phase modulation on an optical signal that has been subjected to optical intensity modulation based on an optical phase modulator drive signal synchronized with the optical intensity modulator drive signal; A discriminator that generates a light intensity modulator drive signal for driving the light intensity modulator based on the data signal and a clock signal input from a clock signal input terminal; and a light intensity modulator generated by the discriminator. An adder that adds a drive signal and the clock signal and generates an optical phase modulator drive signal that drives the optical phase modulator only when an optical signal is input to the optical phase modulator. Optical transmitter 12. A return-to-zero conversion circuit which converts a light intensity modulator drive signal generated by a discriminator into a return-to-zero code and sends it to the light intensity modulator. The optical transmitter according to any one of claims 1 to 11. 13. A light source that generates an optical signal; a light intensity modulator that modulates an optical signal output from the light source based on a light intensity modulator drive signal; and a cascade connection to the light intensity modulator. An optical phase modulator that performs optical phase modulation on an optical signal that has been subjected to optical intensity modulation based on an optical phase modulator drive signal synchronized with the optical intensity modulator drive signal; A discriminator that generates a light intensity modulator drive signal for driving the light intensity modulator based on the data signal and a clock signal input from a clock signal input terminal; and a light intensity modulation output from the discriminator. A return-to-zero conversion circuit that converts a device drive signal into a return-to-zero code and sends the return-to-zero code to the optical intensity modulator, and multiplies the frequency of a clock signal input from the clock signal input terminal. An optical transmitter that includes a frequency multiplier for generating an optical phase modulator drive signal for driving the optical phase modulator. 14. A return-to-zero conversion circuit for converting a light intensity modulator drive signal output from a discriminator into a return-to-zero code and sending the return-to-zero code to the light intensity modulator, wherein the adder includes: The optical intensity modulator drive signal converted to the return-to-zero code by the two-to-zero conversion circuit and the clock signal are added, and only when the optical signal is input to the optical phase modulator, The optical transmitter according to claim 11, wherein the optical transmitter generates an optical phase modulator drive signal for driving the phase modulator. 15. A light multiplier having a frequency multiplier for multiplying the frequency of a clock signal input from a clock signal input terminal, wherein the adder converts the light intensity into a return-to-zero code by a return-to-zero conversion circuit. A light for driving the optical phase modulator only when an optical signal is input to the optical phase modulator by adding a modulator drive signal and a clock signal whose frequency has been multiplied by the frequency multiplier. 15. The optical transmitter according to claim 14, wherein the optical transmitter generates a phase modulator drive signal. 16. The optical transmitter according to claim 1, wherein a polarization scrambler is used as the optical phase modulator. 17. A λ / 4 plate connected to the output of the optical phase modulator to rotate the polarization state of the input optical signal by 90 degrees, and connected to the output of the λ / 4 plate to be input. 17. A Faraday rotator that rotates a polarization state of an optical signal according to a drive signal, and a Faraday rotator drive circuit that supplies a drive signal to the Faraday rotator and drives the Faraday rotator. Any one of
The optical transmitter according to the item. 18. The apparatus according to claim 1, further comprising two or more optical phase modulators connected in cascade.
The optical transmitter according to claim 1. 19. The optical transmitter according to claim 18, wherein a polarization scrambler is used for at least one of the two or more optical phase modulators connected in cascade.