JPH10148801A - Optical modulation device by external modulation system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simple system using an external modulation system which causes no deterioration in transfer characteristics. SOLUTION: Light outputs of a LD(semiconductor laser) 21 and a LED(light emitting diode) 22 differing in wavelength are coupled by a wavelength- multiplexing coupler 23, supplied to a Mach Zender optical modulator 24, from whose output the component of the wavelength of the LED 22 is branched to be taken out and photo-electrically convereted by a PD(photodiode) 211. The light output of the LBD 22 is applied with low-frequency intensity modulation in advance and electric signals photoelectrically converted by the PD 211 are adjusted in gain by a gain adjusting circuit 212 to be inputted into a differential amplifier 213. On the other hand, the output of a low-frequency oscillator 26 is phase-adjusted by a phase-shifter 214 and inputted into the differential amplifier 213 to output the difference of these two signals, which is synchronously detected by a synchronous detection circuit 215 to control the bias voltage by its low-frequency component.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator using an external modulation system which can be used in an optical transmitter or an optical repeater of an optical communication system and can maintain an optical output waveform in a good state.

[0002]

2. Description of the Related Art As is well known, in an optical communication system, with the recent increase in the amount of information, a dramatic increase in communication capacity is desired, and transmission experiments at 10 Gbps are being conducted. . On the other hand, from the viewpoint of reducing the line cost, the relay interval is increased, and non-relay transmission of about 100 km has been confirmed by experiments.

In such a long-distance, large-capacity optical communication system, in a method of directly modulating a laser, an optical waveform at a receiving end is affected by a change in a laser oscillation wavelength known as chirping and a wavelength dispersion of a fiber. to degrade. For this reason, optical transmission using an external modulator that is less affected by chirping is being studied.

As an external modulator, a Mach-Zehnder (MZ) type optical modulator using lithium niobate is well known. FIG. 8A shows the input / output characteristics of the MZ optical modulator.
FIG. 8C shows an output optical signal when the electric signal shown in FIG. 8B is input to the modulator.

In FIG. 8A, Pmax is the maximum value of the light output from the modulator and is smaller than the input light by the amount corresponding to the loss of the modulator. In an ideal MZ optical modulator, almost no Pmax is obtained when no voltage is input. When a half-wave voltage (indicated by Vπ in FIG. 8A) is input to the modulator, almost no light is output as shown in the figure.

When an electric signal as shown in FIG. 8B is added as an input signal, an optical signal as shown in FIG. 8C is output. That is, in order to obtain such an optical signal, a DC component (bias voltage Vb) is necessary for the voltage signal.

In an MZ optical modulator using lithium niobate, its input / output characteristics drift due to temperature, humidity, voltage applied to the modulator, distortion, and the like. For example, when the input / output characteristic drifts as shown in FIG. 9A, when the electric signal shown in FIG. 9B is added, the output waveform of the optical signal is folded as shown in FIG. 9C. Be able to be seen. Therefore, it is necessary to shift the bias voltage in order to compensate for the drift of the input / output characteristics.

As a method for compensating for drift in input / output characteristics, Japanese Patent Publication No. 3-251815 discloses a method in which a low frequency is superimposed on an input electric signal to detect drift. FIG. 10 shows the configuration of an external modulator control device according to this method.

In FIG. 10, coherent light emitted from a light source 101 is input to an external modulator (here, an MZ optical modulator is used) 102. An input electric signal to be a modulated signal is a variable gain (AGC) amplifier 103.
Is amplitude-modulated with a low-frequency signal from the oscillator 104 using The electric signal subjected to the amplitude modulation is input to the drive amplifier 105 and amplified to a level necessary for driving the external modulator 102, and then the external modulator 10
2 is sent. A bias voltage is applied to the external modulator 102 using the bias T107.

A part of the output light from the external modulator 102 is extracted by an optical splitter 109 and detected by a photoelectric converter (photodiode) 1010. Photoelectric converter 101
The signal detected at 0 includes a low-frequency signal superimposed by the AGC amplifier 103. After being amplified by the low noise amplifier 1011, the detection signal is mixed with the reference low frequency from the oscillator 104 by the phase comparator 1012 and subjected to phase detection.

The output of this phase detection is a low-pass filter (L
The signal is converted into a DC voltage signal by a PF 1013, amplified as appropriate by a bias control circuit 1014, and sent to a bias T 107 as a bias voltage. One output terminal of the bias T107 is grounded via a terminating resistor 108, and a bias voltage is applied to the external modulator 102 from the other output terminal.

In the above configuration, assuming that the optimum bias voltage is applied to the external modulator 102 having the input / output characteristics (state in which no drift occurs) shown in FIG. )), The output optical signal becomes as shown in FIG. 11C when the electric signal on which the low-frequency signal (frequency f0) shown in FIG.
f0 is superimposed.

Here, the input / output characteristics of the external modulator 102 are changed from the normal state A to B and C as shown in FIG.
12 (b), the output optical signal has a phase difference with respect to the input electrical signal shown in FIG. 12 (b) according to the drift direction as shown in FIGS. 12 (c) and 12 (d). A low frequency component of the inverted frequency f0 appears.

From the above, from the electric signal converted by the photoelectric converter 1010, a signal of the frequency f0 is not detected when the optimum bias voltage is supplied, and when a drift of the input / output characteristics occurs, the signal of the frequency f0 is detected. A signal is detected, and the phase of the detected signal is inverted due to the difference in the drift direction. This electric signal is phase-detected by the original low-frequency signal, and the bias voltage is controlled so that the result of the phase detection becomes zero, whereby the drift of the input / output characteristics can be compensated.

However, the above apparatus has the following disadvantages. First, since the above-described device inputs an electric signal on which a low frequency is superimposed to the external modulator 102, the same low frequency intensity modulation is applied to the output optical signal. For this reason, the transmission characteristics are degraded.

Second, variable gain (AGC) amplifier 103
And the drive amplifier 105 needs to have a wideband characteristic equal to or higher than the bit rate of the input electric signal.
It is very difficult to realize an object on which a low-frequency signal can be superimposed. In particular, the drive amplifier 105 needs to perform a large amplitude operation to drive the optical modulator, which has made it more difficult to realize.

[0017]

As described above, the conventional optical modulator is of the type in which a low frequency is superimposed on an input electric signal, so that the transmission characteristics are deteriorated and that an optical modulator for driving an external modulator is required. Since a high bit rate characteristic is required for the variable gain (AGC) amplifier and the driving amplifier, there is a problem that the realization is very difficult.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical modulation device using an external modulation system with a relatively simple configuration that does not cause deterioration in transmission characteristics. It is in.

[0019]

In order to achieve the above object, the present invention uses the following means (1) to (6). (1) In order to achieve the above object, the present invention provides an external modulation method including a light source for generating a carrier optical signal, and an optical modulator for intensity-modulating and outputting the carrier optical signal according to an external electric signal. In the optical modulation device, a light emitting element that outputs light in a wavelength band different from the carrier optical signal, a low frequency oscillator that generates a low frequency signal whose frequency is sufficiently lower than the electric signal, and output from the low frequency oscillator By driving the light emitting element according to the low frequency signal, a light emitting element driving means for generating an optical signal intensity-modulated by the low frequency signal from the light emitting element, and intensity modulated output from the light emitting element An optical signal is coupled to the carrier optical signal, and an optical coupling unit that is input to the optical modulator, and a light component in a wavelength band output by the light emitting element in the optical signal output from the optical modulator is branched. Out An optical splitter for converting the optical signal split by the optical splitter into an electrical signal; a frequency component of a low-frequency signal included in the electrical signal output from the photoelectric converter; It is provided with phase comparing means for comparing the phase with the low-frequency signal output from the oscillator, and optical modulator driving means for driving the optical modulator based on the comparison result of the phase comparing means.

With this configuration, since the low frequency component is not superimposed on the electric signal input to the optical modulator, the deterioration of the transmission characteristics does not occur. (2) Further, according to the present invention, a plurality of light sources for generating a carrier light signal and each of the plurality of light sources are provided, and each of the carrier light signals is intensity-modulated and output according to an external electric signal. In an optical modulation device based on an external modulation method including a plurality of optical modulators, a light emitting element that outputs light of a wavelength band different from any of the respective wavelength bands of a carrier optical signal generated by the plurality of light sources; A low-frequency oscillator that outputs a low-frequency signal whose frequency is sufficiently lower than that of the signal; and driving the light-emitting element in accordance with the low-frequency signal output from the low-frequency oscillator. Light emitting element driving means for generating a modulated optical signal, and combining the intensity-modulated optical signal output from the light emitting element with each of the carrier optical signals generated by each of the plurality of light sources, A plurality of optical coupling means for inputting to each of the plurality of optical modulators; and a plurality of optical coupling means provided corresponding to each of the plurality of light sources, among the optical signals output from each of the plurality of optical modulators, A plurality of optical branching means for branching and outputting light components in a wavelength band output by the element; and a plurality of optical branching means provided for each of the plurality of optical branching means for converting each of the branched optical signals into an electric signal. And the phase of the frequency component of the low-frequency signal included in the output electric signal and the low-frequency signal output from the low-frequency oscillator are provided for each of the plurality of photoelectric converters. A plurality of phase comparing means, and a plurality of optical modulator driving means provided for each of the plurality of optical modulators and driving the optical modulator based on a comparison result of each of the plurality of phase comparing means. And is provided.

With this configuration, in addition to having the advantage shown in the above (1), since optical modulation can be performed on optical signals having a plurality of different wavelengths, it is possible to provide an optical modulator for a wavelength division multiplexing system.

(3) Further, according to the present invention, in the optical modulator using the external modulation method according to any one of the above (1) and (2), the optical modulator driving means is used as a comparison result of the phase comparing means. Based on this, the value of the bias voltage applied to the optical modulator is changed.

With this configuration, it is possible to guarantee a deviation in the characteristics of the optical modulator due to an external factor such as a temperature change. (4) Further, according to the present invention, in the optical modulation device based on the external modulation method according to any one of (1) and (2) above, the optical modulator driving unit is configured to control the optical modulator driving unit based on a comparison result of the phase comparison unit. The shift amount in the intensity direction of the electric signal input to the optical modulator is changed.

With this configuration, the same operation and effect as the above (3) can be obtained. (5) Further, according to the present invention, in the optical modulation device according to any one of the above (1) to (4), the optical coupling means and the optical branching means are wavelength multiplex couplers.

With such a configuration, the present invention can be applied to a wavelength division multiplexing system. (6) Further, according to the present invention, in the optical modulation device according to any one of the above (1) to (5), the optical modulator is a Mach-Zehnder optical modulator. With this configuration, it is possible to provide an optical modulation device capable of performing optical modulation on an input electric signal having a high bit rate.

[0026]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 is a principle block diagram showing a first embodiment of an optical modulation device using an external modulation system according to the present invention.

In FIG. 1, a carrier light output from a light source 11 is input to an optical modulator 14 via an optical coupler 13 and is intensity-modulated in accordance with an input data signal.
Is output as modulated light. At this time, the output light of the light emitting element 12 is given to the optical coupler 13. The low frequency signal generated by the low frequency oscillator 16 is given to the light emitting element driving section 15, whereby the low frequency signal is superimposed on the output light of the light emitting element 12.

On the other hand, the optical splitter 18 splits and extracts the output light wavelength component of the light emitting element 12 from the output of the optical modulator 14, and the split optical signal is photoelectrically converted by the photoelectric converter 19. It is input to the phase comparison unit 110.

The phase comparing section 110 compares the phase of the low-frequency component of the photoelectrically converted optical signal with the phase of the low-frequency signal output from the low-frequency oscillator 16, and outputs a control signal corresponding to this. This is given to the modulator driving unit 17.

FIG. 2 is a circuit block diagram showing a first embodiment of an optical modulation device using an external modulation system according to the present invention. In FIG. 2, optical signals output from a semiconductor laser (LD) 21 and a light emitting diode (LED) 22 are combined by a wavelength multiplexing coupler 23 and input to an MZ optical modulator 24. The LD 21 outputs carrier light, and the output light wavelength is 1550 nm. The output light wavelength of the LED 22 is 1310 nm. Note that a semiconductor laser may be used instead of the LED 22.

The driving circuit 25 for driving the LED 22 includes:
A low frequency signal generated by the low frequency oscillator 26 is input.
As the drive circuit 25, for example, a variable gain (AGC)
An amplifier is used.

A data signal is input to one modulation input terminal of the MZ light modulator 24, and a low-pass filter (LPF) 27 and an adder connected in series are connected to the other modulation input terminal of the MZ light modulator 24. 28, and a bias T29. Note that these LPF 27, adder 28,
And the bias T29 correspond to the driving unit of the MZ optical modulator 24.

The MZ light modulator 24 is bias-controlled by this drive, and the combined 1550 nm and 1310 nm
The optical signal having a wavelength of nm is intensity-modulated according to the input data signal and output. This intensity-modulated light is input to a wavelength multiplexing coupler 210 as an optical splitter. The wavelength multiplex coupler 210 converts the output light of the MZ optical modulator 24 into 1310
An optical signal having a wavelength of nm is branched and guided to a photodiode (PD) 211 as a photoelectric converter. It should be noted that the PD 211 may be one that is sufficiently slow with respect to the bit rate of the input data signal.

The output of the PD 211 is connected to the gain adjustment circuit 2
The signal is input to one input terminal of the differential amplifier 213 through the input terminal 12. A low-frequency signal generated by the low-frequency oscillator 26 is supplied to the other input terminal of the differential amplifier 213.
14. Further, the output of the differential amplifier 213 is input to the synchronous detection circuit 215 together with the low frequency signal via the phase shifter 214.

The gain adjustment circuit 212 is inserted to make the output of the PD 211 and the output amplitude of the phase shifter 214 equal to each other at the optimum bias point.
Is provided to compensate for the time delay of the bias control loop. The gain adjustment circuit 212, the differential amplifier 213, the phase shifter 214, and the synchronous detection circuit 21
5 corresponds to the phase comparison unit 110.

The output of the synchronous detection circuit 215 is input to an adder 28 via an LPF 27, added to a preset initial setting voltage, and then applied to the other modulation input of the MZ optical modulator 24 via a bias T29. Input to terminal.

In FIG. 2, the positions at which the gain adjustment circuit 212 and the phase shifter 214 are provided do not necessarily have to be exactly as described above.
4 may be connected in series and provided on the output side of the PD 211 or for the output of the low-frequency oscillator 26. Further, the LPF 27 may be provided at the output stage of the adder 28.

The operation of the above configuration will be described below. First, the output light (wavelength 1550 nm) of the LD 21 and the output light (wavelength 1310 nm) of the LED 22 are coupled by the wavelength multiplexing coupler 23 and input to the MZ optical modulator 24.
Here, it is assumed that the output light of the LED 22 is unmodulated.

FIG. 3 shows an applied voltage-transmittance characteristic of the MZ optical modulator 24 with respect to the input light wavelength. In FIG. 3, the dotted line shows the applied voltage-transmittance characteristic for an optical signal having a wavelength of 1550 nm, and the solid line shows the applied voltage-transmittance characteristic for an optical signal having a wavelength of 1310 nm. In FIG. 3, it is assumed that no drift occurs with respect to the applied voltage-transmittance characteristic. Generally, in the MZ optical modulator, the transmittance characteristic with respect to the applied voltage periodically changes. It is proportional to the wavelength of the optical signal. That is, assuming that the half-wave voltage for an optical signal having a wavelength of 1550 nm is Vπ, the half-wave voltage for an optical signal having a wavelength of 1310 nm is Vπ × (1310 ÷ 1550).

A data signal is input from one modulation input terminal to the MZ optical modulator 24 having such characteristics.
According to this input data signal, 1550 nm and 13
Light having a wavelength of 10 nm is modulated and output. Here, it is assumed that the input data signal is a sine wave high frequency voltage as shown in FIG. 3, but the same discussion can be applied to the case where this is a binary pulse signal. In the equation representing the high-frequency voltage V, since the voltage amplitude and the bias voltage value are each Vπ / 2 on the assumption that no drift occurs,

[0041]

(Equation 1) Becomes Therefore, the optical output Iout from the MZ optical modulator 24 for the optical signal having the wavelength of 1310 nm is

[0042]

(Equation 2) It is expressed as In the above equation (2),

[0043]

(Equation 3) Where Iin is the input optical signal strength and ω is the angular frequency of the high frequency voltage V. Α is a bias point with respect to the light having a wavelength of 1310 nm when no drift occurs in the MZ optical modulator 24, and is a variable that changes as the drift occurs.

The optical signal modulated by the MZ optical modulator 24 is split by the wavelength division multiplexing coupler 210 and 1550n.
m wavelength light to the transmission line, while 1310 nm wavelength light
D211 is input. Here, the low frequency component Ioutav of the optical signal that is photoelectrically converted is obtained by integrating the above (3) over one cycle of ω,

[0045]

(Equation 4) It is expressed as Note that J0 (A) is a 0th-order Bessel function, which is a constant here.

As can be seen from the above (4), Ioutav is α
Monotonically increases or monotonically decreases. Therefore, this I
By monitoring outav, the position of α can be obtained, and based on this, the bias voltage to be applied to the MZ optical modulator 24 can be controlled. However Iouta
When v is monitored in a DC manner, an error accompanies the dark current flowing through the PD 211 and the like.

To avoid this, the output light of the LED 22, ie, Iin, is subjected to low-frequency intensity modulation, and the resulting change in the output of the PD 211 is monitored. This is performed by changing the amplification gain of the drive circuit 25 in accordance with the output of the low-frequency oscillator 26.
From 211, an electric signal whose intensity is modulated according to the low frequency generated by the low frequency oscillator 26 is output. This PD21
The output from 1 is input to the differential amplifier 213 together with the output of the phase shifter 214 after the gain is controlled by the gain adjustment circuit 212.

At this time, the gain adjustment circuit 212 and the phase shifter 2
The output waveforms of the differential amplifier 14 and the differential amplifier 213 change depending on whether drift occurs. This is shown in FIG. FIG.
9A shows the output waveform of the gain adjustment circuit 212, FIG. 9B shows the output waveform of the phase shifter 214, and FIG. 9C shows the output waveform of the differential amplifier 213. When no drift occurs in the MZ optical modulator 24, each output waveform is represented by a solid line in the figure. In this case, since the waveforms (a) and (b) are set in advance so as to match, no low-frequency component appears in the waveform (c).

On the other hand, as shown in FIG.
When a drift occurs between // 2 and Vπ, α also increases accordingly and from equation (4), Iouta
v changes to be smaller. Therefore, as shown by the dashed line in FIG. 4, the amplitude of the output of FIG.
A low frequency component appears in the waveform of (c). Conversely, when drift occurs such that the bias point becomes smaller, the amplitude of the output in (a) increases, although not shown, and a low-frequency component having an opposite phase appears in the waveform in (c).

The low frequency component generated in (c) is synchronously detected by the synchronous detection circuit 215, and the resulting DC voltage is fed back to the MZ optical modulator 24 to optimally control the bias voltage. Synchronous detection circuit 21
FIG. 6 shows output waveforms of the LPF 27 and the LPF 27.

As shown in FIG. 6D, the synchronous detection circuit 2
The output of the phase shifter 214 and the differential amplifier 213 is supplied to 15. The synchronous detection circuit 215 outputs a product of the input waveforms by, for example, a multiplier, and outputs a waveform as shown in FIG. This is LPF27
, An output as shown in (f) is obtained.
That is, a DC voltage corresponding to the degree of the drift is obtained. By feeding this back to the MZ optical modulator 24, the bias voltage can be optimally controlled. In the above configuration, the output of the LPF 27 and the preset initial setting voltage are added by the adder 28 and then fed back. This makes it possible to appropriately set the initial operating point of the MZ optical modulator 24. The bias voltage is fed back via the bias T29 so that the input data signal can be terminated.

As described above, in the present embodiment, the LED 22 for outputting an optical signal having a wavelength different from that of the LD 21 for outputting the carrier light is provided, and the output light of the LED 22 is intensity-modulated. The transmission characteristics are not degraded.
Further, since the output light wavelengths of the LD 21 and the LED 22 are set to be different from each other, there is no interference in the device. In addition, since the LED 22 is modulated at a low frequency, the drive circuit 25 may be of a sufficiently low speed as compared with the bit rate of the input data signal, thereby making it possible to easily configure an optical modulation device using an external modulation method. .

(Second Embodiment) FIG. 7 is a circuit block diagram showing a second embodiment of an optical modulation device using an external modulation system according to the present invention. In FIG. 7, the same parts as those in FIG. 2 are denoted by the same reference numerals, and overlapping description will be omitted.

In FIG. 7, the LD 21, the wavelength multiplexing coupler 23, the MZ optical modulator 24, the wavelength multiplexing coupler 210, and the portion related to the bias voltage control (dotted line in FIG. 2) shown in FIG. An LED 22, a drive circuit 25, and a low-frequency oscillator 26 are provided as pilot light sources common to each system.

The output lights of the LDs 221 to 22n are modulated and output according to the input data signals 71 to 7n, respectively. At this time, the MZ light modulators 241 to 242 are operated in the same manner as in the first embodiment.
The 4n bias point is optimally controlled. LED2
2, a drive circuit 25 and a low-frequency oscillator 26 are provided in common for each system. Thus, according to the present embodiment, there is an effect that the configuration can be simplified when the present invention is applied to a wavelength division multiplexing system.

The output light wavelengths of the LDs 221 to 22n may be different from each other or may be the same, but the output light wavelengths of the LEDs 22 must be different from each other as in the first embodiment. is there.

[0057]

As described in detail above, according to the present invention, M
Since the low frequency for monitoring the drift of the Z light modulator 24 is superimposed on the output of the LED 22, the waveform of the input data signal is not changed, and thus the transmission characteristics are not deteriorated. Further, since the drive circuit 25 and the PD 211 need only have a sufficiently low speed as compared with the bit rate of the input data signal, the configuration can be simplified. When the present invention is applied to a wavelength division multiplexing system, the LED 22, the driving circuit 25, and the low-frequency oscillator 26 can be provided in common, so that the configuration can be simplified.

[Brief description of the drawings]

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

FIG. 2 is a circuit block diagram showing a first embodiment of an optical modulation device using an external modulation method according to the present invention.

FIG. 3 is a diagram showing an applied voltage-transmittance characteristic of the MZ optical modulator 24 when no drift occurs.

FIG. 4 is a diagram showing output waveforms of a gain adjustment circuit 212, a phase shifter 214, and a differential amplifier 213.

FIG. 5 shows an MZ optical modulator 24 when a drift occurs.
FIG. 4 is a diagram showing an applied voltage-transmittance characteristic of FIG.

FIG. 6 is a diagram showing output waveforms of a synchronous detection circuit 215 and an LPF 27;

FIG. 7 is a circuit block diagram showing a second embodiment of the optical modulation device using the external modulation method according to the present invention.

FIG. 8 is a diagram showing input / output characteristics of an external modulator and input / output changes when the characteristics are normal.

FIG. 9 is a diagram showing a change in input / output when the input / output characteristics of the external modulator drift.

FIG. 10 is a circuit block diagram showing an example of a conventional control device for an external modulator.

FIG. 11 is a diagram showing an input / output change when an optimum bias voltage is applied to an external modulator in the control device of FIG. 10;

FIG. 12 is a diagram showing an input / output change when the input / output characteristics of the external modulator drift in the control device of FIG. 10;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 11 ... Light source 12 ... Light emitting element 13 ... Optical coupler 14 ... Optical modulator 15 ... Light emitting element drive part 16 ... Low frequency oscillator 17 ... Optical modulator drive part 18 ... Optical splitter 19 ... Photoelectric converter 110 ... Phase comparison part DESCRIPTION OF SYMBOLS 21 ... Semiconductor laser (LD) 22 ... Light emitting diode (LED) 23 ... Wavelength multiplexing coupler 24 ... Mach-Zehnder optical modulator (MZ optical modulator) 25 ... Drive circuit 26 ... Low frequency oscillator 27 ... Low pass filter (LPF) 28 ... Addition Device 29 Bias T 210 Wavelength multiplex coupler 211 Photodiode (PD) 212 Gain control circuit 213 Differential amplifier 214 Phase shifter 215 Synchronous detection circuit 221-22n Semiconductor laser (LD) 231-23n Wavelength multiplexing couplers 241 to 24n: Mach-Zehnder optical modulator (MZ optical modulator) 2101 to 210n: wavelength multiplexing coupler L 71-7n ... input data signal

──────────────────────────────────────────────────続 き Continued on front page (51) Int.Cl. ⁶ Identification code FI // H01S 3/18

Claims (6)

[Claims] An optical modulation device based on an external modulation method, comprising: a light source for generating a carrier optical signal; and an optical modulator for intensity-modulating and outputting the carrier optical signal in response to an electric signal from the outside. A light emitting element that outputs light in a wavelength band different from the light signal; a low frequency oscillator that generates a low frequency signal having a frequency sufficiently lower than the electric signal; and the light emitting element according to a low frequency signal output from the low frequency oscillator. A light emitting element driving unit that generates an optical signal intensity-modulated by the low frequency signal from the light emitting element, and couples the intensity-modulated optical signal output from the light emitting element to the carrier optical signal. An optical coupling unit that inputs the optical modulator; an optical branching unit that branches and outputs a light component in a wavelength band output by the light emitting element in an optical signal output from the optical modulator; A photoelectric converter that converts the optical signal branched by the branching unit into an electric signal, a frequency component of a low-frequency signal included in the electric signal output from the photoelectric converter, and a low-frequency signal output by the low-frequency oscillator. An optical modulation device based on an external modulation method, comprising: a phase comparison unit that compares the phases of the optical modulators; and an optical modulator driving unit that drives the optical modulator based on a comparison result of the phase comparison unit. 2. A plurality of light sources for generating a carrier light signal, and a plurality of light sources provided for each of the plurality of light sources and for intensity-modulating and outputting each of the carrier light signals according to an external electric signal. An optical modulation device based on an external modulation method including a modulator, a light-emitting element that outputs light of a wavelength band different from any of the respective wavelength bands of the carrier light signals generated by the plurality of light sources, and more than the electric signal. A low-frequency oscillator that outputs a low-frequency signal having a low frequency; and by driving the light-emitting element according to the low-frequency signal output from the low-frequency oscillator, the intensity is modulated by the low-frequency signal from the light-emitting element. A light-emitting element driving means for generating an optical signal; combining the intensity-modulated optical signal output from the light-emitting element with each of the carrier optical signals generated by each of the plurality of light sources; A plurality of light coupling means for inputting to each of the number of light modulators; and a light emitting element provided for each of the plurality of light sources, of the light signals output from each of the plurality of light modulators. A plurality of optical branching means for branching and outputting the optical component of the wavelength band output by the plurality of optical branching means provided for each of the plurality of optical branching means and converting each of the branched optical signals into an electric signal A photoelectric converter, provided for each of the plurality of photoelectric converters, for comparing the phase of the frequency component of the low-frequency signal included in the output electric signal with the phase of the low-frequency signal output by the low-frequency oscillator; A plurality of phase comparing means, provided for each of the plurality of optical modulators, and a plurality of optical modulator driving means for driving the optical modulator based on a comparison result of each of the plurality of phase comparing means; External modulation method characterized by having Light modulation device by the formula. 3. The optical modulator driving means according to claim 1, wherein said optical modulator driving means changes a value of a bias voltage applied to said optical modulator based on a comparison result of said phase comparing means. 3. An optical modulation device based on the external modulation method according to any one of 2. 4. An optical modulator driving means for changing a shift amount of an electric signal input to the optical modulator in an intensity direction based on a comparison result of the phase comparing means. An optical modulation device using an external modulation method according to claim 1. 5. An optical modulation apparatus according to claim 1, wherein said optical coupling means and said optical branching means are wavelength division multiplexing couplers. 6. An optical modulator according to claim 1, wherein said optical modulator is a Mach-Zehnder type optical modulator.