JP2001244896A - Optical transmission device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a bias voltage applied to an external optical modulator precisely follow the optimum bias variation caused by the DC drift as to an optical transmission device equipped with the external optical modulator which modules the carrier light from a light source with an electric signal. SOLUTION: A bias voltage control circuit 150 controls the bias voltage that a bias application unit 105 applies to the external optical modulator 110 so that the quantity of secondary distortion that is generated at the light signal outputted from the external modulator 110 becomes minimum.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter, and more particularly, to an optical transmitter using an external optical modulator, wherein a bias voltage is applied to the external optical modulator.
The present invention relates to an optical transmission device capable of performing bias voltage control for causing the applied bias voltage to follow an optimum bias voltage change due to a DC drift.

[0002]

2. Description of the Related Art In an optical transmitting apparatus used in a conventional optical communication system, a current injected into a laser diode constituting a light source is directly modulated by an input signal, so that the optical signal modulated by the input signal is converted. The output modulation method was adopted. However, in this modulation method, since the current injected into the laser diode changes, a phenomenon occurs in which the oscillation wavelength changes due to the chirp characteristics of the laser diode. When an optical signal output from such a laser diode is transmitted over a long distance, there is a problem that the waveform of the optical signal is deteriorated due to the influence of chromatic dispersion in the optical fiber, and signal characteristics are deteriorated.

For future mobile communications, the use of a high frequency (particularly, a millimeter wave band), which can easily secure a band, has been studied in order to greatly increase the transmission rate. When a coaxial cable is used as a transmission line, the loss in the transmission line is very large.
Amplifiers are required at 0 m intervals. When actually constructing a system, this is a problem in terms of cost and reliability.
From such a viewpoint, it is essential to use an optical fiber having a characteristic of a small loss for signal transmission in the millimeter wave band. However, currently commercially available laser diodes have a frequency response characteristic of about 10 GHz and do not respond to very high frequencies such as a millimeter wave band, so that it is impossible to directly modulate these laser diodes. Was.

Therefore, when performing long-distance transmission or transmitting a signal with a high frequency such as a millimeter wave, chirping does not occur in principle, and a Mach-Zehnder type that can respond to a signal with a high frequency is used. It has been proposed to use an optical transmission device with an external optical modulator.

FIG. 11 shows a configuration of a Mach-Zehnder type external optical modulator (hereinafter referred to as an MZ type optical modulator). The carrier light output from the light source is input to the MZ type optical modulator, and is branched toward two optical waveguides. When an electric field is generated by applying a voltage to an electrode provided on a crystal substrate, the refractive index in the waveguide changes, and as a result, the phase of light propagating in the waveguide changes. FIG. 11 shows a configuration in which only light passing through one optical waveguide is subjected to phase modulation. Light from each optical waveguide is multiplexed with each other, and an optical signal is output from the MZ type optical modulator. The optical electric field of the optical signal thus output is represented by the following equation.

[0006]

(Equation 1)

However,

(Equation 2) And V indicates a bias voltage, m indicates a phase modulation degree, and ωf indicates an angular frequency of an analog signal. Using this optical electric field, the optical signal power output from the MZ optical modulator is given by the following equation.

[0008]

(Equation 3)

FIG. 12 shows the relationship between the bias voltage and the light output at this time. In FIG. 12, the horizontal axis represents the bias voltage, and the vertical axis represents the output power of the optical signal. in this way,
The power of the optical signal output from the MZ optical modulator is MZ
5 shows a sine wave characteristic with respect to a bias voltage applied to the optical modulator.

However, in the MZ type optical modulator, the above-mentioned phenomenon occurs in which the relationship between the bias voltage and the optical output fluctuates from the initial state due to various conditions such as aging and temperature change. This phenomenon is called DC drift. This DC drift phenomenon is shown in FIG.

When the DC drift phenomenon as shown in FIG. 13 occurs, the phase state determined by the bias voltage fluctuates from the initial phase state, so that the power of the optical signal output from the MZ optical modulator changes. However, the signal characteristics deteriorate. Note that the bias voltage in the initial phase state is defined as the optimum bias voltage (Vb in the figure), and will be described as follows.

In a conventional optical transmission device using an external optical modulator, the power of an optical signal output from the external optical modulator (this is an optical signal The bias voltage applied to the external optical modulator is controlled based on the DC component of the electrical signal obtained by the electrical conversion, which is obtained by measuring the power. More specifically, in the initial state where the bias voltage is optimally set, the power of the output optical signal is measured in advance, and the value (reference value) is stored. Thereafter, the power of the output optical signal is monitored, and the applied bias voltage is controlled so that the power of the optical signal becomes equal to the reference value.

[0013]

However, the amount of change in the power of the output optical signal is very small with respect to the amount of change in the applied bias voltage from the optimum bias voltage. Therefore, it is extremely difficult to make the applied bias voltage follow the fluctuation of the optimum bias voltage with high accuracy based on the power of the output optical signal.

Here, the relationship between the variation of the applied bias voltage from the optimum bias voltage and the variation of the power of the output optical signal will be described using mathematical expressions. Using the result of the above equation (3), the power Pdc of the output optical signal can be expressed by the following equation (4).

(Equation 4)

The optimum bias voltage is generally when the initial phase state is π / 2. If the amount of phase change caused by the bias voltage fluctuating due to DC drift is φ, the phase state at that time is:

(Equation 5) It can be expressed as.

From the above equations (4) and (5), when the variation of the power of the output optical signal with respect to the variation of the applied bias voltage from the optimum bias voltage is obtained, the following relationship is obtained.

(Equation 6) However,

(Equation 7)

Since the fluctuation of the bias voltage due to the DC drift is very small instantaneously,

(Equation 8) Becomes

(Equation 9) Can be approximated.

Therefore, the amount of change of the optical signal power with respect to the amount of phase change, which is expressed by the above equation (6), is very small.
Therefore, it can be seen that there is only a slight change in the power of the output optical signal with respect to the change in the applied bias voltage from the optimum bias voltage. Therefore, as in the conventional case, when controlling the bias voltage based on the power of the output optical signal,
It is difficult to increase control accuracy.

When transmitting a baseband digital signal, it is not a problem that the control accuracy of the bias voltage is low. However, when transmitting an analog signal, especially a frequency-multiplexed multi-channel signal, a slight fluctuation in the bias voltage is required. Increases the second-order distortion (second-order intermodulation distortion = IM2).

FIG. 14 shows the relationship between the bias voltage applied to the external optical modulator shown in FIG. 11 and the amount of secondary distortion (hereinafter referred to as the amount of distortion) generated in the optical signal output from the external optical modulator. FIG. As shown in FIG. 14, the distortion amount has a quadratic function characteristic with an optimum bias voltage (Vb in the figure) as a target axis with respect to the bias voltage. That is, the amount of distortion is
As the bias voltage deviates from the optimum bias voltage, it increases quadratically. Therefore, for a given amount of bias voltage fluctuation, if the bias voltage is near the optimum bias voltage, the distortion amount does not change much, but as the deviation from the optimum bias voltage increases, the distortion for the same amount of bias fluctuation changes. The change in the amount is significant.

Hereinafter, the relationship between the bias voltage and the amount of distortion will be described using mathematical expressions. When two analog signals are input to the external optical modulator, the power of the output light from the external optical modulator can be expressed by the following equation. Here, ω1 and ω2 are the angular frequencies of the analog signal.

[0022]

(Equation 10)

In the above equation (10), terms including ω 1 and / or ω 2 represent analog signals, and the other terms are distortion components generated by nonlinearity of the external optical modulator. Where the analog signals and IM2 and IM3
The ratio to (third-order intermodulation distortion) is represented by the following equations.

[0024]

[Equation 11]

[0025]

(Equation 12)

As can be seen from the above equations (11) and (12), the value of IM2 depends on the bias voltage and the phase modulation degree.
IM3 depends on the degree of phase modulation. on the other hand,
As described above, in the initial state, the bias voltage applied to the external optical modulator is set to the optimum bias voltage Vb. That is, the relationship between the bias voltage and the phase is

(Equation 13) To be satisfied. At this time, as can be seen from the above equation (11),
IM2 does not occur.

However, after that, D
Due to the C drift characteristic, the relationship between the bias voltage and the phase does not satisfy the above expression (13). Therefore, the following calculation was performed for the phase shift due to the DC drift and the resulting distortion amount.

When the bias voltage V is the optimum bias voltage Vb
, Can be expressed by the following equation:

[Equation 14] Here, Vπ represents a half-wave voltage of the external optical modulator.

When the above equation (14) is expressed by using the phase shift Δx (%),

(Equation 15) Becomes

By substituting equation (15) into equation (11), IM2 and CSO (combined quadratic distortion) with respect to the deviation of the bias voltage phase from the optimum bias voltage phase were calculated. In this calculation, m = 0.04. Also,
In the conversion from IM2 to CSO, power addition was performed with the maximum number of composites being 60 at the time of 60 ch transmission. FIG. 15 shows the calculation result.

At the time of analog 60-channel transmission, CSO is-
A specification of 60 dBc or less is required, but in order to satisfy the requirement, as can be seen from FIG. 15, the deviation of the bias voltage phase from the optimum bias voltage phase must be suppressed to about ± 0.03% at the maximum. Must. It is difficult to perform such high-precision control by monitoring changes in the power of the optical signal.

Therefore, an object of the present invention is to provide an optical transmitting apparatus having an external optical modulator for modulating carrier light from a light source with an electric signal, wherein a bias voltage applied to the external optical modulator is controlled by a DC drift. , It is possible to accurately follow the fluctuation of the optimum bias due to the above-mentioned characteristics. As a result, even if DC drift occurs due to aging or temperature change, the power fluctuation and the increase in the amount of distortion are suppressed. An object of the present invention is to realize an optical transmission device capable of transmitting an optical signal.

[0033]

A first aspect of the present invention is an optical transmission apparatus for externally modulating a carrier light with an electric signal and transmitting the same, wherein the light source outputs the carrier light and the light source outputs the light. An external light modulator that modulates the carrier light with an electric signal, a bias voltage applicator that applies a bias voltage to the external light modulator, and a bias voltage that the bias voltage applicator applies to the external light modulator. Bias voltage control means for controlling based on the amount of secondary distortion generated in the optical signal output from the optical signal.

In the first aspect of the present invention, the bias voltage applied to the external optical modulator (hereinafter referred to as an applied bias voltage) is changed to the amount of secondary distortion (hereinafter referred to as distortion) generated in the optical signal output from the external optical modulator. ), The applied bias voltage can accurately follow the fluctuation of the optimum bias due to the DC drift. as a result,
Even if a DC drift occurs due to aging or temperature change, an optical transmission device capable of transmitting an optical signal having good characteristics with suppressed power fluctuation and increase in distortion is realized.

Here, the amount of change in the distortion with respect to the amount of change in the bias voltage will be described using mathematical expressions. Second-order distortion (I
M2) is given by the above equation (11), and its exact value is

(Equation 16) Becomes

By partially differentiating the above equation (16),
The following equation (17), that is, the fluctuation amount of IM2 with respect to the fluctuation amount of the bias voltage can be obtained.

[0037]

[Equation 17]

The amount of change in the power of the optical signal with respect to the amount of change in the bias voltage and the amount of I.sub.
Comparing with the variation of M2, the variation of the optical power is almost 0 as shown in the above equation (6), while the variation of IM2 is
Becomes infinite as shown in the above equation (17).
For this reason, controlling the bias voltage by monitoring the amount of distortion generated in the optical signal enables higher-precision control than controlling the bias voltage by monitoring the power of the optical signal.

According to a second aspect of the present invention, in the first aspect, the bias voltage control means includes an optical splitter for bifurcating the optical signal output from the external optical modulator, and one optical signal output from the optical splitter. A light-to-electricity converter that converts a light into an electric signal, extracts a component within a specific band from the electric signal output by the light-to-electrical converter, and measures the level of the component, so that the light output from the external light modulator is output. Controls the amount of secondary distortion that occurs in the signal and the bias voltage applied by the bias voltage applicator to the external optical modulator so that the amount of distortion detected by the distortion detector is minimized. Bias voltage control circuit.

In the second aspect, the applied bias voltage is controlled so that the amount of distortion is minimized.

In a third aspect based on the second aspect, the bias voltage control circuit increases or decreases the bias voltage applied to the external optical modulator by the bias voltage applicator, and controls the distortion detected by the distortion amount detector. Determining whether the amount has increased or decreased before and after the increase or decrease of the bias voltage, and determining, based on the determination result, whether to control the bias voltage to increase or decrease in the next direction. It is characterized by.

Here, the amount of distortion has a quadratic function characteristic with the optimum bias voltage as a target axis with respect to the applied bias voltage (see FIG. 14). Therefore, when the optimum bias voltage fluctuates due to the DC drift, it is necessary to determine whether to control the applied bias voltage to increase or decrease the applied bias voltage.

Therefore, in the third aspect of the invention, first, the applied bias voltage is increased or decreased, and before and after that, it is determined whether the distortion amount has increased or decreased. Then, based on the result of the determination, it is determined whether the control is to be performed in the direction of increasing or decreasing the bias voltage next. Specifically, if the amount of distortion is reduced by increasing (or decreasing) the applied bias voltage, control is performed in the direction of increasing (or decreasing) the bias voltage next, and if the amount of distortion increases, Next, control is performed in a direction to decrease (or increase) the bias voltage.

Alternatively, as in the following fourth invention, the power of the optical signal output from the external optical modulator is further detected,
Based on the optical power, it may be determined whether to control the applied bias voltage to increase or decrease the applied bias voltage. Specifically, it is determined that the control is to be performed in a direction to increase the applied bias voltage when the optical power increases, and to a direction to decrease the applied bias voltage when the optical power decreases.

Alternatively, when the external optical modulator has two output ports (first and second ports) as in a fifth aspect of the present invention, an optical signal output from the second port is generated. The amount of secondary distortion may be detected, and the applied bias voltage may be controlled so that the amount of distortion matches the reference value.
The reference value is preferably the first value, as in the sixth invention described below.
When the bias voltage (the optimal bias voltage on the first port side) that minimizes the amount of secondary distortion generated in the optical signal output from the port is applied to the external optical modulator (that is, in the initial state) ), The amount of distortion detected by the distortion amount detector (the amount of distortion on the second port side).

In a fourth aspect based on the second aspect, the bias voltage control means measures the power of the electrical signal output from the opto-electrical converter to thereby determine the power of the optical signal output from the external optical modulator. The bias voltage control circuit controls the bias voltage applying unit to increase the bias voltage applied to the external optical modulator based on the optical power detected by the optical power detector. Or control to reduce the direction.

In a fifth aspect based on the first aspect, the external optical modulator comprises two waveguides for guiding the carrier light from the light source into two branches, and mutually couples the optical signals guided by the respective waveguides. And a first and a second port for outputting two optical signals output from the optical coupler, and the bias voltage control means converts the optical signal output from the second port into an electric signal. An optical-electrical converter to be converted, a component within a specific band is extracted from an electric signal output from the optical-electrical converter, and the level of the component is measured, thereby obtaining an optical signal output from the second port. The amount of distortion detected by the distortion detector and the bias voltage applied by the bias voltage applicator to the external optical modulator match the amount of distortion detected by the distortion detector with a previously stored reference value. Bias voltage control Including the road.

In the fourth and fifth aspects of the present invention, whether the applied bias voltage should be increased or decreased can be immediately determined. Therefore, it is possible to accurately and promptly follow the fluctuation of the optimum bias due to the DC drift. It becomes possible.

In the fourth aspect, whether to increase or decrease the applied bias voltage is determined based on the increase or decrease of the optical power. On the other hand, in the fifth invention, the second
Is determined based on the amount of distortion on the port side of the second port, so that it is only necessary to monitor the amount of distortion on the second port side, and the configuration of the device is simpler than in the fourth invention (the bias voltage control process is simpler). Is easier).

In a sixth aspect based on the fifth aspect, the bias voltage control circuit is configured to generate a bias voltage that minimizes the amount of secondary distortion generated in the optical signal output from the first port. The distortion amount detected by the distortion amount detector is stored as a reference value when the applying unit applies the voltage to the external optical modulator.

A seventh invention is a method for controlling a bias voltage applied to an external optical modulator in an optical transmission device including an external optical modulator for modulating carrier light from a light source with an electric signal, A distortion amount measuring step of measuring the amount of secondary distortion generated in the optical signal output from the external optical modulator, and a bias voltage applied to the external optical modulator is adjusted so that the distortion amount measured in the distortion amount measuring step is minimized. And a bias voltage control step of controlling the bias voltage.

In an eighth aspect based on the seventh aspect, the bias voltage control step is detected by a bias voltage increase / decrease step for increasing or decreasing a bias voltage applied to the external optical modulator, and a distortion amount detection step. It is determined whether the amount of distortion has increased or decreased before and after the step of increasing / decreasing the bias voltage. Based on the result of the determination, it is determined whether to control the bias voltage to be increased next or to be decreased. Including the step of:

In a ninth aspect based on the seventh aspect, the method further comprises an optical power detection step of detecting the power of the optical signal output from the external optical modulator, and the bias voltage control step is performed by the optical power detection step. Determining whether to control the bias voltage applied to the external optical modulator to increase or decrease the bias voltage based on the obtained optical power.

In a tenth aspect based on the seventh aspect, the external optical modulator comprises two waveguides for guiding the carrier light from the light source in two branches, and mutually couples the optical signals guided by the respective waveguides. And a first and a second port for outputting two optical signals output from the optical coupler, and the bias voltage control step includes the step of controlling a secondary voltage generated in the optical signal output from the second port. A distortion amount detecting step for detecting the amount of distortion, and a bias voltage for controlling the bias voltage applied to the external optical modulator so that the amount of distortion detected in the distortion amount detecting step matches a previously stored reference value. A voltage control step is included.

According to an eleventh aspect, in the tenth aspect,
The bias voltage control step includes the first step of minimizing the amount of secondary distortion generated in the optical signal output from the first port.
In the initial state where the optimum bias voltage on the port side is applied to the external optical modulator, a step of storing the distortion amount detected in the distortion amount detecting step as a reference value is included.

[0056]

Embodiments of the present invention will be described below in detail with reference to the drawings. (First Embodiment) FIG. 1 is a block diagram showing a configuration of an optical transmission apparatus according to a first embodiment of the present invention. In FIG. 1, the optical transmission device includes a light source 100, a bias voltage applicator 105, an external optical modulator 110, and an optical splitter 120.
And a photoelectric converter 130 and a distortion detector 140. The bias voltage applicator 105 includes a bias voltage control circuit 150.

The light source 100 outputs carrier light. The bias voltage applicator 105 applies a bias voltage. The external light modulator 110 receives the carrier light output from the light source 100 and an electric signal in a state where the bias voltage is applied by the bias voltage applicator 105 and responds to the biased electric signal to the carrier light. And modulates the intensity to output an optical signal. Here, the electric signal input to the external optical modulator 110 is a signal obtained by frequency-multiplexing a plurality of analog signals.

The optical splitter 120 splits the optical signal output from the external optical modulator 110. One optical signal output from the optical splitter 120 is transmitted to a receiving device (not shown) through an optical transmission line. The other optical signal output from the optical splitter 120 is input to the photoelectric converter 130,
It is converted into an electric signal by the photoelectric converter 130.

The electric signal output from the photoelectric converter 130 is given to the distortion detector 140, and the distortion detector 140
Is the amount of secondary distortion contained in the optical signal output from the external optical modulator 110 based on the electric signal (hereinafter, the amount of distortion)
Is detected. The amount of distortion detected by the distortion amount detector 140 is supplied to the bias voltage control circuit 150, and the bias voltage control circuit 150 causes the bias voltage application unit 105 to transmit the signal to the external optical modulator 110 so that the amount of distortion is minimized. The applied bias voltage (hereinafter, applied bias voltage) is controlled.

The distortion amount detector 140 includes, for example, a filter for extracting only a component within a desired frequency band from an electric signal, a circuit for measuring the level (voltage) of the component, and an optical power based on the level. Is calculated. The distortion amount detector 140 typically extracts components within one band in which second-order distortion occurs. If the frequency of each analog signal included in the electric signal is known, the frequency of the generated secondary distortion can be calculated in advance. Therefore, a filter that transmits only components near that frequency may be selected.

As shown in FIG. 2, when several second-order distortions (10 to 12) having different frequencies are generated, a filter having a transmission characteristic indicated by reference numeral 13 is used to make the highest level. Is preferably extracted. In this case, the maximum level of the secondary distortion 10 is
The distortion amount is output from the distortion amount detector 140 as the distortion amount.

Alternatively, similarly to the case of the second-order distortion 10, the levels of the second-order distortions 11 and 12 may be measured, and the measured plural levels may be added to each other. in this case,
The total level of the plurality of secondary distortions 10 to 12 is output from the distortion amount detector 140 as the distortion amount.

The operation of the optical transmission device configured as described above will be described below. Light output from the light source 100 is input to the external light modulator 110. In general, light input to the external optical modulator 110 is limited to only one of the TE mode and the TM mode by adjusting the polarization plane. This adjustment is performed by inserting a polarization controller between the light source 100 and the external light modulator 110,
There are a method of adjusting the polarization plane by a polarization controller, a method of using a polarization maintaining fiber as a fiber between the light source 100 and the external optical modulator 110, and the like. FIG. 1 does not specify a polarization controller or a polarization maintaining fiber that adjusts the polarization plane.

The external optical modulator 110, to which a bias voltage has been applied by the bias voltage applicator 105, modulates the intensity of input light in accordance with an input electric signal, and outputs an optical signal. This output optical signal is split by the optical splitter 120, and one of the split optical signals is output from the photoelectric converter 13.
At 0, it is converted to an electrical signal. The electric signal output from the photoelectric converter 130 is input to the distortion detector 140.

The distortion amount detector 140 has a function of extracting a component of a desired band from an AC component included in the electric signal and measuring the level thereof, and performs a predetermined calculation based on the measured level. Thus, the amount of distortion generated in the optical signal output from the external optical modulator 110 is detected. The desired band is not particularly limited as long as it is a band in which second-order distortion occurs, but is desirably set to a frequency band in which a large amount of distortion occurs.

The signal output from the distortion detector 140 is
Bias voltage control circuit 1 in bias voltage applicator 105
50 is input. The bias voltage control circuit 150 controls the bias voltage applicator 105 so that the amount of distortion detected by the distortion amount detector 140 is minimized.
The bias voltage (applied bias voltage) to be applied is controlled. With such a configuration, the control accuracy of the bias voltage can be improved by a relatively simple control method, and the applied bias voltage is always maintained at a bias voltage (optimal bias voltage) that minimizes the amount of distortion. It becomes possible.

Here, the bias voltage control processing performed by the bias voltage control circuit 150 will be described in detail. As in the conventional case, the bias voltage applied by the bias voltage applicator 105 to the external optical modulator 110 (applied bias voltage) in response to the fluctuation of the optimum bias voltage due to the DC drift
Is the process of following. The difference from the conventional technique is that the applied bias voltage is controlled so that the amount of distortion generated in the optical signal is minimized, not by controlling the applied bias voltage so that the power (that is, the DC component) of the optical signal is constant. The only difference is that the applied bias voltage follows the fluctuation of the optimum bias voltage.

FIG. 3 shows the bias voltage control circuit 15 of FIG.
FIG. 5 is a diagram visually illustrating a process performed by No. 0; FIG.
5 is a flowchart describing processing performed by the bias voltage control circuit 150 of FIG. The relationship between the bias voltage and the amount of distortion shown in FIG. 3 is the same as that shown in FIG. 14 (see the column of the prior art).

3 and 4, the bias voltage control circuit 150 is supplied with the distortion amount m0 detected by the distortion amount detector 140. First, the bias voltage control circuit 150 increases the applied bias voltage by a predetermined voltage (step S101). Next, the distortion amount (m1) after the applied bias is increased is given from the distortion amount detector 140, and accordingly, the bias voltage control circuit 150 compares the distortion amounts (m0 and m1) before and after the applied bias increase with each other. It is determined whether or not the change in the amount of distortion is greater than a predetermined threshold (step S102). If the determination result is negative, the process proceeds to step S107.

If the determination result in step S102 is affirmative, the bias voltage control circuit 150 determines whether the amount of distortion has decreased (step S103). When the determination result is affirmative, steps S101 to S103 are repeatedly performed. That is, the bias voltage control circuit 150 increases the applied bias voltage again, and then the distortion amount detector 1
The amount of distortion (m
2) is compared with the distortion amount (m1) before the increase, and it is determined whether or not the change in the distortion amount is larger than a threshold value. If the determination result is negative, the process proceeds to step S107. If the determination result is affirmative, the determination in step S103 (determination whether the amount of distortion has decreased) is performed. If the determination result is positive, the process is repeated. , Steps S101 to S103 are repeatedly performed.

If the decision result in the step S103 is negative, the bias voltage control circuit 150 reduces the applied bias voltage by a predetermined voltage (step S10).
4) Then, it is determined whether or not the change in the distortion amount (change from m1 'to m0 in the figure) is larger than a threshold value (step S105). And if the judgment result is negative,
Proceed to step S107.

If the result of the determination in step S105 is affirmative, it is determined whether or not the amount of distortion has decreased (step S1).
06). If the determination is positive, step S104
To S106 are repeatedly performed. That is, the bias voltage control circuit 150 reduces the applied bias voltage again, and then changes the distortion amount (m1) after the applied bias voltage reduction supplied from the distortion amount detector 140 to the distortion amount (m0) before the reduction.
Then, it is determined whether or not the change in the amount of distortion is larger than a threshold value. If the determination result is negative, step S1
07, but if affirmative, the determination of step S106 (determination of whether or not the amount of distortion has been reduced) is performed. If the determination result is positive, steps S104 to S10 are performed again.
6 will be repeated.

In step S107, the bias voltage control circuit 150 holds the applied bias voltage at the current value for a certain period. Thereafter, it is determined whether or not to continue the processing. If the determination result is positive, step S10
Returning to 1, the same processing as described above is repeatedly performed, and if negative, the processing is terminated.

By performing the above processing by the bias voltage control circuit 150, even if aging or temperature changes occur, the bias voltage applying unit 105 changes the bias voltage applied to the external optical modulator 110 to DC. It is possible to follow the fluctuation of the optimum bias voltage due to the drift.

In the above processing, the distortion amounts before and after the bias voltage is increased (or decreased) are compared with each other to determine whether or not the change in the distortion amount is larger than the threshold value.
If the change in strain is greater than the threshold, increase (or decrease) the bias voltage until the change in strain is below the threshold
I let it. In addition to such processing, for example, the following processing may be performed.

In the initial state in which the applied bias voltage is equal to the optimum bias voltage, the amount of distortion given from the distortion detector 140 is measured in advance, and the value (reference value) is stored in the bias voltage control circuit 150. . Thereafter, the bias voltage control circuit 150 controls the applied bias voltage so that the distortion amount given from the distortion amount detector 140 becomes equal to the reference value. That is, the bias voltage control circuit 150
Compares the distortion amount given from the distortion amount detector 140 with a reference value, and increases (or decreases) the applied bias voltage when the distortion amount becomes larger than the reference value.

(Second Embodiment) FIG. 5 shows a second embodiment of the present invention.
It is a block diagram which shows the structure of the optical transmission apparatus which concerns on embodiment. In FIG. 5, the optical transmission device includes a light source 200, a bias voltage applicator 205, an external optical modulator 210, an optical splitter 220, a photoelectric converter 230, and a distortion detector 2
40 and an optical power detector 250. The bias voltage applicator 205 includes a bias voltage control circuit 260. The light source 200, the bias voltage applicator 205, the external light modulator 210, the optical splitter 220, the photoelectric converter 230, and the distortion detector 240 are the same as those in FIG.

The electric signal output from the photoelectric converter 230 is supplied to the optical power detector 250, and the optical power detector 250 measures the power of the electric signal (that is, the DC component included in the signal). Thus, the power of the optical signal output from the external optical modulator 210 is detected.

The amount of distortion detected by the amount of distortion detector 240 is given to the bias voltage control circuit 260. The bias voltage control circuit 260 controls the bias voltage applying unit 205 to control the external light modulation so that the amount of distortion is minimized. The bias voltage applied to the device 210 is controlled. At that time, the optical power detector 2
The optical power detected by 50 is supplied to the bias voltage control circuit 260, and the bias voltage control circuit 260
It is determined whether to increase or decrease the applied bias voltage based on the light power.

The operation of the optical transmission device configured as described above will be described below. The light output from the light source 200 is input to the external light modulator 210. As described in the first embodiment, the polarization plane is generally adjusted so that the optical signal input to the external optical modulator 210 is limited to only one of the TE mode and the TM mode. It is. FIG. 5 does not specify a polarization controller or a polarization maintaining fiber for adjusting the polarization plane.

The external light modulator 210, to which a bias voltage has been applied by the bias voltage applicator 205, modulates the intensity of input light according to an input electric signal and outputs an optical signal. This output optical signal is split by the optical splitter 220, and one of the split optical signals is output by the photoelectric converter 23.
At 0, it is converted to an electrical signal. The electric signal output from the photoelectric converter 230 includes a DC component and an AC component, and is input to the optical power detector 250 and the distortion amount detector 240. The optical power detector 250 is provided for the photoelectric converter 23.
It has a function of measuring the power (DC component) of the electric signal output from 0, and detects the power of the optical signal output from the external optical modulator 210 by performing a predetermined operation based on the measured power. I do.

On the other hand, the distortion amount detector 240 has a function of extracting a component of a desired band from an AC component included in the electric signal and measuring the level thereof. Is performed, the external light modulator 21
The amount of distortion generated in the optical signal output from 0 is detected. The desired band is not particularly limited as long as it is a band in which second-order distortion occurs, but is desirably set to a frequency band in which a large amount of distortion occurs.

The signal output from each detector is
It is input to the bias voltage control circuit 260. The bias voltage control circuit 260 adjusts the bias voltage applied by the bias voltage applicator 205 to the external optical modulator 210 so that the amount of distortion generated in the optical signal output from the external optical modulator 210 is minimized in the following procedure. Is controlled so as to be equal to a certain voltage (optimum bias voltage at that time).

The bias voltage control circuit 260 compares the signal input from the optical power detector 250 with a reference signal (that is, a signal indicating the optical power in the initial state where the applied bias voltage is set to the optimum bias voltage). Then, the increase or decrease of the applied bias voltage is determined according to the comparison result.

For example, if the relationship between the bias voltage applied to the external optical modulator 210 and the power of the optical signal output therefrom has the characteristic shown in FIG. When the signal level provided from the power detector 250 is higher than the reference signal, control is performed to increase the applied bias voltage. On the other hand, when the signal level given from the optical power detector 250 is lower than the reference signal, control is performed so as to reduce the applied bias voltage. As described above, the bias voltage control circuit 260 first controls or decreases the applied bias voltage in the direction to increase based on the signal provided from the optical power detector 250, that is, the output optical power of the external optical modulator 210. Decide whether to control the orientation. Then, the distortion amount detector 24
Fine adjustment of the applied bias voltage is performed so that the signal value given from 0 (the amount of distortion generated in the output optical signal of the external optical modulator 210) is minimized.

That is, the bias voltage applied to the external optical modulator 210 and the amount of distortion generated in the optical signal output from the external optical modulator 210 are as shown in FIG. Since it has a symmetrical relationship with the optimal bias voltage, just observing the variation in the amount of distortion,
The increase or decrease in the bias voltage cannot be determined immediately.
Therefore, for example, complicated processing as described in the first embodiment is required. However, by adopting a configuration as in the present embodiment, control accuracy can be improved by relatively simple processing. , The applied bias voltage is always
It is possible to maintain a bias voltage (optimal bias voltage) that minimizes the amount of distortion.

Here, the bias voltage control processing performed by the bias voltage control circuit 260 will be described in detail. This process is a process in which the applied bias voltage follows the fluctuation of the optimum bias voltage by controlling the applied bias voltage so as to minimize the amount of distortion given from the distortion amount detector 240, as in the first embodiment. is there. The difference from the first embodiment is that, before starting the bias voltage control, the external optical modulator determines whether to control the applied bias voltage to increase or decrease the applied bias voltage. The only difference is that it is determined based on the power of the optical signal output from 210. Specifically, the current optical power is compared with a reference value. If the former is larger than the latter, the applied bias voltage is changed in the increasing direction, and if the former is smaller, the applied bias voltage is changed in the decreasing direction.

On the other hand, in the first embodiment, the applied bias voltage is actually changed in the increasing direction, the increase or decrease of the distortion amount due to the change is detected, and the change direction is determined based on the result. (See FIG. 3). If the amount of distortion is reduced as a result of changing the applied bias voltage in the increasing direction, the change in the increasing direction may be continued, but if the amount of distortion is increased, the applied bias voltage is optimized. This means that the bias voltage has been changed in a direction away from the bias voltage. At that point, the direction in which the applied bias voltage is changed must be reversed in the decreasing direction (the process indicated by the dotted arrow in FIG. 3). Therefore, it sometimes takes time to follow up.

FIG. 6 shows the bias voltage control circuit 26 of FIG.
10 is a flowchart describing the processing performed by the C.O. In addition,
FIG. 3 visually shows the processing performed by the bias voltage control circuit 150 of FIG. 1 (see the first embodiment).
However, in the processing performed by the bias voltage control circuit 260 in FIG. 5, in the control processing indicated by a series of arrows in FIG.
The processing at the dotted arrow is unnecessary. That is, in the first embodiment, the applied bias voltage is changed in the increasing direction, and as a result, if the amount of distortion increases, the applied bias voltage is changed in the direction at which the applied bias voltage is changed, and the applied bias voltage is reduced. Although the processing for changing the direction is performed, in the present embodiment, such useless processing is eliminated.

In FIG. 6, bias voltage control circuit 26
To 0, the distortion amount m0 detected by the distortion amount detector 240 and the optical power detected by the optical power detection circuit are given. The bias voltage control circuit 260 first determines whether the optical power has changed by a predetermined threshold value or more by comparing the applied optical power with a reference value (step S201). If the result of the determination is negative, the process proceeds to step S207.

If the result of the determination in step S201 is affirmative, the bias voltage control circuit 260 determines whether the optical power has increased (step S202). If the determination result is affirmative, the bias voltage control circuit 260 increases the applied bias voltage by a predetermined voltage (step S203). Next, the distortion amount (m1) after the applied bias is increased from the distortion amount detector 240, and the bias voltage control circuit 260 compares the distortion amounts (m0 and m1) before and after the applied bias increase with each other. Thus, it is determined whether or not the change in the amount of distortion is greater than a predetermined threshold (step S204). If the determination result is negative, the process proceeds to step S207.

When the result of the determination in step S204 is affirmative, steps S203 and S204 are repeatedly performed. That is, the bias voltage control circuit 260 increases the applied bias voltage again, and then the distortion amount detector 240
(M2) after applied bias voltage is increased
Is compared with the distortion amount (m1) before the increase, and it is determined whether or not the change in the distortion amount is larger than a threshold value. When the determination result is negative, the process proceeds to step S207, but when the determination result is positive, steps S203 and S204 are repeatedly performed again.

If the decision result in the step S202 is negative, the bias voltage control circuit 260 reduces the applied bias voltage by a predetermined voltage (step S2).
05). Next, the distortion amount (m1) after the applied bias is reduced is given from the distortion amount detector 240, and the bias voltage control circuit 260 responds accordingly to the distortion amount (m1) before and after the applied bias is reduced.
By comparing 0 and m1) with each other, it is determined whether or not the change in the amount of distortion is greater than a predetermined threshold (step S206). If the determination result is negative, the process proceeds to step S207.

If the result of the determination in step S206 is affirmative, steps S205 and S206 are repeatedly performed. That is, the bias voltage control circuit 260 reduces the applied bias voltage again, and then the distortion amount detector 240
(M2) after applied bias voltage is reduced
Is compared with the distortion amount (m1) before reduction to determine whether or not the change in the distortion amount is larger than a threshold value. If the determination result is negative, the process proceeds to step S207. If the determination result is affirmative, steps S205 and S206 are repeated.

In step S207, the bias voltage control circuit 260 holds the applied bias voltage at the current value for a certain period. Thereafter, it is determined whether or not to continue the process (step S208). If the determination result is affirmative, the process returns to step S201 and the same process as above is repeatedly performed. To end.

[0096] The bias voltage control circuit 260 performs the above-described processing, whereby the bias voltage
Changes the bias voltage applied to the external optical modulator 210 to DC
It is possible to follow the fluctuation of the optimum bias voltage due to the drift. Also, when the optimal bias fluctuates,
Following can be started more quickly than in the first embodiment.

(Third Embodiment) FIG. 7 shows a third embodiment of the present invention.
It is a block diagram which shows the structure of the optical transmission apparatus which concerns on embodiment. 7, the optical transmission device includes a light source 300, a bias voltage applicator 305, an external optical modulator 310, a photoelectric converter 320, and a distortion detector 330. The bias voltage applicator 305 includes a bias voltage control circuit 340. Light source 300, bias voltage applicator 30
5. The photoelectric converter 320 and the distortion detector 330 are the same as those in FIG.

The external light modulator 310, similarly to the external light modulator 110 of FIG. 1, receives the carrier light output from the light source 300, Is input, the intensity of the carrier light is modulated according to the electric signal, and an optical signal is output (the electric signal input to the external optical modulator 310 is a signal obtained by frequency-multiplexing a plurality of analog signals). External light modulator 31
0 differs from the external optical modulator 110 of FIG. 1 in that it has first and second two output ports. The optical signal output from the first port is transmitted to a receiving device (not shown) through an optical transmission line. The optical signal output from the second port is input to the photoelectric converter 320, and is converted into an electric signal by the photoelectric converter 320.

FIG. 8 is a diagram showing the configuration of the external optical modulator 310 of FIG. FIG. 8 shows an external optical modulator that further includes an X-branch directional optical coupler 20 in the external optical modulator of FIG. 11 (see the related art). In FIG. 8, the optical signals from the two waveguides are input to the X-branch directional optical coupler 20, where they are mutually coupled and then output through the first and second ports.
At this time, the optical signal output from the first port and the optical signal output from the second port are generally out of phase with each other in the bias voltage-distortion characteristic.

FIG. 9 shows a first example of the external optical modulator 310 of FIG.
And for each optical signal output from the second port,
FIG. 4 is a diagram illustrating a relationship between a bias voltage and a distortion amount in an initial state. As shown in FIG. 9, in the initial state, the optical signal output from the first port and the optical signal output from the second port have their respective optimum bias voltages Vo.
1, Vo2 (Vo1 <Vo2) are connected to a certain voltage (=
ΔVo).

Thereafter, these two optimum bias voltages V
o1 and Vo2 fluctuate according to a DC drift due to a temperature change or the like. At this time, the difference between the two voltages (= Vo2-Vo)
1) is kept at the same constant voltage (= ΔVo) as in the initial state. At this time, even if the bias voltage is controlled so that the amount of distortion generated in the optical signal output from the port 2 is minimized, the amount of distortion generated in the optical signal output from the port 1 and traveling toward the receiving apparatus is not minimized.

In a state where the applied bias voltage is equal to the optimum bias voltage (= Vo1) on the first port side (that is, the initial state shown in FIG. 9), the amount of distortion generated in the optical signal output from the second port is reduced. D0, the applied bias voltage is changed to the optimum bias voltage Vo1 on the first port side.
In order to follow the fluctuation of the optical signal, the amount of distortion generated in the optical signal output from the second port may be monitored, and the applied bias voltage may be controlled so that the amount of distortion is maintained at D0. This is because the two optimum bias voltages Vo1 and Vo2 occur in the optical signal output from the second port because the difference between the two voltages (= Vo2−Vo1) fluctuates while maintaining a constant (= ΔVo) relationship. This is because if the amount of distortion is kept at D0, the amount of distortion occurring in the optical signal output from the first port is kept to a minimum.

Therefore, the bias voltage control circuit 340
The bias voltage applicator 3 is set so that the amount of distortion given from the distortion amount detector 330 (that is, the amount of distortion generated in the optical signal output from the second port of the external optical modulator 310) becomes D0.
05 controls the bias voltage applied to the external light modulator 310. Specifically, the bias voltage control circuit 340
The amount of distortion given from the distortion amount detector 330 is compared with D0. When the amount of distortion becomes larger than D0, the applied bias voltage is increased, and when the amount of distortion becomes smaller, the applied bias voltage is decreased.

The operation of the optical transmission device configured as described above will be described below. The external light modulator 310 has two output ports. Optical signals output from these first and second ports ideally have the same characteristics. However, in practice, generally, there is a difference in characteristics between the optical signal on the first port side and the optical signal on the second port side due to various factors.

For example, as shown in FIG. 9, the bias voltage-distortion characteristic is shifted between ports. For this reason,
The bias voltages that minimize the amount of distortion contained in the optical signal output from each port are different from each other Vo1 and Vo
It becomes 2. In the present embodiment, the bias voltage −
Utilizing the fact that the distortion characteristics are different, the increase / decrease of the bias voltage is determined.

That is, the optical signal output from the first port is transmitted to the receiving device side, and the optical signal output from the second port is used for controlling the bias voltage. The bias voltage (the optimal bias voltage on the first port side) that minimizes the amount of distortion generated in the optical signal output from the first port is:
The amount of distortion generated in the optical signal output from the second port coincides with the bias voltage at which the optical signal becomes D0, and the distortion is monotonically reduced as the bias voltage increases. Therefore, by detecting the amount of distortion generated in the optical signal output from the second port and comparing it with D0, it is possible to instantaneously determine whether to increase or decrease the bias voltage. Accordingly, the bias voltage can be quickly adjusted only by monitoring the amount of distortion.

Here, a case where an optical signal output from the first port is transmitted to the receiving device side and an optical signal output from the second port is used for bias voltage control will be described.
The operation of the optical transmission device will be described. Light output from the light source 300 is input to the external light modulator 310. As described in the first embodiment, the polarization plane is generally adjusted so that the optical signal input to the external optical modulator 310 is limited to only one of the TE mode and the TM mode. It is. FIG. 9 does not specify a polarization controller or a polarization maintaining fiber for adjusting the polarization plane.

The external light modulator 310 is applied with a bias voltage by the bias voltage applicator 305, modulates the intensity of the input light according to the input electric signal, and outputs light from the first and second ports. Output a signal. The optical signal output from the first port is transmitted to the receiving device through the optical transmission path.

[0109] The optical signal output from the second port is converted to an electrical signal by the photoelectric converter 320. The electric signal output from the photoelectric converter 320 is output to the distortion detector 330.
Is input to The distortion amount detector 330 has a function of extracting a component of a desired band from an AC component included in the electric signal and measuring the level thereof, and by performing a predetermined calculation based on the measured level, The amount of distortion generated in the optical signal output from the external optical modulator 310 is detected. The desired band is not particularly limited as long as it is a band in which second-order distortion occurs, but is desirably set to a frequency band in which a large amount of distortion occurs.

The signal (distortion amount on the second port side) output from the distortion amount detector 330 is input to the bias voltage control circuit 340 in the bias voltage applicator 305. The bias control circuit 340 controls the bias voltage applicator 305 so that the amount of distortion on the second port side becomes D0.
Control of the bias voltage (applied bias voltage) applied to 0 is performed.

That is, as shown in FIG. 9, when the applied bias voltage is controlled to a bias voltage at which the amount of distortion on the second port side is minimized, the amount of distortion on the first port side is not minimized. Therefore, in an initial state in which a bias voltage that minimizes the amount of distortion generated in the optical signal output from the first port is applied, the amount D0 of distortion generated in the optical signal output from the second port is measured in advance. Then, the value of D0 is stored. Thereafter, the bias voltage control circuit 340 controls the applied bias voltage so that the amount of distortion generated in the optical signal output from the second port is equal to the stored amount of distortion D0. By adopting such a configuration, it is possible to improve control accuracy by relatively simple processing, and to always apply an applied bias voltage to a bias that minimizes the amount of distortion generated in an optical signal transmitted to a receiving device. Voltage (optimal bias voltage).

Here, the above-described bias voltage control processing will be described in detail. This processing is performed by the distortion amount detector 330.
(Ie, the amount of distortion generated in the optical signal output from the second port) is D0 (D0 is the amount of distortion in the initial state where the applied bias voltage is equal to the optimum bias voltage Vo1 on the first port side). This is a process of controlling the applied bias voltage so that the applied bias voltage becomes equal to the amount of distortion applied from the detector 330, thereby causing the applied bias voltage to follow the fluctuation of the optimum bias voltage Vo2 on the first port side.

With this control, the distortion amount detector 330
It is possible to quickly determine whether to increase or decrease the applied bias voltage depending on whether the amount of distortion given by is larger or smaller than D0. That is, the bias voltage control circuit 340 compares the distortion amount given from the distortion amount detector 330 with D0, increases the applied bias voltage when the distortion amount becomes larger than D0, and increases the applied bias voltage when the distortion amount becomes smaller. , The applied bias voltage may be reduced.

On the other hand, in the first embodiment, the applied bias voltage is actually changed in the increasing direction, the increase or decrease in the amount of distortion due to the change is detected, and the change direction is determined based on the result. Was. For this reason, at the start of tracking, the applied bias voltage may be changed in a direction away from the optimum bias voltage, and tracking may take time.

FIG. 10 shows the bias voltage control circuit 3 of FIG.
4 is a flowchart describing a process performed by the control unit 40. FIG.
At 0, the bias voltage control circuit 340 first applies a bias voltage equal to the optimum bias voltage Vo1 on the first port side to the external optical modulator 310 via the bias voltage applicator 305 (step S301). And
At this time, the distortion amount D0 given from the distortion amount detector 330 is stored (step S302).

Thereafter, the amount of distortion detected by the distortion amount detector 330 is given to the bias voltage control circuit 340. The bias voltage control circuit 340 first determines whether the distortion amount has changed from D0 by a predetermined threshold value or more by comparing the applied distortion amount with the distortion amount D0 stored in step S302. (Step S303). If the result of the determination is negative, the process proceeds to step S307.

If the determination result in step S303 is affirmative, the bias voltage control circuit 340 determines whether or not the amount of distortion has increased (step S304). When the determination result is affirmative, the bias voltage control circuit 340 increases the applied bias voltage so that the distortion amount matches D0 while monitoring the distortion amount provided from the distortion amount detector 330 (step S305). ). Then, the process proceeds to step S307.

If the decision result in the step S304 is negative, the bias voltage control circuit 340 sets the distortion amount detector 330
While monitoring the amount of distortion given from the above, the applied bias voltage is reduced so that the amount of distortion matches D0 (step S306). Then, the process proceeds to step S307.

In step S307, the bias voltage control circuit 340 holds the applied bias voltage at the current value for a certain period. Thereafter, it is determined whether or not to continue the process (step S308). If the determination result is affirmative, the process returns to step S303, and the same process as described above is repeatedly performed. To end.

The bias voltage control circuit 340 performs the above-described processing, whereby the bias voltage
Allows the bias voltage applied to the external optical modulator 310 to follow the fluctuation of the optimum bias voltage on the first port side due to a temperature change or the like. Further, when the optimum bias voltage fluctuates, the following can be started more quickly than in the first embodiment.

Note that this quick start of tracking is the same as in the second embodiment, but in this embodiment,
Since the bias voltage is controlled so that the amount of distortion given from the distortion amount detector 330 matches D0 (constant), only the amount of distortion needs to be monitored, and the second embodiment monitors the amount of distortion and the optical power. In comparison, the configuration of the device is simple (control processing is simpler).

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating a secondary distortion component included in an optical signal output from an external optical modulator 110 of FIG. 1 and an example of characteristics of a filter used in a distortion amount detector 140.

FIG. 3 is a diagram visually illustrating a process performed by a bias voltage control circuit 150 of FIG. 1;

FIG. 4 is a flowchart describing a process performed by a bias voltage control circuit 150 of FIG. 1;

FIG. 5 is a block diagram illustrating a configuration of an optical transmission device according to a second embodiment of the present invention.

FIG. 6 is a flowchart describing a process performed by a bias voltage control circuit 260 of FIG. 5;

FIG. 7 is a block diagram illustrating a configuration of an optical transmission device according to a third embodiment of the present invention.

8 is a diagram showing a configuration of the external light modulator 310 of FIG.

9 is a diagram illustrating a relationship between a bias voltage and a distortion amount in an initial state for each optical signal output from the first and second ports of the external optical modulator 310 of FIG. 7;

FIG. 10 is a flowchart describing a process performed by a bias voltage control circuit 340 of FIG. 7;

FIG. 11 shows a conventional external optical modulator (Mach-Zehnder type)
FIG. 3 is a diagram showing the configuration of FIG.

12 is a diagram illustrating a relationship between a bias voltage applied to the external optical modulator of FIG. 11 and the power of an optical signal output from the external optical modulator.

13 is a diagram showing a DC drift phenomenon occurring in the external optical modulator of FIG.

14 is a diagram illustrating a relationship between a bias voltage applied to the external optical modulator of FIG. 11 and an amount of secondary distortion generated in an optical signal output from the external optical modulator.

FIG. 15 shows a calculation result of a secondary distortion (IM2) and a composite secondary distortion (CSO) with respect to a deviation of the phase of the bias voltage applied to the external optical modulator of FIG. 11 from the phase of the optimum bias voltage. FIG.

[Explanation of symbols]

 20 X-branch directional optical coupler 100, 200, 300 Light source 105, 205, 305 Bias voltage applicator 110, 210, 310 External light modulator 120, 220 Optical splitter 130, 230, 320 Opto-electrical converter 140, 240 , 330 Strain detector 150, 260, 340 Bias voltage control circuit 250 Optical power detector

Claims (11)

[Claims] 1. An optical transmitter for externally modulating a carrier light with an electric signal and transmitting the same, comprising: a light source that outputs the carrier light; an external light modulator that modulates the carrier light output from the light source with an electric signal; A bias voltage applying unit that applies a bias voltage to the external light modulator; and a bias voltage applied by the bias voltage applying unit to the external light modulator, the secondary voltage being generated in an optical signal output from the external light modulator. An optical transmission device including a bias voltage control unit that performs control based on an amount of distortion. 2. An optical splitter for splitting an optical signal output from the external optical modulator into two, and a light for converting one optical signal output from the optical splitter into an electric signal. An electrical converter, extracting a component within a specific band from the electrical signal output by the opto-electric converter and measuring the level of the component, thereby generating a secondary signal generated in the optical signal output from the external optical modulator. A distortion amount detector for detecting the amount of distortion, and a bias for controlling the bias voltage applied by the bias voltage applicator to the external optical modulator so that the amount of distortion detected by the distortion amount detector is minimized. The optical transmitter according to claim 1, further comprising a voltage control circuit. 3. The bias voltage control circuit increases or decreases a bias voltage applied by the bias voltage applicator to the external optical modulator, and the amount of distortion detected by the distortion amount detector is the bias voltage. Determine whether the increase or decrease before and after the increase or decrease, the control results in the direction to increase the bias voltage next, or to control the direction to decrease, according to the determination result, The optical transmission device according to claim 2. 4. An optical power detector for detecting a power of an optical signal output from the external optical modulator by measuring a power of an electrical signal output from the photoelectric converter, The bias voltage control circuit, based on the optical power detected by the optical power detector, the bias voltage applicator controls to increase the bias voltage applied to the external optical modulator, The optical transmission device according to claim 2, wherein it is determined whether control is performed in a direction of decreasing the light transmission. 5. The external light modulator, comprising: two waveguides for guiding the carrier light from the light source in two branches; an optical coupler for mutually coupling optical signals guided by each of the waveguides; A first and a second port for outputting two optical signals output from the optical coupler, wherein the bias voltage control means converts an optical signal output from the second port into an electrical signal A converter, by extracting a component within a specific band from the electric signal output from the photoelectric converter and measuring the level of the component, to obtain a second-order distortion generated in the optical signal output from the second port. The amount of distortion to detect the amount of, and the bias voltage applied to the external optical modulator by the bias voltage applicator, the amount of distortion detected by the distortion amount detector, a reference value stored in advance and Bar to control to match Including Ass voltage control circuit, an optical transmitter according to claim 1. 6. The bias voltage control circuit according to claim 1, wherein the bias voltage applicator controls the bias voltage so that the amount of secondary distortion generated in the optical signal output from the first port is minimized. 6. The optical transmission device according to claim 5, wherein the distortion amount detected by the distortion amount detector is stored as the reference value when the distortion is applied to the optical transmitter. 7. A method for controlling a bias voltage applied to an external light modulator in an optical transmission device including an external light modulator that modulates carrier light from a light source with an electric signal, the method comprising: Measuring the amount of secondary distortion generated in the optical signal output from the optical device, the bias voltage applied to the external optical modulator, so that the amount of distortion measured in the distortion amount measuring step is minimized A bias voltage control method, comprising a bias voltage control step of controlling. 8. The bias voltage control step includes: a bias voltage increasing / decreasing step of increasing or decreasing a bias voltage applied to the external optical modulator; and the distortion amount detected in the distortion amount detecting step is the bias voltage. Determining whether the bias voltage has increased or decreased before and after the increasing / decreasing step, and determining, based on the determination result, whether to control the bias voltage in the direction of increasing or decreasing in the next direction. Item 8. A bias voltage control method according to Item 7. 9. An optical power detection step for detecting a power of an optical signal output from the external optical modulator, wherein the bias voltage control step includes: based on the optical power detected in the optical power detection step; The bias voltage control method according to claim 7, further comprising a step of determining whether to control the bias voltage applied to the external optical modulator to increase or decrease the bias voltage. 10. The external light modulator, comprising: two waveguides for guiding the carrier light from the light source in two branches; an optical coupler for mutually coupling optical signals guided by each of the waveguides; A first port and a second port for outputting two optical signals output from the optical coupler, wherein the bias voltage control step includes the step of determining an amount of secondary distortion generated in the optical signal output from the second port. And a bias voltage control for controlling a bias voltage to be applied to the external optical modulator so that a distortion amount detected in the distortion amount detecting step matches a reference value stored in advance. The bias voltage control method according to claim 7, comprising a step. 11. The external optical modulator according to claim 11, wherein the bias voltage controlling step includes: setting an optimum bias voltage on the first port side such that an amount of secondary distortion generated in the optical signal output from the first port is minimized. When the initial state is applied to
The bias voltage control method according to claim 10, further comprising a step of storing a distortion amount detected in the distortion amount detecting step as the reference value.