JP2002221698A - Optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an optical transmitter which is equipped with an external optical modulator for modulating the carrier light from a light source by two RF signals obtained by branching one RF signal multiplexed with plural analog signals and is capable of suppressing the fluctuation in the average power of the light signal outputted from the external modulator and an increase of the distortion quantity included in the light signal in spite of generation of a DC drift and is capable of preventing the level fluctuation of the RF signals included in the output light signal. SOLUTION: The external optical modulator 110 receives the impression of first and second bias voltages and modulates the carrier light with the first and second RF signals. At this time, a bias voltage control circuit 150 controls the first bias voltage so as to make the average power of the light signal outputted from the external optical modulator 110 coincident with reference power and controls the second bias voltage so as to make the level of the RF signals included into light signal coincident with a reference level.

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 a bias voltage control for causing the applied bias voltage to follow a fluctuation of an optimum bias voltage 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. It is known that when an optical signal output from such a laser diode is transmitted over a long distance, the waveform of the optical signal is deteriorated due to the influence of chromatic dispersion in the optical fiber, thereby causing a problem that signal characteristics are deteriorated. .

Therefore, when performing long-distance transmission, an optical transmission device provided with a Mach-Zehnder type external optical modulator (hereinafter referred to as an MZ type external optical modulator) which does not generate chirping in principle is used. Has been proposed.

FIG. 14 shows a configuration example of an MZ type external optical modulator. The carrier light output from the light source is input to the MZ type external optical modulator using one MZ interferometer, 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. 14 shows a configuration in which phase modulation is performed only on light passing through one optical waveguide. Light from each optical waveguide is multiplexed with each other, and an optical signal is output from the MZ type external optical modulator. The optical electric field of the optical signal thus output is represented by the following equation.

[0005]

(Equation 1)

However,

(Equation 2) And V is the bias voltage, m is the degree of phase modulation, ωf is R
The angular frequency of the F signal is shown. Using this optical electric field,
The optical signal power output from the MZ type external optical modulator is given by the following equation.

[0007]

(Equation 3)

FIG. 15 shows the relationship between the bias voltage and the optical output power at this time. In FIG. 15, the horizontal axis represents the bias voltage, and the vertical axis represents the average power of the output optical signal. As described above, the average power of the optical signal output from the MZ type external optical modulator has a sine wave characteristic with respect to the bias voltage applied to the MZ type external optical modulator. M having this property
When optically transmitting a frequency-division multiplexed signal using a Z-type external modulator, the bias voltage is set to a point (a) where the characteristic becomes the most linear. The bias voltage corresponding to this point (a) is called an optimum bias voltage.

However, in the MZ type external optical modulator,
Due to various conditions such as a change over time and a change in temperature, a phenomenon occurs in which the relationship between the bias voltage and the optical output varies from the initial state as described above. This phenomenon is called DC drift. This DC drift phenomenon is shown in FIG. Figure 1
In 6, the characteristic (curve indicating the relationship between the bias voltage and the optical output power) moves along the horizontal axis (bias voltage) direction.

In FIG. 16, it is assumed that a bias voltage corresponding to the point (a) is applied to the MZ type external optical modulator. At this time, assuming that the optimum bias changes from the point (a) to the point (b) due to the DC drift phenomenon, the bias voltage when the bias voltage corresponding to the point (a) is applied to the MZ type external optical modulator is obtained. The optical output power from the MZ type external optical modulator decreases from P0 to P1.

[0011] In addition, the amount of secondary distortion (hereinafter, distortion amount) included in the optical signal output from the MZ type external optical modulator increases. FIG. 17 shows the relationship between the bias voltage and the amount of distortion. In FIG. 17, the amount of distortion rapidly increases when the applied bias voltage slightly deviates from the optimal bias voltage (this is a voltage corresponding to the optimal bias point (a) in FIG. 15).

In a conventional optical transmitter using an MZ type external optical modulator, the fluctuation of the optical average power and the increase of the distortion amount are prevented by following the fluctuation of the optimum bias voltage due to the DC drift. Therefore, based on the average power of the optical signal output from the MZ type external optical modulator (this is obtained by measuring the DC component of the obtained optical signal and measuring the power). Thus, the bias voltage applied to the external optical modulator has been controlled.

More specifically, in the initial state where the bias voltage applied to the MZ type external optical modulator is optimally set, the average power of the output optical signal is measured in advance, and the value (reference power) is stored. Keep it. Thereafter, the average power of the output optical signal is monitored, and the applied bias voltage is controlled so that the average power of the optical signal becomes equal to the reference power.

On the other hand, the frequency-division multiplexed RF signal is represented by M
In the case of transmission using a Z-type external optical modulator, as shown in FIG. 15, since the optical output power exhibits a sine wave characteristic with respect to the bias voltage, the third-order distortion based on this nonlinearity causes the RF signal to have a third-order distortion. Limit the dynamic range. As one of the methods for improving the third-order distortion due to the nonlinearity of the MZ type external optical modulator, an MZ type external optical modulator having a configuration in which two MZ interferometers are connected in cascade is used (for example, the document “ AN OPTICALLY LINEARRIZED
MODULATOR FOR CATV APPLIC
ATIONS "Proc. SPIE Vol. 229
1, pp. 227-238 (1994)).

FIG. 18 shows an example of the configuration of an MZ external optical modulator having a configuration in which these two MZ interferometers are connected in cascade (hereinafter, a two-stage MZ external optical modulator). In FIG. 18, the two-stage MZ type external optical modulator has two MZ interferometers 20 and 21 cascaded with each other, and directional couplers 22 and 23 are inserted before and after the rear MZ interferometer 21 respectively. Configuration. When a carrier light is input from the input port, the carrier light is modulated through the two MZ interferometers 20 and 21, and an optical signal is output from each of the two output ports.

At this time, the first RF signal is input to the MZ interferometer 20 with the first bias voltage (Vb1) applied. The second RF signal is input to the MZ interferometer 21 with the second bias voltage (Vb2) applied. Here, the first RF signal and the second RF signal are an RF signal output from one signal generator (not shown) (this RF signal is a frequency multiplexed signal obtained by multiplexing a plurality of analog signals having different frequencies from each other). (Which is a signal). The first and second RF signals have the same frequency and different amplitudes and phases. The relationship between the amplitude and the phase between the two signals is adjusted so that the third-order distortion included in the optical signal output from the external optical modulator is minimized. Then, optical signals modulated by the first and second RF signals are output from the two output ports.

[0017]

The transfer function Gt of the two-stage MZ type external optical modulator is represented by G1 and G2 of the transfer functions of the MZ interferometers 20 and 21, and transfer functions of the directional couplers 22 and 23, respectively. If Gc,

(Equation 4) It can be expressed as.

However,

(Equation 5)

[0019]

(Equation 6)

[0020]

(Equation 7) In the above equation (7), γ indicates a coupling coefficient of the directional coupler.

In the above equations (5) to (7), x,
y is a signal input to each of the MZ interferometers 20 and 21;
It is represented by the following equations (8) and (9).

(Equation 8)

[0022]

(Equation 9) In the above equations (8) and (9), Vb1 and Vb2 are bias voltages applied to the MZ interferometers 20 and 21, respectively.
1 and m2 represent a phase modulation factor, and ωf represents an angular frequency of one of the frequency-division multiplexed RF signals.

When Gt is calculated using the above equations (4) to (9), the following equation (10) is obtained.

(Equation 10)

From this equation (10), the input / output function F (x) of one output port is

[Equation 11] become that way. However, Gt (1,1), Gt (1,2)
Represents the terms in the first row and first column and the first row and second column of the matrix Gt, respectively.

Using the above equation (11), the second bias voltage (Vb2) is kept constant and the first bias voltage (Vb1)
FIG. 19 shows the result of calculating the characteristics of the optical output power with respect to FIG.
Shown in FIG. 19 shows two curves A and B. In these curves A and B, the second bias voltage (Vb
The values of 2) are different from each other. In the two-stage MZ type external optical modulator, it can be considered that the characteristic changes from A to B due to the DC drift phenomenon.

Here, in the case of a one-stage MZ type external optical modulator,
As shown in FIG. 15, the applied bias voltage-optical output power characteristic shows a sine wave shape. This characteristic is shown in FIG.
As shown in FIG. 6, the DC drift caused a parallel movement in the horizontal axis (bias voltage) direction, and as a result, the optimum bias point changed from (a) to (b) in the figure.

On the other hand, in the case of the two-stage MZ type external optical modulator, as shown in FIG. 19, the first bias voltage-optical output power characteristic shows a sine wave shape as in the case of the one-stage type. And
This characteristic shifts in parallel in the horizontal axis (bias voltage) direction due to DC drift, and as a result, the first optimum bias point changes from (a) to (b) in the figure, as compared with the case of the one-stage MZ type. Is the same. The difference between the two-stage MZ type and the one-stage MZ type is that the DC drift causes the characteristic curve to not only move in parallel in the horizontal axis direction but also expand and contract in the vertical axis direction (that is, the amplitude of the curve changes).

Here, as shown in FIG. 16, when the characteristic curve only moves in the horizontal direction, the slope of the characteristic curve (solid line) at the optimum bias point (a) before DC drift occurs (here, The slope of the characteristic curve represents the degree of light modulation in the external light modulator) and the slope of the characteristic curve (dotted line) at the optimum bias point (b) after the DC drift has occurred. On the other hand, as shown in FIG. 19, when the characteristic curve moves in the horizontal axis direction and expands / contracts in the vertical axis direction, the characteristic curve (A) at the optimum bias point (a) before the DC drift is generated. ) And the slope of the characteristic curve (B) at the optimum bias point (b) after the DC drift,
The values are different from each other.

Therefore, suppose that in the optical transmitter using the two-stage MZ type external optical modulator, the same process as the bias voltage control process performed by the one-stage MZ type external optical modulator, ie,
As the optimal bias point changes from (a) to (b) in FIG. 19, the first bias voltage is changed from Vb1_a to Vb.
If the processing for changing to 1_b is performed, an increase in second-order distortion can be prevented, but the level of the RF signal included in the output optical signal is changed because the degree of light modulation of the external optical modulator is changed. Fluctuations, ie, fluctuations in the carrier level, occur. Thus, D occurring in the two-stage MZ type external optical modulator
The fluctuation of the carrier level due to the C drift causes an inconvenience that the optical transmitter cannot obtain a stable C / N characteristic.

Therefore, an object of the present invention is to convert a carrier light from a light source into one RF signal in which a plurality of analog signals are multiplexed.
An optical transmitter including an external optical modulator that modulates a signal with two RF signals obtained by splitting a signal. Even if a DC drift occurs due to aging or temperature change, an output from the external modulator is generated. It is possible to suppress the fluctuation of the average power of the received optical signal and the increase in the amount of distortion included in the optical signal, and also to prevent the level fluctuation (carrier level fluctuation) of the RF signal included in the output optical signal. An object of the present invention is to realize an optical transmission device having stable transmission characteristics and C / N characteristics.

[0031]

According to a first aspect of the present invention, a carrier light is generated by converting a carrier light into a plurality of multiplexed analog signals.
First and second RF signals obtained by splitting two RF signals
An optical transmitter for externally modulating a signal and transmitting the signal, comprising: a light source for outputting carrier light; an external light modulator for modulating carrier light output from the light source with first and second RF signals; A first bias voltage applied to the external optical modulator by the bias voltage applicator, and an average power of an optical signal output from the external optical modulator. And the second bias voltage is controlled based on RF included in the optical signal.
A bias voltage control means for controlling based on a signal level is provided.

In the first aspect, the external optical modulator (preferably, a two-stage MZ type external optical modulator as in the third aspect described below) receives the application of the first and second bias voltages,
The carrier light is modulated with the first and second RF signals. At this time, a first bias voltage (hereinafter, referred to as a first bias voltage) applied to the external optical modulator.
The first bias voltage is controlled based on the average power of the optical signal output from the external optical modulator, and the second bias voltage (hereinafter, the second bias voltage) applied to the external optical modulator is controlled by the external bias. Since the control is performed based on the level of the RF signal included in the optical signal output from the optical modulator, even if a DC drift occurs due to aging or temperature change, the optical signal output from the external optical modulator And the increase in the amount of second-order distortion (hereinafter referred to as distortion) included in the output optical signal, and the level fluctuation of each RF signal included in the optical signal output from the external optical modulator. (Carrier level fluctuation) can be prevented. Thereby, an optical transmission device having stable transmission characteristics and C / N characteristics is realized.

In a second aspect based on the first aspect, the bias voltage control means includes an optical splitter for splitting the optical signal output from the external optical modulator into two, and one optical signal output from the optical splitter. By converting the power of the electrical signal output by the photoelectric converter to convert the electrical signal into an electrical signal,
An optical power detector that detects the average power of the optical signal output from the external optical modulator, by extracting a component within a specific band from the electrical signal output by the photoelectric converter, and measuring the level of the component. An RF signal level detector for detecting a level of an RF signal included in an optical signal output from an external optical modulator, and an optical power detector for applying a first bias voltage applied to the external optical modulator by a bias voltage applicator. Voltage control circuit that controls the light average power detected by the RF signal level to match the reference power, and controls the second bias voltage so that the level detected by the RF signal level detector matches the reference level. including.

In the second aspect, the first bias voltage is controlled so that the average power of the optical signal output from the external optical modulator matches the reference power, and the level of the RF signal included in the optical signal is controlled. Second so that
Control the bias voltage. Note that the reference power is the average power of the optical signal output from the external optical modulator in the initial state, and the reference level is the average power of the RF signal included in the optical signal output from the external optical modulator in the initial state. Level.

According to a third aspect, in the first aspect, the external optical modulator is a two-stage MZ type external optical modulator having first and second Mach-Zehnder interferometers cascaded to each other. The first Mach-Zehnder interferometer receives a first RF signal in a state where a first bias voltage is applied by a bias voltage applicator, and the second Mach-Zehnder interferometer has a second bias voltage applied by a bias voltage applicator. Is applied and the second RF signal is input.

The third invention (or the eighth invention described below)
Then, since the third-order distortion is suppressed, the dynamic range of each RF signal can be increased.

According to a fourth aspect of the present invention, there is provided an optical transmitter for externally modulating a carrier light with first and second RF signals obtained by branching one RF signal in which a plurality of analog signals are multiplexed, and transmitting the resultant signal. A light source that outputs a carrier light; an external light modulator that modulates the carrier light output from the light source with first and second RF signals; and a first and a second bias voltage applied to the external light modulator. A bias voltage applicator, and a first bias voltage applied by the bias voltage applicator to the external optical modulator.
The optical signal output from the external optical modulator is controlled based on the amount of secondary distortion caused by nonlinearity of the external optical modulator, and the second bias voltage is included in the optical signal. Bias voltage control means for controlling based on the level of the RF signal to be applied.

In the fourth aspect, the external optical modulator (preferably, a two-stage MZ type external optical modulator as in the eighth aspect described below) receives the application of the first and second bias voltages,
The carrier light is modulated with the first and second RF signals. At this time, a first bias voltage (hereinafter, referred to as a first bias voltage) applied to the external optical modulator.
The first bias voltage is controlled based on the amount of secondary distortion (hereinafter referred to as distortion amount) included in the optical signal output from the external optical modulator, and the second bias voltage is applied to the external optical modulator. (Hereinafter, the second bias voltage) is controlled based on the level of the RF signal included in the optical signal output from the external optical modulator. Therefore, even if a DC drift occurs due to aging or temperature change. In addition, it is possible to suppress the fluctuation of the average power of the optical signal output from the external optical modulator and the increase in the amount of distortion included in the output optical signal. Level fluctuation (carrier level fluctuation) can also be prevented. Thereby, an optical transmission device having stable transmission characteristics and C / N characteristics is realized.

Further, since the first bias voltage is controlled based on the amount of distortion, control accuracy of the first bias voltage is improved as compared with the first invention in which the first bias voltage is controlled based on the optical average power. More stable transmission characteristics can be obtained. Note that controlling the first bias voltage based on the amount of distortion is
The fact that the control can be performed with higher precision than the control based on the optical power is, for example, the inclination of the first bias voltage-distortion characteristic curve shown in FIG. 17 near the optimum bias voltage shown in FIG. It can be easily understood by comparing the slope of the first bias voltage-output light power characteristic curve near the optimum bias voltage.

In a fifth aspect based on the fourth aspect, the bias voltage control means includes an optical splitter for splitting the optical signal output from the external optical modulator into two, 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, whereby the light output from the external light modulator Included in the signal, a distortion amount detector that detects the amount of secondary distortion caused by the nonlinearity of the external optical modulator, extracting a component within another specific band from the electric signal output by the photoelectric converter. An RF signal level detector that detects the level of the RF signal included in the optical signal output from the external optical modulator by measuring the level of the component, and a bias voltage applicator applies the RF signal to the external optical modulator. First bias voltage Bias voltage for controlling the amount of distortion detected by the amount of distortion detector to be minimum and controlling the second bias voltage so that the level detected by the RF signal level detector matches the reference level. Including control circuit.

In the fifth aspect, the first bias voltage is controlled so as to minimize the amount of distortion, and the second bias voltage is controlled such that the level of the RF signal included in the optical signal matches the reference level.
Control the bias voltage. Note that the reference level is the level of the RF signal included in the optical signal output from the external optical modulator in the initial state.

Here, the amount of distortion has a left-right symmetric characteristic with respect to the first bias voltage, with the first optimal bias voltage at which the amount of distortion is minimized as the axis of symmetry (see FIG. 9). Therefore, when the first optimal bias voltage fluctuates due to DC drift, it is necessary to determine whether to control the first bias voltage to increase or decrease the first bias voltage.

Therefore, in the sixth invention described below, first, the first invention
The bias voltage is increased or decreased, and before and after, it is determined whether the distortion amount has increased or decreased. Then, based on the result of the determination, it is determined whether the first bias voltage is to be controlled to increase or to decrease. Specifically, if the amount of distortion is decreased by increasing (or decreasing) the first bias voltage, control is also performed in the direction of increasing (or decreasing) the first bias voltage, and if the amount of distortion is increased, Then, control is performed in a direction to decrease (or increase) the first bias voltage.

Alternatively, as in a seventh aspect described below, the average power of the optical signal output from the external optical modulator is further detected, and control is performed to increase the first bias voltage based on the average power of the optical signal. It is also possible to determine whether to perform the control or the control in the direction in which the reduction is performed. Specifically, when the first bias voltage applied to the external optical modulator and the average power of the optical signal output from the external optical modulator have a relationship as shown in FIG. 19, the optical average power increases ( That is, when the optical average power decreases (that is, when it becomes smaller than Po), the control is performed in such a direction that the first bias voltage is increased. Decide to do it.

In a sixth aspect based on the fifth aspect, the bias voltage control circuit increases or decreases the first bias voltage applied to the external optical modulator by the bias voltage applying device,
It is determined whether the amount of distortion detected by the distortion amount detector has increased or decreased before or after the increase or decrease of the first bias voltage, and control is performed in a direction to increase the first bias voltage next based on the determination result. Or control to reduce the direction.

In a seventh aspect based on the fifth aspect, the bias voltage control means measures the average of the optical signal output from the external optical modulator by measuring the power of the electrical signal output from the photoelectric converter. An optical power detector for detecting power; wherein the bias voltage control circuit increases the first bias voltage applied to the external optical modulator by the bias voltage applicator based on the average optical power detected by the optical power detector. It is characterized in that it is determined whether the control is to be performed in the direction in which it is performed or in the direction in which it is reduced.

In the seventh aspect of the present invention, whether the first bias voltage should be increased or decreased can be immediately determined, so that the variation of the first optimum bias due to the DC drift can be accurately and promptly followed. It becomes possible.

In an eighth aspect based on the fourth aspect, the external optical modulator is a two-stage MZ type external optical modulator having first and second Mach-Zehnder interferometers cascaded to each other. The first Mach-Zehnder interferometer receives a first RF signal in a state where a first bias voltage is applied by a bias voltage applicator, and the second Mach-Zehnder interferometer has a second bias voltage applied by a bias voltage applicator. Is applied and the second RF signal is input.

[0049]

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.
, Photoelectric converter 130 and RF signal level detector 14
0 and an optical power detector 160. 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 first and second bias voltages to the external optical modulator 110. External light modulator 110
Has a configuration as shown in FIG. 2, for example. That is, FIG.
The external light modulator of FIG. 18 has the same configuration as that of the external light modulator of FIG. 18 except that it has no directional coupler 23 behind the rear MZ interferometer 21 and has only one output port. (See description).

Note that, instead of the external optical modulator of FIG.
Eight external light modulators may be used. The two-stage MZ type external optical modulator shown in FIG. 18 has one input port and two output ports. When a carrier light is input from the input port, the carrier light is transmitted to the two MZ interferometers 20 and 20. The optical signal is modulated through 21 and optical signals are output from two output ports.
When the two-stage MZ type external optical modulator shown in FIG. 18 is used, one optical signal is transmitted to the receiving device side through an optical transmission path,
Since the other optical signal may be input to the photoelectric converter 130, the optical splitter 120 among the constituent elements shown in FIG.
Becomes unnecessary.

The external modulator 110 receives the carrier light output from the light source 100, the first RF signal and the second RF signal in a state where the first bias voltage and the second bias voltage are applied by the bias voltage applicator 105. Is entered,
The carrier light is intensity-modulated according to the first RF signal and the second RF signal biased by the first bias voltage and the second bias voltage, respectively, and outputs an optical signal. Here, the first RF signal and the second RF signal input to the external optical modulator 110
The signal is obtained by branching an RF signal output from one signal generator (not shown) (this RF signal is a frequency multiplexed signal in which a plurality of analog signals having mutually different frequencies are multiplexed). Signal. The first and second RF signals have the same frequency and different amplitudes and phases. The relationship between the amplitude and the phase between the two signals is adjusted so that the third-order distortion included in the optical signal output from the external optical modulator 110 is minimized.

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.

An electric signal output from the photoelectric converter 130 is supplied to an RF signal level detector 140 and an optical power detector 160, and the RF signal level detector 140 performs external light modulation based on the electric signal. The level of the RF signal included in the optical signal output from the detector 110 is detected. The optical power detector 160 detects the average power of the optical signal output from the external optical modulator 110 by measuring the power or current of the electric signal (that is, a DC component included in the signal).

The level detected by the RF signal level detector 140 and the optical average power detected by the optical power detector 160 are given to the bias voltage control circuit 150, and the bias voltage control circuit 150 determines the level based on the levels. The bias voltage applicator 105 is connected to the external optical modulator 1
A first bias voltage (hereinafter, referred to as a first bias voltage) applied to the power supply 10 is controlled, and based on the optical average power,
The bias voltage applicator 105 controls a second bias voltage (hereinafter, a second bias voltage) applied to the external optical modulator 110.

The above-mentioned RF signal level detector 140 is realized by, for example, a filter for extracting only a component within a desired frequency band from an electric signal, and a circuit for measuring the level (voltage) of the component. Here, the first RF
A filter having a characteristic of transmitting only a component within a band including the frequency of the signal is selected.

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. There are two ways to make this adjustment:
A method of inserting a polarization controller between the light source 100 and the external light modulator 110 and adjusting the polarization plane by the polarization controller, or a method of using a polarization maintaining fiber in the fiber between the light source 100 and the external light modulator 110 And the like. FIG. 1 does not specify a polarization controller or a polarization maintaining fiber for adjusting the polarization plane.

The external light modulator 110 is applied with the first bias voltage and the second bias voltage by the bias voltage applicator 105, and modulates the intensity of the input light according to the input first RF signal and second RF signal. Output an optical signal. This output optical signal is split by the optical splitter 120,
One of the branched optical signals is converted into an electrical signal by the photoelectric converter 130. The electric signal output from the photoelectric converter 130 is input to the RF signal level detector 140 and the optical power detector 160.

The RF signal level detector 140 extracts a component within a desired band (here, a band including the frequency of the first RF signal) from the AC component included in the electric signal, and extracts the component. Measure the level of the component. The optical power detector 160 has a function of measuring the power or the current (DC component) of the electric signal, and performs a predetermined calculation based on the measured power, thereby obtaining the external light modulator 11.
The average power of the optical signal output from 0 is detected.

The signal output from the RF signal level detector 140 and the signal output from the optical power detector 160 are input to a bias voltage control circuit 150 in the bias voltage applicator 105. In response, the bias voltage control circuit 150 applies the bias voltage to the external light modulator 110 by the bias voltage applicator 105 so that the average optical power detected by the optical power detector 160 matches the reference power Po (described later). One bias voltage is controlled. With such a configuration, the first bias voltage can always be maintained at a bias voltage (hereinafter, referred to as a first optimum bias voltage) such that the optical average power matches the reference power Po.

Further, the bias voltage control circuit 150
The second voltage applied by the bias voltage applicator 105 to the external optical modulator 110 so that the level detected by the RF signal level detector 140 matches a reference level Co (described later).
Controls the bias voltage. With such a configuration, the second bias voltage is always set to a bias voltage (hereinafter, referred to as a reference voltage Co) such that the RF signal level matches the reference level Co.
(The second optimum bias voltage).

Here, the first bias voltage control processing and the second bias voltage control performed by the bias voltage control circuit 150 will be described in detail. In the first bias voltage control process, the bias voltage applicator 105 controls the external optical modulator 110 by controlling the first bias voltage so that the optical average power is constant, so that the fluctuation of the first optimal bias voltage due to the DC drift is caused. This is a process of following the first bias voltage applied to the first control signal.

On the other hand, the second bias voltage control process
By controlling the second bias voltage so that the signal level becomes constant, the bias voltage applicator 105 follows the second bias voltage applied to the external optical modulator 110 by the fluctuation of the second optimum bias voltage due to the DC drift. This is the process to make it.

The first bias voltage control process is the same as the conventional process. However, by adding the second bias voltage control process, even if a DC drift occurs, the level of the RF signal included in the output optical signal is increased. (Career level)
Does not decrease, and as a result, always good C /
An N ratio is obtained.

FIG. 3 shows the bias voltage control circuit 15 of FIG.
10 is a flowchart describing the processing performed by the C.O. 3, first, the bias voltage control circuit 150 stores the reference power Po (Step S11), and then stores the reference level Co (Step S12). The execution order of steps S11 and S12 may be reversed.

The reference power Po and the reference level Co stored in steps S11 and S12 correspond to the RF signal level detector 140 and the optical power in the initial state where the first bias voltage and the second bias voltage are respectively adjusted optimally. These are signal values output from the detector 160.

Next, the bias voltage control circuit 150 converts the reference power Po and the reference level Co stored in steps S11 and S12 and the signal values output from the RF signal level detector 140 and the optical power detector 160, respectively. First, the first bias voltage is controlled (step S13), and then the second bias voltage is controlled (step S14). The execution order of steps S13 and S14 may be reversed.
Thereafter, it is determined whether or not to continue the processing (step S).
15) If the judgment result is affirmative, each control process of steps S13 and S14 is repeated. On the other hand, if the determination is negative, the process ends.

FIG. 4 is a flowchart showing the details of step S13 of FIG. 3 (first bias voltage control processing performed by the bias voltage control circuit 150 of FIG. 1). In FIG. 4, an optical average power detected by the optical power detector 160 is given to the bias voltage control circuit 150. The bias voltage control circuit 150 first compares the given optical average power with the reference power Po stored in step S11 in FIG. 3 so that the optical average power has changed from Po by a predetermined threshold or more. It is determined whether or not (step S101). And if the judgment result is negative,
Proceed to step S105.

If the determination result in step S101 is affirmative, the bias voltage control circuit 150 determines whether or not the optical average power has increased (step S102). If the determination result is positive, the bias voltage control circuit 150
While monitoring the optical average power given from the optical power detector 160, the first power is adjusted so that the power coincides with Po.
The bias voltage is increased (Step S103). Then, the process proceeds to step S105.

If the decision result in the step S102 is negative, the bias voltage control circuit 150 sets the optical power detector 1
While monitoring the optical average power given from 60, the first bias voltage is reduced so that the power matches Po (step S104). Then, step S1
Proceed to 05.

In step S105, the bias voltage control circuit 150 holds the first bias voltage at the current value for a certain period. The time for holding the first bias voltage is:
It is set shorter than the period during which aging or temperature changes occur. The above is the first bias voltage control processing.

The bias voltage control circuit 150 performs the above-described processing, whereby the bias voltage
Is the first bias voltage applied to the external light modulator 110,
It is possible to follow the fluctuation of the first optimum bias voltage due to aging, temperature change, and the like. Thereafter, returning to the flow of FIG. 3, the second bias voltage control processing of step S14 is executed.

FIG. 5 is a flowchart showing the details of step S14 of FIG. 3 (the second bias voltage control process performed by the bias voltage control circuit 150 of FIG. 1). In FIG. 5, an RF signal level detected by an RF signal level detector 140 is supplied to a bias voltage control circuit 150.
The bias voltage control circuit 150 initially receives the applied RF
By comparing the signal level with the reference level Co stored in step S12 of FIG.
It is determined whether or not the value has changed from o by a predetermined threshold value or more (step S201). If the result of the determination is negative, the process proceeds to step S205.

If the determination result of step S201 is affirmative, the bias voltage control circuit 150 determines whether or not the RF signal level has increased (step S202). If the determination result is affirmative, the bias voltage control circuit 150
Monitors the RF signal level supplied from the RF signal level detector 160, and increases the second bias voltage so that the level matches Co (step S2).
03). Then, the process proceeds to step S205.

If the decision result in the step S202 is negative, the bias voltage control circuit 150 monitors the RF signal level supplied from the RF signal level detector 160, and adjusts the second bias so that the level coincides with Co. The voltage is decreased (step S204). Then, the process proceeds to step S205.

In step S205, the bias voltage control circuit 150 holds the second bias voltage at the current value for a certain period. The time for holding the second bias voltage is:
It is set shorter than the period during which aging or temperature changes occur. The above is the second bias voltage control processing.

The bias voltage control circuit 150 performs the above-described processing, whereby the bias voltage
Is the second bias voltage applied to the external optical modulator 110,
It is possible to follow a change in the second optimum bias voltage due to aging, temperature change, or the like. Thereafter, returning to the flow of FIG. 3, the determination in step S15 is performed, and when the processing is continued, the same processing as in steps S13 and S14 is repeated.

Here, as described above, after the first bias voltage is optimally controlled in step S13, the process proceeds to step S14.
In the case where the second bias voltage is controlled in the first step, the first bias voltage may deviate from the optimum value due to the influence of the latter control processing. Alternatively, contrary to the above, step S1
After optimally controlling the second bias voltage in step 4,
When the control of the first bias voltage is performed in step 13, the second bias voltage may deviate from the optimum value due to the influence of the latter control processing.

However, the deviation from the optimum value caused by the first bias voltage control and the second bias voltage control affecting each other is described in steps S13 and S13.
The problem can be solved by repeatedly executing step 14.

As described above, according to this embodiment, the carrier light from the light source 100 is converted into the first and second RF signals obtained by branching one RF signal in which a plurality of analog signals are multiplexed. An optical transmission device having an external optical modulator 110 for modulating a signal.
Even if the drift occurs, it is possible to suppress the fluctuation of the average power of the optical signal output from the external modulator 110 and the increase in the amount of distortion included in the optical signal, and further, the RF signal included in the output optical signal. Stable transmission characteristics and C that can also prevent level fluctuations (carrier level fluctuations)
An optical transmission device having the / N characteristic is realized.

(Second Embodiment) FIG. 6 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. 6, the optical transmitter includes a light source 200, a bias voltage applicator 205, an external optical modulator 210, an optical splitter 220, an optical-electrical converter 230, an RF signal level detector 240, a distortion amount, And a detector 260.
The bias voltage applicator 205 is connected to the bias voltage control circuit 2
50.

The light source 200, the bias voltage applicator 205,
External optical modulator 210, optical splitter 220, photoelectric converter 2
30 and the RF signal level detector 240 are the same as those in FIG. 1, and the description is omitted (see the first embodiment). The configuration of the external light modulator 210 is shown, for example, in FIG. 2 (see the section of the prior art).

The electric signal output from the photoelectric converter 230 is supplied to the distortion detector 260, and the distortion detector 260
Is the amount of secondary distortion contained in the optical signal output from the external optical modulator 210 based on the electric signal (hereinafter, the amount of distortion)
Is detected.

The level detected by the RF signal level detector 240 and the distortion amount detected by the distortion amount detector 260 are given to the bias voltage control circuit 250, and the bias voltage control circuit 250 determines the level based on the distortion amount. And the first
Controls the bias voltage and, based on its level,
Controlling the second bias voltage (this second applied bias control process is similar to that of the first embodiment;
(See FIG. 5).

The above-mentioned distortion detector 260 is realized, for example, by a filter for extracting only a component within a desired frequency band from an electric signal, and a circuit for measuring the level (voltage) of the component. The distortion amount detector 260 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. 7, when several second-order distortions (10 to 12) having different frequencies from each other occur, a filter having a transmission characteristic as indicated by reference numeral 13 is used to obtain 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 260.

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. In general, the light input to the external optical modulator 210 is limited to only one of the TE mode and the TM mode by adjusting the polarization plane. There are two ways to make this adjustment:
A method of inserting a polarization controller between the light source 200 and the external optical modulator 210 to adjust the polarization plane by the polarization controller, or a polarization maintaining fiber in the fiber between the light source 200 and the external optical modulator 210 And the like. FIG. 6 does not specify a polarization controller or a polarization maintaining fiber for adjusting the polarization plane.

The external light modulator 210 is applied with the first bias voltage and the second bias voltage by the bias voltage applicator 205, and modulates the intensity of the input light in accordance with the input first RF signal and second RF signal. Output an optical signal. This output optical signal is split by the optical splitter 220,
One of the branched optical signals is converted into an electrical signal by the photoelectric converter 230. The electric signal output from the photoelectric converter 230 is input to the RF signal level detector 240 and the distortion amount detector 260.

The RF signal level detector 240 extracts a component within a desired band (here, within a band including the frequency of the first RF signal) from the AC component included in the electric signal, and extracts the component. Measure the level of the component. The distortion amount detector 260 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 included in the optical signal output from the external optical modulator 210 is detected. The desired band is not particularly limited as long as it is a band in which secondary distortion occurs, but it is desirable to set the desired band to a frequency band having a large amount of distortion.

The signal output from the RF signal level detector 240 and the signal output from the distortion detector 260 are:
Bias voltage control circuit 2 in bias voltage applicator 205
50 is input. Accordingly, the bias voltage control circuit 25
0 controls the first bias voltage applied by the bias voltage applicator 205 to the external optical modulator 210 so that the amount of distortion detected by the distortion amount detector 260 is minimized. With such a configuration, the control accuracy of the first bias voltage can be improved by a relatively simple control method. (1 optimal bias voltage).

Further, the bias voltage control circuit 250
The bias voltage applicator 205 controls the second bias voltage applied to the external optical modulator 210 so that the level detected by the RF signal level detector matches the reference level Co (described above). With such a configuration, the second bias voltage can be maintained at a bias voltage at which the RF signal level matches the reference level Co (hereinafter, a second optimum bias voltage).

Here, the first bias voltage control processing and the second bias voltage control performed by the bias voltage control circuit 250 will be described in detail. The second bias voltage control process controls the second bias voltage so that the RF signal level becomes constant, thereby controlling the second bias voltage due to the DC drift.
The bias voltage applicator 205 responds to the fluctuation of the optimum bias voltage.
Is a process for following the second bias voltage (second bias voltage) applied to the external optical modulator 210, and is a process similar to that of the first embodiment.

On the other hand, the first bias voltage control process
For the purpose of causing the first bias voltage to follow the fluctuation of the first optimum bias voltage due to drift, it is the same as the first embodiment, but since control is performed based on the amount of distortion,
Compared to the first embodiment in which the control is performed based on the light average power, the tracking can be performed with higher accuracy. By controlling the first bias voltage based on the amount of distortion,
The fact that the control can be performed with higher precision than the control based on the optical power is, for example, the inclination of the first bias voltage-distortion characteristic curve shown in FIG. 17 near the optimum bias voltage shown in FIG. It can be easily understood by comparing the slope of the first bias voltage-output light power characteristic curve near the optimum bias voltage.

FIG. 8 shows the bias voltage control circuit 25 of FIG.
10 is a flowchart describing the processing performed by the C.O. 8, first, the bias voltage control circuit 250 stores the reference level Co (Step S21). The reference level Co stored in step S21 is a signal value output from the RF signal level detector 240 in an initial state in which the first bias voltage and the second bias voltage are each adjusted optimally.

Next, based on the reference level Co stored in step S21 and the signal values output from the RF signal level detector 240 and the distortion amount detector 260, the bias voltage control circuit 250 Control of one bias voltage is performed (step S22), and then control of the second bias voltage is performed (step S23). The execution order of steps S22 and S23 may be reversed. Thereafter, it is determined whether or not to continue the process (step S24). If the determination result is positive, each control process of steps S22 and S23 is repeated. On the other hand, if the determination is negative, the process ends.

FIG. 9 is a diagram visually illustrating step S22 of FIG. 8 (first bias voltage control processing performed by the bias voltage control circuit 250 of FIG. 6). FIG. 10 is a flowchart describing details of step S22 in FIG. The relationship between the bias voltage and the amount of distortion shown in FIG. 9 is the same as that shown in FIG. 17 (see the column of the prior art).

9 and 10, the bias voltage control circuit 250 supplies a distortion amount m0 detected by the distortion amount detector 260 to the bias voltage control circuit 250.
Is given. First, the bias voltage control circuit 250
The first bias voltage is increased by a predetermined voltage (Step S301). Next, the distortion amount (m1) after the first applied bias is increased from the distortion amount detector 260, and the bias voltage control circuit 250 accordingly calculates the distortion amounts (m0 and m1) before and after the first applied bias is increased. By comparing with each other, it is determined whether or not the amount of distortion has decreased (step S302). When the determination result is affirmative, steps S301 and S302 are repeatedly performed. That is, the bias voltage control circuit 250 increases the first bias voltage again, and then sets the distortion amount (m2) after the first bias voltage increase supplied from the distortion amount detector 260 to the distortion amount (m1) before the increase. In comparison, it is determined whether or not the amount of distortion has decreased. If the determination result is negative, step S30
3, but if affirmative, step S30 is performed again.
1 and S302 will be repeated.

If the result of the determination in step S302 is negative (ie, if the amount of distortion after increasing the first applied bias in step S301 is not m1 but m1 '), the bias voltage control circuit 250 The bias voltage is reduced by a predetermined voltage (step S303),
Thereafter, it is determined whether or not the distortion amount has decreased (step S
304). If the determination result is affirmative, step S30
3 and S304 are repeatedly performed. That is, the bias voltage control circuit 250 decreases the first bias voltage again, and then the first bias voltage supplied from the distortion amount detector 260.
The distortion amount (m1) after the bias voltage is reduced is changed to the distortion amount (m1) before the reduction.
By comparing with 0), it is determined whether or not the amount of distortion has decreased. If the determination result is affirmative, steps S303 and S304 are repeatedly performed again.

If the decision result in the step S304 is negative, the bias voltage control circuit 250 returns the first bias voltage to a value immediately before the current value and holds the first bias voltage for a certain period (step S305). The time during which the first bias voltage is held is set shorter than the period during which aging or temperature changes occur. The above is the first bias voltage control processing. Thereafter, returning to the flow of FIG. 8, the second bias voltage control processing of step S23 is executed.

When the bias voltage control circuit 250 performs the first bias voltage control processing as described above, the bias voltage applicator 205 applies the voltage to the external light modulator 110 even if aging or temperature changes occur. The first bias voltage can follow the fluctuation of the first optimum bias voltage due to the DC drift.

In the above-described processing, it is determined whether the distortion amount has increased or decreased before and after the first bias voltage is increased (or decreased), and the first bias voltage is then increased based on the determination result. It was decided whether to control in the direction or to decrease the direction. In addition to such processing, for example, the following processing may be performed.

In the initial state where the first bias voltage is equal to the first optimum bias voltage, the amount of distortion given from the distortion detector 260 is measured in advance, and the value (reference distortion amount) is sent to the bias voltage control circuit 250. Remember. Thereafter, the bias voltage control circuit 250 controls the first bias voltage so that the distortion amount given from the distortion amount detector 260 becomes equal to the reference distortion amount. That is, the bias voltage control circuit 250 compares the distortion amount given from the distortion amount detector 260 with the reference distortion amount, and increases (or decreases) the first bias voltage when the former becomes larger than the latter. .

The details of step S23 in FIG. 8 (the second bias voltage control process performed by the bias voltage control circuit 250 in FIG. 6) are the same as those shown in the flowchart in FIG. 5 (first embodiment). ), And the description is omitted.

Here, as described above, after the first bias voltage is optimally controlled in step S22, the process proceeds to step S23.
In the case where the second bias voltage is controlled in the first step, the first bias voltage may deviate from the optimum value due to the influence of the latter control processing. Alternatively, contrary to the above, step S2
After optimally controlling the second bias voltage in step 3,
When the control of the first bias voltage is performed in step 22, the second bias voltage may deviate from the optimum value due to the influence of the latter control process.

However, the deviation from the optimum value caused by the influence of the first bias voltage control and the second bias voltage control on each other is described in steps S22 and S22.
The problem can be solved by repeatedly executing step 23.

As described above, according to the present embodiment, the carrier light from the light source 200 is converted into the first and second RF signals obtained by branching one RF signal in which a plurality of analog signals are multiplexed. An optical transmission device having an external optical modulator 210 for modulating a signal.
Even if the drift occurs, it is possible to suppress the fluctuation of the average power of the optical signal output from the external modulator 210 and the increase in the amount of distortion included in the optical signal, and further, the RF signal included in the output optical signal. Stable transmission characteristics and C that can also prevent level fluctuations (carrier level fluctuations)
An optical transmission device having the / N characteristic is realized.

Further, the optical transmitter of the present embodiment controls the first bias voltage based on the amount of distortion, so that the optical transmitter of the first embodiment controls the first bias voltage based on the average optical power. The control accuracy of the first bias voltage is improved, and more stable transmission characteristics can be obtained.

(Third Embodiment) FIG. 11 is a block diagram showing a configuration of an optical transmission device according to a third embodiment of the present invention. In FIG. 11, an optical transmission device includes a light source 300
, A bias voltage applicator 305 and an external light modulator 310
, An optical splitter 320, an optical-electrical converter 330, an RF signal level detector 340, an optical power detector 360, and a distortion detector 370. Bias voltage applicator 3
05 includes a bias voltage control circuit 350.

The light source 300, the bias voltage applicator 305,
External optical modulator 310, optical splitter 320, photoelectric converter 3
30, the RF signal level detector 340, and the optical power detector 360 are the same as those in FIG. 1, and description thereof will be omitted (see the first embodiment). FIG. 6 shows the distortion amount detector 370.
The description is omitted (see the second embodiment). The configuration of the external light modulator 310 is shown, for example, in FIG. 2 (see the first embodiment).

The level detected by the RF signal level detector 340, the average optical power detected by the optical power detector 360, and the distortion detected by the distortion detector 370 are given to the bias voltage control circuit 350. Can be

The bias voltage control circuit 350 controls the first bias voltage based on the given amount of distortion. At that time, the bias voltage applicator 305 determines whether to increase or decrease the first bias voltage based on the given optical average power. Also, the bias voltage control circuit 3
50 controls the second bias voltage based on the given level.
This is the same process as that of the embodiment; see FIG. 5).

The operation of the optical transmission device configured as described above will be described below. Light output from the light source 300 is input to the external light modulator 310. Generally, the light input to the external optical modulator 310 is limited to only one of the TE mode and the TM mode by adjusting the polarization plane. There are two ways to make this adjustment:
A method of inserting a polarization controller between the light source 300 and the external light modulator 310 to adjust the polarization plane by the polarization controller, or a method of using a polarization maintaining fiber in the fiber between the light source 300 and the external light modulator 310 And the like. FIG. 11 does not specify a polarization controller or a polarization maintaining fiber for adjusting the polarization plane.

The external light modulator 310 is applied with the first bias voltage and the second bias voltage by the bias voltage applicator 305, and modulates the intensity of the input light according to the input first RF signal and second RF signal. Output an optical signal. This output optical signal is split by the optical splitter 320,
One of the branched optical signals is converted into an electrical signal by the photoelectric converter 330. The electric signal output from the photoelectric converter 330 is input to the RF signal level detector 340, the optical power detector 360, and the distortion detector 370.

The RF signal level detector 340 extracts a component within a desired band (here, within a band including the frequency of the first RF signal) from the AC component included in the electric signal, and extracts the component. Measure the level of the component. The optical power detector 360 has a function of measuring the power or the current (DC component) of the electric signal, and performs a predetermined calculation based on the measured power to thereby output the external light modulator 31.
The average power of the optical signal output from 0 is detected.

The distortion amount detector 370 has a function of extracting a component of a desired band from an AC component included in the electric signal and measuring the level, and performs a predetermined calculation based on the measured level. By doing so, the external light modulator 310
The amount of distortion contained in the optical signal output from the device is detected. The desired band is not particularly limited as long as it is a band in which secondary distortion occurs, but it is desirable to set the desired band to a frequency band having a large amount of distortion.

The signal output from the RF signal level detector 340, the signal output from the optical power detector 360, and the signal output from the distortion detector 370 correspond to the bias voltage in the bias voltage applicator 305. It is input to the control circuit 350. Accordingly, the bias voltage control circuit 350
The bias voltage applicator 305 is connected to the external optical modulator 31 so that the amount of distortion detected by the distortion amount detector 370 becomes minimum.
Control of the first bias voltage applied to 0 is performed.

When executing the first bias voltage control process, the bias voltage applicator 305
0 is the reference power Po
(Ie, the optical average power in the initial state where the first and second bias voltages are set to the first and second optimum bias voltages), and based on the comparison result, the control direction of the first bias voltage, that is, It is determined whether to increase or decrease the first bias voltage.

For example, the relationship between the first bias voltage applied to the external optical modulator 310 and the average power of the optical signal output therefrom has the characteristic shown in FIG. 19 (see the column of the prior art). When the signal level is provided, when the signal level given from the optical power detector 360 is larger than the reference value Po,
Control is performed to increase the first bias voltage. on the other hand,
When the signal level provided from the optical power detector 360 is smaller than the reference value Po, control is performed to reduce the first bias voltage.

As described above, the bias voltage control circuit 350
Is a signal provided from the optical power detector 360,
That is, based on the average power of the output light of the external light modulator 310, it is determined whether to control the first bias voltage to increase or decrease the first bias voltage. Then, the signal value given from the distortion amount detector 370 (the external light modulator 3
Fine adjustment of the first bias voltage is performed so that the amount of distortion included in the ten output optical signals is minimized.

That is, the first bias voltage applied to the external optical modulator 310 and the amount of distortion included in the optical signal output from the external optical modulator 310 are shown in FIG. 17 (see the section of the prior art). As shown, since it has a symmetrical relationship with respect to the optimum bias voltage, it is not possible to immediately judge whether the first bias voltage has increased or decreased simply by observing the variation in the amount of distortion. Therefore, for example, complicated processing as described in the second embodiment is required. However, by adopting the configuration as in the present embodiment, control accuracy of the first bias voltage is improved by relatively simple processing. It is possible to always maintain the first bias voltage at a bias voltage that minimizes the amount of distortion (hereinafter, a first optimum bias voltage).

Further, the bias voltage control circuit 350
The bias voltage applicator 305 controls the second bias voltage applied to the external optical modulator 310 so that the level detected by the RF signal level detector 340 matches the reference level Co. With such a configuration, it is possible to always keep the second bias voltage at a bias voltage at which the RF signal level matches the reference level Co (hereinafter, the second optimum bias voltage).

Here, the first bias voltage control processing and the second bias voltage control processing performed by the bias voltage control circuit 350 will be described in detail. This second bias voltage control processing is the same processing as that of the first embodiment. On the other hand, in the first bias voltage control process, as in the second embodiment, the first bias voltage is controlled so that the amount of distortion given from the distortion amount detector 370 is minimized, so that the first optimal bias voltage is controlled. This is processing for causing the first bias voltage to follow the fluctuation.

The first bias voltage control process is different from that of the second embodiment in that the first bias voltage control should be performed in a direction to increase the first bias voltage before starting the first bias voltage control, or The only point to determine whether the control should be performed in the decreasing direction is based on the average power of the optical signal output from the external optical modulator 310. Specifically, when the first bias voltage applied to the external optical modulator 310 and the average power of the optical signal output from the external optical modulator 310 have a relationship as shown in FIG.
The current light average power is compared with the reference power. If the former is larger than the latter, the first bias voltage is changed in the increasing direction, and if the former is smaller, the first bias voltage is changed in the decreasing direction.

On the other hand, in the second embodiment, the first 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. (See FIG. 9). As a result of changing the first bias voltage in the increasing direction, if the amount of distortion is reduced, the change in the increasing direction may be continued. If the amount of distortion is increased, the first bias voltage is changed. Is changed in a direction away from the first optimum bias voltage, and at that time,
The direction in which the first bias voltage is changed must be reversed in the decreasing direction (the process indicated by the dotted arrow in FIG. 9). Therefore, it sometimes takes time to follow up.

FIG. 12 is a flowchart describing the processing performed by the bias voltage control circuit 350 of FIG. In FIG. 12, first, the bias voltage control circuit 350
The reference power Po is stored (step S31), and then
The reference level Co is stored (Step S32). In addition,
The execution order of steps S31 and S32 may be reversed.

The reference power Po and the reference level Co stored in steps S31 and S32 are equal to the RF signal level detector 340 and the optical power in the initial state in which the first bias voltage and the second bias voltage are respectively optimally adjusted. These are signal values output from the detector 360.

Next, the bias voltage control circuit 350 compares the reference power Po and the reference level Co stored in steps S31 and S32 with the signal values output from the RF signal level detector 340 and the optical power detector 360, respectively. First, the first bias voltage is controlled (step S33), and then the second bias voltage is controlled (step S34). The execution order of steps S33 and S34 may be reversed.
Thereafter, it is determined whether or not to continue the processing (step S).
35) If the determination result is affirmative, each control process of steps S33 and S34 is repeated. On the other hand, if the determination is negative, the process ends.

FIG. 13 is a flowchart showing the operation of step S33 in FIG.
5 is a flowchart describing a first bias voltage control process performed by one bias voltage control circuit 350. In addition,
FIG. 9 visually shows the first bias voltage control processing performed by the bias voltage control circuit 350 in FIG. 11 (see the second embodiment), but is performed by the bias voltage control circuit 350 in FIG. In the process, in the control process indicated by a series of arrows in FIG. 9, the process of the dotted arrow portion is not required. That is, in the second embodiment, the first bias voltage is changed in the increasing direction, and as a result, if the amount of distortion increases, the direction in which the first bias voltage is changed at that time is inverted, and the first bias voltage is changed. Although the process of changing the voltage in the decreasing direction is performed, such a useless process is eliminated in the present embodiment.

In FIG. 13, bias voltage control circuit 3
To 50, the distortion amount m0 detected by the distortion amount detector 370 and the optical average power detected by the optical power detector 360 are given. The bias voltage control circuit 350 first determines whether or not the optical average power has changed by a predetermined threshold value or more by comparing the applied optical average power with the reference power Po (step S401). If the result of the determination is negative, the process proceeds to step S407.

When the result of the determination in step S401 is affirmative, the bias voltage control circuit 350 determines whether or not the optical average power has increased (step S402). If the determination result is positive, the bias voltage control circuit 350
The first bias voltage is increased by a predetermined voltage (Step S403). Next, the distortion amount (m1) after the increase of the first applied bias is given from the distortion amount detector 370, and the bias voltage control circuit 350 accordingly calculates the distortion amounts (m0 and m1) before and after the first applied bias is increased. By comparing with each other, it is determined whether or not the change in the distortion amount is larger than a predetermined threshold value (step S40).
4). If the result of the determination is negative, the process proceeds to step S407.

If the decision result in the step S404 is affirmative, the steps S403 and S404 are repeated. That is, the bias voltage control circuit 350 increases the first bias voltage again, and then the distortion amount detector 370
(M2) after the increase of the first bias voltage given by
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 S407. If the determination result is affirmative, steps S403 and S404 are repeated.

If the decision result in the step S402 is negative, the bias voltage control circuit 350 reduces the first bias voltage by a predetermined voltage (step S4).
05). Next, the distortion amount (m1) after the first applied bias is reduced is given from the distortion amount detector 370, and the bias voltage control circuit 350 accordingly calculates the distortion amounts (m0 and m1) before and after the first applied bias is reduced. By comparing with each other,
It is determined whether or not the change in the amount of distortion is greater than a predetermined threshold (step S406). If the result of the determination is negative, the process proceeds to step S407.

If the decision result in the step S406 is affirmative, the steps S405 and S406 are repeated. That is, the bias voltage control circuit 350 reduces the first bias voltage again, and then the distortion amount detector 370
(M2) after the first 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 S407. If the determination result is positive, steps S405 and S406 are repeated.

In step S407, the bias voltage control circuit 350 holds the first bias voltage at the current value for a certain period. The time for holding the first bias voltage is:
It is set shorter than the period during which aging or temperature changes occur. The above is the first bias voltage control processing. Thereafter, returning to the flow of FIG. 12, the second bias voltage control processing of step S34 is executed.

The bias voltage control circuit 350 performs the above-described first bias voltage control processing, so that the bias voltage applied by the bias voltage applicator 205 to the external optical modulator 210 changes the optimum bias voltage due to the DC drift. Can be followed. Further, when the optimum bias fluctuates, the following can be started more quickly than in the first embodiment.

The details of step S34 in FIG. 12 (the second bias voltage control process performed by the bias voltage control circuit 350 in FIG. 11) are the same as those shown in the flowchart in FIG. 5 (first embodiment). ), And the description is omitted.

Here, as described above, after the first bias voltage has been optimally controlled in step S33, the process proceeds to step S34.
In the case where the second bias voltage is controlled in the first step, the first bias voltage may deviate from the optimum value due to the influence of the latter control processing. Alternatively, contrary to the above, step S3
After optimally controlling the second bias voltage in step 4,
When the control of the first bias voltage is performed at 33, the second bias voltage may deviate from the optimum value due to the influence of the latter control processing.

However, the deviation from the optimum value caused by the influence of the first bias voltage control and the second bias voltage control on each other is described in steps S33 and S33.
The problem can be solved by repeatedly executing 34.

As described above, according to the present embodiment, the carrier light from the light source 300 is converted into the first and second RF signals obtained by branching one RF signal in which a plurality of analog signals are multiplexed. An optical transmission device provided with an external optical modulator 310 for modulating a signal.
Even if the drift occurs, it is possible to suppress the fluctuation of the average power of the optical signal output from the external modulator 310 and the increase in the amount of distortion included in the optical signal, and further, the RF signal included in the output optical signal. Stable transmission characteristics and C that can also prevent level fluctuation (carrier level fluctuation)
An optical transmission device having the / N characteristic is realized.

Further, since the present optical transmitter controls the first bias voltage based on the amount of distortion, the optical transmitter controls the first bias voltage based on the average optical power (see the first embodiment).
The control accuracy of the first bias voltage is improved compared to
More stable transmission characteristics can be obtained.

Further, the optical transmission apparatus determines whether to control the first bias voltage to increase or to decrease the first bias voltage based on the optical average power, so that the first bias voltage is controlled based only on the amount of distortion. Unlike an optical transmitter that controls one bias voltage (see the second embodiment), it is immediately known whether the first bias voltage should be increased or decreased. As a result, it is possible to accurately and promptly follow the fluctuation of the first optimum bias due to the DC drift.

In the above embodiments, the first MZ type optical modulator (see FIG. 2) on the input port side of the two-stage MZ type external optical modulator is used.
The first bias voltage input to the Z interferometer 20 is controlled based on the optical average power and / or the amount of distortion, and the second bias voltage input to the second MZ interferometer 21 on the output port side is converted to an RF signal. Although the control is performed based on the level, the reverse may be applied, that is, the first bias voltage may be controlled based on the RF signal level, and the second bias voltage may be controlled based on the optical average power and / or the amount of distortion.

[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 an example of a configuration of an external light modulator 110 of FIG.

FIG. 3 is a flowchart describing a process performed by a bias voltage control circuit 150 of FIG. 1;

4 is a flowchart showing details of step S13 in FIG. 3 (first bias voltage control processing performed by the bias voltage control circuit 150 in FIG. 1).

5 is a flowchart showing details of step S14 in FIG. 3 (second bias voltage control processing performed by the bias voltage control circuit 150 in FIG. 1).

FIG. 6 is a block diagram illustrating a configuration of an optical transmission device according to a second embodiment of the present invention.

7 is a diagram illustrating a secondary distortion component included in an optical signal output from the external optical modulator 210 of FIG. 6 and an example of characteristics of a filter used in a distortion amount detector 260.

FIG. 8 is a flowchart describing a process performed by the bias voltage control circuit 250 of FIG.

9 is a diagram visually illustrating step S22 of FIG. 8 (first bias voltage control processing performed by the bias voltage control circuit 250 of FIG. 6).

FIG. 10 is a flowchart describing details of step S22 in FIG. 8;

FIG. 11 is a block diagram illustrating a configuration of an optical transmission device according to a third embodiment of the present invention.

FIG. 12 is a flowchart describing processing performed by a bias voltage control circuit 350 of FIG. 11;

13 is a step S33 in FIG. 12 (first bias voltage control processing performed by the bias voltage control circuit 350 in FIG. 11);
9 is a flowchart describing the above.

FIG. 14 is a diagram showing a configuration of a conventional external optical modulator (one-stage MZ type including one MZ interferometer).

15 is a diagram illustrating a relationship between a bias voltage applied to the external optical modulator of FIG. 14 and an average power of an optical signal output from the external optical modulator.

FIG. 16 is a diagram showing a DC drift phenomenon occurring in the external optical modulator of FIG.

FIG. 17 is a diagram showing a relationship between a bias voltage applied to the external optical modulator of FIG. 14 and an amount of secondary distortion included in an optical signal output from the external optical modulator.

FIG. 18 is a diagram showing an example of the configuration of a new conventional external optical modulator (a two-stage MZ type having a configuration in which two MZ interferometers are cascaded).

19 is a diagram showing a relationship between a first bias voltage (Vb1) applied to the external optical modulator of FIG. 18 and an average power of an optical signal output from the external optical modulator (note that FIG.
In the curves A and B, the value of the second bias voltage (Vb2) is different from each other, and the relationship can be seen to change from A to B due to the DC drift phenomenon.

[Explanation of symbols]

 20, 21 MZ interferometer 100, 200, 300 Light source 105, 205, 305 Bias voltage applicator 110, 210, 310 External light modulator 120, 220, 320 Optical splitter 130, 230, 330 Optoelectric converter 140, 240 , 340 RF signal level detector 150, 250, 350 Bias voltage control circuit 160, 360 Optical power detector 260, 370 Strain detector

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/142 10/04 10/06 F term (Reference) 2H079 AA02 AA12 BA01 CA04 EA05 FA01 FA04 HA11 HA23 5K002 AA01 CA01 CA02 CA08 CA14 DA21 FA01

Claims (8)

[Claims] 1. A first and a second carrier light obtained by branching a single RF signal in which a plurality of analog signals are multiplexed.
An optical transmitter that externally modulates and transmits the RF signal with a light source, a light source that outputs a carrier light, an external optical modulator that modulates the carrier light output from the light source with the first and second RF signals, A bias voltage application unit that applies first and second bias voltages to the external light modulator, and a first bias voltage that the bias voltage application unit applies to the external light modulator is output from the external light modulator. An optical transmission device comprising: a bias voltage control unit that controls based on an average power of an optical signal to be performed and controls a second bias voltage based on a level of an RF signal included in the optical signal. 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, an optical power detector that detects the average power of the optical signal output from the external optical modulator by measuring the power of the electrical signal output by the photoelectric converter, RF signal level detection for extracting a component within a specific band from the output electrical signal and measuring the level of the component to detect the level of an RF signal included in the optical signal output from the external optical modulator And a first bias voltage applied by the bias voltage applicator to the external optical modulator so that an average optical power detected by the optical power detector matches a reference power. And, the second bias voltage comprises a bias voltage control circuit for controlling such that the level detected by the RF signal level detector matches the reference level, the optical transmission apparatus according to claim 1. 3. The external optical modulator is a two-stage MZ type external optical modulator having first and second Mach-Zehnder interferometers connected in cascade with each other. A state in which a first RF signal is input in a state where a first bias voltage is applied by the bias voltage applicator, and a state in which a second bias voltage is applied to the second Mach-Zehnder interferometer by the bias voltage applicator The optical transmission device according to claim 1, wherein the second RF signal is input at the input. 4. A first and a second carrier light obtained by branching one RF signal in which a plurality of analog signals are multiplexed.
An optical transmitter that externally modulates and transmits the RF signal with a light source, a light source that outputs a carrier light, an external optical modulator that modulates the carrier light output from the light source with the first and second RF signals, A bias voltage application unit that applies first and second bias voltages to the external light modulator, and a first bias voltage that the bias voltage application unit applies to the external light modulator is output from the external light modulator. And controlling the second bias voltage based on the level of the RF signal included in the optical signal. An optical transmission device, comprising: 5. 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, measuring the level of the component, included in the optical signal output from the external optical modulator, A distortion amount detector that detects the amount of secondary distortion caused by the nonlinearity of the external optical modulator, extracting a component within another specific band from the electric signal output by the photoelectric converter, and An RF signal level detector that detects a level of an RF signal included in the optical signal output from the external optical modulator by measuring a level, and a bias voltage applying unit that applies the bias voltage to the external optical modulator. 1 via The voltage is controlled so that the amount of distortion detected by the distortion amount detector is minimized, and the second bias voltage is controlled so that the level detected by the RF signal level detector matches a reference level. The optical transmission device according to claim 4, further comprising a bias voltage control circuit for controlling. 6. The bias voltage control circuit increases or decreases a first 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 It is determined whether the first bias voltage has increased or decreased before and after the increase or decrease of the first bias voltage. Based on the determination result, it is determined whether the first bias voltage is to be controlled to increase or decrease in the next direction. The optical transmission device according to claim 5, wherein: 7. An optical power detector for detecting an average 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 further includes a bias voltage control circuit configured to increase a first bias voltage applied to the external optical modulator by the bias voltage applicator based on an average optical power detected by the optical power detector. Control or
The optical transmission device according to claim 5, wherein it is determined whether the control is to be performed in a direction of decreasing the light transmission. 8. The external optical modulator is a two-stage MZ external optical modulator having first and second Mach-Zehnder interferometers connected in cascade with each other. A state in which a first RF signal is input in a state where a first bias voltage is applied by the bias voltage applicator, and a state in which a second bias voltage is applied to the second Mach-Zehnder interferometer by the bias voltage applicator The optical transmission device according to claim 4, wherein the second RF signal is input at the input.