JPH11326850A - Light external modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce changes with time of an operating point of a light external modulator and surely set it to an optimal bias point in a short time after a power source is switched on. SOLUTION: This light external modulator shifts an operating point to an optimal bias point by monitoring a light output from LD 10 by an LD light receiving part 11, setting an initial bias setting circuit 17 to an initial control state until a light output reaches a fixed output level, and setting the bias voltage to an initial value consisting of 0 volt or a predetermined voltage, preventing an excessive bias voltage from being applied on a light external intensity modulating element 12, and moreover, after the LD turns on and then the light output reaches a fixed output level, putting a switch to a feedback control state and making the initial bias voltage setting circuit set the bias voltage to a comparison output value with a comparator 15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an external optical modulator for setting a bias voltage of an external optical intensity modulation element to a predetermined initial value when power is turned on.

[0002]

2. Related Background Art Conventionally, in this type of external optical modulator, it is necessary to superimpose a bias voltage on a main signal in order to control an operating point of an external optical intensity modulation element to a certain position. However, this operating point fluctuates with time due to temperature, applied voltage, and the like. Here, since the relationship between the intensity (output signal) of the output light of the optical external intensity modulation element and the bias voltage (input signal) has a periodicity as shown in FIG. 8, there are a plurality of stable points of the operating point. However, the higher the applied voltage, the higher the power consumption and the greater the fluctuation of the operating point. Therefore, the stable point closest to the zero voltage at which the applied voltage is as small as possible is the optimum bias point.

[0003] Further, since the bias voltage is limited only by the power supply voltage and the like, it changes only within a finite range. Therefore, in the initial state when the power is turned on, the bias voltage is applied to a stable point near the upper and lower limit voltage values. Then, the stable point fluctuates slightly, and the signal may be distorted just beyond the range.

Therefore, conventionally, for example, Japanese Patent Application Laid-Open No. 8-76
No. 068, when performing bias control,
In some cases, an initial bias setting circuit is provided so that the bias voltage is controlled to an optimum bias point, and the initial setting bias voltage is controlled to a predetermined voltage level when power is turned on.

[0005]

However, in the above-mentioned optical external modulator, the initial bias setting circuit performs the initial setting corresponding to the ON / OFF of the power supply voltage. If the initial bias setting circuit is operated when there is no light output, the bias voltage moves toward the plus side or the minus side of the voltage. That is, as shown in the waveform diagram of the light output level and the bias voltage of the light source in FIG. 9, since the bias voltage is always applied even when there is no light output, the aging of the operating point of the external optical modulator increases. There was a problem.

In the light output control of the light source, a slow starter circuit is provided in order to avoid an overcurrent at the start of normal light emission to delay the time when the light output level becomes a steady state. The light output is not always stable. Therefore, when the power of the light source LD is turned on,
Since the initial bias setting circuit is controlled once to a voltage corresponding to the light output at the start of light emission and then shifts to the optimum bias voltage, there is a problem that it takes time to reach the optimum bias voltage.

Further, in the optical external modulator, a problem is avoided by configuring a circuit to return the bias voltage to an initial value when the bias voltage is out of a certain range. However, the bias voltage operates normally. Sometimes it drifts and moves, so it is difficult to determine whether it is normal or abnormal in the voltage range, and there is a problem of lack of certainty. For example, even if the voltage range is set to a value that is significantly different from the bias voltage during normal operation and controlled, when the optical output is off when feedback control operates abnormally, a bias voltage control loop cannot be avoided, and Since the initial value of the bias voltage when light is actually input is undefined, an increase in the time required for the stable point to become stable and an increase in power consumption are unavoidable as compared with the normal operation.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide an optical external modulator that can set the operating point of the optical external modulator to an optimum bias point in a short time after power-on and reliably. Aim.

Another object of the present invention is to reduce the aging of the operating point of the optical external modulator.

[0010]

In order to achieve the above object, according to the present invention, a bias voltage is initially set by initial setting means comprising a switch in an initial bias setting circuit based on an optical output from an optical external intensity modulation element. A light output from a light source, such as a laser diode (LD), is detected by an external optical modulator that sets a value and superimposes the bias voltage on the main signal and adds a modulation signal obtained from the main signal to the external optical intensity modulation element A light detecting means including, for example, an LD light receiving unit;
For example, comparing the light output from the light external intensity modulation element with the light output from the light source, an output comparing means including a comparator, and an operation control of an initial setting means according to the detected light output level. And an operation control means for setting the bias voltage to the initial value or the output value based on the comparison, for example, a switch control circuit in an initial bias setting circuit.

That is, the light output from the LD is monitored by the LD light receiving section, and the switch is initially controlled to a bias voltage of 0 V until the light output reaches a constant output level.
Alternatively, by setting the initial value to a predetermined voltage and preventing an unnecessary bias voltage from being applied to the external light intensity modulation element, the secular variation of the operating point is reduced. Further, after the LD is turned on, when the output reaches a certain output level, the switch is set to the feedback control state, the bias voltage is set to the output value obtained by comparison with the comparator, and the operating point is moved to the optimum bias point. The time required to change to the optimum bias point becomes shorter.

Further, according to the present invention, a bias voltage is set to an initial value by initial setting means comprising, for example, a switch similar to the above, and the bias voltage is superimposed on a main signal based on an optical output from an optical external intensity modulation element. Determining means for determining an output level of light from the optical external intensity modulation element by an optical external modulator for applying the modulated signal obtained to the optical external intensity modulation element to the optical external intensity modulation element; and An output comparing means for comparing a light output from a modulation element with a light output from the light source, for example, a comparator, and an initial setting means in accordance with the determined output level of the light; Setting the initial value or the output value by the comparison, for example, comprising an operation control means comprising a switch control circuit, and the output level of the optical signal from the optical external intensity modulation element By constant, the switch may be switched controlled to the initial control state or a feedback control condition.

According to the present invention, the bias voltage is set to an initial value by initial setting means comprising a switch in an initial bias setting circuit, based on the optical output from the optical external intensity modulation element, and the bias voltage is set to the main signal. A light detecting means for detecting a light output from a light source such as a laser diode (LD) by an external light modulator for adding a modulation signal obtained by superimposing the light to the external light intensity modulating element. And an output comparing means comprising, for example, a comparator for comparing the optical output from the optical external intensity modulation element with the optical output from the light source, and holding at least one output value before the output value by the comparison. For example, a holding unit including a voltage holding circuit and an output level of light from the optical external intensity modulation element are determined, and an output held in the holding unit is determined based on the determination result. For example, a determination unit including a level determination circuit for outputting the output voltage detected by the light detection unit and an initial setting unit in accordance with the determination result of the determination unit to control the operation of the bias voltage to the initial value. An operation control unit comprising, for example, a switch control circuit for setting a voltage value based on the comparison output or an output value from the holding unit, and switching the switch to the initial control state or the feedback control state by an optical output from an LD. At the same time, switching control may be performed so as to output, for example, the immediately preceding output value from the comparator held in the voltage holding circuit according to the judgment result from the level judgment circuit.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of an optical external modulator according to the present invention will be described with reference to FIGS.

FIG. 1 shows a first example of an optical external modulator according to the present invention.
FIG. 2 is a configuration diagram illustrating a configuration block of an embodiment. In the figure, the optical external modulator includes an LD light receiving section 11, an optical external intensity modulation element 12, an optical splitter 13, an optical / electrical converter 14, which constitutes the light detecting means of the present invention. And an amplifier 16, an initial bias setting circuit 17 having initial setting means and operation control means of the present invention, an adder 18, and a level determination circuit 25. Note that the optical external intensity modulator 12, the optical splitter 13, the optical / electrical converter 14, the comparator 15, the amplifier 1
6. The loop of the initial bias setting circuit 17 and the adder 18 is for feedback control of the bias voltage.

The LD light receiving section 11 detects the light emitted from the LD 10 and supplies it to the comparator 15 and the initial bias setting circuit 17 as an LD light monitor signal (voltage).

The optical external intensity modulating element 12 is, for example, of an interference type, modulates the intensity of the light from the LD 10 based on the modulating signal given from the adder 18, and outputs the light to the optical splitter 13 as an optical signal. The output optical signal is split by the optical splitter 13, and one of the split optical signals is given to the optical / electrical converter 14. In the optical / electrical converter 14,
The optical signal is optically / electrically converted and output to the comparator 15 as an electric signal (voltage).
Is compared with the electric signal from the converter 14, and a signal having a voltage level corresponding to the difference is output to the amplifier 16. The amplifier 16 amplifies the signal given from the amplifier 16 and outputs the amplified signal to the initial bias setting circuit 17.

As shown in FIG. 2, the initial bias setting circuit 17 includes a level judgment circuit 25 for judging the level of the LD light monitor signal, and a switch 21 which is connected to the adder 18 and constitutes the initial setting means of the present invention. And a switch control circuit 22 which constitutes an operation control means of the present invention for controlling the switch 21 to be switched to the terminal a or the terminal b, and a voltage source 23 connected to the terminal b. Have been.

The voltage source 23 supplies a bias voltage consisting of 0 V (ground level) or a predetermined voltage (an initial voltage optimum for operating the optical external intensity modulation element). The switch control circuit 22 uses the LD
Until the output level of the LD light monitor signal provided from the light receiving section 11 reaches a predetermined constant output level, the switch 21 is connected to the terminal b to enter the initial control state, and the bias voltage applied to the adder 18 Is set to 0V or an initial value consisting of a predetermined voltage, and when the output level of the LD light monitor signal becomes equal to or higher than a predetermined output level in the level determination circuit 25, the switch 21 is connected to the terminal a to enter a feedback control state, and the amplifier 16 Is applied to the adder 18 as a bias voltage.

The adder 18 generates a modulation signal by superimposing the bias voltage from the initial bias setting circuit 17 on the main signal, and outputs the modulation signal to the external optical intensity modulation element 12.

Next, the operation of the optical external modulator having the above configuration will be described based on the waveforms of the optical output level and the bias voltage of the LD in FIG. In this embodiment, the constant output level preset in the switch control circuit 22 is:
For example, the output level is set slightly lower than the light output level of the LD 10 in the steady state.

First, before the power is turned on, the LD light monitor signal corresponding to the light output level of the LD 10 does not reach the set constant output level, so the switch control circuit 22 moves the switch 21 to the terminal b side. The control is switched to the initial control state, and the bias voltage (normally the ground level) from the voltage source 23 is applied to the adder 18.

Next, when the power is turned on and the LD 10 is driven, the light output level of the LD 10 rises and the output level of the LD light monitor signal supplied from the LD light receiving section 11 becomes higher than a certain output level, the switch control is performed. The circuit 22 is
The switch 21 is switched to the terminal a to enter the feedback control state, and the bias voltage from the amplifier 16 (the voltage level corresponding to the difference between the LD light monitor voltage and the electric signal from the converter 14) is applied to the adder 18 to perform the bias control. And move the operating point to the optimal bias point.

As described above, in the present embodiment, the light output of the LD is detected and the operation of the switch of the initial bias setting circuit is controlled in accordance with the light output level. The operating point of the external modulator can be set to the optimum bias point.

Further, in this embodiment, the bias voltage is set to the ground level until the light output level of the LD reaches a constant output level, so that no extra bias voltage is applied to the optical external intensity modulation element, and the operating point becomes lower. Aging fluctuations can be reduced.

FIGS. 4 and 5 are a configuration diagram showing a configuration block of a second embodiment of the optical external modulator according to the present invention and a configuration diagram showing a configuration of an initial bias setting circuit thereof. In the following drawings, the same components as those in the first embodiment are denoted by the same reference numerals for convenience of explanation.

FIGS. 4 and 5 are different from the first embodiment in that the operation of the switch of the initial bias setting circuit is controlled in accordance with the voltage level of the electric signal supplied from the optical / electrical converter 14. That is, the optical-electrical converter 14
Is determined by a level determination circuit 19 and output to a switch control circuit 22. The switch control circuit 22 determines that the voltage level of the electrical signal is a constant output level instead of the LD light monitor signal. , The switch 21 is in the initial control state, the voltage from the voltage source 23 is used as a bias voltage, and when the voltage level of the electric signal reaches a constant output level, the switch 21 is set in the feedback control state, and the voltage from the amplifier 16 is biased. The voltage is applied to the adder 18 as a voltage.

As described above, in this embodiment, the level of the optical output of the optical external intensity modulation element is determined, and the operation of the switch of the initial bias setting circuit is controlled according to the optical output level. Similarly, in a short time after the power is turned on, the operating point of the optical external modulator can be set to the optimum bias point reliably, and no extra bias voltage is applied to the optical external intensity modulation element, so that the aging of the operating point is reduced. it can.

The present embodiment is particularly effective when the voltage characteristic of the electric signal from the optical-electrical converter 14 is superior to the optical monitor signal of the LD. Further, it is also possible to connect the level determination circuit to the monitoring system and output the voltage level determined by the level determination circuit to the monitoring system. In this case, it is possible to output the voltage level from the optical / electrical converter. The electric signal can be monitored, and maintenance of the optical external modulator is facilitated.

By the way, in such an optical external modulator,
Even if the LD or optical external intensity modulator is operating normally,
The electric converter may fail.

Therefore, in the third embodiment, as shown in FIGS. 6 and 7, a voltage holding circuit 24 for holding the voltage level of the electric signal from the optical / electrical converter 14 in the feedback control state is provided. The circuit 24 is connected to the terminal c. Further, the level determination circuit 19 gives the determined voltage level to the voltage holding circuit 24 together with the switch control circuit 22, and the switch control circuit 22
Is controlled by an LD light monitor signal (level determination circuit 25).
Is performed in accordance with the voltage level of the electrical signal from the optical-electrical converter 14 (determined by the level determining circuit 19), and the switch 21 is controlled to switch to the terminals a to c.

That is, the switch control circuit 22 includes the LD
When both the voltage level of the optical monitor signal and the voltage level of the electric signal from the optical-electrical converter 14 are equal to or higher than a certain output level, the switch 21 is switched to the terminal a to enter a feedback control state. When only the voltage level of the signal is lower than the fixed output level, it is determined that the optical / electrical converter 14 has failed, and the switch 21 is switched to the terminal c.

At this time, since the voltage level of the electric signal in the feedback control state immediately before switching is held in the voltage holding circuit 24, the voltage holding circuit 24
Outputs the held voltage when the voltage level from the level determination circuit 19 falls below a predetermined value. Therefore, a voltage having the same level as the bias voltage in the feedback control state is applied to the adder 18.

As described above, in this embodiment, the operation of the switch of the initial bias setting circuit is controlled in accordance with the light output level of the LD and the light output level of the external light intensity modulation element. In addition to having the same effect, the voltage from the voltage holding circuit can be supplied as a bias voltage even when the optical-electrical converter fails, so that the external optical modulator can be driven in the same way as the feedback control state Becomes

Also, in this embodiment, as in the second embodiment, if the level determination circuit is connected to the monitoring system, the failure of the optical / electrical converter can be notified to the monitoring system, and the monitor can repair the device. During this time, the transmission of the optical signal can be performed without stopping, and the stability and reliability of the operation can be improved.

The present invention is not limited to these embodiments, and various modifications can be made without departing from the gist of the present invention.

[0037]

As described above, according to the present invention, the bias voltage is set to the initial value by the initial setting means based on the optical output from the external optical intensity modulator, and the bias voltage is obtained by superimposing the bias voltage on the main signal. In the optical external modulator for applying the modulated signal to the optical external intensity modulation element, the operation of the initial setting means is controlled according to the optical output level from at least one of the light source and the optical external intensity modulation element. Until a constant output level is reached, the initial setting means is set to the initial control state, the bias voltage is set to the initial value, and an unnecessary bias voltage is prevented from being applied to the optical external intensity modulation element.
The secular variation of the operating point of the optical external modulator can be reduced, and after reaching a certain output level, the initial setting means is set in a feedback control state, and the operating point is moved to the optimum bias point, so that the power supply is turned on in a short time. In addition, the operating point of the external optical modulator can be reliably set to the optimum bias point.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing configuration blocks of a first embodiment of an optical external modulator according to the present invention.

FIG. 2 is a configuration diagram illustrating a configuration of an initial bias setting circuit illustrated in FIG. 1;

FIG. 3 is a waveform diagram showing a temporal change of an optical output level and a bias voltage of an LD according to the present invention.

FIG. 4 is a configuration diagram showing configuration blocks of a second embodiment.

FIG. 5 is a configuration diagram illustrating a configuration of an initial bias setting circuit illustrated in FIG. 4;

FIG. 6 is a configuration diagram showing the configuration blocks of a third embodiment.

FIG. 7 is a configuration diagram illustrating a configuration of an initial bias setting circuit illustrated in FIG. 6;

FIG. 8 is a relationship diagram showing a relationship between the intensity of output light and a bias voltage.

FIG. 9 is a waveform diagram showing changes over time in the light output level of the light source and the bias voltage.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 LD light receiving part 12 Optical external intensity modulation element 13 Optical splitter 14 Optical / electrical converter 15 Comparator 16 Amplifier 17 Initial bias setting circuit 18 Adder 19, 25 Level judgment circuit 21 Switch 22 Switch control circuit 23 Voltage source 24 Voltage holding circuit

Claims (7)

[Claims] 1. A modulation signal obtained by setting a bias voltage to an initial value by initial setting means based on an optical output from an optical external intensity modulation element and superimposing the bias voltage on a main signal is output from the optical external intensity modulation element. An optical external modulator added to the above, comprising: a light detection unit for detecting a light output from a light source; and an operation control unit for controlling the operation of an initial setting unit in accordance with the detected output level of the light. And an optical external modulator. 2. The optical external modulator includes an output comparing unit that compares an optical output from the optical external intensity modulation element with an optical output from the light source, and the operation control unit includes: The optical external modulator according to claim 1, wherein the operation is controlled to set the bias voltage to the initial value or the output value by the comparison. 3. A modulation signal obtained by setting a bias voltage to an initial value by initial setting means based on an optical output from the optical external intensity modulation element and superimposing the bias voltage on a main signal is output from the optical external intensity modulation element. A light external modulator to be added to the light emitting device; a determination unit that determines an output level of light from the external light intensity modulation element; and an operation control unit that controls operation of an initial setting unit according to the determined light output level. An optical external modulator comprising: 4. The external optical modulator includes an output comparing unit that compares an optical output from the optical external intensity modulator with an optical output from the light source, and the operation control unit includes an output setting unit that sets the initial setting unit. 4. The external optical modulator according to claim 3, wherein operation control is performed to set the bias voltage to the initial value or the output value based on the comparison. 5. A modulation signal obtained by setting a bias voltage to an initial value by initial setting means based on an optical output from the optical external intensity modulation element, and superimposing the bias voltage on a main signal. An optical external modulator to be added to: an optical detection unit that detects an optical output from a light source; an output comparing unit that compares an optical output from the optical external intensity modulation element with an optical output from the light source; Holding means for holding at least one previous output value among the output values according to the following formulas: determining an output level of light from the optical external intensity modulation element; and an output value held by the holding means from the determination result. And an operation control means for controlling the operation of the initial setting means according to the output level detected by the light detection means and the judgment result of the judgment means. 6. The initialization means sets the bias voltage to 0 until the light from the light source reaches a constant output level.
6. The external optical modulator according to claim 1, wherein the external modulator is set to an initial value consisting of V or a predetermined voltage. 7. The operation control unit controls the operation of the initial setting unit to set the bias voltage to the initial value, a voltage value based on the comparison output, or an output value from the holding unit. The optical external modulator according to claim 5, wherein: