JP2010080557A - Optical transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To avoid unstability of an APC before ABC control comes to be stable at the time of starting ABC. <P>SOLUTION: An optical transmitter of external modulation method is equipped with: an ABC part 113 which superposes a predetermined ratio of low frequency signal on a voltage data signal which comes from an optical modulation driver 103 for driving an LiNbO<SB>3</SB>optical modulator 112; a first I-V conversion part 108 for converting the average value of optical currents from a first PD114 that receives optical signal using the optical modulator into a voltage; a first voltage comparing part 107 which compares the voltage form the first I-V conversion part with a reference voltage for forming an error signal; a second I-V conversion part 111 which converts the average value of optical currents from a second PD109 that receives rear output light output from a semiconductor laser into a voltage, a second voltage comparing part 110 which compares the voltage from the second I-V conversion part with the reference voltage for forming an error signal; and a laser current control part 105 which makes laser current control to an DC current source 102, based on the error signal of the first voltage comparing part when notified of stable state from the ABC part, based on the error signal of the second voltage comparing part at the time of start-up of the ABC part. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

The present invention relates to an external modulation type optical transmitter using a semiconductor laser and a LiNbO ₃ optical modulator. In particular, the present invention relates to an optical transmitter that enables automatic output control (APC) to function effectively when the automatic bias control (ABC) of a LiNbO ₃ optical modulator is activated, thereby enabling optical output stability control.

2. Description of the Related Art An external modulation type optical transmitter using a semiconductor laser and a LiNbO ₃ optical modulator has been widely used in trunk optical communication systems and the like with the recent increase in speed and distance of optical communication systems.
In order to realize high speed and long distance, stabilization of optical output from the optical transmitter is indispensable.

FIG. 4 is a block diagram showing a schematic configuration of an optical transmitter as a premise of the present invention. In addition, the same code | symbol is attached | subjected to the same component throughout a figure.
As shown in the figure, a semiconductor laser 101 is provided in an optical transmitter 100 of an external modulation system, and the semiconductor laser 101 outputs DC light.
A direct current source 102 is connected to the semiconductor laser 101, and the direct current source 102 supplies a direct current to the semiconductor laser 101.

A LiNbO ₃ optical modulator 112 is connected to the semiconductor laser 101, and the LiNbO ₃ optical modulator 112 receives the DC light output from the semiconductor laser 101, modulates it, and outputs the modulated optical signal to the optical fiber 200. .
LiNbO ₃ optical modulator 112 is connected an optical modulator driver 103, an optical modulator driver 103 inputs the data signal, and outputs a voltage data signal to the LiNbO ₃ optical modulator 112 as a modulation signal LiNbO ₃ optical modulator 112 is driven.

A photodiode (PD) 114 is provided at the output end of the LiNbO ₃ optical modulator 112. The photodiode 114 receives the optical signal generated by the LiNbO ₃ optical modulator 112 and converts it into a photocurrent.
A current-voltage (IV) converter 117 is connected to the photodiode 114, and the current-voltage converter 117 is composed of an operational amplifier and converts the average value of the photocurrent converted by the photodiode 114 into a voltage.

Further, an automatic bias control (ABC) unit 113 is provided, and the automatic bias control (ABC) unit 113 is set to an optimum DC control voltage against a DC (direct current) drift occurring in the LiNbO ₃ optical modulator 112. In general, a low frequency signal of a certain ratio is superimposed on a voltage data signal that is an optical data signal from the optical modulation driver 103.
A voltage comparator 116 is connected to the current-voltage converter 117. The voltage comparator 116 is composed of an operational amplifier, compares the voltage converted by the current-voltage converter 117 with the reference voltage Vref, and based on the error signal. Feedback control is performed on the DC current source 102 to control the laser current.

That is, in the automatic output control (APC) including the photodiode 114, the current-voltage conversion unit 117, the voltage comparison unit 116, and the DC current source 102, the semiconductor laser 101 and the LiNbO ₃ optical modulation are caused by the fluctuation of the semiconductor laser 101 itself. To the DC current source 102 based on the error signal due to the optical connection loss with the optical device 112 and the insertion loss variation of the LiNbO ₃ optical modulator 112 and due to the low frequency component amount superimposed by the ABC unit 113. Feedback control is performed, and the laser output is controlled to stabilize the light output.

However, in the automatic bias control (ABC) unit 113, when the automatic bias control (ABC) unit 113 starts up, the input optical power to the LiNbO ₃ optical modulator 112 is constant during the period until the automatic bias control (ABC) is stabilized. Even so, the average light output from the LiNbO ₃ optical modulator 112 is changed.
In addition, when the input light power to the LiNbO ₃ optical modulator 112 changes, the absolute amount of the low-frequency signal detected by the photodiode 114 varies even with the same shift amount.

As a result, in automatic output control (APC), feedback control is performed only with the average light output from the LiNbO ₃ optical modulator 112, and therefore the laser current is largely varied during the period until the control of the ABC unit 113 is stabilized and becomes unstable. , Does not pull in stably. For this reason, it becomes a problem to stabilize the laser current for a period until the control of the ABC unit 113 is stabilized.
There are the following conventional techniques related to the output stability of such an optical transmitter.
2. Description of the Related Art Conventionally, a Mach-Zehnder type optical modulator using a laser element as a light source in order to provide a Mach-Zehnder type optical modulator capable of feedback compensation with respect to fluctuations due to optical coupling loss and characteristic changes specific to a functional device, and a control method thereof. In the control method, a step of modulating light emitted from the laser element and extracting it as output light; a step of detecting monitor light separately from the output light out of the light after light modulation; and the monitor light And a step of performing feedback control of the light output intensity of the laser element based on the above (see, for example, Patent Document 1).

However, Patent Document 1 does not improve instability of automatic output control (APC) during a period until the automatic bias control (ABC) is stabilized when the automatic bias control (ABC) unit is started.
Another object of the present invention is to provide an optical transmitter that can keep the power of output light below a specified value in a transient state such as when the power is turned on in an optical transmission system. LD that emits light, an LD drive circuit that keeps the output of the LD constant, an LN modulator that modulates CW light, and a modulator drive circuit that outputs a drive signal to the LN modulator according to an input data signal A bias control circuit for preventing DC bias drift in the LN modulator, a bias monitor circuit for monitoring the bias voltage of the LN modulator, and a bias current to the LD in a normal state when the optical transmitter is in a transient state. Some include an output control circuit that controls the LD drive circuit so as to be set lower than a predetermined value (see, for example, Patent Document 2).

However, Patent Document 2 does not improve instability of automatic output control (APC) during a period until the automatic bias control (ABC) is stabilized when the automatic bias control (ABC) unit is started.
Further, in the conventional external modulation system using an optical modulator, even when there is a change in the optical connection between the LD and the modulator and the insertion loss of the modulator, the optical output level output from the optical modulator In order to keep the average value constant, there is an operational amplifier that converts the photocurrent flowing through the semiconductor optical modulator into a voltage, and a negative feedback circuit that compares the output voltage of the operational amplifier with the reference voltage Vref and reduces the difference. In some cases, when the photocurrent flowing through the optical modulator decreases, the optical output of the LD, which is a light source, is controlled to keep the average value output from the optical modulator constant (for example, see Patent Document 3).

However, Patent Document 3 does not improve the instability of automatic output control (APC) during the period until the automatic bias control (ABC) is stabilized when the automatic bias control (ABC) unit is started.
Conventionally, the mark ratio detection circuit converts the input into light by the drive circuit in order to stabilize the optical output by enabling the optimum operating point of the external modulator to follow the fluctuation of the mark ratio of the input data signal. The average value of the data signal is detected and given to the comparison circuit, and the average value of the power of the branched light output signal detected by the monitoring photodiode is given to the comparison circuit. The average value is compared, and the comparison result is given to the automatic output constant control circuit. The automatic output constant control circuit, based on the comparison result, powers the semiconductor laser via the drive circuit in order to stabilize the optical output signal. As a result, the operating point of the external modulator can be always kept optimal (for example, Patent reference 4).

  However, Patent Document 4 does not improve instability of automatic output control (APC) during a period until the automatic bias control (ABC) is stabilized when the automatic bias control (ABC) unit is started.

JP 2007-093717 A JP 2007-123747 A JP 05-226751 A JP 08-163043 A

  Therefore, in view of the above problems, the present invention provides an optical transmitter capable of avoiding instability of automatic output control (APC) during a period until automatic bias control is stabilized when the automatic bias control (ABC) unit is started. The purpose is to provide.

In order to solve the above problems, the present invention provides an external modulation type optical transmitter using a semiconductor laser and a LiNbO ₃ optical modulator, a direct current source for supplying a direct current to the semiconductor laser, and a data signal enter a, and the optical modulator driver outputs a voltage data signal to the LiNbO ₃ optical modulator for driving the LiNbO ₃ optical modulator as a modulation signal, a low a percentage to the voltage data signal from the optical modulator driver The frequency signal is superimposed and set to the optimum DC control voltage against the DC drift that occurs in the LiNbO ₃ optical modulator. After the start-up period when the control is stable, the control state becomes stable. an automatic bias control unit to notify, located at the output end of the LiNbO ₃ optical modulator, converts the optical signal generated by the LiNbO ₃ optical modulator to the light receiving and light current A first photodiode; a first current-voltage converter that converts an average value of the photocurrent converted by the first photodiode into a voltage; and a voltage converted by the first current-voltage converter. Is compared with the reference voltage Vref1 to form an error signal, a second photodiode that receives the backward output light output from the semiconductor laser and converts it into a photocurrent, and the first voltage comparator. A second current-voltage conversion unit that converts an average value of the photocurrent converted by the photodiode of 2 into a voltage, a voltage converted by the second current-voltage conversion unit, and a reference voltage Vref2, The second voltage comparison unit that forms the error signal, the error signal from the first voltage comparison unit, and the error signal from the second voltage comparison unit are input, and the automatic bias control unit stabilizes at startup. Received a notification of the condition, When starting the dynamic bias control unit, laser current control is performed on the DC current source based on the error signal of the second voltage comparison unit, and when a notification of a stable state is received from the automatic bias control unit, There is provided an optical transmitter comprising a laser current control unit that performs laser current control on a DC current source based on an error signal of the first voltage comparison unit.

Further, a mark rate detection unit for detecting a signal of a mark rate in the voltage data signal output from the light modulation driver is provided, and the mark rate detection unit is configured to detect the first current-voltage conversion unit according to the detected mark rate. The reference voltage Vref1 is varied. For example, when the mark rate is small, the reference voltage Vref1 is decreased, and when the mark rate is large, the reference voltage Vref1 is increased. The fluctuation amount and fluctuation direction of the reference voltage Vref1 depend on the module used and the circuit constant.
Further, the first photodiode is incorporated in the LiNbO ₃ optical modulator.

Further, the first photodiode is installed after the optical signal output from the LiNbO ₃ optical modulator is branched by an optical coupler.
Further, the semiconductor laser and the second photodiode are incorporated in a laser module.
Further, the mark ratio detection unit is provided on the input side of the light modulation driver, or is provided in the differential drive unit when the light modulation driver takes the form of a differential drive unit.

As described above, according to the present invention, a DC current source that supplies a DC current to the semiconductor laser and a data signal are input, and a voltage data signal is output as a modulation signal to the LiNbO ₃ optical modulator to output LiNbO _3. An optical modulator driver that drives the optical modulator, and a low frequency signal of a certain ratio superimposed on the voltage data signal from the optical modulation driver, and set to an optimum DC control voltage against DC drift that occurs in the LiNbO ₃ optical modulator and, control is passed period to stabilize after startup, the automatic bias control unit for notifying the stable state when the control enters a stable state, located at the output end of the LiNbO ₃ optical modulator, LiNbO ₃ optical modulator A first photodiode that receives an optical signal generated by the device 103 and converts it into a photocurrent, and a first current that converts an average value of the photocurrent converted by the first photodiode into a voltage The voltage conversion unit, the voltage converted by the first current-voltage conversion unit and the reference voltage Vref1 are compared, the first voltage comparison unit that forms an error signal, and the backward output light output from the semiconductor laser A second photodiode that receives light and converts it into a photocurrent, a second current-voltage conversion unit that converts an average value of the photocurrent converted by the second photodiode into a voltage, and a second current-voltage conversion The voltage converted by the first section is compared with the reference voltage Vref2, and an error signal from the second voltage comparison section, the first voltage comparison section, and the error signal from the second voltage comparison section are formed. The automatic bias control unit receives a notification of the stable state at the start-up, and when the automatic bias control unit starts up, the laser current control is performed on the DC current source based on the error signal of the second voltage comparison unit. Stable state from the bias controller And a laser current control unit that performs laser current control on the basis of the error signal of the first voltage comparison unit with respect to the DC current source when receiving the knowledge, so that the automatic bias control (ABC) unit Since automatic output control (APC) is performed by the backward output light output from the semiconductor laser at startup, even if the average light output fluctuation of the LiNbO ₃ optical modulator occurs until the automatic output control (APC) is stabilized. The automatic output control (APC) is not affected, and as a result, there is no oscillation due to the automatic bias control (ABC), and it is possible to start up stably.
Furthermore, it is possible to prevent the transmission characteristics from being deteriorated without being affected by fluctuations in the mark rate of the data signal.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing a schematic configuration of an optical transmitter according to the present invention. As shown in this figure, the optical transmitter 100 according to the present invention adopts an external modulation method using a semiconductor laser 101 and a LiNbO ₃ optical modulator 112, and compared with FIG. 4, a current-voltage conversion unit. 117. Instead of the voltage comparison unit 116, a current-voltage conversion unit 108 and a voltage comparison unit 107 are provided.

The voltage comparison unit 107 compares the voltage converted by the current-voltage conversion unit 108 with the reference voltage Vref1, and feedback control is performed on the DC current source 102 based on the error signal, thereby controlling the laser current. Done.
Here, the current-voltage conversion unit 108 and the voltage comparison unit 107 may be formed of an analog circuit similarly to the current-voltage (IV) conversion unit 117 and the voltage comparison unit 116, or a CPU (Central Processing Unit). ) Etc. may be used for digital control.

Further, the laser module 115 includes a semiconductor laser 101 and a photodiode (PD) 109, and the photodiode 109 receives the backward output light output from the semiconductor laser 101 and converts it into a photocurrent.
A current-voltage conversion unit 111 is connected to the photodiode 109, and the current-voltage conversion unit 111 converts an average value of the photocurrent converted by the photodiode 109 into a voltage.

A voltage comparison unit 110 is connected to the current-voltage conversion unit 111. The voltage comparison unit 110 compares the voltage converted by the current-voltage conversion unit 111 with the reference voltage Vref2, and based on the error signal, the direct current source 102 Is subjected to feedback control, and laser current is controlled.
The automatic bias control (ABC) unit 113 indicates that the automatic bias control (ABC) is in a stable state with respect to a laser current control unit 105 (to be described later) when a period until the automatic bias control is stabilized after the startup has elapsed. Make a notification.

  A laser current control unit 105 is connected to the automatic bias control (ABC) unit 113, the voltage comparison unit 107, and the voltage comparison unit 110, and the automatic bias control (ABC) unit 113 is connected to the automatic bias control (ABC). In response to the notification that the current is in a stable state, error signals are input from the voltage comparison unit 107 and the voltage comparison unit 110, respectively, and the following laser current control is performed on the DC current source 102 based on such information. .

The current-voltage conversion unit 111, the voltage comparison unit 110, and the laser current control unit 105 may be formed by analog circuits, or may be digital control using a CPU (Central Processing Unit) or the like.
Further, although the example in which the photodiode 114 is built in the LiNbO ₃ optical modulator 112 is shown, it may be installed after the optical signal output from the LiNbO ₃ optical modulator 112 is branched by an optical coupler.

FIG. 2 is a flowchart for explaining an operation example of the laser current control unit 105 in FIG.
In step S <b> 301, the laser current control unit 105 performs laser current control using the error signal from the voltage comparison unit 110 to the DC current source 102 when the automatic bias control (ABC) unit 113 is started, and the voltage comparison unit 107. This error signal is not used for laser current control. Thus, until the ABC stability of the automatic bias control (ABC) unit 113 is obtained, only the fluctuation of the semiconductor laser 101 itself is stabilized. That is, until the automatic bias control (ABC) is stabilized, the control for setting the optimum DC control voltage against the DC (DC) drift occurring in the LiNbO ₃ optical modulator 112 is not performed. Along with this, control due to the optical connection loss between the semiconductor laser 101 and the LiNbO ₃ optical modulator 112 and the insertion loss variation of the LiNbO ₃ optical modulator 112 is not performed.

In step S302, the laser current control unit 105 determines whether or not the automatic bias control (ABC) unit 113 has received a notification that the ABC stable state has been obtained. If the ABC stable state notification cannot be obtained, the laser current control unit 105 proceeds to step S301. Return.
In step S303, when the laser current control unit 105 receives a notification of the ABC stable state, the laser current control using the error signal from the voltage comparison unit 110 is stopped for the DC current source 102, and the voltage comparison unit 107 is stopped. The error signal is used for laser current control. As a result, automatic output control (APC) enables stable pull-in without greatly changing the laser current, and the optical connection between the semiconductor laser 101 and the LiNbO ₃ optical modulator 112 due to fluctuations in the semiconductor laser 101 itself. losses and due to the insertion loss variation of LiNbO ₃ optical modulator 112, and an automatic bias control (ABC) 113 for setting the optimum DC control voltage to DC (direct current) drift occurring in LiNbO ₃ optical modulator 112 The laser current is controlled by feedback control on the direct current source 102 based on the error signal caused by the low frequency component amount superimposed by the above, so that the optical output can be stabilized.

Therefore, according to the present invention, when the automatic bias control (ABC) unit 113 is started, automatic output control (APC) is performed by the backward output light output from the semiconductor laser 101. Therefore, automatic output control (APC) is performed. Even if the average optical output fluctuation of the LiNbO ₃ optical modulator 112 until it stabilizes, it does not affect the automatic output control (APC), and as a result, there is no oscillation by the automatic bias control (ABC), and it can be started stably. Became.

  FIG. 3 is a block diagram showing a schematic configuration of an optical transmitter according to a modification of FIG. As shown in this figure, compared to FIG. 2, a mark rate detection unit 106 is provided, the mark rate detection unit 106 is located on the output side of the light modulation driver 103, and voltage data output from the light modulation driver 103. A mark ratio signal in the signal is detected and output to the voltage comparison unit 107. Here, the mark ratio detection unit 106 may be located on the input side of the light modulation driver 103, or provided in the differential drive unit when the light modulation driver 103 takes the form of differential drive. Also good.

The voltage comparison unit 107 varies the reference voltage Vref1 according to the mark ratio of the detected signal. For example, the voltage conversion unit 107 decreases the reference voltage Vref1 when the mark ratio is small, and increases the reference voltage Vref1 when the mark ratio is large. The fluctuation amount and fluctuation direction of the reference voltage Vref1 depend on the module used and the circuit constant.
The error signal extracted by the voltage comparison unit 107 based on the reference voltage Vref1 thus changed is output to the laser current control unit 105 and used to control the laser current.

  Conventionally, a transmitter using automatic output control (APC) is susceptible to fluctuations in the mark rate of a data signal. For example, when 0 “space” signals are continuous, the average optical output is independent of loss. As a result, the automatic output control (APC) differentially tries to keep the average optical output constant based on the signal, which increases the laser current excessively and increases the optical output of 1 “mark” signal. As a result, it becomes susceptible to nonlinear effects of the fiber such as SPM (self-phase modulation) depending on the optical peak power, which deteriorates the transmission characteristics, while 1 “mark” signal is continuous. Will increase the average optical power regardless of loss and excessively reduce the laser current. As a result, the optical output of 1 “mark” will be decreased, and the receiving side characteristics such as code error rate degradation will be reduced. Although degradation, according to the present invention, without the influence of mark ratio variation of such data signals, it is possible so as not to degradation of transmission characteristics.

It is a block diagram which shows schematic structure of the optical transmitter which concerns on this invention. 2 is a flowchart for explaining an operation example of a laser current control unit 105 in FIG. 1. It is a block diagram which shows schematic structure of the optical transmitter which concerns on the modification of FIG. It is a block diagram which shows schematic structure of the optical transmitter used as the premise of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 ... Optical transmitter 101 ... Semiconductor laser 102 ... DC current source 103 ... Optical modulation driver 105 ... Laser current control part 106 ... Mark rate detection part 107 ... Voltage comparison part 108 ... Current-voltage (IV) conversion part 109 ... Photodiode (PD)
110 ... voltage comparator 111 ... current over voltage (I-V) conversion unit 112 ... LiNbO ₃ optical modulator 113 ... automatic bias control (ABC) 114 ... photodiode (PD)
115 ... Laser module 200 ... Optical fiber

Claims (6)

In an optical transmitter of an external modulation system using a semiconductor laser and a LiNbO ₃ optical modulator,
A direct current source for supplying a direct current to the semiconductor laser;
Enter the data signal, and the optical modulator driver outputs a voltage data signal to the LiNbO ₃ optical modulator for driving the LiNbO ₃ optical modulator as a modulation signal,
A low frequency signal of a certain ratio is superimposed on the voltage data signal from the optical modulation driver and set to an optimum DC control voltage against DC drift that occurs in the LiNbO ₃ optical modulator, and a period in which the control is stabilized after startup has elapsed. And an automatic bias control unit that notifies the stable state when the control becomes stable,
A first photodiode for converting said LiNbO ₃ located at the output end of the optical modulator, an optical signal generated by the LiNbO ₃ optical modulator to the light receiving and light current,
A first current-voltage converter that converts an average value of the photocurrent converted by the first photodiode into a voltage;
A first voltage comparison unit that compares the voltage converted by the first current-voltage conversion unit with a reference voltage Vref1 to form an error signal;
A second photodiode that receives backward output light output from the semiconductor laser and converts it into photocurrent;
A second current-voltage conversion unit that converts an average value of the photocurrent converted by the second photodiode into a voltage;
A second voltage comparison unit that compares the voltage converted by the second current-voltage conversion unit with a reference voltage Vref2 to form an error signal;
The error signal from the first voltage comparison unit and the error signal from the second voltage comparison unit are input, and the automatic bias control unit is notified of the stable state at the time of activation, and the automatic bias control unit is activated. Sometimes the laser current control is performed on the DC current source based on the error signal of the second voltage comparison unit, and when the notification of the stable state is received from the automatic bias control unit, the DC current source An optical transmitter comprising: a laser current control unit that performs laser current control based on an error signal of the first voltage comparison unit. A mark rate detection unit that detects a signal of a mark rate in the voltage data signal output from the light modulation driver is provided, and the mark rate detection unit is a reference for the first current-voltage conversion unit according to the detected mark rate. The optical transmitter according to claim 1, wherein the voltage Vref1 is varied. The optical transmitter according to claim 1, wherein the first photodiode is built in the LiNbO ₃ optical modulator. 2. The optical transmitter according to claim 1, wherein the first photodiode is installed after the optical signal output from the LiNbO ₃ optical modulator is branched by an optical coupler. 3. The optical transmitter according to claim 1, wherein the semiconductor laser and the second photodiode are built in a laser module. The mark ratio detection unit is provided on the input side of the light modulation driver, or is provided in the differential drive unit when the light modulation driver is in the form of a differential drive unit, The optical transmitter according to claim 1.

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