JP2007123747A - Optical transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmitter of an optical communication system capable of suppressing a power of an output light down to not more than the predetermined value in the transient state of a power-on or the like. <P>SOLUTION: The optical transmitter comprises an LD 1 for emitting a CW light, an LD driving circuit 2 for holding the output of the LD 1 constant, an LN modulator 4 for modulating the CW light, a modulator drive circuit 5 for outputting a driving signal to the LN modulator 4 in response to an input data signal, a bias control circuit 6 for preventing a DC bias drift of the LN modulator 4, a bias monitoring circuit 7 for monitoring a bias voltage of the LN modulator 4, and an output control circuit 3 for controlling the LD driving circuit 2 so as to set the bias current to the LD 1 down to not more than the normal predetermined value at the time of the transient state of the optical transmitter. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a control technique for an optical transmitter used in an optical transmission system.

In a conventional optical transmission system, in particular, an optical transmitter used in a wavelength division multiplexing optical transmission system, and an optical transceiver including the optical transmitter, a modulation method according to the transmission distance has been applied.
For example, for a short distance, a direct modulation method for turning ON / OFF the optical output by applying a data signal to the bias current is applied to a semiconductor laser diode (hereinafter referred to as LD). For long distances, it is common to apply an external modulation system that combines an LD and an external modulator.

In particular, as an external modulator for a long distance, in addition to a modulator built-in LD in which an electroabsorption optical modulator (EA: Electro Absorption, hereinafter abbreviated as EA modulator) is integrated in the LD, lithium niobate (LiNbO) is used. ₃ : LN modulator using LN crystal is generally used. Since the LN modulator is less wavelength-dependent than the EA modulator, it can be combined with a tunable LD that is used in a wide wavelength range, or in combination with an LD that emits light at a predetermined wavelength, thereby providing a DWDM (Dense Wavelength). Often used for optical transmitters in Division Multiplex systems.

In an optical transmitter using an LN modulator, the LD keeps the output of the LD constant by controlling the bias current to the LD by an automatic power control (hereinafter referred to as APC) circuit. The LN modulator that has received the output light from the LD changes the refractive index of the light and changes the phase by applying a voltage based on the data modulation signal to the internal LN crystal. The output signal is turned ON / OFF by the mutual interference.
In addition, in order to solve the problem of DC bias drift (drift of the operating characteristic curve) due to temperature fluctuations and long-time bias application, a bias control circuit that applies a DC bias to keep the output characteristics of the LN modulator constant is provided. (For example, refer to Patent Document 1). The operating characteristic curve is expressed as the output optical power periodically increasing / decreasing with increasing voltage.

However, when the power is turned on or when no data signal is input, the LD is disabled and the modulated light cannot be monitored, or when a failure occurs, the DC bias control by the bias control circuit becomes indeterminate and applied. DC bias becomes the maximum value or the minimum value.
At these bias points, the output light from the LD (Continuous Wave: hereinafter referred to as CW light) cannot be modulated, and there is a possibility that the CW light is output without being modulated.
In the case of normal modulated light, the ON / OFF ratio of light is 1: 1, so the average power is an output that is half that of CW light, that is, 3 dB lower. However, unmodulated CW light may be output with 3 dB higher than the transmission power specified for the optical transmitter, and may exceed the rated power of the light receiving element connected to the optical transmitter. . As a result, there is a problem that the reliability of the entire optical transmission system is reduced.

JP 2000-180804 A

  Since the conventional optical transmitter is configured as described above, there has been a problem that excessive optical output is performed from the optical transmitter when the power is turned on, when no data signal is input, or when the device is faulty. .

  The present invention has been made to solve the above-described problems. In an optical transmission system, an optical transmitter capable of holding the power of output light below a specified value in a transient state such as when the power is turned on is provided. The purpose is to provide.

  An optical transmitter according to the present invention includes a semiconductor laser diode that emits CW light, an LD drive circuit that maintains a constant output of the semiconductor laser diode, an LN modulator that modulates CW light from the semiconductor laser diode, A modulator driving circuit that outputs a driving signal to the LN modulator according to an input data signal, a bias control circuit that prevents a DC bias drift in the LN modulator, and an optical transmitter in a transient state And an output control circuit for controlling the LD drive circuit so as to set a bias current to the semiconductor laser lower than a normal predetermined value.

  According to the present invention, when the optical transmitter is in a transient state, it is possible to prevent failure of the light receiving element connected to the optical transmitter by suppressing the output light from the semiconductor laser diode to a specified value or less. As a result, a highly reliable optical transmission system can be provided.

Embodiment 1 FIG.
Embodiment 1 of the present invention will be described below. 1 is a block diagram showing an example of an optical transmitter according to Embodiment 1 of the present invention.
In FIG. 1, the present optical transmitter outputs LD 1 and LD 1 that control LD 1 so that the output power of LD 1 and LD 1 that emit CW light by a bias current from a power source becomes a constant value. The output control circuit 3 that determines the value, the LN modulator 4 that modulates the CW light from the LD 1, the modulator drive circuit 5 that inputs the drive signal to the LN modulator 4, and the DC bias drift of the LN modulator 4 are canceled ( A bias control circuit 6 for preventing) and a bias monitor circuit 7 for monitoring a bias control value (DC bias signal) of the bias control circuit 6.
The LN modulator 4 includes a PD (Photo Detector) 8 that detects an output value of the CW light after modulation, and a signal electrode 9 to which a drive signal from the modulator drive circuit 5 is input.

  Next, the operation will be described. As shown in FIG. 1, the output control circuit 3 supplies a constant voltage serving as a reference (hereinafter referred to as a reference voltage) to the LD drive circuit 2 so that a constant output CW light is emitted from the LD 1.

  In the LD drive circuit 2, the monitor current of the optical output from the LD 1 is converted into a monitor voltage by current / voltage conversion, and the monitor voltage is compared with the reference voltage. Based on the comparison result, the bias current of the LD 1 is controlled by APC control so that the output of the CW light from the LD 1 is kept constant.

  By configuring the output control circuit 3 with a D / A converter, an electronic variable resistor, or the like, the output can be changed by external control. Therefore, it is possible to arbitrarily vary the output of the CW light from the LD 1 by varying the reference voltage according to the status of the monitor voltage.

The modulator driving circuit 5 converts the input data signal into an electric signal having an electric field necessary for operating the LN modulator 4 (hereinafter referred to as a data modulation signal). A drive signal for driving the LN modulator 4 is generated by superimposing the DC bias signal generated by the bias control circuit 6 on the data modulation signal.
Here, the DC bias signal is a signal for controlling the DC bias drift in the LN modulator 4 in order to always stabilize the modulated light.
The LN modulator 4 obtains modulated light necessary for transmission by the drive signal from the modulator drive circuit 5.

On the other hand, when the bias control circuit 6 sends a DC bias signal to the modulator drive circuit 5 and the LN modulator 4 modulates the CW light from the LD 1 with the drive signal on which the DC bias signal is superimposed, LN modulation is performed. A part of the modulated light is branched by an optical coupler or the like inside the device 4, converted into current by the PD 8, and fed back to the bias control circuit 6.
Based on this feedback, the bias control circuit 6 controls the modulator drive circuit 5 with a DC bias signal so that no DC bias drift occurs.
The bias monitor 7 constantly monitors a DC bias signal sent from the bias control circuit 6 to the modulator drive circuit 5, that is, a DC bias (bias voltage) applied to the LN modulator 4.

Here, when the power is turned on or when reset is performed to enable the LD 1, the output control circuit 3 of the LD 1 does not set the reference voltage as the original optical transmission output, but outputs the output for a predetermined time. Lower the setting. Therefore, even when the bias control to the LN modulator 4 becomes indefinite in the transient state and the CW light input by the LN modulator 4 is output without being modulated, the CW light has a specified value. The power will not exceed.
After the predetermined time has elapsed, the output control circuit 3 sets the reference voltage to the original value in a stepwise manner, so that the output value of the optical transmitter is independent of the state of the DC bias control in the LN modulator 4. No longer becomes excessive.

FIG. 2 is a diagram schematically showing temporal changes from when the power is turned on for the LD output, the reference voltage for determining the LD output, the LN applied bias voltage in the LN modulator, and the optical transmitter output.
In FIG. 2, when the power is turned on, the reference voltage and the LD output are set lower than predetermined values. However, when the optical output is output without being lowered by 3 dB during modulation due to the state of the DC bias control in the LN modulator 4, the output of the optical transmitter becomes a specified value.
Next, when the bias of the LN modulator 4 is normally controlled, the output of the optical transmitter is lowered by 3 dB due to the modulation effect (initialization is completed). After the voltage drops by 3 dB, the reference voltage is changed and set to a point where a desired optical transmitter output is obtained (adjustment completed).
Therefore, as shown in FIG. 2, it can be seen that there is no excessive state of the optical transmitter output.

  Further, the DC bias drift of the LN modulator 4 continuously drifts in units of several minutes. Therefore, when the bias control becomes impossible due to some failure, the time change of the bias voltage applied from the bias control circuit 6 is sufficiently smaller than the time change due to the bias drift. Therefore, when the change of the bias voltage value with time becomes much larger in the bias monitor 7 than in the normal monitoring, the reference voltage is changed from the operating state to the output control circuit 3 of the LD. By changing to the above power-on value lower than a predetermined value, it is possible to prevent an optical output from the optical transmitter from becoming excessive even when a failure occurs.

  As described above, according to the first embodiment, the bias control circuit 6 that can set a plurality of LD biases in advance, the bias monitor circuit 7 that monitors a sudden change in the DC bias in the LN modulator 4, and the monitoring result. By providing the output control circuit 3 for supplying the reference voltage to the LD drive circuit 2, the output light of the optical transmitter can always be controlled within a predetermined range, and 2 of the optical receiver connected to the optical transmitter. The next failure can be prevented. As a result, a highly reliable optical transmission system can be provided.

It is a block diagram which shows an example of the optical transmitter which concerns on Embodiment 1 of this invention. It is the figure which showed typically the time change from the time of power activation about each of the reference voltage which determines LD output, LD output, the LN application bias voltage in an LN modulator, and an optical transmitter output.

Explanation of symbols

1 LD (semiconductor laser diode), 2 LD drive circuit, 3 output control circuit, 4 LN modulator, 5 modulator drive circuit, 6 bias control circuit, 7 bias monitor circuit, 8 PD (Photo Detector), 9 signal electrode.

Claims (4)

A semiconductor laser diode emitting CW light;
An LD driving circuit for keeping the output of the semiconductor laser diode constant;
An LN modulator for modulating CW light from the semiconductor laser diode;
A modulator driving circuit that outputs a driving signal to the LN modulator in response to an input data signal;
A bias control circuit for preventing DC bias drift in the LN modulator;
An optical transmitter comprising: an output control circuit that controls the LD driving circuit so that a bias current to the semiconductor laser is set lower than a normal predetermined value when the optical transmitter is in a transient state. A bias monitor circuit for monitoring a bias voltage applied to the LN modulator;
2. The optical transmitter according to claim 1, wherein the output control circuit performs control for setting the bias current to the semiconductor laser diode to be lower than a normal predetermined value in accordance with a monitoring state of the bias voltage. . The output control circuit outputs a reference voltage to the LD drive circuit,
3. The optical transmitter according to claim 1, wherein the LD driving circuit controls the output of the semiconductor laser diode based on the reference voltage. The bias control circuit can set a plurality of DC bias signals in advance,
The modulator driving circuit prevents a DC bias drift in the LN modulator by the driving signal generated by superimposing the DC bias signal from the bias control circuit on an input data signal. The optical transmitter according to any one of claims 1 to 3.

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