JP2006080262A - Automatic power control method of optical modulation circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To keep constant extinction ratio of laser diode output light which is on/off modulated by digital signals. <P>SOLUTION: The output light of laser diode 1 is monitored with a photodiode 2. The level corresponding to the output light "0" level of its monitor current Imon is detected with a DC current detecting circuit 3. The bias current of the laser diode is so controlled that its detection level is constant, resulting in keeping "0" level of the output light constant. The average current of the monitor current Imon is detected with a detection circuit 4, and "1" level of the output light is so controlled that its detection level is constant, resulting in keeping the on/off ratio, or extinction ratio, constant. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an automatic power control method for an optical modulation circuit, and more particularly to an automatic power control method for an optical modulation circuit suitable for stabilizing the extinction ratio of an optical modulation output.

  In a light modulation circuit that controls the drive current of a laser diode in accordance with a digital signal and generates an optical modulation signal by turning on (emitting) and turning off (extinguishing) its optical output, Measures are taken to ensure communication quality by preventing the average output power and extinction ratio (on / off ratio) from changing due to the temperature and aging of the laser diode.

  As a conventional technique for this purpose, there is a technique in which an optical modulation signal output from a laser diode is detected by a monitoring photodiode and the driving current of the laser diode is controlled so that the average power is constant. Here, the drive current is given as the sum of the bias current and the modulation current that changes according to the digital signal, and the control of the average power is constant by controlling the bias current, and the modulation current remains constant. However, this method may decrease the extinction ratio.

Further, in the “APC method for preventing extinction ratio degradation” disclosed in Patent Document 1, the laser diode drive current is also given as the sum of the bias current and the modulation current. Then, “1” level (light emission level) and “0” level (extinction level) are detected from the light modulation signal detected by the photodiode, and modulation is performed so that the detected “1” level becomes a predetermined value. Control the current. Further, by comparing the detected “1” level with the detected “0” level using a voltage obtained by dividing the detected “1” level at a predetermined ratio as a reference voltage, the difference between the detected “1” level and the “0” level is obtained. The bias current is controlled to be constant. According to this prior art, the laser diode emission level and the extinction ratio are maintained at predetermined values even when there is a change in the characteristics of the laser diode due to changes over time or temperature, or variations in characteristics of individual laser diodes. The same effect can be obtained even if the mark ratio of the digital signal changes.
Japanese Patent Laid-Open No. 10-144986

  In data transmission using optical transmission, a transmission code having a good DC balance (mark ratio is approximately 0.5) is often used. The present invention realizes an automatic power control method for an optical modulation circuit capable of a simple configuration that can stabilize a light emission level and an extinction ratio when such a DC balanced transmission code is used. For the purpose.

  The present invention monitors a laser diode output light modulated on / off by a digital signal by a photodiode, detects a level corresponding to the output light off level of the monitor output, and detects the level at a predetermined first reference level. The laser diode output light is controlled such that the level corresponding to the average output light level of the monitor output is detected and the detected level becomes a predetermined second reference level. An automatic power control method for an optical modulation circuit, characterized in that the on level of the optical modulation circuit is controlled.

  The off level of the laser diode output, that is, the extinction level, is directly controlled by detecting the corresponding level of the monitor output. The on level, that is, the level at the time of light emission is controlled according to the average output of the monitor output. However, if a transmission code having a DC balance of almost half of the digital signal is used, the constant average output control is performed at the time of light emission. The effect is almost the same as the on-level constant control, and therefore the extinction ratio is kept constant even if the laser diode characteristic changes.

  Embodiments of the present invention will be described below. First, it is assumed that a transmission code having a good DC balance is used in the optical transmission line targeted by the present invention. When a transmission code having a good DC balance is used, a DC component can be cut by a capacitor to connect to a logic IC, and a DC bias that is different for each logic IC need not be considered. In addition, a phase-locked loop (PLL) is used for clock synchronization or the like, but this PLL may be extremely difficult to design in the case of a code string that is not DC balanced. From these points, for example, 1000Base-F Ethernet, which is one of Ethernet (registered trademark), employs an “8B10B code” with good DC balance.

  In such an optical modulation circuit used in the optical transmission system, the “1” level of the optical modulation signal and the average power are in a substantially proportional relationship. Therefore, in the present invention, the average power of the optical modulation signal is detected, Thus, the modulation current in the laser diode drive current is controlled to keep the “1” level constant. Further, the “0” level of the optical modulation signal is directly detected and controlled so that the value becomes a predetermined value. Thus, both the “1” and “0” levels can be stabilized, so that the extinction ratio can be stabilized together with the output during light emission.

  FIG. 1 is a block diagram showing a configuration example of an optical modulation circuit provided with an automatic power control method according to the present invention. In the figure, a monitoring photodiode 2 detects the output light of a transmitting laser diode 1, and these laser diode 1 and photodiode 2 are usually combined into one module. The drive current of the laser diode 1 is the sum of the modulation current Imod modulated by the digital signal and the bias current Ib, and the output light of the laser diode 1 is “0” level (extinction level) and “1” level (light emission level). The signal turns on and off. A monitor current Imon of this output light flows through the photodiode 2, and this monitor current Imon is proportional to the output light level of the laser diode 1. The digital signal detection circuit 3 and the average current detection circuit 4 detect the “0” level and the average current of the monitor current Imon, respectively.

  FIG. 2 is a configuration example of the DC current detection circuit 3. A circuit that detects the level of the monitor current Imon with the resistor 21 and the differential amplifier 22 and holds the level corresponding to the “0” level to the capacitor 24 via the diode 23 and outputs an average value (approximation) of the “0” level. It is. Instead of the diode 24, a MOS switch that is turned on only at the “0” level may be used.

  FIG. 3 shows a configuration example of the average current detection circuit 4. The level of the monitor current Imon is detected by the resistor 31 and the differential amplifier 32, and the average current of the monitor current Imon is obtained by an integrating circuit formed by the resistor 33 and the capacitor 34. Obtain a voltage that is proportional to the average current.

  The comparison circuit 5 obtains the difference between the output voltage of the DC current detection circuit 3 and the reference voltage ref1, and gives this to the bias current control circuit 6. The bias current control circuit 6 is a circuit for controlling the bias current Ib, and determines the “0” output of the laser diode 1, that is, the level at the time of extinction. Therefore, if the output polarity of the comparison circuit 5 is determined so that the bias current Ib decreases as the “0” level of the monitor current Imon detected by the DC current detection circuit 3 increases, there is a change in the characteristics of the laser diode 1. Also, the “0” level of the output light is maintained at a level corresponding to the reference voltage ref1.

  On the other hand, the comparison circuit 7 obtains the difference between the output voltage of the average current detection circuit 4 and the reference voltage ref2, and supplies this to the modulation current control circuit 8. The modulation current control circuit 8 sets the modulation current Imod to 0 when the digital signal D is “0”, and sets the modulation current Imod to a value corresponding to the output of the comparison circuit 7 when it is “1”. At this time, if the output polarity of the comparison circuit 7 is determined so that the modulation current Imod becomes smaller as the detected average current is larger, the average output power is kept constant even if the characteristics of the laser diode 1 change. It is. Here, if the digital signal D is a transmission code having a good DC balance, the average output power is constant, which means that the “1” level, that is, the light emission level is also kept constant. Thus, according to the embodiment of FIG. 1, both the emission level and the extinction ratio of the laser diode output light are stably controlled.

  FIG. 4 shows another example of the configuration of an optical modulation circuit equipped with the automatic power control method of the present invention. WDM (Wavelength) in which transmission and reception are performed using a single optical fiber in different optical wavelength regions. This is a configuration example suitable for a division multiplexing system. In this bidirectional WDM system, in the case of a simple configuration in which transmitted light and received light are separated by an optical filter, a part of the received light enters the monitoring photodiode, which may cause an error in laser diode control. There is sex. FIG. 4 is obtained by adding a circuit for removing such an error to the embodiment of FIG.

  In FIG. 4, the photodiode 2, the DC current detection circuit 3, and the average current detection circuit 4 are the same as those in FIG. 1, and other portions in FIG. 1 are omitted. On the other hand, in the receiving circuit, a received current Ir proportional to the received light flows through the photodiode 10 to which the bias current is applied from the bias current control circuit 11, and this is output as a received signal through the amplifier 12. Further, a current ΔI proportional to the reception current Ir is applied to the monitor current Imon detected by the transmission unit monitoring photodiode 2 via the DC amplifier 13 and the variable resistor 14. Here, if the magnitude and polarity of the current ΔI cancel the error of the monitor current Imon generated when the received light enters the photodiode 2, the error of laser diode control due to the received light can be suppressed. it can.

It is a block diagram which shows the structural example of the optical modulation circuit which comprised the automatic power control method of this invention. It is a structural example of a DC current detection circuit. It is a structural example of an average current detection circuit. It is a block diagram which shows another structural example of the optical modulation circuit which comprised the automatic power control method of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser diode 2 Photodiode 3 DC current detection circuit 4 Average current detection circuit 5, 7 Comparison circuit 6 Bias current control circuit 8 Modulation current control circuit

Claims (1)

The laser diode output light that is on / off modulated by the digital signal is monitored by a photodiode, and a level corresponding to the output light off level of the monitor output is detected so that the detection level becomes a predetermined first reference level. The bias current of the laser diode is controlled, the level corresponding to the average output light level of the monitor output is detected, and the on level of the laser diode output light is set so that the detected level becomes a predetermined second reference level. An automatic power control method for an optical modulation circuit, characterized by being controlled.

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