Coherent light generators – Particular beam control device – Modulation
Reexamination Certificate
2003-01-24
2004-09-21
Leung, Quyen (Department: 2828)
Coherent light generators
Particular beam control device
Modulation
C372S038020
Reexamination Certificate
active
06795458
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser diode control circuit and a laser diode control method.
2. Related Background Art
An optical transmitter comprises a semiconductor laser and a laser driving circuit. In the field of optical communications, a semiconductor laser is driven by a laser driving circuit to generate signal light corresponding to a transmission signal.
SUMMARY OF THE INVENTION
The present inventors have performed research into the control of semiconductor lasers. In this research, the inventors investigated semiconductor laser control methods. As a result of this investigation, the following clarifications were made.
In a first technique, the optical output power and extinction ratio of a semiconductor laser are controlled by detecting the average value and peak value of a monitoring photodetector. However, according to the investigations of the inventors, it was learned that when the optical signal transmission rate increases, it becomes difficult to operate the monitoring photodetector at a high enough speed to detect the peak value in the received optical signal.
In a second technique, a control circuit converts the output of the monitoring photodetector into a digital value using an A/D converter and subtracts this digital value from a reference value stored in memory. The control circuit converts the subtracted value into an analog value using a D/A converter and controls the semiconductor laser on the basis of this analog value. In this controlling technique, an optimum modulation current value for each temperature is stored in memory and the extinction ratio is held at a constant level on the basis of this value. In this technique, for the purposes of control, the modulation current value is determined uniquely using a signal from a thermistor for detecting ambient temperature, and the bias current is determined such that the average power is maintained at a constant level. However, it was learned from the investigations of the inventors that in this technique, when the luminous efficiency decreases due to deterioration in the laser light generating portion over time, only the bias current is increased. Thus the extinction ratio cannot be maintained at a constant level.
In a third technique, the bias current, the modulation current, and an initial value for the monitoring photodetector in respect of the usage range temperature are stored in memory, and the modulation current and bias current are determined using a signal based on the result of a comparison between a signal from the monitoring photodetector and the values stored in the memory. In this technique, the modulation current and bias current are determined using a signal from a temperature sensor. However, according to the investigations of the inventors, it is impossible with this technique to compensate for changes due to deterioration of the semiconductor laser over time.
According to these investigations, it was discovered that technical problems exist in conventional techniques such that a technique for controlling the optical output power of a semiconductor laser toward a predetermined value and a technique for controlling the extinction ratio of a semiconductor laser toward a predetermined value have not been achieved.
It is therefore an object of the present invention to provide a laser diode control circuit and laser diode control method for performing control of the optical output power and extinction ratio of a laser light generating portion having a semiconductor light source.
A first aspect of the present invention relates to a laser diode control circuit. The laser diode control circuit comprises a load portion, a control portion, a bias current circuit portion, and a modulation current circuit portion. The load portion generates a signal corresponding to a photocurrent which is generated by a photodetector in accordance with the optical power of the light received from a laser light generating portion. The control portion generates a first control signal for modifying one of a bias current and a modulation current and a second control signal for modifying the other of the bias current and modulation current from among a value X, a value Y, an amount of change &Dgr;X in relation to the value X, and an amount of change &Dgr;Y in relation to the value Y. The first control signal is generated from &Dgr;Db and a value corresponding to Db. The second control signal is generated from &Dgr;Dm and a value corresponding to Dm. The amount of change &Dgr;Y is generated by a function f(X, &Dgr;X)=&Dgr;Y which is defined such that the light emission power displays a predetermined dependence and the extinction ratio displays a predetermined dependence. A set of variables (X, Y, &Dgr;X, &Dgr;Y) is either (Db, Dm, &Dgr;Db, &Dgr;Dm) or (Dm, Db, &Dgr;Dm, &Dgr;Db), the symbol Db indicating a value corresponding to a bias current Ib, the symbol Dm indicating a value corresponding to a modulation current Im, the symbol &Dgr;Db indicating a value corresponding to an amount of change &Dgr;Ib in the bias current Ib, and the symbol &Dgr;Dm indicating a value corresponding to an amount of change &Dgr;Im in the modulation current Im. The bias current circuit portion is connected to the laser light generating portion and generates a bias current in accordance with the first control signal. The modulation current circuit portion is connected to the laser light generating portion and generates a modulation current in accordance with the second control signal.
In order to control the bias current and modulation current in this control circuit, the amount of change &Dgr;Y is generated by a function f(X, &Dgr;X)=&Dgr;Y in relation to a set of variables (X, Y, &Dgr;X, &Dgr;Y)=(Db, Dm, &Dgr;Db, &Dgr;Dm) or (X, Y, &Dgr;X, &Dgr;Y)=(Dm, Db, &Dgr;Dm, &Dgr;Db), and hence a control circuit which is capable of control such that the light emission power displays a predetermined dependence and the extinction ratio displays a predetermined dependence is provided.
In the laser diode control circuit, the function f may be constituted as an approximated function of a function defined such that the light emission power becomes constant and the extinction ratio becomes constant.
In the laser diode control circuit, the control portion may be constituted so as to comprise first means, second means, and third means. The first means generates the amount of change &Dgr;Y in accordance with the function f(X, &Dgr;X)=&Dgr;Y and in accordance with the amount of change &Dgr;X which is determined according to the result of a comparison between a value corresponding to the signal which corresponds to the photocurrent and a reference value. The second means generates the first control signal from values corresponding to &Dgr;Db and Db. The third means generates the second control signal from values corresponding to &Dgr;Dm and Dm.
In the laser diode control circuit, the first means may comprise nonvolatile memory. The laser diode control circuit may further comprise an interface portion for communicating with an external device and means connected to the interface portion for performing a write operation to the nonvolatile memory. According to this constitution, the laser diode control circuit can be adjusted via the interface portion. For example, by providing the interface portion, the amount of labor required for each individual adjustment operation in the laser diode driving circuit can be reduced in comparison with manual adjustments. Further, when the characteristic data for each laser diode are stored in an external device which is a computer, adjustments can be performed in alignment with the characteristic data.
In the laser diode control circuit, first storage means may store a constant (&Dgr;Y/&Dgr;X)
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in relation to the entire range of the variable X. This constitution is easily realized and allows a reduction in the circuit scale and the number of processing steps.
In the laser diode control circuit, the first storage means may store a con
Leung Quyen
Sumitomo Electric Industries Inc.
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