1. Field of the Invention
The present invention relates to a drive circuit and a drive method of a semiconductor laser module that externally modulates carrier light output from a semiconductor laser using an electro-absorption type optical modulator to output the modulated light, in particular, to a drive control technique for compensating for temperature characteristics of the semiconductor laser and the electro-absorption type optical modulator.
2. Description of the Related Art
In various optical communication systems for transmitting optical signals over a long distance, there is sometimes used a semiconductor laser module in which a semiconductor laser and an external modulator of electro-absorption type are integrated with each other, since such a semiconductor laser module enables to increase a transmission distance compared with a semiconductor laser of direct modulation type. In the aforementioned semiconductor laser module, operation characteristics of the semiconductor laser and the electro-absorption type optical modulator (hereinafter referred to as EA modulator) each has dependence on temperature. Therefore, in order to hold optical output power in stable, it is necessary to stabilize a module temperature or to control a drive condition according to a temperature change.
In a conventional drive control of a semiconductor laser module having an EA modulator, such a method is typical in that, for example, by using an electronic cooling device such as a Peltier device and a temperature detection device such as a thermistor integrated with each other in a module, a current to be supplied to the Peltier device is controlled so that a resistance value of the thermistor is maintained to be constant, to hold the temperature of a semiconductor laser and the EA modulator to be constant. In this case, the EA modulator is driven by a constant modulated electric signal and at the same time, a constant drive current or a drive current to make a back power monitor current constant is supplied to the semiconductor laser, thereby achieving a stable optical output. The drive control method described above, however, has a disadvantage that since the Peltier device needs to be driven, the power consumption is increased and also a package size of the module is made to be large.
To cope with this problem, a semiconductor laser module using an EA modulator which is not provided with a Peltier device, has recently been under development. According to this semiconductor laser module, both the reduction of power consumption and the miniaturization can be achieved by eliminating the need of a Peltier device, although there is a need of stabilizing optical output power by controlling a bias voltage of the EA modulator or a drive current of the semiconductor laser according to a temperature change. Also, since this module is of an external modulation system using the EA modulator, it is possible to obtain an optical output having a small amount of chirping.
As a conventional drive control technique applicable to such a semiconductor laser module without the Peltier device as described above, a drive circuit for an EA modulator is disclosed in Japanese Unexamined Patent Publication No. 11-119176. In this drive circuit, an anode voltage of the EA modulator is detected and according to the detection result, a bias voltage of the EA modulator is controlled, so that an applied voltage to the EA modulator is maintained to be constant even if a temperature change or a change with age occurs.
However, in the case where the semiconductor laser module provided with EA modulator without the Peltier device is driven by applying the conventional control technique as described above, although the bias voltage of the EA modulator is controlled according to the temperature change and the like, since the EA modulator is driven by a constant modulated electric signal, an optical output waveform of the semiconductor laser module is considerably deteriorated.
FIG. 16 is a diagram showing temperature dependence of operating characteristics of a typical EA modulator. In FIG. 16, each characteristic curve represents a relation between power Pf of an optical signal output from the EA modulator and an applied voltage Vea to the EA modulator at each of temperatures 0° C., 25° C. and 75° C. In FIG. 16, if a modulated electric signal of a waveform as shown in the lower left part is applied to drive the EA modulator, since the optical output power Pf is changed along each characteristic curve, an optical waveform at 0° C. output from the EA modulator is considerably deteriorated as compared with an optical waveform at 75° C. as shown in the upper right part. Even if the bias voltage is controlled according to the temperature change as in the conventional technique so that the EA modulator operates in a region where the characteristic curve has the large inclination and is changed substantially linearly, since the inclination or the distortion of the characteristic curve in each operating region differs from each other depending on the temperature, an extinction ratio or a duty of the optical output waveform is changed depending on temperature when the EA modulator is driven by the modulated electric signal in which a modulated amplitude and a cross point are set to be constant.
Further, since a current generated by absorption of carrier light (hereinafter referred to as a photocurrent) flows through the EA modulator, there is caused a problem in that the applied voltage to the EA modulator deviates, if the photocurrent is changed when controlling a current source for drive controlling the EA modulator. Therefore, for the semiconductor laser module without the Peltier device, it is critically important to realize a drive control method that is not affected by a change in the photocurrent due to the electro-absorption effect, when the temperature change or the change with age is compensated by controlling the electrical drive signal of the EA modulator.