1. Field of the Invention
The present application relates to a light source driving apparatus that adjusts a bias current to be supplied so as to control intensity of a light source, and a light source driving method.
2. Description of the Related Art
Conventionally, an LD (Laser Diode) which outputs light according to an electric input is used for light sources such as light transmitting apparatuses in optical communication systems. Further, auto power control (APC) is used. With the auto power control, output light from the LD is received by a PD (Photo Diode), and intensity of the output light from the LD is controlled based on an electric signal output from the PD.
The auto power control is normally made by supplying a bias current obtained by multiplying an intensity control signal based on the electric signal output from the PD by a reference voltage to the LD (for example, see Japanese Patent Application Laid-Open Nos. 5-335664, 9-326522 and 9-246646). The light source, which outputs a light signal, uses extinction ratio control for steadily controlling an extinction ratio of the output light from the LD based on the electric signal output from the PD. The extinction ratio control is normally made by supplying a pulse current obtained by multiplying an extinction ratio control signal based on the electric signal output from the PD by a reference signal to the LD.
In the above conventional technique, however, an intensity characteristic of light of a driving current of the LD (hereinafter, “differential efficiency”) changes according to temperature fluctuation of the LD and fluctuation due to aging such as deterioration of elements. Therefore, a unit change amount of the intensity of the output light from the LD at the time of changing the intensity control signal changes due to factors such as the temperature fluctuation of the LD. Furthermore, a unit change amount of the extinction ratio of the signal light at the time of changing the extinction ratio control signal changes according to the temperature of the LD. For this reason, the intensity control and the extinction ratio control of the LD cannot be stabilized. This problem is described in detail below.
FIG. 11 is a graph illustrating a relationship between the driving current of the LD and the intensity of the output light. In FIG. 11, an abscissa axis shows the driving current [mA] to be supplied to the LD. An ordinate axis shows the intensity Pf [μW] of the output light from the LD. Characteristic lines 1111, 1112, and 1113 represent characteristics of the intensity Pf with respect to the driving current at the time when the temperature of the LD is −5° C., 25° C., and 80° C., respectively.
As shown by the characteristic lines 1111 to 1113, when the temperature of the LD changes, the intensity characteristic with respect to the driving current of the LD changes. A thick line 1120 represents a desirable average intensity (Pave) of the output light from the LD. A dotted line 1131 shows desirable intensity (Plow) at the time when the output light is “0” (extinction state). A dotted line 1132 represents desirable intensity (Phigh) at the time when the output light is “1” (light-emitting state).
The driving current to be supplied to the LD includes a bias current and a pulse current. The bias current is a driving current which determines the average intensity of the output light from the LD. A reference symbol 1141 represents the bias current which changes the average intensity of the output light from the LD into desirable average intensity 1120 when the temperature of the LD is 80° C.
The pulse current is a driving current which determines the intensity at the time when the output light from the LD is “0” and the intensity at the time when the output light from the LD is “1” (extinction ratio). A reference symbol 1142 represents a pulse current which changes the intensity at the time of the output light “0” and the intensity at the time of the output light “1” into desirable intensity 1131, and into desirable intensity 1132 when the temperature of the LD is 80° C.
FIG. 12 is a graph illustrating control of the driving current for stabilizing the average intensity and the extinction ratio. In FIG. 12, an abscissa axis represents the temperature [° C.] of the LD. An ordinate axis represents the driving current [mA] to be supplied to the LD. A characteristic line 1210 represents a relationship between the temperature of the LD and a control value of the bias current according to the temperature of the LD. A characteristic line 1220 represents a relationship between the temperature of the LD and a control value of the pulse current according to the temperature of the LD. As shown by characteristic lines 1210 and 1220, the bias current and the pulse current to be supplied to the LD are controlled according to the temperature fluctuation of the LD, so that the average intensity and the extinction ratio of the output light from the LD are stabilized.
FIG. 13 is a graph illustrating a relationship between a control signal and the intensity of the output light. In FIG. 13, an abscissa axis represents a code DAC_code [LSB] of a control signal. Characteristic lines 1311, 1312, and 1313 represent characteristics of the intensity Pf with respect to the control signal at the time when the temperature of the LD is −5° C., 25° C., and 80° C., respectively.
The control signal is a digital signal which controls the output light from the LD. The control signal includes an intensity control signal which controls the average intensity of the output light from the LD, and an extinction ratio control signal which controls the extinction ratio of the output light from the LD. The driving current to be supplied to the LD is generated by multiplying a constant reference voltage by the control signal.
As shown in FIG. 11, when the temperature of the LD changes, the intensity characteristics with respect to the driving current of the LD changes. Therefore, as shown by characteristic lines 1311 to 1313, when the temperature of the LD changes, the intensity change amount of the output light from the LD at the time of changing the control signal changes merely by a minimum unit (1LSB: Least Significant Bit).
Specifically, as shown by the characteristic line 1311, when the temperature of the LD is low (−5° C.), the change amount per the control signal 1LSB is larger than the case where the temperature of the LD is high (80° C.) as shown by the characteristic line 1313. For this reason, accuracy of the intensity control using the intensity control signal and the extinction ratio control using the extinction ratio control signal change according to the temperature fluctuation of the LD, and thus the intensity control and the extinction ratio control of the LD cannot be stabilized.
In order to improve the accuracy of the intensity control, an analog converter which multiples the reference voltage by the control signal is provided with a high resolution so that the unit change amount of the control signal can be reduced. However, since the analog converter having the high resolution is expensive, the cost of the apparatus increases.
When the extinction ratio control is conducted, information about differential efficiency of the LD is may be acquired from temperature information about the LD acquired from a temperature sensor. In this case, a value of the extinction ratio control signal that makes the extinction ratio of the output light constant is calculated according to the acquired information of the differential efficiency of the LD. In this case, however, the information of the temperate characteristics of the differential efficiency of the respective LDs should be acquired in advance. For this reason, the steps of manufacturing the apparatus increases and thus manufacturing becomes complicated. Since a temperature sensor needs to be provided, the apparatus is enlarged which increases costs.
In order to address the above problems of the conventional technique, it is an aspect of the present invention to provide a light source driving apparatus and a light source driving method which can provide stable control of the intensity of a light source.