JPH07336305A - Light transmitter - Google Patents

Abstract

PURPOSE:To fixedly hold a bias current and a light modulation degree even when using environment conditions such as an ambient temperature or the like fluctuate by detecting a current value made flow to a light emitting element and controlling the bias voltage of the light emitting element and an input signal level. CONSTITUTION:When a voltage between the base and the emitter of a transistor 2 and the value of a resistor 3 fluctuate due to ambient temperature fluctuation or the like and the bias current value to be supplied to the light emitting element 1 increases, the output of a current value detection means 5 and a DC level detection means 6 increases. A difference voltage detection means 8 obtains the difference of the output of the means 6 and a reference voltage 7, addes it to a bias voltage control means 9, decreases the base voltage of the transistor 2 and absorbs the increased portion of a bias current to be supplied to the element 1. Also, when a signal current value to be supplied to the element 1 increases, the output of a peak detection means 10 and a difference voltage detection circuit 11 increases, a difference voltage with the reference voltage 12 is added through the difference voltage detection circuit 13 to a variable gain amplifier 14 and the increased portion of a signal current to be supplied to the element 1 is absorbed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter in which the bias current supplied to a light emitting element and the degree of optical modulation are unadjusted on the transmitting side.

[0002]

2. Description of the Related Art As an example of a conventional light emitting element drive circuit,
Optical communication technology, published by Ohmsha, edited by the Television Society, P1
There is a light emitting element drive circuit of FIG.
In FIG. 2, the resistor VR1 determines each bias potential of the transistor Q1, and the emitter potential of the transistor Q1 and the resistors R3 and R4 determined by this determine the bias current flowing through the light emitting element L1. Further, the signal current superimposed on the bias current is determined by the signal amplitude input from the signal input terminal and the resistor R3. The bias current and the signal current determine the bias light power emitted by the light emitting element L1, the Po value, and the peak power value Pp value of the light modulation signal, and the optical modulation degree OMD is determined by the Po value and the Pp value.
Was decided as follows.

OMD = (Pp-Po) / Po That is, the degree of light modulation is determined by the bias current supplied to the light emitting element by the light emitting element drive circuit and the level of the signal current.

[0004]

The above-mentioned prior art adjusts the resistor VR1 in response to changes in the bias current due to ambient temperature fluctuations and power supply voltage fluctuations, and further superimposes them on the bias current due to fluctuations in the input signal amplitude. There is a problem in that the value of the resistor R3 must be changed in response to the change in the signal current generated, and the resistor VR1 must be adjusted in response to the change in the bias current due to the change.

In order to solve the above problems, the present invention provides a light source that keeps the bias current supplied to the light emitting element and the light modulation degree constant even when there are ambient temperature fluctuations, power supply voltage fluctuations, or input signal amplitude fluctuations. An object is to provide a transmitter.

[0006]

To achieve the above object, in the present invention, in a light emitting element drive circuit for supplying a signal current corresponding to an input signal voltage and a bias voltage and a bias current to a light emitting element, A means for detecting a current value supplied to the light emitting element, a means for controlling a bias voltage applied to the means for driving the light emitting element based on a signal from the means for detecting the current value, and a gain control for controlling a signal voltage level. And a means.

[0007]

The current value flowing through the light emitting element is detected, the direct current level is detected from the detected current value, the bias current flowing through the light emitting element is controlled, and the peak level is detected from the detected current value. , The DC level detected previously is subtracted, and the input signal level is controlled from the subtraction result. Therefore, even if the bias current and the signal current flowing through the light emitting element are about to change due to a change in the ambient temperature, a change in the power supply voltage, etc., the bias current and the optical modulation degree are kept constant.

[0008]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a first embodiment of an optical transmitter according to the present invention, in which 1 is a light emitting element, 2 is a transistor, 3 is a resistor, and 1 to 3 are signal voltages input to the base terminal of the transistor 2, And a signal current corresponding to the bias voltage and a light emitting element drive circuit 4 for supplying a bias current to the light emitting element, 5 is a current value detecting means for detecting the current value supplied to the light emitting element 1, and 6 is a current value detection. DC level detecting means for detecting a DC level from the current value detected by the means 5, 7
Is a first reference voltage corresponding to a desired bias current, 8 is a first differential voltage detecting means for detecting a difference between the direct current level detected by the direct current level detecting means 6 and the first reference voltage 7, and 9 is a first differential voltage detecting means. 1, bias voltage control means for controlling the bias voltage applied to the base of the transistor 2 by the output from the differential voltage detection means 8, 10 is peak detection means for detecting the peak level from the current value detected by the current value detection means 5, 11 Is a second differential voltage detecting means for subtracting the DC level detected by the DC level detecting means 6 from the peak level detected by the peak detecting means 10, 12 is a second reference voltage corresponding to a desired optical modulation degree, and 13 is The third difference voltage detecting means 13 and 14 for detecting the difference between the output of the second difference voltage detecting means 11 and the second reference voltage 12 have the level of the input signal voltage according to the output from the third difference voltage detecting means. Variable gain increase to control Circuit, 15 is an input terminal.

The operation of this embodiment will be described with reference to FIG.
The output of the light emitting element and the light modulation degree are determined by the bias current value and the signal current value supplied to the light emitting element 1 as described in the above-mentioned conventional technique. Therefore, a bias voltage is applied to the base terminal of the transistor 2 in the light emitting element drive circuit 4 so that the light emitting element drive circuit 4 supplies the light emitting element 1 with a signal current having a desired bias current and a desired light modulation degree. The bias voltage from the control means 9 and the signal voltage input from the input terminal 15 are applied with their voltage levels controlled by the variable gain amplifier circuit 14. At this time, the current value supplied to the light emitting element 1 is detected by the current value detecting means 5. Then, in the detected current value, the direct current level is detected by the direct current level detecting means 6, and then the difference from the desired direct current level is detected by the first difference voltage detecting means 7 with respect to the first reference voltage 7. The first differential voltage detecting means 7
The light emitting element drive circuit 4 always uses the output of
The bias voltage control means 9 is operated so as to supply a desired bias current. Further, a peak level is detected by the peak detection means 10 from the current value detected by the current value detection means 5, and the DC level detected by the DC level detection means 6 is detected as a second difference from the detected peak level. The signal level is detected by subtraction by the voltage detection means 11, and the difference between the signal current level and the signal current level that achieves the desired modulation degree is detected by the third difference voltage detection means 13.
The third difference voltage detecting means
Using the output of 13, the light-emitting element drive circuit 4 operates to control the variable gain amplifier circuit 14 so that the light-emitting element 1 is supplied with a desired signal current level at all times.

Here, as an example when the current value specifically changes, the operation when the current value supplied to the light emitting element 1 increases will be shown. It is assumed that the Vbe of the transistor 2 and the value of the resistor 3 change due to the ambient temperature change, and the bias current value supplied to the light emitting element 1 increases. Then, the output from the DC level detecting means 6 increases, and the increase is detected by the first difference voltage detecting means 8 by taking a difference from the first reference voltage 7, and this is detected. The increment is added to the bias voltage control means 9, and the bias voltage control means 9
By decreasing the voltage applied to the base of the transistor 2 in accordance with the added amount, the bias current supplied to the light emitting element drive circuit 4 is reduced, and the increase amount of the bias current supplied to the light emitting element 1 is increased. Absorbs. Further, for the same reason as above, even if the signal current value supplied to the light emitting element 1 increases, the output from the above-mentioned peak detection means 10 and the output from the second differential voltage detection means 11 increase,
The increased amount is detected by the third difference voltage detection means 13 by taking the difference from the second reference voltage 12, and the detected increased amount is added to the variable gain amplifying circuit 14 to obtain the variable gain amplifying circuit.
By decreasing the gain of 14, the signal current supplied to the light emitting element drive circuit 4 is decreased to operate the light emitting element 1
Absorbs an increase in the signal current supplied to.

Further, when the current value supplied to the light emitting element 1 is reduced, the same operation is performed to absorb the reduced amount.

FIG. 3 shows an example of the structure of the current value detecting means 5,
Is a resistor and 17 is a buffer amplifier. As the resistor 16, a resistor whose value hardly changes due to changes in the surrounding environment and whose value is known accurately is used. The current supplied to the light emitting element drive circuit 4 is passed through the resistor 16, converted into a voltage, and the value is buffer amplifier 17. The current value is detected by outputting via the.

FIG. 4 shows an example of the structure of the peak detecting means, in which 18 is a transistor, 19 is a capacitor, and 20 is a buffer amplifier. The peak voltage of the signal applied to the base of the transistor 18 is detected by charging the capacitor 19 with the signal applied to the base of the transistor 18, and the peak voltage is output by outputting the value via the buffer amplifier 20. To detect.

FIG. 5 shows one of the first to third differential voltage detecting means.
In the configuration example, 21 is an operational amplifier, 22 to 25 are resistors, 26 is a-input terminal, and 27 is a + input terminal. By equalizing the values of the resistors 22 to 25, a value obtained by subtracting the signal voltage value input to the-input terminal 26 from the signal voltage value input to the + input terminal 27 is output.

FIG. 6 shows an example of the structure of the DC level detecting means 6, in which 28 is a buffer amplifier, 29 is a resistor, 30 is a capacitor, and the resistor 29 and the capacitor 30 constitute a low-pass filter. By sufficiently increasing the time constant of the low-pass filter composed of the resistor 29 and the capacitor 30,
The DC level is detected by smoothing the input signal and outputting the value via the buffer amplifier 28.

FIG. 7 shows an example of the configuration of the bias voltage control means 9, 31 is a difference voltage detection means having the same configuration as shown in FIG. 5, 32 is a bias reference voltage, and 33 is a buffer amplifier. The signal input to the bias voltage control means 9 is subtracted from the bias reference voltage by the differential voltage detection means 31, and the value is output via the buffer amplifier 33.

[0018]

According to the present invention, there are provided means for controlling the bias voltage applied to the means for driving the light emitting element and means for controlling the signal voltage level based on the current value detected by the means for detecting the current value supplied to the light emitting element. Therefore, even if the bias current and signal current supplied to the light emitting element fluctuate due to fluctuations in ambient temperature, fluctuations in power supply voltage, etc., the fluctuations are absorbed,
This has the effect of keeping the bias current and the degree of optical modulation constant.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of an optical transmitter of the present invention.

FIG. 2 is a conventional light emitting element drive circuit diagram.

FIG. 3 is a diagram showing one configuration example of a current value detection means.

FIG. 4 is a diagram showing a configuration example of a peak detection unit.

FIG. 5 is a diagram showing one configuration example of first to third differential voltage detection means.

FIG. 6 is a diagram showing one configuration example of a DC level detecting means.

FIG. 7 is a diagram showing a configuration example of bias voltage control means.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light emitting element, 2 ... Transistor, 3 ... Resistor, 4 ... Optical element drive circuit, 5 ... Current value detection means, 6 ... DC level detection means, 7 ... 1 reference value generation circuit, 8 ... 1st difference voltage Detecting means, 9 ... Bias voltage controlling means, 10 ... Peak detecting means, 11 ... Second difference voltage detecting means, 12 ... Second reference value generating circuit, 13 ... Third difference voltage detecting means, 14 ... Variable gain Amplifier circuit, 15 ... Input terminal.

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Claims (1)

[Claims] 1. A light emitting element drive circuit for supplying a signal current and a bias current corresponding to an input signal voltage and a bias voltage to a light emitting element, a current value detecting means for detecting a current value supplied to the light emitting element, and A direct current level detecting means for detecting a direct current level from the output from the current value detecting means, a first differential voltage detecting means for detecting a difference between the level detected by the direct current level detecting means and a reference value, and the first differential voltage detecting means. Bias voltage control means for controlling the bias voltage applied to the light emitting element drive circuit according to the output of the differential voltage detection means, peak detection means for detecting a peak level from the output from the current value detection means, and the peak detection means. Second difference voltage detecting means for subtracting the output from the DC level detecting means and the output from the second difference voltage detecting means and a third difference for detecting the difference between the output from the second difference voltage detecting means and the reference value. A pressure detecting means, the optical transmission apparatus characterized by comprising a variable gain amplifier circuit for controlling the input signal voltage according to the output of the differential voltage detecting means of the third.