JPS5928076B2 - Laser diode bias circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a bias circuit for a laser diode that can stabilize the light emission output of a laser diode and monitor the output waveform of the light emission output.

A laser diode emits light (oscillates) when a current exceeding its threshold current flows through the laser diode.

Since the value of this threshold current varies greatly depending on temperature, if a bias circuit that simply keeps the value of the bias current (drive current) constant is used, the light emission output will vary greatly due to temperature changes. FIG. 1 shows a conventional bias circuit, in which 1 is a laser diode, 2 is a photodiode that receives a portion of the light output from laser diode 1 through an optical waveguide 9, and 3 and 10 are Darlington-connected NPN transistors. The collector of the subsequent stage transistor 3 is connected to the cathode of the laser diode 1 via a resistor 4, and the base of the first stage transistor 10 is connected to the cathode of the photodiode 2 and a resistor 5.
One end of each capacitor 6 is connected to each other.

7 is a DC power supply, and a laser diode 1 is connected to its positive side.
The anode of the photodiode 2, the emitter of the transistor 3, and the other end of the capacitor 6 are connected to the negative side.

Reference numeral 8 denotes a modulation signal input terminal connected to the cathode side of the laser diode 1.

In such a configuration, it is assumed that the laser diode 1 receives its bias current from the transistor 3 and performs a light emitting operation in accordance with a modulation signal inputted from the input terminal 8 at a certain operating level.

A part of the light emitting output of the laser diode 1 is transmitted through the optical waveguide 9.
The photodiode 2 outputs a received light output corresponding to the amount of received light, that is, the light emitting output of the laser diode 1, and inputs this into the base of the transistor 10. In this case, transistor 10
Since the base of is grounded in an alternating current manner by the capacitor 6, the alternating current component of the light receiving current output from the photodiode 2 (i.e., the modulation signal component input from the input terminal 8)
is smoothed by a capacitor 6, and only the smoothed current, ie, the DC component, is input to the base of the transistor 10 as a control signal. The amplifier consisting of transistors 10 and 3 controls the bias current of laser diode 1 according to its input control signal, and operates so that its average light emission output is constant. By the way, in the conventional circuit, the capacitor 6 is directly connected in parallel to the photodiode 2 and the received light current is averaged.
It is not possible to monitor the light receiving current of the laser diode 1, that is, the light emission output waveform of the laser diode 1, and if you want to monitor the light emission output waveform, a dedicated waveform monitor circuit is required in addition to the bias circuit described above, which makes the circuit complicated and expensive. There were some drawbacks such as:

This invention was made to eliminate the drawbacks of the conventional ones as described above.
To provide a bias circuit for a laser diode capable of monitoring the light emitting output waveform of the laser diode without impairing the stability of the light emitting output of the laser diode with an inexpensive and simple circuit configuration by providing a resistive element for detecting the light receiving current. It is an object.

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

In FIG. 2, numerals 1 to 11 are the same as those in the conventional FIG. 1. 12 is a resistor inserted in series between the cathode of the photodiode 2 and the base of the transistor 10, and 13 is a capacitor connected between the monitor terminal 14 and the cathode of the photodiode 2.

First, the monitor operation in FIG. 2 will be described.

Similar to the conventional circuit operation, a part of the light emission output of the laser diode 1 is transmitted to the photodiode 2 via the optical waveguide 9.
The photodiode 2 outputs a light receiving current corresponding to the amount of light received, that is, the light emission output of the laser diode 1 . This light-receiving current is smoothed by a capacitor 6 through a resistor 12, and its DC component is input to the base of a transistor 10. In this case, a voltage drop due to the light receiving current of the photodiode 2, that is, a voltage waveform similar to the light emission output waveform of the laser diode 1 can be obtained across the resistor 12. In addition, since the negative end of this resistor 12 is grounded in an AC manner by the capacitor 6, if the other end of the resistor 12 is monitored through the capacitor 13, the AC component of the light emission output waveform of the laser diode 1 can be monitored. It is possible to monitor the extraction.

Next, the stabilizing operation of the light emission output during this monitoring will be explained.

Since the monitor terminal 14 is separated from the light emitting output stabilizing circuit in terms of direct current by the capacitor 13, its direct current operation, that is, the stabilizing operation is similar to that of the conventional circuit. In other words, it is equivalent to increasing the internal resistance of photodiode 2 by the resistance value of resistor 12. However, photodiode 2 generally has a very large internal resistance and can be regarded as a constant current source (infinite internal resistance). The resistor 12 connected in series with photodiode 2 has little effect on the operation of photodiode 2.

Therefore, the operation of the circuit shown in FIG. 2 is the same as the conventional circuit shown in FIG. 1 in terms of direct current, and the light emission output of the laser diode 1 is stabilized. As described above, the difference between the present invention and the prior art is that the resistor 12 inserted in series with the photodiode 2 is used to pick up the alternating current component of the current flowing through the photodiode 2 as a monitor voltage.

Therefore, the resistor 12 does not necessarily need to be connected to the cathode side of the photodiode 2, but may be connected to the anode side of the photodiode 2 as shown in FIG. In either case, an AC voltage waveform similar to the light emission output waveform of the laser diode 1 can be monitored from the connection point between the photodiode 2 and the resistor 12 through the DC blocking capacitor 13. In the above embodiment, the negative side of the power source 7 is set to the ground potential for convenience, but any point that can be considered to have the same potential in terms of AC may be set to the ground potential. For example, the positive side of the power source 7 may be set to the ground potential. You can also.

Also, the number of connected stages of transistors 3 and 10 is not limited to two stages,
It goes without saying that the present invention also includes a case where the structure is configured in one stage or three or more stages, and a case where a PNP transistor is used instead of an NPN transistor and the polarities of the power supply 7 and the diodes 1 and 2 are reversed. do not have. As described above, according to the present invention, the light emitting output waveform of the laser diode can be monitored as a voltage waveform without impairing the stabilizing operation of the average light emitting output of the laser diode with an extremely simple circuit configuration in which a resistive element is inserted in series with the photodiode. The advantage is that it can be done.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional laser diode bias circuit, FIG. 2 is a circuit diagram showing a laser diode bias circuit according to an embodiment of the present invention, and FIG. 3 is a circuit diagram showing another embodiment of the present invention. FIG. In the figure, 1 is a laser diode, 2 is a photodiode, and 3 is a laser diode.
10 is a transistor, 6 is a capacitor, 12 is a resistor, and 14 is a monitor terminal.

Claims (1)

[Claims] 1. A bias current is supplied to a laser diode from an amplifier whose input side is grounded in an alternating current manner, a part of the light emission output of the laser diode is fed back to a photodiode provided on the input side of the amplifier, and the received light current is sent to the amplifier. What is claimed is: 1. A bias circuit for a laser diode, which stabilizes the average light emission output of the laser diode by inputting an input signal to the photodiode, the bias circuit comprising a resistive element connected in series with the photodiode and detecting only the light-receiving current.