KR100649467B1 - Driving circuit of laser diode - Google Patents

Abstract

An object of the present invention is to provide a driving circuit of a laser diode that can reliably remove noise without adversely affecting the current amplifying action of the driving circuit of the laser diode. The first current path P1 from the power source, the laser diode, and the first transistor FET1 to ground, and the second current path P2 from the power source, resistor R, and the second transistor FET2 to ground are provided. The FET1 and FET2 are supplied with input signals of opposite phases to control the current flowing through the current paths P1 and P2, respectively, and the filter circuit 2 is a common contact between a power supply, a laser diode, and a resistor R. It is inserted between (a).

Description

Driving circuit of laser diode {DRIVING CIRCUIT OF LASER DIODE}

1 is a circuit diagram showing a driving circuit of a laser diode of the present invention.

2 is a graph showing waveforms of a drive current I of a laser diode.

3 is a graph showing a burst signal waveform;

4 is a circuit diagram illustrating a specific configuration example of the filter circuit 2.

Fig. 5 is a circuit diagram showing a driving circuit of a laser diode having a bias current variable.

6 is a circuit diagram showing a driving circuit of a conventional laser diode.

7 is a circuit diagram of an essential part for explaining the voltage drop by the filter circuit 10. FIG.

8 is an essential part circuit diagram when a filter circuit is applied to a driving IC of a laser diode.

<Explanation of symbols for the main parts of the drawings>

1, 1 ': driving circuit of LD

2, 10: filter circuit

P1: first current path

P2: second current path

P3: third current path

P4: fourth current path

a: common contact

b: common terminal

The present invention relates to a driving circuit of a laser diode used in an optical transmission device using a semiconductor laser (called a laser diode).

In recent years, the optical communication speed is further increased, and accordingly, the speed of the pulse signal composed of 0 and 1 digital signals supplied to the driving circuit of the laser diode and used for communication has also increased. Specifically, the bit intervals of 0 and 1 digital signals in the pulse signal are shortened to about 1 to 10 nanoseconds. The driving circuit of the laser diode converts the electrical amplitude of this pulse signal into the strength of the driving current of the laser diode. When the drive current of this laser diode drive circuit is noisy and the optical signal waveform is disturbed, it is necessary to put a filter circuit for noise removal in the current path.

Conventionally, when removing the noise of the driving current of a laser diode, the noise removal filter is inserted in the part with little current change so that the amplitude of a pulse current may be stabilized and the signal component may not be removed as noise.

Specifically, as shown in FIG. 6, a filter circuit 10 such as ferrite beads is inserted between the source electrode of the transistor FET1 for turning on / off the pulse current and the ground, where a normal resistance is placed. have.

[Patent Document 1] Japanese Patent Application Laid-Open No. 6-164038

By the way, when the filter circuit 10 is provided between the source electrode of the transistor FET1 and the ground, as shown in FIG. 7, the source voltage V of the FET1 is reduced due to the resistance component of the filter circuit 10. There arises a problem that the voltage difference with the gate voltage rises and the amplitude of the current output I decreases. The same applies to the case where the transistor is a bipolar type instead of a field effect type.

In addition, in the case of using a commercially available laser diode driving IC, the internal structure may be private, and there may be a case in which the terminal for installing the filter is unknown.

In the case of using a commercially available laser diode driving IC, as shown in FIG. 8, the ground terminal b may be common in the laser diode driving IC. In this case, even if the noise is removed by the filter circuit 10, since the impedance of the ground terminal b of the laser diode driving IC is low, the noise flowing through the laser diode driving IC may not be removed.

Therefore, the present invention provides a laser diode drive circuit capable of reliably removing noise and thereby transmitting a high quality optical signal without adversely affecting the current amplification effect of the laser diode drive circuit. It aims to do it.

The driving circuit of the laser diode of the present invention is provided with a current path from a power source, a laser diode, and a transistor to ground, and the transistor is supplied with an input signal for controlling a current flowing through the current path, and the filter circuit is connected to a power source. The laser diode is inserted between the laser diodes.

According to this structure, since the filter circuit is inserted between the power supply and the laser diode, the impedance of the filter circuit has little influence on the amplifying action, and thus the variation in the amplitude of the current output can be prevented.

In addition, the driving circuit of the laser diode of the present invention is provided with a first current path from a power source, a laser diode, a first transistor to ground, and a second current path from a power source, a resistor, and a second transistor from ground. The first and second transistors are supplied with mutually opposite input signals for controlling currents flowing through the first and second current paths, respectively, and the filter circuit is inserted between a common contact between a power supply, a laser diode, and a resistor. will be.

According to this configuration, the sum of the currents flowing through the first and second current paths is constant regardless of the state of the input signal, and the filter circuit is inserted between the power supply, the common contact of the laser diode and the resistor, The current flowing through the circuit also becomes constant. Therefore, the impedance of the filter circuit has little influence on the amplifying action, thereby making it possible to reliably remove noise and to prevent variations in the amplitude of the current output.

In addition, if the circuit configuration is provided with a third current path for controlling the bias current flowing through the laser diode and a fourth current path for compensating the increase and decrease of the bias current flowing through the third current path, the third and third The sum of the currents flowing through the four current paths is also constant, and the current flowing through the filter circuit does not fluctuate. Therefore, the influence of the impedance of the filter circuit on the amplifying action is less, and the variation in the amplitude of the current output can be prevented.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a circuit diagram showing a drive circuit 1 of a laser diode (hereinafter referred to as "LD") of the present invention.

The driving circuit 1 of the LD includes an amplifier circuit composed of two transistors FET1 and FET2, a third transistor FET3 for controlling the amplitude of the pulse modulation, and a fourth transistor FET4 for setting the bias current of the LD. And resistors R and LD respectively inserted in the load of the amplifier circuit, and filter circuits 2 connected in common with the resistors R and LD.

The path P1 from the power supply, LD, and the first transistor FET1 to ground is called the first current path P1, and the current path from the power supply, resistor R, and the second transistor FET2 to ground is defined. 2 is called the current path P2.

A burst signal for turning on and off the LD at high speed is input from a signal circuit (not shown) to the gates of the two FETs 1 and FET 2 constituting the amplifier circuit. This burst signal is composed of two signals applied to the gate of FET1 and a signal applied to the gate of FET2, and these two signals are supplied out of phase with each other. The pulse current control signal is input to the gate of the FET3 for modulation amplitude control, and the bias setting signal is input to the gate of the FET4 for bias setting.

The filter circuit 2 is a circuit provided to remove noise superimposed on the LD drive current signal, and is characterized in that it is commonly connected between the power supply and the resistors R and LD.

2 is a graph showing the relationship between the drive current I and the time t of the LD. The width of the amplitude of the LD driving current I is represented by I _W , and the bias current value is represented by I _B. The width I _W of the amplitude is determined according to the magnitude of the pulse current control signal, and the bias current value I _B is determined according to the magnitude of the bias setting signal.

The burst signal is a signal which repeats 1 and 0 in a very short period (for example, 1 to 10 nanoseconds) as shown in "signal input" in FIG. The drive circuit 1 of the LD generates a drive current I for driving the LD based on the burst signal. The output light of the LD is intensity modulated with this drive current signal. The intensity modulated light is incident on a transmission optical fiber (not shown) to propagate the optical fiber.

4 is a circuit diagram showing a specific configuration of the filter circuit 2. The filter circuit 2 includes an inductor L made of ferrite beads inserted in series between a common contact a between the resistors R and LD and a power supply, and between the common contact a and ground. It consists of the resistor R1 and the capacitor C1 connected in series.

The effect of installing this filter circuit 2 between the common contact a with the resistors R and LD and a power supply is demonstrated.

Since the sum of the amounts of current flowing through FET1 and FET2 in FIG. 1 is constant, the filter circuit 2 is provided between the common contact a with the resistors R and LD and the power supply. Can be made constant regardless of fluctuations in the input signal. Therefore, the voltage drop by the filter circuit 2 also becomes constant, and the fluctuation of the electric potential of the common contact a can be prevented. Therefore, there is no variation in the source voltage of FET1, so that the gate-source voltage that determines the load current amount can also maintain a stable value. This eliminates the conventional drawbacks of fluctuations in current gain and instability in amplification characteristics.

Fig. 5 is a circuit diagram showing the driving circuit 1 'of the LD capable of switching the bias current in accordance with the input. This circuit is used, for example, as a drive circuit of the LD for a passive optical network (PON) system optical network unit.

In this circuit, the fifth transistor FET5 for controlling the bias current flowing through the LD is provided on the cathode side of the LD. The bias current is controlled by controlling the gate voltage of FET5.

In addition, a compensation circuit for compensating the increase and decrease of the bias current of the LD is composed of a resistor R 'and a sixth transistor FET6. The path from FET5 to ground is called third current path P3, and the current path from FET6 to ground is called fourth current path P4.

The gate voltage applied to the gate of FET6 is directly out of phase with the gate voltage of FET5. Therefore, the sum of the amount of current flowing through the LD and the amount of current flowing through the resistor R 'becomes constant, and the current flowing through the filter circuit 2 is kept constant. As a result, there is no fluctuation in the source voltage of the FET1 as in the LD driving circuit of FIG.

As mentioned above, although the Example of this invention was described, the implementation of this invention is not limited to said example. For example, it is possible to reverse the positive and negative poles of the power supply and reverse the polarity of the LD. In addition, the filter circuit 2 is not limited to the inverted L type circuit which consists of LCR shown in FIG. 4, The filter circuit generally used can be employ | adopted. In addition, although the transistor used so far functioned as a switching switch, you may operate as a variable resistor which changes resistance analogously by setting the operating point of a semiconductor element suitably. In addition, it is possible to make various changes within the scope of the present invention.

As described above, according to the present invention, even if the input signal changes at high speed, the effect of the laser diode driving circuit on the output of the current is small and the noise can be reliably removed.

Claims (3)

In the driving circuit of a laser diode provided with a filter circuit for removing noise mixed in an output current signal for driving a laser diode, A current path is provided from the power source, the laser diode, and the transistor to ground, and the transistor is supplied with an input signal for controlling the current flowing through the current path, and the filter circuit is inserted between the power supply and the laser diode. A driving circuit for a laser diode, characterized in that. In the driving circuit of a laser diode provided with a filter circuit for removing noise mixed in an output current signal for driving a laser diode, A first current path from the power source, laser diode, first transistor to ground, A second current path is provided from the power source, resistor, second transistor to ground, The first and second transistors are supplied with mutually opposite input signals for controlling currents flowing through the first and second current paths, respectively. And said filter circuit is inserted between a power supply, a common contact of a laser diode and a resistor. The method of claim 2, further comprising: a third current path P3 for controlling the bias current flowing through the laser diode; A fourth current path P4 is provided to compensate for the increase and decrease of the bias current flowing through the third current path P3. The third current path P3 is provided between the connection point of the laser diode LD and the transistor FET5 and ground. And said fourth current path (P4) is provided between the connection point of resistor (R ') and transistor (FET6) and ground.

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