JPH077929B2 - Optical transmitter - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical transmitter for transmitting an optical signal by a laser element.

[Conventional technology]

FIG. 3 is an electric circuit diagram of a conventional optical transmitter based on the configuration disclosed in Japanese Patent Laid-Open No. 60-121834. In FIG. 3, 1 is an LD (Laser D) as a semiconductor laser device.
iode) 8 is a modulation circuit that supplies a modulation current to the iode 8 and stops the modulation current by an optical output inhibition signal. This modulation circuit 1 comprises transistors 2, 3, a current source 4 and a current switch 5. Reference numeral 9 denotes a bias current circuit which is connected so as to supply a direct current bias current to the LD8 and stop the bias current by the optical output prohibition signal so that the modulation current from the modulation circuit 1 and the self output current are added. is there.
This bias current circuit 9 includes transistors 10 and 11 and a current source 1
2, current switch 13, photodiode 14 and capacitor 1
Composed of 5. Reference numeral 17 is a resistor connected between an addition point a of the modulation current and the bias current from the bias current circuit 9 and an output point b of the bias current circuit 9. Reference numeral 18 is a buffer amplifier circuit that monitors the voltage generated across the resistor 17. This buffer amplifier circuit 18 includes resistors 19, 20, 2
1, 22 and operational amplifier 23. 6 is a high-speed optical modulation signal input terminal, 7 is a threshold value input terminal, 16 is an optical output inhibition signal input terminal, 24 is a power supply REF that supplies a reference voltage, 25 is a buffer amplifier circuit output terminal, 26 is a power supply VCC, 27 Is the power supply VEE.

Next, the operation of the conventional optical transmitter will be described. The modulation circuit 1 includes a transistor 2, a transistor 3, a current source 4 and a current switch 5 which form a pair. For the digital high speed optical modulation signal input from the high speed optical modulation signal input terminal 6, the current supplied from the current source 4 via the current switch 5 is switched using the voltage of the threshold value input terminal 7 as a switching threshold. Switch operation. The LD8 is connected to the collector of the transistor 3 and is optically modulated at high speed by a pulse current switched.

Bias current circuit 9 is transistor 10, transistor 1
1, a current source 12, a current switch 13, a photodiode 14, and a capacitor 15. The transistor 10 and the transistor 11 are Darlington-connected and amplifies the difference current between the current supplied from the current source 12 via the current switch 13 and the output current of the photodiode 14 and supplies the amplified current to the LD8. The photodiode 14 monitors the output light of the LD8. The output current of the photodiode 14 is short-circuited by the capacitor 15 for the high frequency signal and only the low frequency component is output. Bias current circuit 9
The loop consisting of LD8 and LD8 is a negative feedback control loop and is automatically controlled so that the output current of the photodiode 14 becomes substantially equal to the output current of the current source 12. The current source 12 is a current source that determines the light output.

By the way, it is necessary to stop the output of the optical transmission device at some time in order to check a line break or the like by passing another optical signal through the optical fiber for system maintenance. In order to stop the output of the optical transmitter, the current switch 5 and the current switch 13 are opened by the optical output inhibition signal input from the terminal 16, so that neither the modulation circuit 1 nor the bias current circuit 9 outputs current.
This completely stops the optical output of LD8.

In addition, the bias current flowing in the LD8 is monitored for system maintenance, and when the current value becomes abnormal, it may be detected and an alarm may be issued.

In the conventional optical transmitter shown in FIG. 3, a simple bias current monitor system composed of a resistor 17 and a buffer amplifier circuit 18 is used. The buffer amplifier circuit 18 includes the resistor 19, the resistor 20, the resistor 21, the resistor 22 and the operational amplifier as described above.
Composed of 23. Resistor 19, resistor 20, resistor 21, and resistor 22
Have the same resistance value and cause the buffer amplifier circuit 18 to perform a recoil operation, and their gain is "1". In this circuit system, a voltage having the same value as the voltage generated across the resistor 17 is generated between the power supply REF24 and the output terminal 25. This circuit system operates normally when the light output is not prohibited. When the light output is prohibited, the potential difference across the resistor 17 becomes zero and the voltage between the terminals of the LD8 also becomes zero, so that the input common mode voltage of the buffer amplifier circuit 18 becomes equal to the power supply VCC26. When the input common mode voltage becomes equal to the power supply VCC26, the output of the buffer amplifier circuit 18 may drop to the power supply VEE27 side as if a current is flowing though the voltage between the input terminals is zero.

[Problems to be Solved by the Invention]

Since the conventional optical transmitter is configured as described above,
When the optical output is prohibited, the input voltage of the buffer amplifier circuit 18 is out of the common mode operation range, the normal operation cannot be performed, and an erroneous alarm may be issued.

The present invention has been made to solve the above problems, and an optical transmitter that can accurately monitor a bias current by a simple method even when an optical output is prohibited and can prevent an erroneous alarm from being issued. The purpose is to provide.

[Means for Solving the Problems]

An optical transmitter according to the present invention includes a modulation circuit 1 for supplying a modulation current to a laser element (LD8) and stopping the modulation current by an optical output inhibition signal, and a direct current bias current for the laser element. The bias current circuit 9 connected so that the bias current is stopped by the optical output inhibition signal and the modulation current from the modulation circuit 1 and the self output current are added, and the modulation current and the bias current circuit 9 A resistor 17 connected between the addition point a of the bias current circuit 9 and the output point b of the bias current circuit 9 and an amplifier circuit 18 for monitoring the voltage generated across the resistor 17.
And a current source 28 connected to the addition point a of the modulation current and the bias current to set the current, and a current switch 29 connected in series with the current source 28 to cut off the current. To do.

[Action]

The modulation circuit 1 supplies a modulation current to the laser element (LD8) and stops the modulation current by an optical output inhibition signal. The bias current circuit 9 supplies a direct current bias current to the laser element, stops the bias current by an optical output inhibition signal, and adds the modulation current from the modulation circuit 1 to the self output current. The resistor 17 generates a voltage which is the difference between the potential at the addition point a and the potential at the output point b. The amplifier circuit 18 monitors the voltage generated across the resistor 17. The current source 28 sets the current at the addition point a (current of the laser element). The current switch 29 shuts off the direct current of the current source 28.

〔Example〕

FIG. 1 is an electric circuit diagram of an optical transmitter according to an embodiment of the present invention. In FIG. 1, components corresponding to those shown in FIG. 3 are designated by the same reference numerals, and their description will be omitted. In FIG. 1, 28 is a current source connected to the addition point a of the modulation current from the modulation circuit 1 and the bias current from the bias current circuit 9 to set the current, and 29 is connected in series with the current source 28 to supply the current. It is a front flow switch that shuts off.

Next, the operation of this embodiment will be described. Here, differences from the conventional device shown in FIG. 3 will be described.

Before describing the embodiment of the present invention shown in FIG. 1, the characteristics of the LD8 will be described with reference to FIG. Second
The figure shows the general optical output-current / voltage characteristics of InGaAs-LD. LD8 is a current above the oscillation threshold (10m in Fig. 2)
A) emits a light output, but the LD8's terminal voltage is a steady current (1 mA in Fig. 2) even if it is a very small current (1 mA in Fig. 2).
It turns out that V) is reached. Therefore, the light output does not occur, but the voltage between the terminals of LD8 is close to the steady value.
It's easy to flush to D8.

In FIG. 1, the input common mode voltage of the buffer amplifier circuit 18 is in an appropriate range (a distance of about 1V from the normal power supply VCC26) even when the optical output is prohibited, that is, when no current flows from the modulation circuit 1 and the bias current circuit 9. In order to keep it at this value, the voltage between the terminals of LD8 should not be set to zero. This can be easily achieved by setting the current value of the current source 28 to, for example, about 1 mA and closing the current switch 29 when the light output is prohibited.

Even if the oscillation threshold of LD8 becomes low due to low temperature, if the value of the current source 28 is set to a sufficiently low current value, the extinction ratio does not deteriorate.

In the above embodiment, the bias current circuit 9 is a case where a current amplifier by Darlington connection is used, but the same effect is obtained when a voltage amplifier and a voltage / current conversion circuit (for example, grounded emitter transistor circuit) are used.
Switch 13 for inhibiting light output is transistor 10 and resistor
The same effect is obtained when connected between 17 and so on. In addition, when connecting a resistor in parallel with LD8, the current source 28
If the current value of is set to be slightly larger, the same effect can be obtained.

〔The invention's effect〕

As described above, according to the present invention, the current source connected to the addition point of the modulation current and the bias current to set the current, and the current switch connected in series with the current source to cut off the current are configured. In addition, the bias current can be accurately monitored even when the light output is prohibited, and it is possible to prevent an erroneous alarm from being issued, thus improving the reliability.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram of an optical transmitter according to an embodiment of the present invention, FIG. 2 is a diagram showing the optical output vs. current / voltage characteristics of InGaAs-LD in this embodiment, and FIG. It is an electric circuit diagram of a transmitter. 1 ... Modulation circuit, 8 ... LD (semiconductor laser element), 9 ...
... bias current circuit, 17 ... resistance, 18 ... buffer amplifier circuit, 28 ... current source, 29 ... current switch, a ... addition point, b ... output point.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tadayoshi Kitayama 5-1-1, Ofuna, Kamakura-shi, Kanagawa Mitsubishi Electric Corporation Communication Systems Laboratory (56) Reference JP-A-61-262327 (JP, A)

Claims (1)

[Claims] 1. A modulation circuit for supplying a modulation current to a laser element and stopping the modulation current by an optical output prohibition signal; and a DC bias current for supplying the laser element to the laser element and the bias current for the optical output prohibition signal. And a bias current circuit connected so that the modulation current from the modulation circuit and the self-output current are added, an addition point of the modulation current and the bias current from the bias current circuit, and the bias current circuit. A resistor connected between the output points of the amplifier, an amplifier circuit that monitors the voltage generated across the resistor, a current source that is connected to the summing point of the modulation current and the bias current to set the current, and this current An optical transmitter, comprising: a current switch connected in series with the power source to cut off an electric current.