JPH03141738A - optical transmitter - Google Patents

Abstract

PURPOSE:To monitor a bias current when an optical output is inhibited and to prevent an erroneous warning by providing a current switch to interrupt a current between an adding point, where a modulating current and the bias current is added, and a current source to apply a set current to this adding point. CONSTITUTION:A modulation circuit 1 supplies the modulating current to a laser element 8 and stops the modulating current by an optical output inhibiting signal. A bias current circuit 9 supplies the direct current bias current to the element 8, stops the bias current by the optical output inhibiting signal and adds the current from the circuit 1 and the output current of the circuit 9 itself. A resistor 17 generates the voltage of difference between the potential of an adding point (a) and the potential of an output point (b) and an amplifier circuit 18 monitors the voltages at the both ends of the resistor 17. A current source 28 sets the current of the element 8 and between this current source 28 and the adding point (a), a current switch 29 is provided and closed when the optical output is inhibited. Thus, the bias current is monitored and the erroneous warning is prevented from being generated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical transmitter that transmits an optical signal using a laser element.

[Conventional technology]

FIG. 3 is an electrical circuit diagram of a conventional optical transmitter based on the configuration disclosed in, for example, Japanese Patent Laid-Open No. 60-121834. In FIG. 3, l is L as a semiconductor laser element.
This is a modulation circuit that supplies a modulation current to the D (Laser Diode) 8 and stops the modulation current by an optical output prohibition signal. This modulation circuit 1 consists of transistors 2, 3, a current source 4 and a current switch 5. Reference numeral 9 denotes a bias current circuit connected to supply a direct current bias current to the LD 8, stop the bias current by the optical output prohibition signal, and add the modulation current from the modulation circuit 1 and the self-output current. be. This bias current circuit 9 includes a transistor 10.11, a current source 12, a current switch 13, a photodiode 14, and a capacitor 15.
Consists of. Reference numeral 17 denotes a resistor connected between the addition point a of the modulation current and the bias current from the bias current circuit 9 and the output point b of the bias current circuit 9. 18 is a buffer amplifier circuit that monitors the voltage generated across the resistor 17. This buffer amplifier circuit 18 has a resistor 1
9.20, 21.22 and an operational amplifier 23. 6
is a high-speed optical modulation signal input terminal, 7 is a threshold input terminal, and 1 is a high-speed optical modulation signal input terminal.
6 is an optical output prohibition signal input terminal, 24 is a power supply REF that supplies a reference voltage, 25 is a buffer amplifier circuit output terminal, 26
The power supply VCC127 is the power supply VEE.

Next, the operation of the conventional optical transmitter will be explained.

Modulation circuit 1 consists of a pair of transistors 2 and 3.
, current -a4 and current switch 5.

The current supplied from the current source 4 via the current switch 5 is switched using the voltage at the threshold input terminal 7 as a switching threshold for the digital high-speed optical modulation signal input manually from the high-speed optical modulation signal input terminal 6. Operate switching.

The LD 8 is connected to the collector of the transistor 3 and subjected to high-speed optical modulation by a switched pulse current.

The bias current circuit 9 includes a transistor IO, a transistor 11, and a current a! 12, a current switch 13, a photodiode 14, and a capacitor 15. Transistors 10 and 11 are Darlington connected and amplify the difference between the current supplied from the current source 12 through the current switch 13 and the output current of the photodiode 14.
It is supplied to D8. The photodiode 14 monitors the output light of the LD8. The output current of the photodiode 14 is connected to a capacitor 15, which short-circuits the high frequency signal and outputs only the low frequency component. The loop consisting of the bias current circuit 9 and the LD 8 is a negative feedback control loop, and the output current of the photodiode 14 is automatically controlled to be approximately equal to the output current of the current source 12.

Current #12 is a current source that determines the optical output.

Incidentally, for system maintenance purposes, it is sometimes necessary to stop the output of the optical transmitter in order to check for line breakage by passing another optical signal through the optical fiber. To stop the output of the optical transmitter, the current switch 5 and the current switch 13 are opened by the optical output prohibition signal inputted from the terminal 16, and the modulation circuit 1 is turned off.
No current is output from either the bias current circuit 9 or the bias current circuit 9. As a result, the optical output of the LD8 is completely stopped.

Also, for system maintenance purposes, the bias current flowing through the LD' 8 is monitored, and if the current value becomes abnormal, this may be detected and an alarm may be issued.

The conventional optical transmitter shown in FIG. 3 uses a simple bias current monitoring system configured by a resistor 17 and a buffer amplifier circuit 18. The buffer amplifier circuit 18 is composed of the resistor 19, the resistor 20, the resistor 21, the resistor 22, and the operational amplifier 23 as described above. resistance 19, resistance 20
, the resistor 21, and the resistor 22 have the same resistance value and perform a reaction operation of the buffer amplifier circuit 18, and their gain is rlJ. In this circuit system, a voltage having the same value as the voltage generated across the resistor 17 is generated between the power supply REF 24 and the output terminal 25. This circuit system operates normally when optical output is not inhibited. When optical output is prohibited, the potential difference between both ends of the resistor 17 becomes zero and L
Since the voltage between the terminals of D8 also becomes zero, the buffer amplifier circuit 1
The input common mode voltage of 8 becomes equal to 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 even though the voltage between the input terminals is zero.

[Problem to be solved by the invention]

Since the conventional optical transmitter was configured as described above,
When optical output is inhibited, the input voltage of the buffer amplifier circuit 18 is outside the common mode operating range, making it unable to operate normally and causing a problem in that a false alarm may be issued.

This invention was made to solve the above problems, and provides an optical transmitter that can accurately monitor bias current in a simple manner even when optical output is prohibited, and can prevent false alarms from being issued. The purpose is to provide

[Means to solve the problem]

The optical transmitter according to the present invention includes a modulation circuit 1 that supplies a modulation current to a laser element (LD8) and stops the modulation current by an optical output prohibition signal, and a modulation circuit 1 that supplies a DC bias current to the laser element and a bias current circuit 9 connected so that the bias current is stopped by the optical output prohibition signal and the modulation current from the modulation circuit 1 and the self-output current are added; a resistor 17 connected between the bias current addition point a and the output point of the bias current circuit 9;
An amplifier circuit 18 that monitors the voltage generated across the resistor 17, a current source 28 that is connected to the addition point a of the modulation current and the bias current, and that sets the current, are connected in series with the current source 28. Current switch 29 that cuts off the current
It is characterized by having the following.

[Effect]

The modulation circuit 1 supplies a modulation current to the laser element (LD8) and stops the modulation current by an optical output prohibition signal. The bias current circuit 9 supplies a DC bias current to the laser element, stops the bias current in response to an optical output prohibition signal, and adds the modulation current from the modulation circuit 1 and its own output current. The resistor 17 generates a voltage that is the difference between the potential at the addition point a and the potential at the output point S. The amplifier circuit 18 has a resistor 1
Monitor the voltage developed across 7. A current source 28 sets the current at the addition point a (current of the laser element). The current switch 29 cuts off the direct current of the current source 28.

〔Example〕

FIG. 1 is an electrical 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 explanations will be omitted. In FIG. 1, 28 is the addition point a of the modulation current from the modulation circuit 1 and the bias current from the bias current circuit 9.
29 is a current switch connected in series with the current a28 to cut off the current.

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

Before explaining one embodiment of the present invention shown in FIG. 1, the characteristics of the LD 8 will be explained using FIG. 2. FIG. 2 shows the general optical output versus current/voltage characteristics of an InGaAs-LD. LD8 has a current above the oscillation threshold (
In Figure 2, it emits light output for 10mA), but LD8
It can be seen that the voltage between the terminals reaches a steady voltage (IV) even if the current is minute (1 mA in FIG. 2). Therefore, although no optical output is emitted, the voltage between the terminals of LD8 takes a value close to the steady value.When the Mon mode voltage is prohibited from outputting light, that is, when no current is flowing from the modulation circuit 1 and the bias current circuit 9. In order to keep the voltage within an appropriate range (approximately IV away from the normal power supply VCC26), the voltage between the terminals of the 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 optical output is prohibited.

Even if the temperature becomes low and the oscillation threshold of the LD 8 becomes low, if the value of the current source 28 is set to a sufficiently low current value, deterioration of the extinction ratio will not occur.

In addition, is the bias current circuit used in the above embodiment? Although I have described the case where a current amplifier with a Darlington connection is used, the same effect can be obtained when a voltage amplifier and a voltage/current conversion circuit (for example, a common emitter transistor circuit) are used. The switch 13 for inhibiting light output is a transistor 10
A similar effect can be obtained when the resistor 17 is connected between the resistor 17 and the resistor 17.

Furthermore, when a resistor is connected in parallel with the LD 8, the same effect can be obtained by making the current value of the current source 28 slightly larger.

〔Effect of the invention〕

As described above, according to the present invention, the configuration includes a current source connected to the addition point of the modulation current and bias current to set the current, and a current switch connected in series with the current source to interrupt the current. The bias current can be accurately monitored even when optical output is prohibited, preventing false alarms from being issued, and thus improving reliability.

[Brief explanation of the drawing]

FIG. 1 is an electrical circuit diagram of an optical transmitter according to an embodiment of the present invention, and FIG.
FIG. 3 is an electrical circuit diagram of a conventional optical transmitter. l...Modulation circuit, 8...LD (semiconductor laser element), 9...Bias current circuit, 17
...Resistor, 18...Buffer amplifier circuit, 28...Current source, 29...Current switch, a...Addition point, b ...Output point.

Claims (1)

[Claims] a modulation circuit that supplies a modulation current to the laser element and stops the modulation current with an optical output prohibition signal; and a modulation circuit that supplies a DC bias current to the laser element and stops the bias current with the optical output prohibition signal, and modulates the modulation circuit. A bias current circuit connected so that the modulation current from the circuit and the self-output current are added, and a point between the addition point of the modulation current and the bias current from the bias current circuit and the output point of the bias current circuit. A connected resistor, an amplifier circuit that monitors the voltage generated across this resistor, a current source connected to the addition point of the modulation current and the bias current to set the current, and connected in series with this current source. An optical transmitting device characterized by comprising: a current switch that cuts off current.