JPH0364134A - Optical transmission circuit - Google Patents

Abstract

PURPOSE:To decrease the deterioration in a drive current waveform, to simplify the circuit constitution and to reduce the cost and size of an entire device by using a control voltage from an automatic power control(APC) circuit and varying the output voltage of a voltage source so as to allow a driving active element to supply a driving current and a DC bias current to a light source. CONSTITUTION:A control voltage is applied from an APC circuit to a voltage source connected to the source terminal of a FET 2 and a control voltage outputted from the APC circuit 7 follows the forward current change of a photodiode 5 monitoring the output light of a laser 3. When a fluctuation occurs in the output light of the laser 3, the voltage of a FET driving voltage source is changed by a control voltage outputted form the APC circuit 7, the source potential of the FET 2 is changed to control a DC bias current to the laser 3 to attain a constant optical output. Thus, it is possible to supply a DC bias current together with a driving current without special addition of a DC bias circuit of a light emitting element. Thus, the deterioration in the driving current waveform is small, the circuit constitution is simplified and the cost and size of the entire device are reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a drive circuit for a light emitting element in an optical communication device.

(Prior technology) An optical communication system that uses a semiconductor laser that can easily obtain modulated light by changing the drive current (direct intensity modulation method) and an optical fiber that has excellent characteristics such as low loss and broadband performance. Until now, it has been mainly applied to the trunk lines of public communication networks, and 1.6 gigabit/second long-distance, high-capacity systems have already been put into practical use. (For example, described in Nikkei Electronics no. 432.) Furthermore, in recent years, optical transmission technology has been applied to optical subscriber systems for the purpose of providing broadband communication services, but For mass popularization, miniaturization and cost reduction are important issues.

Conventionally, when a semiconductor laser biased below a threshold current is directly modulated, a delay time occurs in the generation of laser light because laser oscillation does not occur unless a drive current above the threshold current is supplied. Furthermore, since the refractive index inside the laser resonator changes during high-speed modulation, wavelength chirping, in which the oscillation wavelength is modulated, also occurs. This phenomenon reduces the signal discrimination ability on the receiving side, so by applying a DC bias current higher than the oscillation threshold to the laser in advance,
A bias circuit is required to suppress these phenomena, which tends to increase the size and cost of the entire device.

FIG. 2 shows an example of an optical communication circuit in a conventional large-capacity transmission optical communication system. Hereinafter, the operation of the conventional example will be explained using FIG. 2 and FIG. 3 showing an example of the FET characteristics in FIG. 2.

In FIG. 2, a pulse signal input to input terminal 1 is shifted to a desired gate potential by level shift diode D2 (301 in FIG. 3). Here, the semiconductor laser 3
is assumed to have the characteristics as shown in FIG. - When using such a laser for boat fishing, the oscillation threshold current ■
L), (for example, 20 mA) or more offset current must be supplied. However, if only a driving current (302 in Fig. 3) with a peak current value of 11 (for example, 30 mA) is applied to the laser 3 without supplying this offset current, the receiving side will not be able to receive the signal due to the effects of light emission delay time and wavelength chirping. signal discrimination ability is reduced. Therefore, the laser drive current flowing from the ground point of the laser 3 to the FET 2 (302 in FIG. 3)
), it is essential to further supply a DC bias current of at least Ith or more using the DC bias circuit 4 and the DC bias current source Ib. Furthermore, when a conventional optical transmission circuit is driven using a high-speed pulse of gigabit or higher, there is a risk that the drive current waveform may deteriorate due to the parasitic elements (capacitor C, inductance L) of this DC bias circuit.

Note that a light emitting element such as the laser 3 has an unstable characteristic in that the optical output fluctuates when the ambient temperature etc. change, but the optical output can be monitored using the monitoring photodiode 5 and the DC bias current can be controlled. The optical transmitter circuit includes an auto power control (APC) circuit 7 that performs feedback control so that the optical output remains at a constant value, and an auto thermo control (ATC) circuit 8 that maintains the temperature of the light emitting element at a constant value. By adding this to , it is possible to stabilize the optical output. (For example, it is stated in "New Edition Optical Fiber Communication J (Telecommunications Technology News Company)" supervised by Mitsuji Maeda.) (Problem to be solved by the invention) An optical transmission circuit that directly modulates the intensity of a semiconductor laser is circuit and a DC bias current circuit necessary to reduce the light emission delay time and wavelength chirping amount of the modulated output light.Therefore, in conventional optical transmitter circuits, by adding the DC bias circuit, This has disadvantages such as a complicated circuit configuration and an increase in the cost and size of the entire device.

Furthermore, when a conventional optical transmission circuit is driven using high-speed pulses of gigabit or higher, there is a risk that the drive current waveform will deteriorate due to parasitic elements in the DC bias circuit.

An object of the present invention is to eliminate such conventional drawbacks, reduce deterioration of the drive current waveform, simplify the circuit configuration, and reduce the cost and size of the entire device. An object of the present invention is to provide an optical transmission circuit that can also be applied to transmission systems.

(Means for Solving the Problems) In an optical transmission circuit having a light source and means for modulating the output of the light source, a driving active element supplies a driving current to the light source, and an input pulse signal is transmitted to the driving active element. A level shift circuit that shifts the voltage to a desired level within a linear operating region and applies it to the driving active element, a monitoring photodiode that monitors a portion of the output light of the light source, and a light monitor current of the photodiode that is input. and an auto power control (APC) circuit that generates a control voltage for the voltage source of the driving active element, and the A
The driving active element supplies a driving current and a DC bias current to the light source by changing the output voltage of the voltage source using a control voltage output from a PC circuit.

(Function) In the present invention, as an active element for supplying a drive current to a light emitting element, one having a current supply capability greater than the sum of an oscillation threshold current and a necessary drive current is used. Further, the power supply voltage of the active element is set so that a "0" signal current equal to or higher than the oscillation threshold is supplied.

This makes it possible to supply a DC bias current as well as a drive current without adding a special DC bias circuit for the light emitting element to the optical transmitter.

(Example) FIG. 1 shows an embodiment of an optical transmission circuit using the present invention.

The operation will be explained below using FIG. 1 and FIG. 3 showing an example of FET characteristics in FIG. 1.

In FIG. 1, a pulse signal input to input terminal 1 is shifted to a constant gate potential (303 in FIG. 3) by level shift diode D1. At this time, the power supply voltage V-〇 supplied to the source end of FET2 has a drive current amplitude of F
Lax needs to be within the linear region of ET2. This makes it possible to supply a constant offset current to the laser 3 even at the FET input signal power ('0' level). Therefore, if the optical transmitter circuit shown in Fig. 1 is used, the input pulse code will be Even in the case of 'O',
A DC bias current (for example, 30 mA) greater than the oscillation threshold current (20 mA, for example) of the laser 3 is constantly supplied to the laser 3 . In other words, the DC bias current and peak current value ■ can be easily adjusted without adding a special DC bias circuit as an optical transmission circuit. (e.g. 60mA) drive current (
304) in FIG. 3 can be simultaneously supplied to the laser 3. Furthermore, a light emitting element such as the laser 3 has unstable characteristics in that its optical output fluctuates when the ambient temperature or the like changes. However, in the optical transmission circuit according to the present invention, FET
A control voltage is supplied from an APC circuit 7 to a voltage source connected to the source end of the circuit 2. (Note that ■AP. is the reference voltage of the APC circuit.) The control voltage output from the APC circuit 7 follows the change in the forward current of the photodiode 5 that monitors the output light of the laser 3. (In addition, vPD
is the voltage source for the monitor photodiode. >here,
When a fluctuation occurs in the output light of the laser 3, the voltage of the FET driving voltage source changes due to the control voltage output from the APC circuit, and the source potential of the FET 2 changes, so that the light output from the laser 3 is changed to maintain a constant optical output. DC bias current can be controlled. That is, it is possible to stabilize the optical output as in the conventional case.

In the above explanation, the characteristics of the FET 2 and the laser 3 used are as shown in Figures 3 and 4, respectively, but in reality they do not need to have exactly the same characteristics, and the oscillation threshold current It is sufficient if the current supply capacity is greater than the sum of the required drive current and the required drive current.

(Effects of the Invention) As described above, according to the present invention, the deterioration of the drive current waveform is small, the circuit configuration is simple, and the cost and size of the entire device can be reduced. It is possible to obtain an optical transmission circuit that can also be applied to.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing an example of a conventional optical transmitter circuit, and FIGS. 3 and 4 explain the operation of the optical transmitter d between the conventional and the present invention. FE used to
It is an example of the characteristic diagram of T and a semiconductor laser. In the figure, 2...FET, 3... semiconductor laser,
4... DC bias circuit, 5... Monitor photodiode, 6... Peltier element, 7... Auto power control, 8... Auto thermo control, R1, R2, R3゜R4...Resistance, R1...
Thermistor, Dl, D2...Level shift diode.

Claims (1)

[Claims] An optical transmission circuit having a light source and a means for modulating output light from the light source, a driving active element that supplies a driving current to the light source, and a driving active element that adjusts an input pulse signal to a desired level within a linear operating region of the driving active element. a level shift circuit that shifts the voltage to the driving active element, a monitoring photodiode that monitors a part of the output light of the light source, and a voltage of the driving active element that inputs the light monitoring current of the photodiode. and an auto power control (APC) circuit that generates a control voltage for the drive source, and by changing the output voltage of the voltage source using the control voltage output from the APC circuit, An optical transmission circuit characterized in that an active element supplies a drive current and a DC bias current to the light source.