JPS6170777A - Driving circuit for laser diode - Google Patents

Abstract

PURPOSE:To keep the ratio of bias currents to threshold currents constant at all times, and to enable transmission, through which a symbol error rate is reduced, by using bias currents through an APC circuit as an input to an ATC circuit. CONSTITUTION:An optical output from a LD is detected by an optical output detecting element 3, and transmitted over an APC circuit 4, the APC circuit 4 compares the optical output and the reference value of a previously set optical output, and the APC circuit 4 reduces bias currents IB when the optical output is larger than the reference value. When the optical output is smaller than the reference value, inversely, the APC circuit 4 operates so as to increase bias currents IB. Bias currents IB are added to an ATC circuit 5, bias currents IB and the reference value of previously set IB are compared, and the ATC circuit 5 drives a thermoelectric cooling element 2 so as to cool a LD1 when bias currents IB are larger than the reference value. When bias currents IB are smaller than the reference value, inversely, the ATC circuit 5 drives the thermoelectric cooling element 2 so as to heat the LD1.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a driving circuit for a laser diode (hereinafter abbreviated as LD).

Conventional configurations and their problems LDs have a wide modulation band, and are therefore widely used for high-speed transmissions of several hundred Mbit/s. In order to use LDs in these fields, stabilization of optical output characteristics is desired.

To this end, conventional LD drive circuits are provided with an APC circuit that keeps the optical output constant and an ATC circuit that keeps the ambient temperature of the LD constant.

Figure 1 shows the forward current of the LD! The F vs. optical output P characteristics are shown.

As shown in the figure, the LD has an ambient temperature of T1. The threshold current th changes depending on T2.

At this time, in order to keep the optical output P0 constant, the bias current IB should be changed according to the threshold current "th", such as B1, or 2. In this way, the signal current 1s is kept constant, and the bias current APC circuits that change the size of IB are widely used because of their simple circuit configuration.

However, even if an APC circuit is provided, when the threshold current Ith of the LD starts to increase due to a rise in ambient temperature or aging of the LD, the bias current IB increases due to the action of the APC circuit, and the power consumption of the LD decreases. As the temperature increases, the temperature of the LD increases due to self-heating. Therefore, there is a risk of falling into a vicious cycle in which the threshold value Ith further increases. In order to prevent such thermal runaway, keep the ambient temperature of the LD constant, and stabilize and cool the optical output characteristics of the LD to extend its life, an ATC circuit is often used in combination.

The ATC circuit detects the ambient temperature of the LD using a thermistor or the like, and controls the temperature to be constant using a thermoelectric cooling element.

By the way, in PCM optical transmission, bias current! B
From the ratio of the threshold current and the threshold current, th is 0.9 to 1.
It is known that when the bit error rate is 0, the bit error rate of the received signal is the minimum.

However, in the APC circuit described above, since the bias current changes, the ratio of t/th changes.

Furthermore, the ATC circuit is only intended to keep the ambient temperature of the LD constant, and there is a problem in that there is no guarantee that the threshold current of the LD will be constant.

Purpose of the Invention In view of these problems, the present invention has always been developed to
An object of the present invention is to provide a laser diode drive circuit that can maintain a constant ratio of .

Structure of the Invention The laser diode drive circuit of the present invention includes an APC circuit and a bias current controlled by the APC circuit.
It is composed of an ATC circuit which serves as an input signal to the C circuit.

DESCRIPTION OF EMBODIMENTS FIG. 2 shows an embodiment of the present invention. In the figure, 1 is a laser diode (LD), 2 is a thermoelectric cooling element, 3 is an optical output detection element, 4 is an automatic gain control circuit (APC circuit),
6 is an automatic temperature control circuit (ATC circuit).
□The optical output of the LD is detected by the optical output detection element 3,
It is added to the APC circuit 4. The APC circuit 4 compares the optical output with a preset optical output reference value, and if the optical output is larger than the reference value, the APC circuit 4 changes the bias current I.
Decrease B0 Conversely, if the optical output is smaller than the reference value,
APC circuit 4 works to increase bias current IB.

Also, the bias current is applied to the ATC circuit 5, and the bias current is compared with a preset reference value of IB, and if the bias current IB is larger than the reference value, the ATC circuit 6 increases the LDl. On the other hand, when the IA current that drives the thermoelectric cooling element 2 to cool the LD1 is smaller than the reference value, the ATC circuit 5 drives the thermoelectric cooling element 2 to heat the LD1.

In the laser diode drive circuit configured as above, when the threshold current th of LDl starts to increase, A
The bias current fiIB increases due to the action of the PC circuit 4. At the same time, LDl is cooled by the action of ATC circuit 5,
The threshold current Ith of LDl is suppressed from increasing and is kept more constant than the bias current. Therefore, IB/Ith
Effects of the Invention According to the present invention, since the input of the ATC circuit is the bias current via the APC circuit, the ratio of the bias current IB to the threshold current This enables transmission with a low bit error rate.

[Brief explanation of drawings]

FIG. 1 is a current vs. light output characteristic diagram of an LD, and FIG. 2 is a configuration diagram of an embodiment of the present invention. 1... Laser diode, 2... Thermoelectric cooling element. 3... Optical output detection element, 4... APC
Circuit, 5...ATC circuit.

Claims (1)

[Claims] A laser comprising an APC circuit that adjusts a bias current of a laser diode and controls the optical output of the laser diode to a constant value, and an ATC circuit that controls an ambient temperature of the laser diode according to changes in the bias current. Diode drive circuit.