JPH04101481A - Optical transmitter - Google Patents

Abstract

PURPOSE:To hold an optical output of an optical fiber constant even if a tracking error exists by controlling a bias current from a driving circuit based on a sensed result of a photodiode for sensing an output of a rear light of a laser diode and a resistance value of a thermistor for sensing a peripheral temperature of the diode. CONSTITUTION:When a peripheral temperature of a laser diode 11 rises, an optical output 113 of an optical fiber 12 is increased. In this case, a resistance value of a thermistor 44 of an automatic optical output controller 4 is reduced, and a potential of a non-inverting input terminal of an operational amplifier 41 is reduced. Thus, since the base potential of a transistor 45 is lowered, its collector current is reduced. Accordingly, the bias current of the diode 11 is reduced, and its output is reduced. Accordingly, if the output of the diode 11 is reduced in response to its peripheral temperature by suitably selecting a constant indicating the ratio of variation in the resistance value of second thermistor 44 with respect to its temperature, the output 113 of the fiber 12 can be maintained constant irrespective of the peripheral temperature of the diode 11.

Description

[Detailed description of the invention] Industrial applications The present invention relates to an optical transmitter used for optical communications and the like.

BACKGROUND OF THE INVENTION The circuit configuration of a conventional optical transmitter is shown in FIG. In FIG. 3, 1 is a laser diode module, and this laser diode module 1 includes a laser diode 11 as a light emitting element, an optical fiber 12 as an optical transmission line,
a photodiode 13 for monitoring the output of the laser diode 11; a first thermistor 14 for monitoring the temperature of the laser diode 11;
Based on the monitoring result of step 4, the laser diode 11
It is constituted by a Peltier element 15 for keeping the temperature at a constant temperature.

Reference numeral 2 denotes a laser diode driving circuit, which applies a driving pulse current to the laser diode 11 in response to a pulse signal applied via an input terminal 5 from a driving source for the laser diode 11 (not shown).

3 is an automatic temperature control circuit which, based on the result of monitoring the temperature of the laser diode 11 by the first thermistor 14 of the laser diode module 1, controls the flow to the Peltier element 15 to keep the temperature of the laser diode 11 constant. It controls the current.

4 is an automatic light output control circuit, and this automatic light output control circuit 4 maintains the output of the laser diode 11 at a constant level based on the result of monitoring the output of the laser diode 11 by the photodiode 13. an operational amplifier 41 that supplies a drooping output, a resistor 42 for applying negative feedback to the operational amplifier 41, and the operational amplifier 41.
Resistor 43.47 for defining the potential on the non-inverting input side of
It is equipped with

Further, the automatic light output control circuit 4 includes an operational amplifier 41
The laser diode 11 is provided with a transistor 45 which supplies the collector current to the laser diode 11 as a bias current via the inductance 6 based on the output of the laser diode 11 .

On the other hand, FIG. 4 shows a specific configuration of the optical system of the laser diode module 1. As shown in FIG. In FIG. 4, 16 is the forward light of the laser diode 11, 17 is
The backward light 103 of the laser diode 11 focuses the forward light 16 of the laser diode 11 and connects it to the optical fiber 12.
For example, a laser diode 1"
The focal point is set to be located at the end face of the optical fiber 12 when the surrounding temperature of the optical fiber 11 is normal temperature.

Reference numeral 113 denotes a light output transmitted on the optical fiber 12 based on the forward light 16 of the laser diode 11.

Next, the operation of the above conventional example will be explained. In FIG. 3, when a pulse current from the laser diode drive circuit 2 is applied to the laser diode 11, the laser diode
1''11 emits light, and the accompanying heat generation causes the temperature of the laser diode 11 to rise.

Then, the resistance value of the first thermistor 14 changes with this temperature rise, so that the automatic temperature control circuit 3 maintains the temperature of the laser diode 11 constant (for example, room temperature) according to the change in the resistance value of the thermistor. The current value supplied to the Peltier element 15 is controlled so that the Peltier element 15 absorbs the heat generated by the laser diode 11 and keeps the temperature of the laser diode 11 constant.

In this way, by keeping the temperature of the laser diode 11 constant, the threshold current and differential efficiency of the laser diode 11 are kept constant, so that the output of the laser diode 11 changes due to the heat generated by the laser diode 11. I try not to have anything to do.

In this state, when the laser diode 11 emits light, its forward light 16 is focused by the lens 103 and input to the optical fiber 12, as shown in FIG. 4, and its backward light 17 is received by the photodiode 13. .

The forward light 16 input into the optical fiber 12 becomes an optical output 113 and is transmitted through the optical fiber 12, while the backward light 17 received by the photodiode 13 represents the output of the laser diode 11 here. Converted to monitor current.

The monitor current proportional to the output of the laser diode 11 converted by the photodiode 13 is determined by the voltage dividing resistance value of the resistor 43, 47 connected to its non-inverting input side and the negative feedback resistor 42 from its output side. The operational amplifier 41 converts the current into voltage based on the resistance value of the transistor 4.
5 as the base potential.

As a result, the collector current of the transistor 45 is determined, and the collector current of the transistor 45 is given to the laser diode 11 as its bias current via the inductance 6, and the laser diode 11 emits light with an output based on this bias current. do.

In the conventional optical transmitter with the above configuration, by keeping the temperature of the laser diode 11 constant as described above, the threshold current and differential efficiency of the laser diode 11 are kept constant, and the optical output is For example, if the ambient temperature of the laser diode 11 changes and the threshold current and differential efficiency deteriorate, the output of the laser diode 11 will decrease.

Therefore, conventionally, the decrease in the output of the laser diode 11 is detected by monitoring the photodiode 13 of the light 17 behind it, and as the monitor current of the photodiode 13 decreases, the output of the operational amplifier 41, that is, the output of the transistor 4 is detected.
By increasing the base potential of 5, the laser diode 11
By increasing the bias current of the laser diode 11, a decrease in output due to the laser diode 11 is suppressed.

Problems to be Solved by the Invention However, in the conventional optical transmitter described above, as shown in FIG. 3, when the temperature around the laser diode 11 changes,
Optical axis deviation occurs in the optical system such as the lens 103, and the amount of forward light 16 input to the optical fiber 12 decreases, and accordingly, the optical output 113 from the optical fiber 12 also decreases.

In other words, by monitoring the rear light 17 of the laser diode 11 by the photodiode 13, even if the monitor current representing the output of the rear light 17 is constant, the laser diode 1
The optical output 113 on the optical fiber 12 through which the forward light 16 of the laser diode 11 is focused and transmitted by the lens 103 fluctuates as the ambient temperature of the laser diode 11 changes, causing a tracking error.

Here, an example of the above tracking error is shown in FIG. Since the above optical system is adjusted to maximize the efficiency with which the forward light 16 is input to the optical fiber 12 at room temperature TO, the optical output 113 on the optical fiber 12 is maximized.

When the ambient temperature of the laser diode 11 changes, a shift occurs in the optical axis of the convergence of the forward light 16 input to the optical fiber 12 by the lens 103, and the input efficiency of the forward light 16 to the optical fiber 12 decreases. Therefore, the optical power 113 on the optical fiber 12 also decreases.

When looking at the change in the optical output 113 on the optical fiber 12 when the bias current of the laser diode 11 is controlled so that the average output of the backward light 17 is constant, as shown in FIG. optical output 113 (Pf
) is maximum when the ambient temperature (T) of the laser diode 11 is room temperature, and decreases when the temperature is low or high.

In this way, in the conventional optical transmitter described above, in order to keep the output of the laser diode 11 constant, the bias current of the laser diode 11 is controlled so that the output of the backward light 17 is always constant. Due to this control of the bias current, there is a problem in that the optical output 113 on the optical fiber 12 that transmits the forward light 16 of the laser diode 11 fluctuates by the tracking error.

The present invention solves these conventional problems,
It is an object of the present invention to provide an optical transmitter that can maintain a constant optical output on an optical fiber even if a tracking error exists.

Means for Solving the Problems In order to achieve the above object, the present invention includes a laser diode, a laser diode drive circuit that supplies a driving bias current to the laser diode, and a laser diode drive circuit that inputs the forward light of the laser diode. an automatic temperature control circuit that controls the transmission optical fiber and the temperature of the laser diode to maintain a constant temperature by supplying a current to a Peltier element according to the resistance value of a first thermistor that detects the temperature of the laser diode; The output of the laser diode on the optical fiber is determined based on the detection result of the photodiode that detects the output of the backward light of the laser diode, and the resistance value of the second thermistor that detects the ambient temperature of the laser diode. The structure includes an automatic light output control circuit that controls the magnitude of the bias current from the laser diode drive circuit so that the current is constant.

Therefore, according to the present invention, even if the proportion of forward light input to the optical fiber decreases due to tracking error, the output of the laser diode increases to compensate for the decrease, so that the optical output on the optical fiber decreases. can be kept constant.

Embodiment FIG. 1 shows a circuit configuration of an embodiment of the present invention. It should be noted that elements in FIG. 1 that are similar to those in the above-mentioned FIG. 3 are given the same reference numerals as those in FIG. 3, and their explanations will be omitted.

Reference numeral 44 in FIG. 1 denotes a second resistor, which is provided in the automatic optical output control circuit 4 to define the potential on the non-inverting input side of the operational amplifier 41, and constitutes a voltage dividing resistor together with the resistor 43.
This is a thermistor whose resistance value changes depending on the ambient temperature of the laser diode 11.

Next, the operation of the above embodiment will be explained.

The configuration of the laser diode module 1 in this embodiment is the same as that shown in FIG. 4 above, and further explanation of the configuration will be omitted. In the optical system of this embodiment, the position of the lens 103 is adjusted to such a position that the optical output on the optical fiber 12 is maximized at high temperatures.

In the optical transmitter of this embodiment, the temperature of the laser diode 11 is adjusted by the same operation as a conventional optical transmitter, thereby keeping the threshold current and differential efficiency of the laser diode 11 constant. This ensures that there is no drop in output due to this.

Then, the ambient temperature of the laser diode 11 changes, causing deterioration in the threshold current and differential efficiency of the laser diode 1''11, resulting in a tracking error in the optical output 113 on the optical fiber 12. When this occurs, it can be approximated that this tracking error changes linearly as shown by straight line a in FIG. 2 in response to the ambient temperature.

That is, in the laser diode module 1, when the ambient temperature (T) of the laser diode 11 increases, the optical output 113 (P) on the optical fiber 12 increases.

At this time, the resistance value of the thermistor 44 of the automatic optical output control circuit 4 decreases, and the potential of the non-inverting input terminal of the operational amplifier 41 decreases. Correspondingly, the output voltage of the operational amplifier 41, that is, the base potential of the transistor 45 decreases, so that the collector current of the transistor 45 decreases. Therefore, the bias current of the laser diode 11 decreases, and thus the output of the laser diode 11 decreases.

For this reason, the constant of the second thermistor 44 of the automatic light output control circuit 4 (a constant indicating the rate of change in resistance value with respect to temperature of the thermistor) is appropriately selected so that the output (P) of the laser diode 11 is controlled by the laser diode. 1 ■ ambient temperature (
If the optical output 113 (P) on the optical fiber 12 decreases as shown in the straight line in FIG.
) can be kept constant regardless of the ambient temperature (T) of the laser diode 11, as shown by straight line C in FIG.

Effects of the Invention As is clear from the above embodiments, the present invention includes a laser diode, a laser diode drive circuit that supplies a driving bias current to the laser diode, and a light that inputs and transmits the forward light of the laser diode. an automatic temperature control circuit that controls the temperature of the laser diode to be kept constant by supplying a current according to the resistance value of a first thermistor that detects the temperature of the fiber and the laser diode to a Peltier element; The output of the laser diode on the optical fiber is kept constant based on the detection result of a photodiode that detects the output of the rear light of the laser diode and the resistance value of a second thermistor that detects the ambient temperature of the laser diode. The structure includes an automatic light output control circuit that controls the magnitude of the bias current from the laser diode drive circuit.

Therefore, even if the proportion of forward light input to the optical fiber decreases due to tracking error, the output of the laser diode increases to compensate for the decrease, making it possible to keep the optical output on the optical fiber constant. can.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an optical transmitter according to an embodiment of the present invention, FIG. 2 is an explanatory diagram showing the characteristics of optical output on an optical fiber with respect to temperature, and FIG. 3 is a diagram of a conventional optical transmitter. The circuit diagram, Fig. 4, is a diagram showing the configuration of the optical system of the laser diode module in Figs. 1 and 3, and Fig. 5 shows the characteristics of the optical output on the optical fiber in a state where a dragging error occurs. FIG. 2...Laser diode drive circuit, 3...Automatic temperature control circuit, 4...Automatic light output control circuit, 11...Laser diode, 12...Optical fiber, 13.Fumeto diode, 14... First thermistor, 15... Peltier element, 16... Front light, 17... Back light, 44
...Second thermistor, 103...Lens, 113
・Light output. Name of agent Patent attorney Shige Awano Takahaka 1 figure T [0C] Figure 4, fl-11 ki = Hiro.

Claims (1)

[Claims] A laser diode, a laser diode drive circuit that supplies a driving bias current to the laser diode, an optical fiber that inputs and transmits the forward light of the laser diode, and a laser diode that controls the temperature of the laser diode. an automatic temperature control circuit that controls the temperature of the laser diode to be kept constant by supplying a current corresponding to the resistance value of the first thermistor to be detected to the Peltier element, and detects the output of the backward light of the laser diode. Based on the detection result of the photodiode and the resistance value of the second thermistor that detects the ambient temperature of the laser diode, the laser diode drive circuit controls the laser diode so that the output of the laser diode on the optical fiber is constant. and an automatic optical output control circuit that controls the magnitude of the bias current.