JPH04311115A - Optical output compensating circuit - Google Patents

Abstract

PURPOSE:To prevent the quenching ratio of an outputted optical signal from being varied due to a change in the duty of an input signal in a light transmitter using a semiconductor laser. CONSTITUTION:This optical output compensating circuit is provided with a photodiode(PD) for monitoring an optical signal outputted from a semiconductor laser, an integrator (y) including an operational amplifier for receiving the output of the PD to its inverted input as a voltage signal through a resistor, a detection circuit (z) for detecting the duty of an input signal to a semiconductor laser driving circuit (d) as a voltage signal and impressing its inverted signal to the non-inverted input of the operational amplifier, and a current source transistor Q1 for receiving the output of the integrator (y) to a control terminal and controlling a bias current to be supplied to the semiconductor laser LD.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser drive circuit. More specifically, the present invention relates to the configuration of a novel optical output compensation circuit for stabilizing the extinction light of an output optical signal of a semiconductor laser drive circuit used in an optical communication system or the like.

0002

[Background Art] In an optical transmitter used for optical communication, etc., an optical signal corresponding to the data to be transmitted is generated by a semiconductor laser drive circuit that supplies a semiconductor laser with a drive current modulated in accordance with the data to be transmitted. are doing. Furthermore, suppressing fluctuations in optical signal output due to the temperature characteristics of the semiconductor laser or deterioration over time is one of the important functions of the semiconductor laser drive circuit that is actually used. Furthermore, on the receiving side, this modulated optical signal is demodulated to reproduce data, and the extinction ratio of the optical signal is a characteristic that deeply affects the sensitivity on the receiving side. Extinction ratio means the ratio of two types of light intensities corresponding to each binary signal.In order to effectively detect an optical signal consisting of two types of intensities on the receiving side, the light output on the transmitting side must be It is extremely important that the extinction ratio of the signal is stable.

FIG. 4 shows a typical example of an optical output compensation circuit that can be suitably used in the semiconductor laser drive circuit as described above, and is a typical example of an optical output compensation circuit that can be suitably used in the semiconductor laser drive circuit as described above.
11 is a diagram showing the configuration of a circuit described on page 116 of the book written by Kaiser/translated by Eikichi Yamashita, Sangyo Tosho).

As shown in the figure, this optical output compensation circuit includes a photodiode PD that monitors the rearview mirror light emitted from the rear surface of the semiconductor laser LD, and converts the monitor current generated by the photodiode PD into a DC voltage. After that, a differential amplifier IC10 receives the non-inverting input, and a transistor Q4 whose base receives the output of the differential amplifier IC10.
It is mainly composed of. A reproduced signal, which is the source of the transmitted optical signal, is input to the inverting input of the differential amplifier IC10. The collector of transistor Q4 is connected to the current path of semiconductor laser LD.

[0005] The optical output compensation circuit configured as described above adjusts the output power of the semiconductor laser L so as to cancel out the fluctuation when the output optical signal differs from the reproduced signal to be transmitted.
The DC bias current applied to D is changed.

[0006]

However, the function of the optical output compensation circuit as described above has the following problems.

That is, the conventional optical output compensation circuit shown in FIG. 4 monitors the output optical signal and performs feedback control so that the average level of the optical signal output is stabilized. When this changes, the extinction ratio of the output optical signal changes instead of keeping the optical output constant.

[0008] Therefore, when the duty of the transmission signal changes, there is a problem in that receiving sensitivity deteriorates on the receiving side.

SUMMARY OF THE INVENTION Therefore, the present invention provides a novel optical output compensation circuit that solves the problems of the prior art described above and can effectively stabilize the optical signal output even when the signal duty changes. That is its purpose.

0010

[Means for Solving the Problems] According to the present invention, there is provided a semiconductor laser, a drive circuit that supplies a current modulated in accordance with an input signal to the semiconductor laser, and a drive circuit that bypasses the drive circuit and supplies the semiconductor laser with a current modulated in accordance with an input signal. An optical output compensation circuit for stabilizing an optical signal output of the semiconductor laser in a semiconductor laser drive circuit comprising a current source transistor inserted in a current path of a bias current to be supplied, the optical output compensation circuit stabilizing the optical signal output of the semiconductor laser. an integrator including an operational amplifier that receives the output of the photodiode as a first voltage signal at its inverting input; detects the duty of the input signal to the drive circuit as a second voltage signal; and a detection circuit that applies an inverted signal of the signal to a non-inverting input of the operational amplifier, and is configured such that the output of the integrator is applied to a control terminal of the current source transistor. A compensation circuit is provided.

[0011]

[Operation] The main feature of the optical output compensation circuit according to the present invention is that it has a unique configuration that stabilizes the optical signal output while referring to the duty of the optical signal input to the semiconductor laser drive circuit. It is said that

That is, the conventional optical output compensation circuit performs feedback control of the bias current with reference to the optical signal output.
In other words, it had a function of keeping the average output of the output optical signal constant.

On the other hand, the optical output compensation circuit according to the present invention controls the bias current supplied to the semiconductor laser by referring to the optical signal output, and also refers to the duty of the input signal to compensate for variations in the duty. Correspondingly, the amount of bias current supplied to the semiconductor laser is corrected. Therefore, even if the duty of the input signal to the semiconductor laser drive circuit changes, the ratio signal output can be stabilized without deteriorating the extinction ratio of the output optical signal.

That is, the optical output compensation circuit according to the present invention includes a photodiode that receives a part of the output optical signal of a semiconductor laser, and a transistor that receives a monitor current output from the photodiode at one input and serves as a bias current source. The semiconductor laser drive circuit includes a differential amplifier for controlling the semiconductor laser drive circuit, and the other input of the differential amplifier receives a voltage signal corresponding to the duty of the input signal to the semiconductor laser drive circuit. Therefore, the differential amplifier responds to variations in the duty of the input signal by
The amount of bias current supplied can be changed.

[0015] Hereinafter, the present invention will be explained in more detail with reference to Examples. However, the following disclosure is merely an example of the present invention, and is not intended to limit the technical scope of the present invention in any way.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a circuit diagram showing the configuration of an optical output compensation circuit according to the present invention when connected to a semiconductor laser transmitter.

As shown in the figure, the optical transmitter a includes an operational amplifier IC1 having a pair of complementary inputs and a pair of complementary outputs, and a driving signal modulated by a signal supplied from the operational amplifier IC1. It mainly consists of a semiconductor laser drive circuit d that supplies current to the semiconductor laser LD, and a semiconductor laser LD driven by the semiconductor laser drive circuit d.

The operational amplifier IC1 receives the input signal SD and its inverted signal SD*, shapes the input signal, and supplies it to the semiconductor laser drive circuit d. The semiconductor laser drive circuit d supplies a modulated current modulated based on an input signal to the semiconductor laser LD, and the semiconductor laser LD outputs an optical signal in response to a change in the supplied modulated current.

On the other hand, the optical output compensation circuit b includes a photodiode P that receives a part of the output optical signal of the semiconductor laser LD.
D, an operational amplifier IC2 which receives the output of the photodiode PD at its non-inverting input, an operational amplifier IC4 which receives one output of the operational amplifier IC1 at its non-inverting input via a voltage divider circuit constituted by resistors R1 and R2, and an operational amplifier. Operational amplifier IC3 receives the outputs of IC2 and IC4 at its inverting input and non-inverting input, respectively.
and a transistor Q1 whose base receives the output of the operational amplifier IC3.

The operational amplifier IC2 which receives the output of the photodiode PD has its output and inverting input short-circuited to form a voltage follower circuit x. Therefore, the operational amplifier IC2 outputs a voltage signal that changes in response to the current output by the photodiode PD upon receiving the optical signal.

On the other hand, the inverted output of the operational amplifier IC1 is
The voltage is divided by a voltage divider circuit composed of resistors R1 and R2, and then connected to the operational amplifier IC4 via resistor R3.
is input to the inverted input of The output of the operational amplifier IC4 is short-circuited to the inverting input via the resistor R4.
It operates as an inverting amplifier z. Therefore, the operational amplifier IC
4 outputs a voltage signal having a phase opposite to the drive current supplied to the semiconductor laser drive circuit d.

The operational amplifier IC3 has its output and inverting input short-circuited by a resistor R6 and a capacitor C.
Together with the resistor R5, it constitutes an integrator y. Further, as described above, the inverting input receives the output of the operational amplifier IC2, and the non-inverting input receives the output of the operational amplifier IC4. Further, its output is applied to the base of a transistor Q1 that controls the bias current of the semiconductor laser LD. Note that the transistor Q1 has its collector connected to the anode of the semiconductor laser, and its emitter grounded via a resistor R8.

In the optical output compensation circuit configured as described above, if the positive phase input potential of the operational amplifier IC2 is V1, the amount of change ΔV1 in the voltage V1 when the duty of the input signal changes from α1 to α2 is as follows. It can be expressed as Equation 1. ΔV1 = R7 kηId (α2 − α1)...
・Equation 1 However, R7 is the resistance value of resistor R7, and k is,
The sensitivity of the photodiode PD, η is the semiconductor laser L
D represents the external quantum differential efficiency, and Id represents the modulation current of the semiconductor laser, respectively.

Further, assuming that the positive-phase input potential of the operational amplifier IC3 is V2, the amount of change ΔV2 in the voltage V2 when the duty of the input signal changes from α1 to α2 can be expressed as in the following equation 2. ΔV2 = [(VL-VH)R2/(R1+R2)]
・[(α2-α1)R4/R3]...Equation 2 However, R1, R2, R3 and R4 are the resistance values of the resistors R1, R2, R3 and R4, respectively, and VL is the "L" of the operational amplifier IC1. ” The level output voltage, VH, is the “H” level of the operational amplifier IC1.
Each level represents the output voltage.

Here, the constants of each element can be selected so as to satisfy Equation 3 below. R4 /R3 =R7kηId・[(R1+R2)/R
2]・[1/(VL-VH)]...Equation 3 In the circuit shown in FIG. 1, which is constructed of elements having constants that satisfy Equation 3, for arbitrary extinction ratios α1 and α2, , ΔV1 = ΔV2 always holds true. That is, in such a circuit, the duty of the input signal varies from α1 to α
2, the positive phase input potential V of operational amplifier IC2
1 changes by ΔV1, the change ΔV2 in the positive-phase input potential V2 of the operational amplifier IC3 becomes the same as ΔV1.

Therefore, in this circuit, even if the duty of the input signal changes, the output of the operational amplifier IC3,
That is, the base voltage of the transistor Q1 becomes stable, and the bias current supplied to the semiconductor laser LD becomes constant.

FIG. 2 is a diagram showing the operation of the optical output compensation circuit according to the present invention as described above.

As shown in the figure, in this circuit, even if the duty of the input signal changes, the bias current and modulation current supplied to the semiconductor laser LD do not change.
The extinction ratio of the output optical signal is held constant.

FIG. 3 is a diagram showing an example of the configuration of a system for confirming the operation of the specific output compensation circuit according to the present invention shown in FIG. 1.

As shown in FIG. 3(a), this system is equipped with an optical output compensation circuit according to the present invention and converts a signal generated by a signal generator 31 that can arbitrarily change the duty of the output signal. Semiconductor laser transmitter circuit 3
2 and output as an optical signal from the semiconductor laser LD.

The optical signal generated by the semiconductor laser LD is input to the optical/electrical signal converter 34 via the optical fiber 33, and the electrical signal output from the optical/electrical signal converter 34 is DC-coupled. It is configured such that the waveform and signal level can be monitored by an oscilloscope 35.

Using the system configured as described above, we observed the operation of a semiconductor laser transmitter equipped with an optical output compensation circuit configured as shown in FIG. Even if the signal duty is changed, in the optical signal waveform observed by the oscilloscope 35, the high level P1 and low level P2 of the optical signal relative to the zero level of the optical output as shown in FIG. 3(b) do not change. , and the extinction ratio of the output optical signal P1 /
It was confirmed that P2 did not change.

[0033]

As explained above, the optical output compensation circuit according to the present invention has a unique configuration that controls the bias current supplied to the semiconductor laser while referring to the duty of the input signal. In addition to stabilizing the output, it also effectively compensates for fluctuations in the extinction ratio due to changes in the duty of the input signal.

Therefore, it can be advantageously used in an optical system for optically transmitting signals whose duty is unknown or variable.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the basic configuration of an optical output compensation circuit according to the present invention.

FIG. 2 is a diagram for explaining the operation of the circuit shown in FIG. 1;

FIG. 3 is a diagram showing the configuration of a system for checking the operation of the circuit shown in FIG. 1;

FIG. 4 is a diagram showing a typical configuration of a conventional optical output compensation circuit.

[Explanation of symbols]

31 signal generator, 32 semiconductor laser transmitter, 33 optical fiber, 34 optical/electrical converter, 35 oscilloscope, C capacitance,
IC1 to IC4 operational amplifier, Q1 transistor, R1 to R8 resistor, LD semiconductor laser, PD photodiode, a optical transmitter, b optical output compensation circuit, d semiconductor laser drive circuit, x voltage follower circuit,
y integrator, z inverting amplifier

Claims (1)

[Claims] 1. A semiconductor laser; a drive circuit that supplies the semiconductor laser with a current modulated in accordance with an input signal;
an optical output compensation circuit for stabilizing an optical signal output of the semiconductor laser in a semiconductor laser driving circuit comprising: a current source transistor inserted in a current path for a bias current that bypasses the driving circuit and supplies a bias current to the semiconductor laser; an integrator including a photodiode that receives an output optical signal of the semiconductor laser; an operational amplifier that receives the output of the photodiode as a first voltage signal at an inverting input; and a duty ratio of an input signal to the drive circuit. 2
a detection circuit that detects the second voltage signal as a voltage signal and applies an inverted signal of the second voltage signal to a non-inverting input of the operational amplifier, and is configured such that the output of the integrator is applied to a control terminal of the current source transistor. An optical output compensation circuit characterized in that: