JPH10190575A - Automatic optical output control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an automatic optical output control circuit whereby a constant optical output is obtained without taking a high frequency countermeasure as against a high bit rate signal with the large fluctuation of a signal mark rate m. SOLUTION: A driving part 5 driving a light emitting element LD based on an input signal so as to execute light emitting, a photodetector PD monitor- receiving light from the light emitting element LD and converting a light quantity into an electric signal and a driving control part 3 comparing the electric signal from the photodetctor PD with a previously set reference electric signal and outputting a control signal for controlling the driving part 5 based on a comparison result are provided in the automatic optical output control circuit 1. The circuit 1 is constituted in such a way that a gain variable differential amplifier circuit 3 varying gain based on the fluctuation of the signal mark rate m in the input signal and a definitive convertion means for making tour gain inside a negative feedback loop definite is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of digital optical communication, and more particularly, to a device using a laser diode as a light emitting element.

[0002]

2. Description of the Related Art In a digital optical communication, a laser diode (LD) is used as a light emitting element on a transmitter side in order to stabilize an optical output level against a temperature change, an aging change, and the like. Automatic output control (hereinafter referred to as AP
C: Auto Power Control) circuit is generally provided. FIG. 3 is a block diagram showing a configuration of a conventional APC circuit. As shown in FIG. 3, the APC circuit 11 includes a reference voltage source 12, a comparator 13, and a current-voltage conversion circuit 14.
And a laser diode drive circuit 15. In the figure, LD indicates a laser diode, and PD indicates a photodiode.

A reference voltage source 12 applies a reference voltage _Vref to a _positive input terminal of a comparator 13, and the comparator 13 is inputted from a current-voltage conversion circuit 14 to a negative input terminal. The DC voltage V _P is compared with the reference voltage V _ref, and the difference voltage is compared with the AP voltage for the laser diode drive circuit 15.
Output as C control voltage _Vapc . The current-voltage conversion circuit 14 converts a current flowing based on the amount of light received by the photodiode PD into a voltage, and the laser diode drive circuit 15 generates a drive signal modulated by an input electric signal, and generates a laser signal. A drive signal is output to the diode LD to make the laser diode LD blink.

In the above configuration, the APC circuit 11
By blinking the laser diode LD by the laser diode drive circuit 15, and outputs an optical output P _F to the outside. At this time, the back light of the laser diode LD is detected by the photodiode PD, and the detected current is equal to the current −
By the voltage circuit 14, current - as well as voltage conversion, and inputs to the other input of the comparator 13 as a direct current voltage V _P of this according to the level of the optical output P _F. The comparator 13 outputs a APC control voltage V _apc to the laser diode drive circuit 15 to the DC voltage V _P is compared with a reference voltage V _ref, to form a negative feedback loop.

Here, the light output P of the laser diode LD
_F (peak value) is represented by the following equation [Equation 1].

[0006]

## EQU1 ## P _F = (x K K V _ref ) / (1 + x K L L R _T ) where x: APC control voltage V _apc → light output P _F conversion coefficient, K: differential amplifier circuit gain , L: Photodiode light receiving sensitivity, R _T : Current-voltage conversion coefficient, x · K · L · _RT : Loop loop gain

[0007] [Equation 1], when the _{x · K · L · R T} »1, coefficients indicating the relationship between the output light P _F of the laser diode drive circuit 15 to supply the control voltage and the laser diode LD x Regardless of the value, the light output P _F FV determined by the photodiode light receiving sensitivity L and the current-voltage conversion coefficient _RT
shows that to obtain _ref / a (L · R _T). And
As a loop round gain x · K · L · R _T is increased, but it decreases the variation of the optical output P _F to variations in the coefficient of x, the negative feedback loop Extremely high loop round gain becomes unstable, oscillating, etc. Is more likely to cause trouble. For this reason, in general, an appropriate value is set in consideration of the stability of the negative feedback loop.

Here, when the signal mark rate m indicating the rate at which the optical output is on is fixed, that is, when the on / off rate per unit time is constant, the current-voltage conversion circuit 14 For example, as shown in FIG. 4, a simple DC averaging circuit 21 composed of a parallel circuit of a resistor R and a capacitor C can be used. This circuit is
When the voltage conversion circuit 14 is used, the above equation [Equation 1] is expressed by the following equation [Equation 2].

[0009]

## EQU2 ## P _F = (x · K · V _ref ) / (1 + m · x · K)
· L · r) where, x: APC control voltage V _apc → optical output P _F transform coefficients, K: a differential amplifier circuit gain, L: a photodiode light receiving sensitivity, r: resistance of the resistor R, m: signal mark ratio

On the other hand, when the signal mark rate m fluctuates, the above equation (2) is applied, for example, by averaging the DC of the electric signal input and setting the reference voltage _Vref to a DC value proportional to the signal mark rate m. if configured to output a voltage V _P, it may correspond to vary somewhat. Light output P _{F in} this case
(Peak value) is represented by the following equation [Equation 3].

[0011]

## EQU3 ## P _F = ( _mxKKVref ) / (1 + mx
· K · L · r) where, x: APC control voltage V _apc → optical output P _F transform coefficients, K: a differential amplifier circuit gain, L: a photodiode light receiving sensitivity, r: resistance of the resistor R, m: signal Mark rate

On the other hand, in a conventional APC circuit,
For a signal whose signal mark rate m fluctuates extremely, the loop gain (mxxKLr) greatly fluctuates, so that the signal mark rate m is always highly stable with respect to the fluctuation range of the signal mark rate m. A negative feedback loop could not be formed. Therefore, for example, time division multiple access (TDMA)
In le Access) transmission, etc. of the transmission capacity variable burst optical signal as seen by the system, it will also vary the optical output P _F in accordance with the variation of the signal mark ratio m.

[0013] Therefore, when the signal mark ratio m varies greatly, conventionally, the current - voltage conversion circuit 14, a peak value hold circuit 31 which detects the level peak value of the optical output P _F as shown in FIG. 5 Had been added. Thus, from the peak value holding circuit 31, without depending on the signal mark ratio m, it is possible to output a DC voltage V _P of each instantaneous.

[0014]

However, in the method using the conventional peak value hold circuit 31 described above,
It is necessary to instantaneously and accurately detect and hold a peak value even for a random signal pattern. That is, specifically, the voltage waveform at point A in FIG. 5 must rise within one bit of the signal. When the bit rate is high, the photodiode PD for detecting the optical output and the current-voltage Since it is necessary to employ a component and a circuit method excellent in high frequency identification for the conversion circuit 14,
A new problem arises in that the circuit becomes complicated and expensive.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems in the conventional APC circuit, and a simple circuit can be used to control a signal at a high bit rate in which the signal mark rate m fluctuates greatly. It is an object to provide an APC circuit capable of obtaining an output.

[0016]

In order to achieve the above object, an APC circuit according to the present invention comprises a driving section for driving a light emitting element based on an input signal to emit light, and a monitor receiving light from the light emitting element. A light-receiving element that converts the amount of light into an electric signal;
A drive control unit for comparing an electric signal from the light receiving element with a preset reference electric signal and outputting a control signal for controlling the drive unit based on the comparison result; , The drive control unit includes a variable gain differential amplifier circuit that varies a gain based on a change in a signal mark ratio of an input signal, and includes a constant unit that fixes a loop gain in a negative feedback loop. .

In this case, it is effective that a reference value for the drive control unit is an output value obtained by DC-averaging a current in an input signal, the light emitting element is a laser diode, and the light receiving element is It is preferably a photodiode.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on one illustrated embodiment. FIG. 1 shows the APC of the present invention.
FIG. 3 is a block diagram illustrating a configuration of an example of a circuit. In FIG. 1, reference numeral 1 denotes an APC circuit according to the present embodiment, which is a DC averaging circuit 2, a variable gain type differential amplifier circuit (drive control unit) 3, a current-voltage conversion circuit (conversion unit) 4, a laser And a diode drive circuit (drive unit) 5. In the figure, LD indicates a laser diode (light emitting element), and PD indicates a photodiode (light receiving element). The DC averaging circuit 2 DC-averages the electric signal input branched from the input terminal side, and outputs the output m · V _ref to the + side input terminal of the variable gain type differential amplifier 3 and the gain control signal input terminal (CON).
T).

The variable gain type differential amplifier circuit 3 inputs a voltage obtained by DC-averaging an electric signal input to a positive input terminal, that is, a voltage m · _Vref proportional to a signal mark rate m, and a _negative input terminal. DC to end - inputs a DC voltage V _P from the voltage conversion circuit 4, to form a negative feedback loop by comparing the DC voltage m · V _ref and the DC voltage V _P. here,
The variable gain type differential amplifier circuit 3 has a variable type differential amplifier circuit gain K as shown in FIG. 2, and is set so that the gain decreases as the control signal output from the DC averaging circuit increases. ing. FIG. 2 shows a variable characteristic of the differential amplifier circuit gain K with respect to the control signal of the variable gain differential amplifier circuit.

The current-voltage conversion circuit 4 converts a current flowing according to the amount of light received by the photodiode PD into a voltage. In this embodiment, the DC averaging circuit shown in FIG. 4 is used. Therefore, the DC voltage V _P of the circuit output, the DC voltage m · proportional to the signal mark ratio m
The P _F · L · r. The laser diode drive circuit 5
A drive signal is output to the laser diode LD to cause the laser diode LD to emit light, and is controlled by an APC control voltage V _apc output from the variable gain differential amplifier circuit 3.

Next, the operation of the APC circuit in the above embodiment will be described. First, the laser diode driving circuit 5
P _F from laser diode LD driven by
When the light of the light is emitted, the back light is
Then, a current corresponding to the amount of light generated thereby is converted into a voltage in the current-voltage conversion circuit 4. This DC voltage _VP is input to the variable gain differential amplifier circuit 3 and based on a comparison with the output voltage m · _Vref of the DC averaging circuit 2, the APC control voltage VP is supplied to the laser diode drive circuit 5.
Output _apc . Light output P of laser diode LD
_F (peak value) is represented by the following equation [Equation 4].

[0022]

[Number 4] _{P F = (m · x ·} K · V ref) / (1 + m · x · K · L
· R) where, x: APC control voltage V _apc → optical output P _F transform coefficients, K: a differential amplifier circuit gain, L: a photodiode light receiving sensitivity, r: resistance of the resistor R, m: signal mark ratio

Here, in the same manner as in [Equation 1], the above equation [Equation 4] is changed to a laser diode LD → photodiode PD → current-voltage conversion circuit 4 → variable gain type differential amplifier circuit 3 → laser diode drive circuit 5 Loop gain m ·
When x · K · L · r»1, also counts x showing the relationship between the optical output P _F of the laser diode driving circuit 5 controls the voltage supplied to the laser diode LD is varied, a photodiode light receiving sensitivity L This shows that an optical output P _F ≒ V _ref / (L · r) determined by the current-voltage conversion coefficient _RT can be obtained. Further, if the signal mark rate m fluctuates, the signal mark rate m is corrected by the differential amplifier gain K of the variable gain differential amplifier circuit 3, so that the loop gain mx.times. KLr does not fluctuate greatly.

That is, when the signal mark ratio m of the input signal increases, the value of the output m · V _ref of the DC averaging circuit 2 increases, but the gain of the variable gain type differential amplifier circuit 3 is as shown in FIG.
As shown in (1), it becomes smaller as the control signal (CONT) increases. Therefore, the relationship between the output m · V _ref of the DC averaging circuit 2 accompanying the change in the signal mark rate m and the change in the gain of the variable gain differential amplifier circuit 3 using the output m · V _ref as a control signal cancels out each other. If set, the output V _apc of the differential amplifier circuit 3 can be _kept constant _irrespective of the change in the signal mark rate m, so that a highly stable negative feedback loop is always formed with respect to the change in the signal mark rate m. And a constant light output P _F can be obtained. In this case, since each circuit block forming the above-described negative feedback loop can use a conventional circuit, it is possible to cope with an increase in the signal bit rate without specially devising a high frequency.

As described above, in this embodiment, a constant light output can be obtained stably with respect to the fluctuation of the signal mark rate m without devising a high-frequency circuit. It is possible to cope with the burst signal optical communication and the like. In the above embodiment, the case where the laser diode LD is used as the light emitting element is described as an example. However, the present invention is not limited to this, and can be applied to various light emitting elements.

[0026]

As is apparent from the above description, according to the present invention, a constant light output can be obtained without taking a high-frequency countermeasure even for a signal of a high bit rate in which a signal mark rate greatly varies. Can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an APC circuit according to an embodiment.

FIG. 2 is a diagram illustrating a variable characteristic of a differential amplifier circuit gain with respect to a control signal of a variable gain differential amplifier circuit.

FIG. 3 is a block diagram showing a configuration of a conventional APC circuit.

FIG. 4 is a circuit diagram showing an example of a DC averaging circuit included in the current-voltage conversion circuit shown in FIG.

FIG. 5 is a circuit diagram illustrating an example of a peak value detection circuit.

[Explanation of symbols]

 Reference Signs List 1 APC circuit 2 DC averaging circuit 3 Variable gain type differential amplifier circuit (drive control unit) 4 Current-voltage conversion circuit (conversion unit) 5 Laser diode drive circuit (drive unit) 11 APC circuit 12 Reference voltage source 13 Comparator 14 Current-voltage conversion circuit 15 Laser diode drive circuit LD Laser diode (light emitting element) PD Photodiode (light receiving element)

──────────────────────────────────────────────────続 き Continued on front page (51) Int.Cl. ⁶ Identification code FI H04B 10/28 10/26

Claims (3)

[Claims] 1. A driving unit for driving a light emitting element based on an input signal to emit light, a light receiving element for monitoring and receiving light from the light emitting element and converting a light quantity into an electric signal, and an electric signal from the light receiving element And a preset reference electrical signal,
A drive control unit that outputs a control signal for controlling the drive unit based on the comparison result, wherein the drive control unit obtains a gain based on a change in a signal mark rate in an input signal. An automatic optical output control circuit, comprising: a variable gain differential amplifier circuit for varying the gain; and a stabilizing means for stabilizing the loop gain in the negative feedback loop. 2. The automatic optical output control circuit according to claim 1, wherein an output value obtained by direct current averaging of a current in the input signal is used as a reference value for said drive control unit. 3. The light emitting device is a laser diode.
2. The automatic light output control circuit according to claim 1, wherein said light receiving element is a photodiode.