JPS6245085A - Laser diode drive circuit - Google Patents

Abstract

PURPOSE:To contrive the reduction of the deterioration of extinction coefficient and that of light output a high-velocity operation by taking out an average value signal from a light accepting element output by means of a d.c. monitor circuit and compar ing it with an average value of the input signal and controlling a d.c. bias current value of laser diode corresponding to the differential quantity. CONSTITUTION:The output of a level recognition circuit 5 is input in a rising delay detecting circuit 6 and a lowering delay detecting circuit 7 together with an input signal 1. An addition device 8 applies a constant offset voltage VB to the output of the rising delay detecting circuit 6 in order to give a contant delay time tX to a rising of the wave of light output. The signal of the output of the rising delay detecting circuit 6 added with the offset voltage VB and the output of the lowering delay detecting circuit 7 are input in a differential amplifier 9. The bias current source 11 for controlling the magnitude of a modulation current IP through a filter 10 is supplied. Meanwhile, the output of the light accepting element 3 is detected its average value by a d.c. monitor circuit and it is input in a differential amplifier 15 together with the output of a low-pass filter 14. It is input in a bias power source 17 for supply ing a d.c. bias current IB through a filter 16, thereby controlling the d.c. bias current IB.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a laser diode drive circuit for an optical transmission device that performs optical communication using a radar diode.

(Prior Art) A conventionally widely used method for laser diode drive circuits is a method of driving a laser diode by superimposing a constant modulation current and a variable DC bias current. For example, as shown in the injection laser control circuit (Japanese Patent Publication No. 59-38756), an attempt was made to maintain a constant optical output by detecting threshold changes due to temperature and laser element deterioration and changing the DC bias current. It is something.

FIG. 2 is a block diagram showing a configuration example of a conventional laser diode drive circuit. In this configuration example, input signal 1 modulates fixed modulation peak current IP. Further, a part of the optical output of the laser diode is received by the light receiving element 3 as monitor light, and the average value of the received light power is obtained by the DCC unicircuit 13. At the same time, the input signal is also integrated by the low-pass filter 14, both signals are compared and amplified by the differential amplifier 15, and this output passes through the filter 16 to control the DC bias current source 17. In other words, the modulation current generator is fixed and the DC bias current generator is controlled to keep the peak value of the optical output constant.

(Problem that the invention sought to solve) However, in the above method, since the modulation current is kept constant, it is not possible to compensate for changes in differential efficiency due to temperature or deterioration of the laser diode 2, and the optical output waveform changes during high-speed operation. Degradation, fluctuations and extinction ratio degradation occur. This situation is shown in FIG. As shown in the figure, the extinction ratio deteriorates at high temperatures.

In addition, there is a delay time in laser emission, and this is a function of the DC bias current and the threshold current when the drive current rises from below the threshold.9 This has the disadvantage of causing deterioration of the optical output waveform during high-speed operation. The present invention has been made in view of these drawbacks, and provides a laser diode drive circuit that exhibits less extinction ratio deterioration and optical output deterioration during high-speed operation due to threshold current fluctuations or differential efficiency fluctuations due to temperature or laser diode deterioration. It is something to do.

(Means for solving the problem) A laser diode drive circuit that causes a laser diode to emit and extinguish light in response to a dinotal drive input signal, and has a light receiving element that receives light from the laser diode, and from the output of the light receiving element. a level identification circuit that extracts a DC reproduced waveform by an ACC uni-circuit and slices it at the center value of the waveform; a rise delay detection circuit that detects the rise of the output of the rep/I/identification circuit and the delay time of the input signal; It has a falling delay detection circuit that detects the falling edge of the identification circuit and the delay time of the input waveform, or if the falling time is constant, a circuit that generates an output voltage corresponding to it,
The output of the 9-rise delay detection circuit and the output of the fall delay detection circuit are compared, and the bias current source is controlled based on the difference value, thereby controlling the modulation current value of the laser diode. Or control the DC bias current value. - An average value signal is taken out from the output of the photodetector by a DCC uni-circuit, and compared with the average value of the input signal to control the DC bias current value of the laser diode or the modulation current value according to the difference. This is a laser diode drive circuit with a current source.

(Operation) The output of the light-receiving element is converted into an electric signal waveform by the ACC unicircuit, and the waveform is DC-regenerated by the level discrimination circuit and sliced at the center value of the waveform. The rise delay detection circuit and the fall delay detection circuit detect the rise delay time and the fall delay time between the output of the level identification circuit and the input signal waveform, and adjust the modulation current (I, ) or direct current bias current (more).

(Embodiment) The present invention provides a mark rate compensation type APC (Automatic
Control the DC bias current I or modulation current by the power control circuit, detect the rise delay time and fall delay time of the level discriminator output waveform of the monitor waveform of the laser diode optical output and the drive input waveform, By controlling the modulation current or the DC bias current (8) so that the delay time difference becomes a constant value, deterioration of the extinction ratio and optical output waveform due to fluctuations in the laser diode is prevented.

The rise delay time d of the optical output waveform with respect to the drive input signal is generally the DC bias current 1. is expressed as a function of threshold current Ith by the following equation.

Here, τn is carrier lifetime, IP is modulation current (peak)
It is. The rise delay time is caused by the difference in time until the threshold injection carrier density is reached. Its characteristics are shown in FIG. 5, and the relationship between the laser diode drive current and the optical waveform is shown in FIG. Note that such a time difference does not occur regarding the fall of the optical output. Therefore, as shown in FIG. 5, the DC bias current IB is changed to the threshold current IL
If it is set to less than h, the rise time will be delayed.

In order to set the DC bias current IB to a point Ix that is slightly lower than the threshold value, the delay of the rise time and fall time of the optical output relative to the drive input waveform may be controlled to tx. In order to provide the time difference tx in an actual circuit, an offset voltage corresponding to the rise delay detection output and the fall delay detection output should be provided when comparing the rise delay detection output, or a drive input waveform in which the rise of the optical output corresponds to 9. Either method of advancing the waveform by tx compared to the drive input waveform corresponding to the falling edge is used. By controlling both the DC bias current IB and the modulation current IP using the method described above, the threshold current due to laser diode variations, temperature fluctuations, and deterioration can be reduced.
Regardless of differential efficiency fluctuations, the DC bias current IB can always be set near the threshold current without adjustment. As a result, a laser diode drive circuit with less waveform deterioration, extinction ratio deterioration, and peak optical output change can be realized.

FIG. 1 is a block diagram showing an embodiment of the present invention. In the same figure (a), input signal 1 is input to AC modulation circuit 12,
The laser diode 2 is modulated with the peak modulation current IP.

A part of the light emitting output of the laser diode 2 is received by a light receiving element 3, and the output of the light receiving element is converted into an electric signal waveform by an AC monitor circuit 4, and after DC reproduction is performed by a level discrimination circuit 5, a maximum value and a minimum value are determined. Slice at the intermediate value and output as a logic level. The output of the level discrimination circuit 5 is input together with the input signal 1 to a rise delay detection circuit 6 and a fall delay detection circuit 7. The adder 8 adds a constant offset voltage V to the output of the rise delay detection circuit 6 in order to give a constant delay time Ex to the rise of the optical output waveform.

A signal obtained by adding an offset voltage VB to the output of the rise delay detection circuit 6 and the output of the fall delay detection circuit 7 are input to a differential amplifier 9, and the magnitude of the modulation current is controlled through a filter 10. The bias current source 11 is supplied with the bias current.

Note that when the output of the fall delay detection circuit 7 is a constant value, the same result can be obtained even if a circuit is provided in place of the fall delay detection circuit 70 to apply a voltage or voltage corresponding to the output to the differential amplifier 9. can get.

On the other hand, the average value of the output of the light receiving element 3 is detected by a DCC unicircuit 13, and is inputted to a low pass filter (LPF) 14 and an bias current source 17 to control the DC bias current generator.

Next, FIG. 1(b) shows another method for giving a constant delay tx to the rise of the optical output waveform. In order to advance only the input waveform corresponding to the rise time, a pulse width expansion circuit 18 is provided as shown in FIG. In this case, adder 8 is not necessary. Input signal 1 is connected to the noggle width extension circuit, line 18.
By applying direct current to the AC modulation circuit 12 via
It becomes possible to control the Asumi flow IB to a constant value below the threshold current.

FIG. 4 is a block diagram showing a second embodiment of the present invention. The figure (A) shows the DC bias current due to the delay difference between the rise and fall of the optical output waveform! , and operates to control the modulation current I by the mark rate compensated optical output, and those having the same reference numerals as in FIG. 1(A) have similar functions. That is, the output obtained by adding the offset voltage VB to the output of the rise delay detection circuit 6 and the output of the fall delay detection circuit 7 are input to the differential amplifier 9, and the filter 1
6 to a bias current source 17 that supplies a DC bias current I, and controls the value of the DC bias current IB.

On the other hand, the output of the DC monitor circuit 13 and the input signal 1 are input to the differential amplifier 15 via a low-pass filter (LPF) 14,
It is supplied through a filter to a bias current source 1 which controls the magnitude of the modulation current I.

Figure 4 (b) shows a certain delay tx at the rise of the optical output waveform.
In other ways of giving
? A pulse width expansion circuit 18 is provided, and the input signal 1 is applied to the AC modulation circuit 12 through the pulse width expansion circuit 18.

Figure 7 shows the specific circuit configuration of the rising delay detection circuit and the falling delay detection circuit.
61, 71, NOR gates 62, 72, and buffer gates 63, 73.

FIG. 8 shows a time chart of the delay detection circuit.
τ8 indicates the delay in the rising edge and τ8 indicates the delay in the falling edge.

(Effects of the Invention) As explained above, the present invention provides a mark rate compensation type APC circuit that keeps the peak value of the optical output constant, and a constant difference between the rise delay time and the fall delay time of the level identification output of the optical monitor waveform. By controlling the DC bias current (8) and the modulation current (IP or Il) using the control circuit, there is no extinction ratio deterioration due to fluctuations in the differential efficiency of the laser diode and fluctuations in the threshold value, and the optical output peak can be maintained. A laser diode drive circuit without value fluctuation or waveform deterioration can be provided.

It goes without saying that the present invention can also be applied to cases where the input signal is a balanced code, etc., and no mark rate compensation circuit is required.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing a laser diode drive circuit according to the present invention, Fig. 2 is a block diagram showing a conventional laser diode drive circuit, Fig. 3 is a current-light output characteristic diagram, and Fig. 4 is a block diagram showing a conventional laser diode drive circuit.
The figure is a block diagram showing another embodiment of the laser diode drive circuit according to the present invention, FIG. 5 is a delay time-current characteristic diagram,
FIG. 6 is a drive current and optical waveform diagram, FIG. 7 is a delay detection circuit diagram, and FIG. 8 is a time chart. l...Input signal, 2...Laser diode (LD)
, 3... Light receiving element, 4... AC monitor circuit, 5...
...Level identification circuit, 6...Rise delay detection circuit,
7...Falling delay detection circuit, 8...Adder, 9,
15...Differential amplifier, 10.16...Filter, 1
1.17...Bias current source, 12...AC modulation circuit,
13.-DC monitor circuit, 14...low-pass filter, 1
8...t4 Lux width extension circuit, 61.71...Inverter dart, 62°72-NORe-to, 63.73.
Patent applicant: Oki Electric Industry Co., Ltd. Conventional n laser "Old Ling Figure Figure 6 Excessive Species Recirculation Figure 7 F Im Naito Figure 8 Figure 1, Incident Display 1985 Patent Application No. 183009 2, Title of Invention Laser Diode Drive Circuit 3, Amendment Relationship with the case of a person who commits a patent application Colonel Tokoro (105) 1-7-12-5 Toranomon, Minato-ku, Tokyo
, Subject of amendment The column of "Claims" in the specification, the column of "Detailed Description of the Invention" and the drawing "Figure 6" 6 Contents of amendment (1) The "Scope of Claims" in the specification Correct as shown in the attached sheet. (2) In the same book, page 3, lines 15 to 16, the phrase "try to keep it constant" is amended to "try to keep it constant." (3) In the first line of page 8 of the same book, the phrase ``It will get slower.'' will be corrected to ``It will get bigger.'' (4) The drawing "Figure 6 has been amended to the sentencing paper. Claim 1: In a laser diode drive circuit that emits and extinguishes a laser diode in response to a dinotal drive input signal, a laser diode having a light receiving element and , comprising level identification means for extracting a DC reproduced waveform from the output of the light receiving element and slicing it at the center value of the waveform, and a rising delay detection circuit and a falling delay detection circuit for detecting the delay time between the output of the level identification means and the input signal. a modulation current control means for controlling the modulation current value of the laser diode based on the difference value between the output of the rise delay detection circuit and the output of the fall delay detection circuit; 2. When the output of the fall delay detection circuit is constant, the delay The laser diode drive circuit according to claim 1, characterized in that the voltage generation circuit corresponds to the output of the detection circuit. 3. A laser diode that causes the laser diode to emit and extinguish light in response to a digital drive input signal. The drive circuit includes a level identification means for slicing at the center value of a laser diode output waveform having a light receiving element, and a rising delay detection circuit and a falling delay detection circuit for detecting the delay time between the output of the level identification means and the input signal. , direct bias current control means for controlling the DC bias current value of the laser diode based on the difference value between the output of the rise delay detection circuit and the output of the fall delay detection circuit; A laser diode 9 drive circuit characterized by having a modulation current value control means for controlling the modulation current value of the laser diode according to the difference between the laser diode and the laser diode 9.4. A laser diode drive circuit according to claim 3, characterized in that the voltage generation circuit corresponds to the output of the delay detection circuit. Denso L l5 -Illll Imama En

Claims (1)

[Scope of Claims] 1. In a laser diode drive circuit that emits and extinguishes a laser diode in response to a digital drive input signal, a laser diode having a light receiving element and a DC reproduced waveform are extracted from the output of the light receiving element and a waveform center value is provided. It has a level identification means for slicing at a rate of 1, and a rising delay detection circuit and a falling delay detection circuit for detecting the delay time of the output of the level identification means and the input signal, and the output of the rising delay detection circuit and the falling delay detection circuit output. a modulation current control means for controlling a modulation current value of a laser diode based on a difference value; and a modulation current control means for extracting an average signal of the output of the light receiving element and controlling a DC bias current value of the laser diode according to a difference from the average value of the input signal. A laser diode drive circuit characterized by having a DC bias current control means. 2. If the output of the falling delay detection circuit is constant,
2. A laser diode drive circuit according to claim 1, characterized in that the voltage generation circuit corresponds to the output of said delay detection circuit. 3. In a laser diode drive circuit that emits and extinguishes a laser diode in response to a digital drive input signal, a laser diode having a light-receiving element and a level identification device that extracts a DC reproduction wave from the output of the light-receiving element and slices it at the center value of the waveform. and a rising delay detecting circuit and a falling delay detecting circuit for detecting the delay time between the output of the level identifying means and the input signal, and detects the laser diode by the difference value between the output of the rising delay detecting circuit and the output of the falling delay detecting circuit. direct bias current control means for controlling a direct current bias current value of the laser diode, and a modulation current value for extracting an average signal of the output of the light receiving element and controlling a modulation current value of the laser diode according to a difference from the average value of the input signal. A laser diode drive circuit characterized by having a control means. 4. The laser diode drive circuit according to claim 3, wherein when the output of the fall delay detection circuit is constant, a voltage generation circuit corresponding to the output of the delay detection circuit is used.