JP2004056830A - Burst signal extinction ratio control circuit and integrated circuit thereof, burst signal extinction ratio control method, computer program, and laser diode drive circuit - Google Patents

Abstract

To maintain a constant extinction ratio of a laser diode that converts a high-speed burst signal into an optical signal.
SOLUTION: The difference between the average optical power of burst # 1 where modulation current Im is a normal value and the average optical power of burst # 2 where modulation current is slightly increased is equal to a reference value. Control is performed so that the average light power of burst # 3 with Im being a normal value is equal to the reference value.
[Selection diagram] Fig. 1

Description

The present invention relates to a burst signal extinction ratio control circuit for controlling an extinction ratio of a laser diode used in optical transmission of digital data in packet communication, an integrated circuit thereof, a burst signal extinction ratio control method, a computer program, and a laser diode driving circuit.

FIG. 4 shows typical characteristics of a laser diode used for optical transmission of digital data in packet communication. The horizontal axis is the current Id flowing through the laser diode, and the vertical axis is the optical output power Pout from the laser diode. The instep is a characteristic at a certain temperature. When Id is gradually increased from 0, initially, the optical output power Pout hardly increases, but the optical output power Pout relatively linearly increases from the point where the threshold current value is exceeded (point X). This linear region is used to convert digital electrical signals into optical signals. To this end, when transmitting "L", the bias current Ib is supplied, and when "H" is transmitted, the modulation current Im is supplied in addition thereto. As a result, the optical output power becomes PL when it is “L” and PH when it is “H”.
At this time, the extinction ratio is defined by PH / PL. If the mark ratio of digital signals (the ratio of “1” in a plurality of bits) is 0.5, the average output power (also referred to as the average light power) is represented by (PH + PL) / 2.

However, assuming that the characteristics change as shown by the line due to temperature change and time-dependent change, in order to keep the extinction ratio and the average light power equal, the bias current Ib and the modulation current Im are changed to Ib 'and Im' as shown in the figure. Must be changed. Thus, a control circuit for controlling the extinction ratio and the average light power to be constant has already been proposed.

FIG. 5 shows a first configuration example of a conventional control circuit disclosed in the following Patent Document 1. FIG. 5 shows, in addition to the control circuit 5, a laser diode 512, a modulation current source 515, a bias current source 516, and a monitor photodiode 511 for converting a part of light of the laser diode 512 into an electric signal. During the burst output, the switch 513 is connected, and the bias current Ib always flows through the laser diode 512. During the burst, when the data is “L”, only the current of Ib flows through the laser diode 512, but when the data is “H”, the switch 514 is connected, and the current of Ib + Im flows through the laser diode 512. Flows. In practice, a pre-bias operation for connecting the switch 513 slightly earlier than the start of the burst is often performed. However, since the pre-bias operation is not directly related to the present invention, the description is omitted.

(4) When the optical burst signal is emitted from the laser diode 512, a part of the signal is converted by the monitor photodiode 511 into a current. The current signal is converted into a voltage signal by a current-voltage converter (I / V) 51. The maximum value detection unit 52 and the minimum value detection unit 53 detect the maximum value and the minimum value of the voltage signal, respectively. The Im / Ib control unit 54 calculates the modulation current Im and the bias current Ib so that those values match PH and PL in FIG. 4, and sets those values in the modulation current source 515 and the bias current source 516. Thus, the extinction ratio is controlled to be constant.

FIG. 6 shows a second configuration example of a conventional control circuit disclosed in Patent Document 2 below. FIG. 6 shows, in addition to the control circuit 6, a laser diode 612, a pilot current source 614, a modulation current source 615, a bias current source 616, and a monitor photodiode 611 for converting a part of light of the laser diode 612 into an electric signal. ing. When the data is “L”, the switch 613 is opened, and a current of Ib + Ip flows through the laser diode 612. When the data is “H”, the switch 613 is connected, and a current of Ib + Im + Ip flows through the laser diode 612.

The sine wave signal is output from the pilot oscillator 64, and its frequency is selected to be sufficiently lower than the data frequency band. The pilot current source 614 supplies a sine wave current Ip according to the sine wave signal, but the amplitude is smaller than the modulation current Im.

When an optical signal is emitted from the laser diode 612, a part thereof is converted by the monitor photodiode 611 into a current. The current signal is converted into a voltage signal by the current-voltage converter 61. The electric signal includes a frequency component of data and a pilot signal. The low-pass filter 62 extracts only the pilot signal from the low-pass filter 62. The control unit 63 determines the modulation current Im and the bias current Ib so that the amplitude of the pilot signal is constant. Since the control section 63 has a role of keeping the average light power of the laser diode 612 constant, it also inputs an electric signal before passing through the low-pass filter 62. If the amplitude of the pilot signal and the average optical power are constant, the extinction ratio is kept constant.
JP-A-03-209890 Japanese Patent No. 2932100

However, in the above-described conventional first control circuit, when the data speed increases, the monitor photodiode 511, the current-voltage converter 51, the maximum value detector 52, and the minimum value detector 53 also need to operate at high speed. You. That is, a band in which the optical waveform emitted from the laser diode 512 can be accurately traced is required. This causes a problem that an optical module on which the monitor photodiode and the laser diode are mounted and a control circuit are complicated and expensive.

Further, in the above-mentioned conventional second control circuit, since only the low frequency pilot signal and the average optical power are handled, high speed of the circuit is not required, but the transmission data is assumed to be continuous, and the signal is assumed to be continuous. There is a problem that the extinction ratio cannot be controlled for intermittent burst signals. Further, there is a problem that the pilot signal becomes noise with respect to the data signal, and deteriorates transmission quality.

An object of the present invention is to solve the above-mentioned conventional problems, and to provide an excellent control circuit capable of controlling the extinction ratio to a constant value for a high-speed burst signal.

In order to solve the above problem, the present invention provides a function of slightly increasing the modulation current in burst units, and calculates the difference between the average optical power at a normal modulation current and the average optical power when the modulation current is increased. The control is performed so as to match the reference value. Further, the average light power at the normal modulation current is controlled to be equal to the reference value.

That is, according to the present invention, there is provided a burst signal extinction ratio control circuit that supplies a control signal to a driving unit that supplies a bias current and a modulation current to a laser diode and drives the laser diode.
Measuring means for measuring the average optical power for each burst of the laser diode,
Modulation current control means for controlling the modulation current Im of the laser diode based on the average light power measured by the measurement means,
Bias current control means for controlling a bias current Ib of the laser diode based on the average optical power measured by the measurement means,
A burst signal extinction ratio control circuit is provided.

According to this configuration, the extinction ratio can be controlled to be constant for a high-speed burst signal at a small cost, and the transmission quality does not deteriorate.

Further, according to the present invention, there is provided a burst signal extinction ratio control method for supplying a control signal to a drive unit that supplies a bias current and a modulation current to a laser diode and drives the laser diode.
Measuring the average optical power for each burst of the laser diode,
A modulation current control step of controlling a modulation current Im of the laser diode based on the average light power measured in the measurement step,
A bias current control step of controlling a bias current Ib of the laser diode based on the average optical power measured in the measurement step,
A burst signal extinction ratio control method is provided.

According to this configuration, the extinction ratio can be controlled to be constant for a high-speed burst signal, and the transmission quality does not deteriorate.

Further, according to the present invention, there is provided a burst signal extinction ratio control method for supplying a control signal to a drive unit that supplies a bias current and a modulation current to a laser diode and drives the laser diode.
Measuring the average optical power for each burst of the laser diode,
A modulation current control step of controlling a modulation current Im of the laser diode based on the average light power measured in the measurement step,
A bias current control step of controlling a bias current Ib of the laser diode based on the average optical power measured in the measurement step,
A computer program for causing a computer to execute the burst signal extinction ratio control method is provided.

According to this configuration, the extinction ratio for a high-speed burst signal can be controlled to be constant by signal processing by software, and the transmission quality does not deteriorate.

Further, according to the present invention, a bias current source for supplying a bias current to the laser diode,
A modulation current source that supplies a modulation current to the laser diode;
Measuring means for measuring the average optical power for each burst of the laser diode,
Modulation current control means for controlling the modulation current Im of the laser diode based on the average light power measured by the measurement means,
Bias current control means for controlling a bias current Ib of the laser diode based on the average optical power measured by the measurement means,
There is provided a laser diode driving circuit having a constant laser diode averaging power and an extinction ratio.

According to this configuration, the extinction ratio can be controlled to be constant for a high-speed burst signal at a small cost, and the transmission quality does not deteriorate.

{Circle over (2)} In the present invention, the modulation current is slightly changed in burst units, and the extinction ratio is kept constant from the amount of change in the optical output power. A method of superimposing an alternating current on a modulation current is equivalent to adding noise to a signal and causes a decrease in transmission quality. On the other hand, in the present invention, a direct current is superimposed, so that there is no decrease in transmission quality. In the present invention, the signal amplitude slightly changes for each burst, but the burst receiver generally controls the gain for each burst, so that there is no problem.

FIG. 1 is a block diagram showing an embodiment of the control circuit of the present invention. FIG. 1 shows, in addition to the control circuit 1, a laser diode 112, a modulation current source 115, a bias current source 116, and a monitor photodiode 111 for converting a part of the light of the laser diode 112 into an electric signal. During the burst output, the switch 113 is connected and the bias current Ib always flows through the laser diode 112. During the burst, when the data is “L”, only the current of Ib flows through the laser diode 112, but when the data is “H”, the switch 114 is connected and the current of Ib + Im flows through the laser diode 112. Flows. Note that the pre-bias operation is omitted as in the conventional example. Here, the mark ratio of burst data is assumed to be 0.5.

The control circuit 1 of FIG. 1 includes a power measurement unit 11 that measures power based on a signal from the monitor photodiode 111, a storage unit 12 that stores a value measured by the power measurement unit 11 as data, and a power measurement unit 11 that measures the value. A difference detection unit 13 for detecting a difference between the stored value and a value previously stored in the storage unit 12, a ΔPref storage unit 14 in which a reference value ΔPref is stored in advance, an output signal of the difference detection unit 13 and a reference value A comparison unit 15 for comparing ΔPref; a modulation current (Im) setting unit 16 for setting a modulation current Im according to an output signal of the comparison unit 15; a Pref storage unit 17 in which a reference value Pref is stored in advance; A comparing section 18 for comparing the output signal of the section 11 with the reference value Pref; a bias current (Ib) setting section 19 for setting a bias current Ib according to the output signal of the comparing section 18; And a section 110.

{Operation of this control circuit will be described with reference to FIGS. After the start-up, the control circuit 1 sets the reference values ΔPref and Pref in the ΔPref storage unit 14 and the Pref storage unit 17, respectively. Subsequently, the initial values of the modulation current Im and the bias current Ib are set in the modulation current setting unit 16 and the bias current setting unit 19, respectively. Details of these setting operations are omitted.

When transmitting the first burst # 1, the power measuring unit 11 measures the average optical power of the burst based on the signal from the monitor photodiode 111. The result is temporarily stored in the storage unit 12. Thereafter, the modulation current setting unit 16 increases the current ΔIm corresponding to, for example, 1% of the current modulation current Im. During the transmission of the next burst # 2, the power measuring unit 11 measures the average optical power of the burst. After that, the difference detector 13 obtains a difference from the value previously stored in the storage 12, and the comparator 15 compares the difference with the reference value ΔPref. As a result, if the difference is greater than ΔPref, the modulation current setting unit 16 reduces the modulation current Im by the specified amount A. Conversely, if the difference <ΔPref, the modulation current setting unit 16 increases the modulation current Im by the specified amount A.

In the next burst # 3, the output of the power measuring unit 11 is compared with the reference value Pref by the comparing unit 18, and if the measured power is greater than Pref, the bias current setting unit 19 reduces the bias current Ib by the specified amount B. . Conversely, if the measured power <Pref, the bias current setting unit 19 increases the bias current Ib by the specified amount B.
These processes need not be performed for successive bursts if they are sufficiently frequent compared to the speed of the characteristic change of the laser diode. That is, in FIG. 2, there may be bursts that do not contribute to control between bursts # 1, # 2, and # 3.

The reason why the extinction ratio can be controlled to be constant by the above operation will be described with reference to FIG. FIG. 3 shows the current Id versus the optical output power Pout characteristic of the laser diode. If data is "L", Id = Ib
Then, the optical output power at that time is PL. If the data is "H", Id = Ib + Im
Then, the optical output power at that time is PH. If the mark ratio is 0.5, Id = Ib + Im / 2 on average
And the optical output power at that time is Pave.

Now, if the modulation current is increased by ΔIm, on average, Id = Ib + (Im + ΔIm) / 2
And the optical output power at that time is Pave +. The power increase at this time is defined as ΔP.

From these, the extinction ratio ExR is obtained. The characteristics of the laser diode in the linear region can be regarded as a straight line, and the equation is expressed as Pout = K × Id + J
far. At this time, the inclination K is, as is apparent from FIG. 3, K = ΔP / (ΔIm / 2)
It is. The average optical power is Pave = K (Ib + (Im / 2)) + J

further,
PL = K × Ib + J
PH = K (Ib + Im) + J
It is. From the above, ExR = PH / PL
= (Pave + (ΔP / ΔIm)) / (Pave− (ΔP / ΔIm))

Here, assuming that ΔIm is a value proportional to Im, C × Im (C is a constant), ExR = PH / PL
= (Pave + (ΔP / C)) / (Pave− (ΔP / C))
It can be seen that the extinction ratio can be kept constant by controlling the variation ΔP of the average optical power when the average optical power Pave and the modulation current are changed to be constant.

Note that this control circuit may be realized by individual components, or may be entirely or partially realized by an integrated circuit.

Further, in the above-described embodiment, each unit operates under the control of the control unit 110. However, the output of the monitor photodiode 111 is AD-converted and taken into a CPU (Central Processing Unit) by software. It is also possible to perform these processes. The processing flow in that case is as shown in FIG. That is, reference values ΔPref and Pref are set in step S1, and initial values of the modulation current Im and the bias current Ib are set in step S2. Next, the first burst is transmitted in step S3. In step S4, the average optical power of the burst is measured based on the signal from the monitor photodiode 111. The result is stored in step S5. After that, in step S6, the current ΔIm corresponding to, for example, 1% of the current modulation current Im is increased. Next, the next burst is transmitted in step S7, and the average optical power of the burst is measured in step S8.

Then, in step S9, the difference between the previously stored value and the current measured value is obtained, and this difference is compared with the reference value ΔPref in step S10. As a result, if the difference is greater than ΔPref, the modulation current Im is reduced by the specified amounts A and ΔIm in step S11. Conversely, if the difference <ΔPref, the modulation current Im is increased by the specified amount A and decreased by ΔIm in step S12. The reason why the modulation current Im is decreased by ΔIm in steps S11 and S12 is to reduce the modulation current Im by ΔIm increased in step S6 to return to the original value. When step S11 or step S12 ends, the process goes to step S13 to transmit the next burst. In step S14, the average optical power is measured, and the value is compared with the reference value Pref in step S15. If the measured power is greater than Pref, the bias current Ib is reduced by the specified amount B in step S16. Conversely, if the measured power <Pref, the bias current Ib is increased by the specified amount B in step S17.

After step S16 or S17, the process returns to step S3, and thereafter, steps S3 to S16 or S17 are repeated. That is, step S3 to step S11 or step S12 and step S13 to step S16 or step S17 are executed alternately.

As described above, according to the present invention, so-called DC is superimposed, so that transmission quality does not decrease, and in the present invention, the signal amplitude slightly changes for each burst. Since there is no problem caused by this, a burst signal extinction ratio control circuit for controlling the extinction ratio of a laser diode used in optical transmission of digital data in packet communication, an integrated circuit thereof, and a burst signal It can be used for an extinction ratio control method, a computer program, a laser diode drive circuit, and the like.

FIG. 1 is a block diagram showing an embodiment of a control circuit of the present invention. Flow chart showing the flow of processing in an embodiment for realizing the control circuit of the present invention by software Graph showing current Id vs. optical output power Pout characteristics of a laser diode Graph showing typical characteristics of a laser diode FIG. 2 is a block diagram showing a first configuration example of a conventional control circuit. FIG. 2 is a block diagram showing a second configuration example of a conventional control circuit.

Explanation of reference numerals

1, 5, 6 control circuit 11 power measurement unit 12 storage unit 13 difference detection unit 14 ΔPref storage unit 15, 18 comparison unit 16 modulation current setting unit 17 Pref storage unit 19 bias current setting unit 51, 61 current-voltage conversion unit ( I / V)
52 Maximum value detection unit 53 Minimum value detection unit 54 Im / Ib control unit 62 Low-pass filter 63, 110 Control unit 64 Pilot oscillator 111, 511, 611 Monitor photodiode 112, 512, 612 Laser diode 113, 114, 513, 514, 613 Switch 115, 515, 615 Modulation current source 116, 516, 616 Bias current source 614 Pilot current source

Claims (13)

A burst signal extinction ratio control circuit that supplies a control signal to a drive unit that supplies a bias current and a modulation current to the laser diode and drives the laser diode,
Measuring means for measuring the average optical power for each burst of the laser diode,
Modulation current control means for controlling the modulation current Im of the laser diode based on the average light power measured by the measurement means,
Bias current control means for controlling a bias current Ib of the laser diode based on the average optical power measured by the measurement means,
Burst signal extinction ratio control circuit. 2. The burst signal extinction ratio control circuit according to claim 1, wherein the modulation current control means has a function of increasing the modulation current Im by a specified value ΔIm. 3. The burst signal extinction ratio control circuit according to claim 2, wherein the specified value ΔIm is an amount proportional to the modulation current Im. The modulation current control means,
Means for taking the difference between the average optical power P1 of burst # 1 when the modulation current is Im and the average optical power P2 of burst # 2 when the modulation current is Im + ΔIm;
Means for decreasing the modulation current when the value of the difference is larger than a predetermined reference value ΔPref, and increasing the modulation current when the value of the difference is smaller than the reference value ΔPref. ,
2. The burst signal extinction ratio control circuit according to claim 1, comprising: 5. The burst signal extinction ratio control circuit according to claim 4, wherein the increase / decrease amount of the modulation current is a predetermined constant value A. The bias current control means,
Means for comparing the average light power P3 of burst # 3 when the bias current is Ib with a predetermined reference value Pref;
Means for decreasing the bias current when the average light power P3 is greater than the reference value Pref, and increasing the bias current when the average light power P3 is less than the reference value Pref. To
2. The burst signal extinction ratio control circuit according to claim 1, comprising: 7. The burst signal extinction ratio control circuit according to claim 6, wherein the amount of increase or decrease of the bias current is a predetermined constant value B. A burst signal extinction ratio control method for supplying a control signal to a drive unit that supplies a bias current and a modulation current to a laser diode and drives the laser diode,
Measuring the average optical power for each burst of the laser diode,
A modulation current control step of controlling a modulation current Im of the laser diode based on the average light power measured in the measurement step,
A bias current control step of controlling a bias current Ib of the laser diode based on the average optical power measured in the measurement step,
And a burst signal extinction ratio control method. Taking the difference between the average optical power P1 of burst # 1 when the modulation current is Im and the average optical power P2 of burst # 2 when the modulation current is Im + ΔIm;
When the value of the difference is larger than a predetermined reference value ΔPref, the modulation current is decreased, and when the value of the difference is smaller than the reference value Pref, the modulation current is increased. ,
A modulating current control step having
Comparing the average light power P3 of the burst # 3 when the bias current is Ib with a predetermined reference value Pref;
When the average optical power P3 is higher than the reference value Pref, the bias current is decreased when the average optical power P3 is lower than the reference value Pref, and the bias current is increased when the average optical power P3 is lower than the reference value Pref. To
Having a bias current control step,
9. The burst signal extinction ratio control method according to claim 8, comprising a step of alternately performing the modulation current control step and the bias current control step. A burst signal extinction ratio control method for supplying a control signal to a drive unit that supplies a bias current and a modulation current to a laser diode and drives the laser diode,
Measuring the average optical power for each burst of the laser diode,
A modulation current control step of controlling a modulation current Im of the laser diode based on the average light power measured in the measurement step,
A bias current control step of controlling a bias current Ib of the laser diode based on the average optical power measured in the measurement step,
A computer program for causing a computer to execute the burst signal extinction ratio control method. Taking the difference between the average optical power P1 of burst # 1 when the modulation current is Im and the average optical power P2 of burst # 2 when the modulation current is Im + ΔIm;
When the value of the difference is larger than a predetermined reference value ΔPref, the modulation current is decreased, and when the value of the difference is smaller than the reference value ΔPref, the modulation current is increased. ,
A modulating current control step having
Comparing the average light power P3 of the burst # 3 when the bias current is Ib with a predetermined reference value Pref;
When the average optical power P3 is higher than the reference value Pref, the bias current is decreased when the average optical power P3 is lower than the reference value Pref, and the bias current is increased when the average optical power P3 is lower than the reference value Pref. To
Having a bias current control step,
The computer program according to claim 10, further comprising: alternately executing the modulation current control step and the bias current control step. A bias current source for supplying a bias current to the laser diode;
A modulation current source that supplies a modulation current to the laser diode;
Measuring means for measuring the average optical power for each burst of the laser diode,
Modulation current control means for controlling the modulation current Im of the laser diode based on the average light power measured by the measurement means,
Bias current control means for controlling a bias current Ib of the laser diode based on the average optical power measured by the measurement means,
A laser diode drive circuit having a laser diode having an average light power and an extinction ratio that are constant. An integrated circuit comprising the burst signal extinction ratio control circuit according to claim 1.

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