JPH0584073B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a driving method for stabilizing optical output in an optical network system, and particularly to a driving method for stabilizing burst optical output generated from a semiconductor light emitting element.

[Technical background of the invention]

Bus-type network systems in which terminal devices such as personal computers are connected in parallel to a common bus, or star-type network systems in which terminal devices are connected in a tree-like manner have been developed as inexpensive configuration methods for office automation systems and the like. There is.

The signals handled by such network systems are generally burst signals in which a data pulse train of 10 MHz or more is transmitted only for a certain period of time, for example.

However, in the optical transmission technology used in such network systems, problems that did not occur in the conventional transmission technology using coaxial cables have arisen when transmitting burst signals. One of the reasons for this is that the luminous efficiency of a semiconductor light emitting element, such as a semiconductor laser (hereinafter abbreviated as LD), reacts very sensitively to temperature.

In other words, semiconductor light emitting devices are affected by their own heat generation during burst signal transmission, and for example, LD
In this case, the optical output decreases due to the increase in threshold current,
This is a cause of thermal sag phenomenon during burst signal transmission.

As a method for suppressing such thermal sag phenomena caused by semiconductor light emitting devices, an analog control method has been proposed that stabilizes the burst light output by increasing or decreasing the drive current of the semiconductor light emitting device using a feedback loop of the burst light output. Proposed.

[Problems with background technology]

However, in such an analog control method, since the drive current to the semiconductor light emitting element is increased and decreased, oscillation occurs due to delays in the photodetector elements, transistors, etc. in the circuit, making the circuit itself unstable. There was a problem with becoming.

[Purpose of the invention]

The present invention solves the above problems and provides a burst light output stabilization drive system that operates stably and provides burst light output with a constant peak value.

[Summary of the invention]

The present invention detects a part of the burst light output of a semiconductor light emitting element with a semiconductor detection element, compares this light detection signal with a reference signal, and controls the bias current of the semiconductor light emitting element according to the comparison result. A triangular wave-shaped error signal having a level corresponding to the difference between the reference signal and the photodetection signal is generated for each pulse of a burst signal for generating a burst optical output, and a pulse width corresponding to this error signal is generated. The above object is achieved by controlling the bias current with a pulse signal.

[Embodiments of the invention]

FIG. 1 is a block diagram showing an embodiment of the present invention, in which a pulse drive section 2 having a burst signal input terminal 1 and a bias application section 4 having a transmission request input terminal 3 are provided. A signal obtained by superimposing the outputs of the pulse drive section 2 and the bias application section 4 is input to the semiconductor light emitting device 5 consisting of the above. Also, a semiconductor photodetector element (hereinafter abbreviated as PD) 6, which is composed of, for example, a PIN photodiode, detects the optical output of the semiconductor light emitting element 5, and a reference signal generator that generates a reference signal for stabilizing the burst optical output. 7, and a comparison detection unit 8 that compares the deviation signal between the photodetection signal of the PD 6 and the reference signal generated from the reference signal generation unit 7 with a predetermined reference level and outputs a control signal with a pulse width corresponding to the deviation signal. is provided. The control signal output from the comparison detection section 8 is input to the bias application section 4, and controls the output signal of the bias application section 4.

FIG. 2 is an enlarged view of the main part of the bias application section 4, in which two constant current sources 40, 41 and high-speed differential switches 42, 43 are provided. , 43 controls the charging and discharging of the bias control capacitor 44, and according to the charging voltage of this capacitor 44.
It is configured to control the bias current to LD5. In this case, the high-speed differential switches 42, 43
is switched by the bias signal control signal A and the burst signal control signal B.

In such a configuration, a burst signal as shown in FIG. A transmission request signal as shown in FIG. 3b is inputted from the input terminal 3 to the bias applying section 4.

The output signal of the comparison detection section 8 is input to the bias application section 4 in addition to the transmission request signal. When a burst signal is input, this comparison detection section 8 compares the reference signal outputted from the reference signal generation section 7 and the photodetection signal outputted from the PD 6, and sends out an output signal with a pulse width proportional to the deviation between the two signals. However, when the transmission request signal is input before the burst signal is input, for example, if the photodetection signal is smaller than the reference signal and the deviation between the two is larger than the reference level, an "L" output signal is sent,
If the photodetection signal is larger than the reference signal and the deviation between the two is smaller than the reference level, an "H" output signal is sent.

On the other hand, when the reference signal generating section 7 receives the transmission request signal before inputting the burst signal, the reference signal generating section 7
A first reference signal corresponding to the threshold value is generated. Furthermore, the LD5 does not emit light before the transmission request signal is input.

Therefore, when the transmission request signal is input, the photodetection signal becomes smaller than the first reference signal corresponding to the threshold value of the LD 5, and the output signal of the comparison detection section 8 becomes "L".

By outputting an "L" output signal from the comparison detection section 8 and inputting the transmission request signal to the bias application section 4, a bias signal is generated in the bias application section 4 to turn on the high-speed differential switch 42. Control signal A and one high-speed differential switch 43
A burst signal control signal B is generated which turns OFF. As a result, the high-speed differential switch 42 is turned on.
One high-speed differential switch 43 is turned off, charging the capacitor 44, controlling the transistor 45 by the charging voltage of the capacitor 44, and applying the bias current of the LD5. LD5
The bias current increases sequentially as the charging voltage of the capacitor 44 increases, and as the bias current increases, the bias light of the LD 5 also increases. As the bias light increases, the photodetection current of PD6 decreases to LD5.
is larger than the first reference signal corresponding to the threshold value. Then, the output signal of the comparison detection section 8 becomes "L"
The signal changes from “H” to “H”. As a result, charging of the capacitor 44 is stopped, and the increase in the bias current to LD5 is stopped at this point. As a result, the bias current of LD5 is controlled to the threshold value.

Here, when applying such a bias current, the LD
When the thermal sag phenomenon of No. 5 occurs and the photodetection signal of the PD 6 becomes smaller than the first reference signal, the output signal of the comparison detection section 8 becomes "L" again and the bias application section 4 operates. The bias current of LD5 is maintained at the threshold value.

In this way, LD5 is biased to the threshold value by inputting the transmission request signal. After that, when the burst signal is input to the LD5 via the pulse driver 2,
LD5 emits light in response to the burst signal. In this case, the burst signal is superimposed on the bias signal from the bias application section 4 and applied to the LD 5.

On the other hand, in response to the input of the burst signal, the reference signal generating section 7 generates a second reference signal for the optical output when the thermal sag phenomenon of the LD 5 does not occur due to the burst signal, in accordance with the first reference signal corresponding to the threshold value. It will start outputting a signal.

As a result, the comparison detection section 8 compares the detection signal of the burst light with the second reference signal, and when the burst light output decreases due to the thermal sag phenomenon of the LD5, the light detection signal is detected by the PD6. Since the signal becomes smaller than the second reference signal, the comparison detection section 8 outputs an "L" output signal. As a result, the comparison detection section 8 operates again,
The bias current for the LD 5 is increased in accordance with the decrease in optical output caused by the thermal sag phenomenon. This prevents the burst light output from decreasing due to the thermal sag phenomenon of the LD 5.

Here, the "L" or "H" output signal output from the comparison detection section 8 is configured to be output as a signal with a pulse width proportional to the deviation between the second reference signal and the burst light detection signal. There is. this is,
This is to perform more accurate control to keep the burst light constant. That is, the magnitude relationship between the burst light detection signal and the second reference signal is simply compared, a deviation signal as shown in FIG. Depending on whether the
If the bias current applied to the fourth output is controlled, even if the burst optical output is slightly small, the bias current will be increased to the same extent as when it is considerably large, and the burst optical output will increase due to the response of the feedback loop. The result is a sawtooth shape as shown in Figure d.

On the other hand, for example, the second reference signal generated from the reference signal generating section 7 in synchronization with the burst signal input may be integrated through an integrating circuit, or
By using PD6 with a low response frequency, etc.
A deviation signal between the second reference signal and the burst light detection signal is made into a triangular wave-like signal having a level proportional to the deviation between the two as shown in FIG. 3c, and this deviation signal is compared with a predetermined reference level. Therefore, if a bias control signal with a pulse width proportional to the deviation as shown in FIG. As shown in Figure g, a constant burst light output can be obtained. In other words, initially the compensation for the decrease in burst light due to the thermal sag phenomenon is increased, and the second
As the deviation between the reference signal and the burst light output signal decreases, the comparator 8 generates an output signal with a smaller pulse width, and this signal charges the capacitor 44 of the bias applying section 4. As a result, the bias current applied to the LD 5 changes as shown in FIG. 3f, and when considering the burst light output characteristics as shown in FIG. 3e when feedback control of the bias current is not performed, As a result, a burst light output with a constant peak value can be obtained as shown in FIG. 3g.

In summary, the bias current is not increased or decreased by a predetermined amount by simply comparing the magnitudes of the second reference signal and the burst light detection signal, but the bias current is increased or decreased by a triangular wave-like deviation with a level proportional to the deviation from the second reference signal. A signal is generated, a bias signal control capacitor 44 is charged for a period proportional to the magnitude of this deviation signal, and the bias current is controlled by this charging voltage.

Thereby, a burst light output with a constant peak value is obtained.

On the other hand, in this embodiment, the bias applying section 4 operates only in the direction of increasing the bias current. Therefore, the oscillation phenomenon that has been a problem with conventional negative feedback loops having an increase/decrease function does not occur, and the bias current can be controlled extremely stably.

In addition, although the case where a semiconductor laser was used was explained in the above-mentioned Example, it is applicable also when a light emitting diode is used.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, the bias current to the semiconductor light emitting element is controlled in an increasing direction only for a period proportional to the deviation between the burst light detection signal and the reference signal. Compared to the case where the bias current is controlled based on the magnitude comparison result signal with the reference signal, highly accurate bias current control can be performed stably, and a burst light output with a constant peak value can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention;
FIG. 2 is a diagram showing the configuration of the main parts of the bias application section,
FIG. 3 is a diagram showing output waveforms of each part in the embodiment, and FIG. 4 is a waveform diagram for explaining problems with the conventional system. DESCRIPTION OF SYMBOLS 1...Burst signal input terminal, 2...Pulse driver, 3...Transmission request signal input terminal, 4...Bias application unit, 5...Semiconductor light emitting element, 6...Semiconductor photodetector, 7...Reference Signal generation section, 8... Comparison detection section, 40, 41... Constant current source, 42, 43
...high-speed differential switch, 44...bias signal control capacitor, 45...transistor.

Claims (1)

[Claims] 1. Detecting a part of the burst light output of a semiconductor light emitting element with a semiconductor photodetecting element, comparing the detection signal with a reference signal, and when the photodetection signal is smaller than the reference signal. burst light output stabilization, which stabilizes the burst light output by increasing the bias signal of the semiconductor light emitting element and holding the bias signal at a constant value when the light detection signal is larger than the reference signal; In the optical drive method, a triangular wave-shaped error signal having a level corresponding to the difference between the reference signal and the optical detection signal is generated for each pulse of a burst signal for generating a burst optical output, and this error signal A burst light output stabilization drive method characterized in that the bias signal is controlled by a pulse signal having a pulse width corresponding to the above. 2. The burst light output stabilization drive system according to claim 1, wherein the reference signal corresponds to a burst signal and a bias signal for generating burst light output.