JPS62151039A - Optical output stabilizing system - Google Patents

Abstract

PURPOSE:To stabilize the light irradiated from a light emitting element by using an output of the 1st comparator to control an output bias current of a bias application section and using an output of the 2nd comparator to control a pulse peak value of a pulse drive section output pulse. CONSTITUTION:Part of an output light of a semiconductor laser 11 is detected by a photo diode 15 for monitor and its electric signal is amplified by an amplifier 16. Part of the amplified signal is integrated by an integration device 17. The output represents a mean value of the optical output, the output of the integration device and a reference potential 18 are compared by a comparator 19, the output controls the bias application section to change the bias of the semiconductor laser. On the other hand, another output of the amplifier 16 is given to a band pass filter 20 filtering the oscillation frequency of the low frequency oscillator 14 and an amplitude detector 21 detects the low frequency signal amplitude. Since the output of the amplitude detector is similar to the peak value of the optical pulse by the data input, the output is compared with the specific reference potential 22 by a comparator 23, the output controls the pulse current peak value of the pulse drive section to stabilize the peak value of the optical pulse.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a light output stabilization method for stabilizing the output light intensity of a light emitting element.

[Technical background of the invention and its problems]

Among light-emitting elements, especially semiconductor lasers, the relationship between injection current and optical output changes greatly depending on temperature.

The relationship between this current and optical output is shown in Figure 'g2. When the ambient temperature of the semiconductor laser is low, the threshold current is low and the differential quantum efficiency (the slope of the current vs. optical output characteristic in the figure) is high above the threshold current, but when the ambient temperature is high In this case, the threshold current I+h' increases and the differential quantum efficiency decreases. In an optical communication system in which a semiconductor laser is driven in a pulsed manner to transmit signals, the above-mentioned change in the characteristics of the semiconductor laser poses a major problem. For this purpose, a part of the light from the semiconductor laser is received by a photodiode, and the light output of the semiconductor laser is stabilized by the received light signal. One method is to average the output photocurrent of the monitoring photodiode and control the bias current of the semiconductor laser using the averaged output. According to this method, it is possible to always keep the average optical output of the semiconductor laser constant.

However, since the differential quantum efficiency of a semiconductor laser changes depending on the ambient temperature in the current versus light output characteristic shown in FIG. 2, the on-off ratio of the optical pulse deteriorates as the ambient temperature increases. This is not a big problem for short-wavelength A-tGaAs semiconductor lasers, whose differential quantum efficiency changes little with temperature, but it is a problem for optical communication systems that use long-wavelength InGαAsP semiconductor lasers, whose differential quantum efficiency changes largely with temperature. Now that's a big problem.

In order to avoid this deterioration of the on-off ratio of the optical pulse, there is an optical output stabilization method shown in FIG. After the output of the monitor photodiode (34) is amplified by the amplifier (35), a part of the output is input to the off-level detection s (36). The off-level detection section detects the optical level of the off portion of the data pulse, and can be realized by a minimum value hold circuit of the pulse signal. The output of this off-level detection section controls the bias application section (33) to control the bias current of the semiconductor laser.

On the other hand, the output of the amplifier is clamped at the minimum value of the optical pulse train by a clamp circuit (37), and a peak detector (38)
The output light pulse amplitude is made constant by detecting the peak and controlling the pulse driver (32).

In Figure 3, which shows this method, the pulse current value changes due to the mark rate fluctuation of the laser drive pulse, but this is done by detecting the peak of the drive pulse and using it as a reference voltage to reduce the control error due to the mark rate fluctuation. can.

According to the above method, even if the ambient temperature rises, the threshold current of the semiconductor laser increases, and the differential quantum efficiency decreases, the light on/off ratio can always be kept constant, and the light output during the pulse space is also constant. It can be done.

In this method, when the pulse repetition frequency, that is, the bit rate is low, the monitoring photodiode can sufficiently follow the output light of the semiconductor laser, and the optical output can be stabilized. Furthermore, when the bit rate is low, the shape of the monitoring photodiode can be made large, the amount of semiconductor laser light can be sufficiently reduced, and the feedback gain of the control loop shown in FIG. 3 can be increased. However, as the bit rate increases, in order to reduce the capacitance of the monitor photodiode and enable high-speed response, it is necessary to reduce the size of the monitor photodiode, and the feedback gain accordingly decreases. In order to compensate for this, it is necessary to increase the gain of the amplifier, but the configuration of a high-gain, high-bandwidth amplifier becomes very complex. Furthermore, the noise of the amplifier increases, leading to an increase in control errors.

Furthermore, the clamp circuit, peak detection section, and off-level detection section are also required to have high-speed response, making the configuration complicated.

[Purpose of the invention]

The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide an optical output stabilization method that can stabilize high bit rate optical output pulses even if the response speed of a monitor photodiode is slow.

[Summary of the invention]

In the present invention, a pulse current of a semiconductor laser is changed in advance by a low frequency oscillator, the semiconductor laser is driven together with a bias current by a bias applying section, and a portion of the optical output of the semiconductor laser is controlled by a monitoring photodiode. receives light,
The received optical signal is amplified by the amplifier.

After integrating the output of the amplifier, a first comparator compares it with a first reference potential to control the bias of the semiconductor laser.

The above-described control is such that the average value of the optical pulses is always constant. After passing the output of the amplifier through a bandpass filter that filters the oscillation frequency of the low-frequency oscillator, a second comparator compares the detected amplitude signal with a second reference potential to generate a semiconductor laser. control the pulse and drive unit.

The above-mentioned control is to control the amplitude of the optical pulse, and the amplitude of the optical pulse is controlled using a low frequency signal as a pilot.

〔Effect of the invention〕

According to the present invention, even in high bit rate optical transmission,
The monitor photodiode does not need to detect optical pulses and does not need to have a high response speed. Therefore, the shape of the photodiode can be increased, and the feedback gain can also be increased. Furthermore, the amplifier and amplitude detector do not need high-speed response and are simple in construction.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the optical output stabilization method of the present invention. A bias current for the semiconductor laser (11) is supplied by a bias application section (13) that can be controlled from the outside. Further, a pulse for driving the semiconductor laser is supplied from a pulse driving section (12). The pulse driver changes the pulse amplitude using a low frequency oscillator (14), and is also capable of changing the pulse amplitude from the outside.

A part of the output light of the semiconductor laser is detected by a monitoring photodiode (15), and the electrical signal is sent to an amplifier (15).
6). A part of the amplified signal is passed through an integrator (1
7). This output indicates the average value of the optical output, and the integrator output and the reference potential (18) are connected to the comparator (1).
9), and the bias application unit is controlled by this output to change the bias voltage of the semiconductor laser.

On the other hand, the other output of the amplifier (16) is a band filter (2o) that filters the oscillation frequency of the low frequency oscillator (14).
After passing through, the amplitude detector (21) detects the low frequency signal amplitude. This amplitude detector can be realized, for example, by a diode detector and an integrator. or,
It can also be realized using a DC clamp circuit and an integrator.

The output of this amplitude detector is similar to the peak value of the optical pulse generated by the data input, so it is compared with a specific reference potential (22) and a comparator (23), and this output is used to generate the pulse current waveform of the pulse driver. By controlling the high value, the peak value of the optical pulse can be stabilized.

If the mark rate of the data signal input to the pulse drive unit is constant, the reference potentials (18) and (22) may be voltage sources. Also, when the mark rate changes, the reference potential (18)
is a potential obtained by integrating a part of the output of the pulse drive unit, and the reference potential (22) is a potential that is integrated with a part of the output of the pulse drive unit. After passing through, it is sufficient to use the potential whose amplitude is detected. As a result, stable optical pulses can be generated even when mark rate fluctuations occur.

In the above embodiment, transmitter light source stabilization in PCM transmission was shown, but this method is not only applicable to PCM transmission.
It can also be applied to transmitter light source stabilization in pulsed analog transmission systems.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing changes in the injection current versus optical output characteristics of a semiconductor laser due to temperature, and FIG. 3 is a diagram showing a conventional example. 11...Semiconductor laser 15...Monitor photodiode 16...Amplifier 17...Integrator 20...Band filter 19.23...Comparator 18.22...Reference potential substitute People Patent Attorneys Nori Chika Yudo Kikuo Takehana

Claims (6)

[Claims] (1) A light emitting element, a bias application unit that drives the light emitting element, a pulse driving unit that changes the magnitude of the light emitting element drive pulse using a low frequency oscillator, and a monitoring light receiver that receives a part of the output light of the light emitting element. an amplifier that amplifies the output of the element and the light receiving element, and an integrator that integrates a part of the output;
a first comparator that compares the output of the integrator with a first reference potential; a band filter that filters the oscillation frequency of the low frequency oscillator from the output of the amplifier; and a band filter that detects the amplitude of the output signal. It consists of an amplitude detector and a second comparator that compares the output of the amplitude detector with a second reference potential.The output of the first comparator controls the output bias current of the bias application section, and the second comparison An optical output stabilization method characterized in that light emitted from a light emitting element is stabilized by controlling the pulse peak value of the pulse driver output pulse using the output of the device. (2) The optical output stabilization method according to claim 1, wherein the oscillation frequency of the low-frequency oscillator is within a control band of an automatic gain control circuit of a receiver that receives an optical signal. (3) The optical output stabilization method according to claim 1, wherein the first and second reference potentials are constant voltages from a power source. (4) The first reference potential is a potential obtained by integrating a part of the output of the pulse drive unit, and the second reference potential is a potential that integrates a part of the output of the pulse drive unit to oscillate the low-frequency oscillator. 2. The optical output stabilization method according to claim 1, wherein the potential is the amplitude of which is detected by an amplitude detector after passing through a bandpass filter that filters the frequency. (5) The optical output stabilization method according to claim 1, wherein the amplitude detector is composed of a wave detector and an integrator. (6) The optical output stabilization method according to claim 1, wherein the amplitude detector is comprised of a DC clamp circuit and an integrator.