JPS63216395A - Injection current setting circuit of semiconductor laser - Google Patents

Abstract

PURPOSE:To facilitate keeping the gain of optical amplification always constant even if the intensity of applied light or an ambient temperature is changed by controlling the injection current of a semiconductor laser by feedback to make the minimum value of the optical output of the semiconductor laser the same intensity of the optical output by the oscillation threshold value of the semiconductor laser when light is not applied or less value. CONSTITUTION:The means of externally applying modulated light (input light) L3 in a semiconductor laser 13 and a photodetector 14 which receives optically amplified output light (amplified light) L4 from the above-mentioned semiconductor laser 13 and converts to an electric signal are provided. The means of controlling the input current of the semiconductor laser 13 by feedback are also provided to make the minimum value of the output current (optical current) I1 of the photodetector 14 equal to the value of the output current of the photo detector 14 when the quantity of emitted light near the oscillation threshold value of the semiconductor laser 13 when no light is applied or less quantity is received. Or further, the optically amplified output light L4 from the above- mentioned semiconductor laser 13 is divided into two, one is given to the photodetector 14 and the means of sending out the other externally are provided.

Description

[Detailed description of the invention] [Technical field to which the invention pertains] The present invention relates to a circuit for setting the injection current of a semiconductor laser used for optical amplification, optical wavelength band pass filters, etc. in the field of optical communications. be.

[Conventional technology]

FIGS. 6 and 7 are diagrams of examples of conventional injection current setting circuits (for example, as described in the Proceedings of the 1985 National Conference of the Institute of Electronics and Communication Engineers, Volume 8, Page 282).

First, the circuit shown in FIG. 6 is a circuit that sets the injection current of the semiconductor laser so that the average output destination of the semiconductor laser is constant.

In FIG. 6, the optical output L1 of the semiconductor laser 1 is outputted to the outside via the optical fiber 11. Further, the monitoring output light L2 from the semiconductor laser 1 is received by the light receiving element 2, and the output voltage due to the photocurrent I2 is averaged by the low pass filter 3. Then, the average output and the base P3 voltage given from the reference voltage circuit 6 are given to the differential amplifier 4, and the difference is amplified.
By controlling , the injection current of the semiconductor laser 1 is controlled.

In this circuit, the average output light of the semiconductor laser 1 can be made constant.

Next, FIG. 7 shows a circuit for setting the injection current of the semiconductor laser so as to keep the maximum peak value of the optical output constant.

In FIG. 7, a modulation signal S1 is applied to a semiconductor laser 1 via a capacitor 5. The output light from the semiconductor laser 1 is modulated and becomes larger or smaller. Of the output light, the light output L2 for monitoring is sent to the light receiving element 2.
and generates a photocurrent I. Therefore, a photocurrent (1) flows depending on the magnitude of the optical output. The voltage generated by this photocurrent I is amplified by a differential amplifier 8, and a diode 9
After performing half-wave rectification, by charging the capacitor 10, a voltage proportional to the maximum value of the photocurrent, that is, the maximum peak value of the optical output, is obtained.

This voltage is compared with the voltage given from the reference voltage circuit 6 by the differential amplifier 4, and the transistor 7 is controlled according to the difference.

The injection current of the semiconductor laser 1 is controlled.

In this circuit, the maximum peak value of the optical output of the semiconductor laser 1 can be kept constant.

[Problem that the invention seeks to solve]

However, the conventional circuit as described above is mainly a circuit for making the output optical power constant when a semiconductor laser is used as a light emitting source.

This circuit is inappropriate when a semiconductor laser is used as an optical amplification element or an element that amplifies light and has optical wavelength band pass filter characteristics.

That is, when a semiconductor laser is used as an optical amplification element or an element that amplifies light and has optical wavelength band pass filter characteristics, depending on the magnitude of the input light injected into the semiconductor laser, that is, the injected light, The magnitude of the light that is amplified and output changes, but in conventional circuits like the one described above, when the power of the injected light changes, the gain of the optical amplification is changed to control the injected current so that the output light remains constant. Resulting in. However, when using a semiconductor laser as an optical amplification element or an element that amplifies light and has optical wavelength band pass filter characteristics, it is necessary to always set the injection current so that the gain of optical amplification is constant. There is.

Furthermore, since the conventional circuit described above is a circuit for a light emitting source, it is not possible to output the optical reception signal of the input light to the outside, and there is only one entrance/exit for light to the outside of the semiconductor laser. Therefore, there was a drawback that it was not possible to inject light from the outside and output the amplified light to the outside.

The present invention aims to solve the problems of the prior art as described above.

[Means to solve the problem]

In order to achieve the above object, the present invention includes a means for injecting modulated light from the outside into a semiconductor laser, and a means for receiving optically amplified output light from the semiconductor laser and converting it into an electrical signal. A light-receiving element and a minimum value of the output current of the light-receiving element when the light-receiving element receives light with an amount of light emitted near or below the oscillation threshold of the semiconductor laser in the absence of injected light. and a means for feedback controlling the injection current of the semiconductor laser so that the magnitude of the output current is equal to the magnitude of the output current of the semiconductor laser. It consists of

With the above configuration, in the present invention, the gain of optical amplification can always be kept constant even if the magnitude of the injected light or the ambient temperature changes.

In addition to the above configuration, another configuration of the present invention includes means for branching the output light amplified and output from the semiconductor laser, giving one side to the light receiving element and outputting the other to the outside. By providing this, the structure is such that light can be injected from the outside and the optically amplified light can be outputted to the outside.

That is, in the present invention, when using a semiconductor laser as an optical amplification and optical wavelength band pass filter,
In order to be able to always set the injection current to a value near or below the oscillation threshold current, the lowest value of the optical output of the half-8-day laser injected with digital intensity modulated light is detected, and the The present invention is characterized in that the injection current of the semiconductor laser is feedback-controlled so that the magnitude is the same as the optical output at or below the oscillation threshold of the semiconductor laser when no light is injected.

As described above, in the present invention, in order to detect the output of the semiconductor laser when the input light becomes zero (the optical output corresponding to the oscillation threshold current, which is a type of noise light), the input light is pulsed. And it needs to be turned off. Therefore, digital intensity modulated light is used as human power.

〔Example〕

FIG. 1 is a diagram showing a first embodiment of the present invention, and FIG.
The figure is a diagram showing the potentials of main parts in the circuit of FIG. 1.

In FIG. 1, digital intensity-modulated input light L3 sent from the outside via an optical fiber 12 is injected into a semiconductor laser 13.

This input light (that is, injection light) L3 is shown in Fig. 2 (
As shown in a), two types of pulses with different widths and duties, for example 47i1, are set.

The above-mentioned injected light is optically amplified by the semiconductor laser 13 to become amplified light L4, which is incident on the light receiving element 14 (for example, a pin photodiode) and photoelectrically converted. This is shown in FIG. 2(b). This causes the resistor R1 to receive a photocurrent
, flows, and a potential Va is generated.

The potential Va generated by the photocurrent described above is input to the lowest peak value detection circuit 15.

In this lowest peak value detection circuit 15, first, the potentials V and a are amplified by the differential amplifier 24 to become the potential at point A, that is, as shown in FIG. 2(c).

Next, the potential at point A is half-wave rectified by diode 16,
By charging the capacitor 19, there is a voltage at point F.
The maximum value of the potential at the point is output [Fig. 2 (h)]. This value corresponds to the peak ■ or v2 of the pulse in FIG. 2(C).

On the other hand, by determining the alternating current component of the potential at point A using the capacitor 20, the potential at point B can be obtained [Fig.
)].

The potential at point B has no direct current component, that is, the potential at point B is zero when averaged.

Next, the potential at point B is half-wave rectified by the diode 17 and the capacitor 21 is charged, thereby obtaining the maximum negative value of the potential at point B. This is the potential at point C [Figure 2 (e)]
It is. At the same time, the potential at point B is half-wave rectified by the diode 18 in the opposite direction to that of the diode I7, and the capacitor 22 is charged, thereby obtaining the maximum positive value of the potential at point B. This is the potential at point D [Fig. 2(f)].

Next, the difference between the potential at point C and the potential at point D is determined by the differential amplifier 25 with an amplification factor of 1. This is the potential at point E [second
Figure (g)].

The potential at point E is the difference between the maximum and minimum potentials at point A, that is, the peak-to-peak value of the amplitude of the change eJj in the potential at point A.

On the other hand, the maximum value of the potential at point A output from the capacitor 19 is obtained as the potential at point F. The difference between the potential at point F and the potential at point E is determined by a differential amplifier 26 with an amplification factor of 1. This is the potential at point G [Fig. 2(i)].

The potential at point G is the maximum value of the potential at point A minus the peak-to-peak value of the amplitude of the potential fluctuation at point A, and this is the result when no light is injected into the semiconductor laser 13. It is proportional to the amount of light emitted near or below the oscillation threshold.

That is, as shown in Fig. 2 (i), at the potential at point G, the left half of the figure is v 1- (v , + v ,
), of which v and +v are the pulse amplitude portions of the potential of A shown in FIG. 2(c), as can be seen from FIG. 2(d). Therefore, subtracting this from v4 becomes the potential Pa corresponding to the oscillation threshold. The same applies to the right half of the potential at point G.

Next, the potential at point G and the base 1! given from the reference voltage circuit 28! GI! The difference in potential is amplified by a differential amplifier 27 and output.

Next, the output of the differential amplifier 27 is input to the base of the transistor 29, and the amplified and output collector current is used as the injection current of the semiconductor laser 13. Note that R2 and R6 are resistors.

The injection current setting circuit described above is a circuit that controls the amount of laser light emission when no light is injected into the semiconductor laser so that it is constant at a value near or below the one-oscillation threshold.

In this circuit, as shown in Figure 3(a), even if the ambient temperature changes from 20°C to 40°C,
The injected current changes from Ia to Ia' so that the optical output becomes constant at the value of Pa, and the optical output can be kept close to or below the oscillation threshold.

Furthermore, even if the magnitude of the incident light changes when the digital signal light is incident on the injected light, as shown in FIG. 3(b), in the present invention, the power Pa corresponding to the oscillation threshold of the semiconductor laser Since this value is detected, the value is the optical output when there is no injected light, and can be kept close to or below the oscillation threshold as shown in FIG. 3(a).

Next, FIG. 4 is a diagram showing a second embodiment of the present invention.

In this embodiment, a capacitor 30 is added to the circuit shown in FIG. 1, and the alternating current component of the output of the light receiving element 14 is taken out and made into an optical reception signal S2.

In FIG. 4, a portion 15 surrounded by a broken line is a lowest peak value detection circuit similar to that in FIG. 1.

In the circuit shown in Figure 4, even if the average optical power or maximum peak power of the digital signal light of the injected light changes, or the ambient temperature changes, the semiconductor laser is always kept close to or below the oscillation threshold. can be set,
Moreover, an optical reception signal can be obtained.

Next, FIG. 5 is a diagram showing a third embodiment of the present invention.

In FIG. 5, input light L3 is injected into the semiconductor laser 13 via the optical fiber 31. In FIG. The amplified light L4 optically amplified by the semiconductor laser 13 is split into two by a semi-transparent mirror 33 that reflects a part of the incident light and transmits most of it. Of these, the monitoring light L5 is transmitted to the light receiving element 1.
4, and the injection current of the semiconductor laser 13 is controlled by a circuit similar to that shown in FIG.

On the other hand, the output light L6 that has passed through the semi-transparent fi 34 is output to the outside via the optical fiber 32. This optical output is sent to the next receiving device as, for example, optically amplified light. That is, optical signals can be relayed.

Although this embodiment also includes a capacitor 30 for taking out the optical reception signal S2, this can be omitted.

〔Effect of the invention〕

As explained above, according to the present invention, the injection current of a semiconductor laser used for optical amplification and optical wavelength band pass filters can be kept close to the oscillation threshold current even if the magnitude of the injected light or the ambient noise changes. Since the gain can be set to , or lower, the gain of optical amplification can be kept constant even if the size of the injected light or the ambient temperature changes. Effects such as being able to output light to the outside can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing the potential at each point in the circuit of FIG. 1, FIG. 3 is a characteristic diagram of the circuit of FIG. 1, and FIGS. 4 and 5 6 and 7 are diagrams of other embodiments of the present invention, respectively, and FIGS. 6 and 7 are diagrams of examples of conventional devices, respectively. Explanation of symbols> 1... Semiconductor laser 2... Light receiving element 3...
Low pass filter 4... Differential amplifier 5... Capacitor 6...
・Reference voltage circuit 7...Transistor 8...Differential amplifier 9...Diode 10...Capacitor II, 12...Optical fiber 13...Semi-dedicated laser 14...Photodetector 15. ...Lowest peak value detection circuit 16,, 17.18...Diode 19.20.21.22...Capacitor 24.25.
26.27...Differential amplifier 28...Reference voltage circuit
29...Transistor 30...Capacitor
31.32...Optical fiber 33...Semi-transparent mirror Patent applicant Nippon Telegraph and Telephone Corporation Patent attorney Junnosuke Nakamura Figure 23 and 6 [1

Claims (2)

[Claims] (1) A means for injecting modulated light from the outside into a semiconductor laser, a light-receiving element that receives the optically amplified output light from the semiconductor laser and converts it into an electrical signal, and an output current of the light-receiving element. The minimum value of is set to be equal to the output current of the light receiving element when receiving light with an emission amount near or below the oscillation threshold of the semiconductor laser in the absence of injected light. An injection current setting circuit for a semiconductor laser, comprising means for feedback controlling the injection current of the semiconductor laser. (2) A means for injecting modulated light from the outside into the semiconductor laser, a light receiving element that receives the optically amplified output light from the semiconductor laser and converts it into an electrical signal, and an output current of the light receiving element. The minimum value of is set to be equal to the output current of the light receiving element when receiving light with an emission amount near or below the oscillation threshold of the semiconductor laser in the absence of injected light. and means for feedback controlling the injection current of the semiconductor laser, and means for branching the output light amplified and output from the semiconductor laser, giving one to the light receiving element and outputting the other to the outside. An injection current setting circuit for a semiconductor laser, characterized in that: