JPS5939914B2 - Semiconductor laser output control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical communication device using a semiconductor laser, and more particularly to a semiconductor laser output control circuit within the optical communication device.

With the progress of semiconductor lasers in recent years, optical communications in combination with optical fibers, such as optical PCM communications, are being put into practical use.

When realizing an optical communication device using a semiconductor laser, the most troubling problem is that the output of the semiconductor laser changes greatly with temperature fluctuations. That is, when trying to obtain a large or small laser output by setting a large or small bias current to a semiconductor laser element, the threshold value of the bias current changes significantly with temperature fluctuations. Therefore, in an optical communication device using a semiconductor laser, a semiconductor laser output control circuit is essential to obtain a substantially constant laser output regardless of temperature fluctuations.

As will be detailed later, the most common method for realizing this semiconductor laser output control circuit is to provide a comparator and apply the peak value of the current output level of the semiconductor laser to the first comparison input of the comparator. On the other hand, a method can be considered in which a predetermined reference level is applied to the second comparison input, and the bias current to be applied to the semiconductor laser element is changed according to the level difference between these levels in the comparator. However, for example, in optical PCM communication, the PC to transmit
Since the laser output is turned on or off in response to ``1'' or 0'' of the M signal, when the mark rate of ``0'' in the PCM signal becomes high, the semiconductor laser element actually turns the laser at a predetermined level. Despite having the ability to transmit output, the comparator assumes that the laser output of the semiconductor laser device has extremely decreased, and increases the bias current to an infinite extent. It is clear that this results in the semiconductor laser element being destroyed. For this reason, the mark rate of 60° in the PCM signal increases and when N "0"s (N is 15, for example) are sent out consecutively, it corresponds to the PCM signal "1" regardless of the presence or absence of the PCM signal. It is possible to forcibly generate the laser output from the outside so that even if "0" continues, the laser output is not considered to be extremely low. however,
The laser output (“1”) is forcibly generated from the outside.
The signal representing ゛) (signal to the first comparison input) is CR
It is held for a predetermined time via a circuit. However, the method described above has the disadvantage that when the PCM signal is interrupted, the bias current increases infinitely, resulting in destruction of the semiconductor laser element. The operation of counting N and the operation of generating the above-mentioned pseudo laser output when the content of the count reaches N causes the circuit to become complicated and large. Furthermore, if the setting of N is incorrect, there is a drawback that the PCM signal may contain an error. Therefore, it is an object of the present invention to always maintain an appropriate constant power without applying an excessive bias current to the semiconductor laser device, regardless of the pattern of the signal to be transmitted, and even when the signal is disconnected. It is an object of the present invention to provide a semiconductor laser output control circuit that can obtain a laser output having a laser output of 100%, and to achieve this object with an extremely simplified and therefore small-sized circuit.

In accordance with the above object, the present invention provides that the reference level to be applied to the second comparison input of the comparator is variable according to the pattern of the signal to be transmitted, including when the signal is disconnected. This is a characteristic feature.

The present invention will now be described with reference to the drawings.

However, the case where a PCM signal is transmitted will be described as an example.
FIG. 1 is a block diagram showing an optical communication device equipped with a semiconductor laser output control circuit realized by the most general method. In the figure, reference numeral 11 denotes a semiconductor laser element, which oscillates with a bias current supplied from a laser bias current control circuit 12 to generate a laser output. On the other hand, the laser element 11 is simultaneously controlled by the laser and drive circuit 13, and it is determined at what timing the laser output should be sent out (radiated). Furthermore, the transmission timing of this laser output is determined by the transmission signal supply circuit 14. That is, the transmission signal supply circuit 14 transmits P to be transmitted.
A CM signal is supplied to the laser 1 drive circuit 13. Here, the laser element 11 sends out a binary laser output P (and P') corresponding to "1" and "0" of the PCM signal to be transmitted. However, the laser output P is the laser front output, and the laser output P' is the laser rear output. Of these, the laser front output P is irradiated onto the end of an optical fiber (not shown) and transmitted as an optical PCM signal to a receiving station at the other end of the optical fiber. The above configuration is the basics of optical communication equipment.
A semiconductor laser output control circuit 10 is further added to compensate for temperature fluctuations in the laser output.

A typical semiconductor laser output control circuit 10 consists of parts 15, 16, 17, 18 and 19 described below.
First, the photodetector 15 receives the laser rear output P', performs optical/electrical conversion on its level, and sends it to the amplifier 16. Note that the magnitude of the laser rear output P' and the magnitude of the laser front output P are not the same (although they are not the same, but when P changes, P' also changes in a certain proportional relationship. Also, the photodetector 15 For example, when a PCM signal consisting of a photodiode is sent out from the transmission signal supply circuit 14, the laser output P (and P7) is emitted from the laser element 11 in response to the PCM signal "1". Therefore, the peak 1 direct of the level of the laser output P, therefore, the laser output P
'0) level peak value is detected in the form of an electrical signal via the aforementioned photodetector 15 and amplifier 16. This detection is performed in the peak value detection circuit 17. As already mentioned, the laser element 11 is extremely susceptible to temperature fluctuations, so even if the PCM electric signal 1 is supplied from the laser 1 drive circuit 13 at a constant level, the corresponding laser output (P and P peaks) The value changes from time to time.The change in this peak value is the first comparison input 18-1 of the comparator 18.
is applied as voltage Vp to , while its second comparison input 18
−2 is connected to a predetermined reference voltage, E, through a semi-fixed resistor 19.
is applied. This reference voltage VrO, has a voltage Vp of V
re, the desired level of laser output P(
and Pl). Comparator 18
outputs a differential voltage proportional to the difference between the voltage V and the reference 0P standard voltage Vre, and controls the bias current from the bias current control circuit 12 so as to lower the laser output when Vp>Ref, while V, When Vre is reached, the bias current is controlled so as to increase the laser output.

Thus, the system of the semiconductor laser output control circuit 10 has Vp=R.
It is stable at ef, and a desired level of laser output is always obtained.

However, if the PCM signal from the transmission signal supply circuit 14 has a pattern such as (1011000...0010) that continuously includes 60°, then the voltage from the peak value detection circuit Fjl7 becomes
p becomes almost zero level, and it is regarded as if the peak value of the laser output corresponding to the PCM signal "1" has dropped to zero.

For this reason, the comparator 18 drives the bias current control circuit 12 in a direction that increases the bias current infinitely, resulting in the destruction of the expensive laser element 11. The peak value detection circuit 17 is equipped with a p circuit that holds the voltage at a predetermined time so that the above-mentioned problem does not occur even if a pattern of continuous 0s is included. It is invalid for 10' continuous patterns exceeding 10'.

For this reason, every time the number of 0's in a 10' continuous pattern exceeds a predetermined constant value, for example 15, that is, every time the above holding time is exceeded, a laser output equivalent to the PCM signal 1 is forced from the outside. A method was proposed to cause this to occur. However, this requires the addition of a relatively complicated new circuit, and this cannot be said to be an effective method. If the number of 0's is set too large, it will be difficult to determine whether the voltage V represents the held voltage p pressure or the peak value of the laser output corresponding to the PCM signal '1' actually generated during the holding period. It becomes impossible to distinguish whether the voltage represents a voltage corresponding to , and in the worst case, even though the peak value of the actual laser output has decreased considerably, the bias current may be erroneously controlled not to be increased.

Furthermore, if the PCM signal is temporarily interrupted, the clock timing to count the number of "0" continuous patterns of "0" cannot be obtained, and the laser output corresponding to the PCM signal "1" is zero. It exhibits a state completely equivalent to that of
This also results in the destruction of the laser element 11. Therefore, the present invention solves the above-mentioned disadvantages,
In addition, we have proposed a semiconductor laser output control circuit that is inexpensive, simple, and allows the dimensions of the optical communication device circuit to remain almost the same as conventional ones. An example of this circuit is shown in FIG.

The fact that this semiconductor laser output restraining circuit solves the above-mentioned disadvantages and is still inexpensive and compact is that it is a repeater (
This is extremely important considering that it is used as a repeater. In this figure, the same reference numbers (symbols) as in FIG. 1 indicate the same components. In the figure, 20 indicates a semiconductor laser output control circuit based on the present invention. As is clear from comparing FIGS. 1 and 2, the semiconductor laser output control circuit 20 (1, first
A peak value detection circuit 21 is simply added to the one shown in the figure. This peak value detection circuit 21 is connected to a PCM electric signal ``1'' obtained by branching from a part of the transmission signal supply circuit 14.
The peak value of V/Re is detected, and the relative position of the entire variable resistor 22 (corresponding to the semi-fixed resistor 19 in FIG. 1) that supplies the reference voltage V/Re is changed in accordance with the detected peak value voltage. That is, when the PCM electric signal includes a continuous pattern of 0, the output voltage of the peak value detection circuit 21 connected to the signal supply circuit 14 also decreases accordingly, and therefore the second comparison input of the comparator 18 Reference voltage to be applied to 18-2'
ROf also decreases. As a result, even if the PCM electrical signal includes a long "O" continuous pattern and the voltage V from the peak value detection circuit 17 is almost at zero level, it should be compared with the voltage V) P reference voltage 'Re , are also at zero level, and the output of the comparator 18 is also approximately zero.

Therefore, the bias current will not be increased excessively. In addition, by adopting such a configuration, the circuit always operates properly no matter how many "0"s there are in the "0" continuous pattern, and furthermore, even when the PCM signal is disconnected, the circuit always operates properly. In other words, even if the disadvantage that the output of the comparator 18 excessively increases the bias current is resolved, the laser element 11 will not be destroyed.The configuration of the peak value detection circuit 21 described above is similar to that shown in FIG. It may have the same configuration as the peak value detection circuit 17, and can be realized by a combination of a diode and a CR circuit (time constant circuit) as described later.However, it should be noted here that all CR circuits have the same time constant. If there is a difference in the time constant between the two, the voltage and the reference voltage ' will change as the discharge time elapses after sampling one peak value.
This is because the difference between the bias current and the bias current increases (or decreases) unnecessarily. and both CRs
It is also necessary to align the phases of the outputs from the circuit. FIG. 3 is a diagram showing a part of the block shown in FIG. 2 replaced with a concrete circuit.

The laser element 11 is driven by a laser drive circuit 13 consisting of a current switch circuit. On the other hand, a direct current bias is applied to the laser element 11 from the bias current control circuit 12. L is a coil that cuts off the signal. The current switch circuit 13 is turned on and off by a transmission signal supply circuit 14 consisting of an identification circuit (F/F). Identification circuit (F/F
) is given 1n PCM signals (PCM.).

The peak value detection circuit 21, which is a feature of the present invention, is composed of a diode D and a capacitor C, and the input to the diode D is the Q output of the identification circuit (F/F) or the R point of the current switch circuit 13. given from. As explained above, according to the present invention, a simple, compact, and inexpensive peak value detection circuit is simply added to the semiconductor laser output control circuit, which is an essential component in an optical communication device using a semiconductor laser. This reliably prevents the bias current from running out of control when there is no continuous signal or when the signal is cut off, which has been a problem in the past.

In addition, in the above explanation, the method of controlling the laser bias current has been explained as an example of the method of controlling the laser output, but the same method can also be applied to the method of controlling the laser 1 drive signal current. Applicable.

[Brief Description of the Drawings] Fig. 1 is a block diagram showing an optical communication device equipped with a general semiconductor laser output control circuit, and Fig. 2 is a block diagram showing an optical communication device equipped with a semiconductor laser output control circuit based on the present invention. The block diagram shown in FIG. 3 is a block diagram in which part of the block in FIG. 2 is replaced with a concrete circuit. In the figure, 11 is a semiconductor laser element, 12 is a laser bias control circuit, 14 is a transmission signal supply circuit, 18 is a comparator, 20 is a semiconductor laser output control circuit according to the present invention, 2
1 is a peak value detection circuit, 22 is a variable resistor for supplying a reference voltage, P is a laser front output, and P' is a laser rear output.

Claims (1)

[Claims] 1 Semiconductor laser element 11 that sends out a laser output signal
, a laser drive system consisting of a transmission signal supply circuit 14 and a laser drive circuit 13 that output a "1""0" pattern signal for driving and controlling the timing of transmitting laser output from the semiconductor laser element 11; It has a laser bias current control circuit 12 that controls the bias current to be applied to the semiconductor laser element 11, and a comparator 18 that generates a control signal to be supplied to the laser bias current control circuit 12. The output from the peak value detection circuit 17 is received at its first comparison input 18-1, and a predetermined reference voltage is received at its second comparison input 18-2 to form a difference voltage between them, and the peak value The detection circuit 17 is
It is provided with a function of detecting the peak value of the laser output from the semiconductor laser element 11 converted into an electric signal via a photodetector 15 and an amplifier 16 and holding it with a predetermined time constant. In the semiconductor laser output control circuit, a peak value detection circuit 21 is provided that changes the level of the reference voltage to be received at the second comparison input 18-2, and the peak value detection circuit 21 detects the "1" level. It has a function of detecting the peak value of the "0" pattern signal and holding it with a predetermined time constant, and is characterized in that the time constants of the peak value detection circuits 17 and 21 are substantially the same. Semiconductor laser output control circuit.