JPS58124286A - Driving circuit for semiconductor laser diode - Google Patents

Abstract

PURPOSE:To make the gradient rate of current versus light output constant equivalently, by supplying the current corresponding to the variation in the gradient rate of the current versus light outut of an LD (semiconductor laser diode) to a current switch circuit from a variable current source, and variably controlling the driving current. CONSTITUTION:The light signal outputted from the LD11 is transduced from the light to electricity by a PD17, and converted into the voltage by a resistor 18. The DC component of the detected voltage generated across the resistor 18 is removed by a capacitor 21, and only the variable component is extracted. The output voltage from the capacitor 21 is integrated in an LPF23, further integrated and amplified in an active filter 24, and supplied to the base of a transistor 27 as a control voltage. The collector current of the transistor 27, i.e. a supply current of the current source 26 with respect to the current switch circuit 12, is varied by the output voltage (control voltage) of the active filter 24. In this way, the collector current of a transistor 13, i.e. a the driving current I0 is varied.

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD OF THE INVENTION The present invention relates to improvements in semiconductor radar diode drive circuits.

Technical Background of the Invention In optical transmission in which data is transmitted using an optical fiber cable, a semiconductor laser diode (hereinafter referred to as LD) is generally used as a light source (light emitting element). FIG. 1 shows the current (I) vs. light output (L) characteristics of the LD. As is clear from the figure, when the current (2) flowing through LDi exceeds the threshold current Ith, Raded oscillation of D starts, and the optical output (light intensity) L rapidly increases. Therefore, in the LD (semiconductor laser diode) drive circuit, the second
As shown in the LD operation explanatory diagram in the figure, the threshold current It
Let h be a reference current Ib, and a drive current I corresponding to an input signal as a transmission signal to this reference current b. The weight is T,
By supplying D, the optical output P. I'm doing V so that I can get it.

By the way, the threshold current Ith of the LD varies depending on the product, the operating environment temperature, and even the deterioration of the LD itself, as shown in Fig. 3.
) The light output (L) varies as shown in the characteristics. When the threshold current Ith fluctuates in this way, the method of supplying a constant reference current Ibr to the LD has the disadvantage that the optical output fluctuates. Therefore, in the conventional LD drive circuit, L
By electrically detecting the optical output from D and controlling the reference current Ib in accordance with this detected output.

It was designed to completely prevent fluctuations in optical output (L) due to fluctuations in threshold current Ith. FIG. 4 shows the configuration of such a conventional LD drive circuit, in which 11 is an LD (semiconductor laser diode). 12 is the drive current ■ to LDJ 1. Transistor (second transistor) 1 that supplies
3, and a transistor (first transistor 1) that drives this transistor 13 according to the signal level of the input signal IN.
This is a current switch circuit having 4 and 4 parts. A constant pace voltage is applied to the base of the transistor 13. A resistor 15 serves as a constant current power supply and is commonly connected to the emitters of the transistors 13 and 14. Thus, when the transistor 14 turns on/off in accordance with the signal level of the input signal IN, the transistor 13 conversely turns off/on. Then, while the transistor 13 is in the on state,
A constant drive current iIo limited by the resistor 15 is supplied to the LDII. 16 is a transistor that supplies reference current Ib to LDII. LDII is the reference current from transistor 16 and the drive current from transistor 13.

By supplying each of them, oscillation is performed in response to the input signal IN. This causes the LDll to output an optical signal, that is, a modulated optical signal.

(A part of the optical signal) output from the LDll is received by a light receiving element, such as a photodiode (hereinafter referred to as PD) 17, and converted into an n5 electric signal. This electrical signal is subjected to flow/force conversion by a resistor 18, and is then filtered through a low-pass filter (hereinafter referred to as F).

(referred to as LPF) 19 converts it into a direct current. Low pass filter 1
The output voltage of the transistor 16 is supplied to the output voltage of the transistor 16, so that the collector direct current of the transistor 16, that is, the reference current (2) is varied. In other words, in the LD drive circuit shown in FIG. 4, the LD
When the threshold current Ith of ll changes and its optical output changes, by detecting the output change and controlling the base voltage of the transistor 16, the reference voltage KIb is variably controlled to follow the fluctuation of the threshold current Ith. This makes it possible to stabilize the optical output.

Problems with the background art - LD current CI) Fluctuations in light characteristics (L) include, in addition to fluctuations in the threshold current Ith mentioned above, current-light output slope rate (hereinafter referred to as I-L slope rate). Measure the fluctuation of α. This IL gradient rate α varies depending on various factors such as the slope dL of the current (I) vs. light characteristic (L) curve in the oscillation region, the slope dL of the current (I) vs. light characteristic (L) curve, and the deterioration of the LD. FIG. 5 shows the current (1) vs. light output characteristic (L) of the LD when the L gradient rate α fluctuates, and the light output also fluctuates in this case.

Therefore, as described above, the base voltage of the transistor 16 is controlled according to the fluctuation of the optical output, and the reference current Ib is varied. For example, if the IL gradient rate α decreases,
The optical output decreases, and the reference current Ib increases accordingly. In this way, the reference current Ib becomes the threshold current I of the LD.
When the setting formula n is set larger than th, the optical output L (corresponding to Pb in FIG. 2) of the LDll when the transistor 13 is in the off state, that is, the off output becomes large. As a result, the optical output L of LDII when the transistor 13 is in the on state
(Corresponds to Po in Figure 2) That is, the on output is X-t,
&j Even if the reference current Ib decreases below a predetermined value due to a decrease in the distribution ratio α, the effective value of the light output is kept constant7.

However, in such a conventional LD drive circuit, when the IL gradient rate α fluctuates, although the effective value of the optical output is stabilized, the LDI Since the difference in strength between the OFF output and the OFF output becomes small, there is a drawback that it becomes difficult for the receiving side to identify and reproduce all of the received signals. In other words, in the conventional LD drive circuit that aims to prevent instability of the optical output due to fluctuations in the threshold current Ith, when the I-L gradient rate α of the LD fluctuates, this fluctuation cannot be efficiently dealt with. However, the problem was that it was not possible to output a good modulated optical signal.

OBJECT OF THE INVENTION The present invention has been developed in view of the above circumstances, and its purpose is to improve the performance of semiconductor devices that change depending on various factors.
The object of the present invention is to provide an entire semiconductor laser diode 9 drive circuit that can make the gradient of current P versus optical output substantially constant, thereby outputting a well-modulated optical signal with constant optical intensity.

Summary of the Invention The current source of a current switch circuit equipped with a pair of transistors (first and second transistors) that operates according to the signal level of a human input signal is a variable current source equipped with a third trunk transistor, and an LD (semiconductor) After converting the optical signal output from the LED diode into a total electric signal and removing the DC component of this electric signal, the electric signal from which the DC component has been removed is fully integrated, and according to the integration result, the third transistor By fully controlling the variable current source, a direct current is supplied to the current switch circuit according to the variation in the gradient rate of the current versus light output of the LD, and the drive current supplied from the current switch circuit to the LDK is variably controlled. It is something.

Embodiment of the Invention An embodiment of the present invention will be described below with reference to the drawings. Note that the same parts as in FIG. 4 are given the same reference numerals and detailed explanations will be omitted. In the T, D (semiconductor radar diode) drive circuit shown in FIG. 6, 21 is a capacitor that removes the DC component of the voltage generated across the resistor 18. 22 is LPF
(Low pass filter) 23 and active filter 24?
The LPF 23 has a well-known function of dividing the output of the capacitor 2I.
) 25, and has the well-known function of further integrating and amplifying the output of the LPF 23. 26 is a current source of the current switch circuit 12. The current source 26 consists of a transistor (third transistor) 27 and a resistor 28. The collector of the transistor 27 is commonly connected to each emitter of the transistors 13 and 14, and the emitter of the transistor 27 is connected to one end of the resistor 28. There is.

The other end of the resistor 28 is connected to the negative power supply -E. The integrated and amplified output of the r-active filter 24 is supplied to the transistor 27 as a control voltage.

Next, the entire operation of one embodiment of the present invention will be explained. LDll is the reference current Ib supplied from the transistor 16 and the drive current ■ supplied from the transistor 13. It outputs an optical signal with an intensity of n indicated by its Flu(1) vs. optical output (L) characteristic. The optical signal (-9 part) output from the LDll is converted into light/drowsiness by the PDI 7, and converted into voltage by the resistor 18. FIG. 7(a) shows the signal voltage wave engraving of which resistor 18, with time t on the horizontal axis and voltage V on the vertical axis. Here, the voltage Vlμ is the detection voltage corresponding to the optical output (off output) of the LDII when the trunk transistor 13 is in the off state (that is, the transistor 14 is in the on state). That is,
Transistor 1 is connected to LDll regardless of the input signal IN.
Since a reference current is always supplied from the transistor 6, the optical output of L'DII does not become zero even when the transistor 13 is off, and a slight voltage v1 is generated in the resistor 18. Further, the breakdown voltage v3 is LD'l when the transistor 13 is in the on state (that is, the transistor 14 is in the off state).
This is the detection voltage corresponding to the optical output (on output) of l.

Of the detected voltage generated at the resistor 18, the DC component of the voltage Vl is removed by the capacitor 21, and only the changed portion is outputted. As a result, the output voltage waveform of the capacitor 21 is shown in Fig. 7 (
As shown in b). The output voltage of the capacitor 21 is integrated by the LPF 23, and then further integrated and amplified by the active filter 24.
The voltage is supplied to the transistor 270 as a voltage. FIG. 7(c) shows the waveform of this control voltage. The collector current of the transistor 27, that is, the current supplied by the current source 26 to the current switch circuit 12, is varied by the output voltage (control voltage) of the active filter 24. As a result, the collector current of the transistor 13, that is, the drive current ■. is made variable.

These operations will be explained in more detail by taking as an example the case where the IL gradient rate α of LDII increases. i-4, L
When I-1 of DII and the slope rate α increase, the drive current before the increase ■. Same drive current as■. is from transistor 13'
Even if the light is supplied to L and D11, the optical output (ON output) of LDII increases. Note that the threshold current Ith of LDJ 1
If no change occurs, the off output of LDJ 1 will not change.

When the on-output of LDll increases, the signal voltage corresponding to the on-output of LDII of the detection voltage generated at the resistor I8 increases as shown by the broken line in FIG.
2, the voltage becomes V/2. Then, the DC component of the detection voltage generated across the resistor 18 is cut off by the capacitor 2 (+a line in FIG. 7(b)), and as described above, the LPF
23, and is integrated and amplified by an active filter 24.

As a result, the integrated and amplified output of the active filter 24 decreases from voltage v3 to voltage v3 as indicated by the broken line n in FIG. 7(c). As the output 'voltage of the active filter 24, that is, the control voltage decreases, the transistor 2
The pace potential of transistor 27 decreases, and the collector current of transistor 27 decreases. As a result, the collector current of the transistor 13, that is, the drive current I. As n decreases, the optical output L (ON output) of LDII decreases. As a result, resistance 18
The signal voltage corresponding to the on-output of LDJ 1 of the detection voltage generated at is infertile, and becomes voltage 2 in the state before the IL gradient rate α increases from voltage v2. That is, I
-The optical output is stabilized with respect to an increase in the L gradient rate α. This is the same even when the IL gradient rate α decreases. However, in this case, the driving heart flow technique. is controlled so that it increases.

By the way, the transistor 27 of the current source 26 detects the optical output of the LDll, and the PD 17° resistor 18, the capacitor 21, and the low-pass filter circuit 2
The control system consisting of the LPF 23 and the active filter 24 constitutes a closed loop system having a second-order loop transfer function. Then, the drive d flow I in a steady state by this second-order roots transfer function. , the transient response of the system is determined, and the electronic constants of each circuit are also determined.

In addition, in the above explanation of the operation, the I-L gradient rate α of LDll is
Although we have explained the case where only the threshold current Ith fluctuates due to changes in temperature, etc., even if the threshold cardiac current Ith fluctuates due to temperature changes, etc., it can still be achieved by controlling the 1-drive cardiac current described above and controlling the reference current described in the conventional example. It is clear that the optical output of LDll can be further stabilized.

Effect 13 of the Invention As detailed above, according to the semiconductor laser diode drive circuit of the present invention, the slope rate of the current versus light output of the semiconductor laser diode can be kept constant under equal constraints, so that when a certain amount of light is A modulated optical signal with excellent intensity can be output.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the current vs. optical output characteristics of a semiconductor laser diode, Figure 2 is a diagram used to explain the operation of a semiconductor laser diode, and Figure 3 is a diagram showing the DC vs. optical output characteristics of a semiconductor laser diode when the threshold current varies. Diagram showing optical output characteristics, 4th
The figure is a circuit configuration diagram showing a conventional example, Figure 5 is a diagram showing the current vs. light output characteristics of a semiconductor laser diode when the current-acid pressure gradient rate changes, and Figure 6 is a diagram showing an embodiment of the present invention. The circuit configuration diagram and FIGS. 7(a) to 7(c) are signal voltage waveform diagrams for explaining the present invention in detail. 11...Semiconductor laser diode (LD), 12...
Current switch circuit, 13, J 4.16°27
...Transistor, 15.18.28...Resistor, 14
- l7... Photodiode (PD), 19.23...
- Expanded acid Noirta (L, PF), 2z...con answer. 22...Low pass filter circuit bf5, 26...Current source. Junjin's agent Patent attorney Takehiko Suzu 15-III Figure 2 Figure 5 Figure 6 1711

Claims (1)

[Claims] A semiconductor radar diode, a first transistor that turns on/off depending on the signal level of an input signal, and a drive mechanism that turns off/on according to the on/off of the first transistor, and that drives the semiconductor radar diode in an on state. #, a current switch circuit including a second transistor that supplies a current, a current source that includes a third transistor and supplies the entire current to the current switch circuit, and a reference current supply circuit that supplies a reference current to the semiconductor laser diode. , an optical/electrical converter that converts the optical signal output from the semiconductor radar diode into an electrical signal, a capacitor that removes the DC component of this electrical signal, and an integrated circuit that integrates the output of this capacitor and responds to the integration result. and a low-pass filter circuit that controls the third transnoster, and the current supplied from the current source to the current switch circuit is varied according to the integration result of the low-pass filter circuit. Semiconductor laser diode drive circuit.