GB1566809A - Creación de Inventos y Patentes con Inteligencia Artificial

GB1566809A – Driving circuitry for driving light emitting semiconductor devices
– Google Patents

GB1566809A – Driving circuitry for driving light emitting semiconductor devices
– Google Patents
Driving circuitry for driving light emitting semiconductor devices

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Publication number
GB1566809A

GB1566809A
GB22171/77A
GB2217177A
GB1566809A
GB 1566809 A
GB1566809 A
GB 1566809A
GB 22171/77 A
GB22171/77 A
GB 22171/77A
GB 2217177 A
GB2217177 A
GB 2217177A
GB 1566809 A
GB1566809 A
GB 1566809A
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GB
United Kingdom
Prior art keywords
light emitting
circuitry
driving
signal
semiconductor device
Prior art date
1976-05-25
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)

Expired

Application number
GB22171/77A
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)

Fujitsu Ltd

Original Assignee
Fujitsu Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
1976-05-25
Filing date
1977-05-25
Publication date
1980-05-08

1977-05-25
Application filed by Fujitsu Ltd
filed
Critical
Fujitsu Ltd

1980-05-08
Publication of GB1566809A
publication
Critical
patent/GB1566809A/en

Status
Expired
legal-status
Critical
Current

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Classifications

H—ELECTRICITY

H01—ELECTRIC ELEMENTS

H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL

H01S5/00—Semiconductor lasers

H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium

H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes

H01S5/06209—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes in single-section lasers

H01S5/06216—Pulse modulation or generation

H—ELECTRICITY

H01—ELECTRIC ELEMENTS

H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10

H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof

H—ELECTRICITY

H01—ELECTRIC ELEMENTS

H01S5/00—Semiconductor lasers

H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium

H01S5/068—Stabilisation of laser output parameters

H01S5/0683—Stabilisation of laser output parameters by monitoring the optical output parameters

H01S5/06835—Stabilising during pulse modulation or generation

H—ELECTRICITY

H04—ELECTRIC COMMUNICATION TECHNIQUE

H04B—TRANSMISSION

H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication

H04B10/50—Transmitters

H04B10/501—Structural aspects

H04B10/503—Laser transmitters

H04B10/504—Laser transmitters using direct modulation

H—ELECTRICITY

H04—ELECTRIC COMMUNICATION TECHNIQUE

H04B—TRANSMISSION

H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication

H04B10/50—Transmitters

H04B10/58—Compensation for non-linear transmitter output

H—ELECTRICITY

H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR

H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL

H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]

H05B45/30—Driver circuits

H05B45/31—Phase-control circuits

H—ELECTRICITY

H01—ELECTRIC ELEMENTS

H01S5/00—Semiconductor lasers

H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams

H01S5/042—Electrical excitation ; Circuits therefor

H01S5/0427—Electrical excitation ; Circuits therefor for applying modulation to the laser

H—ELECTRICITY

H01—ELECTRIC ELEMENTS

H01S5/00—Semiconductor lasers

H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium

H01S5/068—Stabilisation of laser output parameters

H01S5/06812—Stabilisation of laser output parameters by monitoring or fixing the threshold current or other specific points of the L-I or V-I characteristics

Description

PATENT SPECIFICATION ( 11) 1 566 809
2 ( 21) Application No 22171/77 ( 22) Filed 25 May 1977 ( 31) Convention Application No 51/060769 ( 19) ( 32) Filed 25 May 1976 in ( 33) Japan (JP) ( 44) Complete Specification published 8 May 1980 ( 51) INT CL 3 H 04 B 9/00 HOIS 3/10 HO 5 B 43/00 ( 52) Index at acceptance GIA AB C 10 C 13 Cl C 6 FA G 16 R 7 512 55 ( 54) DRIVING CIRCUITRY FOR DRIVING LIGHT EMITTING SEMICONDUCTOR DEVICES ( 71) We, FUJITSU LIMITED, a Japanese Corporation of No 1015, Kamikodanaka, Nakahara-ku, Kawasaki, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:-
This invention relates to driving circuitry for driving light emitting 5 semiconductor devices.
Semiconductor lasers, light emitting diodes and other such light emitting semiconductor devices can have merits such as small size, high efficiency, and easy direct modulation, for example, and they are considered to be promising for use as optical sources in optical communications systems At present, optical 10 communication systems employing such light emitting devices are being developed A possible demerit of such light emitting devices has been considered to be apparently short working lifetimes, but there have recently been reported a semiconductor laser having a lifetime of ten or several thousand hours and a light emitting diode of a longer lifetime still Further, it is reported that lifetimes for 15 semiconductor lasers and for light emitting diodes, estimated from the result of aging in a high temperature atmosphere, greater than one hundred thousand hours, and greater than one million hours, respectively, can be expected to be achieved.
Thus, light emitting devices such as semiconductor lasers, light emitting diodes, and so on, have been subject to rapid improvement and, with such 20 improvement in light emitting devices, performance of optical repeaters, for example, in optical communication systems, which repeaters are formed with such light emitting devices has also been improved, and quality requirements placed on those devices have also become severe Especially in relation to competition against wire communication systems employing coaxial cables, for example, optical 25 communication systems using light emitting devices, optical cables, etc must be constructed economically so far as possible, for example by decreasing the number of parts used for optical repeaters and other system components so as to achieve reduced power consumption.
On the other hand, for modulating light from light emitting devices such as 30 semiconductor lasers, light emitting diodes, or the like, in response to an input modulation signal, it is necessary to sufficiently grasp the dynamic characteristics of the light emitting devices Heretofore, there have been proposed some solutions to problems resulting from deterioration of optical waveforms, which deterioration is caused by phenomena such as relaxation oscillation and lasing delay time in the 35 light emitting devices These problems are encountered in the case of relatively high modulation speeds However, even in the case of medium and lower modulation speeds, and even if the peak value of a drive current is constant, it is found that the level of an optical output waveform does not remain unchanged.
Especially in cases where it is desired to obtain a continuous optical pulse output, 40 there is observed a phenomenon of a gradual optical output level decrease and in a case where the pulse width of the pulse drive current is relatively large, an output sag occurs in each optical pulse output of the output waveform Where such an optical pulse train is used, for example, in PCM communication, an eye diagram of received signals is degraded and the result is an increased error rate In other 45 words, transmission performance of the optical communication system is degraded.
According to the present invention, there is provided driving circuitry, for employing an input modulation signal to control electrical driving power supplied to a light emitting semiconductor device, including compensating means for counteracting thermal effects tending to alter the input/output characteristics of the device, the said compensating means being such that the waveform of a driving signal delivered to the device when the circuitry is in use is a modified version of that of the input modulation signal, whereby the optical output waveform of the device is rendered more similar to the waveform of the input modulation signal 5 than it would be if the waveform of the driving signal were to resemble that of the input modulation signal more closely.
Driving circuitry embodying the present invention can be constructed for a light emitting device so as to provide, in operation, an optical output waveform from the emitting device which faithfully corresponds to an input modulation 10 signal.
Driving circuitry embodying the present invention can compensate for deterioration in modulation characteristics of a light emitting device resulting from thermal phenomenon in the light emitting device.
Briefly stated, by the use of driving circuitry embodying the present invention, 15 a drive current, for example, supplied to a light emitting device has a waveform modified over that of an input modulation signal, corrected by the compensating means in the circuitry in order to correct for changes in optical output of the light emitting device with which the circuitry is used due to the thermal constant of the light emitting device The light emitting device is a semiconductor laser, or light 20 emitting diode for example The light emitting device can thus be driven by the drive current from the driving circuitry to obtain an optical output waveform proportional to the input modulation signal waveform.
For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the 25 accompanying drawings, in which:Figures 1 and 2 are graphs explanatory of the relationship between optical output from a light emitting device and drive current supplied to the device; Figures 3 to 5 are graphs explanatory of the optical output characteristics of a semiconductor light emitting device with respect to drive current thereof, in more 30 detail; Figure 6 A is a diagram of a thermal equivalent circuit of a semiconductor light emitting device; Figure 6 B is a graph illustrating optical output power and voltage characteristics of a semiconductor light emitting device in relation to the device 35 drive current; Figure 7 is a graph illustrating calculated values and measured values for optical output of a semiconductor light emitting device; Figure 8 is a graph illustrating calculated temperature variations of a junction portion of a semiconductor light emitting device; 40 Figures 9 A and 9 B are diagrams of equivalent circuits explanatory of step response of a semiconductor light emitting device; Figure 10 is a graph illustrating a transfer function.
Figures 1 IA and l l B illustrate the form of compensating circuits, for use in explanation of embodiments of the present invention; 45 Figure 12 is a graph explanatory of the relationship between compensating drive current and optical output, for use in explanation of embodiments of the present invention; Figures 13 A and 13 B illustrate respective forms of driving circuitry embodying this invention; 50 Figure 14 is a block diagram schematically illustrating another embodiment of this invention; Figure 15 is a diagram of a specific operative circuit in accordance with the embodiment exemplified in Figure 14; Figures 16 (a) to 16 (e) are waveform diagrams explanatory of the operation of 55 the embodiment of Figure 14; Figure 17 is a block diagram schematically illustrating yet another embodiment of this invention, and Figure 18 is a diagram of a specific operative circuit in accordance with the embodiment depicted in Figure 17 60 As shown in the graphs of Figure 1, even if the peak value of a drive current I for a light emitting device is constant, when the light emitting device is repeatedly driven at short time intervals, its optical output P gradually decreases in level.
Further, as depicted in the graphs of Figure 2, when the pulse width of the drive current I for a light emitting device is relatively large, a sag occurs in each pulse of 65 1,566,809 the optical output P of the light emitting device from the peak pulse value, and if the light emitting device is repeatedly driven with pulses of such relatively large width, the peak value of the pulses of the optical output P also gradually decreases.
A semiconductor laser has a drive current I vs optical output P characteristic such as is shown by way of example in Figure 3, and emits light when supplied with a drive current I According to our experimental results, a threshold current Ith is given by the following equation:
T 11 Ith=Ithexp(-) To ( 1) where T is the temperature of the junction portion of the semiconductor laser, and To and iths are constants and To 80 to 120 l Kl Further, Tj is expressed by Ti=Tjo+AT, ( 2) where AT is a temperature rise of the junction portion during operation.
Accordingly, the equation (I) becomes as follows:1 = I exp( j OT j) th ths TO = Ih exp(JJ/O T O ths I texp’Oh) expr»‘l) TO To A Tj 4-1 (A) 4 – Tn 21 TO Letting To= 100 l Kl and A Tj= 10 l Cl, AT/To=O 1 and (AT/To 0)2 = O 01 Accordingly, it follows that rjof A Tj Ith- Ith exp( -) ( 1 +T-) To TO Letting a threshold current at the time when Tj=Tjo be represented by Itho, it follows that -‘ths’ exp(TD/JO ths = It exp(TJ TO 0) = I th O O/TO ( 1 Aj) TO +I ( Tj) th O TO If Ith=Itho+Alth, the following equation is obtained from the above equation:
A Ti A Ith=Ith O () To TJO Itho=Iths exp(-) To where ( 3) 1,566,809 T AT 4 I O’) ( VT 0) ,,,e)p ( 30,,-,TO t 101 10 Assuming that the threshold current Itho before the temperature change is 150 m A, that the temperature change AT, of the junction during operation is 10 C and that To= 1000 K, then, from the equation ( 3), A Ith= 15 m A The inclination of the drive current I vs optical P characteristic above the threshold current does not change within the abovesaid temperature variation range 5 Accordingly, assuming that the drive current I is, for instance, 170 m A, that the optical output P is 7 5 m W and that an increase in the threshold current caused by a temperature change in the junction, A Ith, is 15 m A, the optical output from the light emitting device, when considered as excluding spontaneous emission light, falls to about 25 % of the optical output before the temperature change, as 10 illustrated in Figure 3.
As illustrated in Figure 4, in a case where pulses of drive current I above the threshold current are supplied to a semiconductor laser to derive therefrom the optical output P in the form of pulsed light, if the forward voltage of the semiconductor laser is 1 8 V and if the peak value of a modulation drive current is 15 m A, the following power 1.8 x 0 170 = 0 306 lWl is consumed at the junction portion of the semiconductor laser, as compared with the case where the drive current is zero.
The heat generated by the power dissipation is released to the outside But, 20 assuming that a thermal resistance of 300 C/W, the temperature rise A Tj of the junction portion is as follows:ATJ= 0 306 x 30 = 9 2 lPCl With such a temperature rise of the junction portion, the optical output changes as much as approximately 75 %, as described previously in connection with Figure 3 25 As shown in Figure 5, the optical output P provided when a DC drive current I rising up at the moment t is supplied to a semiconductor laser presents a simple exponential response, and the time constant r in this case is about 200 n S and the difference between the initial value and the convergent value of the optical output P is AP 30 Figure 6 A shows a thermal equivalent circuit of a semiconductor laser, in which the temperature of the junction portion is indicated by Ti; the temperature of a mount portion of the semiconductor laser is designated by Tm; the thermal capacity and the thermal resistance between the junction portion and the mount portion are identified by C, and by R,, respectively; and the thermal resistance 35 between the mount portion and the air is represented by Rm Fig 6 B shows a drive current I vs optical output P characteristic curve (P-I) and a drive current I vs.
voltage V characteristic curve (V-I) of the semiconductor laser The characteristic curve (P-I) rises up at the threshold current Ith and the characteristic curve (V-I) is nearly a straight line at a voltage VD 40 In Figure 7, there are shown calculated values (full lines) and measured values (broken lines) of relative values of the optical output obtained when a drive current having a modulation speed of 6 3 Mb/S and a pulse width of duty 50 % was supplied in a certain pattern to a semiconductor laser assumed to have a thermal equivalent circuit as shown in Figure 6 A and to have characteristics as shown in Figure 6 B 45 The results of calculation of temperature variations of the junction portion are shown in Figure 8.
As illustrated in Figure 7, the calculated values and the measured values are well coincident with each other and Figure 8 clarifies the pattern of temperature variation of the junction portion of the light emitting device in response to the 50 application of a pulse pattern as shown in Figure 7 Thus, it has been found that the pattern effect of temperature variation is due to thermal effects due to changes in modulation power, and it has been shown that the model assumed in Figures 6 A and 6 B is correct Further, the generation of sag in the optical output waveform, such as shown in Figure 2, can also be explained in terms of a thermal equivalent 55 circuit having a given thermal time constant In other words, (I) when the modulation output varies, ( 2) the temperature of the junction portion changes ( 3) to raise the threshold value, ( 4) causing a change in the optical output These changes in terms of time are dependent mainly upon the thermal resistance R, and the thermal capacity C, between the junction portion and the semiconductor laser 60 mount portion Accordingly, the abovesaid changes are determined and 1,566,809 characterised by a thermal time constant dependent upon the thermal resistance Rj and the thermal capacity C.
Then, in accordance with an embodiment of the present invention, the variation in the optical output response resulting from the above described thermal causes is suppressed by means of control of drive current supplied to the light 5 emitting device, thereby to obtain a constant optical output response characteristic As depicted in Figure 5, the optical output P, when a rectangular drive current I is supplied to a semiconductor laser, is such that it gradually decreases from its initial value to the convergent value (i e decreases by AP) An electric circuit representing the change in the waveform of the optical output is 10 such as is shown in Figures 9 A and 9 B That is, in Figure 9 A, a resistor R Ia is connected in series with a parallel circuit consisting of a resistor R 2 a and a capacitor C 2 a, and supplied with a current Pl from a constant voltage source la, and a change in the current Pl corresponds to the change in the optical output P In Figure 9 B, a resistor Rlb is connected in parallel with a series circuit consisting of 15 an inductance L 2 b and a resistor R 2 b and a current P 2 flowing in the resistor R Ib from a constant current source lb corresponds to the change in the optical output P.
The transfer function H(jw) of the current Pl or P 2 flowing from the voltage source la or the current source lb to the resistor Rla or Rib is given as follows: 20 1 +jiwo T 2 H(jco)=k 1 ( 4) In Fig 9 A, Rla+R 2 a Rla R 2 a T 2 =C 2 aR 2 a and T 1 =C 2 a Rla+R 2 a R 2 b L 2 b L 2 b and, in Figure 9 B, k= T 2 = and T 1 = Rlb+R 2 b R 2 b Rlb+R 2 b In either case, the relation T 2 >T 1 holds 25 In Figure 10, the solid line indicates the abovesaid transfer function H(j W) and the abscissa represents an angular frequency a, 1 Rlb+R 2 b 1 R 2 b a, and co 2 T, L 2 b T 2 L 2 b Such a characteristic can be compensated for by adding a circuit having a characteristic as indicated by the broken line in Figure 10 Circuits of 30 characteristics as indicated by the broken line in Figure 10 are shown in Figures ll A and 1 l B, corresponding to Figures 9 A and 9 B, respectively In Figures 1 l A and Il B, reference characters Rlc and R 2 c indicate resistors; L 2 c designates an inductance; Rld and R 2 d identify resistors; and C 2 d denotes a capacitor; and la and lb are, respectively, a constant voltage source and a constant current source 35 The following will describe how circuitry embodying the present invention can be constructed in connection with the case of Figures 9 B and l IB In order that the variation in the optical output P with respect to the drive current I such as shown in Figure 5 may be caused to have a characteristic as depicted in Figure 12, it is sufficient only to select the initial value of the drive current I smaller than a 40 convergent value 10 by AI The ratio AP/PO of a change value AP to the initial value PO of the optical output in Figure 5 is as follows:AP R 2 b ( 5) PO Rlb+R 2 b 1,566,809 Is 6 1,566,809 6 The time constant T is as follows:1 L 2 b T=T, ( 6) co 1 Rlb+R 2 b Further, as is evident from Figure 10, if R 2 b R 1 +R 2 b and 6 ol are firstly determined, then the constants for use in the compensation 5 circuit are dependent upon the following formulae:Rld R 2 b AP ( 7) Rld+R 2 d Rlb+R 2 b PO L 2 b C 2 d(Rld+R 2 d) x ( 8) Rlb+R 2 b That is, by measuring AP and T in the step response of Figure 5, relationship equations for the three constants of a compensation circuit following the formulae 10 ( 7) and ( 8) can be obtained.
The incapability here of determining all the constant absolutely is a natural consequence of the simulation between different physical constants (the optical output and current), and does not prevent the construction of circuitry for improving modulation characteristic Namely, the example of Figure 1 l B indicates 15 that desired one of three unknown circuit constants can be determined by the designer at will; it can be determined as desired in accordance with other conditions of the circuit to be realized, and that the other constants may then be determined by the formulae ( 7) and ( 8) Also, in Figure 1 IA, the constants of the compensation circuit can be determined by means similar to those described 20 above.
Figures 13 A and 13 B illustrate specific embodiments relating to combinations of Figures 9 A and 11 A, and Figures 9 B and 1 B, respectively In each Figure reference character IN indicates a modulation signal input terminal; Q designates a transistor; D identifies a light emitting element; and CP denotes a compensation 25 circuit, which in Figures 13 A and 13 B are respectively identical with the structures of Figures 1 IA and 1 l B Reference characters +E and -E represent power sources.
Figure 13 A illustrates a case where the light emitting element D is driven in a constant voltage source configuration The initial value of the drive current is controlled by the compensation circuit CP as shown in Figure 12, by means of 30 which the optical output P is provided corresponding to the waveform of a modulation signal applied to the input terminal IN Figure 13 B shows a case where the light emitting element D is driven in a constant current source configuration, and the modulation characteristic is improved as is the case with Figure 13 A.
Figure 14 is a block diagram illustrating another embodiment of this invention 35 Reference numeral 10 indicates a modulation signal input terminal; 11 designates a phase shifter, for example, a phase control circuit; 12 is an amplifier; 13 is an adder:
14 is an integration circuit; 15 and 16 are amplifiers; and 17 is a light emitting device (a semi-conductor light source or light emitting device) A modulation signal such as a PCM signal, for example, is applied from the input terminal 10 to the 40 phase control circuit 11 and the integration circuit 14, and a phase and amplitude controlled signal and an integrated and amplified signal are added together in the adder 13 The resulting added output is amplified by the amplifier 16, and applied to the light emitting element 17.
The time constant of the integration circuit 14 is selected in agreement with 45 the time constant which is dependent upon the thermal resistance R, and the thermal capacity C, of the thermal equivalent circuit shown in Figure 6 A relating to the light-emitting device used Such a time constant can be obtained easily by observing the step response of the optical output from the light emitting device 17 by means of an oscilloscope for example 50 Figure 15 illustrates a specific operative circuit corresponding to the embodiment depicted in block diagrammatic form in Figure 14 Reference characters Qi and Q 2 indicate transistors: Rll to R 18 are resistors; Cll to C 15 are capacitors; L is a coil; Dl is a diode; and LD is a light emitting element The functions of the phase control circuit 11 and the amplifiers 12 and 15 are 5 accomplished by the transistor Q 1, and the adder 13 is formed with the resistor R 17, the coil L and connection wirings The integration circuit 14 is made up with the resistor R 13 and the capacitor C 14, and the amplifier 16 is formed with the transistor Q 2.
Figure 16 is explanatory of the operation of the circuit shown in Figure 15 10 Assuming that a modulation signal was as depicted in Figure 16 (a), the optical output, which would be produced when the light emitting device was driven by a drive current proportional to the modulation signal waveform, would take the form shown in Figure 16 (b), as described previously with regard to Figures 1 and 2 Now, in the circuit of Figure 15, the output resulting from integration of the abovesaid 15 modulation signal by the integration circuit 14 is as shown in Figure 16 (c) The output signal from the amplifier 12 amplifying the output from the phase control circuit 11, and the output signal from the amplifier 15 amplifying the output from the integration circuit 14, are added together in the adder 13 to provide a signal waveform as shown in Figure 16 (d) This added output signal is amplified by the 20 amplifier 16 to provide a drive current for the light emitting element 17.
The part 18 of the signal waveform shown in Figure 16 (d) is an inclined part which compensates for the sag of the output waveform of Figure 16 (b), and the part 19 is a part for averaging the temperature variations of the junction portion to suppress fluctuation of the optical output By driving the light emitting device 17 25 with such a drive current as is shown in Figure 16 (d) an optical output shown in Figure 16 (e) is obtained.
Figure 17 illustrates in block form another embodiment of this invention.
Reference numeral 20 indicates a modulation signal input terminal; 21 is a phase shifter for example, a phase control circuit; 22, 25, 26, 30 and 32 are amplifiers; 23 30 is an adder; 24 is an integration circuit; 27 is a light emitting device; 28 is a light receiving element (an optical detector or a light detector); 29 is an integration circuit; and 31 is a subtractor The present embodiment is provided by adding a negative feedback loop to the embodiment of Figure 14, for stabilizing the optical output One portion of the optical output from the light emitting device 27 is 35 directed to the light receiving element 28, the output from which is integrated by the integration circuit 29 The time constant of the integration circuit 29 is selected to be sufficiently larger than the time constant of the integration circuit 24 for integrating the input modulation signal The integrated output from the integration circuit 29 is amplified by the amplifier 30, and then applied to the subtractor 31 to 40 obtain the difference between the amplified input and a signal from the amplifier 25 amplifying the modulation signal integrated output The difference signal thus obtained is amplified by the amplifier 32, and then applied to the adder 23 With such a negative feedback loop, the optical output from the light emitting device 27 is stabilized with respect to ambient temperature change Moreover, by integrating 45 the modulation signal and driving the light emitting device 27 with a signal waveform in which the integrated output is added to the modulation signal, the modulation characteristic can be improved.
Figure 18 shows a specific circuit construction corresponding to the embodiment of the present invention depicted in block diagrammatic form in 50 Figure 17 Reference characters R 21 to R 33 indicate resistors; C 21 to C 29 are capacitors; Q 21 to Q 23 are transistors; DFA is a differential amplifier; LD is a light emitting element; PD is a photodiode or like light receiving element; D 21 is a diode; and L is a coil The functions of the phase control circuit 21 and the amplifiers 22 and 25 of Figure 17 are all performed by the transistor Q 21, and the 55 amplifier 26 of Figure 17 for supplying the drive current to the light emitting device LD is formed with the transistor Q 22 The adder 23 is made up of the coil L, the resistor R 28 and connection wirings The integration circuit 24 is formed of resistors R 31 and R 32 and the capacitor C 28 The subtractor 31 and the amplifiers 30 and 32 comprise the transistor Q 23, the differential amplifier D} A the resistors 60 R 30 to R 32 and the capacitor C 28 The integration circuit 29 is composed of the resistor R 33 and the capacitor C 29 The light receiving element PD is disposed in the vicinity of the light emitting device LD, and supplied with one portion of the optical output from the light emitting device LD Where the light emitting device LD is a semiconductor laser, it may be disposed so that an optical output on the 65 a 7 _ 1,566809 opposite side from a main optical output is incident upon the light receiving element PD.
In the foregoing embodiments, the light emitting device is driven by a drive current which is compensated in correspondence to the integrated value of the modulation signal It is additionally possible to superimpose DC of a suitable 5 magnitude so that the drive current is made up of a signal current (which is compensated as described above) with a superimposed DC current.
As has been described in the foregoing, an optical output waveform which is proportional to an input modulation signal waveform can be obtained by controlling the drive current for a light emitting device, which device is a 10 semiconductor device such as a semiconductor laser, a light emitting diode or the like, in such a manner as to compensate for a change in the optical output caused attributable to the operational thermal time constant of the light emitting device.
Accordingly, the modulation characteristic in a system in which the light emitting device is modulated directly by a drive current can be improved 15 Thus, driving circuitry embodying the present invention, for driving a light emitting device such as a semiconductor laser, a light emitting diode, or the like, is, for example, operable to supply drive current to the device which is modulated so as to modulate directly the optical output of the light emitting device, in response to an input modulation signal The drive current waveform is derived so as to 20 correspond to an integrated value of the input modulation signal, thereby to compensate for deterioration of the modulation characteristics of the light-emitting device due to thermal causes, whereby a desired optical output waveform can be obtained.

Claims (14)

WHAT WE CLAIM IS: 25

1 Driving circuitry, for employing an input modulation signal to control electrical driving power supplied to a light emitting semiconductor device, including compensating means for counteracting thermal effects tending to alter the input/output characteristics of the device, the said compensating means being such that the waveform of a driving signal delivered to the device when the 30 circuitry is in use is a modified version of that of the input modulation signal, whereby the optical output waveform of the device is rendered more similar to the waveform of the input modulation signal than it would be if the waveform of the driving signal were to resemble that of the input modulation signal more closely.

2 Circuitry as claimed in claim 1, wherein the said compensation means 35 include an input signal integration circuit, the circuitry being operable to deliver driving current, constituting the said driving signal, derived from a combination of the said input modulation signal with an integrated version thereof provided by the said input signal integration circuit.

3 Circuitry as claimed in claim 2, wherein the said driving current comprises a 40 DC component of selected value superimposed upon a signal current derived from the said combination.

4 Driving circuitry as claimed in claim 1, 2 or 3, in combination with, and operatively connected to, such a semiconductor device.

5 Circuitry as claimed in claim 4, read as appended to claim 2, wherein the 45 said input signal integration circuit has a characteristic time constant substantially equal to the thermal time constant, as hereinbefore defined, of the lightemitting semi-conductor device.

6 Circuitry as claimed in claim 4 or 5, wherein the said compensating means include a light detecting device arranged to receive light from the optical output of 50 the light emitting device and to deliver an electrical detection signal, dependent upon the intensity of the received light, to a detection signal integration circuit of the compensating means, the said driving signal being dependent upon an output from the detection signal integration circuit.

7 Circuitry as claimed in claim 6, constructed in such a manner that, when it is 55 in use, the said driving signal is dependent upon the difference between two signals that are produced respectively by integration of the input modulation signal and the said electrical detection signal.

8 Circuitry as claimed in either of claims 6 and 7 when they are read as appended to claim 2, wherein the said detection signal integration circuit has a 60 characteristic time constant larger than that of the input signal integration circuit.

9 Circuitry as claimed in claim 4, comprising a constant current source circuit connected to supply driving current to the light emitting semiconductor device by way of a resistor that is connected in series with the device and forms part of the 1.566 809 said compensating means which include, connected in parallel with the series combination of the said resistor and light emitting semiconductor device across the output of the constant current source circuit, a further resistor and a capacitor connected in series therewith.

10 Circuitry as claimed in claim 4, comprising a constant voltage source 5 circuit connected to supply driving current to the light emitting semiconductor device by way of first and second resistors that are connected in series with the device and form part of the said compensating means, which further include an inductor connected in parallel with the said first resistor.

11 Driving circuitry as claimed in any preceding claim, wherein the said light 10 emitting semiconductor device is a semiconductor laser.

12 Driving circuitry as claimed in any one of claims I to 10, wherein the said light emitting semiconductor device is a light emitting diode.

13 Driving circuitry, for employing an input modulation signal to control electrical driving power supplied to a light emitting semiconductor device, 15 substantially as hereinbefore described with reference to Figure 1 IA or Figure 13 A, or as described with reference to Figure 1 l B or Figure 13 B, or as described with reference to Figures 14 and 15 or Figures 17 and 18, of the accompanying drawings.

14 Driving circuitry as claimed in claim 13, in combination with, and 20 operatively connected to, such a light emitting semiconductor device.
HASELTINE, LAKE & CO, Chartered Patent Agents, Hazlitt House, 28, Southampton Buildings, Chancery Lane, London WC 2 A IAT also Temple Gate House, Temple Gate, Bristol BSI 6 PT and 9, Park Square, Leeds LSI 2 LH, Yorks.
Agents for the Applicants.
Printed for Her Majesty’s Stationery Office, by the Courier Press, Leamington Spa, 1980 Published by The Patent Office, 25 Southampton Buildings London WC 2 A l AY, from which copies may be obtained.
1,566,809

GB22171/77A
1976-05-25
1977-05-25
Driving circuitry for driving light emitting semiconductor devices

Expired

GB1566809A
(en)

Applications Claiming Priority (1)

Application Number
Priority Date
Filing Date
Title

JP51060769A

JPS5851435B2
(en)

1976-05-25
1976-05-25

Light emitting element driving method

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GB1566809A
true

GB1566809A
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1980-05-08

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ID=13151808
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Title
Priority Date
Filing Date

GB22171/77A
Expired

GB1566809A
(en)

1976-05-25
1977-05-25
Driving circuitry for driving light emitting semiconductor devices

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(1)

US4149071A
(en)

JP
(1)

JPS5851435B2
(en)

CA
(1)

CA1075316A
(en)

DE
(1)

DE2723419C2
(en)

FR
(1)

FR2353198A1
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GB
(1)

GB1566809A
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patent/JPS5851435B2/en
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1977

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US
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Expired – Lifetime

1977-05-20
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Granted

1977-05-24
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CA279,041A
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1977-05-24
DE
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Also Published As

Publication number
Publication date

FR2353198A1
(en)

1977-12-23

US4149071A
(en)

1979-04-10

CA1075316A
(en)

1980-04-08

DE2723419C2
(en)

1985-05-09

JPS5851435B2
(en)

1983-11-16

JPS52143790A
(en)

1977-11-30

DE2723419A1
(en)

1977-12-01

FR2353198B1
(en)

1981-11-13

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