AU618339B2 - Processes and devices to control a laser diode fitted in an optical transmitter of a telecommunication system - Google Patents

Description

619339 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION NAME ADDRESS OF APPLICANT: Krone Aktiengesellschaft Beeskowdamm 3-11 D-1000 Berlin 37 Federal Republic of Germany NAME(S) OF INVENTOR(S): Sungur AYTAC Hon HUYNH ADDRESS FOR SERVICE: DAVIES COLLISON Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED: Processes and devices to control a laser diode fiited in an optical transmitter of a telecommunication system The following statement is a full description of this invention, including the best method of performing it known to me/us:j_ 1_ ~I -2- The invention concerns a process to control laser diodes used in an optical transmitter of a telecommunication system, as well as the _.arrangements to conduct such a process.

Nowadays, telecommunication systems often use light guides for optical transmission. The optical transmitter uses multi-mode or single-mode laser diodes as light sources. For reasons of operating stability through suppression of the modal noise, laser diodes are preferred for multi-mode transmission lines which emit a laser light with a multitude of spectral o lines. Multi-mode laser diodes with passive wave guidance K.ndex-guided laser) are preferred for reasons of stability and long service life.

However, these generate only a very narrow-band spectrum. This is not favourable due to the high sensitivity of the entire transmission towards reflected light waves and intense modal noise which adversely influence transmission quality.

A customary method (see DE-34 29 255-Al) to artificially widen the emitted spectrum is the superimposition mode. It involves superimposing a high- -frequency current on the direct current that is supplied to the laser diode for its operation. This modulates the laser process with high speed in the sense of an ON/OFF operation, and the longitudinal mode of the laser becomes multi-mode. For practical operation of a corresponding optical transmitter in a telecommunications system, the intelligence signal fed to the laser diode is superimposed by a high-frequency signal.

However, this results in a difficulty resulting from the fact that the output signal, obtained in this manner, is subject to such intense jitter that, in many cases, dependable transmission is no longer possible.

The situation can be improved somewhat by increasing the frequency of the superimposed high-frequency signal. But this soon results in limits that must not be exceeded. The reason for this is that the frequency of the high-frequency signal should not exceed the twofold of the resonant frequency of the given laser diode to ensure an adequate degree of modulation, and thus sufficient noise reduction.

-3 Consequently, the purpose of the invention is to present a course, based on the previously described process, which will secure adequate jitter freedom of the modulation signal, even when high demands are expected of the transmission quality.

According to the present invention, there is provided a process to control a laser diode used in an optical transmitter of a telecommunicatios system, in which the modulation signal for a laser diode that is to be controlled is obtained by superimposing on an intelligence signal a high-frequency signal with a frequency that is less than approximately double the value of the resonant frequency of the laser diode, |11 I C i C• 15 characterised by the fact that, a fixed phase relationship is maintained between the intelligence signal and the high-frequency signal, whereby, in use, said fixed phase relationship is effective to q k 9,/U r-iCy superimpose the in'igene,~ignal on a pulse of the intelligence signal for a period of time less than a pulse-width of the pulse such that the high-frequency signal is suppressed adjacent each edge of the pulse of the intelligence signal.

The invention will now be explained by way of example only on the basis of drawings which present the preferred configuration examples that clearly indicate the characteristics and advantages of the invention. The following is shown by t' drawings: Fig. 1 A block diagram for the construction of an arrangement to contrA a laser diode according to a first configuration example of the invention.

Fig. 2 A corresponding block diagram for an arrangement to control a laser diode according to a second configuration example of the invention.

Fig. 3 A corresponding block diagram for an arrangement to control a laser diode according to a third configuration example of the invention.

Fig. 4 The possibility to implement a signal conditioning circuit, within the framework of the block diagrams of Figs. 2 or 3, on the basis of a block diagram.

Fig. 5 Time diagrams to illustrate the signals arising at various points of the circuit diagram of Fig. 4.

Fig. 6 A corresponding diagram presentation with special consideration given )to the configuration example in Fig. 3.

910715,vrsspe.010,krone,3 0 7- 4. The block diagram of the first configuration example of the invention, shown' in Fig. 1, is based on the cognition that jitter freedom of a modulation signal of high frequency fHF compared with the frequency fD of the intelligence signal SD, obtained by superimposing a highfrequency signal SHF on a intelligence signal SD, can be achieved if there is a fixed phase relationship between the intelligence signal SD and the high-frequency signal SHF* The circuit shown in Fig.l maintains such a fixed phase relationship between intelligence signal SD on the one hand, and high-frequency signal SHF on the other.

The arrangement shown in Fig. 1 is intended for the control of a laser diode L that forms part of an optical transmitter for a telecommunication system. For this purpose, the arrangement in Fig. 1 features a laser driver circuit 1 which supplies the laser diode L with the corresponding S" control currents.

f, The laser driver circuit is supplied at a first input with the highfrequency signal SHF from a high-frequency signal generator 2, while O* an intelligence signal S D is applied to the second input of the laser t driver circuit 1. The high-frequency signal SHF is superimposed by use of a mixer on the intelligence signal SD in the laser driver circuit 1 to obtain the modulation signal for the control of the laser diode L.

The fixed phase relationship between the signals supplied to the laser driver circuit 1, namely between the intelligence signal S D and the high-frequency signal SHF from the high-frequency signal generator 2, is adjusted in the described configuration example by an appropriate control of the high-frequency signal generator 2 designed in this case as a voltage-controlled oscillator with the help of a phasecontrol circuit. This phase-control circuit contains a frequency divider 3 that is supplied with the high-frequency signal SHF from the high-frequency signal generator 2, and of a phase detector 4 connected to the latter by a first input. This phase detector 4 is supplied at a second input with a clock signal S T derived from the intelligence signal SD.

The frequency divider 3 is designed in such a manner that it divides the high-frequency signal SHF it receives from the high-frequency signal generator 2 by the factor 1/N down to the frequency of the clock signal ST. The signal SN, derived in this manner from the high-frequency signal SHF' is supplied to the phase detector 4 at the first input connected with the frequency divider 3. i The phase position of the signals SN and Sr is compared in the phase detector 4. The result of this phase comparison is supplied, via the output of the phase detector 4, to the high-frequency signal generator 2 as a control signal in the form of a control voltage U. The control voltage U is therefore a function of the difference in phase between the signals SN and Sr and may be expressed as U This ensures a fixed phase relationship between the intelligence signal S. and the high-frequency signal Sp supplied by the high-frequency signal generator 2 to the laser driver circuit 1. This is because the high-frequency signal generator 2 is rigidly controlled in the cycle of the intelligence signal SD from the phases of the frequency divider 3 and the phase detector 4 supplied by way of the phase control circuit.

The intelligence signal S D is supplied to the second input of the laser driver circuit 1 via an interposed time-delay circuit 5. Its delay T ensures a compensation of the operating time in the phase control circuit from the frequency divider 3 and the phase detector 4.

15 The block diagram shown in Fig. 2 presents a configuration example of the 7 invention which operates by controlling with start/stop pulses.

The arrangement shown in Fig. 2 is intended for the operation of a laser diode L in an optical transmitter of a telecommunications system. Once again, to supply the laser diode I, with the necessary control currents a laser driver circuit 1 with a mixer is used which is supplied at its first input with the high-frequency signal delivered from the high-frequency signal generator 2. A second input of the laser driver circuit 1 is supplied with the intelligence signal SD at a frequency fD that is small compared with the frequency f. of the high-frequency signal SHF.

|1 In the configuration example of Fig. 2, the high-frequency signal generator 2 is operated under the control of a start/stop pulse generator 7; a high-frequency signal generator 2, connected on the line side, suppiesses at least one pulse of the high-frequency signal Sjm at the output of the high-frequency signal generator 2, following each positive or leading edge of the intelligence signal So, and in front of each negative or trailing edge of the intelligence signal SD, thereby making it ineffective for signal superimposition in the laser driver circuit 1. Fig. 2. indicates the start/stop pulses with the symbol 0 910715,vrapc)10, LonO5 i i I The start/stop pulses SI are generated in the start/stop pulse generator 7 which is controlled with the help of the intelligence signal SD; a signal conditioning circuit 6 is connected on the line side of the start/ stop pulse generator 7; it provides the necessary reference of the start/stop pulses SI to the given edges of the intelligence signal S

D

The corresponding control signals for the start/stop pulse generator 7 from the signal conditioning circuit are indicated in Fig.2 by the symbol S The intelligence signal SD is fed to the second input of the laser driver circuit 1 by way of a time-delay circuit 5' which provides for a compensation of the operating time in the signal conditioning circuit 6.

S* Fig. 4 shows a block diagram for the implementation of the arrangement of the signal conditioning circuit 6 and the connected start/stop pulse generator 7 of Fig. 2; the signals at the input and output of the individual module stages have also been entered.

The circuit example, shown in Fig. 4, has an input that is supplied with the intelligence signal SD, and two outputs where the start/stop pulses SI and S can be tapped for the control of the high-frequency t. signal generator 2 in Fig. 2.

The input supplied with the intelligence signal S D in Fig. 4 is followed by ar inverter 11 which supplies an OR circuit 12 directly at a first input, and indirectly at a second input, by interposing a time-delay circuit 13 which causes a delay 1 corresponding to doubl, the period 2THF of the high-frequency signal SHF with the inverted ir -elligence signal SD and SD The OR circuit 12 supplies to its inverted output a signal SD* to the first input of an exclusive-OR circuit 15; it is also supplied at a second input, by way of a time-delay circuit 14 with 'a delay 2 of a single period of time T of the high-frequency signal SHF, with the intelligence signal SD from the corresponding circuit input, i.e. with a signal S D 2 Start/stop pulses nd ~d can be tapped at both outputs of the exclusive-OR circuit 15 to control the high-frequency signal generator 2 in Fig. 2.

O "7 n i -:IJ-c i i i- ii-: ii-- i i. The time relationship between the different signals in Fig. 4 is shown as a diagram in Fig. 5; the latter also shows the period of time THF p o se Lo"A K

HF

of the high-frequency signal SHF, and the e 4 ed T D of the intelligence signal

SD.

Fig. 3 shows a block diagram of the third configuration example of an arrangement, in conformity with the invention, to control a laser diode L, that is fed with an intelligence current IL, and forms an integral part of an optical transmitter for a telecommunication system; for this purposes it uses a control current I which is coupled, by way of a a capacitor C, in the line conducting the current I L to the laser diode L.

Tnis third variant of the invention operates according the method of mixing an intelligence signal SD on the one hand, and the high-frequency o0°0, signal SHF on the other; the latter signal is generated by a high-fre-

HE

°o quency signal generator 2; with this configuration example, however, the high-frequency signal SF is applied to the input of a mixer 8, Sdesigned as a ring mixer. By way of a second input, the mixer 8 is sup- 99 g4 Splied with the signal SD* from a signal conditioning circuit the output of the mixer S supplies a mixed signal S to a time-delay cira cuit 9, from where this signal is transferred, via a matching network t~ 4 o, to the capacitor C; from here it is coupled in the form of the current I a into the line conducting the current I L to the laser diode L.

An input of the signal conditioning circuit 6' in Fig. 3 is supplied with the intelligence signal SD; its principal purpose is to shorten the pulse width of this intelligence signal S in order to generate D

C

the signal SD*. Fig. 4 and the diagrams in Fig. 5 indicate the details concerning the shortening of the pulse-width for the intelligence signal SDI inasmuch as these relate to the generation of the signal SD or refer to the signal SD The production of the mixed signal S from the pulse-width shortened a intelligence signal SD of the signal conditioning circuit 6' on the one hand, and from the high-frequency signal SHF of the high-frequency signal generator 2 on the other hand, is performed in the customary manner, and can be seen from the diagram in Fig. 6 without any further explanations. The time-delay circuit 9, series-connected on the load side of the mixer 8 and supplied with the mixed signal Sa, causes a signal delay by a period of time T 1 F of the high-frequency signal SHF.

i -8- This results in a phase centering of the mixed signal S a in relation to the intelligence signal SD. This is indicated in the diagram of Fig. 6 through the corresponding position of the current pulses for the two superimposed currents I and I

L

The purpose of the matching network 10 in Fig. 3 is to increase the mixed signal S from the mixer 8 to a processable signal magnitude, a for instance by corresponding amplification, before this mixed signal S is coupled in the conducting line to the laser diode L in the form a of the current Ia a The reference numerals in the following claims do not in any way limit the scope of the respective claims.

i-:i

Claims (7)

1. A process to control a laser diode used in an optical transmitter of a telecommunications system, in which the modulation signal for a laser diode that is to be controlled is obtained by superimposing on an intelligence signal a high-frequency signal with a frequency that is less than approximately double the value of the resonant frequency of the laser diode, characterised by the fact that, a fixed phase relationship is maintained between the intelligence signal and the high-frequency signal, whereby, in use, said fixed phase relationship is effective to ~iik 4"CrGlue--c/ superimpose the ntelligenc.signal on a pulse of the intelligence signal for a period of time less than a pulse-width of the pulse such that the high-frequency signal is suppressed adjacent each edge of the pulse of the intelligence signal.

2. Process according to claim 1, whereby the high-frequency signal superimposed on the intelligence signal is suppressed adjacent each edge of the pulse for a short period of time, in comparison with the pulse-width.

3. Process according to claim 1, characterised by the fact that it is controlled for the achievement of a fixed phase relationship between the intelligence signal and the high-frequency signal, the generation of which is the result of a phase comparison between a signal, derived from the high-frequency signal, and a clock signal taken from the intelligence signal.

4. Process according to claim 1, characterised by the fact that the high-frequency signal is suppressed after each positive pulse edge, and in front of each negative pulse edge of the intelligence signal, on each occasion for the period of time of a pulse of the high-frequency signal.

5. Process according to claim 1, characterised by the fact that the intelligence signal is initially reduced in its pulse-width by a small fixed value, and is then mixed with the high-frequency signal. 91715,vrsspe.010,1o i9 10

6. Arrangement to conduct the process according to claim 1 or 2, with a high-frequency signal generator to generate the high-frequency signal, and with a laser driver circuit which is supplied by way of a first input with the intelligence signal, and by way of a second input with the high-frequency signal from the high-frequency signal generator, and which delivers the modulation signal, gained from the superimposition of the intelligence signal and the high-frequency signal, to the laser diode that is to be controlled, characterised by the fact that a frequency divider is connected on the load side of the high-frequency signal generator; it divides the frequency of the high-frequency signal down to the frequency of the clock signal from the intelligence signal; the signal derived in this manner is fed to a phase detector, that is additionally supplied with the clock signal, and which supplies the result of a phase 91071,vsp01 OIronc,1 -ll- comparison between the signals (SN and S T it has received to the high- -frequency signal generator as a control voltage and that a time-delay circuit is connected on the line side of the input of the laser driver circuit that is fed with the intelligence signal which delays the intelligence signal (SD) by a fixed period (t) before it is fed into the laser driver circuit for superinosition with the high-frequency signal (SHF); the delay corresponds with the modulation time of the high-frequency signal generator by way of the phase detector 1 5' Arrangement to conduct the process according to Claim 1 or 3, with a high-frequency signal generator to generate the high-frequency signal, and with a laser driver circuit which is supplied by way of a first input with the intelligence signal, and by way of a second input with the high-frequency signal from the high-frequency signal generator, and which supplies the modulation signal gained from the superimposi- tion of the intelligence signal and the high-frequency signal to the laser diode that is to be controlled, characterized by the fact that a signal conditioning circuit and a start/stop pulse generator (7) are connected on the line side of the high-frequency signal generator of which the signal conditioning circuit is supplied with the d intelligence signal and delivers it with shortened pulse width to the start/stop pulse generator which, in turn, supplies correspond- ing start/stop pulses in the width of the shortened pulse duration, but in reversed polarity, to the high-frequency signal generator S here it causes a pulse suppression for the high-frequency signal (SHF both after each ieading edge, and in front of each trailing edge, of the intelligence signal (SD) and that a time-delay circuit is connected on the line side of the input of the laser driver circuit that is fed with the intelligence signal which delays the intelligence signal (S D by a fixed period before it is fed into the laser driver circuit for superimposition with the high-frequency signal (SHF); the delay corresponds with the modulation time of the high-frequency signal generator by way of the signal conditioning Scircuit and the start/stop pulse generator 7 0* C i:t K~i -12-- }7 Arrangement to conduct the process according to Claim 1 or 4 with a high-frequency signal generator to generate the high-frequency signal, characterized by the fact that a mixer is connected on the load side of the high-frequency signal generator it is supplied by way of a second input and via the signal conditioning circuit that is fed with the intelligence signal (SD), with an intelligence signal that is shortened in its pulse width by a fixed period of time 2 TRF); the mixed signal gained by mixing the shortened intelligence signal with the high-frequency signal (SHF), is conducted by way of a time-delay circuit for a fixed signal delay (THF) and a matching network and coupled via a capacitor and superimoosed, in the form of a current (I on the aer on- current (I L by the laser diode which is to be controlled. 4 4 a t fl S# i' 11 -i I

13- 9. A method of operating a laser diode for use in an optical telecommunications transmitter, comprising the step of superimposing high frequency modulation on an input signal to the laser diode so as to achieve a multimode laser diode output, wherein a predetermined phase relationship is maintained between the modulation and the input signal, such that the modulation signal is suppressed after a leading edge of a pulse of the input signal and in front of a trailing edge of said pulse. Apparatus for use in a method according to either ene ef claim 9-e 9 comprising superimposition means for superimposing high frequency modulation on an input signal to a laser diode, comparison means for comparing the relative phases of the modulation and the input signal, and phase adjustment means for adjusting the phase relationship between the modulation and the input signal to a predetermined phase relationship such that the modulation signal is suppressed after a leading edge of a pulse of the input signal and in front of a trailing edge of said pulse. 11. A process to control a laser diode, substantially as hereinbefore described with reference to any one of the accompanying drawings. 12. An apparatus for use in operating a laser diode, substantially as hereinbefore described with referenre to any one of the accompanying drawings. DATED this 15th day of July, 1991 KRONE AKTIENGESELLSCHAT By its Patent Attorneys DAVIES COLLISON -91071,vrsspe.010,krone,13 nl