GB2219165A

GB2219165A - Optical transmission systems

Info

Publication number: GB2219165A
Application number: GB8812647A
Authority: GB
Inventors: Adrian Perrin Janssen; Herbert Hayeem Nathan
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-05-27
Filing date: 1988-05-27
Publication date: 1989-11-29
Also published as: GB2219165B; GB8812647D0

Abstract

Automatic feedback control of the output power of a semiconductor laser employed in a transmitter of an optical transmission system, which also includes a receiver, involves use of both the transmitter and the receiver. The mean value of the power, as transmitted via a fibre (23), received by a photodiode (27) is compared with a predetermined value and the output of the laser (25) adjusted to achieve equalisation therewith. The necessary return link between the photodiode and the laser may comprise a second fibre, particularly the other fibre of a two-fibre duplex optical link, or may use the same fibre (23). The two fibres of such a link transmit respective data signals, output from respective semiconductor lasers, in opposite directions between the ends of the link. Each transmitted data signal is modulated at a low frequency whose value is indicative of the necessary change in the output of the semiconductor laser, at the other end of the link to the laser that is transmitting the respective data signal, which is required to equalise the received power with the predetermined level. <IMAGE>

Description

OPTICAL TRANSMISSION SYSTEMS This invention relates to optical transmission systems and in particular to control of an optical signal source, especially a semiconductor laser optical source, therein.

One of the complications of using a semiconductor laser as an optical source for fibre-optic communications arises from the -non-linear optical power {/current I characteristic L, which consists of a substantially linear region beyond a lasing threshold current, as opposed to the more linear overall characteristic D of an LED (light emitting diode) (Fig. 1). To control the optical power from a semiconductor laser and to sensibly modulate the output, the laser must be biassed above the threshold current.

Further complications arise in that the threshold current, and the dependence of output power on forward (drive) current, is generally very temperature sensitive; in that the laser may gradually degrade over its lifetime; and in that the optical coupling of fibre to the laser ay not remain constant.

A widely adopted solution to some of the above difficulties is to provide a monitoring photodiode at one, the rear, facet of the laser to measure the optical output. A laser can emit light from its front and rear facets but only that from its front facet is employed for transmission. Using such a monitoring photodiode provides a means of feedback control for laser operation. The disadvantages with this system are that a separate detector (monitoring photodiode) is used with attendant cost of incorporation in the laser package; that the laser may degrade such that the ratio of power from front and rear facets is not constant; and that any change in fibre coupling due to alignment change between the laser and fibre cannot be compensated.

According to one aspect of the present invention there is provided an automatic feedback control of the output power of an optical signal source employed in a transmitter of an optical transmission system which also includes a receiver, the automatic feedback control involving both the transmitter and the receiver.

According to another aspect of the present invention there is provided an optical transmission system including a transmitter, a receiver and an optical path therebetween, wherein the transmitter includes a semiconductor laser, and wherein automatic feedback control of the output power of the semiconductor laser involves components of both the transmitter and the receiver.

According to a further aspect of the present invention there is provided a method of automatically controlling the output power of a semiconductor laser in a transmitter of an optical transmission system, which system also includes a receiver and an optical path between the transmitter and the receiver, including the steps of driving the semiconductor laser in accordance with a signal to be transmitted, transmitting the semiconductor laser output to the receiver via the optical path, at the receiver comparing the received power level with a standard, comprising a predetermined power level, and transmitting a signal from the receiver to the transmitter whereby to adjust the output power of the semiconductor laser in the sense of equalising the received power level and the standard.

According to another aspect of the present invention there is provided a receiver module including a photodiode and associated circuitry for determining the mean power received by the photodiode, comparing the received mean power with a predetermined power level and producing an output signal which in use serves to adjust the output power of a semiconductor laser producing the power received by the photodiode whereby to equalise the received power with the predetermined power level.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates the optical power versus drive current characteristics for a semiconductor laser and a light emitting diode.

Fig. 2 illustrates a conventional optical fibre transmission system.

Fig. 3 illustrates an optical fibre transmission system as proposed by an embodiment of the present invention.

Fig. 4 illustrates schematically an embodiment of an optical fibre transmission system according to the present invention and including a duplex optical fibre link, and Fig. 5 illustrates more specifically a transmission system of the Fig. 4 type.

A conventional optical fibre transmission system illustrated in Fig. 2 includes a transmitter 1 and a receiver 2 which are linked by an optical fibre 3 that can be single or multimode. The transmitter 1 includes an optical source 4, typically a semiconductor laser, and a monitor 5, typically a photodiode. The optical output from the rear facet of the semiconductor laser 4 is detected by photodiode 5 and the electrical output thereof input to a laser mean power control unit 6, whose output is a d.c. bias current, approximately equal to the laser threshold current, used to drive the semiconductor laser. The unit 6 is designed to keep the laser biased to the correct operating part of its transfer characteristic even as the lasing theshold varies with temperature or ageing.The semiconductor laser 4 is modulated in accordance with data to be transmitted by adding a modulation signal to the d.c.

bias current. This is indicated in Fig. 2 by a data input, which may be analogue or digital, to a data interface 7 which provides a suitable modulation signal, illustrated as a series of rectangular pulses. The receiver 2 comprises a receiver photodiode 8 and a pulse regenerator 9 which are coupled by a capacitor 10 and an amplifier 11 so that the regenerator 9 receives an amplified version of the high frequency component of the photodiode output from which it produces a pulsed data output signal. As illustrated a negative bias is provided for the photodiode. The present invention is not concerned with the actual regeneration of the transmitted data, which is well known in itself, and thus description thereof is considered unnecessary.

In the conventional optical fibre transmission system illustrated in Fig. 2, feedback control of the semiconductor laser 4 is achieved locally within the transmitter 1.

The present invention, however, is concerned with providing feedback control of the laser power by means of a total communication system (transmit and receive) rather than locally within the transmitter.

The advantage of using the total system lies in the fact that, in principle, all factors contributing to variation in received signal can be compensated for.

Fig. 3 illustrates a basic version of the system of the invention which includes a transmitter 21 and a receiver 22 which are linked by an optical fibre 23. A d.c. bias current output from a laser controller 24 is employed to drive a semiconductor laser 25. A data modulation signal, indicated as a series of rectangular pulses, is also applied to the laser 25.

The data modulation signal is derived by a data interface 26 from a data input which may be analogue or digital in form. The optical output of the laser 25 is transmitted to the receiver 22 via the optical fibre 23, where it is detected such as by a negatively biased photodiode 27 whose corresponding electrical output signal is applied to a regenerator 28 coupled thereto by capacitor 29 and amplifier 30, as described with reference to Fig. 2, to produce a pulsed data output signal. The system as described so far is similar to the conventional system of Fig. 2, except that there is no monitoring photodiode at the semiconductor laser.

The laser controller 24 differs from the laser mean power control 6 in that it also serves to modulate the data transmitted in fibre 23 at a low frequency ( < lOKHz) at constant amplitude, this low frequency modulation frequency being dependent on a bias level set in a modulator of the laser controller 24. At the receiver 22 the electrical output signal of the photodiode 27 is also low-pass filtered at 31 and decoded at 32. An output from the decoder 32 is applied as an input to the laser controller 24. If the power of the optical signal received by the photodiode 27 was too low or high compared to a standard level, the receiver 22 changes (increases or decreases) the low frequency modulation frequency and this is supplied to the laser controller, as the input from the decoder, to adjust the bias level set therein.Thus a control frequency signal is supplied from the receiver to the transmitter to control the semiconductor laser output power. A connection between the receiver and the transmitter in addition to that provided between the transmitter and the receiver by optical fibre 23 is thus required and may be comprised by another optical fibre.

A block diagram of a practical system involving a duplex (two-way) optical link is illustrated in Fig.

4. Associated with one optical fibre 41 is an optical transmitter 42 at one end of the link (left) and an optical receiver 43 at the other end of the link (right), and associated with another optical fibre 44 is an optical transmitter at the other end of the link 45 and an optical receiver 46 at the one end of the link.

A pulsed data input (data 1) to be transmitted from the one end to the other end of the link is modulated at low frequency at constant amplitude in modulator 47 and a corresponding optical signal transmitted by transmitter 42 via fibre 41 to optical receiver 43 from which the pulsed data signal (data 1) can be recovered and regenerated as conventional. In addition the electrical output of the receiver 43 is compared in amplitude with a standard (STD) at converter 49 and an appropriate control frequency signal then supplied to a modulator 48 associated with transmitter 45. The control frequency is increased or decreased as appropriate at the amplitude/frequency converter 49 in dependence on the power of the received signal, as described above.A pulsed data input (data 2) to be transmitted from the other end (right) to the one end (left) of the link is modulated at 48 with the control frequency and a corresponding optical signal transmitted by transmitter 45 to receiver 46 via fibre 44. The pulsed data signal (data 2) can be recovered and regenerated as in conventional practice. In addition the electrical output of receiver 46 is compared in amplitude with a standard at converter 50 and an appropriate control frequency signal supplied by converter 50 to modulator 47. Each receiver uses the mean received power to control the semiconductor laser which transmitted to it by encoding a control error with the transmitted data and returning the control feedback signal along the other fibre link.

In Fig. 5 there is illustrated a more specific version of the basic duplex (two-way) link arrangement of Fig. 4 although it is still somewhat schematic. At end A of the duplex link comprising two optical fibres 51 and 52, a semiconductor laser 53 produces an output corresponding to a pulsed data signal (data 1) modulated at a low frequency. This optical output is transmitted to a photodiode 54 over fibre 51. The data signal (data 1) is recovered and regenerated from the electrical output of photodiode 54 by means (not shown) and as described above.In addition the photodiode output is low pass filtered at 55 and decoded at 56, as appropriate to the employed low frequency modulation, to produce a d.c. signal, comprising the mean received power level, which signal is compared with a standard level and then the low frequency modulation frequency required to modulate a data signal (data 2), to be sent from end B to end A of the link over fibre 52, is generated at converter 57, this modulation frequency providing a feedback control error signal to laser 53 in order to change its output power accordingly.The low frequency modulation frequency extracted by decoder 56 and output as a d.c. signal is employed to control the output power of the semiconductor laser 58 at end B of the link and is applied to a laser control 59 which may be such as to change the temperature of the laser 58 in order to change the output power, such as by means of a Peltier cooler arrangement 60 as illustrated, or alternatively (and not shown) such as to correspondingly alter the d.c. drive current of the laser. Means 61 may be incorporated such that in the event of no low frequency modulation being present on the received signal the laser 58 is controlled to operate at a preset power level and an alarm is activated.

The optical signal transmitted from end B of the link to end A via fibre 52 thus corresponds to data 2 modulated at a low frequency, the value of the low frequency being indicative of any change required to the laser control 62 of laser 53 to control its output power. At end A of the link the transmitted optical signal is received by photodiode 63 and the data signal (data 2) recovered and regenerated as conventional practice. The electrical output of photodiode 63 is low pass filtered at 64 and decoded at 65 as appropriate to the employed low frequency modulation. The mean recovered optical power is compared with a standard level and the required low frequency modulation frequency for data signal 1 is generated at converter 66 and applied to semiconductor laser 53, this modulation frequency providing feedback to laser 58 to change its output accordingly.The low frequency modulation frequency is extracted by decoder 65 and output therefrom as a d.c. signal which is employed to control the output power of semiconductor laser 53 via laser control 62. The latter may be such as to change the temperature of laser 53 by means of a Peltier cooler arrangement 67 as appropriate or to correspondingly alter the d.c. drive current. Means 68 may be incorporated such that in the event of no low frequency modulation being present on the received signal the laser 53 is controlled to operate at a preset power level and an alarm is activated.

The low frequency modulation can be FM, PFM, low frequency AM or any such modulation scheme, and may be at less than one hertz. No monitoring photodiodes are required at the semiconductor lasers, instead the receivers use mean received power to change the temperature of the transmitting lasers or alter the d.c.

drive current of the transmitting lasers. The mean optical received power is maintained independently of fibre to source optical coupling, optical loss, temperature change and laser degradation. The data transmission system as such is unchanged by the automatic feedback control provided by the present invention, but a cheaper optical transmitter package (semiconductor laser) can be employed since a monitoring photodiode is not required. Furthermore, the associated electronics to carry out the functions described here may be produced as part of an integrated silicon chip (integrated circuit). The automatic feedback control of the present invention may be used in many data transmission applications. It is particularly useful in situations where operating electrical power needs to be conserved, for example in battery operated systems such as battlefield fibre-optic links.

The hitherto conventional feedback control involves a photodiode 5 at the rear facet of the semiconductor laser 4 and suitable circuitry comprising the laser mean power control 6 and modules comprising the photodiode, semiconductor laser and laser mean power control circuitry are commercially available. As has been stated above, incorporating a photodiode in a semiconductor laser package increases the cost thereof and in addition such photodiodes may be found to be unreliable. Furthermore the hitherto conventional feedback control does not compensate for all possible variations in received signal, rather it just compensates for variations in the semiconductor laser.

The automatic feedback control provided by the invention does not require an additional monitoring photodiode, only the already necessary receiver photodiode is required. A photodiode receiver module may thus be envisaged which includes the necessary low pass filtering and decoding circuitry for automatic feedback control as described above. An example of such a module 70 is illustrated schematically in Fig. 6. The photodiode 71 has an output lead 72 for data and an output lead 73 for coupling to a compatible semiconductor laser module (not shown) which includes a laser controller with a modulator. A single module, transmitter/receiver, for each end of a duplex link can also be envisaged which includes both a photodiode, a semiconductor laser and the associated circuitry equivalent to that illustrated at A or B of Fig. 5.

The receiver module of Fig. 6 is illustrated as having a lens 74 but is physically separated from a fibre 75. It is envisaged that the module could include an integral fibre tail for direct splicing to a system fibre.

Whereas the duplex (two-way) optical links described above involve the use of two optical fibres, such as 41 and 44, the automatic feedback control of the present invention can also be used with duplex optical links involving a single optical fibre. In this case transmission in one direction over the fibre is at a first wavelength, whilst transmission in the other direction over the fibre is at a second wavelength.

Examples of two suitable wavelengths are 1300nm and 1550nm. Such working over a single fibre can be achieved by splitting the light with either a semi-reflecting mirror or a wavelength filter.

Furthermore, the automatic feedback control of the present invention can also be employed with free space optical transmittion between the ends of an optical link.

Claims

CLAIMS:-

1. Automatic feedback control of the output power of an optical signal source employed in a transmitter of an optical transmission system which also includes a receiver, the automatic feedback control involving both the transmitter and the receiver.

2. An optical transmission system including a transmitter, a receiver and an optical path therebetween, wherein the transmitter includes a semiconductor laser, and wherein automatic feedback control of the output power of the semiconductor laser involves components of both the transmitter and the receiver.

3. An optical transmission system as claimed in claim 2 wherein the optical path is comprised by optical fibre.

4. An optical transmission system as claimed in claim 2 wherein the optical path is comprised by free space.

5. An optical transmission system as claimed in any one of claims 2 to 4, wherein the transmitter includes a laser controller, wherein the receiver includes an optical responsive element, wherein in an electrical output circuit of the optically responsive element is a means for determining the mean power received following transmission in use of the system between the transmitter and the receiver via the optical path, including means for transmitting a signal, from the determining means to the laser controller, indicative of the adjustment required in the semiconductor laser output power to equalise the mean received power with a predetermined power level, the laser controller incorporating means responsive to said signal to adjust the semiconductor laser output power accordingly.

6. An optical transmission system as claimed in claim 2 wherein the said optical path is a first optical fibre of a duplex optical link, wherein the said transmitter is at one end of the optical link and the said receiver is at the other end of the optical link, wherein the duplex optical link includes a second optical fibre, a second transmitter being disposed at the other end of the optical link for transmission of optical signals via the second optical fibre to a second receiver at the one end of the optical link, wherein each transmitter includes a respective laser controller, wherein each receiver includes a respective optically responsive element, wherein in an electrical output circuit of each optically responsive element is a respective means for determining the mean received power transmitted in use of the system from the respective transmitter over the respective optical fibre, and wherein in use a signal from each determining means is transmitted to the laser controller at the opposite end of the link whereby to adjust the semiconductor laser thereat so that the mean received power at the output end of the respective optical fibre is at a predetermined level, which signals are transmitted via the other optical fibre.

7. An optical transmission system as claimed in claim 2 wherein the said optical path is a single optical fibre of a duplex optical link, transmission of optical signals is opposite directions over the single optical fibre being at different wavelengths, wherein the said transmitter is at one end of the single optical fibre and the said receiver is at the other end of the single optical fibre, wherein a second transmitter is disposed at the other end of the single optical fibre for transmission of optical signals via the single optical fibre to a second receiver at the one end of the single optical fibre, wherein each transmitter includes a respective laser controller, wherein each receiver includes a respective optically responsive element, wherein in an electrical output circuit of each optically responsive element is a respective means for determining the mean received power transmitted in use of the system from the respective transmitter over the single optical fibre, and wherein in use a signal from each determining means is transmitted to the respective laser controller at the opposite end of the single optical fibre whereby to adjust the semiconductor laser threat so that the mean received power is at a predetermined level, which signals are transmitted over the single optical fibre.

8. An optical transmission system including automatic feedback control of a semiconductor laser substantially as herein described with reference to Fig. 3, Fig. 4 or Fig. 5 of the accompanying drawings.

9. A method of automatically controlling the output power of a semiconductor laser in a transmitter of an optical transmission system, which system also includes a receiver and an optical path between the transmitter and the receiver, including the steps of driving the semiconductor laser in accordance with a signal to be transmitted, transmitting the semiconductor laser output to the receiver via the optical path, at the receiver comparing the received power level with a standard, comprising a predetermined power level, and transmitting a signal from the receiver to the transmitter whereby to adjust the output power of the semiconductor laser in the sense of equalising the received power level and the standard.

10. A method as claimed in claim 9 wherein the optical path is comprised by optical fibre.

11. A method as claimed in claim 9 wherein the optical path is comprised by free space.

12. A method as claimed in any one of claims 9 to 11, wherein the semiconductor laser is driven by a laser controller including a modulator, wherein the semiconductor laser output is modulated at first frequency determined by a bias level set in the modulator, and wherein the signal transmitted from the receiver to the transmitter is modulated at a second frequency determined by the difference between the received power level and the standard, and including the step of adjusting the bias level set in the modulator in accordance with the second frequency and thus correspondingly adjusting the output power of the laser.

13. A method as claimed in any of claims 9 to 12 wherein the output power of the semiconductor laser is adjusted by changing its temperature.

14. A method as claimed in any one of claims 9 to 12 wherein the output power of the semiconductor laser is adjusted by changing a d.c. current bias applied thereto.

15. A method as claimed in claim 9 wherein the said optical path is one optical fibre of a duplex optical link, wherein the said transmitter is at one end of the optical link and the said receiver is at the other end of the optical link, and wherein the duplex optical link includes another optical fibre, a second transmitter being disposed at the other end of the optical link for transmission of a respective optical signal via the other optical fibre to a second receiver at the one end of the optical link, and including the steps of driving a semiconductor laser of the second transmitter in accordance with a signal to be transmitted, transmitting the output of the second transmitter semiconductor laser to the second receiver, at the second receiver comparing the received power level with a standard comprising a respective predetermined power level, and transmitting a signal from the second receiver to the second transmitter via said one optical fibre whereby to adjust the second transmitter semiconductor laser in the sense of equalising the received power level at the second receiver with the respective standard, the signal from the receiver to the transmitter associated with the one optical fibre being transmitted via said other optical fibre.

16. A method as claimed in claim 9 wherein the said optical path is a single optical fibre of a duplex optical link, transmission of optical signals in opposite directions over the single fibre being at different wavelengths, wherein the said transmitter is at one end of the single optical fibre and the said receiver is at the other end of the single optical fibre, wherein a second transmitter is disposed at the other end of the single optical fibre for transmission of optical signals via the single optical fibre to a second receiver at the one end of the single optical fibre, and including the steps of driving a semiconductor laser of the second transmitter in accordance with a signal to be transmitted, transmitting the output of the second transmitter semiconductor laser to the second receiver via the single optical fibre, at the second receiver comparing the received power level with a standard comprising a respective predetermined power level, and transmitting a signal from the second receiver to the second transmitter via the single optical fibre whereby to adjust the second transmitter semiconductor laser in the sense of equalising the received power level at the second receiver with the respective standard.

17. A method as claimed in claim 15 or claim 16, wherein each semiconductor laser is driven by a respective laser controller including a respective modulator, wherein the laser output of each semiconductor laser is modulated at a respective first frequency determined by a respective bias level set in the respective modulator, and wherein the signal transmitted from each receiver to the respective transmitter is modulated at a respective second frequency determined by the difference between the respective received power level and a respective standard, and including the step of adjusting the bias level set in each modulator in accordance with the respective second frequency and correspondingly adjusting the output power of the respective laser.

18. A method as claimed in claim 17 wherein the output power of the respective laser is adjusted by changing its temperature.

19. A method as claimed in claim 17 wherein the output power of the respective laser is adjusted by changing a d.c. current bias applied thereto.

20. A method as claimed in claim 17 wherein the first frequencies are low compared with the transmission rate of the transmission system and including the steps of low pass filtering the electrical output of a respective photodiode of each receiver, and decoding the filtered output, the decoded output being employed to modulate the output of the semiconductor laser at the same end of the link as the receiver, whereby to feedback to the semiconductor laser controller at one end of the link a control signal to adjust the output power as necessary, and being employed to drive the semiconductor laser controller at the other end of the link in accordance with the mean received power.

21. A method of automatically controlling the output power of a semiconductor laser in an optical transmission system substantially as herein described with reference to and as illustrated in Fig. 3, Fig. 4 or Fig. 5 of the accompanying drawings.

22. A receiver module including a photodiode and associated circuitry for determining the mean power received by the photodiode, comparing the received mean power with a predetermined power level and producing an output signal which in use serves to adjust the output power of a semiconductor laser producing the power received by the photodiode whereby to equalise the received power with the predetermined power level.

23. A module as claimed in claim 22 and for use at one end of a duplex optical link with said semiconductor laser being disposed at the other end of the link, the module further including a respective semiconductor laser for transmitting an optical signal from said one end of the link to the other end of the link, said optical signal including said output signal.

24. A receiver module substantially as herein described with reference to and as illustrated in Fig. 6 with or without reference to Fig. 5.