NZ240524A

NZ240524A - Optical signal transmitter: service signals modulate pump source light. (51)

Info

Publication number: NZ240524A
Application number: NZ24052491A
Authority: NZ
Inventors: Rolf Heidemann
Original assignee: Alcatel Australia
Priority date: 1991-11-08
Filing date: 1991-11-08
Publication date: 1994-02-25

Description

<div id="description" class="application article clearfix"> 240524 Priority Dsts^}: JS . J\-.SQ Compi&Ui Specification Filed: . S'.IV.^. Class: Publication Date: P.O. Joums!, No: ..... \3>7)7t,, NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION u.z. p^TEJirer-FicE ■8 NOV 1991 HECEiy l "OPTICAL COMMUNICATION SYSTEM" WE, ALCATEL AUSTRALIA LIMITED, A Company of the State of New South Wales, of 280 Botany Road, Alexandria, New South Wales, 2015, Australia, hereby declare the invention for which wc pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly dcscribcd in and by the following statement: 1 240524 The present invention relates to a system for transmitting an optical signal through an optical waveguide containing a fibre-optic amplifier which includes an amplifying length of fibre, a pump source, and a wavelength-selective fibre-optic coupler for coupling the pump source to the amplifying length of fibre. 5 Such a transmission system is known from "ECOC '89", Fifteenth European Conference on Optical Commnication, September 10-14, 1989, Gothenburg, Sweden, Proceedings, vol. 1, Regular Papers, TuA 5-7, pages 86 to 89. In the above-referenced article, two different system configurations are explained. In the first ("copropagating configuration"), the light from the pump source 10 is launched into the Er3 ' -doped length of fibre via a coupler which, as viewed in the direction of transmission of the optical signal to be amplified, is located in front of the amplifying length of fibre. In the second system configuration ("counterpropagating configuration"), the couplcr, as viewed in the direction of transmission of the signal to be amplified, is located behind the length of fibre. IS Sometimes it is desirable or necessary in transmission systems to transmit, in addition to the first optical signal, a second optical signal, eg., a service-channel signal, from the location of the fibre-optic amplifier in one direction or the other. It is the object of the invention to provide a system suitable for the above purpose. 20 According to the invention there is provided a system of the aforementioned type, wherein in order to transmit an additional information signal through the optical waveguide, - at the pump source a modulator is provided which modulates the pump light generated by the pump source with a modulating signal containing the additional in-25 formation signal, 2 240524 - at a point remote from the location of the pump source the optical waveguide contains a second wavelength-selective fibrc-optic coupler which couples pump light received from the pump source out of the optical waveguide, and - an optical receiver is connccted to the sccond wavelength-selective fibre-optic 5 coupler for receiving the information signal contained in the pump light. The invention wil now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows an embodiment for transmitting the sccond optical signal in a direction opposite to that of the first optical signal, and 10 Figure 2 shows an embodiment for transmitting the sccond optical signal in the same direction as the first optical signal. Figure 1 shows a transmission system for transmitting an optical signal from a point A to a point B. At the end point A there is an optical transmitter (not shown), and at the end point (B) there is an optical receiver (not shown). The optical signal 15 to be transmitted from A to B has a wavelength of 1540 nm. The transmission link is implemented with a single-mode optical waveguide 15 which has sufficiently good transmission properties at the wavelength X(. The system of Figure 1, like the prior art system referred to above, includes a fibre-optic amplifier 10 consisting of an Er1 '-doped length of fibre (1, a wavelength-20 selective fibrc-optic couplcr 12, and a pump source 13. The coupler 12 has four ports 1 to 4. Port 1 is connectcd to the Er31 -doped length of fibre 11, port is connected via the optical waveguide 15 to the end point B of the optical transmission link, and port 3 is connected via an optical waveguide to the pump-source 13. The coupler 12 is a wavelength-selective coupler which has the property of coupling the optical signal of 25 wavelength A., which is to be transmitted from A to B, from port 1 to port 2 with minimum loss, and of coupling the pump light generated by the pump source 13, 3 240 5 2 4 which has a wavelength X of 980 nm, from port 3 to port 1 with minimum loss. Port 4 is unused. To be able to transmit an additional information signal, eg., a service-channel signal or signals for monitoring the fibrc-optic amplifier itself, from the location of the 5 pump source to the end point A of the optical transmission link, according to the invention, a modulator 14 is provided at the pump source 13. This modulator modulates the pump light generated by the pump source 13 with a modulating signal containing the additional information signal. It is preferably a frequency modulator which modulates an analog or digital additional signal applied at its modulation input 10 ME, eg., a signal at baseband, onto a carrier wave of frequency f from a generator 18. The output of the frequency modulator 14 thus provides the modulating signal for the pump source, ie., the carricr wave frequency-modulated with the additional signal. The pump sourcc 13 must be thought of as containing the necessary control circuits for its laser, ie. the so-called laser driver, and a control circuit for the DC bias 15 to be applied to the laser. The modulating signal is injected into the laser driver of the pump source 13, thereby modulating the intensity of the light generated by the pump source 13. In another embodiment, the pump sourcc generates unmodulated light, and the modulating signal is used to modulate the pump light in a modulator following the 20 pump source. In that case, too, the pump light generated by the pump source is modulated. During normal operation, the intensity of the pump light is so high that a considerable portion which is not absorbed in the length-of fibre 11 is transferred from the end of the length of fibre 11 remote from the coupler 12 into the optical 25 waveguide 15 and passes down the latter in the direction of the end point A. In this manner, the additional signal can be transmitted in the direction of the end point A 4 240524 up to the point where the pump light can be detected with a level sufficient for signal transmission. At that point, the optical waveguide 15 contains a fibre-optic wavelength-selective couplcr 16 which extracts the pump light from the optical waveguide. The output port of the couplcr 16 is connected via an optical waveguide 5 19 to an optical receiver 17 whose output provides the electric modulating signal impressed on the pump light. This modulating signal is finally demodulated in an FM demodulator 20, so that the additional signal appears at the output of the demodulator. Thus, according to the invention, the portion of the pump light which 10 unavoidably emerges from the length of fibre 11 and could only be suppressed with filters is utilised to transmit an additional signal from the location of the pump source over the length of fibre 11 to the distant end point of the optical transmission link. To prevent the modulation of the pump light from modulating the amplification brought about by passing the optical signal to be transmitted from end point A to end iS point B through the amplifying length of fibre 11, a suitable value is chosen for the frequency f of the carrier used in the FM modulator 14. A suitable frequency fQ has a value which is substantially greater than the reciprocal of the lifetime of those energy states of the Er3+-dopcd material of the length of fibre 11 which are excitable by the pump light, ie., a value above 1 MHz. 20 One application of the novel communication system shown in Figure 1 is in a cable television distribution system in which an electric frequency-division multiplex signal with a bandwidth of 450 MHz has to be transmitted over the optical transmission link, and in which front-end equipment, which distributes the optical signal to a plurality of optical waveguides running to individual subscribers, contains the 25 fibre-optic amplifier 10, which then serves to amplify the optical signal before it is distributed to the optical waveguides. In such a system, as shown in Figure 1, a re- 5 240524 verse channel can be installed from the location of the pump source to the end point A, ie. the head end, for transmitting an additional signal, eg., a servicc-channel signal. The system described has the so-called counterpropagating configuration, in 5 which the coupler coupling the pump light into the doped length of fibre, as viewed in the direction of transmission of the optical signal to be amplified, is located behind the amplifying length of fibre 11. With the aid of Figure 2, a system configuration, the so-called copropagating configuration, will now be explained in which the coupler coupling the pump light into the doped length of fibre, as viewed in the direction of 10 transmission of the optical signal to be amplified, is loeated before the amplifying length of fibre. In such a configuration, too, the pump light which is not absorbed in and emerges from the doped length of fibre can be used to transmit an additional signal. In the system according to the invention shown in Figure 2, parts having the 15 same functions as in Figure 1 are designated by similar reference characters and hence need not be explained again. The coupler 12, as viewed in the direction of transmission of the signal to be transmitted from A to B, is located before the Er3+-doped length of fibre 11. As in Figure 1, it couples the signal from port 1 to port 2 and the pump light from port 3 to the length of fibre 11, the latter via port 2 instead 20 of port 1 as in Figure 1. In this configuration of Figure 2, the pump light emerging from the doped length of the fibre 11 travels in the direction of the end point B of the transmission system, ie., in the same direction as the first optical signal of wavelength in the system of Figure 1, it travels in the opposite direction. Thus, the embodiment of the invention shown in Figure 1 can be used if an ad-25 ditional signal has to be transmitted from the location of the pump source of the transmission link in the direction of the source of the first optical signal, ie., the end 6 240524 point A, and the system of Figure 2 can be used if an additional signal has to be transmitted from the location of the pump source of the fibre-optic amplifier in the direction of the sink of the first optical signal, ie., to the end point B. By combining the two embodiments, a fibre-optic amplifier is obtained whose 5 doped length of fibre is fed from a pump source at each of its two ends, so that two additional signals can be transmitted from the location of the fibre-optic amplifier in different directions of the transmission system. The wavelengths mentioned in the foregoing description are only examples of wavelengths for which the available system components are suitable. It is, of course, 10 possible to use other wavelengths, with the signal wavelength lying in the range between 1520 and 1570 nm, and the pump wavelength being 532 nm, 800 nm, 980 nm or 1480 nm. 20 </div>

Claims

<div id="claims" class="application article clearfix printTableText"> 240524 What we claim is:

1. A system for transmitting a first optical signal through an optical waveguide containing a fibre-optic amplifier which includes an amplifying length of fibre, a pump source, and a wavelcngth-sclectivc fibrc-optic coupler for coupling the pump 5 source to the amplifying length of fibre, wherein in order to transmit an additional information signal through the optical waveguide, - at the pump source a modulator is provided which modulates the pump light generated by the pump source with a modulating signal containing the additional information signal, 10 - at a point remote from the location of the pump source the optical waveguide contains a second wavelcngth-sclcctive fibre-optic coupler which couples pump light received from the pump source out of the optical waveguide, and - an optical receiver is connected to the sccond wavelength-selective fibre-optic coupler for receiving the information signal contained in the pump light. 15

2. A system as claimed in claim 1, wherein the fibre-optic coupler, as viewed in the direction of transmission of the first optical signal, is located behind the amplifying length of fibre, so that the additional information signal is transmitted through the optical waveguide in a direction opposite to that of the first optical signal.

3. A system as claimed in claim 1, wherein the fibre-optic coupler, as viewed in 20 the direction of transmission of the first optical signal, is locatcd before the amplifying length of fibre, so that the additional information signal is transmitted through the optical waveguide in the same direction as the first optical signal.

4. A system as claimed in any one of claims 1 to- 3, wherein the modulator modulates the additional information signal onto a carrier wave whose frequency f is 25 substantially higher than the reciprocal of the lifetime of those energy states of the 8 24 0 5 2 4 5 light-amplifying material of the length of fibre which are excitable in the length of fibre by the pump light.

5. A system for transmitting a first optical signal through an optical waveguide, substantially as herein described with rcfcrcncc to Figures 1 - 2 of the accompanying drawings. ALCATEL AUSTRALIA LIMITED 10 B. O'Connor Authorized Agent P5/1/1703 </div>