ARRANGEMENT AND METHOD AT AN OPTICAL FIBRE

Abstract: The present invention relates to an arrangement and a method for selectively switching wavelength channels to and from an optical fibre (1). The arrangement according to the invention comprises at least two fibre couplers (2, 3) and one or more fibre gratings (4) arranged between these. The first fibre coupler (2) is adapted to take off a component of the signal and to switch another component of the signal to a fibre grating. The said grating is adapted to function as a band reflex filter and to prevent selected wavelengts from continuing on the fibre (1) and to reflect these back in the fibre (1). Other wavelengths are allowed to pass further in the arrangement. The second fibre coupler (3) is arranged in a position on the other side of the fibre grating (4) along the optical fibre (1). The said fibre coupler (3) is adapted for introduction of the wavelength removed in the first fibre coupler (2) or for introduction of any other wavelength which is not already present on the fibre.

Description: ι Arrangement and method at an optical fibre TECHNICAL SPHERE The present invention relates to an arrangement and a method for selective switching of wavelength channels to and from an optical fibre. PRIOR ART In the sphere of telecommunications there is, in many cases, a need for very high transmission capacity. By utilising optical transmission by way of modulated optical signals, very rapid data transmission can be achieved. In order to transmit a plurality of optical signals over a common optical medium- wavelength division multiplexing (WDM) is used. The signals are transmitted by way of independent wavelength channels, which can exist simultaneously in an optical fibre. For selectively switching a wavelength channel to and from an optical fibre, that is introducing or removing a wavelength channel on an optical fibre, optical multiplexers and wavelength-selective demultiplexers are used. An optical demultiplexer comprising waveguide coupler and waveguide grating is known from US-A-5 195 161. A waveguide coupler arranged between two parallel waveguides connects to waveguide gratings, so-called Bragg reflectors, connected to two output ports on the coupler.; By means of the said waveguide grating all light of a desired wavelength can in principle be reflected to an outlet port parallel with the inlet port by constructively using interference. The disadvantage to this multi- plexer is that all light of a desired wavelength is only fed out if a very precise ratio exists between the length of waveguide branches between the coupler and the two reflectors. The difference in optical wavelength must be measured to within fractions of a light wavelength; otherwise total extinguishing can occur at the outlet. This type of precision is not practically feasible, however, in the case of optical fibres, but can only be achieved in integrated optics. Demultiplexers are consequently both expensive and difficult to use in a fibre-optic network. A fibre coupler for selectively switching a wavelength channel to or from an optical fibre is already known from US-A-4 673 270. This fibre coupler comprises two optical fibres with different refractive index profiles, which are arranged adjacent to one another in a switching area. In order to connect a certain wavelength channel between the fibres, the connection coefficient must be periodically varied along the switching area. This adding dropping coupler is difficult to implement since high precision is required in the switching area between the fibres in order to permit switching of a certain wavelength channel in the fibre coupler. For this reason this does not represent an economically or practically feasible solution in many contexts. DESCRIPTION OF THE INVENTION The object of the present invention is to provide a wavelength-selective multiplexer and demultiplexer, that is an adding/dropping multiplexer (ADM), by means of which wavelength channels can be switched to and from an optical fibre and blocked at arbitrary positions along this in a system with wavelength division multiplexing (WDM). This is achieved by the arrangement according to the invention, which comprises two fibre couplers and one or more fibre gratings arranged between them. The first fibre coupler is designed to divide up an incoming optical signal, which comprises a plurality of wavelength channels, into a first and a second branch in the fibre coupler. The strength ratio between the component signals can be varied arbitrarily. The component signal in the first branch is allowed to pass through the fibre grating, with the exception of one or more wavelength channels which are reflected in one or more gratings and consequently prevented from continuing on the fibre. Each wave¬ length channel which is reflected in a grating is allowed to pass to a third branch in the first fibre coupler. Other wavelength channels are allowed to pass further in the arrangement. The second or third branch of the first fibre coupler is connected to one or more receivers, preferably intended for the wavelengths blocked in the fibre grating. The second fibre coupler is arranged in a position on the other side of the fibre grating along the optical fibre. The said fibre coupler is adapted for introducing one or more wavelengths removed on the first fibre coupler or for introducing wave¬ lengths which are not already present on the fibre. The invention also relates to a method for selectively switching wavelength channels to and from an optical fibre. In the method according to the invention optical signals, comprising a plurality of wavelength channels, are carried to a first fibre coupler. The signal is divided up into two component signals, one of which is fed to a fibre grating, which reflects a certain wavelength channel and prevents this from continuing downstream along the optical fibre. The wavelength channel corresponding to the blocking area of the fibre grating is taken off to a receiver connected to the first fibre coupler. One or more wavelength channels are thereafter introduced on the optical fibre in a second fibre coupler. One advantage to the arrangement according to the invention is that it is fibre-based and simple in its structure and therefore inexpensive to implement. DESCRIPTION OF FIGURES Figure 1 shows a schematic diagram of a wavelength-selective adding/dropping multiplexer (ADM) for selective switching of a wavelength channel; Figure 2 shows a schematic diagram of a second embodiment of a wavelength- selective ADM for selective switching of a wavelength channel; Figure 3 shows a schematic diagram of an ADM for selective switching of four wavelength channels; Figure 4 shows a schematic diagram of a second embodiment of an ADM for selective switching of four wavelength channels; and Figure 5 shows a schematic diagram of a third embodiment of an ADM for selective switching of four wavelength channels. PREFERRED EMBODIMENTS The invention will now be described in detail with reference to the figures for the various embodiments of adding'dropping multiplexers (ADM). Figure 1 shows an adding dropping multiplexer for selective switching of a single specific wavelength channel. A optical signal comprising a plurality of wavelength channels is carried to a fibre coupler 2, which is adapted to switch an incoming optical signal, irrespective of wavelength, to a first fibre branch 21 and a second fibre branch 22 in accordance with a predetermined switching ratio of the fibre coupler 2, so that a component signal is obtained on each branch. Each component signal comprises all wavelength channels: division in the fibre coupler only involves a strength division of the signal between the two branches 21, 22. The component signal in the second branch 22 is allowed to pass to a bandpass filter 5. through which only the reception wavelength channel is allowed to pass. The component signal in the first branch 21 is allowed to pass to a band reflex filter, which is formed by a fibre grating 4. The said fibre grating is designed to block and reflect the reception wavelength channel, other wavelength channels being propagated unmodified along the optical fibre 1 even downstream of the fibre grating 4. The adding^dropping multiplexer shown in the figure is further provided with a second fibre coupler 3 with freely selectable switching ratio, which is adapted to introduce wavelength channels on the optical fibre 1. The band reflex filter in the form of a fibre grating facilitates the re-introduction into the other fibre coupler 3 of the wavelength channel which is removed in the first fibre coupler and blocked in the grating 4. In the embodiment shown in the figure, wavelength channels can be removed and blocked independently of one another. The arrangement shown in the figure can therefore be used to block a wavelength channel on an optical fibre 1, at the same time that a completely different wavelength channel is being removed from the fibre. It is also possible to introduce into the other fibre coupler 3 any other wavelength channel which is not already present on the optical fibre. Figure 2 shows a second possible embodiment of the adding/dropping multiplexer. As in the embodiment shown in Fig.1, the first fibre coupler 2 is adapted to connect an incoming optical signal to a first fibre branch 21 and a second fibre branch 22 of the fibre coupler according to a predetermined switching ratio. The component signal which is connected to the second branch 22 is not subjected to any further processing. The component signal in the first branch 21 is allowed to pass to a fibre grating, which is adapted to entirely reflect a specific wavelength channel, whilst allowing other wavelength channels to pass through the fibre grating 4 and further on the optical fibre 1. The wavelength channel reflected in the fibre grating is returned to the fibre coupler 2 and switched to a third fibre branch 23, which in turn connects to a receiver Rx The bandpass filter 5 required in the embodiment according to Figure 1 can consequently be omitted in this embodiment, since filtering out of the wavelength intended for reception occurs through the fibre grating 4, which reflects the desired wavelength channel to the third fibre branch 23 and on to the receiver Rx. This wavelength channel is therefore blocked for further transmission on the fibre 1. Other wavelength channels are allowed to pass through the fibre grating 4, further on the optical fibre 1. The first fibre coupler 2 and second fibre coupler 3 are arranged, as in the embodiment shown in Fig. 1, on either side of the fibre grating 4 and a wavelength channel removed in the fibre coupler 2 can be reintroduced into the second fibre coupler 3. The adding/dropping multiplexer shown in Figure 1 can easily be adapted for introducing or removing a plurality of channels. Fig. 3 shows an example for 4 channels. Since each fibre grating constitutes a band reflex filter for a wavelength range or a certain special wavelength, a plurality of fibre gratings must be arranged in an adding dropping multiplexer for multiple wavelengths. In the example shown in Figure 3 for four channels, four different fibre gratings 4a, b, c, d must be arranged between fibre coupler 2 to receiver Rx and fibre coupler 3 for the transmitter Tx. The embodiment shown in the figure essentially corresponds to the embodiment in Figure 1. The function of a first fibre coupler 2 is to deflect a part of the signal to an optical fibre branch, which is connected to a number of bandpass filters 5. Each of these bandpass filters 5 is adapted to allow a certain wavelength channel to pass to a receiver Rx, provided for this wavelength channel, whilst the other wavelength channels are prevented from passing. A second branch 22 of the first fibre coupler 2 is connected to the receivers. This branch 22 is further divided by the use of a star coupler or cascade-coupled 2 x 2 coupler to four branches. The four fibre gratings 4a. b, c, d between the first fibre coupler and the second fibre coupler 3 are adapted to prevent the wavelengths which it is intended to remove, from progressing to the second fibre coupler 3, in which the same wavelengths are preferably reintroduced from the optical fibre 1 with a new content. In the embodiment shown in the figure, wavelength channels can be removed and blocked independently of one another, it being possible, therefore, to remove channels other than those which are blocked in the respective fibre gratings. On the transmitter side the signals from four transmitters Tx are combined and carried to a fibre branch in the second fibre coupler 3. The wavelength channels are then carried to the optical fibre 1 by switching from the fibre coupler 3. In the same way the embodiment shown in Figure 2 can be adapted to switching a plurality of wavelength channels to or from an optical fibre 1. Figure 4 shows this addin^dropping multiplexer adapted to four wavelength channels. Four fibre gratings 4a, b, c, d are designed so that a first fibre coupler 2 reflects four specific wavelengths. A second branch 22 of the fibre coupler 2, to which the reflected signal is switched, contains a node in which the reflected signal is distributed on the same number of fibre branches as the number of channels, that is four in this case. Each branch comprises a bandpass filter 5a, b, c, d, by means of which wavelength channels to be received are allowed to pass to a receiver Rx adapted to the wave¬ length. Since the adding ropping multiplexer shown in Figure 2 is adapted to a plurality of channels, the bandpass filters 5a, b, c, d can not be avoided in connection with removal of the signal, but the double filtering which is obtained gives a better filtered signal to the receivers. As in earlier examples, the second fibre coupler 3, that is that for connecting the transmitters Tx, is arranged along the optical fibre 1, at a position in the transmission direction of the signal on the optical fibre 1 which is located downstream of the fibre grating. The embodiment shown in Figure 5 finally shows a third way of using an adding dropping multiplexer for switching a plurality of wavelength channels to and from an optical fibre 1. In this embodiment four separate first fibre couplers 2a, b, c, d are used for taking off four wavelength channels to four different receivers Rx. Fibre gratings 4 a, b, c, d are arranged after each of the said fibre couplers, in the downstream direction along the optical fibre 1. The said gratings are designed to reflect a selected signal, in the same way as in the embodiment shown in Figure 2, down to a receiver Rx for the wavelength channel corresponding to the signal. Optical isolators 6 a, b, c are arranged between respective fibre gratings 4a, b, c, and on fibre couplers 2b, c, d following these, in order to prevent the reflected signal from continuing upstream in the fibre. The introduction of four new wavelength channels is achieved in the same wav as in the embodiment shown in connection with Figure 3 and Figure 4. Since a large number of first fibre couplers 2a b. c. d is used in the embodiment shown in this figure, the switching ratio between the various branches of the fibre couplers is of great importance.

Claims: CLAIMS 1. Arrangement for switching one or more wavelength channels to and from an optical fibre (1) characterised in that it comprises - at least one first fibre coupler (1), arranged connected to the fibre, the said coupler being adapted to remove one or more wavelength channels from the optical fibre (1); - at least one second fibre coupler (3), arranged connected to the fibre (1) downstream of this in relation to the first fibre coupler (2), the said second coupler being adapted to carry one or more wavelength channels from a transmitter (Tx) to the optical fibre (1); and - one or more fibre gratings (4), arranged connected to the fibre (1) between the said fibre couplers (2,3), the said grating being adapted to reflect and prevent one or more wavelength channels from continuing on the optical fibre (1). 2. Arrangement according to claim 1, characterised in that a first branch (21) on the first fibre coupler (2) is connected to a fibre grating (4) and that a second branch (22) or third branch (23) on the first fibre coupler (2) is connected to one or more receivers (Rx). 3. Arrangement according to claim 2, characterised in that one or more fibre gratings (4) are adapted to reflect one or more wavelength channels to the third branch (23) of the fibre coupler. 4. Arrangement according to claim 2 or 3, characterised in that a bandpass filter (5), adjusted to a certain wavelength, is arranged between the second branch (22) or third branch (23) of the fibre coupler and each receiver (Rx). 5. Arrangement according to claim 1, characterised in that the fibre gratings (4a, b, c, d) arranged between the first fibre coupler (2) and second fibre coupler (3) are adapted so that their number corresponds to the number of wavelengths to be removed. 6. Arrangement according to claim 1, characterised in that the number of fibre couplers (2a, b, c. d) for removing a wavelength channel from the optical fibre (1 ) corresponds to the number of wavelengths to be removed, that fibre gratings (4a b. c, d) intended for reflecting the respective wavelength are arranged immediately after each fibre coupler (2a b, c, d) intended for taking off. and that optical isolators (6a b, c) arranged between the fibre couplers (2a b, c, d) are adapted to prevent the wavelengths reflected in the fibre gratings (4a, b, c. d) from passing upstream in the optical fibre (1). 7. Method for switching one or more wavelength channels to and from an optical fibre (1), characterised by the following steps: - carrying a first optical signal, comprising a plurality of wavelength channels, to a first fibre coupler (2); - dividing the optical signal up into a first and a second component signal in the first fibre coupler (2); - carrying the first of the component signals to one or more fibre gratings (4), one wavelength channel being reflected in each fibre grating and prevented from continuing on the optical fibre (1); - taking off of wavelength channels, corresponding to the blocking range of the fibre grating, to receivers (Rx) connected to the first fibre coupler (2); and - introducing one or more wavelength channels on the optical fibre (1) in a second fibre coupler (3). 8. Method according to claim 7, characterised in that the wavelength channels which are taken off in the first fibre coupler (2) are reintroduced into the second fibre coupler (3). 9. Method according to claim 7 or 8, characterised in that the second component signal is carried to one or more bandpass filters (5a, b, c, d) which allow the wave- length channels blocked in the fibre gratings (4a, b, c, d) to pass to receivers (Rx) connected to the first fibre coupler. 10. Method according to claim 7 or 8. characterised in that one or more wavelength channels blocked in the fibre gratings (4a b. c. d) are carried to one or more receivers (Rx) connected to the first fibre coupler.

Inventors: BONNEDAL DAG, EGNELL LARS, FRANSSON OWE, JOHANSEN LARS, JOHANSSON BENGT, LAGERSTROEM BO, OEBERG MAGNUS, SANDELL JOHAN, BONNEDAL, DAG, EGNELL, LARS, FRANSSON, OWE, JOHANSEN, LARS, JOHANSSON, BENGT, LAGERSTROEM, BO, OEBERG, MAGNUS, SANDELL, JOHAN
Applicants: BONNEDAL DAG, EGNELL LARS, ERICSSON TELEFON AB L M, FRANSSON OWE, JOHANSEN LARS, JOHANSSON BENGT, LAGERSTROEM BO, OEBERG MAGNUS, SANDELL JOHAN, BONNEDAL, DAG, EGNELL, LARS, FRANSSON, OWE, JOHANSEN, LARS, JOHANSSON, BENGT, LAGERSTROEM, BO, OEBERG, MAGNUS, SANDELL, JOHAN, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1996
Main classification: G02B6
IPC codes: G02B6, H04B10, H04J14