Many situations exist in which an optical fibre is employed to carry a number of signals at different wavelengths, for example in wavelength division multiplexed systems for telecommunications applications, and in sensing applications where light of different wavelengths is used to carry signals with information about the measurand and reference signals. A number of arrangements have been proposed in order to provide the necessary wavelength demultiplexing in such systems, but all such arrangements are relatively complex and therefore expensive. It is therefore an object of the invention to provide an arrangement in which the signals can be demultiplexed relatively inexpensively.

According to the present invention, there is provided an arrangement which comprises an optical fibre that has a Bragg grating written on an end face thereof, and detecting means located at the end face of the fibre to detect electromagnetic radiation emerging from the end face at different angles with respect to the axis of the fibre, the fibre carrying optical signals at at least two different wavelengths, which signals are diffracted by the Bragg grating so that different signals are detected by the detecting means.

The arrangement according to the invention has the advantage that it is possible to separate and detect light of different wavelengths directly from the end of the optical fibre using a single Bragg grating simply by appropriate positioning of the detecting means.

The detecting means may, for example, comprise a plurality of detectors oriented at different angles with respect to the axis of the fibre or may, for example, comprise an array of detecting elements (e. g. an array of CCD detecting elements).

The grating may be formed by exposing the optical fibre to beams of ultraviolet radiation that may interfere with one another so as to generate a physical periodic structure or a periodic variation of refractive index of the fibre core across its face.

The gratings may be formed by a number of methods, for example by a light induced method e. g. as described in US patent No. 4,474,427, a two beam interferometry method e. g. as described in international patent application No. WO 86/01303 or a phase mask method e. g. as described in US patent No. 5,367,588, the disclosures of which are also incorporated herein by reference. Alternatively, the gratings may be written by a point-by-point method in which the radiation is focused to a spot that is moved along the end face of the fibre, thereby enabling gratings with longer pitches to be written.

The arrangement according to the invention may be employed with silica (multimode) fibres or with plastics fibres, but is particularly applicable to use with plastics fibres since, among other reasons, it is difficult in the extreme at the present date to couple plastics fibres with other fibres. The fibre is typically one based on polymethyl methacrylate (PMMA), for example having a PMMA core (ncore = 1. 492) surrounded by a cladding layer of fluorinated PMMA (nclad = 1.405). Such an arrangement may for example, be used in monitoring equipment, for example for monitoring temperature, strain, humidity, presence of specific chemicals, (the measurand) in which the optical fibre transmits one signal that carries information about the measurand, and another signal at a different wavelength for reference purposes in order to compensate the measurand signal for changes in the intensity of the light source, macrobending of the fibre, temperature, strain or any other variation that would affect the output intensity.

The angle at which the radiation leaves the fibre is given by the Bragg condition for diffraction: mA = 2d. sinS where: d is the grating period, 0 is the diffracted angle, m is the order of diffraction, and A is the wavelength of the radiation.

The arrangement according to the invention can, however, be employed in telecommunications applications for wavelength demultiplexing of signals.

Several forms of arrangement according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic view of the end of a plastics optical fibre having a Bragg grating written thereon; Figure 2 is a schematic view of the end of the optical fibre of figure 1 when light of a single wavelength is sent along it together with a graphical indication of the light intensity; Figure 3a is a schematic view of a measuring system that employs an arrangement according to the invention; Figure 3b is a schematic view of a measuring system which is a modification of the system of Fig. 3a; Figure 4a is a schematic view of a system showing the principle of wavelength demultiplexing according to the invention; and Figure 4b is a schematic view of a measuring system which is a modification of the system of Fig. 4a.

Referring to the accompanying drawings, figure 1 shows the end of a plastics optical fibre I having a core 2 of diameter of 980 llm formed from polymethyl methacrylate (PMMA) and a 10 Rm thick cladding 4 thereon formed from fluorinated PMMA. The end face 6 of the fibre has a Bragg grating written thereon by shining ultraviolet radiation of wavelength 248 nm from an excimer laser at the face through a phase mask. The phase mask has a relief pattern etched written thereon to cause the ultraviolet radiation to form an intensity modulated interference pattern on the end face of the fibre. The period of the interference pattern depends only on the period of the phase mask, so that the resultant photo induced grating has a period equal to half that of the phase mask.

Figure 2 shows the effect of transmitting monochromatic light at a wavelength of about 500 nm along the fibre. Most of the light that exits the optical fibre at the end face 6 thereof falls within a light cone 12 whose shape is defined by the numerical aperture of the fibre, while a small part of the light, corresponding to first order diffraction, will fall within a further cone 14 located on either side of the light cone 12 and can be picked up by detectors (not shown). Figure 2 shows, in addition, the intensity of the light that exits the fibre with the intensity shown on the horizontal axis and with the lateral dimensions shown on the vertical axis with the dimensions corresponding to the fibre as shown.

Figure 3a shows a measuring system that employs an optical fibre arrangement according to the invention. The system comprises a pair of plastics optical fibres 20 and 21, and a detection zone 22 which contains material to be detected. One of the fibres 20 launches light into the detection zone 22, while the second fibre 21, which is of the type shown in figure 1 having a Bragg grating written on the end thereof remote from the detection zone, collects the light from the detection zone. In the detection zone 22, the fibres are cleaved and kept aligned with a lmm distance between the fibre ends. Several detectors (in this case detectors 24 and 26) are placed at the end of the second fibre at angles that match the diffracted angles of each of two wavelengths of the light sent along the fibres. If the material to be detected in the detection zone 22 absorbs light at one of the wavelengths but not at the other, the output of one of the detectors 24 and 26 will depend on the concentration of the material that is being detected together with other parameters such as light intensity, macrobending of the fibres, temperature etc. while the output of the other detector will depend only on the other parameters, and can be used as a reference. As an example, the system was employed to measure the concentration of methylene blue by employing radiation at wavelengths of 633 nm (to detect the methylene blue) and radiation at a wavelength of 800 nm as a reference.

The measuring system of Fig. 3b is very similar to that of Fig. 3a and the same reference numerals have been used for the same features. The modification is that the discrete detectors'24, 26 of Fig. 3a have been replaced with an array 30 of photo detecting elements 32, for example CCD (charge coupled device) photo detectors.

Figure 4a shows the principle of wavelength demultiplexing according to the invention in which three signals 41,42 and 43 which are transmitted along the plastics optical fibre 1 at three different wavelengths. By setting photodetectors 44,45 and 46 at appropriate angles with respect to the axis of the fibre 1, the signals in the fibre may be demultiplexed to separate the three original signals 41,42 and 43.

The measuring system of Fig. 4b is very similar to that of Fig. 4a and the same reference numerals have been used for the same features. The modification is that the discrete detectors 44,45 and 46 of Fig. 4a have been replaced with an array 30 of photo detecting elements 32, for example CCD (charge coupled device) photo detectors.

The invention is not restricted to the details of the foregoing embodiments. For example, any number of wavelengths may be transmitted along the optical fibre and subsequently detected after diffraction at the fibre end, subject to the appropriate number of detectors 24,26,44,45 46 or subject to the provision of an appropriate array of detecting elements 32.