GB2380257A

GB2380257A - Monitor for an optical fibre

Info

Publication number: GB2380257A
Application number: GB0123367A
Authority: GB
Inventors: Ian Peter Giles; Rodney Babcock
Original assignee: PROTODEL INTERNAT Ltd; British Technology Group Inter Corporate Licensing Ltd
Current assignee: PROTODEL INTERNAT Ltd; British Technology Group Inter Corporate Licensing Ltd
Priority date: 2001-09-28
Filing date: 2001-09-28
Publication date: 2003-04-02
Anticipated expiration: 2021-09-28
Also published as: GB0123367D0; GB2380257B

Abstract

A monitor for monitoring the optical signal parameters in an optical fibre comprises an optical fibre 1 having an access region 9 of reduced cladding thickness sufficient to allow access to the evanescent field of the optical fibre, and an optical element mounted adjacent to the access region to enable use to be made of data therein. The system can establish whether there is light in the fibre and monitor its properties, including its polarisation state, wavelength, power or the information it carries. Preferably the optical element comprises a photodetector 11. A lens 15 and, additionally or alternatively, a polariser or wavelength filter may be interposed between the access region and the photodetector. Alternatively the optical element may comprise a second optical fibre (see figure 12). Also described is an arrangement for controlling the power in a optical fibre comprising an optical attenuator upstream of a monitor as described above (see figure 14).

Description

MONITOR FOR AN OPTICAL FIBRE This invention relates to a monitor for an optical fibre for monitoring properties thereof.

The propagating wave in an optical fibre is contained within the Silica core and is guided along the fibre with very little loss. The primary requirement from the fibre is to transmit the light within to the required destination, with low loss and no interference from the outside environment, which may corrupt the information carried by the light. These qualities, which make the fibre an ideal transmission medium make the optical signal difficult to access from outside the fibre without either interrupting the light path or bending the fibre to allow light to escape, In both cases considerably enhancing the possibility of corrupting the signal. Both methods introduce high loss and potential mechanical reliability problems In many fibre applications it is desirable to establish whether there is light in the fibre and monitor some of its properties ; such as polarisation state, wavelength, power or information being carried.

The optical signal in a fibre can, at present, be sampled from the fibre by : 1. Bending the fibre which encourages light to escape so that the escaped light can be detected and evaluated However, this method is unsatisfactory since it induces losses and the escaping light is quite dispersed once it penetrates the cladding thickness 2. 'Tapping'a small amount of power from the optical fibre using a directional coupler (power splitter) which diverts a small portion ( < 1 %) of the light into another fibre This method suffers from the disadvantage that it induces losses equal to the level of power tapped and also losses due to the directional coupler itself.

In both of the above cases an optical detector IS required to covert the optical parameter being measured to an electronic signal, e g power It will now be considered the way In which the light propagates within an optical fibre A portion of the field of the propagating wave extends Into the cladding and rapidly decays exponentially through the cladding. This field is an integral part of the propagation and, if it can be accessed, it allows the light wave parameters to be measured. In order to achieve this, It is necessary to remove at least the major part of the cladding at which the propagating wave is to be accessed Two options are available, to remove cladding material until the field is reached or to extend the field beyond the fibre cladding. The cladding can be removed by either, grinding and polishing or etching with acid and the field can be extended whilst reducing the cladding by heating the fibre and tapering.

Although all methods are feasible for the invention discussed here, the preferred approach is to grind and polish one side of the fibre which has advantages of providing a flat exposed side to access the field, and in that less material needs to be removed leaving a robust component Also, better control of exposed length can be achieved and this method is suitable for high yield manufacture.

Using this known method, the fibre IS ground to remove the appropriate amount of cladding material and then polished to provide a good quality optical surface.

In one method, as shown in figure 1, the fibre 1 comprising a core 3 In a cladding 5 is mounted in a substrate block 7 In an arc and'flat'polished or polished over a rotating wheel. Alternatively the fibre 1 can be suspended over a polishing wheel to give a surface finish shown In figure 2 As can be seen, the fibre 1 has a length L of reduced outer cladding 5, the cladding over this length L having a

residual cladding depth d. The method used to produce the form shown schematically in figure 2, allows control of, the length of the exposed region, and the thickness of the remaining cladding The invention seeks to provide a monitor for monitoring the optical signal parameters in an optical fibre which enables low losses to be achieved, both induced and polarisation dependent and to provide access to the fibre without the use of additional fibre paths.

According to a first aspect of the invention there is provided a monitor for monitoring the optical signal parameters in an optical fibre comprising a fibre having a region of reduced cladding sufficient to allow access to the evanescent field of the optical fibre and an optical element mounted adjacent to the said region of reduced cladding to obtain access to the evanescent field so as to enable use to be made of the data therein.

The optical fibres may be a single mode, multimode or polarisation maintaining fibre.

Preferably, the optical element is a photo detector arranged to access the evanescent field a produce an electrical signal related thereto Means may be provided for maintaining the photo detector and the access region in a fixed relationship which includes the photo detector being in contact with the access region of the fibre A lens may be interposed between the access region and the photo detector. Additionally or alternatively, a polariser or a wavelength filter may be interposed between the access region and the photo detector

A plurality of photo detectors may be provided, each with a different polariser or wavelength filter for detecting different polarising fields or wavelengths An array of photo detectors may be provided with an array of elements provided between the detector array and the fibre, the elements being selected from one or more of polarisers and wavelength filters A plurality of fibres may be arranged in parallel and have aligned access areas and a photo detector array may span all of the access regions Alternatively, the optical element may comprise a second optical fibre, the end of which IS located adjacent to the access region for capturing light output from the evanescent field and a lens may be interposed between the access region and the end of the second fibre According to a second aspect of the invention, a channel monitor for a multichannel optical fibre comprises means for splitting an input fibre into a plurality of fibres each having an aligned access regions and each carrying a single channel, an array of photo detectors spanning the access regions of the said plurality of fibres, and means for combining the plurality of fibres into a single output fibre The optical fibres may be a single mode, multimode or polarisation maintaining fibre According to a third aspect of the invention, a control arrangement for controlling the power in an optical fibre comprises a monitor as above described, a variable optical attenuator upstream of the monitor and control means for controlling the attenuator including an input for setting the desired power and means for comparing the output from the monitor with the desired power input

According to a fourth aspect of the invention, a control arrangement for providing constant optical attenuation in an optical fibre comprises a vanable optical attenuator controlling the attenuation of the fibre, a first monitor as descnbed above upstream of the attenuator, a second monitor as descnbed above downstream of the attenuator and control means for controlling the attenuator including means for determining the attenuation in the fibre from the outputs of the two monitors an input for setting the desired attenuation and means for comparing the determined attenuation with the desired attenuation and controlling the attenuator accordingly. power input The invention will now be described in greater detail, by way of example, with reference to the drawings, in which Figure 1 shows schematically the mounting of an optical fibre for polishing Figure 2 shows schematically the result of a second method of optical fibre polishing : Figure 3 shows schematically a first embodiment of the invention, Figure 4a shows schematically one arrangement for the mounting of the photo detector of figure 3 : Figure 4b shows schematically a second arrangement for mounting the photodetector of figure 3; Figure 5 shows schematically the use of a polariser with the photo-detector; Figure 6 shows schematically the use of two spaced photo detectors and polarisers

Figure 7a shows schematically the use of a wavelength filter with the photo detector Figure 7b shows graphically the transmitted power and wavelength using the set up of figure 7a, Figure 8a shows schematically a set up using a number of photo detectors to measure different wavelength of transmitted power, Figure 8b is a graph similar to figure 7b showing the results of the use of the set up of figure 8a; Figure 9 shows the use of a multi-filter array, Figure 10 shows schematically an arrangement of a number of fibres mounted with their reduced cladding regions aligned ; Figure 11 shows schematically the use of a detector array with the mounting arrangement shown in figure 10, Figure 12 shows schematically an arrangement for capturing light Into a second fibre.

Figure 13 shows schematically an arrangement for detecting the direction of propagation in a fibre ; Figure 14 shows schematically an arrangement for controlling the power in an optical fibre ;

Figure 15 shows schematically an arrangement for maintenance of constant attenuation In a fibre ; Figure 16 shows schematically an arrangement for monitoring individual channels in an optical fibre, and Figure 17 shows schematically an alternative arrangement for channel monitonng Referring firstly to figure 3, a first embodiment of the invention is shown The figure shows an optical fibre 1 having a cladding 3 and core 5 with an access region 9 in which the cladding 3 is reduced by the method discussed above In relation to figure 2. An optical detector 11 provided with electronic output leads 13 is positioned adjacent to the optical fibre 1 in the region 9 so that it detects the evanescent field and converts this into an electrical signal on the leads 13. A lens 15 may be provided between the fibre 1 and the photo detector 11 if desired.

In practice, an holding mechanism (not shown) IS used for holding the optical fibre with the exposed face vertical, such as a V-groove etched or machined into a suitable mounting material to hold the fibre firmly and allow It to be fixed permanently. The photo detector 11 IS likewise mounted in the mounting material with its active area in close proximity to the exposed face of the access region 9 so as to mechanically hold it in a fixed position. The detector 11 is held vertically above the fibre 1 with its active surface parallel to the polished face or at an angle to the surface appropriate for the light In radiation modes escaping the fibre. The level of light reaching the detector 11 can be modified by altering the remaining cladding thickness or adjusting the distance between the fibre 1 and the detector 11. The lens 15, if used, is placed between the fibre surface and the detector to concentrate the light onto the detector active surface area. The whole assembly is packaged for mechanical rigidity.

Figure 4a shows an arrangement to fix the detector In the form of a package 17 directly positioned directly on the optical to the fibre surface In the access region 9 Thus the detector is pre-mounted In a housing 19 with a glass or lensed window and the access region of fibre 1 IS fixed permanently to the window using for example an optical epoxy.

A more compact version is of this arrangement is shown in figure 4b Here a chip detector 21 IS used without housing and fixed directly to the fibre 1 in the access region 9. For this embodiment the level of power (number of photons) reaching the detector active surface can be optimised by varying the remaining cladding thickness. The optimisation will ensure sufficient detected power with low insertion loss The evanescent field approach is generally applicable to all known optical fibre types and dielectric waveguides In the next embodiment of the invention (Figure 5) Information IS detected in relation to a polarisation maintaining optical fibre in which two linear polarisation states are defined in the fibre.

The embodiments so far described have monitored optical power level, but other information about the light signal is often required. Placing an optical element between the access region of the fibre and the detector can select the specific charactenstic sought.

Figure 5 shows an aligned polariser 23 placed between the detector 21 and the access region 9 of the fibre. This will enable the power in a selected polarisation state to be monitored. This is particularly important for PM fibres in which the two polarisation states may have different power levels.

Figure 6 shows an arrangement for detecting the power in two orthogonal polarisation states simultaneously. For this purpose, two detectors 21 are used and the polarising elements 25 and 27 between the two detectors 21 and the access region 9 are set at right angles relative to one another In a similar fashion, wavelength filters can be used to select a specific wavelength (Figure 7a). Thus the arrangement is similar to that of figure 5 except that the polariser 23 is replaced with a wavelength filter 29. The filter 29 can be designed to filter specific Dense Wavelength Division Multiplexer (DWDM) channels In a communication network for example, to detect the power level or assess whether the channel is'lit'. The filters can, as shown, be placed between fibre 1 and detector 21, formed on the surface of the access region 9 of the fibre 1 or formed on the surface of the detector 21. A typical output from the detector 21 is shown in figure 7b.

In the embodiment of figure 8a several detectors 21 are used with different wavelength selecting filters 31. These detectors 21 and their associated filters 31 are placed along the surface of the access region 9 to access a number of channels at once. Figure 8b shows the type of out put which can be obtained from such a detector system.

In an alternative embodiment shown in figure 9, a linear detector array 33 IS used together with a series of discrete filters or a graded filter 35. Several of these devices can be cascaded to cover the full channel range for a network Multi-channel communication systems demand multiple components in a package. All of the previously discussed embodiments can be adapted for use In multi-fibre environment.

Figure 10 shows a way In which a number of optical fibres 1 can be positioned In parallel. If these fibres 1 have been treated to reduce the cladding thickness at

certain points to produce access areas 9 then the fibres can be placed In a carrier 39 and held with their access regions 9 transversely aligned. Then several linear arrays 41 (figure 11) or a single two dimensional array could be used across all or, in any event, several fibres. In this case, the power in each fibre is detected by addressing the appropriate detector element Several such arrays used together enable multi-channel versions of the other components to be realised For remote detection, as shown in figure 12, an additional fibre 43 can be placed in close proximity to the exposed surface of the access region 9 to guide a portion of the light to a detector (not shown). A lens 45 at or on the fibre end 47 will enhance the level of power launched Into the sampling fibre 43.

In some applications, directionality along the fibre of the optical signal is important. This can be detected using the arrangement shown in figure 13 The relative power level detected by detectors is a function of the angle between the detector and the fibre with a maximum when the detector is angled to match the exit angle of the light. Two detectors 21 placed optimally for each direction enable the levels of power transmitted in each direction to be detected and thus the directionality determined Figure 14 shows an application of the invention used for power control by control of a power level controlling variable optical attenuator 51 The power level In the

optical fibre 1 after the attenuator 51, is detected by a photo detector 53, constructed in accordance with any suitable preceding embodiment An electronic conditioning circuit 55 gives an output voltage proportional to the sampled power level. The voltage is compared in a control circuit 57 to a set voltage level provided by input 59 and an error signal generated. The error signal controls the attenuator 51 to maintain the power level detected by the monitor 53 and consequently the power level in the fibre 1

Similarly, for example, the power from a laser can be controlled by feedback to the laser power control circuitry Figure 15 shows the use of an attenuator 61 to provide a fixed attenuation. In this case the circuit is similar to that of figure 14 but with an additional detector 63 on the other side of the attenuator 61 to the detector 53. Here, the control circuit 65 generates an error signal to control the attenuator 61 which is derived by taking the ratio of the two detected voltages and comparing it with the input set voltage on the input line 59.

Placed in a fibre the detector will produce an output current when there is light in the fibre and no current when light IS absent This provides a low loss method of checking for signals in fibre lines Figure 16 shows a channel monitor In which the optical channels carried by a single fibre 71 in a DWDM network are split into individual channels in individual fibres 73 through a Wavelength Division Multiplexer (WDM) 75 and the relative power levels of each channel can be monitored and adjusted if necessary using an attenuator. Individual detectors or a detector array 77 can be used. The channels are then recombined by a second WDM 79 Into a single output fibre 81 Figure 17 shows an alternative channel monitor In which no splitting of the fibre IS required. A single fibre 1 IS used and a line of detectors 21 are used, each of the detectors having different filtering characteristics along the access region of the fibre surface.

It will be appreciated that the above described monitors can have many other applications, including, for example, spectral analysis

Claims

CLAIMS :A monitor for monitoring the optical signal parameters in an optical fibre comprising a fibre having an access region of reduced cladding sufficient to allow access to the evanescent field of the optical fibre and an optical element mounted adjacent to the access region to obtain access to the evanescent field so as to enable use to be made of the data therein.
2. A monitor as claimed in claim 1 wherein the fibre is a single made fibre
3 A monitor as claimed in claim 1 wherein the fibre is a multimode fibre
4. A monitor as claimed in claim 1 wherein the fibre is a polarisation maintaining fibre
5 A monitor as claimed in claim 1 to 4, wherein the optical element is a photo detector arranged to access the evanescent field and produce an electrical signal related thereto
6. A monitor as claimed in claim 5, wherein means are provided for maintaining the photo detector and the access region In a fixed relationship
7. A monitor as claimed In claim 5 or 6, wherein the photo detector IS In contact with the access region of the fibre
8 A monitor as claimed In claim 5 or 6, wherein a lens Is Interposed between the access region and the photo detector
9 A monitor as claimed In claim 6 or 8, wherein a polariser IS Interposed between the access region and the photo detector

<Desc/Clms Page number 13>
10 A monitor as claimed in claim 9, wherein a plurality of photo detectors are provided, each with a different polariser for detecting different polarizing fields
11. A monitor as claimed in claim 6 or 8, wherein a wavelength filter IS interposed between the access region and the photo detector
12 A monitor as claimed in claim 10, wherein a plurality of photo detectors are provided, each with a different wavelength filter for detecting different wavelengths.
13 A monitor as claimed in any one of claims 5 to 12, wherein an array of photo detectors are provided with an array of elements are provided between the detector array and the fibre, the elements being selected from one or more of polansers and wavelength filters
14. A monitor as claimed in any preceding claim wherein a plurality of fibres are arranged in parallel and have aligned access areas and a photo detector array spans all of the access regions.
15 A monitor as claimed In claim 1,3 or 4, wherein the optical element comprises a second optical fibre, the end of which is located adjacent to the access region for capturing light output from the evanescent field.
16. A monitor as claimed in claim 15, wherein a lens is interposed between the access region and the end of the second fibre.
17 A channel monitor for a multi-channel optical fibre comprising means for splitting an input fibre into a plurality of fibres each having an aligned access regions and each carrying a single channel, an array of photo

<Desc/Clms Page number 14>

detectors spanning the access regions of the said plurality of fibres, and means for combining the plurality of fibres into a single output fibre
18 A monitor according to claim 17 wherein the fibre is a single mode fibre
19. A monitor according to claim 17 wherein the fibre is a multimode fibre.
20. A monitor according to claim 17 wherein the fibre is a polarisation maintaining fibre.
21. A control arrangement for controlling the power in an optical fibre comprising a monitor as claimed in claim 2,3 or 4, a variable optical attenuator upstream of the monitor and control means for controlling the attenuator including an input for setting the desired power and means for comparing the output from the monitor with the desired power input
22. A control arrangement for providing constant optical attenuation in an optical fibre comprising a variable optical attenuator controlling the attenuation of the fibre, a first monitor as claimed in claim 5,6 or 7 upstream of the attenuator, a second monitor as claimed in claim 5, 6 or 7 downstream of the attenuator and control means for controlling the attenuator including means for determining the attenuation in the fibre from the outputs of the two monitors an input for setting the desired attenuation and means for comparing the determined attenuation with the desired attenuation and controlling the attenuator accordingly
23. A monitor for monitoring the optical signal parameters in an optical fibre substantially as described herein with reference to the drawings
24. A channel monitor for a multi-channel optical fibre substantially as described herein with reference to the drawings

<Desc/Clms Page number 15>
25. A control arrangement for controlling the power in an optical fibre substantially as described herein with reference to the drawings.
26. A control arrangement for providing constant optical attenuation In an optical fibre substantially as described herein with reference to the drawings.