GB2180641A - Optical fibre end face finish assessment - Google Patents

Abstract

Light from l.e.d. 3 is split by beam splitter 8 into two sub-beams which are directed respectively to the fibre end face 2 to be assessed and to a high finish quality reference block 9. Reflected and/or scattered light is directed to detector 10. The amount of light is first detected with no fibre present, and then with the fibre present. A ratio, e.g. of the two amounts, gives an indication of the quality of the end face finish of the fibre. The beam splitter may be a polarizing beam splitter, a quarterwave plate 15 then being positioned in the incident and reflected beam to the reference block 9. <IMAGE>

Description

SPECIFICATION Optical fibre end face finish assessment This invention relates two an apparatus and method forassessingthefinishofa prepared end of an optical fibre.

The optical power loss through a fibre optic connectordepends upon the alignment, proximity and quality of finish ofthefacingfibre end faces. Align- ment and proximity of the end faces are determined by the mechanical design ofthe connector butthe quality of the end face finish is determined bythepre- paration ofthe end face during assembly ofthefer- rule containing thefibre end intotheconnector.

Currently, connector-terminated optical fibres are finished using either'crimp and cleave' techniques or'grind and polish'techniques. Intheformer,the bentand/ortensionedfibre is nicked buy a diamond edge which initiates fracture. In the latter, the fibre end is ground and polished until a satisfactory finish is achieved. At present the quality of the end facefinish is normally assessed visually by the use of a microscope and is purely a qualititive, subjective assessment based on the experience of the person undertaking the assessment. Alternatively, interferometric assessment techniques can be empioyed.

The current assessment techniques has a number of disadvantages: relatively skilled people are required to undertake the assessment and even with a skilled opeative the actual power loss through similarconnectors on the same linkor networkcan be highly variable. In many situations, physical access limitations can prevent the use of optical microscopy for fibre end assessment. It can be difficult to measure the actual power loss in an installed fibre optic link or network since this involves the measure mentofoptical power launched into a fibre atone end and measurement of output poweratthe farend.

In many situations, such as a factory, process plant or aircraft, access to each end ofthe link or network simulataneously may be difficult or impossible and will involve more than one person.

The object of the invention is to overcome these problems by providing a quantitative assessment of the fibre wend finish from one end of the fibre.

Viewed from one aspect, the invention provides a method of assessing the finish quality of a prepared end of an optical fibre, comprising providing a beam of light, splitting the beam into two sub-beams, directing tlie sub-beams onto the fibre end and a refer encesurfaceofhighfinish quality of the same material as the fibre, respectively, successively detecting with the same detector the amounts of light reflected and/or scattered back from said end and surface, or from one of these and from both of them, and providing an indication ofthe finish quality on the basis of the relation between the two detected amounts.

In one form of the invention the total intensities of the light reflected and scattered back from the fibre end and the reference surface are detected. In another form the apparatus is arranged to detectthe intensities of lightthe plane of polarisation of which is rotated through 90" by double scattering at the fibre end and reference surface.

Preferably the ratio of the two detected amounts is determined and the indication provided on the basis of the ratio. A numerical value ofthe ratio may be provided or a binary (accept/reject) indication may be given in dependence on the ratio being with a predetermined range.

Preferably the detector detects the amount of light reflected and scattered back from said surface, and the sum ofthis amount and the amount reflected and scattered back from the fibre end.

Viewed from another aspect, the invention provides apparatus for assessing the finish quality of a prepared end of an optical fibre, comprising a source of light, a beam splitter arranged to splitthe light from the source into two sub-beams and to direct them onto said fibre end and a surface of high finish quality of the same material as the fibre, respectively, and a detector arranged to receive light reflected and/ orscattered backfrom saidfibre end and said surface.

Preferably the light is formed by a lens arrangement into axially symmetric beams which are directed with their axes at substantially normal incidence onto the fibre end and the surface, and preferably a further lens arrangement collects the reflected and scattered light and directs itto said detector. An aperture arrangement is preferably provides to define the area and convergence angle ofthe light incident on thefibre end. The light source is preferably a lightemitting diode, and may be modulatedto enable enhanced rejection ofthe effects of spurious lightand ofthermal and temporal driftofthe processing circuitry.

The apparatus preferably includes an electronic circuit arranged to store an indication ofthe amount of light reflected and scattered from said surface, and the sum ofthis amount and the amount reflected and scattered from said end face, and to provide an indication offinish quality in dependence on these amounts.

The invention enables a quantitative assessment of the quality of finish of a prepared end of an optical fibre and thus allows the preparation of repeatable high quality fibre end terminations by relatively unskilled operatives and the construction of a fibre optic linkor network having low optical power loss.

Preferably the wavelength and/or the modulation characteristics of the light used differsfrom that of the signal-carrying light. This enables measurementsto be made offibres connected in 'live' systems.

Certain embodiments ofthe invention will now be described by way of example and with reference to the accompanying drawing in which the sole Figure is a schematic diagram of an optical fibre end face finish measurement apparatus in accordance with the invention.

Referring to the drawing, a fibre optic end face finish measuring instrument 1 includes a precision hole for receiving a connectorferrule 2 containing the prepared end of an optical fibre. Mechanical arrange mentsonthefrontpanel of the instrument and within the instrument allow different connectorferrules, and hence different fibre sizes, to be accurately located within the precision hole.

An optical arrangement within the instrument comprises a light emitting diode 3 which is provided with a modulated drive signal, e.g. at 2kHz. Lenses 4 and 5focusthe light from the light emitting diode onto the end face of the optical fibre. The diameter of the beam of light is set at 90% of the fibre corediame- ter by aperture 6 and the core angle is set at 90% of the nominal fibre core angle by aperture 7. Disposed between the aperture 6 and the fibre end face is a 50% beam splitter 8. This may be a split cube type with antireflection coated faces.The beam splitter splits the light beam from the light emitting diode so that one half is transmitted to fall on the optical fibre end face and the other half is reflected to fall on a highly polished reference block 9 of the same material as the optical fibre, e.g. silica. Light reflected or scatteredfromthe optical fibre orthe block9 returnstothe beam splitter 8 and one half of each passes to an electro-optical detector 10 via a high-aperture lens 11.

The other half passes to the right towards the light emitting diode and has no effect.

The electronic control circuits of the instrument in clude an LED drive circuit 12 and amplifier and pro cessing circuitry 13. A synchronisation signal is supplied fromthe drive circuit 12 to the processing circuit 13 and is used for filtering the signal received from the detector 10 to eliminate the effects of any background light.

The instrument may operate underthe control of a microprocessor. When the instrument is switched on and with no connectorferrule located in the precision hole, the output ofthe detector 10 is sampled by the processing circuit 13. The value of the output is sto red and is referred to as Rs. This value relates directly to the total reflected and back scattered light from the reference silica block 9. When the ferrule with the prepared fibre is located in the precision hole, the re flected and back scattered light adds to that from the reference block. The electronics detect this change and stores the new detected output as Rf.The circu itrythen calculates the ratio Rf/Rs and displays it as a four-digit numberon a suitable display 14; the user oftheinstrumentcanthen determinefromthisvalue whetherthefinish quality is acceptable. Alternati vely, orin addition, a 'go' indicator, e.g. a green LED, may be illuminated if the detected light from the fibre satisfies suitable criteria e.g. 1 .8Rs < Rf < 2.2Rs, or a 'no go' indicator (e.g. a red LED) outside these criteria, e.g. Rf > 2.2Rs. As will be clearfrom the above,the instrument determines quantitivelythe amountof reflected and back scattered light from the prepared fibre end.If the fibre/air interface were per fect,the normal incidence reflection coefficient for silica core fibre would be typically 3.5%. Expected variations of the refractive index would vary this re flection coefficient by less than 0.5%. Therefore, if light is launched into a perfectly polished, concentric fibre core, less than 5% is reflected back. An imper fect end finish will reduce the coupling of the light into the fibre and thus cause both increased reflec tion and scattering. A serious imperfection in the end face, such as the fibre being missing or badly pitted, will cause a substantial reduction in the amount of light reflected and scattered.

The light emitted by the LED 3 preferably has a wavelength removed from the wavelengths normally used in fibre optic communications. Wavelengths commonly used are 850nm and 1300nm, and a suitable wavelength for the output of the LED 3 is 660nm.Thisallowstheinstrumenttooperatewhilst the fibre optic network is operational and whilst light may be emitted by the fibre. Alternatively or in addition the modulation characteristics ofthe light output by LED 3may be distinguishably different from those ofthe light in the network.

An alternative form of the invention makes use of thefactthat rougher surfaces produce an increassed proportion of light which is scattered back after two or more reflections. The plane of polarisation of such multiply scattered light may be orthogonal to that of the incident beam. In thisform a polarising beam splitter8 is used and a polarising plate, e.g. a 1/4 wave plate is positioned at 15 in the incident andreflected beam to the reference block 9. Unpolarised Iightfrom LED 3 is reflected bythe beam splitter8 towards block9 and light which has its plane of polarisation rotated 90" passes backthrough the beam splitter 8. The beam splitter 8 also acts as a polariser for light passing to the fibre end and then selects orthogonal polarisation light reflected from the fibre end face and reflects ittothe detector.

Claims (18)

1. A method of assessing the finish quality of a prepared end of an optical fibre, comprising providing a beam of light, splitting the beam into two subbeams, directing the sub-beams onto the fibre end and a reference surface of high finish quality ofthe same material as the fibre, respectively, successively detecting with the same detectorthe amounts of light reflected and/or scattered back from said end and surface, or from one of these and from both of them, and providing an indication of the finish quality on the basis of the relation between the two detected amounts.

2. A method as claimed in claim 1 wherein the total intensities ofthe light reflected and scattered back from the fibre end and the reference surface are detected.

3. A method as claimed in claim 1 wherein are detected the intensities of lightthe plane of polarisation of which is rotated through 90" by double scattering atthefibre end and referencesurface.

4. A method as claimed in claim 1, 2 or 3 wherein the ratio ofthe two detected amounts is determined and the indication provided on the basis of the ratio.

5. A method as claimed in claim 4 wherein an indication is provided as a numerical value of the ratio.

6. A method as claimed in claim 4wherein a bi nary indication is given in dependence on the ratio being within a predetermined range.

7. A method as claimed in any preceding claim, wherein the detector detects the amount of light re flected and scattered back from said surface, and the sum ofthis amount and the amount reflected and scattered backfrom the fibre end.

8. A method as claimed in any preceding claim, wherein the wavelength and/orthe modulation char acteristics of the light used differs from that ofthe signal-carrying light.

9. Methodsofassessingthefinish quality of a prepared end of an optical fibre, substantially as hererin before described with reference to the accompanying drawings.

10. Apparatusforassessingthefinish qualityofa prepared end of an optical fibre, comprising a source of light, a beam splitter arranged to split the light from the source into two sub-beams and to direct them onto said fibre end and a surface of high finish quality of the same material as the fibre, respectively, and a detector arranged to receive light reflected and/ orscattered backfrom saidfibre end and said surface.

11. Apparatus as claimed in claim 1Owhereinthe lightisformed bya lens arrangement into axially symmetric beams which are directed with their axes at substantially normal incidence onto the fibre end and the surface.

12. Apparatus as claimed in claim 11 wherein a further lens arrangement collects the reflected and scattered light and directs itto said detector.

13. Apparatus as claimed in claim 10,11 or 12, wherein an aperture arrangement is provided to definethe area and convergence angle ofthe lightincident on the fibre end.

14. Apparatus is claimed in any of claims lOto 13 wherein the light source is a light emitting diode.

15. Apparatus as claimed in anyofclaims lotto 14 whrein the light source is modulated.

16. Apparatus as claimed in any of claims lotto 15 including an electronic circuit arranged to store an indication of the amount of light reflected and scattered from said surface, and the sum of this amount and the amount reflected and scattered from said end face, and to provide an indication of finish quality in dependence on these amounts.

17. Apparatus as claimed in any of claims lotto 16 wherein the beam splitter is a polarising beam splitter and a quarter-wave plate is positioned in the incident and reflected beam to and from said surface.

18. Apparatusforassessingthefinish qualityofa prepared end of an optical fibre, substantially as her einbefore described with referencetotheaccompanying drawings.