GB2179145A - Optical fibre end-face assessment - Google Patents

Abstract

A method for assessing the quality of an optical fibre cleaved end-face (41) comprises using a discard portion (4a) of the optical fibre (4) with a corresponding end- face (41a) produced on cleaving of the optical fibre (4) and measuring the optical power reflected from the corresponding end-face (41a) relative to a predetermined optical power threshold. Apparatus for performing the method comprises a measuring unit (1) joined by a length of optical fibre (11c) to a coupling (2). <IMAGE>

Description

SPECIFICATION Optical fibre end-face assessment This invention relates to assessment of optical fibre end-faces, and has an important application in the preparation of optical fibres for connection in optical communication systems.

In optical fibre communication systems fibreto-fibre splices are often required. Traditionally, the fibre-fibre splices required are made by fusion splicing.

Fibre ends are usually prepared for this splicing by cleaving, which provides the fibre with the flat and clean end-face needed for a good splice connection. Only a few centimetres of fibre are normally removed from the end of a fibre to be used, and discarded. The end-face of another fibre is similarly prepared and the two fibres are then aligned under a microscope and fused. The loss introduced by a splice depends critically on the quality of the fibre end-faces.

To result in a good, low-loss splice the endface produced by cleaving should be both perpendicular to the fibre axis and flat. For a good connection the plane of the end-face angle should not deviate by more than approximately 1.0 from this perpendicularity. At greater deviations the potential splice loss increases markedly. Additionally, if the end-face is not flat, for example owing to chipping or fracturing during cleaving, the subsequent fused splice is also likely to show an unsatisfactorily high loss level.

In practice, fewer than half of the end-faces produced by cleaving have been found to be acceptable. Field operators have heretofore relied on visual inspection of a fibre through a microscope, and on experience to judge whether or not a cleave is of adequate quality.

Badly chipped ends are usually readily apparent, but the subjective assessment of a cleaved end-face angle to within 1.0 is not consistently reliable.

Methods of, and apparatus for accurately measuring the cleaved end-face angle are available. One such, for example, is disclosed in published European Patent Application FP 167269. However, specialised training is needed both to operate such apparatus and to interpret the results therefrom. Presently therefore, such equipment is not suited to everyday field use.

It is an object of the present invention to provide a method of assessing the quality of an optical fibre cleaved end-face, which method is readily adaptable for field use.

It is a further object of the present invention to provide an apparatus adapted to perform the method.

According to one aspect of the present invention a method of making an assessment of an optical fibre cleaved end-face comprises assessing the quality of a corresponding cleaved end-face of a discard portion produced as a result of cleaving said optical fibre.

The quality of the corresponding cleaved endface of the discard portion is evaluated by measuring the optical power reflected therefrom relative to a predetermined power threshold using optical power measuring means.

Conveniently the method includes coupling the optical fibre to the power measuring means before cleaving.

Preferably the reflected power is determined as a ratio of reflected power to incident or input power. The power threshold represents the allowable limit of deviation of the measured ratio relative to the same ratio for an ideal or known best quality cleaved end-face.

The further the deviation the lower is the quality of the cleave. The threshold is therefore set at a limit of deviation beyond which the cleave quality is considered unacceptable and below which the cleave quality is considered acceptable for splicing.

According to a second aspect of the present invention apparatus for assessing the quality of an optical fibre cleaved end-face comprises a connector to receive an original end of a discard portion of an optical fibre, means for measurement of the optical power reflected from a cleaved end-face of said discard portion, after cleaving thereof from said optical fibre, relative to a predetermined optical power threshold, and means to indicate the result of the said measurement.

Preferably the reflected and threshold powers are measured as ratios of incident or input power to reflected power.

The apparatus may conveniently include an optical fibre cleaver.

Preferably, the means for measurement comprises an optical source, a directional coupler and an optical detector.

The optical source may, for example, be a light emitting diode.

The measuring means may further comprise means to modulate the optical source and means, for example, a phase locked loop, to synchronise the detector with the modulation.

Conveniently, the coupling means comprises an optical fibre connector attached by a length of optical fibre to the optical power measuring means.

The present invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram showing an apparatus according to the invention; Figure 2 is a schematic sectional view of a connector suitable for providing the coupling means of the invention; and Figure 3 is a graph of the reflected power loss from a cleaved fibre end versus fibre end-face angle.

Referring first to Figure 1, an apparatus for assessing the quality of a cleaved fibre end face comprises a measuring unit 1 joined by a length of optical fibre 1 1c to a coupling 2.

This apparatus is provided in combination with a fibre cleaver 3. The construction of a cleaver is well known and will not be described in detail here. The representation of the cleaver 3 in Figure 1 shows typical common features, which include an anvil 33, fibre clamps 32 and a scriber blade 31.

In operation, an optical fibre 4, to be cleaved, is positioned on the cleaver 3. A short length 4a, usually a few centimetres, forming the discard end portion, of the fibre 4 is allowed to extend beyond the cleaver and is inserted in the coupling 2. In the cleaver 3, the scriber blade 31 scores the fibre 4 and initiates crack propagation which splits the fibre 4 providing a cleaved end-face 41 on the fibre which may then be spliced to another similarly prepared fibre (not shown), and providing a corresponding end-face 41a on the separated discard portion 4a.

In the past, assessment of the quality of the cleaved end-face 41 relied on visual and microscopic examination of that end-face at this stage to determine whether or not this prepared end was suitable for splicing. In contrast, the present invention relies on the appreciation by the applicants that a particular result of the cleaving process, namely that the two so-called 'cleaved' ends 41 and 41a, of the fibre 4 and fibre discard portion 4a respectively, are expected to have matching characteristics, and importantly, identical endface angles (an assumption consistently borne out by field experience), can be used to permit a better, less subjective assessment of cleave quality.Measurements can be made on the discard portion 4a and end-face 41a that it may not generally be possible to make on the main length of fibre 4, (for example, because the fibre 4 may already be connected to optical apparatus at the end remote from the cleave). The identity of character of ends 41 and 41a created by the cleave means that an assessment of the end 41, which will eventually be connected or spliced into an optical system can thus be made by conducting measurements with respect to the end 41 a on the short discard end portion 4a, which has hitherto been ignored.

After the cleaving, and after releasing any clamps -32, on the cleaver, which might otherwise induce spurious reflections, for example, owing to microbending, measurements are made as follows using the measuring unit 1.

Light from an optical source 10, which may be a light emitting diode, passes via fibre 1 lea and Y-coupler 11 to fibre 1 1c. The light then enters the fibre discard portion 4a via the coupling 2. Reflections at this interface, which would be detrimental to the ultimate quality assessment, may be substantially eliminated by dipping the original end-face of the fibre discard portion 4a into index matching gel before insertion in the coupling 2.

A proportion of the light which enters the discard portion 4a is reflected at the end-face 41a and is then routed back via the directional Y-coupler 11 into fibre 1 1 b. (It should be observed that a typical Y-coupler of this form will provide at least 65dB optical isolation between fibres 1 1a and 1 1 b, such that negligible optical power is coupled directly from the source 10 into fibre 1 1b.) It is the proportion of optical power reflected at the end-face 41 a which provides a measure of the quality of the cleave. This proportion is very small (less than 4% of incident power), and the measurement apparatus must therefore be adapted where necessary to handle low optical powers.

The reflected light is then incident on detector 12, which may, for example, comprise an avalanche photodiode with associated Peltier cooler to reduce the intrinsic noise level.

To further improve the detector resolution, a modulator 14 may be provided to modulate the source. A suitable synchronisation circuit 15, for example, a phase locking amplifier circuit is then required to synchronise the detector operation.

The detector photocurrent provides a measure of the reflected optical power. The reflected power measured will depend on the quality of the reflecting cleaved end face as already described and also on the original launch power of source 10. If this launch power is highly stable and closely controlled, which is achievable using known techniques, then an absolute measurement of the reflected power may be used as to indicate the relative quality of a cleaved end face. However, source stabilisation may require more circuitry than is desirable for a field unit according to the present invention (eg. for cost).

In the alternative case where the source 10 has unstabilised launch power, absolute measurements of reflected power will not be repeatable, and relative measurements must be made. This involves taking the ratio of the reflected power (P2) to the launch power (P1).

(The reflected power loss is absolutely determined from the equation: Power Loss = l0Log(P2/P1) dB.) The magnitude of the ratio is then compared with an predetermined GO/NOGO threshold and an assessment of the relative quality of the cleave is thereby obtained. The threshold is conveniently determined and set at a shop calibration, for example, as described in more detail below.

An indication of whether the loss is above or below the threshold is provided on display 13, for example, as a green light for loss below threshold or red light for loss above.

Figure 2 shows a suitable coupling 2 in more detail. The coupling is effectively a conventional butt connector with only one fibre 1 1c permanently mounted. This fibre 1 1c may conveniently be a multimode fibre of large core diameter (e.g. 50mm) to provide a large coupling cross section and to reduce misalignment problems when the fibre end portion 4a is inserted. The fibre 1 1c is fixed in a ferrule 21 which is held in an alignment sleeve 20. A second ferrule 22 is provided with an aperture through which the original end of the discard portion 4a of the fibre to be tested is inserted.

The end 23 of fibre 1 Ic may be carefully prepared to have an end-face of high quality in order to provide a suitable reference for the shop calibration of the apparatus as mentioned above. For the calibration the end 23 of fibre 1 1c must be clean and dry in air (ie.

without index matching fluid in coupling 2 and without the end of fibre discard portion 4a present, but otherwise with the arrangement of Figure 1). The apparatus is then in a minimum loss configuration. The total loss in such a situation comprises Y-coupler loss (approximately 3dB), reflection loss at the end 23 fibre-air interface (approximately 14dB) and other losses due to fibre splices and connections (approximately 1dB). Thus a theoretical minimum loss of approximately 18dB is expected in this arrangement. The actual minimum for a given apparatus, of course, is determined from practical measurements under the conditions described. The GO/NOGO threshold is then set at a desired level, corresponding to the extra loss introduced at an end-face with the maximum end-angle permissible, above the minimum.

Figure 3 is a graph of reflected power (as measured on the apparatus of Figure 1) versus end-face angle (measured independently) for a number of cleaved fibres, showing the GO/N OGO resolution available. Fibres with an angle below 1.0 were considered acceptable for the purposes of the experiments from which these data were derived. In this case a loss greater than 20dB implies that the end-face angle is unacceptable and likely to produce a poor splice. Thus the threshold would be set at a loss level approximately 2dB greater than the minimum (nominally 18dB). A measurement showing a loss over the threshold indicates that the fibre should be re-cleaved.Although insertion losses of around 0.2 to 0.5dB, not related to the quality of the cleave, may be associated with the introduction of fibre end portion 4a into the coupling 2, these extra losses will lead to rejection of cleaves which might otherwise give a satisfactory loss below threshold, and not to acceptance of undesirably bad cleaves. Thus there is inherent protection in the system, since any operational deficiencies are likely to increase rather than decrease the measured loss.

It should be noted that although the data of Figure 3 are shown with respect to end-face angle, the measurements made by the apparatus of the present invention are not solely dependent on this end-face angle. The overall quality of a fibre end-face, including the extent of any cracks and chipping, for example, will also influence the reflectivity. Thus the measurements do not indicate what exactly is 'bad' about a cleaved end-face, but merely allow a 'good' end face to be differentiated from a 'bad' one. Since a 'bad' cleave will have to be repeated whatever the actual reason for it being 'bad', this GO/NOGO resolution is normally quite adequate for field purposes.

Claims (9)

1. A method of making an assessment of an optical fibre cleaved end-face comprising assessing the quality of a corresponding cleaved end-face of a discard portion produced as a result of cleaving said optical fibre.

2. A method according to claim 1 wherein the quality of the corresponding cleaved endface of the discard portion is evaluated by measuring the optical power reflected therefrom relative to a predetermined power threshold.

3. Apparatus for assessing optical fibre cleaved end-face quality by assessing the quality of a corresponding end-face of a discard portion produced as a result of cleaving said optical fibre, said apparatus comprising a connector to receive an original end of said discard portion, means for measurement of optical power reflected from the cleaved endface of said discard portion, after cleaving thereof from said optical fibre, relative to a predetermined optical power threshold, and means to indicate the result of said measurement

4. Apparatus according to claim 3 further comprising a cleaver for cleaving said optical fibre to produce said end-face and said discard portion with said corresponding end-face.

5. Apparatus according to claim 3 or 4 wherein the means for measurement of optical power comprises an optical source, a directional coupler and an optical detector.

6. Apparatus according to claim 5 wherein the means for measurement further comprises means for modulating the optical source and means for synchronising the optical detector with the modulation.

7. Apparatus according to any one of claims 3 to 6 wherein the connector is coupled to the means for measurement of optical power by a length of large core multimode optical fibre.

8. A method of assessing optical fibre cleaved end-face quality substantially as hereinbefore described and with reference to the accompanying drawings.

9. Apparatus for assessing optical fibre cleaved end-face quality substantially as hereinbefore described and with reference to the accompanying drawings.