GB2405488A - Device for controlling the mode distribution in multimode optical fibre - Google Patents

Abstract

A device 1 for controlling the mode distribution in an optical fibre, comprising a mode-scrambler followed by a mode-filter. The mode-scrambler, which comprises a single loop of fibre 3 with a controlled load applied at the cross-over position 12 in the loop, creates a mode distribution that is slightly more fully-filled than the distribution that is required by application of the device. The mode-scrambler directs light towards a mode-filter, which comprises a controlled gap 7 between two sections of fibre, and serves to further modify the mode distribution by selective attenuation of higher-order modes. Adjustment of the applied load to the loop 3 and the size of the gap 7 enables a controlled mode distribution to be launched into a fibre which is attached to the output connector 9 of the mode control device.

Description

DESCRIPTION

FIELD OF THE INVENTION

This,nvent,u,, relates to a means u' mn.,o,,iny the moue distribution in a multimode optical fibre.

BACKGROUND OF THE INVENTION

An optical fibre is a transparent dielectric waveguide which provides a means of transmitting light signals at high data-rates over distances of many kilometres Furthermore, optical fibres can be bent round corners enabling the optical signal to be routed as required.

An optical fibre consists of a transparent central region, known as the core, surrounded by a protective transparent layer, known as the cladding. The refractive index of the core is normally slightly higher than that of the cladding, permitting light transmission along the fibre by the process of Total Internal Reflection (TIR). Further protective layers are usually applied to the fibre to protect it against damage and degradation of performance caused by handling during installation and environmental effects such as temperature and humidity.

To transmit information along the fibre, the light signal is typically switched on and off in a binary-encoded fashion, at rates of up to billions of times per second. In a typical multimode fibre it is possible for light to take a variety of routes along the fibre by the process of TIR depending on the angle with which the light was launched into the fibre.

Electromagnetic theory shows, however, that only certain angles, known as modes, are permitted in the fibre. A typical graded-index multimode fibre with a core diameter of 62.5um and a Numerical Aperture of 0.275 will potentially support about 1000 individual modes at an optical wavelength of 850nm.

Light travailing at steeper angles will clearly traverse a longer route than rays travailing along the axis of the fibre. This effect, known as modal dispersion, causes the pulses of light to spread out temporally and, in the worst case, overlap with neighbouring pulses, thus corrupting the transmitted information. To overcome this limitation the core region in a multimode fibre is normally manufactured with a graded refractive index profile, such that light travailing at steeper angles travels more quickly in the region near to the core/cladding boundary compared to light travelling along the axis of the fibre. With the correct choice of refractive index profile the propagation time for each angle can be equalised, thus limiting the amount of pulse spreading that occurs in the fibre.

Furthermore, higher-order modes, those corresponding to steeper propagation angles, experience a greater attenuation than iower-order modes, those corresponding to smaller propagation angles. This is particularly the case where the fibre is subject to bending, such as sharp corners in the installation ducting.

Thus both the attenuation and modal dispersion depend on the particular set of modes, modal distribution, that is launched into the fibre by the optical source.

The amount at model dispersion experit:nct:u by the optics, signal depends very much on the accuracy of the refractive index profile in the core. In so-called 'legacy' fibre, the refractive index profile of the core often deviates from the optimum shape and it has been found that the use of controlled modal launch conditions can result in reduced modal dispersion, and thus increased fibre bandwidth. Recent work by the Institute of Electrical and Electronic Engineers (IEEE) has produced standards on the modal distributions that are required for Optical Ethernet systems operating at 1 GbiVsec and Gbit/sec.

The loss of a fibre which is tested using a light source that excites all of the possible modes in the fibre, known as an 'overfilled launch' is generally higher than that obtained using a low angle source, such as a laser, that excites only low-order modes. Thus the attenuation of an installed fibre link will depend on the particular characteristics of the source that is used to test it. This situation can, therefore, lead to poor reproducibility when comparing measurements made with different test equipment, and is undesirable both contractually and metrologically. Likewise, the modal dispersion will also depend on the particular characteristics of the light source and the characteristics of the test equipment may not be representative of the source used in an installed fibre system.

There exists, therefore a need, to standardise launch conditions so that reproducible and representative measurements of both attenuation and dispersion may be made.

One such approach is to utilise what is known as an 'Equilibrium Mode Distribution' (EMD) launch. The EMD corresponds to a stable modal distribution that usually results from a process of mode-coupling in long lengths of fibre of, for example, several kilometres. Several prior art methods for creating an EMD have been described. One such method consists of overfilling the modal distribution in the fibre with a 'modescrambler' and then filtering out a proportion of the higher-order modes using a mode- hiter.

Many prior art methods exist for creating an over-filled launch using mode-scrambling techniques. Historically, on over-filled modal distribution has been used for the measurement of fibre dispersion, or bandwidth, as it has been found to give the most reproducible, if not representative, results. Examples of prior art are found in US4934787 where the fibre is wrapped in a figure-of-eight configuration around two cylinders, US4229067 where a series of alternating step-index and gradedindex fibres are joined together, DE3411272 where the fibre is passed in and out between a series of cylinders, JP54068254 where a series of fibre sections are laterally offset with respect to each other, JP60178409 where the fibre is passed in and out of two series of cylinders orientated at right angles to each other, US4877305 where the fibre is wound in a spiral fashion with turns of differing separation, JP53011141 where the fibre is pressed against a rough surface. As previously stated, these methods are generally designed for the purpose of over-filling the fbre under test and, as such, are not easily adjustable in the modal distributions they produce.

A particular example of a mode-filter is described in ANSI/EIA/TIA-45550B and IEC 61300-3-34 and consists of wrapping the;ibre 5 turns of fibre around a mandrel o, a given diameter, for example 20mm diameter for an uncoated fibre with a core diameter of 62.5um. The mandrel wrapping technique is said to preferentially attenuate high-order modes leading to a modal distribution that is representative of an EMD.

Another prior art method consists of directly launching an EMD distribution into the fibre under test by controlling both the spot-size and Numerical Aperture of the light source.

This method is, however, a bit cumbersome, making use of bulk optical components,

and is not practical for field use.

Another prior art method of achieving an EMD, described in EP0442731, consists firstly of filtering out all but the lowest-order modes that are present in the fibre and then introducing a means of mode-scrambling the remaining light such that it is redistributed to give an EMD. In this method, the initial mode-filtering is achieved by inserting a small longitudinal gap between two sections of fbre such that only light travailing at small angles, corresponding to very low-order modes, can pass between the cores of the two fibres. Mode-scrambling is then achieved by inserting a few turns of fibre around a mandrel, typically of similar dimension to that described above for the mode-filter, positioned immediately following the longitudinal gap. It is claimed that the mandrel redistributes the light into higher-order modes thus creating an EMD. It has, however, been found by the authors of the present invention that mandrel-wrapping has little effect on the modal distribution in the fbre if it is under-filled to start with. This is because mandrel wrapping does not generally introduce sufficient mode-coupling to excite the mid-order modes that are present in an EMD and so the method does not lead to an EMD being produced.

A second drawback with this method is that it is very inefficient optically if the fibre preceding the gap has been excited with a fullyfilled launch, as only the lowest-order modes are able to propagate across the gap, leading to substantial loss of light.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a compact and optically efficient means of controlling the mode distribution in a multimode optical fbre.

The object of the invention is achieved by means of a mode-scrambler, comprising a single loop of optical fibre which is mechanically stressed, and a mode-filter, comprising a longitudinal gap between two fibre sections. Both the mode-scrambler and mode-filter components are independently adjustable such that any desired modal distribution may be obtained, including an under-filled distribution, an over-filled distribution, or an EMD.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1(aA) iS a plot of the modal distrlouilun in a Fibre attached to a commercial LED test source.

Fig.2 is a plot of modal distributions in fibres which are severely underfilled and well- filled. The effect of applying a mandrel mode-filter is demonstrated.

Fig.3(a) is a schematic of the present invention, comprising a modescrambler followed by a mode-filter.

Fig.3(b) shows detail of the fibre loop.

Fig.4 shows the effect on an under-filled modal distribution of applying pressure to the cross-over point in a single loop of fibre.

Fig.5 shows the effect on an over-filled modal distribution of introducing a gap between two fibre sections.

Fig.6(a) shows examples of three types of modal distribution: underfilled, medium-filled and almost fully-filled.

Fig.6(b) and Fig.6(c) show examples of output modal distributions from a mode control device according to the present invention for the three modal distributions shown in Fig.6(a).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Fig1.(a) a modal distribution in a 62.5um fibre connected to a typical commercial LED test source is shown. Here the relative intensity in each mode group is plotted as a function of the normalised mode group number. A mode group corresponds to a group of modes that all have the same propagation velocity. Typically, a 62.5um core diameter would support approximately 32 mode groups. Small values of normalised mode group number correspond to low-order modes and larger values to high- order modes. in a fibre where all the modes are equally excited the modal distribution will show a unity-valued horizontal line. The modal distribution may be measured by any established method, such as that described in TlAfEIA ITM-3.

The distribution shown in Fig.1 (a) therefore indicates a predominance of low-order modes, tailing off monotonically towards high-order modes. s !

Fig.2 shows the effect of introducing a mandrel wrap in fibres having either an under- filled or an over-filled modal distribution. The mandrel wrap is constructed according to the mode-scrambler described in EP0442731 and comprises 5 turns around a 20mm diameter mandrel. It can be see that the mandrel wrap has a slight effect in the well- filled case, by attenuating higher-order modes. The effect of the mode- filter for the under-filled case is, however, negligible. Thus the use of a mandrel wrap for creating an EMD in an uru'er-fi'led,,bre is ine',ectua, in this example.

It is the object of the present invention to provide a means of accurately controlling the modal distribution in a fibre such that an EMD may be readily created. Furthermore, the present invention also permits a plurality of modal distributions to be created, ranging from under-filled to over-filled. In addition, the present invention permits a range of modal distributions to be created that is independent of the input modal distribution.

Fig.3(a) and Fig.3(b) show schematics of the present invention, known as a 'mode control device'. The mode control device 1 comprises a section of fibre 2 into which is inserted a single loop 3. At the crossover region 12 in loop 3 a load is applied by means of fixed block 10 and weight 11. The input end of fibre 2 is fitted with an optical connector 8 for attachment to a light source such as an LED, a laser or an Optical Time Domain Reflector (OTDR). The distal end 12 of fibre 2 is terminated with a flat perpendicular surface produced by established methods such as cleaving or polishing.

The proximal end 13 of a second fibre 4 is positioned co-axially with fibre end 12 such that there is a variable gap 7 between the two ends. The distal end of the second fibre 4 is connected to a third fibre 5 using, for example, a fusion splice 6. The distal end of third fibre 5 is terminated by an optical connector 9 for connection to the fibre under test.

In this manner the modal distribution exiting the mode control device at the output connector 9 may be made adjustable and independent of the modal distribution launched at the input connector 8.

In a particular embodiment of the mode control device the first fibre 2 and the second fibre 4 will both be constructed with a stepped refractive index profile with a core diameter of typically 50 to 100 um. The loop 3 inserted into the first fibre has a typical diameter of between 10mm and 20mm. The loading to the crossover point 12 in the loop 3 may be applied by using static weights or preferably using a controllable clamping mechanism which may be mechanically or electro- mechanically operated. The action of applying a load 11 to the crossover point 12 in loop 3 is to provide local distortion to the fibre 2 and is found to produce a controllable degree of mode-scrambling in fibre 2.

Fig.4 shows the effect on a particular modal distribution measured at the output connector 9 of the device by applying an increasing load to the crossover point 12 in loop 3 in fibre 2. As the load is increased the modal distribution becomes increasingly fully-filled.

The purpose of the variable gap 7 between fibres 2 and 4 is to introduce selective mode- filtering whereby only light travailing at small angles, the low-order modes, is able to pass between the cores of the two fibres. The size of the variable gap 7 may be in the range of zero to 2000um, depending on the amount of mode-filtering that is required.

The variable gap may comprise, for example, aligning the fibre ends 12, 13 inside a small bore capillary tube and adjusting the gap between them by mechanical or electro- mechanical means. Furthermore, if required, the fibres may be permanently fixed in position by potting them in epoxy.

Fig.5 shows the effect on a pa,-ticular modal distribution measu,ed at the output connector 9 of the device as the gap 7 is increased. The modal distribution becomes increasing under-filled.

The third fibre 5 is typically a graded index fibre and would typically have a similar numerical aperture and core diameter to the fibre under test.

By careful control of the amount of loading 11 on the crossover point 12 and the size of the variable gap 7 a range of modal distributions may be produced in the third fibre 5 and subsequently launched into the fibre under test at the output connector 9. The present invention may be used in an optically efficient manner by introducing just enough mode- scrambling by control of the load 11 so that the modal distribution is only slightly more fully-filled than the modal distribution that is required at the output connector 9. Final adjustment of the modal distribution is then achieved by adjustment of the variable gap 7. In this manner light loss due to mode-filtering at the variable gap is minimised.

Fig.6(a) shows modal distributions that are respectively under-filled, medium-filled and almost fully-filled. These distributions are in turn connected to the input connector 8 of the mode control device 1 and Fig. 6(b) shows the modal distributions at the output connector 9, for each of the above cases, where the load 11 and the variable gap 7 have been set at a particular values. It can be seen that the output modal distribution is independent of the launched modal distributions.

Fig.6(c) shows the output modal distribution for the launch distributions shown in Fig.6(a) for another combination of load 11 and gap 7. It can be seen that the output modal distribution is again independent of the launched modal distributions.

Variations may be made to the disclosed method and embodiment without departing from the scope of the invention. For example, the mode control device described herein may be manufactured for field use as a preset component whereby the load 11 and the gap 7 are of fixed and stable magnitude.

It is to be expressly understood that the invention is not to be limited only to the specific examples and embodiments disclosed herein.

Claims (7)

CLAI MS

1. Apparatus for controlling the mode distribution of light in a multimode optical fibre, comprising: means of directing light through a mode-scrambler to redistribute said light into higher order modes, ar-u means of directing said mode-scrambled light through a mode-filter to eliminate a controlled proportion of the higher-order modes.

2. Apparatus as claimed in claim 1 wherein said mode-scrambler consists of a single loop of fibre with a load applied to the cross-over position of said loop.

3. Apparatus as claimed in claims 1 and 2 wherein the applied load to said mode- scrambler may be static or adjustable.

4. Apparatus as claimed in claim 1 wherein said mode-filter consists of two portions of fibre separated by a gap.

5. Apparatus as claimed in claims 1 and 4 wherein the gap in said modefilter may be static or adjustable.

6. An apparatus is any preceding claim which consists of fibres having either step or graded refractive index profiles.

7. An apparatus substantially as herein described above and illustrated in the accompanying drawings.