PROCESO DE PRODUCCIÓNGENERAL

Directional couplers are necessary to divide and combine light efficiently in an all optical fibre system. A singlemode optical fibre uses evanescent wave coupling. A wave propagating in the core of a fibre decays as an evanescent wave in the cladding. If this evanescent wave is allowed to overlap the core of a second fibre then coupling will occur and light will propagate in the second fibre. A multimode fibre coupler works by converting high order modes to cladding modes

which are subsequently recaptured by a second fibre. Splitting ratios

in multimode fibre couplers are therefore a function of the degree of 4

1

mode filling. i

The first all fibre couplers to be made had a variable coupling ratio and were described by Sheem and Giallorenzi [4.1]. Two optical fibres were tw isted together and placed in a container filled w ith acid. The fibre cladding was removed by etching allowing the two fibre cores to come close enough for evanescent coupling to occur. The acid was removed and replaced w ith an index m atching fluid. The coupling ratio was controlled by the tension in the two twisted fibres.

A second method of producing singlemode optical fibre couplers w ith variable splitting ratio was described by Bergh et al. [4.2]. This

method is to embed an optical fibre in each of two glass blocks and

remove the exposed cladding by polishing until very close to the core.

The polished fibres are th en placed together w ith some index

m atching fluid. Adjusting the relative positions of the fibres gives some control over the splitting ratio.

The most common couplers are directional couplers m ade by fusing together two optical fibres, first described by V illarruel and Moeller [4.3]. Two glass fibres are heated together at around 1500*^0, in order to soften and fuse them , and th en stretched whilst hot. This decreases the core diam eter and causes the evanescent wave to

spread further from the core. This allows coupling to occur when the cores are further ap art th an would otherwise be possible. H eating and stretching the fibre is continued until the required splitting ratio has been achieved. Fused fibre couplers known as star couplers are now available with 3,4,8 or even 16 input and output ports. Most fibre couplers do not preserve the polarisation when the light is split or combined. A fused polarisation m aintaining coupler is described by N ayar and Smith [4.4].

As the coupling ratio depends on the wave guided in the fibres the coupling ratio is wavelength dependant. A device which exploits this c h a ra c te ris tic is sa id to be a w a v e le n g th d iv isio n

m ultiplexer/demultiplexer. A device which showed unity coupling at

1300nm but close to zero coupling at 1550nm is described by Lawson et al. [4.5].

Although for our purposes light can be coupled between fibres using lenses and beam splitters it was decided to try and m ake 2 by 2 couplers (i.e. 2 input ports and 2 output ports) by fusion using single and multimode optical fibre.

The multimode fibre had 50/125 micron core / cladding diam eters.

The fibre is covered w ith a protective outer coating which brings the

fibre to about 200 microns in diameter. This outer layer was removed

from a 5cm length at the centre of each of two Im long fibres by a loop

of cotton pulled along the fibre. The stripped regions of the fibres were th en tw isted together w ith one or two tu rn s and fixed to two adjustable mounts, figure 4.23. Light from a chopped HeNe laser was coupled into one of the fibres. The output from both fibres was

monitored using two photodiodes. The fibres were th en softened using a propane flame and tapered by winding the mounts slowly apart. The tw ist in the fibres caused the two cores to be pulled together as the cladding softened. Care had to be taken not to try and taper the fibres too quickly as the fibres would snap. Overheating the fibres distorted them and caused large losses in the coupler.

The heating and tapering was continued until coupling occurred between the fibres w ith the required splitting ratio. If the tapering was continued then the splitting ratio was seen to oscillate. There is therefore a beatlength associated w ith the fabrication of these couplers. Coupling was easily achieved using the multimode fibre and 50/50 splitting ratios were obtained. The process was repeated w ith singlemode fibres. These have a core diam eter of about 8 microns and a cladding of 125 microns. So the separation of the two singlemode cores is much greater th an in the case of the multimode fibres. The couplers are therefore much more difficult to make. The fibres were prepared in the same m anner and stretched and heated

u n til coupling occurred. The losses for both m ultimode and

singlemode couplers were typically greater than 6dBs. Commercially available couplers may have losses as low as 0.1 dBs.

The optical fibre couplers then had to be packaged. The best solution available was found to be supporting the coupler on a glass slide and the covering it in epoxy. However, couplers packaged in this m anner

different temperature coefficients of expansion of the silica fibre and the epoxy. This problem was overcome by loading the epoxy w ith silica powder.