Fibre-Qptic Couplers

This invention relates to Fused Tapered Fibre-optic couplers and in particular to directional couplers of this type such as beam splitters and combiners.

There are two main types of single-mode fibre optic couplers available, the polished coupler and the fused tapered coupler.

Polished directional couplers are made by polishing away the cladding of two single mode fibres to within about one micron of their respective cores. The coupler is then formed by placing the two polished half couplers together with an index-inatching oil or UV curable epoxy resin between them. The optical characteristics of the resulting directional coupler can be tuned by sliding the two polished halves relative to each other and then fixing them, if desired, by curing the epoxy resin.

The fused tapered single mode directional coupler is made by a quite different technique and a section through a typical fused directional coupler is shown in figure 1. Typically, two single-mode fibres are intertwined at the coupling location and then held by two movable supports on either side of the intertwined section. A small oxy-butane flame is applied to the fibres so that they fuse together at the intertwined section. At the same time the two supports are moved apart such that an elongate fused section 1 is formed between two fused tapered sections 3 from which the two separate fibres 5 emerge at both ends of the coupler. It is important to move the supports apart in a straight line so that a low-loss coupler will be produced. The speed of separation of the supports is also .Important since this determines the shape of the resulting taper which has a

significant effect on any losses in the coupler.

Although both types of directional coupler can be made with losses less than 0.05dB, the fused directional coupler can be made much more quickly and cheaply and its power splitting ratio can be monitored during fabrication. Conversely the polished coupler's power splitting ratio can be measured after fabrication but it can then be tuned to a desired ratio by sliding the two halves over each other.

In accordance with one aspect of the present invention there is provided a twisted fused tapered coupler. Further aspects of the invention are defined in claims 2 to 9 to which reference should now be made.

The invention is now described in more detail by way of example with reference to the drawings in which:

Figure 1 is a longitudinal section through a typical fused tapered directional coupler as described above;

Figure 2 is a longitudinal section through an adjustable fused tapered directional coupler assembly in accordance with the invention;

Figure 3 is a graph showing the variation of power-splitting ratio for the coupler shown in Figure 2 as a function of the axial twist angle; and

Figure 4 is a graph of the spectral power splitting ratio for increasing amounts of twist angle.

A twistable optic fibre directional coupler assembly is shown in figure 2 and comprises a silica tube 2 contø_ining a fused

directional coupler 4. The tube has two rotatable plastic end caps 6 each having a hole therein, through which the optic fibres 8, leading to and from the coupler pass. The holes in this case are 1mm diameter to make threading of the fibres t±irough the end caps relatively simple. The fibres are secured to the end caps with epoxy resin 10 whilst the coupler is kept taut between them. The relatively rotatable end caps thus provide a means for twisting the coupler 4. The end caps may have a graduated scale upon them to indicate the power splitting ratio available at any angular position.

For the particular coupler used in this example, two complete revolutions of the end piece were necessary to tune across the full range of splitting ratios (0%-100%) at a wavelength of 850nm.

With the coupler tuned to give power from both output fibres the assembly was subjected to external temperature changes. A change from 0° Celsius to 60° Celsius gave a 0.45% change in the fraction of the input power coupled, whilst the coupler insertion loss of 0.2dB suffered negligible change.

The coupler assembly was also struck vigourously and dropped onto a bench from a height of 20mm. This caused a momentary coupled power variation of typically 0.4% and at no stage greater than 0.9%

A similar assembly was filled with a viscous uncured silicon elastomer (e.g. Dow-Corning Sylgard 182) to give extra mechanical protection to the coupler. This had little effect on the tunability of the coupler but the temperature sensitivity of the coupler increased to a 0.8% change in coupled power per 20° Celcius change in temperature. This increase is due to the thermally induced change in the

- ⁱ -v-

elastαmer refractive index.

The reason twisting induces a change in the power splitting ratio of the coupler is that the effective refractive index across the transverse cross section of the coupling region 1 of Figure 1 changes with twisting. This accounts for the decoupling observed in twisted directional couplers since the effective index induced by the twist increases with distance from the longitudinal axis of the coupler. The relative increase in the effective index at the edges of the coupler causes a redistribution of the modal fields in the coupler away from the axis. This reduces the field overlap between the two sides of the coupler and hence causes a degree of decoupling. It can be shown by calculation that although this change in the effective index is small it is sufficient to account for the halving of the coupling strength observed in twisted couplers.

Experiments have been conducted to quantify the effect of axial twisting upon the optical characteristics of a fused tapered directional coupler made in the conventional manner. The coupler was fabricated to give 0% coupling at 870πm i.e. the optical power returns completely to the t_hroughput fibre at 870πm. To avoid the effects any bends in the coupler have on the power splitting ratio, the coupler was kept taut and tabs were fixed along the untapered fibres to confirm that any twisting was absorbed by the tapered and fused region. The variation of the logarithmic power splitting ratio with twist angle for the coupler is shown in figure 3 at a wavelength of 870nm. The ratio varies across all possible values from no power coupled to all the power coupled at 480Deg of twist angle. The variation is due to a decoupling effect which varies with the twist angle. The variation in power splitting ratio was reversed when the coupler was

untwisted and no significant change in the coupler insertion loss of O.ldB was detected at any stage.

The variation in power splitting ratio with different wavelengths is shown for three different wavelengths in figure 4. Curve a. is for 0° of twist, curve b. for 240°, and curve c. for 420°. The graph shows that the wavelengths where no power is coupled and where all the power is coupled increase with increasing twist angle. Thus curves b and c are approximately shifted versions of curve a. This is the opposite of what happens during coupler fabrication and it can therefore be concluded that twisting a coupler induces decoupling rather than further coupling. It should be noted that the coupling strength at 870πm is halved as the power transfers from one fibre to the other over the 480° twist.

The coupler was twisted and untwisted through 480° many times without suffering any degradation in performance. The coupler was finally destroyed after eight consecutive revolutions in the same direction.

A number of similar couplers were also tested and these exhibited similar properties to those described above thus demonstrating the reproducibility of the tuning effect.

The twisting of fused tapered directional couplers provides a method of tuning them across all splitting ratios with low losses. The process is reversible and repeatable and does not degrade the coupler performance.