Fused-tapered Coupler/Splitter

In this type of device, light is coupled from one fibre to another along a short section where the two fibre cores are fused together. It is generally found that higher-order modes couple most efficiently and so the split ratio of this type of device is dependent on the launch mode distribution.

In the plots shown below, the MTFs of the two output ports of a fused-tapered splitter (blue and red) are compared with the launched distribution (green). Three cases are considered where the input distribution is well-filled, medium-filled and under-filled. It can be seen that the output from port 1 is predominantly low-order modes whereas the output from port 2 has a lack of low-order modes. The effect of the mode-filtering that is occurring is to make the split ratio dependent on the launched distribution.

For the well-filled case, the split ratio is 1.12, which corresponds approximately to the manufacturer's specification of this 50/50 splitter. A s the input distribution becomes less well-filled the split ratio increases to 1.35 for the under-filled case. This corresponds to a 57/43 split ratio.

Commercial LEDs and OTDRs

The plot below shows measured MPD curves for a variety of commercial LED sources and OTDRs. The red areas, top left and right, and lower centre correspond to a pass/fail template which is specified in ISO/IEC link testing standard 14763-3.

It can be seen that OTDRs 1 and 3 have predominantly low-order modes. This is due to the low numerical aperture of the laser sources in the OTDRs. In contrast,OTDR 2 features some internal mode-conditioning and passes the template test. The two LEDs (LSPM 1 and 2) are very much at the borders of the template at both the low-and high-order extremes.

So, it can be seen that there is a real need to characterise all light sources to ensure that they comply with the new standard. The MPX provides a real-time display of MPD, so that adjustments to the light source, such as alignment and mode-conditioning, may be carried out in real-time to ensure compliance.

To assist both manufacturers and installers to comply with the new standard, Arden Photonics Ltd has developed a mode conditioning device, known as the Modcon, that simply fits between the light source, LED or OTDR, and provides instant compliance with the standard whatever the input distribution is. In the plots below, the LEDs and OTDRs have been connected to a Modcon and their output MPDs re-measured. Now, they all pass the template. For more details of the Modcon click here.

Long Length Equalisation

It is often thought that a long length of fibre automatically leads to an Equilibrium Mode Distribution (EMD). This may be true if sufficient mode coupling is present, caused either by internal imperfections in the fibre or by external effects such as microbending, but to be sure, a direct measurement of MPD is necessary. The MPD curves shown below are the measured output from a 202m length of tight-buffered cable for four different launch distributions, and the corresponding MPDs after transmission through only 1 metre.

It can be seen that the peak of the low NA  launch (red)  moves to the right slightly as mode coupling takes place in the fibre. In contrast, the peak of the well-filled launch (magenta) moves slightly to the left, due to preferential attenuation of higher-order modes in the fibre.

So, while there is a tendency for the MPDs to converge to an equilibrium state, similar to that of the mid-launch  in green, the fibre is clearly producing an stable EMD for all launch states. To achieve this, a longer fibre might be tested or else some external  mechanism might be used to induce more mode-coupling.

220 metres