Fibre couplers are used to combine the optical power from multiple input fibres (“ports”) to one common output port. They can also be operated as a fibre splitter, where optical power from the input ports is split between the output ports.
Three technologies are available: Fused biconic tapered, thin filter coupler, and integrated waveguide directional coupler.
Fused biconic tapered
A fused biconic tapered coupler consists of two, parallel optical fibres that have been twisted, stretched and fused together so that their cores are very close to each other. This forms a coupling region, the length of which determines the coupling ratio from one fibre to the other.
These couplers provide low cost coupling. As port counts increase, they can become unmanageably large and bulky.
Thin film filter coupler
A thin film filter coupler processes light output from an optical fibre in free space using miniature optics and reinjects the processed light back into an optical fibre.
Integrated waveguide directional coupler
Consists of an optical waveguide that is grown or etched onto a glass substrate. The waveguide is a physical structure that guides optical electromagnetic waves. The waveguide can be constructed to combine or split optical fibre inputs at the output optical fibres. Complex structures can be built that provide multiple different splitting/coupling configurations. Frequently referred to as Planar Lightwave Circuit (PLC) splitters, they provide a low cost light distribution solution with small form factor and high reliability.
PLCs provide higher channel count splits (upto 64 outputs) from smaller and more compact packages than other technologies. They are also highly controlled in manufacturing to deliver precise split ratios.
Fibre couplers/splitters can take on many different physical forms. Broadly speaking, these include:
- Star couplers, N x M ports (multiple inputs, multiple outputs)
- Tree couplers, 1 x N ports (one input, multiple outputs)
The splitting ratio is the output power of an individual output port divided by the total output power of all output ports. This ratio can be precisely controlled during the manufacturing process.
Temperature Dependent Loss
Due to the manufacturing process and to the sensitivity of the fused region and of the splices integrated in the device, Fused coupler manufacturers have to specify also the TDL value. for a 1×2 Fused coupler, a typical value is +/10.15dB for a temperature range from -5 to +75 centigrade . At the first sight, it could look good, but we have here again to take into account the cascading effect. To make the comparison with 1×8 PLC splitter we have to multiply 0.15 by 3 (3 1×2 for each arm) to finally obtain 0.45dB.
PLC splitter works from -40 to 85 centigrade with a typical TDL of out +/- 0.25dB (-5 to 75 centigrade:+/-0.15dB
Polarisation dependent loss
An lon-exchange PLC splitter shows a PDL much less than 0.2 dB independently from the split-ratio. A 1×2 fused coupler PDL ranges from 0.1 to 0.15dB.Also in this case, we have to cascade discrete 1*2 Fused coupler to obtain the desired split-ratio, Then also PDL will be increased.
A 1×8 fused coupler will show up to 0.45dB PDL, what is more than the double of a 1×8 PLC splitter
A 1 × 2 splitter is typically used to access a small amount of signal power in a live optical network. In this way, network traffic can be “tapped” to monitor the performance of the optical network (e.g. packet loss), or for optical spectrum analysis.
Star couplers are typically used to distribute a common signal to several locations simultaneously. This is the principle behind a local area network (LAN).