In sight – faster data transmission

Materials World magazine
1 Jan 2010
Examples of organic semiconductor-doped fibres with different colours excited by daylight or a table lamp. Image courtesy POLYCOM partner LUCEAT SpA, Dello, Italy

European scientists claim to have demonstrated the feasibility of doping plastic optical fibres (POFs) with conjugated oligomers or co-polymer F8BT to create all-optical switching. This may enhance data transmission in short haul POF networks.

Plastic optical fibres cannot compete with silica fibres in long-distance transmission, but their low cost makes them useful for local area communication networks, sending signals to onboard entertainment systems in vehicles, sensors and intelligent clothing.

The EU-funded POLYCOM consortium says it has achieved optical and net gain properties of 0.07dB in two-millimetre POFs by doping them with 0.02wt% of oligofluorenes. Although the net gains are still low and have yet to be replicated over longer lengths of fibres, the team believes the research is a proof of concept for achieving optical fibre amplifiers for all-optical resonant switching in organic photonics, with high on/off ratios and fast response times. The researchers calculate that, in principle, the achievable switching rate could reach one giga hertz, which would be suitable for short-haul transmissions and could enable time division multiplexing to increase the bandwidth of optical networks.

The 1D nature of the conjugated oligomers enables the ultrafast resonance mechanism, with a recovery time below one picosecond and 100% modulation. ‘This is because 1D confined carriers have very short lifetimes,’ explains Guglielmo Lanzani, a researcher at Milan Technical University, Italy, and coordinator of the programme. ‘POLYCOM explores the electronic state dynamics typical of linear conjugated chains. From 3D to 1D, recombination gets an order of a magnitude faster. This is also true for charge transfer states.’

Furthermore, these dopants are said to withstand POF fabrication. The oligofluorene chromophores are dissolved in liquid MMA before polymerisation and then subject to preform and POF manufacture. The 0.02wt% of the active material has been set to inhibit aggregation and ensure solubility of the molecules in the MMA. The doped POFs have attenuation below 5,000dB/km at equilibrium mode distribution, indicating low aggregation between the molecules for higher optical gain.

In testing different chemical doping agents, however, the researchers have found that F8BT, unlike the oligoflurenes, can also obtain amplification and switching at the right wavelength of 520nm and works effectively even when aggregated, due to a localised excited state. The concentration of active material in the POF could therefore be increased for enhanced optical gain, without aggregation causing optical losses and needing a high pumping intensity.

Better than dye

Lanzani explains that the organic semiconductors they used overcome the limitations of previous research that involved adding laser dyes, such as rhodamine, to POFs. ‘These dyes do not stand continuous pumping in a steady state. They need to be circulated [because] a population of dark states (triplet) builds in time and degrades the properties. Alternatively, they can be used under pulse excitation, yet the rate of excitation cannot be too high to avoid the same problem of triplet build-up,’ explains Lanzani. ‘Organic semiconductors are less affected by this problem – the yield of triplet formation is negligibly small.’

Dr Periklis Petropoulos of the Optoelectronics Research Centre at the University of Southampton, UK, comments that the research is still in its early days but shows potential. ‘The objectives of the programme have been forward-looking, not only because they are addressing amplification and switching in systems which have only been simple point-to-point links in the past, but also because they aspire to operate these devices in time-scales that are much faster. Advances in device engineering and material science will have to be achieved before such devices can be deployed in the field’.

Moving forward, Lanzani foresees that ‘the best configuration for amplification and switching is an external photonic chip coupled to the POF transmission network’.