Polarisation transparency in microphotonic devices

MIT researchers have found a way of overcoming a fundamental problem with the use of photonics in telecommunications: processing light of mixed polarisation with no loss of signal. The development brings us a step closer to integrating photonics and electronics on a single chip.

Polarisation sensitivity is a big issue in microphotonics. Light passing through optical fibres undergoes random changes in polarisation, and this creates difficulties when it comes to processing the output of the fibres. On-chip photonic structures are suited to processing only particular polarisations, and if mixed-polarisation light is input to such structures, heavy signal loss is the result.

Integrated polarisation diversity scheme (taken from: Barwicz et al., Nature Photonics 1, 57, 2006)

Instead of feeding light through multiple devices, in order to process all polarisations, the MIT engineers split the input into separate vertical and horizontal polarisation components, and then rotate one of these into the other form. Identical photonic structures are then used to process the light, which is combined back into a mixed-polarisation signal for transmission elsewhere.

As a test case for their technique, the MIT group created an add-drop filter made from microring resonators, and achieved an almost complete elimination of polarisation sensitivity over the 60-nanometre bandwidth measured.

‘To date there has been no way to incorporate polarisation transparency into densely integrated photonic circuits that might contain many different filters on a tiny semiconductor chip,’ says team member Erich Ippen. ‘Making densely integrated photonic circuits compatible with real world fibre-optics is very important for increasing the performance and lowering the cost of optical communication networks.’

Ippen’s colleague Tymon Barwicz, who is first author of the Nature Photonics paper in which the work is reported, adds: ‘The demonstrated scheme enables the application of a new category of micro-optical devices to optical networks, spectroscopy and remote sensing.’

The research was funded by leading telecommunications company Pirelli, which is developing the technology for commercial purposes.

Figure: An arbitrary input polarisation is split into orthogonal components, and one of these is rotated into the other form to achieve a single on-chip polarisation state. At the output, the two components are recombined after one has been rotated to prevent interference between the two signals. Taken from: Barwicz et al., Nature Photonics 1, 57, 2006.

Article first published in Nanomaterials News.