An optical chemical sensor developed at the Optoelectronics Research Centre (ORC) at the University of Southampton can extend the sensitivity of optical fibre-based bio sensors to be able to detect one molecule in 10 million.

The key feature is that the sensor uses exceptionally thin optical fibre. With a diameter of 400nm it is up to 1,000 times smaller that the fibre typically used on comms systems.

“Other optical sensors currently under investigation or on the market have detection limits of around 1 in 1 million,” said Dr Gilberto Brambilla, a senior research fellow at the ORC.

Because the fibre’s diameter is smaller than the wavelength of the light it is transmitting, some of the light propagates outside the physical boundaries of the fibre. This is known as the evanescent field. When this evanescent field interacts with the target chemical or analyte, its propagation speed changes.

It is these small changes that give information about the concentration of the analyte.

 “To make the ultra-thin optical fibre, we take a standard telecoms fibre, heat it up and stretch it,” said Brambilla. “This results in a thin piece of fibre in between two tapered ends which can be used to couple light into the device with very low loss.”

The fibre in the sensor, which is known as a coated all-coupling nanowire microcoil resonator, is wrapped around a channel through which the analyte flows. The fibre is coated with a polymer such as Teflon to protect the sensor and light is launched into one end of the fibre using standard telecoms equipment. This then travels along the fibre and around the analyte a million times before being detected at the other end of the coil.