Water and light flow likeness leads to ultrasensitive sensor

07 Sep 2016

US-Singapore partnership develops microfiber sensor 20x better than conventional optical biochemical devices.

Water flow and light flow are similar in many ways, and this inspired a group of researchers from Nanyang Technological University in Singapore and the University of Nebraska–Lincoln to explore whether light flow within a waveguide also behaves in the same manner.

As the group reports in Applied Physics Letters, they tested the theory by studying the infrared sensing performance of optical microfiber coupler sensors and ended up creating a new sensor, which is about 20 times more sensitive than conventional optical-fiber-sensor-based biochemical sensors.

By increasing the sensitivity of the optical fibers to small index changes within the surrounding environment, small concentrations of molecules on or near the fiber’s surface can be detected. While the general idea is well known, this approach solves an existing sensitivity problem.

The concept behind the researchers’ new sensor is “the interference of light, which travels along an optical microfiber coupler,” said Lei Wei, an assistant professor in the School of Electrical & Electronic Engineering, College of Engineering, Nanyang Technological University. “Two different optical guided modes in the microfiber coupler can be excited and interference occurs when they propagate along the fiber.”

Biological applications

Optical interference-based sensors are currently used for many applications within the field of biological measurement, and the use of optical fibers makes such devices extremely compact and economic.

“But a downside to most of the optical fiber biochemical sensors reported to date is that they exhibit high sensitivity only when the refractive index of the surrounding medium approaches that of an optical fiber [typically around 1.44],” said Wei. “In practice, biomolecules under detection are virtually prepared in the form of aqueous solutions that have a RI quite close to that of water [~1.33], so achieving high sensitivity for detection of trace analyte and small molecule is fundamentally challenging for fiber-based sensors.”

During the group’s study, they created an optical microfiber coupler sensor capable of achieving ultrahigh sensitivity “at low RI – around 1.33 – by precisely designing the parameter to minimize the difference between the effective group indexes of the ‘even supermode’ and ‘odd supermode’,” he added.

'Supermode'

A typical optical microfiber coupler comprises two parallel, closely-packed microfibers with two input and two output ports, two transition tapers, and a central uniform waist-shaped region (see graphic, above). The modal fields injected from the input modes are known as supermodes, which include odd and even distributions. When even and odd supermodes propagate along the optical microfiber coupler, a power exchange occurs and the resulting interferometric spectrum can be obtained at its output ports.

The group’s experimental results revealed that their sensor is significantly more sensitive, by a factor of ~20x, than conventional optical-fiber-sensor-based biochemical sensors.

“The resulting ultrasensitive mechanism not only creates opportunities for a wide range of applications within chemical and biological sensing fields, but also broadens the repertoire of other interferometric fiber-optic sensors, and even acoustic sensors,” said Wei. “The next step is to make the sensor into arrays and further explore its sensing performance for cancer biomarkers.”