Compact photonic sensors can provide accurate readings

In a recent example of their work, Ji Fang Tao from A*STAR's Institute of Microelectronics and co-workers developed a tiny optical temperature sensor just 120 by 80 micrometers and modeled on a classic experimental design called the Michelson Interferometer.

The device splits light into two beams, which travel through different materials and are then brought back together. The resulting interference pattern gives information about the light and the surrounding temperature.

"Changes in temperature can change the optical properties of some materials," explains Tao. "In our sensor, the temperature changes the refractive index of the waveguides. The Michelson Interferometer is a powerful design that allows us to pick up these refractive changes, and therefore work out the temperature changes."

One of the standout features of the device is that it does not require mirrors, and instead guides light through loops to bring it back to the interference point. The researchers found that it could measure a wide range of temperatures, and was also around 20 times more sensitive to small temperature changes than existing fiber-optic sensors.

Recently, they also developed a tiny photonic hygrometer which identifies the local dew-point temperature by recognising the refractive properties of water when droplets condense in the path of the waves. This information can then be combined with the temperature sensor to calculate relative humidity with a very high level of accuracy.

The applications of such versatile, compact and low-cost environmental sensors could be unlimited, and Tao and co-workers from Singapore and China are only at the beginning of their journey. "Silicon photonics and optical MEMS are two amazing technologies for sensor development," he says. "Now we can measure temperature, dew-point and humidity with ultrahigh accuracy, the next step is to design on-chip photonic gas sensors."