Developing Fibre Optic Sensors

ISP research team have long been working on fibre optic sensors. Our research focuses on developing different types of extrinsic and intrinsic fibre optic sensors and materials for sensor applications. See the details of some of the sensors that we have designed and developed in our laboratories. 

Recently we have developed a highly sensitive evanescent wave fibre optic sensor for the measurement of pH of a solution. This is done by immobilizing a pH sensitive dye (BCP as well as BCG) on the unclad portion of the optic fibre in a sole-gel matrix. The sol-gel technique essentially relies on a process involving a solution or sol that undergoes a sol-gel transition. The sensitivity of some of the recently developed fibre optic pH sensors, using the sol-gel route, has been found to be low, as there is only a 20% fractional change in output intensity when a single dye is used . So in order to enhance the sensitivity, we propose a new method, called the multiple sol-gel coating technique. In this technique, different layers of sol-gel thin films containing the pH sensing dye are coated one over the other on the unclad region of the fibre. The sensitivity of the device is found to increase by 70% when multi-layer sol-gel coatings are employed. Using our design, the pH value of the fluid can be monitored continuously and the operating range of the sensor can also be increased by mixing various dyes in the earlier stages of sol preparation.

Microbend sensors are based on coupling and leakage of modes that are propagating in a deformed fibre. Usually one achieves this deformation by employing corrugated plates that deforms the fibre into a series of sharp bend with small bending radii. In our laboratories we have developed a highly sensitive chemical sensor by inducing permanent microbends on a bare plastic optical fibre. The output intensity is found to be linearly dependent on the logarithm of concentration of the absorbing species surrounding the the bent portion of the fibre. This sensor can even detect very low concentrations , of the order of nanomoles per liter. and the dynamic range of the sensor is found to 6 order of magnitude. By carefully choosing the reagents this microbend  sensor can be used to detect different chemical species.

Pulsed laser deposition is a very convenient technique for the film preparation. Online measurement of the deposition rate of abalated material is essential to establish optimal film growth as well as for the continuous monitoring of the deposition process. To achieve this goal, we have developed an evanescent wave fibre optic sensor. A multimode optical fibre with uncladed sensing region is used in our design. As the deposition proceeds, the thickness of the thin film in this uncladed region increases. Now the laser beam guided through this fibre encounters evanescent wave absorption in this region of the fibre and the output power decreases. Using this sensor we can monitor the deposition rate of different materials. Using the same principle we have also developed a fibre optic sensor to monitor the deposition rate of thin films of organic materials produced by AC plasma polymerisation. 

Some of the other sensors developed are: Sensor for etch rate measurement, chemical reaction rate measurement, viscosity and surface tension measurement (extrinsic type), trace gas detection, water pollution monitoring and glucose level detection.
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