Fiber optic refractometers are a valuable analytical tool for chemical analysis, food and pharmaceutical testing, biological sensing, and environmental monitoring. For quality assurance, they can be useful in determining the purity of liquid samples such as honey, milk, fruit juices, seawater, solvents, aromatic chemicals, and petroleum products.
The team from the biosensors lab, IIT Madras, which include Mr. Allwyn S. Rajamani, Mr. Hariharan Manoharan, and Prof. V. V. Raghavendra Sai from the Biosensors Laboratory, Department of Applied Mechanics, Indian Institute of Technology Madras, Chennai, Tamil Nadu, India, Ms. Christina Grace Danny from the Department of Electronics and Instrumentation Engineering, M. S. Ramaiah Institute of Technology, Bengaluru, Karnataka, India, and Mr. P. V. N. Kishore from the Department of Physics, Lakireddy Bali Reddy College of Engineering, Mylavaram, Andhra Pradesh, India, have developed a handheld refractometer which consists of a LED (Light Emitting Diode)-photodetector and a U-shaped optical fiber as a sensing element. This sensor works on the principle of refractive losses. The U-shaped fiber optic probe is referred to as the “U-bent” fiber optic probe that offers advantages such as easy fabrication and design. In the lab, the U-region was chemically etched to give a tapered geometry. The etched probe was found to be twice sensitive to RI (Refractive Index) changes compared to the unetched probe. Furthermore, the handheld refractometer was able to measure liquid concentrations in the narrow (1.333–1.348) and wide (1.33–1.40) Refractive Index range. The refractometer was tested for liquid samples such as glycerol, sulphuric acid, ethylene glycol and ethanol with different concentrations. In future, the sensitivity of these probes can be further improved using plasmonic nanoparticles or graphene-coating. The results of this study can pave the way for several refractive index based chemical and biochemical sensing applications.
Mr. Allwyn S. Rajamani 
Mr. Hariharan Manoharan 
Prof. V. V. Raghavendra Sai 
Ms. Christina Grace Danny 
Mr. P. V. N. Kishore
Prof. Nirmal Punjabi from the Indian Institute of Technology Bombay, India, who is also the Founder of the Optical Sensing and Monitoring Community, greatly appreciated the work done by this team by giving the following comments: “Fiber-optic sensors (FOS) have been widely used for monitoring physical parameters in various industries. Among different types of FOS, U-shaped multimode fiber-based sensors have shown potential to be used in many applications ranging from physical measurements to biosensing. For successful commercialization of the technology, optimization of various parameters of such sensors is critical. This study by Prof. Sai’s group at IIT Madras provides a ray theory model to select the optimum U-shaped bent fiber-optic probe for different fiber diameters. Further, the HF etched fiber helps combine the high sensitivity of lower core diameter along with the light coupling tolerance of the large-core multimode fiber. The prototype of a pen-type refractometer shows a promising solution for compact refractometers, as the same sensor can be used to measure the refractive index for various liquids, including organic solvents and acids.”
Article by Akshay Anantharaman
Here is the original link to the paper:
https://www.sciencedirect.com/science/article/abs/pii/S0924424722002539
