We propose and demonstrate a novel temperature-insensitive bio-sensor for accurate and quantitative detection of Escherichia coli (E. coli) bacteria in water. Surface sensitivity is maximized by operating the long-period fiber grating (LPFG) closest to its turnaround wavelength, and the temperature insensitivity is achieved by selectively exciting a pair of cladding modes with opposite dispersion characteristics. Our sensor shows a nominal temperature sensitivity of ∼1.25  pm/°C, which can be further reduced by properly adjusting the LPFG lengths, while maintaining a high refractive index sensitivity of 1929 nm/RIU. The overall length of the sensor is ∼3.6  cm, making it ideally suitable for bio-sensing applications. As an example, we also show the sensor's capability for reliable, quantitative detection of E. coli bacteria in water over a temperature fluctuation of room temperature to 40°C.

We propose and demonstrate a technique easy to fabricate and measure water depth based on selective mode excitation using single-multi-single (SMS) mode structure. The high extinction ratio has been achieved by equally exciting preselected modes of the multi-mode fibre (MMF) by optimizing the core offset at both the input/output splices of the SMS structure, and a wavelength shifting has been achieved by varying the ratio of major to minor axis of the MMF loop. A theoretical analysis of the observed behaviour is also presented showing an excellent agreement between the theoretical and experimental results.Index Terms-Single-modemultimodesingle-mode (SMS) structure, graded index multimode fiber, modal interference, optical fibre components.

An ultra-sensitive sensor based on long-period fiber graings (LPFGs) has been fabricated for the detection of methanol and water content in ethanol. Our sensor is very compact in size, highly accurate and easy to fabricate making it very useful than the conventional surface plasmon resonance (SPR) sensor, which is bulky and expensive. We show that our sensor is capable to achieve an ultra sensitivity of 696.34 pm/ V% methanol and 655.3 pm/ V% water in presence of methanol and water in ethanol respectively. Our sensor is capable of detecting minimum 1.5 × 10 −3 V% methanol and water in ethanol, which is far better than all the methods reported till now.

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An ultra-sensitive sensor based on dual resonance long-period fiber gratings has been fabricated for the detection of methanol and water content in ethanol. The developed sensor is compact in size and light weight and employs a highly accurate spectral interrogation technique for adulterant detection, increasing its applicability compared to conventional surface plasmon resonance based sensors, which are generally expensive, as they require metal film deposition. We demonstrate that the sensor is capable of achieving sensitivity of 802.66 pm/V% methanol and 749.06 pm/V% water in the ethanol solution. The estimated detection limit using the experimental data and spectral resolution of the interrogator is found to be ∼ 1.3 × 10 − 3 V % in the 1300–1700 nm wavelength range. We also present the sensor’s theoretical study, and good agreement is found between theoretical and experimental results.