Label Free Sensing with gratings

Optical fiber sensors were seeing a groundbreaking development in the last twenty years, not only in both the fundamental and applied researches but also in the industry.

Thanks to the peculiarities of optical fibers, optical fiber gratings (LPG and FBG) have recently been suggested as alternative optical platforms for chemical and biochemical label-free sensing, on the basis of the measurement of refractive index changes induced by an interaction.

Projects

Bilateral Project S&T Programme (Joint Research Proposal under CNR, Italy – CSIR, India)

Date: 2012-2014.

Reference number: 22/EU/Italy/CNR/proj./2012.

Activity period: from 2013 up to 2015.

Main Publications on the subject

Farnesi D, Chiavaioli F, Righini GC, Soria S, Trono C, Jorge P, Nunzi Conti G (2014). “Long period grating-based fiber coupler to whispering gallery mode resonators”. Optics Letters, vol. 39 (22), p. 6525-6528.

Abstract:We present a new method for coupling light to high-Q silica whispering gallery mode resonators (WGMs) that is based on long period fiber gratings (LPGs) written in silica fibers. An LPG allows selective excitation of high-order azimuthally symmetric cladding modes in a fiber. Coupling of these cladding modes to WGMs in silica resonators is possible when partial tapering of the fiber is also implemented in order to reduce the optical field size and increase its external evanescent portion. Importantly, the taper size is about one order of magnitude larger than that of a standard fiber taper coupler. The suggested approach is therefore much more robust and useful especially for practical applications. We demonstrate coupling to high-Q silica microspheres and microbubbles detecting the transmission dip at the fiber output when crossing a resonance. An additional feature of this approach is that by cascading LPGs with different periods, a wavelength selective addressing of different resonators along the same fiber is also possible.

F. Baldini, M. Brenci, F. Chiavaioli, A. Giannetti, C Trono: Optical fibre gratings as tools for chemical and biochemical sensing,Anal. Bioanal. Chem., 402, pp. 109-116, 2012

Abstract:Optical fibre gratings have recently been suggested as optical platforms for chemical and biochemical sensing. On the basis of the measurement of refractive index changes induced by a chemical and biochemical interaction in the transmission spectrum along the fibres, they are proposed as a possible alternative to the other label-free optical approaches, such as surface plasmon resonance and optical resonators. The combination of the use of optical fibres with the fact that the signal modulation is spectrally encoded offers multiplexing and remote measurement capabilities which the other technology platforms are not able to or can hardly offer. The fundamentals of the different types of optical fibre gratings are described and the performances of the chemical and biochemical sensors based on this approach are reviewed. Advantages and limitations of optical fibre gratings are considered, with a look at new perspectives for their utilization in the field.

C. Trono, F. Baldini, M. Brenci, F. Chiavaioli and M. Mugnaini, Flow cell for strain- and temperature-compensated refractive index measurements by means of cascaded optical fibre long period and Bragg gratings, Meas Sci Technol, 22, pp. 075204, 2011

Abstract:An optical fibre sensing system based on a hybrid cascaded long period grating (LPG) and fibre Bragg grating configuration and a thermo-stabilized flow cell for refractometric measurements is proposed. The system makes it possible to measure, and thus to cancel, the LPG cross-sensitivities to strain, temperature and fibre bending. The experimental results show that the proposed system provides satisfactory performances as far as the refractive index sensitivity and resolution are concerned. The maximum sensor sensitivity and resolution are 3120 nm/RIU and 2 × 10−5 RIU, respectively. The whole system with its flow cell and the gratings fabrication are extensively described, together with the acquisition and data processing. The stability of the sensor for several hours was also tested. We believe that the proposed system can be successfully used for label-free chemical/biochemical sensing.