1240 IEEE SENSORS JOURNAL, VOL. 15, NO. 2, FEBRUARY 2015 Sensitivity Enhancement of Turn-Around-Point Long Period Gratings By Tuning Initial Coupling Condition Palas Biswas, Member, IEEE, Nandini Basumallick, Member, IEEE, Sankhyabrata Bandyopadhyay, Kamal Dasgupta, Member, IEEE, Ajay Ghosh, and Somnath Bandyopadhyay, Member, IEEE Abstract— Long period grating (LPG) at turn-around- point (TAP) has been studied with a view to enumerate the dependence of sensitivity of a particular resonant mode at the TAP to surrounding refractive index on the initial coupling strength. It has been shown theoretically and also validated experimentally that sensitivity can be enhanced significantly by tailoring the coupling strength of the cladding mode at the resonant wavelength near the TAP. Sensitivity characteristics have been studied for surrounding refractive index in the range 1.335–1.360, which is of interest in the field of biosensors, where the sensitivity of conventional LPGs is relatively small. We could attain a sensitivity of ∼1850 nm/RIU using a TAP-LPG with ∼3-dB attenuation at resonance. Index Terms— Long period grating, turn-around-point, refractive index sensing. I. I NTRODUCTION L ONG period gratings (LPGs) are nowadays extensively used as refractive index sensors. In LPG power in the core mode is coupled to several cladding modes at some specific resonant wavelengths. The resonant wavelength depends on the period and the difference of effective indices of the core and the cladding modes. A change in surrounding refractive index alters the effective indices of the cladding modes which in turn induces a change in the resonant wavelengths and forms the basis of refractive index sensing using LPG [1]. Sensitivity characteristics of different resonant modes of LPG have been investigated in detail [2]. It was revealed that the highest sensitivity can be achieved by designing the LPG resonant mode around the turn-around-point (TAP) where the general sensitivity factor (γ) was found to be appreciably high. Manuscript received August 7, 2014; revised September 26, 2014; accepted September 27, 2014. Date of publication October 2, 2014; date of current version December 3, 2014. This work was supported by the Network Project under Grant ESC-102 and Grant ESC-110 through the CSIR India, under the 12th five year plan. The associate editor coordinating the review of this paper and approving it for publication was Dr. Anna G. Mignani. P. Biswas, N. Basumallick, S. Bandyopadhyay, K. Dasgupta, and S. Bandyopadhyay are with the Fiber Optics and Photonics Division, Council of Scientific and Industrial Research-Central Glass and Ceramic Research Institute, Kolkata 700032, India (e-mail: palas@cgcri.res.in; nandini_b@cgcri.res.in; sankhya.brata@gmail.com; kamal@cgcri.res.in; somnath@cgcri.res.in). A. Ghosh is with the Department of Applied Optics and Photonics, Univer- sity of Calcutta, Kolkata 700098, India (e-mail: aghosh.cu@gmail.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2014.2361166 There has been a gush of research reports recently where this TAP-LPG sensor and several modifications thereof were used successfully in bio-sensing and chemical sensing applications, where high sensitivity refractive index (RI) measurement is a necessity [3]–[11]. In general there are two distinct methodologies based on which TAP-LPGs are used for refractive index sensing. The resonance between the core mode and a specific higher order cladding mode at TAP of its phase matching curve (PMC) pro- duces a single broadband attenuation around the TAP. An LPG with a single attenuation band centred around the TAP can be so designed that any change of surrounding index modifies the peak attenuation at the resonant wavelength without altering the wavelength itself [7]–[12]. The advantage is that the sensor is usable exactly at the TAP and thus possesses a very high sensitivity in theory but often plagued with the limitation of intensity based measurement in reality. On the other hand a TAP-LPG with a single attenuation band in air when immersed in an environment having refractive index higher than that of air, the single wide band resonant peak splits into two in either side of the resonant wavelength at the TAP. Shift of either of these two resonant wavelengths or both are usually monitored for subsequent change in surrounding RI. This is important to note that the resonant wavelength that we monitor under this circumstance is far from the TAP and thus should be having a much reduced sensitivity factor. A simple example will elucidate the fact. If we inscribe a TAP-LPG (with a resonant wavelength at the TAP of the PMC of LP 0,12 cladding mode of SMF28e) in air and then immerse it in water (RI ≈ 1.333), the split peaks are typically separated by about 130 nm from the TAP in either side. This is of course an estimate that may easily be obtained by simulation and may change with mode order and variation in the fiber parameters. But the fact is that, the dual resonant wavelengths which now form the basis for measuring infinitesimal RI change around 1.333 have lost the essence of working around TAP and the parameter γ becomes much smaller as compared to what would have been for the same resonant mode near the TAP. So far this has not been explicitly addressed in the literature where TAP-LPGs have been used and therefore, the results available so far could have been improved if the TAP was designed optimally. Recently there is an interesting proposal of designing the TAP-LPG using two LPGs with a precise phase shift in between so that 1530-437X © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.