Saurabh Mani Tripathi

Strain and Temperature Sensing Characteristics of Single-Mode–Multimode–Single-Mode Structures

We present a comprehensive study of the strain and temperature-sensing characteristics of single-mode-multimode-single-mode (SMS) structures based on the modal interference of guided modes of graded index multimode fiber (MMF) section spliced in between two single-mode fibers. A detailed theoretical study of the structures in terms of the refractive index distribution, effect of dopant and their concentrations, and the variation of core diameter has been carried out. Our study shows that for the SMS structure with a GeO2-doped MMF there exists a critical wavelength on either side of which the spectrum shows opposite spectral shift with a change in temperature/strain, whereas for structures with a P2O5-doped MMF it shows monotonic red shift with increasing temperature/strain. It has been found that the critical wavelength shifts toward higher wavelengths with decreasing ldquoqrdquo value/doping concentration. Using different MMFs, both the red and blue spectral shifts have been observed experimentally. It has also been found that the SMS structure has higher sensitivity toward this critical wavelength. The study should find application in designing strain-insensitive high-sensitive temperature sensors or vice versa.

Long period grating based biosensor for the detection of Escherichia coli bacteria

In this paper we report a stable, label-free, bacteriophage-based detection of Escherichia coli (E. coli) using ultra sensitive long-period fiber gratings (LPFGs). Bacteriophage T4 was covalently immobilized on optical fiber surface and the E. coli binding was investigated using the highly accurate spectral interrogation mechanism. In contrast to the widely used surface plasmon resonance (SPR) based sensors, no moving part or metal deposition is required in our sensor, making the present sensor extremely accurate, very compact and cost effective. We demonstrated that our detection mechanism is capable of reliable detection of E. coli concentrations as low as 10(3)cfu/ml with an experimental accuracy greater than 99%.

Side-Polished Optical Fiber Grating-Based Refractive Index Sensors Utilizing the Pure Surface Plasmon Polariton

In this paper, we present the ambient refractive index (ARI) sensing characteristics of metal-coated side-polished optical fiber gratings based on the excitation of the pure surface plasmon polariton. The resonance wavelength shift as a function of the ARI and the grating lengths required for a fixed minimum transmittivity (30%) for the TM-like mode are obtained for different metal and residual cladding thicknesses. It is found that a long-period grating (LPG)-based sensor is about 5-20 times more sensitive to the change in the ARI and requires much shorter grating lengths for a given sensitivity than the one based on fiber Bragg grating (FBG). Further, unlike a FBG-based device, an LPG-based sensor is found to have maximum sensitivity at an optimum value of metal thickness, the reason for which is also explained. This paper should find application in the design of sensitive fiber-optic bio/chemical sensors.

Temperature insensitive high-precision refractive-index sensor using two concatenated dual-resonance long-period gratings

In this Letter we report on fabricating and analyzing a temperature insensitive refractometer based on two concatenated dual-resonance long-period gratings (LPGs) with an appropriate inter-grating space (IGS) in between. The IGS provides a temperature-dependent extra phase difference between the core and cladding modes, making the refractometer similar to a Mach-Zehnder interferometer with its arms phase shifted. We demonstrate that an appropriate IGS can produce temperature-insensitive resonance wavelengths. The interferometer is highly stable over a wide range of temperature (20°C-100°C). The measured refractive index sensitivity for aqueous solutions (1.333-1.343) is ~2583 nm/RIU, which is the highest reported so far for biological samples. The interferometer can be used for various other temperature-immune sensing applications also.

Highly Sensitive Miniaturized Refractive Index Sensor Based on Au-Ag Surface Gratings on a Planar Optical Waveguide

We present the theoretical study of a novel highly sensitive, miniaturized, integrated optic refractive index sensor based on a Au-Ag surface grating. The grating is considered to be made of alternate layers of equi-thick Au and Ag regions along the direction of propagation, on the surface of the waveguide. Due to the same thickness of both the metals, the surface plasmon polaritons (SPP) for both metals have their field maxima at the same transverse distance, leading to an increased modal overlap in the grating region and hence a reduced grating length. An exact coupled-mode-theory based on the local mode matching has been used to analyze the mode coupling between the guided mode and the SPP. It has been shown that the proposed design requires nearly one fourth of the grating length as compared to the corrugated metal grating for the same metal thickness. Further, for co-propagating mode coupling (LPG based sensor) the structure is found to be maximum sensitive at an optimum metal thickness, however, such an optimum metal thickness does not exist for counter-propagating coupling (FBG based sensor).

Critical Wavelength in the Transmission Spectrum of SMS Fiber Structure Employing GeO

We demonstrate and explain the existence of a critical wavelength in the transmission spectrum of single-multi-singlemode fiber structures, employing GeO2-doped multimode fibers (MMFs), such that the spectrum shows opposite spectral shift on either side of this wavelength with increasing temperature. Further, the shift is maximum for peaks/dips nearest to this wavelength. The variation of critical wavelength with temperature is also measured. A theoretical account of the observed behavior has been presented and the results are found to be in excellent agreement with the experiments. It is shown theoretically that for a given MMF there should be two critical wavelengths of the opposite nature separated by a large wavelength difference.

_{2}

THz Bragg gratings were fabricated by using CO2 laser inscription. The simulated and experimental results demonstrate potential of such gratings in paper thickness monitoring, with experimental spectral sensitivities of ~ -0.67 GHz / 10 μm.

-Doped Multimode Fiber

We present a simple, efficient and easy to fabricate single-multi-single mode (SMS) fiber-based tunable bandwidth optical bandpass/bandstop filter. The device exploits the transmission characteristics of an SMS structure near its critical wavelength. Using both temperature and strain tuning, we show that the device can be switched between band pass to band stop modes and that the filter bandwidth in each mode can be dynamically tuned. We present a theoretical analysis of the observed behavior and obtain excellent agreement with the experimental observations.

Resonant THz sensor for paper quality monitoring using THz fiber Bragg gratings.

We present a theoretical study of the ambient refractive index sensing characteristics of long period gratings in bare and metal-coated D-shaped fibers. An equivalent rectangular core waveguide method based on the first-order perturbation theory has been used to study the modal behavior of the waveguide. Power coupling corresponding to dual resonance in both cases has been investigated, and an optimum metal thickness giving maximum sensitivity has been found to exist. The study shows that the dual resonances can be shifted to lower wavelengths by increasing (decreasing) the metal thickness (core to flat surface separation). Further, an optimum combination of metal thickness and core to flat surface separation, corresponding to maximum sensitivity, has been presented for different cladding modes and their relative performance has been discussed. It has been shown theoretically that detection of refractive index changes as small as 1.67x10(-7) RIU in the ambient region is possible using the optimized parameters. The study should find application in realizing highly sensitive biochemical sensors.

Single-Multi-Single Mode Structure Based Band Pass/Stop Fiber Optic Filter With Tunable Bandwidth

A novel biochemical sensor based on a submicrometer size, high core-cladding index difference, silica core Si-SiO(2) waveguide with a Bragg grating written in its cladding region is proposed and analyzed. Waveguide parameters are optimized to obtain maximum sensitivity, and for lower refractive index samples, an optimum core width is found to exist for both the TE and the TM mode configurations. Owing to the high index contrast at the Si-SiO(2) interface, the structure is much more sensitive while operating in the TM mode configuration, showing extremely high sensitivity [200-740 nm refractive index units (RIU)] for the ambient refractive indices between 1.33 and 1.63, which is of the order of most surface plasmon polariton (SPP) based biosensors. Further, unlike SPP based sensors, the proposed structure is free from any metallic layer or bulky prism and hence easy to realize. Owing to its simple structure and small dimensions, the proposed device could be easily integrated with planar lightwave circuits and could be used for lab-on-a-chip applications.