Phase-shifted fiber Bragg grating filters based on counter-propagating cladding modes coupling.

In this paper, we comprehensively analyze counter-propagating cladding mode assisted phase-shifted fiber Bragg gratings (FBGs) and propose an ultra-narrow bandwidth laser line filter based on such gratings. Full vector modal analysis has been used to obtain the mode guiding and its coupling characteristics. We show that the transmission spectrum of the counter-propagating cladding mode assisted FBGs can be tailored by incorporating single or multiple phase shifts along the grating length. Phase shifts open up narrowband transmission windows inside the stop band of the Bragg grating, and the transmitted wavelength can be altered by controlling the amount of phase shift. Unlike conventional FBGs, the grating discussed here has access to the evanescent field of the excited counter-propagating cladding mode, which opens up the possibility of refractive index based tuning of resonance wavelength. Further, the bandwidth of the proposed grating is two orders of magnitude smaller than that of the conventional LPGs. As an alternative application, we also show that with the right length ratio, the multiple phase shifts can also be used to create a very flattened single transmission peak inside the stop band. The effects of grating regulating factors such as the grating length, grating strength, location of the phase shift, and index apodization on the linewidth of the narrow central peak of π PS-FBG are also investigated. We present a clear physical explanation of various factors involved in the counter-propagating cladding mode coupling in such phase-shifted Bragg grating. Due to its extremely narrow transmission window, our study can find application in developing sensors for measuring parameters with greater accuracy. Such phase-shifted Bragg gratings can also be used to create an all-fiber demultiplexer for multichannel systems and fiber optic sensors as a particular application.

Ultra-narrow linewidth transmission filters based on the cladding mode assisted Fabry-Perot effect in a planar waveguide.

We propose and analyze a counterpropagating cladding mode assisted tunable frequency Fabry-Perot interferometer formed by a Bragg grating (BG) cavity in a liquid crystal coated planar optical waveguide. A full vector modal analysis has been used to obtain the transmission spectra of the individual Bragg reflectors, and the cavity effects have been incorporated by employing a suitable phase matrix. We show that the cavity resonances that appear from two fiber BGs forming a resonator can be efficiently explained by incorporating appropriate phase shifts in one BG grating period. We further show that utilizing the cladding mode evanescent field, a liquid crystal overlay can be used to tune the cavity resonance over the entire free-spectral range of the cavity transmission spectra. Our study should find application in designing highly tunable integrated optical Fabry-Perot interferometers.

Sensing characteristics of grating assisted counterpropagating cladding modes in optical fibers.

In this paper, using full vector modal field analysis, we present physical parameter sensing, e.g., temperature, axial strain, bulk refractive index, and affinity sensing characteristics of counterpropagating cladding modes in optical fibers. Counterpropagating cladding modes (HEmn) are considered to be excited by resonant power coupling from the fundamental core mode using a suitably designed fiber Bragg grating (FBG). We show that for such couplings, the bandwidths of reflection spectra are much smaller than those obtained for conventional core mode reflecting FBGs. Next, we also show that the reflection bandwidth increases linearly with increasing grating strength, whereas it decreases exponentially with increasing grating length. The reflection spectra of such FBGs, as a function of different ambient physical perturbation parameters, are obtained, and then the grating sensitivity corresponding to those parameters is established. Interestingly, we have noticed that despite having the same evanescent field in the analyte region, unlike the long-period grating assisted co-propagating cladding modes (HEmn), the same cladding modes while propagating counter to the core mode are weakly sensitive to changes in the ambient refractive index. The reason behind this observation is discussed.

Ultrahigh sensitive long-period fiber grating-based sensor for detection of adulterators in biofuel.

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 ∼1.3×10-3V% 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.

Compact and cost-effective temperature-insensitive bio-sensor based on long-period fiber gratings for accurate detection of E. coli bacteria in water.

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.

Temperature insensitive single-mode-multimode-single-mode fiber optic structures with two multimode fibers in series.

We propose and demonstrate a temperature insensitive single-mode-multimode-single-mode fiber optic structure consisting of two in-series multimode fibers of appropriate lengths and of opposite temperature sensitivities. A simple approximate expression to estimate the required length ratio of the multimode fiber sections has also been derived whose prediction is found in good agreement with the experiment. The study should be useful in realizing various fiber optic devices based on multimode interference with zero temperature cross sensitivity.

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.

Temperature-insensitive fiber-optic devices using multimode interference effect.

We show theoretically that the fiber-optic devices using single-multi-single mode fiber structures can be made temperature insensitive by properly adjusting the concentration of P(2)O(5) in the core region of the multimode fiber used. Taking an example of a parabolic index multimode fiber, we obtain the temperature-insensitive transmission spectrum and fiber-optic lens action for a core composition of 1.57 mol. % P(2)O(5) and 13.5 mol. % GeO(2) in the SiO(2) host.

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%.

Tunable wavelength division multiplexing channel isolation filter based on dual chirped long-period fiber gratings.

We propose and demonstrate a wavelength tunable wavelength division multiplexing channel isolation filter based on two concatenated chirped long-period fiber gratings (LPGs). An intergrating space (IGS), deliberately introduced between the two gratings, provides an extra phase difference between the core and cladding modes. Changing this phase by heating the IGS without affecting the gratings tunes the channels. A theoretical account of the filter action is also presented and the results are found to be in excellent agreement with the experiments. Unlike the filters based on normal concatenated chirped LPGs without an IGS, the current filter shows a linear tuning over an increased spectral range.

Bragg grating based biochemical sensor using submicron Si/SiO2 waveguides for lab-on-a-chip applications: a novel design.

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.