Predrag Mikulic

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%.

Detection of bacteria using bacteriophages as recognition elements immobilized on long-period fiber gratings

The paper presents for the first time a study of long-period gratings (LPGs) applied for label-free detection of specific bacteria using physically adsorbed bacteriophages. For the purposes of the experiment a number of highly sensitive LPGs working at the turning point of phase matching curve was fabricated in SMF28 fiber using UV exposure. We show that the device allows for real-time monitoring of phenomena taking place on the sensors surface, including phage-bacteria interactions. For the applied conditions a resonance wavelength shift of ~1.3 nm induced by bacteria binding was observed.

Increasing sensitivity of arc-induced long-period gratings—pushing the fabrication technique toward its limits

This paper presents an investigation of the sensing properties of long-period gratings (LPGs) written with the electric-arc technique in commonly used standard germanium-doped Corning SMF28 and boron co-doped Fibercore PS1250/1500 fibers. In order to increase the sensitivity of the LPGs, we studied and established for each fiber the writing parameters allowing for the coupling of the highest possible order of cladding modes at a resonance wavelength around λ = 1550 nm. The sensitivity of the LPGs to refractive index, to temperature and to hydrostatic pressure was investigated. The experimental results were supported by extensive numerical simulations. Thanks to the well-established and precisely controlled arc-writing process, we were able to reduce the minimum period of the gratings down to 345 and 221 µm, respectively, for LPGs based on the SMF28 and PS1250/1500 fibers. To the best of our knowledge, these are the shortest periods ever achieved for these fibers using the arc-manufacturing technique. The pressure sensitivities of 13 and 220 pm bar−1 are the highest ever measured for LPGs written in the SMF28 and PS1250/1500 fibers, respectively. Moreover, a reduction in the diameters of the SMF28 fiber induced by the arc was found, which significantly affected the distribution of resonances generated by the coupled cladding modes.

A Novel Fiber-Optic Tapered Long-Period Bragg Grating Sensor for Pressure Monitoring

The method and the required installations for fabricating tapered long-period fiber gratings can be simpler than that of the standard fiber Bragg gratings, and the fabrication process is faster. To our knowledge the fiber-optic tapered long-period fiber grating pressure sensor is presented here for the first time. In this paper the fabrication method for tapered long-period fiber gratings, the sensing principle, the sensor structure, the measurement set-up and the preliminary results are presented and discussed. The pressure sensitivity of the sensor is as high as 5.1 pm/bar

Pressure sensing using periodically tapered long-period gratings written in photonic crystal fibres

This paper reports on the first application of tapered long-period gratings (TLPGs) written in photonic crystal fibres (PCFs) using a cost-effective computer-assisted precision arc-discharge apparatus for conducting direct measurements of hydrostatic pressure up to 180 bar. The developed TLPG-PCF device displays significant potential as a highly sensitive and cost-effective pressure sensor, with a pressure sensitivity of 11.2 pm bar?1. A negligible temperature sensitivity of about 0.3 pm ?C?1 of the sensor output signal was observed within the temperature range from 5 ?C to 65 ?C.

An Inline Core-Cladding Intermodal Interferometer Using a Photonic Crystal Fiber

We propose a simple all-fiber structure based on intermodal interference between a core and a cladding mode of an endlessly single-mode photonic crystal fiber (PCF) section sandwiched between a lead-in and lead-out SMF-28 fiber. We have measured the intermodal dispersion and the polarization-dependent loss (PDL) of the structure. The sensitivities to strain, pressure, and temperature and external refractive index are studied. The interferometer is compared to an interferometer based on LP01-LP02 SMF-28 fiber and to a PCF-based tapered LPG.

Label-free sensitivity of long-period gratings enhanced by atomic layer deposited TiO 2 nano-overlays

In this paper, we discuss an impact of thin titanium dioxide (TiO(2)) coatings on refractive index (RI) sensitivity and biofunctionalization of long-period gratings (LPGs). The TiO(2) overlays on the LPG surfaces have been obtained using atomic layer deposition (ALD) method. This method allows for a deposition of conformal, thickness-controlled, with well-defined optical properties, and high-RI thin films which are highly desired for optical fiber sensors. It has been found that for LPGs working at a dispersion turning point of higher order cladding modes only tens of nanometers of TiO(2) overlay thickness allow to obtain cladding mode transition effect, and thus significant improvement of RI sensitivity. When the TiO(2) overlay thickness reaches 70 nm, it is possible to obtain RI sensitivity exceeding 6200 nm/RIU in RI range where label-free sensors operate. Moreover, LPGs with TiO(2)-enhanced RI sensitivity have shown improved sensitivity to bacteria endotoxin (E. coli B lipopolysaccharide) detection, when TiO(2) surface is functionalized with endotoxin binding protein (adhesin) of T4 bacteriophage.

Tuned Pressure Sensitivity of Dual Resonant Long-Period Gratings Written in Boron Co-Doped Optical Fiber

This paper presents a pressure sensor based on a long-period grating (LPG) written in boron co-doped photosensitive fiber and operating at the phase-matching turning point. It is shown that the pressure sensitivity can be tuned by varying the UV exposure time during the LPG fabrication process as well as by varying ambient temperature during pressure measurements. The achieved pressure sensitivity in certain pressure range can reach over 1 nm·bar- 1, and is at least four times higher than for previously presented gratings working away from the double-resonance regime. In terms of an intensity-based measurement, the sensitivity at the turning point can reach 0.212 dB ·bar - 1.

Comparative study of long-period gratings written in a boron co-doped fiber by an electric arc and UV irradiation

The paper presents for the first time a comparative study of long-period gratings (LPGs) written by point-by-point UV irradiation and by electrical arc discharges. These gratings were inscribed in a highly photosensitive boron co-doped fiber that can be considered as a suitable platform for LPG writing using either technology. The experimental transmission data for the manufactured LPG devices fit well when compared to the simulations we carried out in parallel. As a result of each of these writing processes, we were able to obtain a remarkably good quality of grating. Two reasons could explain the observed small differences between the spectra: a slight mismatch of the period of the gratings and an unintentional tapering of the fiber during the arc-based processes. We also found that the UV irradiation at λ = 248 nm can cause clearly visible damage to the fibers surface. As a result of the UV writing, a coupling to the asymmetrical cladding modes can take place. Moreover, the gratings written using the two technologies show a very similar refractive index and temperature-sensing properties. The only differences between them can come from a physical deformation of the fiber induced by the electric arc discharges.

Refractive-Index Sensing With Inline Core-Cladding Intermodal Interferometer Based on Silicon Nitride Nano-Coated Photonic Crystal Fiber

This paper presents a modification of the refractive-index (RI) response of an intermodal interferometer based on photonic crystal fiber (PCF) using a thin plasma-deposited silicon nitride (SiNx) overlay with a high refractive index. We show that the film overlay can effectively change the distribution of the cladding modes and thus tune the RI sensitivity of the interferometer. Thanks to the nano-coating we were able to increase the RI sensitivity eightfold in the range required for biosensors (nD ~ 1.33). Due to the extreme hardness of SiNx films and their excellent adhesion to the fiber surface, we believe that after the film deposition the device will still maintain its advantages, i.e., lack of degradation over time or with temperature.