Quasi-distributed and wavelength selective addressing of optical micro-resonators based on long period fiber gratings.

A novel all-in-fiber method for coupling light to high-Q silica whispering gallery mode (WGM) optical micro-resonators is presented, which is based on a pair of long period fiber gratings (LPGs) written in the same silica fiber, along with a thick fiber taper (15-18 µm in waist) in between the LPGs. The proposed coupling structure is robust and can be replicated many times along the same fiber simply cascading LPGs with different bands. Typical Q-factors of the order of 10(8) and total coupling efficiency up to 60% were measured collecting the resonances of microspheres or microbubbles at the fiber end. This approach uniquely allows quasi-distributed and wavelength selective addressing of different micro-resonators along the same fiber.

Long period grating-based fiber coupler to whispering gallery mode resonators.

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.

Fiber ring laser for intracavity sensing using a whispering-gallery-mode resonator.

Whispering-gallery-mode (WGM) microresonators are used as optical transducers for sensing applications. The typical detection scheme is based on tracking the WGM resonance shift, by scanning with a tunable laser, when a change of the refractive index in the region probed by the WGM takes place. We propose a sensing approach based instead on monitoring the position of the laser line of a fiber ring laser having a WGM microsphere in its loop. We have demonstrated that the induced shift is the same for the ring laser line and for the microsphere resonance. The proposed method requires simpler, cheaper equipment and may also improve the sensor resolution because the ring laser line is much narrower than the microsphere WGM resonance.

Optical fibre gratings as tools for chemical and biochemical sensing.

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.

A new procalcitonin optical immunosensor for POCT applications.

A new immunosensor for the determination of procalcitonin was developed. A sandwich assay format was implemented on a polymethylmetacrylate optical biochip, opportunely shaped in order to obtain several flow channels and potentially suitable for point of care testing applications. The sandwich format makes use of two new rat monoclonal antibodies. The capture antibody was covalently immobilised on the surface of the plastic chip, and the detection antibody was labelled with DY647 dye. Different combinations of capture and detection antibodies were investigated, and particular attention was devoted in order to avoid the non-specific adsorption. A limit of detection of 0.088 mg L(-1) was achieved within the working range of 0.28-50 mg L(-1) in buffer samples. The assay was also implemented in human serum, and 0.2 and 0.7-25 mg L(-1) were the attained limit of detection and working range, respectively.

A new optical platform for biosensing based on fluorescence anisotropy.

A novel fluorescence-based optical platform for the interrogation of an optical biochip was designed and developed. The optical biochip was made of poly(methyl methacrylate) (PMMA) formed by two pieces of PMMA appropriately shaped in order to obtain four microchannels that are 500-microm wide and 400-microm high. The lower part includes the microchannels and the inlet and outlet for the fluidics, while the sensing biolayer was immobilized on the upper part. The optical signal comprised the fluorescence emitted by the biolayer, which was anisotropically coupled to the PMMA cover and suitably guided by the PMMA chip. The potentiality of the optical chip as a biosensor was investigated by means of a direct IgG/anti-IgG interaction carried out inside the flow channels. The mouse-IgG was covalently immobilized on the internal wall of the PMMA cover, and the Cy5-labelled anti-mouse IgG was used for the specific interaction. Several chemical treatments of the PMMA surface were investigated, poly(L: -lactic acid), Eudragit L100 and NaOH, in order to obtain the most effective distribution of carboxylic groups useful for the covalent immobilisation of the mouse-IgG. The treatment with Eudragit L100 was found to be the most successful. Limits of detection and quantification of 0.05 microg mL(-1) and 0.2 microg mL(-1), respectively, were obtained with the configuration described.

Towards sensitive label-free immunosensing by means of turn-around point long period fiber gratings.

Long period fiber gratings have been effectively used in the field of biochemical sensing since a few years. Compared to other well-known label-free optical approaches, long period gratings (LPGs) take advantage of the typical peculiarity of optical fibers. Coupling the propagating core mode with a high-order cladding mode near its turn-around point (TAP) was the strategy adopted to achieve good performances without additional coatings, except for the sensing and selective biolayer deposited on the fiber. Both the modeling and manufacturing of TAP LPGs were discussed. After the functionalization of the fiber surface with the deposition of a Eudragit L100 copolymer layer followed by immunoglobulin G (IgG) covalent immobilization, an IgG/anti-IgG bioassay was implemented along the grating region and the kinetics of antibody/antigen interaction was analyzed. A quantitative comparison between a TAP LPG and a non-TAP LPG was carried out to highlight the improvement of the proposed immunosensor. The real effectiveness and feasibility of an LPG-based biosensor were demonstrated by using a complex matrix consisting of human serum, which also confirmed the specificity of the assay, and a limit of detection of 70 µg L(-1) (460 pM) was achieved.

Optofluidic microsystems with integrated vertical one-dimensional photonic crystals for chemical analysis.

In this work, we report all-silicon, integrated optofluidic microsystems (OFMs) fabricated by electrochemical micromachining (ECM) technology, in which high aspect-ratio (HAR) photonic crystal (PhC) devices (i.e. micromirrors, optical cavities) are integrated by one-etching-step, together with microfluidic reservoirs/channels, for the infiltration of liquids in the PhC air gaps, and with fiber grooves for alignment/positioning of readout optical fibers in front of the PhC, on the same silicon die. This has not previously been reported in the literature, and opens up new ground in, though not limited to, the optofluidics field, due to the low-cost and high-flexibility of the ECM technology that allows optofluidic microsystem fabrication to be performed in any lab. Optofluidic characterization of PhC-OFMs by both capillary-action and pressure-driven operations is carried out through the measurement of the reflectivity spectra of HAR-PhCs upon injection of liquids featuring different refractive index values in the HAR-PhC air gaps, by using readout optical fibers positioned in the on-chip fiber grooves. High sensitivity and good limit of detection of PhC-OFMs are obtained for both capillary-action and pressure-driven operations. A best sensitivity value of 670 nm/RIU and a worst-case limit of detection of the order of 10(-3) RIU are measured, the former being comparable to state-of-the-art integrated refractive index sensors and the latter being limited by constraints of the experimental setup. The proof of concept about the biosensing potential of PhC-OFMs is given by successfully carrying out a sandwich assay based on antigen-antibody interactions for the detection of the C-reactive protein (CRP) at a concentration value of 10 mg L(-1), which represents the boundary level between physiological and pathological conditions.