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Microbubble-enabled photonic crystal fiber-based sensors
Abstract
A review of photonic crystal fiber sensors with microbubble is presented. The work concentrate in three sensing fields: the first reports on temperature sensing; the second describes on strain sensing; the last part focuses on refractive index sensing and bio-sensing. The method of fabricating the microbubble is also briefly discussed.
The microhole collapsing effect technique is a relatively simple fabrication process that produces Photonic Crystal Fiber (PCF) refractometer using modal interferometry in the range of 10^−5 refractive index resolution. The repeatable method preserves the same 125 μm structural integrity of the optical fiber for various applications such as multi-parameter sensing and bioaffinity. Compared to previous reports for conventional strain or temperature sensing using a single microbubble, the use of two microbubbles in the in-line microbubble structure significantly increases the light-molecule interaction for developing ultralow concentration biosensor. It has also been demonstrated as a potential reusable and label-less PCF biosensor platform. For temperature sensing, another low-complexity approach for fabricating a PCF directional coupler structure, without costly masking or precision marking laser, is also discussed. Numerical simulations have also been investigated on the PCF directional coupler structure to validate experimental result and on the microfluidic optical fiber device to rapidly find optimal fabrication-sensitivity design.
We present a cascaded fiber device based on photonic crystal fiber (PCF) interferometers for refractive index (RI) sensing. The PCF modal interferometers have microbubbles at both sides of the splice regions. The microbubbles act as thick diverging lens that scatter light for efficient excitation of higher order cladding modes and attenuation of the core modes in the transmitted spectrum. Three resonance wavelengths are monitored, and their corresponding RI sensitivities are found to be 252, 187, and 207 nm/refractive index unit (RIU). The results, to our best knowledge, demonstrate that PCF interferometers with microbubbles are repeatable for high RI sensitivity, and the best crosstalk achievable is 0.295 nm/RIU.
We present a fiber-optic Fizeau interferometric strain sensor consisting of an in-fiber spherical microcavity of 39 mum in diameter. The spherical microcavity was formed by splicing a normal single-mode fiber with a hollow-core photonic crystal fiber. We demonstrate that strain sensing can be realized by using the interference between the light signals reflected by the front and rear surfaces of the sphere. Experiments have shown that the strain sensor has a strain sensitivity of 3.36 pm/muε and a temperature sensitivity of 1.35 pm/°C.
A fiber-optic strain sensor is demonstrated by using a short length of polarization-maintaining photonic crystal fiber (PM-PCF) as the sensing element inserted in a Sagnac loop interferometer. Spectrum shift in response of strain with a sensitivity of 0.23 pm/με is achieved, and the measurement range, by stretching the PM-PCF only, is up to 32 mε. Due to the ultralow thermal sensitivity of the PM-PCF, the proposed strain sensor is inherently insensitive to temperature, eliminating the requirement for temperature compensation.
In this paper we review the fabrication and characterisation techniques of microstructured optical fibre (MOF) tapers, their fundamental waveguiding properties and potential applications. We fabricate photonic crystal fibre tapers without collapsing the air-holes, and confirm this along the taper with a non-invasive probing technique. We then describe the fundamental property of such tapers associated with the leakage of the core mode that leads to long wavelength loss. We also revisit the waveguiding properties in another form of tapered MOF photonic wires, which transition through waveguiding regimes associated with how strongly the mode is isolated from the external environment. We explore these regimes as a potential basis for evanescent field sensing applications, in which we can take advantage of controlled airhole collapse as an extra dimension to these photonic wires.
In this paper we present an interferometer based on photonic crystal fiber (PCF) tip ended with a solid silica-sphere for refractive index sensing. The sensor is fabricated by splicing one end of the holey PCF to a single mode fiber (SMF) and applying arc at the other end to form a solid sphere. The sensor has been experimentally tested for refractive index and temperature sensing by monitoring its wavelength shift. Measurement results show that the sensor has the resolution of the order of 8.7 × 10 − 4 over the refractive index range of 1.33–1.40, and temperature sensitivity of the order of 10 pm / ° C in the range of 20–100 °C.
An extremely short long-period grating (LPG) with strong resonance has been developed in a large-mode-area photonic crystal fiber (PCF) by use of the heat source of a CO 2 laser. We believe that such a LPG in pure silica PCF is the first example to be obtained with the point-by-point technique. The fabrication method is simple and repeatable. The resulting LPG has 8 periods, written by a CO 2 laser, within a 2.8-mm length of fiber, which yields a deep notch of core–cladding mode coupling of - 31.5 dB at the telecommunication wavelength of 1529.2 nm, with a FWHM of ∼ 0.7 nm . The principal advantages of this LPG are that it is practical, cost effective, and compact.
A tunable photonic crystal fiber (PCF) coupler, which couples part of the optical power in one PCF with that in another PCF, has been made by side polishing. We fabricated the PCF coupler by mating two side-polished PCFs. We achieved evanescent field coupling between the core modes of the two PCFs by using side polishing to bring the cores close to each other. By adjusting the mating angle between the two side-polished PCFs we obtained as much as 90% tunability in the coupling ratio. The spectrum of the coupling ratio was almost flat, with small ripples, over a 400-nm wavelength range.
We report about fiber Bragg gratings (FBGs) inscribed in two different types of small-core Ge-doped photonic crystal fibers with a UV laser. Sensing properties of the FBGs were systematically investigated by means of demonstrating the responses of Bragg wavelengths to temperature, strain, bending, and transverse-loading. The Bragg wavelength of the FBGs shifts toward longer wavelengths with increasing temperature, tensile strain, and transverse-loading. Moreover, the bending and transverse-loading properties of the FBGs are sensitive to the fiber orientations. The reasonable analyses for these sensing properties also are presented.
Microstructured optical fibres (MOFs) have attracted much interest in recent times, due to their unique waveguiding properties that are vastly different from those of conventional step-index fibres. Tapering of these MOFs promises to significantly extend and enhance their capabilities. In this paper, we review the fabrication and characterisation techniques of these fibre tapers, and explore their fundamental waveguiding properties and potential applications. We fabricate photonic crystal fibre tapers without collapsing the air-holes, and confirm this with a non-invasive probing technique that enables the characterisation of the internal microstructure along the taper. We then describe the fundamental property of such tapers associated with the leakage of the core mode that leads to long-wavelength loss, influencing the operational bandwidth of these tapers. We also revisit the waveguiding properties in another form of tapered MOF photonic wires, which transition through waveguiding regimes associated with how strongly the mode is isolated from the external environment. We explore these regimes as a potential basis for evanescent field sensing applications, in which we can take advantage of air-hole collapse as an extra dimension to these photonic wires.
We report the fabrication of a highly sensitive refractometer based on photonic crystal fiber (PCF) interferometry. Using fusion arc splicing technique, a short length of PCF (~3 mm) was spliced to conventional single mode fibers (SMF) on both sides. The voids of the PCF were completely collapsed in the splice region, which acted as mode splitter and combiner on both sides. Microbubbles were formed in the splice region and acted as diverging lens for exciting cladding modes efficiently. The transmission spectrum exhibited detectable dips for refractive index (RI) sensing. The bulk liquid sensitivity can reach up to 320 nm/RIU (refractive index unit) based on the linear curve fit for RI ranging from 1.33 to 1.34. Biosensing experiment was carried out using streptavidin and biotin as respective analyte and receptor. To the best of authors knowledge, this is the first time of reporting bio-affinity test on the external surface of PCF using modal interferometry.