Our group has integrated surface-enhanced Raman scattering (SERS) silver coated gold nanostars on an optical fiber. Fiber-based sensors are an in-situ technology that can simultaneously bring the sensor and light to the sample without disturbing the environment. This technology is a multi-use method that does not require complex sample preparation. Fiber sensors or optrodes, enable the detection of analytes in samples that are difficult to access. Additionally, optrodes allow for specific detection while evading background signals from non-target regions. The fiber-optrode was used to detect miRNA and illegal food additives.
Molecular analysis has revolutionized many applications, including bio-safety, bio-engineering and biofuel research; however, there are limited practical tools for in situ detection during field work. New technology is needed to translate molecular advances from laboratory settings into the practical realm. The unique characteristics of plasmonic nanosensors have made them ideal candidates for field-ready sensing applications. Herein, we discuss the development of a fiber-based plasmonic sensor capable of direct detection (i.e., no washing steps required) of miRNA targets, which are detected by immerging the sensor in the sample solution. This sensor is composed of an optical fiber that is decorated with plasmonic nanoprobes based on silver-coated gold nanostars to detect target nucleic acids using the surface-enhanced Raman scattering sensing mechanism of nanoprobes referred to as inverse molecular sentinels. The fiber sensors were tested in extracts from leaves of plants that were induced to have different miRNA expression levels. The results indicate that the fiber sensors developed have the potential to be a powerful tool for field analysis.
As one of the most proven fiber optic sensors, novel fiber Bragg gratings are continually investigated to extend their roles in extreme environments. In this paper, a newly found “secondary Bragg grating” (SBG) is proposed. The presence of SBG occurs in the case of the type-IIa Bragg grating inscribed in small active fibers, where an additional resonance appears at the shorter wavelength. The SBG provides a variety of interesting characteristics, such as the dip integration, high temperature resistance and high reflectivity, showing promising potential in high temperature sensing.
Label-free fiber optical biosensor has a promising prospect in “point-of-care” (POC) test for disease diagnosis. A sensitive label-free fiber-optic based immunosensor for quantitative Cardiac Troponin I (cTn-I) testing has been proposed by using a phase-shifted Bragg grating directly inscribed in microfiber. The fine notch signal in the grating spectrum remarkably enhances the ability of the sensor in detecting an extremely small amount of immune binding events, which is essential for AMI diagnosis at very early stage. A cTn-I concentration of 6 pg/mL is enough to arouse the response of the sensor with high specificity. According to the log-linear range of the concentration between 0.1-10 ng/mL, measurements with shorter detection time are analyzed to demonstrate the potential of the sensor in the fast screen of the high-risk patients. The proposed sensing probe is compact and feasible, easy to handle, fabricate and network, making itself a competitive candidate in POC diagnosis of AMI.
A high sensitivity biosensor based on graphene coated silica fiber taper interferometer is presented. Thank to the combination of graphene coating and the optical fiber taper interferometer structure, the biosensor demonstrates improved DNA concentration sensitivity of 0.4 nm/log M and good linearity, yielding the lower detection limit of 10 pM. This high sensitivity and biocompatibility enable the biosensor in precision in-situ DNA detection, even in ultra-diluted DNA solution. Based on our work, the graphene coating could convert the concentration information of target molecular to the RI variation, and further to light signals by the taper.
In this paper, an abnormal grating evolution was recorded during microfiber Bragg grating (mFBG) inscription under 193nm excimer laser. Within 20 minutes exposing, a Type IIa FBG could be obtained with above 20dB strength in 8.5 μm microfiber. This regenerated mFBG had good survival ability against high temperature up to 800 °C. Moreover, the strain response of the regenerated grating was enlarged by the microfiber structure. Thus, highly sensitive strain sensor with considerable temperature resistance could be obtained, which had potential applications in gas/oil and aerospace territory.
We demonstrate an all-solid photonic bandgap fiber modal interferometer by concatenating two tapers separated with a middle section of the fiber. Unlike the conventional devices, our structure has a lower effective index in the core and a higher effective index in the cladding, which produce novel sensing characteristics. The measured sensing sensitivities are ~63pm/°C for temperarure and ~1.74nm/N for the axial stress, respectively.
Microfiber Bragg gratings (mFBGs) can be used as cost-effective and relatively simple-to-implement biosensors for monitoring DNA interactions in situ. The sensors are functionalized by a monolayer of poly-L-lysine (PLL) with the specific molecular recognition probe DNA sequences to bind with high specificity to a given target. By recording the wavelength seperation between the two resonant peaks of a single mFBG, the mFBG biosensor is capable of detecting the presence of specific target DNA in situ.
We demonstrate an acid-based sensor from the biofuncationalized microfiber Bragg grating. By electrostatic selfassembly layer-by-layer technique, the film consisting of sodium alginate which has hygroscopic response to the potential of hydrogen is coated on the fiber surface. Consequently, the refractive index variation of the sensing film caused by water absorption can be measured by mFBG’s higher order mode peak which can be translated into pH value information. The sensitivity of the sensor is received as high as 265pm/pH.
We demonstrate an ultrasensitive temperature sensor by sealing a highly-birefringent microfiber into an alcoholinfiltrated copper capillary. With a Sagnac loop configuration, the interferometric spectrum is strongly dependent on the external refractive index (RI) with sensitivity of 36800nm/RIU around RI=1.356. As mainly derived from the ultrahigh RI sensitivity, the temperature response can reach as high as −14.72 nm/°C in the range of 30.9-36.9 °C. The measured response time is ~8s, as determined by the heat-conducting characteristic of the device and the diameter of the copper capillary. Our sensor is featured with low cost, easy fabrication and robustness.
We demonstrate a temperature-independent displacement sensor by inscribing a periodic grating in a microfiber taper with assistance of the 193-nm ultraviolet exposure technique. The obtained bandwidth is as large as 29.64nm for the grating with diameter of 3.8~6.38μm and length of 6.2mm, respectively. When the displacement is increased from 0 to 1.08mm, the reflecting bandwidth reduces to 3.38nm gradually, producing an average sensitivity of around −22.8nm/mm. The minimum displacement of measurement is ~4.39×10−4mm considering the wavelength resolution of 10pm in the optical spectrum analyzer. Moreover, the temperature-cross sensitivity is suppressed.
We demonstrated a novel method for temperature-independent refractive index measurement by use of a Bragg grating
fabricated in a highly birefringent rectangular microfiber. The two reflective peaks corresponding to two polarization
axes exhibit almost identical temperature sensitivity of 12.01 pm/°C and different responses to ambient refractive index
of 38.9 and 46nm/RIU at RI of 1.36, respectively. By monitoring the wavelength separation between the two peaks,
temperature-independent refractive index measurement can be achieved.
In this paper, temperature compensated microfiber Bragg grating (mFBG) is realized by use of a liquid with a negative
thermo-optic coefficient. The effects of grating elongation and the index change of silica glass are compensated by the
liquid through evanescent-field interaction. As a result, the reflective wavelength shifts by only 30 pm when the
temperature varies from 15 to 60°C. The proposed method is promising due to the compactness and high flexibility of
the device.
Orientation-recognized two-dimensional vibration sensor based on a polarization-controlled cladding-to-core recoupling is demonstrated experimentally. A compact structure in which a short section of multi-mode fiber stub containing a weakly tilted fiber Bragg grating (TFBG) is spliced to another single-mode fiber without any lateral offset. Several well defined lower-order cladding resonances in reflection show different polarization dependence due to the tilted grating vector excitation. Both orientation and amplitude of the vibration can be determined unambiguously via dual-path power detection of the orthogonal-polarimetric odd-cladding-modes. Meanwhile, the unwanted power fluctuations and temperature perturbations can be definitely removed via core mode monitoring.
The inscription of fiber Bragg gratings was demonstrated by 193 nm ArF excimer laser in microfibers drawn from the
standard single mode telecommunication fiber. Fiber Bragg gratings are directly inscribed in a series of microfibers with
diameter ranged from ~10μm to 3.3 μm without any treatment to photosensitize the microfibers such as hydrogen
loading, etc. Four reflection peaks are observed where three correspond to high order mode resonances. The resonance
wavelength depends on the fiber diameter and it sharply blueshifts when the diameter is decreased below 10 μm. The
gratings are characterized for their response to ambient refractive index. The higher order mode resonance exhibits
higher sensitivity to refractive index.
By analyzed the two-step cascaded step-changed LPGs which have been already used as band filters for flattening the
gain of EDFAs, an improved method of introducing three or more steps cascade has been advanced in order to optimize
the transmission spectra of the filters. The calculation and simulation of the cascaded LPGs' transmission spectra are
given. The simulation results show that the transmission spectral profile can be tailored into an adoptive shape and
simultaneously the effective bandwidth would become wider by adjusting the two parameters, the length and the index
perturbations of each segment in cascade. This method is demonstrated to be an effective way to flatten the gains profile
of EDFAs.
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