Thin-core Fiber Research Articles

Thin-Core Fiber Sandwiched Photonic Crystal Fiber Modal Interferometer for Temperature and Refractive Index Sensing

A photonic crystal fiber modal interferometer (PCFMI) with two segments of thin-core fiber functioning as a mode exciter and a re-coupler was proposed, analyzed, and experimentally demonstrated for temperature and refractive index (RI) sensing. Theoretical analysis and simulations were first conducted, to investigate the mode propagating behavior and the phase matching condition. Experimental results showed that RI sensitivity of 48.9 nm/RIU within the range of 1.34 ~ 1.40, and temperature sensitivity of 1.7 pm/°C can be obtained, which are comparable to those of the normal PCFMI structure. The liquid with RI of 1.40 was filled into the PCF to further improve the sensing performance. The RI and temperature sensitivities were enhanced to 283.9 nm/RIU and −354.1 pm/°C, respectively. Such good performance renders our proposed structure a promising platform for refractive index and temperature sensing applications, with the features of high sensitivity, compact size, easy fabrication, and low cost.

Label-Free Detection of DNA Hybridization Utilizing Dual S-Tapered Thin-Core Fiber Interferometer

A thin-core-fiber-based (TCF-based) dual S-tapered fiber sensor for label-free DNA hybridization detection has been proposed and experimentally demonstrated. The proposed biosensor is constructed by splicing a TCF of about 645 μ m in length with two segments of single-mode fibers, and the two S-tapers are fabricated at the splicing joints by using a commercial fusion splicer. By functionalizing the fiber surface with monolayer poly-L-lysine and single-stranded DNA (ssDNA) probes, label-free detection of target ssDNA is achieved. The sensor functionalization and the hybridization processes for 1 nM target ssDNA are monitored through real-time analysis of the transmission spectral evolution. Owing to its compactness, low cost, fast response, real-time detection, and good specificity and repeatability, the proposed biosensor is expected to find potential applications in label-free biomolecule detection.

Highly sensitive refractive index sensor based on novel Mach–Zehnder interferometer with multimode fiber–thin core fiber–multimode fiber structure

A highly sensitive refractive index (RI) sensors based on a fiber in-line Mach–Zehnder interferometer (MZI) with a multimode fiber–thin core fiber–multimode fiber (MMF–TCF–MMF) structure was proposed and demonstrated. This MZI has shown a good interference visibility as high as 16 dB in air. Experimentally, the sensor exhibited high RI sensitivity, with a max sensitivity of up to 453.99 nm/RIU in the RI range of 1.3330–1.3904. Furthermore, the dependences of RI sensitivity and TCF length were also analyzed. In addition, the proposed sensor has the advantages of simplicity of fabrication, low cost, compact size, and high sensitivity, which are beyond what conventional RI sensors can offer.

Simultaneous measurement of temperature and refractive index based on a spherical structure cascaded with thin-core fiber

A novel fiber-optic sensor for simultaneous measurement of temperature and refractive index (RI) has been proposed. The sensor consists of a structure of lead-in single-mode fiber (SMF)–spherical structure–middle section SMF–thin-core fiber–lead-out SMF. The spherical structure acts as optical coupler, and excites the cladding modes. The extinction ratio of the transmission spectra can reach up to 20 dB. The experimental results indicate that the temperature sensitivities at interference dip1, dip2 and dip3 are 0.037 nm °C−1, 0.046 nm °C−1 and 0.048 nm °C−1 within the temperature range from 25 °C to 70 °C, respectively, and the RI sensitivities at interference dip1 are −22.111 nm/refractive index unit (RIU) within the RI range from 1.3330 to 1.3496 and −72.584 nm/RIU within the RI range from 1.3496 to 1.3757. The RI sensitivities at interference dip2 and dip3 are 52.679 nm/RIU and −39.803 nm/RIU, respectively, within the RI range from 1.3330 to 1.3797, respectively. Given the large difference in RI between interference dip2 and dip3, simultaneous measurement of temperature and RI can be achieved by monitoring the wavelength shifts of interference dip2 and dip3. The proposed sensor has the advantages of easy fabrication, low cost, and good mechanical strength, and has potential applications in physical, biological, and chemical sensing.

Strain and temperature discrimination using a fiber Bragg grating with off-axis inscription

A fiber configuration for discriminating measurement of strain and temperature using a thin core fiber Bragg grating (FBG) with off-axis inscription is demonstrated. A group of cladding resonances located in short wavelength of Bragg resonance. For grasping its sensing property, a set of numerical simulation models is built by COMSOL soft and their coupling mechanism is discussed and surmised. Utilizing the different thermal and tension sensitivities from cladding resonance and Bragg resonance, the temperature and strain could be simultaneously measured with a matrix. The experimental results present that the cladding resonance have 0.983 ± 0.020 pm/με and 11.34 ± 0.24 pm/°C for strain and temperature, respectively. They are different with Bragg resonance. This approach in this paper exploits a novel grating inscription to discriminate temperature and strain.

Simultaneous measurement of refractive index and temperature based on down-taper and thin-core fiber

A Mach–Zehnder interferometer (MZI) based on the cascaded structure of a single-mode fiber (SMF) down-taper and thin-core fiber (TCF) is proposed and experimentally demonstrated. The down-taper acts as mode coupler to excite cladding modes. The extinction ratio of the transmission spectra can reach up to 20dB. By monitoring the wavelength shifts of interference dip1 and dip2, simultaneous measurement of refractive index (RI) and temperature can be achieved. The experimental results show that the wavelengths at interference dip1and dip2 have blue shifts with the increase of RI and red shifts with the increase of temperature. The RI sensitivities at interference dip1 and dip2 are -30.290 nm/RIU and -79.335nm/RIU, respectively. The temperature sensitivities at interference dip1 and dip2 are 0.065 nm/ºC and 0.053 nm/ºC, respectively. The proposed MZI exhibits the advantages of easy fabrication, high extinction ratio, low cost, and simultaneous measurement of RI and temperature, which will make a significant contribution to RI measurement.

Refractive index and temperature-sensing characteristics of a cladding-etched thin core fiber interferometer

A high refractive index (RI) sensor based on an in-line Mach–Zehnder mode interferometer (MZI) is proposed. The sensor was realized by splicing a 2-cm length of cladding-etched thin core fiber (TCF) between two single mode fibers (SMFs). The TCF-structured MZI exhibited good fringe visibility as high as 15 dB in air and the high RI sensitivity attained a value of 1143.89 nm/RIU at a RI of 1.447. The experimental data revealed that the MZI has high RI sensitivity after HF etching realizing 2599.66 nm/RIU. Studies were performed on the temperature characteristics of the device. It is anticipated that this high RI sensor will be deployed in new and diverse applications in the chemical and biological fields.

Intensity-demodulated torsion sensor based on thin-core polarization-maintaining fiber.

An intensity-demodulated torsion sensor is designed and realized, which consists of a polarization ring as the sensing part and a section of thin-core polarization-maintaining fiber as the demodulation part. An intensity map of a sinusoidal change can be obtained at some specific wavelengths, and the experimental results correspond to the theoretical analysis well. The maximum sensitivity is about 0.29dB/deg at the wavelength of 1584.6nm, and the minimum sensitivity is about 0.10dB/deg at the wavelength of 1510.2nm. Meanwhile, the temperature characteristic is measured in the experiment. More broadly, the proposed structure can be used in an integrated smart device for loose-screw detection in devices in aeronautics and astronautics.

Research on dual-parameter optical fiber sensor based on thin-core fiber and spherical structure

A novel dual-parameter optical fiber sensor is proposed and experimentally demonstrated. The proposed sensor is based on a fiber in-line Mach–Zehnder interferometer, which is fabricated by sandwiching a section of thin-core fiber between two spherical structures made of single-mode fibers. The transmission spectrum exhibits the response of the interference between the core and the different cladding modes. Due to the different wavelength shifts of the two selected dips, the simultaneous measurement of temperature and the surrounding refractive index can be achieved. The measured temperature sensitivities are 0.067 nm/°C and 0.050 nm/°C, and the refractive index sensitivities are −119.9 nm/RIU and −69.71 nm/RIU, respectively. In addition, the compact size, simple fabrication and cost-effectiveness of the fiber sensor are also advantages.

Reversible and Fast Responsive Optical Fiber Relative Humidity Sensor Based on Polyelectrolyte Self-Assembly Multilayer Film

A high-performance fiber-optic relative humidity (RH) sensor was prepared by layer-by-layer self-assembly of polycation, poly (4-vinylpyridinium chloride), and polyanion, sodium alginate, on the surface of a thin-core fiber modal interferometer. The process of layer-by-layer self-assembly was monitored by UV-vis spectroscopy and surface profilometer. The morphology of the fabricated multilayer film was characterized by atomic force microscopy and field emission scanning electron microscopy. A fast, reversible, and linear response to RH with a high resolution of 0.1 nm/1% RH has been experimentally demonstrated by the proposed fiber-optic sensor.

Highly Sensitive Strain Sensor Based on a Novel Mach-Zehnder Interferometer with TCF-PCF Structure.

A highly sensitive strain sensor based on a novel fiber in line Mach-Zehnder interferometer (MZI) was demonstrated experimentally. The MZI was realized by splicing a section of photonic crystal fiber (PCF) with the same length of thin core fiber (TCF) between two single mode fibers (SMFs). The fringe visibility of MZI can reach as high as 20 dB in air. In particular, the strain sensitivity of −1.95 pm/με was achieved within a range from 0 to 4000 με. Furthermore, the strain properties of different length of MZI was investigated. It was found that the sensitivity was weekly dependent on the length of MZI. The strain sensitivities corresponding to the MZI with 35 mm PCF, 40 mm PCF and 45 mm PCF at 1550 nm band were −1.78 pm/με, −1.73 pm/με and −1.63 pm/με, respectively. Additionally, the sensor has advantages of simple fabrication, compact size and high sensitivity as well as good fringe visibility.

Fiber ring laser based on SMF–TCF–SMF structure for strain and refractive index sensing

An erbium-doped fiber ring laser with embedded Mach–Zehnder interferometer (MZI) is constructed and experimentally demonstrated for strain and refractive index (RI) measurement. The MZI consists of a segment of thin-core fiber sandwiched between two single-mode fibers and acts as both the sensing component as well as a bandpass filter to select the lasing wavelength. The strain sensitivity of ∼-0.97 pm/μϵ and RI sensitivity of ∼44.88 nm/RIU are obtained in the range of 0 to 1750 μϵ and 1.3300 to 1.3537, respectively. The high-optical signal-to-noise ratio of >50 dB and narrow 3-dB bandwidth of <0.11 nm obtained indicate that the fiber ring laser sensor is promising for high-precision strain and RI measurement.

Long-period fiber grating cascaded to thin-core fiber for simultaneous measurement of liquid refractive-index and temperature

PurposeThe purpose of this paper is to demonstrate a simple fiber sensor for simultaneous measurement of liquid refractive-index (RI) and temperature.Design/methodology/approachThe sensor structure is formed by a long period fiber grating cascaded with a section of thin-core fiber. The long period fiber grating is fabricated on single mode fiber, followed by a section of 20-mm length thin-core fiber which is a modal interferometer.FindingsCladding mode interference between long period fiber grating and thin-core fiber modal interferometer is weak in the experimental investigation. Both of these two cladding mode type fiber devices are sensitive to surrounding RI and temperature. So the RI and temperature can be measured simultaneously by monitoring the spectral characteristics of the compound sensor. The sensitivity is calibrated and sensor matrix is provided in the experiment.Originality/valueThis proposed fiber sensor is simple, tough, cost-effective and suitable for discriminate the liquid RI and temperature with high sensitivity.

Thin-core fiber structures with overlays for sensing applications.

We investigate thin-core fiber-optic structures with film overlays that can be used for sensing applications. The structures are formed by a section of thin-core fibers (SM630 or SM450) spliced between standard SMF-28 fibers. The fibers are coated with overlays using the layer-by-layer assembly technology based on sequential alternating adsorption of polymer monolayers via electrostatic attraction. Transmission spectrum of the structures exhibits resonance dips caused by interaction between cladding modes. We measure the shifts of spectra with increasing thickness of the overlay and with pH value of the external medium. We calculate the shift of resonance wavelengths, which we compare with the experiment.

Off-axis ultraviolet-written thin-core fiber Bragg grating for directional bending measurements

A directional bending sensor based on thin-core fiber Bragg grating is proposed and demonstrated experimentally. It is inscribed by off-center technique and exposed by 193 nm ArF excimer laser through a phase mask. A series of cladding modes are excited and their intensities are enhanced to about 10 dB. The formation mechanism of those cladding modes is discussed and analyzed. The intensities of these cladding mode resonances is detected for bending and direction with maximum sensitivity 1.93 dB/m1 at 0° to −1.95 dB/m1 at 180°under the curvature varied from 0 m−1to 2.5 m−1. The sensitivity of surrounding temperature is 11.3pm/°C ranging from 25 °C to 60 °C. This all-fiber structure has a great advantage for fiber orientation identification sensor with more convenient manufacture and needless de-localize FBGs.

Carbon-nanotube / Polyvinyl alcohol coated thin core fiber sensor for humidity measurement

A Carbon-nanotube (CNT)/Polyvinyl alcohol (PVA) composite film-coated thin core fiber (TCF) optics sensor is proposed for monitoring relative humidity (RH). The unique Michelson type fiber interferometer sensor consists a short segment of TCF, the tip of which is melted into a convex shape and coated with a hydroscopic CNT/PVA film. The proposed sensor is linearly responsive to relative humidity (RH) beyond the humidity range 70%RH, with maximum sensitivity of −0.4573dB/%RH. The advantages of this sensor are its compact size, excellent stability and facile fabrication process.

High temperature high sensitivity Mach-Zehnder interferometer based on waist-enlarged fiber bitapers

An optical Mach-Zehnder interferometer (MZI) composed of a short section of thin-core fiber (TCF) which inserted into a standard single-mode fiber (SMF) with two waist-enlarged bitapers was proposed for high temperature measurement. MZIs with different lengths were fabricated, and the relationship between length and interference period was studied. The fast Fourier transform (FFT) method was used to determine the transmitted spectra of MZIs, and the results indicate that LP01 and LP02 modes are the main modes. The MZI has a high temperature sensitivity of 0.087nm/°C with a good linearity of 99.8% from 30 to 1000°C. Because it has advantages of compact size, easy fabrication, high sensitivity, high mechanical strength and wide sensing range, the MZI can be used for high sensitivity temperature measurement.

Novel Compact and Low-Cost Ultraweak Fabry–Perot Interferometer as a Highly Sensitive Refractive Index Sensor

A novel compact refractive index (RI) sensor based on an ultra-weak intrinsic fiber Fabry-Perot interferometer (FPI) is proposed and demonstrated, which is simply fabricated by splicing a tiny section of thin-core fiber to a single-mode fiber. Such an FPI exhibits an average RI sensitivity of 240dB/RIU over a wide RI range of 1.3326–1.4305, with a maximum sensitivity of 1110.7dB/RIU at the RI of 1.4305. In addition, the FPI can also achieve the simultaneous measurement of the RI and temperature.

Temperature Sensor Employed TCF-PMF Fiber Structure-Based Sagnac Interferometer

A cost-efficient all fiber temperature sensor based on a Sagnac interferometer is proposed and experimentally demonstrated. The sensor proposed comprises a section of thin core fiber (TCF) in combination with Panda type polarization maintaining fiber (PMF). The key point is that the core of TCF is intentionally aligned to one of two stress areas of Panda type PMF in order to change the mode transmission. Then the prepared TCF-PMF fiber structure is built into a Sagnac loop. A well-defined spectrum is obtained and the fringe contrast reaches up to 28 dB. The dip wavelengths are employed to demonstrate the temperature characteristics. The experimental results show the transmission spectrum blue-shift with the temperature and sensitivity of -1.54 nm/°C is obtained. The proposed sensor has potential application in temperature measurement.

Trace hydrogen sulfide gas sensor based on tungsten sulfide membrane-coated thin-core fiber modal interferometer

A novel fiber-optic hydrogen sulfide sensor based on a thin-core Mach-Zehnder fiber modal interferometer (TMZFI) is demonstrated and fabricated. This in-line interferometer is composed of a short section of thin-core fiber sandwiched between two standard single mode fibers, and the fast response to hydrogen sulfide is achieved via the construction of tungsten sulfide film on the outside surface of the TMZFI using the dip-coating and calcination technique. The fabricated sensing nanofilm is characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) spectrometer, Fourier transform infrared (FTIR) and spectroscopic analysis technology, etc. Experimental results showed that the WS2 sensing film has a hexagonal structure with a compact and porous morphology. The XPS and FTIR indicate that the existence of two elements (W and S) is demonstrated. With the increasing concentration of hydrogen sulfide, the interference spectra appear blue shift. In addition, a high sensitivity of 18.37 pm/ppm and a good linear relationship are obtained within a measurement range from 0 to 80ppm. In addition, there is an excellent selectivity for H2S, which has also been proved by the surface adsorption energy results of tungsten sulfide with four gases (H2S, N2, O2 and CO2) by using the density functional theory calculations. This interferometer has the advantages of simple structure, high sensitivity and easy manufacture, and could be used in the safety monitoring field of hydrogen sulfide gas.