Single-mode-no-core-single-mode Research Articles

Role of lateral Core-Offset in the sensitivity enhancement of single Mode-No core-Single mode fiber cascaded structure

In this study, an extensive examination of the influence of core-offset on different sensitivities of a single-mode-no-core-single-mode (SNS) fiber sensor probe in a variety of external disturbances, including refractive index changes, strain, and temperature, has been presented and thoroughly investigated. To achieve this, a consistent sensing length of 5 cm no-core fiber (NCF) was utilized between two single-mode fibers (SMFs) with different offset values, facilitated by the built-in options of a fusion splicer. The theoretical and experimental principles of sensitivity are discussed in detail. Key aspects such as spectral analysis and fast Fourier transform of the disclosed transmission spectrum underwent meticulous scrutiny. The outcome of the experiment indicates that the SNS fiber sensor exhibits an increasing sensitivity to refractive index (RI) over the 1.333–1.398 range when the offset value increases, with a maximum of 201.221 nm/RIU sensitivity. On the other hand, the RI sensitivity decreases with increasing offset value in the range of 1.402–1.441; the highest sensitivity attained is 656.641 nm/RIU. The strain sensitivity shows a decreasing trend with increasing offset, with the maximum sensitivity reaching −1.59 pm/µε in a wide range of 0 to 2000 µε. For temperature sensing, the sensitivity remains unaffected with a core offset; consistently, the achieved sensitivity is approximately 11.51 pm/°C over a broad range of temperature of 30 °C to 120 °C Compared with RI, it is found that the reliance of core-offset on strain sensitivity is very low whereas on temperature sensing almost insensitive, provided that the sensing length and diameter remain constant. These results provide insightful direction for the creation of extremely sensitive sensors for a variety of applications.

Simultaneous displacement, temperature and strain sensing system based on fiber macro-bend loss and edge filter

We proposed a multi-parameter fiber sensing system. The macro-bending loss of cascaded single-mode-no-core-single-mode (SNS) fiber was used for displacement sensing, and the combination of the SNS and fiber loop mirror (FLM) was constructed to improve the dynamic range of measurement. We designed the single-mode-dispersion-compensation-single-mode (SDS) fiber edge filter for fiber bragg grating (FBG) demodulation. The steep linear transmission region of the edge filter was used to improve the sensitivity of FBG for measurement of temperature and strain. The two measurement methods were applied the multi-parameter fiber sensing system to realize simultaneous measurement of displacement, temperature and strain. The system can overcome the crosstalk between displacement and temperature.

A Temperature-Insensitive Multipoint Displacement Sensing System Based on Fiber Macro-Bending Loss

We proposed a novel, temperature-insensitive, low-cost multipoint displacement sensing system. The system includes three parts: fiber displacement sensor, fiber loop mirror (FLM) and optical time domain reflectometer (OTDR). The fiber displacement sensor is designed by cascading single-mode-no-core-single-mode (SNS) fiber, the SNS is attached to FLM that is constructed by a 3 dB coupler to improve measurement dynamic range. The variation of displacement will significantly change the value of SNS macro-bending loss, which cause the variation of FLM reflected energy to achieve displacement measurement. Each FLM constitutes a displacement sensing unit, only 5% pulsed light energy is input for each sensing unit to increase the number of reusable sensors. The results show that each sensor can maintain a similar sensing effect, the measurement dynamic range can reach 15.613 dB. The peak energy of each FLM is almost unaffected by changes in external temperature. The proposed system is an ideal solution to engineering problems of displacement measurement, which can be widely used in the field of structural health monitoring.

The improvement of temperature sensitivity by controlling the thermal expansion coefficient of no-core fiber

In this paper, a liquid sealed temperature sensor is assembled based on single-mode-no-core-single-mode (SNCS) optical fiber structure. Theoretical calculations and experiments verify the sensing characteristics of this structure. The structure improves the temperature sensitivity to −355 pm/°C by encapsulating an organic solvent with large thermal optical coefficient and using packaging material with large thermal expansion coefficient. The theoretical results are in good agreement with the experimental results. Theoretical simulations are completed on MATLAB using far from the cut-off condition. The SNCS structure is encapsulated in a microfluidic chamber by applying microfluidic processing technology. The axial thermal expansion coefficient of the no-core fiber (NCF) is increased and radial thermal expansion coefficient is suppressed by the polydimethylsiloxane (PDMS) mold stretching NCF to improve the temperature sensitivity. The enhancement effect of PDMS stretching NCF on temperature sensitivity is verified by contrast experiment. Due to the wrapping of PDMS, the sensor has strong resistance to mechanical damage and external interference.

Numerical and experimental study on temperature measurement performance of SNS fiber optic sensor with liquid-sealed

Single-mode-no-core-single-mode (SNS) optical fiber structures have valuable potential to encapsulate as high-precision temperature sensors, due to their great sensitivity. This paper is to improve the temperature sensitivity of SNS fibers by studying the parameters of fiber length, diameter, and refractive index of the seal liquid. Transmission spectra of the fibers are given in both aspects of experiment and numerical simulation. We find that using a slim no-core fiber section sealed with liquid of high refractive index can effectively improve the sensitivity, while the choice of fiber length mainly affects the width of transmission peak. The results may contribute to the development of high-sensitivity optic temperature sensor.

Temperature sensor of single-mode-no-core-single-mode fiber structure coated with PDMS

In this article, a temperature sensor of single-mode-no-core-single-mode (SNCS) fiber structure coated with polydimethylsiloxane (PDMS) is proposed and investigated. Due to the large thermal expansion coefficient and negative thermal optical coefficient of PDMS, PDMS is used to increase the thermal expansion coefficient of no-core optical fiber (NCF) to improve the temperature sensitivity of the sensor. The temperature sensitivity of the sensor is −260 pm/℃. The simulation is completed on MATLAB. The experimental results and theoretical results are in good agreement. The advantages of simple manufacture, low cost, strong resistance to environmental impact and mechanical damage make it possible to be used in a variety of scenarios.

Sensing properties of graphene-oxide-functionalized single-mode–no-core–single-mode fiber structure

A series of single-mode–no-core–single-mode (SNS) optical fiber sensing structures with different thicknesses of graphene oxide (GO) coating were fabricated and tested with glycerin and magnetic fluid solutions. Owing to the two-dimensional network structure of GO, the GO coating improved the performance of the sensor for the structure immersed in glycerin solution. The highest refractive-index (RI) sensitivity (in RI units or RIU) of the GO-coated sensor is 496.823 nm/RIU. However, the GO coating reduced the magnetic field sensitivity of the sensor for the structure immersed in magnetic fluid solution. The mechanism of enhanced RI sensitivity and decreased magnetic field sensitivity is clarified.

Graphene Oxide Sensitized No-Core Fiber Step-Index Distribution Sucrose Sensor

By coating graphene oxide (GO) onto the surface no-core fiber (NCF), we designed a single-mode no-core single-mode (SNS) fiber Step-Index Distribution sucrose sensor. With wavelength demodulation and the beam propagation method (BPM), the sensor without a GO coating was studied in the low RI range of 1.33~1.389, and the high RI range of 1.389~1.4185. The experiments show that the RI sensitivity of the sensor respectively reaches 132.9 nm/RIU and 292.22 nm/RIU. Both the numerical simulation and the experiments are highly consistent with the theoretical analysis results. Especially, having coated GO on the NCF for sensitization, a high sensitivity was achieved for the response to sucrose concentration solutions. The sensor’s RI sensitivity was increased from 132.9 nm/RIU up to 1348.67 nm/RIU in the ultra-narrow range of 1.33 to 1.3385. This result provides a theoretical and experimental basis for the enrichment and development of sensor detection with a low threshold sucrose concentration.

Temperature Self-Compensation Biosensor Based on LPG Concatenated With SNCS Structure

A biosensor based on long period grating (LPG) concatenated with single-mode-no-core-single-mode (SNCS) fiber structure is designed and demonstrated both theoretically and experimentally. By analyzing the opposite shift of transmission dips formed by LPG and SNCS, the RI and temperature variations can be demodulated to realize the temperature self-compensation. The RI sensitivity of 90.56 nm/RIU is obtained by experiment at 1.333-1.399 with temperature compensation coefficient of -1.33× 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> /°C. The biosensing performance of the sensor is evaluated by detecting porcine hemoglobin (Hb) solutions with different concentrations and the sensitivity of 0.0165 nm/mg/mL is obtained. The designed sensor with simple structure, high sensitivity, noninvasive and label-free detection capability shows great potential in biosensing applications.

Nanostructured coating of graphene nanoparticles deposited onto a cladding etched no-core optical fiber for temperature measurement.

We report on a temperature sensor based on an optical fiber with nanostructured cladding fabricated by graphene nanoparticles (G-NPs) deposited onto a chemically etched no-core fiber (NCF). The basic structure of this sensor comprises a single-mode-no-core-single-mode (SNCS) fiber section concatenation. The sensing head is the NCF segment where the modal interference is highly sensitive to temperature variations. The influence of the NCF diameter on the sensitivity of the sensor has been investigated. The obtained results revealed that the sensor with an etched NCF diameter of 60 µm coated with G-NPs embedded in polyvinyl alcohol exhibits a maximum sensitivity of -0.104nm/∘C in the temperature range of 25-235°C, which is approximately six times higher than that of the basic structure without coating. To the best of our knowledge, this is the first demonstration of a temperature sensor based on an etched NCF coated with graphene-polyvinyl alcohol thin film.

Effect of graphene nanoparticle coating on the detection performance of cladding etched no-core fiber interferometer sensor for relative humidity measurement

Abstract We report on relative humidity (RH) sensing probe based on a cladding etched no-core fiber (NCF) multimode interferometer. The sensor structure was constructed by fusion splicing a section of NCF between two segments of single-mode fibers (SMF). Then a part of the NCF’s cladding was chemically etched with hydrofluoric acid (HF) solution until it reaches 50 μm in diameter. Thin-film of graphene-polyvinyl alcohol (G-PVA) was coated onto the etched zone of the bare NCF. The experimental results revealed that sensitivity enhanced more than 3 times compared to bare fiber at the proposed diameter. The strong interaction of G-PVA with the evanescent field of the etched NCF results in higher sensitivity. The maximum sensitivity of 0.412 nm/RH% with good stability at the RH range of 40%–100% was obtained. To the best of our knowledge, this is the first demonstration of an RH sensor based on an etched NCF coated with G-PVA film. This work presents a method to improve the sensitivity of a single-mode- no-core -single-mode (SNCS) fiber structure based RH sensor featuring a low- cost, compact size, and good reversibility.

基于单模光纤传输的单模-无心-单模光纤型表面等离子体共振传感器（特邀）

基于单模光纤传输的单模-无心-单模光纤型表面等离子体共振传感器（特邀）

All-fiber-optic vector magnetic field sensor based on side-polished fiber and magnetic fluid.

A kind of compact all-fiber-optic vector magnetic sensor is proposed and demonstrated. The sensor consists of a side-polished-fiber (SPF)-integrated with singlemode-no core-singlemode (SNS) fiber structure. A section of side-polished fiber breaks the axially symmetry of the composite structure. The as-fabricated sensor supports vector sensing and has a magnetic field strength sensitivity of up to -2370 pm/mT over 2-6 mT range. The physical mechanism is that the modal interference is strongly influenced by the refractive index (RI) near the side-polished surface. The advantages of the proposed sensor lie in low cost, simple structure and easy manufacture, which make it attractive in the field of magnetic field vector sensing.

SNS optical fiber sensor for direct detection of phase transitions in C18H38 n-alkane material

A single-mode-no-core-single-mode (SNS) fiber optical sensor for the detection of solid-liquid and liquid-solid phase changes in C18H38 n-alkane (n-octadecane) is proposed and demonstrated. The transmission-type sensor probe consists of a short section of no-core fiber sandwiched between two sections of a single-mode fiber. Phase changes in n-octadecane are accompanied by large step-like variations of its refractive index (RI). Such a large discontinuous change of the n-octadecane’s RI during its phase transition leads to the corresponding step-like change in the transmitted optical power that can reliably indicate the phase change of the sample in the vicinity of the sensor. The proposed sensor probe is simple, accurate and is capable of detecting the material’s phase based on a single measurement. The results of this work suggest that the proposed sensor is potentially capable of detecting liquid-solid phase changes in other materials whose thermo-optic properties are similar to those of n-octadecane.

Temperature self-compensative all-fiber magnetic field sensing structure based on no-core fiber cascaded with fiber Bragg gratings

A kind of fiber-optic magnetic field sensing structure based on no-core fiber (NCF) cascaded with fiber Bragg gratings (FBGs) is proposed. The NCF is sandwiched between two pieces of singlemode fibers to form singlemode-no-core-singlemode (SNS) interferometric fiber structure, which is cladded with the magnetic field sensitive material (i.e. magnetic fluid). Measurement is done by monitoring the differential intensity of the signal at the two Bragg peak wavelengths, which increases the magnetic field sensitivity and can decrease the cost of the pragmatic sensing system. Due to the similar thermal effect of NCF and FBG, the temperature cross-sensitivity is greatly reduced. The achieved magnetic field sensitivity is 0.1774 dB/mT at 0–6 mT and 1.9296 dB/mT at 6–18 mT. The temperature sensitivity is only 0.0623 dB/ °C in the range of 21–35 °C. The corresponding device does not necessitate intensity-stabilized broadband light source as well.

High sensitivity temperature sensor based on singlemode-no-core-singlemode fibre structure and alcohol

A high sensitivity temperature sensor based on a singlemode-no-core-singlemode (SNCS) fibre structure and surrounded with alcohol within a silica capillary is described. In this investigation, no-core fibre (NCF) is used as the multimode waveguide and alcohol is chosen as the temperature sensitive medium. By packaging the alcohol solution with a short length of NCF enclosed within a silica capillary, the surrounding temperature can be detected by monitoring the variations of transmission loss at a specific wavelength. The theoretical analysis predicts this temperature sensor can provide high sensitivity, and the experimental results support this. The maximum temperature sensitivity of the sample is 0.49 dB/°C with a potential temperature resolution of 0.02 °C at the operating wavelength of 1545.9 nm. In addition, the repeatability and response time of the sensor of this investigation are investigated experimentally.

A fiber-optic sensor based on no-core fiber and Faraday rotator mirror structure

An optical fiber sensor based on the single-mode/no-core/single-mode (SNS) core-offset technology along with a Faraday rotator mirror structure has been proposed and experimentally demonstrated. A transverse optical field distribution of self-imaging has been simulated and experimental parameters have been selected under theoretical guidance. Results of the experiments demonstrate that the temperature sensitivity of the sensor is 0.0551 nm/°C for temperatures between 25 and 80 °C, and the correlation coefficient is 0.99582. The concentration sensitivity of the device for sucrose and glucose solutions was found to be as high as 12.5416 and 6.02248 nm/(g/ml), respectively. Curves demonstrating a linear fit between wavelength shift and solution concentration for three different heavy metal solutions have also been derived on the basis of experimental results. The proposed fiber-optic sensor design provides valuable guidance for the measurement of concentration and temperature.

Dual-point reflective refractometer based on parallel no-core fiber/FBG structure

Abstract A novel dual-point reflective fiber-optic refractometer based on multimode interference (MMI) effect and fiber Bragg grating (FBG) reflection is proposed and experimentally demonstrated, which adopts parallel structure. Each point of the refractometer consists of a single mode-no core-single mode fiber (SNS) structure cascaded with a FBG. Assisted by the reflection of FBG, refractive index (RI) measurement can be achieved by monitoring the peak power variation of the reflected FBG spectrum. By selecting different length of the no core fiber and center wavelength of the FBG, independent dual-point refractometer is easily realized. Experiment results show that the refractometer has a nonlinear relationship between the surrounding refractive index (SRI) and the peak power of the reflected FBG spectrum in the RI range of 1.3330–1.4086. Linear relationship can be approximately obtained by dividing the measuring range into 1.3330–1.3611 and 1.3764–1.4086. In the RI range of 1.3764–1.4086, the two sensing points have higher RI sensitivities of 319.34 dB/RIU and 211.84 dB/RIU, respectively.

Humidity sensor based on intracavity sensing of fiber ring laser

A humidity sensor based on the intracavity sensing of a fiber ring laser is proposed and experimentally demonstrated. In the fiber ring laser, a humidity-sensitive fiber-optic multimode interferometer (MMI), fabricated by the single-mode-no-core-single-mode (SNCS) fiber coated with Agarose, works as the wavelength-selective filter for intracavity wavelength-modulated humidity sensing. The experiment shows that the lasing wavelength of the fiber laser has a good linear response to ambient humidity from 35%RH to 95%RH. The humidity sensitivity of pm/%RH is obtained with a narrow 3 dB bandwidth less than 0.09 nm and a high signal-to-noise ratio (SNR) ∼60 dB. The time response of the sensor has been measured to be as fast as 93 ms. The proposed sensor possesses a good stability and low temperature cross-sensitivity.

Fiber laser refractometer based on tunable bandpass filter tailored FBG reflection

A fiber laser refractometer based on single-mode-no-core-single-mode (SNS) structure cascaded with a FBG is proposed and experimentally demonstrated. The output wavelength of the fiber laser keeps constant because the oscillating wavelength is only determined by the central wavelength of the FBG which is insensitive to the surrounding refractive index (SRI). However, the output power is sensitive to the SRI because the intracavity loss of the fiber laser varies with the SRI. A cost-effective power detection refractometer with reflective operation can be realized through measuring the variation of the fiber laser’s output power. The refractometer has a sensitivity of 195.52dB/RIU and 365.52dB/RIU in the RI range of 1.3330–1.3687 and 1.3687–1.4135, respectively. Moreover, the refractometer can also be used for temperature measurement through discriminating the output wavelength of the fiber laser.