Lead-in Single Mode Fiber Research Articles

Miniature Optical Fiber Fabry-Perot Interferometer Based on a Single-Crystal Metal-Organic Framework for the Detection and Quantification of Benzene and Ethanol at Low Concentrations in Nitrogen Gas.

This study reports for the first time, to the best of our knowledge, a real-time detection of ultralow-concentration chemical gases using fiber-optic technology, combining a miniaturized Fabry-Perot interferometer (FPI) with metal-organic frameworks (MOFs). The sensor consists of a short and thick-walled silica capillary segment spliced to a lead-in single-mode fiber (SMF), housing a tiny single crystal of HKUST-1 MOF, imparting chemoselectivity features. Ethanol and benzene gases were tested, resulting in a shift in the FPI interference signal. The sensor demonstrated high sensitivity, detecting ethanol gas concentrations (EGCs) with a sensitivity of 0.428 nm/ppm between 24.9 and 40.11 ppm and benzene gas concentrations (BGCs) with a sensitivity of 0.15 nm/ppm between 99 and 124 ppm. The selectivity study involved a combination of three ultralow concentrations of ethanol, benzene, and toluene gases, revealing an enhancement factor of 436% for benzene and 140% for toluene, attributed to the improved miscibility of these conjugated ring molecules with the alkane chains of the ethanol-modified HKUST-1. Experimental tests confirmed the sensor's viability, demonstrating significantly improved response time and spectral characteristics through crystal polishing, indicating its potential for quantifying and detecting chemical gases at ultralow concentrations. This technology may prevent energy resource losses, and the sensor's small size and robust construction make it applicable in confined and hazardous locations.

Vernier Effect-Based Displacement Sensor Using In-Fiber Hollow Microsphere With a Hole

We demonstrate a compact subnanometer resolution displacement senor based on Vernier effect, which consists of a section of capillary fusion spliced with a single-mode fiber (SMF) and an in-fiber hollow microsphere with a hole (HMH). Two concave erbium-doped fibers (EDFs) obtained by chemical etching are fusion spliced for the hollow microsphere, which further has a hole by vertical polishing and chemical etching. The Newton’s rings phenomenon observed on the endface of EDF is the judgment condition of the hollow microsphere’s ultrathin wall via polishing, and then of the beginning of etching. Consequently, there is no concave area around the hole on the EDF endface, which significantly enhances the endface reflectivity. Combined with the lead-in SMF endface and the inner surface of the HMH, three-beam interference is generated from beams reflected from the three mirrors, which can produce two kinds of Vernier effect. They are analyzed theoretically and thus distinguishable in spectral patterns. Due to the low loss of light transmission in the hollow microsphere, the wider reflection spectra of identifiable interference pattern with Vernier effect are obtained, and the maximum displacement sensitivity can be further improved to 451.5 pm/nm with a subnanometer resolution of 0.044 nm. Plus the high robustness resulting from the in-fiber structure of the hollow microsphere, the displacement sensor shows prospects in health monitoring, surface topography, precision machining and other fields.

Ultrasensitive refractive index fiber sensor based on high-order harmonic Vernier effect and a cascaded FPI.

This paper proposes and demonstrates an ultrasensitive refractive index (RI) sensor based on harmonic Vernier effect (HEV) and a cascaded Fabry-Perot interferometer (FPI). The sensor is fabricated by sandwiching a hollow-core fiber (HCF) segment between a lead-in single-mode fiber (SMF) pigtail and a reflection SMF segment with an offset of 37 µm between two fiber centers to form a cascaded FPI structure, where the HCF is the sensing FPI, and the reflection SMF is the reference FPI. To excite the HEV, the optical path of the reference FPI must be multiple times (>1) that of the sensing FPI. Several sensors have been made to conduct RI measurements of gas and liquid. The sensor's ultrahigh RI sensitivity of up to ∼378000 nm/RIU can be achieved by reducing the detuning ratio of the optical path and increasing the harmonic order. This paper also proved that the proposed sensor with a harmonic order of up to 12 can increase the fabricated tolerances while achieving high sensitivity. The large fabrication tolerances greatly increase the manufacturing repeatability, reduce production costs, and make it easier to achieve high sensitivity. In addition, the proposed RI sensor has advantages of ultrahigh sensitivity, compactness, low production cost (large fabrication tolerances), and capability to detect gas and liquid samples. This sensor has promising potentials for biochemical sensing, gas or liquid concentration sensing, and environmental monitoring.

Highly sensitive gas pressure sensor based on the hollow core Bragg fiber and harmonic Vernier effect.

A highly sensitive inline gas pressure sensor based on the hollow core Bragg fiber (HCBF) and harmonic Vernier effect (VE) is proposed and experimentally demonstrated. By sandwiching a segment of HCBF between the lead-in single-mode fiber (SMF) and the hollow core fiber (HCF), a cascaded Fabry-Perot interferometer is produced. The lengths of the HCBF and HCF are precisely optimized and controlled to generate the VE, achieving a high sensitivity of the sensor. Meanwhile, a digital signal processing (DSP) algorithm is proposed to research the mechanism of the VE envelope, thus providing an effective way to improve the sensor's dynamic range based on calibrating the order of the dip. Theoretical simulations are investigated and matched well with the experimental results. The proposed sensor exhibits a maximum gas pressure sensitivity of 150.02 nm/MPa with a low temperature cross talk of 0.00235 MPa/ ∘C. All these advantages highlight the sensor's enormous potential for gas pressure monitoring under various extreme conditions.

Fabrication and Sensing Characteristics of Long-Period Fiber Grating in Capillary Fiber

We propose a novel fiber device based on arc-induced long-period fiber gratings (LPFGs) in a hollow core silica capillary. This device is made up of a lead-in single-mode fiber (SMF), LPFGs in capillary (CLPFGs), and a lead-out SMF, with a tapered air cavity (TAC) connecting SMF and capillary. The transmissive characteristics of capillary with and without LPFGs are analyzed theoretically and experimentally. Mode coupling of CLPFGs at longer wavelength could be enhanced, as well as the sensing performance increases times over the capillary without gratings. The refractive index (RI) sensitivity can reach to be 15 713.5 nm/RIU in the RI range from 1.451 to 1.456. Moreover, with the asymmetric index distribution induced in the capillary, the CLPFGs present vector bending properties that the bending sensitivities are 79.4 and −8.8 nm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-{1}}$ </tex-math></inline-formula> , for bending directions at 0° and 180°, respectively. Furthermore, the proposed device shows negligible change to the external temperature variation, which could solve the cross-sensitivity problem. With the advantages of compactness, low cost, and ease of fabrication, the proposed device is potential in high-sensitivity RI sensing fields and vector bending measurement.

Temperature-immune Fabry-Perot cavity sensor based on an opened hollow-core anti-resonant fiber.

A new approach to conquer the thermal phase drift of an optical fiber Fabry-Perot interferometer (FPI) sensor is proposed and experimentally demonstrated. By employing a hollow-core anti-resonant fiber (HC-ARF) and optimizing the fusion splicing (includes mode field adaptation) between the lead-in single-mode fiber (SMF) and the HC-ARF, a high spectral resolution (λ/Δλ ≈ 3.8 × 104) optical fiber air-cavity FPI sensor with a fringe visibility higher than 7 dB is constructed. To eliminate the thermal phase drift (i.e. temperature crosstalk) of the sensor that originates from the intrinsic thermal expansion effect of the silica material of the HC-ARF, the FPI air cavity is connected to the external environments, by which the effect of air expelling from the cavity with temperature increasing can well compensate the temperature-induced cavity elongation. As a result, the thermal phase drift of the FPI is reduced to zero at a temperature range of ∼ 80-110 °C and within the temperature range of 40-80 °C, the thermal phase drift is still halved compared with the sealed FPI cavity. The nearly zero thermal phase drift of a FPI at such a temperature range has never been achieved before, to our best knowledge. As a proof of concept, a temperature-immune fiber-optic strain sensor is demonstrated. This work offers a new and efficient approach to eliminate the thermal phase drift (i.e. temperature crosstalk) of a fiber-optic device, which may significantly improve the measurement accuracy and detection limit of fiber-optic FPI sensors. Furthermore, the principle and schema can be generalized to a wide variety of fiber-optic devices.

High-sensitivity salinity sensor based on etched C-type micro-structured fiber sensing structure

A salinity sensor based on the principle of Mach-Zehnder interferometer (MZI) is presented and verified in this letter, which has certain advantages in the sensitivity and cost of the salinity sensor. This structure consists of a lead-in single-mode fiber (SMF), a small section of C-type micro-structured optical fiber (CMOF), a multi-mode fiber (MMF), and a lead-out single-mode fiber (SMF), which can be referred to as an SCMS structure for short. According to the experiment results, the salinity sensitivity of the sensing probe can reach − 3.25 nm/‰ with linearity of 0.999 when the CMOF is about 500 µm of the length. In addition, its refractive index (RI) sensitivity can be more than 10000 nm/RIU. Moreover, the sensing probe has a relatively good detection limit, namely 0.1‰. At the same time, it also provides a low-cost fabrication way to realize the direct contact with and exchange of the fluid being measured, and the sensor proposed has the prospect of monitoring ocean salinity.

Research on temperature sensing characteristics of fiber side-open cavity structure

A sensing probe based on fiber side-open cavity structure is proposed for the measurement of seawater temperature, which is mainly composed of a lead-in single mode fiber (SMF), a side-open cavity C-type microstructure fiber (SOCTF), a multimode fiber (MMF), a lead-out SMF, and a layer of polydimethylsiloxane (PDMS). It could effectively reduce the transmission loss, and promote the temperature sensitivity, with a micron-level sensing area. The relative simulations and experiments were carried out to verify the feasibility. The sensitivity could be up to 12.32 nm/°C with a temperature resolution of 0.0016 °C. Considering comprehensively, the sensing structure proposed in this paper could provide an effective measurement method for seawater temperature measurement.

High-sensitive fiber-optic pressure sensor based on Fabry-Perot interferometer filled with ultraviolet glue film and Vernier effect

A high sensitivity fiber-optic pressure sensor based on Fabry-Perot interferometer filled with ultraviolet (UV) glue film and Vernier effect was proposed and demonstrated. The sensor is composed of a sensing Fabry-Perot (FP) cavity and a reference FP cavity in parallel. The sensing cavity is fabricated by splicing a section of silica tube (ST) to the lead-in single-mode fiber (SMF), and filling a thin layer of UV glue at the end-face of the ST. The reference cavity is made by sandwiching a section of ST between two SMF. The parallel cavities generate the Vernier effect due to a similar free spectrum range, which significantly improves the sensitivity of the sensor. The proposed sensor with good repeatability and reliability was used to monitor the change of air pressure, and provided a high sensitivity of −38.3 nm/MPa. The sensor exhibited the potential applications in the fields of aerospace industry, biological diagnostics and environmental monitoring.

All-Fiber Fabry-Perot Interferometer Gas Refractive Index Sensor Based on Hole-Assisted One-Core Fiber and Vernier Effect

A novel high sensitivity all-fiber Fabry-Perot interferometer (FPI) gas refractive index (RI) sensor based on hole-assisted one-core fiber (HAOCF) and Vernier effect was proposed and demonstrated. The sensor is fabricated by splicing a section of HAOCF to a section of silica tube (ST) fused with the lead-in single-mode fiber (SMF), ensuring that the sensor is composed of an air-cavity FPI and a silica-cavity FPI. The cascaded dual-cavities generate the Vernier effect due to a similar free spectrum range (FSR), which significantly improve the sensitivity of the sensor. Owing to the air hole in the cladding of the HAOCF, gases with different RI can enter or leave the in-fiber air cavity, which makes the device usable as a gas RI sensor. The sensor was used to monitor the RI changes of air with different pressures and provided a high sensitivity of -9462.4 nm/RIU. The proposed sensor has a potential to find further applications in the fields of gas composition detection, medical diagnostics and environmental monitoring.

Determination of temperature and strain by a compact optical fiber Mach-Zehnder interferometer (MZI) composed of a single-mode fiber (SMF), seven core fiber (SCF), and multimode fiber (MMF) with a fiber Bragg grating (FBG)

A compact optical fiber Mach-Zehnder interferometer (MZI) sensor is reported for temperature and strain. The MZI sensor consists of a single-mode fiber (SMF), a seven core fiber (SCF) and a multimode fiber (MMF) with a fiber Bragg grating (FBG). The intensity distribution of the sensor is calculated by the beam propagation method. The light is coupled from the lead-in SMF into the SCF through the MMF. The highest transmission efficiency was obtained when the length of MMF is 2 mm with a diameter of 105 μm. The temperature sensitivity is up to 55.06 pm/°C from 130 °C to 250 °C, while the maximum extinction ratio was 27.4 dB. The measurements were divided into two intervals to address cross coupling to provide sensitive temperature and strain measurements.

A novel temperature insensitive refractive index sensor based on dual photonic crystal fiber

A novel dual photonic crystal fiber (DPCF) temperature insensitive refractive index sensor is proposed in this article. By sequentially splicing lead-in SMF (single mode fiber), PCF_1, PCF_2, and lead-out SMF constitute the Mach-Zehnder interferometer (MZI). The refractive index sensitivity (RIS) of the proposed sensor is improved by making an abrupt-taper on the fusion point of two PCFs. The proposed sensor is compared with a sensor without an abrupt-taper, the RIS value of the proposed sensor is increased by ∼1.9 times. The collapsed area of two photonic crystal fiber air holes is used as a beam splitter and combiner of Mach-Zehnder interferometer. Experimental analysis indicates that the RIS is 152.97 nm/RIU (1.330–1.383 RIU). Besides, the proposed sensor is independent of temperature and its sensitivity towards temperature is only -0.0045 nm/°C (30−80 °C). This sensor has the characteristics of relatively high sensitivity, ease of fabrication, and compact size.

Intensity-modulated bend sensor by using a twin core fiber: theoretical and experimental studies.

A new optical fiber bend sensor is proposed and demonstrated based on a sandwich structure created by splicing a segment of twin core fiber (TCF) between two segments of single mode fibers (SMFs). One core of the TCF is aligned with the cores of two segments of SMFs. An incident beam is directed into the TCF by the lead-in SMF. Light couples back and forth between two cores. The bend sensing performance of the sensor is investigated by intensity-modulated method. The intensity of the operation wavelength is modulated by the change of refractive index and geometrical deformation in the bent TCF. Experimental results show such a bend sensor achieves sensitivity of +0.671 /m-1 and resolution of 0.003 m-1 in the range of 0 to 1.25 m-1. In addition, the sensitivity and bend measurement range can be flexibly adjusted through selection of the length of TCF and sensing configuration. As such, the proposed sensor can be further developed for large or small bend ranges measurement.

Fiber-interface directional coupler inscribed by femtosecond laser for refractive index measurements.

A novel fiber-interface directional waveguide coupler was inscribed on the surface of a coreless fiber by femtosecond laser, and was successfully applied to highly sensitive refractive index (RI) measurements. The primary arm was first inscribed to couple light from a lead-in single mode fiber to the fiber interface, then back to a lead-out single mode fiber. A side arm was inscribed parallel and in close proximity to the primary arm. Light propagating in the primary arm could then be efficiently coupled into the side arm when a phase-matching condition was met, which produced a dramatic spectral dip at the coupling wavelength. The proposed device achieved a sensitivity as high as ∼8249 nm/RIU over an RI range of 1.44-1.45, due to strong evanescent fields excited in fiber-interface waveguides. The proposed in-fiber directional coupler exhibits high mechanical strength, a compact configuration, and excellent RI sensitivity. As such, it has significant potential for practical applications in biochemical sensing.

High-temperature fiber sensor based on two paralleled fiber-optic Fabry–Perot interferometers with ultrahigh sensitivity

A high-temperature fiber sensor based on two paralleled fiber-optic Fabry–Perot interferometers (FPIs) with ultrahigh sensitivity is proposed and experimentally demonstrated. Unlike the structures of the traditional Vernier effect composed of the cascaded components, the proposed fiber sensor is made up of two paralleled FPIs for high-temperature sensing with advantages of simple fabrication, high sensitivity, and low noise. One FPI for sensing is obtained by fusing a short section of polarization-maintaining photonic crystal fiber into the lead-in single-mode fiber (SMF). The other for reference is obtained by fusing a short section of hollow core silica tube between two SMFs. The two FPIs have similar free spectral range, with the spectral envelope of the paralleled sensor shifting much more than the single-sensing FPI. Experimental results indicate that the proposed sensor possesses considerable temperature sensitivities of −45 and −92 pm / ° C, respectively, in the measurements of 100°C to 300°C and 300°C to 800°C.

High temperature Vernier probe utilizing photonic crystal fiber-based Fabry-Perot interferometers.

This study proposes a highly sensitive and stable optical fiber probe based on Vernier effect for high temperature measurement (up to 1000 °C), utilizing photonic crystal fiber (PCF)-based Fabry-Perot interferometers (FPIs). The cascaded FPIs are fabricated by fusion splicing a section of polarization maintaining PCF to a lead-in single-mode fiber, and then a section of temperature-insensitive hollow core PCF is spliced between the PMPCF and a multi-mode fiber. The shift of the spectral envelope is monitored to measure the temperature variation. Experimental results show that the sensitivities of three fabricated probes are as high as 173.43 pm/ °C, 230.53 pm/ °C and 535.16 pm/ °C when operating from room temperature to 1000 °C, which are consistent with theoretical results. The sensitivities are magnified about 13, 19 and 45 times compared with the single FPI. The linearity of the temperature response is as high as 99.73%. The proposed probe has great application prospects due to compactness, high sensitivity and low cost.

Hollow-Core Fiber-Based All-Fiber FPI Sensor for Simultaneous Measurement of Air Pressure and Temperature

In paper, an all-fiber Fabry-Perot interferometer (FPI) sensor with cascaded micro-cavities is presented to measure air pressure and temperature simultaneously. The proposed sensor mainly consists of two tiny segments of hollow-core fiber (HCF) located at the end of lead-in single mode fiber (SMF), and there is a misalignment fusion splicing between the two HCFs with different core diameter. The sensor has minor length of about $210~\mu \text{m}$ since the two HCFs lengths are about $80~\mu \text{m}$ and $130~\mu \text{m}$ , respectively. Because the reflection spectrum of the sensor is formed by the fiber cavity, air cavity and their combined cavity, which can be processed by methods of fast Fourier transform (FFT) and Fourier band-pass filter (FBPF), so we can analyze the corresponding responses of every cavity from the reflection spectrum of sensor for temperature and pressure variations. The experiment results show that the sensor has not only two different linear temperature sensitivities but also two different linear pressure sensitivities, which can be used to distinguish air pressure and temperature simultaneously. The validity experiment shows the relative errors of 1.0% and 1.4% are obtained in simultaneous measurements of air pressure and temperature. The advantages of the presented sensor than other structure sensors are miniaturization, easier to manufacture and with lower fabrication cost, which is promising used in the harsh environment monitoring.

Sensitivity-enhanced humidity sensor based on helix structure-assisted Mach-Zehnder interference.

A relative humidity (RH) sensor based on helix structure-assisted Mach-Zehnder interferometer (HSA-MZI) with graphene oxide (GO) coated was proposed and experimentally demonstrated in this research. The sensor was composed of a lead-in single mode fiber (SMF), a lead-in multimode fiber (MMF), a triangular four core fiber (TFCF) with a twisted structure, a lead-out MMF, and a lead-out SMF. The twisted structure was prepared by twisting under continuous CO2 laser heating. The TFCF was coated with graphene oxide (GO) via natural evaporation. The torsional region in the TFCF provided an effective means for exciting the higher order cladding modes, and strong evanescent fields were introduced from the higher order cladding modes to improve their sensitivity to the changes of the refractive index (RI) in the surrounding environment. The effective RI of the GO film changes was caused by the GO absorption of water molecules, so the intensity of the transmission signal could be adjusted accordingly. The proposed RH sensor had a high RH sensitivity of -0.885 dB/%RH in the RH ranges of 70% and 80%. Additionally, the respiratory rise and fall time were 0.42 s and 6.54 s, respectively.

High-sensitivity hydraulic pressure sensor based on Fabry-Perot interferometer filled with polydimethylsiloxane film.

A high-sensitivity hydraulic pressure sensor is proposed, which consists of a Fabry-Perot interferometer (FPI) filled with a polymer film of polydimethylsiloxane (PDMS). The FPI structure is fabricated by splicing a section of hollow core fiber (HCF) to the end-face of a lead-in single mode fiber (SMF). Then, the PDMS is filled into the HCF which acts as a light reflector and a diaphragm to detect external pressure variation. As a result, the length of the FPI cavity and the thickness of the PDMS are 137.8 µm and 33.8 µm, respectively. Experimental results indicate that the sensor's wavelength exhibits a linear response to the hydraulic pressure, which function is described as y = -7.35 × 10-3x + 1536.395. Here, x and y represent the hydraulic pressure and the wavelength, respectively. The pressure sensitivity is up to -7.35 nm/kPa. Besides, a temperature compensation method based on a fiber Bragg grating is proposed to eliminate the influence of temperature. Experiments show that the scheme can effectively eliminate the influence of temperature and achieve accurate measurement of hydraulic pressure.

High-sensitive all-fiber fabry-perot interferometer gas refractive index sensor based on lateral offset splicing and Vernier effect

A high sensitivity all-fiber open-cavity Fabry-Perot interferometer gas refractive index (RI) sensor based on lateral offset splicing and Vernier effect is proposed and demonstrated. The sensor is fabricated by splicing a short section of single mode fiber (SMF) between the lead-in SMF and a section of SMF with a specific length, ensuring that the sensor consists of an open-cavity FPI and a silica-cavity FPI. The reflection beam from the last end face of the sensor is used to generate the Vernier effect, which significantly improves the sensitivity of the sensor. The sensor was successfully used to monitor changes of air RI as a function of pressure and provided a high sensitivity of -11368.5 nm/RIU with good linearity by tracing the shift of the reflection spectrum envelope caused by Vernier effect. This sensor has potential applications in fields such as gas composition detection, environmental monitoring, and biological sensing.