Piece Of Single-mode Fiber Research Articles

Large measurement range curvature sensor based on all-fiber Mach-Zehnder interferometer using triple-ring-core few-mode fiber.

This study experimentally demonstrates a large measurement range curvature sensor based on a Mach-Zehnder interferometer (MZI) in a triple-ring-core fiber (TRCF). The sensor is fabricated by fusion splicing a segment of TRCF between two pieces of single-mode fiber (SMF), forming the SMF-TRCF-SMF sandwich structure. Since the TRCF can support the propagation of a few guided modes, the fundamental mode interferes with high-order modes in the sensing part to produce a periodic interference spectrum. Bending causes the effective refractive index of the mode to change, which shifts the interference spectrum. The bending measurement is achieved by monitoring the wavelength variation of interference dips. The proposed sensors can realize curvature measurements up to 100 m-1, with a bending sensitivity of -225.9 pm/m-1. It is believed that the proposed MZI sensor may have potential applications in robot hand gesture recognition and control.

Strain and curvature optical fiber laser sensor based on an accurate RI-regulated FMF with high modal power balance

In this paper, a ring fiber laser sensor based on a three layered-core few modes fiber (TL-FMF) is proposed. The home-made TL-FMF is spliced with two pieces of single mode fiber (SMF) at both sides as the sensing and filtering component. In the TL-FMF, an intermediate with higher refractive index (RI) is sandwiched between a central dip and a sub-high RI layer. Due to the special RI, the TL-FMF has an equidistant modal effective RI difference and a balanced modal power of different modes, enabling a highly uniform interference spectrum. Then applying the TL-FMF in proposed ring fiber laser sensor, the sensing experimental results show that its sensitivities in strain and curvature are −2.78 pm/με and −6.17 nm/m−1, respectively, and shows an excellent characteristic of temperature insensitivity. This fiber laser sensor implements the dual-parameter measurement with anti-cross sensitivity, which offers a promising candidate in fiber fabrication monitoring, deformation measurement and bending detection.

Strain sensing with parallel air-cavity Fabry-Perot interferometers based on Vernier Effect

We proposed and investigated Vernier Effect (VE) based strain sensor, which comprises two parallel air cavity Fabry-Perot Interferometers (FPI). The sensor involves simple fabrication techniques, such as cleaving and fusion splicing. A small section of Double Side Hole Fiber (DSHF) is cleaved and spliced between two pieces of Single Mode Fiber (SMF). The core of the SMF-1 and the air core of the DSHF are aligned by a minor lateral offset to make an air cavity FPI. During the strain measurement, the sensor exhibits the strain sensitivity of 36.7 pm/με. The sensor improves the sensitivity with the highest amplification factor of 10. In addition, the proposed strain sensor shows the temperature sensitivity of only 1 pm/°C in the temperature range of 30 °C–100 °C, which has temperature cross-sensitivity of 2.72 × 10−2 με/°C, and appear to be insensitive to temperature. The proposed device has high strain sensitivity, low temperature cross-sensitivity, simple fabrication, and low cost. Therefore, it has many applications in different engineering fields, for example, structural health monitoring and civil engineering.

High Sensitivity Hollow-Core Fiber Strain Sensor Based on Signal Processing Assisted Vernier Effect

A fiber strain sensor with improved sensitivity is proposed and experimentally demonstrated, based on a single Fabry–Perot interferometer (FPI) using a signal processing assisted Vernier effect. The FPI structure is simply fabricated by splicing a segment of hollow-core fiber (HCF) between two pieces of single-mode fiber (SMF) with a commercial fusion splicer. Through data processing, we do not need to precisely manufacture two interferometers with very small detuning between their optical path differences (OPDs), as in the traditional schemes based on Vernier effect, since it is difficult to control the detuning strictly. The optical spectra of the reference interferometer (RIM) are replaced by an artificial reference spectrum (ARS), which is produced by spatial frequency down-conversion of the interference spectra of the sensing interferometer (SIM) in the initial state. Then, the optical spectrum envelope of Vernier-effect can be obtained by multiplying the SIM intensity with the constructed ARS intensity. When the length of HCF is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1201\mu \text{m}$ </tex-math></inline-formula> , the sensitivity can reach 315pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> with the modulation length of 5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> m. Compared with the sensitivity without signal processing, the sensitivity magnification is 188.6 times. The proposed scheme to realize sensitivity-enhancement using a single FPI based on the HCF, could simplify the sensor structure and device fabrication.

Ring core few-mode fiber sensor for curvature measurement.

An all-fiber Mach-Zehnder interferometer (MZI) using ring core few-mode fiber (RC-FMF) for curvature sensing is proposed and experimentally demonstrated. The MZI was fabricated by splicing a segment of RC-FMF between two pieces of single-mode fiber (SMF). With the benefit of a RC of the central axis of the RC-FMF, the sensor is more sensitive to curvature compared to other fiber sensors based on ordinary SMF or FMF. Curvature measurement can be achieved by monitoring the wavelength shift of interference dips. Experimental results have shown that the sensitivity of curvature sensing can reach up to -4.370nm/m-1, within the range of 1.199-1.549m-1. Also, the temperature sensing characteristics of the sensor are measured, and the maximum temperature sensitivity is 57.6 pm/°C, ranging from 25°C to 45°C. The proposed MZI sensor has excellent potential for curvature measurement of building structural health monitoring, bridge engineering, and more.

Two-Dimensional Tapered Optical Fiber Core for Whispering Gallery Mode Excitation

A two-dimensional tapered fiber core mimicking a commonly used tapered fiber coupler is proposed and experimentally demonstrated in a piece of single-mode fiber (SMF) for whispering gallery mode (WGM) excitation. Tapering the fiber core with femtosecond laser ablation can simultaneously enhance the evanescent field and decrease the effective refractive index of the core mode, which lowers the phase mismatch between the core mode and the WGM in a spherical cavity with a diameter less than the fiber cladding radius. Experimental results show that symmetric Lorentzian dip with a quality factor (Q-factor) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.6\times 10 ^{3}$ </tex-math></inline-formula> and asymmetric Fano-like lineshape can be achieved. Such a tapered fiber core provides an effective coupling method for a single WGM based photonic device or WGM array-based lab in-fiber due to its inherent compatibility with fiber optic hardware, high integration, and robustness.

Mid-Infrared Random Fiber Laser Assisted by the Passive Feedback

Random fiber lasers (RFLs), featuring a simple open cavity but with unique lasing characteristics, have captured considerable attention in the past several decades. Particularly, RFLs assisted by the passive feedback exhibit excellent robustness and compactness owing to the combination of active amplification and passive feedback from random distributed Rayleigh scattering. However, RFLs with the aid of the passive feedback reported previously are primarily operating in the near-infrared wavelength region of 1.1 μm or 1.5 μm based on the ytterbium or erbium doped active fibers. Here, we propose and demonstrate a random fiber laser configuration combining the passive feedback from a piece of single mode fiber with the active gain based on the thulium doped fiber, and extend the operation wavelength to the critically important mid-infrared wavelength band of ~2 μm. The spectral and temporal characteristics of the lasing output are evaluated and compared with those generated in a single pass amplification structure and a conventional resonant structure. Our results show that the passive feedback assisted RFL has a stabilized lasing wavelength and notably suppressed temporal dynamics. This work not only advances the development of RFLs, but also provides a promising strategy to achieve mid-infrared laser emission with a comparatively low relative intensity noise.

All-Fiber Liquid-Level Sensor Based on In-Line MSM Fiber Structure

We propose and demonstrate an all-fiber liquid-level sensor using an in-line multimode-single-mode-multimode (MSM) fiber structure. A piece of single-mode fiber (SMF) is spliced to two sections of equivalent multimode fiber (MMF) which are used as both mode splitter and mode coupler. The cladding mode will be excited when the light propagates from MMF to SMF, and then it will be combined with fundamental mode to form a Mach-Zehnder interferometer (MZI) when the light propagates from SMF to the other MMF. The liquid level is detected by the selected resonant dips shift of the transmission spectrum. A sensing sensitivity of 264.6 pm/mm is achieved for the proposed sensor with an SMF length of 26mm. Due to its compact structure, easy fabrication, and high sensitivity, the proposed liquid-level sensor is attractive for practical applications in a variety of fields, such as marine detection and chemical processing.

Underwater Pressure and Temperature Sensor Based on a Special Dual-Mode Optical Fiber

In this paper, an all fiber optic sensor based on a Mach-Zehnder interferometer (MZI) has been proposed for the simultaneous measurement of underwater pressure and temperature, utilizing a dual-mode fiber (DMF) which has been specially designed, and supporting only the LP 01 and LP 02 modes propagating in the fiber. In this design, an in-line MZI sensor that was constructed by splicing a DMF between two pieces of single mode fibers, shows a critical wavelength (CWL) which exists in the transmission spectrum of the LP 01 -LP 02 mode interference. Since the two peaks, located closest to the CWL (and from both lower and higher wavelengths), shift in opposite directions and show different sensitivities under temperature and water pressure variations, the DMF-MZI sensor is capable of measuring both the water pressure and the temperature simultaneously. The CWL-based interference spectrum is stable with the variation of underwater salinity or impurities seen around the fiber surface and independent of the polarization states of the transmission light. As a result, in the operation of the DMF-MZI sensor, underwater pressure and temperature sensitivities increase significantly, when the peak wavelengths are close to that of the CWL. A theoretical analysis has been developed and used to predict that the sensitivities of this specific DMF-MZI structure which can be further improved by increasing the physical length of the DMF and by adjusting the position of the first left/right peak to be closer to the critical wavelength. This co-located, multi-parameter all-fiber sensor developed in this way and showing relatively high sensitivity is easy to implement in the underwater environment. It does not require a complex shell design and the peaks nearest to a CWL are convenient, allowing easy identification and detection, thereby providing a large measurement range to satisfy the requirements of practical marine and fresh water measurements.

Distributed fiber-optic pressure sensor based on Bourdon tubes metered by optical frequency-domain reflectometry

We report a distributed fiber-optic pressure sensor based on Bourdon tubes using Rayleigh backscattering metered by optical frequency-domain reflectometry (OFDR). In the proposed sensor, a piece of single-mode fiber (SMF) is attached to the concave surfaces of Bourdon tubes using a thin layer of epoxy. The strain profiles along the concave surface of the Bourdon tube vary with applied pressure, and the strain variations are transferred to the attached SMF through the epoxy layer, resulting in spectral shifts in the local Rayleigh backscattering signals. By monitoring the local spectral shifts of the OFDR system, the pressure applied to the Bourdon tube can be determined. By cascading multiple Bourdon tubes and correspondingly attaching SMF sections (i.e., a series of SMF-modified Bourdon tubes), distributed pressure measurements can be realized. Three Bourdon tubes are employed to demonstrate the proposed spatially distributed sensing scheme. The experimental results showed that linear relationships between spectral shift and pressure were obtained in all three SMF-Bourdon tubes (i.e., at three spatial locations). It is expected that the proposed sensing device, the SMF-Bourdon tube, can be used in applications where distributed/multipoint pressure measurements are needed.

Studies on Temperature and Strain Sensitivities of a Few-Mode Critical Wavelength Fiber Optic Sensor

This paper studied the relationship between the temperature/strain wavelength sensitivity of a fiber optic in-line Mach–Zehnder interferometer (MZI) sensor and the wavelength separation of the measured wavelength to the critical wavelength (CWL) in a CWL-existed interference spectrum formed by interference between LP01 and LP02 modes. The in-line MZI fiber optic sensor has been constructed by splicing a section of specially designed few-mode fiber (FMF), which supports LP01 and LP02 modes propagating in the fiber, between two pieces of single-mode fiber. The propagation constant difference, $\Delta \beta $ , between the LP01 and LP02 modes, changes non-monotonously with wavelength and reaches a maximum at the CWL. As a result, in the sensor operation, peaks on the different sides of the CWL then shift in opposite directions, and the associated temperature/strain sensitivities increase significantly when the measured wavelength points become close to the CWL, from both sides of the CWL. A theoretical analysis carried out has predicted that with this specified FMF sensor approach, the temperature/strain wavelength sensitivities are governed by the wavelength difference between the measured wavelength and the CWL. This conclusion was seen to agree well with the experimental results obtained. Combining the wavelength shifts of the peaks and the CWL in the transmission spectrum of the SFS structure, this paper has shown that this approach forms the basis of effective designs of high sensitivity sensors for multi-parameter detection and offering a large measurement range to satisfy the requirements needed for better industrial measurements.

Miniature all-fiber axicon probe with extended Bessel focus for optical coherence tomography.

The trade-off between lateral resolution and depth of focus (DOF) severely limits the capability of endoscopic optical coherence tomography (OCT) for high-resolution deep-tissue imaging. To address this issue, we developed a novel miniature all-fiber axicon OCT probe by inserting a segment of gradient-index (GRIN) fiber between a piece of single-mode fiber (SMF) and an axicon polished from a no-core fiber. The GRIN lens served as a beam expander extending the probe DOF by 5.2 times while maintaining a high lateral resolution of 2 μm. The DOF extension was experimentally verified by measuring the axial profile of the probe output beam and further by imaging multilayered polymer tapes and onion samples. The designed probe with a tight focus over a large DOF holds great potential in endoscopic OCT imaging of deep tissues at the cellular level.

Highly-sensitive optical fiber temperature sensors based on PDMS/silica hybrid fiber structures

It is very important to accurately measure temperature in industrial production and scientific research. In this work, the high-sensitivity temperature sensors are proposed based on the modal interference principle. To increase the sensitivity, polydimethylsiloxane (PDMS) as the temperature sensitive material in filled into the hollow core fiber to fabricate the PDMS/silica hybrid fiber structures. The hybrid fiber structure is inserted between two pieces of single mode fiber as the sensing section. The environmental temperature can modify the phase delays of the modes in the hybrid fiber with different values. According to the principle of modal interference, the resonance wavelength of the transmission spectrum is shifted with temperature variation. The experimental results showed that the sensitivity of the sensor was −384 pm/°C in the range of 25 °C–80 °C when the core diameter of the hollow core fiber was 30 μm. The proposed temperature can be used to monitor the temperature in real time accurately in fields of biological detection, chemical and pharmaceutical industries.

Research on photonic generation of quadrupling triangular-shaped waveform using external modulation

Abstract We propose an improved approach to generate frequency-quadrupled triangular-shaped waveform signals based on two cascaded modulators. A dual-parallel Mach-Zehnder modulator (DP-MZM) is employed to provide the quadrupling RF modulation, after which two primary optical sidebands (±2nd) are generated in spectrum. Then a dual-drive Mach-Zehnder modulator (DD-MZM) is followed to perform the optical double sideband (ODSB) modulation and signal with four sidebands (±2nd and ±6th) is obtained. By adjusting modulation index carefully, the power ratio of ±2nd and ±6th sidebands can be tuned to 9.5 dB. A piece of single mode fiber (SMF) is applied to remove the undesired harmonic. The expression of optical intensity is approximately equal to the Fourier expansion of ideal triangular-shaped waveform. The principle is illustrated by theory, simulation, and experiment. Finally, triangular-shaped waveform signals with repetition rate of 8 GHz, 12 GHz, 16 GHz and 20 GHz are generated.

Common difference temperature compensation based fiber refractive index sensor through asymmetrical core-offset splicing

In this paper, an in-fiber Mach–Zehnder interferometer (MZI) is developed for refractive index (RI) sensing through core-offset splicing. In MZI, two ends of a section erbium-doped fiber are asymmetrically spliced with two pieces of single-mode fiber in order to get large contrast ratio of fringes. The comprehensive experimental measurements are then performed in terms of temperature and ambient RI. Owing to high sensitive consistency in temperature, a novel common difference compensation (CDC) method is introduced to achieve the self-elimination of cross-sensitivity. Thus the measurement error (∼1.2%) can be deleted by CDC and the corrected sensitivity (-74.22nm/RIU) is obtained without the crosstalk from temperature In addition, high working stabilities are demonstrated in the fabricated RI sensor in terms of wavelength (±0.02 nm) and intensity (±0.03 dB).

High sensitivity strain sensors based on single-mode-fiber core-offset Mach-Zehnder interferometers

Mach–Zehnder interferometers make highly sensitive sensors of physical quantities based on the accumulated phase difference between two or more optical paths. In this work, the application of single-mode-fiber core-offset Mach–Zehnder interferometers as strain sensors is reported. Three in-line modal Mach–Zehnder interferometers were manufactured by splicing three pieces of single mode fiber with well-defined offset and orientation of the cores and a well-defined length of its middle section. These interferometers were characterized as strain sensors, and a maximum sensitivity of 7.46 pm/µε was obtained over the linear range 0–1754 µε. Compared to other strain sensors, the ones reported in this work achieve a high sensitivity using inexpensive single mode fiber and an easy manufacturing process.

Ultrasensitive Magnetic Field Sensing Based on Refractive-Index-Matched Coupling.

An ultrasensitive magnetic field sensor is proposed and investigated experimentally. The no-core fiber is fusion-spliced between two pieces of single-mode fibers and then immersed in magnetic fluid with an appropriate value of refractive index. Under the refractive-index-matched coupling condition, the guided mode becomes leaky and a coupling wavelength dip in the transmission spectrum of the structure is observed. The coupling wavelength dip is extremely sensitive to the ambient environment. The excellent sensitivity to the refractive index is measured to be 116.681 μm/RIU (refractive index unit) in the refractive index range of 1.45691–1.45926. For the as-fabricated sensors, the highest magnetic field sensing sensitivities of 6.33 and 1.83 nm/mT are achieved at low and high fields, respectively. The sensitivity is considerably enhanced compared with those of previously designed, similar structures.

Fiber loop ring down for static ice pressure detection

Abstract We studied static ice pressure detection of hydraulic structures based on fiber loop ring down technology. Our detection system consists of a mode-locked laser, two 2 × 1 optical fiber couplers, a piece of single-mode fiber, the on-line micro-bend fiber sensor and a photo-detector. The static ice pressure was monitored based on measuring the change of the ring down time induced by external ice pressure on on-line micro-bend fiber sensor. We monitored the static ice pressure during ice growth and melting from −10 °C to 6 °C. The detection sensitivity is 0.00998/(μs·kPa). The proposed pressure sensing method has many advantages such as free from the fluctuation of the light power, simple structure, fast response and high sensitivity.

A Core-Offset Mach Zehnder Interferometer Based on A Non-Zero Dispersion-Shifted Fiber and Its Torsion Sensing Application

In this paper, an all-fiber Mach-Zehnder interferometer (MZI) based on a non-zero dispersion-shifted fiber (NZ-DSF) is presented. The MZI was implemented by core-offset fusion splicing one section of a NZ-DSF fiber between two pieces of single mode fibers (SMFs). Here, the NZ-DSF core and cladding were used as the arms of the MZI, while the core-offset sections acted as optical fiber couplers. Thus, a MZI interference spectrum with a fringe contrast (FC) of about 20 dB was observed. Moreover, its response spectrum was experimentally characterized to the torsion parameter and a sensitivity of 0.070 nm/° was achieved. Finally, these MZIs can be implemented in a compact size and low cost.

Ytterbium Fiber Laser Based on a Three Beam Optical Path Mach-Zehnder Interferometer

In this letter, a switchable ytterbium doped double cladding photonic crystal fiber (Yb-doped-DCPCF) laser based on a three optical path Mach-Zehnder interferometer (MZI) is presented. Here, the MZI with three-beam path was achieved by fusion splicing a segment of an Yb-doped-DCPCF between two pieces of single mode fibers. Moreover, in the proposed laser arrangement, the Yb-doped-DCPCF segment is acting simultaneously as the MZI and also as the gain medium. This laser can be switched to emit a single or double line by controlling the polarization state and it operates within the range from 1028 to 1033 nm. In addition, the laser emission has a linewidth of 0.07 nm and a single-mode suppression ratio of 40 dB. Finally, it is shown that the fiber laser arrangement is compact and robust and that requires a relativity simple fabrication procedure.