Single-mode-multimode-single-mode Research Articles

Optical Fibers Characterization for Macrobending Sensors

The article delves into characterizing macrobending losses in optical fiber coils for use in diverse sensors. It examines Single-Mode Fiber (SMF) and Multimode Fiber (MMF) with step and graded index profiles incorporated into sensor coils of varying diameters and numbers of turns. The experimental configuration involves compressing the coils to monitor optical power loss caused by attenuation. The findings reveal that SMFs experience more significant macrobending losses than MMFs, while the graded index fiber exhibits notable resistance to bending. These results offer valuable guidance for fiber selection and sensor design considerations.

Spectral Characteristics and Displacement Sensing of U-Shaped Single-Mode-Multimode-Single-Mode Fiber Structure.

The U-shaped fiber configuration represents the elementary form of micro-displacement sensing, characterized by its exceptional freedom and flexibility. The study proposes the U-shaped bent single-mode-multimode-single-mode (SMS) fiber structure that integrates the multimode interference (MMI) effect for enhanced mode dispersion and the Mach-Zönder interference (MZI) effect for spectral sensitivity improvement. The transmission spectral properties of the U-shaped SMS fiber structure with a bent radius over 1 cm are experimentally measured as the change in displacement varied within the range of 5 mm in this work. As the radius decreases, the spectrum shows redshift, which is related to the central wavelength of the peak or dips-a smaller wavelength results in a stronger redshift for the same displacement change. The average sensitivity of micro-displacement measurement within a range of 5 mm is 5.41 pm/μm, and the linearity is 99.62%. The maximum sensitivity of U-shaped SMS fiber structure is 34.46 pm/μm, with the minimum displacement change of approximately 5.804 nm. The transmission spectral properties of the U-shaped SMS fiber structure within the ranges of 50 μm, 500 μm, and 5 mm are experimentally measured in this work. This experiment observed a relatively uniform spectral drift pattern in a large range of micro-displacement sensing. The measurement range is limited by the limited spectral range of the light source and the discontinuous variation in the effective refractive index. This provides an experimental reference for further understanding the characteristics of U-shaped fiber structures and applying its application in micro-displacement sensing.

Sensitivity-enhanced strain sensor based on a shape-modulated multimode fiber.

In this Letter, we demonstrate a sensitivity-enhanced strain sensor based on a shape-modulated multimode fiber (MMF). In contrast to conventional single-mode-multimode-single-mode (SMS) fiber structures, which typically contain a single cylindrical homogeneous MMF section, the shape of the MMF section in this investigation is modulated by lateral offset splicing of multiple MMF segments. Simulation results show that the designed shape-modulated MMF has a higher peak mechanical strain than that of a cylindrical MMF. Experimental results demonstrate that the strain sensitivity achieved by the shaped-modulated MMF-formed SMS fiber structure is as high as -55.63 pm/µε, which is 33 times higher than that for a cylindrical MMF-formed conventional SMS fiber structure at -1.65 pm/µε. This high sensitivity and low-fabrication cost SMS fiber sensor has the potential to be a promising candidate in precise strain measurement applications.

Advanced Integration of Glutathione-Functionalized Optical Fiber SPR Sensor for Ultra-Sensitive Detection of Lead Ions.

It is crucial to detect Pb2+ accurately and rapidly. This work proposes an ultra-sensitive optical fiber surface plasmon resonance (SPR) sensor functionalized with glutathione (GSH) for label-free detection of the ultra-low Pb2+ concentration, in which the refractive index (RI) sensitivity of the multimode-singlemode-multimode (MSM) hetero-core fiber is largely enhanced by the gold nanoparticles (AuNPs)/Au film coupling SPR effect. The GSH is modified on the fiber as the sensing probe to capture and identify Pb2+ specifically. Its working principle is that the Pb2+ chemically reacts with deprotonated carboxyl groups in GSH through ligand bonding, resulting in the formation of stable and specific chelates, inducing the variation of the local RI on the sensor surface, which in turn leads to the SPR wavelength shift in the transmission spectrum. Attributing to the AuNPs, both the Au substrates can be fully functionalized with the GSH molecules as the probes, which largely increases the number of active sites for Pb2+ trapping. Combined with the SPR effect, the sensor achieves a sensitivity of 2.32 × 1011 nm/M and a limit of detection (LOD) of 0.43 pM. It also demonstrates exceptional specificity, stability, and reproducibility, making it suitable for various applications in water pollution, biomedicine, and food safety.

Dual-parameter fiber sensor for temperature and strain based on SMS-FP composite structure

A dual-parameter sensor for simultaneous strain and temperature measurement is proposed and demonstrated based on the introduction of higher-order modes in a single-mode-multimode-single-mode (SMS) structure cascaded with a Fabry–Perot (FP) cavity as a compact and low-cost fiber optic sensor. In this structure, a multimode fiber is spliced between two single-mode fibers, and the single-mode fiber at the other end is fusion spliced to the HF-corroded single-mode fiber to form the SMS-FP cascade structure. Light transmitted from the multimode fiber to the FP cavity can couple light into the cladding, introducing higher-order modes into modal interference. Moreover, multiple-beam interference generates spectra. The experimental results indicate that the sensor performs effectively within the range of 150∘C–250∘C and 0–875[Formula: see text][Formula: see text]. The strain sensitivities of SMS and FP were found to be −2.45 and −2.34[Formula: see text]pm/[Formula: see text], respectively, and their temperature sensitivities are 11.85 and 5.59[Formula: see text]pm/∘C, respectively. The interesting features of the sensor include good operating linearity, compact size, high sensitivity, simple structure and low cost.

Monitoring of Water Freeze–Thaw Cycle by Means of an Etched Single- Mode–Multimode–Single-Mode Fiber-Optic Refractometer

As an alternative to the different technologies that permit the detection of in-situ ice formation on different surfaces, this contribution proposes the design of an etched single-mode – multimode – single-mode (E-SMS) fiber-optic-based structure as a multimode interference refractometer. This sensor provides enhanced properties with respect to a basic SMS structure, including a higher sensitivity and periodical interferometry bands that can measure surrounding refractive indices with repeatability and robustness. Since ice and water refractive indices are sufficiently different, this structure has been used to detect the freezing - thawing process of water taking place inside a freezer between -20°C and +20°C. Also, this work intends to show a proof of concept of a simple technology that can be applied in different situations, such as in smart cities, avionics, structural health monitoring or even to avoid a cold chain breakage. Inside, novel developments to better understand the working operation of the E-SMS structure are shown, together with a study on how to correlate optical and thermal measurements from a refractive index point of view.

Experimental demonstration of offset-induced sensitivity enhancement in SMS-based temperature and strain sensing

A simple, inexpensive, and high-sensitivity temperature and strain sensor based on a single-mode–multimode–single-mode (SMS) structure with core offset is developed and experimentally characterized. This sensor does not require specialty fibers and can be fabricated using a standard fiber fusion splicer. The dependencies of the temperature and strain sensitivities on the core-offset amplitudes at the input and output single-mode/multimode fiber boundaries are investigated. The results indicate that the maximum temperature and strain sensitivities are two times and eight times higher than those of the standard SMS structure, respectively. The limit of the sensitivity enhancement by core offset is also revealed.

Highly Efficient Refractive Index Sensor Based on a Dual-Side Polished SMS Fiber Enabled by Femtosecond Laser Writing.

Fiber-optic refractive index (RI) sensors based on wavelength-shift-based interrogation continue to present a challenge in achieving high sensitivity for a wide detection range. In this paper, we propose a sensor for determining the RI of liquids based on femtosecond laser (fs-laser) writing of a dual-side polished singlemode-multimode-singlemode (SMS) fiber. The proposed sensor can determine the RI value of a surrounding liquid by detecting the dip wavelength in the transmission spectrum of the light propagating through the sensing area. The high RI sensitivity is attributed to the increased interaction area established by the fs-laser, which creates hydrophilic surfaces and maintains the wide detection range of the SMS structure. The results of the wavelength-shift-based interrogation reveal that the fabricated device exhibited a high sensitivity of 161.40 nm per refractive index unit (RIU) over a wide RI detection range of 0.062 RIU. The proposed device has high processing accuracy and a simple manufacturing process. Hence, it has the potential to be used as a lab-on-fiber sensing platform in chemical and biotechnological applications.

Vector dark–bright pulses from a ytterbium doped fiber laser mode-locked by nonlinear multimode interference

We report dark–bright pulse pair generation from the ytterbium doped fiber laser mode-locked by cascaded nonlinear multimode interference (MMI) effect under an all-normal dispersion configuration. The MMI is implemented in a step index single mode–multimode–single mode fiber structure. The combination of two MMI structures, acting like a saturable absorber, has been used for stable mode-locking operation in the resonator. By adjusting the polarization controller attached to one of the MMI structure, stable dark–bright pulses are observed at fundamental, second and third harmonic repetition rates. The optical spectrum of the dark–bright pulses is doubly peaked at 1036.6 and 1040.6 nm with a spectral width ∼0.7 nm. The width of the dark pulses varied from 72 to 50 ns and that of the bright pulses from 30 to 14 ns when the pump power is increased from 102 to 170 mW at the fundamental repetition rate mode-locking. The darkness of the dark and intensity of bright pulses increase with the pump power. Dark and bright pulses in the pair are resolved by placing a polarizing component confirming their vector nature.

Three-Dimensional-Printed Fabrication of POFs Using Different Filaments and Their Characterization for Sensing Applications.

This paper presents the development and sensor applications of 3D-printed polymer optical fibers (POFs) using commercially available filaments. The well-known intensity variation sensor was developed using this fiber for temperature and curvature sensing, where the results indicate a linear response in the curvature analysis, with a coefficient of determination (R2) of 0.97 and sensitivity of 4.407 × 10-4 mW/∘, whereas the temperature response was fitted to an R2 of 0.956 with a sensitivity of 5.718 × 10-3 mW/∘C. Then, the POF was used in the development of a modal interferometer by splicing the POF in-between two single-mode fibers (SMFs), which result in a single-mode-multimode-single-mode (SMS) configuration. The such interferometer was tested for temperature and axial strain responses, where the temperature response presented a linear trend R2 of around 0.98 with a sensitivity of -78.8 pm/∘C. The negative value of the sensitivity is related to the negative thermo-optic coefficient commonly obtained in POFs. Furthermore, the strain response of the SMS interferometer showed a high sensitivity (9.5 pm/μϵ) with a quadratic behavior in which the R2 of around 0.99 was obtained. Therefore, the proposed approach is a low-cost, environmentally friendly and straightforward method for the production of highly sensitive optical fiber sensors.

Characterization of single-mode multimode single-mode fiber optic sensors for steel rebar corrosion monitoring in NaCl and simulated concrete pore solutions

Single-mode multimode single-mode (SMS) fiber optic sensors are characterized for corrosion monitoring of steel rebars in NaCl and simulated concrete pore solutions. Three diameters of the multimode fiber (MMF) are considered including 125 µm, 90 µm, and 60 µm. Steel rebar specimens were first cast in epoxy resin and then polished to expose a rectangular section. The SMS fiber optic sensors are directly attached on the polished surface of the steel rebar and then immersed in solutions for corrosion monitoring. Two types of solution are prepared, including 3.5 wt.% NaCl solution and simulated concrete pore solutions. During the immersion tests, the corrosion evolution of the steel rebar was measured with open circuit potential and linear polarization resistance. The change of the transmission spectrum of the SMS fiber optic sensors is recorded with an optical spectra analyzer. The change of the characteristic wavelength is correlated with the corrosion-induced mass loss of the steel rebars. Results show that linear relationships are present between the corrosion-induced mass loss of the steel rebars and the shift of the characteristic wavelength of the SMS fiber optic sensors. The corrosion monitoring sensitivity of SMS fiber optic sensors increases with a decrease of the diameter of the MMF.

Analyzing two hetero-core spliced low-cost set-up in assessing temperature

Accurate measurement of temperature is very essential in laboratory scale to industrial sector. Extensive research is under way in achieving higher accuracy. In this direction, we present a comprehensive and procedural performance analysis of two hetero-core spliced system in measurement of temperature in identical domain. Employing a single mode-multimode-single mode (SMS) and its’ reverse arrangement multimode-single mode-multimode (MSM), we measure temperature in the range of 30°–80 °C. Through a facile intensity interrogation mechanism, the two arrangements are compared in terms of their sensing characteristics. It is found that the latter outperforms the former in terms of linearity. Apart from this, the resolution is also higher in case of the latter. All these findings are procedurally investigated and interpreted.

Highly sensitive temperature sensor based upon a multimode interference structure filled with an ethanol-glycerol solution

A temperature sensor based upon a single mode - multimode-single mode – multimode - single mode (SMSMS) fiber structure is experimentally demonstrated. The SMSMS structure is covered by ethanol-glycerol solution, and changing the ratio of ethanol to glycerol changes the temperature sensitivity. A theoretical calculation was developed to characterize the influence of the ethanol-glycerol content upon the sensor performance,. Multimode interference (MMI) occurs between single mode fibers (SMF) and multimode fibers (MMF), and temperature measurements were achieved by monitoring the movement of the interference profile in the transmission spectrum. The results show that covering the SMSMS structure with an ethanol-glycerol solution improves the temperature sensitivity of the sensor. The temperature sensitivity was is 0.700 nm/°C from 30 °C to 80 °C.

A label-free biosensor based on E-SMS optical fiber structure for anti BSA detection

In this work, a novel, compact and highly sensitive Bovine Serum Albumin (BSA)–anti-BSA biosensor based on an etched single-mode–multimode–single-mode (E-SMS) optical fiber structure is presented. This biosensor based on the multimode interference produced into an E-SMS structure was fabricated by chemical etching of the cladding region of the multimode fiber, followed by the functionalization of this etched region with BSA. The functionalized E-SMS biosensor provided high selectivity in the detection of anti-BSA, with the ability to detect biological interactions up to 100 ng/mL and a sensitivity of 2.3 nm/(µg/ml). This sensitive label-free biosensor has potential to be widely applied to various areas such as disease detection, medical diagnosis, and food safety inspection.

Modal dispersion characteristics of LP0m modes in a step-index multimode fiber.

A modal interferometer method (MIM) is applied to measure the differential mode delay (DMD) between the L P 0m modes traversing a step-index multimode fiber (SI-MMF). Only linearly polarized radial modes, i.e.,L P 0m modes, are excited and transmitted in the SI-MMF by using a single-mode-multimode-single-mode (SMS) fiber structure. The measurement principle is based on investigating a transmitted spectrum through temporal decomposition by means of a Fourier transform. The Fourier-transform-based MIM provides simultaneous measurements of the DMD between the L P 0m modes. The wavelength dependence of the DMD is estimated experimentally in both the 1260-1360nm and 1450-1625nm telecommunication bands. The normalized frequency dependence of the DMD is also investigated theoretically. The result suggests that the 1260-1360nm band is preferable to the 1450-1625nm band for a mode-division multiplexing (MDM) transmission employing an SI-MMF in terms of realizing a smaller DMD.

Improvement of Temperature Performance of Singlemode-Multimode-Singlemode Fiber Structure.

A theoretical model for studying the temperature properties of singlemode-multimode-singlemode (SMS) fiber structure fabricated by absorptive multimode fiber (MMF) cladding is established. Moreover, an SMS-based temperature sensor is fabricated and experimentally demonstrated. Experimental results show that the dip wavelength of the transmission spectrum changes linearly with temperature, which is in good agreement with the simulated results obtained by using the model. Further, a comprehensive study of temperature characteristics affected by the thermo-optic effect, thermal expansion effect, and thermal effect of absorption characteristics is performed for SMS fiber optic structures with different refractive indexes, thermo-optic coefficients, and absorption properties of MMF cladding, MMF core diameters, and thermal expansion coefficients of packaging shell. According to the obtained rules, investigations are carried out into the thermal response of an SMS fiber structure resulting from combined thermal effects for temperature performance optimization. Excellent temperature stability with a temperature sensitivity of 0 pm/°C or good temperature sensitivity of -441.58 pm/°C is achieved accordingly.

Temperature sensing based on multimode interference in polymer optical fibers: sensitivity enhancement by PC-APC connections

A simple, stable, and high-sensitivity temperature sensor based on multimode interference in a polymer optical fiber (POF) with higher-order mode excitation has been developed. In a single-mode–multimode–single-mode (SMS) structure, one end of the multimode POF with physical-contact (PC) connectors is connected to a silica single-mode fiber with an angled-PC (APC) connector. We compare the temperature sensing characteristics of the three configurations (no PC-APC, PC-APC at input, and PC-APC at output) and obtain the highest temperature sensitivity of 219.2 pm °C−1, which is more than double the value of the standard (no PC-APC) SMS structure.

Fe-C-Coated Single-Mode–Multimode–Single- Mode Optical Fiber Sensor for Steel Corrosion Monitoring

Fe-C-coated single-mode–multimode–single-mode (SMS) optical fiber sensor is fabricated and characterized for steel corrosion monitoring in 3.5 wt.% NaCl solution. The multimode fiber (MMF) is a none core fiber and has a length of 30 mm. A 100-nm-thick layer of the gold film is first sputter deposited on the surface of the SMS fiber and then the Fe-C layer is electroplated with a thickness of around 20 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> . The microstructure of both the inner gold film and the outer Fe-C layer is characterized by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy. The corrosion evolution of the Fe-C layer over time in 3.5 wt.% NaCl solution is measured with open circuit potential (OCP), linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS). In the meantime, the change in the transmission spectrum is recorded with an optical spectra analyzer during the corrosion test. The shift in the characteristic wavelength of the Fe-C-coated SMS fiber sensor is extracted and correlated with the corrosion-induced mass loss of the Fe-C layer. Regression analysis demonstrates that the corrosion-induced mass loss of the Fe-C layer exponentially increases with an increase in the characteristic wavelength of the Fe-C-coated SMS optical fiber sensor.

Enhanced Mach-Zehnder interferometer multimode–single-mode–multimode fiber optic refractive index sensor based on surface plasmon resonance

In this paper, an all-fiber Mach-Zehnder interferometer (MZI) sensor for refractive index (RI) measuring is presented, which is based on Multimode–Single-mode–Multimode (MSM) fiber. The effects of both reducing the radius of the sensing part and the surface plasmon resonance (SPR) on its efficiency are investigated. Increasing the interaction of high-order modes with external media, caused by etching the cladding layer of the single-mode fiber part, significantly improves the sensitivity. Both wavelength and intensity interrogation approaches are employed to study the Multimode–etched Single-mode–Multimode (MESM) fiber sensor. The intensity and the wavelength sensitivities for the RI measurement in the range of 1.428–1.458 are obtained as −2306.754 %/RIU and 1315.74 nm/RIU, respectively. Finally, the MESM-SPR sensor is proposed and characterized. Results exhibit high performance in the RI range of 1.333 to 1.357, in which the sensitivity of 1437.056 nm/RIU is achieved. The advantages like low cost, high sensitivity, and simple fabrication methods make these sensors promising devices for chemical, food industry, and biosensing applications.

Mode Transitions and Thickness Measurements During Deposition of Nanoscale TiO2 Coatings on Tilted Fiber Bragg Gratings

The mode transition is a phenomenon observed in thin film coated long period fiber gratings (LPGs) and single-mode multimode single-mode (SMS) fibers for certain values of the coating thickness and refractive index, resulting in increased sensitivity for sensing applications. It is shown here that mode transitions occur simultaneously for a large number of mode resonances in the transmission spectra of tilted fiber Bragg gratings (TFBG) measured during the deposition of ∼350 nm thick TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> coatings by Atomic Layer Deposition (ALD). In TFBGs, the mode transition shows up as an acceleration of the resonance wavelength shift vs thickness, but without fading of the resonance amplitude. Furthermore, the results show that the mode transition for cladding modes with predominantly “TE” polarization at the cladding boundary is significantly sharper than that of predominantly “TM” polarized modes and that it occurs at a smaller coating thickness (<100 nm vs >200 nm). Finally, using a separately determined coating refractive index (2.14, by ellipsometry on witness flats deposited simultaneously) and simulations of the resonance shifts of the TFBG with coating thickness, it is demonstrated that a TFBG connected to a spectral interrogation system can be used to measure the growth of a coating on the surface of the fiber in real time.