Section Of Multimode Fiber Research Articles

Directional bending sensor based on Mach-Zehnder interferometer formed by side-polished multimode fiber

This work proposes a fiber bending sensor based on the Mach-Zehnder interferometer (MZI) constructed using a side-polished multimode fiber (MMF) with bending direction recognition. The sensor is fabricated by directly fusing a section of side-polished MMF sandwiched between two no-core fibers (NCFs) to an input and output single-mode fibers (SMFs). As the applied curvature varies from 0 m−1 to 1.64 m−1, the transmission spectra of the sensor exhibit distinct and linear wavelength shift in opposite directions. The maximum sensitivity observed in 0° and 180° bending is 3.252 nm/m−1 and −1.146 nm/m−1, respectively. The maximum temperature sensitivity is 0.0621 nm/°C and this effect can be compensated by using the sensitivity matrix. The advantages of the proposed sensor include its ability to accurately quantify the direction and magnitude of bending, temperature crosstalk elimination, and cost-effectiveness. These features make the sensor highly desirable for a wide range of practical directional bending applications, such as medical wearable devices for human joint range of motion monitoring.

SCF-MMF based Michelson interferometer for ethanol multi-parameter measurements

A optical fiber Michelson interferometer (MI) for multi-parameter ethanol measurements is proposed and experimentally demonstrated. The sensor consists of a brief seven-core fiber (SCF) and a section of multimode fiber (MMF). With the SCF, multiple high-order modes are coupled into MMF, while the core fundamental mode and high-order modes are reflected back by the MMF end face and recoupled to the SMF by the SCF. When the surrounding ethanol changes, the reflection spectrum of the MI shift for the intermodal interference strongly integrates with the surrounding medium. Experimental results show that the sensor has an ethanol concentration sensitivity of −0.041 dB/% over the range of 0–90 %. The level sensitivities are −0.0795 nm/mm and −0.1052 nm/mm in the 0–5 mm range for 5 % and 10 % ethanol solutions, respectively, while in the 5–20 mm range, the sensitivities are −0.2773 nm/mm and −0.2243 nm/mm, respectively. Moreover, the volatilization time and volatilization stability are tested under different concentrations of ethanol, and the volatilization sensitivity is 0.071 dB/% in the range of 0–60 %. Response and recovery time of 1.008 s and 0.514 s for 15 % ethanol volatilization molecules is achieved. This MI has advantages of compact structure, easy fabrication, good stability and dominant ability to achieve multi-parameter ethanol measurement via reflective operation.

Highly sensitive and quick response carbon dioxide sensor based on sol–gel film coated four-leaf clover fiber mode interferometer

A highly sensitive and quick response gaseous carbon dioxide (CO2) fiber sensor based on sol–gel film coated four-leaf clover fiber (FLCF) mode interferometer (MI) is proposed. The MI is constructed by using porous polymer tube to connect two sections of multi-mode fiber (MMF) with FLCF. By depositing thymol blue sol–gel film on the cladding wall of the FLCF, interference spectrum caused by the core mode and cladding mode of the MI can be modulated by CO2 concentration through the reaction between sol–gel film and CO2. By observing the shift of peak wavelength at 1555.7 nm, 1552.9 nm and 1549.3 nm, it is found that the interference spectrum changes linearly with CO2 concentration. Experimental results show that the FLCF-MI CO2 sensor has high wavelength sensitivity of 445 pm/% and short response time of 24/28 s. Besides, the sensor exhibits good temperature, humidity stability, and well selectivity, which shows great prospect in detecting CO2 concentration in carbon capture, utilization and storage (CCUS) project.

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.

Learning-enabled recognition of LG beams from multimode fiber specklegrams

The use of orbital angular momentum (OAM) beams, like Laguerre Gaussian (LG) beams is a significant advancement in optical communications, enhancing versatility in data encoding. In optical communications, especially OAM multiplexing, previous studies have successfully recognized LG beams in free space. However, free space propagation presents challenges including power loss, beam distortion, wandering, and physical obstacles. To overcome these hurdles, we introduce a novel method using speckle patterns (specklegrams) within a multimode fiber (MMF) for LG beam recognition through deep learning. Our study focuses on recognizing 15 LGp|l| beams (0≤p,|l|≤3; excluding LG00) generated with a spatial light modulator (SLM) and launched into a standard silica MMF with a 200 µm core diameter, producing corresponding speckle patterns. We utilize the widely-used deep convolutional neural network (CNN) model, AlexNet, to train these specklegram images, achieving an impressive 91 % recognition accuracy. Furthermore, we assess the recognition accuracy of 15 LGp(-l) modes, obtaining an 86.5 % accuracy rate. We also explore the impact of external perturbations, such as transverse weights (ranging from 0 to 3 kg in 1 kg increments) and temperature variations (ranging from 50 °C to 150 °C in 10 °C steps) on a section of MMF. In both scenarios, recognition accuracies are > 80 %. These findings highlight the effectiveness of our proposed approach, promising high-capacity data transmission in optical communications.

Multi-tapered polarization-maintaining fiber-optic sensor for refractive index and temperature measurements

In this paper, a fiber-optic refractive index and temperature sensor based on Mach-Zehnder interferometer (MZI) is designed and fabricated. The sensor structure consists of a section of polarization-maintaining fiber (PMF) with multi-tapers and sandwiched by two sections of multi-mode fiber (MMF). Due to the changes of ambient refractive index and temperature, the effective refractive index of the core and cladding modes of the multi-tapered PMF will be changed, which will cause the interference spectral shift of core mode and cladding mode. The series experiments show that the structure has better repeatability, the maximum refractive index sensitivity is 250.861 nm/RIU, the maximum temperature sensitivity is −78 pm/°C. As the sensitivity matrix is used to realize the two-parameter distinguishing measurement, the resolutions of 1.2 × 10−5 RIU and 0.38 °C for refractive index and temperature are achieved. The structure has superior mechanical properties, and has broad application prospects in petrochemical industry, underwater exploration and other fields.

Refractometer based on extinction ratio demodulation using no-core and dispersion-compensating fiber structure

A simple and cost-efficient fiber sensor based on multi-mode interference and core diameter mismatch technologies is proposed to detect the refractive index (RI) in this paper. The refractometer is fabricated by embedding a section of no-core fiber between a section of multimode fiber and dispersion-compensating fiber. Since the power distribution in the fiber will change with the effective refractive index of the cladding, the extinction ratio can be used to characterize the RI sensitivity. The experimental results show that a linear response is obtained with a RI sensitivity of 392.585 dB/RIU in the range of 1.333–1.374. Featured with high sensitivity, simple construction, easy fabrication, low cost and good repeatability, the novel refractometer has potential applications in practical chemical detecting and portable biological sensing.

A highly sensitive strain fiber sensor based on waved core structure

Strain and temperature, as two important parameters to evaluate structural health, is increasingly paid numerous attentions for years. In this article, we developed and experimentally tested a highly sensitive strain sensor based on SMS structure with waved core multi-mode fiber (MMF), which has ability to monitor temperature simultaneously. By introducing a section of bent-core fiber into the portion of MMF, the higher-order cladding mode is excited to enhance the sensitivity. Furthermore, the portion of bent-core displays significant influence on effective refractive index (ERI) of the MMF section as strain applied, which brings about higher strain sensitivity. The experimental results display that the highest sensitivity of the sensor reaches 125.5 pm/με within 0–200 με and the measuring range achieves 0–1200 με. By demodulating two dips, simultaneous measurement for strain and temperature is achieved by solving the equation matrix. This simple method displays minor influence on mechanical property and enhances the strain sensitivity considerably. It is strongly believed the proposed sensor brings about a competitive choice in engineering fields.

A Machine Learning Specklegram Wavemeter (MaSWave) Based on a Short Section of Multimode Fiber as the Dispersive Element

Wavemeters are very important for precise and accurate measurements of both pulses and continuous-wave optical sources. Conventional wavemeters employ gratings, prisms, and other wavelength-sensitive devices in their design. Here, we report a simple and low-cost wavemeter based on a section of multimode fiber (MMF). The concept is to correlate the multimodal interference pattern (i.e., speckle patterns or specklegrams) at the end face of an MMF with the wavelength of the input light source. Through a series of experiments, specklegrams from the end face of an MMF as captured by a CCD camera (acting as a low-cost interrogation unit) were analyzed using a convolutional neural network (CNN) model. The developed machine learning specklegram wavemeter (MaSWave) can accurately map specklegrams of wavelengths up to 1 pm resolution when employing a 0.1 m long MMF. Moreover, the CNN was trained with several categories of image datasets (from 10 nm to 1 pm wavelength shifts). In addition, analysis for different step-index and graded-index MMF types was carried out. The work shows how further robustness to the effects of environmental changes (mainly vibrations and temperature changes) can be achieved at the expense of decreased wavelength shift resolution, by employing a shorter length MMF section (e.g., 0.02 m long MMF). In summary, this work demonstrates how a machine learning model can be used for the analysis of specklegrams in the design of a wavemeter.

Reflective Optical Tactile Sensor Based on Fiber Specklegram Analysis With the Capability of Contact Position Identification

A reflective tactile sensing scheme based on fiber specklegram analysis is proposed in this study. The specially designed sensor probe with a reflector inside was 3D printed from elastic material. A section of single mode fiber (SMF) and a section of multimode fiber (MMF) were embedded under the reflector. The light emitted from the SMF is reflected by the reflector and the reflected light illuminates the MMF. At the output facet of the MMF, specklegram is detected by a camera. The reflector consists of two soft mirrors that are perpendicular to each other originally. The contact force on the sensor probe causes the variation of the angle between the two mirrors. The transmission path of the light from SMF to MMF changes accordingly, which results in a changed specklegram. In this way, the zero-mean normalized cross-correlation coefficients of the fiber specklegrams decrease linearly with the increase of contact force. The sensitivity and measurement range of contact force are examined with MMFs of different diameters under two operating wavelengths. The negligible effect of fiber bending on the force sensing was also experimentally confirmed. In addition, the convolutional neural network is used to classify the specklegrams of three contact forces applied at nine positions of the sensor probe, and the average accuracy of classification is 100%, which shows its capability to identify the contact position on the probe. On the whole, the proposed tactile sensor is confirmed with a high sensitivity of force sensing in a proper measurement range and a capability to localize the contact position. The reflective structure also facilitates the integration with robot arm and other intelligent structures.

High-sensitive and temperature-immune curvature sensor based on bitaper sandwiching in SMS fiber structure

A high-sensitive and temperature-immune curvature sensor based on all fiber Mach–Zehnder interferometer (MZI) is proposed and experimentally investigated. The sensor is constructed by sandwiching a short section of multimode fiber (MMF) between two single-mode fibers (SMFs) with two bitapers. Experimental results show that the sensor can achieve a maximum sensitivity of -34.3935 nm/m−1 in the curvature range of 2.96-4.19 m−1 and a low temperature cross-sensitivity of -0.0003 m−1/∘C in the temperature range of 30-100 °C. The stability test reveals that the standard error of instability is 0.0004 m−1, indicating the sensor’s capacity of long-term operation. The sensor has advantages of low cost, easy fabrication, large curvature detection range, high curvature sensitivity and temperature immunity, makes it has broad application prospects in the curvature sensing field.

Tapered MMF sensor fabrication using SnO2-NPs for alcohol sensing application

Metal oxide coated fiber optic sensor is proposed and experimentally characterized for alcohol sensing. The sensor probe design is based on the cladding removed methodology, done via tapering of the fiber. Chemical etching process was used to taper a section of multimode fiber (MMF) and the region was coated by tin dioxide nanoparticles (SnO2-NPs), which were synthesized by the sol–gel method. The sensing principle relies on the refractive index (RI) modification of SnO2 after being in contact with isopropanol solution leading to modulated output response. XRD structurally characterized SnO2-NPs and their crystallite size was found to be 19.88 nm. The prepared fiber dimension, nanoparticles morphology and the elemental composition were confirmed by FESEM and EDX characterization. A maximum average sensitivity with good linear behavior was found to be 22 counts/ppm for the wide range of concentrations (0–500 ppm). Along with excellent sensitivity, sensor shows nice reversibility and repeatability characteristics. A sensor combining optical fiber and metal oxide as sensing elements provides advantages of a miniaturized probe, low cost, ruggedness, online monitoring and remote sensing.

A Simple Optical Sensor Based on Multimodal Interference Superimposed on Additive Manufacturing for Diameter Measurement.

In many areas, the analysis of a cylindrical structure is necessary, and a form to analyze it is by evaluating the diameter changes. Some areas can be cited: pipelines for oil or gas distribution and radial growth of trees whose diameter changes are directly related to irrigation and the radial expansion since it depends on the water soil deficit. For some species, these radial variations can change in around 5 mm. This paper proposes and experimentally investigates a sensor based on a core diameter mismatch technique for diameter changes measurement. The sensor structure is a combination of a cylindrical piece developed using a 3D printer and a Mach–Zehnder interferometer. The pieces were developed to assist in monitoring the diameter variation. It is formed by splicing an uncoated short section of MMF (Multimode Fiber) between two standard SMFs (Singlemode Fibers) called SMF-MMF-SMF (SMS), where the MMF length is 15 mm. The work is divided into two main parts. Firstly, the sensor was fixed at two points on the first developed piece, and the diameter reduction caused dips or peaks shift of the transmittance spectrum due to curvature and strain influence. The fixation point (FP) distances used are: 5 mm, 10 mm, and 15 mm. Finally, the setup with the best sensitivity was chosen, from first results, to develop another test with an optimization. This optimization is performed in the printed piece where two supports are created so that only the strain influences the sensor. The results showed good sensitivity, reasonable dynamic range, and easy setup reproduction. Therefore, the sensor could be used for diameter variation measurement for proposed applications.

Investigation on the Dependence of Directional Torsion Measurement on Multimode Fiber Geometry

The torsion measurement and torsion direction discrimination characteristics of a multimode fiber (MMF) geometry in a singlemode-multimode-singlemode (SMS) fiber structure are investigated theoretically and experimentally. Three different geometries of MMFs, including circular symmetric MMF, axisymmetric MMF, and asymmetric MMF were fabricated based on a fiber polishing technique. The torsion response characteristics of a MMF depends on the birefringence within the MMF, as birefringence is related to the twist rate of the fiber. The experimental results demonstrate that the unpolished MMF with a circular asymmetric refractive index (RI) distribution has no wavelength response to torsion, and the single-side polished MMF with an axisymmetric RI distribution can measure torsion, but it does not have the ability to discriminate torsion direction. It is found that an MMF section with two polished surfaces at an angle to each other results in a departure from the symmetrical RI distribution, which introduces elliptical birefringence into the MMF. As a result, directional torsion measurement is achieved with sensitivities of 0.162 nm/(rad/m) clockwise and -0.139 nm/(rad/m) counterclockwise. The MMF geometry presented provides a new approach to torsion direction discrimination and shows good application potential in practical torsion vector measurement.

Fiber optic high temperature sensor based on ZnO composite graphene temperature sensitive material

The working temperature of the cable is particularly important for the safe operation of the power system, which requires high temperature detection for its working environment, but the sensitivity of the ordinary optical fiber temperature sensor in the high temperature state cannot meet this demand. For this purpose, an optical fiber high temperature sensor based on ZnO composite graphene temperature sensitive material is developed in this paper. MMF-TSCF-MMF temperature sensing unit is composed of two sections of multimode fiber (MMF) and a tapered seven core fiber (TSCF). Three different shapes of ZnO@Gr (ZnO composite graphene) temperature-sensitive materials were prepared by hydrothermal method. The experimental results show that the sensor has high stability and good interference effect. Meanwhile, the wavelength sensitivity reached 0.33268 nm/°C in a temperature measurement range of 89–98 °C. The fiber sensor is used for precision temperature measurement of cable temperature in fields such as electrical engineering due to its high temperature sensitivity, small size and low cost.

Narrow spectral linewidth and tunable erbium-doped fiber ring laser using a MZI based on CHCF

In this paper, a narrow spectral linewidth and wavelength-tunable erbium-doped fiber laser based on a Mach-Zehnder interferometer (MZI) as a filter is proposed and experimentally demonstrated. The filter is fabricated by splicing a short length of capillary hollow-core fiber (CHCF) between two small sections of multimode fibers (MMF). The interference pattern generated by the filter has a free spectral range of 10.5 nm and a maximum extinction ratio of 36 dB at 1560 nm. Then, by incorporating the filter inside a ring cavity laser, a single laser emission centered at 1566.46 nm with a spectral width around 0.03 nm and a high SNR of 66 dB (also has a SMSR of 53 dB) is obtained. By changing the temperature of the filter, the wavelength emission of the laser can be tuned from 1566.46 to 1570.09 nm. Considering that the fabrication process of the filter is relatively simple and can be easily implemented in an EDFL, we strongly believe that it can have a great deal of interest in telecom applications.

An Intensity-Modulated In-Fiber Liquid Lever Sensor Based on Intermodal Michelson Interferssometer

A compact in-fiber Michelson interferometer (MI) for liquid level measurement is proposed and demonstrated. The interferometer is fabricated by simply splicing a section of multimode fiber (MMF) with a standard singlemode fiber (SMF). The reflection spectrum of the MI is dependent on the portion of the length of the MMF which is immersed in liquid. The sensing characteristics of the liquid level and temperature are theoretically and experimentally investigated. The experimental results show that the liquid level sensitivity is −1.20 dB/mm when the liquid is water, and the liquid level measurement error induced by temperature is 0.004 mm/°C. The advantages of temperature-immune, cost-effective and reflective intensity demodulation offer the prospective application in chemical and petroleum industrial sensing.

Micro-bending sensing based on single-mode fiber spliced multimode fiber Bragg grating structure

Fiber Bragg grating (FBG) is a commonly used optical fiber sensing structure. FBG bent sensors in multimode fiber have been achieved. However, the structure and manufacturing process are complex. In the present study, we designed a simple structure that composed of an ordinary single-mode fiber (SMF) and a section of multimode fiber (MMF) with a FBG. It can realize the dual-parameter sensing of micro-bending and temperature simultaneously. The experimental results suggested that the structure is sensitive to micro-bending and temperature, and the sensitivities do not interfere with each other. The temperature sensitivity is 13.4 pm/∘C, and the maximum bending sensitivity is 23.03 dB/m−1.

Highly Sensitive Dual Parameter Sensor Based on a Hybrid Structure with Multimode Interferometer and Fiber Bragg Grating Fabricated by Femtosecond Laser

A hybrid sensing configuration for simultaneous measurement of strain and temperature based on fiber Bragg grating (FBG) written in an offset multimode fiber (MMF) interferometer using femtosecond laser pulse is proposed and demonstrated. A Mach–Zehnder interferometer is formed by splicing a section of MMF between two single-mode fibers (SMFs) and a high interference fringe of up to 15 dB is achieved. The sensing experimental results show a strain sensitivity of −1.17 pm/με and 0.6498 pm/με for the dip of MZI and Bragg peak, while a temperature sensitivity of 42.84 pm/°C and 19.96 pm/°C is measured. Furthermore, the matrix analysis has found that the strain and temperature resolution of the sensor are as high as ±12.36 με and ±0.35 °C, respectively. In addition, the sensor has merits of simple fabrication, good spectral quality, and high resolution, which shows attractive potential applications in dual-parameter sensing.

Supercontraction of spider dragline silk for humidity sensing.

The spider dragline silk (SDS) has a supercontraction characteristic, which may cause the axial length of the SDS to shrink up to 50% when the SDS is wet or the relative humidity is higher than 58% RH. In this manuscript, we employ the supercontraction characteristic of the SDS to measure relative humidity. We connect two sections of a single-mode fiber (SMF) and a section of multimode fiber (MMF) with a sandwich structure to fabricate a single-mode-multimode-single-mode (SMS) interferometer. Then we fix the SDS on two SMFs to configure a bow-shaped sensing unit. The increase of environmental humidity will cause the supercontraction of the SDS, which will cause the change of the SDS length. The excellent mechanical properties of the SDS will generate a strong pulling force and change the bending of the arch, whose interference spectrum will shift correspondingly. In this way, we may perform relative humidity sensing. In the relative humidity range of 58% RH to 100% RH, the average sensitivity is as high as 6.213 nm/% RH, higher than most fiber-based humidity sensors. Compared with the traditional sensing structure with humidity-sensitive materials, the proposed sensor improves the sensitivity with environmental friendliness. The results suggest that the SDS can be used for high-sensitivity humidity sensors, and its degradability and biocompatibility also have a vast development space in biochemical sensors.