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In this paper, we report a novel and compact sensor based on an optic microfiber coupler interferometer (OMCI) for seawater salinity application. The OMCI device is fabricated by connecting Faraday rotating mirrors to the two out-ports of the microfiber coupler, respectively. The sensor signal processing is based on a wavelength demodulation technique. We theoretically analyze the sensing characteristics with different device structure parameters. Besides, the results show that the date reading error decreases with the thinner waist region and longer arm difference. Through the experiment, the reflection spectra red-shifted as the sea water salinity increased; the highest response sensitivity of the OMCI salinity sensor reached 303.7 pm/‰ for a range of 16.6–23.8‰, and the resolution was less than 0.03‰. This study provides a new technical solution for the development of practical optical fiber seawater salinity sensors.

Wavelength tracking methods are widely employed in fiber-optic interferometers, but they suffer from the problem of fringe order ambiguity, which limits the dynamic range within half of the free spectral range. Here, we propose a new sensing strategy utilizing the unique property of the dispersion turning point in an optical microfiber coupler mode interferometer. Numerical calculations show that the position of the dispersion turning point is sensitive to the ambient refractive index, and its position can be approximated by the dual peaks/dips that lay symmetrically on both sides. In this study, we demonstrate the potential of this sensing strategy, achieving high sensitivities of larger than 5327.3 nm/RIU (refractive index unit) in the whole refractive index (RI) range of 1.333–1.4186. This sensor also shows good performance in narrow RI ranges with high resolution and high linearity. The resolution can be improved by increasing the length of the coupler.

Optical microfiber biosensors demonstrate exceptionally ultra-high sensitivity at the dispersion turning point (DTP). However, the DTP is highly susceptible to variations in dimensional and external environmental factors, and the spectral response is mismatched from preparation in air to application in a liquid environment, making the DTP difficult to control effectively. In this work, we propose a method that bridges the relationship between the interference spectra of air and aqueous environments. By counting the interference peaks in air, we can accurately predict the DTP position in liquids. Meanwhile, it provides a new balance between sensitivity and wide linear dynamic range, achieving wide dynamic range detection across various concentrations. The optical microfiber coupler (OMC) is fabricated using the hydrogen–oxygen flame melting tapering method. In addition, the concentration, temperature, and solvent used for the sensor’s biofunctional layer are optimized. Finally, in refractive index sensing, a maximum sensitivity of 1.17 × 105 ± 0.038 × 105 nm/RIU is achieved. For biosensing, a wide dynamic range detection of cardiac troponin I (cTnI) is realized at concentrations of 12–48 ng/mL, 120–480 pg/mL, and 120–480 fg/mL.

In this study, a graphene and optical microfiber coupler (OMC) integrated device (GOMC) was proposed and fabricated. After its characteristic analysis and testing, it was applied to the development of adjustable multi-wavelength fiber lasers. By integrating the OMC with graphene, the polarization dependence of OMC was enhanced. Meanwhile, the novel GOMC was given the capabilities of filtering, coupling, beam splitting, and polarization correlation. When the GOMC was integrated as a filter and beam splitter into the ring cavity of the fiber laser, the proposed GOMC-based fiber laser could achieve single-wavelength and multi-wavelength regulated output. The laser had a 3 dB linewidth of less than 30 pm, a signal-to-noise ratio of approximately 40 dB, and an output power fluctuation of less than 1 dB. The GOMC could also be used for the development of functional devices, such as adjustable mode lockers and mode coupling selectors, which provide an excellent experimental platform for new fiber lasers and the research of multi-dimensional light-field manipulation.

In this paper, a novel and compact magnetic field sensor based on the combination of an optical microfiber coupler interferometer (OMCI) and magnetic fluid (MF) is proposed. The sensor is made up of an OMCI cover with polydimethylsiloxane (PDMS) and MF, and it uses MF as a material for adjusting the magnetic refractive index and magnetic field response. The sensing characteristics of the sensor are analyzed, and the experimental test is carried out. Under the condition of the same OMC waist length, the sensor sensitivity increases with the decrease of the OMC waist radius. The sensitivity of 54.71 and 48.21 pm/Oe was obtained when the OMC waist radius was set at 3.5 and 4 μm, respectively. In addition, we also tested the sensing response time and vector response characteristics of the sensor. At the same time, we discuss the demodulation idea about the cross-sensitivity of the magnetic field and temperature. The sensor has the advantages of high sensitivity, low cost, small size, optimized performance, and convenient integration. It has huge application potential in the fields of navigation and industrial intelligent manufacturing.

In this paper, a novel high-sensitivity temperature sensor with two sensing regions based on optical microfiber coupler interferometer (OMCI) for ocean application is proposed. The OMCI sensor is constructed by connecting Faraday mirrors to the two ports of the microfiber coupler respectively. Its sensing characteristics analysis and experimental test are conducted. Using a broad-spectrum light source as input light, temperature sensor demodulation can be implemented by tracking the drift of the characteristic wavelength (dip wavelength or peak wavelength) of the reflection spectrum. Experimental results show that the temperature sensitivity of the OMCI sensor can reach 1007.4 pm/°C and the detection dynamic range up to 17.6 °C. Besides, due to the two sensing regions in OMCI, self-calibration of seawater temperature sensing and optimization of multi-parameter cross-sensitive demodulation are performed by affecting the non-equal-arm interferometer through a specific package design of the external environment. The sensor has the advantages of high sensitivity, large dynamic range, small size, easy to manufacture, which will play an important role in the practical application of marine environment monitoring.

Flexible strain sensors are essential components of wearable smart devices that perceive and respond to strain stimulations. However, the sensitivity and response time of most sensors require further improvement to detect subtle strains related to human bodies. Herein, an ultrasensitive flexible optical sensor with fast response time has been built based on a microfiber coupler encapsulated in polydimethylsiloxane. Benefiting from dramatic change of coupling ratio of the microfiber coupler under weak strain, this flexible strain sensor exhibits ultrahigh strain sensitivity (gauge factor, GF = 900), low detection limit (0.001%), ultrafast response time (<0.167 ms), wide sensing range (0.45%), and superior durability and stability (>10 000 cycles). Real‐time capturing and recognizing of respiration, broadband sound signals, and pulse waves at different sites of human body have been demonstrated based on this highly sensitive microfiber coupler sensor. Moreover, simultaneous detection of the wrist pulse and human voice has been achieved based on the frequency division multiplexing technology. This flexible photonics strain sensor could serve as the prototype of ultrasensitive flexible optical sensors with fast response time for the development of high performance and wearable healthcare devices. An ultrasensitive flexible optical sensor with fast response time is built based on a microfiber coupler encapsulated in polydimethylsiloxane. This flexible sensor realizes the monitoring of weak motion, sound signals, and pulse waves. The proposed flexible microfiber coupler sensor shows great potential for human–machine interfaces and soft robots.

An optical fiber coupler is a simple and fundamental component for fiber optic technologies that works by reducing the fiber diameter to hundred nanometers or several micrometers. The microfiber coupler (MFC) has regained interest in optical fiber sensing in recent years. The subwavelength diameter rationales vast refractive index (RI) contrast between microfiber “core” and surrounding “cladding”, a large portion of energy transmits in the form of an evanescent wave over the fiber surface that determines the MFC ultrasensitive to local environmental changes. Consequently, MFC has the potential to develop as a sensor. With the merits of easy fabrication, low cost and compact size, numerous researches have been carried out on different microfiber coupler configurations for various sensing applications, such as refractive index (RI), temperature, humidity, magnetic field, gas, biomolecule, and so on. In this manuscript, the fabrication and operation principle of an MFC are elaborated and recent advances of MFC-based sensors for scientific and technological applications are comprehensively reviewed.

Integrating materials onto optical fibers, enabling optical signal tuning during low-loss light transmission, is essential in optical communications, biosensors, and implantable devices. Such tuning, based on light-matter interaction, requires tight physical and optical contact between materials and fibers. However, large surface curvature (>10 5  m −1 ) of fiber makes it challenging for most materials to transfer onto fibers with tight contact, due to insufficient small-range forces. This induces weak light-matter interaction and ineffective optical coupling. Here, we propose a general strategy for conformal integration of nanomembranes—including metals, oxides, semiconductors, and polymers—onto microfibers. This integration relies on engineered elastic and surface energies between nanomembranes and fibers, enabling tight wrapping. We demonstrate homogeneous and inhomogeneous nanomembranes conformally integrated on microfibers, which are further developed into sensors, modulators, filters, and photodetectors as plug-and-play devices. Our study provides a versatile platform for integrating multifunctional materials on fibers, enabling health monitoring and on-fiber photonic computing. Integrating functional materials with microfibers for optical tuning remains challenging. Here, the authors present a conformal strategy to integrate homogeneous and inhomogeneous nanomembranes—including metals, oxides, semiconductors and polymers—onto microfibers, which are further developed into on-fiber sensors, modulators, filters, and photodetectors.

A highly sensitive refractive index sensor with low detection limit based on an asymmetric optical microfiber coupler is proposed. It is composed of a silica optical microfiber and an As2Se3 optical microfiber. Due to the asymmetry of the microfiber materials, a single-notch transmission spectrum is demonstrated by the large refractive index difference between the two optical microfibers. Compared with the symmetric coupler, the bandwidth of the asymmetric structure is over one order of magnitude narrower than that of the former. Therefore, the asymmetric optical microfiber coupler based sensor can reach over one order of magnitude smaller detection limit, which is defined as the minimal detectable refractive index change caused by the surrounding analyte. With the advantage of large evanescent field, the results also show that a sensitivity of up to 3212 nm per refractive index unit with a bandwidth of 12 nm is achieved with the asymmetric optical microfiber coupler. Furthermore, a maximum sensitivity of 4549 nm per refractive index unit can be reached while the radii of the silica optical microfiber and As2Se3 optical microfiber are 0.5 μm and a 0.128 μm, respectively. This sensor component may have important potential for low detection-limit physical and biochemical sensing applications.