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In this paper, we realize tunable Fano and electromagnetically induced transparency (EIT)-like resonances in the outputs of a nested feedback ring resonator. Compared with the traditional add-drop ring resonator, the feedback arm introduces interference between different resonance modes. Using coupled mode theory, we reveal the output Fano resonances are the interference of discrete modes and continuum modes, whereas the EIT-like resonance only appears in the over-coupled state. We experimentally confirm the two effects using optical fiber ring resonators, finding that the Fano effect can be easily controlled with phase shifts in the feedback arm. Moreover, we achieve the double Fano resonance, which displays an X type lineshape utilizing one ring resonator, predicting it to be a good candidate in wavelength monitor. The proposed simple configuration provides a platform to study Fano and EIT-like effect, which is useful in the optical filter, fast and slow light, optical switching, and optical sensing

The dual-parameter sensors based on topological ring structure are proposed and experimentally demonstrated, which has obvious advantages in relative humidity (RH) and displacement measurement. The topological ring structures are composed of two separated optical microfiber couplers (OMCs) as independent sensing units. The two OMCs are connected by two 30 cm length single-mode fibers (SMFs). The first sensor is formed by introducing OMC into the loop resonator (LR). The second sensor is constructed by introducing OMC into the Sagnac interferometer (SI). The LR-OMC and SI-OMC structures are made by splicing the two ports of OMC with two ports on one side of the different central axis. The coupling coefficient and effective refractive index (ERI) of OMC vary with RH and displacement. The reason for the wavelength shift caused by RH is that water molecules trigger the change ERI of the OMC evanescent. Furthermore, the RH measurement is realized without any humidity-sensitive material. Here, we present a topological ring structure that can measure two parameters discretely, eliminating the problem of RH and displacement crosstalk. The LR-OMC structure and SI-OMC structure have different RH and displacement sensitivity. The proposed topology sensors could find applications in the multi-parameter RH and displacement sensing due to their simple fabrication and low cross-sensitivity.

A polarization-controlled strain fiber sensorwith a low detection limit and high sensitivity is proposed and demonstrated using a helical birefringence fiber loop mirror (HBFLM). The HBFLM is consists of a micro-helix region, a macro-helix region, and a fiber ring mirror (FLM). The two standard single-mode fibers (SMFs) are pre-surrounded by each other and use hydrogen flame heating and stretching techniques to prepare the micro-helix region (MIHR). The pre- rotation turns of the MIHR determine the coupling length. The MIHR is influenced by a twisting force. The purpose of the macro-helix region (MAHR) is to regulate the x- and y-polarizationstates in the fiber (applying stress inducesdouble refraction). At the same time, the envelope of the transmission spectrum is modulated to ultimately control the strain sensor. The polarization control of the strain sensor is realized by using the principle of stress birefringence of the SMF. The reasons for the polarization control spectrum are analyzed theoretically. The strain and temperature characteristics were indicated in the experiment. Polarization-controlled strain sensors are expected to be used in high-precision and varying strain range measurements.

A compact, high-performance bending sensor based on fiber microspheres array (FMA) Michelson interferometer (MI) was experimentally proved. The sensor consists of a refractive index modulation region (RIMR) fused by three microspheres, interference arm and end face protection. The RIMR acts as a modal coupling device between core and cladding modes. Due to the different stress for the RIMR in the middle (lowest point) of the stepper motor or the side (high point) during the bending process, the mode field distribution excited to the cladding is inconsistent which shows a discrepancy in sensitivity. The results show that the bending sensitivity at the lowest point is lower than the high point, which is consistent with the theory. Meanwhile, the variable range method is used to realize the repeatability verification of the sensor. Besides, the relative sensitivity (the ratio of wavelength sensitivity to intensity sensitivity) is defined to realize the resolution of strain and bending.

A high-sensitivity optical fiber sensor for low refractive index (RI) measurement is proposed and experimentally demonstrated. It is fabricated by fused splicing a section of multimode fiber (MMF) between two single-mode fibers (SMFs) with a slight offset at two splicing joints, then the MMF is tapered using fused tapering technology. The offset splicing between SMF and MMF can effectively excite more high-order modes. The tapered multimode fiber (TMMF) can enhance the strength of the evanescent field and promote the interplay between the high-order modes and the surrounding medium. The experimental results indicate that the offset value and the waist diameter of the TMMF are two determinants for RI sensitivity enhancement. The RI sensitivity up to 12145 nm/RIU is achieved over the low RI range of 1.3345-1.339. This proposed high-sensitivity fiber-optic RI sensor may provide a low-cost and effective approach for low RI measurement in many chemical and biological applications.

A large-loop microfiber structure based on an adiabatic microfiber is proposed. By introducing bending into the microfiber, the fundamental mode can be coupled into high-order modes. The intensities of the high-order modes can be enhanced by increasing bending curvature, leading to a multimode interference (MMI) with tunable intensity. The transmission spectrum characteristics are analyzed, and the relative humidity (RH) sensing application of this structure is demonstrated without any sensitive coating materials. The experimental result shows that the wavelength shifts nonlinearly with RH varying from 30% to 80% RH, and the maximum average sensitivity of 205 pm/%RH is obtained in the range of 70% to 80% RH. The large-loop microfiber structure with high RH sensitivity, low loss, and simple fabrication could provide a cost-effective platform for integrating functional materials or micro/nano-structures for constructing high-performance fiber devices of various applications.

A configuration based on a parallel double Fabry–Perot interferometers (FPIs) is proposed and experimentally demonstrated to enhance the temperature sensitivity by Vernier-effect. Two separated FPIs constructed by suspended-core microstructured optical fiber (SCMF) are connected by a 3-dB coupler to form a parallel structure. FPI-2 is used for temperature sensing, while FPI-1 as a reference. With the temperature increasing from 40 °C to 220 °C, both the fine interference fringes and envelope have a linear redshift. This parallel double-FPIs configuration shows an enhanced temperature sensitivity of 153.8 pm/°C with an amplification factor ${M} =14.6$ . Moreover, since the free reference FPI-1 is away from the temperature sensing environment, this simple and novel parallel double-FPIs configuration provides good reversibility and stability during the heating and cooling processes.

This paper presents a low-cost and high-sensitivity temperature sensor based on an isopropanol-sealed multimode microfiber (MMMF). Isopropanol is a kind of material with a high thermo-optic coefficient. Refractive index (RI) of isopropanol changes with the surrounding temperature variation allowing high-sensitivity temperature sensing. By inserting the MMMF into an isopropanol-sealed capillary, a simple and highly sensitive fiber temperature sensor is implemented. Theoretical and experimental results demonstrate that the tapered multimode fiber can efficiently excite high-order cladding modes, the MMMF can enhance the multimode interference and increase the interaction between the evanescent wave and the isopropanol. The characteristics of the measured transmission spectra under different conditions are evaluated. Experimental results demonstrate that the temperature sensitivity increases with the decreasing diameter of MMMF. The maximum temperature sensitivity of −4.12 nm/°C is obtained over the range of 30 °C–50 °C. This packaging technology not only offers a more stable structure but also improves the temperature sensitivity for the thermo-optic effect of isopropanol and the thermal expansion effect of packaging materials.

Tunable coupled resonator induced transparency (CRIT) is observed based on two asymmetrical self-interference ring resonators (SIRRs). The phase shift of the left feedback arm will regulate the output CRIT lineshape, whereas the right FA does not influence the output lineshape. The transmission spectra of the left SIRR will be greatly reshaped with different cross-coupling coefficients, this characteristic is of great use both for optical sensing and designing output lineshape. Instead of adding ring resonators or tuning the uncontrolled multimodes of the whisper gallery resonator, we realize two transparency windows via changing the cross-coupling coefficients of couplers, which provides a convenient and controllable approach for the multichannel filter in optical devices. Moreover, the number of transparency window can be extended via adding more resonators. The proposed structure provides a potential application in the optical filter, optical sensing, slow light, and optical storage.

In this Letter, Autler–Townes splitting and induced transparency windows are observed in a multimode microfiber knot. The microfiber knot is fabricated using tapered single-mode fiber, with the knot position located at the transition area of the tapered fiber. The spectrum, in analogy to Autler–Townes splitting, derives from the mode splitting of two high-order excited modes, which is theoretically explained by the multimode transfer matrix method. Moreover, without adding resonators, two induced transparency windows are realized with the tunable coupling coefficients and phase difference of excited knot modes. The tunable, easily fabricated, compact, and robust microfiber knot has potential applications in optical sensing, filters, slow light, and optical switching.

We propose a method of magnetic-field tuning whispering gallery modes (WGMs) based on a hollow microbubble resonator (HMBR) with Terfenol-D-fixed. WGMs are excited by the evanescent field from a tapered fiber coupling with an HMBR. Both ends of the HMBR are fixed with Terfenol-D and vary with different lengths of the Terfenol-D. The length of the Terfenol-D varies with the external magnetic field for the high magnetostriction coefficient of Terfenol-D. The magnetic field sensitivity of 0.081 pm/mT in the magnetic field range of 0.14 mT–21.8 mT is achieved. The $Q$Q-factor of the HMBR can be regulated up to ${2.07} \times {{10}^4}$2.07×104 with physical stretching HMBR. This work provides a novel tuning whispering gallery mode scheme and a broad application prospect in the fields of optical measurement and precise optical clocks in the future.

We propose a high-temperature sensor based on a suspended-core microstructured optical fiber (SCMF). The sensor is constructed by fusion splicing a piece of SCMF between two sections of multimode fibers (MMFs) which act as light beam couplers. The multimode interference is formed by the air cladding modes and the silica core modes in the SCMF. Fast Fourier transform is adapted to filtering the raw transmission spectra of the MMF-SCMF-MMF structure. The wavelength shift of the dominant spatial frequency is monitored as the temperature varies from 50 °C to 800 °C. The sensitivities of 31.6 pm/°C and 51.6 pm/°C in the temperature range of 50 °C-450 °C and 450 °C-800 °C are respectively achieved. Taking advantage of the compact size, good stability and repeatability, easy fabrication, and low cost, this proposed high-temperature sensor has an applicable value.

A mechanical strain tuning of whispering gallery mode (WGM) in a hollow-peanut-shaped optical microresonator (HPSOM) is demonstrated. The HPSOM is fabricated by fusion splicing two hollow bubbles. The central resonator is formed by the solid structure in the middle, and the near end (far end) resonator is formed by the hollow structure on both sides. The hollow microresonator can improve the strain response-ability compared with the solid micro resonant. The resonator structure and strain response characteristics of the HPSOM are researched theoretically and experimentally researched. It is worth noting that due to the elliptical effect of the central resonator during the manufacturing process, the central resonator cannot confine the light field to the equator and form the WGM. In the coupled spectrum, the WGMs of bilateral HPSOM with low Q-mode are easier to distinguish and monitor. Therefore, a low Q-mode is selected for strain measurement. The testing highest sensitivity is 6.96 pm/μɛ in the strain range 0–25 μɛ. Relative to the large-strain dynamic testing, low-strain testing has broad application prospects in geotechnical engineering testing, structural testing, and engineering geophysical prospecting. Being of general interest, our findings pave the way for improved perception of strain.

A simple, high-sensitivity strain sensor based on a sealed optical microfiber coupler (OMC) was proposed and demonstrated. Affected by the strain, the effective refractive index (RI), microfiber length, microfiber diameter changes the coupling coefficient of the OMC. Due to the uneven force of the port3 and port4, the strain sensitivity after encapsulation will improve, which is proved in the experiment. The mechanical simulation demonstrates that there is a dependence between strain sensitivity and waist diameter. The sensor with a 2.3 μm waist diameter presents a sensitivity of −18.351 pm/μe regarding the variation of axial strain from 10 to 1200μe, which was one order of magnitude higher than that of optical strain sensors based on fiber Bragg gratings. Experimental results demonstrate that the OMC sensor exhibits high sensitivity, strong stability and the capability of simultaneous measurement of strain and temperature.

A hybrid structure sensor based on Michelson and Fabry-Perot fiber-optic interferometer for simultaneous measurement of transverse load and temperature is proposed and experimentally investigated. This structure has the advantages of high sensitivity, low cost and easy fabrication. It consists of lateral offset splicing of single-mode fibers (SMFs) and air-cavity of silica microsphere. The temperature sensitivities of the sensor are 41.5 pm/oC and 12.5 pm/oC in 35 oC-60 oC at two tracking points, respectively. The transverse load sensitivities of the sensor are 0.85 nm/N and 1.09 nm/N in 0 N–3.40 N at two tracking points, respectively. The sensor can be used to concurrently measure transverse load and temperature owing to the different sensitivities of them to the two wavelengths.

We studied a miniature optical fiber sensor based on in-line Fabry-Perot interferometer for strain measurement. The sensor was fabricated by splicing a section of hollow core tube (HCT) between two single mode fibers. The reflective spectrum of the interferometer exhibits a number of resonance wavelength dips owing to destructive interference of two reflective lights. The experiment results demonstrate that, as the strain increased, the resonance wavelength exhibits a redshift behavior. The maximal strain sensitivity obtained by linear fitting is 4.55pm/μe. The device proposed has high potential in construction engineering, industry production, and high speed rail due to its merits of electromagnetic immunity, miniature structure and ease of fabrication.

We provide a novel arithmetic for the calculation of the transmission equation in a structure based on the add-drop resonator. Moreover this method can be applied on all the structures containing add-drop resonators or analogous configurations to simplify the calculation and the light path analysis. The characteristics of the transmission spectra in add-drop interferometer with two different kinds of curves are analyzed. We find out the reason for the different transmission spectra and draw an analogy with the under coupling, over coupling, and critical coupling. These results will provide some useful information and a new track to analyze the structures in this field.

We theoretically demonstrate the transmittion spectra and dispersion characteristics based on the electromagnetically induced transparency like effect in the nested fiber double-ring resonator with the transfer matrix theory; the system which are connected by three directional couplers consists of two inner rings, one outer ring and one straight waveguide. The simulation results show that the tunable group delay can be realized by changing the coupling coefficients. In the NDRR coupled Mach-Zehnder interferometer system, we obtained fast light and slow light simultaneously. By adjusting appropriate parameters, we can archive flat band group delay curve that has a profound application in optical interferometer, optical buffer, optical filter, optical modulator, dynamic or static optical sensing field.

We investigate the phase and group delay in eye-like ring resonator with the effect of different coupling coefficients and attenuation factors. The eye-like structure is composed of two bus waveguides coupling with the outer ring and the two rings coupling together which have the same perimeters. The eye-like ring resonator has two outputs which have different transmission characteristics. In this paper, we measure the group delay of the two outputs through changing the coupling coefficients and the attenuation factors of the inner and the outer ring. The result shows that the two outputs have reverse group delay (superluminal and slow light) which will have potential use in slow light fiber, optical buffers and optical switches.

A stress-insensitive vector curvature sensor based on a single fiber Bragg grating (FBG) is proposed and experimentally demonstrated. The sensor is easily fabricated by encapsulating an FBG on a thin steel plate with ultraviolet glue. When the FBG deviates from the neutral plane, its effective refractive index and grating constant are changed by bending, therefore, the sensor can realize the curvature measurement. The sensitivity of the sensor in convex bending is 553.65 pm/m−1 in the range of 0.4798–1.3569 m−1. The sensor can realize vector measurement of curvature due to the opposite stress direction on the two sides of the neutral plane during bending. The concave curvature sensitivity of the sensor is −811.19 pm/m−1 in the range of 0.4798–1.3569 m−1. Due to the large stiffness of the thin steel plate, the package of the FBG makes it insensitive to small axial stress. The favorable stability of the sensor facilitates its practical application in the measurement of curvature and inclination. This sensor has the advantage of simple manufacture, low cost, and easy industrial production.

A tip packaged high-temperature miniature sensor constructed by a suspended-core optical fiber (SCF) is presented and experimentally demonstrated. An SCF with a short length spliced to the single-mode fiber (SMF) is utilized to build the Fabry-Perot interferometer (FPI). A section of SMF acting as a tip package is spliced to the end of the SCF, which gives the sensor immunity of the external refractive index (RI). The test processes of heating up and cooling down are repeated three times to evaluate the repeatability and hysteresis. The arithmetic means temperature sensitivity increases from 9.42 pm/ °C to 12.51 pm/ °C over the temperature range of 50–800 °C. Moreover, the maximum stability error is 5 °C within the high-temperature 800 °C duration period. This tip packaged sensor with simple fabrication, good repeatability, and good stability within a large dynamic range is advantageous to practical temperature measurement and massive production.

We theoretically investigate an all-fiber interleaver consisting of a two-stage cascaded Mach-Zehnder interferometer (MZI). The simulation results of the all-fiber interleaver presented symmetrical and asymmetrical output spectra and a uniform flat-top spectral response by changing the coupling coefficient of the couplers. In addition, the interleaver proposed can realize arbitrary passband width ratio between the two output ports by adjusting the coupling coefficient of the couplers involved. A near box-shaped spectral response can be generated through setting suitable parameters, at the same time, the passband and stopband of optical interleaver are improved significantly. This interleaver proposed should be useful to realize the deployment of flat-top all-fiber asymmetric interleaver in dense wavelength division multiplexing (DWDM) networks of high spectral efficiency.

A miniature fiber-optic sensor fabricated by fuse splicing a section of hollow silica tube between two sections of single-mode fiber is proposed and experimentally demonstrated for simultaneous measurement of the liquid refractive index (RI) and temperature. The sensitivities of RI and temperature are measured by linear fitting, respectively, and corresponding detecting resolutions are obtained by the sensitivity matrix. The different response sensitivities of two dips to RI and temperature manifest that the structure proposed can be used for simultaneous measurement. The experimental evidence indicates that the miniature fiber sensor with advantages of easy fabrication, ultracompactness, and robustness is insensitive to axial strain.

In this paper, we propose an add-drop resonator Sagnac interferometer filter to achieve tunable dual-channel function. Not only is just one ring utilized to control two channels but also the reconfiguration is viable by phase difference control. Three methods are put forward to tune the bandwidth, the extinction ratio, and the reconfiguration of the channels. The reconfiguration of the dual channel is realized via controlling the phase difference between the two arms of the Sagnac interferometer, avoiding the traditional method of shifting the resonance wavelength. Furthermore, the channels can be extended by adding add-drop interferometers. The flexible and compact filter can provide potential implications in dense wavelength division multiplexing systems and optical switches.