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An optical fiber directional-bend sensor based on an inline Mach–Zehnder interferometer is proposed and demonstrated. The device consists of a piece of a multimode fiber (MMF) splicing with a polarization-maintaining photonic crystal fiber (PMPCF) and sandwiched by lead in/out single-mode fibers (SMFs). Owing to the larger diameter of the MMF, some high-order modes in fiber are efficiently coupled and transmitted through the PMPCF, and finally interfere with each other in the output SMFs. The experimental results show that a well-defined interference fringe envelope can be obtained in the transmitted spectrum and, when the fiber is bent, both the intensity and the fringe visibility of the interference pattern are changed with the bending curvature. Meanwhile, the bend sensitivities are varied with different bending directions, and the maximum sensitivity is achieved up to − 8.33 d B / m − 1 within the bend range from 0 to 1.7 m − 1 . The proposed device also demonstrates a very low-intensity cross-talk of environment temperature.

We propose a method for simultaneous measurement of strain, temperature, and surrounding refractive index (SRI) using a fiber Bragg grating (FBG) inscribed in dual-mode fiber by femtosecond laser. This FBG not only possesses two Bragg resonances corresponding to LP01 and LP11 modes, respectively, but also shows many strong cladding mode resonances, which are sensitive to SRI. Due to the different responses, the strain, temperature, and SRI can be discriminated simultaneously by measuring the two Bragg resonance wavelengths and the normalized area of cladding mode resonances. The estimated resolutions of this FBG in measuring strain, temperature, and SRI are $\pm 6.3~\mu \varepsilon $ , ±0.8 °C, and ±2.7 $\times 10^{-5}$ RIU, respectively.

A 10 mm-long three-dimensional shape sensor in a single-mode fiber is described and demonstrated experimentally. The sensor is based on a pair of fiber Bragg gratings inscribed at the same location along the fiber axis but offset along different radial directions away from the fiber center. Each offset grating generates cladding mode resonances over a ∼ 20 n m -wide spectral bandwidth, and the two gratings are also offset in period so that their transmission spectra are separated by 40 nm, and thus non-overlapping and fully distinguishable. Directional bending sensitivity results from the differential amplitude response of the cladding mode resonances from the two gratings, depending on the relative orientation of the bend with the azimuthal direction of the grating offsets. It is further demonstrated that both axial deformation and temperature have no influence on the shape measurement as they both only cause a global wavelength shift of the spectra without amplitude change. The experimental results demonstrate that the shape orientation of an object can be unambiguously determined for bend directions covering the full 360° range around the fiber axis with sensitivities of the order of 1 d B / m − 1 and small curvatures between 0 and 1 m − 1 .

A new method for phase-shifted fiber Bragg grating (PS-FBG) inscription in single mode fiber by fusion splicing technique and femtosecond laser is presented. The PS-FBG is produced by exposing the fusion spliced fiber with femtosecond laser through a uniform phase mask. The transmission spectrum of the PS-FBG shows a nonlinear red shift during the inscription process, and two or three main dips can be observed due to the formation of one or two FBG-based Fabry–Perot structures by controlling the exposure intensity and time of the laser. For a peak power density of 4.8×1013 W/cm2, the induced refractive index modulation can reach to 6.3×10−4 in the fiber without sensitization. The PS-FBG's temperature, strain and pressure characteristics are also experimentally studied. These PS-FBGs can be potentially used for multiple wavelength fiber lasers, filters and optical fiber sensors.

A temperature-insensitive miniaturized all-fiber Fabry–Perot interferometer strain sensor was proposed and experimentally studied. This sensor with an air cavity was made by splicing a short section of hollow-core fiber with two standard single mode fibers. The experimental results showed that these sensors have an fringe contrast exceeding 22 and 13 dB for the air cavity with a length of 167.08 and 557.11 μm, and its reflection spectrum shifts linearly to long wavelength by increasing the strain from 0 to 3000 μe with a strain sensitivity of 1.8 and 1.7 pm/μe, respectively. This strain sensor also showed good repeatability and long time stability, while its temperature sensitivity was only about 0.95 pm/°C in the range of 0°C–180°C for the 557.11 μm air cavity, so there is less strain–temperature cross-sensitivity. Moreover, this sensor was easy to be fabricated and there was no requirement for chemical etching, careful cleaving, and special fusion. It may be potentially used for strain sensing applications with high resolution and miniaturized size. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1510–1514, 2016

A method for precise measurement of the liquid-level by using a fiber loop ring-down technique incorporating a short section of etched fiber is proposed and experimentally demonstrated. The change of liquid-level is obtained by recording the ring-down time τ of light in the fiber loop. The experimental results show that the liquid-level has a well defined linear relationship with the inverse of τ over the range of 1 cm; the relative standard deviation of τ is about 0.21% and correspondingly the least detectable liquid-level change is about 4 µm by using an etched fiber with diameter of 25.1 µm. Moreover, this method has low temperature cross-sensitivity when the refractive index of liquid is higher than that of the fiber cladding, so it can be employed for liquid-level measurement with high resolution.

The lower order cladding mode resonances of a fiber Bragg grating (FBG) are sensitive to fiber bending but their spectral density makes their response to bending very complex. In this work we present a simple method to reduce and control the number of low order cladding mode resonances via FBGs written in a two-mode fiber (TMF) with an ultrafast laser. Owing to the larger core size of the TMF, a slight break of the cylindrical asymmetry of the grating patterns can be induced when using femtosecond side-irradiation with a small change in the writing condition. This allows us to control the mode families coupled by the grating, and in particular to those modes that have positive or negative bending responses along certain bend directions. Experimental results demonstrate that several lower-order neighboring-cladding mode pairs coupled by the asymmetric TMFBG have antagonistic loss responses (by several dB) for different bending directions, thus allowing full 2D bending measurements with many applications in shape sensing. Finally, this device has similar advantages as tilted FBGs, i.e. temperature de-correlation and the possibility of increasing the signal to noise ratio by averaging simultaneous measurements on several pairs of resonances.

We present and experimentally demonstrate a highly sensitive sensor for simultaneously measuring the refractive index (RI) and temperature based on a multipath fiber Mach-Zehnder interferometer. The sensor is fabricated by sandwiching a segment of weak-coupling seven-core fiber (SCF) with two short multimode fibers, and then splicing it with lead-in and lead-out single-mode fibers, respectively. Six outer cores of the SCF are half-etched chemically for enhancing the interaction between light and matter. A high-quality transmission spectrum with 23 dB fringe visibility is obtained. Due to the strong interaction between the outer core modes and cladding modes with the surrounding medium, the proposed fiber structure exhibits not only an extremely high RI sensitivity of -1802.26 nm/RI unit from 1.427 to 1.442, but also a superior temperature sensitivity of 82 pm/°C from 10°C to 90°C. Moreover, RI and temperature can be discriminated simultaneously by measuring the central wavelength shifts of two transmission notches. This sensor has outstanding advantages of high sensitivity, easy fabrication, simple structure, and low cost, and may find applications in multiparameter highly sensitive sensing.

Cylindrical-vector beams (CVBs) with axial symmetry in polarization and field intensity are gathering increasing attention from fundamental research to practical applications. However, a majority of the CVBs are generated by modulating light beams in free space, and the temporal durations are far away from the ultrafast regime. Here, an ultrafast all-fiber based CVB laser is demonstrated via intermodal coupling in two mode fibers. In the temporal domain, chirp-free pulses are formed with combined actions of the ultrafast saturable absorption, self-phase modulation, and anomalous dispersion. In the spatial domain, the lateral offset splicing technique and a two mode fiber Bragg grating are adopted to excite and extract CVBs, respectively. The ultrafast CVB has an annular profile with a duration of 6.87 ps and a fundamental repetition rate of 13.16 MHz, and the output polarization status is switchable between radially and azimuthally polarized states. This all-fiber-based ultrafast CVB laser is a simple, low-cost source for diversified applications of nanoparticle manipulation, high-resolution imaging, material processing, spatiotemporal nonlinear optics, etc.

Based on the theory of Jones matrix and the condition of eigenmode self-consistency, we analyze the effect of linear stress birefringence of prismon the beam polarization (described with the ellipticity and loss of the eigenmodes) and null drift (described with the output frequency difference of the eigenmodes) in a TRP resonator. It is found that the linear stress birefringence of prisms can cause larger polarization ellipticity for both CW (clockwise) and CCW (counterclockwise) beams. The results may be useful for designing and optimizing the structure of super high precision TRP ring laser gyroscopes.

The deduction of the augmented 5×5 matrix describing all the possible angular and decentration misalignment perturbation sources on a spherical prism plane during the refraction is presented in detail for the first time. Combining the augmented matrices and the condition of eigenmode self-consistency, the optical axis perturbation properties (described with the optical axis perturbation sensitivity factor SD1, ST1, SD2, and ST2) in a total reflection prism ring resonator are also analyzed. The singular points of the optical axis perturbation sensitivity factor are found at L/R=0.417 for the cases where the misalignments may originate from distinguished type of prisms P1 and P4 in the resonator, respectively. It is shown that the small errors of the prism can cause large optical axis disturbance in the position of the perturbed singular point and its nearby area, and the disturbance for the case where the misalignments originate from prism P1 is more violent than that of prism P4. With some upgrades to find and avoid the singularity of the optical axis in the total reflection prism ring resonator structure successfully, we expect this work will have great potential for designing and optimizing the structure of super-high-precision ring laser gyroscopes.

We present a theoretical model to quantitatively analyze the influence of the eigenmode polarization deviation (described with ellipticity parameter) on the output Sagnac frequency difference in two nonplanar ring resonators with different mirror arrangements. The theoretical and numerical analysis results prove that the deviation of the eigenmode polarization from the ideally circular polarized state will produce a drop of the output Sagnac frequency difference. Furthermore, for the identical ellipticity parameter and incident angles, the decline values along the rotation axis A(2) is more obvious than that along axis A(1). In other words, these results show that the output Sagnac frequency difference can be improved effectively by restraining the eigenmode polarization deviation quantitatively and that these analysis findings are useful for designing and optimizing the structure of laser gyroscopes with super-high precision.

Phase-shifted fiber Bragg grating (PS-FBG) inscription in nonphotosensitive single mode fiber (SMF) by the fusion splicing technique and femtosecond laser is reported. Two SMFs are fusion spliced to introduce a refractive index modulation point which acts as a phase shift, then exposing the fusion spliced fiber with femtosecond laser and a uniform phase mask. Two dips can be observed in the transmission spectrum of inscribed grating, and the max induced refractive index modulation can reach to 4.2×10−4 without any fiber sensitization for a peak power density of 4.5×1013 W/cm2. The annealing tests show that type I PS-FBG is successfully inscribed. This type of grating also shows good strain and pressure characteristics. Such PS-FBGs can be potentially used for optical fiber lasers, filters and sensors.

A novel method for simultaneous measurement of temperature and strain using a single phase-shifted fiber Bragg grating (PS-FBG) is proposed. The PS-FBG is produced by exposing the fusion-spliced fiber with a femtosecond laser and uniform phase mask. Due to the non-uniform structure and strain distribution in the fusion-spliced region, the phase-shift changes with different responses during increases to the temperature and strain; by measuring the central wavelengths and the loss difference of two transmission dips, temperature and strain can be determined simultaneously. The resolutions of this particular sensor in measuring temperature and strain are estimated to be ±1.5 °C and ±12.2 µe in a range from −50 °C to 150 °C and from 0 µe to 2070 µe.

A novel method for simultaneous measurement of strain and high temperature using a Type II fiber Bragg grating (FBG) and a miniature fiber Fabry-Perot interferometer (MFFPI) is proposed. The MFFPI is produced by fusion splicing a short section of quartz capillary tube with two single-mode fibers, and then it is exposed by a focused femtosecond laser and a phase mask to inscribe a Type II FBG nearby. The reflection spectrum of this sensor is the superposition of the reflection spectrum of the FBG and the interference fringe of the MFFPI. This sensor shows perfect high-temperature and strain responses. Because of the different responses to the uniform variations of strain and temperature, by measuring the reflection peak of FBG and one of the interference dips of the MFFPI, strain and temperature can be simultaneously determined. The resolutions of this particular sensor in measuring strain and temperature are estimated to be ±8.4 μϵ and ±3.3°C, respectively, in the range from 0 to 1122 μϵ and from 23°C to 600°C.

The ultralong time (a few hours) response properties of magnetic fluid using etched optical fiber are visualized and investigated experimentally. The operating structure is made by injecting magnetic fluid into a capillary tube that contains etched single-mode fiber. An interesting extreme asymmetry is observed, in which the transmitted light intensity after the etched optical fiber cannot reach the final steady value when the external magnetic field is turned on (referred to as the falling process), while it can reach the stable state quickly once the magnetic field is turned off (referred to as the rising process). The relationship between the response times/loss rates of the transmitted light and the strength of the applied magnetic field is obtained. The physical mechanisms of two different processes are discussed qualitatively.