An analytical approach about the general type of step-index optical fiber modes and their variations is presented in this paper. The theoretical model is solved through our analytical method. With ...the boundary conditions, a composite equation set is established and solved. The precise results of all modes are achieved with a high-precision numerical solution. In this paper, the key modal qualities contain electromagnetic fields' distributions, the effective refractive index, and its tendencies. And the modal sensitivity is based on the partial derivative of the effective refractive index (<inline-formula><tex-math notation="LaTeX">\partial</tex-math></inline-formula>n eff /<inline-formula><tex-math notation="LaTeX">\partial</tex-math></inline-formula>n i and <inline-formula><tex-math notation="LaTeX">\partial</tex-math></inline-formula>n eff /<inline-formula><tex-math notation="LaTeX">\partial</tex-math></inline-formula>r i ). The derivative rule of special function allows analyzing the sensitivity of modes. Furthermore, four qualitative deductions are derived during the process, including the qualitative analysis of modes and their variation, several of which are supported with previous experiments. Our work is beneficial for exploiting new fiber and researching relative fiber sensors.
In this letter, a strain sensor with high sensitivity enhancement using a special case of Vernier effect is presented. The sensor configuration is composed of two-fiber loop mirrors in a cascaded ...configuration with opposite strain responses when individually characterized. Thus, the enhanced Vernier effect is explored, which is the most sensitive of three possible cases Vernier effect. Here, the Vernier response depends on the difference between the sensitivities of each Hi-Bi optical fiber. In addition to this, the fundamental and the first harmonic were also explored. The results obtained are a strain sensitivity of (13.3 ± 0.3) pm/<inline-formula> <tex-math notation="LaTeX">\mu \varepsilon </tex-math></inline-formula> for the carrier, (80.0 ± 0.3) pm/<inline-formula> <tex-math notation="LaTeX">\mu \varepsilon </tex-math></inline-formula> or the Vernier envelope of the fundamental case and (120 ± 1) pm/<inline-formula> <tex-math notation="LaTeX">\mu \varepsilon </tex-math></inline-formula> for the Vernier envelope of the first harmonic. The first harmonic could achieve a magnification factor of 8. Considering that the optical interrogation system allows a minimum resolution of 0.02 nm, the minimum measurement step achievable is <inline-formula> <tex-math notation="LaTeX">0.2~\mu \varepsilon </tex-math></inline-formula>. This work proves the possibility of applying the concept of enhanced Vernier effect to fiber loop mirrors, obtaining higher sensitivity than a standard fiber loop mirror alone. Besides, the sensitivity can be increased through the usage of harmonics of the Vernier effect. Moreover, the use of large interferometers allows a better discretization of the envelope, which implies a greater ease of analysis.
We propose a novel polarization-maintaining supermode fiber containing quasi-elliptically arranged high-index cores. Numerical simulation indicates that such a fiber could support up to 20 distinct ...modes with different polarization states and spatial orientations. At 1550 nm, all the 20 modes are separated from their adjacent modes with effective refractive differences beyond 10- 4 . We also investigate the influence of parameters on mode number and effective area of the proposed fiber. Wavelength-dependent performance is analyzed subsequently ranging from 1500 to 1630 nm. The proposed fiber could guide 20 modes with low dispersion over the entire C-band. This letter illustrates that the proposed fiber might be a promising candidate for mode division multiplexing to enhance optical transmission capacity.
In this letter, we demonstrate the enhancement of the Rayleigh back-scattering signal in germanium-doped-core silica fibers, using high resolution Rayleigh fiber sensor. Experiments are performed to ...estimate the thermo-optic coefficients <inline-formula> <tex-math notation="LaTeX">\text{C}_{\mathrm {T}} </tex-math></inline-formula> for different germanium dioxide (GeO 2 ) concentrations. It was demonstrated that, the thermo-optic coefficients increase with the (GeO 2 ) doped concentrations.
In this letter, a nonlinear semi-analytical model (NSAM) for simulation of few-mode fiber transmission is proposed. The NSAM considers the mode mixing arising from the Kerr effect and waveguide ...imperfections. An analytical explanation of the model is presented, as well as simulation results for the transmission over a two mode fiber (TMF) of 112 Gb/s using coherently detected polarization multiplexed quadrature phase-shift-keying modulation. The simulations show that by transmitting over only one of the two modes on TMFs, long-haul transmission can be realized without increase of receiver complexity. For a 6000-km transmission link, a small modal dispersion penalty is observed in the linear domain, while a significant increase of the nonlinear threshold is observed due to the large core of TMF.
Reflector-less biosensors based on optical fibers enable the detection of the refractive index (RI) variations, in reflection mode, without the need for any reflective element (such as a grating or ...an interferometer). The sensing system works by interrogating the wavelength shift occurring in the Rayleigh scattering signal at each location of a thinned fiber through an optical backscatter reflectometer (OBR); a high-scattering fiber is required in order to compensate for the propagation losses. The approaches reported so far make use of etched fibers, which are however fragile and require tens of minutes of fabrication, and U-bent fibers which are incompatible with in-situ devices. In this article, we present the working principle and main results of reflector-less biosensors fabricated through shallow tapering; this concept allows the fastest fabrication (∼20 seconds through a CO 2 automated laser splicer) reported so far, and allows maintaining a compact and robust form factor as the fiber maintains a diameter >28 μm (>22% of the initial thickness). The fabrication process allows achieving a peak sensitivity up to 1.33 nm/RIU (1.05 nm/RIU average over 5 tapers), using MgO-nanoparticle doped fibers. The tapered biosensors have been functionalized through the silanization method, in order to detect CD44 protein (from 100 fM to 10 nM), a breast cancer biomarker. The results show a successful detection with a low limit of detection (16.4 pM) and high specificity over two controls.
It is essential to carry out real-time position and speed monitoring of trains to ensure the safety of railway operation. In this letter, the phase-sensitive optical-time-domain reflectometry ...(Φ-OTDR) is demonstrated to overcome the drawbacks of the track circuit, which is widely used and has the risk to be damaged by lightening, for the first time. A long sensing cable, buried nearby to two parallel railways, is used to detect the vibration signals generated by trains with Φ-OTDR and the real-time vibration signals of the trains are extracted/quantified by wavelet denoising. With the edge detection by normalized sliding variance, the operation status of two nearby trains, including their relative position and speed, are obtained over a 10.2-km measurement length in real time. This letter offers a new passive way for safety monitoring of railway operation.
Universal fiber is an optical fiber that supports both multimode and single-mode transmission. It is a multimode fiber with a mode field diameter of the fundamental mode roughly matching that of a ...standard single-mode fiber. In today's short-distance communications, both multimode fiber and single-mode fiber are used. Universal fiber can accommodate the needs of single-mode and multimode transmission so that end users can take advantage of the cost and performance benefits of each transmission type. In this paper, we present the design and properties of universal fiber, as well as its transmission performance for 100G systems. We also explore several application scenarios where the fiber can be utilized. In particular, we illustrate how the fiber can be used in 5G wireless fronthaul applications to meet the current needs while providing a path for future upgrades. Testing results and discussions of practical issues are also presented.
Twist detection is of great significance in industrial production, aerospace, biomedicine, and other fields. Aiming at the problem of the low sensitivity of the twisted sensor, this article proposed ...and prepared a kind of fiber twist sensor with high sensitivity based on Sagnac interference. The sensitive area of the sensor is a helical elliptical-core polarization maintaining (HEPM) fiber, the optical fiber with an elliptical core has linear birefringence property, and when the fiber is rotated, it has circular birefringence property. The HEPM fiber is plugged into the light loop to form Sagnac interference. Since circular birefringence is proportional to the degree of fiber distortion, the interference spectral trough will respond to different angles of fiber torsion. In addition, the sensor can not only identify the twist direction but also maintain good stability in different twist directions. The experimental results show that the free spectral range (FSR) of the interference spectrum increases and the sensitivity of twist sensing increases as the length of the used HEPM fiber decreases. The maximum twist sensitivity that can be achieved is 8740.64 pm/(rad/m) when the length of HEPM fiber is 15 cm in the twist range of plus or negative 120°, which is higher than the sensitivity of existing studies. The proposed sensor can be used for high-sensitivity distortion sensing. The temperature stability of the sensor is studied experimentally, and a sensing matrix is established that can simultaneously measure the torsion and temperature to eliminate the error caused by temperature. In summary, the proposed sensor not only has high sensitivity but also has an excellent linear response and can identify the twist direction, which has great potential in practical distortion detection.
This paper presents a linear least squares method for fiber-longitudinal power profile estimation (PPE), which estimates the optical signal power distribution throughout a fiber-optic link at a ...coherent receiver. The method finds the global optimum in the least squares estimation of the longitudinal power profiles; thus, its results closely match the true optical power profiles and locate loss anomalies in a link with high spatial resolution. Experimental results show that the method achieves accurate PPE with an RMS error of 0.18 dB from OTDR. Consequently, it successfully identifies a loss anomaly as small as 0.77 dB, demonstrating the potential of a coherent receiver in locating even splice and connector losses. The method is also evaluated under WDM conditions with optimal system fiber launch power, highlighting its feasibility for use in practical operations. Furthermore, the fundamental limit for stable estimation and the spatial resolution of least-squares-based PPE are quantitatively discussed in relation to the ill-posedness of the PPE by evaluating the condition number of the nonlinear perturbation matrix.
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