Temperature-insensitive intensity-modulation liquid refractive index sensor based on fiber-optic Michelson probe structure

A novel liquid refractive index sensor based on the connected single-mode fiber (SMF), no-core fiber (NCF), four-core fiber (FCF), and silver mirror (SM) to form an SMF–NCF–FCF–SM Michelson probe structure is proposed and fabricated. The change of light field in the probe structure has been simulated by the light-beam propagation method. The theoretical results show that light is excited in the NCF and couples into the cores and cladding of FCF at the junction of NCF and FCF. The interference fringes are generated between the cladding modes and core modes of FCF. The sensitivities of the probe in NaCl, sucrose, and glycerol are 171.75 dB/RIU, 121.41 dB/RIU, and 207.50 dB/RIU, respectively. The temperature sensitivity is 0.05 nm/°C, and the intensity change of temperature (≤0.046 dB/°C) is very small and has little effect on the liquid refractive index. Thus, the cross-sensitivity of temperature for the liquid refractive index can be removed. The proposed probe structure has the advantages of easy fabrication, good stability, and linear response, having potential application in the liquid refractive index monitoring environments.

Ultrasensitive Gas Refractometer Using Capillary-Based Mach–Zehnder Interferometer

In this paper, we report a capillary-based Mach–Zehnder (M–Z) interferometer that could be used for precise detection of variations in refractive indices of gaseous samples. The sensing mechanism is quite straightforward. Cladding and core modes of a capillary are simultaneously excited by coupling coherent laser beams to the capillary cladding and core, respectively. An interferogram would be generated as the light transmitted from the core interferes with the light transmitted from the cladding. Variations in the refractive index of the air filling the core lead to variations in the phase difference between the core and cladding modes, thus shifting the interference fringes. Using a photodiode together with a narrow slit, we could interrogate the fringe shifts. The resolution of the sensor was found to be ~5.7 × 10−8 RIU (refractive index unit), which is comparable to the highest resolution obtained by other interferometric sensors reported in previous studies. Finally, we also analyze the temperature cross sensitivity of the sensor. The main goal of this paper is to demonstrate that the ultra-sensitive sensing of gas refractive index could be realized by simply using a single capillary fiber rather than some complex fiber-optic devices such as photonic crystal fibers or other fiber-optic devices fabricated via tricky fiber processing techniques. This capillary sensor, while featuring an ultrahigh resolution, has many other advantages such as simple structure, ease of fabrication, straightforward sensing principle, and low cost.

Analysis of FEAST Spectral Approximations Using the DPG Discretization

A filtered subspace iteration for computing a cluster of eigenvalues and its accompanying eigenspace, known as “FEAST”, has gained considerable attention in recent years. This work studies issues that arise when FEAST is applied to compute part of the spectrum of an unbounded partial differential operator. Specifically, when the resolvent of the partial differential operator is approximated by the discontinuous Petrov–Galerkin (DPG) method, it is shown that there is no spectral pollution. The theory also provides bounds on the discretization errors in the spectral approximations. Numerical experiments for simple operators illustrate the theory and also indicate the value of the algorithm beyond the confines of the theoretical assumptions. The utility of the algorithm is illustrated by applying it to compute guided transverse core modes of a realistic optical fiber.

Electric Field and Dispersion Characteristic Calculations of Glass Tube Waveguides Filled with Biological Substances

This study presents calculation of dispersion characteristics in the frequency range 1–100 GHz as well as electric field distributions in an open cylindrical waveguide with a central channel. The waveguide is made of glass material. The channel can be either empty or filled with blood plasma or blood cells. We investigated two kinds of electromagnetic (ЕМ) waves, the “tube” and “core” modes, each having a different structure of their electric fields. In the current study, the analysis focused on the fundamental and the first higher hybrid magnetic and electric “tube” modes. The fundamental “tube” mode that propagates in the waveguide filled with blood plasma is characterized by a very small loss at frequencies above 65 GHz. Meanwhile, the first higher mode suffers from strong attenuation in the same frequency range. This calls for finding ways to improve the waveguide’s broad-bandwidth. Our approach involves determining the dependence of this parameter on the inner radius of the waveguide. Extremes of the waveguide’s broad-bandwidth are observed at certain values of its inner radius. When the waveguide is filled with blood plasma or blood cells, the electric fields of the magnetic “tube” mode concentrate around the channel, and the electric field intensity decreases with the propagation of this mode along the waveguide, i.e., with increase of coordinate z. If the channel is filled with blood cells, the electric field of the hybrid magnetic “core” mode is concentrated in the center of the waveguide. This mode is characterized by a large attenuation h”, which reaches 500 m−1 at 30 GHz.

Controllable all-fiber generation/conversion of circularly polarized orbital angular momentum beams using long period fiber gratings

Mode-division multiplexing (MDM) is a promising technology for increasing the data-carrying capacity of a single few-mode optical fiber. The flexible mode manipulation would be highly desired in a robust MDM network. Recently, orbital angular momentum (OAM) modes have received wide attention as a new spatial mode basis. In this paper, we firstly proposed a long period fiber grating (LPFG) system to realize mode conversions between the higher order LP core modes in four-mode fiber. Based on the proposed system, we, for the first time, demonstrate the controllable all-fiber generation and conversion of the higher order LP core modes to the first and second order circularly polarized OAM beams with all the combinations of spin and OAM. Therefore, the proposed LPFG system can be potentially used as a controllable higher order OAM beam switch and a physical layer of the translating protocol from the conventional LP modes communication to the OAM modes communication in the future mode carrier telecommunication system and light calculation protocols.