Improved Numerical Calculation of the Single-Mode-No-Core-Single-Mode Fiber Structure Using the Fields Far from Cutoff Approximation

Abstract

Multimode interferometers based on the single-mode-no-core-single-mode fiber (SNCS) structure have been widely investigated as functional devices and sensors. However, the theoretical support for the sensing mechanism is still imperfect, especially for the cladding refractive index response. In this paper, a modified model of no-core fiber (NCF) based on far from cut-off approximation is proposed to investigate the spectrum characteristic and sensing mechanism of the SNCS structure. Guided-mode propagation analysis (MPA) is used to analyze the self-image effect and spectrum response to the cladding refractive index and temperature. Verified by experiments, the performance of the SNCS structure can be estimated specifically and easily by the proposed method.

Discrete Self-Imaging in Small-Core Optical Fiber Interferometers

“…It should be noted that achieving 100% perfect self-imaging (the normalized power is equal to 1) is impossible due to phase mismatches [36]. These features are consistent with the results in [34], [35], indicating that an explicit self-imaging of the input field is formed and the self-imaging period LZ for SNCS is equal to 58.64 mm. The result of LZ = 58.64 mm is in reasonable agreement with the result of LZ = 58.24 mm calculated by (2).…”

“…All the simulations were performed in the scalar mode, under the assumption that the linear polarization (LP) approximation is valid for the fiber due to a relatively small difference between the core and cladding nco (1.451) and ncl (1.445). The LP approximation is also suitable for the NCF (SNCS) due to the mode fields far from cutoff [35], although the difference between ncl and nair is relatively large. In the BPM simulation of the neff, TIP, LID and on-axis intensity, the mesh size is 0.05 µm along the X and Y directions and 1 µm along the Z direction.…”

“…The derivation of (2) and (19) is based on an approximate expression for the neff of the core modes related to the roots of equation 0 = 0 [33]. Although there were some improvements, the simulation of SMS and SNCS in most publications are still based on approximate expressions for the core modes [35], [37]. To the best of our knowledge, there has been no (approximate) analytical expression for the cladding modes in optical fiber to date.…”

Composite Modal Interferometer for Simultaneous Triple-Parameter Measurement Based on Cascaded No-Cladding Fiber and Thin-Core Fiber Structure

“…It is noted that the thermo-optic coefficient of silica is greater than its thermal expansion coefficient. Thus the contribution of fiber length on temperature response is generally ignored, and the above wavelength sensitivities are mainly caused by the thermo-optic effect of fibers [32]. Moreover, it is noticed that the intensity (visibilities) of fringes is positive (negative) proportional to the variation of temperature.…”

Optical Sensing Using Fiber-Optic Multimode Interference Devices: A Review of Nonconventional Sensing Schemes

“…These quantities, by definition, are implicit functions of the RI of the cladding of the MMF. Recently, an effort has been made to provide more refined analytical expressions for the effective propagation constant of each mode in the MMF, e.g., using the so-called “far from cutoff” approximation [ 16 , 17 ]. These new expressions are equivalent to the ones described here, but the RI of the cladding of the MMF appears explicitly in the estimations of .…”