Experimental Analysis of Optical Fiber Multimode Interference Structure and its Application to Refractive Index Measurement

Abstract

We investigated a fiber-based multimode interference phenomenon in the wavelength domain by using a white light source and an optical spectrum analyzer. This phenomenon was produced by a larger-core optical fiber joined at both ends with smaller-core optical fibers. We examined the variation of interference wavelength with changes in the length of the larger-core fiber. The interference wavelengths were blue-shifted and the interference signals were sharpened with an increase in the length of the larger-core fiber. The calculated results agreed well with the measured results. Next, we investigated how the input and output fibers with a small core influence the interference signal characteristics. By comparing the amplitude differences of the interference signal we find the conditions of input and output (I/O) fibers for higher sensitivity. In addition, an interference-signal shift was observed by changing the medium surrounding an multimode interference (MMI) structure. The amount of shift increased at a longer wavelength. This leads to the sensitive detection of the refractive index. Finally, a demonstration of the optical fiber refractometer with a multimode interference structure was given by refractive-index measurements of ethanol/water solutions.

High-Sensitivity, Short-Length Optical Fiber Refractive-Index Sensor using a Multimode Interference Structure with an End-Face Mirror

“…Because the RI difference between water and ethanol is 0.0266, the sensitivity estimated using the wavelength resolution of the optical spectrum analyzer (0.05 nm) is 3.4 × 10 -4 , and using the optical power resolution of the optical power meter (0.01 dB), the sensitivity is 1.7 × 10 -5 at this refractive index range. These sensitivities are almost comparable or slightly better than those of conventional two-port MMI RI sensors [3,4]. This high sensitivity clearly indicates that the MMI phenomenon is maintained in almost the same condition as with a normal MMI structure, even with light reflection from a sputtered gold mirror.…”

“…An optical fiber multimode interference (MMI) structure is very simple and enables the sensitive detection of the change in the refractive index of the surrounding medium when using an unclad fiber as a sensor [1][2][3][4]. However, a conventional sensor with an MMI structure needs two ports for light input and output.…”

Multimode-interference-structure optical-fiber temperature sensor with high sensitivity

“…The important characteristic of MMI is that λ 0 can be easily designed to be in the wavelength region used in the corresponding optical-fiber communication system [8]. The intended MMI signal at any λ can be utilized by specifying the MMF length.…”

Investigation of Total Energy Density and Power Flow for Propagating Modes of Square-Core Optical Fiber

“…Optical fibers are used in many well-known fields such as telecommunications [1], [2], computer networks [3], [4] and medicine [5]- [7]. Also, these optical devices have many uses in sensors [8], [9], fiber lasers [10], [11], highfrequency optical scanners [12], scanning nearfield optic/atomic force microscopes (SNOAM) [13], optical recording [14], measurement of the degree of coherence of light [15], optical imaging transport [16], [17], photolithography techniques [18], threedimensional optical memory [11], sensitive detection of the refractive index [19], measurement of piezoelectric vibration [20] and others.…”