This paper proposes a novel detection scheme simultaneously to measure strain and temperature, based on a simple to construct device using a section of a specially designed few-mode fiber (FMF). The parameters and index profile of the FMF used as the key sensor element are such that the fiber supports only the LP01 and LP02 modes. The propagation constant difference between LP01 and LP02 modes Δβ has a maximum corresponding to the critical wavelength (CWL) in the fiber transmission spectrum. Because the two peaks located closest to the CWL from both sides, left peak 1 and right peak 1, shift in opposite directions, with different sensitivities under axial strain and temperature variations, the FMF device is capable of measuring the strain and temperature simultaneously. A theoretical analysis has been carried out as part of the design process and the experimental results obtained are found to agree well with the theoretical predictions. The characteristics of this sensor scheme are discussed in light of other competing approaches to simultaneous temperature and strain monitoring and are found to show advantages that suit several practical applications including compactness, ease of fabrication and implementation, relatively high sensitivities, and low cost.

In this paper, an all fiber optic sensor based on a Mach-Zehnder interferometer (MZI) has been proposed for the simultaneous measurement of underwater pressure and temperature, utilizing a dual-mode fiber (DMF) which has been specially designed, and supporting only the LP ₀₁ and LP ₀₂ modes propagating in the fiber. In this design, an in-line MZI sensor that was constructed by splicing a DMF between two pieces of single mode fibers, shows a critical wavelength (CWL) which exists in the transmission spectrum of the LP ₀₁ -LP ₀₂ mode interference. Since the two peaks, located closest to the CWL (and from both lower and higher wavelengths), shift in opposite directions and show different sensitivities under temperature and water pressure variations, the DMF-MZI sensor is capable of measuring both the water pressure and the temperature simultaneously. The CWL-based interference spectrum is stable with the variation of underwater salinity or impurities seen around the fiber surface and independent of the polarization states of the transmission light. As a result, in the operation of the DMF-MZI sensor, underwater pressure and temperature sensitivities increase significantly, when the peak wavelengths are close to that of the CWL. A theoretical analysis has been developed and used to predict that the sensitivities of this specific DMF-MZI structure which can be further improved by increasing the physical length of the DMF and by adjusting the position of the first left/right peak to be closer to the critical wavelength. This co-located, multi-parameter all-fiber sensor developed in this way and showing relatively high sensitivity is easy to implement in the underwater environment. It does not require a complex shell design and the peaks nearest to a CWL are convenient, allowing easy identification and detection, thereby providing a large measurement range to satisfy the requirements of practical marine and fresh water measurements.

This paper reported a label free, all-fiber static pressure sensor constructed by cascading two single mode fiber(SMF) -- few-mode fiber(FMF) – single mode fiber(SFS) critical wavelength structures. Vernier effect was adopted to magnify the static pressure wavelength sensitivity of the interference peaks and the superimposed transmission spectrum has an envelope which the free spectrum range varies with wavelength and reaches maximum when approaching the envelope critical wavelength (CWLE). Thus, the envelope peaks near the CWLE in the transmission spectrum with the modified Vernier effect is label free, which is easy to recognize and entirely different from the envelope peaks in a transmission spectrum with uniform free spectrum range. Both theoretical and experimental results show that the envelope peaks near the CWLE exhibit different static pressure and temperature wavelength sensitivities, and the wavelength sensitivity increase significantly when the envelope peak approaching the CWLE. The experimental results show that the left envelope peak 1 and the left envelope peak 2, which numbered from the peak on the left side and closest to the CWLE, have different static pressure sensitivities of 4.072 nm/MPa and 2.649 nm/MPa, respectively, and temperature sensitivities of 1.753 nm/°C and 1.045 nm/°C, respectively. The performances of this proposed approach can be further improved by optimizing the physical lengths of the FMF employed in the sensing and reference SFS structures, to satisfy the requirements in practical applications, especially the extreme conditions and two-parameter measurement of ultra-high pressure and high temperature.

A novel scheme for the detection of refractive index change based on the mode interference in a special few mode fiber (FMF) is proposed in this paper. The parameters and index profile of the FMF used in the measurement are specifically designed so that only LP ₀₁ and LP ₀₂ modes are transmitted within the operation wavelength, and the propagation constant difference Δβ between these two modes reaches maximum at critical wavelength (CWL). An inline Mach-Zehnder interferometric sensor for the detection of surrounding refractive index (SRI) is constructed by splicing FMF with two sections of conventional single mode fiber. We demonstrated at the first time that the CWL will change monotonically with the change of SRI in the measurement range of 1.316-1.439. The experimental sensitivity of the CWL increases with the increase of SRI. A sensitiviy of 140.626 ± 12.560 nm/RIU in the range of 1.316-1.383 and a maximum sensitivity of 2489.796 ± 190.179 nm/RIU in the range of 1.433-1.439 were obtained, respectively, in the experiment. At present parameters of the etched FMF, the limit of detection of the refractive index is 3.413 × 10 ^-4 RIU in the SRI range of 1.316-1.383, and 1.928 × 10 ^-5 RIU in the SRI range of 1.433-1.439, respectively. The detection of SRI via monitoring the shift of CWL is superior to the traditional detection method via monitoring merely the shift of peak or dip wavelength of the transmission spectrum, since the latter usually suffers from limited measurement range and multivalue problem because it is difficult to identify and follow one specific peak or dip from the transmission spectrum especially if the system needs to reset in practical application. The novel detection scheme based on monitoring the shift of the exclusive CWL of FMF is promising in practical applications.

The intermodal interference between LP ₀₁ and LP ₀₂ modes of few mode fiber (FMF) under bending is studied theoretically and experimentally. The shift of critical wavelength caused by fiber curvature is measured in the experiment and analyzed theoretically for the first time. The sensitivity of the critical wavelength shift to curvature variation has been obtained from the measurement and calculation results. Based on the wavelength shift effect induced by the curvature of FMF, a novel displacement sensor employing a helix coil, as an example of the application, is proposed in this letter. With present fiber and structure parameters, 120-mm displacement with 0.172-nm/mm maximum sensitivity has been achieved by changing the fiber curvature through adjusting the pitch of the helix coil. The effect of curvature-induced wavelength shift provides various potential applications of FMF such as displacement and curvature sensors.

Sensing characteristics of a special few mode fiber (FMF) which supports only the fundamental mode LP ₀₁ and LP ₀₂ mode under bending are theoretically and experimentally studied in this paper. The mode field distributions, effective indices, and propagation constant differences of the FMF under bending are calculated. With the specifically designed fiber parameters, the transmission spectrum of such FMF will show a critical wavelength at 1556.220 nm. We observed at the first time in experiment and later verified theoretically that the critical wavelength will shift with the change of fiber curvature. The sensitivity of the critical wavelength to the curvature has been calculated and measured, respectively. Also the curvature induced wavelength shifts of the interferometric peaks appeared on both sides of the critical wavelength are analyzed theoretically and measured experimentally. There is a good agreement between theoretical calculation and experimental measurement. The behaviors of curvature depended critical wavelength and shifts of the peaks close to the critical wavelength can be utilized in the design of curvature or curvature relevant displacement and vibration sensors, etc.