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.

An ultra-high sensitivity refractive index (RI) sensor, based on an in-line fiber Mach-Zehnder interferometer (MZI) and utilizing a special 40 μm fiber which has been specifically designed for the purpose, supporting only the LP ₀₁ and LP ₁₁ modes propagating in the fiber, has been proposed and numerically demonstrated in this work. The all-fiber MZI, based on LP ₀₁ and LP ₁₁ mode interference, was fabricated by utilizing a special design with two gradually tapered joints at both sides of this multi-clad thin diameter fiber (TDF). When the TDF diameters varied from 40 μm to 5 μm, a sudden change in the direction of the wavelength shift in the transmission spectrum of the TDF-MZI devices was observed both in liquid RI and air relative humidity (RH) monitoring experiments, as the surrounding RI or RH was seen to increase monotonically. Ultra-high sensitivity values of 5942.8nm/RIU (1.335-1.342) and 21292.2nm/RIU (1.4000-1.4025) were obtained in the experiments carried out, through the use of a tapered TDF-MZI device with TDF diameters of ~5 μm and ~9 μm, respectively. The maximum RH sensitivity of 1.084nm/%RH was obtained by the use of the TDF-MZI device with a TDF diameter of ~12 μm. A compact all-fiber TDF-MZI sensor was thus created with an overall sensing length of <; 4mm, showing the advantages of high sensitivity, low loss, and flexibility in the tunable monitoring direction of the wavelength shift. This design is well suited to various applications, where the high sensitivity RI and RH measurements are required at precise locations.

This paper studied the relationship between the temperature/strain wavelength sensitivity of a fiber optic in-line Mach-Zehnder Interferometer (MZI) sensor and the wavelength separation of the measured wavelength to the critical wavelength (CWL) in a CWL-existed interference spectrum formed by interference between LP01 and LP02 modes. The in-line MZI fiber optic sensor has been constructed by splicing a section of specially designed few-mode fiber (FMF), which support LP01 and LP02 modes propagating in the fiber, between two pieces of single mode fiber. The propagation constant difference, Δβ, between the LP01 and LP02 modes, changes non-monotonously with wavelength and reaches a maximum at the CWL. As a result, in sensor operation, peaks on the different sides of the CWL then shift in opposite directions, and the associated temperature/strain sensitivities increase significantly when the measured wavelength points become close to the CWL, from both sides of the CWL. A theoretical analysis carried out has predicted that with this specified FMF sensor approach, the temperature/strain wavelength sensitivities are governed by the wavelength difference between the measured wavelength and the CWL. This conclusion was seen to agree well with the experimental results obtained. Combining the wavelength shifts of the peaks and the CWL in the transmission spectrum of the SFS structure, this study has shown that this approach forms the basis of effective designs of high sensitivity sensors for multi-parameter detection and offering a large measurement range to satisfy the requirements needed for better industrial measurements.

This paper presents a compact fiber-optic sensor based on a fiber Bragg grating (FBG), partially bonded with thick polyimide (PI) films, for the simultaneous measurement of humidity and temperature. Half of the grating was attached to the PI film, while the rest of the grating remained bare. When the sensor was exposed to humidity and temperature changes, a significant non-uniform stress distribution was introduced along the entire grating. The chirp effect occurs, which causes the main peak of the spectrum to split into different peaks. Depending on the wavelength shift of these different peaks, simultaneous measurement of temperature and humidity could be achieved by solving a matrix equation. This is the first time that such a thick PI film-based FBG sensor has been used for humidity and temperature monitoring. The benefit of the approach is that the sub-millimeter thickness of the PI film is at least one order of magnitude greater than that of the traditional PI coating used in such sensors. The proposed sensing structure with PI films enhances the sensor response to both humidity and temperature significantly. The humidity and temperature sensitivity are achieved as 13.70 pm/%RH and 19.12 pm/°C, respectively, in the experiment.

Cryogenic technologies are becoming essential in future applications in aerospace, quantum computing, cryogenic computing, and superconducting research. A highly sensitive temperature sensor is critical for such cryogenic applications. In this paper, we propose an extrinsic Fabry–Perot interferometer (EFPI) for cryogenic temperature measurement, leveraging the high thermal deformation of the copper sleeve to overcome the sensitivity drop issue in temperatures below 50 K for conventional cryogenic temperature sensors. Experimental results exhibit a temperature sensitivity of 2.836 nm/K throughout a temperature range of 5 K-50 K with a 15.82 μm long EFPI cavity, which is over three orders higher than the recently reported metal-coated FBG. The high sensitivity holds for the entire temperature range from 5 K to 295 K with excellent sensing repeatability. Our device, featuring a compact structure, and capability for multiplexing, is with great promise in cryogenic applications.