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High-sensitivity fiber-optic interferometric refractive index sensors (FIRSs) have been of interest to researchers due to their potential to fabricate specific physical, chemical, and biological sensors. Fabricating interferometers working near the dispersion turning point (DTP) is an effective approach to improve the sensitivity of FIRSs. However, the group effective refractive index (RI) difference approaching 0 and the ratio of the variation of the effective RI difference to the external RI change being −1 cannot be simultaneously realized in low RI sensing, which restricts the further improvement of sensing sensitivity. Here, dual-path Mach-Zehnder interferometers (MZIs) with unequal geometrical path length are proposed for ultrasensitive RI sensing. The dual-path MZIs contain fiber path and sample path of different geometrical lengths to form the optical path difference. The dual-path MZIs can not only show a turning point where the sensing sensitivity tends to be infinite, which is similar to the previously reported DTP, but also get the constant value −1 for the RI response factor, leading to the result that the dips within an over-500-nm band width around the turning point can achieve high sensitivity reaching 105 nm/RIU level or higher. Ultrahigh sensitivity of −1.26 × 106 nm/RIU has been experimentally demonstrated at the RI around 1.35022. The dual-path MZIs proposed here may enlighten new ideas for developing high-sensitivity FIRSs.

An ultrasensitive Panda-microfiber (PMF)-based multimode interferometer for salinity measurement is proposed and demonstrated. By inserting a section of PMF supporting a few modes into a conventional optical fiber system, an inline interference between the high-order modes of PMF, i.e., ${\rm{TE}}_{{\rm{01}}}$ and ${\rm{TM}}_{{\rm{01}}}$ , is realized. The sensitivity is as high as 2938.16 pm/‰ and its resolution is up to 0.0014‰ (0.0001 S/m), which is two magnitudes higher than other fiber salinity sensors. With the features of compactness, reconfigurability, stability, and compatibility with other fiberized components, PMF has great potential in tunable filtering, multiwavelength lasing, etc.

This paper investigates the transmission spectral characteristics of microfiber MZ interferometer with a knot resonator (MZIKR). The MZIKR exhibits a combined effect of resonance and interference in its transmission spectra. By adjusting the coupling coefficients to convert the relative strength of resonance and interference, the transmission spectra show various shapes, mainly reflected in the direction of fringes. The obtained upward fringe exhibits an extinction ratio of 17 dB and a FWHM of 0.3 nm. The tunable transmission spectrum demonstrated here may have great potential for narrow-band filtering, and the coexisting resonance and interference effect also allows the MZIKR to perform dual-parameters sensing.

Dual-points temperature measurement in seawater is demonstrated based on a seawater temperature sensor assembled by the highly-birefringent elliptical-core fiber (HBECF). The theoretical and experimental transmission spectra of the sensor are studied respectively, which shows good agreement with each other. And a matrix equation is established by analyzing the experiment results, by which the temperature changes can be obtained by measuring the wavelength shifts of spectra. The minimum temperature change that can be detected is 0.016 °C and the measurement error is about 0.1%. The advantages of simple structure, high accuracy, high sensitivity, fast response and anti-electromagnetic interference make the sensor meet the demands of seawater temperature measurement.

It is significant to develop new methods for temperature and salinity measurement in seawater since they are two important parameters in physical oceanography application. In this paper, a new approach for simultaneous measurement of temperature and salinity in seawater based on microfiber Mach-Zehnder (MZ) interferometer with a knot resonator is proposed. The typical transmission spectra show the combined effect of MZ interference and knot resonance. When the temperature and salinity change, both the interference dip and the resonance dip shift linearly. Based on the linear relation, the measurement matrix is established, and the temperature and salinity of seawater sample can be obtained simultaneously by collecting the transmission spectra. The hybrid structure demonstrated here shows advantages of high sensitivity and good accuracy, which may have great potential for temperature and salinity measurement in ocean and other liquid surroundings.

Combined with non-adiabatic tapering and mode field mismatch between two different fibers, a splicing point tapered fiber Mach-Zehnder interferometer is proposed. Theoretically, two sensing units are carefully designed for dual-parameter sensing, and mode exciting and evolution in fibers are analyzed and shown dynamically. In experiment, transmission spectrum is obtained with two sets of clear interference as designed. After a simple capsulation, simultaneous measurement of seawater temperature and salinity are realized with sensitivities of −994.83pm/°C and 290.47pm/‰, respectively. Additionally, short response time of 33ms and good repeatability are also demonstrated. And effects of encapsulation on avoiding strain and press are verified experimentally. The MZI demonstrated here shows advantages of low cost, easy fabrication, simple construction, compact and robust structure, and dual-parameter sensing with high sensitivity and fast response.

A simple, high-sensitivity, and low-cost optical sensor for the measurement of temperature and salinity in seawater is demonstrated based on a microfiber directional coupler. The shifts of sensing dips are highly sensitive to the change of salinity and temperature in seawater with highest sensitivities at 930 pm/‰ and $-$ 160 pm/ $^{\circ}$ C for coupler assembled by 3.79- and 2.20- $\mu\text{m}$ - diameter fibers. The temperature sensitivity can be further improved to $-$ 1130 pm/ $^{\circ}$ C for smaller fibers with diameters of 3.50 and 1.35 $\mu\text{m}$ . To keep the stable structure in liquid, a methodology of coating using ethyl cellulose ethoce film is proposed. By ethyl cellulose ethoce solvent coating on the coupling area, the coupler can be operated stably in seawater. The sensor demonstrated here is easy to construct, is low cost, is high sensitivity, and is compatible with the optical fiber system, which may find applications in developing miniature and multifunction optical sensors used in seawater and other liquid surroundings.

In this paper, an ultrahigh sensitivity sensing in aqueous solution for microfiber modal interferometer (MMI) is realized. The group refractive index (RI) difference between HE11 and HE12 mode will come down to 0 at a certain wavelength, resulting in ultrahigh sensitivity near this wavelength. MMI with different diameters have their individual ultrasensitive wavelength band, which indicates the broad range of optional probing wavelength and more liberal diameter condition on fiber fabrication. In the experimentation, infrared absorption band in water around 1360-1600nm is easy to keep away by adjusting the microfiber diameter. As a result, an ultrahigh sensitivity of 14.95 pm/ppm is realized for sodium nitrate at 1320nm, whose equivalent sensitivity is about 1.26 × 105-nm/RIU for RI, which is much higher than most of the existing naked sensors with magnitude of 102-104nm/RIU.

A new method for seawater temperature sensing based on a high-birefringence elliptic fiber (HBEF) Sagnac loop is proposed in this letter due to the linear relationship between the change of temperature and the shift of interference peak. The transmission spectra of HBEF Sagnac loop at different seawater temperatures are studied theoretically and experimentally, which show good agreement with each other. The results show that sensing sensitivity increases with the increasing HBEF length and the decreasing wavelength, which is just opposite to the dependence of free spectrum range on the fiber length and the probing wavelength. Thus, there may exist an optimal fiber length and probing wavelength for the highest sensitivity and the largest detection range. By optimizing the parameters of probing system, the sensitivity of −472 pm/°C is obtained for temperature sensing, and the detection range from 10 °C to 30 °C is kept with the wavelength of 1310 nm (communication window) and the fiber length of 98.9 cm. Seawater temperature sensors demonstrated here show the advantages of small size, low cost, easy fabrication, high sensitivity, and easy integration with fiber systems.

The microfiber knot-type ring resonator is widely used in sensing, and the resonant mode is an essential concern. In this paper, resonant mode characteristics of microfiber knot-type ring resonator are studied theoretically. Results show that resonant mode split occurs when the deviation angle is large enough under the condition that the input is linearly or elliptically polarized light. Moreover, in the case that the input is partially polarized light, the resonant peak positions of split modes are identical with those in the case of inputting linearly or elliptically polarized light under the same deviation angle. The split mode characteristic is further studied by salinity sensing simulation and experiment. The split modes have the same free spectral range, and their resonant peak shifts have the same response to salinity changing. The experimental split mode characteristic is in good agreement with theoretical simulating, which demonstrates the resonant mode split theory of the microfiber knot-type ring resonator. The analysis on resonant mode presented here may offer useful references for sensing applications.

Ocean internal-wave phenomena occur with the variation in seawater vertical temperature, and most internal-wave detections are dependent on the measurement of seawater vertical temperature. A seawater temperature sensor based on a microfiber knot resonator (MKR) is designed theoretically and demonstrated experimentally in this paper. Especially, the dependences of sensing sensitivity on fiber diameter and probing wavelength are studied. Calculated results show that sensing sensitivity increases with the increasing microfiber diameter with the range of 2.30–3.91 μm and increases with the increasing probing wavelength, which reach good agreement with results obtained by experiments. By choosing the appropriate parameters, the maximum sensitivity measured can reach to be 22.81 pm/°C. The seawater temperature sensor demonstrated here shows advantages of small size, high sensitivity, easy fabrication, and easy integration with fiber systems, which may offer a new optical method to detect temperature of seawater or ocean internal-wave phenomenon and offer valuable reference for assembling micro sensors used for other parameters related to seawater, such as salinity, refractive index, concentration of NO3− and so on.

The salinity and temperature of seawater are important parameters in oceanography. Based on the directional coupler assembled by polyimide-coated micro/nanofibers, optical sensors with high sensitivity for simultaneous salinity and temperature sensing in seawater are proposed. Dependences of sensitivities on wavelength, salinity, and temperature are investigated theoretically, with which performances of such sensor under general sea conditions can be evaluated. Results show that salinity and temperature sensitivities can reach levels of nm/‰ and nm/°C, which are much higher than those of fiber Bragg gratings, knot resonators, and photonic crystal fibers. Other considerations for system design such as the length of the coupling area, the diameter difference between two fibers, and the thickness of polyimide coatings are also discussed. Sensors proposed here suggest a simple approach to realize high-sensitivity micro/nanofiber optical sensing of salinity and temperature in seawater simultaneously and may find applications in developing miniature sensors used in seawater.

An all-fiber Mach-Zehnder interferometer (MZI) for two quasi-continuous points' temperature sensing in seawater is proposed. Based on the beam propagation theory, transmission spectrum is designed to present two sets of clear and independent interferences. Following this design, MZI is fabricated and two points' temperature sensing in seawater are demonstrated with sensitivities of 42.69pm/°C and 39.17pm/°C, respectively. By further optimization, sensitivity of 80.91pm/°C can be obtained, which is 3-10 times higher than fiber Bragg gratings and microfiber resonator, and higher than almost all similar MZI based temperature sensors. In addition, factors affecting sensitivities are also discussed and verified in experiment. The two points' temperature sensing demonstrated here show advantages of simple and compact construction, robust structure, easy fabrication, high sensitivity, immunity to salinity and tunable distance of 1-20 centimeters between two points, which may provide references for macroscopic oceanic research and other sensing applications based on MZIs.