Detection of Liquid Level With an MI-Based Fiber Laser Sensor Using Few-Mode EMCF

A novel sensitivity-enhanced optical fiber sensor has been presented and demonstrated to measure liquid temperature and refractive index (RI) based on the reflective single-mode–multimode–single-mode fiber structure. The sensing part is packaged in a segment of capillary tube by hot glue. Due to the high coefficient of thermal expansion of hot glue, the increasing temperature bends the fiber in the tube. The measurement is achieved via monitoring the spectral shift of the modal interference between the core and the cladding modes in the ended single-mode fiber, which is well carved to provide a mirror interface. The Fresnel reflection occurred in the interface is utilized to measure liquid RI and temperature. The highest sensitivity of −92.6 pm/°C and −67.9 dB/RIU are achieved, respectively. This structure has many advantages, such as high sensitivity, expectation for simultaneous measurement of liquid temperature and RI, convenient operation, and low cost of fabrication.

We report on a novel all-fiber refractometer sensor based on multimode interference (MMI) effects. The operating mechanism is based on the self-imaging phenomena that occur in the multimode fiber (MMF) section, which basically replicates the field at the input of the MMF on the output of the MMF for a specific wavelength. However, the longitudinal position of this image is highly dependent on the MMF diameter (D), since there is D² dependence on the longitudinal position of this image. For the refractive index measurement a section of no-core multimode fiber, whose cladding is air, is surrounded by the liquid sample. The liquid sample now works as the cladding medium and as a result of the Goes-Hanchen shift the effective width (fundamental mode width) of the No-Core fiber is increased. As a result, the maximum coupling resulting from the imaging phenomena occurs at a different wavelength, and this can be used to measure the refractive index of the liquid. Using this scheme we can achieve a resolution on the order of 1x10^-5 for a refractive index range from 1.333 to 1.434. The device was used here to measure refractive index in liquids, but can also be applied for measuring concentration of liquids. These sensors are promising and attractive in chemical and biotechnological applications because of their high sensitivity, immunity to electromagnetic interference, and compact size.

A method is described for measuring the refractive index of a liquid using only simple apparatus found in any laboratory together with two specimens of known refractive index.

The mechanism of capillary tube interferometer is expected to be two-beam interference by ray tracing. A computer program to simulate the interference fringe pattern was established. By comparing the simulated fringe pattern and experimental fringe pattern, the refractive index of the liquid can be given when the two fringes coincide best. The results of this method are very near the Abbe refratometer. In the first time, the refractive indices of the low transparent extraction of source rocks were measured by capillary tube interferometer. A curve between the refractive indices of the diluted extraction of source rocks and corresponding vitrinite reflectance Ro was established. When Ro is in the range of 0.36-1.25, the refractive indices of the extraction increase with Ro, or the maturity of the source rocks. Good correlation was observed between the refractive indices and vitrinite reflectance Ro. The refractive index of the extraction of source rock is valuable for determining the degree of maturity of source rock. This technique is promising to measure the refractive indices of low transparent liquids and could be used to estimate the maturity of source rock.

In this paper, another approach to measure the refractive indices of liquids is proposed. The method is based on lens–fibre interference. This uses a capillary tube containing the measured liquid instead of the fibres. A part of the capillary tube is empty to calibrate the system. A simple mathematical expression has been derived to get the liquid refractive index by measuring only the deviation angle from the interferogram. The constants that relate the liquid refractive index to the deviation angle have been determined using a standard set of liquids. This method can measure liquids' refractive indices in a wide range with an error less than ±1 × 10−3, which is expected in the case of two-beam interference.

A miniature fiber sensor tip with both high refractive index (RI) and temperature sensitivities was proposed for measuring the temperature and liquid RI simultaneously. A two-photon polymerization technique was used to fabricate a ⊓-shaped polymer micro-structure on the end-face of an optical fiber. A three-wave Fabry–Pérot interferometer was formed. The size of the proposed FPI can be precisely controlled and the total height is less than 30 μm. Temperature and RI variations can be detected simultaneously using the sensitivity matrix method. The sensor tip can be used in harsh environments, as cross-linking SU-8 has moderate acid and alkali resistance and good mechanical strength.

An optical waveguide for measuring the change of refractive index by using a photonic crystal fiber is designed. The simulation results show that a variation of 0.032 refractive index unit (RIU) could shift the resonant wavelengths of 245nm and 68nm when the refractive indices of liquid are 1.5 and 1.7, respectively. The sensitivity for the refractive index measurement of about 1.3×10 -6 RIU is achieved.

A liquid level optical fiber sensor with refractive index compensation through cascading double multi-mode interferences

High sensitivity photonic crystal fiber sensor based on dual-core coupling with circular lattice

The refractive index of seawater is one of the important optical properties of the air-sea interface, which is of great significance to the study of ocean optics, ocean dynamics, and air-sea interaction. The refractive index of seawater refers to the ratio of the propagation speed of light waves in a vacuum to that in seawater. In this paper, the refractive index of seawater is measured quickly and accurately by a spectrophotometer combined with the double triangular prism method. By comparing with the measured data of the Abbe refractometer, the measured data of this experimental method are accurate and reliable, which can be extended to the rapid measurement of the refractive index of other liquids by simple instruments. The double-prism method for rapid measurement of seawater refractive index is to use the existing simple equipment to measure the liquid refractive index. It provides accurate optical parameters for investigating the optical properties of the ocean or the propagation law of light in the sea and provides accurate optical parameters for studying marine optics.

In this paper it is shown that temperature, refractive index and density and also convective heat transfer coefficient around a vertical axisymmetric cylindrical wire can be measured by the Michelson Interferometer. In experimental setup, a vertical wire has been put in one of the arms of the Michelson Interferometer. By applying voltage to the wire, temperature gradient is created around the wire. These phenomena curved the linear Michelson fringes. By measuring the fringes shift and applying the Abel transform, the distribution of refractive index can be evaluated. This distribution gives the density distribution. Minimum value for refractive index and density is found on the surface of the wire. For the far distance from the wire, density and refractive index approaches to the room values. Air density was evaluated experimentally, which is compatible with its reported value at definite weather conditions. Considering high reflectance of the wire, its thermal radiation is low and heat mostly transfers by convection. Convective heat transfer coefficient is measured experimentally which agrees with last results.

On the concept of precrystallisation, i. e. , on considering the supercooled liquid as a solution of its metastable solid-phase, the variation of refractive index of supercooled liquids has been found dependent on the specific surface free energy of the liquid-solid interface. Calculations have been made for the refractive index of supercooled water by using the derived formulae. In view of the uncertainty in the value of the water-ice interface surface free energy, σ, two sets of σ have been taken for calculations. Experimentally known values have been found to agree with those of our calculated ones for which the value of σ is the McDonald's (using Volmer's rule). It has been shown that an experimental determination of refractive index of the supercooled water at lower temperatures will be useful to obtain correct values of σ at different temperatures.

A high-sensitivity Mach–Zehnder interferometer (MZI) based on the cascaded core-offset and macrobending fiber structure is proposed for refractive index (RI) measurement. The core-offset structure makes the fiber core mode couple to the cladding modes, and some of them recouple back to the fiber core at the macrobending structure forming a model interference effect. The liquid RI can be measured by monitoring the spectral shift of the modal interference. The RI sensing performances for the interferometers with different macrobending radii and core offsets are investigated experimentally. Experimental results show that when the core offset is 2 μm and the macrobending radius is 5.5 mm, the sensitivity can reach 699.95 nm/RIU for the RI of 1.43. The temperature dependence for the proposed sensor is also tested, and a temperature sensitivity of 0.112 nm/°C is obtained.

Michelson interferometer-based fiber-optic sensing of liquid refractive index

In this paper, a method for measuring the liquid refractive index based on liquid crystal on silicon (LCoS) is presented. The liquid measuring chamber is composed of two optical lenses, an inlet, and an outlet. By filling the liquid with different refractive indices, the liquid measuring chamber has different focal lengths according to geometric optics. As a reflective micro-display device, the LCoS is another key component, in which the liquid crystal (LC) molecules are arranged in a nematic phase. The programming function of the LCoS, based on the interaction of incident light and the LC molecules in the LCoS, enables the LCoS to form as a digital lens. By using the measuring chamber in the optical system and adjusting the focal lengths of the digital lens, which is loaded on the LCoS, the refractive index of the liquid can be calculated accurately when the sensor receives the focused spot. The experimental results verify the feasibility of our proposed method.