Design and demonstration of a liquid level fiber sensor based on self-imaging effect

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

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.

Fiber laser sensor based on multimode interference (MMI) effect can achieve high sensitivity with lower temperaturestrain cross sensitivity by designing optimally the MMI structure. An erbium-doped fiber laser for temperature, stress and refractive sensing based on single mode fiber spliced a short section of no-core fiber was proposed in this paper. The optical field characteristics of this MMI structure have been analyzed, indicating the spectral filtering effect existed. An erbium-doped fiber laser with the MMI structure inserted as the sensor element, the enhanced sensing sensitivity is achieved on the sensing experiments of ambient temperature, mechanical stress and liquid refractive index. The sensitivity of this MMI-based fiber laser sensor is up to 13.24 pm/°C with the measured range from 30 °C to 220 °C, 1.29pm/μe from 0 to 7776.05μe, 180.21nm/RIU from 1.34 to 1.42, respectively.

This paper shows mainly the performance study on tapered section of Multimode Interference (MMI) coupler having either parabolic or exponential taper section. The comparative study of the 1 × 2 MMI coupler with proposed structure, such as the normalized width variation, and variation of MMI length are investigated by the 2D finite difference beam propagation method (BPM). We have found that exponential tapered MMI coupler has advantageous compared to parabolic tapered MMI coupler. The relation between numbers of allowed modes with power transmission has been predicted along with pulse propagation through MMI coupler. At the output of proposed structure, S-bend waveguides used to provide sufficient separation between the two output waveguides. At the end of paper, compared the excess loss of proposed structure. Our results are unique, because, we have analyzed relatively a new structure compared to existing structure found in literature. Keywords: Linear waveguide, Parabolically tapered waveguide, Exponentially tapered waveguide, S-bend Arc waveguide

We report on a novel tuning mechanism to fabricate an all-fiber tunable laser based on multimode interference (MMI) effects. It is well known that the wavelength response of MMI devices exhibits a linear dependence when the length of the multimode fiber (MMF) section. Therefore, tuning in the MMI filter is achieved using a ferrule (capillary tube of 127 μm diameter) filled with a liquid with a higher refractive index than that of the ferrule, which creates a variable liquid MMF. This liquid MMF is used to increase the effective length of the MMI filter and tuning takes place. Using this simple scheme, a tuning range of 30 nm was easily achieved, with very small insertion losses. The filter was tested within a typical Erbium doped fiber (EDF) ring laser cavity, and a tunable EDF laser covering the full C-band was demonstrated. The advantage of our laser is of course the simplicity of the tunable MMI filter, which results in an inexpensive tunable fiber laser.

We demonstrate a high refractive index (RI) liquid level sensor based on coreless multimode fiber (CMF) which is capable of measuring the liquid level and discriminating the high RI. The direct exposure of the CMF enables a strong interaction between the high-order modes guided in it and the surrounding medium through evanescent wave. Part of the energy leak out at the fiber/liquid interface when the RI of the liquid exceeds that of the CMF. The transmission of the single-mode fiber (SMF)-CMF-SMF structure is a function of both the liquid level and RI which experiences leakage loss with the increasing liquid level. Liquid level information can be determined by simply selecting a wavelength range with linear power variation. The proposed sensor with 4.1-cm CMF has approximately linear responses of −0.223, −0.253, −0.316, −0.392, and −0.461 dB/mm for glycerol solution concentrations of 98%, 96%, 92%, 88%, and 84% in a 2-nm wavelength range, respectively. Such a simple and low-cost sensor may be integrated as biochemical probes, indicating preferable sensing performance and practicability.

Fiber-optic magnetic field sensing is an important method of magnetic field monitoring, which is essential for the safety of civil infrastructures, especially for power plant. We theoretically and experimentally demonstrated an optical fiber magnetic field sensor based on a single-mode-multimode-single-mode (SMS) structure immersed into the magnetic fluid (MF). The length of multimode section fiber is determined based on the self-image effect through the simulation. Due to variation characteristics of the refractive index and absorption coefficient of MF under different magnetic fields, an effective method to improve the sensitivity of SMS fiber structure is realized based on the intensity modulation method. This sensor shows a high sensitivity up to 0.097 dB/Oe and a high modulation depth up to 78% in a relatively linear range, for the no-core fiber (NCF) with the diameter of 125 μm and length of 59.8 mm as the multimode section. This optical fiber sensor possesses advantages of low cost, ease of fabrication, high sensitivity, simple structure, and compact size, with great potential applications in measuring the magnetic field.

High sensitivity seawater temperature sensor based on no-core optical fiber

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.

We propose and demonstrate a hybrid fiber-based sensor combining a multimode interference (MMI) structure and a surface plasmon resonance (SPR) structure for simultaneous measurement of temperature and refractive index (RI) of a liquid sample. We configure the MMI structure by connecting a single-mode fiber, a no-core fiber, and a single-mode fiber sequentially. We set up the SPR structure by coating a gold film with a thickness of 50 nm on the surface of the no-core fiber. We measure the sensitivity of RI and the temperature of the MMI and SPR structure, respectively. Then we obtain the coefficient matrix to simultaneously measure the temperature and RI of a liquid sample and obtain the highest RI sensitivity of 2061.6 nm/RIU and temperature sensitivity of 37.9 pm/°C. We verify the feasibility of the sensor in liquid alcohol. The testing results indicate that the proposed sensor and testing method are feasible, accurate, and convenient.

In this paper, the simulation and design of a waveguide for water turbidity sensing are presented. The structure of the proposed sensor uses a 2x2 array of multimode interference (MMI) coupler based on micro graphene waveguide for high sensitivity. The beam propagation method (BPM) are used to efficiently design the sensor structure. The structure is consist of an array of two by two elements of sensors. Each element has three sections of single mode for field input tapered to MMI as the main core sensor without cladding which is graphene based material, and then a single mode fiber as an output. In this configuration MMI responses to any change in the environment. We validate and present the results by implementing the design on a set of sucrose solution and showing how these samples lead to a sensitivity change in the sensor based on the MMI structures. Overall results, the 3D design has a feasible and effective sensing by drawing topographical distribution of suspended particles in the water.

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.

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.

In order to meet the actual requirements for sensor miniaturization and integration, we propose a compact dual-parameter surface plasma resonance (SPR) fiber sensor, which can simultaneously measure the refractive index and temperature of liquid. The sensor is composed of input-output couplers and two sensing arms. The no-core fibers (NCFs) and microstructured optical fiber (MOF) with deposited films are used as input-output couplers and sensing arms, respectively. One sensing arm is the MOF with silver film for refractive index measurement, and another is that with silver film and PDMS for temperature measurement. Numerical simulation results show its cladding mode is more advantageous in stimulating the SPR effect. Experimental results demonstrate the maximum sensitivity of liquid refractive index is 4139 nm/RIU in the range of 1.333-1.365, and that of liquid temperature is 4.7 nm/°Cin the range of 20-70 °C. The compact structure makes the proposed sensor unique and has a promising application in refractive index and temperature monitoring.