All-fiber multimode interference refractometer sensor

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.

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

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.

Lossy mode resonance refractometer operating in the 1.55 µm waveband based on TiOxNy thin films deposited onto no-core multimode fiber by DC magnetron sputtering

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.

Refractometer based on extinction ratio demodulation using no-core and dispersion-compensating fiber structure

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

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.

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.

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.

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 simple fiber optic sensing method is reported for measuring refractive index of clear liquid samples. The refractometer is designed by introducing longitudinal displacement in between two multimode optical fibers, which are inserted inside a hollow glass chamber. Optical power coupling between the displaced fibers is found to vary with refractive index of liquid present inside the glass chamber when the fiber displacement is kept constant. Experimental observations have shown that the proposed sensing method can measure refractive index of clear and transparent liquid up to the resolution of $10^{-4}$ .

Mid-infrared refractive index sensor with high sensitivity and wide detection range based on multimode interference in tellurite optical fibers

Exposed-core fiber multimode interference sensor