High Sensitive Modal Interferometer for Temperature and Refractive Index Measurement

PurposeThe purpose of this paper is to demonstrate a simple fiber sensor for simultaneous measurement of liquid refractive-index (RI) and temperature.Design/methodology/approachThe sensor structure is formed by a long period fiber grating cascaded with a section of thin-core fiber. The long period fiber grating is fabricated on single mode fiber, followed by a section of 20-mm length thin-core fiber which is a modal interferometer.FindingsCladding mode interference between long period fiber grating and thin-core fiber modal interferometer is weak in the experimental investigation. Both of these two cladding mode type fiber devices are sensitive to surrounding RI and temperature. So the RI and temperature can be measured simultaneously by monitoring the spectral characteristics of the compound sensor. The sensitivity is calibrated and sensor matrix is provided in the experiment.Originality/valueThis proposed fiber sensor is simple, tough, cost-effective and suitable for discriminate the liquid RI and temperature with high sensitivity.

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.

An all fiber Mach–Zehnder interferometric (MZI) sensor for measuring temperature and refractive index (RI) is proposed and experimentally demonstrated. The proposed sensor is fabricated with a coreless–few mode–coreless fiber structure. Variations in ambient temperature and RI cause changes in phase differences between the fundamental mode and the higher-order modes, leading to shifts in interference spectrum. The two resonance dips shifts within the wavelength spectrum are used to investigate the temperature and RI characteristics of the sensor. Experimental results show that the two dips have different responses to temperature and RI, indicating that the sensor can realize simultaneous measurement of temperature and RI; the maximum sensing sensitivities are 0.0739 nm/°C and −25.29 nm/RIU, respectively. The proposed sensor has potential applications in physical, biological, and chemical sensing owing to its low cost, good linearity, and simple fabrication.

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.

This letter firstly and experimentally demonstrated a fiber laser sensing system using Michelson interferometer (MI) based on few-mode elliptical multilayer-core fibers (EMCFs) for liquid level and refractive index (RI) measurement. The fiber sensing system has high optical signal-to-noise ratio (~45 dB) and narrow 3-dB bandwidth (<;0.1 nm). Four water-glycerin solutions of different RI are used for this experiment, and the higher sensitivity is acquired for the higher RI liquid. The sensitivities for the liquids with indices of 1.333, 1.353, 1.373, and 1.393 are 30.5, 35.6, 41.1, and 50.6 pm/mm within a depth range of 0-45 mm, respectively. Moreover, the RI of liquid can also be measured, and a sensitivity of 329.222 pm/mm/RIU is experimentally achieved.

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.

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.

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.

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 new type of evanescent field sensors based on a doubly cladding single mode fiber is proposed. The theoretical analysis shows that the measurable refractive index (RI) ranges for the sensors are limited in the single mode in the fiber becoming leaky and the range is less than the RI difference between the core and the cladding of the fibers. Two temperature sensors and a humidity sensor were fabricated. One temperature sensor showed a good linear response in a range of 60°C, the modulation depth of 40 dB, and the insertion loss of -1.4 dB. The humidity sensors fabricated by agarose gel had a measurable moisture range of 16 ~ 100% of RH. Its optical power variation with humidity is 8.5 dB and insertion loss is 3.9 dB. We confirmed that the DC single mode fiber sensors became a new type of intensity sensors characterized in good linear response, low insertion loss, high modulation depth, and low cost in fabrication.

Multi-modes interferometer for magnetic field and temperature measurement using Photonic crystal fiber filled with magnetic fluid

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.

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.

We propose and demonstrate a compact all-fiber sensor for simultaneous measurement of temperature and refractive index (RI), which is based on a no-core fiber (NCF) and fiber Bragg grating (FBG). Two segments of NCF used as beam splitter and combiner are embedded on the two ends of an FBG, which constitute a Mach-Zehnder (MZ) interferometer susceptible to the temperature and an ambient RI. With the change of RI and temperature, MZ interference and Bragg reflection result in the variation of transmission spectrum. The experimental results show that the sensitivity of RI is -109.573 nm/RI in the range from 1.333 to 1.398, and the sensitivity of temperature ranging from 10 °C to 70 °C is 0.014 nm/°C, respectively. The feasibility of simultaneous measurement of an external RI and the temperature with this configuration has been demonstrated experimentally.