Liquid Refractive Index Sensor Research Articles

Random Raman Fiber Laser as a Liquid Refractive Index Sensor

In this paper, a new concept of forward-pumped random Raman fiber laser (RRFL)-based liquid refractive index sensing is proposed for the first time. For liquid refractive index sensing, the flat fiber end immersed in the liquid can act as the point reflector for generating random fiber lasing and also as the sensing head. Due to the high sensitivity of the output power of the RRFL to the reflectivity provided by the point reflector in the ultralow reflectivity regime, the proposed RRFL is capable of achieving liquid refractive index sensing by measuring the random lasing output power. We theoretically investigate the effects of the operating pump power and fiber length on the refractive index sensitivity for the proposed RRFL. As a proof-of-concept demonstration, we experimentally realize high-sensitivity half-open short-cavity RRFL-based liquid refractive index sensing with the maximum sensitivity and the sensing resolution of–39.88W/RIU and 2.5075×10−5 RIU, respectively. We also experimentally verify that the refractive index sensitivity can be enhanced with the shorter fiber length of the RRFL. This work extends the application of the random fiber laser as a new platform for highly-sensitive refractive index sensing in chemical, biomedical, and environmental monitoring applications, etc.

New oil derivative refractive index sensor using ribbon of multi-walled carbon nanotubes

In this work, we propose a new liquid refractive index (RI) sensor based on one-dimensional (1D) regular array of multi-walled carbon nanotubes (MWCNTs) for sensing oil derivatives (ODs), for the first time. The proposed sensor is a ribbon consisting of 10 MWCNTs grown inside an AAO (Al2O3) template as the host material, which can sense the change of host material RI with high precision up to △n = 0.0015 corresponding to the sensitivity S = 930 nm/RIU. Replacing the host material with ODs changes the reflection in the range of visible wavelengths. The reflection spectrum of sensor is calculated for the RI range of 1.3888–1.5306 belonging to various ODs. The RI difference △n = 0.1418 between Butanol (C4H10O) and Nitrobenzene (C6H5NO2) host materials results in 130 nm wavelength shift. By removing one or two cylinders of MWCNTs and creating a point defect within the sensor structure, whereas ODs are injected into the defect area, we achieved a sensitivity up to 430 nm/RIU. The proposed sensor with un-reactive nature of MWCNTs can be used in environments with high temperature and pressure such as oil refinery industry.

Compact and Highly Sensitive Refractive Index Sensor Based on Embedded Double-Ring Resonator Using Vernier Effect

A compact and highly sensitive liquid refractive index (RI) sensor using Vernier effect achieved with embedded double-ring resonator is proposed and demonstrated. Unlike these conventional sensors using cascaded double-ring resonators to achieve Vernier effect, in our proposed embedded double-ring sensor (EDRS) the sensing ring is embedded on the reference ring, which makes the sensor more compact while achieving high sensitivity. Our typical fabricated EDRS with polymer material, which has a footprint of 1500 μm × 850 μm, achieves the RI sensitivity as high as 7390 nm/RIU in aqueous medium, which is in excellent agreement with the theoretical one.

High stability microfiber Sagnac loop liquid refractive index sensor with FBG mode selecting

As a high-resolution sensor, it is very important to improve the spectral stability of microfiber Sagnac loop interferometer. A fiber Bragg grating (FBG) mode selecting method is proposed and demonstrated. The method can filter out some cladding modes by FBG in the Sagnac loop and make working modes energy more concentrated, which shows that the interference spectrum becomes more uniform. At the same time, the sensitivity and linearity of the sensor can be improved slightly by this method. In our experiment, the resolution of sensor can get 1.41 × 10-5 ∼ 7.49 × 10-6 in the liquid refractive index range of 1.3330∼1.38481. Moreover, this method can effectively improve the spectral stability of the interferometer, which shows that the dip wavelength absolute error of time fluctuation is improved from ±0.358 nm to 0.021 nm and from ±0.116 nm to 0.000 nm at 1536.6 nm and 1559.9 nm initial dip wavelength, respectively. This method also has advantages of compact structure, good mechanical properties, and easy preparation. On this basis, the FBG also can be replaced by various modes filters to develop more multifunctional high stability interferometers according to the detection needs.

Ultra compact and sensitive optical fiber interferometric refractive index sensor

An ultra-compact and sensitive Fabry-Perot interferometer liquid refractive index sensor is presented. The device consists of an inner reflection mirror in single-mode fiber written by femtosecond laser and a hollow-core fiber. Two micro-holes are drilled on the side wall of the hollow-core fiber by femtosecond laser micromachining, which enables the liquid freely enters or leaves the hollow-core cavity. An enhanced and precisely controllable amplification factor can be obtained for the Vernier effect used, which provides an ultra-high refractive index sensitivity of − 73417.9 nm/RIU (refractive index unit). The device has ultra-compact size, simple structure, high sensitivity, and is featured with easy fabrication and good reproducing capability, thus exhibiting high potential in many sensing applications in biomedicine and chemical industry.

Temperature-insensitive intensity-modulation liquid refractive index sensor based on fiber-optic Michelson probe structure

Abstract A novel liquid refractive index sensor based on the connected single-mode fiber (SMF), no-core fiber (NCF), four-core fiber (FCF), and silver mirror (SM) to form an SMF–NCF–FCF–SM Michelson probe structure is proposed and fabricated. The change of light field in the probe structure has been simulated by the light-beam propagation method. The theoretical results show that light is excited in the NCF and couples into the cores and cladding of FCF at the junction of NCF and FCF. The interference fringes are generated between the cladding modes and core modes of FCF. The sensitivities of the probe in NaCl, sucrose, and glycerol are 171.75 dB/RIU, 121.41 dB/RIU, and 207.50 dB/RIU, respectively. The temperature sensitivity is 0.05 nm/°C, and the intensity change of temperature (≤0.046 dB/°C) is very small and has little effect on the liquid refractive index. Thus, the cross-sensitivity of temperature for the liquid refractive index can be removed. The proposed probe structure has the advantages of easy fabrication, good stability, and linear response, having potential application in the liquid refractive index monitoring environments.

Cost-Effective Mach-Zehnder Interferometer Liquid Refractive Index Sensor Based on Conventional Polymer Strip Waveguide

We propose and demonstrate a cost-effective liquid refractive index (RI) sensor in polymer materials by employing a Mach-Zehnder interferometer (MZI) formed with the conventional strip waveguide. The proposed sensor can attain high sensitivity by choosing a suitable reference arm length to reduce optical path difference (OPD) between the two arms of the employed MZI. Our fabricated devices, which have the same sensing arm length of 7900 µm but different reference arm lengths of 7900.0, 7942.5, 7950.9, 7962.2, and 7969.5 µm, respectively, exhibit the maximum RI sensitivities of 33662.8 nm/RIU, and almost the same detection range (DR) of 0.0041 ± 0.0002 RIU.

A low-cost liquid refractive index sensor based on plastic optical fibre and CCD array

A wireless refractive index sensor for in-line measurement in liquids was developed. It is based on the Fresnel reflection on a prism–liquid interface, where the critical angle of the totally reflected light in the interface of two media is detected by a charge-coupled device (CCD) array. A LED is used as a light source and a plastic optical fibre, with a well-established numeric aperture, guides the light to the prism interface. Both the LED and the CCD are controlled with a Raspberry Pi 3 Model B board with wireless communication. Resolutions with values lower than 1.0 × 10−4 RIU can be achieved with this potential low-cost configuration.

Surface plasmon resonance-induced high sensitivity refractive index sensor with adjustable measurement range based on an evanescent field-enhanced D-shaped five-hole photonic crystal fiber

A simple configuration of a D-shaped five-hole photonic crystal fiber (DFPCF)-based surface plasmon resonance (SPR) refractive index (RI) sensor is proposed. A full vector finite element method is applied to simulate the resonant coupling characteristics of the core modes and the surface plasmon polariton (SPP) modes. In addition to its simple structure, a very small polishing depth can introduce a sufficiently strong evanescent field to excite surface plasmon waves, thus ensuring the mechanical strength of the polished fiber. These two key factors lay the foundation for mass production of the sensors. Further, two air holes above the core induce a tunable evanescent field from the core leakage to the polished surface that adjusts the resonance between core modes and SPP modes. A thin layer of gold film is applied to the polished surface to provide a detection platform for liquid analytes without filling pores, forming the SPR effect. Numerical simulation results show that the proposed DFPCF-based SPR RI sensor exhibits sensing performance that is tunable with the film thickness. A maximum wavelength sensitivity of 20 786 nm RIU−1 and widest detection range of 1.30–1.50 are achieved. Due to its improved sensing performance, the liquid RI sensor could be a potential candidate for use in biochemical analyte detection, industrial production, and food and environmental quality monitoring.

Graphene-Enhanced Surface Plasmon Resonance Liquid Refractive Index Sensor Based on Photonic Crystal Fiber.

A surface plasmon resonance (SPR) liquid refractive index sensor based on photonic crystal fiber (PCF) is proposed. The PCF is made of the exposed core structure, and the gold film is formed by electron beam evaporation within its defects. The sensitivity of the sensor is improved by coating graphene on the surface of the gold film. The experimental results show that the sensitivity of the sensor is increased by 390 nm/RIU after the introduction of graphene, and finally to 2290 nm/RIU. The experiment and simulation have a good consistency. Significantly, the sensor can be reused, and the measurement accuracy can be maintained.

A liquid refractive index sensor based on 3-core fiber Michelson interferometer

A liquid refractive index (RI) sensor based on in-fiber Michelson interferometer (MI) is proposed and experimentally demonstrated, which consists of a segment of 3-core fiber (3CF) fusion spliced with a short piece of multimode fiber (MMF). The operation principle of the MI lies in the intermodal interference of the 3CF via employing MMF to excite high-order modes, and the 3CF end face reflects the interference spectrum which loaded the surrounding RI information. By measuring the reflection spectrum variation, the RI can be determined. Experimentally, a linearly RI sensitivity of -151.56 dB/RIU is achieved in the sensing range of 1.3335-1.3720 RIU, and the temperature sensitivity in water is 0.048 nm/°C.

Liquid Surface Tension and Refractive Index Sensor Based on a Side-Hole Fiber Bragg Grating

A novel side-hole fiber Bragg grating (SHFBG)-based sensor is proposed and realized surface tension, refractive index (RI), and temperature of liquid monitoring simultaneously. A segment of side-hole fiber (SHF) is spliced of a single mode fiber (SMF), and a 10 mm long fiber Bragg grating (FBG) is inscribed within the core of the open end of the SHF. Due to the capillary effect, the liquid level in the SHF changes as a function of the surface tension of the liquid, which can be measured by monitoring the liquid level in the hole. The reflectivity of the FBG is affected by the fraction length immersed in the liquid, while the Bragg wavelength is affected by the refractive index and the temperature of the liquid. The absolute shift and relative shift of the Bragg wavelength can be used to measure the temperature and the refractive index of the liquid, respectively. The sensor described in this paper can measure the surface tension and the refractive index easily taking advantage of the simple structure and bring, an additional benefit of the “internal thermometer” provided by the absolute value of the Bragg wavelength.

Low-Cost and Highly Sensitive Liquid Refractive Index Sensor Based on Polymer Horizontal Slot Waveguide

We analyze and explore the potential of using a polymer horizontal slot waveguide as light-analyte interactive region to implement a low-cost and highly sensitive liquid refractive index sensor. Numerical analysis shows that the optimized polymer horizontal slot waveguide is able to realize high waveguide sensitivity. With the optimized horizontal slot waveguide, polymer liquid refractive index sensors based on Mach-Zehnder interferometer (MZI) and microring resonator (MRR) are then investigated numerically, and the results show that the MZI-based sensor can achieve high sensitivity of 17024nm/RIU and low limit of detection (LOD) of 1.76×10−6 RIU while the MRR-based sensor can achieve the sensitivity of 177nm/RIU and the LOD of 1.69×10−4 RIU with a very small footprint. Compared with the sensors employing conventional silicon or silicon nitride vertical slot waveguide, the sensors employing polymer horizontal slot waveguide exhibit comparable performances but simpler and lower fabrication costs.

Liquid refractive index and temperature sensor using multimode interference-based corroded polarization-maintaining fiber

A cost-efficient multimode interference-based sensor with corroded polarization-maintaining fiber (PMF) is proposed and experimentally demonstrated for the measurement of liquid refractive index (RI) and temperature. The sensor consists of a segment of corroded PMF and two segments of multimode fiber (MMF). Two segments of MMF, which are used as beam splitter and combiner are embedded on both ends of the PMF. The all-fiber sensor is put in the middle of the single-mode fiber. The RI and temperature characteristics of the sensor are investigated experimentally. The results show that two resonant peaks have different spectral responses of RI and temperature changes, which indicate that the sensor can realize simultaneous RI and temperature measurements. The experimental result shows that the high RI sensitivity is up to 318.279 nm / RIU in the RI range from 1.333 to 1.350 and the high temperature sensitivity is 0.188 nm / ° C in the range from 20°C to 40°C. This sensor exhibits the advantages of low cost and high sensitivity and may have potential application in RI and temperature measurements.

基于游标效应增敏的全光纤液体折射率传感器

基于游标效应增敏的全光纤液体折射率传感器

Optimization of a liquid refractive index sensor based on an integrated optic slot-waveguide directional coupler

An integrated optic silica slot-waveguide directional coupler is investigated here for its ability to measure the refractive index (n) of a liquid biochemical species. The geometrical dimensions of the directional coupler are optimized so that the output power of the directional coupler is linear when refractive index of the liquid species changes from 1.33 to 1.38. The best geometrical dimensions achieved here from a simulation with the beam propagation method (BPM) were a waveguide width of 6um, a gap of 2um, a slot-widht of 0.5um, and a channel length of 2,000um. The proposed directional coupler type of biochemical sensor changes its output power linearly from 2 to 24% when the refractive index of the liquid species varies from 1.33 to 1.38.

High sensitivity and low loss open-cavity Mach-Zehnder interferometer based on multimode interference coupling for refractive index measurement

An open-cavity Mach-Zehnder interferometer (MZI) liquid refractive index (RI) sensor based on multimode interference (MMI) coupling is proposed and demonstrated. The sensor is fabricated by embedding a small section of no core fiber (NCF) into two sections of multimode fiber (MMF) with large lateral-offset splicing. In term of MMI effect, the lengths of the MMFs are optimized to enhance the exciting and coupling efficiencies of the interference lights. Therefore, this coupling method reduces the insertion loss obviously and allows bigger tolerance of offset distance. The simulated and experimental results all prove it has the most optimized loss compared with other coupling ways that we know. A sensor with 1 mm sensing length is fabricated and its loss is only −11.9 dB which is prominently reduced compared with other open cavity MZIs with the similar cavity length. This sensor has a high RI sensitivity of −1364.343 nm/RIU and shows a good linear response in the measurement range. The temperature sensitivity is about 33 pm/°C, so the cross-sensitivity is only 2.419 × 10−5 RIU/°C. The proposed sensor can promote the development of the open-cavity MZI to satisfy practical demands.

Temperature and liquid refractive index sensor using P-D fiber structure-based Sagnac loop.

A cost-efficient P-D fiber structure-based Sagnac loop sensor is proposed and experimentally demonstrated for measuring temperature and liquid refractive index (RI). The P-D structure is fabricated by fusion splicing a section of polarization-maintaining fiber (PMF) to a piece of multimode D-shaped optical fiber (MMDF). Then the P-D structure is built into a Sagnac loop using a 3dB coupler. The temperature and RI characteristics of the sensor are investigated experimentally. The results show that two resonant dips have different spectral responses of temperature and RI, which indicate that the sensor can realize simultaneous temperature and RI measurement. The high sensitivities of -1.804nm/°C and -131.49nm/RIU are achieved. The obtained resolutions of temperature and RI of the proposed sensor can reach 0.01°C and 2.46 × 10-4RIU, respectively. The proposed sensor has the potential application in biological and chemical fields.

Optical fiber liquid refractive index sensor based on Fresnel reflection of anti-Stokes light

An optical fiber liquid refractive index (RI) sensing method based on Fresnel reflection of Raman anti-Stokes light is firstly proposed and demonstrated. A specially fabricated sensor head formed by a fiber end and a fiber coil is designed, and they are used to sense RI and temperature respectively. Experimental results show that the sensor has a RI resolution up to 0.0002 RIU (Refractive Index Unit) and a temperature accuracy of ±1 °C within a measurement range of 10 km. The performance confirms that the sensor has potential to act as a remote refractometer and thermometer. Its case of simple fabrication of using only one multimode fiber offers attractive remote RI and temperature sensing applications involving chemical and biological fields.

Liquid surface tension and refractive index sensor based on a tilted fiber Bragg grating

A refractive index (RI) and liquid surface tension (ST) (at the level of 10−3 N) measurement system has been proposed by using a 1.80-cm-long, 8°-tilted fiber Bragg grating (TFBG). When the TFBG is pulled up from the liquid into the air at a fixed height (corresponding to a fixed contact angle between the fiber and the liquid), the ST acting upon the TFBG depends on the length of the TFBG covered with the liquid film along the fiber axis direction. The liquid film covering the grating results in the shifts of the resonant cladding modes of the TFBG. For the liquid with an RI in the range from 1.3341 to 1.4415, its RI and ST can be obtained through the measurements of shifts in the resonant wavelengths of the cutoff mode and changes in the intensities of the resonant wavelength dips in the TFBG spectrum by using the known ST and RI of water as a reference.