Refractive Index Of Liquid Research Articles

Optical fiber multimode interference sensors using spatial multiplexing

Multimode fiber (MMF) sensors based on intermodal interferences have been widely studied due to their advantages of easy manufacture and high sensitivity. We introduced the scheme of spatial multiplexing to MMF sensors for improvement of resolution. A seven-core fiber (SCF) was fused with the NCF sensing head with a slight misalignment. Therefore, each core of the SCF has different coupling coefficients with the NCF due to its particular location at the cross section of the fusion point. The NCF sensor head was then shared by multiple cores and thus can be considered to be multiplexing. In addition, traditional demodulation method based on transmission spectrum mainly depends on the tracking spectral peaks or dips, where only a portion of spectrum data were utilized. To address this limitation, we further proposed a deep convolutional network to interrogate the spectrum that can exploit the whole spectrum data. The measurements of temperature and liquid refractive index were demonstrated in experiments. With the help of spatial multiplexing and deep learning algorithm, the resolution of temperature and refractive index of 0.0054°C and 2.25×10-5 RIU were obtained, about ten times improvement of the traditional NCF sensor. Our proposed spatial multiplexing offers a simple way to enhance the resolution without extra processing of the NCF.

Simultaneous measurement of liquid refractive index and temperature based on the birefringence response of a water-filled D-shaped photonic crystal fiber

In this paper, a water-filled D-shaped photonic crystal fiber (WFDSPCF) sensor based on the birefringence response for simultaneously measuring the liquid refractive index (LRI) and liquid temperature (LT) is proposed. We undertake theoretical and simulation calculations, and the results show that the proposed WFDSPCF can achieve ultrahigh LRI and LT sensitivities when operating around the group birefringence turning point. By optimizing the structural parameters, the proposed WFDSPCF sensor can achieve LRI and LT sensitivities of -25642.86 nm/RIU and 12.8 nm/°C, respectively. In addition, in order to eliminate the sensing crosstalk between the LRI and LT, a parallel Lyot interferometer structure is also proposed. The proposed WFDSPCF sensor is expected to have potential use in biochemical sensing, measurement science, and environmental monitoring.

Highly sensitive liquid refractive index sensor based on a polymer two-mode ridge waveguide interferometer

In this paper, we propose and experimentally demonstrate a highly sensitive liquid refractive index (RI) sensor based on a two-mode interferometer (TMI) formed with a polymer ridge waveguide. Compared with the rectangular waveguide TMI sensor, our proposed ridge waveguide TMI sensor enables a higher waveguide sensitivity and hence device sensitivity. Our experimental results show that the fabricated ridge waveguide TMI sensor achieves a sensitivity of 6980.62 nm/RIU, indicating a significant improvement over the fabricated rectangular waveguide TMI sensor with almost identical dimensions; the latter exhibits a sensitivity of 3753.81 nm/RIU. Our proposed sensor is simple in structure but has high sensitivity and low cost.

Regulation and Liquid Sensing of Electromagnetically Induced Transparency-like Phenomena Implemented in a SNAP Microresonator.

Optical microresonators supporting whispering-gallery modes (WGMs) have become a versatile platform for achieving electromagnetically induced transparency-like (EIT-like) phenomena. We theoretically and experimentally demonstrated the tunable coupled-mode induced transparency based on the surface nanoscale axial photonics (SNAP) microresonator. Single-EIT-like and double-EIT-like (DEIT-like) effects with one or more transparent windows are achieved due to dense mode families and tunable resonant frequencies. The experimental results can be well-fitted by the coupled mode theory. An automatically adjustable EIT-like effect is discovered by immersing the sensing region of the SNAP microresonator into an aqueous environment. The sharp lineshape and high slope of the transparent window allow us to achieve a liquid refractive index sensitivity of 2058.8 pm/RIU. Furthermore, we investigated a displacement sensing phenomenon by monitoring changes in the slope of the transparent window. We believe that the above results pave the way for multi-channel all-optical switching devices, multi-channel optical communications, and biochemical sensing processing.

Temperature and RI sensing based on micro-structured optical fiber surface plasmon resonance

In this paper, we propose a surface plasmon resonance sensor based on micro-structured optical fiber capable of simultaneous sensing of a liquid refractive index and temperature. The fiber structure comprises four large air holes, with side polishing and rectangular groove processing on one side without air holes. Ag film and α-Fe2O3 film are coated inside the rectangular groove for refractive index sensing. Au film is coated on the inner walls of the two side air holes and filled with polydimethylsiloxane (PDMS) for temperature sensing. The results show that the sensor can achieve sensing of two parameters in two polarization directions. With a refractive index ranging from 1.35 to 1.42 and temperature ranging from 10°C to 100°C, the maximum refractive index sensitivity reaches 28225.7 nm/RIU and the maximum temperature sensitivity reaches −3.64nm/∘C. The sensor features simple structure, easy implementation, and high sensitivity, making it applicable in fields such as biosensing, chemical sensing, and environmental monitoring.

A fiber optic refractive index sensor with temperature compensation based on cascaded Mach-Zehnder interference and Intermodal interference

Optical fiber refractive index (RI) sensor with temperature (T) compensation has important application values in biomedicine and environmental monitoring. In order to solve the problem that the detection accuracy of liquid RI is affected by T in practical applications, a T-compensated RI fiber sensor based on cascaded Mach-Zehnder interference (MZI) and multimode interference (IMI) is proposed in this paper. Experimental results showed the proposed sensor achieved a sensitivity of 797.41 nm/RIU for the measurement of RI in the range of 1.3333–1.3640, and a sensitivity of 0.120 nm/°C for the measurement of T in the range of 0–60 °C. The resolution (R) of RI and T measurements reached 2.5 × 10−5 RIU and 0.167 °C, respectively. The detection limits (DL) of RI and T measurements are 3.135 × 10−8 RIU and 1.392 °C, respectively. A matrix is obtained to demodulate the cross-sensitivity in RI and T measurements. The proposed sensor shows the advantages of simple structure, low hysteresis, and good stability.

Elucidating nanostructural organization and photonic properties of butterfly wing scales using hyperspectral microscopy.

Biophotonic nanostructures in butterfly wing scales remain fascinating examples of biological functional materials, with intriguing open questions with regard to formation and evolutionary function. One particularly interesting butterfly species, Erora opisena (Lycaenidae: Theclinae), develops wing scales that contain three-dimensional photonic crystals that closely resemble a single gyroid geometry. Unlike most other gyroid-forming butterflies, E. opisena develops discrete gyroid crystallites with a pronounced size gradient hinting at a developmental sequence frozen in time. Here, we present a novel application of a hyperspectral (wavelength-resolved) microscopy technique to investigate the ultrastructural organization of these gyroid crystallites in dry, adult wing scales. We show that reflectance corresponds to crystallite size, where larger crystallites reflect green wavelengths more intensely; this relationship could be used to infer size from the optical signal. We further successfully resolve the red-shifted reflectance signal from wing scales immersed in refractive index liquids with varying refractive index, including values similar to water or cytosol. Such photonic crystals with lower refractive index contrast may be similar to the hypothesized nanostructural forms in the developing butterfly scales. The ability to resolve these fainter signals hints at the potential of this facile light microscopy method for in vivo analysis of nanostructure formation in developing butterflies.

Resolving the Refractive Indices of Transparent and Translucent Liquids from the Spacings, Spatial Frequencies, and Directions of Interference Fringes

In this work, we present a novel approach to resolve the refractive indices of transparent and translucent liquids from straight interference fringes. The optical path difference between the two arms of the Mach–Zehnder interferometer is first derived by assuming a reference plane wave interfering with a plane wave passing through a rectangular cuvette. The analytic expressions for the liquid refractive indices are then deduced, describing how the refractive index is related to the fringe spacings, spatial frequencies, and directions. The structure coefficients in the above formulas are determined from the fringe spacings and directions of the interference patterns of the empty cuvette and the cuvette filled with a liquid of a known refractive index. The NaCl solution and Coca Cola are adopted as the test examples to show experimentally the validity of the proposed method. There is good agreement between the refractive indices obtained from the fringe spacings and direction of a single interference pattern. The sensitivity and resolution of this method are dependent on the structure of the experimental systems and thus can be adjusted in a controlled manner. The proposed method is simple to implement and can be easily extended to other high precision optical interferometer systems.

SPR based dual parameter wide range curling pot shaped photonic crystal fiber sensor

This article proposes a curling pot shaped photonic crystal fiber (PCF) sensor based on surface plasmon resonance (SPR), which utilizes two parallel polished surfaces in the cladding to achieve dual parameter measurements of liquid refractive index (RI) and temperature. The mode characteristics and sensing performance of the designed PCF sensor are studied using the finite element method, and the effects of changes in structural parameters such as pore radius, spacing, and gold film thickness on the resonance spectrum are analyzed. The sensing accuracy of the sensor is insensitive to the change of structural parameters, and it has the characteristics of a wide detection range, high sensitivity, and easy manufacture. When the measured RI is in the range of 1.33∼1.42, the maximum RI sensitivity is 20400 nm RIU−1, and the maximum FOM is 483.3 RIU−1. When the temperature ranges from −10 °C to 100 °C, the maximum sensitivity is 15.4 nm °C−1, and the maximum FOM is 0.43 RIU−1. The tight structure design of the sensor core close to the polishing surface and the anti-spill light design with a uniform arrangement of air holes enhance the SPR effect, which is the essential reason for achieving a wide detection range and high sensitivity.

Deep-learning approach to measuring the refractive index of transparent liquids.

A deep-learning approach is introduced to determine the refractive index of transparent liquids based on variations in the displacement of ultra-smooth interference fringes. The phase characteristics of these fringe variations captured in video data were analyzed and modeled using group-phase fitting. A neural network model, integrating a dense convolutional network with a long short-term memory network, was then developed and trained for high-precision liquid refractive index measurements. Experiments demonstrated an R2 accuracy of 99.70% and a mean squared error of 0.0003. This methodology has been confirmed to be temperature-dependent, considerably stable against external disturbances, highly accurate, and capable of real-time processing.

Simultaneous measurement of temperature and liquid refractive index based on fiber open Fabry-Pérot cavity and Bragg grating

Simultaneous measurement of temperature and liquid refractive index based on fiber open Fabry-Pérot cavity and Bragg grating

Highly sensitive surface plasmon resonance sensor based on the anti-resonant fiber with six nested tubes

On the heels of the continuous development of optical fiber sensing technology, optical fiber sensors based on surface plasmon resonance (SPR) have attracted widespread attention. Herein, an SPR sensor based on the six nested anti-resonant fiber (ARF) is designed and analyzed by the finite element method (FEM). All the structural parameters are optimized to achieve high-sensitivity liquid refractive index detection. Filling the anti-resonant tube with plasmonic materials to excite SPR can further reduce the manufacturing complexity. The optimal ARF-SPR sensor has excellent characteristics, including an average wavelength sensitivity of 42,000 nm/RIU, maximum sensor length of 0.08 cm, and resolution of 1.61×10−6RIU. The ARF-SPR sensor has great potential in chemical analysis, biomedicine, and other fields.

Fano resonance in a microcylinder-taper coupling system for liquid refractive index sensing via axial separation method

We propose and realize an axial separation detection scheme to measure liquid refractive index by utilizing a microcylinder-taper coupling system. Profiting from the clean resonance dip and distinctive light field distribution, a high sensitivity of 1029.6pm/RIU for detecting the liquid refractive index is achieved. As a good candidate for generating Fano resonance, asymmetric Fano linearity amplifies the detection sensitivity by 8.9 times compared to Lorentz dip by evaluating the small variation in the interaction area between the analyte and the mode field. This axial separation detection method can avoid the interference of the target analyte on the coupling region, making it attractive in practical sensing applications, especially in unstable biochemistry environments.

Refractive index measurements of liquids from 0.5 to 2 µm using Rayleigh interferometry

There is growing interest in the refractive index of liquids beyond the visible and into the short-wave infrared (SWIR) for applications such as the study of liquid-core fibers and supercontinuum generation. However, most of the data reported are in the visible. For liquids with a wide transmission window in the SWIR region, refractive index data are sparse. We present a Rayleigh interferometry-based refractometer to characterize the refractive index relative to standard materials at seven different wavelengths (543.4, 632.8, 780, 973, 1064, 1550, and 1970 nm) at a temperature of ~ 21.3 ± 0.6 °C. We also show Sellmeier fits using our results juxtaposed with previously published data. Our data extends previous work to the SWIR.

A method of liquid refractive index measurement based on image correlation coefficient

According to the difference of speckle image caused by the axial displacement due to the refraction of a transparent medium, the liquid refractive index is measured by using the image correlation coefficient. An optical measurement system based on laser speckle is built into the experiment. Firstly, the CCD camera is adjusted to move along the optical axis in air, and multiple speckle images at different positions are collected, which are correlated with the speckle image at zero position to obtain the image correlation coefficients, and then the exponential calibration relation between the correlation coefficient and the axial displacement can be obtained. Secondly, two speckle images are collected when the laser speckle spreads respectively through the liquid to be measured and water as the internal standard, and the correlation coefficient between them is calculated. Finally, according to the above calibration relation, the corresponding axial displacement between the liquid to be measured and water can be derived, and then the refractive index of the liquid to be measured is calculated using the axial displacement formula. The results show that this method can realize the measurement of liquid refractive index, and the optical structure is simple and easy to operate.

Vernier-effect-based fiber microcoupler for highly sensitive liquid refractive index sensing

An orthogonal mode interferometer (OMI) is proposed and experimentally demonstrated for liquid refractive index sensing using the optical Vernier effect. The OMIs are based on weakly fiber microcouplers, which are fabricated by fusing single mode fiber and coreless fiber together. Owing to the birefringent characteristic of the hybrid coupler, the optical Vernier effect is dependent on the overlap of mode interference between the x and y polarizations. Compared to the response of the individual resonance dip, the signal demodulation of the Vernier envelope exhibits more excellent signal amplification capability. Experimental results show that the Vernier envelope of the OMI achieves a refractive index (RI) sensitivity of 22 427.03 nm/RIU near the RI of 1.33 with a magnification factor of 4.1. Moreover, with its high sensitivity, flexible design and simplified configuration, our proposed OMI based on the optical Vernier effect is well suitable for a wide range of biosensing applications.

Simultaneous measurement of liquid level and refractive index based on a sandwich multimode optical fiber structure

Simultaneous measurement of liquid level and refractive index based on a sandwich multimode optical fiber structure

LiqDetector

With the advancement of wireless sensing technologies, RF-based contact-less liquid detection attracts more and more attention. Compared with other RF devices, the mmWave radar has the advantages of large bandwidth and low cost. While existing radar-based liquid detection systems demonstrate promising performance, they still have a shortcoming that in the detection result depends on container-related factors (e.g., container placement, container caliber, and container material). In this paper, to enable container-independent liquid detection with a COTS mmWave radar, we propose a dual-reflection model by exploring reflections from different interfaces of the liquid container. Specifically, we design a pair of amplitude ratios based on the signals reflected from different interfaces, and theoretically demonstrate how the refractive index of liquids can be estimated by eliminating the container's impact. To validate the proposed approach, we implement a liquid detection system LiqDetector. Experimental results show that LiqDetector achieves cross-container estimation of the liquid's refractive index with a mean absolute percentage error (MAPE) of about 4.4%. Moreover, the classification accuracies for 6 different liquids and alcohol with different strengths (even a difference of 1%) exceed 96% and 95%, respectively. To the best of our knowledge, this is the first study that achieves container-independent liquid detection based on the COTS mmWave radar by leveraging only one pair of Tx-Rx antennas.

Tamm Plasmon Polariton Biosensors Based on Porous Silicon: Design, Validation and Analysis.

Tamm Plasmon Polariton (TPP) is a nanophotonic phenomenon that has attracted much attention due to its spatial strong field confinement, ease of mode excitation, and polarization independence. TPP has applications in sensing, storage, lasing, perfect absorber, solar cell, nonlinear optics, and many others. In this work, we demonstrate a biosensing platform based on TPP resonant mode. Both theoretical analyses based on the transfer matrix method and experimental validation through nonspecific detection of liquids of different refractive indices and specific detection of SARS-CoV-2 nucleocapsid protein (N-protein) are presented. Results show that the TPP biosensor has high sensitivity and good specificity. For N-protein detection, the sensitivity can be up to 1.5 nm/(µg/mL), and the limit of detection can reach down to 7 ng/mL with a spectrometer of 0.01 nm resolution in wavelength shift. Both nonspecific detection of R.I. liquids and specific detection of N-protein have been simulated and compared with experimental results to demonstrate consistency. This work paves the way for design, optimization, fabrication, characterization, and performance analysis of TPP based biosensors.

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.