Liquid Refractive Index Measurement Research Articles

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.

Drift Error Compensation Algorithm for Heterodyne Optical Seawater Refractive Index Monitoring of Unstable Signals.

The refractive index measurement of seawater has proven significance in oceanography, while an optical heterodyne interferometer is an important, highly accurate, tool used for seawater refractive index measurement. However, for practical seawater refractive index measurement, the refractive index of seawater needs to be monitored for long periods of time, and the influence of drift error on the measurement results for these cases cannot be ignored. This paper proposes a drift error compensation algorithm based on wavelet decomposition, which can adaptively separate the background from the signal, and then calculate the frequency difference to compensate for the drift error. It is suitable for unstable signals, especially signals with large differences between the beginning and the end, which is common in actual seawater refractive index monitoring. The authors identify that the primary cause of drift error is the frequency instability of the acousto-optic frequency shifter (AOFS), and the actual frequency difference was measured through experimentation. The frequency difference was around 0.1 Hz. Simulation experiments were designed to verify the effectiveness of the algorithm, and the standard deviation of the optical length of the results was on the scale of 10-8 m. Liquid refractive index measurement experiments were carried out in a laboratory, and the measurement error was reduced from 36.942% to 0.592% after algorithm processing. Field experiments were carried out regarding seawater refractive index monitoring, and the algorithm-processing results are able to match the motion of the target vehicle. The experimental data were processed with different algorithms, and, according to the comparison of the results, the proposed algorithm performs better than other existing drift error elimination algorithms.

Measurement of liquid refractive index by quantitative phase reconstruction of single frame dual-wavelength digital hologram

A measurement method of liquid’s refractive index by means of single-shot dual-wavelength off-axis digital holographic microscopy system and a microfluidic chip is presented. The refractive index of liquid filled in the microchannel of a microfluidic chip can be calculated directly with the unwrapped phase map obtained at the synthetized wavelength, without requirement of knowing another liquid’s refractive index as a measurement reference. Moreover, it also avoids the influence of the accuracy of the known refractive index on the refractive index to be measured. The refractive indices of salt solutions and sucrose solutions in the several concentrations are achieved in the experiments, which verifies that the measurement of liquid’s refractive index is accurate and reliable. With the merit of accurate measurement of liquid’s refractive index based on the one-frame dual-wavelength hologram, it is expected that this approach will be employed to detect in real-time the fluid’s parameters related to its refractive index.

Error Analysis and Improvement of Liquid Refractive Index Measurement Using Laser Speckle Interferometry

Error Analysis and Improvement of Liquid Refractive Index Measurement Using Laser Speckle Interferometry

Frequency-splitting effect of fiber laser for liquid refractive index-measurement

Liquid refractive index is a crucial parameter for precision biochemistry and environmental monitoring. Here, we propose a liquid refractive index-measurement method in the near-infrared region based on the frequency-splitting effect of all-fiber lasers. Combining the non-circular symmetric fiber and the longitudinal frequency-splitting effect in our proposed laser, the synthesis model of external refractive index modulated birefringence and the original intracavity birefringence is obtained, and their relationship with the frequency-splitting frequency difference is established. Experiments are conducted to study the concentration–refractive-index relationship of a glucose solution with a mass fraction in the range of 0–40% as well as the thermo-optical coefficient of different concentration solutions in the temperature range of 5–35 °C. In this method, the refractive index is obtained directly from the laser-output wavelength and frequency change, and the refractive index measurement is traceable to the basic physical quantity. The results show that our method has a broad application prospect in the field of accurate measurement and sensing of liquid refractive indices.

High-consistency fiber-optic Fabry-Perot sensor based on MEMS for simultaneous temperature and liquid refractive index measurement.

We proposed a hybrid Fabry-Perot fiber-optic sensor based on the microelectromechanical system (MEMS) technique for measuring temperature and liquid refractive index simultaneously, and we verify the consistency of four sensors in the same batch. The sensor consists of a groove-array structured glass wafer and two silicon wafers, which are connected by double-sided anodic bonding. The three parts form two independent Fabry-Perot cavities for temperature and liquid refractive index sensing, respectively. We randomly selected three sensors in the same batch and conducted temperature and refractive index experiments to establish the sensing equation. The experimental results demonstrate their high consistency with temperature sensitivities of 81.6, 81.8, and 81.4 pm/°C in the range of 10°C to 80°C, and refractive index sensitivities of 1040.11, 1044.24, 1042.91 nm/RIU in the range of 1.333-1.374. The sensors have low cross-sensitivities that are less than 5.86×10-6 RIU/°C and high precisions of 0.047°C, 2.14×10-6RIU, respectively. To verify the validity of the sensing equation, we made another sensor in the same batch and got maximum errors of 0.36°C and 7.7×10-5RIU, respectively.

Theoretical and experimental study of liquid refractive index measurement method based on the double external-cavity-laser feedback effect

A liquid refractive index (RI) measurement method based on the double external-cavity laser feedback effect is proposed. The method uses a laser beam to simultaneously enter the top and bottom surfaces of the liquid to be tested, with the light reflected from the two surfaces returning to the laser cavity along the original path, forming a double external-cavity system. When the liquid level rises, the self-mixing interference signal, caused by the optical path from the top and bottom liquid surfaces, superimposes on the tuning intensity of the laser, and the RI of the liquid can be obtained by demodulating the amplitude-frequency feedback tuning curve. This method offers the following innovative performances. Firstly, the structure of the system is simple and the measuring range is large. Secondly, the measurement results can be traced back to one physical quantity, i.e. the RI of air. Thirdly, it has been proved that the laser feedback effect is ubiquitous in all types of lasers, and the change of wavelength does not affect the feedback phenomenon, so that the method is not limited by the type and wavelength of the laser.

Liquid Refractive Index Measurement System Based on Electrowetting Lens

In this paper, a liquid refractive index (LRI) measurement system based on an electrowetting lens was proposed. The system is composed of a light source, a collimating lens, a liquid measurement chamber (LMC), an electrowetting lens and an image sensor, which is integrated into a cylindrical cavity. The refractive index of the LMC changes with the addition of the measured liquid, and the incident light cannot be focused on the image plane. By adjusting the driving voltage of the electrowetting lens, the curvature of the liquid-liquid interface changes to focus the incident light onto the image plane. The refractive index of the liquid could be measured according to the voltage value. The proposed LRI measurement system has no mechanical moving parts, and the imaging surface remains stationary, which can make the measurement simply and correctly. The experiments show that the refractive index measurement range of the system can be turned from ~1.3300 to ~1.4040, and the measurement accuracy is 10−4. The system can be used to measure the optical properties of liquids and has broad potential applications in chemical reagent detection and pharmaceutical testing.

Integrated Fiber-Optic Fabry-Pérot Interferometer Sensor for Simultaneous Measurement of Liquid Refractive Index and Temperature

In this paper, an integrated all-fiber Fabry–Perot interferometer (FPI) sensor is proposed and fabricated for the simultaneous measurement of liquid refractive index (RI) and temperature. The sensor structure consists of an open micro-hole and a tiny section of single-mode fiber. Since, the micro-hole that is machined by 193 nm excimer laser, the micro-fiber, and the integrated micro-hole-fiber, respectively, act as different FPI, consequently, the regular interference spectrum is produced and used in simultaneous measurement based on analyzing wavelength shifts of interference fringes with Fourier band-pass filtering method. Experiments indicate the low-frequency interference fringes resulting from micro-hole cavity is sensitive to liquid RI and temperature changes, and the response sensitivities are 1143.0 nm/RIU and −0.1805 nm/°C, respectively. However, the high-frequency interference fringes of micro-fiber cavity is only sensitive to temperature change, and the obtained temperature sensitivity of 0.0115 nm/°C is obviously superior to that of integrated micro-hole-fiber cavity. Utilizing the different response coefficients, we not only construct a measurement matrix to realize simultaneous liquid RI and temperature measurement successfully, but also achieve resolutions of 0.1 °C and $1.4\boldsymbol {\times } 10^{-5}$ RIU. The excellent merits such as all-fiber, miniature size, simple structure, and wavelength-sensing structure make the FPI sensor promise for high precision bio/chemical sensing applications.

All-Fiber Fabry–Perot Interferometer for Liquid Refractive Index Measurement

We propose and design an all-fiber Fabry–Perot interferometer (FPI) for liquid refractive index (RI) measurements. The sensor comprises a short section of a single-mode fiber (SMF) fused between the end sections of two longer SMFs with a large intentional lateral offset. The open FPI cavity is obtained by adjusting the lateral offset. Experimental results show that the liquid to be measured can freely enter and leave the cavity without residual bubbles. The RI sensitivity of this sensor is 1,013.8 nm/RIU in the RI range from 1.3464 to 1.3777 with good linearity. Only cleaving and fusion splicing are used in its making; thus the fabrication of the sensor is relatively simple and cost effective. This sensor has potential applications in the chemical and biomedical domains.

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.

Research on Measurement of Liquid Refractive Index Using Fem to Second Laser Frequency Comb

In this paper, we develop a method to measure the refractive index of liquid at different temperatures applying the femtosecond laser frequency comb. The measurement principle is to test the phase changes of the measuring signal caused by light traveling through the liquid, then calculating the refractive index according to the phase difference between water and air. The experiment on measuring the water refractive index at temperature from 20oC to 60oC was performed, and the measuring results show a satisfactory accuracy.

Dual-resonance long-period grating in fiber loop mirror structure for liquid refractive index measurement

An interferometric structure based on a Dual-Resonance Long-Period Grating (DRLPG) within a Fiber Loop Mirror (FLM) is presented in this paper. Its purpose is to measure the refractive index (RI) of liquid analytes. The grating is the RI sensing probe, while the FLM serves as a band-pass filter. Due to the high extinction ratio of the FLM, amplitude measurements can be obtained, allowing implementation of the differential interrogation method to establish the sensitivity of the device. The use of a polarization controller makes it possible to fine-tune the interferometric peaks with respect to the two notches of the DRLPG. Precisely aligned configuration produces a maximum sensitivity of 3871.5 dB/RIU within the RI range of 1.3333 up to 1.3419 with linear sensor response.

Refractive Index Measurement of Liquids by Double-Beam Laser Frequency-Shift Feedback

The liquid refractive index measurement has become an important process in many research and industrial applications. However, the measurement results using traditional methods cannot be calibrated, and the corresponding accuracy is limited, since those methods cannot trace to laser wavelength. This letter describes a novel method to measure the liquid refractive index based on the double-beam laser frequency-shift feedback. The two beams are incident on the liquid surface and the mirror under liquid, respectively. Utilizing the high sensitivity of the frequency-shift feedback of solid lasers and high stability of heterodyne interference, the height variation of the liquid surface and the depth variation of the mirror under liquid are measured. Addressing these two variations, the traceable measurement of liquid refractive index is realized. The measurement results of five different liquid samples and NaCl solution with various concentrations are presented, which proved the system has high repeatability of better than 0.00005.

A polarization-maintaining fiber loop mirror based sensor for liquid refractive index absolute measurement

A fiber sensor based on polarization-maintaining fiber loop mirror (FLM) for liquid refractive index absolute measurement is described. Two sections of polarization maintaining fibers (PMFs) with different length were inserted into the FLM. One section of PMF was chemical etched for liquid refractive index measurement and the other one for reference. Liquid refractive index absolute measurement can be obtained by detecting the interference wavelength difference corresponding to the two sections of PMFs. High sensitivity of 63.82±0.04nm/RIU (refractive index unit) and a resolution of 3.14×10−4RIU were obtained for the proposed liquid refractive index sensor.

Michelson interferometer-based fiber-optic sensing of liquid refractive index

A fiber-optic sensor for liquid refractive index measurement based on a Michelson interferometer is described. The interferometer is achieved between the Fresnel reflections from the fiber ends of the coupler. The fiber ends are then in contact with the solvent and solution samples to provide reference and refractive index measurement, respectively. The refractive index of the solution is determined according to the fringe contrast of the interference fringes in the reflective spectrum of the sensor. Its ease of fabrication and low-cost offer the attractive applications in chemical and biological sensing.

Full-field surface plasmon resonance sensor for high resolution refractive index measurement using common-path heterodyne interferometer

This study presents a full-field surface plasmon resonance (SPR) sensor induced by attenuated total reflection (ATR)-couple for liquid refractive index measurement. The system adopts a common-path heterodyne interferometer to measure the phase difference between P- and S-wave after passing through the SPR sensor. In order to realize the full-field measurement, it adopts a three-frame integrating-bucket method. The experimental results show great consistency profile between single point and full-field liquid refractive index measurement from 1.330 to 1.340 RIU. It shows that the best sensitivity and resolution of a single pixel in charge couple device (CCD) for liquid refractive measurement are 3.3 × 10 4 (deg/RIU) and 3.53 × 10 −6 (RIU), respectively. As compared with traditional single-point method, the proposed method with a regular CCD has no degradation. Therefore, the system has many applications in chemistry and biology.

Fiber-Optic Probe Based on a Bifunctional Lensed Photonic Crystal Fiber for Refractive Index Measurements of Liquids

We report a fiber-optic sensor for measuring the refractive index (RI) of a liquid sample. The sensing probe is constructed from an extrinsic cavity formed by a micromirror and a lensed photonic crystal fiber (PCF) tip. We show that PCF lens with an unconventionally large radius of curvature is bifunctional; an effective reflector as well as a collimator and thus suitable for realizing long single-arm interferometers, with a cavity length of the order of few mm. A sensing head suitable for measuring the refractive index (RI) of liquids is constructed by encapsulating the bifunctional lens tip and a micromirror in a glass tube. A hole is made in the glass-tube to allow free-flow of the liquid sample in-and-out of the cavity. The cavity length was about 1 mm. The group index of the liquid samples is obtained from the Fourier peak position of the interference signal measured in wavelength with optical spectrometer. The RIs of distilled water, acetone and ethanol (at 829 nm, 20°C) were measured to be 1.32822, 1.35416, and 1.35715, respectively. The resolution of the sensor was analytically found to be 2.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> and the response was experimentally shown to be linear.

Microfiber-probe-based ultrasmall interferometric sensor

We report an ultrasmall microfiber-probe-based reflective interferometer for highly sensitive liquid refractive index measurement. It has a 3.5 microm micronotch cavity fabricated by focused ion beam micromachining. A sensitivity of 110 nm/RIU (refractive index unit) in liquid is achieved with over 20 dB extinction ratio. Theoretical analysis shows this kind of device is a hybrid of Fabry-Perot and modal interferometers. In comparison with normal fiber interferometers, this probe sensor is very compact, stable, and cheap, offering great potentials for detecting inside sub-wavelength bubbles, droplets, or biocells.