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Tomographic coherent imaging requires the reconstruction of a series of two-dimensional projections of the object. We show that using the solution for the image of one projection as the starting point for the reconstruction of the next projection offers a reliable and rapid approach to the image reconstruction. The method is demonstrated on simulated and experimental data. This technique also simplifies reconstructions using data with curved incident wavefronts.

International audience; Evanescent-wave scattering is a topic in classical electrodynamics and in the study of colloidal particles near a boundary. However, how such near-surface scattering at subcellular refractive-index heterogeneities degrades the excitation confinement in biological total internal reflection fluorescence microscopy has not been well studied. An elegant theoretical work by Axelrod and Axelrod now addresses this very relevant question and reveals that-even when scattered-evanescent light preserves some of its surprising optical properties.

Critical angle refractometry is an established technique for determining the refractive index of liquids and solids. For transparent samples, the critical angle refractometry precision is limited by incidence angle resolution. For lossy samples, the precision is also affected by reflectance measurement error. In the present study, it is demonstarted that reflectance error can be practically eliminated, provided that the sample’s extinction coefficient is a priori known with sufficient accuracy (typically, better than 5%) through an independent measurement. Then, critical angle refractometry can be as precise with lossy media as with transparent ones.

Quantification of molecular interactions on a surface is typically achieved via label-free techniques such as surface plasmon resonance (SPR). The sensitivity of SPR originates from the characteristic that the SPR angle is sensitive to the surface refractive index change. Analogously, in another interfacial optical phenomenon, total internal reflection, the critical angle is also refractive index dependent. Therefore, surface refractive index change can also be quantified by measuring the reflectivity near the critical angle. Based on this concept, we develop a method called critical angle reflection (CAR) imaging to quantify molecular interactions on glass surface. CAR imaging can be performed on SPR imaging setups. Through a side-by-side comparison, we show that CAR is capable of most molecular interaction measurements that SPR performs, including proteins, nucleic acids and cell-based detections. In addition, we show that CAR can detect small molecule bindings and intracellular signals beyond SPR sensing range. CAR exhibits several distinct characteristics, including tunable sensitivity and dynamic range, deeper vertical sensing range, fluorescence compatibility, broader wavelength and polarization of light selection, and glass surface chemistry. We anticipate CAR can expand SPR′s capability in small molecule detection, whole cell-based detection, simultaneous fluorescence imaging, and broader conjugation chemistry., Here, the authors present a method for quantifying molecular interactions on a glass surface, based on measuring surface refractive index changes via the reflectivity near the critical angle. They demonstrate tunable sensitivity and dynamic range, deep vertical sensing range, also for intracellular signals.

When developing oil fields, there is an urgent task to quickly determine the type of pumped formation fluid, which includes formation gas, formation oil and formation water. In this paper, we propose a new type of a sensor element designed for flow refractometry of formation fluid based on the effect of total internal reflection. The sensor element is a taper tip of a conical shape made of sapphire and is 20 mm in length and 20 mm in diameter. The original shape of the sensor element is determined by a modified ray tracing method, taking into account analytical relations that determine the conditions for providing a larger dynamic range of measurements under specified physical, technological and design constraints. The conversion dependence of the tip is obtained for the wavelengths of 405 nm, 1064 nm and 3300 nm and allows determining the type of formation fluid (gas/water/oil). The proposed method enables the development of conical sensor elements based on the total internal reflection for downhole monitoring systems and optical threshold sensors of the refractive index.

In modern large-sized tokamaks and those under construction, for example, in ITER, when measuring the average plasma electron density using the refractometry technique, it becomes necessary to study different effects that affect the measurement accuracy. This paper presents the results from studies of the high-temperature effects that can affect the interpretation of delay time measurements of the probing microwave signal in the ITER refractometry diagnostics. The criteria for applicability of different plasma models (“cold” plasma, modified “cold” plasma, and “hot plasma”) are obtained for calculating the plasma refractive index and interpreting the delay times that will be measured using the refractometry diagnostics of the ITER plasma. In the second article, based on the results obtained in this article, we will consider the influence of the high-temperature effects on the accuracy of determining the mean density using the refractometry diagnostics.

In this work, the splitting of resonance lines in a Fabry-Perot resonator formed by a section of a standard curved single-mode fiber with metal-coated ends is investigated numerically and experimentally. It is shown that this splitting is similar to the Autler-Townes splitting and results from a strong coupling between the fundamental mode of the core and the whispering gallery mode of the cladding of the curved fiber. The influence of all basic parameters of the curved resonator on the splitting of its resonance lines in the reflection and transmission spectra is considered. Prospects for the practical application of the effects studied for high-resolution optical refractometry, as well as the direction of further research are outlined.

In this study, a continuous cell-imaging system with subcellular resolution was developed by integrating a microfluidic platform with lattice lightsheet microscopy (LLSM). To reduce aberrations of the lightsheet propagating into the device, a microfluidic channel sealed with a water refractive index-matched thin film was fabricated. When the lightsheet emerged from the water-immersed objectives and penetrated through the water refractive-matched thin film into the microfluidic channel at an incident angle, less light scattering and fewer aberrations were found. Suspended cells flowed across the lattice lightsheet, and an imaging system with the image plane perpendicular to the lightsheet was used to sequentially acquire cell images. By applying a thinner lattice lightsheet, higher-resolution, higher-contrast images were obtained. Furthermore, three-dimensional cell images could be achieved by reconstructing sequential two-dimensional cell images.

We present a numerical and experimental study of resonance splitting in the transmission and reflection spectra of the Fabry–Perot resonator made of a bent section of a conventional single-mode optical fiber with metallized end faces. We show the splitting to be similar in nature to the well-known Autler–Townes effect and to arise from the strong coupling between the core mode and cladding whispering gallery mode of the bent fiber. The effect of all major parameters of the bent resonator on the splitting of its resonances is demonstrated. Potential applications of the observed effects in the field of precision optical refractometry as well as further research directions are discussed.

Herein, we demonstrate a cavity-enhanced hyperspectral refractometric imaging using an all-dielectric photonic crystal slab (PhCS). Our approach takes advantage of the synergy between two mechanisms, surface-enhanced fluorescence (SEF) and refractometric sensing, both based on high-Q resonances in proximity of bound states in the continuum (BICs). The enhanced local optical field of the first resonance amplifies of 2 orders of magnitude the SEF emission of a probe dye. Simultaneously, hyperspectral refractometric sensing, based on Fano interference between second mode and fluorescence emission, is used for mapping the spatially variant refractive index produced by the specimen on the PhCS. The spectral matching between first resonance and input laser is modulated by the specimen local refractive index, and thanks to the calibrated dependence with the spectral shift of the Fano resonance, the cavity tuning is used to achieve an enhanced correlative refractometric map with a resolution of 10-5 RIU within femtoliter-scale sampling volumes. This is experimentally applied also on live prostate cancer cells grown on the PhCS, reconstructing enhanced surface refractive index images at the single-cell level. This dual mechanism of quasi-BIC spatially variant gain tracked by quasi-BIC refractometric sensing provides a correlative imaging platform that can find application in many fields for monitoring physical and biochemical processes, such as molecular interactions, chemical reactions, or surface cell analysis.

Digital holographic microscopy (DHM) is an important technique that may be used for quantitative phase imaging of unstained biological cell samples. Since the DHM technology is not commonly used in clinics or bioscience research labs, at present there is no well-accepted focusing criterion for unstained samples that users can follow while recording image plane digital holograms of cells. The usual sharpness metrics that are useful for auto-focusing of stained cells do not work well for unstained cells as there is no amplitude contrast. In this work, we report a practical method for estimating the best focus plane for unstained cells in the digital hologram domain. The method is based on an interesting observation that for the best focus plane the fringe pattern associated with individual unstained cells predominantly shows phase modulation effect in the form of bending of fringes and minimal amplitude modulation. This criterion when applied to unstained red blood cells shows that the central dip in the doughnut-like phase profile of cells is maximal in this plane. The proposed methodology is helpful for standardizing the usage of DHM technology across different users and application development efforts. LAY DESCRIPTION: Digital holographic microscopy (DHM) is slowly but steadily becoming an important microscopy modality and gaining acceptability for basic bio-science research as well as clinical usage. One of the important features of DHM is that it allows users to perform quantitative imaging of unstained transparent cells. Instead of using dyes or fluorescent labelling, DHM systems use quantitative phase as a contrast mechanism which depends on the natural refractive index variation within the cell samples. Since minimal wet lab processing is required in order to image cell samples with a DHM, cells can be imaged in their natural state. While DHM is gaining popularity among users, the imaging protocols across the labs or users need to be standardized in order to make sure that the same quantitative phase parameters are used for tasks such as quantitative phased based cell classification. One of the important operational tasks for any microscopy work is to focus the sample under study. While focusing comes naturally to users of brightfield microscopes based on image contrast, the focusing is not straightforward when samples are unstained so that they do not offer any amplitude contrast. When performing quantitative phase imaging, defocus can actually change the phase profile of the cell due to near-zone (Fresnel) diffraction effects. So unless a standardized focusing methodology is used, it will be difficult for multiple DHM users (potentially at different sites) to agree on quantitative results out of their phase images. DHM literature has prior works which perform numerical focusing of recovered complex wave-field in the hologram plane to find the best focus plane. However such methods are not user friendly and do not allow user the same focusing experience as in a brightfield microscope. The numerical focusing is therefore a reasonably good method for an optics researcher but not necessarily so for a microscopy technician looking at cell samples with a DHM system in a clinical setting. The present work provides a simple focusing criterion for unstained samples that works directly in the hologram domain. The technique is based on an interesting observation that the when an unstained cell sample is in the best-focus plane, its corresponding hologram (or fringe pattern) predominantly shows phase modulation manifested by bending of fringes at the location of the cell. This criterion can be converted into a simple numerical method as we have used to find the best-focus plane using a stack of through focus holograms. We believe that the technique can be used manually by visually observing the holograms or can be converted to an auto-focus algorithm for a motorized DHM system.

We propose and experimentally demonstrate a highly flexible and applicable configuration, namely, the “ v ertical-fluid-array i nduced o ptic microfiber l ong per i od grati n g” (VIOLIN), for fully exploring the characteristics of liquids. The presence of flowing liquid in the periodically carved grooves of the designated substrate can activate a long-period grating resonance, as the microfiber is attached. Since the features of the resonance depend strongly on the refractive index of the fluid, the VIOLIN can be utilized as a refractometer with a high sensitivity of 2443 nm/RIU. Modeling and simulation are employed to investigate the mechanism of the sensor, which can be described as follows briefly: “Liquid renders the resonance to light. Light deciphers the secret of liquid.” Moreover, due to the highly elastic control of the fluidic array and the detachable configuration, multiple sensing and easy reconfiguration of the VIOLIN can be realized. The VIOLIN is expected to be highly suitable for microfiber-based chemo/biosensing applications.

In this research work, an anisotropic photonic crystal fiber (PCF) biosensor working on a refractive index (RI) variation and based on surface plasmon resonance (SPR) is presented. Liquid analytes (LA) having a RI within the range of 1.340 to 1.380 RIU are investigated from the proposed biosensor. Spectroscopy analysis of LA having RI values of 1.340 RIU, 1.360 RIU, and 1.380 RIU is performed from the developed sensing setup for modeling an ultrasensitive biosensor. The numerical analysis of the sensing parameters for the proposed sensor presents a maximum wavelength sensitivity (WS) of 20000 nm/RIU for x- polarization (x − pol.) and 18000 nm/RIU for y- polarization (y − pol.), respectively, using the wavelength interrogation technique. Maximum amplitude sensitivity (AS) of 2158 RIU−1 and 3167 RIU−1 is obtained for x − pol. and y − pol., respectively, using the amplitude interrogation technique. Maximum sensor resolution (SR) of 5.00 × 10−6 RIU and 5.55 × 10−6 RIU is obtained for x − pol. and y − pol., respectively. The linear relationship of the resonant wavelength (RW) with the RI produces R2 = 0.9972 and R2 = 0.9978, corresponding to a degree (2) for x − pol. and y − pol., respectively. The figure of merit (FOM) for x − pol. and y − pol. are 93.45 RIU−1 and 105.88 RIU−1, respectively. The sensing parameters have obtained the maximum value for the LA having a RI value of 1.375 RIU.

Fourier ptychographic diffraction tomography (FPDT) is a recently developed label-free computational microscopy technique that retrieves high-resolution and large-field 3D tomograms by synthesizing a set of low-resolution intensity images obtained with a low numerical aperture (NA) objective. However, in order to ensure sufficient overlap of Ewald spheres in 3D Fourier space, conventional FPDT requires thousands of intensity measurements and consumes a significant amount of time for stable convergence of the iterative algorithm. Herein, we present accelerated Fourier ptychographic diffraction tomography (aFPDT), which combines sparse annular LED illuminations and multiplexing illumination to significantly decrease data amount and achieve computational acceleration of 3D RI tomography. Compared with existing FPDT technique, the equivalent high-resolution 3D RI results are obtained using aFPDT with reducing data requirement by more than 40 times. The validity of the proposed method is experimentally demonstrated on control samples and various biological cells, including polystyrene beads, unicellular algae, and clustered HeLa cells in a large field of view. With the capability for high-resolution and high-throughput 3D imaging using small amounts of data, aFPDT has the potential to further advance its widespread applications in biomedicine. This article is protected by copyright. All rights reserved.

Rainbow refractometry has been used in the past to measure size and refractive index of spherical particles, typically droplets in a spray. In the present study, conventional optical configurations for point measurements or line measurements have been extended to allow also the particle position in a plane to be determined, and hence, the designation planar rainbow refractometry. However, this extension introduces challenges in accurately calibrating the 2D scattering angles with the image coordinates. This challenge has been met using a novel calibration method, employing a monodispersed droplet stream traversed through the measurement plane. Experiments confirm achievable horizontal and vertical position accuracies of 0.42 mm and 0.36 mm, respectively, and a refractive index uncertainty of 2 × 10 − 4 .

In modern 3D microscopy, holding and orienting arbitrary biological objects with optical forces instead of using coverslips and gel cylinders is still a vision. Although optical trapping forces are strong enough and related photodamage is acceptable, the precise (re-) orientation of large specimen with multiple optical traps is difficult, since they grab blindly at the object and often slip off. Here, we present an approach to localize and track regions with increased refractive index using several holographic optical traps with a single camera in an off-focus position. We estimate the 3D grabbing positions around several trapping foci in parallel through analysis of the beam deformations, which are continuously measured by defocused camera images of cellular structures inside cell clusters. Although non-blind optical trapping is still a vision, this is an important step towards fully computer-controlled orientation and feature-optimized laser scanning of sub-mm sized biological specimen for future 3D light microscopy., Optical manipulation of large objects is challenging as optical trap positions are blindly chosen. Here, the authors present off-focus interferometric tracking, which localises optimal grabbing positions with increased refractive index, by analysing the beam deformations of several holographic optical traps.

The optical properties of biochemical compositions of microalgae are vital for the improvement of biosensor design, photobioreactor design, biofuel, and biophotonics techniques. A combination method using both the double optical pathlength transmission method (DOPTM) and the ellipsometry method (EM) is called DOPTM-EM, and it is used to acquire the optical constants of protein, lipid, and carbohydrate of Haematococcus pluvialis, Nannochloropsis sp., and Spirulina in both a solid state and a solution state within the visible and near-infrared spectral range. For different types of microalgae, the refractive indices of protein and carbohydrate in the solid state are similar to each other, but show an observed difference from lipid in the solid state. The refractive indices of protein and carbohydrate in the solution state presents a visible distinction in the researched spectral range. The absorption indices of protein, lipid, and carbohydrate in the solid state for these three types of microalgae are close to each other in the spectral range of 300–500 nm. However, an observed difference is shown in the spectral range of 500–1700 nm. For ease of application, the refractive index of biochemical composition of microalgae was fitted based on the Sellmeier equation. We believe this work can provide a reference to obtain the optical properties of biomaterial with high accuracy.

The SPR phenomenon results in an abrupt change in the optical phase such that one can measure the phase shift of the reflected light as a sensing parameter. Moreover, many studies have demonstrated that the phase changes more acutely than the intensity, leading to a higher sensitivity to the refractive index change. However, currently, the optical phase cannot be measured directly because of its high frequency; therefore, investigators usually have to use complicated techniques for the extraction of phase information. In this study, we propose a simple and effective strategy for measuring the SPR phase shift based on phase-shift interferometry. In this system, the polarization-dependent interference signals are recorded simultaneously by a pixelated polarization camera in a single snapshot. Subsequently, the phase information can be effortlessly acquired by a phase extraction algorithm. Experimentally, the proposed phase-sensitive SPR sensor was successfully applied for the detection of small molecules of glyphosate, which is the most frequently used herbicide worldwide. Additionally, the sensor exhibited a detection limit of 15 ng/mL (0.015 ppm). Regarding its simplicity and effectiveness, we believe that our phase-sensitive SPR system presents a prospective method for acquiring phase signals.

The optical fiber grating sensors have strong potential for the detection of biological samples. However, a careful effort is still in demand to enhance the performance of existing grating sensors especially in biological sensing. Therefore, in this work, we have introduced a novel plus shaped cavity (PSC) in optical fiber model and used it for the detection of haemoglobin (Hb) refractive index (RI). The numerical analysis of designed model is done by the testing of single and double vertical slots cavity in optical fiber core structure. The testing of designed sensor model is done at the wavelength of 800 nm at which the RI of oxygenated and deoxygenated Hb is 1.392 and 1.389, respectively. The analysis of reported PSC sensor model is done in the wide range of Hb RI from 1.333 to 1.392. The tested range of RI corresponds to the Hb concentration from 0 to 140 gl-1. The obtained results states that for the tested range of RI, the autocorrelation coefficientt of R2 = 99.51 % is achieved. The analysis of projected work is done by using finite difference time domain (FDTD) method. The introduction of PSC can increase in sensitivity. In proposed PSC, the length and width of created slots are 1.8 μm and 1 μm, respectively, which is quite enough to observe the response of analytes RI. This can minimize the creation of multiple gratings required for observing the analyte response.

Optical fiber ball resonators based on single-mode fibers in the infrared range are an emerging technology for refractive index sensing and biosensing. These devices are easy and rapid to fabricate using a CO2 laser splicer and yield a very low finesse reflection spectrum with a quasi-random pattern. In addition, they can be functionalized for biosensing by using a thin-film sputtering method. A common problem of this type of device is that the spectral response is substantially unknown, and poorly correlated with the size and shape of the spherical device. In this work, we propose a detection method based on Karhunen−Loeve transform (KLT), applied to the undersampled spectrum measured by an optical backscatter reflectometer. We show that this method correctly detects the response of the ball resonator in any working condition, without prior knowledge of the sensor under interrogation. First, this method for refractive index sensing of a gold-coated resonator is applied, showing 1594 RIU−1 sensitivity, then, this concept is extended to a biofunctionalized ball resonator, detecting CD44 cancer biomarker concentration with a picomolar-level limit of detection (19.7 pM) and high specificity (30–41%).

In this paper, a novel refractometer based on Mach-Zehnder Interferometer (MZI) is proposed and experimentally investigated. The MZI is composed of 2 micro bending cores (MBCs), one of which excites the cladding modes and the other couples the modes back. This structure is formed by high-frequency CO2 laser polishing and oxyhydrogen flame heating. With the unique deformation method, the interaction between the fiber core and the external status gets enhanced, moreover, higher modes in the cladding are excited, which leads to a high refractive index (RI) sensitivity. Due to the high temperature of the oxyhydrogen flame, the core of CO2 polished fiber is modulated, furthermore, the cladding shape of MBC tends to be circular. Hence, relatively small modulating regions of 500 μm can form for interference. In the experiment, 2 transmission dips are chosen for RI measuring, which possesses the wavelength of 1530.4 nm and 1600.8 nm, respectively. The RI sensitivities of the 2 transmission dips are -271.7 nm/RIU and -333.8 nm/RIU with the RI range of 1.33-1.42. The temperature characteristic is also experimentally analyzed and the temperature sensitivities of which are 0.121 nm/℃ and 0.171 nm/℃ in the range of 34℃-154℃. By solving the matrix equation, the proposed sensor can be applied for simultaneous measurement of RI and temperature.

PURPOSE Optical rehabilitation for hemianopia includes the prescription of partial aperture prismatic lenses. Fresnel lenses for this purpose have poor optical quality, while bonded lenses have poor cosmesis and can only be made in glass, creating a potential hazard. Here we present an alternative lens type, which does not reduce contrast sensitivity and which can be made in plastic materials. METHODS A rotated Franklin split lens was prescribed for the right eye only to provide a full aperture prismatic lens without using a Fresnel prism or bonded lenses. Using different refractive indices in each lens provided a minimal transition in thickness and an acceptable cosmetic appearance. This lens was prescribed to a 34-year-old woman with homonymous right hemianopia. Her contemporaneous comments are presented. RESULTS The rotated Franklin split lens provided 18Δ over the temporal edge of the lens. The wearer noticed chromatic aberration with this lens, but was able to cycle comfortably when wearing the lens without any reduction in contrast. CONCLUSIONS A rotated Franklin split lens can be used for the optical rehabilitation of hemianopia. In this case study, cycling was possible with this lens.

We explore the refractive index (RI) modulation of bioreaction induced self-assembled magnetic fluid (SAMF) so as to facilitate the design of sensitive biosensors. Bovine serum albumin (BSA) is taken as a model protein to induce self-assembly of carboxylated magnetic nanoparticles (MNPs). A bidirectional magnetophoresis (BDMP) system is designed to generate uniform and large RI modulation to SAMF. A robust Mach–Zehnder interferometer compatible to the BDMP system is used for the RI detection. It is found that BDMP has greater ability in modulating the RI of magnetic fluid than the alignment of MNPs in a uniform magnetic field. The modulation depth of the RI increases with the increase of BSA concentration when applying a certain external magnetic field for a period of time. A larger magnetic field has greater ability to modulate the RI of SAMF. However, it may generate an over-modulation.

In oil and gas production, it is important to determine the type and chemical composition of the formation fluids in the pipeline in real time. Different types of the hydrocarbon formations present in the downhole environment have distinct optical characteristics, such as spectral transmittance, diffusion, fluorescence and refractive index. In the formation sampling and testing, knowing the refractive index of a downhole fluid makes it possible to identify the fluid (water, gas or oil), estimate various properties, such as salinity and crude oil density, and monitor the cleanup processes. The paper describes a new design for an on-axis inline process refractometer for continuous fluid measurements using a defocusing imaging technique. Design principles for the refractometer are discussed, the dependence of the defocus effect on the refractive index is calculated. Experimental energy distributions for different liquid samples are obtained and analysed.

Endoscopic optical coherence tomography of large airways poses unique challenges. A hybrid lens is described that consists of a section of coreless fiber and graded index fiber (GIF), followed by a ball lens section. This design produces low numerical aperture beams better suited for large airway imaging. The performance of this lens is compared against conventional GIF and ball lens designs. Forward- and side-viewing probes were modeled, fabricated, and tested. The impact of a sheath on the beam profile was also investigated. Probes with working distances larger than 10 mm and depth-of-focus exceeding 12 mm are demonstrated with the proposed design.

This paper is devoted the base principles of the method of laser refractography and the results of its application in experimental visualization of optically inhomogeneous liquids. Laser refractography is a novel information-measurement technique based on the probing of fluid flows with a structured laser beams, digital recording of the refraction images obtained, and their computer processing with a view to reconstructing the parameters of the flows. Block diagrams of measuring laser systems are shown, and typical refracrograms are presented.

A novel demodulation method for tilted fiber Bragg grating (TFBG) refractometry is proposed and experimentally demonstrated. This “integrated differential area” method consists of calculating the difference between measured spectral transmission over a selected bandwidth and a reference spectrum and then integrating the absolute value of the result. This has the effect of multiplying the effect of the individual cladding mode resonance shifts relative to the background noise. The method further allows the use of spectrum analyzers with low resolution, including portable ones with spectral resolution of the order of 150–200 pm without loss of sensitivity. In practical measurements, a reflection configuration is demonstrated using a TFBG cascaded by a chirped-fiber Bragg grating with bandwidth of 19 nm. A refractive index sensitivity of 7.19% transmission change per refractive index unit is achieved for refractive indices ranging from 1.32 to 1.38, along with a limit of detection of 0.00026 refractive index units.

Infrared refractive index is an indispensable parameter for various fields including infrared photonics. To date, critical-angle refractometers, V-block refractometers, and spectroscopic ellipsometry have been commonly used to measure the refractive index. Although every method has an accuracy of four decimal places for the refractive index, a measurable wavelength region is limited up to about 2 µm. In this study, we demonstrated a metamaterial infrared refractometer for determining broadband complex refractive index. Using the device, a broadband (40–120 THz; wavelength 2.5–7.5 µm) and high-precision(< 5 ×10−3) complex refractive index of polymethyl methacrylate was measured for the first time.

The reflectance of a hybrid fiber Fabry–Perot interferometer modulated by the Fresnel reflection at the fiber tip was used to assess the refractive index (RI) of liquid samples. Notwithstanding the closeness of RI values between samples and fiber core, well-defined interference spectra in Fourier domain were obtained after a robust signal processing algorithm. Then, the analysis of these spectra allowed us to derive a mathematical expression to obtain the RI value of the fiber tip surrounding media. Next, to assess the fiber refractometer measuring capability a series of liquid samples, typically used for RI calibration, with values ranging from 1.3924 to 1.5882 were measured. A standard deviation of 7 × 10−4 between the measured and actual RI values was estimated. Finally, the refractometer was used to monitor the RI of a sample, with a high thermo-optic coefficient, while it was heated. The accurate and compact all-fiber refractometer proposed here exhibited immunity to optical power fluctuations and one of the largest dynamic ranges reported. This set of combined features make it very attractive for real-time label-free biosensing applications.

The refractive index of a biological tissue is required for investigating the tissue's optical properties. Efforts have been made to characterize the refractive indices of biological tissues at a single wavelength, but it is more convenient to know the Cauchy's coefficients, which provide refractive index over a wide range of wavelengths. We demonstrate a method to noninvasively provide the Cauchy's dispersion coefficients of biological tissues using a micro-optical coherence tomography. Using the short-frequency Fourier transforms, the relative optical thickness of the sample in the wavelength range of the broadband source was obtained from interferograms. The coefficients of the Cauchy's equation were estimated based on the wavelength-dependent sample thickness. We validated the proposed method using distilled water and fresh rat cornea ex vivo , and the experimental results were consistent with the reference data.

We present a spatial light modulator (SLM) based phase shifting interferometry (PSI) using an optical vortex for sensing the refractive index (RI) of liquids. The vortex phase rotates according to the phase shift introduced by a medium, and its rotation angle quantifies the index of refraction of the sample. An interferometer modified with the SLM is designed to obtain reliable results. We experimentally demonstrate the stability of the SLM-based system by measuring interference and phase fluctuation. Experimental results show that the proposed system keeps the fluctuation small. The vortex probe PSI combined with the SLM-based interferometer enables not only detecting extremely small changes in concentration but also precisely measuring the RI with high resolution of ∼10−5 refractive index unit.