Precise Measurement Of Refractive Index Research Articles

Polarization-maintaining fiber based macehead shaped interferometric sensor for accurate measurement of refractive index and temperature

A macehead-shaped bent polarization-maintaining fiber-based interferometric sensing structure called MBPIS is described and experimentally demonstrated for precise temperature and refractive index measurement. The sensor’s working principle is explained by simulating the spatial distribution of the field intensity in straight and bending PANDA fibers. A maximum extinction ratio (∼21 dBm) for the interference dip wavelength (1527.825 nm) in the sensor’s output spectrum was optimized by manipulating the birefringence of propagating fiber modes by adjusting PMF’s bending diameter from 17 to 11 mm. The phase difference changes between these fiber modes due to temperature and RI-induced birefringence cause a shift in the interference spectrum. The sensor’s highest RI sensitivity has been seen at −259.32 nm/RIU for a wide range of analytes from 1.3333 to 1.3579. In contrast, the highest temperature sensitivities evaluated for the temperature range of 0 ∼ 100 ℃ are −220 pm/℃ and −0.139 dBm/℃, respectively.

Precise refractive index measurement of fused silica optics

Abstract We have developed an experimental platform that non-destructively and precisely measures the Refractive Index (RI) and dispersion of ultra-polished fused silica optics. Using Total Internal Reflection Digital Holographic Microscopy (TIRDHM), we exploit the phase change of reflected light in Total Internal Reflection (TIR) mode. This phase change depends on the incident angle at the TIR interface and the refractive indices of the involved media. We have optimized a combination of higher TIR phase sensitivity, considerable penetration depth, and minimized phase measurement inaccuracies through simulations to design our experiment. Key features include a custom-made precision Right-Angle Prism (RAP) of Astrositall material, a seamless interface with fused silica optics on TIR interface through optical contact, and single-shot measurement. We have demonstrated the accuracy of measuring fused silica optics through proof of concept and experimental results. Our measurements on two different samples show accuracy better than ±3 × 10−4 compared to those obtained using a commercially available critical angle Refractometer (Metricon). Importantly, the setup offers the advantage of spatially mapping the refractive index, unlike point measurements by available Refractometers.

Investigation of polymer template removal techniques in three-dimensional thin-shell nanolattices

Recent advanced in nanofabrication has enabled various opportunities for research and development in photonic crystals, integrated circuits, and nanostructured materials. One interesting class of emerging materials is nanolattices, which consist of hollow-core, thin-shell elements fabricated using thin-film deposition on three-dimensional polymer templates. While many applications of nanolattices have been demonstrated, the residual polymer in the nanolattice can be problematic and is not well understood. This research investigates the effectiveness of different template removal techniques, including oxygen plasma etching, solvent dissolution, and thermal desorption. The rates and effectiveness of resist removal for the different techniques are quantified using spectroscopic ellipsometry, which enables precise measurement of the effective refractive index and calculation of the residual polymer. A three-phase Maxwell–Garnett effective medium model is used to calculate the residual polymer in the nanolattices. This work demonstrates that the temperature treatment is most effective at template removal, which can be used to improve the fabrication of nanolattices for mechanical, optical, and thermal applications.

Precise measurement of refractive index in the ambient environment using continuous-wave terahertz frequency-domain spectroscopy (THz-FDS)

In this paper, we present the application of THz frequency-domain spectroscopy (THz-FDS) for determining the refractive index in an ambient environment. The signal phase is extracted from the periodically oscillated photocurrent by a magnitude normalization method. The proposed method is demonstrated using experimental data of polytetrafluoroethylene and α-lactose monohydrate as reference materials collected at a relative humidity of around 17.0% ± 2.0%. A detailed comparison to the existing algorithms, including the Hilbert transform and extreme points analysis, reveals the remarkable reliability of our proposed method. This study expands the characterization capabilities of THz-FDS and furthers the development of practical terahertz spectroscopy applications.

Ultrasensitive refractometer based on helical long-period fiber grating near the dispersion turning point.

Precise and accurate measurements of the optical refractive index (RI) for liquids are increasingly finding applications in biochemistry and biomedicine. Here, we demonstrate a dual-resonance helical long-period fiber grating (HLPFG) near the dispersion turning point (DTP), which exhibits an ultrahigh RI sensitivity (∼25546 nm/RIU at ∼1.440). The achieved RI sensitivity is, to the best of our knowledge, more than one order of magnitude higher than a conventional HLPFG. The ultrahigh RI sensitivity can improve the RI measurement precision and accuracy significantly. Furthermore, ultralow wavelength shifts (nearly zero) with temperature and strain ranging from 20 to 100°C and 0 to 2226 µε, respectively, are also demonstrated for the proposed HLPFG, which may be a good candidate for developing new low-cross-talk sensors.

Application of strongly overcoupled resonant modes of long-period fiber gratings to measure the adulteration of olive oil.

We have shown that strongly overcoupled resonant modes of long-period fiber gratings (LPFGs) can be used to assess the adulteration of olive oil. In this background, we investigate the response characteristics of strongly overcoupled resonant modes of different orders to a surrounding medium, the refractive index (RI) of which is greater than that of the fiber cladding, and in the range where a precise refractive index measurement is immensely useful for inspecting the quality of olive oils and other edible oils. A theoretical simulation that would help in designing a sensor with suitable sensitivity and range of measurement has been presented in detail and also validated with experimental results. It was interesting to observe that in a high RI surrounding, a lower order overcoupled resonant mode is much more sensitive as compared to a higher-order one having a similar coupling coefficient. A quantitative analysis demonstrates that for a particular LPFG, the sensitivity of a strongly overcoupled LP06 mode was found to be ∼2000 dB/RIU, while that of the LP07 mode having similar coupling strength was ∼550 dB/RIU in the surrounding refractive index range from 1.458 to 1.520. The results have been validated experimentally.

Novel techniques for high precision refractive index measurements, and application to assessing neutron damage and dose in crystals

In this work we present novel techniques for high precision index of refraction measurements for transparent crystals, and demonstrate a change from neutron irradiation. Radiation damage affects the structure of material, which can be read out nondestructively in transparent crystals. There is some difference in gamma-ray and neutron interactions which may be useful in characterization. Ionization from gamma rays produces color centers in the material, producing distinct spectral absorption, and some small shift in the index of refraction. Neutrons produce atomic recoils and, while the recoils do some ionization, they have a much greater efficiency for lattice displacement than do gamma rays, and these displacements can have a greater effect on the index of refraction. Using CaF2 crystals exposed to neutron radiation, together with a new high precision technique of detecting changes of index of refraction, we establish proof that this type of measurement can be used to monitor neutron exposure. This can provide a basic study of material changes with radiation and, with calibration of material in known neutron fields, this may even find application to neutron dosimetry.

Picometer Metrology for Precise Measurement of Refractive Index, Pressure, and Temperature

Fabry-Perot interferometers can be used for very precise measurement of the refractive index of gasses. This can enable increased accuracy of interferometer-based length measurement. In addition, b...

Spectroscopic method for measuring refractive index

A method for routine, but precise measurements of refractive index is described. The method is fast and accurate. It is based on the analysis of interference fringes, and uses positions of the fringe maxima and/or minima and a precise measurement of sample thickness to extract refractive index. An extremely dense dataset of refractive index values over the entire spectral range of interest can be routinely obtained.

Strong Enhancement of Light–Matter Interaction in Graphene Coupled to a Photonic Crystal Nanocavity

We demonstrate a large enhancement in the interaction of light with graphene through coupling with localized modes in a photonic crystal nanocavity. Spectroscopic studies show that a single atomic layer of graphene reduces the cavity reflection by more than a factor of one hundred, while also sharply reducing the cavity quality factor. The strong interaction allows for cavity-enhanced Raman spectroscopy on subwavelength regions of a graphene sample. A coupled-mode theory model matches experimental observations and indicates significantly increased light absorption in the graphene layer. The coupled graphene-cavity system also enables precise measurements of graphene's complex refractive index.

Comment on “Precision refractive index measurements of air, N_2, O_2, Ar, and CO_2 with a frequency comb”

The measured oxygen refractive index formulas are still not sufficient for precise metrological applications. Thus, a new dispersion formula for the molecular oxygen refractive index at standard conditions is derived. The agreement of the standard air refractive index obtained from indices of its components with recent dry air refractive index formulas is within a few billionths.

Reply to “Comment on the precision refractive index measurements of air, N_2, O_2, Ar, and CO_2with a frequency comb”

In reply to this issue’s comment paper on our previous paper [Appl. Opt.47, 3143 (2008)], we clarify several points concerning the validity of the dispersion formula for molecular oxygen.

Proposed design for high precision refractive index sensor using integrated planar lightwave circuit

A high precision and compact refractive index sensor is proposed. The combination of coarse measurement utilizing the change of the angle of refraction and fine measurement utilizing the phase change is newly proposed to measure absolute refractive index precisely. The proposed method does not need expensive optical measurement equipment such as an optical spectrum analyzer. The integrated planar lightwave circuit (PLC) technology enables us to obtain a compact sensor that is preferable for the practical use. The principle, design, and some configurations for precise refractive index measurement are described.

Precise measurement of refractive index spectral dependence of optical materials for laser, fiber, and integrated optics

An interferometric technique for measuring the spectral dependence of the refractive index of solid-state materials to an accuracy of 10−5 over the entire transparency region is described. The possibilities of the technique are demonstrated by the example of new phosphate glasses formulated for laser and fiber optics. It is shown that, in the presence of absorption bands in the transmission spectrum, the technique allows their associated jumps in the refractive index values to be recorded on the order of 10−4−10−5 and below. The shift in the spectral dependence of the refractive index in the region of the absorption band with intensity at 1.7 cm−1 is measured for the first time.

Precision refractive index measurements of air, N_2, O_2, Ar, and CO_2 with a frequency comb

We report precision measurements of the refractive indices of dry air, N(2), O(2), Ar, and CO(2), performed by using a frequency comb as the light source in a Mach-Zehnder interferometer setup. Improved dispersion formulas for all gases are derived with a sensitivity level of 10(-9). These results are valid for a wavelength range from 740 to 860 nm and are in good agreement with measurements from other groups.

Measurement of droplet interfacial phenomena by light‐scattering techniques

AbstractTwo light‐scattering techniques have been applied to study the kinetics of droplet evaporation and attendant interfacial phenomena. The first technique, which is particularly useful for noisy data, utilizes the fast Fourier transform of phase function data (intensity vs. scattering angle) to obtain the size by comparing the phase function with Lorenz‐Mie theory. The second method applies optical resonance measurements to obtain very precise measurements of size and/or refractive index as functions of time. The techniques have been used to measure the slow evaporation of multicomponent and single‐component organic microdroplets, rapid evaporation of water droplets, and rapid evaporation of droplets of aqueous solutions and surfactant solutions. The experiments were performed in an electrodynamic balance, which was used to suspend single microdroplets in a polarized laser beam. Rapid evaporation rate data yield information on the surface temperature, and data for surfactant solution evaporation show that insoluble monolayer formation at the microdroplet surface reduces the evaporation rate by a factor of 200. In addition, it is shown that droplet explosion due to surface charge effects can occur well below the Rayleigh limit of charge.

Correcting for environmental conditions in precise refractive index measurement

Correcting for environmental conditions in precise refractive index measurement

Precise Millimeter-Wave Measurements of Complex Refractive Index, Complex Dielectric Permittivity and Loss Tangent of GaAs, Si, SiO/sub 2/, A1/sub 2/O/sub 3/, BeO, Macor, and Glass

Highly accurate continuous spectra of the complex refractive index and complex dielectric permittivity are given in the millimeter range for a variety of potentially useful materials. The absorption coefficient is found to increase monotonically with increasing frequencies. Small amounts of glassy inclusions or water were found to increase losses at all frequencies, but impurities and radiation damage (except in semiconductors) have not yet proved to be detrimental to performance. Materials have been found for which the millimeter-wave losses can be tolerated when used as dielectric waveguide, high-power windows, and other applications. Nominal consideration must be given, however, to the conditions of preparation and the nature of contaminants, The measurements were made in a modular, polarizing, dispersive Fourier-transform spectrometer.

Equipment for precise measurements of the complex refractive index of gases as a function of wavenumber, temperature and pressure

The apparatus in precise scanning-wavelength interferometry with gases is described. The dispersion of the refractive index is measured with the aid of an evacuated Michelson interferometer in the wavelength range 400<or= lambda <or=700 nm with precision mod Delta (n-1) mod =2*10-8 and an uncertainty in the absolute values of mod Delta (n-1)/(n-1) mod =2*10-4. The gas pressure is determined absolutely using a repeatedly calibrated pressure transducer to about mod Delta p mod =5 Pa or mod Delta p/p mod =10-4 in the pressure range 0-3*105 Pa. The temperature is measured with precision mod Delta T mod =1 mK and uncertainty mod Delta T mod =10 mK using platinum resistance thermometers and voltage ratio measurements with the aid of a digital voltmeter. The system of automatic data acquisition is also described.