Refractive index mapping of layered samples using optical coherence refractometry

Optical path length, temperature, and wavelength effects simulation on ozone gas absorption cross sections towards green communications

We previously proposed a system based on a combination of a confocal microscope and a wavelength scanning heterodyneinterferometer, for separate measurement of refractive index and geometrical thickness of lens and plates. However, we usedtwo optical systems successively although they share the same optical system. In this paper, we propose a new measurementscheme which enable us to measure both the confocal profile and the optical path difference of the interferometer by singlescanning of an object. We describe here the measurement principle, the apparatus, signal processing means, and some experi-mental results.Keywords: Interferometry, microscopy, thickness measurement, refractive index measurement, wavelength tuning 1. INTRODUCTION Optical measurement of thickness usually gives the optical thickness that corresponds to a product of refractive index andgeometrical thickness. We have proposed recently separate measurement of refractive indices and geometrical thicknesses ofmultiple layers.1 This technique was based on a combination of a low-coherence interferometer (LCI) and a confocal micro-scope. Similar technique, in which a heterodyne microscope is used instead of the confocal microscope for focus detection, hasbeen proposed independently by Ohmi et aL2 For quicker measurement we further promoted to replace the LCI, which requiresmechanical scanning, by a wavelength scanning heterodyne interferometer (WSHI). We have already shortened the measure-ment time for optical path length to about 1 s. from 60 s. taken in the low-coherence interferometry. For separate measurement,however, we used the confocal microscope and the interferometer successively although they share one optical system and arepetition of sample movement was required for each measurement. Here, we propose a new measurement scheme whichenable us to measure both confocal profile and optical path difference (OPD) of the interferometer by single movement of asample.

Detecting cellular activity in sub-millisecond timescale and micrometer resolution without using invasive means has been a long standing goal in the study of cellular networks. Here, we have employed phase sensitive low coherence interferometry for detecting optogenetically stimulated activity of cells. Nanoscale changes in optical path length (due to change in refractive index and changes in cell thickness) occur when cells are activated, which we aim to detect by phase sensitive low coherence interferometry. A low coherence interferometry and patch-clamp electrophysiology systems were integrated with an inverted fluorescence microscope. Blue laser beam was coupled to the electrophysiology-interferometric detection system for optogenetic stimulation. The phase-sensitive measurements were carried out on Channelrhodopsin-2 sensitized cells (identified by YFP fluorescence) as well as control cells in reflection mode for different intensities and exposures of optogenetic stimulation beam. This method offers good temporal and spatial resolution without using exogenous labeling. Results of studies on all optical stimulation and detection of cellular activity will be presented. Interpretation of the optical activity signals will be discussed in context with changes in cell physiology during stimulation. We will also discuss the potential sources of various artifacts in optical/electrical detection of cellular activity during optical stimulation.

Optical ring or disk resonators can trap traveling optical waves in a closed path, for example, in a waveguide loop or silicon disk cavity. Energy can be added to or extracted from the cavity with the help of waveguide directional couplers. Ring/disk resonators can be fabricated with high quality factor (Q) in diverse material platforms. The high Q of these devices makes them very sensitive to small variations in (i) optical path length (phase), (ii) optical path loss (attenuation), and (iii) coupling coefficient. Any change in the optical path length can cause a shift in the resonance frequency of the ring. Whereas, the change in path loss and coupling coefficient can lead to a change in the Q of the resonator. The micro or nano opto-electro-mechanical systems (MOEMS/NOEMS) provide unique flexibility with a wide change in the tuning parameters. The tuning is obtained by either (i) displacement of the detached parts or (ii) strain in the waveguide/disk. Displacement of a structure detached from the ring/disk closer to the evanescent field can increase the effective refractive index or attenuation. The displacement of a waveguide in the directional coupler altering the gap can change the coupling coefficient. Strain from various sources can lead to refractive index (phase) and absorption changes, in addition to changing the dimensions. Mechanical, electrothermal, piezoelectric, electrostatic, and magnetic actuators have been demonstrated with MOEMS. Mechanical latching has been used to achieve nonvolatile tuning. Multi-beam supported, freestanding, and diaphragm supported structures have shown applications in sensing, optical switching, and energy conversion. MOEMS/NOEMS has recently been shown to provide the desired properties of modern sensors and photonic integrated circuits (PICs) such as low optical power consumption, high resolution, wide tuning range, low complexity, small footprint, low actuator power consumption, and compatibility with standard fabrication technologies. This chapter will start by briefly covering the basics of MOEM tuned ring/disk resonators. Then, the current technology aspects such as tuning mechanism, actuator types, and the fabrication technology will be discussed along with recent innovations. Furthermore, the evolution into NOEMS with specialized applications and materials will be emphasized along with emerging trends.

Cellular automata ray tracing in flow field near the optical window of the optical dome

This paper presents an all-optically controlled, all-fiber optical path-length modulator. The presented system takes advantage of the heating effect induced within vanadium-doped fiber through laser excitation. It can be applied in various applications, for example in white-light interferometry. The system consists of a Michelson interferometer with vanadium-doped fiber in one arm, a 980 nm excitation high-power laser diode, and 1310/1550 nm signal sources or channels. Due to the spectral-absorption properties of vanadium ions in silica, the absorbed optical power emitted by the 980 nm source is mostly converted via a non-radiative relaxation process into heat within the vanadium-doped fiber. A rise of fiber core temperature causes the fiber core refractive index to change and consequently, a change in the optical path difference of the interferometer. The extinction laser diode operates in pulse mode for continuous scanning of the white-light interferometer. The vanadium-doped fiber is, therefore, periodically heated and self-cooled. The optical path difference of the scanning interferometer is simultaneously measured using a high-coherence source that provides the needed reference trace. The achieved modulated optical path is over 150 μm, with a system time constant of below 1 s. This all-optical configuration of the scanning interferometer allows for the remote and electrically passive control of the optical path length differences in various fiber-optic systems. In particular, the proposed design would be suitable as an interrogation system for various sensors, where an absolute optical path length variation/measurement is required.

A multipass trace gas cell with 24 m optical path length and 80 cc volume was developed and built. A direct ray tracing and genetic algorithm was used to determine optimal mirror arrangement. © 2019 The Author(s)

Two different sinusoidal wavelength-scanning (SWS) interferometers with a SWS light source using a superluminescent laser diode are proposed for step-profile measurement and real-time distance measurement, respectively. An optical path difference (OPD) longer than a wavelength is measured from detection of sinusoidal phase- modulation amplitude Zb of the interference signal that is proportional to the OPD and the scanning width 2b. In step-profile measurement, if measurement error in the OPD obtained from Zb is smaller than a half wavelength, this measured value of the OPD is combined with a fractional value of the OPD obtained from the conventional phase of the interference signal. This combination enables us to measure the OPD longer than a wavelength with a high accuracy of a few nm. In real-time distance measurement, the amplitude Zb is kept at a specified constant value for changes of OPD by controlling the scanning width 2b of the wavelength with a feedback system. The amplitude Zb is detected by processing the interference signal with electric circuits in real-time. The value of b is easily controlled in the SWS light source, and an OPD longer than a wavelength is measured from the value of b with an accuracy of about a wavelength.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optical coherence tomography (OCT) is fast emerging as an additional non-interventional modality for skin tumor detection and diagnosis. A master/slave flying spot OCT configuration was assembled to detect periocular basal cell carcinomas (BCC). A swept source at 1300 nm and sweeping speed of 50 kHz were used. A three-step process was involved. First, 384 channeled spectra using a mirror were stored for 384 optical path differences at the master stage. Then, the stored channeled spectra (masks) were correlated with the channeled spectrum from the BCC tissue to produce 384 en face OCT images (200×200 pixels) for the optical path difference values used to acquire the masks. Finally, these en face slices were stacked to form a volume to cross-reference BCC tumor margins in the orthogonal plane. Per each eyelid sample, several en face images of 200×200 lateral pixels are produced in the time to scan laterally a complete raster of 1.6 s. Combination of the en face views with the cross-sectioning views allow for better discrimination of BCCs comparable to using cross-sectional imaging alone, as previously reported using the conventional fast-Fourier-transform-based OCT techniques.

Optical measurement technique is widely used in length measurement, such as laser interferometers and electronic distance meters. The correction of air refractive index is necessary because the optical path length should be converted to the geometrical length in most vases. In order to obtain an average refractive index through the optical path, Edlen's formulae have been widely used with environmental measurements of air temperature, pressure, humidity and carbon dioxide concentration. In recent days, more precise formulae for calculating a refractive index of air have been desired in IR region. However, the measurement accuracy for near IR region is not sufficient.

With the Fizeau interferometer method, the change in optical path length with temperature is measured at six wavelengths for twenty new Corning optical glasses. The sample, a polished block with a few seconds of wedge, is held in a double-walled chamber with temperature control provided over the range -20 degrees C to +80 degrees C by a circulating water-glycol mixture. A Gaertner interferometer viewing apparatus is modified by addition of a monitor head that permits simultaneous photoelectric fringe detecting and viewing. Measurement of thermal expansion with the Plummer-Hagy high precision dilatometer permits the extraction of the temperature coefficient of absolute refractive index from the optical path measurement. The coefficient of refractive index relative to standard air at the temperature of the glass is found by further data reduction using Edlén's equations. The experimental values of the index change as a function of temperature are curve-fitted by a FORTRAN program, and the slopes are computed to give instantaneous values of the coefficients. The average residual for the fit is about 1.0 x 10(-6). The precision of the absolute coefficient values is estimated to be +/-0.05 x 10(-6). Representative data for one glass are presented to illustrate the results available from this work.

Direct optical path measurement in fibre-optic gyroscopes: A potential method for compensating slow-drifting errors

Optical methods of measurement do not require contact of a probe and the object under study, and thus have found use in a broad range of applications such as nondestructive testing (NDT), where noninvasive measurement is crucial. Measuring the refractive index of a material can give a valuable insight into its composition. Low‑coherence radiation sources enable measurement of the sample’s properties across a wide spectrum, while simultaneously measuring the absolute value of optical path difference between interfering waves, which is necessary to calculate the refractive index. The measurement setup used in this study consists of a fiber‑based Fabry‑Perot interferometer, illuminated by a low‑coherence infrared source. The samples under measurement are located in the cavity of the interferometer, and their transmission spectra are recorded using an optical spectrum analyzer. Additional reference measurements are performed with the cavity filled with air, in order to precisely measure the geometrical length of the cavity. The purpose of the study was to develop a digital signal processing algorithm to improve the resolution of analysis of the spectra of radiation measured at the output of the interferometer. This goal was achieved by decreasing the broadening of the signal in the Fourier domain caused by dispersion of the medium filling the cavity. The Fractional Fourier Transform is a generalization of the Fourier transform allowing arbitrary rotation of the signal in the time-frequency domain, allowing more precise analysis of signals with variable frequency. This property makes this transformation a valuable tool for the analysis of interferometric signals obtained from measurements of dispersive media, as the variable rate of change of the optical path length with respect to wavenumber in such media results in varying frequency of the modulation of measured spectra. The optical path difference inside the material under measurement is used together with the geometrical length obtained from the reference measurement in order to determine the refractive index. The parameters of the transformation are found by iterative adjustment to the signal under analysis. The developed algorithm was tested using both real measured spectra and simulated signals based on a theoretical model of the interferometric setup, and its effectiveness was compared to previously used methods of analysis. It was found to increase the resolution of analysis up to the Fourier limit that occurs in signals with no dispersion.

Method of autocorrelation low coherence interferometry (LCI) is investigated in the work and proposed for diagnostic of layered media inner structure. The autocorrelation LCI is non reference beam method [1–3]. A sample beam directs to the interferometer where optical path difference between the waves reflected from different layers within the sample is compensated. In these methods an object is out of interferometer. So any distance to object is suitable and movements of object do not influence on interference signal. The output signal represents sample field autocorrelation function. Variable component of output signal I(2Δz M ) is: given equation where Δt=2Δz M /c, Δt ij =2d ij n ij /c, d ij n ij - is the optical thickness of layer contained by i-th and j-th boundaries, λ 0 =2πc/ω 0 [4,5]. There is a central pulse which corresponds to zero path difference between two sample fields in the interferometer. The side pulses are located symmetrically about the central. Location of side pulses in interference signal shows optical thicknesses of layers and serial combinations of theirs within the sample. Total number of side pulses is N = 1/2*(m2 − m), where m - is the number of reflecting boundaries. Several technical and biological objects (including skin abruption and nanorods of gold [6]) were investigated using the method. Result of scanning of skin abruption is shown on fig. 1. The scheme of the object (fig. 1a) and interference signal of it (fig. 1b) are presented.

In clinical settings, optical coherence tomography (OCT) is an indispensable tool for the diagnosis of anterior and fundus abnormalities and decision making in the management of ophthalmic diseases. OCT imaging provides live analysis of cross-sectional ocular tissues of the anterior and posterior segments. A conventional OCT is commonly used for preoperative and postoperative macular assessments for the surgical treatment of macular diseases, such as a macular hole or an epiretinal membrane (ERM); it is not used intraoperatively because of its large size and difficulty maintaining sterility in surgical fields. Intraoperative OCT (iOCT) resolves these concerns. To date, three types of iOCT have been developed: portable iOCT, which includes a handheld and microscope-mounted OCT [1–3], fiber-optic OCT probe [4, 5], and microscope-integrated iOCT (MIOCT). The fiber-optic OCT probe is limited to development stage in the field of ophthalmology [5]. This chapter introduces the current clinical usage of the MIOCT for the treatment of macular diseases in clinical cases.