Spectral Low Coherence Interferometry: A Complete Analysis of the Detection System and the Signal Processing

It is demonstrated that, by obstructing half of one of the two beams from a low coherence interferometer before it is incident on the diffraction grating in a spectral interferometry setup, an asymmetric profile can be generated for the visibility of the channelled spectrum (V) with optical path difference (OPD). Together with a lateral shift of the beam, as inspired by Talbot bands studies, this can be used to optimise V(OPD). The model for the visibility of Talbot bands is improved by considering the spectrometer resolution and an improved Talbot band experiment is demonstrated. It is also shown that it is possible to obtain regions of no interference around zero OPD.

We present a full-field reflection phase microscope that combines low-coherence interferometry and off-axis digital holographic microscopy (DHM). The reflection-based DHM provides highly sensitive and a single-shot imaging of cellular dynamics while the use of low coherence source provides a depth-selective measurement. The setup uniquely uses a diffraction grating in the reference arm to generate an interference image of uniform contrast over the entire field-of-view albeit low-coherence light source. We have measured the path-length sensitivity of our instrument to be approximately that makes it suitable for nanometer-scale full-field measurement of membrane dynamics in live cells.

Confocal microscopy is a powerful technique that permits three-dimensional (3D) imaging of thick objects. Recently, however, two new techniques have been introduced, which are rivals for 3D imaging in reflection mode. Coherence probe microscopy (CPM) is used for surface inspection and profiling, particularly in the semiconductor device industry [1–10]. Optical coherence tomography (OCT) is used for medical diagnostics, particularly in ophthalmology and dermatology [11,12]. These are both types of low coherence interferometry (LCI), in which coherence gating is used for optical sectioning. LCI has many advantages over the conventional (narrow-band source) interferometric techniques, including the ability to reject strongly light that has undergone scattering outside of a small sample volume, thus allowing precise non-invasive optical probing of dense tissue and other turbid media. LCI can be used to investigate deep, narrow structures. This feature is particularly useful for in-vivo measurement of deep tissue, for example, in transpupilliary imaging of the posterior eye and endoscoping imaging. Another application area of LCI is in optical fibre sensors [13–15].KeywordsOptical Coherence TomographyReference BeamGeometric PhaseMicroscope ObjectiveFringe VisibilityThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The capability offered by the low-coherence interferometry in the measurement of extremely small-optical path changes is well recognized.1 This technique is at the basis of attosecond chromatic-dispersion measurements. Figure 1 reports the realized optical circuit.2 Light from a superluminescent source is coupled to the fiber under test (FUT). The FUT is finished by two reflective splices and acts as a Fabry-Perot cavity. This cavity presents an optical-path unbalance that is fully compensated by the fiber-loop insert into the Mach-Zehnder cavity set in the receiver apparatus. When the compensating loop is kept in quiet conditions, any optical-path change in the FUT causes a fringe-shift that is measurable after C2. The double-cavity structure guarantees that any other optical-path change caused by the leading fiber is not revealed. In other words, the FUT is completely isolated from the unwanted perturbations. To increase the phase-reading precision, a carrier at frequency fc is provided by an acousto-optic modulator. The dispersion is measured by using four bandpass optical filters whose center wavelength spans the source spectrum and by reading the interference through the filters. As an example, consider the first two filters, F1 and F2. The same interference signal, coming from the coupler C2, is read with a different phase after the two filters because of the different center wavelengths. This difference in phase gives the measurement of the group velocity in the considered spectrum region. A symmetric measurement is performed in the source complementary spectrum region by the other two filters, F3 and F4. The difference between the two readings is, by definition, the measurement of the dispersion (in units of phase) in the considered spectrum zone.

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.

Clinical Imaging with Optical Coherence Tomography

Optical coherence tomography (OCT), a technique for the noninvasive imaging of turbid media, based on low-coherence interferometry, was originally developed for the imaging of biological tissues. Since the development of the technique, most of its applications have been related to the area of biomedicine. However, from early stages, the vertical resolution of the technique has already been improved to a submicron scale. This enables new possibilities and applications. This article presents the possible applications of OCT in paper industry, where submicron or at least a resolution close to one micron is required. This requirement comes from the layered structure of paper products, where layer thickness may vary from single microns to tens of micrometers. This is especially similar to the case with high-quality paper products, where several different coating layers are used to obtain a smooth surface structure and a high gloss. In this study, we demonstrate that optical coherence tomography can be used to measure and evaluate the quality of the coating layer of a premium glossy photopaper. In addition, we show that for some paper products, it is possible to measure across the entire thickness range of a paper sheet. Furthermore, we suggest that in addition to topography and tomography images of objects, it is possible to obtain information similar to gloss by tracking the magnitude of individual interference signals in optical coherence tomography.

Growing requirements for in situ metrology during manufacturing have led to an increased interest in optical coherence tomography (OCT) configurations of low coherence interferometry (LCI) for industrial domains. This paper investigates the optimisation of spectral domain OCT hardware and signal processing for such implementations. A collation of the underlying theory of OCT configured LCI systems from disparate sources linking the journey of the light reflected from the object surface to the definition of the measurand is presented. This is portrayed in an applicable, comprehensible design framework through its application to profilometry measurements for optimising system performance.

We present accurate signal processing techniques based on the continuous wavelet transform (CWT) for nonstationary optical coherence tomography (OCT) interference signals. OCT is attracting considerable attention in the clinical, scientific, and engineering fields. However, the temporal or spatial period of the detected interference signal is unequal because of the nonlinearity of the scanning device or the inverse proportionality of the phase to the wavelength. If these nonstationary interference signals are acquired at equal sampling intervals and processed using fast Fourier transform (FFT), measurement errors occur. We first propose techniques that compensate for the fringe intervals of the temporal interference signal based on the CWT. The accurate position measurement of a reflective surface is demonstrated. Next, we describe a novel signal processing technique that accurately determines the positions of the fringe peak and reference mirror for TD-OCT. An accurate step-height measurement is demonstrated in this study. Finally, we describe a threedimensional profile measurement method. The optical path difference can be determined through a simple relationship between the wavelength and the corresponding phase obtained by the CWT. Measurements of a step-height configured with a pair of gauge blocks and the thickness distribution of a thin glass plate are performed. In the latter case, we demonstrate that the accuracy is improved in the pulse-driven technique.

We present optical coherence quantitation technique to monitor the redox state of mitochondria enzyme Cytochrome oxidase (CytOx) in bone tissue by the use of optical coherence tomography (OCT) system. Superluminescent diode (SLD) with its peak emission wavelength ((lambda) = 820nm) on the absorption band of oxidized form of CytOx was used in the experiments. The reflectance returning from the liquid phantoms (naphthol green B with intralipid) and bone tissue specimens (periosteum of calvaria from newborn rats) as a function of penetration depth was used to quantify the absorption changes of the sample. Absorption coefficients of naphthol green B were accurately quantified by the linear relationship between attenuation coefficients from the slopes of the reflected signals and naphthol green B concentration. The results show that the attenuation coefficient decreases in periosteums as CytOx is reduced by sodium dithionite, demonstrating the feasibility of this method to quantify the redox state of tissues studied. A 70% +/- 7% (n=4) reduction of attenuation coefficients in periosteums was clearly observed with redox change of CytOx after 5 min reduction. In addition, the results demonstrate that the OCT system is also capable of imaging the calvaria tomographically with a resolution at 9 microns, which could only be previously obtained by the conventional excisional biopsy.

Optical coherence tomography (OCT) is a new technique for micron scale, cross-sectional, tomographic imaging.1 In biology and medicine, OCT can be a powerful technique for optical biopsy because it permits the direct imaging of cross-sectional tissue architectural morphology without the need to excisionally remove specimens. OCT imaging is somewhat analogous to radar or ultrasound, except that OCT uses a low coherence optical source and interferometry to perform time delay measurement of light which is backreflected or backscattered from optically turbid specimens. Superluminescent diodes can provide 15-20 micron resolution imaging at several fixed wavelengths, including 800 and 1300 nm, but output powers are limited to the order of a milliwatt. Studies at these wavelengths have demonstrated that 1300 nm wavelengths can achieve greater imaging penetration depths in biological tissues because of reduced scattering. Ultrahigh resolution imaging has been demonstrated using the fluorescent emission from solid state laser media2 and femtosecond pulses from a mode locked Ti:Al2O3 laser.3 OCT using mode locked solid state lasers3 is especially promising because the high output powers available enable higher signal to noise levels as well as faster data acquisition. Using a mode locked source, OCT imaging rates approaching real time can be achieved. In addition, the spectral bandwidth of femtosecond pulses can be broadened by using self phase modulation in an optical fiber thus yielding significant increases in imaging resolution. In this paper we present a Kerr lens mode locked Forsterite laser (at 1.3 micron wavelength) which is spectrally broadened through self-phase modulation in a dispersion shifted fiber. With this source 5.7 micron resolution OCT images with 116 dB signal to noise are demonstrated. Applications of mode locked solid state technology for OCT biomedical imaging is discussed.

Non-invasive investigation of rice leaf specimens to characterize the morphological formation and particular structural information that is beneficial for agricultural perspective was demonstrated using a low coherence interferometric method called swept source optical coherence tomography (SS-OCT). The acquired results non-invasively revealed morphological properties of rice leaf, such as bulliform cells; aerenchyma, parenchyma, and collenchyma layer; and vascular bundle. Beside aforementioned morphologic characteristics, several leaf characteristics associated with cytological mechanisms of leaf rolling (leaf inclination) were examined for the pre-identification of inevitable necrosis and atrophy of leaf tissues by evaluating acute angle information, such as angular characteristics of the external bi-directional angles between the lower epidermis layer and lower mid-vein, and internal angle of lower mid-vein. To further assist the pre-identification, acquired cross-sections were employed to enumerate the small veins of each leaf specimen. Since mutants enlarge leaf angles due to increased cell division in the adaxial epidermis, healthy and abnormal leaf specimens were morphologically and quantitatively compared. Therefore, the results of the method can be used in agriculture, and SS-OCT shows potential as a rigorous investigation method for selecting mutant infected rice leaf specimens rapidly and non-destructively compared to destructive and time consuming gold-standard methods with a lack of precision.

Several earlier publications have addressed the theory of optical coherence tomography (OCT) imaging. These include original articles [1–12], reviews [13, 14], and books/book chapters [15, 16]. Many of these publications were authored before the major revolution that Fourier domain techniques (here termed FDOCT) brought to OCT in the last few years, and thus were written primarily from the perspective of time-domain OCT (TDOCT). Also, relatively few prior publications have addressed lateral resolution in OCT systems, which, from an end-user perspective is of equal importance to the axial resolving power derived from low-coherence interferometry. The aim of this chapter is to present a unified theory of OCT, which includes a discussion of imaging performance in all three dimensions, and treats both Fourier and time domain OCT on equal footing as specializations of the same underlying principles.

A thermal light source technique for optical coherence tomography

Optical Coherence Tomography (OCT) is a new type of an imaging technique of the internal microstructure of the material. Among several detection techniques which can be used to implement OCT, Optical Low-Coherence Reflectometry (OLCR) is the most common and the most promising one. OLCR uses a broadband source, which parameters: the spectral characteristics and the noise property, determine metrological abilities of the designed OCT system. Authors measured the power spectral density of the intensity noise, as well as the spectral characteristic of various broadband sources. Obtained results has been used to determine metrological properties of the design OCT system.