There have been three basic approaches to optical tomography since the early 1980s: diffraction tomography, diffuse optical tomography and optical coherence tomography (OCT). Optical techniques are of particular importance in the medical field, because these techniques promise to be safe and cheap and, in addition, offer a therapeutic potential. Advances in OCT technology have made it possible to apply OCT in a wide variety of applications but medical applications are still dominating. Specific advantages of OCT are its high depth and transversal resolution, the fact, that its depth resolution is decoupled from transverse resolution, high probing depth in scattering media, contact-free and non-invasive operation, and the possibility to create various function dependent image contrasting methods. This report presents the principles of OCT and the state of important OCT applications. OCT synthesises cross-sectional images from a series of laterally adjacent depth-scans. At present OCT is used in three different fields of optical imaging, in macroscopic imaging of structures which can be seen by the naked eye or using weak magnifications, in microscopic imaging using magnifications up to the classical limit of microscopic resolution and in endoscopic imaging, using low and medium magnification. First, OCT techniques, like the reflectometry technique and the dual beam technique were based on time-domain low coherence interferometry depth-scans. Later, Fourier-domain techniques have been developed and led to new imaging schemes. Recently developed parallel OCT schemes eliminate the need for lateral scanning and, therefore, dramatically increase the imaging rate. These schemes use CCD cameras and CMOS detector arrays as photodetectors. Video-rate three-dimensional OCT pictures have been obtained. Modifying interference microscopy techniques has led to high-resolution optical coherence microscopy that achieved sub-micrometre resolution. This report is concluded with a short presentation of important OCT applications. Ophthalmology is, due to the transparent ocular structures, still the main field of OCT application. The first commercial instrument too has been introduced for ophthalmic diagnostics (Carl Zeiss Meditec AG). Advances in using near-infrared light, however, opened the path for OCT imaging in strongly scattering tissues. Today, optical in vivo biopsy is one of the most challenging fields of OCT application. High resolution, high penetration depth, and its potential for functional imaging attribute to OCT an optical biopsy quality, which can be used to assess tissue and cell function and morphology in situ. OCT can already clarify the relevant architectural tissue morphology. For many diseases, however, including cancer in its early stages, higher resolution is necessary. New broad-bandwidth light sources, like photonic crystal fibres and superfluorescent fibre sources, and new contrasting techniques, give access to new sample properties and unmatched sensitivity and resolution.

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Optical coherence tomography - principles and applications

Optical Wave Propagation In Random Media - Background Review Optical Scintillation Modelling Theory Of Scintillation - Plane Wave Model Theory Of Scintillation - Spherical Wave Model Theory Of Scintillation - Gaussian-Beam Wave Model Aperture Averaging Optical Communication Systems Fade Statistics For Lasercom Systems Laser Radar Systems - Scintillation Of Return Waves Laser Radar Systems - Imaging Through Turbulence.

Laser Beam Scintillation with Applications

In recent years, free space optical (FSO) communication has gained significant importance owing to its unique features: large bandwidth, license free spectrum, high data rate, easy and quick deployability, less power, and low mass requirements. FSO communication uses optical carrier in the near infrared band to establish either terrestrial links within the Earth’s atmosphere or inter-satellite/deep space links or ground-to-satellite/satellite-to-ground links. It also finds its applications in remote sensing, radio astronomy, military, disaster recovery, last mile access, backhaul for wireless cellular networks, and many more. However, despite of great potential of FSO communication, its performance is limited by the adverse effects (viz., absorption, scattering, and turbulence) of the atmospheric channel. Out of these three effects, the atmospheric turbulence is a major challenge that may lead to serious degradation in the bit error rate performance of the system and make the communication link infeasible. This paper presents a comprehensive survey on various challenges faced by FSO communication system for ground-to-satellite/satellite-to-ground and inter-satellite links. It also provides details of various performance mitigation techniques in order to have high link availability and reliability. The first part of this paper will focus on various types of impairments that pose a serious challenge to the performance of optical communication system for ground-to-satellite/satellite-to-ground and inter-satellite links. The latter part of this paper will provide the reader with an exhaustive review of various techniques both at physical layer as well as at the other layers (link, network, or transport layer) to combat the adverse effects of the atmosphere. It also uniquely presents a recently developed technique using orbital angular momentum for utilizing the high capacity advantage of optical carrier in case of space-based and near-Earth optical communication links. This survey provides the reader with comprehensive details on the use of space-based optical backhaul links in order to provide high capacity and low cost backhaul solutions.

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Optical Communication in Space: Challenges and Mitigation Techniques

Here we present new technology for optical coherence tomography (OCT) that enables ultrahigh-resolution, non-invasive in vivo ophthalmologic imaging of retinal and corneal morphology with an axial resolution of 2–3 μm. This resolution represents a significant advance in performance over the 10–15-μm resolution currently available in ophthalmic OCT systems and, to our knowledge, is the highest resolution for in vivo ophthalmologic imaging achieved to date. This resolution enables in vivo visualization of intraretinal and intra-corneal architectural morphology that had previously only been possible with histopathology. We demonstrate image processing and segmentation techniques for automatic identification and quantification of retinal morphology. Ultrahigh-resolution OCT promises to enhance early diagnosis and objective measurement for tracking progression of ocular diseases, as well as monitoring the efficacy of therapy.

Current clinical practice emphasizes the development of techniques to diagnose disease in its early stages, when treatment is most effective and irreversible damage can be prevented or delayed. In ophthalmology, the precise visualization of pathology is especially critical for the diagnosis and staging of ocular diseases. Therefore, new imaging techniques have been developed to augment conventional fundoscopy and slit-lamp biomicroscopy. Ultrasonography is routinely used in ophthalmology, but requires physical contact with the eye and has axial resolutions of approximately 200 μm (ref. 1). High-frequency ultrasound enables approximately 20 μm axial resolutions, but due to limited penetration, only anterior eye structures can be imaged2. Confocal microscopy has been used to image the cornea with sub-micrometer transverse resolution3. Scanning laser ophthalmoscopy enables en face fundus imaging with micron-scale transverse and approximately 300-μm axial resolution4,5. None of these techniques, however, permits high-resolution, cross-sectional imaging of the retina in vivo.

Recently, optical coherence tomography (OCT) has emerged as a promising new technique for high-resolution, cross-sectional imaging6,7. OCT is attractive for ophthalmic imaging because image resolutions are 1–2 orders of magnitude higher than conventional ultrasound, imaging can be performed non-invasively and in real time, and quantitative morphometric information can be obtained. OCT is somewhat analogous to ultrasound imaging except that it uses light instead of sound. High-resolution, cross-sectional images are obtained by measuring the echo time delay of reflected infrared light using a technique known as low coherence interferometry8,9. OCT imaging was first demonstrated in the human retina in vitro6 and in vivo10,11. Recently, it has been extended to a wide range of other non-transparent tissues to function as a type of optical biopsy12–15. To date, however, the most important clinical applications of OCT have been retinal imaging in ophthalmic diagnosis7,16–22. Current ophthalmic OCT systems have 10–15-μm axial resolution and provide more detailed structural information than any other non-invasive ophthalmic imaging technique6,7,10,11. However, the resolution of current clinical ophthalmic OCT technology is significantly below what is theoretically possible. Improving the resolution of OCT ophthalmic imaging would enable structural imaging of retinal pathology at an intraretinal level, as well as improve the accuracy of morphometric quantification.

The axial resolution of conventional ophthalmic OCT systems is limited to 10–15 μm by the bandwidth of light sources used for imaging. Short-pulse, solid-state lasers can generate ultrabroad bandwidth, low-coherence light13. A broadband Cr:Forsterite laser operating in the near infrared (1,300 nm) has permitted cellular-level OCT imaging in developmental biology specimens with 6-μm axial resolution13. For ophthalmic imaging, light sources operating at 800 nm are necessary to avoid absorption in the ocular media. Recently, a state-of-the-art broadband Ti:Al2O3 laser has been developed for ultrahigh (~1 μm) axial resolution, spectroscopic OCT imaging in non-transparent tissue at 800-nm center wavelength23,24. We describe here ultrahigh-resolution ophthalmic OCT based on this state-of-the-art optical technology and demonstrate its potential to provide enhanced structural and quantitative information for ophthalmologic imaging.

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Ultrahigh-resolution ophthalmic optical coherence tomography

Localized fluid flow measurements with an He-Ne laser spectrometer

We have developed a new theoretical description of the optical coherence tomography (OCT) technique for imaging in highly scattering tissue. The description is based on the extended Huygens–Fresnel principle, valid in both the single- and multiple-scattering regimes. The so-called shower curtain effect, which manifests itself in a standard OCT system, is an inherent property of the present theory. We demonstrate that the shower curtain effect leads to a strong increase in the heterodyne signal in a standard OCT system. This is in contrast to previous OCT models, where the shower curtain effect was not taken into account. The theoretical analysis is verified by measurements on samples consisting of aqueous suspensions of microspheres. Finally, we discuss the use of our new theoretical model for optimization of the OCT system.

Analysis of optical coherence tomography systems based on the extended Huygens-Fresnel principle

On the basis of the extended Huygens-Fresnel principle, a general expression is derived for the mutual coherence function (MCF) of a finite optical beam propagating in a weakly inhomogeneous medium. The results obtained here for the beam MCF are valid both in the near and far field of the laser transmitting aperture and for an arbitrary complex disturbance in the exit pupil of the aperture. A general expression is also derived for the propagation distance z(B) such that for distances much less (greater) than z(B), the MCF is well approximated by the plane (spherical) wave results. An analytic expression is pre-sented for a Gaussian beam such that a numerical error in previous results is corrected. Finally, some comments regarding higher order statistical moments of the field are given.

Mutual coherence function of a finite cross section optical beam propagating in a turbulent medium.

20We describe a novel formulation of light beam propagation through any complex optical system that can be described by an ABCD ray-transfer matrix. Within the paraxial approximation, optical propagation can be formulated in terms of a Huygens principle expressed in terms of the ray-transfer ABCD matrix elements of the optical system. We extend and generalize previous treatments to include the effects of finite-sized limiting apertures (i.e., diffractive screens) in the optical train, tilt and random jitter of the optical elements, and distributed random inhomogeneities along the optical path (e.g., clear air turbulence and aerosols). In the presence of limiting apertures the ABCD matrix elements of the optical system are complex. For the case of laser beam propagation and Gaussian-shaped limiting apertures in the optical train, we obtain analytical expressions for both the spot radius and the wave-front radius of curvature at an arbitrary observation plane and give illustrative examples of practical concern. In particular, analytical expressions for the fringe visibility obtained in a coherent laser interferometric system are presented. An analytical expression for the mean spot radius of a laser beam propagating through an optical system in the presence of tilt and random jitter is obtained. We also consider the propagation of partially coherent light through optical systems. In particular, we derive a generalized van Cittert–Zernike theorem that is valid for an arbitrary optical system that can be characterized by an ABCD ray-transfer matrix. Finally, the propagation of laser beams through a general optical system in the presence of distributed random inhomogeneities is considered. An explicit expression for the mean irradiance distribution of a Gaussian-shaped beam is derived that is valid for an arbitrary optical system. In addition, we derive an expression for the mutual-coherence function for wave propagation through an arbitrary optical system. In all cases the results are expressed in terms of the ABCD matrix elements of the complete optical system. The formulation of optical propagation presented here is a rather simple and straightforward way of determining the effects of finite-sized optical elements, tilt and random jitter, and distributed random inhomogeneities along the optical path. It is merely necessary first to multiply the relevant ray matrices together to find the complete system matrix and then to substitute this matrix into the expressions given in this paper.

Optical beam wave propagation through complex optical systems

Statistical estimates of selected scintillation parameters for an infrared laser ground-to-space communication system are presented for a point-receiving aperture. The quantities estimated here are the fraction of time that the signal power is both above and below a given value, the mean number of times per second the signal power crosses a given signal level, and the mean duration of both surges and fades for a given log-irradiance variance.

Optical scintillation statistics for IR ground-to-space laser communication systems.

A Monte Carlo (MC) method for modeling optical coherence tomography (OCT) measurements of a diffusely reflecting discontinuity embedded in a scattering medium is presented. For the first time to the authors' knowledge it is shown analytically that the applicability of an MC approach to this optical geometry is firmly justified, because, as we show, in the conjugate image plane the field reflected from the sample is delta-correlated from which it follows that the heterodyne signal is calculated from the intensity distribution only. This is not a trivial result because, in general, the light from the sample will have a finite spatial coherence that cannot be accounted for by MC simulation. To estimate this intensity distribution adequately we have developed a novel method for modeling a focused Gaussian beam in MC simulation. This approach is valid for a softly as well as for a strongly focused beam, and it is shown that in free space the full three-dimensional intensity distribution of a Gaussian beam is obtained. The OCT signal and the intensity distribution in a scattering medium have been obtained for several geometries with the suggested MC method; when this model and a recently published analytical model based on the extended Huygens-Fresnel principle are compared, excellent agreement is found.

Harold T. Yura

Papers

Analysis of optical coherence tomography systems based on the extended Huygens-Fresnel principle

Mutual coherence function of a finite cross section optical beam propagating in a turbulent medium.

Optical beam wave propagation through complex optical systems

Optical scintillation statistics for IR ground-to-space laser communication systems.

Derivation of a Monte Carlo method for modeling heterodyne detection in optical coherence tomography systems