Optical coherence tomography (OCT) has developed rapidly since its first realisation in medicine and is currently an emerging technology in the diagnosis of skin disease. OCT is an interferometric technique that detects reflected and backscattered light from tissue and is often described as the optical analogue to ultrasound. The inherent safety of the technology allows for in vivo use of OCT in patients. The main strength of OCT is the depth resolution. In dermatology, most OCT research has turned on non-melanoma skin cancer (NMSC) and non-invasive monitoring of morphological changes in a number of skin diseases based on pattern recognition, and studies have found good agreement between OCT images and histopathological architecture. OCT has shown high accuracy in distinguishing lesions from normal skin, which is of great importance in identifying tumour borders or residual neoplastic tissue after therapy. The OCT images provide an advantageous combination of resolution and penetration depth, but specific studies of diagnostic sensitivity and specificity in dermatology are sparse. In order to improve OCT image quality and expand the potential of OCT, technical developments are necessary. It is suggested that the technology will be of particular interest to the routine follow-up of patients undergoing non-invasive therapy of malignant or premalignant keratinocyte tumours. It is speculated that the continued technological development can propel the method to a greater level of dermatological use.

https://link.springer.com/content/pdf/bfm%3A978-3-319-06419-2%2F1.pdf

Optical Coherence Tomography

Publisher Summary This chapter describes the phase-measurement interferometry techniques. For all techniques, a temporal phase modulation is introduced to perform the measurement. By measuring the interferogram intensity as the phase is shifted, the phase of the wavefront can be determined with the aid of electronics or a computer. Phase modulation in an interferometer can be induced by moving a mirror, tilting a glass plate, moving a grating, rotating a half-wave plate or analyzer, using an acousto-optic or electro-optic modulator, or using a Zeeman laser. Phase-measurement techniques using analytical means to determine phase all have some common denominators. There are different equations for calculating the phase of a wavefront from interference fringe intensity measurements. The precision of a phase-measuring interferometer system can be determined by taking two measurements, subtracting them, and looking at the root-meansquare of the difference wavefront. The chapter discusses the simulation results. The elimination of the errors that reduce the measurement accuracy depends on the type of measurement being performed. Phase-measurement interferometry (PMI) can be applied to any two-beam interferometer, including holographic interferometers. Applications can be divided into: surface figure, surface roughness, and metrology.

V Phase-Measurement Interferometry Techniques

We introduce a three-dimensional sensor designed primarily for rough objects that supplies an accuracy that is limited only by the roughness of the object surface. This differs from conventional optical systems in which the depth accuracy is limited by the aperture. Consequently, our sensor supplies high accuracy with a small aperture, i.e., we can probe narrow crevices and holes. The sensor is based on a Michelson interferometer, with the rough object surface serving as one mirror. The small coherence length of the light source is used. While scanning the object in depth, one can detect the local occurrence of interference within the speckles emerging from the object. We call this method coherence radar.

Three-dimensional sensing of rough surfaces by coherence radar

This paper gives an introduction to optical coherence tomography (OCT), explains its basic principles, and
discusses the information content of OCT images. Various interferometric techniques used in OCT are reviewed
and a short survey of results obtained so far in different fields of application and possible future
developments are presented.

https://www.spiedigitallibrary.org/journals/Journal-of-Biomedical-Optics/volume-1/issue-02/0000/Optical-coherence-tomography/10.1117/12.231361.pdf

Optical coherence tomography.

In two-wavelength interferometry, synthetic wavelengths are generated in order to reduce the sensitivity or to extend the range of unambiguity for interferometric measurements. Here a novel optoelectronic technique, called superheterodyne detection, is presented, which permits measurement of the phase difference of two optical frequencies that cannot be resolved by direct optoelectronic heterodyne detection. This technique offers the possibility for operation of two-wavelength interferometry in real time with arbitrary synthetic wavelengths from micrometers to meters in length. Preliminary experimental results are reported. An optical arrangement for absolute range-finding applications using tunable-laser sources (e.g., semiconductor lasers) is proposed.

Two-wavelength laser interferometry using superheterodyne detection

If a rough surface is illuminated by a coherent lightwave of wavelength λ1, it is not possible to determine the surface profile from the phases of the speckle field formed by the scattered light. If the rough surface is illuminated, however, by an additional coherent wave of wavelength λ1, the phase differences between the two speckle fields do contain information about the macroscopic surface profile even if subject to a statistical error. It is shown that (1) the macroscopic surface profile may be determined from the phase differences if the effective wavelength Λ = λ1λ2/|λ1−λ2| is sufficiently larger than the standard deviation of the microscopic profile of the illuminated surface, and (2) the statistical error is reasonably small if the phase measurements are obtained from speckles of sufficient intensity. Using a heterodyne interferometer we demonstrate the feasibility of this technique. In the first experiment we determine the radius of curvature of a rough spherical surface. In the second experiment the macroscopic surface contour on two ophthalmic lenses of the same power variation, one with a grounded surface and the other with a polished surface, was determined.

Rough surface interferometry with a two-wavelength heterodyne speckle interferometer.

This paper is a study of the principles of a new method of rough-surface interferometry (ROSI). We review some properties of the higher-order probability density function (pdf) of correlated speckle fields and discuss some assumptions that are usually made silently. This pdf is used to investigate statistical properties of intensities and phases in a dichromatic speckle field; the results are applied to ROSI. The experiments described confirm the theoretical results.

Higher-order statistical properties of speckle fields and their application to rough-surface interferometry

Two-wavelength speckle interferometry on rough surfaces using a mode hopping diode laser

If a rough surface is illuminated by a coherent light wave of wavelength Al, it is not possible to determine the surface profile from the phases of the speckle field formed by the scattered light. However, if the rough surface is illuminated by an additional coherent wave of wavelength A2, the phase differences between the two speckle fields do contain information about the macro-scopic surface profile even though subject to a statistical error. We present the pertinent statistical properties of dichromatic speckle fields and show that (1) the macroscopic surface profile may be determined from the phase differences if the effective wavelength A = Al X2/I Al - A21 is sufficiently larger than the standard deviation of the microscopic profile of the illuminated surface and (2) the statistical error is reasonably small if the phase measurements are obtained from speckles with sufficient intensity.

Two-Wavelength Speckle Interferometric Technique For Rough Surface Contour Measurement

If a rough surface is illuminated by a coherent light wave of wavelength λ1 it is not possible to determine the surface profile from the phases of the speckle field formed by the scattered light. If the rough surface is illuminated, however, by an additional coherent wave of wavelength λ2, the phase differences between the two speckle fields do contain information about the macroscopic surface profile even if subject to a statistical error. We present the pertinent statistical properties of dichromatic speckle fields and show (1) that the macroscopic surface profile may be determined from the phase differences if the effective wavelength Δ = λ1λ2/|λ1-λ2| sufficiently larger than the standard deviation of the microscopic profile of the illuminated surface and (2), that the statistical error is reasonably small if the phase measurements are obtained from speckles with sufficient intensity.

A. F. Fercher

Papers

Rough surface interferometry with a two-wavelength heterodyne speckle interferometer.

Higher-order statistical properties of speckle fields and their application to rough-surface interferometry

Two-wavelength speckle interferometry on rough surfaces using a mode hopping diode laser

Two-Wavelength Speckle Interferometric Technique For Rough Surface Contour Measurement

Statistical Properties And Application Of Two-Wavelength Speckles