We analyse polarisation effects in spectroscopic optical coherence tomography. Birefringence induced changes in polarisation are wavelength dependent and the spectrum of the interference signal will therefore depend on the polarisation properties of the sample. We have avoided this problem by realising a combination of dual wavelength spectroscopic OCT system and polarisation sensitive detection
We demonstrate that spectroscopic optical coherence tomography can be used for measurement of diffusion. We measured the diffusion coefficient of a Phthalocyanine dye in an Agar gel as a first model of dye diffusing into tissue. We used a two-wavelength interferometer, with one of the wavelengths matched to the absorption peak of the dye at 675nm while the other wavelength at 805nm is not affected by the dye. The diffusion constant of the dye in Agar gel is found by fitting the measured OCT amplitude (depth and time dependent) to a mathematical model for the OCT signal. This method may be used as a tool for dosimetry in in Photo Dynamic Therapy (PDT). In PDT the therapeutic light exposure should be applied at a time when the concentration of sensitizer is optimal in the diseased tissue relative to normal tissue. By studying how the OCT signal changes with time and depth at two wavelengths differently affected by the diffusing dye, it should be possible to extract parameters determining diffusion of the sensitizer in live tissue. In comparison with fluorescence-based methods, this OCT approach has the advantage of better depth penetration and being able to account for attenuation effects due to scattering.
We demonstrate a wavelength multiplexed low coherence interferometer that detects and demodulates four subbands of the source spectrum in parallel. By introducing dispersion into one of the interferometer arms we obtain a wavelength dependent measurement depth in the object. We analyze the influence of the dispersion on the signals and demonstrate simultaneous Doppler measurements of dynamic flow at three different positions within a tube. The method can be used to remove false Doppler signals caused by an unsteady object and therefore has potential in blood flow monitoring.
The basic use of low coherence interferometry is measuring the intensity of light reflected from defined depths in a partially transparent object. The light back-scattered from a certain depth in the object carries information about the medium through which it has passed. Absorption and scattering attenuate the intensity of the light, and will thus reduce the interference signal from a given depth. One single interference measurement will not discriminate between attenuation due to absorption and scattering. However, by measuring the interference signal at several different wavelengths, discrimination between the two parameters is possible, if they vary differently with wavelength. Especially when measurements are performed in a wavelength region where the scattering coefficient is constant, measurements at two different wavelengths will give the difference in absorption at the two wavelengths. We present preliminary measurements on scattering and absorbing solutions showing a qualitative difference in absorption measured at 810 and 830 nm. As an absorbing solution we used a commercially available dye with an absorption maximum at 800 nm while intralipid was used to introduce scattering. The aim of the work is to measure the oxygenation saturation of blood through absorption measurements at different wavelengths using low coherence interferometry.
We describe a technique that is used for the real-time measurement of the vibration of an object point. The technique can be used when the vibration is characterized by a large amplitude, i.e. several millimeters. The technique shows the additional advantages that it requires no special surface treatment and is insensitive to inplane object displacements. In this technique an object point is illuminated by a small diameter beam (at an angle) that is structured with straight parallel fringes. The illuminated object point is then imaged onto a Ronchi ruling. The total light transmitted through the Ronchi ruling is then used to recover the vibration of the object point, in real time, by using well known servo techniques.
The real-time measurement of the vibration of a point in a large vibrating object is an application that is often encountered in industry. Many laser based point vibration measuring devices have been proposed. All have their advantages and disadvantages and which one is used is determined by the application conditions.
We present an outline of vibration analysis techniques that we have used recently which are based on video speckle interferometry. The work described is carried and deals with the following subjects: the determination of composite vibration induced strain, with the use of both (a) speckle shearing interferometry and (b) electronic speckle pattern interferometry (ESPI); (c) two mode vibration analysis based on stroboscopic ESPI and (d) inexpensive techniques for multimode vibration analysis based on hybrid ESPI and laser velocimeter measurements.
The amplitude and phase distributions of sinusoidally vibrating object are measured by use of phase modulation and sub-fringe detection. The resulting high sensitivity enables measurements on objects like high frequency transducers, biomedical membranes and cross sections of sound fields in air and water. For the sound recordings, tomographic reconstruction of several cross-sections produces representation of the sound field in any plane desired. The sub-fringe approach also allows analysis of objects vibrating harmonically. In shear interferometry we measure true derivatives as very small shear may be used. Used in Moire systems, large amplitude vibrations can be measured. The integration of the recording media is the main drawback of the video system when used for general vibration analysis.
TV holography can be used to measure the small variations in refractive index found in acoustic fields in transparent substances. Projections of the amplitude and phase are imaged with an imaging interferometer, and several projections may be combined, using tomographic techniques, to form a volume mapping of the acoustic field. Both projections and reconstructions of acoustic fields in water are presented and discussed. Finally, some of the limitations of this technique are considered.
The pressure variations from a sound source lead to corresponding variations in the refractive index which can be measured using TV holography. We measure cross-sections of the integrated sound fields from different directions. To obtain a complete map of the volume distribution of sound field we use tomographical backprojection of these recordings to reconstruct the amplitude and phase of the sound field in any plane of the volume.
The experimental feasibility study using fiber optic sensors, strain gauges and speckleinterferometry (ESPI), indicates that delamination in FRP-sandwich structures can be detected by monitoring changes in the vibrational resonance frequencies. The frequencies are also determined analytically.
A vibrating sound source causes periodic pressure variations in the air. The pressure variations lead to corresponding variations in the refractive index of the air which can be measured using interferometric techniques like TV- holography. Each measurement maps a cross-section of the integrated sound fields. To obtain a complete map of the volume distribution of sound field we record cross-sections from different directions. Using topographical backprojection of these recordings, we reconstruct the amplitude and phase of the sound field in any plane of the volume.
The basic properties of the laser speckle wave are briefly reviewed. Studies and measurements of test objects are possible by observing either the direct displacement of the speckles (speckle photography) or interferometric variations of brightness by adding a reference wave (speckle interferometry). Speckle interferometry becomes a particularly useful tool combined with direct video-recording and electronic processing. The resulting system--TV-holography (ESPI) is described in greater detail as it is potentially useful for many biomedical applications. It is shown how image processing can be used to increase the measuring accuracy and aid the interpretation of fringe patterns.
The short exposure time and high framing rate of the TV-holography system allow for global interferometric measurements even in industrial environments. In this paper we report some experiences from failure tests on concrete and weak sedimentary rocks. In a set of rock cavity failure tests, TV-holography has been used to monitor the cavity deformations. The technique allows for a complete description of the continuous displacement and deformation of a surface, and has a potential to reveal important information about, e.g., symmetry-breaking processes. Combined with image processing, the technique has been used to study micro-cracks on the surface of concrete blocks during pressure testing. The purpose of these tests was to study the fracture mechanics of concrete, including crack initiation and propagation.
The preceeding paper by von Bally describes a wide range of holographic techniques which may be applied within biomedicine. One of the most promising techniques in terms of research and (potentia) clinical applications was hologram interferometry whereby minute global displacements of surfaces were displayed as contour maps. The noncontact, nondestructive, and high-sensitivity nature of the technique allows studies of even the most delicate specimen. Conventional hologram interferometry is, however, hampered by the necessity of using film or similar media for the registration process. The development introduces a time delay and restricts the sampling rate in the measuring process. TV-holography (ESPI) circumvents these drawbacks by replacing the conventional recording media (film, thermoplastic, etc.) with the photosensitive target of a video-camera. Using analogue or digital electronic processing, the reconstruction process is simulated to give a real-time presentation of interferometric images on the video monitor. The techniques shows promise in the biomedical field, especially as computers are incorporated into the system to aid the operator in analyzing the fringe patterns. The technique and its advantages for biomedical applications are discussed and illustrated with examples of computerized image processing.
The RETRA 1000 TV-holography system has been further developed for analysis of high frequency vibrations, using automatic
data acquisition. Special phase shift algorithms are executed by means of a PC-based image processing system. The phase
shifts are effected by a built in electro-optic modulator, controlled by a two channel high-resolution digital frequency synthesizer.
The optical (speckle) and electronic noise is reduced, using an automatic speckle averaging technique.
Amplitude and phase distributions can be calculated and presented for frequencies up to 10 MHz. The lower limit for
amplitude detection is below 1 nm (for HeNe laser), while the relative phase distribution is given with less than 3° accuracy
under stable conditions.
We show results where the technique has been used to analyze various vibrating test objects like a ceramic "crystal", a high
frequency loudspeaker and finally an underwater acoustic transducer. The crystal had resonant modes from 3,8 to 7,8 MHz
Image processing has been used to analyze TV-holographic fringe patterns of deformation and vibrations. Recent results using such image processing algorithms for deformation and vibration measurements are described. It is shown how these techniques has been used on practical objects such as concrete, high-frequency transducers, and underwater transducers.
Image processing has been used to analyze TV -holographic fringe patterns of
deformations and vibrations. The processed pictures give improved detection and
information for the operator of the interferometer. A lower detection limit of about ?./50
for deformation analysis has been achieved, while amplitudes down to V500 and phase
values with an accuracy about 3° have been measured on vibrating objects. Examples of
applications of the techniques are shown.