KEYWORDS: Cameras, Demodulation, Interferometers, Interferometry, Imaging systems, Inspection, Signal detection, Signal processing, Data processing, Modulation
The paper presents the electro-optical design of an interferometric inspection system for massive parallel inspection of
Micro(Opto)ElectroMechanicalSystems (M(O)EMS). The basic idea is to adapt a micro-optical probing wafer to the
M(O)EMS wafer under test. The probing wafer is exchangeable and contains a micro-optical interferometer array: a low
coherent interferometer (LCI) array based on a Mirau configuration and a laser interferometer (LI) array based on a
Twyman-Green configuration. The interference signals are generated in the micro-optical interferometers and are applied
for M(O)EMS shape and deformation measurements by means of LCI and for M(O)EMS vibration analysis (the
resonance frequency and spatial mode distribution) by means of LI. Distributed array of 5×5 smart pixel imagers detects
the interferometric signals. The signal processing is based on the "on pixel" processing capacity of the smart pixel
camera array, which can be utilised for phase shifting, signal demodulation or envelope maximum determination. Each
micro-interferometer image is detected by the 140 × 146 pixels sub-array distributed in the imaging plane. In the paper
the architecture of cameras with smart-pixel approach are described and their application for massive parallel electrooptical
detection and data reduction is discussed. The full data processing paths for laser interferometer and low coherent
interferometer are presented.
The paper introduces different approaches to overcome the large ratio between wafer size and feature size in micro
production. The EU-project SMARTIEHS develops a new concept for high volume M(O)EMS testing. The design of the
test station is presented and the advancements compared to the state of the art are introduced within the following fields:
micro-optical laser interferometer (LI) design, DOE-based microinterferometer production, smart-pixel camera and
signal processing for resonance frequency and vibration amplitude distribution determination. The first experiments
performed at LI demonstrator are also reported.
We have developed a novel noise model for the analysis of speckle noise and accuracy in birefringence imaging with polarization sensitive optical coherence tomography. Analytical expressions for the signal and noise in Stokes vectors are found, and these are used to investigate the uncertainty in the estimation of amplitude and orientation of birefringence. The important parameter in the model is the correlation between local reflectivity of the two orthogonal polarizations.
We present the first demonstration of measurements of velocity and direction of flow using Transversal Doppler Optical
Coherence Tomography. The experiments are carried out using a four-channel quadrant detector at the output of a freespace
Michelson interferometer. This allows real three dimensional mapping of both flow and velocity with no
limitation on the Doppler angle.
We demonstrate low-coherence interferometry (LCI) for dye diffusion measurements in scattering tissue phantoms. The diffusion coefficient of a phthalocyanine dye in 1.5% agar gel containing scattering Intralipid was measured using a dual-wavelength interfero-meter. One wavelength was matched to the absorption peak of the dye at 675 nm. The other, 805 nm, was not affected by the dye, and was used to correct for varying sample scattering as a function of depth, assuming a constant ratio between scattering at the two wavelengths. The same wavelength dependence of scattering is assumed for the entire sample, but no a priori knowledge about the amount of scattering is needed. The dye diffusion coefficient was estimated by fitting a mathematical model of the interferometer signal to the measured LCI envelope. We compare results obtained using both a constant-scattering and a depth-resolved-scattering approach to determine the sample scattering. The presented method provides robust estimation of the diffusion coefficient when spatial resolution in determining the depth-resolved scattering is varied. Results indicate that the method is valid for samples having continuous spatial variations in the scattering coefficient over lengths as short as the coherence length of the probing light. The method allows in situ characterization of diffusion in scattering media.
We present functional imaging of the concentration of a photodynamic therapy (PDT)-related dye in scattering tissue phantoms based on spatially resolved measurements of optical properties through spectroscopic optical coherence tomography (OCT). Expressions for the OCT signal are developed, enabling estimation of depth-resolved sample optical properties. Based on these expressions, we discuss speckle statistics and speckle correlations of the OCT signal. Speckle noise reduction is performed by spatial filtering and is used to improve accuracy in the estimated optical properties at the expense of spatial resolution. An analytic expression for the precision in the estimated optical properties is derived. This expression shows that axial filtering, and thereby a reduction of axial resolution, gives a larger improvement in precision compared to the same filtering and reduction in the transversal resolution. It also shows that imaging with a shorter coherence length, or a larger numerical aperture, improves precision when the filter length determines the spatial resolution. Good agreement is obtained between experimentally determined and theoretically predicted variance in the estimated attenuation coefficients and dye concentration. Finally, we present guidelines for spectroscopic OCT systems for concentration imaging and discuss application of the method to more realistic phantoms and tissue.
We present a novel OCT (Optical Coherence Tomography) instrument which enables us to detect two orthogonal polarization states at two different wavelengths simultaneously. We have used this instrument to demonstrate, study, and compare the properties of speckle averaging using frequency compounding and polarization diversity separately and in combination. Reductions in speckle contrast obtained by measurements are compared to theoretical values and results from computer simulations of OCT signals.
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 propose and present TDOCT (Transversal Doppler Optical Coherence Tomography), a novel method for measurement of particle flow velocity and direction. A quadrant detector can be employed to enable separate detection of light scattered in four different angles. Transversal flow will induce different Doppler shifts in the four interferometer signals, and suitable processing of the spectra or phase changes can be used to determine the amplitude and direction of the transversal flow component.
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.
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