We show a broad range of swept source performances based on a highly-flexible external cavity laser architecture.
Specifically, we demonstrate a 40-kHz 1300-nm swept source with 10 mm coherence length realized in a compact
butterfly package. Fast wavelength sweeping is achieved through a 1D 20-kHz MEMS mirror in combination with an
advanced diffraction grating. The MEMS mirror is a resonant electrostatic mirror that performs harmonic oscillation only
within a narrow frequency range, resulting in low-jitter and long-term phase-stable sinusoidal bidirectional sweep
operation with an A-scan rate of 40 kHz. The source achieves a coherence length of 10 mm for both the up- and downsweep
and an OCT sensitivity of 105 dB.
KEYWORDS: Signal processing, Field programmable gate arrays, Optical coherence tomography, Imaging systems, Visualization, Data processing, Signal to noise ratio, Data acquisition, Optical simulations, Computer simulations
We have developed a Swept-Source Optical Coherence Tomography (Ss-OCT) system with high-speed, real-time signal processing on a commercially available Data-Acquisition (DAQ) board with a Field-Programmable Gate Array (FPGA). The Ss-OCT system simultaneously acquires OCT and k-clock reference signals at 500MS/s. From the k-clock signal of each A-scan we extract a remap vector for the k-space linearization of the OCT signal. The linear but oversampled interpolation is followed by a 2048-point FFT, additional auxiliary computations, and a data transfer to a host computer for real-time, live-streaming of B-scan or volumetric C-scan OCT visualization. We achieve a 100 kHz A-scan rate by parallelization of our hardware algorithms, which run on standard and affordable, commercially available DAQ boards. Our main development tool for signal analysis as well as for hardware synthesis is MATLAB® with add-on toolboxes and 3rd-party tools.
We demonstrate in-vivo functional imaging of the human retina with Fourier domain optical
coherence tomography employing frequency encoding of an excitation pattern. The
principle is based on projecting a modulated rectangular pattern across the foveal region and
acquiring a time series of B-Scans at the same vertical position across the pattern. The idea is
to modulate the excitation with a frequency that is distinct from the heartbeat and irregular
motion artifacts. Fourier analysis of the time series at each transverse position in the B-scan
series allows assessing the retinal response as change in the FDOCT reflectivity signal exactly
at the pattern modulation frequency. We observe a change in retinal reflectivity within the
region of the outer segment photoreceptor layer exactly at the pattern modulation
frequency.
Fourier domain Optical coherence microscopy (FDOCM) offers excellent sensitivity and high axial resolution to image
the structure of biological tissue. The depth information is extracted in parallel and allows very high volume acquisition
rates. The present system uses a diffractionless beam, produced with an axicon lens, to achieve high lateral resolution all
while maintaining an extended depth of field (xf). The xfOCM signal reveals the spatial distribution of changes of the
refractive index in the sample that scatter the incident light. To identify and validate the functionality of the observed
structures can proof difficult. In this work the xfOCM setup was interfaced with a fluorescent lifetime imaging (FLIM)
system, working in the Fourier domain and measuring the phase offset between the modulated excitation signal and the
returned fluorescence intensity. Both the fluorescence amplitude and lifetime are retrieved. The amplitude contains
important information due to the selective labeling of the tissue. The lifetime is very sensitive to the surrounding
environment and varies for different fluorophores, adding further contrast. The xfOCM tomograms and FLIM images are
acquired in parallel. A complementary view of the sample is obtained that helps to understand and interpret the xfOCM
signal. The lifetime measurement provides further contrast to perform functional imaging on biological samples such as
the rat hair follicle.
We address the problem of exact signal recovery in frequency-domain optical-coherence tomography
(FDOCT). The standard technique for tomogram reconstruction is the inverse Fourier transform. However,
the inverse Fourier transform is known to yield autocorrelation artifacts which interfere with the
desired signal. We propose a new transformation for computing an artifact-free tomogram from intensity
measurements. Our technique relies on the fact that, in the FDOCT measurements, the intensity
of the total signal reflected from the object is smaller than that of the reference arm. Our technique
is noniterative, nonlinear, and it leads to an exact solution in the absence of noise. The reconstructed
signal is free from autocorrelation artifacts. We present results on synthesized data as well as on
experimental FDOCT measurements of the retina of the eye.
Resonant Doppler Fourier Domain Optical Coherence Tomography is a functional imaging modality for quantifying fast
tissue flow. The method profits from the effect of interference fringe blurring in spectrometer-based FDOCT in the
presence of sample motion. If the reference path length is changed in resonance with the Doppler frequency of the
sample flow the signals of resting structures will be suppressed whereas the signals of blood flow are enhanced. This
allows for an easy extraction of vascularization structure. 3D images of blood vessels at the human optic nerve head are
obtained with high axial resolution of 8 μm in air and an imaging speed of 17.400 depth profiles per second. An electro-optic
modulator allows controlled reference phase shifting during camera integration. A differential approach is
presented for the quantification of fast flows that are un-accessible via standard phase sensitive Doppler analysis. Flow
velocity analysis extracts only the axial component which is dependent on the orientation of the vessel with respect to the
optical axis. 3D information of the segmented vessel structure is readily used to obtain the flow velocity vectors along
the individual vessels and to calculate the true angle-corrected flow speed.
KEYWORDS: Blood circulation, Doppler tomography, Signal attenuation, Optical coherence tomography, Doppler effect, Cameras, 3D image processing, Reconstruction algorithms, Image segmentation, Signal to noise ratio
Resonant Doppler Fourier domain optical coherence tomography (FDOCT) is a functional imaging tool for extracting tissue flow. The method is based on the effect of interference fringe blurring in spectrometer-based FDOCT, where the path difference between structure and reference changes during camera integration. If the reference path length is changed in resonance with the Doppler frequency of the sample flow, the signals of resting structures will be suppressed, whereas the signals of blood flow are enhanced. This allows for an easy extraction of vascularization structure. Conventional flow velocity analysis extracts only the axial flow component, which strongly depends on the orientation of the vessel with respect to the incident light. We introduce an algorithm to extract the vessel geometry within the 3-D data volume. The algorithm calculates the angular correction according to the local gradients of the vessel orientations. We apply the algorithm on a measured 3-D resonant Doppler dataset. For validation of the reproducibility, we compare two independently obtained 3-D flow maps of the same volunteer and region.
Fourier Domain OCT proved to be an outstanding tool for measuring 3D retinal structures with
high sensitivity, resolution, and speed. We extended the FDOCT concept towards functional
imaging by analyzing the spectroscopic tissue properties, polarization contrast and Doppler
velocity imaging. Differential spectral contrast FDOCT allows optical staining of retinal
tomograms and to contrast tissue of high pigmentation such as the retinal pigment epithelium
(RPE). The latter shows strong correlation if compared to polarization sensitive OCT images.
First implementations of Doppler FDOCT systems demonstrated the capability of measuring in-vivo
retinal blood flow profiles and pulsatility. We developed a new concept of Doppler FDOCT
that allows measuring also large flow velocities typically close to the optic nerve head. Studies of
retinal perfusion based on Laser Doppler Flowmetry (LDF) demonstrated the high sensitivity of
blood flow to external stimuli. We performed first experiments of studying retinal perfusion in
response to flicker stimulation. An increase in vessel diameter by 11% and of flow velocity by
49% was measured. We believe that a multi-modal functional imaging concept is of high value
for an accurate and early diagnosis and understanding of retinal pathologies and pathogenesis.
Fourier domain optical coherence tomography (FDOCT) is a high speed imaging technique with high axial resolution in
the micro-meter-scale range combined with a high sensitivity allowing to probe 3D volumes of weakly back-scattering
biological tissues in-vivo. Phase shifting techniques allow the reconstruction of the full complex sample signal which
results in an additional suppression of unwanted auto-correlated distortion as well as an extended depth range. Current
complex FDOCT realizations introduce the phase shift via reference path length modulation causing chromatic phase
errors especially if broad bandwidth light sources are employed. Broad optical bandwidth is necessary for ultrahigh
resolution OCT systems. By frequency shifting the light fields with acousto-optic frequency shifters in the reference and
sample arm respectively, a phase-resolved signal at high speed can be registered. Therefore the reference arm does not
rely on arm length changes or delays. The beating signal generated this way shows high phase stability. The phase of this
beating signal is not wavelength-dependent, as the frequency shift applied is the same for all wavelengths. With a
Ti:Sapphire laser at 800nm and a spectral width of 130nm a high speed complex FDOCT system is realized with an axial
resolution of 4μm.
We present an imaging system which could be used for endoscopic topography. Indeed it allows for two dimensional information transmission through a unidimensional imaging channel which is a monomode optical fiber. The principle is the coupling of wavelength multiplexing and spectral interferometry and a special configuration renders this system dispersion self compensated what enables a high signal stability. Principles will be presented as well as results and limits of the system
The combination of wavelength multiplexing and spectral interferometry allows for the encoding of multidimensional information and its transmission over a mono-dimensional channel; for example, measurements of a surface's topography acquired through a monomode fiber in a small endoscope. The local depth of the imaged object is encoded in the local spatial frequency of the signal measured at the output of the fiber-decoder system. We propose a procedure to retrieve the depth-map by determining the signal's instantaneous frequency. First, we compute its continuous, complex-valued, wavelet transform (CWT). The frequency signature at every position is contained in the resulting scalogram. We then extract the ridge of maximal response by use of a dynamic programming algorithm thus directly recovering the object's topography. We present results that validate this procedure based on both simulated and experimental data.
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