A. H. Bachmann

Heterodyne Fourier domain optical coherence tomography for full range probing with high axial resolution

One of the main drawbacks of Fourier domain optical coherence tomography (FDOCT) is the limited measurement depth range. Phase shifting techniques allow reconstructing the complex sample signal resulting in a doubled depth range. In current complex FDOCT realizations the phase shift is introduced via a reference path length modulation, which causes chromatic phase errors especially if broad bandwidth light sources are employed. With acousto-optic frequency shifters in the reference and sample arm, and the detector being locked to the resulting beating frequency, the signal is quadrature detected at high speed. The beating signal frequency is the same for all wavelengths allowing for achromatic complex reconstruction. With a Ti:Sapphire laser at 800nm and spectral width of 130nm, a heterodyne complex FDOCT system is realized with 20kHz line rate and an axial resolution of 4mum.

Resonant Doppler flow imaging and optical vivisection of retinal blood vessels

For Fourier domain optical coherence tomography any sample movement during camera integration causes blurring of interference fringes and as such reduction of sensitivity for flow detection. The proposed method overcomes this problem by phase-matching a reference signal to the sample motion. The interference fringes corresponding to flow signal will appear frozen across the detector whereas those of static sample structures will be blurred resulting in enhanced contrast for blood vessels. An electro-optic phase modulator in the reference arm, driven with specific phase cycles locked to the detection frequency, allows not only for qualitative but also for quantitative flow detection already from the relative signal intensities. First applications to extract in-vivo retinal flow and to visualize 3D vascularization, i.e. optical vivisection, are presented.

Vectorial reconstruction of retinal blood flow in three dimensions measured with high resolution resonant Doppler Fourier domain optical coherence tomography

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.

Continuous wavelet transform ridge extraction for spectral interferometry imaging

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 surfaces 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 signals 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 objects topography. We present results that validate this procedure based on both simulated and experimental data.

NON-ITERATIVE EXACT SIGNAL RECOVERY IN FREQUENCY DOMAIN OPTICAL COHERENCE TOMOGRAPHY

We address the problem of exact signal recovery in frequency domain optical coherence tomography (FDOCT) systems. Our technique relies on the fact that, in a spectral interferometry setup, the intensity of the total signal reflected from the object is smaller than that of the reference arm. We develop a novel algorithm to compute the reflected signal amplitude from the interferometric measurements. Our technique is non-iterative, non-linear and it leads to an exact solution in the absence of noise. The reconstructed signal is free from artifacts such as the autocorrelation noise that is normally encountered in the conventional inverse Fourier transform techniques. We present results on synthesized data where we have a benchmark for comparing the performance of the technique. We also report results on experimental FDOCT measurements of the retina of the human eye

High-speed miniaturized swept sources based on resonant MEMS mirrors and diffraction gratings

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.

FPGA-based real-time swept-source OCT systems for B-scan live-streaming or volumetric imaging

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.

Logarithmic Transformation Technique for Exact Signal Recovery in Frequency-Domain Optical-Coherence Tomography

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.

Measurement of retinal physiology using functional Fourier domain OCT concepts

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.

Complex unltrahigh resolution Fourier domain optical coherence tomography

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.