Néstor Uribe-Patarroyo

Quantitative technique for robust and noise-tolerant speed measurements based on speckle decorrelation in optical coherence tomography

Intensity-based techniques in optical coherence tomography (OCT), such as those based on speckle decorrelation, have attracted great interest for biomedical and industrial applications requiring speed or flow information. In this work we present a rigorous analysis of the effects of noise on speckle decorrelation, demonstrate that these effects frustrate accurate speed quantitation, and propose new techniques that achieve quantitative and repeatable measurements. First, we derive the effect of background noise on the speckle autocorrelation function, finding two detrimental effects of noise. We propose a new autocorrelation function that is immune to the main effect of background noise and permits quantitative measurements at high and moderate signal-to-noise ratios. At the same time, this autocorrelation function is able to provide motion contrast information that accurately identifies areas with movement, similar to speckle variance techniques. In order to extend the SNR range, we quantify and model the second effect of background noise on the autocorrelation function through a calibration. By obtaining an explicit expression for the decorrelation time as a function of speed and diffusion, we show how to use our autocorrelation function and noise calibration to measure a flowing liquid. We obtain accurate results, which are validated by Doppler OCT, and demonstrate a very high dynamic range (> 600 mm/s) compared to that of Doppler OCT (±25 mm/s). We also derive the behavior for low flows, and show that there is an inherent non-linearity in speed measurements in the presence of diffusion due to statistical fluctuations of speckle. Our technique allows quantitative and robust measurements of speeds using OCT, and this work delimits precisely the conditions in which it is accurate.

High-energy pulsed Raman fiber laser for biological tissue coagulation

We demonstrate a high-energy pulsed Raman fiber laser (RFL) with an emission wavelength of 1.44 μm, corresponding to an absorption peak of water. Microsecond pulses with >20 mJ/pulse and >40 W peak power were generated, well above the threshold for tissue coagulation and ablation. Here, we focus on the optical characterization and optimization of high-energy and high-power RFLs excited by an ytterbium fiber laser, comparing three configurations that use different Raman gain fibers, but all of which were prepared with a one-side opened, free-run mode without output mirrors. We show that the free-run configuration can generate sufficiently high energy without requiring a closed cavity design. Experimental RFL characteristics corresponded well with numerical simulations. We discuss the Stokes beam generation process in our system and loss mechanisms mainly associated with fiber Bragg gratings.

Rotational distortion correction in endoscopic optical coherence tomography based on speckle decorrelation

We present a new technique for the correction of nonuniform rotation distortion in catheter-based optical coherence tomography (OCT), based on the statistics of speckle between A-lines using intensity-based dynamic light scattering. This technique does not rely on tissue features and can be performed on single frames of data, thereby enabling real-time image correction. We demonstrate its suitability in a gastrointestinal (GI) balloon-catheter OCT system, determining the actual rotational speed with high temporal resolution, and present corrected cross-sectional and en face views showing significant enhancement of image quality.

Vortex-enhanced coherent-illumination phase diversity for phase retrieval in coherent imaging systems.

We propose a phase-retrieval method based on the numerical optimization of a new objective function using coherent phase-diversity images as inputs for the characterization of aberrations in coherent imaging systems. By employing a spatial light modulator to generate multiple-order spiral phase masks as diversities, we obtain an increase in the accuracy of the retrieved phase compared with similar state-of-the-art phase-retrieval techniques that use the same number of input images. We present simulations that show a consistent advantage of our technique, and experimental validation where our implementation is used to characterize a highly aberrated 4F optical system.

Extended bandwidth wavelength swept laser source for high resolution optical frequency domain imaging

Improving the axial resolution by providing wider bandwidth wavelength swept lasers remains an important issue for optical frequency domain imaging (OFDI). Here, we demonstrate a wide tuning range, all-fiber wavelength swept laser at a center wavelength of 1250 nm by combining two ring cavities that share a single Fabry-Perot tunable filter. The two cavities contain semiconductor optical amplifiers with central wavelengths of 1190 nm and 1292 nm, respectively. To avoid disturbing interference effects in the overlapping spectral region, we modulated the amplifiers in order to obtain consecutive wavelength sweeps in the two spectral regions. The two sweeps were fused together in post-processing to achieve a total scanning range of 223 nm, corresponding to 3.3 µm axial resolution in air. We confirm improved image quality and reduced speckle size in tomograms of swine esophagus ex vivo, and human skin and nailbed in vivo.

Velocity gradients in spatially resolved laser Doppler flowmetry and dynamic light scattering with confocal and coherence gating

Dynamic light scattering (DLS) is widely used to characterize diffusive motion to obtain precise information on colloidal suspensions by calculating the autocorrelation function of the signal from a heterodyne optical system. DLS can also be used to determine the flow velocity field in systems that exhibit mass transport by incorporating the effects of the deterministic motion of scatterers on the autocorrelation function, a technique commonly known as laser Doppler flowmetry. DLS measurements can be localized with confocal and coherence gating techniques such as confocal microscopy and optical coherence tomography, thereby enabling the determination of the spatially resolved velocity field in three dimensions. It has been thought that spatially resolved DLS can determine the axial velocity as well as the lateral speed in a single measurement. We demonstrate, however, that gradients in the axial velocity of scatterers exert a fundamental influence on the autocorrelation function even in well-behaved, nonturbulent flow. By obtaining the explicit functional relation between axial-velocity gradients and the autocorrelation function, we show that the velocity field and its derivatives are intimately related and their contributions cannot be separated. Therefore, a single DLS measurement cannot univocally determine the velocity field. Our extended theoretical model was found to be in good agreement with experimental measurements.

Single-pixel imaging using balanced detection and a digital micromirror device

Over the past decade, single-pixel imaging (SPI) has established as a viable tool in scenarios where traditional imaging techniques struggle to provide images with acceptable quality in practicable times and reasonable costs. However, SPI still has several limitations inherent to the technique, such as working with spurious light and in real time. Here we present a novel approach, using complementary measurements and a single balanced detector. By using balanced detection, we improve the frame rate of the complementary measurement architectures by a factor of two. Furthermore, the use of a balanced detector provides environmental light immunity to the method.

Spectral reconstruction strategies toward generalized-domain optical coherence tomography with a broadband source and a bucket detector(Conference Presentation)

One appealing aspect that compressive sensing offers is the possibility of retrieving a signal’s spectral information using a bucket detector and a characterized measurement matrix. Demonstrations of CS applied to optical coherence tomography (OCT) were performed, however, in the final signal-processing instead of the acquisition end. Here we propose a novel OCT system with a broadband superluminescent excitation and a bucket photodetector where the interferogram is obtained by spectral reconstruction. In particular, this system assumes the same interferometric setup as typical swept-source OCT systems except the excitation is replaced by a broadband source. The interferogram then passes through an off-the-shelf, fast tunable Fabry-Perot filter (FPF) of modest finesse whose free spectral range is designed to be much less than the excitation bandwidth. The spectral response is characterized a priori, before the filtered output is integrated by the photodetector. The spectral sampling measurement is repeated by altering the FPF’s resonant conditions multiple times through the cavity length. Having acquired the integrated photodetector values and the corresponding spectral filter functions, we reconstruct the original interferogram whose Fourier transform generates the tomogram. The sensitivity of this OCT technique is evaluated and compared using simulations with synthetic data. Moreover, B-scan reconstruction of the interferogram due to a fingertip was simulated using our scheme and the resultant image shows excellent reconstruction fidelity compared to the original OCT B-scan. These illustrations point towards a promising future of a new class of tomographic system which combines the respective strengths of swept-source and spectral-domain OCT.

Improving on-axis optical vortex generation by transmissive spatial light modulator using coherent phase diversity

We present a modification of numerical optimization coherent phase diversity in order to detect and correct undesired effects of non-linear and incomplete phase modulation of transmissive spatial light modulators for on-axis optical vortices.

Use of balanced detection in single-pixel imaging

We introduce balanced detection in combination with simultaneous complementary illumination in a single-pixel architecture. With this novel detection scheme we are able to recover a real-time video stream in presence of ambient light.