Meena Siddiqui

A rapid, dispersion-based wavelength-stepped and wavelength-swept laser for optical coherence tomography.

Optical-domain subsampling enables Fourier-domain OCT imaging at high-speeds and extended depth ranges while limiting the required acquisition bandwidth. To perform optical-domain subsampling, a wavelength-stepped rather than a wavelength-swept source is required. This preliminary study introduces a novel design for a rapid wavelength-stepped laser source that uses dispersive fibers in combination with a fast lithium-niobate modulator to achieve wavelength selection. A laser with 200 GHz wavelength-stepping and a sweep rate of 9 MHz over a 94 nm range at a center wavelength of 1550 nm is demonstrated. A reconfiguration of this source design to a continuous wavelength-swept light for conventional Fourier-domain OCT is also demonstrated.

Compensation of spectral and RF errors in swept-source OCT for high extinction complex demodulation

We provide a framework for compensating errors within passive optical quadrature demodulation circuits used in swept-source optical coherence tomography (OCT). Quadrature demodulation allows for detection of both the real and imaginary components of an interference fringe, and this information separates signals from positive and negative depth spaces. To achieve a high extinction (∼60 dB) between these positive and negative signals, the demodulation error must be less than 0.1% in amplitude and phase. It is difficult to construct a system that achieves this low error across the wide spectral and RF bandwidths of high-speed swept-source systems. In a prior work, post-processing methods for removing residual spectral errors were described. Here, we identify the importance of a second class of errors originating in the RF domain, and present a comprehensive framework for compensating both spectral and RF errors. Using this framework, extinctions >60 dB are demonstrated. A stability analysis shows that calibration parameters associated with RF errors are accurate for many days, while those associated with spectral errors must be updated prior to each imaging session. Empirical procedures to derive both RF and spectral calibration parameters simultaneously and to update spectral calibration parameters are presented. These algorithms provide the basis for using passive optical quadrature demodulation circuits with high speed and wide-bandwidth swept-source OCT systems.

Optical-domain subsampling for data efficient depth ranging in Fourier-domain optical coherence tomography

Recent advances in optical coherence tomography (OCT) have led to higher-speed sources that support imaging over longer depth ranges. Limitations in the bandwidth of state-of-the-art acquisition electronics, however, prevent adoption of these advances into the clinical applications. Here, we introduce optical-domain subsampling as a method for imaging at high-speeds and over extended depth ranges but with a lower acquisition bandwidth than that required using conventional approaches. Optically subsampled laser sources utilize a discrete set of wavelengths to alias fringe signals along an extended depth range into a bandwidth limited frequency window. By detecting the complex fringe signals and under the assumption of a depth-constrained signal, optical-domain subsampling enables recovery of the depth-resolved scattering signal without overlapping artifacts from this bandwidth-limited window. We highlight key principles behind optical-domain subsampled imaging, and demonstrate this principle experimentally using a polygon-filter based swept-source laser that includes an intra-cavity Fabry-Perot (FP) etalon.

High-speed optical coherence tomography by circular interferometric ranging

Existing three-dimensional optical imaging methods excel in controlled environments, but are difficult to deploy over large, irregular and dynamic fields. This means that they can be ill-suited for use in areas such as material inspection and medicine. To better address these applications, we developed methods in optical coherence tomography to efficiently interrogate sparse scattering fields, that is, those in which most locations (voxels) do not generate meaningful signal. Frequency comb sources are used to superimpose reflected signals from equispaced locations through optical subsampling. This results in circular ranging, and reduces the number of measurements required to interrogate large volumetric fields. As a result, signal acquisition barriers that have limited speed and field in optical coherence tomography are avoided. With a new ultrafast, time-stretched frequency comb laser design operating with 7.6 MHz to 18.9 MHz repetition rates, we achieved imaging of multi-cm3 fields at up to 7.5 volumes per second.Using an ultrafast, time-stretched frequency comb laser operating with repetition rates from 7.6 MHz to 18.9 MHz, a rapid and large-volumetric-field optical coherence tomography at an imaging rate of up to 7.5 volumes per second is demonstrated.

High-speed subsampled optical coherence tomography imaging with frequency comb lasers

We demonstrate how frequency comb lasers can be used to induce optical-domain compression in optical ranging. In the context of coherent tomography, this compression enables ultra-high speed volumetric microscopy. We describe this concept and a novel high-speed laser based on stretched-pulse mode locking (SPML).

Angiographic imaging using an 18.9 MHz swept-wavelength laser that is phase-locked to the data acquisition clock and resonant scanners(Conference Presentation)

In this study, we present an angiographic system comprised from a novel 18.9 MHz swept wavelength source integrated with a MEMs-based 23.7 kHz fast-axis scanner. The system provides rapid acquisition of frames and volumes on which a range of Doppler and intensity-based angiographic analyses can be performed. Interestingly, the source and data acquisition computer can be directly phase-locked to provide an intrinsically phase stable imaging system supporting Doppler measurements without the need for individual A-line triggers or post-processing phase calibration algorithms. The system is integrated with a 1.8 Gigasample (GS) per second acquisition card supporting continuous acquisition to computer RAM for 10 seconds. Using this system, we demonstrate phase-stable acquisitions across volumes acquired at 60 Hz frequency. We also highlight the ability to perform c-mode angiography providing volume perfusion measurements with 30 Hz temporal resolution. Ultimately, the speed and phase-stability of this laser and MEMs scanner platform can be leveraged to accelerate OCT-based angiography and both phase-sensitive and phase-insensitive extraction of blood flow velocity.

Simultaneous high-speed and long-range imaging with optically subsampled OCT(Conference Presentation)

Current implementations of OCT can either image over long depth ranges with slower imaging speeds, or at high imaging speeds with more limited depth ranges. The simultaneous operation at multi-centimeter depth ranges and MHz-scale A-line rates is challenging due to limitations in the electronic bandwidths of current digitizers and data transfer buses. The lack of multi-cm depth range, MHz-speed OCT hinders the translation of the imaging technology to sites and organs with complex geometries and expansive fields. Here we describe a first demonstration of a simultaneous cm-scale depth range and MHz-scale A-line rate OCT platform. We describe the principles behind data compression by optically subsampled OCT, the development and performance of a novel subsampled OCT wavelength stepped source operating at 19 MHz A-line rates, the extension of passive quadrature demodulation architectures to GHz-scale acquisition bandwidths, and the first ever integration of these technologies into a subsampled OCT system capable of acquiring volume data at video-rates across multi-cm depth ranges. We use this platform to demonstrate depth resolved measurements over large fields that exhibit complex topography such as the face. The performance, limitations, and the next stages of technical development for this optically subsampled OCT platform are summarized. This platform may open new opportunities for camera-like OCT deployments in sites and organs that are inaccessible to current OCT technologies.