Interleaved optical coherence tomography

Solid-state FMCW LiDAR with two-dimensional spectral scanning using a virtually imaged phased array

The beam-steering device is a critical component in LiDAR systems for 3D imaging. Solid-state beam-steering devices attract the most attention for their advantages of robustness, fast beam-steering speed, and stability. However, solid-state beam-steering devices, such as optical phased arrays (OPAs), are challenging to realize 2D scanning ability. Here we employed a virtually imaged phased array (VIPA) in the LiDAR system to realize all solid-state two-dimensional (2D) beam-steering based on dispersion only. A frequency swept laser source is used for performing optical frequency-modulated continuous-wave (FMCW) ranging and 2D beam steering simultaneously. The 2D disperser is compact and can be easily implemented owing to its simple structure. The mechanism of continuous scanning and ranging is beneficial for obtaining high lateral resolution, and a lateral resolution of 0.06° is achieved. 3D maps of the object at a distance of 2 m are obtained with cm-level ranging precision. The frame rate of the proposed LiDAR system only depends on the wavelength-tuning speed of the swept laser source, with the potential to realize ultrafast solid-state LiDAR systems.

Virtually imaged phased-array-based 2D nonmechanical beam-steering device for FMCW LiDAR

Nonmechanical beam-steering devices are of importance to achieve fast, compact, and reliable LiDAR. We propose a 2D nonmechanical beam-steering device based on a virtually imaged phased array (VIPA) for frequency-modulated continuous-wave (FMCW) LiDAR. In the design, 2D nonmechanical beam steering and high-resolution FMCW ranging can be achieved at the same time by wavelength tuning. The design formulas of the VIPA-based 2D disperser are greatly simplified by introducing appropriate approximation, and a feasible design procedure is proposed for the first time, to the best of our knowledge. Based on the proposed method, several design examples with different optimal properties are exhibited.

Wide-field Ophthalmic Space-Division Multiplexing Optical Coherence Tomography

High-speed ophthalmic optical coherence tomography systems are of interest because they allow rapid, motion-free, and wide-field retinal imaging. Space-division multiplexing optical coherence tomography (SDM-OCT) is a high-speed imaging technology which takes advantage of the long coherence length of microelectromechanical vertical cavity surface emitting laser (MEMs VCSEL) sources to multiplex multiple images along a single imaging depth. We demonstrate wide-field retinal OCT imaging, acquired at an effective A-scan rate of 800,000 A-scans/sec with volumetric images covering up to 12.5 mm × 7.4 mm on the retina acquired in less than 1 second. A clinical feasibility study was conducted to compare the ophthalmic SDM-OCT with commercial OCT systems, illustrating the high-speed capability of SDM-OCT in a clinical setting.

Speckle modulation enables high-resolution wide-field human brain tumor margin detection and in vivo murine neuroimaging

Current in vivo neuroimaging techniques provide limited field of view or spatial resolution and often require exogenous contrast. These limitations prohibit detailed structural imaging across wide fields of view and hinder intraoperative tumor margin detection. Here we present a novel neuroimaging technique, speckle-modulating optical coherence tomography (SM-OCT), which allows us to image the brains of live mice and ex vivo human samples with unprecedented resolution and wide field of view using only endogenous contrast. The increased visibility provided by speckle elimination reveals white matter fascicles and cortical layer architecture in brains of live mice. To our knowledge, the data reported herein represents the highest resolution imaging of murine white matter structure achieved in vivo across a wide field of view of several millimeters. When applied to an orthotopic murine glioblastoma xenograft model, SM-OCT readily identifies brain tumor margins with resolution of approximately 10 μm. SM-OCT of ex vivo human temporal lobe tissue reveals fine structures including cortical layers and myelinated axons. Finally, when applied to an ex vivo sample of a low-grade glioma resection margin, SM-OCT is able to resolve the brain tumor margin. Based on these findings, SM-OCT represents a novel approach for intraoperative tumor margin detection and in vivo neuroimaging.

Flexible wide-field optical micro-angiography based on Fourier-domain multiplexed dual-beam swept source optical coherence tomography

Wide-field optical coherence tomography angiography (OCTA) is gaining interest in clinical imaging applications. In this pursuit, it is challenging to maintain the imaging resolution and sensitivity throughout the wide field of view (FoV). Here, we propose a novel method/system of dual-beam arrangement and Fourier-domain multiplexing to achieve wide-field OCTA when imaging the uneven surface samples. The proposed system provides 2 separate FoVs, with flexibility that the imaging area, focus of the imaging beam and imaging depth range can be individually adjusted for each FoV, leading to either (1) increased system imaging FoV or (2) capability of targeting 2 regions of interests that locate at depths with large difference between each other. We demonstrate this novel method by employing 100 kHz laser source in a swept source OCTA to achieve an effective 200 kHz sweeping rate, covering a 22 × 22 mm FoV. The results are verified by a SS-OCTA system employing a 200 kHz laser source, together with the experimental demonstrations when imaging whole brain vasculature in rodent models and skin blood perfusion in human fingers, show-casing the capability of proposed system to image live large samples with complex surface topography.

Wide-field high-speed space-division multiplexing optical coherence tomography using an integrated photonic device

Space-division multiplexing optical coherence tomography (SDM-OCT) is a recently developed parallel OCT imaging method in order to achieve multi-fold speed improvement. However, the assembly of fiber optics components used in the first prototype system was labor-intensive and susceptible to errors. Here, we demonstrate a high-speed SDM-OCT system using an integrated photonic chip that can be reliably manufactured with high precisions and low per-unit cost. A three-layer cascade of 1 × 2 splitters was integrated in the photonic chip to split the incident light into 8 parallel imaging channels with ~3.7 mm optical delay in air between each channel. High-speed imaging (~1s/volume) of porcine eyes ex vivo and wide-field imaging (~18.0 × 14.3 mm²) of human fingers in vivo were demonstrated with the chip-based SDM-OCT system.

High-speed OCT light sources and systems [Invited]

Imaging speed is one of the most important parameters that define the performance of optical coherence tomography (OCT) systems. During the last two decades, OCT speed has increased by over three orders of magnitude. New developments in wavelength-swept lasers have repeatedly been crucial for this development. In this review, we discuss the historical evolution and current state of the art of high-speed OCT systems, with focus on wavelength swept light sources and swept source OCT systems.

Non-moving scanner design for OCT systems

In this work, a novel beam scanner design based on non-moving parts is introduced which will eliminate the phase and inaccuracy problems of the mechanical scanners while providing two times imaging speed improvement for optical coherence tomography systems. The design is comprised of electro-optically activated switches that are placed on the sample arm. For the example considered here, lateral resolution of 20 µm, and lateral scanning range of 1 mm are aimed at which resulted in a scanner size of 1 mm × 9 mm. Due to its compact size, proposed design can also be implemented in forward-looking endoscopic probes.

Micrometer-resolution in-fiber OCT probe with tunable working distance

Optical coherence tomography (OCT) is an attractive modality in biomedical imaging systems due to its non-invasive imaging character. Since the image quality of OCT may be limited by the decrease of transverse resolution away from the focus spot, working distance tunable probe can be a strategy to overcome such limitation and maintain high transverse resolution at different imaging depths. In this paper, a miniature, working distance-tunable in-fiber OCT probe is demonstrated. The influences of the graded index fiber (GIF) length as well as the air cavity length on the working distance and the transverse resolution are simulated and discussed. Experimental results prove that the working distance can be tuned freely from 337.31 μm to 22.28 μm, producing the transverse resolution from 13.86 μm to 3.6 μm, which are in good agreement with the simulated results. The application of the probe in an OCT system for imagining a standard USAF resolution target is investigated in detail. The best resolutions for the standard USAF resolution target imaging are 4.9 μm and 6.9 μm in horizontal and vertical direction, respectively.

Iterative re-weighted approach to high-resolution optical coherence tomography with narrow-band sources

Optical coherence tomography (OCT) is a non-invasive optical imaging modality capable of high resolution imaging of internal tissue structures. It is widely believed that the high axial resolution in OCT systems requires a wide-bandwidth light source. As a result, often the potential advantages of narrow-bandwidth sources (in terms of cost and/or imaging speed) are understood to come at the cost of significant reduction in imaging resolution. In this paper, we argue that this trade-off between resolution and speed is a shortcoming imposed by the-state-of-the-art A-scan reconstruction algorithm, Fast Fourier Transform, and can be circumvented through use of alternative processing methods. In particular, we investigate the shortcomings of the FFT as well as previously proposed alternatives and demonstrate the first application of an iterative regularized re-weighted l(2) norm method to improve the axial resolution of fast scan rate OCT systems in the narrow-bandwidth imaging conditions. We validate our claims via experimental results generated from a home-built OCT system used to image layered phantom and in vivo data. Our results rely on new, sophisticated signal processing algorithms to generate higher precision (i.e., higher resolution) OCT images at correspondingly fast scan rates. In other words, our work demonstrates the feasibility of simultaneously more reliable and more comfortable medical imaging systems for patients by reducing the overall scan time, without sacrificing image quality.

Parallel Imaging of 3D Surface Profile with Space-Division Multiplexing

We have developed a modified optical frequency domain imaging (OFDI) system that performs parallel imaging of three-dimensional (3D) surface profiles by using the space division multiplexing (SDM) method with dual-area swept sourced beams. We have also demonstrated that 3D surface information for two different areas could be well obtained in a same time with only one camera by our method. In this study, double field of views (FOVs) of 11.16 mm × 5.92 mm were achieved within 0.5 s. Height range for each FOV was 460 µm and axial and transverse resolutions were 3.6 and 5.52 µm, respectively.

Polarization-sensitive interleaved optical coherence tomography

We introduce a new strategy for single-mode fiber based polarization-sensitive (PS-) optical coherence tomography (OCT) using orthogonally polarized optical frequency combs (OFC) in the sample arm. The two OFCs are tuned to be interleaved in the spectral domain, permitting simultaneous measurement of both polarization states from the same spatial region C close to the location of zero pathlength delay. The two polarization states of the beam in the sample arm are demultiplexed by interpolation after performing wavelength stabilization via a two-mirror calibration method. The system uses Jones matrix methods to measure quantitatively the round-trip phase retardation B-scans in the sample. A glass plate and quarter-wave plate were measured to validate the accuracy of the birefringence measurement. Further, we demonstrated the potential of this system for biomedical applications by measurement of chicken breast muscle.

Single-shot speckle noise reduction by interleaved optical coherence tomography

Speckle noise is one of the dominant factors that degrade image quality in optical coherence tomography (OCT). Here, we propose a new strategy, interleaved OCT (iOCT), for spatial compounding and angular compounding. We demonstrate the efficiency of compounding with iOCT to restrain speckle noise without compromising imaging speed in phantoms and tissue samples.

Scalable multiplexing for parallel imaging with interleaved optical coherence tomography

We demonstrate highly parallel imaging with interleaved optical coherence tomography (iOCT) using an in-house-fabricated, air-spaced virtually-imaged phased array (VIPA). The air-spaced VIPA performs spectral encoding of the interferograms from multiple lateral points within a single sweep of the source and allows us to tune and balance several imaging parameters: number of multiplexed points, ranging depth, and sensitivity. In addition to a thorough discussion of the parameters and operating principles of the VIPA, we experimentally demonstrate the effect of different VIPA designs on the multiplexing potential of iOCT. Using a 200-kHz light source, we achieve an effective A-scan rate of 3.2-MHz by multiplexing 16 lateral points onto a single wavelength sweep. The improved sensitivity of this system is demonstrated for 3D imaging of biological samples such as a human finger and a fruit fly.