Cubic meter volume optical coherence tomography

Linearity improvement of chirped distributed feedback laser diodes based on an analog electro-optical phase-locked loop

A linearity improvement method for frequency-modulated distributed feedback laser diodes (DFB-LD) is proposed and demonstrated based on a pre-distortion signal and an electro-optical phase-locked loop (EO-PLL). The pre-distortion signal is used to reduce the deterministic frequency errors. The EO-PLL is further used to suppress the stochastic frequency noise and enhance the coherence of the DFB-LD. In the EO-PLL, the DFB-LD output is transmitted through a Mach-Zehnder interferometer (MZI) and detected by a photodetector (PD) to get a beat note signal, which denotes the nonlinearity of the chirp. A mixing signal, achieved by mixing the beat note signal with a fixed frequency reference signal, is then filtered by a proportional integral filter (PIF) and feedback to the DFB-LD to reduce the stochastic frequency noise in the chirp. The EO-PLL bandwidth can be adjusted by tuning the PIF response. Consequently, a linear chirp optical signal with an enhanced linearity is generated from the DFB-LD. In the experiment, 788- and 321-time linearity improvements are implemented when the loop bandwidths are about 100 and 550 kHz, respectively. Correspondingly, residual frequency errors of 1.65 and 4.85 MHz at 52- and 450-THz/s chirp rates are obtained.

On-chip real-time detection of optical frequency variations with ultrahigh resolution using the sine-cosine encoder approach

Real-time measurement of optical frequency variations (OFVs) is crucial for various applications including laser frequency control, optical computing, and optical sensing. Traditional devices, though accurate, are often too large, slow, and costly. Here we present a photonic integrated circuit (PIC) chip, utilizing the sine-cosine encoder principle, for high-speed and high-resolution real-time OFV measurement. Fabricated on a thin film lithium niobate (TFLN) platform, this chip-sized optical frequency detector (OFD) (5.5 mm × 2.7 mm) achieves a speed of up to 2500 THz/s and a resolution as fine as 2 MHz over a range exceeding 160 nm. Our robust algorithm overcomes the device imperfections and ensures precise quantification of OFV parameters. As a practical demonstration, the PIC OFD surpasses existing fiber Bragg grating (FBG) interrogators in sensitivity and speed for strain and vibration measurements. This work opens new avenues for on-chip OFV detection and offers significant potential for diverse applications involving OFV measurement.

Image Quality in Adaptive Optics Optical Coherence Tomography of Diabetic Patients

Background/Objectives: An assessment of the retinal image quality in adaptive optics optical coherence tomography (AO-OCT) is challenging. Many factors influence AO-OCT imaging performance, leading to greatly varying imaging results, even in the same subject. The aim of this study is to introduce quantitative means for an assessment of AO-OCT image quality and to compare these with parameters retrieved from the pyramid wavefront sensor of the system. Methods: We used a spectral domain AO-OCT instrument to repetitively image six patients suffering from diabetic retinopathy over a time span of one year. The data evaluation consists of two volume acquisitions with a focus on the photoreceptor layer, each at five different retinal locations per visit; 7-8 visits per patient are included in this data analysis, resulting in a total of ~420 volumes. Results: A large variability in AO-OCT image quality is observed between subjects and between visits of the same subject. On average, the image quality does not depend on the measurement location. The data show a moderate correlation between the axial position of the volume recording and image quality. The correlation between pupil size and AO-OCT image quality is not linear. A weak correlation is found between the signal-to-noise ratio of the wavefront sensor image and the image quality. Conclusions: The introduced AO-OCT image quality metric gives useful insights into the performance of such a system. A longitudinal assessment of this metric, together with wavefront sensor data, is essential to identify factors influencing image quality and, in the next step, to optimize the performance of AO-OCT systems.

Microscope integrated MHz optical coherence tomography system for neurosurgery: development and clinical in-vivo imaging

Neurosurgical interventions on the brain are impeded by the requirement to keep damages to healthy tissue at a minimum. A new contrast channel enhancing the visual separation of malign tissue should be created. A commercially available surgical microscope was modified with adaptation optics adapting the MHz speed optical coherence tomography (OCT) imaging system developed in our group. This required the design of a scanner optics and beam delivery system overcoming constraints posed by the mechanical and optical parameters of the microscope. High quality volumetric OCT C-scans with dense sample spacing can be acquired in-vivo as part of surgical procedures within seconds and are immediately available for post-processing.

Adaptive contour-tracking to aid wide-field swept-source optical coherence tomography imaging of large objects with uneven surface topology

High-speed and wide-field optical coherence tomography (OCT) imaging is increasingly essential for clinical applications yet faces challenges due to its inherent sensitivity roll-off and limited depth of focus, particularly when imaging samples with significant variations in surface contour. Here, we propose one innovative solution of adaptive contour tracking and scanning methods to address these challenges. The strategy integrates an electrically tunable lens and adjustable optical delay line control with real-time surface contour information, enabling dynamic optimization of imaging protocols. It rapidly pre-scans the sample surface to acquire a comprehensive contour map. Using this map, it generates a tailored scanning protocol by partitioning the entire system ranging distance into depth-resolved segments determined by the optical Raleigh length of the objective lens, ensuring optimal imaging at each segment. Employing short-range imaging mode along the sample contour minimizes data storage and post-processing requirements, while adaptive adjustment of focal length and reference optical delay line maintains high imaging quality throughout. Experimental demonstrations show the effectiveness of the adaptive contour tracking OCT in maintaining high contrast and signal-to-noise ratio across the entire field of view, even in samples with significantly uneven surface curvatures. Notably, this approach achieves these results with reduced data volume compared to traditional OCT methods. This advancement holds promise for enhancing OCT imaging in clinical settings, particularly in applications requiring rapid, wide-field imaging of tissue structures and blood flow.

Wide-field OCT angiography for non-human primate retinal imaging

Optical coherence tomography (OCT) is a well-established research tool for vision research in animal models capable of providing in vivo imaging of the retina. Structural OCT can be enhanced using OCT angiography (OCTA) processing in order to provide simultaneously acquired, automatically co-registered vascular information. Currently available OCT. Currently available OCTA lack either large field of view or high resolution. In this study we developed a wide-field (60-degree), high-resolution (10.5-µm optical transverse) and high-sensitivity (104-dB) OCTA-enabled system for non-human primate imaging and with it imaged multiple disease models, including models of age-related macular degeneration (AMD), Bardet-Biedl Syndrome (BBS), and the CLN7 variant of Batten disease. We demonstrate clear visualization of features including drusen, ellipsoid zone loss, vascular retinopathy, and retinal thinning in these eyes.

High-speed, long-range and wide-field OCT for in vivo 3D imaging of the oral cavity achieved by a 600 kHz swept source laser

We report a high-speed, long-range, and wide-field swept-source optical coherence tomography (SS-OCT) system aimed for imaging microstructures and microcirculations in the oral cavity. This system operates at a scan speed of 600 kHz, delivering a wide imaging field of view at 42 × 42 mm2 and a ranging distance of 36 mm. To simultaneously meet the requirements of high speed and long range, it is necessary for the k-clock trigger signal to be generated at its maximum speed, which may induce non-linear phase response in electronic devices due to the excessive k-clock frequency bandwidth, leading to phase errors. To address this challenge, we introduced a concept of electrical dispersion and a global k-clock compensation approach to improve overall performance of the imaging system. Additionally, image distortion in the wide-field imaging mode is also corrected using a method based on distortion vector maps. With this system, we demonstrate comprehensive structural and blood flow imaging of the anterior oral cavity in healthy individuals. The high-speed, long-range, and wide-field SS-OCT system opens new opportunities for comprehensive oral cavity examinations and holds promise as a reliable tool for assessing oral health conditions.

Electronic frequency shifting enables long, variable working distance optical coherence tomography

Increased imaging range is of growing interest in many applications of optical coherence tomography to reduce constraints on sample location, size, and topography. The design of optical coherence tomography systems with sufficient imaging range (e.g., 10s of centimeters) is a significant challenge due to the direct link between imaging range and acquisition bandwidth. We have developed a novel and flexible method to extend the imaging range in optical coherence tomography using electronic frequency shifting, enabling imaging in dynamic environments. In our approach, a laser with a quasi-linear sweep is used to limit the interferometric bandwidth, enabling decoupling of imaging range and acquisition bandwidth, while a tunable lens allows dynamic refocusing in the sample arm. Electronic frequency shifting then removes the need for high frequency digitization. This strategy is demonstrated to achieve high contrast morphological imaging over a > 21 cm working distance range, while maintaining high resolution and phase sensitivity. The system design is flexible to the application while requiring only a simple phase correction in post-processing. By implementing this approach in an auto-focusing paradigm, the proposed method demonstrates strong potential for the translation of optical coherence tomography into emerging applications requiring variable and centimeter-scale imaging ranges.

Coherent Laser Ranging of Deforming Objects in Fires at Sub-Millimeter Precision

Light Detection and Ranging (LiDAR) is a powerful tool to characterize and track the surface geometry of solid objects. In a fire, however, no method has excelled at measuring three-dimensional shapes at millimeter precision while offering some immunity to the effects of flames. This paper applies coherent Frequency Modulated Continuous Wave Light Detection and Ranging to capture three-dimensional measurements of objects in fire at meters of stand-off distance. We demonstrate that despite the presence of natural gas flame depths up to 1.5 m obscuring the target, measurements with millimeter precision can be obtained. This is a significant improvement over previous work making the technique useful for many fire research applications. An approach to achieve sub-millimeter precision using spatial and temporal averaging during post-processing is presented. The technology is demonstrated in case studies of structural connection and vegetation response in fires.

Automated robot-assisted wide-field optical coherence tomography using structured light camera

Optical coherence tomography (OCT) is a promising real-time and non-invasive imaging technology widely utilized in biomedical and material inspection domains. However, limited field of view (FOV) in conventional OCT systems hampers their broader applicability. Here, we propose an automated system integrating a structured light camera and robotic arm for large-area OCT scanning. The system precisely detects tissue contours, automates scan path generation, and enables accurate scanning of expansive sample areas. The proposed system consists of a robotic arm, a three-dimensional (3D) structured light camera, and a customized portable OCT probe. The 3D structured light camera is employed to generate a precise 3D point cloud of the sample surface, enabling automatic planning of the scanning path for the robotic arm. Meanwhile, the OCT probe is mounted on the robotic arm, facilitating scanning of the sample along the predetermined path. Continuous OCT B-scans are acquired during the scanning process, facilitating the generation of high-resolution and large-area 3D OCT reconstructions of the sample. We conducted position error tests and presented examples of 3D macroscopic imaging of different samples, such as ex vivo kidney, skin and leaf blade. The robotic arm can accurately reach the planned positions with an average absolute error of approximately 0.16 mm. The findings demonstrate that the proposed system enables the acquisition of 3D OCT images covering an area exceeding 20 cm2, indicating wide-ranging potential for utilization in diverse domains such as biomedical, industrial, and agricultural fields.

Quantum Induced Coherence Light Detection and Ranging

Quantum illumination has been proposed and demonstrated to improve the signal-to-noise ratio (SNR) in light detection and ranging (LiDAR). When relying on coincidence detection alone, such a quantum LiDAR is limited by the timing jitter of the detector and suffers from jamming noise. Inspired by the Zou-Wang-Mandel experiment, we design, construct, and validate a quantum induced coherence (QuIC) LiDAR which is inherently immune to ambient and jamming noises. In traditional LiDAR the direct detection of the reflected probe photons suffers from deteriorating SNR for increasing background noise. In QuIC LiDAR we circumvent this obstacle by only detecting the entangled reference photons, whose single-photon interference fringes are used to obtain the distance of the object, while the reflected probe photons are used to erase path information of the reference photons. In consequence, the noise accompanying the reflected probe light has no effect on the detected signal. We demonstrate such noise resilience with both LED and laser light to mimic the background and jamming noise. The proposed method paves a new way of battling noise in precise quantum electromagnetic sensing and ranging.

Long-range frequency domain low-coherence interferometry detector for industrial applications

A low-cost long-range frequency domain low-coherence interferometry (LCI) detector is presented: time Fourier domain LCI (TFD-LCI). Combining ideas of time domain and frequency domain techniques, the TFD-LCI detects the analog Fourier transform of the optical interference signal with no limitation for the maximum optical path, measuring the thickness of several centimeters with micrometer resolution. A complete characterization of the technique is presented with a mathematical demonstration, simulations, and experimental results. An evaluation of repeatability and accuracy is also included. Measurements of small and large monolayer and multilayer thicknesses were done. Characterization of the internal and external thicknesses of industrial products such as transparent packages and glass windshield is presented, showing the potentiality of TFD-LCI for industrial applications.

Continuous-wave parallel interferometric near-infrared spectroscopy (CW NIRS) with a fast two-dimensional camera

Interferometric near-infrared spectroscopy (iNIRS) is an optical method that noninvasively measures the optical and dynamic properties of the human brain in vivo. However, the original iNIRS technique uses single-mode fibers for light collection, which reduces the detected light throughput. The reduced light throughput is compensated by the relatively long measurement or integration times (∼1 sec), which preclude monitoring of rapid blood flow changes that could be linked to neural activation. Here, we propose parallel interferometric near-infrared spectroscopy (πNIRS) to overcome this limitation. In πNIRS we use multi-mode fibers for light collection and a high-speed, two-dimensional camera for light detection. Each camera pixel acts effectively as a single iNIRS channel. So, the processed signals from each pixel are spatially averaged to reduce the overall integration time. Moreover, interferometric detection provides us with the unique capability of accessing complex information (amplitude and phase) about the light remitted from the sample, which with more than 8000 parallel channels, enabled us to sense the cerebral blood flow with only a 10 msec integration time (∼100x faster than conventional iNIRS). In this report, we have described the theoretical foundations and possible ways to implement πNIRS. Then, we developed a prototype continuous wave (CW) πNIRS system and validated it in liquid phantoms. We used our CW πNIRS to monitor the pulsatile blood flow in a human forearm in vivo. Finally, we demonstrated that CW πNIRS could monitor activation of the prefrontal cortex by recording the change in blood flow in the forehead of the subject while he was reading an unknown text.

Dual-comb based time-stretch optical coherence tomography for large and segmental imaging depth

Optical coherence tomography based on time-stretch enables high frame rate and high-resolution imaging for the inertia-free wavelength-swept mechanism. The fundamental obstacle is still the acquisition bandwidth's restriction on imaging depth. By introducing dual-comb with slightly different repetition rates, the induced Vernier effect is found to be capable of relieving the problem. In our work, a dual-comb based time-stretch optical coherence tomography is proposed and experimentally demonstrated, achieving a 1.5-m imaging depth and 200-kHz A-scan rate. Moreover, about a 33.4-µm resolution and 25-µm accuracy are achieved. In addition, by adjusting the frequency detuning of the dual-comb, the A-scan rate can be further boosted to video-rate imaging. With enlarged imaging depth, this scheme is promising for a wide range of applications, including light detection and ranging.

De-aliased depth-range-extended optical coherence tomography based on dual under-sampling

We demonstrate a dual under-sampling (DUS) method to achieve de-aliased and depth-range-extended optical coherence tomography (OCT) imaging. The spectral under-sampling can significantly reduce the data size but causes well-known aliasing artifacts. A change in the sampling frequency used to acquire the interference spectrum alters the aliasing period within the output window except for the true image; this feature is utilized to distinguish the true image from the aliasing artifacts. We demonstrate that with DUS, the data size is reduced to 37% at an extended depth range of 24 mm, over which the true depth can be precisely measured without ambiguity. This reduction in data size and precise measuring capability would be beneficial for reducing the acquisition time for OCT imaging in various biomedical and industrial applications.

Universal photonics tomography

3D imaging is essential for the study and analysis of a wide variety of structures in numerous applications. Coherent photonic systems such as optical coherence tomography (OCT) and light detection and ranging (LiDAR) are state-of-the-art approaches, and their current implementation can operate in regimes that range from under a few millimeters to over more than a kilometer. We introduce a general method, which we call universal photonics tomography (UPT), for analyzing coherent tomography systems, in which conventional methods such as OCT and LiDAR may be viewed as special cases. We demonstrate a novel approach (to our knowledge) based on the use of phase modulation combined with multirate signal processing to collect positional information of objects beyond the Nyquist limits.

Linewidth considerations for MEMS tunable VCSEL LiDAR

The flexible membranes used in MEMS tunable VCSELs are so small and light that thermally induced vibrations can impact laser performance. We measure the thermal vibration spectrum of such a membrane showing peaks at the spatial vibration mode resonant frequencies of the membrane/plate. These vibrations result in a theoretical floor to the linewidth of the VCSEL. Frequency domain LiDAR and optical coherence tomography systems can get around this thermal linewidth limit with adequate clock measurement and processing. Essentially an OCT/LiDAR sweep with a concomitantly measured clock is a feed-forward linewidth reduction scheme. This can be achieved because the membrane resonances are relatively low frequency. LiDAR ranging out to 9 meters has been demonstrated with a resolution of 13 μm, close to the transform limit for the 70 nm sampling range.

Long distance high resolution FMCW laser ranging with phase noise compensation and 2D signal processing

A long distance high resolution frequency-modulated continuous wave (FMCW) laser rangefinder with phase noise compensation and two-dimensional (2D) data processing skills is developed. Range-finding ladar consists of a continuously chirped laser source, an auxiliary reference interferometer, and a monostatic optical transceiver for target illumination and return photon collection. To extend the range unambiguity and lower the electronic processing bandwidth, a two-step laser frequency chirping scheme is adopted, where a long pulse width, small frequency bandwidth laser chirping signal are used in step 1 for coarse distance estimation, and a short pulse width and large frequency bandwidth laser chirping signal are applied afterwards for step 2 high resolution distance realization. An auxiliary reference interferometer is to record the phase noise originated from the laser source to compensate for phase errors induced in the target return photons. The 2D data processing skill helps to coherently sum up all the phase noise removed echo photons to achieve high resolution range peak extraction with high detection sensitivity. Experimental demonstration shows that the proposed FMCW ladar at 1550 nm wavelength with a laser chirping bandwidth of 10 GHz and electronic processing bandwidth of 200 MHz can measure a corner cube test target in an outdoor atmospheric environment, and the measurement results are 12013.905 m with a 2.4 cm range resolution under strong return photon levels and 12013.920 m with a 2.5 cm range resolution under weak return photon levels.

Video-rate high-precision time-frequency multiplexed 3D coherent ranging

Frequency-modulated continuous wave (FMCW) light detection and ranging (LiDAR) is an emerging 3D ranging technology that offers high sensitivity and ranging precision. Due to the limited bandwidth of digitizers and the speed limitations of beam steering using mechanical scanners, meter-scale FMCW LiDAR systems typically suffer from a low 3D frame rate, which greatly restricts their applications in real-time imaging of dynamic scenes. In this work, we report a high-speed FMCW based 3D imaging system, combining a grating for beam steering with a compressed time-frequency analysis approach for depth retrieval. We thoroughly investigate the localization accuracy and precision of our system both theoretically and experimentally. Finally, we demonstrate 3D imaging results of multiple static and moving objects, including a flexing human hand. The demonstrated technique achieves submillimeter localization accuracy over a tens-of-centimeter imaging range with an overall depth voxel acquisition rate of 7.6 MHz, enabling densely sampled 3D imaging at video rate.

Continuous spectral zooming for in vivo live 4D-OCT with MHz A-scan rates and long coherence

We present continuous three-dimensional spectral zooming in live 4D-OCT using a home-built FDML based OCT system with 3.28 MHz A-scan rate. Improved coherence characteristics of the FDML laser allow for imaging ranges up to 10 cm. For the axial spectral zoom feature, we switch between high resolution and long imaging range by adjusting the sweep range of our laser. We present a new imaging setup allowing for synchronized adjustments of the imaging range and lateral field of view during live OCT imaging. For this, a novel inline recalibration algorithm was implemented that enables numerical k-linearization of the raw OCT fringes for every frame instead of every volume. This is realized by acquiring recalibration data within the dead time of the raster scan at the turning points of the fast axis scanner. We demonstrate in vivo OCT images of fingers and hands at different resolution modes and show real three-dimensional zooming during live 4D-OCT. A three-dimensional spectral zooming feature for live 4D-OCT is expected to be a useful tool for a wide range of biomedical, scientific and research applications, especially in OCT guided surgery.

High speed, long range, deep penetration swept source OCT for structural and angiographic imaging of the anterior eye

This study reports the development of prototype swept-source optical coherence tomography (SS-OCT) technology for imaging the anterior eye. Advances in vertical-cavity surface-emitting laser (VCSEL) light sources, signal processing, optics and mechanical designs, enable a unique combination of high speed, long range, and deep penetration that addresses the challenges of anterior eye imaging. We demonstrate SS-OCT with a 325 kHz A-scan rate, 12.2 µm axial resolution (in air), and 15.5 mm depth range (in air) at 1310 nm wavelength. The ultrahigh 325 kHz A-scan rate not only facilitates biometry measurements by minimizing acquisition time and thus reducing motion, but also enables volumetric OCT for comprehensive structural analysis and OCT angiography (OCTA) for visualizing vasculature. The 15.5 mm (~ 11.6 mm in tissue) depth range spans all optical surfaces from the anterior cornea to the posterior lens capsule. The 1310 nm wavelength range enables structural OCT and OCTA deep in the sclera and through the iris. Achieving high speed and long range requires linearizing the VCSEL wavenumber sweep to efficiently utilize analog-to-digital conversion bandwidth. Dual channel recording of the OCT and calibration interferometer fringe signals, as well as sweep to sweep wavenumber compensation, is used to achieve invariant 12.2 µm (~ 9.1 µm in tissue) axial resolution and optimum point spread function throughout the depth range. Dynamic focusing using a tunable liquid lens extends the effective depth of field while preserving the lateral resolution. Improved optical and mechanical design, including parallax “split view” iris cameras and stable, ergonomic patient interface, facilitates accurate instrument positioning, reduces patient motion, and leads to improved imaging data yield and measurement accuracy. We present structural and angiographic OCT images of the anterior eye, demonstrating the unique imaging capabilities using representative scanning protocols which may be relevant to future research and clinical applications.

Optical coherence tomography

Optical coherence tomography (OCT) is a non-contact method for imaging the topological and internal microstructure of samples in three dimensions. OCT can be configured as a conventional microscope, as an ophthalmic scanner, or using endoscopes and small diameter catheters for accessing internal biological organs. In this Primer, we describe the principles underpinning the different instrument configurations that are tailored to distinct imaging applications and explain the origin of signal, based on light scattering and propagation. Although OCT has been used for imaging inanimate objects, we focus our discussion on biological and medical imaging. We examine the signal processing methods and algorithms that make OCT exquisitely sensitive to reflections as weak as just a few photons and that reveal functional information in addition to structure. Image processing, display and interpretation, which are all critical for effective biomedical imaging, are discussed in the context of specific applications. Finally, we consider image artifacts and limitations that commonly arise and reflect on future advances and opportunities.

Single-shot multiple-depth macroscopic imaging by spatial frequency multiplexing

We present a low-coherence interferometric imaging system designed for 3-dimensional (3-D) imaging of a macroscopic object through a narrow passage. Our system is equipped with a probe-type port composed of a bundle fiber for imaging and a separate multimode optical fiber for illumination. To eliminate the need for mechanical depth scanning, we employ a spatial frequency multiplexing method by installing a 2-D diffraction grating and an echelon in the reference arm. This configuration generates multiple reference beams, all having different path lengths and propagation directions, which facilitates the encoding of different depth information in a single interferogram. We demonstrate the acquisition of 9 depth images at the interval of 250 μm for a custom-made cone and a plaster teeth model. The proposed system minimizes the need for mechanical scanning and achieves a wide range of depth coverage, significantly increasing the speed of 3-D imaging for macroscopic objects.

Robotic-arm-assisted flexible large field-of-view optical coherence tomography

Optical coherence tomography (OCT) is a three-dimensional non-invasive high-resolution imaging modality that has been widely used for applications ranging from medical diagnosis to industrial inspection. Common OCT systems are equipped with limited field-of-view (FOV) in both the axial depth direction (a few millimeters) and lateral direction (a few centimeters), prohibiting their applications for samples with large and irregular surface profiles. Image stitching techniques exist but are often limited to at most 3 degrees-of-freedom (DOF) scanning. In this work, we propose a robotic-arm-assisted OCT system with 7 DOF for flexible large FOV 3D imaging. The system consists of a depth camera, a robotic arm and a miniature OCT probe with an integrated RGB camera. The depth camera is used to get the spatial information of targeted sample at large scale while the RGB camera is used to obtain the exact position of target to align the image probe. Eventually, the real-time 3D OCT imaging is used to resolve the relative pose of the probe to the sample and as a feedback for imaging pose optimization when necessary. Flexible probe pose manipulation is enabled by the 7 DOF robotic arm. We demonstrate a prototype system and present experimental results with flexible tens of times enlarged FOV for plastic tube, phantom human finger, and letter stamps. It is expected that robotic-arm-assisted flexible large FOV OCT imaging will benefit a wide range of biomedical, industrial and other scientific applications.

Multi-MHz MEMS-VCSEL swept-source optical coherence tomography for endoscopic structural and angiographic imaging with miniaturized brushless motor probes

Swept source optical coherence tomography (SS-OCT) enables volumetric imaging of subsurface structure. However, applications requiring wide fields of view (FOV), rapid imaging, and higher resolutions have been challenging because multi-MHz axial scan (A-scan) rates are needed. We describe a microelectromechanical systems vertical cavity surface-emitting laser (MEMS-VCSEL) SS-OCT technology for A-scan rates of 2.4 and 3.0 MHz. Sweep to sweep calibration and resampling are performed using dual channel acquisition of the OCT signal and a Mach Zehnder interferometer signal, overcoming inherent optical clock limitations and enabling higher performance. We demonstrate ultrahigh speed structural SS-OCT and OCT angiography (OCTA) imaging of the swine gastrointestinal tract using a suite of miniaturized brushless motor probes, including a 3.2 mm diameter micromotor OCT catheter, a 12 mm diameter tethered OCT capsule, and a 12 mm diameter widefield OCTA probe. MEMS-VCSELs promise to enable ultrahigh speed SS-OCT with a scalable, low cost, and manufacturable technology, suitable for a diverse range of imaging applications.

Increased crystalline lens coverage in optical coherence tomography with oblique scanning and volume stitching

A three-dimensional optical coherence tomography (OCT) crystalline lens imaging method based on oblique scanning and image stitching is presented. The method was designed to increase OCT imaging volume of crystalline lens in vivo. A long-range swept-source (SS)-OCT imaging system, which can measure the entire anterior segment of eye in a single acquisition, is used to acquire one central volume and 4 extra volumes with different angles between optical axis of OCT instrument and the pupillary axis. The volumes are then stitched automatically by developed software. To show its effectiveness and verify its validity, we scanned the subjects before and after pupil dilation drops and compared the experimental results. By determining the number of voxels representing the signal from the crystalline lens in 3-D OCT images, our method can provide around 17% additional volumetric lens coverage compared with a regular imaging procedure. The proposed approach could be used clinically in early diagnosis of cortical cataract. Wider field of view offered by this method may facilitate more accurate lens biometry in its peripheral zones, which potentially contributes to understanding of lens shape modifications of the accommodating eye.

Spectrally sparse optical coherence tomography

Swept-source optical coherence tomography (OCT) typically relies on expensive and complex swept-source lasers, the cost of which currently limits the suitability of OCT for new applications. In this work, we demonstrate spectrally sparse OCT utilizing randomly spaced low-bandwidth optical chirps, suitable for low-cost implementation with telecommunications grade devices. Micron scale distance estimation accuracy with a resolution of 40 μm at a standoff imaging distance greater than 10 cm is demonstrated using a stepped chirp approach with approximately 23% occupancy of 4 THz bandwidth. For imaging of sparse scenes, comparable performance to full bandwidth occupancy is verified for metallic targets.

High-speed range and velocity measurement using frequency scanning interferometry with adaptive delay lines [Invited]

Range (i.e., absolute distance), displacement, and velocity of a moving target have been measured with a frequency scanning interferometer that incorporates a 100,000scans^-1 vertical-cavity surface-emitting laser with 100 nm tuning range. An adaptive delay line in the reference beam, consisting of a chain of switchable exponentially growing optical delays, reduced modulation frequencies to sub-gigahertz levels. Range, displacement, and velocity were determined from the phase of the interference signal; fine alignment and linearization of the scans were achieved from the interferogram of an independent reference interferometer. Sub-nanometer displacement resolution, sub-100-nm range resolution, and velocity resolution of 12µms^-1 have been demonstrated over a depth measurement range of 300 mm.

Three-dimensional imaging through scattering media based on confocal diffuse tomography

Optical imaging techniques, such as light detection and ranging (LiDAR), are essential tools in remote sensing, robotic vision, and autonomous driving. However, the presence of scattering places fundamental limits on our ability to image through fog, rain, dust, or the atmosphere. Conventional approaches for imaging through scattering media operate at microscopic scales or require a priori knowledge of the target location for 3D imaging. We introduce a technique that co-designs single-photon avalanche diodes, ultra-fast pulsed lasers, and a new inverse method to capture 3D shape through scattering media. We demonstrate acquisition of shape and position for objects hidden behind a thick diffuser (≈6 transport mean free paths) at macroscopic scales. Our technique, confocal diffuse tomography, may be of considerable value to the aforementioned applications.

Multi-scale optical coherence tomography imaging and visualization of Vermeer's Girl with a Pearl Earring

We demonstrate multi-scale multi-parameter optical coherence tomography (OCT) imaging and visualization of Johannes Vermeer's painting Girl with a Pearl Earring. Through automated acquisition, OCT image segmentation, and 3D volume stitching we realize OCT imaging at the scale of an entire painting. This makes it possible to image, with micrometer axial and lateral resolution, an entire painting over more than 5 orders of length scale. From the multi-scale OCT data we quantify multiple parameters in a fully automated way: the surface height, the scattering strength, and the combined glaze and varnish layer thickness. The multi-parameter OCT data of Girl with a Pearl Earring shows various features: Vermeer's brushstrokes, surface craquelure, paint losses, and restorations. Through an interactive visualization of the Girl, based on the OCT data and the optical properties of historical reconstructions of Vermeer's paint, we can virtually study the effect of the lighting condition, viewing angle, zoom level and presence/absence of glaze layer. The interactive visualization shows various new painting features. It demonstrates that the glaze layer structure and its optical properties were essential to Vermeer to create an extremely strong light to dark contrast between the figure and the background that gives the painting such an iconic aesthetic appeal.

Swept Source Lidar: simultaneous FMCW ranging and nonmechanical beam steering with a wideband swept source

Light detection and ranging (lidar) has long been used in various applications. Solid-state beam steering mechanisms are needed for robust lidar systems. Here we propose and demonstrate a lidar scheme called "Swept Source Lidar" that allows us to perform frequency-modulated continuous-wave (FMCW) ranging and nonmechanical beam steering simultaneously. Wavelength dispersive elements provide angular beam steering, while a laser frequency is continuously swept by a wideband swept source over its whole tuning bandwidth. Employing a tunable vertical-cavity surface-emitting laser and a 1-axis mechanical beam scanner, three-dimensional point cloud data has been obtained. Swept Source Lidar systems can be flexibly combined with various beam steering elements to realize full solid-state FMCW lidar systems.

Full-range space-division multiplexing optical coherence tomography angiography

In this study, we demonstrated a full-range space-division multiplexing optical coherence tomography (FR-SDM-OCT) system. Utilizing the galvanometer-based phase modulation full-range technique, the total imaging range of FR-SDM-OCT can be extended to >20 mm in tissue, with a digitizer sampling rate of 500 MS/s and a laser sweeping rate of 100 kHz. Complex conjugate terms were suppressed in FR-SDM-OCT images with a measured rejection ratio of up to ∼46 dB at ∼1.4 mm depth and ∼30 dB at ∼19.4 mm depth. The feasibility of FR-SDM-OCT was validated by imaging Scotch tapes and human fingernails. Furthermore, we demonstrated the feasibility of FR-SDM-OCT angiography (FR-SDM-OCTA) to perform simultaneous acquisition of human fingernail angiograms from four positions, with a total field-of-view of ∼1.7 mm × ∼7.5 mm. Employing the full-range technique in SDM-OCT can effectively alleviate hardware requirements to achieve the long depth measurement range, which is required by SDM-OCT to separate multiple images at different sample locations. FR-SDM-OCTA creates new opportunities to apply SDM-OCT to obtain wide-field angiography of in vivo tissue samples free of labeling.

9.4 MHz A-line rate optical coherence tomography at 1300 nm using a wavelength-swept laser based on stretched-pulse active mode-locking

In optical coherence tomography (OCT), high-speed systems based at 1300 nm are among the most broadly used. Here, we present 9.4 MHz A-line rate OCT system at 1300 nm. A wavelength-swept laser based on stretched-pulse active mode locking (SPML) provides a continuous and linear-in-wavenumber sweep from 1240 nm to 1340 nm, and the OCT system using this light source provides a sensitivity of 98 dB and a single-sided 6-dB roll-off depth of 2.5 mm. We present new capabilities of the 9.4 MHz SPML-OCT system in three microscopy applications. First, we demonstrate high quality OCTA imaging at a rate of 1.3 volumes/s. Second, by utilizing its inherent phase stable characteristics, we present wide dynamic range en face Doppler OCT imaging with multiple time intervals ranging from 0.25 ms to 2.0 ms at a rate of 0.53 volumes/s. Third, we demonstrate video-rate 4D microscopic imaging of a beating Xenopus embryo heart at a rate of 30 volumes/s. This high-speed and high-performance OCT system centered at 1300 nm suggests that it can be one of the most promising high-speed OCT platforms enabling a wide range of new scientific research, industrial, and clinical applications at speeds of 10 MHz.

Kalman-Based Real-Time Functional Decomposition for the Spectral Calibration in Swept Source Optical Coherence Tomography

This paper presents a real-time functional decomposition adaptive algorithm for the optimal sampling of the interferometric signal in Swept-Source Optical Coherence Tomography imaging systems, which completely eliminates the input signal dependent nonlinearities that are problematic in current state-of-the-art OCT realizations that use interpolation and resampling. The proposed adaptive calibration algorithm uses the Kalman approach to estimate the wavenumber index parameter k from the Mach-Zender Interferometer signal which is then applied to an adaptive level crossing sampler to generate a sampling clock that k-linearizes the data on real-time during the sampling process. Such a system implements an artifact-free realization of the technology removing the need for classical interpolation and resampling. The new real-time linearization scheme has the additional capability of increasing the imaging acquisition speed by 10X while providing robustness to noise, properties that are demonstrated through mathematical analysis and simulation results throughout the paper.

Single-shot surface 3D imaging by optical coherence factor

We report a single-shot three-dimensional (3D) topographical imaging method, optical coherence factor (OCF) imaging, which uses optical coherence as the contrast mechanism to acquire the surface height (${z}$z-direction) information of an object. A 4-f imaging system records the light field reflected from the surface of the object. The illumination of the imaging system comes from a laser source with the optical coherence length comparable to the depth of field (DoF) of the optical system. Off-axis holographic recording is used to retrieve the coherence factor from the interference fringes, which is then converted to ${z}$z-direction information. In this experiment, we validate our 3D imaging results comparing them to axial scanning full-field optical coherence tomography images. We also analyze the contrast mechanism of OCF and show that it is able to provide additional information over conventional coherent and incoherent imaging using the same imaging setup. This single-shot computationally efficient method may have potential applications in industrial quality control inspection.

Stable multi-megahertz circular-ranging optical coherence tomography at 1.3 µm

In Fourier-domain optical coherence tomography (OCT), the finite bandwidth of the acquisition electronics constrains the depth range and speed of the system. Circular-ranging (CR) OCT methods use optical-domain compression to surpass this limit. However, the CR-OCT system architectures of prior reports were limited by poor stability and were confined to the 1.55 µm wavelength range. In this work, we describe a novel CR-OCT architecture that is free from these limitations. To ensure stable operation, temperature sensitive optical modules within the system were replaced; the kilometer-length fiber spools used in the stretched-pulse mode-locked (SPML) laser was eliminated in favor of a single 10 meter, continuously chirped fiber Bragg grating, and the interferometer's passive optical quadrature demodulation circuit was replaced by an active technique using a lithium niobate phase modulator. For improved imaging penetration in biological tissues, the system operating wavelength was shifted to a center wavelength of 1.29 µm by leveraging the wavelength flexibility intrinsic to CFBG-based dispersive fibers. These improvements were achieved while maintaining a broad (100 nm) optical bandwidth, a long 4 cm imaging range, and a high 7.6 MHz A-line rate. By enhancing stability, simplifying overall system design, and operating at 1.3 µm, this CR-OCT architecture will allow a broader exploration of CR-OCT in both medical and non-medical applications.

Handheld Optical Coherence Tomography Normative Inner Retinal Layer Measurements for Children <5 Years of Age

PURPOSE

Measurements of the ganglion cell complex (GCC), comprising the retinal nerve fiber (RNFL), ganglion cell, and inner plexiform layers, can be correlated with vision loss caused by optic nerve disease. Handheld optical coherence tomography (HH-OCT) can be used with sedation in children who are not amenable to traditional imaging. We report GCC and RNFL measurements in normal children using HH-OCT.

DESIGN

Prospective observational study of normal children ≤5 years of age.

METHODS

Healthy, full-term children ≤5 years of age undergoing sedation or anesthesia were enrolled. Exclusion criteria included prematurity and pre-existing neurologic, genetic, metabolic, or intraocular pathology. Demographic data, axial length (Master-Vu Sonomed Escalon, Lake Success, New York, USA), and HH-OCT macular and optic nerve volume scans at 0° (Bioptigen, Inc., Morrisville, North Carolina, USA) were obtained. Retinal segmentation was completed with DOCTRAP software, creating average volume thickness maps.

RESULTS

Sixty-seven children (67 eyes, 31 males ranging in age from 3.4-70.9 months) were enrolled. Average axial length was 21.2 ± 1.0 mm with mean spherical equivalent +1.49 ± 1.34 diopters (range -2.25 to 4.25). Average GCC volume for the total retina was 0.28 ± 0.04 mm³. Forty-seven of these eyes had RNFL analysis. Average RNFL thickness of the papillomacular bundle was 38.2 ± 9.5 μm. There was no correlation between GCC volume, RNFL thickness, patient age, or axial length.

CONCLUSION

Average GCC volume and RNFL thickness was stable from 6 months to 5 years of age. This study provides normative data for GCC and RNFL obtained by HH-OCT in healthy eyes of young children, to serve in evaluating those with optic neuropathies.

Calibration-free time-stretch optical coherence tomography with large imaging depth

We demonstrate a calibration-free time-stretch optical coherence tomography (TS-OCT), based on an optical higher order dispersion compensation scheme, which substitutes the digital calibration with optical dispersion compensation. As a result, the acquired raw data can directly perform the Fourier transform, and data processing time is greatly reduced by 82%, compared with the digital calibration. Moreover, because of the high-sensitivity and calibration-free characteristics, the high-order dispersion compensation-based TS-OCT can increase sensitivity roll-off by 2.6 times to 6.91 mm/dB and effective imaging depth by 14.2% to 16 mm. The in vivo biological tissue imaging has been demonstrated, with the single-shot A-scan rate approaching 19 MHz. This higher order dispersion compensation scheme could provide a promising solution for the TS-OCT system to realize 3D imaging in real time and enhanced imaging quality.

Automatic proximal airway volume segmentation using optical coherence tomography for assessment of inhalation injury

BACKGROUND

Acute respiratory distress syndrome (ARDS) is a severe form of acute lung injury with a mortality rate of up to 40%. Early management of ARDS has been difficult due to the lack of sensitive imaging tools and robust analysis software. We previously designed an optical coherence tomography (OCT) system to evaluate mucosa thickness (MT) after smoke inhalation, but the analysis relied on manual segmentation. The aim of this study is to assess in vivo proximal airway volume (PAV) after inhalation injury using automated OCT segmentation and correlate the PAV to lung function for rapid indication of ARDS.

METHODS

Anesthetized female Yorkshire pigs (n = 14) received smoke inhalation injury (SII) and 40% total body surface area thermal burns. Measurements of PaO2-to-FiO2 ratio (PFR), peak inspiratory pressure (PIP), dynamic compliance, airway resistance, and OCT bronchoscopy were performed at baseline, postinjury, 24 hours, 48 hours, 72 hours after injury. A tissue segmentation algorithm based on graph theory was used to reconstruct a three-dimensional (3D) model of lower respiratory tract and estimate PAV. Proximal airway volume was correlated with PFR, PIP, compliance, resistance, and MT measurement using a linear regression model.

RESULTS

Proximal airway volume decreased after the SII: the group mean of proximal airway volume at baseline, postinjury, 24 hours, 48 hours, 72 hours were 20.86 cm (±1.39 cm), 17.61 cm (±0.99 cm), 14.83 cm (±1.20 cm), 14.88 cm (±1.21 cm), and 13.11 cm (±1.59 cm), respectively. The decrease in the PAV was more prominent in the animals that developed ARDS after 24 hours after the injury. PAV was significantly correlated with PIP (r = 0.48, p < 0.001), compliance (r = 0.55, p < 0.001), resistance (r = 0.35, p < 0.01), MT (r = 0.60, p < 0.001), and PFR (r = 0.34, p < 0.01).

CONCLUSION

Optical coherence tomography is a useful tool to quantify changes in MT and PAV after SII and burns, which can be used as predictors of developing ARDS at an early stage.

LEVEL OF EVIDENCE

Prognostic, level III.

Laser frequency sweep linearization by iterative learning pre-distortion for FMCW LiDAR

We report on a laser frequency sweep linearization method by iterative learning pre-distortion for frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR) systems. A pre-distorted laser drive voltage waveform that results in a linear frequency sweep is obtained by an iterative learning controller, and then applied to the FMCW LiDAR system. We have also derived a fundamental figure of merit for the maximum residual nonlinearity needed to achieve the transform-limited range resolution. This method is experimentally tested using a commercial vertical cavity surface-emitting laser (VCSEL) and a distributed feedback (DFB) laser, achieving less than 0.005% relative residual nonlinearity of frequency sweep. With the proposed method, high-performance FMCW LiDAR systems can be realized without expensive linear lasers, complex linearization setups, or heavy post-processing.

Evaluating changes of blood flow in retina, choroid, and outer choroid in rats in response to elevated intraocular pressure by 1300 nm swept-source OCT

We report the development of a 1300 nm swept-source optical coherence tomography (SS-OCT) system specifically designed to perform OCT imaging and optical microangiography (OMAG) in rat eyes in vivo and its use in evaluating the effects of intraocular pressure (IOP) elevation on ocular circulation. The swept laser is operated in single longitude mode with a 90 nm bandwidth centered at 1300 nm and 200 kHz A-line rate, providing remarkable sensitivity fall-off performance along the imaging depth, a larger field of view of 2.5 × 2.5 mm2 (approximately 35°), and more time-efficient imaging acquisition. The advantage of the SS-OCT/OMAG is highlighted by an increased imaging depth of the entire posterior thickness of optic nerve head (ONH) and its surrounding vascular anatomy, to include, for the first time in vivo, the vasculature at the scleral opening, allowing visualization of the circle of Zinn-Haller and posterior ciliary arteries (PCAs). Furthermore, the capillary-level resolution angiograms achieved at the retinal and choroidal layers over a larger field of view enable a significantly improved quantification of the response of vascular area density (VAD) to elevated IOP. The results indicate that reduction in perfusion of the choroid in response to elevated IOP is delayed compared to that seen in the retina; while choroidal VAD doesn't reach 50% of baseline until ~70 mmHg, the same effect is seen for the retinal VAD at ~60 mmHg. The superior image quality offered by SS-OCT may allow more comprehensive investigation of IOP-related ocular perfusion changes and their pathological roles in glaucomatous optic nerve damage.

Phase-stability optimization of swept-source optical coherence tomography

Phase-resolved imaging of swept-source optical coherence tomography (SS-OCT) is subject to phase measurement instabilities involved with the sweep variation of a frequency-swept source. In general, optically generated timing references are utilized to track the variations imposed on OCT signals. But they might not be accurately synchronized due to relative time delays. In this research, we investigated the impact of the signal delays on the timing instabilities and the consequent deviations of the measured phases. We considered two types of timing signals utilized in a popular digitizer operation mode: a sweep trigger from a fiber Bragg grating (FBG) that initiates a series of signal sampling actions clocked by an auxiliary Mach-Zehnder interferometer (MZI) signal. We found that significant instabilities were brought by the relative delays through incoherent timing corrections and timing collisions between the timing references. The best-to-worst ratio of the measured phase errors was higher than 200 while only the signal delays varied. Noise-limited phase stability was achieved with a wide dynamic range of OCT signals above 50 dB in optimized delays. This demonstrated that delay optimization is very effective in phase stabilization of SS-OCT.

Recovering distance information in spectral domain interferometry

This work evaluates the performance of the Complex Master Slave (CMS) method, that processes the spectra at the interferometer output of a spectral domain interferometry device without involving Fourier transforms (FT) after data acquisition. Reliability and performance of CMS are compared side by side with the conventional method based on FT, phase calibration with dispersion compensation (PCDC). We demonstrate that both methods provide similar results in terms of resolution and sensitivity drop-off. The mathematical operations required to produce CMS results are highly parallelizable, allowing real-time, simultaneous delivery of data from several points of different optical path differences in the interferometer, not possible via PCDC.

Video-rate centimeter-range optical coherence tomography based on dual optical frequency combs by electro-optic modulators

Imaging speed and range are two important parameters for optical coherence tomography (OCT). A conventional video-rate centimeter-range OCT requires an optical source with hundreds of kHz repetition rate and needs the support of broadband detectors and electronics (>1 GHz). In this paper, a type of video-rate centimeter-range OCT system is proposed and demonstrated based on dual optical frequency combs by leveraging electro-optic modulators. The repetition rate difference between dual combs, i.e. the A-scan rate of dual-comb OCT, can be adjusted within 0~6 MHz. By down-converting the interference signal from optical domain to radio-frequency domain through dual comb beating, the down-converted bandwidth of the interference signal is less than 22.5 MHz which is at least two orders of magnitude lower than that in conventional OCT systems. A LabVIEW program is developed for video-rate operation, and the centimeter imaging depth is proved by using 10 pieces of 1-mm thick glass stacked as the sample. The effective beating bandwidth between two optical comb sources is 7 nm corresponding to ~108 comb lines, and the axial resolution of the dual-comb OCT is 158 µm. Dual optical frequency combs provide a promising solution to relax the detection bandwidth requirement in fast long-range OCT systems.

Fiber-optic transmission and networking: the previous 20 and the next 20 years [Invited]

Celebrating the 20^th anniversary of Optics Express, this paper reviews the evolution of optical fiber communication systems, and through a look at the previous 20 years attempts to extrapolate fiber-optic technology needs and potential solution paths over the coming 20 years. Well aware that 20-year extrapolations are inherently associated with great uncertainties, we still hope that taking a significantly longer-term view than most texts in this field will provide the reader with a broader perspective and will encourage the much needed out-of-the-box thinking to solve the very significant technology scaling problems ahead of us. Focusing on the optical transport and switching layer, we cover aspects of large-scale spatial multiplexing, massive opto-electronic arrays and holistic optics-electronics-DSP integration, as well as optical node architectures for switching and multiplexing of spatial and spectral superchannels.

Cerebral capillary flow imaging by wavelength-division-multiplexing swept-source optical Doppler tomography

Swept-source-based optical coherence tomography (SS-OCT) has demonstrated the unique advantages for fast imaging rate and long imaging distance; however, limited axial resolution and complex phase noises restrict swept-source optical Doppler tomography (SS-ODT) for quantitative capillary blood flow imaging in the deep cortices. Here, the wavelength-dividing-multiplexing optical Doppler tomography (WDM-ODT) method that divides a single interferogram into multiple phase-correlated interferograms is proposed to effectively enhance the sensitivity for cerebral capillary flow imaging. Both flow phantom and in vivo mouse brain imaging studies show that WDM-ODT is able to significantly suppress background phase noise and image cerebral capillary flow down to the vessel size of 5.6 μm. Comparison between the wavelength-division-multiplexing SS-ODT and the spectral-domain ultrahigh-resolution ODT (uODT) reveals that SS-ODT outpaces uODT by extending the capillary flow imaging depth to 1.6 mm in mouse cortex. Thus, for the first time, quantitative capillary flow imaging is demonstrated using SS-ODT in the deep cortex.

Phase-noise model for actively linearized frequency-modulated continuous-wave ladar

Understanding the effects of laser phase noise on frequency-modulated continuous-wave distance measurements is important in evaluating ranging accuracy. The standard white-frequency-noise assumption is commonly used to predict the ranging performance. However, other noise sources are typically present that can further degrade the heterodyne beat signal and make this assumption invalid. In addition, many ranging systems employ active sweep linearization techniques that can impact the phase noise. Here, we present a phase-noise model for assessing the accuracy of a phase-locked swept laser source.

All-depth dispersion cancellation in spectral domain optical coherence tomography using numerical intensity correlations

In ultra-high resolution (UHR-) optical coherence tomography (OCT) group velocity dispersion (GVD) must be corrected for in order to approach the theoretical resolution limit. One approach promises not only compensation, but complete annihilation of even order dispersion effects, and that at all sample depths. This approach has hitherto been demonstrated with an experimentally demanding ‘balanced detection’ configuration based on using two detectors. We demonstrate intensity correlation (IC) OCT using a conventional spectral domain (SD) UHR-OCT system with a single detector. IC-SD-OCT configurations exhibit cross term ghost images and a reduced axial range, half of that of conventional SD-OCT. We demonstrate that both shortcomings can be removed by applying a generic artefact reduction algorithm and using analytic interferograms. We show the superiority of IC-SD-OCT compared to conventional SD-OCT by showing how IC-SD-OCT is able to image spatial structures behind a strongly dispersive silicon wafer. Finally, we question the resolution enhancement of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sqrt{2}$$\end{document}2 that IC-SD-OCT is often believed to have compared to SD-OCT. We show that this is simply the effect of squaring the reflectivity profile as a natural result of processing the product of two intensity spectra instead of a single spectrum.

High-accuracy range-sensing system based on FMCW using low-cost VCSEL

Long-range shape measurement with high accuracy is needed for precision manufacturing of large-scale parts such as turbines, compressors, and trains. We have developed a high-accuracy ranging system based on frequency-modulated continuous-wave (FMCW) technology. Our system has two unique features. First, it achieves high-accuracy range measurement by directly modulating a low-cost vertical-cavity surface-emitting laser (VCSEL) at high sweep rates. The nonlinearity of the optical frequency sweep is compensated for by resampling through a reference interferometer. Second, an optical fiber with multiple fiber Bragg grating (FBG) structures is used for distance calibration. Ranging accuracy better than 10 μm is achieved at 2 m distance. Three-dimensional (3D) imaging of 10-cubic-meter volume has been obtained by combining the FMCW ranging with a galvanometer scanner.

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.