Dr. Kanwarpal Singh

Small leucine-rich proteoglycans inhibit CNS regeneration by modifying the structural and mechanical properties of the lesion environment

Julia Kolb, Vasiliki Tsata, Nora John, Kyoohyun Kim, Conrad Möckel, Gonzalo Rosso, Veronika Kurbel, Asha Parmar, Gargi Sharma, et al.

Extracellular matrix (ECM) deposition after central nervous system (CNS) injury leads to inhibitory scarring in humans and other mammals, whereas it facilitates axon regeneration in the zebrafish. However, the molecular basis of these different fates is not understood. Here, we identify small leucine-rich proteoglycans (SLRPs) as a contributing factor to regeneration failure in mammals. We demonstrate that the SLRPs chondroadherin, fibromodulin, lumican, and prolargin are enriched in rodent and human but not zebrafish CNS lesions. Targeting SLRPs to the zebrafish injury ECM inhibits axon regeneration and functional recovery. Mechanistically, we find that SLRPs confer mechano-structural properties to the lesion environment that are adverse to axon growth. Our study reveals SLRPs as inhibitory ECM factors that impair axon regeneration by modifying tissue mechanics and structure, and identifies their enrichment as a feature of human brain and spinal cord lesions. These findings imply that SLRPs may be targets for therapeutic strategies to promote CNS regeneration.

Ultralong Imaging Range Chromatic Confocal Microscopy

Gargi Sharma, Kanwarpal Singh

Confocal microscopy is regularly used in cellular research but unfortunately, the imaging is restricted to a single plane. Chromatic confocal microscopy (CCM) offers the possibility to image multiple planes simultaneously, thus providing a manifold increase in the imaging speed, whereas eliminating the need for z-axis scanning. Standard chromatic confocal systems have a limited imaging range of the order of a few hundreds of micrometers which limits their applications. Herein, using a single zinc selenide lens, a CCM system that has an imaging range of 18 mm (±68 nm) with an average spatial resolution of 2.46 μm (±44 nm) and another system with a 1.55 mm (±14 nm) imaging range with 0.86 μm (±30 nm) average lateral spatial resolution is demonstrated. In doing so, sevenfold increase in the imaging range for the system with 1.55 mm imaging when compared with previously reported systems with similar lateral spatial resolution is achieved. The proposed approach can be a powerful tool for confocal imaging of biological samples or surface profiling of industrial samples.

Depth of focus extension in optical coherence tomography using ultrahigh chromatic dispersion of zinc selenide

Maria N. Romodina, Kanwarpal Singh

We report a novel technique to overcome<br>the depth-of-focus limitation in optical coherence tomography (OCT) using chromatic<br>dispersion of zinc selenide lens.<br>OCT is an established method of optical<br>imaging, which found numerous biomedical<br>applications. However, the depth scanning range of high-resolution OCT is limited by its depth of focus. Chromatic dispersion of zinc selenide lens allows to get high lateral resolution along extended depth of focus, because the different spectral components are focused at a different position along axes of light propagation. Test measurements with nanoparticle phantom show 2.8 times extension of the depth of focus compare to the system with a standard achromatic lens. The feasibility of biomedical applications was demonstrated by ex vivo imaging of the pig cornea and chicken fat tissue.

A Proposal to Perform High Contrast Imaging of Human Palatine Tonsil with Cross Polarized Optical Coherence Tomography

Gargi Sharma, Asha Parmar, Franziska Hoffmann, Katharina Geißler, Ferdinand von Eggeling, Orlando Guntinas-Lichius, Kanwarpal Singh

The palatine tonsils provide the first line of immune defense against foreign pathogens<br>inhaled or ingested. However, a disruption in the epithelial layer within the tonsil crypts can lead to recurrent acute tonsillitis (RAT). Current imaging techniques suffer from poor resolution and contrast and do not allow a classification of the severity of RAT. We have developed a cross-polarized optical coherence tomography system. The system can detect a change in the polarization of the light after the light-tissue interaction. We demonstrate improved resolution and contrast in tonsil imaging with the developed method. Intensity, as well as retardance images of the excised tonsil tissue, were acquired. Features such as crypt epithelium, lymphoid follicles, and dense connective tissue were observed with improved contrast. Cross polarized optical coherence tomography can be a valuable tool in the clinic to evaluate palatine tonsils as it would allow visualizing common tonsil features without the need for any external contrast agent.

Polarization Independent Optical Coherence Tomography

Asha Parmar, Gargi Sharma, Kanwarpal Singh

Optical coherence tomography (OCT) is a well established imaging modality for high-resolution three-dimensional imaging in clinical settings. While imaging, care must be taken to minimize the imaging artifacts related to the polarization differences between the sample and the reference signals. Current OCT systems adopt complicated mechanisms, such as the use of multiple detectors, polarization-maintaining fibers, polarization controllers to achieve polarization artifacts free sample images.<br>Often the polarization controllers need readjustment which is not suitable for clinical settings. In this work, we demonstrate a simple approach that can minimize the polarization-related artifacts in the OCT systems. Polarization artifact-free images are acquired using two orthogonally polarized reference signals where the orthogonal polarization is achieved using a Faraday mirror. In the current approach, only a single detector is required which makes the current approach compatiblewith swept-source or camera-basedOCT systems. Furthermore, no polarization controllers are used in the system which increases the system stability while minimizing the artifacts related to the sample birefringence, polarization change due to the sample scattering, and polarization change due to the optical fiber movements present in the system.

Cross-Polarized Optical Coherence Tomography System with Unpolarized Light

Georg R. Hartl, Asha Parmar, Gargi Sharma, Kanwarpal Singh

Cross-polarized optical coherence tomography offers improved contrast for samples which<br>can alter the polarization of light when it interacts with the sample. This property has been utilized to screen pathological conditions in several organs. Existing cross-polarized optical coherence tomography systems require several polarization-controlling elements to minimize the optical fiber movement-related image artifacts. In this work, we demonstrate a cross-polarized optical coherence tomography system using unpolarized light and only two quarter-wave plates, which is free from fiber-induced image artifacts. The simplicity of the approach will find many applications in clinical settings.

Depth encoded input polarisation independent swept source cross-polarised optical coherence tomography probe

Katharina Blessing, Judith Schirmer, Asha Parmar, Kanwarpal Singh

Within the last decades, several studies have been published that prove the benefit of polarisation sensitive optical coherence (psOCT) tomography for the field of biomedical diagnostics. However, polarisation sensitive imaging typically requires careful control of the polarisation state of the input illumination, which leads to bulky and delicate systems. While psOCT provides quantitative information, it is mostly sufficient to analyse the images qualitatively in the field of biomedical diagnostics. Therefore, a reduced form of this technique, cross-polarised optical coherence tomography (cpOCT), moves into the focus of interest that serves to visualise the birefringence properties of a sample. Despite the low requirements for the illumination's polarisation, most of the proposed systems still include complex illumination control mechanisms. Here, we propose a common path probe based endoscopic system with an lateral resolution of 30 µm and a sensitivity of 103 dB comprising a commercially available swept-source OCT system and a free-space module which does not require any polarisation controlling elements. A Faraday mirror substitutes the complex polarisation control apparatus. We demonstrate the independence of the approach from the polarisation state of the light source by monitoring the illumination power in the orthogonal channels while varying the source polarisation. Furthermore, we validate the ability of the system to reveal the birefringence properties of different samples, starting from a quarter-wave plate, since its properties are fully characterised. Additionally, we present imaging results from several tissues to demonstrate its feasibility for the field of biomedical diagnostics.

Portable Optical Coherence Elastography System With Flexible and Phase Stable Common Path Optical Fiber Probe

Asha Parmar, Gargi Sharma, Shivani Sharma, Kanwarpal Singh

Biomechanical properties drive the functioning of cells and tissue. Measurement of such properties in the clinic is quite challenging, however. Optical coherence elastography is an emerging technique in this field that can measure the biomechanical properties of the tissue. Unfortunately, such systems have been limited to benchtop configuration with limited clinical applications. A truly portable system with a flexible probe that could probe different sample sites with ease is still missing. In this work, we report a portable optical coherence elastography system based on a flexible common path optical fiber probe. The common path approach allows us to reduce the undesired phase noise in the system by an order of magnitude less than the standard non-common path systems. The flexible catheter makes it possible to probe different parts of the body with ease. Being portable, our system can be easily transported to and from the clinic. We tested the efficacy of the system by measuring the mechanical properties of the agar-based tissue phantoms. We also measured the mechanical properties (Young’s Modulus) of the human skin at different sites. The measured values for the agar phantom and the skin were found to be comparable with the previously reported studies. Ultra-high phase stability and flexibility of the probe along with the portability of the whole system makes an ideal combination for the faster clinical adoption of the optical coherence elastography technique.

Chromatic Dispersion Based Wide-Band, Fiber-Coupled, Tunable Light Source for Hyperspectral Imaging

Gargi Sharma, Sheeza Kainat Naveed, Asha Parmar, Kanwarpal Singh

Hyperspectral imaging is a powerful label-free imaging technique that provides topological and spectral information at once. In this work, we have designed and characterized a hyperspectral source based on the chromatic dispersion property of off-the-shelf lenses and converted a supercontinuum laser light source into a hyperspectral imaging light source for 490 nm to 900 nm wavelength range with a spectral resolution of 3.5 nm to 18 nm respectively. The potential of the source was demonstrated by imaging two color dots with different absorption bands. Further, we generated the hypercube of the lily ovary and dense connective tissue and measured their spectral signature as a function of wavelength. We also imaged the lower tongue of a healthy volunteer at 540 nm, 630 nm, and white light. Our simple hyperspectral light source design can easily be incorporated in a standard endoscope or microscope to perform hyperspectral imaging.

Feasibility and Safety of Tethered Capsule Endomicroscopy in Patients With Barrett’s Esophagus in a Multi-Center Study

Jing Dong, Catriona Grant, Barry Vuong, Norman Nishioka, Anna Huizi Gao, Matthew Beatty, Grace Baldwin, Aaron Bailargeon, Ara Bablouzian, et al.

Tethered capsule endomicroscopy (TCE) involves swallowing small tethered pill that implements optical coherence tomography (OCT) imaging, procuring high resolution images of the whole esophagus. Here, we demonstrate and evaluate the feasibility and safety of TCE and a portable OCT imaging system in patients with Barrett’s esophagus (BE) in a multi-center (5-site) clinical study.

Smartphone‐based multimodal tethered capsule endoscopic platform for white‐light, narrow‐band, and fluorescence/autofluorescence imaging

Gargi Sharma, Oana-Maria Thoma, Katharina Blessing, Robert Gall, Maximilian Waldner, Kanwarpal Singh

Multimodal low‐cost endoscopy is highly desirable in poor resource settings such as in developing nations. In this work, we developed a smartphone‐based low‐cost, reusable tethered capsule endoscopic platform that allows white‐light, narrowband, and fluorescence/autofluorescence imaging of the esophagus. The ex‐vivo studies of swine esophagus were performed and compared with a commercial endoscope to test the white‐light imaging capabilities of the endoscope. The efficacy of the capsule for narrow‐band imaging was tested by imaging the vascularization of the tongue. To determine the autofluorescence/fluorescence capability of the endoscope, fluorescein dye with different concentrations was imaged. Furthermore, swine esophagus injected with fluorescein dye was imaged using the fluorescence/autofluorescence and the white‐light imaging modules, ex‐vivo. The overall cost of the capsules is approximately 12 €, 15 €, and 42 € for the white light imaging, the narrow‐band imaging, and the fluorescence/autofluorescence imaging respectively. In addition, the cost of the laser source module required for the narrow‐band imaging and the fluorescence/autofluorescence imaging is approximately 218 €. This device will open the possibility of imaging the esophagus in underprivileged areas.

Novel input polarisation independent endoscopic cross‐polarised optical coherence tomography probe

Katharina Blessing, Judith Schirmer, Gargi Sharma, Kanwarpal Singh

Lead by the original idea to perform noninvasive optical biopsies of various tissues, optical coherence tomography<br>found numerous medical applications<br>within the last two decades. The interference<br>based imaging technique opens the<br>possibility to visualise subcellular morphology up to an imaging depth of 3 mm<br>and up to micron level axial and lateral<br>resolution. The birefringence properties<br>of the tissue are visualisedwith enhanced<br>contrast using polarisation sensitive or<br>cross-polarised optical coherence tomography (OCT) techniques. Although, it<br>requires strict control over the polarisation states, resulting in several polarisation<br>controlling elements. In this work, we propose a novel input-polarisation independent endoscopic system based on cross-polarised OCT. We tested the feasibility of our approach by measuring the polarisation change from a quarter-wave plate for different rotational angles. Further performance tests reveal a lateral resolution of 30 μm and a sensitivity of 103 dB. Images of the human nail bed and cow muscle tissue demonstrate the potential of the system to measure structural and birefringence properties of the tissue endoscopically.

Paclitaxel Drug-Coated Balloon Angioplasty Suppresses Progression and Inflammation of Experimental Atherosclerosis in Rabbits

Mohammed M. Chowdhury, Kanwarpal Singh, Mazen S. Albaghdadi, Haitham Khraishah, Adam Mauskapf, Chase W. Kessinger, Eric A. Osborn, Stephan Kellnberger, Zhonglie Piao, et al.

Paclitaxel drug-coated balloons (DCBs) reduce restenosis, but their overall safety has recently raised concerns. This study hypothesized that DCBs could lessen inflammation and reduce plaque progression. Using 25 rabbits with cholesterol feeding- and balloon injury-induced lesions, DCB-percutaneous transluminal angioplasty (PTA), plain PTA, or sham-PTA (balloon insertion without inflation) was investigated using serial intravascular near-infrared fluorescence−optical coherence tomography and serial intravascular ultrasound. In these experiments, DCB-PTA reduced inflammation and plaque burden in nonobstructive lesions compared with PTA or sham-PTA. These findings indicated the potential for DCBs to serve safely as regional anti-atherosclerosis therapy.

Input polarization-independent polarization-sensitive optical coherence tomography using a depolarizer

Shivani Sharma, Georg Hartl, Sheeza K. Naveed, Katharina Blessing, Gargi Sharma, Kanwarpal Singh

Polarization-sensitive optical coherence tomography is gaining attention because of its ability to diagnose certain pathological conditions at an early stage. The majority of polarization-sensitive optical coherence tomography systems require a polarization controller and a polarizer to obtain the optimal polarization state of the light at the sample. Such systems are prone to misalignment since any movement of the optical fiber normally coupled to the light source will change the polarization state of the incident beam. We propose and demonstrate an input polarization-independent polarization-sensitive optical coherence tomography system using a depolarizer that works for any input polarization state of the light source. The change in the optical power at the sample for arbitrary input polarized light for the standard polarization-sensitive optical coherence tomography system was found to be approximately 84% compared to 9% for our proposed method. The developed system was used to measure the retardance and optical axis orientation of a quarter-wave plate and the obtained values matched closely to the expectation. To further demonstrate the capability of measuring the birefringent properties of biological samples, we also imaged the nail bed. We believe that the proposed system is a robust polarization-sensitive optical coherence tomography system and that it will improve the diagnostic capabilities in clinical settings.

Swept source cross-polarized optical coherence tomography for any input polarized light

Gargi Sharma, Shivani Sharma, Katharina Blessing, Georg Hartl, Maximilian Waldner, Kanwarpal Singh

Cross polarized optical coherence tomography offers enhanced contrast in certain<br>pathological conditions. Traditional cross-polarized optical coherence tomography systems<br>require a defined input polarization and thus require several polarization controlling elements<br>increasing the overall complexity of the system. Our proposed system requires a single<br>quarter wave plate as a polarization controller thus simplifying the system significantly.<br>Majority of Cross-polarized optical coherence tomography systems are spectrometer based<br>which suffers from slow speed and low signal to noise ratio. In this work, we present a swept<br>source based cross-polarized optical coherence tomography system that works for any input<br>polarization state. The system was tested against known birefringent materials such as quarter<br>wave plate. Furthermore, biological samples such as finger, nail and chicken breast were<br>imaged to demonstrate the potential of our technique.

All fiber polarization insensitive detection for spectrometer based optical coherence tomography using optical switch

David Odeke Otuya, Gargi Sharma, Guillermo J. Tearney, Kanwarpal Singh

Polarization dependent image artifacts are common in optical coherence tomography imaging. Polarization insensitive detection scheme for swept source based optical coherence tomography systems is well established but is yet to be demonstrated for all fiber spectrometer-based Fourier domain optical coherence tomography systems. In this work, we present an all fiber polarization insensitive detection scheme for spectrometer based optical coherence tomography systems. Images from chicken breast muscle tissue were acquired to demonstrate the effectiveness of this scheme for the conventional Fourier domain optical coherence tomography system.

Low cost scalable monolithic common path probe design for the application in endoscopic optical coherence tomography

Katharina Blessing, Shivani Sharma, Alexander Gumann, Kanwarpal Singh

Endoscopic optical coherence tomography is an interference based imaging technique which due to its micron level resolution ability found several applications in medical diagnostics. However, the standard image performance suffers from artefacts caused by dispersion imbalance and polarisation mismatches between reference and sample arm. Such artefacts can be minimised with the use of a special class of probes called common path probes where the reference surface is placed in the vicinity of the sample. Previously reported common path probes suffered from a compromise between sensitivity and resolution. In most cases, proposed probes were not scalable for industrial applications and required sophisticated machines for fabrication, thus limiting their mass production for clinical use. We propose and demonstrate a simple fabrication procedure which would allow small laboratories and industries to mass produce common path probes. Our probe design is based on a thin gold layer within the probe which acts as a reference surface. Low-cost ball lenses were used to focus the signal on the sample. We achieved a sensitivity of 104 dB with the designed probes which is comparable to previously reported common path and non-common path probes. Imaging of biological samples such as pig's oesophagus and pig's coronary artery is also presented.

Imaging the Human Prostate Gland Using 1-µm-Resolution Optical Coherence Tomography

Joseph A. Gardecki, Kanwarpal Singh, Chin-Lee Wu, Guillermo J. Tearney

Context.-The accuracy of needle biopsy for the detection of prostate cancer is limited by well-known sampling errors. Thus, there is an unmet need for a microscopic screening tool that can screen large regions of the prostate comprehensively for cancer. Previous prostate imaging by optical coherence tomography (OCT) has had insufficient resolution for imaging cellular features related to prostate cancer. We have recently developed micro-optical coherence tomography (µ OCT) that generates depth-resolved tissue images at a high frame rate with an isotropic resolution of 1 µ m.<br> Objective.-To demonstrate that optical images obtained with µ OCT provide cellular-level contrast in prostate specimens that will enable differentiation and diagnosis of prostate pathologies.<br> Design.-Fresh prostate specimens obtained from surgical resections were scanned with µ OCT ex vivo. Histologic features in the µ OCT images were correlated to the corresponding conventional histology.<br> Results.-Findings indicate that µ OCT is capable of resolving many of the architectural and cellular features associated with benign and neoplastic prostate.<br> Conclusions.-Because µ OCT can be implemented in a small-diameter flexible probe, this study suggests that high-resolution µ OCT imaging may be a useful tool for needle-based virtual biopsy of the prostate gland.

Optical coherence tomography-guided laser marking with tethered capsule endomicroscopy in unsedated patients

Chia-Pin Liang, Jing Dong, Tim Ford, Rohith Reddy, Hamid Hosseiny, Hamid Farrokhi, Matthew Beatty, Kanwarpal Singh, Hany Osman, et al.

Tethered capsule endomicroscopy (TCE) is an emerging screening technology that comprehensively obtains microstructural OCT images of the gastrointestinal (GI) tract in unsedated patients. To advance clinical adoption of this imaging technique, it will be important to validate TCE images with co-localized histology, the current diagnostic gold standard. One method for co-localizing OCT images with histology is image-targeted laser marking, which has previously been implemented using a driveshaft-based, balloon OCT catheter, deployed during endoscopy. In this paper, we present a TCE device that scans and targets the imaging beam using a low-cost stepper motor that is integrated inside the capsule. In combination with a 4-laser-diode, high power 1430/1450 nm marking laser system (800 mW on the sample and 1s pulse duration), this technology generated clearly visible marks, with a spatial targeting accuracy of better than 0.5 mm. A laser safety study was done on swine esophagus ex vivo, showing that these exposure parameters did not alter the submucosa, with a large, 4-5x safety margin. The technology was demonstrated in living human subjects and shown to be effective for co-localizing OCT TCE images to biopsies obtained during subsequent endoscopy. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

In-line optical fiber metallic mirror reflector for monolithic common path optical coherence tomography probes

Kanwarpal Singh, Rohith Reddy, Gargi Sharma, Yogesh Verma, Joseph A. Gardecki, Guillermo Tearney

Background and ObjectivesEndoscopic optical coherence tomography probes suffer from various artifacts due to dispersion imbalance and polarization mismatch between reference and sample arm light. Such artifacts can be minimized using a common path approach. In this work, we demonstrate a miniaturized common path probe for optical coherence tomography using an inline fiber mirror.<br> Materials and MethodsA common path optical fiber probe suitable for performing high-resolution endoscopic optical coherence tomography imaging was developed. To achieve common path functionality, an inline fiber mirror was fabricated using a thin gold layer. A commercially available swept source engine was used to test the designed probe in a cadaver human coronary artery ex vivo.<br> ResultsWe achieved a sensitivity of 104dB for this probe using a swept source optical coherence tomography system. To test the probe, images of a cadaver human coronary artery were obtained, demonstrating the quality that is comparable to those obtained by OCT systems with separate reference arms. Additionally, we demonstrate recovery of ranging depth by use of a Michelson interferometer in the detection path.<br> ConclusionWe developed a miniaturized monolithic inline fiber mirror-based common path probe for optical coherence tomography. Owing to its simplicity, our design will be helpful in endoscopic applications that require high-resolution probes in a compact form factor while reducing system complexity. Lasers Surg. Med. 50:230-235, 2018. (c) 2017 Wiley Periodicals, Inc.

Estimation and compensation of dispersion for a high-resolution optical coherence tomography system

Kanwarpal Singh, Gargi Sharma, Guillermo J. Tearney

Balanced reference-sample arm dispersion is critical in optical coherence tomography systems in order to attain images with the highest axial resolution. Here, an experimental method for the estimation and correction of dispersion in an optical coherence tomography system is presented. The system dispersion was computed from two optical coherence tomography images of the reference mirror that were symmetrically placed around the zero delay point. The method was tested using a broad bandwidth spectral domain optical coherence tomography system, compensating for the dispersion caused by a 3 mm thick fused silica flat placed in the sample arm. Using our method, dispersion compensation was achieved and the axial resolution was improved from 10.6 mu m to 1.9 mu m in air. Results suggest that this technique can be a simple and effective method for eliminating axial resolution degradation due to dispersion mismatch between the sample and reference arm in high-resolution optical coherence tomography systems.

Astigmatism Corrected Common Path Probe for Optical Coherence Tomography

Kanwarpal Singh, Daisuke Yamada, Guillermo Tearney

Background and Objectives: Optical coherence tomography (OCT) catheters for intraluminal imaging are subject to various artifacts due to reference-sample arm dispersion imbalances and sample arm beam astigmatism. The goal of this work was to develop a probe that minimizes such artifact.<br> Materials and Methods: Our probe was fabricated using a single mode fiber at the tip of which a glass spacer and graded index objective lens were spliced to achieve the desired focal distance. The signal was reflected using a curved reflector to correct for astigmatism caused by the thin, protective, transparent sheath that surrounds the optics. The probe design was optimized using Zemax, a commercially available optical design software. Common path interferometric operation was achieved using Fresnel reflection from the tip of the focusing graded index objective lens. The performance of the probe was tested using a custom designed spectrometer-based OCT system.<br> Results: The probe achieved an axial resolution of 15.6 mu m in air, a lateral resolution 33 mu m, and a sensitivity of 103 dB. A scattering tissue phantom was imaged to test the performance of the probe for astigmatism correction. Images of the phantom confirmed that this common-path, astigmatism-corrected OCT imaging probe had minimal artifacts in the axial, and lateral dimensions.<br> Conclusions: In this work, we developed an astigmatism-corrected, common path probe that minimizes artifacts associated with standard OCT probes. This design may be useful for OCT applications that require high axial and lateral resolutions. (C) 2016 Wiley Periodicals, Inc.

Terahertz Detection Based on Spectral-Domain Interferometry Using Mach-Zehnder Interferometer

Akram Ibrahim, Gargi Sharma, Kanwarpal Singh, Tsuneyuki Ozaki

We demonstrate the use of a Mach-Zehnder interferometer (MZI) to improve the performance of terahertz electric field measurements based on spectral-domain interferometry. The interferometer is introduced into the probe beam line to improve the temporal overlap between the two probe pulses. The probe pulse in the sample arm of the interferometer passes through the detection crystal and overlaps with the terahertz pulse, while the probe pulse in the reference arm does not. We measure the phase change between spectral components of these two pulses using spectral-domain interferometry. Using this new technique, we enable an unlimited temporal scanning window without the loss in the signal-to-noise ratio, thus overcoming the major limitation of conventional spectral-domain interferometry techniques for terahertz electric field detection.

In vivo imaging of airway cilia and mucus clearance with micro-optical coherence tomography

Kengyeh K. Chu, Carolin Unglert, Tim N. Ford, Dongyao Cui, Robert W. Carruth, Kanwarpal Singh, Linbo Liu, Susan E. Birket, George M. Solomon, et al.

We have designed and fabricated a 4 mm diameter rigid endoscopic probe to obtain high resolution micro-optical coherence tomography (mu OCT) images from the tracheal epithelium of living swine. Our common-path fiber-optic probe used gradient-index focusing optics, a selectively coated prism reflector to implement a circular-obscuration apodization for depth-of-focus enhancement, and a common-path reference arm and an ultra-broadbrand supercontinuum laser to achieve high axial resolution. Benchtop characterization demonstrated lateral and axial resolutions of 3.4 mu m and 1.7 mu m, respectively (in tissue). Mechanical standoff rails flanking the imaging window allowed the epithelial surface to be maintained in focus without disrupting mucus flow. During in vivo imaging, relative motion was mitigated by inflating an airway balloon to hold the standoff rails on the epithelium. Software implemented image stabilization was also implemented during post-processing. The resulting image sequences yielded co-registered quantitative outputs of airway surface liquid and periciliary liquid layer thicknesses, ciliary beat frequency, and mucociliary transport rate, metrics that directly indicate airway epithelial function that have dominated in vitro research in diseases such as cystic fibrosis, but have not been available in vivo. (C) 2016 Optical Society of America

Ultra-high dynamic range electro-optic sampling for detecting millimeter and sub-millimeter radiation

Akram Ibrahim, Denis Ferachou, Gargi Sharma, Kanwarpal Singh, Marie Kirouac-Turmel, Tsuneyuki Ozaki

Time-domain spectroscopy using coherent millimeter and sub-millimeter radiation (also known as terahertz radiation) is rapidly expanding its application, owing greatly to the remarkable advances in generating and detecting such radiation. However, many current techniques for coherent terahertz detection have limited dynamic range, thus making it difficult to perform some basic experiments that need to directly compare strong and weak terahertz signals. Here, we propose and demonstrate a novel technique based on cross-polarized spectral-domain interferometry to achieve ultra-high dynamic range electro-optic sampling measurement of coherent millimeter and sub-millimeter radiation. In our scheme, we exploit the birefringence in a single-mode polarization maintaining fiber in order to measure the phase change induced by the electric field of terahertz radiation in the detection crystal. With our new technique, we have achieved a dynamic range of 7 x 10(6), which is 4 orders of magnitude higher than conventional electro-optic sampling techniques, while maintaining comparable signal-to-noise ratio. The present technique is foreseen to have great impact on experiments such as linear terahertz spectroscopy of optically thick materials (such as aqueous samples) and nonlinear terahertz spectroscopy, where the higher dynamic range is crucial for proper interpretation of experimentally obtained results.

µOCT imaging using depth of focus extension by self-imaging wavefront division in a common-path fiber optic probe

Biwei Yin, Kengyeh K. Chu, Chia-Pin Liang, Kanwarpal Singh, Rohith Reddy, Guillermo Tearney

Optical coherence tomography (OCT) is an attractive medical modality due to its ability to acquire high-resolution, cross-sectional images inside the body using flexible, small-diameter, scanning fiber optic probes. Conventional, cross-sectional OCT imaging technologies have approximately 10-µ m axial resolution and 30-µ m lateral resolution, specifications that enable the visualization of microscopic architectural morphology. While this resolution is useful for many clinical applications, it is insufficient for resolving individual cells that characterize many diseases. To address this gap, a supercontinuum-laser-based, µ m-resolution OCT (µ OCT) system and a 500 µ m-diameter, extended depth of focus single fiber optic probe for endoscopic and intravascular imaging were designed and fabricated. At the distal tip of the fiber optic probe, a cylindrical waveguide was used to divide the wavefront to provide multiple circular propagation modes. Once transmitted through a relatively high NA lens (NA >0.1), these modes were projected as multiple coaxial foci (similar to 3 µ m full width at half maximum (FWHM)) over a greatly extended focal depth range. The distal tip of the probe also contained a common-path reference reflectance to minimize polarization and dispersion imbalances between sample and reference arm light. Measurements showed that the probe provides a 20-fold depth of focus extension, maintaining a 3-5 µ m lateral resolution (FWHM of PSF) and a 2 µ m axial resolution over a depth range of approximately 1 mm. These results suggest that this new optical configuration will be useful for achieving high-resolution, cross-sectional OCT imaging in catheter/endoscope-based medical imaging devices. (C) 2016 Optical Society of America

Characterization of Tissue-Engineered Posterior Corneas Using Second- and Third-Harmonic Generation Microscopy

Louis Jay, Jean-Michel Bourget, Benjamin Goyer, Kanwarpal Singh, Isabelle Brunette, Tsuneyuki Ozaki, Stephanie Proulx

Three-dimensional tissues, such as the cornea, are now being engineered as substitutes for the rehabilitation of vision in patients with blinding corneal diseases. Engineering of tissues for translational purposes requires a non-invasive monitoring to control the quality of the resulting biomaterial. Unfortunately, most current methods still imply invasive steps, such as fixation and staining, to clearly observe the tissue-engineered cornea, a transparent tissue with weak natural contrast. Second-and third-harmonic generation imaging are well known to provide high-contrast, high spatial resolution images of such tissues, by taking advantage of the endogenous contrast agents of the tissue itself. In this article, we imaged tissue-engineered corneal substitutes using both harmonic microscopy and classic histopathology techniques. We demonstrate that second-and third-harmonic imaging can non-invasively provide important information regarding the quality and the integrity of these partial-thickness posterior corneal substitutes (observation of collagen network, fibroblasts and endothelial cells). These two nonlinear imaging modalities offer the new opportunity of monitoring the engineered corneas during the entire process of production.

Common Path Side Viewing Monolithic Ball Lens Probe for Optical Coherence Tomography

Kanwarpal Singh, Daisuke Yamada, Guillermo Tearney

Common path probes are highly desirable for optical coherence tomography as they reduce system complexity and cost by eliminating the need of dispersion compensation and polarization controlling optics. In this work, we demonstrate a monolithic ball lens based, common path, side viewing probe that is suitable for Fourier domain optical coherence tomography. The probe design parameters were simulated in Zemax modeling software and the simulated performance parameters were compared with experimental results. We characterized the performance of the probe by measuring its collection efficiency, resolution, and sensitivity. Our results demonstrated that with a source input power of 25 mW, we could achieve sensitivity of 100.5 dB, which is within 7 dB of the maximum possible sensitivity that could be achieved using a separate reference arm. The axial resolution of the system was found to be 15.6 mu m in air and the lateral resolution (full width half maximum) was approximately 49 mu m. The probe optics were assembled in a 1 mm diameter hypotube with a 500 mu m inner diameter. Images of finger skin acquired using this probe demonstrated clear visualization of the stratum corneum, epidermis, and papillary dermis, along with sweat ducts.

Preparing Uniform-Thickness Corneal Endothelial Grafts from Donor Tissues Using a Non-Amplified Femtosecond Laser

Kanwarpal Singh, Nour Haydari, Isabelle Brunette, Santiago Costantino

Corneal grafts for Descemet's Stripping Automated Endothelial Keratoplasty are commonly prepared using mechanical microkeratomes. However, the cuts produced in such way render corneal lenticules that are thinner centrally than peripherally, thus inducing a hyperopic shift. Here we describe a novel device for preparing donor corneal grafts, in which a single low-energy femtosecond laser system is used as both a light source for optical coherence tomography and for cutting the graft illuminating from the endothelial side. The same laser is first utilized to obtain three-dimensional optical coherence tomography images of the donor tissue for guiding the dissection and obtaining grafts of uniform thickness with no applanation or contact. This device allows an optimal procedure for preparing consistently thin posterior grafts for transplantation.

Self-referenced spectral domain interferometry for improved signal-to-noise measurement of terahertz radiation

Gargi Sharma, Kanwarpal Singh, Akram Ibrahim, Ibraheem Al-Naib, Roberto Morandotti, Francois Vidal, Tsuneyuki Ozaki

In this work, we demonstrate self-referenced spectral domain interferometry for the electro-optic sampling of terahertz (THz) electric fields. This technique allows reduction of the phase noise of the measurement, thus increasing the signal-to-noise ratio (SNR). Using the proposed technique, we achieve a more than sixfold improvement in the SNR of the detected THz electric field. (C) 2013 Optical Society of America

Analysis of Pulsatile Retinal Movements by Spectral-Domain Low-Coherence Interferometry: Influence of Age and Glaucoma on the Pulse Wave

Carolyne Dion, Kanwarpal Singh, Tsuneyuki Ozaki, Mark R. Lesk, Santiago Costantino

Recent studies have shown that ocular hemodynamics and eye tissue biomechanical properties play an important role in the pathophysiology of glaucoma. Nevertheless, better, non-invasive methods to assess these characteristics in vivo are essential for a thorough understanding of degenerative mechanisms. Here, we propose to measure ocular tissue movements induced by cardiac pulsations and study the ocular pulse waveform as an indicator of tissue compliance. Using a novel, low-cost and non-invasive device based on spectral-domain low coherence interferometry (SD-LCI), we demonstrate the potential of this technique to differentiate ocular hemodynamic and biomechanical properties. We measured the axial movement of the retina driven by the pulsatile ocular blood flow in 11 young healthy individuals, 12 older healthy individuals and 15 older treated glaucoma patients using our custom-made SD-OCT apparatus. The cardiac pulse was simultaneously measured through the use of an oximeter to allow comparison. Spectral components up to the second harmonic were obtained and analyzed. For the different cohorts, we computed a few parameters that characterize the three groups of individuals by analyzing the movement of the retinal tissue at two locations, using this simple, low-cost interferometric device. Our pilot study indicates that spectral analysis of the fundus pulsation has potential for the study of ocular biomechanical and vascular properties, as well as for the study of ocular disease.

Pulsatile Movement of the Optic Nerve Head and the Peripapillary Retina in Normal Subjects and in Glaucoma

Kanwarpal Singh, Carolyne Dion, Antoine G. Godin, Faezeh Lorghaba, Denise Descovich, Marcelo Wajszilber, Tsuneyuki Ozaki, Santiago Costantino, Mark R. Lesk

PURPOSE. To measure the pulsatile movement of neuroretinal tissue at the optic nerve head synchronous with the cardiac cycle.<br> METHODS. We used a noninvasive imaging device based on Fourier domain low-coherence interferometry to measure the pulsatile movements of the optic nerve head, peripapillary retina, and cornea with submicron accuracy along a line across the fundus. We also measured the change in the Axial Distance between the peripapillary Retina and the base of the optic disc Cup (ADRC) during the cardiac cycle. Twelve normal subjects and 20 subjects with open-angle glaucoma were tested.<br> RESULTS. In normal subjects, the mean fundus pulsation amplitude (defined as the fundus movement minus the simultaneous corneal movement) were 13.0 +/- 2.5 mu m, 9.0 +/- 2.1 mu m, and 8.7 +/- 2.9 mu m at the base of the optic nerve head cup, the nasal peripapillary retina, and the temporal peripapillary retina, respectively, compared with 16.7 +/- 6.8 mu m, 17.3 +/- 10.9 mu m, and 12.7 +/- 6.2 mu m for the corresponding values in the glaucoma group (P = 0.26, P = 0.008, and P = 0.12, respectively). The mean changes in ADRC during the cardiac cycle in normal subjects were 10.7 +/- 2.1 mu m and 11.6 +/- 1.8 mu m for the nasal and temporal side of the optic disc, respectively, compared to 14.9 +/- 5.6 mu m and 14.0 +/- 4.9 mu m in glaucoma subjects (P = 0.03 and P = 0.10, respectively).<br> CONCLUSIONS. There was an approximately 11-mu m pulsatile change in the ADRC in normal subjects, and on the nasal side of the disc, this amount was significantly greater in glaucoma patients. (Invest Ophthalmol Vis Sci. 2012;53:7819-7824) DOI:10.1167/iovs.12-9834

Terahertz detection using spectral domain interferometry

Gargi Sharma, Kanwarpal Singh, Ibraheem Al-Naib, Roberto Morandotti, Tsuneyuki Ozaki

In this work, we present a novel method based on spectral domain interferometry for the electro-optic (EO) sampling of terahertz (THz) electric fields. This technique allows the use of thick crystals without the drawback of the over-rotation that may occur with intense THz sources, allowing longer temporal scans and thus, better spectral resolution. Using this technique, a phase difference of approximately 8898 pi can be measured, which is 18,000 times larger than the phase difference that could be measured using EO sampling. (C) 2012 Optical Society of America

Measurement of Ocular Fundus Pulsation in Healthy Subjects Using a Novel Fourier-Domain Optical Coherence Tomography

Kanwarpal Singh, Carolyne Dion, Marcelo Wajszilber, Tsuneyuki Ozaki, Mark R. Lesk, Santiago Costantino

PURPOSE. Anomalies in the pulsatility of the eye have been associated with many types of ocular pathology. Estimation of ocular pulsatility is usually obtained by measuring the variation in the intraocular pressure using tonometry-based instruments. In this work, the authors present and demonstrate the applicability of a novel and noninvasive Fourier-domain optical coherence tomography (FD-OCT) system to measure pulsatile ocular tissue movements.<br> METHODS. The authors simultaneously measured the longitudinal movement of the cornea and the retina driven by the cardiac cycle in 21 healthy volunteers using their custom-made FD-OCT. They calculated the corresponding fundus pulse amplitude (FPA), which is the variation in the distance between the cornea and the retina.<br> RESULTS. It was found that in young, healthy subjects, the cornea and the retina move axially during the cardiac cycle, with almost equal amplitude but with a phase difference ranging between 1 degrees and 20 degrees. The measured FPA was found to be mostly due to the relative phase difference between corneal and retinal movements, and frequency analysis revealed the presence of the harmonics of heartbeat. The root-mean-square values for cornea, retina, and FPA movements were found to be 28 +/- 9 mu m, 29 +/- 9 mu m, and 4 +/- 2 mu m, respectively. The dominant frequency component in corneal and retinal movement was found to be the second harmonic of the heartbeat.<br> CONCLUSIONS. The technique described here is useful for a precise description of FPA and the movement of ocular tissues. Further investigations and technical improvements will be beneficial for understanding the role of choroidal pulsation in the pathophysiology of ocular diseases. (Invest Ophthalmol Vis Sci. 2011;52:8927-8932) DOI:10.1167/iovs.11-7854

Kinetics of colloidal fractal aggregation by differential dynamic microscopy

F. Ferri, A. D'Angelo, M. Lee, A. Lotti, M. C. Pigazzini, Kanwarpal Singh, R. Cerbino

We study the kinetics of an aggregation process induced by adding salt to a stable colloidal suspension of 73 nm (diameter) particles. Despite the subdiffraction size of the colloidal particles, the process is monitored via optical microscopy, which is used here to obtain time-resolved scattering information about the colloidal aggregates. The radius of the aggregates is determined as a function of time and their fractal dimension is extracted. Our results are compatible with a diffusion limited aggregation process, as independently confirmed by spectral turbidimetry measurements on the same sample.

Analyzing speckle contrast for HiLo microscopy optimization

J. Mazzaferri, D. Kunik, J. M. Belisle, Kanwarpal Singh, S. Lefrancois, S. Costantino

HiLo microscopy is a recently developed technique that provides both optical sectioning and fast imaging with a simple implementation and at a very low cost. The methodology combines widefield and speckled illumination images to obtain one optically sectioned image. Hence, the characteristics of such speckle illumination ultimately determine the quality of HiLo images and the overall performance of the method. In this work, we study how speckle contrast influence local variations of fluorescence intensity and brightness profiles of thick samples. We present this article as a guide to adjust the parameters of the system for optimizing the capabilities of this novel technology. (C) 2011 Optical Society of America

Spectral-domain phase microscopy with improved sensitivity using two-dimensional detector arrays

Kanwarpal Singh, C. Dion, M. R. Lesk, T. Ozaki, S. Costantino

In this work we demonstrate the use of two-dimensional detectors to improve the signal-to-noise ratio (SNR) and sensitivity in spectral-domain phase microscopy for subnanometer accuracy measurements. We show that an increase in SNR can be obtained, from 82 dB to 105 dB, using 150 pixel lines of a low-cost CCD camera as compared to a single line, to compute an averaged axial scan. In optimal mechanical conditions, phase stability as small as 92 mu rad, corresponding to 6 pm displacement accuracy, could be obtained. We also experimentally demonstrate the benefit of spatial-averaging in terms of the reduction of signal fading due to an axially moving sample. The applications of the improved system are illustrated by imaging live cells in culture. (C) 2011 American Institute of Physics. [doi:10.1063/1.3556787]

Development of a novel instrument to measure the pulsatile movement of ocular tissues

Kanwarpal Singh, C. Dion, S. Costantino, M. Wajszilber, M. R. Lesk, T. Ozaki

We demonstrate an optical instrument that can measure the axial displacement of different eye tissues, including the cornea and the fundus. The instrument is based on spectral-domain low-coherence interferometry, which extracts displacement information from sequential axial scans of the eye with 100 Hz sampling frequency and with a precision of 400 nm. Longitudinal retinal and corneal movements were successfully measured in vivo in live rats, and Fourier analysis of the signal revealed the signature of the respiratory and cardiac cycles at 1.0 and 3.5 Hz, respectively. The effective amplitudes of retinal and corneal displacements at the cardiac frequency were found to be about 1.10 and 1.37 mu m, respectively. The synchrony and direction of these two movements relative to the systole and diastole were found to be nearly the same. This novel instrument can be applied to assess biomechanical properties of the eye, which could be important for early diagnosis and for understanding the pathophysiology of glaucoma and other ocular diseases. (C) 2010 Elsevier Ltd. All rights reserved.

Determination of porcine corneal layers with high spatial resolution by simultaneous second and third harmonic generation microscopy

L. Jay, A. Brocas, Kanwarpal Singh, J. -C. Kieffer, I. Brunette, T. Ozaki

We describe a dual, second harmonic generation (SHG) and third harmonic generation (THG) microscope, with the aim to obtain large-scale images of the cornea that can simultaneously resolve the micron-thick thin layers. We use an Ytterbium femtosecond laser as the laser source, the longer wavelength of which reduces scattering and allows simultaneous SHG and THG imaging. We measure one-dimensional SHG and THG profiles across the entire thickness of pig cornea, detected in both the forward and backward directions. These profiles allow us to clearly distinguish all the porcine corneal layers (epithelium, stroma, Descemet's membrane and endothelium). From these profiles, longitudinal cross sectional images of the corneal layers are generated, providing large scale topographic information with high-spatial resolution. The ability to obtain both SHG and THG signals in epi-detection on fresh eyes gives promising hopes for in vivo applications. (c) 2008 Optical Society of America

Arbitrary-order ultrabroadband all-optical differentiators based on fiber bragg gratings

Luis M. Rivas, Kanwarpal Singh, Alejandro Carballar, Jos Azana

We introduce a general approach for designing ultrabroadband arbitrary-order all-optical (all-fiber) time differentiators based on the use of fiber Bragg gratings (FBGs). Specifically, we numerically demonstrate that an Nth-order time differentiator can be implemented using a single specially apodized highly dispersive linearly chirped FBG operated in transmission. A concatenated reflection phase-shifted FBG is also required for implementing any odd-order differentiator. Our numerical simulations show that accurate and efficient time differentiation of optical signals with bandwidths up to a few hundreds of gigahertz can be realized using readily feasible FBG structures.