Fabrication and characterization of indium fluoride multimode fused fiber couplers for the mid-infrared.

Results of the fabrication and characterization of optical fiber couplers made of multimode step-index fluoroindate (InF3) fibers are presented. The fabrication setup was customized for this type of glass with a constant source of controlled nitrogen flow heated to a target temperature with an accuracy ±1°C. Combined with a novel fast fusion approach and with excellent control of the viscosity throughout the process, the clean gas flow and well-controlled temperature enable the fabrication of fused fiber couplers absent of any noticeable crystallization. A coupling ratio of 45/55 was achieved, with an excess loss of 0.35 dB, at 1.7 µm. To the best of our knowledge, this represents the first low excess loss (<1 dB), multimode, InF3 fiber couplers.

Viscosity of fluoride glass fibers for fused component fabrication.

Fluoride glasses show great promise for mid-IR fiber-based applications. Their brittleness and low glass transition temperature have thus far been obstacles towards obtaining low-loss fused components. Here, we suggest a simple method to measure glass viscosity over a range of process temperatures of interest for fused coupler fabrication. We achieved tapers of inverse taper ratio (ITR) 0.12 in multimode fluoroindate fibers. Tapers with loss <0.1dB at ITR 0.3 and no visible defects were fabricated with high repeatability. This work paves the way towards low-loss fused optical couplers in fluoride glass fiber.

Radiometric model for coaxial single- and multimode optical emission from double-clad fiber.

Double-clad fibers (DCFs) are versatile waveguides supporting a single-mode core surrounded by a multimode inner cladding. DCFs are increasingly used for multimodal biomedical applications, such as imaging or therapy, for which the core is typically used for coherent illumination and the inner cladding, to support a concurrent modality. Proper optimization is, however, critical to ensure high optical performance and requires accurate modeling of coaxial single- and multimode output beams. In this paper, we present an approach based on geometrical optics and radiometry, which provides a simple and efficient modeling tool for designing and optimizing DCF-based systems. A radiometric definition of single- and multimode output beams in terms of irradiance and radiant intensity allows for the modeling of the energy distribution along the beams' propagation. We confirmed the validity of the model through comparison with experimental measurements and demonstrate the use of the model for optimizing a catheter for concurrent OCT and laser coagulation.

Endoscopic high-resolution autofluorescence imaging and OCT of pulmonary vascular networks.

High-resolution imaging from within airways may allow new methods for studying lung disease. In this work, we report an endoscopic imaging system capable of high-resolution autofluorescence imaging (AFI) and optical coherence tomography (OCT) in peripheral airways using a 0.9 mm diameter double-clad fiber (DCF) catheter. In this system, AFI excitation light is coupled into the core of the DCF, enabling tightly focused excitation light while maintaining efficient collection of autofluorescence emission through the large diameter inner cladding of the DCF. We demonstrate the ability of this imaging system to visualize pulmonary vasculature as small as 12 µm in vivo.

Double-clad fiber coupler for partially coherent detection.

Double-clad fibers (DCF) have many advantages in fibered confocal microscopes as they allow for coherent illumination through their core and partially coherent detection through their inner cladding. We report a double-clad fiber coupler (DCFC) made from small inner cladding DCF that preserves optical sectioning in confocal microscopy while increasing collection efficiency and reducing coherent effects. Due to the small inner cladding, previously demonstrated fabrication methods could not be translated to this coupler's fabrication. To make such a coupler possible, we introduce in this article three new design concepts. The resulting DCFC fabricated using two custom fibers and a modified fusion-tapering technique achieves high multimodal extraction (≥70 %) and high single mode transmission (≥80 %). Its application to reflectance confocal microscopy showed a 30-fold increase in detected signal intensity, a 4-fold speckle contrast reduction with a penalty in axial resolution of a factor 2. This coupler paves the way towards more efficient confocal microscopes for clinical applications.

MCOCT: an experimentally and numerically validated, open-source Monte Carlo simulator for optical coherence tomography.

Here, we present MCOCT, a Monte Carlo simulator for optical coherence tomography (OCT), incorporating a Gaussian illumination scheme and bias to increase backscattered event collection. MCOCT optical fluence was numerically compared and validated to an established simulator (MCX) and showed concordance at the focus while diverging slightly with distance to it. MCOCT OCT signals were experimentally compared and validated to OCT signals acquired in tissue-mimicking phantoms with known optical properties and showed a similar attenuation pattern with increasing depth while diverging beyond 1.5 mm and proximal to layer interfaces. MCOCT may help in the design of OCT systems for a wide range of applications.

Impact of Cleaning on Membrane Performance during Surface Water Treatment: A Hybrid Process with Biological Ion Exchange and Gravity-Driven Membranes.

In this study, the hybrid biological ion exchange (BIEX) resin and gravity-driven membrane (GDM) process was employed for the treatment of coloured and turbid river water. The primary objective was to investigate the impact of both physical and chemical cleaning methods on ceramic and polymeric membranes in terms of their stabilised flux, flux recovery after physical/chemical cleaning, and permeate quality. To address these objectives, two types of MF and UF membranes were utilised (M1 = polymeric MF, M2 = polymeric UF, M3 = ceramic UF, and M4 = lab-made ceramic MF). Throughout the extended operation, the resin functioned initially in the primary ion exchange (IEX) region (NOM displacement with pre-charged chloride) and progressed to a secondary IEX stage (NOM displacement with bicarbonate and sulphate), while membrane flux remained stable. Subsequently, physical cleaning involved air/water backwash with two different flows and pressures, and chemical cleaning utilised NaOH at concentrations of 20 and 40 mM, as well as NaOCl at concentrations of 250 and 500 mg Cl2/L. These processes were carried out to assess flux recovery and identify fouling reversibility. The results indicate an endpoint of 1728 bed volumes (BVs) for the primary IEX region, while the secondary IEX continued up to 6528 BV. At the end of the operation, DOC and UVA254 removal in the effluent of the BIEX columns were 68% and 81%, respectively, compared to influent water. This was followed by 30% and 57% DOC and UVA254 removal using M4 (ceramic MF). The stabilised flux remained approximately 3.8-5.2 LMH both before and after the cleaning process, suggesting that membrane materials do not play a pivotal role. The mean stabilised flux of polymeric membranes increased after cleaning, whereas that of the ceramics decreased. Enhanced air-water backwash flow and pressure resulted in an increased removal of hydraulic reversible fouling, which was identified as the dominant fouling type. Ceramic membranes exhibited a higher removal of reversible hydraulic fouling than polymeric membranes. Chemical cleaning had a low impact on flux recovery; therefore, we recommend solely employing physical cleaning.

All-reflective tethered capsule endoscope for multimodal optical coherence tomography in the esophagus.

Significance: Esophageal cancer is becoming increasingly prevalent in Western countries. Early detection is crucial for effective treatment. Multimodal imaging combining optical coherence tomography (OCT) with complementary optical imaging techniques may provide enhanced diagnostic capabilities by simultaneously assessing tissue morphology and biochemical content. Aim: We aim to develop a tethered capsule endoscope (TCE) that can accommodate a variety of point-scanning techniques in addition to OCT without requiring design iterations on the optical or mechanical design. Approach: We propose a TCE utilizing exclusively reflective optics to focus and steer light from and to a double-clad fiber. Specifically, we use an ellipsoidal mirror to achieve finite conjugation between the fiber tip and the imaging plane. Results: We demonstrate a functional all-reflective TCE. We first detail the design, fabrication, and assembly steps required to obtain such a device. We then characterize its performance and demonstrate combined OCT at 1300 nm and visible spectroscopic imaging in the 500- to 700-nm range. Finally, we discuss the advantages and limitations of the proposed design. Conclusions: An all-reflective TCE is feasible and allows for achromatic high-quality imaging. Such a device could be utilized as a platform for testing various combinations of modalities to identify the optimal candidates without requiring design iterations.

Capturing reflected cladding modes from a fiber Bragg grating with a double-clad fiber coupler.

We present a novel measurement scheme using a double-clad fiber coupler (DCFC) and a fiber Bragg grating (FBG) to resolve cladding modes. Direct measurement of the optical spectra and power in the cladding modes is obtained through the use of a specially designed DCFC spliced to a highly reflective FBG written into slightly etched standard photosensitive single mode fiber to match the inner cladding diameter of the DCFC. The DCFC is made by tapering and fusing two double-clad fibers (DCF) together. The device is capable of capturing backward propagating low and high order cladding modes simply and efficiently. Also, we demonstrate the capability of such a device to measure the surrounding refractive index (SRI) with an extremely high sensitivity of 69.769 ± 0.035 µW/RIU and a resolution of 1.433 × 10(-5) ± 8 × 10(-9) RIU between 1.37 and 1.45 RIU. The device provides a large SRI operating range from 1.30 to 1.45 RIU with sufficient discrimination for all individual captured cladding modes. The proposed scheme can be adapted to many different types of bend, temperature, refractive index and other evanescent wave based sensors.

Asymmetric double-clad fiber couplers for endoscopy.

We present an asymmetric double-clad fiber coupler (A-DCFC) exploiting a disparity in fiber etendues to exceed the equipartition limit (≤50% extraction of inner cladding multi-mode light). The A-DCFC is fabricated using two commercially available fibers and a custom fusion-tapering setup to achieve >70% extraction of multi-mode inner cladding light without affecting (>95% transmission) single-mode light propagation in the core. Imaging with the A-DCFC is demonstrated in a spectrally encoded imaging setup using a weakly backscattering biological sample. Other applications include the combination of optical coherence tomography with weak fluorescent or Raman scattering signals.

Surface plasmon resonance sensor interrogation with a double-clad fiber coupler and cladding modes excited by a tilted fiber Bragg grating.

We present a novel optical fiber surface plasmon resonance (SPR) sensor scheme using reflected guided cladding modes captured by a double-clad fiber coupler and excited in a gold-coated fiber with a tilted Bragg grating. This new interrogation approach, based on the reflection spectrum, provides an improvement in the operating range of the device over previous techniques. The device allows detection of SPR in the reflected guided cladding modes and also in the transmitted spectrum, allowing comparison with standard techniques. The sensor has a large operating range from 1.335 to 1.432 RIU, and a sensitivity of 510.5 nm/RIU. The device shows strong dependence on the polarization state of the guided core mode which can be used to turn the SPR on or off.

Dual-modality needle probe for combined fluorescence imaging and three-dimensional optical coherence tomography.

To the best of our knowledge, we present the first needle probe for combined optical coherence tomography (OCT), and fluorescence imaging. The probe uses double-clad fiber (DCF) that guides the OCT signal and fluorescence excitation light in the core and collects and guides the returning fluorescence in the large-diameter multimode inner cladding. It is interfaced to a 1310 nm swept-source OCT system that has been modified to enable simultaneous 488 nm fluorescence excitation and >500 nm emission detection by using a DCF coupler to extract the returning fluorescence signal in the inner cladding with high efficiency. We present imaging results from an excised sheep lung with fluorescein solution infused through the vasculature. We were able to identify alveoli, bronchioles, and blood vessels. The results demonstrate that the combined OCT plus fluorescence needle images provide improved tissue differentiation over OCT alone.

Speckle contrast reduction through the use of a modally-specific photonic lantern for optical coherence tomography.

A few-mode optical coherence tomography (FM-OCT) system was developed around a 2 × 1 modally-specific photonic lantern (MSPL) centered at 1310 nm. The MSPL allowed FM-OCT to acquire two coregistered images with uncorrelated speckle patterns generated by their specific coherent spread function. Here, we showed that averaging such images in vitro and in vivo reduced the speckle contrast by up to 28% and increased signal-to-noise ratio (SNR) by up to 48% with negligible impact on image spatial resolution. This method is compatible with other speckle reduction techniques to further improve OCT image quality.

Quantitative distinction of unique vocal fold subepithelial architectures using optical coherence tomography.

OBJECTIVES: The primary objective of this study was to quantitatively analyze ex vivo porcine, fetal human, and adult human vocal folds by use of optical coherence tomography (OCT). A secondary objective was to quantitatively discriminate among 1-, 2-, and 3-layer lamina propria structures. METHODS: We performed an analysis of the vocal folds of 10 adult pig, 3 adult human, and 2 fetal human vocal fold specimens using OCT and histologic techniques. We present a quantitative comparison of the OCT results and histologic findings. RESULTS: We found that OCT allowed for the visualization of the subepithelial vocal fold architecture of all imaged tissue, and that it revealed distinct characteristic signal intensities for each type of specimen. CONCLUSIONS: Optical coherence tomography was developed for in vivo imaging of biological microstructures. This study demonstrates the ability of OCT to differentiate between the vocal fold architectures of 3 histologically distinct types of vocal folds. Future studies aim to develop a quantitative optical imaging algorithm that can be used to facilitate an in vivo longitudinal clinical investigation of the changes that occur in this layered structure over time and maturation.

Double-Clad Fiber-Based Multifunctional Biosensors and Multimodal Bioimaging Systems: Technology and Applications.

Optical fibers have been used to probe various tissue properties such as temperature, pH, absorption, and scattering. Combining different sensing and imaging modalities within a single fiber allows for increased sensitivity without compromising the compactness of an optical fiber probe. A double-clad fiber (DCF) can sustain concurrent propagation modes (single-mode, through its core, and multimode, through an inner cladding), making DCFs ideally suited for multimodal approaches. This study provides a technological review of how DCFs are used to combine multiple sensing functionalities and imaging modalities. Specifically, we discuss the working principles of DCF-based sensors and relevant instrumentation as well as fiber probe designs and functionalization schemes. Secondly, we review different applications using a DCF-based probe to perform multifunctional sensing and multimodal bioimaging.

Calibration procedure for enhanced mirror artifact removal in full-range optical coherence tomography using passive quadrature demultiplexing.

Significance: Passive quadrature demultiplexing allows full-range optical coherence tomography (FR-OCT). However, imperfections in the wavelength- and frequency-response of the demodulation circuits can cause residual mirror artifacts, which hinder high-quality imaging on both sides of zero delay. Aim: We aim at achieving high mirror artifact extinction by calibrated postprocessing of the FR-OCT signal. Approach: We propose a mathematical framework for the origin of the residual mirror peaks as well as a protocol allowing the precise measurement and correction of the associated errors directly from mirror measurements. Results: We demonstrate high extinction of the mirror artifact over the entire imaging range, as well as an assessment of the method's robustness to time and experimental conditions. We also provide a detailed description of the practical implementation of the method to ensure optimal reproducibility. Conclusion: The proposed method is simple to implement and produces high mirror artifact extinction. This may encourage the adoption of FR-OCT in clinical and industrial systems or loosen the performance requirements on the optical demodulation circuit, as the imperfections can be handled in postprocessing.

Morphometric and Microstructural Changes During Murine Retinal Development Characterized Using In Vivo Optical Coherence Tomography.

Purpose: The purpose of this study was to develop an in vivo optical coherence tomography (OCT) system capable of imaging the developing mouse retina and its associated morphometric and microstructural changes. Methods: Thirty-four wild-type mice (129S1/SvlmJ) were anesthetized and imaged between postnatal (P) day 7 and P21. OCT instrumentation was developed to optimize signal intensity and image quality. Semi-automatic segmentation tools were developed to quantify the retinal thickness of the nerve fiber layer (NFL), inner plexiform layer (IPL), inner nuclear layer (INL), and the outer retinal layers (ORL), in addition to the total retina. The retinal maturation was characterized by comparing layer thicknesses between consecutive time points. Results: From P7 to P10, the IPL increased significantly, consistent with retinal synaptogenesis. From P10 to P12, the IPL and ORL also increased, which is coherent with synaptic connectivity and photoreceptor maturation. In contrast, during these periods, the INL decreased significantly, consistent with cellular densification and selective apoptotic "pruning" of the tissue during nuclear migration. Thereafter from P12 to P21, the INL continued to thin (significantly from P17 to P21) whereas the other layers remained unchanged. No time-dependent changes were observed in the NFL. Overall, changes in the total retina were attributed to those in the IPL, INL, and ORL. Regions of the retina adjacent to the optic nerve head were thinner than distal regions during maturation. Conclusions: Changes in retinal layer thickness are consistent with retinal developmental mechanisms. Accordingly, this report opens new horizons in using our system in the mouse to characterize longitudinally developmental digressions in models of human diseases.

All-fiber few-mode optical coherence tomography using a modally-specific photonic lantern.

Optical coherence tomography (OCT) was recently performed using a few-mode (FM) fiber to increase contrast or improve resolution using a sequential time-domain demultiplexing scheme isolating the different interferometric signals of the mode-coupled backscattered light. Here, we present an all-fiber FM-OCT system based on a parallel modal demultiplexing scheme exploiting a novel modally-specific photonic lantern (MSPL). The MSPL allows for maximal fringe visibility for each fiber propagation mode in an all-fiber assembly which provides the robustness required for clinical applications. The custom-built MSPL was designed for OCT at 930 nm and is wavelength-independent over the broad OCT spectrum. We further present a comprehensive coupling model for the interpretation of FM-OCT images using the first two propagation modes of a few-mode fiber, validate its predictions, and demonstrate the technique using in vitro microbead phantoms and ex vivo biological samples.

Double-clad fiber coupler for endoscopy.

We present a double-clad fiber coupler (DCFC) for use in endoscopy to reduce speckle contrast, increase signal collection and depth of field. The DCFC is made by fusing and tapering two all silica double-clad fiber (DCF) and allows achromatic transmission of >95% of core illumination (1265nm - 1325nm) as well as collection of >42% of inner cladding diffuse light. Its potential for endoscopy is demonstrated in a spectrally encoded imaging setup which shows speckle reduction by a factor 5, increased signal collection by a factor 9 and enhanced depth of field by 1.8 times. Separation by the DCFC of single- and multi-mode signals allows combining low-speckle reflectance images (25.5 fps) with interferometrically measured depth profiles (post-processed) for of small three-dimensional (3D) features through an all-fiber low loss instrument.

Rapid spectrally encoded fluorescence imaging using a wavelength-swept source.

We present rapid imaging of fluorescent samples using spectral encoding (SE). A near-IR wavelength-swept source in used to preserve the SE of the position, despite Stokes shifts. To validate this approach, we imaged fluorescent PbS quantum dot solutions at concentrations down to 0.5+/-0.1micromol/L. This simple configuration allowed acquisition rates of up to 9920 lines of 1024 pixels per second to create high-resolution images. This spectrally encoded setup could be easily miniaturized for endoscopy, thus combining high-resolution fluorescence with confocal reflectance imaging at unmatched acquisition speed.