Observing mode-dependent wavelength-to-time mapping in few-mode fibers using a single-photon detector array

Wavelength-to-time mapping (WTM)—stretching ultrashort optical pulses in a dispersive medium such that the instantaneous frequency becomes time-dependent—is usually performed using a single-mode fiber. In a number of applications, such as time-stretch imaging (TSI), the use of this single-mode fiber during WTM limits the achievable sampling rate and the imaging quality. Multimode fiber based WTM is a potential route to overcome this challenge and project a more diverse range of light patterns. Here, we demonstrate the use of a twodimensional single-photon avalanche diode (SPAD) array to image, in a time-correlated single-photon counting (TCSPC) manner, the time- and wavelength-dependent arrival of different spatial modes in a few-mode fiber. We then use a TCSPC spectrometer with a onedimensional SPAD array to record and calibrate the wavelength-dependent and mode-dependent WTM processes. The direct measurement of the WTM of the spatial modes opens a convenient route to estimate group velocity dispersion, differential mode delay, and the effective refractive index of different spatial modes. This is applicable to TSI and ultrafast optical imaging, as well as broader areas such as telecommunications

Sub millimetre flexible fibre probe for background and fluorescence free Raman spectroscopy

Using the shifted-excitation Raman difference spectroscopy technique and an optical fibre featuring a negative curvature excitation core and a coaxial ring of high numerical aperture collection cores, we have developed a portable, background and fluorescence free, endoscopic Raman probe. The probe consists of a single fibre with a diameter of less than 0.25 mm packaged in a sub-millimetre tubing, making it compatible with standard bronchoscopes. The Raman excitation light in the fibre is guided in air and therefore interacts little with silica, enabling an almost background free transmission of the excitation light. In addition, we used the shifted-excitation Raman difference spectroscopy technique and a tunable 785 nm laser to separate the fluorescence and the Raman spectrum from highly fluorescent samples, demonstrating the suitability of the probe for biomedical applications. Using this probe we also acquired fluorescence free human lung tissue data

Demonstrating the Use of Optical Fibres in Biomedical Sensing:A Collaborative Approach for Engagement and Education

This paper demonstrates how research at the intersection of physics, engineering, biology and medicine can be presented in an interactive and educational way to a non-scientific audience. Interdisciplinary research with a focus on prevalent diseases provides a relatable context that can be used to engage with the public. Respiratory diseases are significant contributors to avoidable morbidity and mortality and have a growing social and economic impact. With the aim of improving lung disease understanding, new techniques in fibre-based optical endomicroscopy have been recently developed. Here, we present a novel engagement activity that resembles a bench-to-bedside pathway. The activity comprises an inexpensive educational tool ($70) adapted from a clinical optical endomicroscopy system and tutorials that cover state-of-the-art research. The activity was co-created by high school science teachers and researchers in a collaborative way that can be implemented into any engagement development process

Computational Fluorescence Suppression in Shifted Excitation Raman Spectroscopy

Fiber-based Raman spectroscopy in the context of &lt;italic&gt;in vivo&lt;/italic&gt; biomedical application suffers from the presence of background fluorescence from the surrounding tissue that might mask the crucial but inherently weak Raman signatures. One method that has shown potential for suppressing the background to reveal the Raman spectra is shifted excitation Raman spectroscopy (SER). SER collects multiple emission spectra by shifting the excitation by small amounts and uses these spectra to computationally suppress the fluorescence background based on the principle that Raman spectrum shifts with excitation while fluorescence spectrum does not. We introduce a method that utilizes the spectral characteristics of the Raman and fluorescence spectra to estimate them more effectively, and compare this approach against existing methods on real world datasets.</p

Biomedical fibre optic time-correlated single-photon counting spectroscopy with CMOS SPAD line arrays

In times of economic constraint, an ageing population and antimicrobial resistance, medical challenges must be met by exploring new pathways to improve diagnostics, prevention and specialised treatments. Lung diseases include a wide range of conditions and exhibit high morbidity and mortality rates, especially for actively ventilated patients in intensive care units. The aim of this PhD is to develop systems for endoscopic sensing in size restricted regions such as the alveolar space in the distal lung. The systems are based on miniaturised and disposable single optical fibres and a custom CMOS SPAD line sensor capable of time-resolved single photon detection. Standard optical fibres are suitable for single-use medical devices and allow access to remote locations in the lung. Applying advanced time-resolving detector technology has the potential to overcome common limitations of intensity-based spectroscopy. Two common sensing technologies – Raman and fluorescence spectroscopy – are investigated. Fluorescence spectroscopy is a powerful tool for tissue diagnostics providing insight into the molecular composition and local environment through changes in the fluorescence intensity, spectral properties, and lifetime. Raman spectroscopy reveals the chemical ‘fingerprint’ of molecules under investigation, but is impeded by an inherently weak signal. The visibility of the Raman signal can be significantly enhanced by utilising the different time profiles of unwanted background signals and the Raman signal. The systems are exemplified for differentiating normal and abnormal tissue, detecting pathogens and measuring key physiological parameters such as pH, facilitated by fluorescent markers and Raman reporters. Time-resolved fluorescence lifetime spectroscopy successfully demonstrates its potential for pH-sensing, distinguishing between normal and tumorous tissue, and detecting fluorescent labelled bacteria, all through a miniaturised optical fibre. Time-gated Raman spectroscopy enables pH-sensing with Raman reporters, functionalised gold nanoshells deposited on the fibre tip, with an improved pH sensitivity of 0.06 pH unit

Code and Data accompanying "pH sensing through a single optical fibre using SERS and CMOS SPAD line arrays"

Full exploitation of fibre Raman probes has been limited by the obstruction of weak Raman signals by background fluorescence of the sample and the intrinsic Raman signal of the delivery fibre. Here we utilised functionalised gold nanoshells (NS) to take advantage of the surface-enhanced Raman spectroscopy (SERS) effect to enhance the pH responsive spectrum of 4-mercaptobenzoic acid (MBA), however fibre background is still dominant. Using the photon arrival time-resolving capability of a CMOS single photon avalanche diode (SPAD) based line sensor we recover the SERS spectrum without fibre background in a 10 s measurement. In this manner, pH sensing through a multimode fibre at a low excitation power, that is safe for future in vivo applications, with short acquisition times (10 or 60 s) is demonstrated. A measurement precision of ±0.07 pH units is thus achieved.Katjana, Ehrlich. (2017). Code and Data accompanying "pH sensing through a single optical fibre using SERS and CMOS SPAD line arrays", [dataset]. University of Edinburgh. College of Medicine and Veterinary Medicine. Queen's Medical Research Institute. http://dx.doi.org/10.7488/ds/2241