Raman imaging through a single multimode fibre

Exploiting holographically encoded variance to transmit labelled images through a multimode optical fiber

Artificial intelligence has emerged as promising tool to decode an image transmitted through a multimode fiber (MMF) by applying deep learning techniques. By transmitting thousands of images through the MMF, deep neural networks (DNNs) are able to decipher the seemingly random output speckle patterns and unveil the intrinsic input-output relationship. High fidelity reconstruction is obtained for datasets with a large degree of homogeneity, which underutilizes the capacity of the combined MMF-DNN system. Here, we show that holographic modulation can encode an additional layer of variance on the output speckle pattern, improving the overall transmissive capabilities of the system. Operatively, we have implemented this by adding a holographic label to the original dataset and injecting the resulting phase image into the fiber facet through a Fourier transform lens. The resulting speckle pattern dataset can be clustered primarily by holographic label, and can be reconstructed without loss of fidelity. As an application, we describe how color images may be segmented into RGB components and each color component may then be labelled by distinct hologram. A ResUNet architecture was then used to decode each class of speckle patterns and reconstruct the color image without the need for temporal synchronization between sender and receiver.

Generating Airy beams through multimode fibres

Focussing light through a multimode fibre (MMF) is the basis of holographic endoscopes, which currently enable detailed imaging of deep tissue. Achieving high fidelity and purity diffraction-limited foci has been shown to be possible, when fully controlling the amplitude, phase, and two orthogonal polarisation states of the input field. Yet, generating more complex field distributions with similar performance remains to be assessed. Here, we demonstrate the generation of Airy beams through an MMF containing in excess of 90 % of the optical power delivered by the fibre. We discuss two distinct methods for generating optical landscapes: the direct field and the Fourier domain synthesis. Moreover, we showcase the flexibility of the Fourier domain synthesis to modify the generated beam.

Optimizing the internal phase reference to shape the output of a multimode optical fiber

Pre-shaping light to achieve desired amplitude distributions at the tip of a multimode fiber (MMF) has emerged as a powerful method allowing a wide range of imaging techniques to be implemented at the distal facet. Such techniques rely on measuring the transmission matrix of the optically turbid waveguide which scrambles the coherent input light into an effectively random speckle pattern. Typically, this is done by measuring the interferogram between the output speckle and a reference beam. In recent years, an optical setup where the reference beam passes through the MMF has become an attractive configuration because of the high interferometric stability of the common optical path. However, the merits and drawbacks of an internal reference beam remain controversial. The measurement of the transmission matrix is known to depend on the choice of internal reference and has been reported to result in "blind spots" due to phase singularities of the reference beam. Here, we describe how the focussing efficiency of the calibration can be increased by several percent by optimising the choice of internal reference beam.

Raman imaging of microplastics and nanoplastics released from the printed toner powders burned by a mimicked bushfire

Plastic contamination is a growing global concern, but the characterisation approaches for microplastics are limited so far, and even more lacking for nanoplastics. As another public concern, bushfire has the potential to exacerbate the negative ecological effects of plastic waste. We thus study the release of microplastics and nanoplastics from toner powers printed on a paper sheet following a mimicked bushfire. The results show that, along the fire frontier, there is a charred area first, then a cindered area towards mineralisation via a full combustion. We find that, depending on the extent of burning, the printed toner powers containing microplastics can melt to aggregate, or crack to break down to nanoplastics, which are well characterised by mass spectrometry and Raman imaging combined with algorithms. Overall, the results shed new light on the microplastics and nanoplastics once affected by bushfire.

Development of a coaxial DCF-GRIN fiberoptic Raman probe for enhancing in vivo epithelial tissue Raman measurements

We report on the development of a novel, to the best of our knowledge, coaxial double-clad-fiber (DCF) and graded-index (GRIN) fiberoptic Raman probe for enhancing epithelial tissue Raman measurements in vivo. The ultra-thin (140 µm outer diameter) DCF-GRIN fiberoptic Raman probe is designed and fabricated with an efficient coaxial optical configuration, whereby a GRIN fiber is spliced onto the DCF to enhance both the excitation/collection efficiency and depth-resolved selectivity. We demonstrate that the DCF-GRIN Raman probe can be used to acquire high-quality in vivo Raman spectra from various oral tissues (e.g., buccal mucosa, labial mucosa, gingiva, mouth floor, palate, and tongue) covering both the fingerprint (800-1800 cm^-1) and high-wavenumber (2800-3600 cm^-1) regions within sub-seconds. The subtle biochemical differences between different epithelial tissues in the oral cavity can also be detected with high sensitivity, suggesting the potential of the DCF-GRIN fiberoptic Raman probe for in vivo diagnosis and characterization in epithelial tissue.

Ultrathin, high-speed, all-optical photoacoustic endomicroscopy probe for guiding minimally invasive surgery

Photoacoustic (PA) endoscopy has shown significant potential for clinical diagnosis and surgical guidance. Multimode fibres (MMFs) are becoming increasingly attractive for the development of miniature endoscopy probes owing to their ultrathin size, low cost and diffraction-limited spatial resolution enabled by wavefront shaping. However, current MMF-based PA endomicroscopy probes are either limited by a bulky ultrasound detector or a low imaging speed that hindered their usability. In this work, we report the development of a highly miniaturised and high-speed PA endomicroscopy probe that is integrated within the cannula of a 20 gauge medical needle. This probe comprises a MMF for delivering the PA excitation light and a single-mode optical fibre with a plano-concave microresonator for ultrasound detection. Wavefront shaping with a digital micromirror device enabled rapid raster-scanning of a focused light spot at the distal end of the MMF for tissue interrogation. High-resolution PA imaging of mouse red blood cells covering an area 100 µm in diameter was achieved with the needle probe at ∼3 frames per second. Mosaicing imaging was performed after fibre characterisation by translating the needle probe to enlarge the field-of-view in real-time. The developed ultrathin PA endomicroscopy probe is promising for guiding minimally invasive surgery by providing functional, molecular and microstructural information of tissue in real-time.

All-optical thermometry using a single multimode fiber endoscope and diamond nanoparticles containing nitrogen vacancy centers

Photoluminescence (PL) spectra from diamond nanoparticles containing negative nitrogen vacancy centers were measured by using a single multimode fiber endoscope combined with a high-sensitivity spectroscopy system. A laser light spot was produced at the distal end of the endoscope and the PL spectra from a temperature-controlled ensemble of diamond nanoparticles were measured. After calibrating the sensitivity and wavelength of the spectroscopy system, the temperature dependence of the zero-phonon line peak wavelength similar to those previously reported was obtained.

Investigating kitchen sponge-derived microplastics and nanoplastics with Raman imaging and multivariate analysis

Microplastics can be found almost everywhere, including in our kitchens. The challenge is how to characterise them, particularly for the small ones (<1 μm), referred to as nanoplastics, when they are mixed with larger particles and other components. Herewith we advance Raman imaging to characterise microplastics and nanoplastics released from a dish sponge that we use every day to clean our cookware and eating utensils. The scanning electron microscopy result shows significantly different structures of the soft and hard layers of the sponge, with the hard layer being more likely to shed particles. By scanning the sample surface to generate a spectrum matrix, Raman imaging can significantly improve signal-noise-ratio, compared with individual Raman spectra. Through mapping the characteristic peaks from the matrix that contains hundreds, even thousands of Raman spectra, it is confirmed that the particles released from the soft and hard layers of the sponge are mainly Nylon PA6 and polyethylene terephthalate, respectively. Using principal component analysis (PCA) to decode the spectrum matrix further enhances the signal-noise ratio, which enables mapping the whole set of the spectrum, rather than the selected peaks. By optimising the Raman scanning parameters, the PCA-Raman imaging is able to reliably capture and visualise microplastics and nanoplastics released from both sides of the dish sponge, including a plastic-surrounding-sand composite structure. Overall, PCA-Raman imaging is a holistic and effective approach to characterising miniature plastic particles.

Holographic Manipulation of Nanostructured Fiber Optics Enables Spatially-Resolved, Reconfigurable Optical Control of Plasmonic Local Field Enhancement and SERS

Integration of plasmonic structures on step-index optical fibers is attracting interest for both applications and fundamental studies. However, the possibility to dynamically control the coupling between the guided light fields and the plasmonic resonances is hindered by the turbidity of light propagation in multimode fibers (MMFs). This pivotal point strongly limits the range of studies that can benefit from nanostructured fiber optics. Fortunately, harnessing the interaction between plasmonic modes on the fiber tip and the full set of guided modes can bring this technology to a next generation progress. Here, the intrinsic wealth of information of guided modes is exploited to spatiotemporally control the plasmonic resonances of the coupled system. This concept is shown by employing dynamic phase modulation to structure both the response of plasmonic MMFs on the plasmonic facet and their response in the corresponding Fourier plane, achieving spatial selective field enhancement and direct control of the probe's work point in the dispersion diagram. Such a conceptual leap would transform the biomedical applications of holographic endoscopic imaging by integrating new sensing and manipulation capabilities.

Non-invasive chemically selective energy delivery and focusing inside a scattering medium guided by Raman scattering

Raman scattering is a chemically selective probing mechanism with diverse applications in industry and clinical settings. Yet, most samples are optically opaque limiting the applicability of Raman probing at depth. Here, we demonstrate chemically selective energy deposition behind a scattering medium by combining prior information on the chemical's spectrum with the measurement of a spectrally resolved Raman speckle as a feedback mechanism for wavefront shaping. We demonstrate unprecedented sixfold signal enhancement in an epi-geometry, realizing targeted energy deposition and focusing on individual Raman active particles.

Hybrid confocal Raman endomicroscopy for morpho-chemical tissue characterization

Confocal laser endomicroscopy (CLE) offers imaging of tissue microarchitecture and has emerged as a promising tool for in vivo clinical diagnosis of cancer across many organs. CLE, however, can show high inter-observer dependency and does not provide information about tissue molecular composition. In contrast, Raman spectroscopy is a label-free optical technique that provides detailed biomolecular compositional information but offers limited or no morphological information. Here we present a novel hybrid fiber-optic confocal Raman endomicroscopy system for morpho-chemical tissue imaging and analysis. The developed confocal endomicroscopy system is based on a novel detection scheme for rejecting Raman silica fiber interference permitting simultaneous CLE imaging and Raman spectral acquisition of tissues through a coherent fiber bundle. We show that this technique enables real-time microscopic visualization of tissue architecture as well as simultaneous pointwise label-free biomolecular characterization and fingerprinting of tissue paving the way for multimodal diagnostics at endoscopy.

Near perfect focusing through multimode fibres

Holographic, multimode fibre (MMF) based endoscopes envision high-quality in-vivo imaging inside previously inaccessible structures of living organisms, amongst other perspective applications. Within these instruments, a digital micro-mirror device (DMD) is deployed in order to holographically synthesise light fields which, after traversing the multimode fibre, form foci at desired positions behind the distal fibre facet. When applied in various imaging modalities, the purity and sharpness of the achieved foci are determinant for the imaging performance. Here we present diffraction-limited foci, which contain in excess of 96% of optical power delivered by the fibre which, to the best of our knowledge, represents the highest value reported to date. Further, we quantitatively study the impact of various conditions of the experimental procedure including input polarisation settings, influence of ghost diffraction orders, light modulation regimes, bias of the calibration camera and the influence of noise.

Video-rate dual-modal photoacoustic and fluorescence imaging through a multimode fibre towards forward-viewing endomicroscopy

Multimode fibres (MMFs) are becoming increasingly attractive in optical endoscopy as they promise to enable unparallelled miniaturisation, spatial resolution and cost. However, high-speed imaging with wavefront shaping has been challenging. Here, we report the development of a video-rate dual-modal photoacoustic (PA) and fluorescence microscopy probe with a high-speed digital micromirror device (DMD) towards forward-viewing endomicroscopy. Optimal DMD patterns were obtained using a real-valued intensity transmission matrix algorithm to raster-scan a 1.5  μ m-diameter focused beam at the distal fibre tip for imaging. The PA imaging speed and spatial resolution were varied from  ∼ 2 to 57 frames per second and from 1.7 to 3  μ m, respectively. Further, high-fidelity PA images of carbon fibres and mouse red blood cells were acquired at unprecedented speed. The capability of dual-modal imaging was demonstrated with phantoms. We anticipate that with further miniaturisation of the ultrasound detector, this probe could be integrated into medical needles to guide minimally invasive procedures.

Suppression of the non-linear background in a multimode fibre CARS endoscope

Multimode fibres show great potential for use as miniature endoscopes for imaging deep in tissue with minimal damage. When used for coherent anti-Stokes Raman scattering (CARS) microscopy with femtosecond excitation sources, a high band-width probe is required to efficiently focus the broadband laser pulses at the sample plane. Although graded-index (GRIN) fibres have a large bandwidth, it is accompanied by a strong background signal from four-wave mixing and other non-linear processes occurring inside the fibre. We demonstrate that using a composite probe consisting of a GRIN fibre with a spliced on step-index fibre reduces the intensity of the non-linear background by more than one order of magnitude without significantly decreasing the focusing performance of the probe. Using this composite probe we acquire CARS images of biologically relevant tissue such as myelinated axons in the brain with good contrast.

Confocal 3D reflectance imaging through multimode fiber without wavefront shaping

Imaging through optical multimode fiber (MMF) has the potential to enable hair-thin endoscopes that reduce the invasiveness of imaging deep inside tissues and organs. Active wavefront shaping and fluorescent labeling have recently been exploited to overcome modal scrambling and enable MMF imaging. Here, we present a computational approach that circumvents the need for active wavefront control and exogenous fluorophores. We demonstrate the reconstruction of depth-gated confocal images through MMF using a raster-scanned, focused input illumination at the fiber proximal end. We show the compatibility of this approach with quantitative phase, dark-field, and polarimetric imaging. Computational imaging through MMF opens a new pathway for minimally invasive imaging in medical diagnosis and biological investigations.

Time-of-flight 3D imaging through multimode optical fibers

[Figure: see text].

Focusing light through multimode fibres using a digital micromirror device: a comparison study of non-holographic approaches

Focusing light through a multimode fibre (MMF) has attracted significant research interest, mainly driven by the need for miniature endoscopes in biomedicine. In recent years, digital micromirror devices (DMD) have become increasingly popular as a high-speed alternative to liquid-crystal spatial light modulators for light focusing via wavefront shaping based on binary amplitude modulations. To exploit the potentials and limitations of the state-of-the-art DMD-based wavefront shaping methods, in this study, for the first time, we compared four representative, non-holographic and DMD-based methods that are reported so far in literature with the same experimental and simulation conditions, including a real-valued intensity transmission matrix (RVITM)-based algorithm, a complex-valued transmission matrix (TM)-based algorithm, a conditional probability algorithm and a genetic algorithm. We investigated the maximum achievable peak-to-background ratio (PBR) in comparison to theoretical expectations, and further improved the performance of the RVITM-based method. With both numerical simulations and experiments, we found that the genetic algorithm offered the highest PBR but suffered from the lowest focusing speed, while the RVITM-based algorithm provided a comparable PBR to that of the genetic algorithm, and the highest focusing speed.

Single-shot hybrid photoacoustic-fluorescent microendoscopy through a multimode fiber with wavefront shaping

We present a minimally-invasive endoscope based on a multimode fiber that combines photoacoustic and fluorescence sensing. From the measurement of a transmission matrix during a prior calibration step, a focused spot is produced and raster-scanned over a sample at the distal tip of the fiber by use of a fast spatial light modulator. An ultra-sensitive fiber-optic ultrasound sensor for photoacoustic detection placed next to the fiber is combined with a photodetector to obtain both fluorescence and photoacoustic images with a distal imaging tip no larger than 250 µm. The high signal-to-noise ratio provided by wavefront shaping based focusing and the ultra-sensitive ultrasound sensor enables imaging with a single laser shot per pixel, demonstrating fast two-dimensional hybrid in vitro imaging of red blood cells and fluorescent beads.

Advances in single cell Raman spectroscopy technologies for biological and environmental applications

The increasing sophistication of single cell Raman spectroscopy (SCRS) via its integrations with other advanced analytical techniques and modern data analytics, enable unprecedented exploration of complex biological and environmental samples with significantly improved specificity, sensitivity, and resolution. Because of the merits of being high-resolution, label-free, non-invasive, molecular-specific, culture-independent, and suitable for in situ, in vitro or in vivo analysis, the SCRS-derived techniques offer abilities superior to conventional bulk measurements for environmental and biological studies. Here, we provide a comprehensive and critical review of the most recent advances in the development and application of SCRS-enabled technologies, with focus on those biomolecular and cellular high-resolution applications in environmental and biological fields. The basic principles, unique advantages, and suitable applications, as well as recognized limitations for each technology are recapitulated. The remaining challenges, research needs and future outlook are discussed. We predict that SCRS-enabled technologies are earning its place as a routine and powerful tool in many and rapidly expanding applications across disciplines.

Sensitivity analysis of Raman endoscopy with and without wavefront shaping

Vibrational spectroscopy is a powerful method for the label-free identification of molecules. Spontaneous Raman spectroscopy integrated with an ultra-thin fiber-based endoscope can provide remote, local, and minimally invasive chemical analysis in many fields from biomedical diagnostics to the materials industry. Miniaturization of the probe in combination with a large field of view (FOV) and high sensitivity would be beneficial for a broad class of applications. Here we quantitatively analyze signal-to-noise ratio (SNR) and the sensitivity improvement due to wavefront shaping. We show that wavefront shaping in an ultra-thin single-fiber probe allows to decrease the total measurements time up to several orders of magnitude even without any prior knowledge of the Raman particle location. Such a fiber probe is well suited for minimally-invasive endoscopy in biological and medical applications.

Label-free CARS microscopy through a multimode fiber endoscope

Multimode fibres have recently been employed as high-resolution ultra-thin endoscopes, capable of imaging biological structures deep inside tissue in vivo. Here, we extend this technique to label-free non-linear microscopy with chemical contrast using coherent anti-Stokes Raman scattering (CARS) through a multimode fibre endoscope, which opens up new avenues for instant and in-situ diagnosis of potentially malignant tissue. We use a commercial 125 µm diameter, 0.29 NA GRIN fibre, and wavefront shaping on an SLM is used to create foci that are scanned behind the fibre facet across the sample. The chemical selectivity is demonstrated by imaging 2 µm polystyrene and 2.5 µm PMMA beads with per pixel integration time as low as 1 ms for epi-detection.

Wavelength dependent characterization of a multimode fibre endoscope

Multimode fibres have recently shown promise as miniature endoscopic probes. When used for non-linear microscopy, the bandwidth of the imaging system limits the ability to focus light from broadband pulsed lasers as well as the possibility of wavelength tuning during the imaging. We demonstrate that the bandwidth is limited by the dispersion of the off-axis hologram displayed on the SLM, which can be corrected for, and by the limited bandwidth of the fibre itself. The selection of the fibre is therefore crucial for these experiments. In addition, we show that a standard prism pulse compressor is sufficient for material dispersion compensation for multi-photon imaging with a fibre endoscope.

Applications of Spatial Light Modulators in Raman Spectroscopy

Advances in consumer display screen technologies have historically been adapted by researchers across the fields of optics as they can be used as electronically controlled spatial light modulators (SLMs) for a variety of uses. The performance characteristics of such SLM devices based on liquid crystal (LC) and digital micromirror device (DMD) technologies, in particular, has developed to the point where they are compatible with increasingly sensitive instrumental applications, for example, Raman spectroscopy. Spatial light modulators provide additional flexibility, from modulation of the laser excitation (including multiple laser foci patterns), manipulation of microscopic samples (optical trapping), or selection of sampling volume (adaptive optics or spatially offset Raman spectroscopy), to modulation in the spectral domain for high-resolution spectral filtering or multiplexed/compressive fast detection. Here, we introduce the benefits of different SLM devices as a part of Raman instrumentation and provide a variety of recent example applications which have benefited from their incorporation into a Raman system.

Raman imaging through multimode sapphire fiber

We report on a sapphire fiber Raman imaging probe's use for challenging applications where access is severely restricted. Small-dimension Raman probes have been developed previously for various clinical applications because they show great capability for diagnosing disease states in bodily fluids, cells, and tissues. However, applications of these sub-millimeter diameter Raman probes were constrained by two factors: first, it is difficult to incorporate filters and focusing optics at such small scale; second, the weak Raman signal is often obscured by strong background noise from the fiber probe material, especially the most commonly used silica, which has a strong broad background noise in low wavenumbers (<500-1700 cm^-1). Here, we demonstrate the thinnest-known imaging Raman probe with a 60 μm diameter Sapphire multimode fiber in which both excitation and signal collection pass through. This probe takes advantage of the low fluorescence and narrow Raman peaks of Sapphire, its inherent high temperature and corrosion resistance, and large numerical aperture (NA). Raman images of Polystyrene beads, carbon nanotubes, and CaSO₄ agglomerations are obtained with a spatial resolution of 1 μm and a field of view of 30 μm. Our imaging results show that single polystyrene bead (~15 µm diameter) can be differentiated from a mixture with CaSO₄ agglomerations, which has a close Raman shift.

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, the 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 a 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.

Multimode fibre based imaging for optically cleared samples

Optical clearing is emerging as a popular approach particularly for studies in neuroscience. However the use of corrosive clearing solutions typically requires sophisticated objectives or extreme care with optical components chosen for single- or multi-photon imaging. In contrast to the use of complex, custom-made microscope objectives, we show that the use of a corrected multimode fibre (MMF) offers a route that is resistant to corrosion, can be used in clearing media, is not constrained by the refractive index of the immersion medium and offers flexible working distances. Using a corrected MMF, we demonstrate fluorescence imaging of beads and stained neuroblastoma cells through optically cleared mouse brain tissue, as well as imaging in an extreme oxidative environment to show the versatility of our approach. Additionally, we perform Raman imaging of polystyrene beads in clearing media to demonstrate that this approach may be used for vibrational spectroscopy of optically cleared samples.