We review recent works in optical signal shaping and advanced characterization techniques within the framework of nonlinear fiber propagation. Specifically, we focus on the development of characterization methods based on the dispersive Fourier transform to monitor incoherent spectral broadening processes with enhanced resolution and sensitivity. In this framework, we also discuss recent studies of modulation instability in a noise-driven regime. Paired with suitable optical monitoring techniques, we show that controlled coherent optical seeding can be leveraged via several machine learning approaches to tailor and optimize incoherent spectral broadening dynamics.
We demonstrate the feasibility of multiphoton fluorescence imaging with high spatial resolution using commercially available single-core 50/125 multimode graded-index fiber. Light propagating forward inside the endoscopic fiber undergoes a non-reciprocal propagation exhibiting a robust nonlinear spatial self-cleaning process. Whereas fluorescence from nonlinear interactions with biological samples linearly propagates backward along the same fiber. The scanner head, located at the distal end of the endoscope and suited for multimode fibers, is based on a ceramic tube where the fiber end follows a spiral course to explore the sample. No knowledge of the fiber transfer matrix is required.
We present a flexible, compact, 2photon, 3photon, SHG and CARS nonlinear endo-microscope featuring a 2.2 mm outer diameter and a length of 34 mm. It uses a negative curvature hollow-core double-clad fiber that is scanned with a resonant piezo-scanner. The fiber design allows distortion-less, background-free delivery of femtosecond and picosecond excitation pulses and the back-collection of nonlinear signals through the same fiber. Sub-micron spatial resolution together with >300 microns field of view is made possible using micro-lenses or GRIN based miniature objective lens. We demonstrate 2-photon and 3-photon fluorescence, SHG, THG and CARS imaging at a rate of 10 frames/s.
We analyzed the nonlinear dynamics of pulsed beam self-cleaning in nonlinear tapered Ytterbium doped and Erbium-Ytterbium codoped graded-index multimode optical fibers, with quasi-uniform doping distribution in the core cross-section. By increasing the net gain when operating in active configuration we observed that the output spatial intensity distribution changed from a speckled into a high-quality and bell-shaped beam. By launching pulses in the normal dispersion regime of the taper, from the wider into the smaller core diameter, we generated a supercontinuum emission between 520 nm and 2600 nm. When the laser pulses were launched into the small core side of the tapered fiber or in the Erbium-Ytterbium fiber, self-cleaning was obtained without any self-phase modulation-induced spectral broadening or frequency conversion.
We present a flexible, compact, and multimodal nonlinear endoscope (2.2 mm outer diameter) based on a resonantly scanned negative curvature hollow-core double-clad fiber. The fiber design allows distortion-less, background-free delivery of femtosecond and picosecond excitation pulses and the back-collection of nonlinear signals through the same fiber. Sub-micron spatial resolution together with large field of view is made possible by the combination of a miniature objective lens together with a silica microsphere lens inserted into the fiber core. We demonstrate coherent anti-Stokes Raman scattering, 2-photon fluorescence and second harmonic generation imaging at a rate of 10 frames/s.
We analyse the polarimetric response of fiber Bragg gratings involved in a fiber-based resonant microscanner designed to produce polarimetric images of biological tissues through an endoscope for medical diagnosis. The intrinsic birefringence of the grating is measured and its detrimental consequences on the measured values are discussed. Solutions able to overcome this problem are proposed, paving the way for the fabrication of the very first polarimetric endomicroscope.
Non-linear optical microscopy proves to be an indispensable tool in natural sciences and becomes more and more attractive for clinical applications. Coherent Raman scattering, for instance, has the potential to become an in-vivo fast label-free histology tool as its chemical selectivity provides quantitative information on lipids and proteins locations and concentrations in tissues. Along with these techniques, second-harmonic generation of collagen and 2-photon excitation fluorescence broaden even more the non-linear imaging ability as collagen fibers represent an important role in human body construction. Whilst 2-photon excitation fluorescence allows to study auto-fluorescence (ex. NADH and NADHP molecules), and to excite a vast range of chromophores. However, absorption and scattering limit significantly the nonlinear imaging depth into tissues. As a solution, we offer a flexible, compact, and multimodal nonlinear endoscope (2.2 mm outer diameter, 35 mm rigid length) based on a resonantly piezo scanned hollow-core negative curvature double-clad fiber. The fiber design allows distortion-less, background-free delivery of femtosecond excitation pulses and the back-collection of nonlinear signals through the same fiber. The double-cladding of this fiber attends 10^5μm of silica collection surface which allows for a 4-fold collection improvement compared to previously used Kagomé hollow core fibers. Having a good control on the resonantly scanning fiber the endoscope can perform nonlinear imaging up to 8 frames per second over a field of view of 400μm. We demonstrate 2photon, SHG and CARS imaging in ex vivo gastric human tissue samples and in-vivo 2-photon fluorescence imaging of GFP-labeled neurons in mouse brain.
Recent studies showed that the excitation spectral window lying between 1.6 and 1.8 μm is optimal for in-depth three-photon microscopy of intact tissues due to the reduced scattering and absorption in this wavelength range. Hence, millimeter penetration depth imaging in a living mouse brain has been demonstrated, demonstrating a major potential for neurosciences.
Further improvements of this approach, towards much higher imaging frame rates (up to 15-20 s/frame in previous achievements) requires the development of advanced molecular optical probes specifically designed for three-photon excited fluorescence in the 1.6 -1.8 μm spectral range.
In order to achieve large three-photon brightness at 1700 nm, novel molecular-based fluorescent nanoparticles which combine strong absorption in the green-yellow region, remarkable stability and photostability in aqueous and biological conditions have been designed using a bottom-up route. Due to the multipolar nature of the dedicated dyes subunits, these nanoparticles show large nonlinear absorption in the NIR region.
These new dyes have been experimentally characterized through the measurement of their three-photon action cross-section, fluorescence spectra and lifetimes using a monolithically integrated high repetition rate all-fiber femtosecond laser based on soliton self-frequency shift providing 9 nJ, 75 fs pulses at 1700 nm. The main result is that their brightness could be several orders of magnitude larger than the one of Texas Red in the 1700 nm excitation window.
Ongoing experiments involving the use of these new dyes for in vivo cerebral angiography on a mouse model will be presented and the route towards three-photon endomicroscopy will be discussed.
Fluorescence lifetime imaging microscopy (FLIM) represents a powerful tool for biological studies. Endoscopic FLIM applied to the intracellular native biomarker NADH and FAD represents a promising mean for in vivo in situ malignant tissue diagnosis in the medical field. Else, 2-photon-excited fluorescence (2PEF) provides increased 3D resolution and imaging depth. But very few demonstrations about 2PEF lifetime measurement through a fiber have been reported and none about endoscopic 2P-FLIM through a practical fiber length (< 3m).
Our group has recently demonstrated the possibility to efficiently deliver through a very long optical fiber the short and intense excitation pulses required for 2P-FLIM. Our goal is now to check that collecting fluorescence through the same endoscopic fiber does not deteriorate the lifetime measurement. Relying on the basis previously published in case of 1PEF by P. French and co-workers (J. Biophotonics, 2015), we have experimentally quantitatively evaluated the influence on the lifetime measurement of the fiber chromatic and intermodal dispersions. The main result is that the fiber contribution to the system impulse response function, even in the case of a 3-meter long double-clad optical fiber, does not hinder the separation between free and bound NADH states using FLIM. Related calibrations and measurements will be detailed. Ongoing experiments about the development of a 2P-FLIM endomicroscope on the basis of an previously reported 2P-endomicroscope (Ducourthial et al., Sc. Reports, 2015), used under various configurations (i.e. point measurement in the center of the 2P-endomicroscope image, averaged lifetime, binned endoscopic 2P-FLIM image), will be also presented.
Multiphoton microscopy is a cutting edge imaging modality leading to increasing advances in biology and also in the clinical field. To use it at its full potential and at the very heart of clinical practice, there have been several developments of fiber-based multiphoton microendoscopes. The application for those probes is now limited by few major restrictions, such as the difficulty to collect autofluorescence signals from tissues and cells theses being inherently weak (e.g. the ones from intracellular NADH or FAD metabolites). This limitation reduces the usefulness of microendoscopy in general, effectively restraining it to morphological imaging modality requiring staining of the tissues. Our aim is to go beyond this limitation, showing for the first time label-free cellular metabolism monitoring, in vivo in situ in real time.
The experimental setup is an upgrade of a recently published one (Ducourthial et.al, Scientific Reports, 2016) where femtosecond pulse fiber delivery is further optimized thank’s to a new transmissive-GRISM-based pulse stretcher permitting high energy throughput and wide bandwidth. This device allows fast sequential operation with two different excitation wavelengths for efficient two-photon excited NADH and FAD autofluorescence endoscopic detection (i.e. 860 nm for FAD and 760 nm for NADH), enabling cellular optical redox ratio quantification at 8 frames/s.
The obtained results on cell models in vitro and also on animal models in vivo (e.g. neurons of a living mouse) prove that we accurately assess the level of NADH and FAD at subcellular resolution through a 3-meters-long fiber with our miniaturized probe (O.D. =2.2 mm).
We present the vanadium dioxide (VO2) thin films deposition using e-beam evaporation of a vanadium target under oxygen atmosphere on different substrates (sapphire, Si, SiO2/Si…) and we focus on their electrical and optical properties variations as the material undergoes a metal-insulator transition under thermal and electrical stimuli. The phase transition induces extremely abrupt changes in the electronic and optical properties of the material: the electrical resistivity increases up to 5 orders of magnitude while the optical properties (transmission, reflection, refractive index) are drastically modified. We present the integration of these films in simple planar optical devices and we demonstrate electrical-activated optical modulators for visible-infrared signals with high discrimination between the two states. We will highlight a peculiar behavior of the VO2 material in the infrared and far infrared regions (2- 20 μm), namely its anomalous emissivity change under thermal- end electrical activation (negative differential emittance phenomenon) with potential applications in active coatings for thermal regulation, optical limiting or camouflage coatings.
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