Multiplex Coherent Anti-Stokes Raman Scattering (M-CARS) is an innovative nonlinear spectroscopic approach designed to characterize the vibrational modes of molecular structures. Coherent Raman scattering has been used for the characterization of biomedical targets for about 20 years and the multiplex aspect was introduced 10 years ago thanks to the use of a supercontinuum laser system. For each of these systems, the combination of a pump and a probe wave, driven by an external delay line, is however required to produce the vibrations. In the present work, we propose a new M-CARS system, free of the external delay line. A few-mode microstructured fiber enables merging both wave-packets (pump and supercontinuum) within a single waveguide. We showcase the capability of this system in generating hyperspectral images of biochemically active compounds. Curcumin I, the principal yellow compound isolated from Curcuma longa (Turmeric), is distinguishable by its multiple functional groups that display a nonlinear spectroscopic behavior.
We investigate an original approach for the generation of unequally spaced frequency combs using (2) –(3) nonlinearities in multimode graded-index (MM-GRIN) fiber. In a preliminary step, the MM-GRIN fiber (50 µm of core diameter and 125 μm of cladding diameter) is optically poled with a Nd:YAG sub-nanosecond microchip laser at 1064 nm. As a results, a double periodical inscription of a complex second order non-linearity χ(2) grating was led. The resulting χ(2) inscription allows the generation of second harmonic wave (SH) from a supercontinuum obtained in the infrared domain under the Raman and soliton propagation actions. We then detect the generation of various irregularly spaced spectral peaks surrounding the original SH (532 nm) at the fiber output allowing harmonic generation on more than 100 nm in the visible domain.
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.
Spatial beam self-cleaning in graded-index multimode fibers involves a nonlinear transfer of power among the fiber modes, which leads to robust bell-shaped output beams. The resulting output mode power distribution can be described by statistical mechanics arguments. Although the spatial coherence of the output beam was experimentally demonstrated, there is no direct study of modal phase evolution. Based on a holographic mode decomposition method, we reveal that nonlinear spatial phase-locking occurs between the fundamental and its neighboring low-order modes, in good quantitative agreement with theoretical predictions.
We experimentally reveal a so-far unnoticed high-power limit to spatial beam self-cleaning in graded-index nonlinear multimode optical fibers. As the optical pulse power is progressively increased, we observed that the initial Kerr-induced beam clean-up and associated improvement of the spatial beam quality are eventually spoiled. Based on a holographic mode decomposition of the output field, we show that this beam degradation can be described in a thermodynamic approach to wave propagation as a manifestation of “high-temperature” thermalization, which depletes the fundamental mode in favor of a highly multimodal power distribution.
We report on mid-infrared supercontinuum generation from 4 to 9 µm in orientation-patterned gallium-arsenide waveguides pumped by nanojoule-class ultrafast fiber lasers. The QPM waveguide and the laser source are optimized in tandem to pump the waveguides close to the degeneracy by means of sub-picosecond pulses at 2760 nm. The use of a waveguide geometry drastically reduces the required energy to the nanojoule level, thereby opening supercontinuum generation in GaAs platforms to fiber lasers.
Since its first demonstration, spatial beam self-cleaning has been targeted as a breakthrough nonlinear effect, for its potential of extending to multimode fibers different technologies based on single-mode fibers, such as fiber lasers and endoscopes. To date, most of the theoretical descriptions of beam self-cleaning are based on scalar models. Whereas, in experiments the analysis of the polarization state of self-cleaned beams is often neglected. Here, we fill this gap between theory and experiments, by demonstrating that a self-cleaned beam eventually loses its degree of polarization, as long as linearly polarized light of enough power is injected at the fiber input. Our results are cast in the framework of a thermodynamic description of nonlinear beam propagation in multimode fibers, providing the first experimental proof of the applicability of scalar theories for the description of the spatial beam self-cleaning effect.
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.
High-harmonics generation (HHG) in solids require high-energy few-cycle laser drivers at near- to mid-infrared wavelengths with excellent beam quality to reach fluences of ~1 TW/cm2. Along this line, soliton sources based on large mode area silica-core singlemode fibers produce ultrashort (70 fs) pulses at remote wavelengths with hundreds of nJ, thus providing a new platform for driving HHG in solids. In this communication, we explore the potential of such soliton-based fiber driver for HHG in thin-films of zinc oxide. The laser delivers 41 nJ 70 fs solitonic pulses at 1764 nm and drives harmonics generation up to H7.
The self-channeling of extremely high power laser beams permits the ignition of plasma filaments in dielectrics, such as air and glasses. If no constraints are imposed by the geometry of the material, the plasma appears as a straight bright line. Here, we show that plasma filaments may be ignited in the vicinity of the interface between two materials in optical fibers, i.e., either the core-cladding or the cladding-air interface. In the latter case, helical plasma filaments lead to the emission of rainbow spiral beams. In perspective, our results may pave the way to a novel approach for micro-structuring optical fibers, and for generating light beams with orbital angular momentum.
All-optical poling was demonstrated for the first time in 1986 in single mode fibers: such nonlinear optical process enabled the introduction of a second-order susceptibility (χ(2)) in a doped silica fiber. By simply using an intense laser source, alloptical poling, later theoretically described by Stolen and coworkers, permitted the generation of a second harmonic (SH) signal in an otherwise centrosymmetric doped material. More recently, similar experiments have been carried out by exploiting complex beam propagation in multimode fibers. In this work we reveal, for the first time to our knowledge, the 3D spatial distribution of a χ(2) nonlinearity written in a graded-index (GRIN) multimode (MM) fiber. In particular, the presence of a doubly-periodic distribution of χ(2) is unveiled by means of multiphoton microscopy. The shortest period (tens of micrometers) is due to the beating between the fundamental and the SH beams, and it is responsible for their quasi-phase matching (QPM). Whereas the longest period (hundreds of micrometers) is associated with the periodic evolution, or self-imaging, of the power density of the MM beam along the GRIN MM fiber. The complex modal beating, leading to spatial self-cleaning of the fundamental beam, is thus printed inside the fiber core, and revealed by our measurements. We considered two fibers of similar composition and opto-geometric parameters, and we compared the evolution of the optical poling process with time. Despite the rather similar fiber characteristics, we observed a striking difference in the poling efficiency between the two fibers. Such observation led us to point out the importance of considering the complete fiber fabrication process (both the preform elaboration and the drawing steps) on the final structure and microstructure of optical fibers.
We study multiphoton absorption-induced damages to standard silica multimode optical fibers, induced by means of femtosecond infrared laser beams. During the damaging process, the dynamics of beam propagation turns out to non-trivially evolve over a time scale of several hours. Such a long term evolution produces an irreversible drop of the optical transmission, which is accompanied by a drastic change of the output supercontinuum spectrum. A microscopic analysis of the damages was carried out by means of both optical microscopy and absorption-contrast computed X-ray tomography. This has permitted us to obtain information about the sign of the refractive index variation which is induced by the optical breakdown. Our results will find application in a wide array of emerging technologies employing high-power fiber optic beams, such as fiber lasers and micromachining.
In this study, we report on experiments of spatio-temporal nonlinear frequency conversion in a periodically poled Lithium Niobate (PPLN) crystal designed for second-harmonic generation (SHG). We demonstrated a novel supercontinuum source based on the mixing of second and third-order nonlinearities. We could adjust the (X(2), X(3)) nonlinearities by controlling the input laser polarization orientation, the pulse duration and the PPLN crystal temperature. We obtained an ultra-broadband spectrum, ranging from visible to infrared domains, by pumping a 20-mm-long PPLN crystal with a 3 ps pulse at 1030 nm. This broadband pulse was used to achieve direct multiplex Coherent Anti-Stokes Raman Scattering (M-CARS) imaging, without the need for any optical delay line to temporally synchronize the pump and the Stokes waves. Simultaneous vibrational signatures ranging from -3200 cm-1 to -500 cm-1 were obtained. Several filters were placed on the broadband supercontinuum path, to shape the output spectrum between 1030 nm and 1650 nm, before sending it into the microscope. The output spectral analysis allows for the demonstration of multimodal imaging, by using SHG, M-CARS and multiphoton fluorescence processes.
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.
Beam self-imaging of ultrashort pulses in nonlinear graded-index (GRIN) multimode optical fibers is of interest for many applications, including spatiotemporal mode-locking in fiber lasers. We obtained a new analytical description for the nonlinear evolution of a laser beam of arbitrary transverse shape propagating in a GRIN fiber. The longitudinal beam evolution could be directly visualized by means of femtosecond laser pulses, propagating in the anomalous or in the normal dispersion regime, leading to light scattering out of the fiber core via the emission of blue photo-luminescence. As the critical power for self-focusing is approached and even surpassed, a host of previously undisclosed nonlinear effects is revealed, including strong multiphoton absorption by oxygen-deficiency center defects and Germanium inclusions, splitting and shifting of the self-imaging period, filamentation, and conical emission of the guided light bullets. We discovered that nonlinear loss has a profound influence on the process of high-order spatiotemporal soliton fission. The beam energy carried by the fiber is clamped to a fixed value, and nonlinear bullet attractors with suppressed Raman frequency shift and fixed temporal duration are generated, leading to highly efficient frequency conversion of the input near-infrared femtosecond pulses into mid-infrared multimode solitons.
Nowadays, interactions between pulsed electromagnetic field and biological cells and tissues are particularly investigated to prevent any noxious effects and/or to develop therapeutic processes able to improve cancer treatment. Electrochemotherapy is an example which allows drugs delivery improvement thanks to local pulsed electric field application. Consequently, high voltage electrical nanosecond pulse generation has received great interest from the researchers working in the bioelectromagnetic domain. In that framework, we demonstrated that picosecond kilovolt generator activated by a nanosecond and femtosecond laser sources can be coupled with a Multiplex Coherent Anti- Stokes Raman Scattering system (M-CARS) to study the impact of electric pulses on biological cells. We generated kilovolt picosecond electric pulses by using a frozen-wave generator which integrated silicon PhotoConductive Semiconductors Switches (PCSSs). Because of the linear switching regime no temporal jitter is observed during the optical switching thus, coherent combining of short electric pulses can be obtained. Two semiconductors are activated with adjustable time delay, generating unipolar pulses with less than 60 ps rise time (pulse duration 100 ps). Balanced and unbalanced bipolar pulses have been also obtained with a peak to peak voltage of 1.4 kV and a total duration of 244 ps. The initial optical pulse may be used to produce a supercontinnum extended from the visible and up to infrared domain (2.4 μm). Thus synchronized M-CARS diagnosis can be realized when nanosecond and picosecond electric pulse excitation is applied to biological samples.
Acanthamoeba keratitis is a disease in which amoebae named Acanthamoeba invade the cornea of an eye. To diagnose this disease before it becomes serious, it is important to detect the cyst state of Acanthamoeba in the early stage of infection. In the present study, we explored spectroscopic signitures of the cyst state of Acanthamoeba using multimodal nonlinear optical microscopy with the channels of multiplex coherent anti-Stokes Raman scattering (CARS), second harmonic generation (SHG), and third harmonic generation (THG). A sharp band at around 1603 cm-1 in the CARS (Im[χ(3)]) spectrum was found at the cyst state of Acanthamoeba, which possibly originates from ergosterol and/or 7-dehydrostigmasterol. It can be used as a maker band of Acanthamoeba for medical treatment. Keyword: Acanthamoeba keratitis, coherent anti-Stokes Raman scattering, CARS, second harmonic generation, SHG, microspectroscopy, multiphoton microscopy
We have visualized living cells and tissues using an ultrabroadband multiplex coherent anti-Stokes Raman scattering (CARS) microspectroscopic system by using a sub-nanosecond supercontinuum (SC) light source. Owing to the ultrabroadband spectral profile of the SC, we can generate multiplex CARS signals in the spectral range of 500–3800 cm−1, which covers the whole molecular fingerprint region, as well as the C-H and O-H stretching regions. Through the combination of the ultrabroadband multiplex CARS method with second harmonic generation (SHG) and third harmonic generation (THG) processes, we have successfully performed selective imaging of ciliary rootlet-composing Rootletin filaments in rat retina.
We overview recent advances in the research on spatiotemporal beam shaping in nonlinear multimode optical fibers. An intense light beam coupled to a graded index (GRIN) highly multimode fiber undergoes a series of complex nonlinear processes when its power grows larger. Among them, the lowest threshold effect is the Kerr-induced beam self-cleaning, that redistributes most of the beam energy into a robust bell-shaped beam close to the fundamental mode. At higher powers a series of spectral sidebands is generated, thanks to the phase matching induced by the long period grating due to the periodic self-imaging of the beam and the Kerr effect. Subsequently a broadband and spectrally flat supercontinuum is generated, extending from the visible to the mid-infrared.
The influence of a static electric field on a non-polar molecule has been studied by means of multiplex coherent anti-Stokes Raman scattering (M-CARS). A parallel measurement of electric field induced second harmonic generation (EFISHG) has also been led. Both techniques suggest a reorientation of the molecule due to the presence of an electric field. This phenomenon can be used to increase the chemical selectivity and the signal to non-resonant background ratio, namely, the sensitivity of the M-CARS spectroscopy.
Multiphoton microscopy is a well-established technique for biological imaging of several kinds of targets. It is classically based on multiphoton processes allowing two means of contrast simultaneously: two-photon fluorescence (TPF) and second harmonic generation (SHG). Today, the quasi exclusive laser technology used in that aim is femtosecond titanium sapphire (Ti: Sa) laser. We experimentally demonstrate that a nanosecond supercontinuum laser source (STM-250-VIS-IR-custom, Leukos, France; 1 ns, 600-2400 nm, 250 kHz, 1 W) allows to obtain the same kind of image quality in the case of both TPF and SHG, since it is properly filtered. The first set of images concerns the muscle of a mouse. It highlights the simultaneous detection of TPF and SHG. TPF is obtained thanks to the labelling of alpha-actinin with Alexa Fluor® 546 by immunochemistry. SHG is created from the non-centrosymmetric organization of myosin. As expected, discs of actin and myosin are superimposed alternatively. The resulting images are compared with those obtained from a standard femtosecond Ti: Sa source. The physical parameters of the supercontinuum are discussed. Finally, all the interest of using an ultra-broadband source is presented with images obtained in vivo on the brain of a mouse where tumor cells labeled with eGFP are grafted. Texas Red® conjugating Dextran is injected into the blood vessels network. Thus, two fluorophores having absorption wavelengths separated by 80 nm are imaged simultaneously with a single laser source.
Particle size analyzers based on laser scattering commonly make use of light diffraction and scattering around the particle considered in its medium. For particle size below 50 μm, Fraunhofer theory must be abandoned in favor of Mie model, which requires to know the complex refractive index of both the particle and the medium. In this paper, we demonstrate that particle size characterization can be realized by measuring the macroscopic Raman spectral response of the whole set of particles excited by a laser beam. We use a home-made setup based on coherent anti-Stokes Raman scattering (CARS) and having a 0.36 cm-1 spectral resolution, in which the laser source is a dual-output infrared nanosecond supercontinuum source (1064 nm monochromatic pump wave, 1100-1640 nm broadband Stokes wave). The samples are latex beads in water with different diameters (20 nm, 50 nm, 100 nm, 5 μm). The C-H stretching line around 3050 cm-1 is studied. For this vibration, we study the variation of both the CARS central frequency and linewidth as a function of the particles size. A quasi linear increase of the linewidth with the inverse of the diameter is measured. A difference of 15 cm-1 is obtained between beads with diameters of 5 μm and 20 nm respectively. The physical phenomena at the origin of this difference are discussed, especially considering the contributions of the center and of the boundaries of the object to the global Raman response.
We demonstrate all-normal dispersion supercontinuum generation in the 1080 nm-1600 nm range by propagating subnanosecond pulses in a high numerical aperture standard optical fiber. The extreme saturation of the Raman gain provides a flat spectrum in the considered range, making this broadband source particularly suitable for coherent Raman spectroscopy. This unusual regime of supercontinuum generation (Raman gain saturation regime) is investigated through an experimental spectrotemporal study. The possibility of operating spectrometer-free time-coded coherent Raman spectroscopy is introduced.
In the area of bioelectromagnetic studies there is a growing interest to understand the mechanisms leading to nanosecond
electric fields induced electroporation. Real-time imaging techniques at molecular level could probably bring further
advances on how electric fields interact with living cells. However the investigations are limited by the present-day lack
of these kinds of advanced instrumentations. In this context, we present an innovative electro-optical pump-probe
system. The aim of our project is to provide a performing and compact device for electrical stimulation and multiplex
Coherent anti-Stokes Raman Scattering (M-CARS) imaging of biological cells at once.
The system consists of a 1064 nm sub-nanosecond laser source providing both a monochromatic pump and a
polychromatic Stokes optical beam used in a CARS process, as well as the trigger beam for the optoelectronic switching-based electrical pulse generator.
The polychromatic Stokes beam (from 600 to 1700 nm) results from a supercontinuum generation in a photonic crystal
fiber (PCF). A detailed spectro-temporal characterization of such a broadband spectrum shows the impact of the
nonlinear propagation in the fiber on the Stokes wave. Despite the temporal distortions observable on Stokes pulse
profiles, their spectral synchronization with the pump pulse remains possible and efficient in the interesting region
between 1100 nm and 1700 nm.
The electrical stimulation device consists of a customized generator combining microstrip-line technology and laser-triggered photoconductive semiconductor switches. Our experimental characterization highlights the capability for such
a generator to control the main pulse parameters (profile, amplitude and duration) and to be easily synchronized with the
imaging system. We finally test and calibrate the system by means of a KDP crystal. The preliminary results suggest that
this electro-optical system provides a suitable tool for real-time investigation of bioelectromagnetic interactions in the
nanosecond and sub-nanosecond regime.
Precise identification of eosinophils, basophils, and specific subpopulations of blood cells (B lymphocytes) in an unconventional automatic hematology analyzer is demonstrated. Our specific apparatus mixes two excitation radiations by means of an acousto-optics tunable filter to properly control fluorescence emission of phycoerythrin cyanin 5 (PC5) conjugated to antibodies (anti-CD20 or anti-CRTH2) and Thiazole Orange. This way our analyzer combining techniques of hematology analysis and flow cytometry based on multiple fluorescence detection, drastically improves the signal to noise ratio and decreases the spectral overlaps impact coming from multiple fluorescence emissions.
This paper introduces a supercontinuum (SC) laser source emitting from 400 nm to beyond 1750 nm, with
adjustable pulse repetition rate (from 250 kHz to 1 MHz) and duration (from ~200 ps to ~2 ns). This device
makes use of an internally-modulated 1.06 μm semiconductor laser diode as pump source. The output radiation
is then amplified through a preamplifier (based on single-mode
Yb-doped fibres) followed by a booster (based
on a double-clad Yb-doped fibre). The double-clad fibre output is then spliced to an air-silica microstructured
optical fibre (MOF). The small core diameter of the double-clad fibre allows reducing the splice loss. The strongly
nonlinear propagation regime in the MOF leads to the generation of a SC extending from the violet to the nearinfrared
wavelengths. On the Stokes side of the 1.06 μm pump line, i.e., in the anomalous dispersion regime, the
spectrum is composed of an incoherent distribution of quasi-solitonic components. Therefore, the SC source is
characterised by a low coherence length, which can be tuned by simply modifying pulse duration, that is closely
related to the number of quasi-solitonic components brought into play. Finally, the internal modulation of the
laser diode permits to achieve excellent temporal stability, both in terms of average power and pulse-to-pulse
period.
We demonstrate a low loss microstructured fiber (MOF) with a high nonlinear glass core and silica holey cladding. The
substitution of mostly used silica as core material of microstructured fibers by lanthanum oxide glass promises a high
nonlinear conversion efficiency for supercontinuum (SC) generation. The glass composition is optimized in terms of
thermochemical and optical requirements. The glass for the MOF core has a high lanthanum oxide concentration
(10 mol% La2O3) and a good compatibility with the silica cladding. This is performed by adding a suitable alumina
concentration up to 20 mol%. The lanthanum oxide glass preform rods were manufactured by melting technique. Besides
purity issues the material homogeneity plays an important role to achieve low optical loss. The addition of fluorides
allows the better homogenization of the glass composition in the preform volume by refining. The minimum attenuation
of an unstructured fiber drawn from this glass is about 0.6 dB/m. It is mostly caused by decreasing of scattering effects.
The microstructured silica cladding allows the considerable shifting of dispersive behavior of the MOF for an optimal
pump light conversion. The MOF shows zero dispersion wavelengths (ZDW) of 1140 nm (LP01 mode) and 970 nm (LP11
mode). The supercontinuum generation was investigated with a 1064 nm pump laser (650 ps). It shows a broad band
emission between 500 nm and 2200 nm.
One of the objectives of NextGenPCF European Union R&D project is to develop compact and low-cost white-light supercontinuum sources, based on the use of microchip lasers and air/silica microstructured optical fibres. In this paper, we present some experimental results obtained with doped or undoped highly nonlinear fibres. We also introduce and discuss some ways of getting blue/UV frequency generation, which is particularly useful for certain specific biomedical applications. Finally, the different methods proposed for supercontinuum generation are compared, in terms of spectral power density and spectrum range.
We demonstrate that second harmonic generation obtained in Ge-doped holey fibres can act as a seed for visible supercontinuum generation. This spectral enlargement is obtained by means of a double-pumping system. By using a microchip laser source delivering sub-nanosecond pulses at 1064 nm and a highly Ge-doped fibre, we obtain a second harmonic generation efficiency of 4.8 % after an optical poling process. A white light continuum extending on more than 250 nm is obtained in visible domain.
Some recent developments made at XLIM laboratory in the field of supercontinuum generation in air-silica
microstructured optical fibres are presented in this paper. These results concern the use of non usual nonlinear schemes
in specially designed and home-made holey fibres for the improvement of white light sources. The design of three
different guides and the specific nonlinear processes involved in the spectrum build-up are described. In a first
microstructured fibre, dual-wavelength pumping (532/1064 nm) allows to generate both visible and infrared
broadenings, with possible tunability of the visible spectral range according to the input polarisation. Moreover, a highly
birefringent fibre is pumped at a single wavelength (1064 nm), located in large anomalous dispersion regime. In this
case, a polarisation-controlled FWM process is initiated and permits to obtain very wide spectral broadening (350-
1750 nm) by avoiding the usual critical matching between the zero dispersion wavelength of the fibre and the pump
wavelength. Non conventional second order nonlinearity is also analysed in this pure silica holey fibre. Finally, a specific
air-clad Yb-doped microstructured fibre is used to combine laser amplification and nonlinear gain, resulting in an
increase of the power spectral density level of the generated supercontinuum.
We demonstrate the generation of a low repetition rate picosecond, polarized, visible supercontinuum in a highly
birefringent fibre. The polarization dependence of the supercontinuum spectrum is investigated, and the mechanisms
responsible for the generation of visible light in these pumping conditions are described.
We demonstrate the use of broad super-continua generated in photonic crystal fibers (PCFs) for a novel femtosecond absorption pump-probe experiment that records time- and wavelength resolved data. While such experiments relied up to now on amplified sources for the supercontinuum generation, the non-linearities in the microstructured fibers can be induced with a Ti:sapphire oscillator only. We test the performances of such a compact femtosecond spectrometer for the study of molecules in the liquid phase. The aim is to cover the 400-1100 nm spectral range with a single pulse shorter than 200 fs. A commercial Ti:Sapphire oscillator (KM Labs) cavity, delivering 40 fs pulses, has been extended so as to lower the repetition rate (27 MHz) and to increase the pulse energy. Up to 6 nJ are focussed into 8-mm long pieces of commercial (NL-710, Blazephotonics) and home-made PCFs using a microscope objective. The fiber output spectra are measured from 300-1100 nm with a Peltier-cooled CCD (Spec10, Roper Scientific). Most recently, using the fibers produced at XLIM, we have been able to generate a supercontinuum spectrum extending down to 380 nm. While a full characterization of the temporal properties of the home-built fibers is still in progress, we find a single pulse output for the commercial fibers, with negligible chirp for λ= 600-750 nm. Obviously in these short fiber lengths, soliton fission does not take place. With this system the photo-induced dynamics of malachite green have been studied with a noise-floor of 10-4 relative absorption change.
Optical Coherence Tomography is an emerging technique for biomedical diagnostic help. This is a non-invasive, high resolution, non-destructive mean for some optical biopsy. Since a few years new developments have been undergone in the field of OCT trying to functionalize OCT measurements. One of them is Spectroscopic OCT where simultaneous access to depth resolution as well as spectral features depth resolved in the media are obtained. These spectroscopic OCT system are mainly based on post processing of classical OCT signals what is time consuming and which add numerical noise. We propose an 'all optical' system for real-time direct display of depth-frequency analysis of media.
We achieved the generation of unipolar and bipolar electrical picosecond pulses by using semiconductor switches in linear mode. Electrical pulses of hundreds volts to several thousands volts were obtained: in example unipolar pulses of about 11kV with duration of 300ps (full width at half maximum) were obtained and bipolar pulses of 3kV peak-to-peak with cycle duration of 450ps were done. Besides the linear mode running of these doped silicon semiconductors permitted to synchronise several generators and it allows adjustment of the bipolar pulse spectrum by using the frozen wave generator principle. So we demonstrated the feasibility of high voltage ultra-wideband electrical pulse generation with less optical energy than already published in the linear mode.
The propagation properties of microstructured optical fibers useful for sensing applications are reviewed. The interaction between light and sample can reach 95 % in singlemode hollow core fibers and examples of structures exhibiting such large overlap ratios are described. The generation of 300 nm and 700 nm flat continua of visible and IR light in a single highly nonlinear holey fiber, well suited for the detection of biological species by spectroscopy, is reported. The low temperature sensitivity of long period gratings and of the birefringence in holey fibers is attractive for sensors operating in varying environmental conditions.
Infrared chalcogenide glasses are studied with respect to their non linear optical properties. These glasses are sulfur or selenide glasses synthesized in the binary or ternary systems of the Ge-As-S-Se family and are transparent from the end of the visible region to wavelengths above 10 μm depending on the composition. The non linear optical characteristics are firstly determined through a spatially resolved Mach Zender interferometer with the help of a Nd-YAG laser at 1064 nm. Non linearities three order of magnitude above the non linearity of silica glass are achieved. Then, the non linear imaging technique has been used to characterize the glasses at the telecommunication wavelength of 1.55μm. This one shot technique has allow us to obtain values for the non linear refractive index n2 as high as 14 10-18 m2/W. The non linear absorption at 1.55 μm has also been evaluated and is below 1 cm/GW for all the glasses. These third order non linear optical properties make these glasses suitable candidates for integrated ultra fast all optical devices. On the basis of the GeSe4 vitreous composition, an optical fiber, single mode at 1.55 μm, is achieved.
Chalcogenide glasses in the [Ge-Se-S-As] system have been synthesized and studied with respect to their nonlinear optical properties from third and second order. Z-scan and Mach Zehnder interferometry measurements of the nonlinear refractive index (n2) and nonlinear absorption ((beta) ) have been performed at 1064 nm. Some z-scan measurements have been also realized at 1430 nm. The results have been correlated to the structures of the glasses and the figure of merit has been calculated with the purpose of a potential utilization of these glasses in the realization of ultra- fast all-optical switches. Nonlinearities as high as 850 times the nonlinearity of silica glass have been obtained and some glasses exhibit at 1430 nm nonlinear optical characteristics suitable for telecommunication applications. The all-optical poling of a chalcohalogenide glass has been realized with a Q-switch mode-locked Nd:YAG laser at 1064 nm emitting 45 ps pulses at a repetition rate of 10 Hz with frequency doubling at 532 nm. A nonlinear coefficient deff equals 2.8 10-17 m/V similar to that of the reference glass Schott SF 57 has been obtained. The thermal poling of a chalcogenide glass also has been realized and a transient second order nonlinear susceptibility (chi) (2) has been observed.
Second Harmonic Generation (SHG) using counter-propagating Quasi Phase Matching (QPM) configuration often
requires a fine non-linearity with a period of a fraction of a micrometer. The direct Electron Beam (EB) domain reversal
technique seems to be promising to achieve gratings on LiNbO3 with such a small period compared to other current poling
technologies as it is lithographic free and exhibits a very high theoretical resolution (5 mm).
We present here, a preliminary study concerning the influence of EB irradiation conditions on domain inversion;
SHG is also examined on a 6.58 j.tm inverted periodic domain grating in lithium niobate using a Nd-YaG laser light.
It is well known that LiNbO3 is an attractive material for many optical applications because of its transparency over
a large wavelength band (350 mm to 5000 mm) and its high nonlinear coefficients. EB irradiation is performed using a
Scanning Electron Microscope (SEM) driven by a Computer Aided Design (CAD) application developed in our laboratory.
Various structures such as periodic gratings can be written with the chosen period. The ferroelectric domain inversion is
investigated as a function of the accelerating voltage of the electrons (10 to 30 kV), the scanning speed of the EB (0.97 to
7.76 mm/s) and the current received by the sample (1.6 to 14 nA). The inversion patterns have been revealed by chemical
etching (HF) and they have been observed using SEM. It has been shown that the duty cycle is respected on the irradiated
face. The SHG conversion efficiency is measured as a function of the temperature and the grating period is calculated (value
of6.564 jim).
We report an experimental study of passive mode-locking of a flash-pumped Nd:YAG laser using for the first time a non- linear polarization rotation effect in a type II crystal cut for second harmonic generation. The mode-locking regime is self-starting and produces pulses of 20 ps duration with more than 100 uj per pulse. We developed a numerical model to calculate laser pulse steady-state temporal profile as a function of group velocity mismatch between fundamental and second harmonic pulses and group velocity dispersions in the nonlinear crystal. It takes also into account on laser medium and resonator characteristics insuring perfect second order dispersion compensation during one round trip in the cavity.
Self-guided waves that can be excited in quadratic nonlinear media have been extensively studied for their potential applications in ultra-fast all-optical processing. We have previously reported the use of solitary waves collision in a KTP crystal to experimentally demonstrate all-optical switching of IR picosecond pulses. Up to now, the intensity required to obtain self-trapping of a beam remained at a high level. This has been due to the lack of nonlinear crystals which combine the attributes of a large nonlinearity and phase-matching capability at experimentally convenient wavelengths. The availability of Periodically Poled Lithium Niobate can circumvent this difficulty. 2D spatial solitary waves in PPLN have been predicted theoretically and simulated numerically. In this communication we will report their experimental observation and for the first time their interaction in a 15mm long crystal. Then we will compare solitary wave behavior in KTP and PPLN, in particular self-trapping intensity threshold versus phase mismatch. We will also compare experimental data with the reslut of our computations modeling. In a last part we will show our first experimental result about 2D quadratic soliton collision in a PPLN crystal. Finally we will discuss the advantages of choosing PPLN to realize all- optical devices using solitary wave interactions.
We propose and demonstrate a new and simple technique for characterization of fast third order optical nonlinearity of wave-guides and bulk materials. Z-scan method, proposed and developed or bulk materials, has been preferred by experimentalists. It investigates nonlinear spatial distortion induced on a focused laser beam. In fact the technique that we present here, which we have called 'D- scan', is the temporal analog of the spatial Z-scan. It is based on spectral changes of a femtosecond pulse according to the dispersion preliminary introduced on the input femtosecond pulse by a dispersive delay line. The nonlinear evolution of the output spectrum when the dispersion introduced at the input is varied from negative to positive gives a sensitive measurement of the complex third order nonlinearity. The imaginary part of nonlinear susceptibility can be deduced from the evolution of the total average transmitted power versus the input dispersion. More, the plot of the position of center of the output spectrum may be exploited to measure the refractive index change time response. In order to demonstrate the capability of the proposed method we have experimentally retrieve the standard value of the nonlinear coefficient n2 of silica during characterization of a short samples of a single-mode optical fiber.
We review recent experimental, theoretical and numerical results dealing with the generation and the interaction of solitary waves in second-order nonlinear crystals. Particular emphasis is devoted to the issues related to the collisions between two-dimensional type II second-order solitons of orthogonal polarization in a KTP crystal. We report the experimental evidence that both quasi elastic and inelastic collision (when two solitons at input merge into a single one at output) are feasible, depending on the relative transverse velocity of the interacting beams.
We realized three different configurations for the production of picoseconds pulses in the UV by a diode-pumped solid-state laser. The first one is based on a diode-pumped Cr:LiSAF oscillator delivering tunable picosecond pulses in the near infrared. A Ti:Sapphire regenerative amplifier increased IR pulses energy up to 4.5 (mu) J. After doubling and tripling in two LBO crystals, we obtained pulses with an energy up to 0.14 (mu) J tunable between 273 and 286 nm. In the second scheme, based on a modelocked Nd:YAG laser, the SHG (KTP) and SFG (LBO) crystals are both inserted inside the cavity. The laser operated in a Q-switched mode-locked regime and produced 1.2 mW of UV average power in pulse of 60 picoseconds duration and 4 W peak power. In the last architecture, we used a resonant enhancement with a ring- cavity. We obtained 54% conversion efficiency in the green for picoseconds pulses at 1064 nm at 100 MHz rep. rate. A special arrangement was implemented for compensation of the birefringence and walk-off of the KTP crystal in the resonant cavity. Conversion to UV in the same cavity is in progress.
Passive mode-locking of a cw lamp pumped Nd:YAG laser with the nonlinear mirror technique is reported. Light pulses with 2W of average power and pulse duration shorter than 100 ps at 1064 nm have been obtained. The nonlinear mirror consists of a 3.5 mm long KTP frequency doubling crystal and a dichroic mirror with high reflectivity for the second harmonic and lower reflectivity for the fundamental frequency. The mode-locking process is self-starting.
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