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.
High resolution stellar interferometers are very powerful efficient instruments to get a better knowledge of our Universe through the spatial coherence analysis of the light. For this purpose, the optical fields collected by each telescope T<sub>i</sub> are mixed together. From the interferometric pattern, two expected information called the contrast <i>C<sub>ij</sub></i> and the phase information φ<i><sub>ij</sub></i> are extracted. These information lead to the <i>V<sub>ij</sub></i>, called the complex visibility, with <i>V<sub>ij</sub></i>=<i>C<sub>ij</sub>exp(jφ<sub>ij</sub>)</i>. For each telescope doublet <i>T<sub>i</sub>T<sub>j</sub></i>, it is possible to get a complex visibility <i>V<sub>ij</sub></i>. The Zernike Van Cittert theorem gives a relationship between the intensity distribution of the object observed and the complex visibility. The combination of the acquired complex visibilities and a reconstruction algorithm allows imaging reconstruction.<p> </p>To avoid lots of technical difficulties related to infrared optics (components transmission, thermal noises, thermal cooling…), our team proposes to explore the possibility of using nonlinear optical techniques. This is a promising alternative detection technique for detecting infrared optical signals. This way, we experimentally demonstrate that frequency conversion does not result in additional bias on the interferometric data supplied by a stellar interferometer.<p> </p>In this presentation, we report on wavelength conversion of the light collected by each telescope from the infrared domain to the visible. The interferometric pattern is observed in the visible domain with our, so called, upconversion interferometer. Thereby, one can benefit from mature optical components mainly used in optical telecommunications (waveguide, coupler, multiplexer…) and efficient low-noise detection schemes up to the single-photon counting level.
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.
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
We present theoretical and experimental studies of both scalar and polarization or modal pump-divided parametric
amplification in photonic crystal fibers. In the scalar case, we discuss broadband parametric amplification at telecom
wavelengths near 1550 nm. With a pump-divided scattering process, we discuss the possibility of widely tunable
frequency conversion and four-wave mixing gain at visible wavelengths. We confirmed the theory by experiments where
intense, linearly polarized pump pulses at wavelengths ranging from 532 to 625 nm led to the spontaneous generation of
modulation instability sidebands with frequency shifts ranging from 3 up to 63 THz. The observations were in good
agreement the experimental characterization and theoretical modelling ofthe linear and nonlinear properties of the PCF.
We study the interplay between parametric and Raman gain in photonic crystal fibers by taking into account the vector nature of the electric field, the fiber frequency dependent birefringence, the Kerr nonlinear coefficient, the Raman gain profile and chromatic dispersion. In particular, we show that an accurate representation of the frequency dependence of the nonlinear and dispersive properties of a photonic crystal fiber is essential for correctly describing the overall gain profile for a probe signal at large frequency detuning from a continuous wave or pulsed pump. For example, we found that fourth and higher order dispersion have a striking influence on the spectrum of modulation polarization instability gain in both the high and low birefringence regime, in that the vector parametric gain is suppressed above a critical level of linear birefringence. We validated the theory by experimental observations of vector parametric amplification in high birefringence holey fiber with triple defects.