Rémi Habert

Development of a real-time flexible multiphoton microendoscope for label-free imaging in a live animal

We present a two-photon microendoscope capable of in vivo label-free deep-tissue high-resolution fast imaging through a very long optical fiber. First, an advanced light-pulse spectro-temporal shaping device optimally precompensates for linear and nonlinear distortions occurring during propagation within the endoscopic fiber. This enables the delivery of sub-40-fs duration infrared excitation pulses at the output of 5 meters of fiber. Second, the endoscopic fiber is a custom-made double-clad polarization-maintaining photonic crystal fiber specifically designed to optimize the imaging resolution and the intrinsic luminescence backward collection. Third, a miniaturized fiber-scanner of 2.2 mm outer diameter allows simultaneous second harmonic generation (SHG) and two-photon excited autofluorescence (TPEF) imaging at 8 frames per second. This microendoscope’s transverse and axial resolutions amount respectively to 0.8 μm and 12 μm, with a field-of-view as large as 450 μm. This microendoscope’s unprecedented capabilities are validated during label-free imaging, ex vivo on various fixed human tissue samples, and in vivo on an anesthetized mouse kidney demonstrating an imaging penetration depth greater than 300 μm below the surface of the organ. The results reported in this manuscript confirm that nonlinear microendoscopy can become a valuable clinical tool for real-time in situ assessment of pathological states.

20 THz-bandwidth continuous-wave fiber optical parametric amplifier operating at 1 µm using a dispersion-stabilized photonic crystal fiber

We report the experimental demonstration of a continuous-wave all-fiber optical parametric amplifier in the 1 µm band with a record bandwidth of more than 20 THz and a peak gain of almost 40 dB. This is achieved by using a photonic crystal fiber with a high figure of merit and strongly reduced longitudinal dispersion fluctuations. Due to their unique bandwidth and gain characteristics, fiber parametric amplifiers at 1 µm provide an interesting alternative to solid-state or ytterbium-doped fiber amplifiers for ultrafast optical pulse and signal processing.

Spectral and fluorescence lifetime endoscopic system using a double-clad photonic crystal fiber

We present a customized small-core double-clad photonic crystal fiber for spectral and fluorescence lifetime measurements of human samples. In this Letter, the new fiber has been characterized on different fluorophores and samples of human brain tumor; a comparison to a bi-fiber homemade system and a commercial fiber probe was made.

Widely Tunable Parametric Amplification and Pulse Train Generation by Heating a Photonic Crystal Fiber

We describe a simple technique that allows for the achievement of widely tunable parametric generation in a photonic crystal fiber. This is achieved by heating the fiber and using a specific phase-matching condition in the normal dispersion regime which is highly sensitive to temperature. Experimental results show a sideband tunability of about 17 THz in the 800 nm and 1550 nm spectral bands by heating the fiber from room temperature to 500 , leading to a tuning rate of 34 . By adding a small tunable continuous-wave seed together with the pulsed pump, we further show the generation of sub-nanosecond pulses tunable around 800 nm through parametric amplification.

Characterization of fiber ultrashort pulse delivery for nonlinear endomicroscopy

In this work, we present a detailed characterization of a small-core double-clad photonic crystal fiber, dedicated and approved for in vivo nonlinear imaging endomicroscopy. A numerical and experimental study has been performed to characterize the excitation and collection efficiencies through a 5 m-long optical fiber, including the pulse duration and spectral shape. This was first done without any distal optics, and then the performances of the system were studied by using two kinds of GRIN lenses at the fiber output. These results are compared to published data using commercial double clad fibers and GRIN lenses.

Top-hat beam output with 100 μJ temporally shaped narrow-bandwidth nanosecond pulses from a linearly polarized all-fiber system.

We report on an all-fiber system delivering more than 100 μJ pulses with a top-hat beam output in the few nanoseconds regime at 10 kHz. The linearly polarized flattened beam is obtained thanks to a 3-mm-long single-mode microstructured fiber spliced to the amplifiers output.

Temperature Dependence of the Zero Dispersion Wavelength in a Photonic Crystal Fiber

We present here a measurement of the temperature dependence of the zero dispersion wavelength in an air/silica photonic crystal fiber. This is done through a simple method based on a fourth-order four-wave mixing process by pumping the fiber in a low normal dispersion regime. The zero dispersion wavelength is found to increase by about 10 nm from room temperature to 250°C. This result is in good agreement with numerical simulations which demonstrate that it is mainly due to the thermo-optic effect. We found that the zero-dispersion wavelength evolution is not linear with temperature and that the shift rate is continuously increased from 39 pm/°C at room temperature to 63 pm/°C at 250°C.

Polarization maintaining single-mode fiber delivering a flat top intensity profile

We report, through numerical simulations and experimental data, the first successful fabrication of a polarization maintaining single-mode fiber delivering a flat top intensity profile at 1.05 µm. A high quality flat mode was obtained and single-mode behavior was checked by shifting the injection and by S² imaging method. Numerical investigations were performed to show that it would be possible to increase further the 0.6x10⁻⁴ experimental group birefringence.

Active reduction of fluctuations in fourth-order modulation instability

We experimentally study the fluctuation properties of a scalar fourth-order modulation instability (MI) process obtained by pumping a photonic crystal fiber in the normal dispersion region. We observe large wavelength-dependent pulse-to-pulse fluctuations that cannot be significantly reduced by stimulating the process with a single seed. Their reduction requires two seeds slightly detuned from the maximum gain frequency in order to also stimulate the second-order MI process cascaded from the fourth-order one. This concept is validated by experiments and numerical simulations.

Ultra large mode area pixelated Bragg fiber

We report on the design and the fabrication of a new design of an all-solid Bragg fiber based on the pixelization and heterostructuration of a cladding made of only two high index rings. The thickness of the low index ring as well as the geometry of the heterostructuration (its symmetry and the number of removed pixels) have been chosen to maximize the confinement losses of the Higher Order Modes (HOM) (above 10 dB/m) while keeping the Fundamental Mode (FM) losses low (below 0.1 dB/m). The proposed geometry allows having access to different Mode Field Diameter (MFD) from 54 μm to 60 μm at 1 μm wavelength by drawing the same stack to different fiber (and hence, core) diameters. As a result, a record MFD of 60 μm is reported for a Solid Core Photonic Bandgap Fiber (SC-PBGF) and single-mode behavior is obtained experimentally even for a short fiber length (few tens centimeters) maintained straight.