Towards two-photon excited endogenous fluorescence lifetime imaging microendoscopy.

In situ fluorescence lifetime imaging microscopy (FLIM) in an endoscopic configuration of the endogenous biomarker nicotinamide adenine dinucleotide (NADH) has a great potential for malignant tissue diagnosis. Moreover, two-photon nonlinear excitation provides intrinsic optical sectioning along with enhanced imaging depth. We demonstrate, for the first time to our knowledge, nonlinear endogenous FLIM in a fibered microscope with proximal detection, applied to NADH in cultured cells, as a first step to a nonlinear endomicroscope, using a double-clad microstructured fiber with convenient fiber length (> 3 m) and excitation pulse duration (≈50 fs). Fluorescence photons are collected by the fiber inner cladding and we show that its contribution to the impulse response function (IRF), which originates from its intermodal and chromatic dispersions, is small (< 600 ps) and stable for lengths up to 8 m and allows for short lifetime measurements. We use the phasor representation as a quick visualization tool adapted to the endoscopy speed requirements.

Compact direct space-to-time pulse shaping with a phase-only spatial light modulator.

A very compact and innovative pulse shaper is proposed and demonstrated. The standard architecture for pulse shaping that is composed of diffraction gratings associated with an amplitude-phase spatial light modulator (SLM) is replaced by a single phase-only SLM. It acts as a pulse stretcher and as an amplitude and phase modulator at the same time. Preliminary experiments demonstrate the accurate control of amplitude and phase of shaped pulses.

Coherent combining of 49 laser beams from a multiple core optical fiber by a spatial light modulator.

We report co-phasing of 49 beams from a multicore fiber based on a spatial light modulator using a simple feedback loop control. A specific fiber of 7x7 Germanium doped cores was fabricated. The power carried by the far field main lobe of the phased array, in the continuous wave regime, amounts to 96% of its theoretical value. Femtoseconde pulses were also phase-locked with the same device configuration.

Coherent femtosecond pulse shaping for the optimization of a non-linear micro-endoscope.

A flexible multicore fiber bundle is fed by temporally and spectrally shaped femtosecond pulses allowing for the pre-compensation of both chromatic dispersion and non-linear effects encountered in the bundle. We demonstrate that the pulse duration at the fiber bundle output can be significantly reduced in comparison with linear pre-compensation only. The scheme for femtosecond pulse fiber delivery is applied to the optimization of two-photon fluorescence (TPF) imaging. Experiments and calculations show a five-fold improvement of the TPF signal produced at the end of the fiber bundle in comparison with linear pre-compensation. This is applied to the recording, in real time (12 image/s), of TPF laser-scanning images of human colon cells stained with a fluorescent marker. Further optimizations are discussed.

Coherent measurement of short laser pulses based on spectral interferometry resolved in time.

We propose a new method for measuring the phase and the amplitude of a short laser pulse that is based on shearing interferometry in the spectral domain combined with time gating. The method has several features in common with spectral interferometry for direct electric-field reconstruction, in particular, fast and direct reconstruction of the phase. Accurate measurement of the phase added to an 80-fs pulse by a block of F4 glass demonstrates the technique.

Dispersive-scan measurement of the fast component of the third-order nonlinearity of bulk materials and waveguides.

A new, simple method to measure the nonlinear refraction and absorption of bulk material and waveguides is proposed. The method relies on the evolution of the intensity and width of femtosecond-pulse spectra, owing to self-phase modulation, as a function of variable chirp and stretching introduced at the input.

Self-optimized resonator for optical pumping of solid-state lasers.

We show that the proper use of a self-imaging confocal cavity can be a simple way to optimize the efficiency of optical pumping of solid-state lasers. In this type of resonator the transverse mode self-adjusts to fit the pumped volume optimally. Self-optimization of the mode size of a laser-diode pump Nd:YAG laser (54% slope efficiency) is demonstrated, and an intrinsic efficiency of 68% is measured.

Interferometric measurement of the temporal transfer function of a single-shot picosecond streak camera.

Temporal beats between two separate pulsed lasers of different adjustable frequency are used as a means of measuring the temporal transfer function of a single-shot picosecond streak camera. We describe two parallel pulse generators that use tunable distributed-feedback dye lasers. These lasers produce two pulses of different frequency that interfere at the camera. The temporal fringe contrast at the camera output provides a direct measure of its transfer function. Fringe spacing combined with spectroscopic measure of the laser wavelengths provides a calibration of the temporal scale.

Picosecond streak camera display of an intermodal coupling matrix at a multimode fiber splice.

We have developed a technique that permits direct visualization of the coupling matrix for all guided modes of a moderately multimode fiber optic at a splice or mode coupler. The matrix is formed by an array of spots at the output of a picosecond streak camera. The technique also permits unambiguous determination of the phase velocity and group velocity of the modes.