Raman-induced pulse destabilization and bistability in an all-normal dispersion oscillator.

We report the experimental study of spectral modulations induced by a stimulated Raman scattering process in an all-fiber all-normal dispersion oscillator. With the use of dispersive Fourier transform, we recorded a series of single-shot spectra of consecutive laser pulses. The data indicate that the Raman process destabilizes the long-wavelength part of the laser pulse spectrum without disrupting the single-pulse generation regime. Our experiments revealed also that the oscillator displayed bistable operation for high pump powers. Two stable dissipative soliton mode-locked states were observed, together with output power hysteresis.

Nonlinear polarization evolution of ultrashort pulses in polarization maintaining fibers.

We examine properties of an ultrashort laser pulse propagating through an artificial Saturable Absorber based on Nonlinear Polarization Evolution device which has been realized with Polarization Maintaining fibers only (PM NPE). We study and compare in-line and Faraday Mirror geometries showing that the latter is immune to errors in the PM NPE construction. Experimental results for the transmission measurements of the PM NPE setup for different input linear polarization angles and various input pulse powers are presented. We show that PM NPE topology is of crucial importance for controlling the properties of the output pulse as it rules the contribution of cross-phase modulation to an overall nonlinear phase change. We also demonstrate an excellent agreement between the numerical model and experimental results.

Influence of the excitation light intensity on the rate of fluorescence quenching reactions: pulsed experiments.

The effect of multiple light excitation events on bimolecular photo-induced electron transfer reactions in liquid solution is studied experimentally. It is found that the decay of fluorescence can be up to 25% faster if a second photon is absorbed after a first cycle of quenching and recombination. A theoretical model is presented which ascribes this effect to the enrichment of the concentration of quenchers in the immediate vicinity of fluorophores that have been previously excited. Despite its simplicity, the model delivers a qualitative agreement with the observed experimental trends. The original theory by Burshtein and Igoshin (J. Chem. Phys., 2000, 112, 10930-10940) was created for continuous light excitation though. A qualitative extrapolation from the here presented pulse experiments to the continuous excitation conditions lead us to conclude that in the latter the order of magnitude of the increase of the quenching efficiency upon increasing the light intensity of excitation, must also be on the order of tens of percent. These results mean that the rate constant for photo-induced bimolecular reactions depends not only on the usual known factors, such as temperature, viscosity and other properties of the medium, but also on the intensity of the excitation light.

Ultrafast laser mode-locked using nonlinear polarization evolution in polarization maintaining fibers.

We demonstrate an all-fiber ultrafast ytterbium laser oscillator mode-locked by means of a nonlinear polarization evolution (NPE) method realized in polarization-maintaining (PM) fibers. A sequence of the PM fiber pieces is shown to perform NPE action while maintaining a required temporal overlap of the ordinary and extraordinary pulses propagating through it. We present details of simple numerical simulations showing the advantage of the proposed scheme of segmented PM fibers. The laser utilizing the above mentioned design which generates ultrashort pulses at a 20.54 MHz repetition rate with the dechirped pulse duration around 150 fs and a pulse energy of 0.85 nJ is also presented.

Simple all-PM-fiber laser mode-locked with a nonlinear loop mirror.

In this Letter, we present a figure-eight all-PM-fiber laser oscillator design with a nonlinear optical loop mirror as an artificial saturable absorber. Unlike previous constructions using the same mode-locking technique, our cavity is constructed entirely of polarization-maintaining (PM) fibers, making the oscillator more resistant to thermal and mechanical perturbations. Two simple and robust laser configurations that differ by the output coupling ratio (70% or 30%) are presented. The first configuration delivers high energy pulses of 3.5 nJ, and the second configuration delivers pulses of 1.6 nJ at a common repetition rate of 15 MHz. In either configuration, the pulsed operation is stable, and the laser operates in a single pulse train regime, even for pump powers approaching twice the power required for mode-locking. We have also observed that, at higher intracavity powers, stimulated Raman scattering plays a significant role.