Experimental evolution of the temporal and spectral profiles of noise-like pulses within the mode-locked regions of a figure-eight fiber laser.

We present an experimental analysis of the pulse profile variability within the mode-locked regions of an erbium-doped figure-eight fiber laser (EDFEFL). The tuning of the mode-locked regions was carried out by varying and recording the values of the angle of the polarization controllers in the ring section and in the nonlinear optical loop mirror (NOLM). Within the mode-locked regions, we obtained a large variability of the temporal profile, specifically amplitude and width of the noise-like pulses (NLPs). Subsequently, we recorded and studied the changes in the spectral domain. We identified the mode-locked regions where the temporal profile of the pulse remains constant (stationary state), and where it expels sub-packets (non-stationary state). Finally, a theoretical analysis of the power transmission through the polarizing in the ring section and in the NOLM switching characteristic as a function of wave plate angles is also performed, which allows an understanding of the existence of the multiple mode-locked regions and pulse profile adjustability. We analyze NLPs with a carrier wavelength of 1560 nm with duration of the order of nanoseconds and a repetition rate of 0.9 MHz.

Generation, characterization, and experimental analysis of noise-like pulse envelopes with complex shapes.

We present the generation of 41 noise-like pulse (NLP) envelopes with complex shapes using a passively mode-locked, erbium-doped figure-eight fiber laser (EDFEFL). The tuning of each of the complex forms was carried out by varying the polarization state within the laser cavity, using a quarter-wave retarder (QWR2) inside a nonlinear optical loop mirror (NOLM), which is part of the EDFEFL. The position of the retarder plate was identified and recorded for each of the complex shapes. The temporal and spectral characterization was done using the position of the WQR2 wave plate as an independent variable. We present a single-shot analysis of the dynamics for the temporal amplitude, the full width at half-maximum (FWHM), and the root-mean-square (RMS) width for each of the 360 cycles measured for the 41 complex envelopes. We also perform an analysis for the case in which the pulse is completely divided into subpackets. We analyze the corresponding spectral profile for each of the complex forms generated. Finally, we evaluate the performance of the NOLM theoretical model with our experimental results. The wavelength of the NLPs is 1560 nm; the period of the cavity fiber laser is 1.1 ms; and the temporal FWHM is within the range of nanoseconds.

Universal route to optimal few- to single-cycle pulse generation in hollow-core fiber compressors.

Gas-filled hollow-core fiber (HCF) pulse post-compressors generating few- to single-cycle pulses are a key enabling tool for attosecond science and ultrafast spectroscopy. Achieving optimum performance in this regime can be extremely challenging due to the ultra-broad bandwidth of the pulses and the need of an adequate temporal diagnostic. These difficulties have hindered the full exploitation of HCF post-compressors, namely the generation of stable and high-quality near-Fourier-transform-limited pulses. Here we show that, independently of conditions such as the type of gas or the laser system used, there is a universal route to obtain the shortest stable output pulse down to the single-cycle regime. Numerical simulations and experimental measurements performed with the dispersion-scan technique reveal that, in quite general conditions, post-compressed pulses exhibit a residual third-order dispersion intrinsic to optimum nonlinear propagation within the fiber, in agreement with measurements independently performed in several laboratories around the world. The understanding of this effect and its adequate correction, e.g. using simple transparent optical media, enables achieving high-quality post-compressed pulses with only minor changes in existing setups. These optimized sources have impact in many fields of science and technology and should enable new and exciting applications in the few- to single-cycle pulse regime.

Effects of primary spherical aberration, coma, astigmatism and field curvature on the focusing of ultrashort pulses: homogenous illumination.

We analyze the spatiotemporal intensity of pulses with durations of 20 fs and shorter and a carrier wavelength of 810 nm at the paraxial focal plane of an achromatic doublet lens. The incident pulse is well-collimated, and we use the Seidel aberration theory for thin lenses to evaluate the phase change due to the aberrations of the lens. In a set of cemented thin lenses with the stop at the lens, there is only spherical aberration, coma, astigmatism and field curvature, whereas the distortion aberration in the phase front is zero. We analyze the effect of these aberrations in the focusing of ultrashort pulses for homogenous illumination. We will show that the temporal spreading introduced by these aberrations in pulses shorter than 20 fs at 810 nm is very small but the spatial spreading is not, which reduces the intensity of the pulse considerably.

Effects of primary spherical aberration, coma, astigmatism, and field curvature on the focusing of ultrashort pulses: Gaussian illumination and experiment.

We analyze the spatiotemporal intensity of Gaussian temporal envelope pulses with initial durations of 200 fs and a carrier wavelength of 810 nm at the paraxial focal plane of an achromatic doublet lens for a well-collimated incoming pulse beam by using the Seidel aberration theory for thin lenses with the stop at the lens. We analyze the effect of these aberrations in the focusing of ultrashort pulses for Gaussian illumination and present experimental results for 200 fs pulses focused by a near-IR achromatic doublet.