90 W average power 100 microJ energy femtosecond fiber chirped-pulse amplification system

We report on an ytterbium-doped fiber based chirped-pulse amplification system delivering 100 microJ pulse energy at a repetition rate of 900 kHz, corresponding to an average power of 90 W. The emitted pulses are as short as 500 fs. To the best of our knowledge, this is the highest average power ever reported for high-energy femtosecond solid-state laser systems.     

High-power all-normal-dispersion femtosecond pulse generation from a Yb-doped large-mode-area microstructure fiber laser

We report on an all-normal-dispersion mode-locked fiber laser based on a large-mode-area Yb-doped microstructure fiber and using a high nonlinear modulation depth semiconductor saturable absorber mirror. The laser delivers 3.3 W of average output power with positively chirped 5.5 ps pulses at a center wavelength of 1033 nm. The pulse repetition rate is 46.4 MHz, which results in an energy per pulse of 71 nJ. These pulses are extracavity dechirped down to 516 fs by using bulk gratings. The average power of the dechirped pulses is 2.3 W, which corresponds to a peak power of more than 96 kW.     

Characterization of an ytterbium-doped double-clad fiber laser passively mode-locked by nonlinear polarization rotation

The properties of an ytterbium-doped double-clad fiber laser, passively mode-locked by nonlinear polarization rotation are investigated in this work. Cartographies of mode-locking regime versus halfwave plates orientations are presented for several values of the total cavity dispersion and for different pump powers. Bistability between the continuous and the mode-locking regimes is pointed out. The effect of the total group velocity dispersion is described with a master mode-locking equation. PACS 42.55.Wd; 42.65.Re     

Ultrashort pulse formation and evolution in mode-locked fiber lasers

Passive mode-locking in fiber lasers is investigated by numerical and experimental means. A non-distributed scalar model solving the nonlinear Schr?dinger equation is implemented to study the starting behavior and intra-cavity dynamics numerically. Several operation regimes at positive net-cavity dispersion are experimentally accessed and studied in different environmentally stable, linear laser configurations. In particular, pulse formation and evolution in the chirped-pulse regime at highly positive cavity dispersion is discussed. Based on the experimental results a route to highly energetic pulse solutions is shown in numerical simulations.     

Self-similar low-noise femtosecond ytterbium-doped double-clad fiber laser

We report the generation of self-similar highly stable femtosecond pulses from an ytterbium-doped double-clad fiber laser. Positively chirped parabolic pulses with 6.4-ps duration and more than 3.2-nJ energy are obtained. These pulses are extra-cavity compressed to 140 fs. Radio-frequency noise measurements show that this regime of emission ensures low-noise operation with less than 0.05% amplitude fluctuations. PACS 42.55.Wd; 42.65.Re     

High-energy femtosecond photonic crystal fiber laser

We report the generation of high-energy high-peak power pulses in an all-normal dispersion fiber laser featuring large-mode-area photonic crystal fibers. The self-starting chirped-pulse fiber oscillator delivers 11 W of average power at 15.5 MHz repetition rate, resulting in 710 nJ of pulse energy. The output pulses are dechirped outside the cavity from 7 ps to nearly transform-limited duration of 300 fs, leading to pulse peak powers as high as 1.9 MW. Numerical simulations reveal that pulse shaping is dominated by the amplitude modulation and spectral filtering provided by a resonant semiconductor saturable absorber.     

Polarization‐angle dependence of photoluminescence intensity of ordered GaInP2 layers: observation of polarization memory

We compare measured and calculated polarization‐angle dependencies of the intensity of the photoluminescence emission from MOVPE‐grown GaInP₂ layers with different ordering parameters. We measured the polarization‐angle dependencies of the emission propagating along the [001], [110] and [1 0] directions at room temperature. Symmetry considerations were used to calculate the dependence of the relative intensity of the PL emission which was linearly polarized along different directions and to estimate the value of the valence‐band splitting by fitting the measured dependencies with calculated curves. An intriguing influence of the polarization of the exciting beam on the relative amount of the polarized PL emission was observed in the emission from the (110) plane. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and morphological studies on the deformation behavior of polypropylene/multi‐walled carbon nanotubes nanocomposites prepared through ultrasound‐assisted melt extrusion process

Structural and morphological behavior under stress–strain of polypropylene/multi‐walled carbon nanotubes (PP/MWCNTs) nanocomposites prepared through ultrasound‐assisted melt extrusion process was studied by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, small angle X‐ray scattering (SAXS), and wide angle X‐ray scattering (WAXS). A high ductile behavior was observed in the PP/MWCNT nanocomposites with low concentration of MWCNTs. This was related to an energy‐dissipating mechanism, achieved by the formation of an ordered PP‐CNTs interphase zone and crystal oriented structure in the undeformed samples. Different strain‐induced‐phase transformations were observed by ex situ SAXS/WAXS, characterizing the different stages of structure development during the deformation of PP and PP/MWCNTs nanocomposites. The high concentration of CNTs reduced the strain behavior of PP due to the agglomeration of nanoparticles. A structural pathway relating the deformation‐induced phase transitions and the dissipation energy mechanism in the PP/MWCNTs nanocomposites at low concentration of nanoparticles was proposed. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 475–491     

Full-wave modeling of piezoelectric transducers for SAW acousto-optical interactions

In this paper we present an accurate and efficient numerical method for a rigorous full-wave analysis of interdigital transducers (IDT) for the excitation of surface acoustic waves on the piezoelectric substrate of acousto-optical devices. The problem is formulated in terms of an integral equation that is solved by the method of moments. The transducer input admittance and the power coupling factors to both surface and bulk waves are computed. Numerical results for some configurations of X-Y LiNbO₃ IDT for acousto-optic applications are in very good agreement with measured data. It is pointed out that bulk wave excitation may be a serious limitation in the design of efficient, wide band transducers for acousto-optical devices.     

Role of matrix crystallinity in carbon nanotube dispersion and electrical conductivity of iPP‐based nanocomposites

A homopolymer iPP and a series of propylene‐ethylene random copolymers with a content of ethylene from 7 to 21 mol % were used as matrices to prepare single‐walled carbon nanotube (SWCNT) nanocomposites in a range of SWCNT concentration from 0.15 to 1 wt %. The solution blending and melt‐ compression molding procedures were kept identical for all nanocomposites. The poly(propylenes) have crystallinities ranging from 70 to 10%, and serve to test the role of SWCNTs acting as nucleants to preserve in the nanocomposites the uniform dispersion of SWCNTs after sonication. The major role of polymer crystallinity is to mediate toward a more open and more connected SWCNT network structure. Fast nucleation and growth of high crystalline matrices on multiple sites along the surface of the nanotubes prevents SWCNT clustering, and entraps the SWCNT network between the semicrystalline structure reducing the driving force of nanotubes to curl and twist. Depletion of crystallites in the less crystalline matrices (<35% crystallinity) leads to curled and poorly connected nanotubes. A consequence of the gradual loss of SWCNT connectivity is a decreased electrical conductivity; however, the change with crystallinity is not linear. Conductivity decreases sharply with decreasing crystallinity for SWCNT contents near the percolation region, while for contents approaching the plateau region the electrical conductivity is less sensitive to matrix crystallinity. The percolation threshold decreases rapidly for polymers with <～30% crystallinity and slowly levels off at crystallinities >～40%. At SWCNT concentrations of 0.15 wt %, SEM images of nanocomposites with the highest crystallinity matrix indicate debundled and interconnected nanotubes, whereas more disconnected and curled SWCNTs remain in the lowest crystallinity nanocomposites. Electrical conductivity in the former is relatively high, whereas the latter are insulators. Also discussed is the nucleating effect of nanotubes and restrictions of the filler to polymer chain diffusion in the crystallization of the polymers. SEM images and Raman spectra in the radial breathing modes region (100–400 cm^?1) are complementary tools to extract the quality and details of the SWCNT dispersion in the nanocomposites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2084–2096, 2010