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We demonstrate the use of the Z-scan technique as a method to measure a chemical parameter, the pK(a) value, that characterizes the equilibrium constant in acid-base reactions. The measurements were performed with picosecond pulses at 532 nm in Fe(III)-meso-tetrakis(4-N-methyl-pyridyl) porphyrin. The results were compared with linear absorption measurements and electron paramagnetic resonance, the usual techniques employed to determine pK(a) values in aqueous solutions, and showed excellent agreement.

Nature 368, 436-438 (1994) WE omitted to acknowledge that A.S.L.G. was partially funded by the CNPq Brazilian Agency during this work, undertaken while on sabbatical leave from the Departamento de Fisica, Universidade Federal de Pernambuco, Recife PE 50730, Brazil.

We demonstrate an order-of-magnitude increase in the efficiency for first-Stokes Raman generation in a gas cell (CH(4)) by seeding with spontaneous emission from a dye cell. Second-Stokes and first-anti-Stokes lines were also generated more efficiently. The pump energy required for production of a 1% conversion efficiency for the first-Stokes emission is reduced by a factor of 1.8 below that required without seeding.

An all-optical power-controlled switch is described. It is based on the modulation of self-diffracted beams by a control beam that modifies the third-order grating excited in a nonlinear medium. The experimental results obtained by using Cd(S, Se)-doped glasses are in agreement with the theoretical predictions. For an application, we also report the generation of dark pulses with subnanosecond width based on the studied process.

Wave-mixing and Z-scan experiments were performed in a study of the influence of the electronic trapping states on the nonlinear behavior of semiconductor-doped glasses. The origin of the optical nonlinearity for nanosecond light pulses whose frequencies were smaller than the sample band-gap frequencies was identified, and the densities of deep traps in the studied samples were measured.

We describe an analytical solution of the modified nonlinear Schrodinger equation with the inclusion of the self-steepening term. The results are applied, in particular, to the case of pulse propagation in optical fibers; the role of anomalous dispersion is analyzed, and asymmetric temporal behavior is seen.

A new method for measuring the phase of the third-order susceptibility chi((3)) that is based on the phenomenon of polarization beats is proposed and demonstrated. As a first application, the method was employed to study the phase dispersion of chi((3)) in Cd(S, Se)-doped glasses.

An extra-cavity pulse narrowing technique is demonstrated in which a frequency-swept optical pulse is compressed by the application of a spectral window. The pulses from a cw mode-locked Nd:YAG laser were passed through an optical fibre in a regime where self-phase modulation was the dominant nonlinear mechanism, and group velocity dispersion and stimulated Raman scattering were negligible. A 3-fold compression ratio was obtained, in good agreement with theoritical predictions.

Spectral and temporal investigations of cascade, picosecond, stimulated Raman scattering in SiO2 based single mode optical fibre with a high (∼7 mol%) concentration of P2O5 is reported. At a pump wavelength of 1.06 μm and for peak powers of ∼25 kW, enhancement in the spectrum at a frequency shift ∼ 1300 cm-1 was observed due to the overlap of the 1st Stokes of the P=0 vibrational mode (∼ 1330 cm-1) and the third Stokes (3x440 cm-1) of the Si-O-Si vibrational mode. This process provides the generation of a powerful source of picosecond pulses at 1.24 μm. For higher peak powers launched into the fibre (∼ 60kW) an intense continuum extending up to 1.6 μm was recorded.

Results are presented on the operation and optimization of soliton Raman fibre-ring lasers using a c.w. mode-locked Nd-YAG laser at 1.32μm as the source of 100 ps pump pulses. Various lengths of standard single-mode silica fibre were used, and tunable pulses as short as 100 fs generated.

Through optimisation of the spectrum and dispersion parameters of a synchronously mode locked Rhodamine B dye laser, pulses as short as ∼270 fs have been directly generated without using any saturable absorber or pulse compressor. In addition a system of tunable output coupling with well characterised dispersion has been demonstrated.

The evolution of chirped soliton pulses relaunched into single mode fibres in the region of anomalous dispersion has been examined experimentally under conditions where the bandwidth of the launched soliton pulse was up to seven times that of the transform limit. After relaunch, the soliton rapidly broadened approaching the condition where further broadening was solely due to transmission losses. The results are in good qualitative agreement with recent theoretical predictions.

Temporally compressed pulses from a c.w. mode-locked Nd: YAG laser or Q-switched and mode-locked system have been used as the pump source for a novel optically biased synchronously excited dye laser and a high-power dye laser respectively. The former dye laser produced pulses of 1·5 ps and an average power of ~ 10 mW, while the latter generated 4 ps pulses with peak powers of 20 kW in an unoptimized cavity.

In this Letter, we report on the generation of pulses as short as 18 fs by means of an optimized two-stage optical fibre-grating pair and high-order soliton-effect pulse compressor. These are the shortest pulses yet reported in the near-infrared spectral region and correspond to four optical cycles at 1·32 μm.

We discuss the behavior of a simple, easy to construct, mechanical oscillator subject to local stability. The analysis stresses the physical and intuitive aspects of the problem introducing the concepts of soft modes, order parameter, exchange of stability, and response function used in condensed matter physics.

Using the frequency doubled output of the 3 ps pulses from a temporally compressed cw Q-switched and mode-locked Nd:YAG laser, a simple synchronously pumped dye laser was constructed to give frequency tunable operation with short pulses in the 5–10 ps range and peak powers of ∼ 15 kW. A circularly scanning streak camera operating in stroboscopic mode was also used to examine pulse formation in the dye laser.

An all optical fibre pulse compressor operating in the femtosecond regime is presented for the first time. Commencing with 100 ps pulses from a cw mode locked Nd: YAG laser operating at 1.3 μm, pulses of 130 fs were generated in a standard single mode fibre through a single pass multi soliton Raman compression. These pulses were further compressed in a two stage dispersion shifted fibre arrangement. Pulses as short as 65 fs were generated.

An experimental study of temporally compressed CW Nd:YAG laser pulses using an optical fibre and diffraction grating pair technique has been carried out. The relevant experimental parameters have been varied so that a better insight into the physical processes could be gained and to enable some comparisons to be made with theory. Under optimum conditions an overall 47 × pulsewidth compression and 10 × enhancement in peak power have been achieved. Changes in fibre length, peak power and grating separation gave rise to outputs which showed good qualitative agreement with theoretical predictions.

Intense green and red components generated in a non-polarization preserving optical fibre using a Q-switched and mode-locked cw Nd: YAG laser have been examined spectrally and temporally with picosecond resolution. In addition to the red (four-photon interaction) and green (second-harmonic) components, many other frequency mixing processes were detected. From theoretical considerations, the red component (anti-Stokes) which is generated through parametric four photon mixing should be in the same propagation mode as the Stokes component (ir) and this was confirmed experimentally.

This article reviews the current state-of-the-art of optical pulse compressors based on the optical fibre-grating pair. As this technique has become a standard laboratory procedure providing pulses with duration shorter than those directly generated from mode-locked lasers, we review the basic mechanism of the method, the role played by additional non-linear effects (such as stimulated Raman scattering), describe the current results, with emphasis to our own work, and point out the limitations of the technique.

The power-dependent pulsewidth variations for input 85 ps pulses from a cw mode-locked Nd: YAG laser propagating through 125 m of single-mode optical fibre have been examined using a Synchroscan streak camera. Simultaneous spectral and temporal measurements provide information as to the optimum parameters for pulse compression in fibres.

The 85 ps pulses from a cw mode locked Nd:YAG laser, Q-switched at rates of ≈ 1 kHz have been compressed using a now standard fibre-grating pair arrangement, to give pulses of ≈ 3 ps measured using autocorrelation methods. By employing a circularly scanning streak camera operated in stroboscopic mode, the variation in the compressed pulsed duration through the pulse train has been observed.

Soliton-Raman processes are described in single pass and synchronously pumped ring geometries using standard single-mode, silica based fibres for the generation of kilowatt pulses with durations of 100 fs-200 fs tunable from 1.3-1.45 μm. Using dispersion shifted fibres, similar operation has been demonstrated with a cascade-Raman soliton shaping mechanism in fibre ring oscillators, providing tunability to similar 1.7 μm.

Compression par un facteur de 60. Utilisation d'un laser YAG: Nd. Generation d'impulsions de 1,4 ps a partir d'impulsion de 85 ps

The use of a small ultrafast electrical signal differentiator is described. Its application in conjunction with photoconductive elements in the measurement of mode-locked laser pulses with picosecond resolution has been investigated.

Etude de la nitrification de NH 4 -N en fonction des apports en nitrapyrine, de la temperature du sol, et du temps depuis lequel l'engrais a ete applique

Using a conventional cw mode-locked Nd:YAG laser operating at 1.06 μm, soliton-like pulses with powers ∼kW and durations less than 200 fs have been generated in a high order cascade Raman fibre ring laser operated around 1.5 μm.

Both single-shot and repetitively-operating streak cameras have been used for direct studies of the optical pulses produced by a simultaneously Q-switched and mode-locked cw Nd:YAG laser. Without prelase the mode-locked pulses are observed to be merely highly structured noise-bursts having durations of a few hundred picoseconds while with prelase, short (∼100 ps) stable pulses can be obtained at Q-switch repetition rates in excess of 1 kHz. These pulses are compared to those obtainable from the mode-locked laser without Q-switching.

Le systeme a la composition suivante: 94,5 SiO 2 -5,0 GeO 2 -0,5 P 2 O 5 . Les proprietes du titre sont etudiees par spectrometrie de fluorescence et par microscopie electronique a transmission analytique. Il se forme des clusters lorsque la concentration en Nd(III) est au-dessus de 10 3 ppm

Stimulated Raman scattering of Q -switched and mode-locked Nd:YAG laser pulses at 1.06 μm in a 120-m-long single-mode fiber has been investigated experimentally. A Photochron IIA Synchrosan streak camera with picosecond resolution was used to give time resolved detection of the first four order Stokes-Raman pulses at 1.12, 1.18, 1.24, and 1.31 μm. It has been shown that in traversing the fiber the fundamental and the first two order Stokes were substantially shortened and that the Stokes pulses were asymmetrical with sharper leading edges.

We report the hybrid modelocking of the Rhodamine B dye laser for the first time. Pulses as short as ∼430 fs have been generated from a simple linear cavity with a single dye jet. The demonstration of a new cw saturable absorber is reported.

We have demonstrated the generation of pulses of 1.4 ps duration by the synchronous mode-locking of a Rh6G dye laser using as little as 10 mW average pump power from the frequency doubled output of a CW mode-locked Nd:YAG laser. The dye laser was optically biased using an argon ion laser in a manner analogous to the mode-locking of semiconductor laser diodes. Applications of this technique to the pumping of dyes in different spectral regions are discussed.

The optical susceptibilities of CdSxSe1−x ‐doped glasses were observed up to χ(11) using a spatially resolved phase‐matched multiwave mixing technique. Using a pulsed dye laser tuned near the band gap, measurements were performed for ‖χ(3)‖, ‖χ(5)‖, and ‖χ(7)‖ in four different commercial filter glasses.

Excited by the frequency doubled optically compressed 10‐ps pulses from a cw mode‐locked Nd:YAG laser, the amplified output from a GaAs photoconductive device was used to drive a synchronously operated Photochron IIA streak camera at 300‐MHz sweep rates. A temporal jitter of ±6 ps was measured which was limited by the driving laser pulse width and amplitude stability.

The 90psec pulses from a cw mode-locked Nd:YAC laser operating at 1.32μm have been initially shortened to l.5psec by means of a fibre/grating compressor arrangement and subsequently propagated through a negatively dispersive optical fibre to generate, through high-order soliton effects, pulses as short as 33fsec. Input peak power and fibre length variations have been performed and the results compared with theoretical predictions

The photoaffinity label 8-azido[32P]adenosine 3':5'-monophosphate and affinity chromatography on N6-(2-aminoethyl)-cAMP-Sepharose were used to analyze the cAMP-binding proteins present in cell-free extracts of Blastocladiella emersonii zoospores. In the presence of a mixture of protease inhibitors, 8-azido[32P]cAMP was specifically and quantitatively incorporated into a major protein band of Mr = 58,000, and three minor protein bands of Mr = 50,000, Mr = 43,000, and Mr = 36,000 respectively, after autoradiography following sodium dodecyl sulfate-polyacryl-amide gel electrophoresis. In the absence of the protease inhibitors, the Mr = 58,000 protein band was converted into the lower molecular weight cAMP-binding proteins, indicating a high sensitivity of the intact Mr = 58,000 protein band to endogenous proteases. The Mr = 58,000 protein corresponded to the regulatory subunit (R), of the cAMP-dependent protein kinase of zoospores, as shown by their identical behavior on DEAE-cellulose chromatography. The partially purified protein kinase incorporated 32P from [gamma-32P] ATP . Mg2+ into R as demonstrated by the specific adsorption of the 32P-labeled protein with N6-(2-aminoethyl)-cAMP-Sepharose. The incorporated 32P group was rapidly removed by endogenous phosphoprotein phosphatases in the presence of cAMP, as shown by pulse-chase experiments with [gamma-32P]ATP. Dephosphorylation of R-cAMP and rapid proteolysis may indicate two other mechanisms, in addition to cAMP, for the control of this protein kinase in vivo.

Exploitation of highly efficient stimulated Raman scattering in optical fibers has given rise through a soliton shaping mechanism to a simple source of femtosecond high-power pulses from simple fiber ring laser systems, synchronously pumped by the 100-ps pulses from a cw mode-locked Nd:YAG laser at 1.32 μm.1 Principally, the operation has been limited to the first Stokes band around 1.4 μm (Ar 440 cm-1). The soliton pulse shaping mechanism giving rise to pulses of 100 fs, which as a result of the short interaction length is unable to generate efficiently the higher Raman orders in cascade. By using a dispersion shifted fiber, so that the first Stokes band and the fundamental are in the positive group velocity dispersion regime, highly efficient Raman generation (>50%) is possible in fiber spans 100s of meters. This technique then permits efficient generation of the second Stokes component (centered around 1.5 μm), which if in the region of negative group velocity dispersion can undergo a solitonlike pulse compression.2 In a dispersion shifted fiber ring laser geometry, pulses as short as 170 fs with peak powers of ~4 kW around 1.5 μm have been generated. Further cascade into the third Stokes per mitted similar operation with pulses of ~200 fs.

Extracts of aggregation-competent cells of Dictyostelium discoideum have an S6 protein kinase activity which is inhibited in the presence of the inhibitor of the cAMP-dependent protein kinase. The phosphorylation of S6 is rapid, and decays rapidly. The S6 kinase activity is detectable in the 150,000g supernatant only in the presence of phosphatase inhibitors known for preserving the S6 kinase in other systems, indicating that the activated form of the enzyme is phosphorylated by the cAMP-dependent protein kinase. S6 kinase elutes as a peak from DEAE-Sephacel at 100 mM NaC1, with an activity that is cAMP-dependent.

Since K+-induced germination of Blastocladiella emersonii is accompanied by a rapid decrease of a specific cyclic AMP phosphodiesterase (cAMP PDE) activity and a transient cyclic AMP accumulation, the effects of this compound as well as of some inhibitors of cAMP PDE on the induction of germination were tested. Adenine and caffeine, competitive inhibitors of zoospore cAMP PDE, were able to elicit germination in substitution for K+. Cyclic AMP is a poor inducer, but a synergistic effect was evident when non-effective concentrations of K+ and cyclic AMP were added together to the medium. At the same concentration, cyclic GMP had no effect as compared with cyclic AMP. Lanthanum, a specific antagonist of calcium in several biological systems, completely blocked the germination induced by potassium.

We report the experimental observation of up-conversion in an integrated silica-on-silicon waveguide. Blue light at 455 nm and 475 nm was observed. In addition to the strong blue emission near infrared (∼ 800 nm), visible (648 nm) and UV (366 nm) lines were also detected. We investigated the excitation mechanisms by using different pumping wavelengths.

This paper investigates for the first time the distributed gain of the silica-based thulium-doped fiber amplifiers (TDFA) using coherent optical frequency domain reflectometry (COFDR). Gain spectra and distributed gain are reported for the various pump schemes. Results show that the 805 nm pump has a more pronounced effect when added to 1050 nm than when added to 1426 nm, with a peak gain increase of 0.9 dB (~4% pump power increase gives -20% more gain). This paper also observes that for the 1426-nm upconversion pump, the 805-nm auxiliary pump resulted in a slight gain shift, increasing gain at longer wavelengths (>1470 nm) while reducing at lower wavelengths.

We study the dependence of random laser emission from thin polymer films doped with Rh6G using Ag nanoparticles in several concentrations. We demonstrate that a higher optical gain for lasing is obtained via surface-plasmon resonance.

We present measurements and characterization of stimulated Brillouin scattering (SBS) in multi-mode fiber (MMF) in comparison with single-mode fiber (SMF). As can be expected, the threshold for SBS was measured to be much higher in MMF (105 mW) compared with SMF (15 mW). A difference of 525 MHz was observed in the SBS frequency shift between both types of fibers. An increase in the gain bandwidth, which was measured to be 17 MHz for MMF compared to 8 MHz in SMF and which is due to several modes present in the MMF. Temperature dependence of the frequency shift was also investigated and was shown to be the same (1 MHz/°C) for both types of fibers. Several applications are therefore proposed using SBS in multi-mode fiber., 2011 7th Workshop on Fibre and Optical Passive Components (WFOPC), 13-15 July 2011, Montreal, QC, Series: Fibres and Optical Passive Components, WFOPC, Proceedings of IEEE

We demonstrate random lasing action in a biopolymer that has large potential for medical applications. The novel random laser consists of nanofibers of bacterial cellulose impregnated with silica nanoparticles and Rhodamine 6G.