High power diode-pumped passively Q-switched and mode-locking Nd:GdVO4 laser at 912 nm

A high power diode-end-pumped passively Q-switched and mode-locking (QML) Nd:GdVO₄ laser at 912 nm was demonstrated for the first time, to the best of our knowledge. A Z-type laser cavity with Cr⁴⁺:YAG crystals as the intracavity saturable absorber were employed in the experiments. Influence of the initial transmission (T_U) of the saturable absorber on the QML laser performance was investigated. Using the T_U = 95% Cr⁴⁺:YAG, as much as an average output power of 2.0 W pulsed 912 nm laser was produced at an absorbed pump power of 25.0 W, then the repetition rates of the Q-switched envelope and the mode-locking pulse were ~ 224 kHz and ~ 160 MHz, respectively. Whereas the maximum output power was reduced to 1.3 W using the T_U = 90% Cr⁴⁺:YAG, we obtained a 100% modulation depth for the mode-locking pulses inside the Q-switched envelope.     

Optical pulse compression using the temporal Radon-Wigner transform

The temporal Radon-Wigner transform (RWT), which is the squared modulus of the fractional Fourier transform (FRT) for a varying fractional order p, is here employed as a tool for pulse compression applications. To synthesize the compressed pulse, a selected FRT irradiance is optically produced employing a photonic device that combines phase modulation and dispersive transmission. For analysis purposes, the complete numerical generation of the RWT with 0 < p < 1 is proposed to select the value of p required for pulse compression. To this end, the amplitude and phase of the signal to be processed should be known. In order to obtain this information we use a method based on the recording of two different FRT irradiances of the pulse. The amplitude and phase errors of the recovered signal, which are inherent to the recording process, are discussed in connection with the RWT production. Numerical simulations were performed to illustrate the implementation of the proposed method. The technique is applied to compress signals commonly found in fiber optic transmission systems, such as chirped gaussian pulses, pulses distorted by second and third-order dispersion and nonlinear self-modulated pulses.     

Multipulse irradiation of silicon by femtosecond laser pulses: Variation of surface morphology

The multipulse interaction of ultraviolet femtosecond laser pulses with silicon and generation of surface structures in a large area spot (?1 mm²) has been studied. The evolution of multiscale structures at the constant fluence strongly depends on the number of pulses, N. For N < 200, the “carpet-like” pattern of nano-, and micro-spikes is generated by the bubble explosion in a thin surface foam layer. The accumulation of bubbles and their explosion due to repetition of laser pulses cause damped membrane-like oscillations of the silicon surface. For 200 ≤ N, bifurcation of surface morphology takes place: (i) the surface tension waves of the wavelength ∼200 μm appear in the peripheral region of the spot. Generated by the surface thermal gradient in the liquid foam layer, they spread from the hot centerline towards the periphery of the spot. The change of their wavelength with propagation distance indicates onset of the Eckhaus instability caused by the phase modulation in multipulse interaction. (ii) Deep caverns appear in a highly superheated silicon layer in the central region of the spot due to the fast gas-liquid phase separation and the fragmentation process.     

A fiber-ring laser incorporating dual mode-locking mechanism

A fiber-ring laser incorporating dual mode-locking mechanisms is presented and demonstrated. A multi-quantum well (MQW) semiconductor optical amplifier (SOA) acts as active loss modulator, because the external optical signal injection results in gain depletion. In addition, a reflective SOA (RSOA) and a polarization beam splitter (PBS) introduce nonlinear polarization rotation (NPR), and also function as a saturable absorber. With this dual mode-locking mechanism, 10 GHz repetition-rate stable pulses, with pulse width of 16 ps and a root-mean-square timing jitter of 1 ps, was obtained without the use of external compression approaches.     

Diode-pumped passively mode-locked Nd:GdVO4 laser at 912 nm

We have demonstrated the stable mode-locked Nd:GdVO₄ laser operating on the ⁴F_3/2-⁴I_9/2 transition at 912 nm. With a four-mirror-folded cavity and a semiconductor saturable absorber mirror for passive mode-locking, we have gained 6.5 ps laser pulses at a repetition rate of 178 MHz. The laser is diode-end-pumped, and the total output power from the out coupler is 128 mw at an incident pump power of 19.7 W.     

Suppression of FM-to-AM conversion in phase modulated third-harmonic generation process

Phase modulation to amplitude modulation (FM-to-AM) conversion occurring in phase modulated third-harmonic generation (THG, process 1ω + 2ω → 3ω) process is investigated. To suppress the FM-to-AM conversion, a group-velocity-matching relationship is derived, which appears to be dependent on the ratio of modulation depth of 2ω–1ω pulses. Numerical simulations indicate that the FM-to-AM conversion in phase-modulated THG process can be suppressed effectively by satisfying that group-velocity-matching relationship.     

Passive mode-locking of a ruby laser using low-frequency stimulated brillouin scattering in birefingent crystals

A new technique of mode-locking Q-switched lasers using a birefringent crystal as an intracavity element is reported. Intense elastic waves are generated inside a birefringent crystal through the low-frequency, stimulated Brillouin scattering process and these in turn modulate the refractive index of the crystal and thus provide the necessary phase synchronization for mode-locking. Strong coupling between several neighboring modes of the laser is evidenced by the observation of high pulse repetition rates (several pulses circulating simultaneously within the cavity).     

Generation of widely tunable Fourier-transform-limited terahertz pulses using narrowband near-infrared laser radiation

Widely tunable, Fourier-transform-limited pulses of terahertz (THz) radiation have been generated using (i) crystals of the highly nonlinear organic salt 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST), (ii) zinc telluride (ZnTe) crystals, (iii) gallium phosphide (GaP) crystals, and (iv) low-temperature-grown gallium arsenide (LTG-GaAs) photomixers with THz spiral antennas. Outputs from two narrowband (Δν < 1 MHz, λ ∼ 800 nm) cw titanium-doped sapphire (Ti:Sa) ring lasers with a well-controlled frequency difference were shaped into pulses using acousto-optic modulators (AOM), coupled into an optical fiber, pulse amplified in Nd:YAG-pumped Ti:Sa crystals and used as optical sources to pump the THz emitters. The THz radiation was detected over a broad frequency range and its bandwidth was determined to be ∼10 MHz. The spectroscopic potential of the THz source is illustrated by the absorption spectrum of a pure rotational transition of OCS.     

Magnetic memory effect in chelated zero valent iron nanoparticles

We report the study of nonequilibrium magnetic behavior of air stable zero valent iron nanoparticles synthesized in presence of N-cetyl-N,N,N-trimethyl ammonium bromide chelating agent. X-ray photoelectron spectroscopy study has suggested the presence of iron oxides on nZVI surfaces. Zero-field-cooled and field-cooled magnetization measurements have been carried out at 20–300 K and 100 Oe. For field-cooled measurements with 1 h stops at 200, 100 and 50 K when compared with the warming cycle, we found the signature of magnetic memory effect. A study of magnetic relaxation at the same temperatures shows the existence of two relaxation times.     

Full-field refractive index measurement with simultaneous phase-shift interferometry

Using the phenomenon of total internal reflection and a beam splitting device, a technique of simultaneous phase-shift interferometry is proposed for measuring the full-field refractive index. Because this method applies a beam splitting device that mimics the characteristics of beam splitting and phase modulation, four interferemetric images of various phase distributions can be simultaneously captured. Therefore, this setup can avoid errors caused by non-simultaneous capturing of images and offers the benefits of high stability, ease of operation, and real-time measurement. Furthermore, using the phenomenon of total internal reflection, the phase difference between p- and s-polarized light varies considerably with the refractive index of a tested specimen. This can substantially increase the measurement resolution. The feasibility of this method is verified using an experiment, and the measurement resolution can be higher than 3.65 × 10⁻⁴ RIU.     

CW-pumped polarization-maintaining Brillouin fiber ring laser: I. Self-structuration of Brillouin solitons

Stimulated Brillouin scattering (SBS) of a cw-pump in a long optical fiber ring cavity generates for a given gain G, below a critical feedback R_crit, a backward soliton Stokes pulse at the roundtrip repetition rate, through resonant three-wave interaction between the optical pump wave and backward Stokes wave, and a high frequency (GHz) axial acoustic wave. In Part I, we perform a systematic experimental and numerical exploration of the whole soliton localisation domain for a large set of (G, R) parameters, in a polarization-maintaining Brillouin fiber ring laser in the best conditions of stability and coherence, and we confirm the excellent quantitative agreement of the coherent three-wave model and the experiments.     

Parametric study on femtosecond laser pulse ablation of Au films

Ablation process of 1 kHz rate femtosecond lasers (pulse duration 148 fs, wavelength 775 nm) with Au films on silica substrates has been systemically studied. The single-pulse threshold can be obtained directly. For the multiple pulses the ablation threshold varies with the number of pulses applied to the surface due to the incubation effect. From the plot of accumulated laser fluence N × ?_th(N) and the number of laser pulses N, incubation coefficient of Au film can be obtained (s = 0.765). As the pulse energy is increased, the single pulse ablation rate is increasing following two ablation logarithmic regimes, which can be explained by previous research.     

Tunable 12×10 GHz mode-locked semiconductor fiber laser incorporating a Mach-Zehnder interferometer filter

A stable multiwavelength mode-locked semiconductor fiber ring laser incorporating a fiber Mach-Zehnder interferometer (MZI) filter was proposed and experimentally demonstrated. A semiconductor optical amplifier (SOA) serves as an optically controlled mode-locking element due to gain exhaustion caused by external injected optical pulses. Another SOA severs as a constant-gain medium. A fiber MZI filter with a temperature control is incorporated into the fiber ring cavity to acquire a stable and tunable multiwavelength oscillation. Twelve wavelengths are synchronously mode-locked at 10 GHz, pulse width of mode-locked pulse are about 30 ps. Proposed multiwavelength mode-locked semiconductor fiber ring laser has some distinct advantages, such as simple and compact structure, easy integration, convenient tuning, good stability, potential high repetition rate operating, which has potential application in the future WDM communication system.     

Diode-pumped Nd:BaY2F8 picosecond laser

Picosecond pulse generation at 1050 nm by a diode-pumped Nd:BaY₂F₈ (Nd:BaYF) active crystal has been investigated under low-power pumping (440 mW absorbed). The sample was oriented along the optical axes and cut at Brewster angle for y-axis laser polarization to minimize birefringence effects. Stable passive mode-locking operation has been achieved by inverse saturable absorption with 80-mW output power and 3.7-ps pulses.     

Proton NMR study of pure liquid crystal exhibiting re-entrant phase transition

Proton NMR experiment was carried out on a pure re-entrant nematic liquid crystal OBBC in nematic (N), smectic A(S_A), and re-entrant nematic (RN) phases. The re-entrant phase transition was detected by measuring the rotation pattern (θ₀-dependence) of the spectra in the external field. The dipolar splitting at θ₀=0 showed smooth temperature-dependence through the phase changes, RN?S_A?N. Orientational order of the molecular core is thus hardly affected by the formation of 1-D density modulation. The temperature-dependence of the spin-lattice relaxation rate showed that translational self-diffusion is the predominant mechanism of relaxation in RN, but the director fluctuation is quite significant in N.     

Diode-pumped passively Q-switched mode-locked Nd:YLF laser with uncoated GaAs saturable absorber

We have demonstrated passively Q-switched mode-locked all-solid-state Nd:YLF laser with an uncoated GaAs wafer as saturable absorber and output mirror simultaneously. Q-switched mode-locking pulses laser with about 100% modulation depth were obtained. The average output power is 890 mW at the incident pump power of 5.76 W, corresponding to an optical slop efficiency of 20%. The temporal duration of mode-locked pulses was about 21 ps. At the Q-switched repetition rate of 30 kHz, the energy and peak power of a single pulse near the maximum of the Q-switched envelope was estimated to be about 1.6 μJ and 76 kW.     

Ytterbium femtosecond fiber laser without dispersion compensation tunable from 1015 nm to 1050 nm

We report on a widely tunable ytterbium fs-fiber laser without dispersion compensation. The all-normal dispersion laser contains a spectral filter for wavelength tuning and for generating additional amplitude modulation to support the nonlinear polarization evolution as mode-locking mechanism. By tilting the interference filter the center wavelength of the laser can be tuned from 1015 nm to 1050 nm with a pulse energy up to 2.0 nJ. The pulses can be dechirped externally to 108 fs.     

Quasi Bessel beam by Billet's N-split lens

This study utilizes the focal property of a classical Billet's split lens to create more focal points by splitting the lens. This approach distributes the focal points circularly on the focal plane. This study explores the characteristics of beam propagation and analytically derives the asymptotic characteristics of beam propagation based on the stationary phase approximation and the moment-free Filon-type method. Results show that the unique Billet's N-split lens can generate a quasi Bessel beam if the number of splitting N is large enough, e.g., N ≧ 24. This study also explores the diffraction efficiency of corresponding quasi Bessel beam and the influence of aperture size. The potential advantage of proposed split lens approach is that, unlike the classical means of annular aperture, this simple lens approach allows a much larger throughput in creating the Bessel beam and hence the Bessel beam could have more optical energy.     

μSR investigation of the magnetic ordering in EuAs3

We have investigated the magnetic ordering and the incommensurate-commensurate phase transition in EuAs₃ by zero-field (ZF) and longitudinal-field μSR. In the commensurate phase, stable at temperatures below T_L=10.3 K, the ZF muon signal exhibits oscillations corresponding to four muon precession frequencies the lowest of which behaves anomalously. The muon signal shows no oscillation but exponential decay in the incommensurate phase stable in temperature range from T_L≈10.3 K up to T_N≈11 K. The temperature dependence of the fitted relaxation rate shows divergence-like behaviour at the ordering temperature T_N≈11 K and also at the lock-in transition T_L≈10.3 K. The results are in qualitative agreement with those previously obtained by neutron and X-ray magnetic scattering investigations except for the anomalous temperature dependence of the lowest frequency in the commensurate phase. We propose a model for this anomalous behaviour.