TDLAS–WMS based near-infrared methane sensor system using hollow-core photonic crystal fiber as gas-chamber

A near-infrared methane (CH₄) sensor system was implemented using a hollow-core photonic crystal fiber (HC-PCF) as gas-chamber. Coupling joints including ceramic ferrules and ceramic mating sleeve were used to realize butt coupling between hollow-core fiber and single-mode fiber. A near-infrared distribute feedback laser was used for CH₄ detection based on wavelength modulation spectroscopy technique. CH₄ measurements were conducted to derive the sensor-system performances. Using a 5.3 mW laser power and a 0.8 m-long HC-PCF, a minimum detection limit of ~8.7 ppm at 0.1 s averaging time was obtained and it can be further improved to 1.4 ppm at an averaging time of 10 s. A good linear calibration curve between the amplitude ratio (2f/1f) and the CH₄ concentration was obtained within the concentration range of 0–1000 ppm. This sensor system shows potential applications in distributed field measurements on CH₄ in industrial process control, environmental monitoring, etc.     

Controlled vortex motion in multiple interpenetrating pinning arrays

We study the effect of multiple interpenetrating pinning arrays on the vortex motion in the presence of an ac driving force, f _d (t), by using extensive molecular dynamics (MD) simulations. Firstly, the response to a square ac wave f _d (t) has been explored for the vortices interacting with a periodic square pinning array which has different pinning strengths and sizes. The effect of the type of an ac drive and its amplitude on the oscillatory dynamics of vortices have been investigated in detail. For very low displacements of the vortices, we have found that the single-particle model can produce results analytically similar to the ones obtained by the MD simulations. It is shown that the collective motion of vortices can be controlled easily by varying the number of multiple interpenetrating square pinning lattices (N _SPSL). A regular sequence of peaks has been observed for N _SPSL = 3 in the time evolution of the average velocity of the vortices (i.e., V? _{x - t} curves). The number of peaks (N _peak) strongly depends on the magnitude of f _d (t), and increases with increasing the magnitude of f _d . The close relation between N _peak and f _d is considered as an indication of controlling vortex motion in a multiple periodic pinning structure. Finally, the variation of the power spectrum of noise S(ν) with N _SPSL has been investigated. For N _SPSL = 3, it has been found that the plastic motion of the row of vortices evolves at low frequencies, i.e., 1/ν behavior, whereas, at high frequencies, S(ν) shows a typical behavior of Gaussian white noise.     

The effect of forward flight on the noise and flow field of inverted profile jets

Noise and flow field measurements are reported for an inverted profile coannular jet (where the annular jet speed exceeds the center jet speed) under simulated flight conditions. The annular and center jets were cold and both were operated subsonically. Forward flight was simulated by placing the coannular jet inside a larger open jet. Acoustic measurements show the effects of inverted profile shape and simulated flight on far field directivity, total radiated power, and spectral content. Measurements of total acoustic power demonstrate that the acoustic efficiency of inverted profile jets is about 3 dB less than the efficiency of “top hat” profile jets, and that the noise decreases as the seventh power of the relative jet velocity in the limit of small flight velocity, U_f. Flow measurements demonstrate that the jet spreading parameter λ = (U_j ? U_f)/(U_j + U_f) scales the thickness of the outer shear layer and the passage frequencies of the large turbulence scales. Comparisons between the turbulence time scales and the noise spectra suggest that coherent noise sources may become more important in forward flight.     

Multi-wavelength diode-pumped Nd:LGGG picosecond laser

Laser operation near 1.06 μm by a diode-pumped Nd:(Lu _x Gd_1?x )₃Ga₅O₁₂ (Nd:LGGG) disordered crystal has been investigated. Cw oscillation, with a slope efficiency as high as 61% and 230 mW output power, was achieved with 400 mW absorbed power from a 1-W laser diode. Stable passive mode locking with single- or multi-banded spectrum was obtained with a semiconductor saturable absorber mirror (SAM) and a single-prism, dispersion-compensated cavity. Fourier limited pulses with duration ≈3–9 ps and output power ≈40 mW were generated at three well-defined laser transitions in the range 1062–1067 nm.     

Noise spectral power and non-equilibrium dynamics of vortices in the mixed state of Bi2Sr2CaCu2Oy

A model is proposed to explain the observed noise spectral power generated by the moving vortex lattice in Bi₂Sr₂CaCu₂O_y single crystals. The analysis is based on the observations that the noise generated is due to the vortex velocity rather than the thermodynamic phase of the material. The motion of vortices induces a gradual transition of the vortex lattice from the plastic flow to the uniform flow with increasing magnetic field. The influence of the disorder on the dynamics of vortices is such that a uniform distribution in the activation barrier width will exist. This distribution will affect the attempt frequency of the flux line hopping and consequently the spectral shape of the noise generated. The model provides physical basis to the fact that the observed 1/f-noise power corresponds to the smaller velocity of the vortex lattice, while the deviation from the 1/f-spectrum corresponds to the larger velocity region. The results of the simulation are compared to recent experimental measurements where good agreement has been achieved.     

Performance of a diode end-pumped Cr,Er:YSGG laser at 2.79 μm

We present a diode end-pumped Cr,Er:YSGG laser at 2.79 μm operated in the continuous-wave (CW) and pulsed modes. Er³⁺ ions are directly pumped into the upper laser state ⁴ I _11/2 using a 967-nm diode laser, which avoids various nonradiative losses. The laser produces 50.2 mW of power in the CW mode. An average power of 169 mW is obtained at a repetition rate of 100 Hz and a pulse duration of 2 ms. The maximum laser energy achieved is 1.95 mJ at a repetition rate of 50 Hz, corresponding to a peak power of 970 mW and a slope efficiency is 7.6%. The far-field divergence angle of the laser is measured to be 8 mrad.     

A multichannel Raman spectrometry study of polyacetylene isomerization induced by laser irradiation

The multichannel Raman spectrometry has been used in the study of the isomerization reaction of an 80% cis PA film into a trans PA, using a laser beam for a double purpose. It is employed simultaneously as an activation agent inducing the isomerization reaction and the Raman diffusion. In each experience, the power of the laser beam Pⁱ(λ) was equivalent to the temperature. Twelve spectra have been recorded at different time periods tj = j·dt. The integrations of the Raman intensities related to two selected bands were numerically calculated.We also proposed an original method for the determination of the isomeric composition. A quantitative relationship between the equilibrium temperature and the laser beam power (in the range of laser power: 30 < Pⁱ(λ) < 300 mW) has been found. An estimate number of isomerized molecules N₀ and then a correction factors f_cis and f_trans were also obtained.     

Passively Q-switched Tm,Ho:LuLiF4 laser with near constant pulse energy and duration

A compact diode-pumped passively Q-switched Tm,Ho:LuLiF₄ laser with a Cr:ZnS saturable absorber is demonstrated for the first time. The maximum average output power of 74 mW is achieved at the absorbed pump power of 1.5 W, and the threshold power is only 0.68 W. The laser operates at the wavelength of 2,055 nm and produces pulses with near constant duration of 1.2 μs and pulse energy of 13 μJ at the pulse repetition frequency from 1 to 5.2 kHz. The stable long-pulse laser can be used in laser lidar systems for accurate wind velocity measurements.     

DESIGN OF A 532nm Nd:YVO4/KTP SINGLE-FREQUENCY LASER

A model of the laser-diode pumped solid-state laser is developed to deduce the minimum average radii of the pump beam in the solid medium, since the smaller the cavity waist, the higher the laser output power is expected to be. With an appropriate coupling system consisting of the collimating lens, prism pair and focusing lens, a diode-pumped single-frequency Nd:YVO₄/KTP intracavity frequency-doubling cw laser has been demonstrated through the precise temperature control of the Nd:YVO₄ crystal, the KTP crystal and the diode laser. The 532nm single-frequency output power of 40.4mW (in fact 55mW if the reflection loss of the triangular prism is taken into account) is obtained for an incident power of 515mW. It is derived theoretically and is verified in experiment that the frequency drift of the free-running laser can be reduced by increasing the cavity length.     

Crystal growth, spectroscopy, and diode pumped laser performance of Pr,?Mg:SrAl12O19

Polarization dependent ground state absorption, excited state absorption and emission cross sections of Pr,?Mg:SrAl₁₂O₁₉ are determined in the visible and ultraviolet spectral region. It is shown that excited state absorption to the 4f5d configuration does neither occur on the pump wavelength in the blue region nor on any of the visible laser transitions. Efficient laser action at 643.5?nm is demonstrated with slope efficiencies of up to 47% with respect to the absorbed pump power. Output powers up to 75?mW are achieved by pumping with an InGaN laser diode at 444?nm.     

LiGdF4:Tm3+: spectroscopy and diode-pumped laser experiments

A novel LiGdF₄ crystal doped with Thulium ions has been grown using the Czochralski technique. Three samples with doping concentrations of 0.3 at.%, 8 at.%, and 12 at.% have been extensively spectroscopically analyzed. We also performed room-temperature preliminary laser experiments, pumping the samples with a laser diode at 792 nm obtaining 53% as maximum slope efficiency with a maximum output power of 205 mW and a minimum lasing threshold of 22 mW. The laser emission spectrum in free running condition typically spans between 1990 and 2018 nm.     

Second Stokes 1129 nm generation in gray-trace resistance KTP intracavity driven by a diode-pumped Q-switched Nd:YVO4 laser

A gray-trace resistance KTP (GTR-KTP) second Stokes Raman laser intracavity driven by a diode-pumped acousto-optic Q-switched Nd:YVO₄ laser was first demonstrated in this paper. With an incident pump power of 9.5 W, the intracavity GTR-KTP Raman laser, operating at the repetition rate of 20 kHz, produced the maximum average output power of 860 mW at 1129 nm. The minimum pulse width obtained in this GTR-KTP Raman laser was 10.8 ns. When the GTR-KTP was substituted with a common KTP, a lower average output power of 720 mW and longer pulse width of 15.9 ns were obtained in the common KTP Raman laser under the same pump condition and cavity setup as the GTR-KTP Raman laser. Experimental results indicated that the decreased absorption at the fundamental and Stokes wave in GTR-KTP was beneficial to improve the stimulated Raman scattering performance.     

High repetition rate LiF<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack>: Color center laser

The efficient oscillation of LiF:F₂⁻ color center laser pumped by a compact LD-pumped Nd:YVO₄ acousto optically Q-switched laser with 30 kHz pulse repetition rate was demonstrated. The broadband oscillation with 75 μJ pulse energy and 37 kW peak power with the slope efficiency 20% was obtained. The average output power as high as 230 mW was reached. The narrow line tunable from 1.10 to 1.29 μm laser radiation with 10% conversion efficiency in the maximum of the tuning curve was achieved under pumping with 1.6 W average pump power.     

Two- and three-dimensional waves in falling film flow in the nonlaminar flow regime: an NMR study

Nuclear magnetic resonance (NMR) velocity-encoded imaging results as well as propagators are presented for the nonlaminar flow regime of falling films. The film is generated by a continuous flow of silicon oil along a vertical poly(methyl methacrylate) plate. While the film remains purely laminar for a film Reynolds number Re_f=0.5, it exhibits laminar-wavy behavior for 1.0≤Re_f≤2.5. In this range, a laminar residual film can be distinguished from averaged waves near the surface of the film from measurements of the flow velocity ν₂ along the direction of gravity as a function of the coordinatex normal to the plate. The perpendicular velocity components, ν _x and ν _y are zero within the accuracy of the measurement, indicating that the wave motion is two-dimensional in the laminar-wavy case. For higher Re_f the waves are found to be three-dimensional, a straightforward division into a residual film and waves in ν _z is lost and the waves extend over the whole thickness of the film.     

Quasi-cw laser action in K5Nd(MoO4)4

We report on room temperature quasi-cw laser action in K₅Nd(MoO₄)₄ at a wavelength of 1.067 μm. The crystal was pumped with the 5140 Å line of an argon ion laser. Due to thermal effects actual cw lasing was not possible. Chopping the pump light with a low duty cycle of 1∶1000 leads to quasi-cw laser action and stable output power. We have determined the fundamental laser parameters: threshold pump powerA _T=15 mW, slope efficiency η=22%, and internal lossesL=2.5%. In order to explain the measured relaxation oscillations, diffusion of the excited 4f-excitons is considered. According to a recently developed theory, a diffusion constant ofD=2×10^?6cm²/s explains the observed results.     

A comparative study on diode-pumped single-frequency Tm:YAG and Tm:GdVO<Subscript>4</Subscript> lasers

In this paper, we present experimental results concerning on the laser characteristics of Tm:YAG laser and Tm: GdVO₄ laser. At room temperature, the maximum output power of Tm:YAG laser and Tm:GdVO₄ laser is 210 and 145 mW, respectively. High efficiency can be achieved for both lasers at room temperature. Nevertheless, compared with Tm:GdVO₄ laser, Tm:YAG laser can operate on single frequency with high power easily. As much as 60 mW of 2013.9 nm single-longitudinal-mode (SLM) laser was achieved for Tm:YAG laser. For Tm:GdVO₄ laser 51 mW of 1919.7 nm SLM laser was achieved. The SLM Tm:YAG laser is better for using as a seed laser for coherent wind measurements and differential absorption LIDAR systems.     

LD-pumped passively Q-switched Nd:YVO4 infrared and green lasers

A laser diode directly end-pumped, passively Q-switched Nd:YVO₄/Cr:YAG laser is presented in this paper. With 600 mW incident pump laser, Q-switched 1064 nm laser with an average power of 138 mW, pulse width of 19.8 ns, repetition rate of 170.1 kHz and peak power of 40.96 W is obtained. When a KTP crystal was inserted into the cavity, Q-switched 532 nm laser with an average power of 56 mW, pulse width of 28.4 ns, repetition rate of 118.2 kHz and peak power of 16.7 W is obtained at last.     

Diode-end-pumped frequency-doubled Nd:YAP/Cr:YAG laser using a KTP crystal

We present a design for a compact laser-diode-end-pumped Nd:YAP/Cr:YAG q-switched laser in a plano?Cconcave cavity configuration. With an optical-to-optical efficiency of 35%, this laser provides an average output of 1.2 W with 13 mJ at 1080 nm. After that, the laser is frequency doubled by replacing the output coupler with a flat KTP crystal to achieve an output of 8 mJ in 8 ns at 540 nm. At a peak power of 1 MW, the laser emits linearly polarized output with an average power of 800 mW. The calculated M² value is 1.3, the output is very stable, and the laser operates in the TEM₀₀ mode.     

The so-called 1/Δ f noise

A sinusoidal ac current of frequeencyf _c generates an excess noise in many solidstate conductors. The power spectrum of this noise is calculated starting from the autocorrelation function of the stochastic process. Besides a 1/Δf term the spectrum includes another contribution varying with 1/(f _c+f). In the low frequency range there is white noise.