Laser diode beam shaping with GRIN lenses using the twisted beam approach and its application in pumping of a solid-state laser

We report a novel beam shaping method for laser diodes based on the concept of twisted Gaussian Schell-model beams using GRIN lenses. This method enables a more symmetric pump as well as a compact setup for pumping of solid-state lasers. In the experiment with a standard 2 W diode, the beam quality factors in the two orthogonal directions were equalized to and , respectively, which pumped an intra-cavity frequency-doubled Nd:YAG laser at 946 nm with an output of 28 mW at 473 nm.     

Effects of light on cesium 6S-8S two-photon transition

This work presents a simple approach for determining the contribution of Lorentzian or Gaussian statistics by data fitting the spectrum to a Voigt profile. The fitting result shows that the Lorentzian width remains almost constant (∼1.51 MHz) and the Gaussian width increases (∼1.0-2.0 MHz) while changing the laser intensity or atomic density. The frequency shift associated with the cesium 6S-8S two-photon transition as a function of laser power is approximately ), agrees closely with the theoretical value. These results can be used to improve the optical secondary frequency standard in the near infrared region.     

1.55-μm InGaAs/InGaAlAs MQW vertical-cavity surface-emitting lasers with InGaAlAs/InP distributed Bragg reflectors

High-performance InGaAs/InGaAlAs multiple-quantum-well vertical-cavity surface-emitting lasers (VCSELs) with InGaAlAs/InP distributed Bragg reflectors are proposed for operation at the wavelength of . The lasers have good heat diffusion characteristic, large index contrast in DBRs, and weak temperature sensitivity. They could be fabricated either by metal-organic chemical vapor deposition (MOCVD) or by molecular beam epitaxy (MBE) growth. The laser light-current characteristics indicate that a suitable reflectivity of the DBR on the light output side in a laser makes its output power increase greatly and its lasing threshold current reduce significantly, and that a small VCSEL could output the power around its maximum for the output mirror at the reflectivity varying in a broader range than a large VCSEL does.     

Displacement-noise-free gravitational-wave detection with a single Fabry-Perot cavity: A toy model

We propose a detuned Fabry-Perot cavity, pumped through both the mirrors, as a toy model of the gravitational-wave (GW) detector partially free from displacement noise of the test masses. It is demonstrated that the noise of cavity mirrors can be eliminated, but the one of lasers and detectors cannot. The isolation of the GW signal from displacement noise of the mirrors is achieved in a proper linear combination of the cavity output signals. The construction of such a linear combination is possible due to the difference between the reflected and transmitted output signals of detuned cavity. We demonstrate that in low-frequency region the obtained displacement-noise-free response signal is much stronger than the -limited sensitivity of displacement-noise-free interferometers recently proposed by S. Kawamura and Y. Chen. However, the loss of the resonant gain in the noise cancelation procedure results is the sensitivity limitation of our toy model by displacement noise of lasers and detectors.     

The exact expression of the Voigt profile function

We solve in exact and closed-form a classical problem of mathematical physics of great interest in spectroscopy: the convolution of a Gaussian and a Lorentzian distribution that define the Voigt profile function, V(x). The solution is based in three steps: a power series development following the integral expression for V(x), the ordinary differential equation (ODE) satisfied by that expansion, and the corresponding solution of the ODE. This work converts in obsolete all graphical, numerical and semi-analytical approximations published previously. All results are clearly expressed in terms of the complementary error function Φ_c(a)=1-erf(a), where is, basically, the relation between Lorentzian and Gaussian widths.     

A coupled gain calculation and output characteristics of the two Ne-like 2s and 2p hole X-ray laser lines

In most collisionally pumped X-ray lasers the lasing transitions considered are a result of collisional excitation of a Ne-like ground state 2p electron into a 3p excited level. However, there are suggestions of producing plasma conditions for collisional pumping of the inner-shell 2s electrons into highly excited 3s levels, with lasing arising as a 2p electron fills the 2s hole. Simulations on various Ne-like ions such as germanium, krypton, and yttrium, using collisional pumping, to get gain on the inner-shell transitions at 62, 50.2, and , respectively, have shown gains on the inner-shell transitions up to . It has been suggested that the large Doppler linewidth associated with shorter wavelengths is responsible for the smallness of the small-signal gain in the inner-shell transitions relative to than that predicted for the more usual 2p-hole transitions. However, experimental investigations of this 2s-hole inner-shell laser line, using collisional pumping technique, were unable to register any output. In this paper we report the result of calculations of the gain and the total spontaneous emission rate for the 2s-hole and the 2p-hole X-ray laser lines using a coupled four level model. It is shown that the small-signal gain of the 2s-hole inner-shell transition decreases with increasing pumping rates of the 2p-hole upper and lower levels. The output characteristics of the Ne-like inner-shell transition is simulated using this four-level model and the effects of saturation of the 2p-hole line on the 2s-hole transition is studied showing that the saturation of the former may have a severe effect on the output of the later.     

Output power and degree of transverse coherence of X-ray free electron lasers

The explicit solution of the initial value problem is obtained for a SASE FEL (self-amplified spontaneous emission free electron laser) operating with a large ratio of electron beam emittance to the reduced wavelength, . The output power and the degree of transverse coherence are explicitly calculated for a high-gain linear regime. The degree of coherence is shown to be dependent on the ratio of the number of FEL gain lengths to the parameter . In particular, in the multi-mode limit the radiation from a SASE FEL has by the squared number of gain lengths higher degree of transverse coherence than a synchrotron radiation generated by a beam with the same emittance. An estimate for the degree of coherence of the radiation from X-ray SASE FEL at saturation is presented.     

The influence of the background interference on the escape of photons in atomic absorption measurements

The influence of the background interference on the escape of photons in the determination of Nickel by atomic absorption analysis for a wide range of concentrations between 10 and in an air-acetylene flame have been investigated. Assuming Doppler, Lorentzian and Voigt line profiles for the resonance lines, the resonance escape factor of the Ni-resonance lines at 2320 and have been calculated with and without the effect of the background, in terms of two main parameters, the optical depth in the line center and the number density of absorbing atoms in the ground state. The effect of the background interference due to the presence of the SO₂-molecular band can cause a change in the escape factors values. The dependence of the escape factor for the Voigt distribution on the damping constant is also discussed.     

Problems with power content beam parameter product

The M² beam propagation factor is widely used to characterize the quality of laser radiation and its propagation. When M² is defined by the second-moments, M² ? 1 holds in the paraxial approach. For many applications it is more convenient to use the power content values (normally η = 86.5%), also proposed by ISO. For the corresponding power content factor, it is often assumed that also holds. We have demonstrated previously that for a superposition of two coherent Gauss-Laguerre modes with radial symmetry, the 86.5% value of [6]. In recent years, has also been presented experimentally for a superposition of axially shifted Gaussian beams [7]. The problems with power content for axial superposition of Gaussian beams are discussed. In this paper it is shown that the 86.5% power content value can not be smaller than one for a coherent superposition of axially shifted Gaussian beams with radial symmetry presented in Ref. [7]. A superposition of two Gaussian beams with different waists and without shift is also discussed, and the corresponding of such beam can be smaller than one, depending strongly on the power content value η. For low power content values η and a large (or very small) ratio of the two different waists approaches zero. These investigations demonstrate that is not a suitable parameter to characterize laser radiation.     

Perturbation expansion for option pricing with stochastic volatility

We fit the volatility fluctuations of the S&P 500 index well by a Chi distribution, and the distribution of log-returns by a corresponding superposition of Gaussian distributions. The Fourier transform of this is, remarkably, of the Tsallis type. An option pricing formula is derived from the same superposition of Black-Scholes expressions. An explicit analytic formula is deduced from a perturbation expansion around a Black-Scholes formula with the mean volatility. The expansion has two parts. The first takes into account the non-Gaussian character of the stock-fluctuations and is organized by powers of the excess kurtosis, the second is contract based, and is organized by the moments of moneyness of the option. With this expansion we show that for the Dow Jones Euro Stoxx 50 option data, a -hedging strategy is close to being optimal.     

Two beam coupling gain enhancements in porphyrin:Zn-doped nematic liquid crystals by using grating translation technique with an applied dc field

The two beam coupling (TBC) gains in porphyrin:Zn-doped nematic liquid crystals (NLCs) are greatly enhanced by adopting the grating translation technique with applying dc electric field. The maximum gain coefficient of , whose value is at least three times larger than that of no grating translation was obtained under the applied dc field of and the grating translation speed of . Based on the material and torque balance equations for NLCs taking into account the grating translation, we present the theoretical expressions for TBC gain enhancements, showing excellent agreement with the experimental results.     

Investigation of the water-vapor continuum in the THz region using a multipass cell

Laboratory absorption measurements of the water-vapor continuum in the far infrared region from 12 to (0.4 - 1.83 THz) were obtained using a multipass absorption cell, a Fourier transform spectrometer and a liquid-He-cooled bolometer detector. Measurements were made at a temperature, with water vapor and nitrogen pressures up to 2.2 and , respectively. The effects of the choice of lineshape function and far-wing cut-off factors on the reported continuum absorption are analyzed by modeling the resonant water-vapor spectrum using van Vleck-Weisskopf and Lorentzian lineshapes. Comparisons with available microwave data and model calculations are also presented.     

Membrane fluidity profiles as deduced by saturation-recovery EPR measurements of spin-lattice relaxation times of spin labels

There are no easily obtainable EPR spectral parameters for lipid spin labels that describe profiles of membrane fluidity. The order parameter, which is most often used as a measure of membrane fluidity, describes the amplitude of wobbling motion of alkyl chains relative to the membrane normal and does not contain explicitly time or velocity. Thus, this parameter can be considered as nondynamic. The spin-lattice relaxation rate () obtained from saturation-recovery EPR measurements of lipid spin labels in deoxygenated samples depends primarily on the rotational correlation time of the nitroxide moiety within the lipid bilayer. Thus, can be used as a convenient quantitative measure of membrane fluidity that reflects local membrane dynamics. profiles obtained for 1-palmitoyl-2-(n-doxylstearoyl)phosphatidylcholine (n-PC) spin labels in dimyristoylphosphatidylcholine (DMPC) membranes with and without 50 mol% cholesterol are presented in parallel with profiles of the rotational diffusion coefficient, R_⊥, obtained from simulation of EPR spectra using Freed’s model. These profiles are compared with profiles of the order parameter obtained directly from EPR spectra and with profiles of the order parameter obtained from simulation of EPR spectra. It is shown that and R_⊥ profiles reveal changes in membrane fluidity that depend on the motional properties of the lipid alkyl chain. We find that cholesterol has a rigidifying effect only to the depth occupied by the rigid steroid ring structure and a fluidizing effect at deeper locations. These effects cannot be differentiated by profiles of the order parameter. All profiles in this study were obtained at X-band (9.5 GHz).     

Power transients in time-division multiplexed discrete Raman fibre amplifier

In this paper, the effect of addition and/or dropping of wavelength-division multiplexed channels in an all-optical gain-clamped time-division multiplexed (TDM) pumped discrete Raman fibre amplifier (DRFA) has been investigated experimentally and analyzed by numerical simulation. Nineteen signals were amplified in a counter-directionally pumped DRFA consisting of long dispersion compensating fibre. Light of eight DFB lasers was square-wave modulated at to imitate channel addition/dropping and combined with signals from five 10 Gigabit Ethernet transmitters and with six continuous wave signals. Power transients of the surviving channels caused by cross-gain modulation of the DRFA were monitored at the output of the amplifier. All-optical feedback loop was implemented in the form of a ring laser. Theoretical analysis of the TDM-pumped DRFA was based on numerical solution of coupled propagation equations for backward propagating pumps, signals, and both forward and backward propagating spectral components of amplified spontaneous emission powers.     

Spectroscopic properties of Ba 6pns (J=0,2) autoionizing series

The investigations on Ba and autoionizing states are carried out through a combination of the isolated core excitation method and proper polarization configuration of lasers. With comparison between the 6pns autoionizing states with different J, the spectroscopic properties of both the and series, such as the configuration interactions with the other series, are discussed in detail.