Frequency splitting phenomenon of dual transverse modes in a Nd:YAG laser

We observed frequency splitting phenomenon of dual transverse modes (TEM_00q and TEM_01q) in a Nd:YAG microchip standing wave laser utilizing intracavity stress birefringence effects. Four resonance frequencies (ν_00qe, ν_00qo, ν_01qe, and ν_01qo, respectively) were produced and tuned by changing the diametral compression force applied on the laser crystal. The transverse mode frequency spacing for the same longitudinal mode number (Δν_trans) was 1.16 GHz, and the magnitude of frequency splitting (Δν) ranged from 0 MHz to 1.16 GHz. Based on this phenomenon, a four-mode differential standing wave laser, whose signal characteristics were a little like those of a four-mode differential travelling wave laser gyro however with a much larger frequency splitting range, was produced. The theoretical analysis is in good agreement with the experimental results. This phenomenon not only can be used to make lasers with two or more frequency differences, but also can be used to make high-resolution self-sensing laser sensors (e.g. laser force sensors and laser accelerometers).     

Behavioral studies of gain and saturation energy density in a N2 laser with corona preionization

By the use of an oscillator-amplifier (OSC-AMP) TE N₂ laser system, both operating with corona preionizers, the laser parameters (small-signal gain g ₀ and saturation energy density E _s) have been measured at different N₂ gas pressure and for different states of the AMP preionizer. The details of our measurements are presented. In addition, the effect of He buffer gas on the laser parameters has also been investigated; it was found that both laser parameters remain almost constant up to 150 Torr of He gas pressure, indicating that He mainly affects the discharge uniformity. Finally, with the variation of the N₂ laser gain values in the literature, we found that the g ₀-N₂-laser parameter depends strongly on the length of the laser channel. Based on the most recent measurements, a graph showing this dependency is introduced.     

Frequency measurement of the 496 μm line of a CH3F FIR laser

A direct frequency measurement of the 496 μm, K = 2 line from a CH₃F laser is reported. Our laser is a metallic wave-guide device pumped by a frequency stabilized CO₂ laser; its output is mixed in a CaAs-Au Schottky barrier diode with a 100 GHz klystron and the beat note is observed on a spectrum analyzer. The result is f_CH3F(496 μm, K = 2) = 604 297.5 ± 0.2 MHz.     

Overshoot inversion of a Nd laser system

An important parameter of pulsed solid state lasers is the maximum inversion n_max which has to exceed the threshold inversion n_th to start the laser. The overshoot Δn=n_max-n_th is determined experimentally for a Nd-glass laser, operating in the TEM₀₀ mode and compared with theoretical predictions. Both investigations indicate a relation between normalized pump rate and inversion overshoot of the type Δn ∝ (W/W_th-1)^{$\text{1}\text{2}$}.     

Effects of cross-correlation between the real and imaginary parts of the quantum noise in a two-mode saturation laser system

A two-mode saturation laser model with cross-correlation between the real and imaginary parts of the quantum noise is considered. The laser intensity Langevin equation and corresponding Fokker-Planck equation are derived by the phase-locking method. The effects of the cross-correlation strength λ between the real and imaginary parts of quantum noise and the cavity decay constant K on the steady-state intensity distribution Q(I₁,I₂), the mean light intensity 〈I〉, the normalization autocorrelation λ₁₁(0) and cross correlation λ₁₂(0) are studied by numerical calculation. The results show that as λ increases the Qs(I₁,I₂) show two extrema, and λ almost does not affect the 〈I〉, λ₁₁(0) and λ₁₂(0) when the laser system is operated far above threshold. Nevertheless, when the laser system is operated at and below threshold, λ makes the curves of Qs(I₁,I₂) have the higher peak and drop faster. Furthermore, it enhances the deviation of λ₁₁(0) and λ₁₂(0) and lessens the mean light intensity 〈I〉 when the laser system is operated at and below threshold.     

Nuclear fusion induced by a super-intense ultrashort laser pulse in a deuterated glass aerogel

A SiO₂ aerogel with absorbed deuterium is proposed as a target for the fusion reaction d + d → He³ + n induced by a superintense ultrashort laser pulse. The multiple inner ionization of oxygen and silicon atoms in the aerogel skeleton occurs in the superintense laser field. All the formed free electrons are heated and removed from the aerogel skeleton by the laser field at the front edge of the laser pulse. The subsequent Coulomb explosion of the deuterated charged aerogel skeleton propels the deuterium ions up to kinetic energies of ten keV and higher. The neutron yield is estimated at up to 10⁵ neutrons per laser pulse for ～200–500 ps if the peak intensity is 10¹⁸ W/cm² and the pulse duration is 35 fs.     

Experimental study of nuclear processes in the presence of superstrong fields of a picosecond laser plasma

Nuclear processes in the presence of the superstrong laser fields of a picosecond laser plasma are experimentally studied at a radiation intensity of 2 × 10¹⁸ W/cm² on a Neodim laser setup with a power of 10 TW. Experimental data regarding neutron generation on the surface of a deuterated target (CD₂)_n owing to the thermonuclear fusion ²H(d,n)³He and the neutron generation on the Be target due to the photonuclear reaction ⁹Be(γ,n)2α are presented. Neutron yields Y _n of 10⁶ and 10³ per 4π sr per laser pulse are obtained for the (CD₂)_n and Be targets, respectively. The alpha-particle yield is measured for the first time in the neutron-free thermonuclear reactions ¹¹B + H → 3⁴He in the laser plasma on the surface of the composite B + (CH₂)_n targets. The alpha-particle yield is 10³ per 4π sr per laser pulse.     

Thermocapillary self-focusing of a laser beam: A theoretical analysis

A gaussian laser beam reflected from a thermocapillary liquid surface self-focuses at a distance L from the sample surface at a time-instant t_s after the beginning of the irradiation. The theoretical relation t_s(L) is calculated as a function of the liquid physical properties and the laser power.     

Weak antilocalization in a three-dimensional topological insulator based on a high-mobility HgTe film

The up-conversion luminescence of Er³⁺ from the ²H_11/2, ⁴S_3/2, and ⁴F_9/2 levels in nanocrystals of Y_0.95(1?x)Yb_0.95xEr_0.05PO₄ (x = 0, 0.3, 0.5, 0.7, 1) orthophosphates activated with Er³⁺ ions has been studied under the excitation of Yb³⁺ ions to the ²F_5/2 level by 972-nm cw laser radiation. Broadband radiation in the wavelength range of 370–900 nm has been observed at certain power densities of exciting laser radiation; this broadband radiation is absent in the case of excitation of the powders under study by pulsed laser radiation with a wavelength of 972 nm at a pulse repetition frequency of 10 Hz and a duration of a pulse of 15 ns. Experimental data indicating that this radiation is thermal in nature have been presented.     

Infrared laser-emission study in a resonantly excited sodium vapor

We report on a study of time dependence of ir laser emission at 3.41 μm in a dense Na vapor that is resonantly excited to the 3² P levels by a flashlamp, pumped dye laser. Energy-pooling collisions (excitation transfer) between excited atoms create a large population inversion among the 5² S _1/2 and 4² P _3/2 states which is destroyed by electron collisions as a result of subsequent plasma formation. The ir laser emission may serve as a new diagnostic tool to follow the time evolution of such media.     

Steady state and transient self-interaction of a laser beam in a strongly ionized moving plasma

The steady state and transient self-interaction of a laser beam with a strongly ionized plasma flowing transverse to the direction of propagation have been investigated by a phenomenological approach using perturbation theory, WKB and paraxial ray approximations. The effect of the transverse motion of the plasma has been included by a convection term in the energy balance equation and is found to result in the non-symmetrical heating of electrons. As a result the beam is shifted towards the direction of transverse flow of the plasma by an amount that increases with the flow velocity. The extent of asymmetry in self-focusing along the transverse directions is, however very small. In a typical case of 7.6×10⁵ watt laser of ω=10⁴ GHz and initial beam widthr ₀=0.05 cm the transverse shiftx _p=0.1r ₀ is predicted in a distance of propagationz=0.34 cm in a strongly ionized plasma of electron densityN _e=10¹⁶ cm^?3 and transverse flow velocityW ₀=10⁷ cm/sec.     

Observations of MeV ions emitted from a laser produced plasma

Fast-ion emission from tungsten plasmas has been studied with neodymium laser intensities from 5x10¹² to 2x10¹⁵ W/cm². Ion energies into the MeV region have been observed. The variation of ion energy (E_I) with laser intensity (I) was found to be of the form E_I∞I^0.65.     

Role of auger recombination in the determination of the threshold current density of a green-wavelength laser

The possibility of the creation a green-wavelength laser has been theoretically examined. The gain and threshold current density in the laser based on a (Al_0.5Ga_0.5)_0.49In_0.51P/(Al_0.6Ga_0.4)_0.49In_0.51P double hetero-structure have been calculated. It has been shown that, at a sufficiently high doping of an active region with an n-type impurity, the minimum threshold current density is reached when the coefficient of the eeh process of Auger recombination is larger than the coefficient of the ehh process.     

Laser phase fluctuations in a four-level atomic system in N-configuration

We discuss here the effect of laser phase fluctuations on coherent spectroscopy of four-level N-system interacting with a trichromatic radiation field of frequencies Ω _i , (i = 1?3). The laser phase variables are described by the Wiener-Levy diffusion process to specify the bandwidths (Γ _i ) and cross-correlations (Γ _ij ) that may exist between pairs of laser fields. A general formalism based on the master equation and theory of multiplicative stochastic processes is developed and used to study three-photon and (2+1)-photon absorptive resonances in model N-system of ⁴⁰Ca⁺ ion. It is observed that the resonances are suppressed or broadened by all Γ _i ,(i = 1?3), while their revival is dependent only on Γ ₁₂ and Γ ₂₃, and yet the revival is only partial even when the relevant fields are critically correlated. In contrast Γ ₁₃ is observed to deteriorate the absorptive resonances. The distinctive features of the steady state and time dependent behavior of the system under three-photon and (2 + 1)-photon resonance conditions and for fluctuating fields are discussed.     

Optimization considerations for a SASE free electron laser based on a superconducting undulator

A self amplified spontaneous emission (SASE) free electron laser (FEL) based on a new generation superconducting planar undulator, is optimized. It is shown that the laser wavelength should be down to soft X-rays range (～2–3 nm) of the spectrum via a dedicated undulator driven by a 1 GeV electron linear accelerator (linac). Numerical calculations and simulation results of the three main performance parameters for SASE operation, namely 1D gain length (L_G,1D), saturation power (P_sat) and saturation length (L_sat), are compared and discussed.     

Generation of characteristic x rays by a terawatt femtosecond chromium-forsterite laser

Characteristic K _α x rays arising when a metallic target is irradiated by femtosecond infrared pulses that are generated by a terawatt chromium-forsterite laser system (1240 nm, 90 mJ, 80 fs) are studied. The absolute yield (up to 3 × 10⁸ photons/sr pulse) and the coefficient of the transformation of laser radiation to K _α radiation (maximum value ≈0.03%) are measured for an iron target. The dependence of the radiation intensity on the angle of incidence of p polarized laser radiation is analyzed. The mechanisms of the production of fast electrons responsible for generating characteristic x rays are discussed.     

Analysis of the linear stability of stationary solutions in a circular laser containing a saturable absorber “LSA” for inhomogeneous broadening

In this present paper, the optical bistability in a circular laser containing a saturable absorber LSA for inhomogeneous broadening is studied. A simple mathematical model was developed to describe the action of this type of laser in a circular resonator under two conditions, when the spontaneous emission is neglected or included in the cavity. For both of these two cases, the coefficient of saturation was considered to be equal or different to unity (ζ=1 or ζ≠1). The photons intensities Q_j were determined as function of the pumping rate σ₀ of the active medium and analyzed the linear stability of the stationary solutions obtained in a circular laser containing a saturable absorber “LSA” for inhomogeneous broadening.     

Measurement of the transverse Doppler shift using a stored relativistic7Li+ ion beam

We have performed for the first time precision spectroscopy on a coasting fast⁷Li⁺ ion beam in a storage ring. The ion beam moving with 6.4% speed of light was first electron cooled and then merged with two counterpropagating laser beams acting on two different hyperfine transitions sharing a common upper level (λ-system). One laser was frequency locked to thea ₃ ¹²⁷J₂ hfs frequency component established as a secondary frequency standard at 514 nm. The second laser was tuned over theλ-resonance, which was recorded relative to¹²⁷J₂ hfs components. This experiment is sensitive to the time dilation in fast moving frames and will lead to new limits for the verification of special relatively. The present status of the experiment and perspectives in accuracy are discussed.     

Dependence of impurity binding energy on nitrogen and indium concentrations for shallow donors in a GaInNAs/GaAs quantum well under intense laser field

Within the framework of the effective-mass approximation, using a variational method, we have calculated the effect of intense laser radiation on the binding energy of the shallow-donor impurities in a Ga_{1- x} In _x N _y As_{1- y} /GaAs single quantum well for different nitrogen and indium mole concentrations. Our numerical results show that the binding energy strongly depends on the laser intensity and frequency (via the laser dressing parameter) and it also depends on the nitrogen and indium concentrations. Impurity binding energy under intense laser fields can be tuned by changing the nitrogen and indium mole fraction.     

Non-linear fluorescence in dye solutions induced by a low power laser

Measurements of the fluorescence intensity I_F, excited by two-photon pumping, have been carried out in different dye solutions by utilizing a low power cw laser (I_L ≈ 12 mW) and a particular experimental technique. A considerable departure from the quadratic law I_F ∝ I²_L with the varying of 2hω - ΔE_i quantity has been detected. This behaviour has bee n accounted for by considering the dependence of the non-linear and linear cross sections relative to the S₀ → S₁ and S₀ → S₂ transitions on the laser frequency.