Asynchronously modulated waves in a ring laser cavity

When modulated through the harmonic motion of one mirror, the counterpropagating waves in a ring laser oscillate out of phase. A solution to the wave equation is presented that satisfies both the time-dependent boundary condition and the resonance condition. This theoretical prediction is confirmed experimentally to leading order in terms that are inversely proportional to the speed of light. The method of solution is applicable to arbitrary phase modulation at more than one spatial location in the cavity. Potential uses include the reduction of the locking problem in ring lasers and the testing of higher-order kinematic effects in the theory of relativity.     

Demodulation techniques for the amplitude modulated laser imager

A new technique has been found that uses in-phase and quadrature phase (I/Q) demodulation to optimize the images produced with an amplitude-modulated laser imaging system. An I/Q demodulator was used to collect the I/Q components of the received modulation envelope. It was discovered that by adjusting the local oscillator phase and the modulation frequency, the backscatter and target signals can be analyzed separately via the I/Q components. This new approach enhances image contrast beyond what was achieved with a previous design that processed only the composite magnitude information.     

Transverse intensity distributions of a broadband laser modulated by a hard-edged aperture

By means of the Huygens-Fresnel diffraction integral, the field representation of a laser beam modulated by a hard-edged aperture is derived. The near-field and far-field transverse intensity distributions of the beams with different bandwidths are analyzed by using the representation. The numerical calculation results indicate that the amplitudes and numbers of the intensity spikes decrease with increasing bandwidth, and beam smoothing is achieved when the bandwidth takes a certain value in the near field. In the far field, the radius of the transverse intensity distribution decreases as the bandwidth increases, and the physical explanation of this fact is also given.     

Excitation of capillary waves in strongly absorbing liquids by a modulated laser beam

Several mechanisms for the excitation of capillary waves and for the development of the average deformation of a liquid surface under the action of a modulated laser beam are considered. The amplitude of the capillary wave in a strongly absorbing solution of the dye LDS 751 in ethylene glycol is experimentally studied as a function of laser intensity. Consecutive changes in the predominant mechanism of the excitation with increasing laser intensity are observed and described. At low laser intensities the mechanism connected with the creation of a surface tension gradient prevails. This mechanism becomes nonlinear with increasing influence of the convective motion. In addition, pressure pulsations of the convective flow start to contribute significantly to the generation process. The resonances of capillary waves in a cylindrical container are also investigated and used for determining the surface tension and viscosity of the liquid.     

1053-nm-wavelength selection in a diode-laser-pumped Nd:YLF laser

We report on a Nd:YLF laser that operates at 1053 nm without optical intracavity elements for the suppression of the stronger 1047-nm transition. The Nd:YLF crystal is end pumped by a fiber optically coupled 10-W diode-laser bar. The different thermal-lensing focal lengths of the two main lasing wavelengths in a plane-parallel resonator were used to achieve the selection by tilting the end mirror slightly from its optimum position for maximum output power. With 9.8-W cw diode-laser-pumping power the 1053-nm Nd:YLF laser produces a maximum output power of 1.9 W in cw operation and nearly 1 W of average power at a Q-switch repetition rate of 15 kHz. The highest slope efficiency of 47% achieved in cw operation.     

Energy balance in the discharge circuit of an electric-discharge laser

The possibility of using transient processes in the discharge circuit of an electric-discharge laser to pump the active media is analyzed. It is shown that, due to the nonlinearity of the transient process, under certain conditions, the efficiency of the energy contribution to the alternate load can almost be doubled as compared with the circuit containing a permanent load. It has been established that the process is quasiperiodic when the discharge-plasma resistance changes by an exponential law. The conditions of realization of the maximum energy contribution as well as the regions of stabilization of the optimum parameters have been determined. The necessary calculations have been performed for active media that model solutions of organic dyes under excitation by flashlamps. Deceased. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 73, No. 5, pp. 953–960, September–October, 2000.     

Interferometer for optical coupling and mode selection in a multichannel laser array

A theoretical consideration is presented of the optical coupling and selection of laser array modes by the use of a developed Lummer-Gehrcke interferometer as a resonator reflector. Control of the mirror reflection of the proposed interferometer permits laser power to be redistributed through channels on the outlet resonator mirror; in particular, it makes it possible to lead power out of the resonator by a single beam. In this way it is possible to diminish the sidelobes in the far-field radiation profile of multichannel lasers and to raise the efficiency of the optical coupling of the laser array with waveguides and fibers. This method may be used for the redistribution of laser power on the outlet mirror in striped lasers as well.     

Monte Carlo investigation of an atom laser with a modulated quasi-one-dimensional cavity

Abstract

The dynamics of a recently proposed atom laser scheme based on a modulated quasi-one-dimensional atom cavity are investigated. A three-mode model is developed which includes the effects of dipole–dipole collisions as well as pump and loss mechanisms. It is shown that the Monte Carlo wavefunction simulation technique is superior to a direct solution of the resulting master equation because of the existence of constants of motion which are present in the Monte Carlo wavefunctions but not in the full density operator. Under suitable parameter choices, the solution to the master equation leads to Poissonian atom statistics in the occupation of a single-atomic-cavity mode, analogous to the photon statistics of the optical laser. A threshold behaviour is predicted as the losses are varied relative to the gain for the laser mode.     

Diode laser mode selection using a long external cavity

We describe a simple and practical scheme to control the operation of a diode laser in a specific longitudinal mode through the use of a long external cavity. We show that for a fixed laser current and temperature, the laser can selectively oscillate in several different modes by controlling the external-cavity length. With a simultaneous sweep of the laser current and the cavity length, the laser frequency can be continuously scanned by more than 10 GHz. This laser system has been used successfully in a laser cooling experiment of neutral atoms.     

Performance of a laser Earth-to-satellite link over turbulence and beam wander using the modulated gamma-gamma irradiance distribution

We present an analytical framework for the performance evaluation of laser satellite uplinks over the major probabilistic impairments, i.e., atmospheric turbulence and beam wander. Specifically, we consider a ground-station-to-space laser uplink with a Gaussian beam wave model, and we focus on the particular regime assuming untracked beams where beam wandering takes place. In that regime, the modulated gamma-gamma distribution has been proposed as an effective irradiance model to characterize the combined effect of turbulence and beam wander. First we provide a closed-form expression of the probability density function and deduce the fundamental statistics of the new model. Then we evaluate the performance of the laser system assuming coherent detection for several modulation schemes. An appropriate set of numerical results is presented to verify the accuracy of the derived expressions.     

基于电压灵敏度的模拟电路节点选择策略

测试节点选择问题是当前模拟电路故障诊断领域的研究热点之一。以节点电压灵敏度定义为基础,对电路中的故障状态进行模糊划分,定义测试节点上的电路故障模糊集,提出故障诊断能力指标的概念,并用其描述节点对故障的区分能力。在此基础上给出一种新的测试节点优选方法,最终实现以最少的节点数目覆盖最多的电路故障的目的,应用实例验证了新算法的有效性。     

Noise characteristics of a Nd:YVO(4) laser pumped by laser diode modulated at high frequency

We report, for the first time to our knowledge, on the noise characteristics of a Nd:YVO(4) laser pumped by a laser diode modulated at high frequency. We have investigated noise characteristics of a Nd:YVO(4) laser pumped by a laser diode that is oscillating in a stable multilongitudinal mode because of modulation by a high-frequency (several hundred megahertz) current. As a result, low-noise operations of -130 dB/Hz above frequencies of 1 MHz have been achieved. This noise level is comparable to the level obtained when pumping a Nd:YVO(4) laser with a laser diode that is oscillating in a single longitudinal mode. However, a noise peak corresponding to relaxation oscillations of the Nd:YVO(4) laser has appeared around a frequency of several hundred kilohertz.     

Mode selection in an ultralarge ring laser gyro

A significant operational difficulty with very large ring laser gyroscopes is the length of time required to achieve the desired single-mode configuration. A control technique has been developed where the order of mode splitting between corotating beams is alternated. Theoretical advantages to this are the elimination of noise caused by variations in perimeter and systematic error caused by Adler pulling. External seeding of mode configurations has been proposed to allow the technique to work fast enough to eliminate known sources of perimeter perturbations. While investigating the intensity requirements for this concept, we found that the operating mode of a large ring laser can be successfully self-seeded with seed beams of near (6±3) single photon cavity mode population.     

Delay line length selection in generating fast random numbers with a chaotic laser

The chaotic light signals generated by an external cavity semiconductor laser have been experimentally demonstrated to extract fast random numbers. However, the photon round-trip time in the external cavity can cause the occurrence of the periodicity in random sequences. To overcome it, the exclusive-or operation on corresponding random bits in samples of the chaotic signal and its time-delay signal from a chaotic laser is required. In this scheme, the proper selection of delay length is a key issue. By doing a large number of experiments and theoretically analyzing the interplay between the Runs test and the threshold value of the autocorrelation function, we find when the corresponding delay time of autocorrelation trace with the correlation coefficient of less than 0.007 is considered as the delay time between the chaotic signal and its time-delay signal, streams of random numbers can be generated with verified randomness.     

Unstable periodic orbits in the Lorenz attractor

We apply a new method for the determination of periodic orbits of general dynamical systems to the Lorenz equations. The accuracy of the expectation values obtained using this approach is shown to be much larger and have better convergence properties than the more traditional approach of time averaging over a generic orbit. Finally, we discuss the relevance of the present work to the computation of unstable periodic orbits of the driven Navier-Stokes equations, which can be simulated using the lattice Boltzmann method.     

Probing of a random phase object by a focused spatially modulated laser beam. Integral scanning method

A telescope system incorporating an illuminating spatially modulated laser beam, a diffuser in the entry plane, and a random phase screen in the spatial frequency plane was used to analyze the formation of average-intensity interference fringes in the image plane of the diffuser. It is shown that the system can operate as a shift interferometer where the contrast of the fringes is independent of the diffuser characteristics. Analytic expressions are obtained for the contrast of the fringes as a function of the parameters of the screen and the illuminating beam and it is established that the statistical anisotropy of the screen influences the contrast of the fringes. Pis’ma Zh. Tekh. Fiz. 25, 5–10 (December 26, 1999)     

Nonequilibrium and bolometric responses of YBaCuO thin films to high-frequency modulated laser radiation

Picosecond nonequilibrium and slow bolometric responses to infrared radiation from a patterned high-T _c superconducting (HTS) film in resistive and normal states deposited onto LaAlO₃, NdGaO₃, and MgO substrates were investigated using both pulse and modulation techniques. The response time of 35 ps to a laser pulse of 17 ps FWHM has been observed. The intrinsic response time of the fast process is expected to be about a few picoseconds. The modulation technique, being free from the disadvantages of pulse methods (poor sensitivity, limited dynamic range), makes the detailed study of a number of relaxation processes possible. Besides the nonequilibrium response, two kinds of bolometric processes, namely phonon transport through the film-substrate interface and phonon thermal diffusion in a substrate, manifest themselves in certain frequency dependences.     

Probing of a random phase screen by a focused spatially modulated laser beam: Deflection of interference fringes

Deflection of interference fringes in a diffraction field was investigated under conditions in which a laser beam with a regular interference structure was finely focused onto a random phase screen. It was established that the contrast of the average-intensity fringes depends analytically on the statistical parameters of the screen. Pis’ma Zh. Tekh. Fiz. 25, 56–61 (January 12, 1999)