47-wavelength flat erbium-doped fiber ring laser with reduced operation power

Multi-wavelength laser generated in a piece of erbium- doped fiber (EDF) is attractive because of its high gain and high efficiency with simple structure, which is very useful in wavelength division multiplexing (WDM) sys- tems or fiber sensors. However, the EDF faces a big problem of intrinsic homogeneous line broadening[1] that prevents multi-wavelength oscillation at room tempera- ture. Yamashita et al.[2] have placed the EDF into liquid nitrogen (77 K) to reduce the homogeneous line w…     

An asymptotically flat ℋ space

The null geodesic equation is solved for the space of Sparling and Tod. Bondi coordinates are found and it is verified that the space construction is idempotent, i.e., the -space of this space is itself, symbolically ² = . Properties of the solution are used to motivate a definition of asymptotic flatness.S.R.C. Postdoctoral Research Fellow.     

Free-running emerald laser pumped by laser diode

Free-running emerald laser pumped by 660-nm laser diode (LD) was reported. Free-running output power of 24 mW has been obtained with overall efficiency of 1.4% and slope efficiency of 11.9% when the LD incident power was 2.56 W. The laser threshold value of emerald crystal was estimated to be 0.7 W.     

Quantum electrodynamics in a laser and the electron laser collisionQuantum electrodynamics in a laser and the electron laser collision

Quantum electrodynamics in a laser is formulated, in which the electron-laser interaction is exactly considered, while the interaction of an electron and a single photon is considered by perturbation. The formulation is applied to the electron- laser collisions. The effect of coherence between photons in the laser is therefore fully considered in these collisions. The possibility of y-ray laser generation by use of this kind of collision is discussed.     

Multi-use laser impulse pendulum and laser propulsion parameters measurement

In order to investigate the mechanisms of both the air-breathing and the ablation modes of laser propulsion under laboratory conditions, a multi-use laser impulse pendulum (MULIP) is developed. The measurable impulse range is from 1.0×10-4 to 3.8×10-3 N·s. The experimental calibration data agree well with the theoretical calculated data. With MULIP, the ablation mode has been performed, in which a high power pulsed Nd:glass laser (λ= 1.06μm, τ=20 ns) and a gray PVC film sample are used. The experimental results show that the maximum momentum coupling coefficient Cm is 7.73×10-5 N/W, and the maximum specific impulse Isp is 208.6 s.     

Pulsed laser deposition—UV laser sources and applications

Progress in material research and processing industry is fueled by the technique of pulsed laser deposition (PLD). High energy excimer lasers enable this technique since every material is amenable to their high photon energies. Spectral properties, temporal pulse and laser beam parameters of state of the art excimer lasers will be compared with frequency converted Nd:YAG lasers. Both quality and longevity of the deposited layers strongly depend on the degree of accuracy achieved in the thin film ablation and subsequent deposition process.     

BΛF theory and flat spacetimes

We propose a reduced constrained Hamiltonian formalism for the exactly solubleBF theory of flat connections and closed two-forms over manifolds with topology ³ × (0,1). The reduced phase space variables are the holonomies of a flat connection for loops which form a basis of the first homotopy group ₁( ³), and elements of the second cohomology group of ³ with value in the Lie algebraL(G). WhenG=SO(3,1), and if the two-form can be expressed asB=ee, for some vierbein fielde, then the variables represent a flat spacetime. This is not always possible: We show that the solutions of the theory generally represent spacetimes with global torsion. We describe the dynamical evolution of spacetimes with and without global torsion, and classify the flat spacetimes which admit a locally homogeneous foliations, following thurston's classification of geometric structures.This research is supported in part by the National Science Foundation, Grant No. PHY 89-04035, by CONACyT Grant No. 400349-1714E (Mexico), and by the Association Générale pour la Coopération et le Développement (Belgium).     

Influence of spatial laser energy distribution on evaluated soot particle sizes using two-colour laser-induced incandescence in a flat premixed ethylene/air flame

Time-resolved laser-induced incandescence (LII) has been developed rapidly during the last decade as a useful non-intrusive technique for particle size determination. Still several parameters should be investigated in order to improve the accuracy of LII for particle sizing and the spatial distribution of the laser energy is one of these. Generally a top-hat profile is recommended, as this ensures a uniform heating of all particles in the measurement volume. As it is generally not straightforward to create a uniform beam profile, it is of interest to establish the influence of various profiles on the evaluated particle sizes. In this work we present both an experimental and a theoretical investigation of the influence of the spatial profile on evaluated sizes. All experiments were carried out using a newly developed setup for two-colour LII (2C-LII) which provides online monitoring of both the spatial and temporal profile as well as the laser pulse energy. The LII measurements were performed in a one-dimensional premixed sooting ethylene/air flame, and evaluated particle sizes from LII were compared with thermophoretically sampled soot particles analysed using transmission electron microscopy (TEM). The results show that although there is some influence of the spatial laser energy distribution on the evaluated particle sizes both in modelling and experiments, this effect is substantially smaller than the influence of the uncertainties in gas temperature and the thermal accommodation coefficient.     

Finiteness of Ricci flat supersymmetric non-linear σ-models

Combining the constraints of Kähler differential geometry with the universality of the normal coordinate expansion in the background field method, we study the ultraviolet behavior of 2-dimensional supersymmetric non-linear -models with target space an arbitrary riemannian manifoldM. We show that the constraint ofN=2 supersymmetry requires that all counterterms to the metric beyond one-loop order be cohomologically trivial. It follows that such supersymmetric non-linear -models defined on locally symmetric spaces are super-renormalizable and thatN=4 models are on-shell ultraviolet finite to all orders of perturbation theory.     

Frequency selective surface with a flat topped passband

Two frequency selective surface (FSS) configurations with flat topped passband are presented in this paper. One configuration is single layer FSS with,λ/4 thickness dielectric loaded on both sides,and the other is double layers FSS.Based on the modal matching method,the frequency response properties including angle effect and polarization effect of both FSS configurations are analyzed,and the plots of the frequency versus transmission coefficient are obtained for different incident angles and polarizations.It is shown that the structure with the single layer FSS embedded centrally in the ,k/2 thickness dielectric has a wider flat top bandwidth of 6.8 GHz than that of the double layers FSS of 3 GHz.In addition,the fabrication of single layer is relatively easier than the double layers FSS.     

Optical feedback characteristics in a dual-frequency laser during laser cavity tuning

The optical feedback characteristics in a Zeeman-birefringence dual-frequency laser are studied during the laser cavity tuning in three different kinds of optical feedback conditions: (i) only // -light is fed back; (ii) only ⊥-light is fed back; (iii) both lights are fed back.A compact displacement sensor is designed using the experimental result that there is a nearly 90 degrees phase delay between the two lights' cosine optical feedback signals when both lights are fed back into the laser cavity.The priority order that the two lights' intensity curves appear can be used for direction discrimination. The resolution of the displacement sensor is at least 79 nm, and the sensor can discriminate the target's moving direction easily.     

Multiple photon laser photochemistry

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Investigation of laser diode face-pumped high average power heat capacity laser

The three-dimensional (3D) pump intensity distribution in medium of the laser diode (LD) pumped high average power heat capacity laser is simulated by the ray tracing method, and the divergence characteristics of fast axis and slow axis of LD are simultaneously considered. The transient 3D temperature and stress distributions are also simulated by the finite element method (FEM) with considering the uneven heat source distribution in medium. A LD face-pumped Nd:GGG heat capacity laser is designed. The average output power is 1.49 kW with an optical-optical efficiency of 24.1%.     

Novel laser Doppler tachometer

In order to improve the accuracy and universality of the rotational speed measurement,an instantaneous rotational speed measurement method based on laser Doppler technology is proposed.The composition and working principle of the new system are discussed in detail.Theoretical and experimental results show that the new rotational speed measurement system belongs to the non-contact method,which eliminates the quantization error of the contact method and greatly improves the applicability and accuracy.Compared to the commonly used method,the measurement accuracy of the new rotational speed measurement system will not be affected by the system’s installation angle deviation,and it does not need to measure the radius of the rotating body,so the influence of the radius fluctuation of the rotating body on the measurement accuracy can be avoided while outputting the rotational speed in real time.The relative error of the rotational speed measurement is less than 0.06%(1σ).     

New progress in Ti:Sapphire laser pumped by copper vapor laser

Because the copper vapor laser(CVL)can supply high average output power of severalhundred wattes and it's emission spectra are an excellent match for the absorption band of Ti:sapphire,the potential application of Ti:sapphire laser pumped by CVL is increasing greatly.     

Practical application of pulsed “eye-safe” microchip laser to laser rangefinders

The paper describes practical application of pulsed microchip laser generating at 1535-nm wavelength to a laser rangefinder. The complete prototype of a laser rangefinder was built and investigated in real environmental conditions. The measured performance of the device is discussed. To build the prototype of a laser rangefinder at a reasonable price and shape a number of basic considerations had to be done. These include the mechanical and optical design of a microchip laser and the opto-mechanical construction of the rangefinder.     

Theoretical study on the laser gain of a multi-stage dye laser amplifier

Theoretical studies on laser gain of a multi-stage dye laser amplifier are presented in this paper. The resultsshow that the influences of amplified spontaneous emission(ASE)and nonlinear absorption of excited-state on the gain are different for different input laser energies and gains. A threshold input intensityexists for a specific system. If the imput intensity is higher than the threshold, the nonlinear absorptionof excited-state will be the main cause for gain decrease. Otherwise, the ASE is the main cause. A newscheme is proposed to calculate the gain of the amplifier by the comparison of the input intensity with theeffective saturation intensity and the choice of the calculation gain method.     

High power Nd:YAG slab laser sidepumped by diode laser array

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Conformally flat sources for the Linet–Tian spacetime

We investigate the matching, across cylindrical surfaces, of static cylindrically symmetric conformally flat spacetimes with a cosmological constant $\Lambda $ , satisfying regularity conditions at the axis, to an exterior Linet–Tian spacetime. We prove that for $\Lambda \le 0$ such matching is impossible. On the other hand, we show through simple examples that the matching is possible for $\Lambda >0$ . We suggest a physical argument that might explain these results.     

Study on external-cavity semiconductor laser

In this paper, a narrow-band tunable external-cavity semiconductor laser with the Littman set-up is reported. The laser system consists of a semiconductor laser, a blazed grating and an external mirror. Its sideband suppression ratio over 20 dB was obtained. Conveniently tuning in wavelength region of 797.38 - 807.26 nm was achieved. The laser is operating in single frequency with narrow linewidth smaller than 0.06 nm. The output beam has good directional stability when tuned.