Showing papers in "Laser Physics in 2013"

Vapor-plasma plume investigation during high-power fiber laser welding

Abstract: This work is devoted to the investigation of the laser–matter interaction during up to 20 kW ytterbium fiber laser welding of thick mild steel plates. The plume attenuation of a probe 1.3 μm wavelength diode laser beam as well as of continuous radiation in the 250–600 nm wavelength range was measured during welding with and without Ar shielding gas supply. The measured results allow the calculation of the average size and concentration of fine condensed metal particles in different plume areas using the multi-wavelength method and the Mie scattering theory. The plume temperature, which determines the condensation conditions, was measured by means of Fe I atom spectral line emission registration. The spatial distribution of the extinction coefficient in the welding plume was measured and the plume attenuation of the high-power fiber laser beam during the welding process was estimated.

Optical, spectral and phase-matching properties of BIBO, BBO and LBO crystals for optical parametric oscillation in the visible and near-infrared wavelength ranges

Abstract: The phase-matching properties of BIBO, BBO and LBO crystals for optical parametric oscillators (OPO) with wavelength tuning in the visible and near infrared spectral ranges were numerically investigated. The phase-matching configurations with a pump wavelength of 520 nm that provide the largest effective nonlinearity in each crystal were considered and compared. In addition, dispersive characteristics, including the group velocity mismatch and group velocity dispersion, which are of significant importance in femtosecond OPOs, were calculated. Finally, the attainable gain bandwidths for each crystal were estimated.

A 40 W cw Nd:YAG solar laser pumped through a heliostat: a parabolic mirror system

Abstract: Solar-pumped solid-state lasers are promising for renewable extreme-temperature material processing. Here, we report a significant improvement in solar laser collection efficiency by pumping the most widely used Nd:YAG single-crystal rod through a heliostat?parabolic mirror system. A conical-shaped fused silica light guide with 3D-CPC output end is used to both transmit and compress the concentrated solar radiation from the focal zone of a 2?m diameter parabolic mirror to a 5?mm diameter Nd:YAG rod within a conical pump cavity, which enables multi-pass pumping through the laser rod. 40?W cw laser power is measured, corresponding to 13.9?W?m?2 record-high collection efficiency for the solar laser pumped through a heliostat?parabolic mirror system. 2.9% slope efficiency is fitted, corresponding to 132% enhancement over that of our previous pumping scheme. A 209% reduction in threshold pump power is also registered.

Single- and double-walled carbon nanotube based saturable absorbers for passive mode-locking of an erbium-doped fiber laser

Abstract: The passive mode-locking of an erbium-doped fiber laser (EDFL) with a medium gain is demonstrated and compared by using three different types of carbon nanotubes (CNTs) doped in polyvinyl alcohol (PVA) films. Nano-scale clay is used to disperse the CNTs doped in the PVA polymer aqueous solution to serve as a fast saturable absorber to initiate passive mode-locking. The three types of CNT based saturable absorbers, namely single-walled (SW), double-walled (DW) and multi-walled (MW), are characterized by Raman scattering and optical absorption spectroscopy. The SW-CNTs with a diameter of 1.26 nm have two absorption peaks located around 1550 ± 70 and 860 ± 50 nm. In contrast, the DW-CNTs with a diameter of 1.33 nm reveal two absorption peaks located at 1580 ± 40 and 920 ± 50 nm. By using the SW-CNT based saturable absorber, the passively mode-locked EDFL exhibits a pulsewidth of 1.28 ps and a spectral linewidth of 1.99 nm. Due to the increased linear absorption of the DW-CNT based saturable absorber, the intra-cavity net gain of the EDFL is significantly attenuated to deliver an incompletely mode-locked pulsewidth of 6.8 ps and a spectral linewidth of 0.62 nm. No distinct pulse-train is produced by using the MW-CNT film as the saturable absorber, which is attributed to the significant insertion loss of the EDFL induced by the large linear absorption of the MW-CNT film.

The characteristic time of glucose diffusion measured for muscle tissue at optical clearing

Abstract: The study of agent diffusion in biological tissues is very important to understand and characterize the optical clearing effects and mechanisms involved: tissue dehydration and refractive index matching. From measurements made to study the optical clearing, it is obvious that light scattering is reduced and that the optical properties of the tissue are controlled in the process. On the other hand, optical measurements do not allow direct determination of the diffusion properties of the agent in the tissue and some calculations are necessary to estimate those properties. This fact is imposed by the occurrence of two fluxes at optical clearing: water typically directed out of and agent directed into the tissue. When the water content in the immersion solution is approximately the same as the free water content of the tissue, a balance is established for water and the agent flux dominates. To prove this concept experimentally, we have measured the collimated transmittance of skeletal muscle samples under treatment with aqueous solutions containing different concentrations of glucose. After estimating the mean diffusion time values for each of the treatments we have represented those values as a function of glucose concentration in solution. Such a representation presents a maximum diffusion time for a water content in solution equal to the tissue free water content. Such a maximum represents the real diffusion time of glucose in the muscle and with this value we could calculate the corresponding diffusion coefficient.

A tunable multi-wavelength laser based on a Mach–Zehnder interferometer with photonic crystal fiber

Abstract: In this paper a tunable multi-wavelength erbium doped fiber laser, based on a Mach–Zehnder interferometer, is presented. Here the interferometer is achieved by splicing a piece of photonic crystal fiber (PCF) between two segments of a single-mode fiber. The laser can emit a single, double, triple or quadruple line, which can be tuned from 1530 to 1556 nm by controlling the polarization state. Finally it is shown, by experimental results, that the laser has high stability at room temperature.

A tunable multi-wavelength erbium doped fiber laser based on a Mach–Zehnder interferometer and photonic crystal fiber

Abstract: In this paper a tunable multi-wavelength erbium doped fiber laser, based on a Mach–Zehnder interferometer, is presented. The interferometer is achieved by splicing a piece of photonic crystal fiber between two segments of a single-mode fiber. Here, by changing the curvature radius in the Mach–Zehnder interferometer, the single-, double- or triple-line emissions can be tuned from 1526 to 1550 nm. Finally it is shown, via experimental results, that the laser has high stability at room temperature.

Quantum probabilities of composite events in quantum measurements with multimode states

Abstract: The problem of defining quantum probabilities of composite events is considered. This problem is of great importance for the theory of quantum measurements and for quantum decision theory, which is a part of measurement theory. We show that the Luders probability of consecutive measurements is a transition probability between two quantum states and that this probability cannot be treated as a quantum extension of the classical conditional probability. The Wigner distribution is shown to be a weighted transition probability that cannot be accepted as a quantum extension of the classical joint probability. We suggest the definition of quantum joint probabilities by introducing composite events in multichannel measurements. The notion of measurements under uncertainty is defined. We demonstrate that the necessary condition for mode interference is the entanglement of the composite prospect together with the entanglement of the composite statistical state. As an illustration, we consider an example of a quantum game. Special attention is paid to the application of the approach to systems with multimode states, such as atoms, molecules, quantum dots, or trapped Bose-condensed atoms with several coherent modes.

Effects of crystal length on the angular spectrum of spontaneous parametric downconversion photon pairs

Abstract: We present a theoretical and experimental analysis of the joint effects of the transverse electric field distribution and of the nonlinear crystal characteristics on the properties of photon pairs generated by spontaneous parametric downconversion (SPDC). While it is known that for a sufficiently short crystal the pump electric field distribution fully determines the joint signal–idler properties, for longer crystals the nonlinear crystal properties also play an important role. In this paper we present experimental measurements of the angular spectrum (AS) and of the conditional angular spectrum (CAS) of photon pairs produced by SPDC, carried out through spatially resolved photon counting. In our experiment we control whether or not the source operates in the short-crystal regime through the degree of pump focusing, and explicitly show how the AS and CAS measurements differ in these two regimes. Our theory provides an understanding of the boundary between these two regimes and also predicts the corresponding differing behaviors.

Simulation study of near-field enhancement on a laser-irradiated AFM metal probe

Abstract: A systematic study of near-field enhancement on a laser-irradiated atomic force microscope (AFM) metal probe is reported The dependence of the electric field distribution on various parameters, like laser wavelength, tip curvature radius, half taper angle and tip–substrate distance, is numerically studied using the finite element method in this paper The simulation results show that high field enhancement appears around the tip and is mainly concentrated under the apex of the tip when the incident laser interacts with the AFM metal tip The results indicate that electric field enhancement easily appears when the AFM metal tip is irradiated by a higher frequency incident laser, with a similar phenomenon using a relatively sharp tip In addition, as the tip–substrate distance increases, the peak electric field enhancement underneath the apex of the tip decreases Based on the distribution of electric field enhancement, a new scheme combining an optical fiber probe and an AFM metal probe is proposed for nanolithography

Theory of cold atoms: basics of quantum statistics

Abstract: The aim of this tutorial is to present the basic mathematical techniques required for an accurate description of cold trapped atoms, both Bose and Fermi. The term cold implies that considered temperatures are low, such that quantum theory is necessary, even if temperatures are finite. The term atoms means that the considered particles are structureless, being defined by their masses and mutual interactions. Atoms are trapped in the sense that they form a finite quantum system, though their number can be very large, allowing for the use of the methods of statistical mechanics. This tutorial is the first of several, giving general mathematical techniques for both types of particle statistics. The following tutorials will be devoted separately to Bose atoms and Fermi atoms. Carefully explaining basic techniques is important in order to avoid the numerous misconceptions which propagate in the literature.

Spectroscopic characteristics and laser performance of Tm:CaYAlO4 crystal

Abstract: The spectroscopic characteristics of Tm:CaYAlO4 crystal were studied. Continuous wave laser operation at 2 μm was achieved with a 6 at.% Tm:CaYAlO4 (Tm:CYA) crystal pumped by a fiber-coupled laser diode. The laser emits a maximum output power of 4.3 W with a slope efficiency as high as 46.7%. In addition, a wavelength tuning experiment on Tm:CYA crystal was performed showing the Tm:CYA laser could be continuously tuned from 1861 to 2046 nm using an intracavity CaF2 prism, suggesting the potential of this crystal for ultrashort pulse generation by mode-locking.

A continuous wave Yb:KGW laser with polarization-independent pump absorption

Abstract: A polarization-independent pumping scheme for an end-pumped Yb:KGd(WO4)2 laser oscillator is proposed. It enables the use of compact, high power and high brightness fiber-coupled laser diode modules as pump sources and results in reduced thermal effects, good beam quality and improved output power stability. Based on this design, a continuous wave Yb:KGW laser with polarization-independent pump scheme is demonstrated. The laser delivered 2 W of average output power in TEM00 mode at 1040 nm. The induced thermal lens is reduced by a factor of 2 when compared to conventional diode pumping schemes, which clearly demonstrates the potential of the geometry of this crystal for power scaling.

An L-band graphene-oxide mode-locked fiber laser delivering bright and dark pulses

Abstract: In this work we have demonstrated the generation of L-band bright and dark pulses from a graphene-oxide (GO) mode-locked erbium-doped fiber laser. The polymer-hosted graphene-oxide saturable absorber (SA) was fabricated by a vertical evaporation method, which permitted a flexible choice of GO concentration and SA thickness. Stable fundamental solitons with a width of ~770 fs and repetition rate of ~21.79 MHz could be directly generated from the laser cavity. The corresponding spectrum was centered at ~1596.0 nm with a 3 dB bandwidth of ~4.454 nm. Triple solitons and fundamentally repetitive dark pulses could also be achieved. By changing the laser cavity design, we also achieved the generation of dark pulses, which had one to three times the repetition rate. The multiple soliton and dark pulse generation were experimentally observed for the first time in such a GO mode-locked fiber laser system, to the best of our knowledge. Our experimental results indicate that the GOSA can be successfully utilized to realize different mode-locked states in the L-band.

Near-infrared confocal micro-Raman spectroscopy combined with PCA–LDA multivariate analysis for detection of esophageal cancer

Abstract: The diagnostic capability of using tissue intrinsic micro-Raman signals to obtain biochemical information from human esophageal tissue is presented in this paper. Near-infrared micro-Raman spectroscopy combined with multivariate analysis was applied for discrimination of esophageal cancer tissue from normal tissue samples. Micro-Raman spectroscopy measurements were performed on 54 esophageal cancer tissues and 55 normal tissues in the 400–1750 cm−1 range. The mean Raman spectra showed significant differences between the two groups. Tentative assignments of the Raman bands in the measured tissue spectra suggested some changes in protein structure, a decrease in the relative amount of lactose, and increases in the percentages of tryptophan, collagen and phenylalanine content in esophageal cancer tissue as compared to those of a normal subject. The diagnostic algorithms based on principal component analysis (PCA) and linear discriminate analysis (LDA) achieved a diagnostic sensitivity of 87.0% and specificity of 70.9% for separating cancer from normal esophageal tissue samples. The result demonstrated that near-infrared micro-Raman spectroscopy combined with PCA–LDA analysis could be an effective and sensitive tool for identification of esophageal cancer.

A stable single-longitudinal-mode dual-wavelength erbium-doped fiber ring laser with superimposed FBG and an in-line two-taper MZI filter

Abstract: A simple approach to fabricate a stable single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber ring laser with one superimposed fiber Bragg grating (FBG) is proposed and demonstrated. The superimposed FBG is fabricated by the dual-exposure method for wavelength selection. A home-made in-line two-taper Mach?Zehnder interferometer is incorporated to balance the intra-cavity loss for the wavelengths. The SLM operation is guaranteed by a feedback fiber loop and saturable absorber. A stable dual-wavelength SLM fiber ring laser with a wavelength spacing of approximately 8.53?nm is experimentally realized. The optical signal-to-noise ratio of the laser output is over 50?dB and the linewidth of each wavelength measured by the delayed self-heterodyne method is 9.67?kHz and 9.07?kHz, respectively. The proposed scheme offers the possibility for difference frequency generation of terahertz radiation.

Near-infrared mediated tumor destruction by photothermal effect of PANI-Np in vivo

Abstract: Photothermal therapy is a therapy in which photon energy is converted into heat to kill cancer. The purpose of this study is to evaluate the in vivo efficacy of photothermal therapy, toxicity and hepatic and renal function of polyaniline nanoparticles (PANI-Np) in a tumor-bearing mice model. The in vivo efficacy of nanoparticles, following NIR light exposure, was assessed by examining tumor growth over time compared to the untreated control. Signs of drug toxicity and the histopathology and morphology of tumor and tissues, after intratumoral injection treatment, were examined or monitored. Excellent photothermal therapy efficacy is achieved upon intratumoral injection of PANI-Np followed by near-infrared light exposure. These results suggest that PANI-Np could be considered as an effective photothermal agent and pave the way to future cancer therapeutics.

Thermomechanical effect of pulse-periodic laser radiation on cartilaginous and eye tissues

Abstract: This paper is devoted to theoretical and experimental studies into the thermomechanical action of laser radiation on biological tissues. The thermal stresses and strains developing in biological tissues under the effect of pulse-periodic laser radiation are theoretically modeled for a wide range of laser pulse durations. The models constructed allow one to calculate the magnitude of pressures developing in cartilaginous and eye tissues exposed to laser radiation and predict the evolution of cavitation phenomena occurring therein. The calculation results agree well with experimental data on the growth of pressure and deformations, as well as the dynamics of formation of gas bubbles, in the laser-affected tissues. Experiments on the effect of laser radiation on the trabecular region of the eye in minipigs demonstrated that there existed optimal laser irradiation regimens causing a substantial increase in the hydraulic permeability of the radiation-exposed tissue, which can be used to develop a novel glaucoma treatment method.

Study of the stimulated Brillouin scattering power threshold in high power double-clad fiber lasers

Abstract: In this paper, the effect of stimulated Brillouin scattering (SBS) on increase in the output power of a fiber laser is investigated by solving the steady-state rate equations with the SBS. The effects of fiber core diameter, cavity length, reflectivity bandwidth of rear and front mirrors, and fiber laser pumping configuration on SBS threshold are illustrated. Numerical results show that the reflectivity bandwidth of the front mirror is more effective in suppression of SBS at the fiber laser resonator. On the other hand, increase in the fiber core diameter, reduction of cavity length and using a backward pumping configuration can increase the SBS threshold. In our 220 W experimental fiber laser oscillator setup no SBS effect was observed.

Low threshold and coherent random lasing from dye-doped cholesteric liquid crystals using oriented cells

Abstract: We demonstrate the realization of a coherent random laser in a strongly scattering medium, which contained a dispersive solution of Ag nanoparticles and laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) in cholesteric liquid crystals that was injected into an oriented cell. Multiple scattering of Ag nanoparticles and helical domains greatly enhanced by the oriented cell confinement effect was experimentally verified to account for the coherent lasing observed in our oriented system. Based on non-oriented cells and oriented cells, this paper extends the active and scattering media-based random lasers from the incoherent regime to the coherent regime.

Broadband photoluminescence of Bi2O3-GeO2 binary systems: glass, glass-ceramics and crystals

Abstract: Ultra-broadband emission covering 1000?1800 and 1800?3020?nm of Bi2O3?GeO2 binary system materials, from glass to glass-ceramics to crystals, is presented in this paper. This is the first time, to our best knowledge, that broadband photoluminescence of BGO crystals (including Bi4Ge3O12 and Bi12GeO20) in the range of 1800?3020?nm has been realized. HRTEM, XPS and XANES have been used to investigate the effects of the valence states and the structure environment of bismuth on the emission properties of Bi2O3?GeO2 binary system materials. Bi2+ and Bi+ are proposed as the emission centers of the photoluminescence peaks at 1060 and 1300?nm, respectively. The broadband emission from 1800 to 3020?nm originates from bismuth clusters. Bi2O3?GeO2 binary system materials could be promising laser materials in the field of full-band optical fiber communication amplifiers, ultra-fast lasers and diode pumped solid state lasers, due to their broadband emission spectra and their feasibility of synthesis and drawing into fibers.

Graphene-based passive Q-switching for a 2 μm thulium-doped fiber laser

Abstract: We demonstrate a passively Q-switched Tm3+-doped all-fiber laser (TDFL) at around 2 μm with a graphene-based saturable absorber (SA). The graphene is prepared by the reduction of graphite oxide (GO), deposited on the fiber core and inserted into a linear cavity as the passive Q-switch. A 793 nm laser diode (LD) is used as the pump source, with a maximum power of 200 mW. A maximum output power of ~33.5 mW is obtained at a pump power of 200 mW, with a high optical conversion efficiency of about 17%. The corresponding single pulse energy is 260 nJ and the repetition rate is 128 kHz.

Entropy of a general three-level atom interacting with a two mode

Abstract: A model of a three-level atom in the ?-configuration interacting with a two-mode field under a multi-photon process is considered. The effects of the mean photon numbers, detuning, Kerr-like medium parameters and various forms of the intensity-dependent coupling functional are considered. Analytical expressions for the time unitary operator, the density operator and the final state of the system are analyzed via the framework of the dressed states. The atom is prepared in its upper excited state, and the fields are prepared in binomial states. The general conclusions reached are illustrated by numerical results displaying the effects of photon-number, detuning and nonlinearities of both the field and intensity-dependent atom?field couplings.

Rapid fabrication of rigid biodegradable scaffolds by excimer laser mask projection technique: a comparison between 248 and 308?nm

Abstract: High-resolution photocrosslinking of the biodegradable poly(propylene fumarate) (PPF) and diethyl fumarate (DEF), using pulsed laser light at 248 and 308 nm is presented. The curing depth can be modulated between a few hundreds of nm and a few μm when using 248 nm and ten to a hundred μm when using 308 nm. By adjusting the total fluence (pulse numbers×laser fluence) dose and the weight ratios of PPF, DEF, and the photoinitiator in the photocrosslinkable mixtures, the height of polymerized structures can be precisely tuned. The lateral resolution is evaluated by projecting a pattern of a grid with a specified line width and line spacing. Young's modulus of the cured parts is measured and found to be several GPa for both wavelengths, high enough to support bone formation. Several 2D and 2.5D microstructures, as well as porous 3D scaffolds fabricated by a layer-by-layer method, are presented. The results demonstrate that excimer laser-based photocuring is suitable for the fabrication of stiff and biocompatible structures with defined patterns of micrometer resolution in all three spatial dimensions.

Arbitrary vector beams with selective polarization states patterned by tailored polarizing films

Abstract: A new method of generating arbitrary vector beams with selective polarization states in composite sectors is proposed and experimentally verified by using tailored polarizing film cooperated with a micro-fabricated phase plate Three examples of the beams are demonstrated in terms of quasi-radially, quasi-azimuthally and alternating radially and azimuthally polarized beams Polarization states of the beams are measured by a linear polarizer and the experimental results show excellent agreement with the theoretical predictions The proposed method presents the great advantages of a high degree of freedom in polarization distribution, and easy implementation with low cost

Coherent control of localization of a three-level atom by symmetric and asymmetric superpositions of two standing-wave fields

Abstract: A scheme for one-dimensional localization of a three-level atom is proposed by employing a modified technique for the formation of a standing-wave regime using two standing-wave fields. In the present system, precise position information of the atom can be achieved by measuring the population of the excited state, which can be efficiently controlled by the symmetric and the asymmetric superpositions of two standing-wave fields in the presence of constructive quantum interference. Our results highlight that, depending upon the effect of asymmetric superposition, the proposed scheme may provide a promising way to obtain various types of single-peak and double-peak localizations of the atom either in a one-wavelength range or in a half-wavelength range with appropriate values of the Rabi frequencies, detunings and spatial phase shifts of the coupling fields.

Elastic and inelastic interactions between optical spatial solitons in nonlinear optics

Abstract: Elastic and inelastic interactions between optical spatial solitons in nonlinear optics, which to our knowledge have not been reported before, are investigated in this paper. Analytic soliton solutions for the coupled nonlinear Schrodinger equation are obtained using the Hirota method. The coefficient constraints, which can be used to determine whether the soliton interactions are elastic or inelastic, are presented. With different coefficient constraints, the characteristics of soliton interactions are exhibited. The results of this paper may have applications in the design of directional couplers and all-optical logical gates.

Spectroscopic characterization and pulsed laser operation of Eu3+:KGd(WO4)2 crystal

Abstract: Europium-doped monoclinic potassium gadolinium tungstate KGd(WO4)2 crystals are grown by the top-seeded solution growth (TSSG) technique. Their absorption spectra are studied in detail for principal light polarizations, E ∥ Np, Nm and Ng. It is accompanied by determination of absorption oscillator strengths by means of the theory of f–f transition intensities for systems with anomalously strong configuration interaction. Spectral and temporal characteristics of luminescence associated with 5D0 → 7FJ transitions are analyzed, and luminescence branching ratios and the radiative lifetime of the 5D0 state are determined. Stimulated-emission cross-section spectra are evaluated for Eu:KGd(WO4)2 crystal. Pulsed Eu:KGd(WO4)2 lasers operating at room temperature at the wavelength of 702.8 nm (5D0 → 7F4 transition) are studied at Eu concentration of 10/25 at.%. Under laser pumping at 533.6 nm, a maximum output energy of 570 μJ is obtained. The main limitation for laser operation at 5D0 → 7F2 transition is strong excited-state absorption via the 5D0 → 5F4 channel.

A tunable, linearly polarized Er-fiber laser mode-locked by graphene/PMMA composite

Abstract: An all-polarization maintaining, erbium-doped fiber laser based on a graphene saturable absorber is presented. The graphene layers were grown on copper foils by chemical vapor deposition (CVD) and transferred onto the fiber connector tip. The laser was capable of generating scalar solitons with sub-600 fs duration at 46 MHz repetition rate. The center wavelength of the emission could be continuously tuned from 1556 to 1560 nm by changing the intra-cavity losses. Thanks to the all-polarization maintaining cavity design the laser was self-starting, environmentally stable and could operate for long periods of time. The degree of polarization (DOP) of the output pulses was at the level of 99.8%.

CW and quasi-CW laser performance of 10 at.% Yb3+:LuAG ceramic

Abstract: We have investigated the laser oscillation of 10 at.% Yb3+:LuAG ceramic pumping into the two main absorption peaks in CW and quasi-CW operation modes. Emitting at 1030 and 1046 nm the laser achieves excellent performance, delivering output powers up to 8.8 W. The highest estimated slope efficiencies are 72.6% in CW and 79% in quasi-CW. A tunability range of 57.5 nm is measured.