Continuous-wave Solid-state Lasers Research Articles

ВИЗУАЛИЗАЦИЯ ГОРЕНИЯ НАНОПОРОШКОВ МЕТАЛЛОВ СПРИМЕНЕНИЕМ ИМПУЛЬСНОГО И НЕПРЕРЫВНОГО ЛАЗЕРОВ ПОДСВЕТКИ

The work is devoted to the development of the method for high-speed imaging of high-temperature combustion processes using laser illumination. The object of visualization was a sample of aluminum nanopowder, which combustion temperature reaches 2500 K and is accompanied by a bright glow that varies in a wide range during combustion. It is demonstrated that laser illumination expands the capabilities of a high-speed imaging system and allows a more detailed study of the surface of samples compared to passive imaging by installing a neutral or band-pass filter in front of the high-speed camera lens. Various options for imaging with the installation of light filters with different transmission are considered. To monitor the surface of a burning material with a high-speed video camera in a wide temperature range of an object, it is proposed to use a low-power illumination from a continuous wave solid-state laser or a pulsed laser, as well as a beam expander for uniform illumination of the surface. The results of imaging using illumination from a solid-state laser with a wavelength of 532 nm and a copper bromide vapor laser with a wavelength of 510.6 nm are compared. It is shown that the output power of a solid-state laser of 200 mW with an illumination area diameter of 20 mm is sufficient to visualize “through the flame” the surface of a burning aluminum nanopowder at a low-temperature stage of combustion up to temperatures of ~1950 K. The uniform illumination of the surface can be ensured both by using a diffuser located directly near the sample, and with the use of a telescopic beam expander. Theoretical estimates and experimental results demonstrate that the illumination average power of 200 mW, regardless of the type of laser, is not enough to visualize the surface of a burning aluminum nanopowder at the high-temperature stage. The possibility of complete suppression of illumination during combustion with a temperature above 2500 K will appear when using an illumination laser with an average power of >3,35 W. The proposed illumination technique based on a solid-state laser has advantages in terms of reliability and simplicity compared to known systems based on copper or copper bromide vapor lasers.

(Invited, Digital Presentation) Evaluation of Polycrystalline-Si1-XGex Thin-Film Transistors Grown Laterally on a Glass Substrate Using a Continuous-Wave Laser

System-on-glass requires high-performance thin-film transistors (TFTs). In order to realize high-performance polycrystalline-Si (poly-Si) TFTs, we grew high-quality poly-Si thin films using a continuous-wave solid-state laser, in a method known as continuous-wave laser lateral crystallization (CLC). The grain boundary (G.B.) of CLC poly-Si is characterized by being parallel to the laser scan direction with shape of nearly straight line. Ge-based devices are attractive because of the superior carrier mobility of Ge in comparison to Si; however, as carriers in poly-Ge thin film are high concentration of hole generated from defects, it is difficult to control the level of doping. Moreover, fabrication of stable gate dielectric GeO2 is highly challenging. Addition of Ge to Si is expected to improve carrier mobility in Si, leading to poly-Si1-xGex TFTs with better performance than conventional poly-Si TFTs. Hence, our work here is a step in that direction. The purpose of this study is to understand the performance of poly-Si1-xGex TFTs, and to highlight the issues pertaining to poly-Si1-xGex thin films by comparing the performance of poly-Si1-xGex TFTs with that of poly-Si TFTs.A typical TFT fabrication process is as follows. An amorphous Si1-xGex thin film (~100 nm) with a Ge concentration of 10% is grown on a quartz substrate. After cleaning the substrate surface with hydrogen fluoride solution, CLC is carried out in air without substrate heating, followed by the preparation of transistor islands using reactive ion etching (RIE). A 50-nm-thick SiO₂ film is deposited by plasma enhanced chemical vapor deposition (PECVD), followed by heat treatment in nitrogen atmosphere at 550 °C for 4 h and deposition of Mo film as the top gate. Next, P or BF₂ is ion-implanted into poly-Si1-xGex for source-drain (SD) respectively, at 10 keV and 3×1015 cm-2. After ion implantation is completed, interlayer dielectric SiO2 film is deposited by PECVD and activated by heating in nitrogen atmosphere at 550 °C for 6 h. Subsequently, contact holes and Mo electrodes are formed. Finally, hydrogenation using step-cooling is performed to complete the TFT. N-channel (n-ch) and P-channel (p-ch) poly-Si TFTs are also fabricated using identical processes. Two types of TFTs were fabricated for both poly-Si1-xGex TFTs and poly-Si TFTs: one with the G.B. lying parallel to the SD direction (parallel type) and the other with the G.B. perpendicular to the SD direction (perpendicular type).The transfer characteristics of n-ch TFTs showed that the threshold voltages (Vth) of poly-Si1-xGex shifted in the positive direction in comparison with those of poly-Si TFTs. The mobility and sub-threshold swing (s.s.) values of poly-Si1-xGex were inferior to those of poly-Si TFTs, for both the parallel and perpendicular types.Considering the p-ch TFTs, the transfer characteristics of parallel poly-Si1-xGex TFTs showed higher OFF current and lower ON current than the corresponding poly-Si TFTs. In case of perpendicular types, OFF currents of poly-Si1-xGex TFTs were comparable with that of poly-Si TFTs and ON currents of poly-Si1-xGex TFTs were lower than those of poly-Si TFTs. Moreover, Vth values of poly-Si1-xGex TFTs were shifted in the positive direction in comparison to those of poly-Si TFTs, for both parallel and perpendicular types.The segregation coefficient of Ge in Si is less than 1.0, and it has been shown that Ge segregates to G.B.s in poly-Si1-xGex thin films during solidification. As acceptors are generated in regions with high Ge concentration along G.B.s, p-type quasi-one-dimensional conduction regions were formed in Si1-xGex along G.B.s. Therefore, when SD regions are parallel to a G.B., a current path connecting those regions is formed along G.B.s in p-type poly-Si1-xGex TFTs, leading to enhancement of OFF current. However, when G.B.s lie perpendicular to SD regions, the latter remain unconnected, leading to normal OFF current values.The positive shift in Vth of poly-Si1-xGex TFTs can be explained by the formation of acceptors. It can also be concluded that the decrease of ON current in poly-Si1-xGex TFTs is due to ionized impurity scattering by the acceptors. In addition, the difference in the magnitude of ON current depending on the G.B. direction can be explained as follows: when G.B.s are parallel to the SD direction, ON current is higher than that of the perpendicular type, because G.B.s do not act as strong scattering centers.In summary, the characteristics of CLC poly-Si1-xGex TFTs can be explained by the formation of acceptors. In particular, the large OFF current in parallel p-ch poly-Si1-xGex TFTs is explained by Ge segregation along G.B.s, resulting in formation of quasi-one-dimensional p-type conduction channels and electrical connection between SD region.

High-power continuous-wave diode-side-pumped Nd:YAP/LBO 670.7 nm red laser

In this paper, a high power continuous-wave red laser was reported with LD side-pumped Nd:YAP crystal and type-I critical phase matching of LBO crystal. The thermal focal length of Nd:YAP crystal was measured, and the parameters of laser cavity were selected by computer optimum designing. The output characteristics of line cavity and V-cavity were compared in experiment. By using V-cavity, the red laser output of 9.98W at continuous 670.7nm with near fundamental mode was obtained under the incident pump power of 800W for the Nd:YAP crystal, the divergence of the laser beam was about 10mrad. To the best of our knowledge, these results represent the highest performance at this wavelength for continuous-wave solid-state lasers.

Fully vectorial laser resonator modeling of continuous-wave solid-state lasers including rate equations, thermal lensing and stress-induced birefringence.

The computer-aided design of high quality mono-mode, continuous-wave solid-state lasers requires fast, flexible and accurate simulation algorithms. Therefore in this work a model for the calculation of the transversal dominant mode structure is introduced. It is based on the generalization of the scalar Fox and Li algorithm to a fully-vectorial light representation. To provide a flexible modeling concept of different resonator geometries containing various optical elements, rigorous and approximative solutions of Maxwell's equations are combined in different subdomains of the resonator. This approach allows the simulation of plenty of different passive intracavity components as well as active media. For the numerically efficient simulation of nonlinear gain, thermal lensing and stress-induced birefringence effects in solid-state active crystals a semi-analytical vectorial beam propagation method is discussed in detail. As a numerical example the beam quality and output power of a flash-lamp-pumped Nd:YAG laser are improved. To that end we compensate the influence of stress-induced birefringence and thermal lensing by an aspherical mirror and a 90° quartz polarization rotator.

Study of nonlinear optical properties and optical power limiting of Leishman dye using z-scan technique

The third-order nonlinear optical properties of Leishman dye in ethanol at different concentrations for two different incident laser intensities were investigated. The z-scan technique with continuous wave solid-state laser operating at 532 nm wavelength was employed to measure the sign and magnitude of the nonlinear refractive index (n2) and the nonlinear absorption coefficient (β) of Leishman dye. The real and imaginary parts of the third-order nonlinear optical susceptibility (χ(3)) were determined. The experimental results show that the investigated Leishman dye exhibits saturation absorption and negative nonlinear refraction (self-defocusing). The optical power limiting behavior was also investigated for the Leishman dye solution at different concentrations. The results show that the Leishman dye is a good nonlinear material with large nonlinear response could be used as a potential material for devices in optical applications.

Nonlinear optical properties and optical power limiting of Leishman dye using z-scan technique

The third-order nonlinear optical properties of Leishman dye in ethanol at different concentrations for two different incident laser intensities were investigated. The z-scan technique with continuous wave solid-state laser operating at 532 nm wavelength was employed to measure the sign and magnitude of the nonlinear refractive index (n2) and the nonlinear absorption coefficient (β) of Leishman dye. The real and imaginary parts of the third-order nonlinear optical susceptibility (χ(3)) were determined. The experimental results show that the investigated Leishman dye exhibits saturation absorption and negative nonlinear refraction (self-defocusing). The optical power limiting behavior was also investigated for the Leishman dye solution at different concentrations. The results show that the Leishman dye is a good nonlinear material with large nonlinear response could be used as a potential material for devices in optical applications.

Laser Interaction with Carbon Fibre Reinforced Polymers

In the last decades a strong increase of use in Carbon Fibre Reinforced Polymers (CFRPs) in the production of structures and components occurred. In particular in the automotive and aeronautical sectors several manufacturers already replaced light alloy or high strength steel parts with composites but the use of composite materials is rapidly growing also in other industrial sectors. Composites offer great advantages not only in term of specific proprieties but also in term of “functionalization” of the products by means of the lay-up sequence optimization and consequently providing heterogeneous improved proprieties in the different three-dimensional directions. CFRPs with epoxy resin matrix, among the polymer composites, offer the highest mechanical proprieties in particular when combined with autoclave technology that represents the top standard. On the other side the production of composite materials parts, and particularly the autoclave processing, is still very far respect to conventional metal forming processes in term of production rates. For this reason a considerable effort is oriented towards research and development of the so called “out of autoclave” processing where a trade-off between mechanical proprieties and production rates are investigated. A real optimization of the whole process requires robust and efficient technologies also during those phases able to guarantee high production rates.Laser processing represents a typical technology able to guarantee high flexibility joined with high automation and high production rate not only for small but also for large series manufacturing. For all these reasons, a solid state continuous wave Laser has been used for machining thin sheets made by woven carbon fibre/epoxy resin Pre-preg of 160g/mm2 (HR 3K 200 TEX carbon fibre with a 50% fibres in the warp and 50% in the weft directions). The aim was to investigate the interaction mechanism for removing the resin without damaging the fibres. Preliminary results in terms of process feasibility area were presented and discussed.

Laser-induced optical nonlinearities in Orange G dye: polyacrylamide gel

We observed multiple diffraction rings of a continuous-wave solid-state laser beam from an Orange G dye doped polyacrylamide gel. The number of rings increases almost exponentially with increasing ...

Saturated Absorption Effects in CW-Pumped Solid-State Lasers

Saturation effects in solid-state laser materials are important to accurately model and optimize the performance of laser devices. In this paper, we present analytical and numerical results describing the absorption in continuous wave solid-state lasers with both pump and laser saturation included, for flat-topped and Gaussian spatial distributions. Generalized absorption results are presented as a function of the ratio of pump intensity to pump saturation intensity, laser intensity to laser saturation intensity, optical thickness, and different ratios of laser to pump waists for collimated beams. These results apply to most solid-state laser energy level systems.

Adaptive mode optimization of a continuous-wave solid-state laser using an intracavity piezoelectric deformable mirror

A 19-element piezoelectric deformable mirror has been used as a rear intracavity mirror to adaptively optimize the mode of a flash lamp-pumped continuous-wave solid-state laser. A closed-loop adaptive global genetic algorithm is adopted to search the optimum voltages which are used to control the deformable mirror. A series of experiments have been accomplished and results showed that it is efficient to use such a piezoelectric deformable mirror to compensate the thermal effects in the laser resonators and can successfully change TEM11, TEM10, TEM20 transverse modes into fundamental TEM00 mode. Further more, the output laser powers can also be optimized automatically.

Laser spectroscopy of nanocrystals of aluminum and silicon oxides

The specific features of the spectra of secondary radiation (photoluminescence and Raman scattering) in nanocrystalline samples of aluminum oxide and fused silica are reported. The photoluminescence (PL) spectrum of nanocrystalline aluminum oxide was found to be strongly modified with the wavelength of exciting radiation. Spectra of secondary radiation were excited using a xenon lamp, a pulsed nitrogen laser (with a wavelength of 337.1 nm), a pulse-periodic copper vapor laser (510.6, 578.2 or 255.3 nm), and a solid-state continuous wave laser (532 nm). The numerical calculation of the photoluminescence spectrum of nanocrystalline aluminum oxide doped with chromium under various excitation conditions was performed. The results were compared with the experimental curves. The photoluminescence decay kinetics of nanocrystalline Al2O3 was measured. Modification of the photoluminescence spectra of nanocrystalline Al2O3 was found under the action of ultraviolet and radioactive radiation. The presence of near-ultraviolet bands was found in the photoluminescence spectra of fused silica of the QV type. The form of Raman and photoluminescence spectra of fused silica was shown to be dependent on its microstructure and impurities. Infrared reflectivity spectra of materials of di_erent structures based on SiO2 were obtained. It was shown that the Raman spectra of nanocrystalline samples of aluminum and silicon oxides differed essentially from those of the bulk samples. A characteristic boson peak was revealed in the nanomaterials with the position and width related to the nanoparticles’ sizes.

Up to 30 mW of broadly tunable CW green-to-orange light, based on sum-frequency mixing of Cr4+:forsterite and Nd:YVO4 lasers

Efficient generation of continuous-wave (CW) tunable light in the yellow region is reported. The method is based on sum-frequency mixing of a tunable Cr4+:forsterite laser with a Nd:YVO4 laser. A periodically poled lithium niobate crystal was placed intra-cavity in a Nd:YVO4 laser, and the Cr4+:forsterite laser was single-passed through the non-linear media. With this setup, it was possible to generate up to 3mW of yellow light smoothly tunable from 573 to 587nm. This is the highest output demonstrated to date for a tunable diode pumped solid-state CW laser in this wavelength region. The ways to improve the efficiency further are discussed.

High Performance Low Temperature Polycrystalline Silicon Thin Film Transistors on Non-alkaline Glass Produced Using Diode Pumped Solid State Continuous Wave Laser Lateral Crystallization

High performance low temperature polycrystalline silicon (poly-Si) thin film transistors (TFTs) with large grains were created using diode pumped solid state (DPSS) continuous wave (CW) laser lateral crystallization (CLC), employing fabrication processes at 450°C. Field-effect mobilities of 566 cm2/Vs for the n-channel and 200 cm2/Vs for the p-channel were obtained for a thick Si film (100–150 nm) on a 300×300 mm non-alkaline glass substrate. The high performance of the TFTs is attributed to the predominantly (100)-oriented very large grains. With a decreasing Si-film thickness, the grain size decreases, and the surface orientation of the grain changes from (100) to other orientations. These effects lead to reduced field-effect mobility with decreasing Si-film thickness, but it is easy to obtain a high field-effect mobility of over 300 cm2/Vs, even with a 50 nm thick Si film, without special processing techniques. A complementary metal oxide semiconductor (CMOS) ring oscillator was fabricated using a thin Si film 65 nm thick to demonstrate the high circuit performance of CLC poly-Si TFTs by applying the simplest CMOS process technology. A delay of 400 ps/stage at a gate length of 1.5 µm and a supply voltage of Vdd=5.0 (V) was produced on a large non-alkaline glass substrate utilizing a fabrication temperature of 450°C. This crystallization method will lead to the fabrication of high-performance and cheap Si-LSI circuits on large non-alkaline glass substrates.

Optimal resonators for self-mode locking of continuous-wave solid-state lasers

We propose an effective procedure, based on rather simple mathematical calculations, for optimal cavity design for a variety of laser geometries. In addition to the well-known guidelines from the literature, we suggest ways for the further optimization of a four-mirror cavity. The possibilities of using three- and two-mirror resonators for building compact Kerr-lens lasers are considered.

High-power diode-laser-pumped CW solid-state lasers using stable-unstable resonators

We present a novel design for an efficient, high power (>100 W), continuous-wave (CW) Nd:YAG laser with diffraction-limited performance. It uses side-pumped, sidecooled, zigzag slab which is incorporated in a stable-unstable resonator that has a variable reflectivity output coupler. A geometric magnification of at least 1.3 in the unstable direction can be achieved. Modeled performance characteristics are presented.

Frequency-shift mode locking of continuous-wave solid-state lasers

We present the analysis of a new technique for mode locking cw solid-state lasers. The mode-locking mechanism consists of a combination of a laser carrier frequency shift from the center of gain band caused by insertion of a knife-edge into the intracavity prism section and self-phase modulation. It results in the quasi-soliton formation of ultrashort pulses. An investigation of the soliton parameters versus the laser system parameters and a nonlinear analysis of soliton stability are presented that show new possibilities for highly efficient mode locking of solid-state lasers.

Improvement of coupled-cavity mode locking of continuous-wave solid-state lasers by a resonant nonlinearity

An increase in the efficiency of mode locking achieved with a saturable absorber in the master on an additional cavity is demonstrated on the basis of the fluctuation model of coupled-cavity mode-locked cw solid-state lasers. The increase consists in the broadening of the laser parameter range for which the mode locking occurs, the essential decreasing of the threshold pump intensity, and the reduction of the regime’s sensitivity to phase mismatches of the cavity lengths. The generation of stable pulse trains with a double repetition frequency and coupled-cavity mode locking in the case of the phase mismatch of the cavity lengths and the absence of self-phase modulation in the master cavity are shown to be possible in the system with the absorber in the additional cavity.

Efficient self-mode locking of continuous-wave solid-state lasers with resonant nonlinearity in an additional cavity

Parameters of the efficient mode-locking of cw solid-state lasers with saturable absorber in the additional cavity are found on the basis of the laser generation fluctuation model. These parameters form closed ranges. The main features of the efficient mode- locking are associated with the transmission of the additional cavity as a function of the field intensity. It is shown that due to the resonant nonlinearity the efficient mode-locking takes place in the absence of the interferometrical matching of the lengths of the master and additional cavities. It also takes place in the case of the difference of their lengths of up to the value for which the delay of the pulse from the additional cavity according to generation pulse becomes comparable with generation pulse duration.

Self-mode locking of continuous-wave solid-state lasers by a nonlinear Kerr polarization modulator

A numerical simulation of self-mode locking of cw solid-state lasers with a Kerr polarization modulator is developed. Limited ranges of laser and modulator parameters for which the laser generates a stable train of ultrashort pulses are given.

Novel stability diagrams for continuous-wave solid-state laser resonators

Two novel stability diagrams are proposed, which provide a clear understanding of the behavior of solid-state laser resonators with focusing rods. By means of this graphic analysis, information on the stability boundaries, dynamic stability, misalignment sensitivity, and spot size in the rod can be obtained. On the basis of these diagrams the complicated experimental output power curves of a cw Nd:YAG laser have been easily interpreted.