Thermal Lens Effect Research Articles

Thermal lensing effects and nonlinear refractive indices of fluoride crystals induced by high-power ultrafast lasers.

Thermo-optical and nonlinear property characterization of refractive optical components is essential for endoscopic instrumentation that utilizes high-power, high-repetition-rate ultrafast lasers. For example, ytterbium-doped fiber lasers are well suited for ultrafast laser microsurgery applications; however, the thermo-optical responses of many common lens substrates are not well understood at 1035 nm wavelength. Using a z-scan technique, we first measured the nonlinear refractive indices of CaF2, MgF2, and BaF2 at 1035 nm and found values that match well with those from the literature at 1064 nm. To elucidate effects of thermal lensing, we performed z-scans at multiple laser repetition rates and multiple average powers. The results showed negligible thermal effects up to an average power of 1 W and at 10 W material-specific thermal lensing significantly altered z-scan measurements. Using a 2D temperature model, we could determine the source of the observed thermal lensing effects. Linear absorption was determined as the main source of heating in these crystals. On the other hand, inclusion of nonlinear absorption as an additional heat source in the simulations showed that thermal lensing in borosilicate glass was strongly influenced by nonlinear absorption. This method can potentially provide a sensitive method to measure small nonlinear absorption coefficients of transparent optical materials. These results can guide design of miniaturized optical systems for ultrafast laser surgery and deep-tissue imaging probes.

Seven-rod pumping concept for simultaneous emission of seven TEM00-mode solar laser beams

In present-day single-rod/single-beam solar laser systems, the thermal lens effect is a serious issue that limits its ability for a scale-up to higher powers and improved beam quality. Aiming at resolving this shortcoming, the concept of a seven-rod/seven-beam solar pumping scheme is proposed. This scheme was composed of a first-stage heliostat-parabolic mirror system and a single large laser head. The large laser head consisted of seven fused silica compound parabolic concentrators, which transmitted and focused the concentrated solar radiation to each single-laser rod of small diameter, within a conical cavity, which enabled multiple passages of the pump rays. Consequently, each laser rod was pumped by only one-seventh of the total concentrated solar power, ensuring a significant reduction of the thermal induced effects in the laser rods. 13.3 W/m2 TEM00-mode solar laser collection efficiency was numerically achieved, representing an enhancement of 1.68 times over the experimental record from a single-rod prototype. 1.80 times improvement in solar-to-TEM00-mode laser power conversion efficiency was also registered in relation to the previous experimental record.

Thermal Effects Analysis of High-Power Slab Tm:YLF Laser with Dual-End-Pumped Based on COMSOL Multiphysics

The thermal effects model of slab crystal was established based on the practical operating conditions of the solid-state laser. To our knowledge, this is the first time to use the simulation software of COMSOL Multiphysics to investigate the thermal model of slab crystal with the complex boundary conditions in detail. And this is also the first time to use this theoretical method to a slab Tm:YLF laser. The results showed that thermal effects of Tm:YLF crystal were enhanced with the increase of the pump power. And the thermal effects increased sharply with the decrease of the size of the pump light along the Y-axis or the increase of the crystal doping concentration. Thermal deformation which produced by thermal lens effect played a leading role of Tm:YLF crystal. Finally, under the premise of elevated the laser output efficiency and ameliorated thermal effects, the optimized parameters were chosen which were the pump size of ω1×ω2=1mm × 6mm and doping concentration of 2.5at.%.

Investigation of Anomalous Lasing in Vertical-Cavity Surface-Emitting Lasers of the 850-nm Spectral Range with a Double Oxide Current Aperture at Large Gain-to-Cavity Detuning

Results of investigation of static characteristics of the vertical-cavity surface-emitting lasers (VCSEL) of the 850-nm spectral range based on strained InGaAs/AlGaAs quantum wells in a wide range of current-aperture sizes are presented. The reasons for their anomalous behavior at large design gain-to-cavity wavelength detuning are analyzed. Lateral spreading of carriers in the plane of quantum wells and specific profile of oxide aperture (leading to formation of an effective two-step waveguide) in the studied VCSELs makes possible the existence of higher-order modes localized at the current-aperture periphery. Inhomogeneity of carrier injection across the current aperture in wide-aperture lasers leads to initial onset lasing via the higher-order modes. Subsequent transition to classical lasing via the lower-order modes with increase in current is caused by changes in the gain-to-cavity detuning with increase in internal laser temperature. Anomalous lasing via higher-order modes in the case of narrow-aperture VCSELs becomes possible due to increase in the diffraction losses at the edge of the oxide current aperture for the fundamental mode. In the process, not only a decrease in the gain-to-cavity detuning but also the effect of thermal lens are responsible for subsequent laser hopping to the regime of co-lasing via two modes.

Thermal lens effect and laser characteristics of Ho:YAG ceramic laser with a concave-convex cavity

In this paper, the thermal lens effect of dual-end-pumped Ho:YAG ceramic laser was studied experimentally and numerically. The thermal lens magnitude of Ho:YAG ceramic was obtained by beam quality inverse method. When the pump power increased from 9 to 88 W and the according pump radius increased from 0.2 to 0.25 mm, the thermal focal lengths in X and Y directions were decreased from 102 to 65 mm and 140 to 68 mm, respectively. Considering of the thermal lens, we further optimized the laser resonator with a concave-convex cavity and the slope efficiency was increased by 11.6 percent compared to the plane-concave cavity.

High-power structured laser modes: direct generation of a vortex array.

The frequency degeneracy induced by the astigmatism in a nearly hemispherical cavity is originally exploited to generate vortex array laser modes with the output power up to 300 mW. The inhomogeneous Helmholtz equation is employed to derive the wave function for manifesting the characteristics of the lasing modes. The theoretical wave function explicitly reveals the role of the Gouy phase in the formation of vortex arrays. Numerical analyses are further performed to confirm that the thermal lensing effect in the laser crystal assists the lasing transverse order to increase with increasing pump power. It is believed that the high efficiency enables the present laser modes to be useful in the applications of structured vortex beams.

Highly efficient Er:YAP laser with 6.9 W of output power at 2920 nm.

We report on the efficient high-power operation of a laser-diode-pumped Er3+-doped yttrium aluminum perovskite (Er:YAP) laser in the 3 μm spectral region at room temperature. 6.9 W of continuous-wave (CW) output power was obtained at 2920 nm. The slope efficiency was as high as 30.6% with respect to the absorbed pump power, which is close to the quantum defect limit (33.4%). To the best of our knowledge, this is the highest CW output power generated from 3 μm Er3+-doped solid state lasers at room temperature. Furthermore, our analysis has shown that more than 10 W of output power based on Er:YAP is possible by further mitigating the thermal lens effect.

The thermal lens effect on laser heating of a metal nano-sphere particle immersed in oil

In this work, we revise the model describing laser heating of a spherical metallic nanoparticle (NP) immersed in transparent oil. We study the generated temperature non-uniformity of the ambient medium and its effect on the propagation of the laser beam. Thus, the model describes the influence of thermal non-uniformity on scattering and absorption of light by the heated particle. Recently, different aspects of the heating of NPs by a laser beam have been actively researched. Some experimental works demonstrated an enigmatic dependence of the effective scattering cross-section of the heated particle on the intensity of the laser beam, illuminating it. The authors of these works labeled the observed effects as “saturation,” and “reverse saturation.” In this paper, we show that the propagation of the incident laser beam in a non-uniform oil can by itself be the source of both “saturation” and “reverse saturation” els effects.

Short-pulse-induced thermal lensing effect in 1,2-dichloroethane and 1,2-dibromoethane

1,2-dichloroethane () and 1,2-dibromoethane () show linear absorption (LA) and stimulated light scattering in response to the excitation by 820 nm 18 fs laser pulses. Using the -scan technique, with these pulses separated by 12.2 ns (much shorter than both samples’ thermal diffusivity time constants ’s) and extended over 41 ms (considerably longer than ’s), we investigated the thermal lensing effect of both samples and found that LA dominates their heating. In addition, relative to , shows more/less thermal lensing effect before/after the exposure time passes ’s. This means that the thermal lensing effect is more severely reinforced by heating and stabilized by thermal diffusivity in than in . Notably, the Kerr lensing effect is negligible in this study, although it is noticeable in a -scan study with 17 ps laser pulses.

Experimental and Theoretical Investigations of Beam Deformation Produced by Thermal Lens Effect on a Gaussian Laser Beam in Ethanol

In this paper, influence of thermal lens on propagation of a Gaussian beam through ethanol was investigated experimentally and numerically. The influence of laser power on the thermal lens strength and laser beam characteristics at far field was analyzed. The experimental and numerical results show that a Gaussian laser beam is deformed because of thermal lensing effect. According to these results at enough high laser power a doughnut beam is formed. The deformed beam characteristics i.e. the beam diameter, the size of central dark region of the doughnut beam, and the ratio of peak to valley values were also analyzed. The results show that the numerical and experimental results are in good agreement. The results of this investigation can be very useful for experiments involved to pulse laser ablation in liquid environment (PLAL).

Analysis of a thermal lens in a diamond Raman laser operating at 1.1 kW output power.

We report experimental observations of thermal lens effects in a diamond Raman laser operating up to 1.1 kW output power in a quasi- steady-state regime. Measured changes in the output beam parameters as a function of output power, including beam quality factor and beam divergence after a fixed focusing lens, are compared to modelling enabling us to track the development of a thermal lens up to 16 diopters at maximum output power. Analysis shows that good agreement between model and experiment is obtained by considering the power deposition profile and the spatial overlap with the laser mode. The results clarify previous work that raised questions about thermal lens effects in the diamond gain medium and provides increased confidence in thermal models for determining the power limits for the current design.

A Novel Tool to Investigate the Early and Late Stages of α-Synuclein Aggregation.

The accumulation of an inherently disordered protein α-synuclein (α-syn) aggregates in brain tissue play a pivotal role in the pathology and etiology of Parkinson's disease. Aggregation of α-syn has been found to be complex and heterogeneous, occurring through multitudes of early- and late-stage intermediates. Because of the inherent complexity and large dynamic range (between a few microseconds to several days under in vitro measurement conditions), it is difficult for the conventional biophysical and biochemical techniques to sample the entire time window of α-syn aggregation. Here, for the first time, we introduced the Z-scan technique as a novel tool to investigate different conformations formed in the early and late stage of temperature and mechanical stress-induced α-syn aggregation, in which different species showed its characteristic nonlinear characteristics. A power-dependent study was also performed to observe the changes in the protein nonlinearity. The perceived nonlinearity was accredited to the thermal-lensing effect. A switch in the sign of the refractive nonlinearity was observed for the first time as a signature of the late oligomeric conformation, a prime suspect that triggers cell death associated with neurodegeneration. We validate Z-scan results using a combination of different techniques, like thioflavin-T fluorescence assay, fluorescence correlation spectroscopy, Fourier-transform infrared spectroscopy, and atomic force microscopy. We believe that this simple, inexpensive, and sensitive method can have potential future applications in detecting/monitoring conformations in other essential peptides/proteins related to different neurodegenerative and other human diseases.

Beam waist shrinkage of high-power broad-area diode lasers by mode tailoring.

A new approach was proposed and its role in improvement of the beam quality of high-power broad-area diode lasers was demonstrated, in which a composite arrow array and trench microstructure was used to suppress the beam waist and tailor the high order lateral modes. The beam waist shows a special shrinkage with increasing injection current resulting from the combined effect of mode tailoring and the thermal lens effect. A 58% improvement in lateral beam parameter product was realized compared with conventional broad-area diode lasers.

Design and analysis of thermal stable regenerative cavity with multiple laser heads using 4f system

Regenerative amplifiers containing multiple diverse laser heads have the ability of scaling up output power as well as broadening spectral bandwidth. However, the thermal lensing effect will be more complicated. A method using a 4f system to simplify the thermal lensing effect in a multidisk condition is proposed. Two kinds of Yb-dropped sesquioxide materials are chosen as an example. On the basis of calculating the thermal lensing parameters of the thin-disk laser heads, a feasible cavity design that can minimize the fluctuation of beam size when changing the pump power is given out. The 4f system consisting of concave mirrors will introduce astigmatism in the cavity. The influence of astigmatism and the compensation method are discussed. Astigmatism can be compensated by adding extra convex mirrors and controlling the angle of incidence at each mirror.

Hollow Gaussian beam: Generation, transformation and application in optical limiting

The hollow Gaussian beam can be generated by thermal lens effect for a Gaussian beam propagating through a medium with finite optical absorption. The new model of hollow Gaussian beam (HGB) is proposed by the effective thermal focal length at steady state. The transformation between the HGB and quasi-hollow beam is achieved by tunable input power. Based on the experimental and numerical analysis, the quasi-hollow beam is the superposition of various orders of single HGB. Besides, the property of optical limiting of hollow or quasi-hollow beam is investigated. It shows that the output intensity linearly increases as the input intensity increases when input intensity is below 34 W/cm2, while changes of intensity through the aperture is very low when input intensity is above 34 W/cm2 so that the optical limiting is achieved. Our theoretical and experimental results may have potential applications in optical tweezers, optical limiting, low power all-optical devices and related fields.

High gain fiber-solid hybrid double-passing end-pumped Nd:YVO4 picosecond amplifier with high beam quality**Project supported by the National Natural Science Foundation of China (Grant Nos. 61675009 and 61325021) and Key Program of Beijing Municipal Natural Science Foundation, China (Grant No. KZ201910005006).

We propose a fiber-solid hybrid system which consists of a semiconductor saturable absorber mirror (SESAM) mode-locked fiber seed with a pulse width of 10.2 ps and a repetition rate of 18.9 MHz, a two-level fiber pre-amplifier and a double-passing end-pumped Nd:YVO4 amplifier. In the solid-state amplifier, to enhance the gain and the extraction efficiency, a specially designed structure in which the seed light passes through the gain medium four times and makes full use of population inversion is used as the double-passing amplifier. Besides, the beam filling factor (the ratio of the seed light diameter to the pump light diameter) and the thermal lens effect of the double-passing amplifier are considered and its optical-to-optical conversion efficiency is further improved. To preserve the beam quality of the double-passing amplifier, a new method of spherical-aberration self-compensation based on the principles of geometrical optics is used and discussed. Our system achieves a maximum average power of 9.5 W at the pump power of 28 W, corresponding to an optical-to-optical efficiency of 27%. And the beam quality factor M2 reaches 1.3 at the maximum output power.

Effect of thermal lensing and the micrometric degraded regions on the catastrophic optical damage process of high-power laser diodes.

Catastrophic optical damage (COD) is one of the processes limiting the lifetime of high-power laser diodes. The understanding of this degradation phenomenon is critical to improve the laser power and lifetime for practical applications. In this Letter, we analyze the defect propagation inside the cavity of quantum well (QW) high-power laser diodes presenting COD. For this, we studied the effect of highly localized thermal gradients and degraded regions on the laser field distribution. Finite element method (FEM) simulations are compared to experimental cathodoluminescence (CL) measurements. The presence of micrometric hot spots inside the QW induces the thermal lensing of the laser field. The laser self-focusing inside the cavity eventually generates a new hot spot, and, in a repetitive way, a sequence of hot spots would be created. This would account for the propagation of the dark line defects (DLDs) that are characteristic of this degradation mode.

Simultaneous contribution of saturation in absorption and nonlinear photoacoustic effects in CuTPP under blue laser irradiation

The nonlinear optical responses of CuTPP are studied in toluene under the irradiation of a blue laser beam focused using a convergent lens. The z-scan setup is applied in order to determine nonlinear optical responses. Besides the nonlinear absorption response, which indicates reversed saturable absorption, intensity dependence photoacoustic waves are created in both hearable sound and ultrasonic waves. Hearable sound is recorded by a microphone and analyzed by Fourier transform analyzes in order to calculate the sonic frequencies. Because of the high absorption of dye molecules, a dilute solution is prepared to enhance transmittance from the cell to analyze profile of the passing beam. Diffraction rings pattern is observed with about 30 dense rings, in which the number of rings depends on the beam intensity that could be attributed to the higher frequency photoacoustic waves and the corresponding refractive grating induced by the waves. The nonlinear refraction of the solution is a simultaneous contribution of thermal lensing and photoacoustic effects, in addition to the saturation in absorption process which influences the nonlinear absorption behavior. This compound can be proposed as a good candidate in mode locking, Q-switching devises, and short pulse generators where we need a pattern from the activity of the saturable absorber at the cross section of the beam.

Analysis of laser thermal effect of funnel-shape beam generated by spherical lens pump system

The uniformity of temperature distribution of an end-pumped laser can be improved simply and effectively by introducing the correct amount of spherical aberration into the pump system. An iterative model of optical-thermal coupling is proposed to calculate the temperature distribution of the end-pumped laser amplifier. The results showed that, the thermal effect of the laser gain medium in the coupling system composed of spherical lenses decreased significantly compared with the coupling system composed of aspherical lenses. This is proved by a neodymium-doped yttrium orthovanadate (Nd:YVO4) amplification experimental system, which was end-pumped by a fiber-coupled laser diode (LD) with the optimized wavelength of 878 nm. A comparative study of the two pump systems showed that the decrease in the gain medium thermal wavefront peak-to-valley (PV), the thermal lens effect, and an improvement in beam quality are still observed in the pump amplification experiment with spherical aberration. At last, the relationship between the spherical aberration and the output brightness of the amplifier was established, which provides guidance for further optimization of the spherical aberration of coupling pump systems and improving the beam quality.

Исследование аномальной генерации вертикально-излучающих лазеров спектрального диапазона 850 nm с двойной оксидной токовой апертурой при большой величине спектральной расстройки

The static characteristics of 850 nm-range vertical-cavity surface-emitting lasers (VCSEL) based on strained InGaAs/AlGaAs quantum wells were studied in the wide range of oxide current aperture sizes and the origins of their anomalous behavior at large gain-to-cavity detuning was analyzed. The higher-order modes localized at the periphery of the oxide current aperture can appear in the studied VCSELs due to the lateral carrier spreading in the quantum wells and the specific profile of the oxide aperture (leading to the formation of a two-stage effective waveguide). Inhomogeneous carrier injection over the current aperture area in wide-aperture lasers leads anomalous start of lasing via high-order transverse modes, and the subsequent transition to the classical lasing via low-order modes with an increase in the current is due to a change of the gain-to-cavity detuning with an increase in the internal laser temperature. Anomalous lasing via higher-order modes in the case of narrow-aperture VCSELs becomes possible due to the increase in diffraction losses at the edge of the oxide current aperture for the fundamental mode, while the subsequent switching to the two-mode lasing is due to not only a decrease in the gain-to-cavity detuning, but also the thermal lens effect.