Thermal Lens Effect Research Articles

Fluorescence resonance energy transfer-thermal lens spectrometry (FRET-TLS) as molecular counting of methamphetamine.

A novel and sensitive approach has been presented for the determination of methamphetamine (METH) based on fluorescence resonance energy transfer-thermal lens spectrometry (FRET-TLS). Due to the affinity of fluorescein molecules to the surface of AuNPs through the electrostatic interaction and thereby caused reduction of the distance between fluorescein and AuNPs, the best way for de-excitation of excited fluorescein is FRET. The energy absorbed by fluorescein transferred to AuNPs causes enhancement of the thermal lens effect. The thermal lens of the fluorescence molecule could be enhanced through a proper acceptor. Upon the addition of methamphetamine, the fluorescein molecules aredetached from the surface of AuNPs, due to the stronger adsorption of methamphetamine. As a result, the fluorescence of fluorescein recovered, and the thermal lens effect of fluorescein decreased. The mechanism of energy transfer was evaluated by two different methods including time-resolved spectroscopy and thermal lens spectrometry. Under the optimal conditions, the thermal lens signal was linearly proportional to methamphetamine concentration in the range 5 - 80nM. The limit of detection and limit of quantitation were 1.5nMand 4.5nM, respectively. The detection volume and limit of molecules in the detection volume were 960attoliter and 87 molecules, respectively. The method was successfully applied for the determination of methamphetamine in human blood plasma and urine.

Asymmetric coherent rainbows induced by liquid convection* *Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0301202) and the National Natural Science Foundation of China (Grant Nos. 11674311 and U20A20205)

Coherent rainbows can be formed by focusing white-light laser into liquids. They are bilaterally symmetric interference rings with various shapes. Such interference rings arise from the temperature distribution of the liquid induced by laser heating, i.e., thermal lens effect, which changes the refractive index locally and thus the optical path difference. The up–down asymmetry of the interference rings is caused by convection in the liquid. With the increase of the viscosity, the interference rings change their shape from oval to circular shape. After a shutter is opened and the laser shines into the liquid, the interference rings are circular at the beginning. As time goes on, they gradually turn into anoval shape. Let the liquid go a free-fall at the beginning, the interference rings remain circular. All the three experiments have confirmed that the asymmetric interference rings are due to convection in the liquid associated with thermal lens effect. We also numerically simulate the two-dimensional heat conduction with and without convection, whose results agree well with our experimental observations.

A novel experimental approach for direct observation of magnetic field induced structuration in ferrofluid

This article proposes a novel approach to investigate the field-induced structuration in ferrofluids (FFs) at room temperature. The in-house experimental setup has been fabricated to perceive the influence of dipolar interaction and particle size of dispersed magnetic nanoparticles (MNPs) in FFs on the static magneto-optical (MO) properties under the influence of an external magnetic field. The diffraction fringes are observed in FF when a high-power laser passing from the diluted fluid system which appearing at a specific dilution and varies with the different sizes and volume fraction of MNPs. The change in shape and size of the diffraction fringes is observed with the variation of both, size of dispersed MNPs and the Dilution of the FFs which is used to calculate the refractive index coefficient. The number of diffraction rings increases with an increase in the size of dispersed MNPs and its diameter decrease with an increase in particle volume fraction. The effect of thermal lens and nonlinear optical (NLO) behavior gives the validation of field-induced refractive index coefficient appearing in FFs commutated by the fringe diffraction method. Further, the variation of intensity of transmitted light has been corroborated with the forward scattering resulting from the Brownian motion of the nanoparticle. The experiment validates the chain formation in FFs, as MNPs are working as mesoscopic cylinders resulting in specified scattering patterns. This experimental approach can lead to a new pathway for the development of potential applications such as tunable optical filters, optical limiters, and various photonics fields based on the tunable MO properties of FFs.

Quasi-Gaussian Multibeam Solar Laser Station for a Megawatt Solar Furnace

An alternative multirod solar side-pumping concept for the production of multiple quasi-Gaussian beams is proposed. This scheme was based on the One-Megawatt solar furnace in Odeillo, France, which collected and concentrated the solar light into a multilayered pyramidal pumping cavity placed at the focal zone. Each layer was comprised of a square array of four laser heads, each composed of a biconic surface that reflected the solar rays towards a Nd:YAG rod fixed inside a fused silica flow tube. A pyramidal reflector was placed inside the pumping cavity to close it and maximize the harness of solar energy. Compared to the previous multibeam solar laser station design for the same solar furnace, considerable alleviation of thermal lensing effects was achieved with the present approach, allowing the improvement of the laser beam quality factors and, consequently, the possibility of a 32-laser-beam generation, each with a quasi-Gaussian profile. For this case, 9.44 kW total laser power was calculated. Additionally, 20.01 kW total multimode laser power was numerically determined, which corresponds to a 10.93 W/m2 collection efficiency and a 2.0% solar-to-laser power conversion efficiency.

Quasi-Gaussian Multibeam Solar Laser Station for a Megawatt Solar Furnace

An alternative multirod solar side-pumping concept for the production of multiple quasi-Gaussian beams is proposed. This scheme was based on the One-Megawatt solar furnace in Odeillo, France, which collected and concentrated the solar light into a multilayered pyramidal pumping cavity placed at the focal zone. Each layer was comprised of a square array of four laser heads, each composed of a biconic surface that reflected the solar rays towards a Nd:YAG rod fixed inside a fused silica flow tube. A pyramidal reflector was placed inside the pumping cavity to close it and maximize the harness of solar energy. Compared to the previous multibeam solar laser station design for the same solar furnace, considerable alleviation of thermal lensing effects was achieved with the present approach, allowing the improvement of the laser beam quality factors and, consequently, the possibility of a 32-laser-beam generation, each with a quasi-Gaussian profile. For this case, 9.44 kW total laser power was calculated. Additionally, 20.01 kW total multimode laser power was numerically determined, which corresponds to a 10.93 W/m2 collection efficiency and a 2.0% solar-to-laser power conversion efficiency.

New beam deflection technologies for ultra short pulse lasers

Ultra short pulse lasers pose new challenges on the beam deflection technologies. In this paper we present three recent developments that have an impact on the application of ultra short pulse lasers in industry. The three technologies include:This scanning solution extends a laser scan system‘s working field by simultaneously controlling and moving a scan head and an XY-stage, thus delivering clear laser processing advantages, such as high processing throughput with an unprecedented level of accuracy. Process accuracy significantly improves, too, because a scan system‘s positioning error rises linearly with the image field size. The XL SCAN solution‘s combined motions at higher accuracy will use a smaller image field, thereby reducing this error.This micromachining sub system enables laser micro processing and drilling of flexibly variable geometries as well as laser cutting and structuring. Designed for ultra-short-pulse (USP) lasers, bore holes and cutting edges are exceptionally clean cut and don’t require post-processing.The achieved identical dynamics of all three axes now opens up entirely new processing strategies and opportunities. Furthermore, this new technology uses only reflective optical components. This allows using different wavelengths without dispersion and reduces thermal lens effects at power laser applications.

Continuous-wave Nd:KGd(WO4)2 single-longitudinal-mode laser* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11674273 and 61805209).

A continuous-wave Nd:KGd(WO4)2 single-longitudinal-mode laser is demonstrated with Fabry–Perot etalons in a simple linear cavity. The thermal lens effect is dramatically lowered by propagating the laser beam along the ‘athermal’ direction inside the laser crystal, which is very beneficial to removing the heat generated in the mode selection process. The maximum single-longitudinal-mode output power obtained is 64.8 mW at incident pump power of 4.7 W, corresponding to an optical conversion efficiency of 1.3% and a slope efficiency of 1.7%.

Thermal lens study of the Tris(2,2′-bipyridyl)iron(II) tetrafluoroborate

Experimental results of thermal lens measurements for Tris(2,2 -bipyridyl)iron(II) tetrafluoroborate solution are reported using the dual beam technique. The temporal evaluation of thermal lens was observed at input pump power of I=47 mW. The thermal lens effect was observed with increasing the pump power intensity as well. The evolution time of thermal lens and the fitting of the experimental transient curve were useful to evaluate the parameters of the Tris(2,2′-bipyridyl)iron(II) tetrafluoroborate solution.

The distortion of laser beams in a burst mode dye laser with 10 kHz repetition rates

In this paper, the distortion of laser beams in the burst of a burst mode dye laser at 10 kHz repetition rates is researched dynamically. The distortion caused by the thermal lens effects is shown when the repetition rates surpass 2.5 kHz, and is observed emerging 100 μs after the first pulse in the burst. The deflection angles between the laser beams of the first pulse in the burst and each pulse in the burst are measured to describe the distortion. The deflection angles increase to the steady states 400 μs after the first pulse. The maximum deflection angles increase from 0.5 mrad to 0.85 mrad at 10 kHz with the pump pulse energies increasing from 12 mJ to 30 mJ, and increase from 0.23 mrad to 0.4 mrad at 5 kHz with the pump pulse energies increasing from 12 mJ to 30 mJ. The results show that the distortion increases when the pump pulse energies and the repetition rates of the dye laser increase.

Selective loss tailoring of broad-area diode lasers

High-power high-brightness broad-area diode lasers have poor lateral beam quality, which has limited the application range of this kind of device. To improve the lateral beam quality, a sawtooth microstructure laser was proposed and the microstructures were carefully designed according to the profiles of each lateral mode and their dependence on the self-heating induced thermal lens effect. As a result of selective mode loss adjustment, a 42% improvement in lateral beam quality was achieved under the power-maintained condition. This technology enables us to develop high-brightness direct-diode laser systems.

Experimental investigation of the package of diode laser chip based on lateral heat flow suppression

To improve slow axis beam quality of diode laser (LD) and decrease slow axis divergence angle, a new package with lateral heat flow suppression was designed utilizing the difference in thermal conductivity between air and heat sink. The finite element analysis software was used to analyze the temperature distribution with lateral flow suppression package. It is shown that diode laser chip soldered on trough heat sink with width W=120 μm and length L=4000 μm can reduce slow axis divergence angle about 14%, from 12.25° to 10.49°, when working current was 15A. Correspondingly, beam parameter product (BPP) can reduce from 5.344 mm·mrad to 4.5763 mm·mrad and the brightness of slow axis increased about 5.5% than before. According to the result, the lateral flow suppression package can weaken higher order mode caused by thermal lens effect of diode laser so that decrease slow axis divergence angle effectively.

Six-rod/six-beam concept for revitalizing TEM00 mode lamp-pumped lasers

Low lamp input-to-TEM00 mode laser output conversion efficiency and strong thermal lensing effects are bottleneck issues in commercially available single-rod/single-beam lamp-pumped lasers. A six-rod/six-beam concept is proposed to substantially enhance lamp input-to-TEM00 mode laser output conversion efficiency. A 4-kW-single-thick arc lamp was used to efficiently pump six thin Nd:YAG rods simultaneously within a modified two-ellipse pump cavity. All of the design parameters were numerically optimized by Zemax® and LASCAD® software. Since each rod absorbed only a small part of the lamp power, significantly reduced thermal lensing and thermal stress problems were predicted. Asymmetrical laser resonators were adopted to enable a large mode-matching between the lamp pump mode and the fundamental laser mode within each rod. 34.82-W total TEM00 mode laser power was numerically calculated, corresponding to 0.87% lamp input power-to-TEM00 mode laser power conversion efficiency and a 2.18 times improvement over that of the state-of-the-art lamp-pumped laser. A TEM00 mode power-to-multimode power ratio of 59.0% was also obtained, being 2.40 times more than that of the most efficient single-rod TEM00 mode lamp-pumped laser. Six TEM00 mode laser beams emitting from the single laser head may provide a cost-effective laser solution to several processes in industry and scientific research. For the extraction of a single TEM00 mode laser beam, a laser resonant cavity with folding mirrors was also presented. A 15.34-W laser beam brightness figure of merit was numerically calculated, corresponding to a 13-fold increase over that from each of the six TEM00 mode laser beams.

Investigation of anisotropic thermal lens effect in a dual-polarized Nd:YLF laser.

The anisotropic thermal lens effect of a dual-polarization Nd:YLF laser is experimentally investigated by measuring the transverse beat frequency between TEM0,0 and TEM1,0 modes in the self-pulsing operation, and focal lengths of thermal lensing for both polarizations can be accurately determined. The focal length of the thermal lens for π polarization was observed to be negative and varies from -1.1 to -0.5m for the absorbed pump power increasing from 1.7 to 3.8 W. For σ polarization, the focal length of the thermal lens was determined to be positive and varies from 1.2 to 0.9 m for the absorbed pump power increasing from 8.4 to 10.9 W. The sensitivity factors of the thermal lens for both polarizations were evaluated to be Mπ=-0.54m-1/W and Mσ=0.1m-1/W, respectively.

Laguerre–Gaussian induced temperature and refractive index profiles in thermal lens effect

In this work, we present a theoretical model that describes induced temperature and refractive index profiles generated by the Laguerre–Gaussian (LG) excitation beam. A comparison between LG and conventional fundamental mode T E M 00 is drawn. It indicates that there is a decrease or increase around the optical axis ( z , r = 0 ) in the refractive index by L G 1 0 and L G 0 5 , thus enabling an enhancement of the refractive index gradient along the radius r . The temperature and refractive index profiles produced by the higher-order LG modes are compared with those of a conventional fundamental T E M 00 Gaussian beam. The results indicate that there is an increase along the radius in the refractive index gradient generated by L G 1 0 and L G 0 5 , which could enhance the sensitivity in photothermal lens detection.

Stability analysis and experimental measurements of the corner cube prism solid-state laser

In order to study the stability of corner cube prism solid-state laser under the effect of thermal lens, the resonant equivalence cavity, resonant G-parameter and auxiliary parameter theories were used to model the corner cube prism solid-state lasers containing a thermal lens. The laser stability with resonant cavity length was analyzed. In a stable condition, the beam radius was analyzed with regards to the distance from one of the corner cube prisms to gain medium by calculating the beam radius at each position under the resonator G-parameter theory. On this basis, a V-structured solid-state laser, pumped by a semiconductor laser and two corner cube prisms as a resonant, was built. In consideration of the effect of thermal lens produced by the pumped beam, the experimental measurements of the beam radius at two corner cube prisms and the gain medium were made for the resonant cavity length during stable condition of the laser. The experimental measurements agreed well with the simulation results, and showed that the maximum cavity length depended on thermal lens focal length of the gain medium, variation pattern in the beam radius at the corner cube prism, relationship between cavity length and the beam radius.

Numerical analysis of beam distortion induced by thermal effects in chirped volume Bragg grating compressors for high-power lasers

We conducted a study on the thermal effects in chirped volume Bragg gratings (CVBGs) used as optical compressors in high-power lasers. A simulation code was developed to predict the distortion of the pulses by the thermal effects. The types and significance of the thermal distortion were analyzed using an Yb:YAG laser with an average power of 250 W. The temperature distribution in the CVBG, the thermal lens effect, the beam propagation path changes due to the thermal lens, and the compression efficiency decrease due to the phase delay were analyzed. As a result, we predict that the peak power decreases to 63.9% due to the thermal effects under the operating conditions of 250 W. We also suggest new designs for the CVBG mount that minimize the peak power loss. We expect the peak power loss to be reduced only by 1.1% when using our new mount design for the CVBG.

A method to suppress the distortion of light spot during power amplification

We investigate the prominent role of 4f system in the suppression of beam shape distortion during laser power amplification. We find that 4f system can effectively suppress the distortion of the light spot caused by diffraction and thermal lens effect. In addition, the laser power can be amplified more efficiently by introducing a 4f system. Our results can benefit for applications with high requirements on the spot shape such as optical lattice and optical information processing.

Investigation into the thermal effects of a Nd:YVO4 crystal at 1342 nm using the bistability-like output characteristics

The influence of energy-transfer upconversion (ETU) and excited stimulated absorption (ESA) on the thermal effects of a Nd:YVO4 crystal was investigated in this paper. A stable continuous-wave (CW) single-frequency and intracavity frequency-doubled dual-wavelength 1342/671 nm laser was built, and the output power was manipulated by changing the transmissions of the output coupler. When the output coupler coated with a partial transmission film of 0.5% at 1342 nm was replaced by that of 8% at 1342 nm, the maximum output powers of the 671 nm and 1342 nm lasers ranged from 3.35 W and 1.08 W to 0.43 W and 6.35 W, respectively. By accurately measuring the threshold pump power as the incident pump power increased, a thermal loading of 6.8% caused by the ETU was successfully determined, which was the same for both output couplers. However, the thermal loading caused by the ESA had to be determined by measuring the threshold as the incident pump power decreased, and the measured thermal loading caused by the ESA was 14.03% for the output coupler with a transmission of 0.5%, which was larger than that of 9.13% for the transmission of 8% at 1342 nm. The measured results depicted that the thermal loadings caused by the ETU and ESA depended on the pump power and intracavity power of the 1342 nm laser, respectively. This technique for determining the thermal loadings of the Nd:YVO4 at 1342 nm using the bistability-like output characteristics is a good method of evaluating the thermal loadings and thermal lens effects of other laser crystals.

Compact 50 W All-Solid-State Picosecond Laser System at 1 kHz

Compact, stable, high-power and high repetition rate picosecond laser systems are excellent sources for optical parametric chirped pulse amplification systems and laser satellite ranging systems. Compared with the traditional complex high-power amplifier, this article reports a compact high-power picosecond laser system at a repetition rate of 1 kHz based on Nd:YAG bulk crystal. The thermal lens effect limits the regenerative amplifier to directly output higher energy. For this reason, multi-stage traveling-wave amplifiers are usually used to gradually increase the laser pulse energy. So as to achieve a compact structure, a regenerative amplifier that can output higher power at 1 kHz is designed in the laser system. The regenerative amplifier can output the power of 6.5 W at the pump power of 41.5 W; the beam quality of M2 factor was about 1.3. A more flexible thermal depolarization compensation structure is applied in the side-pumped amplifier, which can effectively compensate for thermal lens effect and thermal depolarization at different pump powers. Finally, the laser pulse can achieve an output power higher than 50 W at 1 kHz after passing through an end-pumped traveling-wave amplifier and a side-pumped traveling wave amplifier.

Investigation of the current influence on near-field and far-field beam patterns for an oxide-confined vertical-cavity surface-emitting laser.

This experiment presents dynamic behaviors between the operating current and the optical beam images of vertical-cavity surface-emitting lasers (VCSELs) with two different aperture diameters of 3 µm (single-mode) and 5 µm (multi-mode). These VCSELs exhibit complex optical phenomena under current injection such as thermal effects, modal competition, carrier distribution, and laser coherence which make the light field distribution difficult to predict. In this report, the DC properties, optical spectrum, and optical images were measured together at different operating currents to accurately evaluate the characteristics of the lasers. Unlike previous works, the variations of the far-field angle were precisely evaluated by the side-mode-suppression ratio (SMSR) of the optical spectrum. In addition to commonly used transform functions such as the Gaussian beam formula, the SMSR provides another tool for the judgment of far-field divergence which could prevent inaccurate analysis. Moreover, the impact of thermal lensing was calculated by the DC measurement and demonstrated by the far-field measurement at high injection current. Through this experiment, the interaction between the injection carrier, thermal lens effect, and current spreading was described as fully as possible.