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Modal instabilities in high power fiber laser oscillators
摘要： Transverse mode competition and instabilities in high-power fiber oscillators have been studied experimentally by monitoring the dynamic power exchanges and characteristic frequencies of the transmitted fundamental mode (FM) and scattered high-order modes (HOMs) of the fiber laser cavity under CW and pulsed pumping. The FM and HOM power evolution indicates the presence of two competing effective laser cavities which result in rich output dynamics and full chaotic operation. The thermal and inversion related contributions to the observed instabilities have been identified by monitoring the associated characteristic instability frequencies under pulsed pumping. It is shown that in the transient regime, both inversion and thermal effects contribute successively to the observed power instabilities. Increasing the pump power leads to full chaotic response through an interplay between transverse and longitudinal mode instabilities.
关键词： Transverse mode instability,inversion effects,thermal effects,high-power fiber lasers,mode competition
更新于2025-09-23 15:19:57
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A study on the heat distribution and oxidative modification of aged dammar films upon Er:YAG laser irradiation
摘要： This work shows the impact of one Er:YAG laser pulse to both the surface and bulk of aged dammar varnishes. Tests were performed in fluences ranging from 0.56 to 2.4J/cm2 and pulse durations of 100 and 300μs. Temperature changes upon irradiation were examined by (a) live recordings with an infrared thermal camera, and (b) determining the optical properties of the varnish, which allowed for estimates of the linear absorption coefficient and the temperature rise associated with the laser irradiation. Transmission and Fourier Transform Infrared (FTIR) spectroscopy studies captured increased energy transmissions through varnish films, as well as registering a reduction of hydroxides and carbon–hydrogen bonding as a function of fluence, respectively. FTIR and Differential Scanning Calorimetry (DSC) experimentally determined the absorption coefficient and the specific heat capacity of dammar films, and an overview of laser spots was carried out with Scanning Electron Microscopy (SEM). Evidence on the varnish response to Er:YAG laser irradiation is provided.
关键词： Er:YAG lasers,transmission,thermal effects,dammar varnish,analysis,modelling
更新于2025-09-23 15:19:57
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[IEEE 2019 41st Annual International Conference of the IEEE Engineering in Medicine & Biology Society (EMBC) - Berlin, Germany (2019.7.23-2019.7.27)] 2019 41st Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC) - Laser ablation in biliary tree: analysis of the intraductal and superficial thermal effects during the treatment
摘要： The treatment of choice for the unresectable cholangiocarcinoma is based on biliary decompression procedures. Despite stent placement is the standard of care, it is related to well-known complications. Hence, alternative techniques were proposed. Ideally, they should guarantee an adequate intraductal disobstruction, without injuring the surrounding tissues. This pre-clinical study aims to investigate the thermal effects of the laser ablation (LA) in the biliary tree, in terms of intraductal and surrounding tissue temperature achieved with different laser settings. The common bile ducts (in their upper and lower portions) of two pigs were ablated for 6 minutes with a diode laser at 3 W and 5 W. A custom-made laser applicator was used to obtain a circumferential ablation within the ducts. The intraductal temperature (Tid) was monitored by means of a fiber Bragg grating (FBG) sensor, while an infrared thermal camera monitored the T distribution in the surrounding tissues (Tsup). A maximum T difference of 65 °C and 57 °C was evidenced between the two power settings for the Tid measured in the upper and lower ducts, respectively. The mean difference between Tid and the averaged Tsup values was evaluated. At 5 W, a difference of 37±3 °C and 44±10 °C were obtained for the upper and lower ducts, respectively. At 3 W, a T difference of 2±1 °C was obtained for the upper biliary duct, while a difference of 8±1 °C was documented for the lower duct. Based on the results obtained in this preliminary study, the possibility to equip the laser probe with temperature sensor can improve the control and the safety of the procedure; this solution will guarantee the monitoring of the treatment while preserving the lumen and the surrounding structures.
关键词： thermal effects,fiber Bragg grating,biliary tree,cholangiocarcinoma,infrared thermal camera,laser ablation
更新于2025-09-19 17:13:59
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Optimizing Settings for Officea??Based Endoscopic CO <sub/>2</sub> Laser Surgery Using an Experimental Vocal Cord Model
摘要： Objectives/Hypothesis: To provide insight in the thermal effects of individual laser settings in target tissues to optimize flexible endoscopic CO2 laser surgery treatment. Study Design: Experimental laboratory study. Methods: Thermal effects of the CO2 laser using a fiber delivery system were visualized using the color Schlieren technique in combination with a polyacrylamide gel tissue model. Variable settings were used for emission mode, power, laser fiber distance, and laser duration, which were evaluated in every possible combination. Collateral thermal expansion and incision depth were measured. To validate the model, the results were compared to histology after CO2 laser irradiation of ex vivo human vocal cords, and the intraclass correlation coefficient was calculated. Thermal damage and incision depth were measured by a blinded pathologist. Results: Of all parameters studied, duration of laser irradiation had the greatest effect on thermal expansion. Increased distance between laser tip and target tissue resulted in significantly reduced incision depth and increased thermal expansion. Pulsed emission modes led to increased incision depths. The intraclass correlation coefficient for consistency between the model setup and the ex vivo human vocal cords was classified as “fair.” Conclusions: By using high-intensity pulsed lasers at minimal distance to the target tissue, exposure times and subsequent damage to surrounding tissue can be reduced. If an evaporation technique is used, lower power in continuous wave at a larger distance to the target tissue will lead to superficial but broader thermal effects. The model setup used in this study is a valid model to investigate laser-induced thermal effects in vocal cord tissue.
关键词： vocal cords,thermal effects,Flexible endoscopic CO2 laser
更新于2025-09-19 17:13:59
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Investigation of spectrally-dependent phonon relaxation mechanism in Yb:YAG gain media and its consequences for thin disk laser performance
摘要： Nonlinear phonon relaxation in Yb:YAG gain media has previously been investigated, and a hypothesis correlating photo-induced free electrons and heat generation in crystals pumped at a wavelength of 940 nm was established (Brandt et al 2011 Appl. Phys. B 102 765–8). Later, we demonstrated efficient suppression of nonlinear phonon relaxation by zero-phonon line (ZPL) pumping of Yb:YAG (969 nm). However, no hypothesis has yet been tested that correlates the heat load and the free electrons in Yb:YAG media. We investigate, both theoretically and experimentally, the behaviour of an Yb:YAG slab pumped by a high-power laser diode at a conventional broadband line (absorption maximum at 941 nm) and a ZPL (absorption maximum at 969 nm), and we compare the generated photocurrent, temperature, and absorbed pump power for those two pump wavelengths. Although the hypothesis established by Brandt et al (2011 Appl. Phys. B 102 765–8) was not confirmed, and the measured photocurrent was identical for the two pump wavelengths, our measurement contributes to the knowledge of the inner processes in Yb:YAG active media. In the following steps we characterized an Yb:YAG thin disk laser, measured the spectrally-resolved absorption and disk surface temperature, and optimised our predictive model of thin disk behaviour. All the experimental and theoretical results are reported.
关键词： numerical modelling,thin disk,thermal effects,Yb:YAG,photocurrent
更新于2025-09-16 10:30:52
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An integrated biological analysis and flow rate sensing for the real-time detection of carcinogen in water based on Co2+-doped optical fibers
摘要： The multi-parameter measurement has significant meaning for biosensor to enhance their practical sensing performance. For the biological detection, simultaneous measurement of biological molecule and flow rate could improve the accuracy and sensitivity of sensors significantly. In this paper, an integrated biological analysis and flow rate sensing based on Co2+ doped tilted fiber Bragg gratings has been proposed and experimentally demonstrated for the detection of the Benzo[a]pyrene molecules. A tilted fiber Bragg grating with a tilted angle of 8o was inscribed into a Co2+ doped fiber. The flow rate and Benzo[a]pyrene molecules can be measured simultaneously by interrogating the resonance wavelength of core mode and wavelength interval between the core mode and cladding mode according to the heat exchange and evanescent field of the cladding mode, respectively. Experimental results show that the real-time measurement sensitivities of 12 pm/pM and -0.13 nm/(μL/s) for the Benzo[a]pyrene detection and flow rate were achieved, respectively. Thus the technique appears to have potential applications in chemistry, medicine, and biology.
关键词： Thermal effects,Co2+ doped tilted fiber Bragg gratings,Flow rate measurement,Benzo[a]pyrene detection
更新于2025-09-12 10:27:22
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Thermal Effects in the Ablation of Bovine Cortical Bone with Pulsed Laser Sources
摘要： Lasers have advantages as bone surgical tools over mechanical methods, but two goals should be achieved to assure its use: Similar ablation rates to those obtained with mechanical tools (1 mm3/s at least) and to avoid thermal damage, a condition that can prevent proper bone healing. We present results of cow femoral bone with a 355 nm nanosecond (ns) and a 1064 nm picosecond (ps) pulsed laser sources that allow us to discuss the influence on the process of pulse duration and the selective ablation through high energy absorption (as bone highly absorbs 355 nm radiation). The treated samples were characterized by scanning electron microscopy (SEM) and Raman spectroscopy. The evaluation of the thermal effects produced in the samples shows clear differences between both laser sources: On one hand, the ns laser allows reaching high ablation rates (around 1 mm3/s); Raman spectra show no signal of bone carbonization, but unavoidable thermal effects in the form of melted and solidified material have been observed by electron microscopy in the samples treated with this laser. On the other hand, ablation without any sign of thermal effects is obtained using the ps laser, but with lower ablation rates, (around 0.15 mm3/s).
关键词： hard tissue,thermal effects,ns-laser,bone osteotomy,ablation rate,laser ablation,ps-laser
更新于2025-09-11 14:15:04
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[IEEE 2018 31st International Vacuum Nanoelectronics Conference (IVNC) - Kyoto, Japan (2018.7.9-2018.7.13)] 2018 31st International Vacuum Nanoelectronics Conference (IVNC) - An impact of thermal electron energy on the field-electron emission from nanosized silicon tips
摘要： Thermal effects associated with the thermal energy of electrons play an important role in the process of field emission. To estimate the contribution of these effects, a self-consistent numerical calculation of the temperature of the semiconductor emitter, heated by the field-emission current, was carried out. It was experimentally obtained that the melting of the silicon field emitter occurs at currents less than 1 μA, for which the calculated temperature of the silicon tip varies only up to 420 K and does not exceed the melting point of Si. For this reason, we suppose that this effect can be caused by the sputtering of the anode material by hot field-emitted electrons that collect a high power density (~300 W/cm2 at 1 μA) on the anode surface. It also can lead to the possible generation of self-sustained plasma in the space gap between anode and cathode. The considered thermal phenomena should be taken into account during the development and predictive modelling of vacuum nanoelectronic devices.
关键词： vacuum nanoelectronics,field-electron emission,emission current,nanoscale emitter,thermal degradation,thermal effects
更新于2025-09-10 09:29:36
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Structural and morphological behavior of bismuth thin films grown through DC-magnetron sputtering
摘要： Bismuth thin films were grown onto glass substrates through the DC magnetron sputtering technique. The effects of substrate temperature on the microstructure of the films were evaluated. The structural behavior was analyzed via X-ray diffraction, and it showed a marked influence of the substrate temperature on the crystallite size and the micro-stress. The morphologies evaluated through Electron Probe Micro-analyzer images indicated a slight difference in the grain size with an increase in temperature as well as a significant increase in surface roughness, according to profilometer measurements.
关键词： microstructure,thin film,Bismuth,sputtering,thermal effects
更新于2025-09-09 09:28:46
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Thermal Effects on Modal Properties of Dual-Core Yb-Doped Fibers
摘要： The impact of thermo-optic refractive-index changes arising from the core heat load in a dual-core fiber amplifier is studied through finite-element based numerical simulations. Trends in coupling lengths, effective area, core overlaps of fundamental and higher-order supermodes, effects of asymmetric heat loads, and thermo-optic mode coupling parameters are quantified. It is concluded that the coupling between cores is only moderately altered by the overall heat load, but can be strongly sensitive to asymmetric loads. The influence of the core coupling strength is demonstrated to be very important on the supermode effective area, besides the shrinking due to thermal effects. While the thermo-optic index perturbations can lead to guidance of higher-order supermodes, it seems realistic to maintain single-mode operation through gain suppression of the higher-order modes. On the other hand, thermo-optic couplings between fundamental supermodes are found to be quite strong.
关键词： dual-core fiber,fiber amplifier,supermodes,thermo-optic,thermal effects
更新于2025-09-04 15:30:14