Fiber lasers, boasting advantages such as excellent beam quality, high energy density, high electro-optical conversion efficiency, good heat dissipation, compact structure, maintenance-free operation, and flexible transmission, have become the mainstream direction in the development of laser technology and the main force in applications. The overall electro-optical efficiency of fiber lasers ranges from 30% to 35%, with most of the energy dissipated in the form of heat. Therefore, the temperature control during the operation of the laser directly determines the quality and service life of the laser. Conventional contact temperature measurement methods will damage the structure of the laser itself, while single-point non-contact temperature measurement methods cannot accurately capture the fiber temperature. Using an infrared thermal imager to detect the entire section of the optical fiber, especially the temperature at the fiber fusion splice, can strongly guarantee the research and development as well as quality control of fiber products. This article introduces infrared solutions in laser processing and laser research and development, including processes such as beam quality analysis, fiber laser temperature detection, thermal imager, laser welding, and marking. 1. Laser Beam Analysis The MAGRay laser beam analyzer series can image and analyze various wavelength beams in the visible, near-infrared, short-wave infrared, mid-wave infrared, long-wave infrared, and far-infrared ranges. It measures spot size, 2D/3D energy distribution, divergence angle, ellipticity, beam pointing stability, waist position and size, as well as the beam quality factor M². MAGRay has a high dynamic range, is compatible with continuous and pulsed lasers, can measure both far-field and near-field spots, and can monitor continuous changes of the beam in real-time, outputting up to 100 image data per second. MAGRay offers two specifications: high sensitivity and high range. The high-sensitivity specification can measure a minimum power of 0.04mW/cm², and the high-range specification can measure a maximum power of 400W/cm². MAGRay is equipped with professional application programs and data analysis software, which can perform contrast adjustment, detail enhancement, selection of multiple color palettes, video recording and playback of beam changes, line intensity analysis, etc. It can also directly export Excel data, facilitating researchers' independent analysis and chart production. Professor Jerome Faist from ETH Zurich is the inventor of the first quantum cascade laser (QCL) in 1994. His research team used MAGRay in cutting-edge research on next-generation lasers. The following figure shows the measured beam energy distribution of a surface垂直发射 long-wave infrared QCL laser. 2. Fiber Laser Temperature Detection Temperature Monitoring of Fiber Lasers In fiber lasers, the fiber may absorb laser energy, causing the temperature to rise, accelerating fiber aging, and reducing the reliability and service life of the laser. During use, using a thermal imager to detect the temperature distribution of the entire fiber and components, especially the temperature at fiber fusion splices and connections, can timely detect abnormalities. The "530 W All-Fiber Structure Continuous Thulium-Doped Fiber Laser" published in the 69th issue of *Acta Physica Sinica* in 2020 used a thermal imager from JuggerTech to observe the temperature of the Tm³⁺-doped fiber. When the maximum output power reached 530 W, the maximum temperature exceeded 60°C, but there were no obvious amplified spontaneous emission and nonlinear effects, and the output power was only limited by the pump power, verifying the reliability of domestic Tm³⁺-doped silica fiber in high-power systems. 3. Laser Welding Closed-loop laser power control systems used in cladding and laser metal deposition processing need to perform real-time measurement and feedback on the morphology and temperature of the molten pool heated by the laser. Using JuggerTech's MAG series high-temperature online thermal imager and image processing algorithms to identify the molten pool width and center point, and real-time control the position and power of the laser can effectively improve welding quality. The F6 scientific-grade thermal imager can provide full-range temperature monitoring from 0 to 2500°C, clearly showing the temperature details of the entire heating and welding process; it supports a maximum data rate of 100Hz, which can record rapid temperature change processes; it provides temperature data stream recording, and can choose slow playback to reproduce complete details. In a high-energy laser environment, conventional thermal imagers themselves are susceptible to the laser heating effect, resulting in temperature measurement errors. The F6 scientific-grade thermal imager can be used in high-energy laser environments with wavelengths less than 2μm while maintaining temperature measurement accuracy. It should be noted that long-wave carbon dioxide lasers with wavelengths greater than 7μm will cause irreversible damage to the thermal imager, and users need to declare in advance to provide laser-safe products. Monitoring of Welding Molten Pool The sensing range of conventional thermal imagers is long-wave infrared of 7.5-14μm. Since the optical system for laser welding includes materials such as quartz glass, long-wave infrared cannot penetrate, so the thermal imager needs to be installed on the side to observe the welding surface without obstruction. To highly integrate the entire monitoring and control system, accurately align the coordinates of the laser and the camera, and improve processing accuracy, the ideal design is to adopt an optical system where the laser beam and the monitoring camera are coaxial. JuggerTech's short-wave infrared thermal imager provides temperature measurement in the 0.9-2.5μm band, which can penetrate quartz glass, so it can be coaxially integrated with the laser optical system. 4. High-Speed Laser Marking Laser marking is widely used in many industries. During marking, the temperature of the material surface rises, and thermal imagers can be used for real-time monitoring of marking quality. To improve efficiency on the production line, marking is usually completed while the target is moving rapidly. Conventional thermal imagers have a long response time, which will cause image trailing when observing moving targets, affecting the recognition effect. JuggerTech's high-speed thermal imager can effectively prevent image trailing and still clearly distinguish thermal distribution details when the target is moving rapidly. 5. Near-Infrared High-Power Semiconductor Laser Chips High-power semiconductor laser chips are the core light sources in contemporary high-energy lasers represented by fiber, solid-state, and direct semiconductor lasers. The power, brightness, and reliability of laser chips, as core indicators, directly affect the performance and cost of laser systems. The main structure of a semiconductor laser chip includes an epitaxial light-emitting layer that provides a laser gain medium, an electrode that injects carriers into the epitaxial light-emitting layer, and a cleaved cavity surface that forms a resonant cavity. The development process of the chip includes epitaxial structure design and material growth, chip structure design and preparation process, cavity surface cleavage passivation treatment and optical coating, chip packaging testing, chip life reliability and performance analysis, etc. Among them, the three key technologies that directly affect the core indicators are epitaxial structure design and material growth, chip structure design and preparation process, and cavity surface cleavage and passivation treatment. (1) Epitaxial Structure Design and Material Growth Epitaxial structure design and material growth involve the gain and pumping of the laser, directly affecting the electro-optical efficiency of the chip. The main factors are heterojunction and bulk material voltage loss, carrier leakage loss, and light absorption loss. According to the energy band analysis of semiconductor materials, the heterojunction voltage mainly comes from the interface between the confinement layer and the substrate and the waveguide layer. The heterojunction voltage of the chip is effectively reduced through interface grading and high doping optimization. The bulk material resistance can be achieved by adjusting the material composition to improve carrier mobility and increasing the doping concentration. Reducing carrier leakage loss requires a sufficient carrier confinement barrier, especially the p-side electron barrier. Therefore, the reduction of bulk material resistance and the improvement of carrier confinement need to be considered comprehensively to optimize the material composition. Light absorption loss can usually be achieved by adopting an asymmetric ultra-large optical cavity waveguide structure design. Under the condition that the total thickness of the waveguide layer remains unchanged, reducing the thickness of the p-side waveguide layer and increasing the thickness of the n-side waveguide layer make the main part of the light field distribute on the n-side with low absorption and low resistance, reducing the overlap between the light field and the high-absorption p-side, reducing the bulk material voltage, and reducing light absorption loss. At the same time, combined with the graded doping distribution design, the simultaneous optimization of bulk material voltage loss and light absorption loss is realized. Laser chips in the 900 nm band usually use InGaAs quantum wells as gain materials, and AlInGaAs quantum wells with high strain are used to improve gain. However, as a quaternary material, AlInGaAs quantum wells have more stringent requirements for material growth control. It is necessary to optimize the atmosphere ratio and growth temperature rate to improve the nucleation energy of quantum well bulk defects, thereby reducing the defect density of quantum wells and growing high-quality high-strain quantum wells. (2) Chip Structure Design and Preparation Process When operating in high-power mode, the intensity of the lateral high-order mode of the chip increases, leading to a sharp increase in the divergence angle and a decrease in brightness. Literature reports generally use absorption and scattering at the waveguide edge to reduce the intensity of high-order modes, but this will also cause additional absorption loss to low-order modes, reducing the total optical power. In addition, when working at high power, the light field intensity of the chip is unevenly distributed in the longitudinal direction, while the carrier concentration generated by current injection in conventional structure chips is uniformly distributed in the longitudinal direction. Therefore, the distribution of light field intensity and carrier concentration cannot match, which will produce longitudinal spatial hole burning effect, leading to power saturation. Adjusting the device structure of carrier injection distribution is a way to solve this problem. (3) Cavity Surface Cleavage and Passivation Treatment The main failure mode of high-power semiconductor laser chips is catastrophic optical damage (COMD) of the cavity surface. COMD comes from light absorption in the cleaved cavity surface and its adjacent areas when the chip works at high power. Surface light absorption is caused by dangling bonds on the cleaved surface, surface oxidation, and surface contamination. However, conventional cavity surface cleavage is carried out in the atmosphere or low vacuum environment, which cannot avoid this problem. The light absorption near the cleaved surface area comes from interband absorption. When the chip works at high power, the temperature in this area increases, leading to a decrease in the material band gap and enhanced interband absorption. The most effective way to reduce such absorption is to form a wide band gap (low absorption) window structure. Through the development of epitaxial structure design and material growth, chip structure design and preparation process, and cavity surface cleavage and passivation treatment, Suzhou Changguang Huaxin Photoelectric Technology Co., Ltd. (hereinafter referred to as "Changguang Huaxin") has launched a 28 W semiconductor laser chip. The power improvement of the chip mainly comes from the optimized design of the chip's epitaxial structure and the improvement of the special cavity surface treatment technology. The output power of semiconductor lasers is mainly affected by factors such as laser threshold, slope, and high-current power bending. Usually, the threshold is reduced and the slope is increased by reducing the doping concentration of the pn junction, but too low a doping concentration will lead to an increase in pn junction resistance and an increase in chip voltage. To solve the problem of optimizing the balance between threshold slope and voltage, Changguang Huaxin optimized the thickness of the waveguide layer of the asymmetric large optical cavity structure and carefully designed the distribution of doping concentration in different regions of the pn junction, achieving the effect of reducing the threshold, improving the slope efficiency, and keeping the voltage basically unchanged. High-current bending is mainly due to the reduction of internal quantum efficiency during high-current injection. Changguang Huaxin optimized the energy band structure of the material near the gain region of the laser structure, improved the confinement ability of electrons injected into the pn junction, and effectively enhanced the quantum efficiency during high-current injection. While optimizing the power of the laser chip, Changguang Huaxin continues to improve the material quality during the special cavity surface treatment process, reduce the proportion of defects, improve the ability of the cavity surface to resist catastrophic optical damage, and ensure that the 28 W high-power laser chip meets the requirements of the industrial market for laser life. 6. Main Application Points of Infrared Thermal Imagers in Fiber Temperature Monitoring Quality Monitoring of Fiber Fusion Splices In the manufacturing process of high-power fiber lasers, there will be certain optical discontinuities and defects at the fiber fusion splices. Severe defects will cause abnormal heating of the fiber fusion splices, which may further damage the laser or burn the heating points. Therefore, temperature monitoring of fiber fusion splices is an important link in the manufacturing process of fiber lasers. Using an infrared thermal imager can realize temperature monitoring at the fiber fusion splices, thereby judging whether the quality of the tested fiber fusion splices is qualified and improving product quality. LD Pump Source The laser power output by a single LD chip is limited. Pumping packages multiple LD chips together to achieve an increase in output power. However, the pump generates a lot of heat, so the temperature directly affects the laser wavelength output by the chip. Using an infrared thermal imager to conduct incoming quality inspection on each pump and return unqualified pumps can ensure the overall quality of the laser. Laser Reflection Protection Verification and Detection Fiber lasers are easily damaged by back-reflected lasers from metal workpieces. Therefore, high-quality fiber lasers need to have a reflection protection mechanism, and before leaving the factory, a certain power of laser is input in reverse simulation to ensure quality. Using an infrared thermal imager for detection makes the laser reflection protection verification accurate and reliable. Beam Combiner The function of the beam combiner is to combine N paths of pump lasers into 1 path of laser to realize high-power output of the laser. Using an infrared thermal imager for factory inspection can effectively reduce the probability of pump returns. 7. Long-Wave Laser-Resistant Thermal Imagers Thermal imagers are often used for temperature monitoring in high-energy processing processes such as laser cutting. In some applications, carbon dioxide lasers are used, whose wavelength is exactly in the sensing range of conventional thermal imagers. Excessive energy (even scattered from the object surface) will instantly cause irreversible damage to the uncooled infrared focal plane detector in the thermal imager. Long-wave laser-resistant thermal imagers can shield specific wavelengths and are safely used in processing occasions with high-energy long-wave infrared such as carbon dioxide lasers. In addition, short-wave thermal imagers are not damaged by long-wave infrared and can be directly used for high-temperature heating process monitoring of long-wave infrared lasers. Article sources: Laser Manufacturing Network, InfiRay, Optoelectronics Network