Analysis and monitoring of laser beam energy distribution
contact processing technology has been used in the past for cutting, welding and marking of steel, alloy steel and other materials. The latest progress in the structural design of high-power (average power above 1kW) CO2 lasers has saved the purchase and use costs of these lasers. Therefore, high-power CO2 lasers have been recognized in many production processes that were originally reserved for other technologies. Laser welding and cutting provide the advantages of non-contact processing, which makes it possible. For example, laser welding can use remote welding joints for large-area processing. Compared with contact machining, the heat affected zone (HAZ) produced by laser machining on the workpiece is much smaller, which reduces the size problem of the processed material and is conducive to the manufacturing of precision parts. As long as the beam is stable and focused on the workpiece, laser processing has a significant cost advantage over non laser processing
laser processing is first a thermal change process . The energy emitted by the laser focuses on a small target area and transfers the heat to the processed material. No wonder many processing processes are highly dependent on the energy absorbed by the material. The efficiency of the processing process is often a function of the square or cube of the irradiance . Therefore, it can be concluded that the total energy and energy spatial distribution of the focal spot on the workpiece are the key to the success of the machining process, and are very sensitive to the deformation of the shape of the spatial energy distribution of the laser beam. Many CO2 lasers not only output single transverse mode beams , so the quality of beam mode is very important
in laser welding, the gap adjustment between parts must be completely maintained, which requires the energy of the laser beam to always aim at the same target area without focal spot drift. If high-speed welding is carried out, poor beam structure can cause poor weld. In laser cutting, the beam quality and focusing ability are very important to the quality of the cut itself. Poor quality beams can cause parts to be scrapped or repaired and increase costs. Despite these limitations, laser processing still has many advantages, enough to become the mainstream technology of material processing in the future
spatial beam energy distribution analysis is a measurement method, which combines all the variables constituting the beam into a clear image. This method is applicable to all lasers, not just CO2 lasers . The most commonly used method for analyzing the beam energy distribution of CO2 lasers is the acrylic acid mode ablation method. In this method, the unfocused beam is directed to an acrylic target. The beam energy vaporizes the acrylic material, and the focal spot profile is proportional to the spatial energy distribution of the beam itself. The profile formed by material gasification describes the spatial energy distribution of the laser beam in the process of irradiating the acrylic target (generally lasting for several seconds)
although this method has been widely used, the accuracy and repetition accuracy largely depend on the skill of the operator. A large number of flammable and toxic vapors are generated in the workshop and must be pumped out. Moreover, the instantaneous response of the laser beam on the optical path cannot be measured by this method, for example, it may mask the change at the beginning of the process. In short, the mode ablation method can only be regarded as an approximate description of the performance of the laser beam
3. Automatic storage: automatic storage of experimental data and conditions
in the past 10 years, some semi electronic diagnostic methods with different effects have been developed. Most of them try to sample unfocused beams, that is, a small part of representative beams are directed to a certain sensor, so as to obtain the spatial energy distribution map of the main beam. As for the application of high-power laser, the sampling is either a micron sized hole on a small hollow tube or a small mirror at the end of a fine metal wire, which directs a small part of the original beam to a thermoelectric single element sensor, and then the sensor converts the absorbed energy into a proportional electrical signal
however, in order to sample the entire beam, the aperture or the radiometer must pass through the beam repeatedly in order to reproduce the entire beam as a composite image. The image thus generated is the cumulative result of time-sharing scanning of the light beam. The scanning time is 2 ~ 10 seconds, which varies with the instrument. This method avoids toxic gases, but like the acrylic acid mode ablation method, it can not provide any information about the instantaneous response of the laser beam. One possible drawback is that since the sampling device passes through the beam, it is impossible to determine with certainty whether the measurement method itself will affect the beam quality
in order to make the electronic beam energy distribution measurement system superior to the traditional method, it must be able to analyze the beam in real time, so that the end user can tune or adjust the beam at any time without waiting for the measurement instrument to respond. The system must be very reliable and can withstand the severe test every day in the production environment that produces dust and smoke. The system must be able to be quickly installed in place, or permanently fixed on the optical path, and easy to operate. Even if it is used by unskilled technicians, it can also provide detailed quantitative information that traditional methods cannot provide. Finally, the system can not interfere with the main beam in any form, otherwise some artifacts will be introduced into the analysis process
in some applications, it is not enough to periodically evaluate the mode quality of laser beam. For example, in the field of medical devices, the verification of mode quality is the key to the successful production of medical equipment parts. It also includes other operations that result in time loss and production reduction due to the recording of nonconforming parts
in order to meet the increasing market demand for high-power CO2 laser beam monitors, spiricon and II-VI jointly developed a powerful and easy to maintain industrial laser beam monitoring system. The system adopts ready-made optical elements, and can be installed on the new laser workstation through the laser system information acquisition card, or it is clearly pointed out to renovate and improve most industrial laser systems in the adhesive industry according to the "1015" development plan program for China's petroleum and chemical industry issued by the China Petroleum and Chemical Industry Association. It provides detailed real-time beam images for end users and records important laser beam parameters. Once any important parameter reaches the preset limit, the system will start the alarm signal to remind the operator that the fault is imminent, so that the operator can take appropriate actions. In addition, the system can be used to diagnose general laser faults, such as aging of output coupler or deviation of laser resonator. Using the diagnostic function of the system, technicians can recover the working state of the laser in a short time, so as to improve the benefit of laser processing
unlike other commonly available beam analysis equipment, this embedded system is completely transparent to the irradiation process, because there are no active devices that interrupt the beam, and all optical elements are liquid cooled passive mirrors. The chassis itself uses the same purified gas as the main optical path, and once the system is installed, it only needs to be maintained like other optical devices. Due to the real-time operation of the instrument, it is very suitable for tuning and adjusting the laser after normal maintenance or diagnosing the actual processing events
remote control laser welding is a new "enabling" technology. Unlike seam welding, it must direct the energy of the laser beam to many points and weld through the shutter switch. For such a weld, the two key measurement results are the spatial energy distribution of the beam in the welding process and the total energy of the beam propagation. Therefore, the new process monitoring system is most suitable for this purpose. If the welding data of short duration are analyzed, the beam width and beam profile change slightly during the welding process (Fig. 1)
Figure 1: real time energy distribution shows that the beam width increases by 10% and the peak energy decreases by 10% during 1/2 second welding
many arguments arise from the theory that the spatial energy distribution of unfocused beams is "copied" onto the focal spot. Although the two sides have argued for many years, so far there are few examples to support either side. During the identification of this system, two different instruments are used to measure the unfocused beam and focal spot simultaneously. The comparison of the results clearly shows that the spatial energy distribution of the unfocused beam and the focused beam of the laser is almost identical. In the process of identification, the focus profile is measured and compared under the two conditions of installing and removing the instrument on the optical path. The results show that the system does not affect the focal spot structure. Figure 2 is an image taken by two systems
Figure 2: the hot spot of the original beam also appears around the focal spot
one side of the argument argues that it is of no practical value to study the spatial energy distribution of the focal spot by studying the spatial profile of the unfocused beam; The other side insists that the poor structure of unfocused beam will be reproduced after focusing. In this example, there is a "hot spot" on one side of the unfocused beam, and the transverse mode structure is a "doughnut", that is, TEM 01 transverse mode, because its energy distribution pattern looks like a typical doughnut. The focal spot measurement also shows the same transverse mode structure, and the "hot spot" also appears on the focal spot. In this case, the spatial energy distribution of the original beam becomes the precursor of the focal spot energy distribution
with the long-term use of the laser, the continuous monitoring process can warn of impending failure, which must be repaired in time. Using the continuous monitor in the system, important beam parameters can be tracked in the machining process
beam energy distribution brings economic benefits to the above measurement methods, and saves costs due to increased productivity, lower scrap rate and reduced downtime. As the processing becomes more rigorous, laser energy distribution and monitoring technology will become more and more cost-effective
larry green is the industrial product sales manager of spiricon (Logan, Utah)
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