Laser Beam Energy Distribution Analysis and Monitoring in Laser Processing

Cutting, welding, and marking of steel, alloy steel, and other materials have traditionally used contact processing technology. Recent developments in the design of high power (average power above 1 kW) CO2 lasers have saved the purchase and use of these lasers. Therefore, high-power CO2 lasers have gained recognition in many production processes originally reserved for other technologies. The advantage of laser welding and cutting to provide non-contact processing makes it possible. For example, laser welding can use a remote welding head for large area processing. Compared to contact machining, laser machining produces a much smaller heat affected zone (HAZ) on the workpiece, which reduces the size of the material being machined and facilitates the manufacture 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.

The laser process is first a thermal process. The energy emitted by the laser focuses on a very small target area and transfers heat to the processed material. It is no wonder that many processes are highly dependent on the energy that the material can absorb. The efficiency of the process is often a function of the square or cubic irradiance. It can be concluded that the total energy and spatial distribution of the focal spot on the workpiece is the key to the success of the machining process, and it is very sensitive to the deformation of the laser beam's spatial energy distribution shape. Many CO2 lasers not only output a single transverse mode beam, so the quality of the beam mode is very important.

In laser welding, the gap adjustment between the parts must be completely maintained, which requires that the energy of the laser beam is always aligned with the same target area without a focal spot drift. If high speed welding is performed, poor beam structure can cause problems with poor welds. In laser cutting, the quality of the beam and the focusing power are critical to the quality of the cut itself. Poor quality beams can cause parts to be scrapped or reworked to increase costs. Despite these limitations, laser processing still has many advantages that will be enough to become the mainstream technology for material processing in the future.

Spatial beam energy distribution analysis is a measurement method that synthesizes all the variables that make up the beam into a single, clear picture. This method is suitable for all lasers, not just CO2 lasers. The most commonly used beam energy distribution analysis method for CO2 lasers is the acrylic pattern ablation method. This method directs the unfocused beam to an acrylic target, the beam energy vaporizes the acrylic material, and the focal spot profile is directly proportional to the spatial energy distribution of the beam itself. The profile of the gasification of the material describes the spatial energy distribution of the laser beam during the irradiation of the acrylic target block (usually lasting for several seconds).

Although this method has been widely used, the accuracy and repeatability rely heavily on the operator's skill, and a large amount of flammable toxic vapors are produced in the workshop and must be pumped out. Moreover, using this method can not measure the transient response of the laser beam in the light path, for example, may mask the change in the beginning of the process. In short, the mode ablation method can only be described as an approximate description of the performance of the laser beam.

In the past decade, several semi-electronic diagnostic methods have been developed with varying results. Most of these methods attempt to sample an unfocused beam, that is, to direct a small, representative beam of light to a sensor. The main beam's spatial energy distribution map. In the case of high power laser applications, sampling is either using tiny holes in tiny hollow tubes or using tiny mirrors at the ends of thin wires to direct a small portion of the original beam to a thermoelectric single element sensor. The sensor converts the absorbed energy into a proportional electrical signal.

However, in order to sample the entire beam, the aperture or radiation must pass through the beam repeatedly in order to reproduce the entire beam as a composite image. The image thus generated is an accumulated result of a time-divisional scanning of the light beam. The scan time is 2 to 10 seconds, depending on the instrument. This method avoids toxic gases, but, like the acrylic mode ablation method, it does not provide any information about the transient response of the laser beam. One possible disadvantage is that since the sampling device passes through the light beam, it cannot be determined with certainty whether this measurement method itself affects the quality of the light beam.

To make the electronic beam energy distribution measurement system superior to conventional methods, 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 robust and withstand the rigors of everyday life in a production environment that generates dust and smoke. The system must be able to be quickly installed in place, or permanently fixed in the light, and easy to operate, even if used by unskilled technicians, can provide detailed quantitative information that cannot be provided by traditional methods. Finally, the system cannot interfere with the main beam in any way, otherwise it will introduce some artifacts into the analysis process.

In some applications, it is not sufficient to periodically evaluate the quality of the laser beam mode. For example, in the field of medical devices, verification of the mode quality is the key to successful production of medical device parts. It also includes other operations that result in lost time and reduced production due to the recording of failed parts.

In order to meet the increasing market demand for high-power CO2 laser beam in-line monitors, Spiricon and II-VI have jointly developed a powerful, easy-to-maintain industrial laser beam monitoring system. The system uses off-the-shelf optics and can be installed on new laser workstations using a laser system information acquisition card or retrofitted to most existing industrial laser systems. It provides the end user with detailed real-time beam images and records important laser beam parameters. Once any important parameter reaches the preset limit, the system activates an alarm signal to alert the operator of the fault, allowing the operator to take appropriate action. In addition, the system can be used to diagnose general laser failures such as aging of the output coupler or deviation of the laser cavity. Using the diagnostic capabilities of the system, technicians can restore the laser's working status in a shorter period of time, thereby increasing the benefits of laser processing.

Unlike other beam analysis devices that are commonly available, this type of embedded system is completely transparent to the illumination process because there is no active device that interrupts the beam and all optical elements are liquid-cooled passive mirrors. The chassis itself uses the same purge gas as the main light path, and once the system is installed, it needs to be maintained just like any other optical device. Because the instrument operates in real time, it is well suited to tune and adjust the laser or diagnose actual processing events after normal maintenance has been completed.

Remote-controlled laser welding is a new type of “enabling” technology. Unlike seam welding, the laser beam energy must be directed to many points and welded through the shutter switch. For such welds, the two most critical measurements are the spatial energy distribution of the beam during the welding process and the total energy delivered by the beam. Therefore, the new process monitoring system is best suited for this purpose. If the short-term continuous welding data is analyzed, the beam width and the beam profile are slightly changed during the welding process.

Many arguments stem from the theory that the spatial energy distribution of an unfocused beam is "replicated" onto the focal spot. Although both parties have been arguing for many years, so far there has been almost no support for either party. In this online system qualification process, two different instruments were used to simultaneously measure the unfocused beam and the focal spot. The results clearly show that the spatial energy distributions of the unfocused and focused beams of the laser are almost identical. During the appraisal process, the focus contours were also measured and compared in both cases where the instrument was installed and removed on the optical path. The results confirmed that the system did not affect the focal spot structure.

One argues that: Studying the spatial profile of the unfocused beam is of no practical value in terms of the spatial energy distribution of the focal spot; on the other hand, it is insisted that the poor structure of the unfocused beam will be reproduced after focusing. In this case, there is a “hot spot” on the side of the unfocused beam, and the transverse mode structure is a “doughnut”, the TEM01 transverse mode, because its energy distribution pattern looks like a typical fried 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 a precursor to the focal spot energy distribution.

With the long-term use of the laser, the continuous monitoring process can warn of impending failure and must be repaired in a timely manner. With the continuous monitor in the system, important beam parameters can be tracked during processing.

The beam energy distribution brings economic benefits to the above measurement methods, and cost savings due to increased productivity, reduced reject rate, and reduced downtime. As processing becomes more stringent, laser energy distribution and monitoring technologies will become increasingly cost-effective.

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