Real-Time Laser Power Measurement

Advanced laser systems usually undergo meticulous testing right up until commissioning. However, this close monitoring often stops once the systems are put into production. Real-time monitoring during normal operation affords enormous economic and ecological optimization potential. In the past few decades, production processes - particularly in manufacturing - have been steadily streamlined and automated. In many areas, laser systems have been replacing mechanical tools because they are not subject to wear and require much less maintenance. Despite these advantages, it is important to regularly verify the laser beam quality via reproducible measurement methods, even during productive operation. Unfortunately, many empirical measurement methods that enjoy widespread use do not provide accurate or standardized results. Only when performing optimally can laser systems guarantee the most cost-effective production of high-quality components. Even the smallest deviations in beam adjustment or focal position may lead to reduced part quality, massive cost increases, and pollution of the environment in a variety of ways, including increased energy consumption and use of process gases.

The distribution of the power and energy density in the beam is considered a key parameter in most laser applications; it determines the effectiveness of laser processing in the material. This parameter is calculated by dividing the emitted power or energy by the cross-sectional area of the focused beam (details regarding such laser parameters are given in Radiometric Measurement). The higher the power or energy density in the focus, the more efficiently the laser processing operation performs. An unexpected increase in focal spot size can severely impact the beam's power density leading to deleterious effects such as:

• The travel speed of the part must be reduced to compensate for reduced power density
• The quality of the machined part in the cutting or welding process suffers
• Production times and power consumption increase, as does the need for expensive gases used in processing
• The HAZ is larger, requiring more post-process finishing treatments like straightening, deburring, or polishing
• Under certain circumstances, an undetected loss in product quality can lead to diminished strength - a defect that, once recognized, can lead to costly recalls

Furthermore, one should not underrate the economic environmental impacts of consuming more processing gases. Significant energy is expended in the production of gases such as argon, which negatively affects the overall sustainability and end price of the manufactured product. At the same time, as more processing gases are consumed, more processing by-products - both gaseous and particulate - are emitted. Such emissions can reduce the quality of the optics, resulting in further reduction in beam quality. Figure 1 depicts the impact of low beam quality on costs per part.

Typically, the first indicator that the performance of a laser system has deteriorated is a decrease of laser power in the focused beam. MKS Ophir has developed Helios, a compact laser measurement system specifically for use in automated industrial cutting and welding processes. The Helios system measures high power industrial lasers - such as diode, fiber, and Nd:YAG lasers - with powers from 100 W to 12 kW and energies from 10 J to 10 kJ. The laser is set to a pulse of between 0.3 seconds and several seconds. The Helios then measures the energy and exposure time of this sample of the power to calculate the average power. The short measurement time obviates the need for water cooling and, therefore, the sensor can be kept to a compact size. Details regarding the basic operation of thermal sensors such as the Helios can be found in Thermopile Sensor Physics. This compact measuring device has a robust metal housing and an automatic motorized shutter that protects the gauge. This ensures that the sensor remains clean in any environment and is also protected against minor physical disturbance. It can be directly integrated into laser systems or production cells to perform automated measurements for the entire optical laser system. By connecting the Helios system via PROFINET¨ industrial Ethernet or RS232 serial communications, all data can be immediately transmitted, analyzed, and centrally stored. By regularly and frequently measuring the laser power, slight deviations from setpoints can be detected immediately. The operator can be directly informed by a pass/fail display. This makes it possible to take remedial measures right away, ensuring consistently high production quality.