Dynamics of molten pool evolution and high-speed real-time optical measurement in laser polishing

Laser polishing (LP) has recently emerged as a promising advanced manufacturing technology for producing ultra-smooth surfaces owing to its non-contact operation, high efficiency, environmental friendliness, and compatibility with complex surfaces and materials. LP has gained attention in various fields, such as additive manufacturing, optical components, mould manufacturing, and biomedical engineering.

 

In a new paper published in Light: Advanced Manufacturing, a team of scientists, led by Professor Du Wang and Professor Cheng Lei of Wuhan University, in collaboration with Professor Zong-qing Zhao of China Academy of Engineering Physics, have developed optical time-stretch quantitative interferometry (OTS-QI) technology, which for the first time recorded the entire evolution of the surface morphology during LP with nanosecond-level temporal resolution, thus providing insights into the mechanisms involved in the surface roughness evolution. The OTS-QI system harnesses the rapid repetition rate of femtosecond lasers, achieving a remarkable measurement speed exceeding 100,000,000 times per second while preserving a measurement accuracy comparable to that of existing white light interferometers (WLIs), setting a new benchmark as the fastest recorded roughness measurement. In addition to LP, the OTS-QI system can be applied for real-time and in situ monitoring of many machining scenarios involving highly dynamic phenomena.

 

Optical time stretch is a data acquisition method that enables continuous ultrafast single-shot spectroscopy, imaging, reflectometry, terahertz and other measurements at refresh rates reaching billions of frames per second with non-stop recording spanning trillions of consecutive frames, presents a significant opportunity for the development of LP. Time stretch techniques are of interest for their unrivalled high throughput and continuous measurement capabilities compared with traditional data acquisition methods. These scientists summarize the operational principle of their OTS-QI system:

 

"The light source is a femtosecond laser, and each pulse from the laser is stretched in the time domain through a 15 km long single mode fibre and amplified by a fibre amplifier. The amplified pulses are split into two optical paths by a beam splitter, forming a signal path and a reference path. In the signal path, the optical signal pulses are spatially dispersed through a diffraction grating to form one-dimensional (1D) rainbow pulses that are focused onto the laser processing area through a lens that acts as an objective lens. This method realises OTS-QI system with high spatial resolution (< 10 µm), high temporal resolution (< 10 ns), and continuous image acquisition capability.”

 

“The primary significance of OTS-QI technology is that it provides a real-time in situ monitoring method, which facilitates directly controlling laser power, scanning speed, and other process parameters based on real-time changes in surface roughness.” they added.

 

“The OTS-QI system allows easy adjustment of the field of view for a single measurement via modifying the basic settings of the imaging optics and scanning strategy, thereby achieving an optimal balance between spatial resolution and sampling rate, making this method particularly well suited for multi-pass and large-area polishing processes involving moving or rotating workpieces. However, ultrahigh inspection speeds generate data streams exceeding 100 Gb/s, placing significant pressure on data transmission and processing equipment. Combining an FPGA with compressed sensing imaging can facilitate rapid data processing and transmission, enabling real-time feedback and regulation of the machining process. Hence, the OTS-QI technology is suitable for LP and offers considerable potential in any machining scenario involving high-speed detection of surface topography.”the scientists forecast. 

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