Laser Auto-Focus Technology for High-Precision Material Processing

01 Introduction
In high-precision material processing, automatic focusing (Auto-Focus, AF) technology for laser systems plays a vital role. With the widespread use of laser technologies in industrial manufacturing—such as micro/nano processing, laser cutting, and laser welding—the demand for precise control of the laser focal position has steadily increased. Traditional manual focusing methods are time-consuming and labor-intensive, often failing to meet high-precision processing requirements. Consequently, automatic focusing technology emerged, significantly enhancing efficiency and quality by swiftly and accurately detecting and adjusting the laser focal position. This paper focuses on various AF methods based on optical signals, image signals, and laser-induced signals, and explores the application of deformable optical elements in focal-position control. By analyzing the principles, performance, advantages, and drawbacks of these technologies, the goal is to provide a comprehensive reference for researchers and advance high-precision laser processing technology.

02 Classification and Characteristics of Auto-Focus
AF technology plays a critical role in high-precision material processing, primarily aiming to rapidly and accurately detect and adjust the laser focal position for optimal processing outcomes. Based on the type of physical signal used to detect focus, AF techniques can be categorized into: optical-signal-based methods, image-signal-based methods, laser-induced-signal-based methods, and deformable-optics-based focal control. Each of these methods is detailed below in terms of principles, features, and practical performance.

2.1 Optical-Signal-Based AF Methods
Stigmatic autofocus analyzes the shape of the reflected laser spot using a cylindrical lens and quad-cell detector (FQD) to extract a focus error signal (FES). At perfect focus, the FQD receives a symmetric circular spot, and the FES equals zero; off-focus, the spot becomes elliptical and FES changes, indicating the deviation. As shown in Figure 1, this method features simple structure, fast response, and high accuracy, making it widely used in high-precision laser processing AF systems.

Figure 1. Correlation between FES and the spot shape on FQD

2.2 Image-Signal-Based AF Methods
Image-based autofocus is a common and flexible focus detection technique used in laser manufacturing. It relies on a CCD camera to capture the bright spot reflected from the target surface. Appropriate optical design then extracts focal-position information from these images to determine optimal focus. Compared to stigmatic methods, image-based AF has no fixed structure and can be flexibly adapted to different requirements.

Figure 2. Image signal experimental setup

2.3 Laser-Induced-Signal-Based AF Methods
In laser-induced-signal autofocus methods, the plasma method is one example. This uses plasma signals generated by laser–material interaction to detect focus, including plasma-emission-intensity and charge-voltage approaches. Although current research on focus-error detection focuses mainly on optical and image methods, this approach shows strong potential for AF system design in various laser-processing applications.

03 Application of Deformable Optical Elements
To overcome the speed limitations of traditional objective-lens movement in focus adjustment, researchers have proposed adaptive-optics control using deformable mirrors. This method adjusts mirror curvature to enable rapid focal shifts, achieving modulation frequencies up to 2 kHz—significantly faster than traditional methods. A monolithic deformable-mirror system can provide large-range focal control, but requires closed-loop control to overcome hysteresis and creep issues, demonstrating strong potential for efficient laser processing applications (see Figure 3).

Figure 3. Setup of the focus shifter for the deformable mirror

04 Conclusion
This paper presents multiple laser auto-focus technologies, including those based on optical signals, image signals, and laser-induced signals, as well as the use of deformable optics for focal control. These techniques hold significant value in high-precision material processing, substantially improving both efficiency and quality. The analysis of each method’s principles, performance, and pros/cons reveals that optical-signal-based AF offers high precision and rapid response, fitting for high-NA systems; image-signal-based AF is suitable for complex surface detection but is sensitive to surface roughness; laser-induced-signal-based AF demonstrates strong anti-interference and roughness immunity, though further work is needed to enhance accuracy. Incorporating deformable optics introduces a novel pathway to rapid and precise focus control with broad application prospects. Future research should focus on improving AF accuracy, interference immunity, and adaptability, as well as developing more intelligent and automated focal-position control systems to meet the demands of high-precision laser processing.

**--Cite the article published by 高能束加工技术 on June 09, 2025, in the WeChat public account "High-Energy Beam Processing Technology and Applications."