In the electronic industry, aerospace, medicine, automobile, and micro-electro-mechanical system, micro-holes are important structural units of devices and functional parts. The current development trend of micro-hole processing is small size, high aspect ratio, high machining accuracy, high machining efficiency, no recast layer, no heat-affected zone, and no micro cracks. At present, several methods, such as mechanical, electro-discharge, electrochemical, and pulse laser drilling, are used for micro-hole machining. Ultrashort pulse laser processing is particularly versatile and can guarantee a high level of controlling the process due to the ultrashort time scale and ultra-high peak power density characteristics. It has been used for micro-hole drilling with limited heat-affected zone to provide high quality and precision, especially for hard and brittle materials. If the ultrashort pulse laser is used as a drilling tool, the beam must be rotated in a circular movement. At present, helical laser drilling technology is mainly based on the rotation of optical components, such as Dove prism, wedge plates, and cylindrical lenses. However, there is still a lack of high stability, small size, and low-cost laser drilling system. In this paper, a helical laser drilling system based on a scanning galvanometer is designed to meet the requirements of precise micro-hole drilling with a diameter ranging from 100 μm to a few hundred microns.

The ultrashort pulse laser drilling system is based on a scanning galvanometer and a helical laser drilling lens. The scanning galvanometer can be rotated in XY direction with high accuracy and high speed. The helical laser drilling lens is made up of a scanning lens, an image transmission lens, and a focusing lens. The basic principle of the helical laser drilling lens is to offer a lateral offset of the laser beam before the focusing lens. Thus, the focusing lens forms an inclined angle on the laser machining surface (Fig. 1). The focal length of the scanning lens is f1. The image transmitting lens is f2, and the focusing lens is f3. Thus, the focal length of the helical laser drilling lens is f, which is expressed as follows f=f3×f1/f2.

In order to achieve high stability and reduce the total length, the telephoto structure and symmetry design are applied to the design of the scanning lens and image transmission lens (Fig. 2). The design specifications of the helical laser drilling lens are listed in Table 1. It is required that the diameter of the beam intersection part should be less than the minimum machining aperture to achieve the desired processing hole aperture range. The processing hole aperture range of this paper is 100–400 μm, and the depth-to-diameter ratio is 10. In order to achieve these goals, the optical system design is carried out by using the optical design software ZEMAX. For ultrashort pulse laser, achromatic design and the best focusing performance should be considered in terms of image quality. Besides, group-velocity dispersion, thermal expansion, and damage threshold of components should all be considered in terms of system performance. Therefore, fused silica is selected as the main positive lens used in helical laser drilling lens because of its relatively high Abbe number, low heat expansion coefficient, and high damage threshold. Dense flint glass (H-ZF13, CDMG) is chosen as the material of the negative lens. The basic idea is that both lenses will compensate for their respective dispersions and cancel each other.

The optical design result of the helical laser drilling lens is shown in Fig. 3, and the distribution of the focusing light field is shown in Fig. 4. The total length of the helical drilling system is 512.5 mm, which is far less than the sum of the focal length of three lenses (f1+f2+f3=950 mm). The telephoto structure and symmetry design are helpful to reduce the total length, thus increasing the system's stability. The spot diagrams and wave aberration results show that the on-axis and off-axis aberrations of the designed lens are almost equal, and the image quality achieves the diffraction limit. The design results show that the system can drill with a diameter of 100-400 μm, and the maximum depth-to-diameter ratio is 10∶1.

The total removal of focused ghost reflections from the optical surfaces of the helical laser drilling lens that can damage optical components is critical. Therefore, lens optimization should be conducted along with an analysis of the focused ghost positions. The main methods that eliminate the focused ghost reflections are to adjust the curvature radius and air space of lenses. The analysis of the back-focused ghost reflections of the helical laser drilling lens is shown in Fig. 6. None of the back-focused ghost reflections are inside the components or on the surfaces.

Tolerance is a critical factor that affects the performance and cost of an optical system. The tolerance of the helical laser drilling lens is analyzed in detail in this paper, as shown in Table 2. The machining and assembly tolerance analysis is put forward according to the actual condition. Analysis results show that the maximum change of spot radius (RMS) is 2.2 μm, much less than the diameter of the airy disk. The Monte Carlo simulation predicts a high-level accuracy performance for the helical laser drilling lens to be manufactured.

According to the design results, the helical laser drilling system is fabricated and assembled. An ultrashort pulse laser is used to drill micro-holes, so as to analyze the performance of the system. The results (Fig. 7) show that the helical laser drilling system can achieve high precision.

In this paper, we have designed a helical laser drilling system based on a scanning galvanometer to meet the requirements of precise micro-hole drilling with a diameter ranging from 100 μm to a few hundred microns. Our comprehensive study has considered the essential issues in designing and optimizing the laser drilling system for high-power and high-precision applications. We have discussed helical laser drilling lens basics, principles of material selection, elimination of focused ghost reflections, and tolerance analysis. The design results show that the system can drill with a diameter ranging from 100 μm to 400 μm, and the maximum depth-to-diameter ratio is 10∶1. The experimental results show that the helical laser drilling system can achieve high precision.