Research on the Marking Effect of Laser Pulses on Orange Peels

With the continuous improvement of living standards, the public's attention to food safety has been increasing. In this context, traceability of fruits, especially green fruits, has become an objective requirement. Currently, pasting paper bar - code labels on the surface of fruits is the main way to achieve fruit traceability. However, some paper labels are prone to falling off, resulting in the loss of key information, while others are so firmly attached that they are often difficult to remove. Therefore, this study selects oranges as the research object and intends to use laser technology to directly engrave two - dimensional barcodes on the fruit surface. By comparing the marking effects of ultra - short - pulse picosecond (ps) laser pulses and short - pulse nanosecond (ns) laser pulses, it aims to provide a marking technology with clear and durable barcodes for fruit traceability.

1 Experimental Instruments and Methods

1.1 Structure and Composition of the Marking Machine

Figure 1 Schematic diagram of laser marking device

1.2 Light Sources

The short - pulse nanosecond laser source uses a self - made Nd:YAG acousto - optic Q - switched laser, with an operating frequency of 3 KHz, a pulse width of 15 ns, and an output wavelength of 532 nm. The ultra - short - pulse picosecond laser source uses an Nd:YVO₄ rod - structured MOPA mode - locked laser (Penny - pico - 10, Ziyun Laser Technology Co., Ltd.), with an output wavelength of 532 nm, a maximum power of 10 W, a pulse width of 5 ps, and an operating frequency of 1 - 100 kHz.

1.3 Experimental Methods

Oranges purchased in the same batch with similar surface smoothness were selected. Picosecond laser pulses and nanosecond laser pulses were used to mark on their surfaces respectively, and each method was repeated 10 times. Then, a polarizing microscope (OLYMPUS - BX51, Olympus Corporation) was used for observation and analysis at 100X magnification to compare and analyze the marking effects of the two lasers on the orange peels.

2 Results and Analysis

2.1 Analysis of Experimental Parameters

Parameters such as the beam quality, pulse width, and energy of the laser will affect the accuracy of laser marking, which in turn affects the quality and effect of marking. The pulse widths of the nanosecond laser and the picosecond laser were adjusted to the optimal state. The output pulse width of the nanosecond laser measured by the laser probe oscilloscope was 8.48 ns, and that of the picosecond laser was 14.2 ps.

Numerous experiments have shown that laser power, marking speed, and line spacing are the main factors affecting barcode quality. The higher the laser frequency and the shorter the pulse time, the faster the marking speed and the smaller the line spacing, resulting in a better marking effect.

2.2 Marking Effect and Analysis

After importing the QR code image into the nanosecond marking machine through the USB interface, the position of the orange peel was adjusted using its translation stage, and then marking began. The results were observed under a 100X polarizing microscope, as shown in Figure 2.

Figure 2 Nanosecond marking results

The QR code graphic was imported into the computer software of the picosecond laser. After turning on the water cooling and adjusting the focal plane, marking was carried out on the orange peel surface. The results were observed under a 100X polarizing microscope, as shown in Figure 3.

Figure 3 Picosecond marking results

Through the observation of the marks on the orange peels under the polarizing microscope, it was found that both picosecond lasers and nanosecond lasers can cause obvious morphological changes on the orange peel surface and print relatively clear marks. However, the picosecond laser prints lines with better straightness and smooth and clear outlines on the orange peel surface, and causes less damage to the epidermis. In contrast, the lines printed by the nanosecond laser on the orange peel surface are less regular, and the outlines of the marks are more blurred.

The most direct parameter affecting the marking effect is the peak power, which is inversely proportional to the laser pulse width. Therefore, as the pulse width decreases, the peak power increases. Compared with the nanosecond - pulse laser, the laser beam output by the picosecond laser has a narrower pulse width, higher energy, and higher precision. The photochemical ablation effect on the orange peel surface is obvious, causing carbonization at the edges of the QR code, with no obvious small cracks or surface debris, so the boundaries are clearer.

3 Conclusions and Discussions

To achieve clear, durable two - dimensional barcodes for fruit traceability without affecting edibility, a comparative experiment was conducted to study the feasibility of laser technology for marking on fruit surfaces. Oranges were selected as the research object, and nanosecond lasers and picosecond lasers were used to print QR codes on the orange peels respectively, and the changes on the orange peel surface were observed. The experimental results show that compared with the nanosecond - pulse laser, the picosecond - pulse laser has higher printing accuracy, deeper marking depth, and clearer marking outlines.