With the development of technology, ultrafast pulse lasers are increasingly used in many fields, such as material processing, imaging, and medical treatments. The precision of these applications often depends on the ability to focus the laser beam into a tight spot with a minimal divergence in a certain range along the optical axis. Therefore, accurate measurement of depth of focus (DOF) is crucial for optimizing the performance of ultrafast laser systems and ensuring the reliability of the results obtained in various experiments and applications. Traditional methods of measuring the DOF mainly rely on directly capturing the beam size, which is impractical in high-intensity environments of ultrafast pulse laser systems due to potential damage to sensors and limitations in measurement accuracy. Furthermore, using autocorrelation or moving sensors to measure DOF in ultrafast pulse lasers introduces complex optical paths, leading to measurement errors and making them unreliable in precise focusing applications.

To solve the problem of the limitations of current DOF measurement techniques for ultrafast pulse laser, in this work we propose a novel method based on Z-scan technique. According to nonlinear optical theory, it is found that the transmittance curves obtained from open-aperture (OA) Z-scan measurements of samples exhibiting two-photon absorption (TPA) all follow a Lorentzian distribution. By fitting this curve by Lorentzian distribution, the DOF of ultrafast pulse lasers and the full widths at half maximum (FWHM) of the OA Z-scan curves can be determined rapidly. The transmittance curves of solid and liquid samples with TPA across different types of lenses and microscope objectives within ultrafast optical systems are measured. The results show that the FWHM of the OA Z-scan curves and the theoretical DOF values are well consistent. This method effectively relates the size of the DOF to the beam waist radius derived from the distribution of the Lorentzian function in the OA Z-scan experimental curves, eliminating the influence of other parameters on the measurement results. In conclusion, a novel method of measuring DOF in ultrafast pulse laser systems by using the OA Z-scan technique is proposed. It provides a rapid, accurate and reliable way for determining the DOF in ultrafast laser focusing systems, thereby precisely controlling the ultrafast laser beam for a wide range of applications.