As technology advances, ultrafast pulse lasers are increasingly used in a wide range of applications, including material processing, imaging, and medical treatments. The precision of these applications often depends on the ability to focus the laser beam to a tight spot with minimal divergence over 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 for DOF measurement primarily rely on direct capturing of 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, employing autocorrelation or moving sensors to measure DOF in ultrafast pulse lasers introduces complex optical paths that can lead to measurement errors, making them unreliable for precise focusing applications.
To address the limitations of current DOF measurement techniques for ultrafast pulse laser, this paper proposes a novel method based on Z-scan technique. According to nonlinear optical theory, we derive that the transmittance curves obtained from open-aperture (OA) Z-scan measurements of samples exhibiting two-photon absorption (TPA) follows a Lorentzian distribution. By fitting this curve by Lorentzian, the DOF of ultrafast pulse lasers can be determined rapidly to the full width at half maximum (FWHM) of the OA Z-scan curves. We conduct experimental measurements of the transmittance curves of solid and liquid samples with TPA across different types of lenses and microscope objectives within ultrafast optical systems. The results demonstrate a significant consistency between the FWHM of the OA Z-scan curves and the theoretical DOF values. This method effectively correlates the size of the DOF with 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 for measuring DOF in ultrafast pulse laser systems using the OA Z-scan technique was proposed. It provides a rapid, accurate and reliable way to determine the DOF in ultrafast laser focusing systems, enabling precise control of the ultrafast laser beam for a wide range of applications.