In high-energy laser systems, the precise localization of the focal plane has become a fundamental challenge, with far-reaching implications across multiple advanced technology fields. In applications such as high-precision material processing, laser-driven nuclear fusion ignition, and advanced defense technologies, the accurate determination of the focal plane directly affects energy transmission efficiency and operational effectiveness. Traditional focal plane detection methods, including mechanical contact-based methods and traditional optical measurement techniques, are often hindered by inherent limitations, such as operational complexity, limited measurement accuracy, and the vulnerability of optical components to damage under high-power irradiation. These challenges become particularly prominent when processing laser beams with non-ideal characteristics such as large divergence angles or irregular beam profiles. This study aims to conceptualize, develop, and comprehensively validate an innovative non-contact detection system specifically designed to overcome these limitations. The main objective is to create a precise measurement platform that can accurately identify the focal plane position even under challenging operating conditions with large beam divergence angles and other non-ideal beam parameters, thereby filling a key technological gap in high-energy laser applications.

A complex polarization-spatial dual modulation detection system was established, incorporating a meticulously designed combination of optical components. The system architecture integrated a half-wave-plate (HWP), two polarizing beam splitters (PBS₁ and PBS₂), a quarter-wave-plate (QWP), an aperture diaphragm, a test lens, and a highly reflective mirror. Its working principle relied on simultaneous modulation of the laser beam’s polarization state and spatial path throughout the measurement process. First, the incident laser beam interacted sequentially with the HWP and the first polarizing beam splitter (PBS₁), which p-polarized light. The modulated beam then passed through the second polarizing beam splitter (PBS₂) and through the QWP, where it was converted from linear to circular polarization. The circularly polarized beam then passed through an aperture diaphragm that effectively filtered stray light and spatial noise, and was subsequently focused by the test lens onto a reflector mounted on a precision translation stage.

The reflected beam precisely retraced its optical path back, passed through QWP again for second polarization conversion, becoming s-polarized light. Due to the inherent polarization selection characteristics of PBS₂ (transmitting p-polarized light while reflecting s-polarized light), the beam was efficiently directed to a high-sensitivity detection unit. The axial position of the mirror was precisely adjusted through a translation stage system with millimeter-level positioning accuracy, while the detector continuously recorded the corresponding irradiance for real-time data acquisition. The focal plane position was determined by detecting the characteristic abrupt change in reflected irradiance when the mirror passed through the focal plane region.

In addition to the experimental setup, simulations were conducted in non-sequential mode, enabling comprehensive simulation of light propagation dynamics, polarization effects, and component interactions under various operational scenarios. Experimental verification was performed utilizing a high-energy laser source with a wavelength of 1064 nm and a divergence angle of 1.12 mrad. The reflector was displaced along the optical axis in precisely controlled 1 mm increments, and irradiance were measured using a calibrated pyroelectric energy meter with high temporal resolution and measurement accuracy.

The simulation results obtained under ideal conditions (assuming zero beam divergence angle and aberration-free lens system) showed a well-defined intensity peak precisely centered at the theoretical focal length of 100 mm (Fig.4), confirming the fundamental validity of the optical design principle. Under more realistic conditions including typical lens aberrations and significant divergence angle of 1.12 mrad, the detected intensity distribution exhibited higher complexity and reduced profile regularity, but still maintained a clearly identifiable and pronounced decline in intensity within the focal region (Fig.5), demonstrating the system’s robustness against common optical defects. High-resolution scanning within the critical 104 mm ~ 106 mm range, with a step size reduced to 0.1 mm, successfully identified the actual focal plane position at 104.7 mm (Fig.6) with sub-millimeter precision, highlighting the system’s capability for fine measurement.

The experimental results obtained from the physical implementation consistently confirmed the changing trends observed in simulations, exhibiting a rapid and clearly detectable intensity drop near the 104 mm position (Fig.8), with slight positional differences attributable to practical implementation factors including mechanical tolerances and alignment precision. Post-measurement data processing included correction for systematic errors caused by beam divergence effects, ultimately producing a corrected focal length value of 100.31 mm. Compared with the nominal focal length of the test lens, the relative error was only 1.33%, indicating a significant improvement over traditional measurement methods under comparable non-ideal conditions. Comprehensive evaluation demonstrated that the system consistently achieved millimeter-level positioning accuracy while exhibiting exceptional robustness against variations in beam collimation quality and the presence of typical optical aberrations.

The high-energy laser beam focal plane detection system adopts an innovative non-contact, polarization-spatial dual modulation architecture, effectively overcoming the inherent limitations of traditional contact-based measurement methods. Even under complex conditions such as large beam divergence angles, typical lens aberrations, and variable beam quality characteristics, the system can still achieve precise, stable, and reproducible focal plane position identification. This technological breakthrough provides a practical and efficient solution for the field of high-power laser application, ensuring stability in system performance even when the beam collimation quality fluctuates. In addition, this technology provides important data support for the performance optimization and operational reliability improvement of industrial and scientific laser systems. The proposed detection method marks a significant advancement in laser metrology technology, especially suitable for situations where traditional measurement techniques are difficult to handle or not applicable.