Compact and Long-Depth-of-Focus Optical Probe Design for Remote LIBS System

Objective Laser-induced breakdown spectroscopy (LIBS) is a powerful technique for real-time, in-situ elemental analysis without sample preparation. It has been widely applied in planetary exploration, geological surveys, metallurgical quality control, and hazardous material detection. To enable long-distance and non-contact operation under extreme conditions, remote LIBS systems have been developed by spatially separating the excitation and detection subsystems. However, the implementation of large-aperture optical probes in such systems often requires the addition of a beam-expanding structure to ensure that the laser beam fully illuminates the secondary mirror and ultimately the entire entrance pupil of the Cassegrain telescope. This introduces severe limitations in system miniaturization due to increased optical path size, limited broadband compatibility, and the difficulty of aligning coaxial excitation and collection paths. Additionally, conventional beam expanders result in a short depth of focus, making the system highly sensitive to focal shifts, which significantly impacts detection efficiency and system stability in complex field conditions. Methods To address these challenges, this paper proposes a compact and long-depth-of-focus optical probe design for remote LIBS systems, wherein a concave lens is embedded into the Cassegrain configuration to replace the traditional standalone beam expander. This embedded lens expands the narrow-diameter laser beam to match the secondary mirror aperture, enabling full-aperture illumination and efficient plasma generation at long distances. Made of fused silica, the lens ensures high transmittance across a wide spectral range and supports coaxial reuse of the optical path for both excitation and collection, reducing optical complexity and system volume. Moreover, the lens design intrinsically enhances the depth of focus, improving robustness against target displacement along the optical axis. A mathematical model of the system geometry is established to derive analytical expressions for focal length, lens position, and depth of field. The optical characteristics of the design are further validated and optimized using Zemax simulations. Results and Discussions To determine the optimal positioning between the concave lens and the Cassegrain system and to characterize the optical properties of the miniaturized probe, a mathematical model of the remote optical system is developed. Detailed theoretical calculations are conducted to derive key parameters, including the system focal length, depth of focus, and inter-element spacing. To evaluate the elemental detection capability of the proposed remote LIBS system, standard copper (Cu) is used as the test sample at a working distance of 8.1 m. A 26 mJ pulsed laser is used for excitation, with a spectral acquisition range of 350 - 690 nm, a trigger delay of 220 µs, and an integration time of 1 ms. The resulting LIBS spectra match well with the NIST copper atomic database, confirming the system's high detection accuracy and spectral fidelity under remote conditions. Additionally, four representative mineral samples—hematite, gold ore, obsidian, and glass meteorite—are analyzed under the same conditions. Characteristic elements such as Fe, Al, and Si are reliably identified, demonstrating the robustness and applicability of the method for complex mineral analysis. Conclusions To meet the growing demand for compact and stable remote LIBS systems, this study proposes a novel miniaturized and long-depth-of-focus optical design that integrates a concave lens into a Cassegrain-based system. By replacing conventional beam expanders, the concave lens enables coaxial integration of the excitation and collection paths, significantly reducing the system size (Φ 120 mm× 190 mm) while maintaining high energy utilization and effective focusing. Experimental results at an 8.1 m distance confirm the system's ability to deliver stable plasma excitation and high-quality spectra, with strong resistance to focal drift. Overall, this work presents a compact, high-stability solution for remote LIBS applications such as planetary exploration, geological mapping, and hazardous material detection.