The term “terahertz radiation” typically refers to the frequency range of 0.1 THz to 10 THz in electromagnetic waves, positioning terahertz waves between microwaves and infrared. Due to the unique frequency band of THz waves, they exhibit several distinctive characteristics. (1) Transience: The signal amplitude of THz pulses is very low, yet they possess a noticeable peak value, making them valuable in time resolution research applications. (2) Spectral resolution function: Experimental THz radiation sources typically consist of only a few pulses, each covering a spectral range containing the vibrational and rotational energy levels of numerous macromolecules, facilitating substance identification. (3) Safety: The photon energy at 1 THz frequency is approximately 4 meV, and terahertz radiation does not easily disrupt the molecular structure of the detected substance when applied in medical imaging. (4) Penetration: With a wavelength falling between microwaves and millimeter waves, terahertz waves can pass through small particles in the air. Given these unique properties of THz rays, THz technology holds significant application prospects in safety inspection, communication technology, terahertz radar, astronomy, biomedical imaging, chemical identification, materials science, and other fields. Consequently, the generation, detection, and application studies of terahertz waves constitute a prominent research area.
Utilizing a double-lens transmission matrix, an initial double-lens structure was designed, encompassing both aperture and thickness considerations. Subsequently, leveraging Gaussian beam transmission characteristics, the size and position of the beam waist were meticulously determined, optimizing the entire optical system. This process enables the creation of a fiber coupling system with a small aperture and high efficiency. The optical simulation software ZEMAX was employed to scrutinize the initial fiber coupling system’s design, aligning with terahertz time-domain spectroscopy (THz-TDS) and fiber coupling technology features. The optical simulation software was further utilized to trace the system’s light, facilitating the preliminary establishment of the placement angle and position between optical components, such as the delay line and light source, ensuring successful light recovery. Concurrently, the delay line and the coupled optical system were configured to avoid mutual interference, allowing for the optimization of the fiber coupling system’s structure. This optimization aimed to achieve higher coupling efficiency and improved beam quality. Considering the practicalities of the experimental installation process, the mechanical structure of the entire module was designed based on the optical specifications. This approach ensures that all system components can be installed and adjusted cohesively. The coupling lens’s mechanical structure was devised as a five-dimensional adjustment structure, characterized by its simplicity, convenient machining and assembly, compactness, high stability without a transmission gap, and five degrees of freedom for three-dimensional translation and two-dimensional angle rotation.
The collimated coupled optical system’s single-mode fiber coupling efficiency is illustrated in Fig. 2. When the system was positioned in front of the rotating delay line, the light followed a path reflected back through the delay line, coupling to the original fiber and rendering the system lens entirely symmetrical, resulting in a high coupling efficiency with a single-mode fiber. Figures. 4 and 5 depict the optimized collimated coupling optical system with its single-mode fiber coupling efficiency and the actual optical path diagram. The observed coupling efficiency with a single-mode fiber was 76.27%, approaching the ideal coupling efficiency of 81.45%. Accounting for Fresnel reflection loss at the incident end face of the fiber, the maximum coupling efficiency was reduced to 78%, closely aligning with the system’s actual coupling efficiency. Simultaneously, the single-mode fiber coupling efficiency reached 97.25% for physical optical propagation. Consequently, following system optimization, the coupling efficiency was markedly high, meeting the specified coupling requirements.
The fiber THz-TDS transceiver-integrated coupling system differs from traditional fiber THz-TDS by incorporating a delay line with the fiber in the coupling aspect, simplifying the structure of the fiber coupling system. An optical system with high coupling efficiency was designed based on the principles of a Gaussian beam relay and the characteristics of a double lens. The single-mode fiber coupling lens model was developed using ZEMAX software, and the system underwent optimization to enhance the coupling efficiency of the single-mode fiber. The results demonstrate that the coupling efficiency reached 97.25%, meeting the high-efficiency coupling requirements for single-mode fibers in terahertz time-domain spectroscopy systems. This not only provides a guiding direction for the design of coupling lenses but also contributes to the advancement of miniaturized terahertz time-domain spectroscopy instruments.
