Influence of cavity length on the output characteristics of an optically pumped pulsed gas terahertz laser

In optically pumped gas terahertz (THz) lasers, the cavity length is closely related to the effective gain length of the gaseous medium and serves as a key structural parameter governing the laser output characteristics. However, the influence of cavity length on THz output performance has rarely been investigated. In this study, the effects of cavity length on the output characteristics and optimal gas pressure of optically pumped pulsed gas THz lasers were systematically investigated experimentally. With D₂O gas as the active medium, pumped with a multi-transverse mode TEA CO₂ laser, the output characteristics of the 385 μm THz laser were investigated at different gas pressures and pump energies within the cavity length range of 35–140 cm. The results show that the THz output energy increases initially and then decreases with increasing gas pressure at a given cavity length and pump energy, indicating the existence of an optimal gas pressure. The optimal gas pressure shifts toward higher values with increasing pump energy, while it shifts toward lower values with increasing cavity length. The maximum output energy and photon conversion efficiency (PCE) increase significantly with increasing cavity length. At the cavity length of 140 cm and pump energy of 1.55 J, the optimum gas pressure was 500 Pa. The maximum pulse energy of 10.12 mJ at 385 μm was achieved, corresponding to a PCE of 54.3%.