Spatial coherence is one of the characteristics of traditional lasers. High spatial coherence enables good directivity in lasers; however, it also results in speckles. Thus, to suppress speckle noise, many methods have been developed to reduce the spatial coherence of lasers, including the mechanical vibration and photoelectric methods. Specifically, the mechanical vibration method is relatively slow and fails to achieve speckle suppression within short periods; alternatively, the photoelectric method suffers from problems pertaining to low threshold power and high cost. Notably, the laser with a degenerate cavity structure proposed by Arnaud in 1969 features low temporal and spatial coherence. Over recent years, factors influencing the spatio-temporal coherence characteristics of 4F degenerate cavity lasers have been studied; however, the influence of the thermal effect in the laser gain medium on the 4F degenerate cavity has been neglected. The thermal effect in solid-state lasers is a problem that cannot be ignored; this thermal lens effect of the laser gain medium degrades the perfect imaging characteristics of 4F degenerate cavity lasers and also alters the degenerate characteristics of the 4F resonant cavity. Accordingly, considering the thermal lens effect in the degenerate cavity gain medium, the influence of cavity length variations on the space – time output characteristics is analyzed and experimentally verified in this work. These results provide theoretical and experimental supports for research on low spacetime coherent lasers.

In this study, the transmission matrix and G-parameter equivalent cavity analysis methods are used for analyzing the spatio-temporal spectrum distribution characteristics of the beam in lasers considering the thermal lens effect in the degenerate cavity. The existence of the thermal lens, which degrade the degeneracy of the transverse mode spectrum in the degenerate cavity, is analyzed. In this experiment, a set of thermal lens measurement optical paths based on the 4F degenerate cavity laser is built. The focal length of the thermal lens, laser beat frequency signal, and maximum transverse mode order of the 4F degenerate cavity laser are measured. Additionally, considering a thermal lens in the cavity, the influence of cavity length variations on the maximum transverse mode order and the transverse mode beat frequency bandwidth in the cavity is theoretically calculated and experimentally verified.

A set of thermal lens measurement optical paths based on the 4F degenerate cavity laser is built in this experiment (Fig. 2). The influence of cavity length variations in the 4F degenerate cavity laser on the focal length of the thermal lens (Fig. 3), laser beat signal (Fig. 4), and maximum transverse mode order (Fig. 5) is studied. Experimental results reveal that the total spectral bandwidth of the degenerate laser cavity does not vary with the focal length and cavity length of the thermal lens. Further, as the cavity length of the thermal lens increases, the frequency interval of the transverse mode increases, the beat frequency bandwidth of the transverse mode fluctuates, and the maximum transverse mode order, N_max, decreases significantly. Moreover, this study theoretically simulates the maximum transverse mode order with cavity length variations under the influence of a thermal lens, and the simulation results in the time domain are consistent with the experimental results.

Here, the influence of cavity length variations on the maximum transverse mode order and spectral structure of a degenerate laser cavity is studied considering the thermal lens effect in the laser gain medium. The existence of a thermal lens degrades the perfect imaging characteristics of the 4F degenerate cavity and the degenerate characteristics at the transverse and longitudinal mode frequencies. In this work, under the influence of a thermal lens, the beat frequency signal and maximum transverse mode order of a degenerate laser cavity are measured considering changes in the cavity length. Results indicate that the total spectral bandwidth of the degenerate laser cavity does not vary with the focal length and cavity length of the thermal lens. As the cavity length under the thermal lens increases, the transverse mode frequency interval increases and the transverse mode beat frequency bandwidth fluctuates. This suggests that changes in the cavity length affect the mode distribution in the spectrum. Further, experimental results for the maximum transverse mode order show that, owing to the existence of the thermal lens, a small distance of cavity mirror from the ideal position has a significant impact on the maximum transverse mode order. The maximum transverse mode order under the influence of a thermal lens is also theoretically simulated with changes in the cavity length. Notably, simulation results in the time domain are consistent with the experimental results, indicating that the theoretical model of the thermal lens has a practical significance for research on the spatio-temporal output characteristics of 4F degenerate cavity lases. Analyses also indicate that adjusting the laser cavity length is an effective method for controlling the spatial coherence of these lasers.