Objective   The cavity length adjustment configuration has important applications in optical resonators. A cavity length adjustment configuration of double optical wedge (DOW) is proposed, which can adjust the cavity length independently of the cavity mirror. DOW configuration is composed of two right-angle wedges with beveled planes placed in parallel opposites. The optical path inside DOW is changed by driving the wedges to move in the vertical direction, and then the optical path in the resonator is changed. The theoretical formula for calculating the change of optical path of DOW configuration is established. According to the formula, the change of optical path is positively correlated with the wedge angle, the refractive index of wedge and the wedge displacement in the vertical direction. The wedge angle and refractive index determine the optical path adjustment efficiency of DOW configuration. According to the theoretical design, the YAG DOW configuration with wedge angle of 29° and refractive index of 1.81 has higher adjustment efficiency and less optical loss, and the adjustment coefficient is 0.53. In the experiment, the double corner cube retroreflector (DCCR) ring cavity is used to verify the cavity length adjustment, and the feasibility and effectiveness of DOW configuration to adjust the cavity length are verified. The deformable structure of DOW configuration is discussed and analyzed. The optical path adjustment properties of DOW configuration with beveled planes placed in parallel opposites, regular optical wedges and cascaded DOW configuration are discussed. The performances of DOW configuration and its deformed configuration in optical path adjustment efficiency, optical loss and the complexity of the optical path construction are compared, and the advantages of these DOW configurations in practical application are determined, which provides a reference for the design and selection of DOW configuration.

  Methods  In theory, by geometric calculation, the calculation formula of the optical path adjustment of the DOW configuration is derived, and the results are shown in Eq.(3). According to Eq.(3), there are three factors affecting the adjustment ΔL, namely wedge angle α, wedge refractive index n, and wedge displacement Δh₁+Δh₂ in the vertical direction. The larger the wedge angle value of α is, the higher the adjustment efficiency is, and the data results are shown (Fig.4); The greater the refractive index n is, the higher the adjustment efficiency is, and the data results are shown (Fig.5). The larger the displacement Δh₁+Δh₂ is, the higher the adjustment efficiency is. According to these factors, a DOW configuration with wedge angle α = 29° and material YAG is designed, and its adjustment coefficient is 0.53.

  Results and Discussions   The adjustment effect of DOW configuration to the cavity length is verified experimentally by using the DCCR ring cavity. The reflector of DCCR ring cavity is corner cube retroreflector, which can not be drived by PZT directly, thus the cavity length adjustment of DCCR ring cavity can be realized with DOW configuration. The experimental setup is shown (Fig.10). DOW configuration is inserted into the DCCR ring cavity, and 1.6 μm laser is injected into the cavity. When DOW configuration is operating, 1.6 μm laser will form a resonance signal and output from M3, through which the cavity length adjustment value ΔL caused by DOW configuration can be determined. The experimental results are shown (Fig.11). The appearance of resonance signal proves that the cavity length changes, and the change value ΔL is consistent with the theoretical expectation. The above experimental results prove that DOW configuration is effective in adjusting the cavity length.

  Conclusions  In this paper, a DOW configuration is proposed, which can be used in the special scenario where the cavity length cannot be adjusted by driving the cavity mirrors. The formula for calculating the adjustment value and adjustment coefficient of DOW configuration is given theoretically. The influence of wedge angle and refractive index on the adjustment efficiency is analyzed. DOW configuration with wedge angle of 29° and matrix of YAG is designed. DOW configuration has a large adjustment efficiency (adjustment coefficient is 0.53) and a small light loss, and is the better choice in various DOW configurations. The cavity length adjustment of the length of DOW configuration is realized experimentally in a double-corner cone ring cavity, which verifies the feasibility and effectiveness of the DOW configuration. Finally, different deformation structures of DOW configurations are given, and the property parameters of each deformation structure are compared. Compared with the traditional cavity length adjustment configuration, cavity length adjustment configurations of DOW has low adjustment efficiency and certain insertion loss, but it provides an adjustment mode independent of the cavity mirror, and provides a new choice for the cavity length adjustment in special application scenarios.