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The noise floor performance of optical frequency transfer system is mainly affected by the change of optical length of the Interferometry and refractive index induced by ambient temperature variations and mechanical vibration effects. At present, the universal installation is connected by fiber components for optical frequency transmission over fiber link. However, for signal detection at the local site, due to the welding technology of fiber, the length between the fiber units is limited to a certain length, which makes it difficult to achieve high-symmetry detection at the local site, and the asymmetric part are susceptible to environmental disturbance. Therefore, in order to minimize the influence of noise base on transmission system, this paper investigates a compact and integrated optical path module based on free space. The spatial optical module integrates beam prism, half wave plate, quarter wave plate, fiber collimator, 0 degree mirror and Faraday mirror in the size of 5 cm×6 cm, which can reduce the connection distance of local devices from 10 cm to 3 cm. At the same time, the local reference light transmission in air medium is more stable than in the fiber medium, which can reduce the noise level of the reference end and improve the stability of the system. In the experiment, the local end based on the spatial optical path and that based on the fiber path is compared and tested in a non-temperature-controlled environment. The experimental results show that the spatial optical path has obvious advantages in both short-term stability and long-term stability. It can decrease the noise floor level of the optical interferometry used for phase noise cancellation in a optical frequency transmission system via fiber, and provide a potential optimization space for long-distance transmission.
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