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.
Ultra-narrow linewidth laser plays a very important role in the high-precision optical frequency transfer via optical fibers. We have designed and realized a compact portable laser system for transportable communication band ultra-narrow linewidth laser. Based on the method of topology optimization design, a stable optical board is developed. Designed and developed a series of small optical grade devices without exercise rules, making the optical system more stable. The optical system we designed is suitable for transportable ultra-narrow linewidth laser.
Frequency modulation technology has been widely used in many fields to get rid of the limitation of the 1/f flicker noise. However, the residual amplitude modulation (RAM) seriously deteriorates the detection accuracy of frequency modulation technology. Especially, for fiber electro-optic modulator (f-EOM), the RAM cannot be minimized by the conventional technique commonly used for free-space EOM such as careful mechanical adjustment or using a wedged crystal EOM. This can limit the use of f-EOM in high-performance detection techniques. In an ultra-stable laser system, the existence of the RAM has become one of the important factors limiting the frequency stability. This paper develops a dual-loop active feedback servo system to minimize the RAM. The voltage and temperature dependence of the EOM are measured to design a suitable active RAM reduction controller. To verify its ability, the active RAM reduction system is placed in an ultra-stable laser system for testing. The RAM noise is suppressed to less than 1 ppm and the corresponding laser frequency stability is 1×10−16. The RAM-reduced frequency stabilization is lower than the thermal noise limit of the reference cavity, so the RAM will not be the main factor affecting the laser frequency stability in the ultra-stable laser system.
As the local oscillator of the space optical clock, the ultra-stable laser determines the short-to-medium-term frequency stability of the space optical clock. Considering the space station’s restrictions on load weight and volume, as well as the impact of vibration and shock during launch, a tunable external cavity diode laser with small size, stable structure and no elastic adjustment device was developed. Optimized the design of the structure of the optical path board, developed small optical components, and developed a double-sided optical path system based on this. Experimental tests show that the free-running line width of the laser is about 175 kHz, which can run stably and reliably for a long time. At the same time, considering the deformation of the optical path substrate in the space microgravity environment, the topology optimization design of the optical board was carried out. Through mechanical simulation analysis, the maximum deformation of the optical path substrate under the influence of gravity is 0.43 μm, which initially meets the requirements of space applications.
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