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• Vibrations make a misalignment for Fabry–Pérot cavities in ultra-stable lasers. • Longer than 100 mm cavities in space and recoupling methods have not been reported. • Detection and compensation of misalignment form a loop for actively recoupling. • Recoupling achieved at 0.9 mm and 0.1° misalignment with 0.03 mm and 0.002° error. • The recoupling method is a new idea for handling cavities' misalignment. An active method for coupling a laser with a high-finesse Fabry–Pérot cavity in ultra-stable lasers is proposed. This method uses the centroid of the reflected laser spot and the power ratio of the resonant modes to detect the misalignment between the laser and the Fabry–Pérot cavity. Subsequently, four actively rotating wedge prisms are used to compensate for the misalignment. Based on ray tracing and table lookup, the detection and compensation systems can form a closed loop for actively recoupling the laser into the Fabry–Pérot cavity when a misalignment occurs. Through experiments, the laser can be re-coupled with the Fabry–Pérot cavity by four 0.1° wedge prisms within a 0.9 mm decentration or a 0.1° tilt. The re-alignment of the laser and Fabry–Pérot cavity by active optical coupling may provide an alternative solution for the issue of support and vibration sensitivity of the Fabry–Pérot cavity. This solution is essential to improve the frequency stability of transportable and space-borne ultra-stable lasers. [ABSTRACT FROM AUTHOR]

Abstract: Polarization properties of single photon states propagating through the non-Kolmogorov turbulence in a slant channel are studied based on the degree of polarization of quantum field. The degree of polarization of single photon states for linearly polarized quantum beam propagation in a slant turbulent channel are developed. Our results show that the effects of the outer scale fluctuations of atmospheric turbulence and the wavelength difference of the beams on the polarization can be ignored and the smaller inner scale of turbulence and larger zenith angle of communication channel will lead to larger fluctuations of the polarization of single-photon states. The effects of the inner scale of turbulence on polarization of single photon states are more significant for bigger refractive-index power and bigger zenith angles. [Copyright &y& Elsevier]

Abstract: The performance of optical wireless communication links depends strongly on the atmospheric conditions and the parameters of the link such as the propagation distance, the operation wavelength, jitter variance, attenuation coefficient and effective beam spot radius at the receiver. The analytical expression for the evaluation of the average capacity of optical wireless communication systems is derived, using the gamma–gamma distribution in the non-Kolmogorov atmosphere turbulence. The impact of atmospheric attenuation, beam wander and pointing errors on the average of the optical wireless communication link is investigated. It is shown that the capacity has a fluctuation curve, when power law α increases, and the power law α of minimum point in fluctuation curve is bigger as the non-Kolmogorov turbulence strength is stronger. [Copyright &y& Elsevier]

Abstract: Nonclassical polarization properties of a quantum field propagating through non-Kolmogorov turbulence are studied in a turbulent atmosphere paraxial channel. The analytic equation for the quantum degree of polarization of linearly polarized light is obtained. It is shown by numerical simulation that the polarization fluctuations of the quantum field are a function of the turbulent strength, the photon number, the propagation distance, the fractal constant α and the coherence length ρ 0. [ABSTRACT FROM AUTHOR]