Optical wireless coherent communication employs optical hybrid to complete the mixing of signal light and local oscillator light, and balance detectors to complete photoelectric conversion. Optical fiber hybrid has been widely applied in optical wireless coherent communication systems due to its advantages such as high integration and compact structure. It is necessary to efficiently couple spatial light into the optical fiber. Adaptive optical technology can improve the coupling efficiency from spatial light into optical fiber by correcting the distorted wavefront, and it is applied to optical wireless coherent communication systems. The existence of non-common optical path aberration between the wavefront sensing branch and the coupling branch leads to the distorted wavefront in the communication branch after the closed loop of the adaptive optical system. The stochastic parallel gradient descent algorithm to correct the non-common optical path aberration is easy to fall into the local optimum, and the phase difference algorithm employed to correct the non-common optical path aberration is only applicable to the field of imaging systems. We propose a reverse transmission calibration algorithm to measure the non-common optical path aberration for the initial calibration of adaptive optical systems in optical wireless coherent communication.

The adaptive optical system in wireless optical coherent communication is shown in Fig. 1. After the laser beam is transmitted through the atmospheric turbulence, the beam is fully reflected by the deformable mirror. The reflected beam is divided into two collimated beams with a power ratio of 1:1 by the beam-splitter. One transmitted beam is to act on the wavefront sensor to monitor the current distorted wavefront. The other reflected beam is directly coupled into a single-mode fiber after being converged by a coupling lens for optical hybrid, coherent detection, and communication. Reverse transmission is sending the same beam from the receiver to the transmitter. A laser beam identical to the transmission source is supposed to be connected to the coupling optical fiber, as shown in Fig. 2. At the same time, the deformable mirror command in this state is cleared so that the deformable mirror is in a completely flat reflection state. The beam output by the coupling fiber is reflected by the beam-splitter and then reflected by the deformable mirror directly into the wavefront sensor to measure the wavefront information. At this time, the measured wavefront information includes both the wavefront phase that can maximize the coupling efficiency and non-common optical path aberration. The coupling efficiency can be improved when the measured wavefront information is converted to the closed-loop control of adaptive optics.

The local oscillator light in the optical wireless coherent communication system is connected to the coupled single-mode optical fiber by the reverse transmission calibration algorithm. The peak-to-valley value of the non-common optical path aberration (Fig. 7) measured by the wavefront sensor is 3.71 μm, with the root mean square value of 1.34 μm. This error is enough to exert a significant impact on coupling efficiency. When the wavefront information is converted into the adaptive optical closed-loop control, coupling efficiency increases from the initial 9.04% to the closed-loop 45.21% (Fig. 8). The self-noise inside the wavefront sensor causes some synaptic data in the wavefront slope measurement and wavefront reconstruction, but does not significantly affect the fluctuation of coupling efficiency (Fig. 8). In turbulent environments, the coupling efficiency increases from 19.72% under uncorrected state to 36.93% under closed-loop state (Fig. 13), and that in complex environments increases from 3.91% in an uncorrected state to 9.13% in the closed-loop state (Fig. 16). This shows that with the increase of communication distance, the influence of atmospheric turbulence on adaptive optical correction effect is more significant than non-common optical path aberration.

Based on the reversibility principle of optical paths, we propose a reverse transmission calibration algorithm to measure and correct non-common optical path aberration for adaptive optical fiber coupling systems in optical wireless coherent communication. This algorithm converts the non-common optical path aberration into closed-loop control and improves coupling efficiency while correcting the distorted wavefront phase. Compared with conventional stochastic parallel gradient descent algorithms, this scheme will not fall into the local optimum, and will not be affected by the external turbulent environments. It can also assist in the position alignment of optical paths, which is simple, feasible, and easy to realize in engineering. Finally, reference significance and practical value are provided for the optical fiber coupling technology of the optical wireless coherent communication system.