###### Abstract  
The reciprocity of the atmospheric turbulence channel in the bidirectional atmospheric laser propagation link is experimentally tested. The bidirectional transceiving coaxial atmospheric laser propagation link is built by using a hot air convection-type atmospheric turbulence emulation device with adjustable turbulence intensity. The influence of different turbulence intensities on the instantaneous-fading correlation of channel is analyzed by the spot characteristics. When there is no atmospheric turbulence in the bidirectional transceiving coaxial atmospheric laser propagation link, the value of channel instantaneous fading correlation coefficient was merely 0.023, which indicates we did not find any reciprocity in the optical channel. With the increment in turbulence intensity, the channel instantaneous fading correlation coefficient presented a constant increasing trend and then tended to be stable around 0.9 in the end. At this moment, the similarity of the instantaneous change trends for these two receiving terminal optical signals, and the consistency of their probability density function, indicates that there is good reciprocity between the bidirectional atmospheric turbulence optical channels. With the increase in the optical signal scintillation factor, we can obtain the result where the correlation coefficient value decreases accordingly. atmospheric turbulence; bidirectional optical channel; correlation coefficient; reciprocity +
Footnote †: journal: Medicalis necessary to establish a dedicated channel state information feedback channel, but this has a large delay and high cost. In a bidirectional atmospheric laser propagation link, two light waves transmitted in the opposite direction pass through the same atmospheric channel. That is, it can be considered that the two channels in the bidirectional atmospheric laser propagation link have the same fading characteristics. That is, the channels have reciprocity. In other words, when the channel is in a fixed state, the channel state of the return propagation is consistent with the channel state of the outbound propagation, and the channel state information (CSI) can be obtained at the transmitting end, so the channel is reciprocal. In this way, the adaptive optics technology can not only correct the wavefront distortion caused by atmospheric turbulence, but also reduce the additional overhead caused by establishing a dedicated channel state information feedback channel. At the same time, the reciprocity of the atmospheric turbulence channel is used for key extraction and other tasks [13]. Therefore, it is significant to measure the related characteristics of the instantaneous fading of the bidirectional atmospheric laser propagation link channel. The theory of atmospheric channel reciprocity was first proposed by [PERSON], and the Helmholtz theory of reciprocity was extended and applied to atmospheric turbulence on a sunny day. Starting from theory, two impulse response functions (Green function) were first defined. Then, a turbulent atmospheric channel model was constructed, and later a bidirectional atmospheric laser propagation was carried out, by which the reciprocity of atmospheric turbulence channel was proved, namely point-source-point-receiver (PSPR) reciprocity. However, experimental results have shown that when the two transceivers have finite apertures, the correlation coefficient between the fluctuations of the luminous flux received at the two link ends may be significantly reduced to below 1 [14]. In a bidirectional FSO link, the received optical signals of the two transceivers composed of a single-mode fiber collimator (SMFC) can always maintain a good correlation [15; 16]. At present, there is existing literature on the correlation of instantaneous fading in bidirectional atmospheric turbulence channels.