## 6 Conclusions  
In this study, an FSO OOK coherent/non-coherent communication receiver was built. To improve receiver sensitivity, non-coherent receiving mode with EDFA and coherent receiving mode without EDFA were selected. With a communication rate of 1 Gbit/s and a BER of \(10^{-3}\), the optical power required by the coherent and non-coherent systems were \(-54.60\) dBm and \(-51.25\) dBm, respectively, both with high sensitivity. The coherent mode was 3.3 dB from the shot noise limit. The influence of AL on OOK coherent and non-coherent receiving systems was theoretically analyzed. For a coherent OOK system, the AL-LO beat noise was mainly introduced by AL. When the AL is sufficiently large, the AL-LO beat noise exceeds the shot noise and dominates. For a non-coherent OOK receiving system with a preamplifier, AL causes new noise. The SSFSD of this noise and the corresponding current variance were reported. An ASE light source was used to simulate AL. The influence of SAL on the OOK coherent and non-coherent receiving systems was experimentally investigated. The relationship curves between the BER and received optical power were presented for different SAL PSDs using the two systems. The experimental results of the influence of AL on the coherent OOK receiving system were consistent with the theoretical results. The optical bandwidth was set to 2 nm for coherent reception and 0.8 nm for non-coherent reception. The experiment demonstrates that the OOK coherent receiver has better anti-AL noise performance than the OOK non-coherent receiver. The experiment was based on the verification completed by the optical fiber system. The main index used to evaluate the intensity of AL was the PSD of the AL (\(\mathrm{W/Hz}\)). To better relate the experimental results with an actual FSO system, the link margin of the FSO system was calculated. Using the sun as the main noise source of the FSO system, the average coupling efficiency of the sunlight to the receiver SMF was analyzed. When the receiver lens diameter was greater than 0.488 mm, the power coupled into the receiver SMF did not increase with an increase in the receiver lens diameter, essentially remaining the same, indicating that AL noise can be suppressed to a certain extent by increasing the receiver lens diameter because the receiver gain increased with an increase in the receiver lens diameter. When the sun shines directly and the receiver lens diameter is greater than 0.488 mm, the coherent OOK receiving system coupled with the SMF and the non-coherent receiving system coupled with the SMF produce sensitivity losses of 1.15 dB and 2.60 dB, respectively, demonstrating that the FSO system can work in direct sunlight. When the BPSK signal or QPSK signal is coherently received, the change in dominant noise caused by AL noise on the corresponding coherent receiver is consistent with that of the OOK coherent receiver. This study is of great significance to the further study of the influence of AL noise on FSO receivers in the future.  
**Author Contributions:** Theoretical derivation and experimental design, [PERSON]; Funding acquisition, [PERSON]; Methodology, [PERSON]; Supervision, [PERSON]; Writing--original draft, [PERSON]; Writing--review and editing, [PERSON] and [PERSON]; Joint discussion, [PERSON], [PERSON], [PERSON] and [PERSON]. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Strategic Priority Research Program of Chinese Academy of Sciences (XDB43030400) and the Key Projects of the Special Innovation Zone of the Science and Technology Commission (19-h863-01-zd-012-001-01).  
**Institutional Review Board Statement:** No applicable.  
**Informed Consent Statement:** No applicable.  
**Data Availability Statement:** No applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.