## 2 Principle and Device of Coherent/Non-Coherent OOK Reception without AL  
In order to study the sensitivity deterioration caused by AL noise, high-sensitivity coherent and non-coherent OOK receiving devices were designed and completed. The information demodulation of the receiver in the FSO OOK system is primarily based on direct detection (DD). APDs are often used in DD receivers for optical-to-electrical conversion [30; 33]. However, the sensitivity of a DD system using an APD is inferior to that of a positive intrinsic negative photodiode (PIN) and a preamplifier. Thus, the latter was selected as the non-coherent reception mode in this study. The preamplifier was an erbium-doped fiber amplifier (EDFA) as the central wavelength of the signal light was 1555.74 nm. The EDFA had a gain of 30 dB and a noise figure of 5.25 dB. A narrowband optical filter is an important device for ensuring high sensitivity in a DD system using preamplifiers. In DD reception experiments, we used a tunable filter and set the full width at half maximum (FWHM) to 0.8 nm, which means the FWHM is from 1555.34 nm to 1556.14 nm. The reason for choosing 0.8 nm is that it is one of the standard wavelengths of dense wavelength division multiplexing (DWDM) and can ensure that the signal light does not appear outside the passband of the optical filter through the influence of Doppler frequency shift. The tunable optical filter in this experiment was the BVF-200 from Alnair Labs. A structural diagram of the non-coherent receiver is shown in Figure 1.  
In the experiment, the signal light entered the light input of the balance detector (BD), which is equivalent to a PIN, after the tunable optical filter. In this experiment, the communication rate was 1 Gbit/s; we obtained the curve between the BER and optical signal power when receiving non-coherently, as shown in Figure 2. When the BER is \(10^{-3}\), the optical signal power was \(-51.25\) dBm. The sampling rate of the high-speed oscilloscope was 5 GSa/s, and the channel analog bandwidth was set to 1 GHz. The offline processing (Figure 1) included low-pass filtering, \(10\times\) resampling, time synchronization, and threshold decisions. Figure 1: Experimental structure of non-coherent OOK receiver. The experimental results for non-coherent reception without AL are shown in Figure 2. The experimental structure for coherent reception of OOK signal light is shown in Figure 3.  
For the coherent reception of OOK, the best LO power was determined to be 12.8 mW by calculating the SNR of the beat frequency signals using the cross-correlation method. The OOK format signal light and the LO light enter the optical 90\({}^{\circ}\) hybrid, and the hybrid outputs four channels of light. Two lights from the in-phase (I) branch enter one BD and two lights from the quadrature (Q) branch enter the other BD. The BD was the same as the BD used for non-coherent reception. The voltage signals generated by the four output lights are presented as [34; 35; 36]  
\[ \begin{split}\vspace{-0.2 cm}\vspace{-0.2 cm}l_{b}(t)& =R\Big{[}\frac{1}{2}(s(t))^{2}(1-k_{S})P_{S}+\frac{1}{2}(1-k_{LO})P_{ \Omega}+\sqrt{1-k_{S}}\sqrt{1-k_{LO}}\sqrt{P_{\Omega}P_{S}}(t)\cos(\omega st- \omega_{LO}t+\varphi_{S}(t)-\varphi_{LO}(t))\Big{]}+n_{\Omega}(t),\end{split}