## 3 Experiment Setup and Results  
The device diagram of the experimental equipment is shown in Figure 9, and the intensity modulation direct detection (IM/DD) system based on 25 km SSMF is established. The attenuation of the SSMF is less than 0.2 dB/km, the dispersion is less than 18 ps/(nm\(\cdot\)km), the effective group refractive index is 1.467, and the cladding diameter is 125.0 \(\pm\) 0.7 \(\upmu\)m. In addition, DSP technology and phase modulator were used to encrypt digital domain and analog domain, respectively. The encrypted signal at the digital end was loaded into arbitrary waveform generator (AWG, TekAWG70002A) with a sampling rate of 15 GSa/s. The analog signal after amplification by an electrical amplifier (EA) was injected into Mach-Zehnder modulator (MZM) to realize photoelectric modulation. The wavelengths and power of the laser were set at 1550 nm and 10 dBm, respectively. The output signal of the MZM was sent to PM 1 for phase encryption perturbation. The half-wave voltage of the PM was 6 V. The modulation depth was equal to 2. The modulation depth is defined as the ratio of the peak-to-peak value of the modulated wave to the half-wave voltage of the PM. The drive signal of PM 1 was generated by the chaotic circuit. After being amplified by an erbium-doped fiber amplifier (EDFA), the modulated optical signal was launched into 25 km SSMF. It is worth mentioning that the radio frequency (RF) signals of the intensity modulator and the phase modulator are independent, so there is no correlation between the rates of the intensity modulator and the phase modulator. The RF signal out by AWG was modulated to the light by the intensity modulator, and the optical carrier carrying the information was modulated by the independent phase modulator for perturbation. The modulation depth has a great influence on the encryption effect. The modulation depth of the proposed scheme reached 2, which can complete the encryption of information. At the receiver, the chaotic circuit 2 was used to drive PM 2 for decryption, and we adjusted the received optical power by a variable optical attenuator (VOA). A photodiode (PD) was used for signal detection. A mixed signal oscilloscope (MSO, TekMSO73304 DX) with a sampling rate of 50 GSa/s was used for analog-to-digital conversion (ADC). DSP technology was used to decrypt signal in the digital domain. The total bit rate of the OFDM signal can be tantamount to the expression of (subcarrier number \(\times\) entropy \(\times\) AWG sampling rate/IFFT size/(1 + CP)). CP is the length of the cyclic prefix to avoid data crosstalk. In this scheme, the length of CP was 1/8 of the data, and the number of IFFT points was 2048. This modulation scheme was 4 bit/symbol, and subcarrier number was 512. So, the scheme can transmit 13.3 Gb/s 16 QAM-OFDM signal. The devices on the transmitter had AWG, Laser, MZM, EA, PM 1, EDFA, and chaotic circuit 1. The devices on the receiver had PM 2, VOA, PD, and MOS. The control power was used to control chaotic circuits. Figure 10 shows the BER curve of the synchronous 16 QAM-OFDM signal and the signal of a 3 s delay decryption under different receiver power after 25 km SSMF. As can be seen from Figure 10, when the receiver and transmitter can synchronize, the signal can be decrypted normally. As the received optical power decreases, the BER of the signal increases. When there is a 3 s delay between the chaotic circuit 2 and chaotic circuit 1, the signal cannot be decrypted normally, and the BER reaches more than 0.3. It is worth mentioning that, when the delay is set to more than 3 s, the information on the receiver cannot be recovered normally. If the chaotic circuit with higher bandwidth is adopted, the synchronization requirement of chaotic circuit 1 and 2 will be stricter. The data after PM 1 are directly recovered, and the decryption of PM 2 and digital end is no longer carried out. The BER after PM 1 is above very high, as can be seen from the experimental results.