With the rapid growth of the demand for wireless communication bandwidth, people's demand for high-speed data access is increasingly urgent. With the aid of visible light communication(VLC) multiplexing techniques, the performance of VLC can overcome the modulation bandwidth constraint and double the transmission capacity of the VLC system, and it is a research hotspot in the field of VLC. This article introduces the basic principles and key technologies of VLC multiplexing techniques, including multi-dimensional multiplexing technique and multiple-in-multiple-out(MIMO) technique proposed in recent years, and looks into the future development based on the research status.

In order to meet the needs of the development of the 5G fronthaul networks and provide greater bandwidth, in this paper, a four pulse amplitude modulation(PAM4) signal with a downlink rate of 50 Gb/s and an non return to zero(NRZ) signal with an uplink rate of 10 Gb/s are used to constitute a two-way communication system in the reflective semiconductor amplifier(RSOA) based on wavelength division multiplexing passive optical network(WDM-PON) system with an asymmetric transmission architecture as a fronthaul network carrying scheme. The simulation software Optisystem is used to simulate and analyze the WDM-PON system based on RSOA remodulation, and analyzes the transmission performance of the system at different transmit optical power; The performance of the uplink and downlink transmission system is analyzed under the conditions of back-to-back and dispersion compensation. It is proved that WDM-PON with downlink 50 Gb/s PAM4 signal and uplink 10 Gb/s NRZ signal can be used as a fronthaul network carrying scheme.

In order to cater to the growing demand for data communication in the 5G and 6G era, aiming at the development trend of ultra high speed Ethernet multi service support, flexible speed and IP optical fusion networking at this stage, this paper studies the protocol specifications of flexible ethernet(FlexE), including FlexE basic structure, overhead frame definition and Calendar mechanism. On this basis, a functional model for mapping user data of different rates in the media access control(MAC) layer to the physical layer is proposed, and a time slot allocation algorithm based on the Calendar mechanism is studied.

Aiming at the problem of communication eavesdropping, based on the research project of "research on self-secured optical communications theories and technologies", the existing technology development of fiber physical layer security transmission is introduced at the information level, carrier level and modulated signal level. This paper proposes a fiber physical layer secure transmission technology based on quantum noise stream cipher. The orthogonal frequency division multiplexing based on DFT-spread(DFTs-OFDM) is introduced into the quantum noise stream cipher transmission system based on quadrature amplitude modulation(QAM), which can effectively resist the eavesdropping of non-partners while ensuring high spectrum utilization. The research results show that the optical transmission rate of 10 Gb/s has no relay transmission distance of 300 km, and the Q factor exceeds 9, and the information interception probability is less than 0.001%.

In order to provide a theoretical reference for the rational application of tilted long period fiber grating in the field of refractive index sensing, the variation rule of effective refractive index and wavelength drift of resonant wave under different cladding modes and different cladding radius are simulated by optical simulation software. The simulation results show that the effective refractive index of each cladding of the grating increases with the increase of the refractive index of the environment, and higher order cladding modes with smaller radius have higher sensitivity. According to the variation rule of the wavelength shift of the resonant wave and the refractive index of the environment, different parameters can be selected in practical applications to meet different sensitivity requirements.

In this paper, a temperature compensated hydrophone based on fiber Bragg grating (FBG) is developed to meet the requirements of sound detection in different temperature in marine environment. The hydrophone is sensitized by the side stretch structure, and the structure of hydrophone is optimized by the finite element analysis method. In this structure, the compensation material Polymethyl methacrylate(PMMA) is used, and its thermal expansion effect is used to modulate FBG, so as to reduce the impact of temperature on FBG and realize temperature compensation. The simulation results show that the structure can achieve higher sensitivity in the range of 10~2000 Hz, and the sensitivity gain spectrum of the hydrophone shows resonance characteristics. Compared with the place far away from the resonance frequency, the sensitivity gain is further improved, reaching 145 dB at most. In the range of ocean temperature (-2 ℃ ~ 30 ℃), the temperature compensation of wavelength shift less than 0.01006 nm can be realized.

In the optical fiber distance measurement based on optical frequency domain reflectometer (OFDR), the nonlinear tuning effect of the light source directly affects the accuracy of the measurement distance, which makes the OFDR measurement result shift. By theoretically analyzing the relationship between the linearity of laser tuning and measurement distance, a method of distance correction is proposed in this paper. An auxiliary interferometer is used to measure the nonlinearity offset of the light source tuning, and the interpolation is directly performed in the signal frequency domain according to the offset. Experiments show that this method can eliminate the influence of the nonlinear tuning of the light source on the measurement distance, improve the accuracy and spatial resolution of the OFDR measurement distance effectively. Since the calculation is completed in a frequency sweep period, it's very efficiency.

Aiming at the problem that the avalanche photo diode(APD) gain changes with temperature under the same voltage, which affects the dynamic range of the optical time domain reflectometer(OTDR), an adaptive precise control scheme of APD bias voltage is proposed. At each suitable operating temperature, This scheme can ensure that the bias voltage is always close to the avalanche voltage, and make APD work stably at the best gain, so as to improve the dynamic index of OTDR, and solve the problem of large dynamic range fluctuation in the high and low temperature test of OTDR, so that the dynamic range fluctuation is controlled within 0.3 dB.

With the rapid development of 5G, the construction of high-speed optical communication network is extremely urgent. Lithium niobate modulator is an important part of signal modulation system in high-speed optical communication. Improving the performance of lithium niobate modulator is the basis of the further development of high-speed signal modulation technology. This paper embarks from the present situation of the application of lithium niobate electrooptic modulator, summarizes the various research institutions in recent years for lithium niobate modulator in all aspects of the improvement and research, compared the various improvement direction of the known research achievements, the advantages and disadvantages of several improving methods are discussed, and the corresponding experimental research is carried out, points out the new silicon-based integrated lithium niobate modulator is the prospect of further development.

The size of the optical device used in the integrated optical path must be small enough. The traditional coupling beam splitting method requires a long coupling distance, the size is not easy to control in a small range. The use of a coupling region to increase the variable dielectric column can effectively reduce the coupling distance. Straight waveguides are introduced into the photonic crystals, and the exit ends of the straight waveguides are designed as three wave exit ports, where the exit ports on both sides have the same structure. Using the finite-difference time-domain analysis, the results show that the purpose of controlling the power distribution of light waves can be achieved by changing the size of the dielectric pillars in the coupling region between the straight waveguides, thereby achieving the splitting ratio of the waveguide transmission process.

Aiming at the distributed sensing system based on multimode fiber is affected by the Brillouin scattering characteristics of multimode fiber, this paper uses finite element simulation to obtain the electric field distributions of six typical transmission modes of multimode fiber, and the Brillouin scattering characteristics of multimode fiber under single mode and mode superposition are simulated and studied from four aspects: Brillouin frequency shift, linewidth, max gain and Brillouin scattering spectrum. The simulation results show that the linewidth and max gain of different transmission modes of multimode fiber decrease slightly with the increase of the mode order, while the Brillouin frequency shift decreases significantly with the decrease of the mode order. Unaffected by strain and temperature, the Brillouin scattering spectrum after mode superposition is broadened compared to the single mode linewidth, and the max gain is greatly reduced.

In the simulation of laser turbulence propagation, with the continuous enhancement of turbulence along the path, the light spot will produce huge distortion, and it is often impossible to have high simulation accuracy and computational efficiency at the same time. In this paper, a method is proposed to simulate the laser propagation in moderate-strong turbulence channel by adopting different sampling mesh spacing in the transmitting and receiving plane. This method can be used to set up different mesh spacing between the transmitting and receiving planes according to the specific situation while keeping the number of sampling points unchanged, to make the simulation results of laser propagation closer to the theoretical value at various turbulence intensities. The simulation results show that, after the mesh spacing of transmitting plane is reduced, the relative error of scintillation index simulation value decreases from more than 20% to less than 10%, and the minimum error reaches 4.15%. The standard deviation of scintillation index simulation value also decreases from more than 0.07 to less than 0.05, and the minimum error reaches 0.015.

Subcarrier index power modulation (SIPM) can use the subcarrier index information to adjust the corresponding power information. It is used in a fiber-visible light fusion system, which can better increase the transmission capacity of the system. A full-duplex fiber-visible hybrid system based on subcarrier index power modulation-orthogonal frequency division multiplexing (SIPM-OFDM) is proposed. The structure of the system is simplified by using the wavelength reuse method, and the SIPM-OFDM signal is established. The model is generated and the feasibility of the system is verified by simulation verification. Simulation results show that after 35 km standard single-mode fiber (SSMF) and 5 m visible light bidirectional transmission, the system power cost is less than 1 dB and 2 dB, respectively, and the constellation diagram is still clear.

Aiming at the scattering effects of seawater optical channel, the Henyey-Greenstein function is selected to generate the scattering angle. Based on the Monte Carlo(MC) simulation method, the effects of transmission distance and field of view (FOV) on the proportion of scattering components of arriving optical signal under caseⅠseawater and caseⅡseawater channels are analyzed successively. The results show that the proportion of scattering components of arrival optical signal under caseⅠseawater is less than 1.2%. The scattering components is significantly increased under caseⅡseawater channels, and the value of FOV has an obvious effect on the proportion characteristics of scattering components. Especially for the caseⅡseawater (harbor) channels, the proportion of scattering components is approaching to 100% when the transmission distance is longer than 6.0 m.