1. Visible light communication technology
Researchers such as Harald Haas of the University of Edinburgh designed a bias optimized visible light communication system (VLC). Most of the nonlinear distortion in VLC system comes from the nonlinear relationship between DC bias current and output optical power in light emitting diode (LED). In order to minimize the impact of nonlinear distortion on VLC system, it is necessary to keep the level change of data bearing signal within the linear dynamic range of LED. They choose two schemes of direct current (DC) bias to find the best bias point. First, when the bias current increases from the midpoint of the dynamic range, keep the amplitude of the modulation signal constant, and some signal components will enter the nonlinear region, but there will be higher bandwidth at higher bias current. The second is to increase the offset point and keep the signal within the dynamic range of the LED; It increases the bandwidth and reduces the signal power. The simulation results show that the optimal offset point is not in the middle of the dynamic range; The VLC experimental device of pam-4 is shown in Figure 1. When the bias current increases from 20mA at the midpoint of the linear region to 30mA at the optimum bias current, the transmission rate can be increased by ~ 36%. The proposed optimization method is combined with pre / post equalization and precoding / predistortion technology to obtain higher gain .
Fig. 1 experimental device
2. Free space optical communication
Toshimasa umezawa and other researchers of the National Institute of information and communication technology in Tokyo, Japan designed an optical radio free space optical communication system (rof-fso), as shown in Figure 2. The system uses a new type of high-speed photodetector array (2-pda) equipment (which can act as both photodetector and photoelectric converter (O / E)). With the help of an external amplifier, a radio frequency signal (RF) output of - 4.4dbm can be obtained in 2d-pda under 2.5 mA input photocurrent. On the 1.5m long free space channel and 1m long radio channel, the researchers evaluated the bit error rate (BER) under the condition of static and dynamic optical beam switching, and realized the rate transmission of 21.1 Gbps using a variety of modulation formats under the condition of static optical path switching. Under the condition of dynamic optical path switching, the vector amplitude error (EVM) in 10 switching cycles is about 9% .
3. Optical fiber communication
Researchers such as VIN í cius oliari of Eindhoven University of technology in the Netherlands designed a new optical fiber transmission model and evaluated its performance in passive optical network system, as shown in Figure 4. The signal propagation in optical fiber can be characterized by nonlinear Schrodinger equation (NLSE); When considering both dispersion and nonlinear effects, NLSE has no known closed solution (analytical solution). The model is an improvement of the regular perturbation (RP) of the group velocity dispersion (GVD) parameters. The results show that when the normalized mean square deviation is 0.1%, the input power of the model is higher (1.5dB) than that of the logarithmic perturbation model (LP) with Kerr nonlinear coefficients. At the same input power, compared with the Kerr nonlinear coefficient LP detector, the detector reduces the uncoded bit error rate by 5.4 times, which means that the input power is reduced by 0.4db at the same information rate .
4. Passive optical network
Researchers such as Robert Borkowski of Bell Laboratories in Germany have designed a flexible rate passive optical network (flcs-pon). The system adjusts the transmission scheme to match the user's channel conditions and optimize the throughput. Flcs-pon adopts technologies such as optical network unit (ONU) grouping, flexible modulation format and forward error correction (FEC) in the 50 Gbit / s PON specified by the communication standardization department of the International Telecommunication Union (ITU-T). The experimental device and test results are shown in Figure 4. For onus with low optical path loss (OPL), the downlink transmission process is twice the PON speed of 50 Gbit / s specified by ITU-T. the network supports onus with high OPL at the same time; When optical preamplifier is used to detect ONU directly, it supports 100 Gbit / s data signal transmission with 31.5 DB loss budget .
5. Semiconductor devices
Kebede ATRA and other researchers of French III-V laboratory designed a new reflective electro absorption modulator (EAM) cascaded with semiconductor optical amplifier (SOA), as shown in Figure 5. The device is fabricated by GaInAsP multiple quantum wells on indium phosphide substrate, semi insulated embedded heterostructure and docking integration technology. eighty μ The frequency response of m-long EAM is still good at 26.5ghz, while 150 μ The 3-dB cut-off bandwidth of m-long EAM is 23 GHz. According to different wavelengths, EAM reverse bias voltage is between - 1.2 ~ 1.5 V to realize zero chirp operation. Under large signal modulation, 80 μ The frequency chirp caused by M EAM is almost 150 μ Half of meam. When running at 25 Gbit / s using a non return to zero code, at 150 μ M and 80 μ High dynamic extinction ratios of about 14.5 and about 8 dB were obtained in EAM of M. Researchers used 150 μ M and 80 μ M EAM achieves 12 km and 16 km colorless transmission on standard single-mode fiber (C-band: between 1530 nm and 1545 nm). (in the case of 10-3 bit error rate, the dispersion loss is 4.5db and 2.5dB respectively) .
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