Research On Few-Mode Optical Fiber Recirculating Loop System

Optical fiber recirculating loop transmission technology widely used in laboratories is a common means to simulate and study optical fiber cascade system,capable of saving resources and easy implementation.The thesis focuses on the few-mode recirculating loop systems.The main contents and innovations are as follows:1.Two architectures of few-mode fiber recirculating loops based on spatial optical path and single-mode optical switch are analyzed,and their advantages and disadvantages are compared.The purpose of the thesis is mainly to study the concatenation characteristics of the self-made FM-EDFA in the recirculating loop system along with the MIMO algorithm.The few-mode fiber recirculating loop transmission system used in the thesis is designed by using the acousto-optic modulators in the all-fiber configuration for the convenient collection of the experimental data.2.A high-speed few-mode communication experiment system is established by photon lanterns available commercially.The end-to-end few-mode experiment is carried out by using LP₀₁ and LP_11b modes.The relationship between bit error rate（BER）and signal-to-crosstalk ratio（SXR）for 2×100 Gb/s DP-QPSK signals is studied.Experimental results show that,the post-correction error-free transmission can be realized when the SXR is greater than 8dB.The BER dependence on the input power imbalance and received optical power is also studied in the case of low crosstalk.When the two channels have the SXRs of 14.25dB and 13.81dB,respectively,compared with the back-to-back transceiver system,the received optical power cost at the post-correction error-free threshold(BER=10^-2)is 1.40dB and 4.76dB,respectively.The effects of fiber attenuation,polarization mode dispersion and mode crosstalk on the high-speed transmission system are analyzed,and the few-mode transmission distance for the system limited by crosstalk is estimated to be about 30 km.3.The principle of optical fiber recirculating loop transmission system is introduced,and the characteristics and differences between the recirculating loop transmission and linear transmission systems are analyzed and compared.A loop controller is developed with an acousto-optic modulator.Field programmable gate array（FPGA）technology and Quartus Ⅱ software are used to generate the control signals.A single-mode recirculating loop system is built up,and the function of cyclic transmission is verified by the experiments.For the recirculating loop system,the signal-to-noise ratio（SNR）can be degraded by 1.57dB after per round trip of40km-long transmission.After 7 round trips（280km）of the recirculating loop system,the BER of the signal is greater than the threshold of 2.4×10^-2 for 20%soft decision forward error correction（SD-FEC）.4.The few-mode erbium-doped fiber amplifier is placed in the fiber loop,and the recirculating loop transmission system with the few-mode amplifier is built up.It consists of the FM-EDFA composed of two self-made few-mode isolated wavelength division multiplexers（FM-IWDMs）and a 12 km-long few-mode fiber.Three-mode and two-mode amplification experiments are carried out,respectively.The results show that,the ASE noise caused by the FM-EDFA is large in the case of three-mode amplification,and especially has an influence on the transmission quality of the input signals with low power,resulting in the transmission of only one round trip（12km）.In the case of two-mode amplification of LP₀₁ and LP_11b,the BER through four round trips（48km）is less than the 20%SD-FEC threshold of 2.4×10^-2.Considering the branch attenuation of 5.54dB,equivalent to the transmission loss of the 27.7km-long single-mode fiber,the total transmission distance is up to 75.7 km.Finally,the mode power balance of the recirculating loop system with the few-mode amplifier under the condition of backward pumping LP_11b,is studied by measuring the optical power of each branch signal using a photoelectric detector（PD）.The experiment shows that,the mode power difference for the 1^st round trip is 3.19dB,and the mode power difference from the 7^th to 17^th round trip is around 4.05dB.In other words,the mode power difference after 17 round trips is about 4.05dB,corresponding to the few-mode transmission distance of 204km.