A Study On Key Technologies For Mode Division Multiplexed Optical Transmission Systems

Space division multiplexing?SDM?has attracted considerable attention in order to overcome the optical transmission capacity crunch of single-mode fiber?SMF?.There are two approaches for realizing space division multiplexing?SDM?:fiber-core multiplexing and mode division multiplexing?MDM?.In fiber-core multiplexing,different cores in multi-core fiber?MCF?are used as independent channels to transmit different signals.On the other hand,MDM system employs different modes in few-mode fiber?FMF?or orbital angular momentum?OAM?fiber as independent channels.Compared with fiber-core multiplexing,MDM is more promising to reduce the cost and power consumption per bit,because it has the advantages of efficient fiber cross-section utilization,sharing of pump light across all the signal modes in optical amplifier,and compatibility with SMF device platforms?e.g.splicer?.For realizing MDM transmission,a series of key technologies are required.This thesis will focus on mode MUX/DEMUX,mode mixer,and optical amplifier for MDM.First of all,this dissertation demonstrates a mode MUX/DEMUX based on orientation-insensitive FMF coupler.The coupling behavior of a weakly-coupled FMF coupler are characterized based on coupled-mode theory.We investigate analytically and numerically the orientation-sensitive coupling issue of the non-circular-symmetric modes.By carefully choosing the fiber core separation and coupler length,we can design orientation-insensitive fiber couplers for non-circular-symmetric modes at arbitrary coupling ratios.Based on the design method,we propose an orientation-insensitive two-mode fiber coupler at 850 nm working as a mode MUX/DEMUX for two-mode transmission using standard single-mode fiber.Within the band from 845 to 855 nm,the insertion losses of LP₀₁and LP₁₁ modes are less than 0.03 dB and 0.24 dB,respectively.When the two-mode fiber coupler is used as mode DEMUX,the LP₀₁/LP₁₁ and LP₁₁/LP₀₁ extinction ratios in the separated branches are respectively above 12.6 dB and 21.2 dB.Our design method can be extended to two-mode communication or sensing system at other wavelengths.After that,this dissertation demonstrates a mode mixer for mixing 10 spatial modes based on multi-plane light conversion?MPLC?.The insertion loss of the mode mixer is about 4 dB for the 10 spatial modes.MPLC allows full control of the mode mixing and operates over a broad bandwidth?200 nm?.This particular device performed a difficult transform using 14phase masks that fully inverted the 10 modes to show the principle,but many other simpler mode mixers could be fabricated with fewer masks.This dissertation then demonstrates a second-order few-mode distributed Raman amplifier?FM-DRA?over 100-km FMF.To minimize the mode dependent gain?MDG?,two backward depolarized pumps at the wavelengths of 1455 nm?75 mW?and 1360 nm?1890 mW?are equally launched both into the LP_11a and LP_11b modes.The Raman on-off gains of the LP₀₁and LP₁₁ modes are both above 8 dB and the MDG is less than 0.7 dB.Amplified spontaneous emission?ASE?noise improvement of 1.8 dB is achieved in the second-order FM-DRA,with respect to the first-order Raman amplification.The effective noise figure of the second-order FM-DRA is about-3 dB,which is¹.4 dB lower than that in the first-order pumping case.We then carry out transmission experiment with 8 wavelength-division multiplexed channels based on the second-order FM-DRA.The bit-error-rates for all the transmission channels are significantly improved due to the low noise figure amplification of the second-order FM-DRA.Subsequently,the dissertation designs and experimentally demonstrates a FM-DRA covering C+L band with a flat on-off gain of⁴ dB.A state-of-the-art gain bandwidth of 75nm?1530-1605 nm?is achieved.The wavelength dependent gain for both LP₀₁ and LP₁₁modes is about 0.6 dB and the maximum MDG is less than 0.3 dB.MDM optical transmission is performed over 75 km FMF assisted by the proposed Raman amplifier.Due to the low noise amplification of the FM-DRA,more than 3.3-dB optical signal-to-noise ratio?OSNR?enhancement has been obtained with significantly improved bit-error rate?BER?of about two orders.Besides linearly polarized?LP?mode basis sets used in FMF,OAM modes also show potential for MDM optical communications.We then propose and demonstrate an orbital angular momentum distributed Raman amplifier?OAM-DRA?over 18-km graded-index ring-core fiber.Within the band from 1530 to 1565 nm,the maximum on-off gains are 3.5 dB for the OAM₊₄ and OAM₊₅ modes,and the MDG is less than 0.3 dB.Wavelength-division multiplexing?WDM?/OAM transmission experiment is performed based on the OAM-DRA.The measured OSNR with DRA is about 1 dB higher than that without DRA for all the multiplexed channels.A total transmission capacity of 640-Gbit/s is achieved by using 2OAM modes and 16 WDM channels.At last,we gather the main conclusions of this dissertation and briefly present some suggestions for the future work.