Digital Signal Processing And Its Applications In Advanced Optical Communications

Fueled by emerging bandwidth-hungry applications such as internet,high-definition video,big data and cloud computing,the internet traffic is growing exponentially and the trend is expected to continue for the foreseeable future.To meet the demand of the explosive growth of internet traffic,the transmission capacity of a single fiber increases by a factor of approximately 10 every four years over the past 40 years.However,over the next decade or so,the commercial capacity will reach its nonlinear Shannon capacity limit.In order to cope with the coming“capacity crunch”,we need to develop the spatial dimension of lightwaves to realize the sustainable growth of communication capacity.On the other hand,with the fast development of the optical fiber communication,and the continuous increasing of the capacity,transmission distance,complexity of the modulation format and the number of multiplexing channel,if the data switching still adopts the traditional technique of optical-electro-optical?O/E/O?conversion,the limited speed on signal processing will become a bottleneck of the future high-speed optical communiations.Therefore,advanced optical communications based on space-division multiplexing?SDM?and advanced all-optical signal processing have become the most promising high-speed optical communication techniques.High order modulation format,coherent detection and DSP technologies are the main technologies of modern optical communications.By integrating DSP with advanced optical communications,it can be used to solve some key problems.In this dissertation,by using DSP technologies and the existing communication optoelectronic devices,a series of theoretical and experimental researches of the SDM based advanced optical transmission and all-optical signal processing are studied.The major contents of this dissertation are summarized as follows:?1?We present a comprehensive characterization of two orbital angular momentum(OAM₊₁ and OAM_-1)modes transmission and multiplexing over long-distance transmission link in few-mode fiber?FMF?.In order to evaluate the performance dependence of long-distance transmission link on inner mode group differential modal delay?DMD?and mode coupling,we compare OAM modes transmission experiments by using four different baud rates of 1-Gbaud,2-Gbaud,5-Gbaud and 10-Gbaud,respectively.Low-density parity-check?LDPC?codes are employed to mitigate the influence of mode crosstalk and DMD for long-distance fiber OAM modes transmission.By evaluating the performance of LDPC-coded single OAM mode transmission over 50-km fiber,significant coding gains of>4 dB,8 dB and 14 dB are demonstrated for 1-Gbaud,2-Gbaud and 5-Gbaud quadrature phase-shift keying?QPSK?signals,respectively.Furthermore,in order to verify and compare the influence of DMD in long-distance fiber transmission,single OAM mode transmission over10-km FMF is also demonstrated in the experiment.Both of the theoretical and experimental results show that inner mode group DMD and mode coupling may limit the capacity of long-distance fiber OAM transmission.Finally,we experimentally demonstrate OAM multiplexing and transmission over a 50-km FMF using LDPC-coded 1-Gbaud QPSK signals to compensate the influence of mode crosstalk and DMD in the 50 km FMF.?2?Space-division multiplexing using multiple-input multiple-output digital signal processing?MIMO-DSP?.?1?Fractional OAM dense mode-division multiplexing?DMDM?in free-space optical communications is experimentally demonstrated.We demonstrate the free-space OAM link with 10-Gbaud QPSK/16-QAM signals over two OAM beams with an OAM channel spacing of 0.6,0.4,0.2 and 0.1 assisted by 2?2 MIMO-DSP and LDPC codes.The experimental results show that the OAM mode crosstalk increases with the decrease of channel spacing.MIMO equalization and LDPC codes can be employed to increase the reliability of the system,and the BER curves are below the 7%FEC threshold.Fractional OAM multiplexing communications even with a small OAM channel spacing of 0.1 is successfully demonstrated in the experiment.?2?An OAM multiplexing scheme over conventional multi-mode fiber?MMF?with less DSP complexity is proposed and experimentally demonstrated.Both of the theoretical and experimental results show that the mode coupling mostly occurs within the mode group,and the mode coupling between different mode groups is negligible.Consequently,instead of using the full MIMO equalization,only partial MIMO equalization is required to mitigate the intra mode-group crosstalk.We demonstrate OAM multiplexing using 6 OAM modes each carrying 20-Gbit/s QPSK signal over 8.8-km OM4 MMF with only 2?2 or 4?4 MIMO-DSP,achieving an aggregated capacity of 120 Gbit/s.We also demonstrate MIMO-free OAM mode group multiplexing transmission in 8.8-km MMF for two OAM modes belonging to different groups.The obtained favorable results show the viability of OAM modes multiplexing transmission over already existing conventional fibers.?3?DSP applied in optical communication systems.?1?We propose a hybrid mode-division multiplexing time-division multiplexing passive optical network?MDM-TDM-PON?scheme based on traditional TDM-PON structure assisted by OAM multiplexing.A 1.1-km FMF is used for short-distance OAM multiplexing transmission.Bidirectional transmissions with 2.5-Gbaud 4-level pulse amplitude modulation?PAM-4?downstream and 2-Gbaud on-off keying?OOK?upstream are demonstrated in the experiment.The observed optical signal-to-noise ratio?OSNR?penalties for downstream and upstream transmissions at a bit-error rate?BER?of 2×10^-3 are less than 2.0 dB and 3.0 dB,respectively.The obtained results show that OAM multiplexing has potential to be combined with other dimension multiplexing techniques to construct hybrid multiplexing PONs.?2?We experimentally demonstrate an adaptive water-air-water data information transfer using OAM.According to the feedback information of the received intensity distribution,the reflection element is adjusted for mitigating the misalignment-induced degradation effect due to water level change.Moreover,we further demonstrate the data-carrying performance of discrete multi-tone modulation?DMT?signal across water-air-water link to verify the viability of the proposed scheme,and the gross bit rate of 1.08 Gbit/s is achieved.By comparing the system performance with and without feedback under two relative surface heights?25 mm/-10 mm?,the measured power penalty at at a bit-error rate?BER?of 3.8×10^-3 shows an improvement of 2.5 dB and 1 dB at relative surface height of 25 mm and-10 mm with feedback,respectively.?3?Based on non-degenerate four-wave mixing?ND-FWM?in a silicon waveguide,we experimentally demonstrate two-input?A,B?simultaneous 5-Gbaud octal computing?A+B,A-B,B-A,-A,-B,2A,2B?in the experiment with 8PSK signals.We evaluate the BER performance of the two input octal computing and plot the symbol sequences and constellations.