The Theory And System Design Of Optical Polarization Division Multiplexing With Direct Detection

The communications and processing of Big Data have brought great pressure to the short reach optical systems,because of its ever-increasing requirement on transmission rate.In consideration of cost and power efficiency,the bottom optical transmission system usually adopts intensity modulation-direct detection(IM-DD)technology.and combined with advanced modulation formats in order to achieve high rate of transmission.However,depending on the high-effective modulation formats only,makes it difficult to increase transmission rate up to 100 Gbit/s or above smoothly and reach the target transmission distance without hardware upgrade innovation.This paper mainly researches on the polarization division multiplexing(PDM)combined with advanced modulation formats in direct detection system.A series of high-speed optical interconnection transmission systems have been designed and some innovative results are obtained.The main work and innovative contributions are listed as follows,(1)The direct detection system of high speed 4 order pulse amplitude modulation(PAM4)signal with low cost commercial device is experimentally studied by using advanced digital signal processing(DSP)algorithms,i.e.least-mean square(LMS)based equalization,direct detection faster than Nyquist(DD-FTN)algorithm,pre-equalization(Pre-EQ),error-table based pre-correction(ETC).In order to obtain these pre-compensated parameters easily,a joint equalization and error table generation(JEEG)module is proposed.Employing the combination of ETC,LMS,and DD-FTN,a single line 112 Gbit/s PAM4 40 km amplifier-less transmission with a record receiver sensitivity of-16.6 dBm(at 7%HD-FEC threshold)is experimentally demonstrated.In addition,the computational complexities of different DSP modules are analyzed and discussed in detail.The receiver computational complexity can be effectively reduced by employing appropriate ETC and Pre-EQ in the transmitter.(2)In view of the low cost and high rate demand in the current short reach optical transmission,a PDM-PAM4 system is proposed based on IM-DD.Employed DSP algorithm,including the state of polarization(SOP)estimation,S2 component elimination and adaptive multiple-input multiple-output(MIMO)filter based on LMS algorithm,the receiver structure is simple and system performance is stable and reliable.Compared with two popular PDM-DD systems in the uniform Stokes vector(SV)space,advantages and disadvantages of three kinds of systems are given.The simulation platform of 224 Gbit/s system is built by Matlab and VPI software,and three PDM-DD systems are analyzed and compared in detail.(3)Based on single sideband(SSB)modulation,a simple PDM-DD system is proposed.Due to the two orthogonal signals have no polarization cross talk effect,two dual-drive Mach-Zehnder modulators(DDMZM)modulated two orthogonal SSB signals with shared guard band.The receiver consists of a single photodiode(PD).The PDM-SSB-DD system is high speed with low cost.Compared with single polarization SSB-DD system,the OSNR requirement is reduced by 3.6 dB at bit error ratio(BER)of 3.8E-3.By using the bit loading algorithm,the PDM-SSB-DD system capacity increased from 80 Gbit/s to 100 Gbit/s.After 80 km transmission,the OSNR requirement is 29.1 dB at BER of 3.8E-3.(4)Based on SSB and DMT hybrid modulation technology,another simple PDM-DD system is proposed.Due to the two orthogonal signals have no polarization cross talk effect,the SSB and DMT signals are modulated on two orthogonal polarizations respectively.The receiver consists of a single PD.The proposed PDM-DMT-SSB system spectral efficiency and chromatic dispersion(CD)tolerance can be optimized.50 Gbit/s PDM-SSB-DMT signals transmission over 40 km without optical amplifier is experimentally realized.It is a promising cost-efficient solution for future short reach optical transmission system.