Research On Optical Field Recovery And Optical Signal-to-noise Ratio Monitoring With Direct Detection

From the perspective of power consumption and cost,direct detection is preferable to coherent detection.In this thesis,direct detection technology is studied in the field of short-reach optical interconnection and optical performance monitoring.The main work and innovative contribution are listed as follows,(1)Self-coherent detection is a promising solution for extending the transmission reach for a direct detection system.For these schemes,a high carrierto-signal power ratio(CSPR)is usually required to mitigate the inherent signalsignal beat interference(SSBI)induced from direct detection.Recently,carrierassisted differential detection(CADD)receiver has been proposed to recover the optical field of complex-valued double sideband(DSB)signals via direct detection.In this thesis,aiming at relaxing the high CSPR requirement for selfcoherent detection systems,an interleaved subcarrier loading scheme is proposed for double-sideband signals.At the transmitter,only odd subcarriers are loaded with data while even subcarriers are left unused.By utilizing the CADD receiver,the signal can be retrieved without the degradation of SSBI,which enables optical field recovery at low CSPR.CADD scheme can improve the electrical spectral efficiency,while interleaved technology can remove SSBI in the system.As a result,the proposed scheme can achieve almost the same electrical spectral efficiency as SSB self-coherent systems,requiring relatively low CSPR.Experimental result demonstrates a successful 100-Gb/s OFDM signal transmission over 160-km SSMF at 3.5-dB CSPR.(2)The accuracy of optical signal-to-noise ratio(OSNR)monitoring scheme based on Stokes parameters is vulnerable to polarization-related impairments,such as polarization mode dispersion(PMD),polarization-dependent loss(PDL),etc.To deal with this problem,a polarization-insensitive OSNR monitoring scheme based on Stokes parameters is proposed in this thesis.Firstly,polarization state tracking is performed in Stokes space and feature parameters representing polarization impairments are extracted from Stokes parameters.Then,a support vector regression(SVR)algorithm is used for nonlinear fitting to correct the monitoring error caused by polarization impairments.The Stokes parameters in the proposed monitoring scheme can be obtained through Stokes vector direct detection(SVDD).The sampling rate of SVDD receiver is only 20 MHz.In addition,the complexity of SVR can be reduced by introducing the kernel trick.As a result,the proposed monitoring scheme has distinguished advantages in cost.The experimental results show that the proposed scheme has a high tolerance to polarization impairment.The maximum error of OSNR monitoring is 0.49 dB and 1.12 dB under the interference of 3-dB PDL and 15-ps first-order PMD,respectively.(3)The existing Stokes parameters-based OSNR monitoring scheme is only applicable for QPSK signal.To overcome such limitation,two modulation format(MF)-transparent OSNR monitoring schemes based on Stokes parameters are proposed in this paper.The high-speed Stokes scheme uses the ratio of the minimum and the maximum variance in the eye diagram of S1 parameter to characterize the MF,while the low-speed Stokes scheme directly uses the variance of S1 parameter as the feature of MF.The SVR algorithm is utilized for nonlinear fitting to realize the MF-transparent OSNR monitoring.The experimental results show that both schemes can achieve MF-transparent OSNR monitoring.Highspeed Stokes scheme is more suitable for the optical networks with severe polarization impairments,while low-speed Stokes scheme is better for optical networks with large residual dispersion.