High Sensitivity Digital Coherent Receiving Technology

In recent years,digital coherent optical communication has gradually become the mainstream optical fiber communication technology due to its high sensitivity,good selectivity,and high spectrum recognition.Digital coherent reception has also become the mainstream receiving method in optical communication systems where the received optical power is limited,such as free-space optical communication and non-relay optical fiber transmission systems,due to its high sensitivity.Higher sensitivity usually means longer transmission distance and lower requirements for received optical power.Among the many modulation formats of coherent optical communication,the m-order pulse position combined quadrature phase modulation format(mPPM-QPSK)has a very high theoretical limit of sensitivity,and has extremely high research value in optical communication systems.This article is based on a high-sensitivity digital coherent optical communication system,with regard to the mPPM-QPSK modulation format,starting from theoretical analysis,high-speed back-to-back digital coherent optical communication experiments and digital signal processing algorithms at the receiving end,and continuously improving the receiver sensitivity of the system to make it further Close to the theoretical limit.The main research contents include:1.Investigated the current research status of high-sensitivity digital coherent receiving technology at home and abroad,introduced the basic principles of mPPM,mPPM-QPSK,and Nyquist-mPPM-QPSK modulation formats,and compared their theoretical performance and advantages and disadvantages.And through theoretical derivation,the signal-to-noise ratio change of different noise figure of optical preamplifier is obtained.2.An experimental platform for mPPM-QPSK and Nyquist-mPPM-QPSK back-to-back coherent optical communication systems with a rate of 10Gb/s was built.The basic flow of digital signal processing at the receiving end is introduced,and the specific algorithms used in each part and the basic principles of the algorithms are introduced in turn.3.From the experimental point of view,the system sensitivity is optimized.For the mPPM-QPSK modulation format of different modulation orders,the adjustable parameters that will affect the sensitivity of the receiver in the experiment are optimized,including the output amplitude of the arbitrary waveform generator.Value,the pump current of the EDFA and the bandwidth of the optical filter.4.Optimized from the perspective of the digital signal processing algorithm at the receiving end.First,by analyzing the signal characteristics of mPPM-QPSK,an improved Gardner clock recovery algorithm with low resources and fast convergence is proposed,and its basic principles are introduced and compared with the traditional Gardner clock recovery algorithm.Secondly,by analyzing the noise distribution of the zero time slot of the received mPPM-QPSK signal,a two-mode equalization algorithm with a non-zero inner ring modulus value is proposed,and various parameters of the algorithm are optimized.Finally,the final experimental sensitivity is given,and the difference between the 2PPM-QPSK sensitivity and the theoretical limit is only 1.3dB.