Researches On Optical Phase-locked Loop Technology And Modulation Format For Optical Communication Systems

With the popularization and the spreading development of Internet,the requirement for data information keeps increasing,bringing ever-growing pressure to the capacity of communication systems.As the core of data networks and the basis of communicative services,optical communication systems are being the scientific research focus and facing enormous challenges.In optical communication systems,the coherent detection could be an optimal solution to achieve high sensitivity and out-of-band noise suppression.Compared with the digital coherent detection which is based on digital signal processing(DSP),the optical phase locked loop(OPLL)technology allows optical carrier phase recovery in locomotion and breaks the cost and power consumption bottlenecks of the digital coherent detection.In addition,coherent optical transmission systems have a four-dimensional(4-D)signal space:two quadratures in two polarization components.The 4-D signal space can be used to create modulation formats that have a higher spectral efficiency or power efficiency.This dissertation is focused on the OPLL technology and modulation formats for optical communication systems and composed of five researches.The contributions and novelties of these researches include:1.The loop parameters designing method and auxiliary pulling technology for an OPLL system were discussed.Considering the impact of the loop parameters on the OPLL's performance,the closed-form solution for the noise performance was derived and those for the tracking and acquisition performance were analyzed,providing a novel theoretical reference of designing the loop parameters to achieve an OPLL system with high quality.The closed-form solution emphasizes the necessity of auxiliary pulling technique,which improves OPLL's acquisition performance observably while maintaining its noise and tracking performance.Then,an easy but powerful auxiliary pulling circuit module was designed and validated in a subcarrier-OPLL system experiment.The experiment successfully demonstrated a pull-in frequency range as wide as 2.4GHz and a frequency accuracy of 2MHz.2.A comprehensive study of optical voltage controlled oscillator(OVCO)was presented.The method to evaluate the OVCO performance was proposed by analyzing the coherent receiving process for the binary phase shift keying(BPSK)signal in a Costas subcarrier-OPLL system.Two possible OVCO configurations realized by Mach-Zehnder modulator(MZM)and phase modulator were discussed.With these two OVCO configurations,the properties and the method to achieve the optimal performance in practical implementations were explained theoretically and experimentally.This work provides useful guides for the OVCO design in a lab experiment or for the production of a commercially integrated OVCO component.3.A novel theoretical description for the behavior of a sampling OPLL in a phase-modulated radio-over-fiber(ROF)system was proposed.It enabled the first-ever stability analysis of a sampling OPLL with a finite sampling duty cycle.The closed-form solutions for the output noise floor and nonlinear distortions of the sampling OPLL were also derived and validated by numerical simulations.The work offers a new theoretical framework of designing the sampling OPLL receiver for a phase-modulated ROF system.Moreover,a sampling OPLL experiment was performed and the results express the validation of optical domain frequency down-conversion by using the sampling OPLL.4.A novel pulsed Sagnac loop phase-modulated ROF system was proposed.Unlike a conventional Sagnac loop,the optical phase offset of the loop can be adjusted to quadrature by applying a synchronized modulation signal to an in-loop phase shifting phase modulator.Thereby,the system exemplifies the fundamental RF response.The proof-of-concept experiment showed a flat frequency response over 1 to 10 GHz.With balanced detection,the experiment demonstrated an output noise floor of-164dBm/Hz,which is?4dB above the shot noise floor,and an SFDR of 104.8dB.Hz2/3.To decrease the impact of the phase modulator insertion loss,a bidirectional erbium doped fiber amplifier(EDFA)was designed.3.1dB SFDR improvement was observed when the bidirectional EDFA was inserted to the pulsed Sagnac loop experiment.5.Theoretical analysis of 6 polarization-quadrature phase shift keying(6P-QPSK)modulation format was accomplished.The principle and bit-to-symbol mapping for the 6P-QPSK modulation format were discussed.With the bit-to-symbol mapping,the way to generate a 6P-QPSK signal with constant amplitude was proposed.Two types of transmitter based on IQ modulator(IQM)and dual-driver MZM(DD-MZM)were studied and the requirements for the driving voltages were analyzed when using these transmitters generating 6P-QPSK signal with constant amplitude.This work can offer a theoretical reference for 6P-QPSK experiments.