Studies On Key Technologies Of 1550nm Optical Transmission In Super-trunk And Broadband Optical Access Network

In the process of switching analog TV to digital TV in China cable society, the optical cable TV network expands rapidly. Two trends appear to further integrate and configure the network resource and to achieve the carrier-level of full service network. One is the network interconnection in metropolitan area. By sharing the same digital headend system, investment can be greatly reduced and effective management can be easily achieved. With some excellent properties like low loss and easy amplification, the 1550nm transmission technology has become the only choice of super-trunk transmission. Another trend is that more and more communities and buildings are connected by optical cable, leading to triple play based on FTTx. The maturity of Ethernet passive optical network (EPON) and the coincidence of network topology between EPON and Cable TV make EPON a promising candidate for the next-generation full service access networks. These two trends raise some new challenges to the 1550nm subcarrier multiplexing optical CATV transmission systems. This thesis focuses on the key technologies of 1550nm optical transmission both in super-trunk and broadband access network, and some theoretical, simulative as well as experimental results are achieved. The contents of the thesis are organized into four parts as follows.The first part discusses the interference between 1490nm data channel and 1550nm CATV channel in a RF Overlay EPON system. By using equivalent system transfer function model, the nonlinear crosstalk from 1490nm to 1550nm lightwave via stimulated Raman scattering (SRS) is presented, the Raman interference is expressed by the relative intensity noise (RIN), based on which the carrier to noise ratio (CNR) of the RF channel is modified. For the first time, the systematic experiment of RF Overlay EPON is carried out. The results show that random Ethernet packet can cause CNR degradation at low end of RF channels, while, Ethernet Idle pattern between packets can cause more interference at 62.5MHz and its harmonics frequencies. The influence of polarization on Raman scatting is also studied. Based on these research works two useful methods are proposed to overcome the interference, one is randomization of Idle stream, another is changing the polarization states of 1490nm or 1550nm lightwave. The linear crosstalk from 1550nm to 1490nm lightwave caused by in-sufficient wavelength isolation of the CWDM coupler is also presented, and the optical power penalty caused by interference is obtained. Studies show that 1490nm/1550nm WDM with wavelength isolation larger than 30dB can eliminate the linear interference.The second part focuses on the composite second order distortion (CSO) as well as its compensation in super-trunk 1550nm CATV transmission system with both self phase modulation (SPM) and fiber dispersion. CSO expression in a cascaded EDFA CATV system is obtained by using weak disturbance method to solve the nonlinear Schrodinger equation (NLSE). This expression is modified to show that a best compensation position exists when chirped fiber grating is used to compensate the self-phase modelation and fiber induced CSO degradation. Furthermore, the simulation result by VPI software agrees well with the calculation result. Some comparison experiments are carried out to show that CSO is degraded by extending the transmission distance and by increasing the input power to fiber. Another experiment shows better CSO performance is obtained when the DCM is placed in the calculated best position. Finally, a 200km experiment system shows that chirped fiber grating can greatly reduce the CSO distortion caused by SPM.The third part studies some related problems when distributed Raman amplifier (RFA) is used in CATV super trunk. After analyzing the gain and noise property in RFA, this chapter point out that the signal to ASE beat noise dominats the carrier to noise ratio (CNR) of RFA when ASE power is filtered by a narrow band optical filter. The CNR expression shows that bi-directional and backward RFA can improve the CNR performance of long distance system. The CSO expression in a RFA system is obtained by solving the NLSE. The calculation results of this expression show that backward RFA has no affect on CSO, and the bi-directional RFA can improve the CSO performance. This chapter also introduces the MER parameter in digital CATV systems, and the expression of the best length of DCF as a compensation module. At last, a 560km super-trunk system including RFA and DCF is analyzed, and the system design, parameter calculation as well as test result are presented.Since the Erbium Doped Fiber Amplifier (EDFA) is the key equipment in super-trunk system, the last chapter addresses the design and fabricate of EDFA. At first, the C+L band EDFA used in DWDM system is introduced. Optimized simulation of Pump power, EDF length and gain equalization filter (GEF) are carried out. Based on these results three kinds of EDFA including preamplifier, line amplifier and boost amplifier are fabricated using split band architecture. Test results show that 70nm amplification bandwidth is obtained. Furthermore, a novel all optical gain-flatten and gain-clamped EDFA is presented. By using a two-stage amplification architecture and adding fiber grating gain flatten filter to the amplifier output, it posesses flat gain spectrum, low NF, and high clamping gain in C band. The simulation and test results shows that this configuration has good gain-clamped characteristic, and can be used in all optical networks and WDM systems. Finally, this chapter introduces the design of a high power pump controller. It uses the pulse width modulation technology to realize the automatic temperature control (ATC) and automatic power control (APC) of EDFA pump. This technology can greatly reduce the power consumption and the size of pump controller compared with the former analog type, and it is already used in our EDFA products.