| Optical transmission has many advantages compared to conventional metal wire(copper wire),such as less expensive,smaller diameter,higher carrying capacity,less signal degradation,light signals,low power consumption,digital signals transmission,non-flammable,lightweight,flexible.In the global long distance transmission system,optical fiber has gradually taken the place of copper wire.In China,the government advocates fiber-instead-cable strategy,many communication companies has upgraded their system to optic fiber transmission system.So far,the optic fiber communication has gone through 4 generations.In the 1st generation,the wavelength is 0.8um with multimode fiber,the bit rate is 20-100Mbit/s.The 2nd generation works at 1.3um,transmits in single mode fiber,the bit rate can reaches to 1.7Gbit/s.In the following 3rd generation,the system works at 1.55 um wavelength,since the lowest attenuation point is near the 1.55 um,the system bit rate can ramp up to 10Gbit/s.The above 3 generation systems all adopted the IM/DD method,that is intensity modulation/direct detection,its capacity has met bottle neck in the end of 1980 s.And in this time,many new techniques has come out such as EDFA(Erbium Doped Fiber mplifier),WDM(Wavelength Division Multiplex),Coherent light systems,these systems have extended the transmission distance and capacity.So the 4rd generation is characterized by the amplifier and wavelength division multiplex technology.So far,the different applications have adopted the different techniques.In short reach,the popular way is take VCSEL as transmitter,and PIN as receiver with MMF as medium.In medium reach,the industries began to take DFB and SMF as transmitter and medium.And in longer distances,APD receivers have been adopted.As the data rate ramps up from 28 Gbps to higher,the industrial field has entered into a cross street.Higher NRZ modulation rate requires more expensive materials,more companies would like to use PAM4 modulation format.Members of the IEEE P802.3bs Task Force agreed during the recent IEEE 802.1/IEEE 802.3 Joint Interim Meeting to adopt a single-mode fiber PMD based on four wavelengths of 100 Gbps each,according to the meeting's host,The Ethernet Alliance.Task Force members voted to adopt a 4x100-Gbps PMD based on PAM-4 modulation for single-mode fiber applications at reaches of 500 m.The PMD would use four fibers,similarly to the way the PSM4 MSA addresses 100 Gigabit Ethernet applications via four 25-Gbps transmissions over individual fibers.The group also voted to adopt an 8x50-Gbps approach for reaches of at least 10 km.This PMD also calls for PAM-4 modulation,but over a single fiber.The paper has firstly introduces the latest research of PAM4 in laboratary and industry,the develop progress of PAM4 protocol on 400 G,and the test method raised by oscilloscope suppliers.Then make some study of existed NRZ transmission system including CDR/driver/PIN and the most 3 popular ways taken by telecommunication companies and data center.The 3rd chapter has discussed the theories of PAM4 modulation and the latest development of PAM4 driver and also the potential 3 system method of PAM4 in the future.In 4th chapter,the author mainly introduces the software of Optisystem and VPItransmission Maker which will be used in this paper.The most important is 5th chapter,it gives some simulation result of SMF/MMF/EML transmission system,and the result shows that the VCSEL+MMF and EML method are not suitable for the PAM4 transmission,and DFB/SMF/PIN has the best performance.But should be noted that its sensitivity is 10 d B higher than NRZ at the same bit rate.Its SER also is higher than the same NRZ modulation format. |
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