Fig. 1. Schematic diagram of UDWDM-PON using Nyquist-ISB modulation^[7]

Fig. 2. UDWDM-PON using seven-core optical fiber transmission^[11]

Fig. 3. Traditional polarization diversity coherent receiver

Fig. 4. Structure diagram of simplified coherent receiver using two BPDs

Fig. 5. Structure diagram of simplified coherent receiver using a BPD

Fig. 6. Low cost polarization independent coherent receiver proposed by Ciaramella^[14]. （a） Type 1； （b） type 2

Fig. 7. Polarization diversity coherent receiver proposed by Tabares et al^[17]

Fig. 8. Model of dual electric absorption modulator(DEML) ^[19]

Fig. 9. UDWDM-PON bidirection transmission completed by DEML^[20]

Fig. 10. Two kinds of carrier recovery algorithms^[30].（a） Traditional carrier recovery algorithm;（b） simplified carrier recovery algorithm

Fig. 11. LUT-free carrier recovery algorithm^[32]