Hi Fayiqa,
OFDM is usually used for more than 10Gbps upstream or downstream bit rate for each user in PONs in papers. I believe you can go with TWDM, as it is the standard scheme for NGPON2 as the following link also states:
https://techzine.alcatel-lucent.com/twdm-pon-taking-fiber-new-wavelengths
Regards