NANOG has some good presentations on optical. "Everything You Always Wanted to Know About Optical":

* https://www.youtube.com/watch?v=nKeZaNwPKPo

Also "Demystifying Submarine Cables":

* https://www.youtube.com/watch?v=Pk1e2YLf5Uc

"Tutorial DWDM & Packet Optical Fundamentals Troubleshooting the Transmission Layer:

* https://www.youtube.com/watch?v=dum8UXtbN3o

APNIC has "DWDM & Packet Optical Fundamentals: How to troubleshoot the Transmission Layer":

* https://www.youtube.com/watch?v=hOgTU80YTDI

For long distance (>100km) transmission, searching for the keywords "coherent optics" is useful.