Optical modem speeds

Digital modems i've worked with have a sampling rate towards the DTE, for example 300bps to 19.2kbps (sync), and they also have a set data rate speed towards the other side of the connection (CDP), meaning 16kbps or 32kbps for example. Optical modems however only show speeds from 19.2kbps to 256kbps, and they don't mention any other speed. I'm guessing these settings are towards the DTE, but if so then is the speed towards the other side the same speed as the sampling rate at the DTE, or is it so fast because it's based on light that the speed isn't mentioned and so the limits are at the sampling rate side towards the DTE, so if you set it to 256kbps then the optical medium could carry alot more data per second but the modem itself works alot slower?