Laser diode bitrate datasheet I'm a student studying communications engineering.  I'm trying to understand why this particular laser diode is made for 155 Mbps and 622 Mbps.  I'm thinking that the bitrate is entirely dependent upon the bandwidth of the photodiode.  I can see from the datasheet that $f_c = 2 \text{ GHz}$ and so what I'm thinking is that you could have, a 1 GHz carrier with around 2 GHz of data.  So wouldn't this allow for a 2 Gbps bitrate?  I'm just wondering if this thinking is correct.

The datasheet can be found here:
Datasheet
I guess I'll convert my comments above into an answer: apart from the theoretical signal processing concepts, physical devices are made to be used in real, physical systems. 155 Mbps and 622 Mbps are common data rates used on fiber-optic SONET networks, specifically OC-3 and OC-12 lines. Therefore, the diode that you referenced is specifically designed and manufactured for use on those systems; it would not be common for such a device to be used for some other nonstandard application (not many people would want to go to the expense of designing their own custom fiber-optic networking standard).

With that said, in theory you could potentially use the devices for higher-bandwidth transmission, barring any specific device-related limitations (I haven't read the full datasheet). As long as the carrier frequency is high enough to accommodate the bandwidth of the signal, then it would be possible (e.g. a 1 GHz carrier can't have any larger than 1 GHz one-sided bandwidth).