While it is physically possible to connect them, the link will not work.
The fiber port on the 1900 is IEEE 802.3u Fast Ethernet 100Base-FX multimode.
The optical GBIC modules in the 3550 are IEEE 802.3z Gigabit Ethernet 1000Base-X (SX, LX).
They are not directly compatible with one another. And the speeds of the fiber interfaces cannot be upshifted or downshifted like 10/100 or 10/100/1000 copper ports: they are what they are, either 100 only or 1000 only.
The connectors used are the same (SC), and the fiber optic cable used is the same (multimode). Even the wavelength of light is the same (1300nm) on the 100Base-FX port and the 1000BASE-LX/LH GBIC module. But the signaling methods are different.
Your best bet if you want to use the 1900's fiber port is to get a Fast Ethernet media converter that converts from 100BASE-FX multimode to 100BASE-TX copper. Then, you can plug the copper into a port and manually set that port to 100-megabit.
If you use the media converter approach, be sure to also match the duplex settings. The 1900's fiber port defaults to half duplex, but can be manually set to full duplex. Use full duplex between two switches, or between the 1900 and a computer (if you plug one in there). Use half duplex if you are connecting to a Fast Ethernet shared media hub or repeater.
Duplex setting also determines how far you can push the 100-megabit traffic: half-duplex is 412m, full-duplex is 2,000m (2km).
Hope this helps.
