Is it possible using only Cisco equipment to link two sites approximately 15km apart using only one single mode fiber strand.
I don't think so. All of the single-strand fiber SFP's from Cisco that I know of are limited to 10km.
The older SFPs are limited to 10KM, however see the list below for longer distance.
- 10KM Single Mode Fiber - GLC-LH-SMD
- 40KM Single Mode Fiber - GLC-EX-SMD
- 70 - 100KM Single Mode Fiber - GLC-ZX-SMD
Part details can be found here