one wavelength mentioned on a Single mode SFP optical transceiver.

I have a cisco optical SFP transceiver which supports Single mode fibre. it is written that it supports 1550nm of wavelength but that confuses me because it was my understanding that Single mode Fibres have Bi directional channels on the same fibres and each channel have a different wavelength like 1550nm Tx and 1490nm Rx. and those SFP transceivers come in pair that is

1. TX: 1550nm, RX: 1490nm
2. TX: 1490nm, RX:1550nm

What does the single wavelength mean on the transceiver ?