In LIGO they use 20W laser source, but then the light gets into Fabry-Perot cavity and this light's power inside it is 100kW .
How is that possible? It seems to me that in order to gather so much power we need highly reflective mirrors of the cavity. On the other hand, the more reflective mirror, the less light can enter the cavity.
 PRL 116, 061102 (2016)