LIGO Phase Shifts

Classical Mechanics Level 3

If the laser light used in Advanced LIGO is $1064 \text{ nm}$ , an arm of the detector is $4 \text{ km}$ long, and the GW150914 system causes strains $h = 10^{-21}$ , what is the phase shift in laser light traversing a detector arm at maximum displacement versus no displacement, in radians?

See Gravitational Waves.