  We have studied spin orbit torque in heavy metal (HM)/ferromagnetic metal
(FM) bilayers using magneto-optical Kerr effect. A double modulation technique
is developed to separate signals from spin orbit torque and Joule heating. At a
current density of ~1x10$^{10}$ A/m$^2$, we observe optical signals that scale
linearly and quadratically with the current density, both in similar magnitude.
The spin orbit torque estimated using this technique is consistent with that
evaluated using spin transport measurements. We find that changes in the
refractive index of the film with temperature is the main source of the heating
induced signal.
