Thickness and refractive-index measurement of birefringent material by laser feedback technique

We introduce a technique for simultaneous measurement of thickness and refractive index of birefringent materials. The principle is based on the laser feedback effect that laser polarization states flip between two orthogonal directions when a birefringent material is placed into the external cavity. The position of polarization flipping is determined by the phase-retardation magnitude of the birefringent material. Some feature points in the laser intensity curve can be used to calculate phase retardation. We derive an expression for phase retardation and rotation angle of a birefringent material to calculate thickness and refractive index. This technique is noncontact and compatible with in situ thickness and refractive-index measurement. The measurement precision of thickness is 59 nm and of refractive index is 0.0006.