Fabry–Perot-cavity-based refractometry without influence of mirror penetration depth

Assessments of refractivity in a Fabry–Perot (FP) cavity by refractometry often encompass step which the penetration depth light into mirrors is estimated to correct for fraction length no gas can penetrate. However, as it currently carried out, this procedure not always coherently performed. Here, we discuss common pitfall that be reason and provide recipe on how perform FP-cavity-based without any influence mirror depth.