A. Garcia-valenzuela et al., Dynamic reflectometry near the critical angle for high-resolution sensing of the index of refraction, SENS ACTU-B, 52(3), 1998, pp. 236-242
We investigate, theoretically and experimentally, a simple technique for se
nsing very small changes on the index of refraction. The technique consists
on measuring the derivative of the reflectivity near the critical angle by
modulating the angle of incidence and measuring the AC amplitude of the re
flectivity variations. Analytical results on the reflection of a Gaussian b
eam near the critical angle are used to derive simple formulas for the sens
itivity, dynamic range, and the optical-limit to the resolution. Experiment
al results validating the theory are presented. We found an experimental re
solution of about 1 x 10(-6), while the ultimate theoretical resolution cal
culated for our experimental setup was 5.7 x 10(-8). This technique may be
a useful alternative in designing a refractive index probe for some applica
tions. (C) 1998 Elsevier Science S.A. All rights reserved.