Optical coherence refractometry

Abstract

We introduce a novel approach to refractometry using a low coherence interferometer at multiple angles of incidence. We show that for plane parallel samples it is possible to measure their phase refractive index rather than the group index that is usually measured by interferometric methods. This is a significant development because it enables bulk refractive index measurement of scattering and soft samples, not relying on surface measurements that can be prone to error. Our technique is also noncontact and compatible with in situ refractive index measurements. Here, we demonstrate this new technique on a pure silica test piece and a highly scattering resin slab, comparing the results with standard critical angle refractometry.

Optofluidic refractometer using resonant optical tunneling effect

“…As one of the most popular commercial refractometers, Abbé refractometer could reach 10 −4 RIU by using the total internal reflection phenomenon. The wave optics based methods exploit the wave nature of light and mainly measure the RI by diffraction, 6 forward/backward scattering 7,8 and interference ͑e.g., Fabry-Pérot, [9][10][11] Mach-Zehnder, 12,13 double slits, 14 Michelson, 15 low coherence, 16 and fiber Bragg gratings 17,18 ͒. These methods improve the sensitivity to 10 −4 -10 −6 RIU since the wavelength acts naturally as the absolute reference of length.…”

Simultaneous thickness and group index measurement with a single arm low-coherence interferometer

“…This feature allowed the development of important applications in optical engineering and biomedical fields, such as the optical coherence tomography (OCT) [1][2][3][4] and the simultaneous measurement of the geometrical thickness and group index of transparent samples [5][6][7][8][9][10]. Although the OCT technique started with the usual dual-arm Michelson interferometer, it evolved to a common-path interferometer, which is more compact and stable [3,4].…”

“…On the other hand, the measurement of thickness and group refractive index of transparent samples has a number of variants, most using the dual-arm low-coherence Michelson interferometer [5][6][7]. Techniques using a combination of Michelson and FabryPérot (FP) interferometers [8], modified OCT [9] and low-coherence interferometry with confocal optics [10] were also introduced, among others. The later has the advantage of measuring, besides the geometrical thickness, both the phase (n p ) and group (n g ) refractive indices that are related according to: n g = n p -λdn/dλ.…”

“…Each of above mentioned methods has its own advantage but most of them have the drawback of needing to move some optical component. For instance, the white-light spectral interferometry of [7] relies on the accurate displacement of the reference mirror of an uncompensated Michelson interferometer while in those techniques using a FP interferometer or the modified OCT, it is necessary the precise rotation of the sample [8,9]. Although the dual-arm white-light Michelson interferometer has no moving parts [5,6], the sample needs to stay close to a reference surface and this inhibits the use of any bulky temperature apparatus such as an oven or cryostat.…”

Design, construction, and performance of a real-time holographic refractometry prototype for liquid analysis

“…[15][16][17][18][19] Among these diffractive and interferometric methods, the holographic ones present interesting characteristics since they usually provide twodimensional interferograms and thus allow for whole-field measurements. 20,21 A novel refractometry concept was recently proposed and demonstrated by obtaining fringe-modulated holographic images of prism-shaped transparent media using a Bi 12 TiO 20 (BTO) crystal and a multimode laser.…”