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Case Study CS-TC07 Rev. A

A Ray Tracing Approach to

Capillary-Based Backscattering

Interferometry

September, 2022

Page 1

ABSTRACT

Capillary-based backscattering interferometry has been used as a highly sensitive refractive index sensing

tool to measure molecular binding. Generally, ray tracing is used to simulate the backscattered light

from the specific capillary. Previous ray tracing models do not account for polarization effects, capillary

dimensions and material choices, and other parameters that could change the interference patterns

captured. Researchers from the United Kingdom have developed a comprehensive ray tracing model

including many parameters not previously reported. The significance of the inner diameter, outer diameter,

and the refractive index of the capillary material are shown to heavily influence the interference pattern.

Potential designs are explored to measure sensitive temperature changes, and therefore refractive index

changes, based on capillary characteristics.

BACKSCATTERING INTERFEROMETRY

Backscattering interferometry (BSI) is a technique of

capturing light from a source (typically a laser) that has

reflected, refracted, or scattered back from a solution under

study. Fringe patterns from the backscattered light, can be

utilized as a refractive index sensor with extremely high

accuracy. Capillary-based BSI is common for measuring

molecular binding through sensing refractive index

changes. Typically, capillaries are hollow glass tubes with a

circular geometry.

Figure 1 shows a typical capillary-based

BSI setup with the different first-order rays that are possible

in the capillary geometry.

Depending on the angle of incident light and the index of

refraction of the capillary material and solution, certain rays

of light may not ever be backscattered to the detector or

camera. This can be seen by the 'x' located in the top

capillary in

Figure 1.

Although capillary-based BSI is very successful in analyzing

different refractive index scenarios, it is generally lacking

in literature for clarification on experimental parameters:

effects of the polarization state of the incident light and

capillary dimensions on the final fringe pattern.1 Some

researchers have developed ray tracing models and

simulations, however, these models can seem contradictory

in specific situations.

Ray tracing is an optical simulation model for determining

the final location and propagation of light rays through a

given system. Amplitude, optical phase, and propagation

direction can be determined analytically with this technique.1

Figure 1 shows the results of propagation in ray tracing.

Figure 1. Capillary-based backscattering interferometry setup with different first-order rays that are possible in the

capillary geometry and the angle of incident vs. intensity of the backscattering signal with different polarizations.1