Chirality

Optical rotation measured via frequency shifts

Schematic of a ring-resonator

Rotation of the plane of polarization is generally determined with polarization optics and intensity measurements. Here we show that the same can be done by observing changes in the resonance frequency of a suitable resonator.

A high-finesse optical interferometer may be built with a closed fiber loop that is in contact with a linear waveguide via a variable (e.g. evanescent-field) coupler as seen in Figure. Resonances are observed as minima in a transmission spectrum whenever an integral multiple of the wavelength equals the circumference of the fiber-loop (ring). The light in the ring acquires a phase that is proportional to the path-length and to the refractive index of the medium it traverses. It follows that the introduction of a sample into the ring will change the round-trip phase and hence will give rise to different resonance frequencies. Refractive indices may now be measured by tuning the frequency of a laser with a sufficiently narrow linewidth. The inherent birefringence of a fiber-loop will in addition give rise to resonant modes with differing polarization states.

We have built a ring-resonator that supports right (+) and left ( -) circularly polarized modes and used these to directly measure the circular birefringence of a chiral liquid. The optically active liquid gives rise to relative frequency shifts and so any common mode noise can be rejected. Promising for the miniaturization of optical activity measurements may be that the frequency shifts of a ring-resonator are scale independent.

This project is the result of a collaboration with Frank Vollmer and has been published: F. Vollmer and P. Fischer, Opt. Lett., 31, (2006), 453.