  Optical rotation, a form of optical activity, is a phenomenon employed in
various metrological applications and industries including chemical, food, and
pharmaceutical. In naturally-occurring, as well as structured media, the
integrated effect is, however, typically small. Here, we demonstrate that, by
exploiting the inherent and stable spin-orbit interaction of orbital angular
momentum fiber modes, giant, scalable optical activity can be obtained, and
that we can use this effect to realize a new type of wavemeter by exploiting
its optical rotary dispersion. The device we construct provides for an
instantaneous wavelength-measurement technique with high resolving power $R
=3.4 \times 10^6$ (i.e. resolution $< 0.3 pm$ at $1-{\mu}m$ wavelengths) and
can also detect spectral bandwidths of known lineshapes with high sensitivity.
