Single-mode and multi-mode fibers to achieve modal splitting and greater
sensitivity in an optical fiber coupler for evanescent-wave biosensor applications.
A source of light having multiple modes is coupled to the input to one of the multi-mode
fibers, with the geometry of necked-down section being such that a limited number
of modes may be carried by the multi-mode fiber as the light emerges from the coupler.
At least one of the single-mode fibers is supported adjacent the multi-mode fiber
to receive and carry one of the limited modes. A biomolecule enveloped by the evanescent
field, exhibits a direct or indirect affinity to a binding partner, such that attachment
of the binding partner is at least partially responsible for the limited number
of modes carried by the multi-mode fiber as the light emerges from the coupler.
A system-level implementation of the invention further includes instrumentation
for receiving the light from the single-mode fiber carrying one of the limited
modes, and determining a characteristic of the binding partner in accordance with
the detected mode. The attachment of the binding partner may also causes a change
in other detectable optical attributes, such as the magnitude or polarization of
the light at one or both of the fiber outputs. The instrumentation may therefore
further include a polarization-sensitive optical element supported at one or both
of the outputs of the optical fibers, for example, in the form of a polarizing
beamsplitter. One or more of the fibers may be pulled substantially without twisting
so as to maintain polarization orientation, and the source of light may randomly,
circularly, or linearly polarized.
