This application describes designs, implementations, and techniques for controlling
propagation mode or modes of light in a common optical path, which may include
one or more waveguides, to sense a sample. As an example, input light in two different
optical propagation modes (e.g., the first and second modes) is directed through
a common input optical path to the optical probe head which sends a portion of
input light in the second mode to the sample. The probe head directs both the light
in the first mode and the returned light from the sample in the second mode through
a common optical path to a detection module.
