  A method for designing physically path length matched, three-dimensional
photonic circuits is described. We focus specifically on the case where all the
waveguides are uniquely routed from the input to output; a problem which has
not been addressed to date and allows for the waveguides to be used in
interferometric measurements. Circuit elements were fabricated via the
femtosecond laser direct-write technique. We demonstrate via interferometric
methods that the fabricated circuits were indeed optically path length matched
to within 45 um which is within the coherence length required for many
applications.
