Angled-MMI-based wavelength splitters on silicon nitride waveguide platforms for fluorescence sensing

Compact wavelength splitters based on angled multimode interferometers (AMMIs) on silicon nitride platforms working in visible lights are reported for fluorescence sensing applications. A diplexer and triplexer with different footprints are designed and experimentally demonstrated. The diplexer and triplexer have the insertion loss of ∼1.7 and ∼2.7  dB/channel with cross talks of less than -22  dB and -17  dB on target wavelengths, respectively. These splitters are used to distinguish the signals collected from two fluorescent dyes that give different emission spectra when excited with an excitation source, due to their different Stokes shifts. In the case of the triplexer, a third port is to collect the excitation light, both to monitor the remaining excitation power and to reduce the interference at the signal ports. A termination structure at the end of the AMMIs and input and output tapering waveguides as a part of the wavelength splitters are designed and their performances are presented.