  We report on the use of silicon nitride (SiN) photonic integrated circuits
(PICs) in high-value instrumentation, namely multi-color laser engines (MLEs),
a core element of cutting-edge biophotonic systems applied to confocal
microscopy, fluorescent microscopy - including super-resolution stimulated
emission depletion (STED) microscopy - flow cytometry, optogenetics, genetic
analysis and DNA sequencing, to name just a few. These have in common the
selective optical excitation of molecules - fluorophores, or, in the case of
optogenetics, light-gated ion channels - with laser radiation falling within
their absorption spectrum. Unambiguous identification of molecules or cellular
subsets often requires jointly analyzing fluorescent signals from several
fluorescent markers, so that MLEs are required to provide excitation
wavelengths for several commercially available biocompatible fluorophores. A
number of functionalities are required from MLEs in addition to sourcing the
required wavelengths: Variable attenuation and/or digital intensity modulation
in the Hz to kHz range are required for a number of applications such as
optical trapping, lifetime imaging, or fluorescence recovery after
photobleaching (FRAP). Moreover, switching of the laser between two fiber
outputs can be utilized for example to switch between scanning confocal
microscopy and widefield illumination modes, for instance, for conventional
fluorescence imaging.
