Currently, most widely tunable lasers rely on an external diffraction grating to tune the laser wavelength. In this paper we present the realization of a chip-scale Vernier tunable racetrack resonator filter on the Ge-on-SOI waveguide platform, that allows for wide tuning (108 nm free spectral range) in the 5 μm wavelength range without any moving parts. The fabricated racetrack resonators have a Q-factor of 20000, resulting in a side-peak suppression of more than 20 dB, which is more than sufficient for wavelength selection in an external cavity lase.
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