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Fabricated cascaded cavity filter: (a) top view of complete device showing integrated waveguide, silicon/air DBR mirrors, and micro-heaters (P = 1.5/2.5/1.5 configuration), (b) cleaved DBR test structure showing etched trenches (P = 2.5), (c) cleaved rib waveguide facet showing silicon device layer and 2 μm thick buried SiO2 layer.
(a)–(c) Cavity 1 tuning, (d)–(f) cavity 2 tuning. Both cavities are matched in resonance for 3 V applied at cavity 2, as shown in case (e). For cases (c) and (f), the two cavities have been tuned far out of resonance with each other resulting in a sharply reduced transmittance. The heater resistance is R = 109 Ω (cavity 1) and R = 103 Ω (cavity 2). All measurements: TM-polarization.
(a) Comparison of measured and transfer matrix calculation for a single- (P = 1.5/1.5) and a cascaded-cavity filter (P = 1.5/2.5/1.5); (b) extinction for the cascaded cavity filter. In contrast to ring resonators, the Si/air DBRs have a large reflectivity bandwidth enabling the filters to achieve a large extinction over > 100 nm wavelength range. Both measurements: TM-polarization.
Cascaded filter with P = 2.5/4.5/2.5: (a) Measured spectrum with and without tuning; due to the narrow linewidth even a small cavity mismatch results in a low filter transmittance; inset: extinction in dB, (b) minimum linewidth in GHz for the resonance near 1613 nm; the FSR ≈ 1000 GHz. Note that the two curves in (a) are offset by 0.04 in the y-axis. All measurements: TE polarization.
Effect of higher-order waveguide modes on the filter transmission spectrum: (a) Excitation of the fundamental waveguide mode by aligning the input fiber at the waveguide center (y 0), (b) excitation of both the fundamental and higher-order waveguide mode by offsetting the input fiber vertically (y 1). The measured spectra clearly show that the spurious resonances are the result of a higher-order waveguide mode. Both measurements are for no cavity tuning, so the filter transmittance is not optimized in (a). All measurements: TM-polarization.
Effect of center mirror reflectivity and mode splitting due to inter-cavity coupling, measurement (top) and transfer matrix calculation (bottom): (a) P = 1.5/1.5/1.5, (b) P = 1.5/2.5/1.5, and (c) P = 1.5/3.5/1.5. All measurements: TE-polarization with optimal thermo-optic tuning.
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