Fig. 1. (a) Schematic configuration of the proposed amplitude equalizer based on MRRs. (b) Top view of an MRR. (c) Cross section of the thermal-tuning region in the MRR.

Fig. 2. (a) Calculated spectral responses of the designed elliptical microring simulated by the 3D-FDTD method. (b) Simulated light propagation in the MRR working at the resonance wavelength near 1535 nm.

Fig. 3. Microscope images of the fabricated amplitude equalizer based on adiabatic elliptical microrings. (a) Eight-channel amplitude equalizer containing eight elliptical microring units. (b) Enlarged view of the focused grating coupler. (c) Enlarged view of the elliptical microring unit.

Fig. 4. (a) Transmission spectra at the through port of the eighth channel of the eight-channel amplitude equalizer as the applied voltage is increased from 0 to 0.8 V with a step of 0.02 V for the initialized state with uniform channel spacing of 200 GHz for clarity. (b) Measured resonance-wavelength shift as the power applied to the microheater increases. Here the chip was placed on the temperature controller set with different temperatures of 25°C, 35°C, 45°C, and 55°C, respectively. (c) Measured transmittances at the through port for the eighth operating wavelength as the heating power increases.

Fig. 5. Measured transmission spectra at the through port of the eight-channel amplitude equalizer with different operating states. (a) Device performance after fabrication when all the eight microheaters are OFF. (b) Nonattenuation state with all the eight resonance wavelengths initialized to be deviated from the operating wavelengths by half of the channel spacing. Full-attenuation states when tuning some resonance peaks to be aligned with some of the operating wavelengths: (c) λ1=λC1; (d) λ1=λC1, λ3=λC3; (e) λ1=λC1, λ3=λC3, λ5=λC5; (f) λ1=λC1, λ3=λC3, λ5=λC5, λ7=λC7.

Fig. 6. Measured transmission spectra at the through port of the eight-channel amplitude equalizer when the resonance peaks of adjacent wavelength-channels are tuned to be overlapped with each other. Attenuation states when tuning some resonance peaks to be aligned with some of the operating wavelengths. (a) λ7=λ8=λC8; (b) λ1=λ2=λC2, λ7=λ8=λC8; (c) λ1=λ2=λ3=λC3, λ7=λ8=λC8; (d) λ1=λ2=λ3=λ4=λC4, λ7=λ8=λC8; (e) λ1=λ2=λC2, λ3=λ4=λ5=λC5, and λ6=λ7=λ8=λC8.

Fig. 7. Measured results T0, T1, and T2 at the through port of the eight-channel amplitude equalizer, where T0 is the measured spectral response at the through port of the equalizer, T1 is the measured transmission at the through port when eight channels of lasers are launched to the equalizer operating at the initialized state, and T2 is the measured transmission at the through port when eight channels of lasers are launched to the equalizer operating to optimally attenuate the channels as desired. (a) Measured results at the nonattenuation state with all the eight resonance wavelengths initialized to be deviated from the operating wavelengths by half of the channel spacing. (b) Measured results when the eight channels are equalized to be with the same amplitude by appropriately tuning the resonance peaks. (c) Measured results when all the eight resonance wavelengths are tuned thermally to be aligned with the operating wavelengths. (d) Measured results when the resonance peaks are tuned thermally so that λ1=λ2=λC2, λ3=λ4=λ5=λC5, and λ6=λ7=λ8=λC8.

Table 1. Summary of the Reported Multichannel Amplitude Equalizers