Fig. 1. (a) Schematic of the UMZI with a length difference of ΔL. MMI, multimode interference coupler. (b) Schematic of the MMI from different view angles. (c) Calculated effective index n_eff of optical modes versus the ridge waveguide width with an air upper cladding. (d) Calculated loss of the MMI with respect to its length. The MMI width is 2.80 µm.

Fig. 2. Experimentally measured EL of the MMI coupler. Inset: optical microscopy image of the MMI coupler.

Fig. 3. (a) Optical microscope image of the UMZI. Insets: SEM images of the grating coupler and MMI components, respectively. (b) False-color SEM image of the cross section of the ridge waveguide. (c) The simulation of the fundamental TE mode at 1550 nm.

Fig. 4. Normalized transmission spectrum of the fabricated UMZI.

Fig. 5. (a) Thermo-optic tuning performance of the UMZI. (b) The FSRs of the UMZI at temperatures from 20°C to 70°C. (c) Wavelength shift and FSR as a function of the temperature. (d) The measured output power varying with temperature and its fitting curve at 1534.34 nm.

Fig. 6. (a) Measured transmitted optical spectrum analyzer (OSA) spectra as a function of input power. (b) Maximum output signal and its wavelength versus input power.

Fig. 7. (a) Electro-optical tuning performance of the UMZI. (b) The FSRs of the UMZI at voltages from 20 V to 45 V. (c) Wavelength shift and FSR as a function of the voltage. (d) The measured output power varying with voltage and its fitting curve at 1549.3 nm.