Fig. 1. (a) 3D schematic configuration of the proposed LNOI photonic chip for DWDM transmitters. $N$ FP cavities with the same MWGs and mode MUXs but in different cavity lengths are cascaded through waveguide bends at the reflected port. (b) Top view of the FP cavity modulator unit, consists of a pair of MWGs with a short straight section in between and a mode (de)multiplexer (inset: cross section of the LNOI ridge waveguide).

Fig. 2. (a) Simulated intensity (blue) and phase (red) response of the reflected ${\mathrm{TE}}_1$ mode (solid line) and transmitted ${\mathrm{TE}}_0$ mode (dotted line) of the MWG with the parameters of $W=2.6\mu \mathrm{m}$, $\mathrm{\Lambda }=430\mathrm{nm}$, ${\delta }_0=1.6\mu \mathrm{m}$, and $N=110$; (b) simulated light propagation in the designed MWG when operating at 1550 nm (around the Bragg wavelength) and 1580 nm (away from the Bragg wavelength), respectively.

Fig. 3. (a) Calculated effective indices $n_{\mathrm{eff}}$ of the ${\mathrm{TE}}_0$ and ${\mathrm{TE}}_1$ modes as the waveguide width $W$ varies [inset: mode profiles $|\mathbf{E}(x,y)|]$. (b) Calculated resonance-wavelength variation $|\delta {\lambda }_{\mathrm{res}}|$ of the FP cavity when assuming that the waveguide width is given as $W=W_{0 }+\delta w$ and the slab thickness is given as $H=H_{0 }+\delta h$. (c) Calculated metal absorption loss of the ${\mathrm{TE}}_0$ and ${\mathrm{TE}}_1$ modes for $W=1.7$, 2.0, 2.3, 2.6, and $2.9\mu \mathrm{m}$. (d) Calculated average modulation efficiency $\mathrm{\Delta }{n}_{\mathrm{eff}}/U$ of the ${\mathrm{TE}}_0$ and ${\mathrm{TE}}_1$ modes for unit applied voltage when the average absorption loss of the ${\mathrm{TE}}_0$ and ${\mathrm{TE}}_1$ modes is $0.5\mathrm{dB}/\mathrm{cm}$ (inset: the static electric field distribution).

Fig. 4. (a) Calculated resonance wavelength shift with the cavity length variation $\mathrm{\Delta }{L}_{\mathrm{wg}}$. (b) Calculated reflective spectrum of the four-channel FP cavity modulators with a channel spacing of 1.6 nm. The inset shows the spectral responses when bias voltages are applied to the second FP cavity (channel #2).

Fig. 5. (a) Microscope image of the fabricated four-channel chip for DWDM transmitters; SEM images of (b) the FP cavities, (c) the MWG, and (d) the mode (de)multiplexer.

Fig. 6. (a) Original spectral response of the fabricated four-channel chip for DWDM transmitters. (b) Calibrated spectral responses with the channel wavelengths aligned to the DWDM grids by controlling the temperature and introducing static electric field individually. (c) Measured resonance wavelength shifts as the applied voltage varies from $-20$ to 20 V. (d) Experiment setup for measurement of small-signal EO responses and interchannel EO cross talk. Measured EO responses $S_{21}$ for all the four channels (e) and interchannel RF cross talk of modulators (f).

Fig. 7. Measured eye diagrams for the four channels where the modulated electrical signals were generated individually and applied to each modulator one by one.

Fig. 8. Numerically calculated eye diagrams for channel #1 and channel #2 (a) without cross talk and (b) with cross talk from other channels, corresponding to the measured results in Fig. 6(e).

Fig. 9. Measured standard deviation $\sigma$ of channel spacings of three groups of FP cavity modulators. In groups A, B, and C, modulators are placed with a separation of 100, 40, and $20\mu \mathrm{m}$ between the adjacent two, respectively.