Fig. 1. Schematic illustration and working principle of the integrated LN OVA system. The SSB modulator consists of a phase modulator for signal upconversion and a tunable flat-top bandpass filter for SSB suppression. Various passive and active DUTs are fabricated on the same chip as the SSB modulator for in situ measurements. Insets (i)–(vi) schematically illustrate the spectra of optical and electrical signals at different locations of the chip.

Fig. 2. Principle and characteristics of LN SSB modulators. (a) Optical image (false-color) of the LN SSB modulator, including a phase modulator and an RAMZI tunable flat-top filter. Left inset: photograph of the chip clamped by tweezers. Middle and right insets: scanning electron micrographs (SEMs) of the slotted modulation electrodes and the TO phase shifter, respectively. (b) Principle of the RAMZI flat-top filter. Top and middle panels show the output optical amplitude transmission and phase response, respectively, of a microring resonator (light blue) and an unbalanced MZI (dark blue), as functions of normalized wavelength detuning (per FSR of MZI). By aligning the resonance notches to the quadrature points of the MZI and choosing a proper coupling state (κ=0.94), the additional phase shift from the microring resonator modifies the linear phase response of MZI, turning the full RAMZI transfer function to a “box-like” flat-top bandpass profile, as shown in the bottom panel. (c) Simulated phase response and output transmission of the RAMZI filter at different coupling states of the microring resonator, showing undesired ripples in the pass/stopbands in the cases of nonideal coupling coefficients. (d) Measured optical transmission spectrum of the RAMZI filter (gray) with a 50 GHz flat-top passband, a 210dB/nm roll-off slope, and a 20-dB extinction ratio, enabling SSB modulation (color-coded output spectra) with 20-dB SSR within 10 to 50 GHz (inset). (e) Measured bandpass profiles when the relative phases of MZI are set at 0, π/2, π, and 3π/2 (from top to bottom). ^#Heater power of ring resonator. *Heater power of MZI.

Fig. 3. In situ OVA for on-chip passive devices. (a) Experimental setup for an in situ OVA system based on integrated SSB modulation. CW, continuous-wave; FPC, fiber polarization coupler; DUT, device under test; EDFA, erbium-doped fiber amplifier; PD, photodetector. (b)–(d) Measured amplitude (blue) and phase (red) responses using OVA, together with laser scanning data (yellow dashed) of a single ring resonator (b), a double ring system (c), and a CROW (d). Black dashed lines correspond to fitted phase responses. Right side of panel (b) shows fine measurement results around one resonance dip, with 40,001 data points and a resolution of 50 kHz. (e) Working principle of a seamless stitching process for broad measurement bandwidth by tuning the position of the bandpass filter and optical carrier. (f) Ultrabroadband OVA from 1500 to 1630 nm for a single microring resonator. Inset at far right shows the zoom-in view of the measured responses between 1626 and 1629 nm, indicating an overcoupling state.

Fig. 4. In situ OVA for on-chip active devices. (a) Schematic illustration of in situ probing of an actively modulated microring resonator. Inset shows the microscope image of the LN resonant EO comb generator. (b), (i) Dispersion diagram of the synthetic frequency crystal, where different pump laser detunings excite Bloch modes with different energies and momenta. a is the lattice constant. (ii) Density of states of the frequency crystal as a function of laser detuning. (iii)–(iv) Coherent addition of the phase responses of individual Bloch modes (iii) leads to the measured collective phase dynamics of the Bloch states (iv). (c), (d) Measured (solid) and fitted (dashed) amplitude (blue) and phase (red) responses of the synthetic frequency crystal at increasing RF modulation power levels, without (c) and with (d) frequency detuning. Amp., amplitude.

Fig. 5. Simulated MAE value as a function of the input RF power. Insets show the simulated amplitude results (blue) and the ideal ones (red) of a single microring resonator at −15dBm (left), 0 dBm (middle), and 19 dBm (right).