J. Eur. Opt. Society-Rapid Publ. 22, 16( 2026) 153
Practical frequency doubling efficiency threshold: g SHG 10 %. When the efficiency is lower than this threshold, the signal-to-noise ratio of the frequency doubling output signal is less than 20 dB, which cannot meet the requirements of microwave photonic signal processing.
Optimal working interval of modulation bandwidth: 30 – 70 GHz. In this interval, the device can achieve high bandwidth and high efficiency synergy. The frequency doubling efficiency remains above 12 %, which is higher than the practical threshold. The designed 3-dB bandwidth of 72 GHz slightly exceeds this threshold, ensuring high efficiency within the entire target communication band.
2.3 Theoretical to design transition: trade-off analysis and parameter mapping
The above phase matching conditions( Formula( 8)) and the correlation model between modulation bandwidth and frequency doubling efficiency provide strict theoretical constraints for the geometric parameter design of the device. In actual structural design, it is necessary to balance three core requirements: the strictness of phase matching, the miniaturization of device integration, and the low loss of high-frequency response. The design of key geometric parameters in Section 3 is all derived from the theoretical constraints in this section. For example, the MRR radius needs to satisfy FSR > 120 GHz to avoid sideband interference, and at the same time, the bending loss must be controlled to ensure a high Q factor; the length of the DD-MZM modulation arm needs to balance the half-wave voltage and microwave transmission loss to realize the compatibility of high bandwidth and low V p L. Based on the above theoretical constraints and design trade-offs, the following section will elaborate on the specific structural parameter optimization design of the Si 3 N 4-TFLN heterogeneous integrated device, and the selection of all geometric parameters is based on the theoretical derivation results of this section.
3 Device structure design and parameter optimization
3.1 Core functional link
Figure 4 shows the complete signal processing functional link of the Si 3 N 4-TFLN heterogeneous integrated device with both phase modulation and frequency doubling functions, which clearly defines the input and output physical quantities of each functional module. The continuous light at 1550 nm( optical communication band) is used as the input optical carrier, and after being equally split by the MMI beam splitter, it enters the multi-channel DD-MZM for electro-optical phase modulation under the drive of RF electrical signals. The modulated multi-channel optical signals are superimposed synchronously in the time / frequency domain by the MMI beam combiner, and the mode field matching is completed to avoid reflection crosstalk. Finally, the signal is input into the racetrack MRR for carrier suppression, sideband resonant enhancement, and
Fig. 3. Trade-off between modulation bandwidth and frequency doubling efficiency.
second-order nonlinear frequency doubling. The final output of the device is a dual-functional optical signal: a high-precision phase-modulated optical signal( phase accuracy ± 0.05 rad) and a frequency-doubled microwave opticalsignal( 2f m, frequency doubling efficiency g SHG = 15 %@ 100 mW fundamental light power), which realizes the onchip integration of two core functions on a single chip. Specifically, the phase modulation function of the integrated device follows the same electro-optic phase control principle as the traditional DD-MZM: it adopts a push-pull driving mode to independently regulate the refractive index of the two modulation arms via electrical signals, thereby introducing a controllable phase difference between the optical paths to achieve high-precision phase modulation of the optical carrier. Meanwhile, the frequency-doublingrelated filtering and resonant enhancement functions rely on the same resonance selection mechanism as the traditional MRR: utilizing the characteristic that only optical signals satisfying the resonant phase condition can be confined and enhanced in the cavity, the device filters out unmodulated optical carriers and amplifies the intensity of sideband components required for frequency doubling. On this basis, this study innovatively integrates the two functional units on a single chip through Si 3 N 4-TFLN heterogeneous integration and optimized cascaded coupling structure, solving the problems of large functional crosstalk, low integration density, and phase mismatch that exist in the combination of traditional discrete DD-MZM and MRR.
The design of this device relies on an optimized integration of materials and structural configurations. At the material level, the low transmission loss of silicon nitride( Si 3 N 4) is synergistically combined with the strong electro-optic effect of LN, thereby achieving superior optical performance and efficient electric field manipulation capability.
Figure 5 presents the multi-layer cross-sectional structure of the Si 3 N 4-TFLN heterogeneous integrated device, and the rational matching of material layers and geometric parameters is the material basis for breaking through the