J. Eur. Opt. Society-Rapid Publ. 22, 16( 2026) 157
Fig. 7. Schematic diagram of the racetrack resonator designed in this article.
suppressing the carrier, with the target coupling efficiency assumed to be j = 0.1 – 0.3 [ 36 ].
The relationship between the coupling coefficient j and the coupling spacing g can be expressed as:
jðÞ¼j g 0 exp ð�agÞ: ð28Þ
Equation( 28) demonstrates the law that the coupling coefficient decays exponentially with increasing spacing. By simulating and fitting the values of j and a, a quantitative relationship between j and g is established.
3.4.2 Cascaded coupling between MZM and MRR
Figure 8 shows the crosstalk simulation results of the cascaded coupling transition structure between DD-MZM and MRR, and the results directly verify the effectiveness of the“ tapered waveguide + straight waveguide” two-stage transition structure in suppressing functional crosstalk. When the total length of the transition segment is 155 lm, the functional crosstalk is suppressed to �38 dB, which is 13 dB lower than that of the traditional direct coupling structure( �25 dB) and far lower than the crosstalk level of existing similar integrated devices(> �5 dB). This significant crosstalk suppression effect not only improves the signal purity of the modulated light entering the MRR but also avoids the mutual interference between the DD-MZM modulation signal and the MRR resonant signal, which is an important reason why the device can realize the synergy of phase modulation and frequency doubling on a single chip.
Conical waveguide section( 55 lm): Gradually change the mode field of the DD-MZM output waveguide from 12 lm 4.7 lm to8lm 3.5 lm, achieving mode field matching with the MRR input waveguide( with mode field overlap efficiency > 95 %) and reducing reflection crosstalk caused by mode mismatch.
Direct waveguide isolation section( 100 lm): Designed with a high refractive index difference( Dn = 0.015), this structure reduces electromagnetic leakage from MZM modulation signal to MRR through mode confinement. Simulation results show that this structure can reduce crosstalk from �22dB in the absence of a transition section to �38dB.
Based on the three-dimensional FDTD method, simulations were conducted in the frequency range of 10 – 100 GHz, with an optical signal wavelength of 1550 nm and a
Fig
. 8. Simulation results of crosstalk under different transition segment lengths.
transition waveguide material of Si 3 N 4-TFLN heterostructure. The curve in Figure 8 clearly reflects the synergistic effect of“ reducing reflection crosstalk with tapered waveguide and enhancing isolation with straight waveguide”– crosstalk is reduced by 6 dB in the first 55 lm( only tapered section), and further reduced by 10dB in the subsequent 100 lm straight waveguide, with a total suppression of 16 dB, and saturates after 155 lm to avoid excessive increase in chip area.
3.4.3
Electric field coupling between T-shaped track electrode and modulation arm
To solve the problem of low electro-optical coupling efficiency caused by the mismatch between traditional straight electrodes and waveguide mode fields, this study innovatively proposes a 30 lm gradient transition structure for the T-shaped track electrode.
The width of the T-shaped signal electrode gradually changes linearly from 10 lm at the input end of the modulation arm, near the beam splitter, to 6 lm at the output end, near the combiner, with a strictly controlled gradient length of 30 lm. The corresponding ground electrode spacing is synchronously reduced from 8 lm to4lm, forming a“ expansion contraction” contour that matches the waveguide mode field( 12 lm 4.7 lm).
Figure 9 shows the 30 lm gradient transition structure of the T-shaped track electrode, and this innovative structural design is the key to breaking through the bottleneck of electro-optical field matching in traditional straight electrodes. By linearly adjusting the electrode width and spacing to match the 12 lm 4.7 lm mode field of the LN waveguide, the structure realizes the“ dynamic matching” between the microwave electric field and the optical field, which increases the electro-optic overlap factor C from 0.516( traditional straight electrodes) to 0.85( a relative increase of 65 %). This improvement directly reduces the half-wave voltage V p from 6.4 V to 5.6 V and the V p L to 2.8 Vcm, effectively reconciling the bandwidth-voltage trade-off that plagues traditional modulators.