Objective  Photonic integrated circuits composed of a variety of integrated functional devices on one chip have become the mainstream of the photoelectric fields due to their low loss, large bandwidth, and anti-electromagnetic interference properties, which are widely applied in optical sensing, radar, photon computing and medical testing. Due to the inherent thermo-optic characteristics of optical waveguide materials, the refractive index of the core and cladding materials will change with the temperature fluctuation, leading to the temperature stability which is one of the main problems in the engineering application of the photonic integrated circuits. Therefore, it is necessary to suppress and compensate for the drift of optical device performance with temperature to improve the temperature stability of the photonic integrated circuits. In this respect, a temperature-insensitive hybrid structure based on silicon nitride and long-range surface plasmon polariton (LRSPP) waveguides was proposed to suppress and compensate for the performance drift caused by temperature variation.

  Methods  For the studies of the proposed temperature insensitive hybrid waveguide based on silicon nitride and LRSPP, the propagation properties, temperature stability and polarization characteristics were investigated. Firstly, considering the coupling efficiency between silicon nitride waveguide and LRSPP waveguide, the propagation model of silicon nitride waveguide and SPP waveguide was established. The mode coupling efficiencies and the optical propagation filed under different waveguide sizes were analyzed by using the finite difference time domain method. Then, the temperature stability was analyzed by the calculation of the phase change when the temperature fluctuated. Moreover, the polarization properties of the fabricated LRSPP waveguide were measured by the output spot and optical power under the transverse magnetic (TM) and the transverse electric (TE) mode.

  Results and Discussions   The mode coupling efficiencies between the silicon nitride waveguide and LRSPP waveguide were more than 99.9% in the optimal cases (Fig.3), resulting in the almost negligible coupling losses. What's more, the proposed hybrid waveguide still showed good propagation characteristics even with 100 nm offset in vertical direction between the silicon nitride waveguide and LRSPP waveguide (Fig.4). For the temperature characteristics of the hybrid waveguide, there was an optimal length ratio of the LRSPP and silicon nitride waveguide for the defined waveguide to realize temperature insensitivity. Specifically, when the thermo-optic coefficient of the LRSPP waveguide was −1.86×10⁻⁴/℃, the optimal length ratio was 0.077, leading to zero phase drift when the temperature changes (Fig.5). However, when the optimal length ratio is not met, the hybrid waveguide can still achieve temperature insensitivity through the active phase compensation performed by the core modulation of the LRSPP waveguide. When a voltage signal was applied directly to the core layer of the LRSPP waveguide, the temperature of the LRSPP waveguide rose and gradually stabilized to the required temperature to compensate for the temperature drift (Fig.6). A voltage of 2.5 V can achieve a temperature rise of 10 °C with the response time of 0.78 ms, which can quickly respond to the tuning needs of the waveguide, resulting in high tuning efficiency. In addition, since the LRSPP waveguide only supports TM polarization state, the proposed hybrid waveguide inherited the single-polarization characteristic. In the TM polarization state, the output of the LRSPP waveguide is good with the output optical power of −46 dBm. While in the TE polarization state there is almost no output and the output optical power is −110 dBm (the lowest detection limit of the detector) (Fig.7). Accordingly, the polarization extinction ratio was calculated as 64 dB, indicating that the LRSPP waveguide has good single-polarization characteristics.

  Conclusions  From the perspective of the basic waveguide, the proposed temperature-insensitive hybrid waveguide has the benefits of active tuning, low loss, single polarization and high universality, which can effectively address the performance drift of the silicon nitride waveguide caused by the temperature change and has broad application prospects in silicon-nitride-based photonic integrated circuits.