Research progress of thin film lithium niobate electro-optic modulator
Electro-optic modulator is the core device of optical communication system and microwave photonic system. It regulates the light propagating in free space or optical waveguide by changing the refractive index of material caused by applied electric field. The traditional lithium niobate electro-optical modulator uses bulk lithium niobate material as electro-optical material. The single crystal lithium niobate material is locally doped to form waveguide through titanium diffusion or proton exchange process. The refractive index difference between the core layer and the cladding layer is very small, and the waveguide has poor binding ability to the light field. The total length of the packaged electro-optic modulator is usually 5~10 cm.
Lithium Niobate on Insulator (LNOI) technology provides an effective way to solve the problem of large size of lithium niobate electro-optic modulator. The refractive index difference between the waveguide core layer and the cladding layer is up to 0.7, which greatly enhances the optical mode binding ability and electro-optical regulation effect of the waveguide, and has become a research hotspot in the field of electro-optical modulator.
Due to the progress of micro-machining technology, the development of electro-optic modulators based on LNOI platform has made rapid progress, showing a trend of more compact size and continuous improvement of performance. According to the waveguide structure used, the typical thin film lithium niobate electro-optic modulators are directly etched waveguide electro-optic modulators, loaded hybrid waveguide modulators and hybrid silicon integrated waveguide electro-optic modulators.
At present, the improvement of dry etching process greatly reduces the loss of thin film lithium niobate waveguide, ridge loading method solves the problem of high etching process difficulty, and has realized the lithium niobate electro-optic modulator with a voltage of less than 1 V half wave, and the combination with mature SOI technology complies with the trend of photon and electron hybrid integration. Thin film lithium niobate technology has advantages in realizing low loss, small size and large bandwidth integrated electro-optic modulator on chip. Theoretically, it is predicted that the 3mm thin film lithium niobate push-pull M⁃Z modulator’s 3dB electro-optical bandwidth can reach up to 400 GHz, and the bandwidth of the experimentally prepared thin film lithium niobate modulator has been reported to be just over 100 GHz, which is still far from the theoretical upper limit. The improvement brought by optimizing the basic structural parameters is limited. In the future, from the perspective of exploring new mechanisms and structures, such as designing the standard coplanar waveguide electrode as a segmented microwave electrode, the performance of the modulator may be further improved.
In addition, the realization of integrated modulator chip packaging and on-chip heterogeneous integration with lasers, detectors and other devices is both an opportunity and a challenge for the future development of thin film lithium niobate modulators. Thin film lithium niobate electro-optic modulator will play a more important role in microwave photon, optical communication and other fields.
Post time: Apr-07-2025