Microcombs are revolutionizing optoelectronics by providing parallel, mutually coherent wavelength channels for time-frequency metrology and information processing. To implement this essential function in integrated photonic systems, it is desirable to drive microcombs directly with an on-chip laser in a simple and flexible way. However, two major difficulties have prevented this goal: (1) generating mode-locked comb states usually requires a significant amount of pump power and (2) the requirement to align laser and resonator frequency significantly complicates operation and limits the tunability of the comb lines. Here, we address these problems by using microresonators on an AlGaAs on-insulator platform to generate dark-pulse microcombs. This highly nonlinear platform dramatically relaxes fabrication requirements and leads to a record-low pump power of <1 mW for coherent comb generation. Dark-pulse microcombs facilitated by thermally controlled avoided mode crossings are accessed by direct distributed feedback laser pumping. Without any feedback or control circuitries, the comb shows good coherence and stability. With around 150 mW on-chip power, this approach also leads to an unprecedentedly wide tuning range of over one free spectral range (97.5 GHz). Our work provides a route to realize power-efficient, simple, and reconfigurable microcombs that can be seamlessly integrated with a wide range of photonic systems.
Optical gyroscope is an important high-precision inertial measurement instrument, which plays significant role in defense, geological exploration and navigation. For high accuracy optical gyroscope system, a high-performance polarizer is strongly needed to filter only one polarization from Amplified Spontaneous Emission (ASE) source. Here, the plasmon-assisted structure is introduced by setting a thin metal layer above the silicon nitride (Si3N4) waveguide. The copper (Cu) is chosen as the metal layer material as it is compatible with the complementary metal-oxide-semiconductor (CMOS) fabrication process. The surface plasmons effect can simply be excited by TM polarization due to the electric field of TM polarization perpendicular to the metal layer, so that it can bring extra propagation loss. The TE polarization, on the contrary, fail to excite the surface plasmons effect as the electric field is parallel to the metal layer. To reduce the reflection caused by mode mismatch, the distance between metal layer and waveguide is increased by inserting a SiO2 layer. The chemical mechanical planarization (CMP) process allows precise thickness control of the SiO2 layer. The spiral waveguide structure is utilized to fully suppress TM polarization while the TE polarization can be well confined in broadened Si3N4 core with negligible propagation loss. The numerical results show that the working wavelength range is as large as 60 nm from 820 nm to 880 nm with the polarization extinction ratio > 30 dB and the insert loss < 0.5 dB. As far as we known, this is the first time to achieve ultra-high extinction ratio, ultra-low insertion loss, ultra-low reflection at the same time, and also achieve a working wavelength range larger than 60 nm at the center of 850 nm. Moreover, the proposed structure doesn’t require high alignment accuracy and is compatible with silicon-on-insulator fabrication technology.
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