An integrated low-cost and low-loss spot size converter (SSC) for thin film lithium niobate (TFLN) is important to support various high-performance hetero-integrated devices, such as electro-optic modulators, optical frequency combs, and optical phased array. The high performance and power consumption restrict the development of hetero-integrated devices consisting of silicon on insulator and lithium niobate, because of a large difference in mode field sizes between TFLN waveguides and standard single mode fibers or semiconductor lasers. Up to now, most low-loss SSCs whose feature sizes range from tens to hundreds of nanometers, are fabricated by high-cost electron beam lithography. Here we theoretically propose an integrated low-cost and low-loss SSC that reduces the end-face coupling losses in integrated system and improve the coupling efficiency, meanwhile especially its feature size of 1 μm can be fabricated by standard lithography. Based on evanescent wave coupling theory, a Si3N4 tapered waveguide surrounded by an amorphous silicon thin layer is used to enhance evanescent wave coupling strength, thereby achieving a high coupling efficiency of 88% between the TFLN and semiconductor lasers. The low-cost and low-loss SSC provides a potential method for hetero-integrated TFLN devices.
Multiple-wavelength laser arrays play important roles in various applications, such as optical communications, optical interconnections, as well as light detection and ranging. A 64-channel laser array with a wavelength grid of 0.8 nm at C+L band is demonstrated. The lasing modes in InP-based multiple quantum wells are built based on lateral α-Si Reconstruction Equivalent Chirp (REC) gratings. By introducing an equivalent λ/4 phase-shift in each lateral α-Si REC grating, the mode degeneracy is eliminated. The equivalent λ/4 phase-shift region has no influences on the zero-order peak but generates a sharp peak in the first order peak, selecting the lasing wavelength. When the seed grating period in lateral α-Si REC grating is fixed at 250 nm and the sampling periods are changed from 6.356 μm to 39.162 μm for 64 channels, the lateral α-Si REC gratings successfully select 64 wavelengths in C+L band at 0.8 nm wavelength interval. Utilizing the sampling periods with several hundred nanometers in the first order resonant peak, we can adjust a sequence of REC gratings more accurate than the seed grating periods with several nanometers in the zero-order resonant peak. The laser in 64-channel laser array with 0.8 nm channel spacing has a threshold of 42 mA, and output power of 74.5 mW. Our work proposes a novel method of multiple-wavelength laser arrays for hetero-integration, which could provide a potential way for the development of Wavelength Division Multiplexing (WDM) system, optical interconnection inside the data center, and photonic switching.
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