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.
32-channel hybrid III-V/silicon laser arrays operating at C-band with 100GHz wavelength spacing are designed and simulated. Each channel of the hybrid III-V/silicon laser arrays includes III-V/silicon waveguide gain region with lateral sampled Bragg grating (LSBG) on silicon waveguide for selecting a single longitudinal mode, tapered III-V/silicon waveguide coupling region and silicon waveguide light output region. Light generated by III-V active region evanescently couples to the silicon waveguide, and outputs from the silicon waveguide. The seed grating’s period of LSBG is fixed and varing the sampled grating period of LSBG for selecting wavelength. The transfer matrix method is used to simulate LSBG’s parameters of hybrid III-V/silicon laser arrays. The simulation results show that 32- wavelengths are selected successfully by LSBG when the sampling grating periods are between 4.6 μm and 6.9 μm while the seed grating is fixed at 251 nm. Besides, the tapered III-V/silicon waveguide coupling region is designed and simulated by eigenmode expansion method of commercial software to convert the light spot size and increase the coupling efficiency from III-V to silicon waveguide. By optimizing the parameters, the coupling efficiency is up to 90% while hybrid III-V/silicon tapered waveguide length is fixed at 400μm
A three-dimensional tapered silicon-based spot-size converter is studied to improve the butt-jointed efficiency between the laser diode and the single-mode silicon waveguide. This kind of the spot-size converter can be fabricated while the ridge waveguide is etched. There is no need to regrow SiN or SiON on silicon. The optimized coupling efficiency between the spot-size converter and laser diode is over 0.8 at the wavelength of 1550 nm. This spot-size converter is useful for silicon photonics.
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