Proceedings Article | 13 December 2024
KEYWORDS: Waveguides, Silicon, Light sources, Beam divergence, Optical transmission, Laser optics, Interfaces, Aspheric lenses, Silicon photonics, Laser bonding
Due to the mismatch between the light output of edge-emitting lasers and the optical field mode transmitted by silicon waveguides, as well as differences in material structure and coupling interfaces, primarily due to spot size and shape, prevent effective direct coupling between the laser and waveguide. Additionally, there is an air gap at the coupling interface. The traditional silicon photonic platforms (such as silica-silicon) have a lower contrast in the core/layer refractive index of the waveguide, which limits the actual coupling efficiency between the laser and the waveguide facet to be even lower than the theoretical value. The initial spot diameter of the laser is about 2~3μm, diverging conically with vertical and horizontal divergence angles of approximately 33° and 21° respectively, forming a vertically elliptical spot profile. The cross-section width and height of the silicon optical waveguide are 0.45*0.22μm, with its mode field being horizontally elliptical in shape. Therefore, in order to solve the coupling problem of these two part, this paper designs and studies a trident end-face coupler, achieving efficient direct proximity coupling between the end-face of silicon waveguide and the laser. Common solutions to these problems include: (1) Designing new coupler structures to improve the match between the laser's output light and the silicon waveguide's mode field. (2) Enhancing the alignment accuracy of the coupling platform to ensure the accuracy of the coupling process. (3) Optimizing the laser design to reduce the divergence angle while ensuring better matching with the silicon waveguide with a width and height of 0.45*0.22μm. (4) Reducing air gaps, such as chip end face flatness and roughness, as well as the parallelism between the two coupling ends, to ensure effective fitting between the two coupling ends. These measures can all enhance the coupling efficiency between the laser and the silicon waveguide. This paper designs and studies a trident structure end-face coupler, which directly couples the laser's output end face with the silicon waveguide. The output power of the laser before and after coupling with the silicon waveguide, minus the transmission loss of the waveguide, represents the coupling loss between the laser and the silicon waveguide. The transmission loss is calculated based on the length of the waveguide, which is 0.78dB (waveguide length of 5.20mm, with a transmission loss of 1.50dB/cm). Under experimental conditions of 23°C and 53% relative humidity, the laser was powered within its rated Iop range. The operating wavelength of the laser was 1342nm, with an operating temperature of 45°C. Five sets of valid data were obtained through testing and measurement, showing coupling losses of 3.12dB, 3.05dB, 3.19dB, 3.15dB, and 2.97dB respectively. After subtracting the transmission loss of 0.78dB, the actual losses were 2.34dB, 2.27dB, 2.41dB, 2.37dB, and 2.19dB respectively. Through comparative analysis, it was concluded that compared to the traditional aspheric lens with a coupling efficiency of 35%~45%, i.e., a coupling loss of about 3.5~4.6dB, the current trident facet coupler has a coupling performance superior to the traditional aspheric lens by approximately 1.1~2.4dB. It also avoids the complex process structure of lens coupling, which inevitably introduces additional insertion loss in the coupling optical path. Moreover, the simplified structure is beneficial for improving system stability. At the same time, this structure can also be extended to the research of silicon photonic integrated chips, reducing the complexity of packaging processes and optics structures, and is also conducive to controlling manufacturing costs for industrialization.
