Asymmetric-ration optical power couplers based on nano-pixel structure

An optical power coupler is one of the most well-used components in integrated photonics. Although couplers with an output power ratio 1:1 have been widely studied in the past, constructing asymmetric-ration optical power couplers is still an issue that is difficult to be addressed by using traditional Y-branch waveguides. Artificial intelligence (AI) assisted design is an effective technique for realizing complex optical structures. In this work, we have designed asymmetric-ration optical power couplers by using AI assisted design. Two couplers with the targeted splitting power ratio as 1:9 and 1:99 have been designed, respectively. In the AI assisted design, the coupler area was divided into discrete nano-pixels in the shape of circular holes with the same dimension. The AI controlled each pixel to be occupied by waveguide or air and trialed the occupation of each pixel one by one. In a 3.4 × 3.2 μm2 area, it took 1452 trials to obtain one optimized coupler. As a result, a splitting power ratio of 1:9.007 and 1:99.004 for the two couplers has been confirmed by using the finite-difference time-domain method. In addition, the waveguide configuration was further modified as the excess loss of the AI-designed coupler was a bit high, more than 3.50 dB. The way to reduce the loss is as follows: 1) positioning optimization of the output waveguide to avoid light scattering at the boundary between the coupler and the output waveguide, and 2) widening the output waveguide width to avoid insufficient light coupling. As a result, a scattering loss reduction of 1.7 dB by position optimization, and a coupling loss reduction of 1.6 dB by width widening were confirmed. The achieved design also exhibited a wide operation wavelength ranging from 1500.1600 nm in addition to sufficient fabrication tolerance of ±10 nm (± 11%).