The flat-top optical filter is an important component in practical optical applications, but it is very challenging for experimental realization in the integrated photonic platform. Here, we propose a robust and high-efficiency optical filter with wide-band flat top, based on the adiabatic conversion of light in coupled waveguides with a chirped modulation of the gap. The bandwidth of the filter can be $110\mathrm{nm}$ for telecom wavelengths, but owing to the adiabatic criteria, the length of the device is about $20\mathrm{mm}$. To narrow the bandwidth and reduce the footprint of the device simultaneously, an additional apodization modulation of the geometry is introduced, and a $6\text{−}\mathrm{nm}$ bandwidth can be realized with a device length of $4\mathrm{mm}$. The influences of structure parameters on the device performance are also analyzed, and the results show that the device is robust to the variation of the structure parameters. The proposed integrated bandpass filter is feasible for experiments, and thus it would be useful in both classical and quantum information processing on photonic chips.

• Received 16 April 2019

DOI:https://doi.org/10.1103/PhysRevA.100.023809