Non-Invasive Calibration Method for Optical Switching Delay Line Based on Iterative Optimization

Optical switching delay line (OSDL) is a structure consisting of cascaded optical switches and optical waveguides with different lengths. Different propagation paths can be selected to obtain the desired optical delay by controlling the optical switches in their “through” or “cross” states. Because of its high delay accuracy, large bandwidth and effective control, OSDL has become one of the key components of the microwave photonic beamforming system. However, calibrations of the “through” and “cross” states of all the optical switches are necessary due to fabrication errors before the OSDL can be used in real applications. At present, four typical calibration methods have been demonstrated: adding optical power monitoring taps, variable optical attenuators (VOAs), or phase shifters (PSs) into the OSDL chip, and measuring the transmission spectra of the chip under different driving voltages. However, adding additional optical power monitoring taps, VOAs, or PSs will increase the insertion loss and complexity of the OSDL chip, while the calibration method based on transmission spectra is still complicated and time-consuming. In this paper, a non-invasive calibration method for OSDL based on iterative optimization is proposed. Only the final output port of the OSDL chip is used as the optical power monitor, and the control voltages for all the optical switches in the “through” and “cross” states are iteratively optimized with the goal of achieving the largest output extinction ratio at the output port. As a proof of concept, we calibrated a 5-Bit silicon OSDL chip by using the proposed iterative optimization method, and all the delay states of the OSDL chip were measured, which were in good agreement with those of ODSL calibrated by using the optical power monitoring taps.