Towards Integrated Optical Feedback FM-to-AM Conversion in Silicon Nitride for Displacement Sensing Applications.

In this paper, we demonstrate the potential of silicon nitride integrated edge filters to perform frequency to amplitude (FM-to-AM) conversion of optical feedback interferometric (OFI) signals to achieve better signal-to-noise ratio than the typical AM OFI signals. Compared to already existing OFI FM-to-AM conversion techniques such as edge filters based on gas cells, and free-space or fiber Mach Zehnder interferometer (MZI), integrated photonic processing of OFI signals can offer greater compactness and design flexibility. In addition, compactness can also indirectly improve noise performances by facilitating temperature control of the chip and allowing a higher immunity to parasitic mechanical vibrations. The FM OFI detection is performed with an integrated MZI with a 2 cm imbalanced path-length implemented on a silicon nitride process. The achieved OFI FM-to-AM conversion factor is 0.83 (GHz)(-1) and the noise equivalent displacement is 16nm for a 1 kHz bandwidth.