Flexible Integrated Photonics for On-Chip Sensing Based on Cascaded FSR-Free Cavities

High spatial resolution mechanical sensing within compact 3D space is important in healthcare, industrial automation, aerospace engineering, and intelligent robotics. Flexible optical sensors have received increasing attention due to their high sensitivity and measurement accuracy, electromagnetic interference immunity, and ability to conform to three-dimensional shapes. Compared to fiber and free-space flexible mechanical sensors, waveguide-integrated sensors exhibit superior performance due to their compact footprint, excellent sensitivity, high spatial resolution, scalability, and integration capability with electronics and other functional components. Here, we present a new flexible photonic integrated sensor concept that utilizes cascaded one-dimensional (1D) photonic crystal (PhC) micro-cavities. Due to the free-spectral-range (FSR) merit of the cascaded cavities, which eliminates restrictions on wavelength spacing, wavelength-division multiplexing technology has been utilized to fully utilize a wide band of optics. Additionally, mechanical sensing signals, like strain and position information, can be embedded into wavelength-dependent resonant dips, facilitating high spatial resolution quasi-distributed-strain sensing and two-dimensional shape sensing. The integrated photonic sensors exhibit ease of scalability, high sensitivity, and compact footprint, holding novel opportunities for on-chip and conformal sensing with high sensitivity and spatial resolution.