Spider Silk-Based Improved Multimode Interference Structure for Humidity Sensing

We propose and demonstrate an improved multimode interference (MMI) structure based on a spider dragline silk (SDS) for relative humidity (RH) sensing. We configure the MMI structure by connecting a section of single-mode fiber, a section of no-core fiber (NCF), and a section of single-mode fiber sequentially, and then obtain the interference spectrum. We spirally wrap the dragline silk on the NCF to improve the MMI and help the interference dips to be significant and identifiable. The refractive index and diameter of the SDS will change due to the change of the RH around the SDS, which will cause the improved interference dips to shift. The experimental results indicate that the average sensitivity of the proposed SDS-based sensor is 1.15 nm/%RH in the range of 33%RH-98%RH and the maximum sensitivity is 2.02 nm/%RH in the range of 83% RH-98%RH. The larger the RH, the larger the sensitivity. The proposed sensor also performs good repeatability with a fluctuation of less than 2.6% and a response time of 558ms. Spider dragline silk has proven itself the right candidate for RH sensing application and promises a way to fabricate environmental-friendly, bio-absorbable and biocompatible protein-based devices for biochemical sensing that uses natural materials as their constituent.