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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. [ABSTRACT FROM AUTHOR]

We analyze the response of surface plasmon (SP) sensors using a transmission line model. We illustrate this analysis with particular reference to a layered structure in which plasmon hybridization occurs. By applying the appropriate resonant condition to the system, we derive a circuit model which predicts the responsivity of different modes. This gives new physical insight into the sensing process. We discuss how the change in the sample region may be modeled as a change in the reactance in the equivalent circuit and from this, it follows that a single parameter can determine the change in resonance position with reactance. This approach is used to predict the response of a generic sensor to binding of an analyte and the bulk change of refractive index. This parameter arises naturally from the circuit representation in a way not readily accessible with the transfer matrix approach. The parameters can be expressed in terms of the ${Q}$ of a resonant circuit and confirms the intuition that a high ${Q}$ is associated with poor responsivity, however, we demonstrate that there is another circuit parameter, the resistance at resonance, that can mitigate this effect, providing a route for optimization of the sensor properties. [ABSTRACT FROM AUTHOR]