Cascaded distributed multichannel fiber SPR sensor based on gold film thickness adjustment approach

We propose and demonstrate a cascaded distributed multichannel fiber surface plasmon resonance (SPR) refractive index (RI) sensor by using a gold film coating thickness adjustment approach. For a multimode fiber-based SPR sensor, different gold film thicknesses produce different resonance wavelengths, different dynamic ranges, and even different testing sensitivities. Therefore, we design and fabricate a cascaded distributed two-channel multimode fiber SPR sensor with the gold film thickness of 30 nm in #1 sub-channel and 60 nm in #2 sub-channel. The corresponding testing sensitivities are 1712 nm/RIU and 3038 nm/RIU with the RI range of 1.333-1.385. The experimental results indicate that, in the range of 20nm-60 nm, the smaller is the gold film thickness, the larger is the RI testing range, and the lower is the testing sensitivity, and the deeper is the resonance dip. In addition, it is worthy to say that, the testing sensitivity and the dynamic range of the cascaded distributed multichannel fiber SPR sensor keep unchanged after multiple sensing sub-channels being cascaded-connected. By using the gold film thickness adjustment approach, we overcome the limit of adjusting and separating the dynamic ranges of the multichannel fiber SPR sensors effectively.