Thin-film tunable bandpass filter for spectral shift detection in surface plasmon resonance sensors

This Letter proposes the use of a thin-film tunable bandpass filter (TTBF) as an accurate absorption spectral shift detector in surface plasmon resonance (SPR) sensors. The passband central wavelength of the TTBF can be tuned by changing the incident angle. A single TTBF is required to filter two passbands with central wavelengths on the falling and rising curves of the SPR absorption spectrum. The spectral shift can then be converted to the differential intensity change of these two TTBF output beams using the differential detectors to eliminate the common-mode noise. This method can perform a high-resolution sensing for the refractive index change in the SPR sensor surface without using high-performance spectrometers. A grating-coupled SPR sensor was used in the experiment system and achieved a detection resolution of 6.99 × 10 − 6 refractive index unit.