Spectral Interrogation Based on Tunable Filters
One of the most successful techniques for spectral interrogation of FFP sensors is based on the use of a tunable narrow bandpass filter , as shown in Figures 5.8 and 5.9. The commonly used filter is a fiber-pigtailed and piezo-electrical transducer (PZT)-driven tunable FP filter (TFPF) . In Figure 5.8, the FFP sensor is illuminated by a broadband light source. The light reflected by the FFP sensor is
Figure 5.8 Schematic diagram of spectrum interrogation by arranging TFPF in front of detector.
Figure 5.9 Schematic diagram of spectrum interrogation by arranging TFPF behind light source.
filtered by the TFPF to allow a narrowband light at a certain wavelength to be received by the detector. The transmission wavelength of the TFPF is controlled by an electrical driving signal. When the pass wavelength of the TFPF scans a wavelength range, the spectrum of the FFP sensor is interrogated. The scheme in Figure 5.9 could perform the same function in Figure 5.8. The only difference is that the TFPF is directly connected to the light source. If the power density of the light source is high, multiple parallel spectra interrogations could be realized by splitting the light after the TFFP. Spectral interrogation scheme based on the TFPF is low cost and simple, but it is difficult to achieve high signal-to-noise ratio and dynamic range due to the relatively low optical power after filtering.