## 2 Microwave Photonic Filters
### Single Bandpass MWP Filter
For Sensor 2, the sensitivities at frequencies of 15.38 GHz and 15.1 GHz are 9\(\cdot\)10\({}^{-4}\)/\({}^{\circ}\)C and \(-\)3\(\cdot\)10\({}^{-4}\)/\({}^{\circ}\)C, respectively. The different sensitivities associated with the two sensors are due to the differences in their corresponding MWP filter responses. In particular, the multiple wavelength optical tap spectra associated with each sensor is different due to differences in the in-fiber FP cavities, and it is known that the shape of the multiple wavelength source will impact the shape of the MWP filter response [15]. Additionally, we monitored the RF power at frequencies located at different points along the MWP filter responses. Figure 5: Measured MWP filter responses from Sensor 1 only, Sensor 2 only, and from both sensors simultaneously (after Reference [12]). Figure 6: (**a**) Variation in normalized RF power as a function of temperature for Sensor 1 at frequencies of 7.24 GHz (\(\blacksquare\)) and 7.48 GHz (\(\blacklozenge\)) and for Sensor 2 at frequencies of 15.38 GHz (\(\blacktriangle\)) and 15.1 GHz (\(\bullet\)). (**b**) Ratiometric measurement for Sensor 1 (\(\blacktriangle\), magnitude of the ratio of power at 7.48 GHz to 7.24 GHz) and for Sensor 2 (\(\bullet\)), magnitude of the ratio of power at 15.38 GHz to 15.1 GHz) (after Reference [12]). When there is no overlap between the MWP filter responses corresponding to each sensor, a ratiometric measurement can be used. We can consider dividing the changes in RF power observed at a frequency located along the rising edge of the MWP filter response by those for a frequency located along the falling edge. Using this ratiometric approach for the frequencies considered previously, the sensitivities for Sensor 1 and Sensor 2 are increased to 7\(\cdot\)10\({}^{-4}\)/\({}^{\circ}\)C and 11\(\cdot\)10\({}^{-4}\)/\({}^{\circ}\)C, respectively, as highlighted in Figure 6b. We now consider operating the two temperature sensors simultaneously. Experiments show that, as we apply temperature changes to one sensor, the MWP filter response corresponding to the other sensor does not shift. For example, as depicted in Figure 7, when the temperature applied to Sensor 2 is varied from 40 \({}^{\circ}\)C to 80 \({}^{\circ}\)C and Sensor 1 is maintained at a constant (room) temperature, the MWP filter response corresponding to the latter does not shift (in particular, the RF power at frequencies 7.48 GHz and 7.24 GHz does not vary). Figure 8 depicts the shift in the two MWP filter responses when different temperature changes are applied simultaneously to the two sensors. For Sensor 1, when the temperature is increased from 25 \({}^{\circ}\)C to 80 \({}^{\circ}\)C, the normalized RF power changes from 0.84 \({}^{\circ}\)C to 0.8 \({}^{\circ}\)C at a frequency of 7.24 GHz. For Sensor 2, when the temperature is increased from 25 \({}^{\circ}\)C to 60 \({}^{\circ}\)C, the normalized RF power changes from 0.66 \({}^{\circ}\)C to 0.67 \({}^{\circ}\)C at a frequency of 15.38 GHz. These changes generally agree with the sensitivities shown in Figure 6. However, they are not the same due to crosstalk in the MWP filter responses. In particular, as observed in Figure 5, the MWP filter response from Sensor 1 extends slightly into the frequency range of that for Sensor 2, while the reverse is not true. As such, for simultaneous multiple sensor operation, there is a small impact in the sensitivity for Sensor 2 and negligible impact in the sensitivity for Sensor 1. Figure 8: Illustrating simultaneous operation of the two sensors. MWP filter responses for Sensor 1 when the temperature is increased from 25 \({}^{\circ}\)C to 80 \({}^{\circ}\)C (**a**) and Sensor 2 when the temperature is increased from 25 \({}^{\circ}\)C to 60 \({}^{\circ}\)C (**b**) (after [12]). Figure 7: MWP filter response for Sensor 1 which is held at constant (room) temperature, while the temperature of Sensor 2 is varied 40 \({}^{\circ}\)C to 80 \({}^{\circ}\)C (after Reference [12]).