Sensing with Fiber Bragg Gratings 443 on the measured parameter to be decoded. Other methods include interfero- metric techniques, such as homodyne detection, which can have extremely high sensitivity. The ultimate choice of the measurement technique depends on the requirements, as this has a large impact on complexity and cost. 10.1 SENSING WITH FIBER BRAGG GRATINGS 10.1.1 Principles of Sensing A sensor is a transducer that responds to the quantity being measured called the measurand. To remain unambiguous, the response of the sensor should be ideally a linear function of the measurand and strictly a single-valued parameter without hysteresis. Therefore, s ¼ kðoÞx; ð10:1:1Þ where s is the output of the sensor, x is the measurand, and k(o) is the sensor's sensitivity or transfer function, which is a function of frequency, o. Any deviation from linearity causes errors and must be quantified for the sensor or the transfer function. However, in most instances, the transfer function is rarely perfectly linear and may wander as a function of time or deviate from an ideal linear