Respiration monitoring with a fibre optic sensor

The continuous monitoring of respiration is an important requirement in intensive care units and for sleep, sport and olfactory studies. Fibre optic (FO) sensors offer an attractive solution to respiration monitoring, as they are relatively non-invasive, intrinsically safe and immune to electromagnetic interference. A variety of FO respiration sensors have been reported in the literature. This research has shown that fibre Bragg gratings (FBGs) can be used to monitor human respiration. A prototype sensor device has been developed using fibres manufactured by a phase mask technique. There are two FBGs used in this prototype. The first grating is referenced to room temperature, while the active grating was mounted inside a breathing mask. A broadband light source and a low power 1550 nm laser diode were used for the initial evaluation. The light reflected from the reference grating provides a probe wavelength for the active grating via a fibre optic circulator. In this way, any shift in the wavelength of the active grating is translated into a change in the transmitted optical power, which is measured with a photodiode. To be able to predict the flow rate, volume and breathing rate, the principle of heat transferred from the ambient environment into the fibre core has been studied, and a thermodynamic model has been established. The recoating material that is used to protect the optical fibre sensor is also considered in the model. Therefore the model can be used in future work to calculate the effect of different materials that may be used for recoating. A breathing simulator was designed to provide stable breathing cycles so that the FBG sensor could be tested under ideal conditions for both quantitative and qualitative analysis. A comparison was made between the FBG sensor, a pressure sensor and a thermocouple in order to show their respective advantages and disadvantages. Human subject testing was then performed in order to access the sensor under realistic operational conditions. This test was also used to assess the potential for predicting peak flow during inspiration as required for practical application in olfactory studies. The results show that the optical sensor can well monitor human respiration. It is possible to make the monitoring system work in a linear response regime, provided that the pair of FBGs is mismatched by a small amount. Based on the thermodynamic model and the results from the breathing simulator and the human subject tests, it is also possible to predict the peak flow rate, volume and the breathing rate using the response of the optical sensor.