A Modal-Noise Insensitive Fibre-Optic Gas Sensor Arman Mohebati And Terence A King

The performance of optical-fibre intensity sensors, particularly those utilising highly coherent diode laser sources and multi-mode fibres, is susceptible to modal-noise in the fibre. This can become a limiting factor in the sensitivity and stability of fibre-optic gas sensors based on absorption measurements where intensity variations of the order of 10-4 need to be measured. For long lengths of fibre (several km), where natural mode scrambling in the fibre has occurred, the modal noise has been shown generally not to be a limiting factor. A signal processing scheme is proposed which is shown to reduce the performance dependence of the sensor on modal noise where short lengths of fibres are used. Data from a fibre-optic methanometer utilising this technique is presented demonstrating the improvement in the sensitivity, reproducibility and long term stability of the sensor.