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

Abstract

ABSTRACT 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 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. 1. INTRODUCTION The need for fully optical, remote gas sensors in industry has stimulated a large amount of research and development in this area over the past several years17. Many of the techniques proposed and demonstrated utilise broadband radiation sources together with wavelength selective components such as diffraction gratings, interference filters and Fabry-Perot interferometers. Since the optical bandwidth of such devices are generally larger than the atmospherically pressure broadened linewidths of gases, which are of the order of a few GHz, the effective absorption coefficient is reduced. This reduces the ultimate sensitivities which can be achieved using such techniques compared with diode laser spectrometers where the radiation linewidth is much narrower than the absorption linewidth of the gas. These gas sensors also suffer from large drifts over relatively short periods of time, mainly due to the instability of the wavelength selection devices utilised. Obviously the elimination of unstable wavelength selectors such as interference filters and Fabry-Perot interferometers would greatly improve the stability of the sensor. The use of diode lasers in optical fibre gas sensors not only eliminates the need for wavelength selective devices but also offers many other advantages such as higher optical power, fast tunability and narrower linewidths. It has recently been demonstrated1 that fibre-optic gas sensors using diode laser sources can achieve much higher sensitivities. In this paper a brief description of the diode laser gas sensor is given and the effects of the fibre speckle noise demonstrated, it is shown howsuch effects can be eliminated to obtain sensitivities which are generally limited by background absorptions.