Optical Fibre Sensors for high Sensitivity Gas Detection

Near infra-red (IR) tuneable diode laser spectroscopy (TDLS) has become a powerful technique in many areas of gas sensing. In particular, interest in TDLS over optical fibre is growing for remote (sometimes multi-point) applications in difficult or hazardous environments / locations. The overall aim of this project is to enhance the sensitivity of TDLS over optical fibre by three to four orders of magnitude to enable its application to the detection of trace gases such as toxins, nerve gases or other species that may represent a security threat and vapour by products of the decomposition of explosives or other hazardous chemicals. A dual approach will be adopted. The first is a relatively low risk approach to gaining two to three orders of magnitude enhancements in the near IR system through careful optical design and enhanced optical and electronic signal processing to eliminate etalon fringes and improve the signal to noise ratios. In the second, more radical, approach we will investigate the use of guided wave, non-linear CW parametric difference frequency generation (DFG) devices at the remote gas cell to generate mid IR idler wavelengths from near IR pump and signal waves transmitted over optical fibre. This will enable, for the first time, the much stronger (by 2 / 3 orders of magnitude) mid IR fundamental absorption lines to be addressed using TDLS over optical fibre. A variation of the basic DFG technique / the singly resonant optical parametric oscillator - will enjoy all the benefits of signal modulation (TDLS techniques), optical fibre transmission, recovery and detection in the near IR, whilst exploiting the sensitivity gains associated with the much stronger fundamental absorption lines of the mid IR. Sensitivities well below the part per billion level are anticipated.