Sensing hazardous gases over kilometer distances using optical fiber systems

Detecting the presence and measuring the concentration of gases is increasingly important as society becomes more and more concerned with safety and security, not to mention climate change. There are many diverse requirements in gas monitoring and detection. These range from environmental control, for example, in offices and laboratories, through hazard detection, most notably for explosive and toxic gases, to the monitoring extensively used by meteorologists and climate scientists. Many approaches exist for the sensing and measurement process, each specifically applicable to a particular measurement requirement. These range from the coal miner's canary (still occasionally used in some countries), to the policeman's breathalyzer, to the electrochemical systems exemplified in domestic carbon monoxide detectors, to a range of optical systems based on absorption spectroscopy.

Absorption spectroscopy depends on the fact that the atomic transitions for particular gas molecules are very precisely defined and correspond to particular, well-characterized wavelengths, usually within the region ranging from the IR to UV. Shining light at one of these particular wavelengths through the gas results in optical attenuation, which, if measured precisely, gives an exact determination of gas concentration. Of course, the absorbed light must go somewhere and usually reappears as heat, most notoriously manifesting as global warming. The measurement systems we describe here exploit exactly these absorption phenomena using precisely engineered optical systems to facilitate sensitive, accurate, and repeatable measurements of gas concentration.

Our work has focused on methane gas, detecting both hazardous concentration levels and volumetric percentages in fuel mixtures. We have used fiber-optic networks, which limits our wavelength range from around 600nm to 2μm. Methane has a particularly useful absorption line at 1.665μm, which combines adequate absorption characteristics with efficient fiber transmission, thereby enabling detection at ranges of up to 10km from the interrogating optical (laser) radiation source. Fiber transmission facilitates other benefits (see Figure 1), most notably accessing up to ~300 sensing points from a single laser using the network concepts illustrated in the figure. Along with fiber distribution comes intrinsic safety. The laser power is well below ignition thresholds even before dividing through the tree network. Furthermore, the detection cells, typically transmitting light through an air path a few centimeters in length, can be operated at extremely high sensitivity (see Figure 2). The detection threshold at each cell is about 50 parts per million. And unlike electrochemical detectors, the measurement capability extends to a methane concentration of 100%, which represents unprecedented dynamic range. Furthermore, other gases may be addressed very simply over the same network by replacing the single laser at the input end with a range of lasers addressed through a wavelength multiplexer or an optical switch (see Figure 3). This enables a single, though extensive, system to address several gases by simple modifications within the control room area. In our work, we have demonstrated useful sensitivity to carbon dioxide, water vapor, and hydrogen peroxide vapor, and we have identified suitable spectral lines for ethylene, ammonia, and a range of other gases.

Figure 1. A schematic diagram illustrating the scope of a passive fiber network for remote near-IR gas sensing.

Figure 2. Calibration characterization for the basic sensor configuration. Concentrations are normalized ppm.m (parts per million.meter), and the measurements refer to concentrations in nitrogen gas (N₂). For example, 100ppm in a 1m-long absorption cell would give a reading identical to 1000ppm in a 10cm absorption line.

Figure 3. Illustrating the principle of multiple gas-species detection in a fiber network using a set of appropriate multiplexed laser diodes. λ: Wavelength.

The interrogation scheme used throughout incorporates tunable-diode laser spectroscopy. The basic concept is simple. The center wavelength of a diode laser can be swept over a wavelength range of around 1nm by varying the drive current. This enables the laser wavelength (for a device nominally tuned to the central wavelength of an absorption line) to pass entirely through the line between two regions at which the absorption is zero. The measurement can also be used as a reference level. A higher frequency current modulation about this sweep signal produces a smaller sinusoidal variation in the laser wavelength, resulting in a signal that is dependent on the slope of the absorption and its second derivative. Consequently, this sweep will provide a full picture of the behavior of the absorption line as a function of wavelength. Full details are available elsewhere.¹

In practice, the scheme is a little more complicated. First, the wavelength modulation signal also produces a similar intensity modulation. Furthermore, there is something of a complex relationship between the phase of the sinusoidal modulation current and the subsequent wavelength modulation of the laser source. However, using suitable signal-processing techniques, we have demonstrated that simple repeatable and accurately calibrated measurements can be made so the depth and breadth of the absorption line can be accurately measured. This not only gives the volumetric concentration of the gas that is present, but it can also be used to derive pressure information for known temperatures or, for some gases, both pressure and temperature.

Figure 4. Typical practical absorption cells. Each assembly encloses an absorption zone 5cm long with packaged application-specific fiber terminations. (Courtesy OptoSci Ltd.)

The system described here has been extensively engineered and tested to assess ventilation systems in tunnels and mine systems and to monitor methane generation in landfill sites used for electrical power generation (see Figure 4). In both contexts, the wide-area access from the single distribution point, the self-calibration and intrinsic reliability within the sensor heads, and the entirely passive sensing network all offer unique and economically compelling benefits. The system concept also shows immense promise in other areas ranging from monitoring sterilization processes in cabinets and rooms to assessing the performance of fuel cells. We are optimistic that this concept will find extensive application as society's monitoring and measurement needs continue to expand. There are particular possibilities in measuring emissions and concentrations of greenhouse gases and in exploiting similar configurations based on these principles to assess leakage from pipelines and storage tanks over areas of vast dimensions. We are currently exploring routes to further expand the range of gases that may be accessed using this technique. We also aim to broaden the spectrum of application through more flexible design of the interaction region.

The work described here results from collaborative activities involving research students and staff at Strathclyde University together with partners in several industrial and government organizations throughout the UK and Europe. A full list of collaborators and sponsors can be obtained on request. Commercial prototypes are currently available for laboratory and field evaluation through OptoSci Limited.