Fine tuning for exploration

Rob Coppinger drills down into the details about OPOs and OPAs for oiI and gas

The oil and gas industry has a hardy image, of ambitious men working in tough environments using rugged equipment to extract the hydrocarbons that fuel the world’s economy. Photonic instruments are not known for having a hardy image or rugged design that can cope with jolts and bumps, but the technology is getting there.

‘It is extremely rugged and one of the big application areas they have targeted is gas sensing,’ says Steve Knight, Laserlines’ photonics sales director. He is referring to a new product from Sweden’s Cobolt, whose laser products Knight’s company distributes.

Different hydrocarbons have different features and the spectral range necessary to analyse them is 3-4µm. An Optical Parametric Oscillator (OPO) combined with a laser oscillator can expand the tuning range from 700-1,000nm to something as broad as 500-4,000nm, or even longer. Another option is to use an amplifier. An amplifier can be altered to vary the pulse duration; the wavelength of the pulses cannot be changed. An Optical Parametric Amplifier (OPA) can be used in combination with an amplifier, in the same way that an OPO can be used with a laser oscillator to give users access to a wider wavelength range. Because of the high peak power possible with pulsed lasers, the range of wavelengths accessible through OPAs is greater than that of OPOs. An OPA in combination with an amplifier can expand the tuning range to 190nm-20μm, easily encompassing the wavelengths needed for hydrocarbon analysis.

Referring to Cobolt’s forthcoming OPO, Knight explains that the prototype hasn’t yet been released, and so, ‘we haven’t shipped any of these yet. We’ve been their [Cobolt’s] partner in the UK for many years and we’re their biggest suppliers. They specialised in diode pumped lasers for commercial application until now; their products have been CW visible lasers. These types of systems, for example, are for the biomedical market, for flow cytometry and confocal microscopy. Until now the OPOs have been large and things that have to be tweaked and aligned. This has been developed to be extremely rugged’.

That tweaking and aligning is needed because the OPO is an intrinsically narrow line device. It is as narrow as the pump laser is narrow; users get a line-width that is equivalent to the laser they pump with. OPOs can be pumped by different types of lasers. By using OPOs chemists are able to produce photons at a wider range of wavelengths, but at a narrow bandwidth at any one time. As well as Cobolt, Berlin-based Angewandte Physik und Elektronik (APE) produces OPOs to alter the wavelength range of pulsed lasers.

In spectroscopy, different wavelengths of light correspond to different types of absorption within a chemical structure. The OPOs can be used for more exotic types of spectroscopy because chemical species at ultra-low temperatures, for example, have very small line-widths for absorption. This is the reason some applications, such as oil and gas detection and identification, need ultra-narrow lasers. And widening the OPO wavelength range does not diminish light quality.

Techniques used with narrow bandwidth light sources include high sensitivity photoacoustic cavity spectroscopy, and ring-down spectroscopy, both of which are able to achieve high-sensitivity detection. This is useful for detecting the small volumes of gas that the petroleum industry can capture. The choice of wavelength range, such as those for hydrocarbons, is important for these high sensitivity techniques.

‘Like all companies they [Cobolt] have to grow so they’ve used some of the technology they have developed to try to get into different application areas. The thing to emphasise is their lasers are not laboratory units which sit on an optical table. They are packaged and extremely rugged,’ says Knight. To prove the point Cobolt provides videos of its lasers being immersed in boiling water and dried and operated.

Boiling water is not something any OPA or OPO will encounter in the oil and gas industry, but the technology’s application to this industry is another example of how every aspect of civilisation can exploit lasers for the benefit of all.