TECHNOLOGY RamaN SpECTROSCOpY
‘We developed a one-of-a-kind laser for both high specification, where narrow linewidth is crucial, and more standard applications in which we can live with medium linewidth,’ says Pfeufer. ‘It’s currently capable of 1.5MHz sharpness. Although spectral linewidths in the range of 1GHz are sufficient for most Raman applications, being sharper than that does not hurt. Our goal was to make a high-resolution Raman instrument in which the laser was never going to be the limiting factor,’ he says, adding that 1MHz is close to the lower limit of what the physics of the laser will allow in terms of linewidth. Coherent does not disclose how it achieves such narrow linewidths, but Pfeufer states that the lasers are based on the same optical-pumped semiconductor lasers (OPSLs) used in telecommunications. OPSLs, says Pfeufer, offer several advantages over diode lasers in these applications: ‘The wavelength of diode lasers fluctuates with current variations, and also with its age and temperature. An imprecise wavelength is a killer for Raman spectroscopy, because it leads to smearing-out of the spectrum.’ In addition, he says, drifting wavelengths could interfere with the filters within the system, which are designed with very narrow tolerances. ‘Diode lasers also have background emissions which the OPSL laser hasn’t got, and so we have to work with filters anyway – these are not cheap. Also, the beam profile of diode lasers is poor; diode lasers need additional optics, which increases costs, and they also need extra control to stabilise the wavelength, such as distributed feedback. For these kinds of application, we’re convinced that OPSL technology is superior to narrow linewidth diode lasers.’ Linewidth aside, the main reason to go with OPSL technology, he says, is the ability to change and customise the excitation wavelength of the system: ‘We chose OPSL over diode and DPSS technologies, mainly because it is wavelength- flexible. DPSS lasers, in contrast, are available in green at 532 and 561nm. Although we do sell these
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for Raman applications, they are not wavelength flexible; if the customer wants to use a 540nm laser instead, the physics of the platform does not allow this. With OPSL technology we have wavelength flexibility,’ he says, explaining that the OPSL lasers are interchangeable in Raman applications. This allows customers to customise their excitation wavelengths. ‘If the customer needs
This system is able to take a Raman spectrum at a focal length of up to ➤ two metres. Image courtesy of McPherson
june 2011 l ELECTRO OpTiCS
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