The mid-infrared region of the spectrum, that is, the wavelengths covering the range between 2 and 20um, offers important benefits in many areas of science and technology, including chemical and biological sensing (e.g. either for environmental monitoring or for bio-agent and medical sensing), spectroscopy and indeed (free-space) communications. There are several reasons behind this. For example, the atmosphere is transparent in regions of the mid-infrared (e.g. between 3 and 5um); a large number of molecules undergo their characteristic vibrational transitions in this part of the spectrum, making mid-infrared spectroscopy a powerful tool; also, Rayleigh scattering decreases with increasing wavelength, making the mid-infrared decidedly preferable for applications requiring propagation of light in turbid media over the shorter wavelengths of the near-infrared. However, there are also several important challenges associated with the implementation of applications in this region of the spectrum. For example, detection of the electromagnetic radiation in the mid-infrared often necessitates the use of bulky devices that need to be cooled to low temperatures. Also, the tremendous advances that have been achieved in the near-infrared through the use of silica optical fibers cannot be transferred directly to these wavelengths due to the strong absorption of silica beyond ~2um. Therefore, optical fibers fabricated from alternative glasses, such as tellurite, lead silicate and – as in this paper by Marandi et al. – chalcogenides, need to be considered for applications in this spectral region. Several of the laser source technologies that are commonplace in the near-infrared are also not available at these longer wavelengths, often necessitating the use of nonlinear effects to convert radiation from the near- to the mid-infrared. This is also true for coherent broadband sources, which are ever so important in many of the applications mentioned earlier.
It is this very problem of the implementation of a broadband supercontinuum source covering the 2-5um region that is the subject of the paper highlighted here. A near-infrared source is first converted to the mid-infrared (using an optical parametric oscillator) and then broadened spectrally using the nonlinear effects generated in a length of chalcogenide fiber. Apart from guiding in the mid-infrared, this material has the additional advantage of being highly nonlinear (possessing around two orders of magnitude higher Kerr nonlinearity than silica), which is crucial for the generation of a spectrally broadened (supercontinuum) source. However, clearly nonlinearity alone is not enough: The onset of strong nonlinear effects that gives rise to a broad supercontinuum spectrum also requires suitable dispersion characteristics in the fiber, the achievement of which is not straight-forward. It is known that it is possible to manipulate the dispersion of an optical fiber to a certain extent by manipulating the dimensions of the guided mode. This is achieved in this work using tapering, and in fact doing this while the optical beam propagates along the fiber length, so as to provide direct feedback for the optimization of the process. As a result, a highly coherent octave-spanning supercontinuum spectrum is generated.
The past few years have already seen a number of significant technological advances that have benefitted tremendously the field of mid-infrared photonics. Advances like this reported in the paper by Marandi et al., allow confidence that it will not be long before practical and reliable sources for the mid-infrared become available, propelling in turn further research in this area. Nevertheless, a number of significant challenges still remain to be faced, particularly when interest shifts to wavelengths ever deeper in the infrared.
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