Supercontinuum Sources

Incoherent thermal light sources, e.g. halogen lamps and globars, have been the dominant broadband source for molecular spectroscopy for several decades. These sources can be combined with a Grating Spectrometer (GS) or a Fourier Transform Spectrometer (FTS). However, these sources are omnidirectional and have low spectral brightness. Therefore, achieving a long interaction length in gas-phase samples is quite a challenge. In addition, especially for FTS, long averaging times are needed to obtain a spectrum with a high resolution and high signal-to-noise ratio (SNR).

Newly developed ultra-broadband MIR supercontinuum (SC) light sources show a strong potential to replace the thermal light sources in Grating Spectrometers and more importantly in Fourier Transform Spectrometers. SC sources are spatially coherent and have the potential to cover a wider spectral wavelength range than thermal sources. They provide a very high spectral brightness, much higher than thermal sources and even exceeding the brilliance of a synchrotron [1-5]. Furthermore, they have the potential to deliver ultra-flat broadband spectra [6], which is highly desirable for Fourier transform spectroscopy to achieve a uniform detection sensitivity over their spectral coverage. Historically, the main drawback of SC sources has been their high Relative Intensity Noise (RIN), due to noise amplification in the nonlinear broadening process [7, 8]. Recently, low-noise near-infrared (NIR) SC sources have been demonstrated using all-normal dispersion photonic crystal fibers [9] and recent advancements in cascading-based MIR SC sources provide reduced RIN noise, due to gain-induced soliton alignment in the in-amplifier SC used as a seed [10]. The technology of MIR SC sources with sub-nanosecond pulse durations and MHz repetition rates has now become sufficiently mature to be used in various applications, such as Optical Coherence Tomography (OCT), spectroscopy, and microscopy [11-14]. The MHz repetition rate can be utilized in synchronous demodulation, to overcome the 1/f noise in the system [15]. In combination with an FTS, the high repetition rate yields a fast demodulation process in the detection system, allowing the FTS to keep a fast scanning speed.

References
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