Implementation of a novel low-noise InGaAs detector enabling rapid near-infrared multichannel Raman spectroscopy of pigmented biological samples

Pigmented tissues are inaccessible to Raman spectroscopy using visible laser light because of the high level of laser-induced tissue fluorescence. The fluorescence contribution to the acquired Raman signal can be reduced by using an excitation wavelength in the near infrared range around 1000nm. This will shift the Raman spectrum above 1100nm, which is the principal upper detection limit for silicon-based CCD detectors. For wavelengths above 1100nm indium gallium arsenide detectors can be used. However, InGaAs detectors have not yet demonstrated satisfactory noise level characteristics for demanding Raman applications. We have tested and implemented for the first time a novel sensitive InGaAs imaging camera with extremely low readout noise for multichannel Raman spectroscopy in the short-wave infrared (SWIR) region. The effective readout noise of two electrons is comparable to that of high quality CCDs and two orders of magnitude lower than that of other commercially available InGaAs detector arrays. With an in-house built Raman system we demonstrate detection of shot-noise limited high quality Raman spectra of pigmented samples in the high wavenumber region, whereas a more traditional excitation laser wavelength (671nm) could not generate a useful Raman signal because of high fluorescence. Our Raman instrument makes it possible to substantially decrease fluorescence background and to obtain high quality Raman spectra from pigmented biological samples in integration times well below 20s.

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