Portable Monochromatic Infrared Light Source Based on a Supercontinuum Laser

Abstract

A portable wavelength tunable light source to calibrate the spectral responsivities of detectors in the infrared region was developed and characterized. It comprises of a supercontinuum laser, laser line tunable filter (LLTF), and output coupling optics, such as a focusing lens and a cut-on filter. This light source improves spectral responsivity calibrations of the detectors as the developed setup is automated. To automate this setup a motorized XY translation stage and a filter wheel was used. In order to characterize the monochromatic light source and its coupling optics, a spectroradiometer was used. The wavelength scale of this spectroradiometer was calibrated using HgAr pen lamps and KrAr+ laser lines to ensure the spectral peaks of LLTF are measured correctly.

Three photovoltaic detectors, of which two were based on indium gallium arsenide and one based on germanium, were calibrated using the automated wavelength tunable light source setup against a pyroelectric radiometer. The pyroelectric radiometer was used as a reference detector due to its spectrally flat responsivity. The aperture diameters of indium gallium arsenide and germanium detectors are 5 mm and 10 mm, respectively. Due to the small active areas of the detectors, the collimated laser beam output from the multimode fiber was focused on the detector plane.

The spectral responsivity calibrations were performed over the spectral range from 800 nm to 2000 nm. As the optimal working spectral range of LLTF is 1000 nm - 2000 nm and depending on the band gap edge of the photovoltaic detectors, the standard deviations of the readings measured at wavelengths below 820 nm and above 1600 nm are slightly higher. The spectral responsivities of the photovoltaic detectors studied were compared to the older calibrations performed using a reference spectroradiometer at the Metrology Research Institute. Through careful analysis of the experiments and comparisons, it is found that this setup decreases the expanded uncertainty in detector calibrations from 4% to approximately 2.2% - 2.7% for the near-infrared region.