Ghost Imaging in the Frequency Domain with a high brilliance Coherent Monochromatic Source: a novel approach to extend Spectroscopy Sensitivity beyond detectors limits

摘要

Ghost imaging is an active technique that implies using a time-varying structured illumination source to image a targetwithout spatially-resolving measurements of the light beam that interacts with the target. Traditionally, a beam splitter isused to create two highly correlated beams, such that the signal interacts with the target and is then measured by a singlepixel detector, while the reference is directly measured by a spatially resolving detector. This approach allows toimplement ghost imaging in the space domain, nevertheless also temporal and frequency domains can be addressed,allowing to extract the pertinent information. In particular, ghost imaging in the frequency domain has been recentlyapplied to extract spectral information from a target object by means of Fourier Transform Interferometry. In this workwe illustrate and discuss the results of interaction-free measurements on an Er~(3+) doped nonlinear crystal, placed in onearm of an interferometer, obtained by using only non-interacting photons. Our equipment is a wave-guided solid statedevice, exploiting an integrated quantum photonic circuit that is equivalent to an Asymmetric Nonlinear Mach-ZehnderInterferometer. The experiment was performed by using a 250mW monochromatic 980 nm laser source that allowedexciting an Er:LiNbO_3 waveguide, placed in one of the arms of the asymmetric interferometer. The interferograms wereobtained by varying the signal in the time domain by using a LiNbO_3 undoped waveguide in the opposite branch of theinterferometer and recorder with a standard Si p-i-n detector, provided with a pass band filter (975nm ± 25nm) thusblocking all photons except the pump ones. The data were analyzed with conventional Fast Fourier TransformTechniques. The application of this approach allowed to recover information in the frequency domain, in particular,despite the monochromatic characteristics of the detected signal, we could recover the whole spectroscopy of the energylevels of the Er~(3+) doped crystal. The role of the converted photons was evidenced by the fact that, by using a radiationsource that does not interact with the dopant (1320nm Laser), only the line of the source is recovered by the FFThandling of the interferograms. An important aspect to remark is that the obtained spectral distribution addressed also theIR part of the spectrum where the applied detector (Si p-i-n) is blind. In this view, this methodology opens the possibilityto extend sensitive spectral measurements in spectral regions where detectors show poor responsivity.