Electro-Optical and Infrared Systems: Technology and Applications
Ghost imaging in the frequency domain with a high brilliance coherent monochromatic source: a novel approach to extend spectroscopy sensitivity beyond detectors limits
Marco Chiarini, Alberto Parini, Francesca Braglia, Lorella Braglia, Sandro Braglia, Sauro Farabegoli, Aldo Artoni, Agostino Desalvo, Gian Giuseppe Bentini
Ghost imaging is an active technique that implies using a time-varying structured illumination source to image a target without spatially-resolving measurements of the light beam that interacts with the target. Traditionally, a beam splitter is used to create two highly correlated beams, such that the signal interacts with the target and is then measured by a single pixel detector, while the reference is directly measured by a spatially resolving detector. This approach allows to implement ghost imaging in the space domain, nevertheless also temporal and frequency domains can be addressed1,2, allowing to extract the pertinent information. In particular, ghost imaging in the frequency domain has been recently applied to extract spectral information from a target object by means of Fourier Transform Interferometry3,4. In this work we illustrate and discuss the results of interaction-free measurements on an Er3+ doped nonlinear crystal, placed in one arm of an interferometer, obtained by using only non-interacting photons. Our equipment is a wave-guided solid state device, exploiting an integrated quantum photonic circuit that is equivalent to an Asymmetric Nonlinear Mach-Zehnder Interferometer. The experiment was performed by using a 250mW monochromatic 980 nm laser source that allowed exciting an Er:LiNbO3 waveguide, placed in one of the arms of the asymmetric interferometer. The interferograms were obtained by varying the signal in the time domain by using a LiNbO3 undoped waveguide in the opposite branch of the interferometer and recorder with a standard Si p-i-n detector, provided with a pass band filter (975nm ± 25nm) thus blocking all photons except the pump ones. The data were analyzed with conventional Fast Fourier Transform Techniques. 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 energy levels of the Er3+ doped crystal. The role of the converted photons was evidenced by the fact that, by using a radiation source that does not interact with the dopant (1320nm Laser), only the line of the source is recovered by the FFT handling of the interferograms. An important aspect to remark is that the obtained spectral distribution addressed also the IR part of the spectrum where the applied detector (Si p-i-n) is blind. In this view, this methodology opens the possibility to extend sensitive spectral measurements in spectral regions where detectors show poor responsivity.
Ghost imaging is an active technique that implies using a time-varying structured illumination source to image a target without spatially-resolving measurements of the light beam that interacts with the target. Traditionally, a beam splitter is used to create two highly correlated beams, such that the signal interacts with the target and is then measured by a single pixel detector, while the reference is directly measured by a spatially resolving detector. This approach allows to implement ghost imaging in the space domain, nevertheless also temporal and frequency domains can be addressed1,2, allowing to extract the pertinent information. In particular, ghost imaging in the frequency domain has been recently applied to extract spectral information from a target object by means of Fourier Transform Interferometry3,4. In this work we illustrate and discuss the results of interaction-free measurements on an Er3+ doped nonlinear crystal, placed in one arm of an interferometer, obtained by using only non-interacting photons. Our equipment is a wave-guided solid state device, exploiting an integrated quantum photonic circuit that is equivalent to an Asymmetric Nonlinear Mach-Zehnder Interferometer. The experiment was performed by using a 250mW monochromatic 980 nm laser source that allowed exciting an Er:LiNbO3 waveguide, placed in one of the arms of the asymmetric interferometer. The interferograms were obtained by varying the signal in the time domain by using a LiNbO3 undoped waveguide in the opposite branch of the interferometer and recorder with a standard Si p-i-n detector, provided with a pass band filter (975nm ± 25nm) thus blocking all photons except the pump ones. The data were analyzed with conventional Fast Fourier Transform Techniques. 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 energy levels of the Er3+ doped crystal. The role of the converted photons was evidenced by the fact that, by using a radiation source that does not interact with the dopant (1320nm Laser), only the line of the source is recovered by the FFT handling of the interferograms. An important aspect to remark is that the obtained spectral distribution addressed also the IR part of the spectrum where the applied detector (Si p-i-n) is blind. In this view, this methodology opens the possibility to extend sensitive spectral measurements in spectral regions where detectors show poor responsivity.
Ghost imaging is a novel non-conventional technique allowing to generate high resolution images by correlating the intensity of two light beams, neither of which independently contains sufficient information about the spatial distribution and shape of the object. The first demonstration of ghost imaging used light in double photon state, obtained from spontaneous parametric down-conversion. Owing to the entanglement of the source photons, the proposed theory required quantum descriptions for both the optical source and its photo-detection statistics1. However, subsequent experimental and theoretical considerations2,3 demonstrated that ghost imaging can be performed also with thermalized light, utilizing either CCD detector arrays or photon-counting detectors, thus admitting to a semi-classical description, employing classical fields and shot-noise limited detectors. This has generated increasing interest4-6 in establishing a unifying theory that characterizes the fundamental physics of ghost imaging and defines the boundary between classical and quantum domains. In this view, we exploited recent progress obtained through the application of Fourier Transform Techniques to demonstrate ghost imaging in the frequency domain, in order to measure a continuous spectrum by using a highly brilliant and coherent monochromatic source. In particular, we demonstrate the application of this ghost imaging technique to broadband spectroscopic measurements by means of interaction free photon detection. The experimental apparatus and the collected data are described in a dedicated work7. In this paper, we consider the theoretical aspects underlying the proposed Spectroscopic technique. In particular, two alternative theoretical models are presented. In one case, a statistical approach (semi-classical) is applied, where the states of the sampling beam are considered, whereas in the other case a pure quantum treatment is carried on, by describing the interaction of vacuum states generated by photon conversion processes. Both theoretical models, though carried on by means of a complementary formalism, lead to equivalent results and offer a physical interpretation of the collected experimental data. The application of these results offer novel perspectives for remote sensing in low light conditions, or in spectral regions where sensitive detectors are lacking.
Most of the experimental interferometric setup are based on the Mach-Zehnder geometry. Usually in the experimental configurations the photons travel in air, at a speed v independent of λ. An integrated optics approach appears necessary both for practical applications and because it can be useful for fundamental optics studies because the transmitted photons are in a spatial coherent state. We exploited a hybrid setup integrating optical fibres and monolithic optical waveguides to implement an asymmetric nonlinear Mach-Zehnder Interferometer (ANLI). One arm of the interferometer was equipped with a single mode Er:LiNbO3 waveguide, acting as nonlinear component whereas the other arm was an undoped LiNbO3 single mode waveguide, used to obtain a phase shift through the application of a voltage ramp. We injected in the ANLI a 980 nm laser radiation to collect interferograms that could be ascribed exclusively to the pump photons, as all frequency conversion events are localized only in one arm of the interferometer. The Fourier Transform analysis of the experimental data allowed us to perform a n Interaction-Free Quantum Optics experiment. This is a novel approach that can be used because in quantum mechanics the wave functions of a state, expressed in terms of two conjugate variables, are Fourier Transform pairs. In fact, operating in the Fourier Transform time-frequency space-domain, we recovered the spectral composition, in the wavenumber space-domain, of the photons generated by the excitation of an Er3+ doped crystal placed in one arm of the interferometer.
Correlated-photon imaging, popularly known as ghost imaging, is a technique whereby an image is formed from light that has never interacted with the object. In ghost imaging experiments two correlated light fields are produced. One of these fields illuminates the object, and the other field is measured by a spatially resolving detector. In the quantum regime, these correlated light fields are produced by entangled photons created by spontaneous parametric down-conversion. To date, all correlated-photon ghost-imaging experiments have scanned a single-pixel detector through the field of view to obtain the spatial information. However, scanning leads to a poor sampling efficiency, which scales inversely with the number of pixels, N, in the image. In this work we overcome this limitation by using a time-gated camera to record the single-photon events across the full scene. We obtain high-contrast images, 90%, in either the image plane or the far-field of the photon pair source, taking advantage of the EPR-like correlations in position and momentum of the photon pairs. Our images contain a large number of modes, >500, creating opportunities in low-light-level imaging and in quantum information processing.
Correlated light (either classical or quantum) can be employed in various ways to improve resolution and measurement sensitivity. In an “interaction-free” measurement, a single photon can be used to reveal the presence of an object placed within one arm of an interferometer without being absorbed by it. This method has previously been applied to imaging. With a technique known as “ghost imaging”, entangled photon pairs are used for detecting an opaque object with significantly improved signal-to-noise ratio while preventing over-illumination. Here, we integrate these two methods to obtain a new imaging technique which we term “interaction-free ghost-imaging” that possesses the benefits of both techniques. While improving the image quality of conventional ghost-imaging, this new technique is also sensitive to phase and polarization changes in the photons introduced by a structured object. Furthermore, thanks to the “interaction-free” nature of this new technique, it is possible to reduce the number of photons required to produce a clear image of the object (which could be otherwise damaged by the photons) making this technique superior for probing light-sensitive materials and eventually biological tissues. If time allows, I will discuss some follow-up works involving partial measurements and remote erasure/completion of images. The latter techniques can help to suppress various types of noise during the imaging process.
Quantum - or classically correlated - light can be employed in various ways to improve resolution and measurement sensitivity. In an "interaction-free" measurement, a single photon can be used to reveal the presence of an object placed within one arm of an interferometer without being absorbed by it. With a technique known as "ghost-imaging", entangled photon pairs are used for detecting an opaque object with significantly improved signal-to-noise ratio while preventing over-illumination. Here, we integrate these two methods to obtain a new imaging technique which we term "interaction-free ghost-imaging" (IFGI). With this new technique, we reduce photon illumination on the object by up to 26.5% while still maintaining at least the same image quality of conventional ghost-imaging. Alternatively, IFGI can improve image signal-to-noise ratio by 18% when given the same number of interacting photons as in standard ghost-imaging. IFGI is also sensitive to phase and polarisation changes of the photons introduced by a structured object. These advantages make IFGI superior for probing light-sensitive materials and biological tissues.
Correlated photon imaging, popularly known as ghost imaging, is a technique whereby an image is formed from light that has never interacted with the object. In ghost imaging experiments, two correlated light fields are produced. One of these fields illuminates the object, and the other field is measured by a spatially resolving detector. In the quantum regime, these correlated light fields are produced by entangled photons created by spontaneous parametric down-conversion. To date, all correlated photon ghost imaging experiments have scanned a single-pixel detector through the field of view to obtain spatial information. However, scanning leads to poor sampling efficiency, which scales inversely with the number of pixels, N , in the image. In this work, we overcome this limitation by using a time-gated camera to record the single-photon events across the full scene. We obtain high-contrast images, 90%, in either the image plane or the far field of the photon pair source, taking advantage of the Einstein–Podolsky–Rosen-like correlations in position and momentum of the photon pairs. Our images contain a large number of modes, >500, creating opportunities in low-light-level imaging and in quantum information processing.