We demonstrate optical coherence tomography based on an SU(1,1) nonlinear interferometer with high-gain parametric down-conversion. For imaging and sensing applications, this scheme promises to outperform previous experiments working at low parametric gain, since higher photon fluxes provide lower integration times for obtaining high-quality images. In this way one can avoid using single-photon detectors or CCD cameras with very high sensitivities, and standard spectrometers can be used instead. Other advantages are: higher sensitivity to small loss and amplification before detection, so that the detected light power considerably exceeds the probing one.
We demonstrate optical coherence tomography based on an SU(1,1) nonlinear interferometer with high-gain parametric down-conversion. For imaging and sensing applications, this scheme promises to outperform previous experiments working at low parametric gain, since higher photon fluxes provide lower integration times for obtaining high-quality images. In this way one can avoid using single-photon detectors or CCD cameras with very high sensitivities, and standard spectrometers can be used instead. Other advantages are: higher sensitivity to small loss and amplification before detection, so that the detected light power considerably exceeds the probing one.
We demonstrate optical coherence tomography based on an SU(1,1) nonlinear interferometer with high-gain parametric downconversion. For imaging and sensing applications, this scheme promises to outperform previous experiments working at low parametric gain, since higher photon fluxes provide lower integration times for obtaining high-quality images. In this way, one can avoid using single-photon detectors or CCD cameras with very high sensitivities, and standard spectrometers can be used instead. Other advantages are higher sensitivity to small loss and amplification before detection so that the detected light power considerably exceeds the probing one.
We demonstrate Fourier-domain optical coherence tomography (FD-OCT) based on a seeded SU(1,1) interferometer. Multilayer objects are probed with broadband light centered at 1550nm, while depth-resolved 3D images are reconstructed from photon flux measurements centered at 810nm. We show that, in the low parametric-gain regime, seeding increases the photon flux, enabling volumetric imaging with a spectrometer rather than single-photon detectors. Our analysis further shows that, under the conditions considered, seeding provides a more effective route to sensitivity enhancement than increasing the parametric gain.
(English) This thesis, mostly experimental, is based on two fundamental pillars: nonlinear interferometers and Optical Coherence Tomography (OCT). Nonlinear interferometers are a class of interferometers that exhibit nonclassical phenomena brought on by nonlinear elements, such as optical parametric amplifiers and parametric down-conversion (PDC) nonlinear crystals. OCT is a non-invasive imaging technique that allows to obtain images with high axial and cross-sectional resolution of a wide variety of samples. The first novel contribution of this thesis is an experimental scheme that combines the two ideas: an OCT scheme based on a nonlinear interferometer. In these new approach the reflectivity of the sample translates in a loss of first-order coherence between two beams, that is the variable that is measured. In addition, it allows probing the sample with a wavelength different from the one that is measured. In this way, the penetration depth in the sample can be enhanced using longer wavelengths while using light at the optimal wavelength for detection. We present and implement two different experimental configurations. The first is a nonlinear interferometer based on induced coherence, or Mandel-type interferometer, that works in the low parametric gain regime of parametric down-conversion (PDC). The results presented here are a proof-of-concept, that can potentially offer new applications for OCT, but that are not meant to substitute traditional OCT systems. The second OCT scheme overcomes some of the limitations of the first scheme discussed above. It is an SU(1,1), or Yurke-type interferometer, that operates in the high parametric gain regime of parametric down-conversion. In addition to taking advantage of the salutary features of this new approach, it also enables obtaining values of power and axial resolution comparable to those of conventional OCT. The second novel contribution of this thesis is related to fundamental aspects at the heart of nonlinear interferometers. We discuss two experiments that study two important concepts behind the idea of induced coherence: quantum distinguishability and parametric amplification (stimulated emission). In the first experiment we propose a new experimental measure of quantum distinguishability and derive a complementarity relation between distinguishability and first-order coherence. In the second experiment, we contribute to the ongoing debate about the true role of quantum distinguishability and stimulated emission in explaining the induced coherence effect. Finally, we put forward theoretically a new scheme to retrieve transverse spatial information of a sample using a nonlinear interferometer, based on projecting the outgoing photons in selected spatial modes. We call this new proposal spatial spectroscopy, and it does not require a physical mechanical scan of the sample. We demonstrate the feasibility of the technique with a simple example. This last contribution constitutes a future proposal to be carried out with nonlinear interferometers, evidencing their great versatility and potential applications in new areas. (Català) Aquesta tesi, majoritàriament experimental, està basada en dos pilars fonamentals: els interferòmetres nolineals i la Tomografia de Coherència Òptica (OCT). Els interferòmetres nolineals són un tipus d'interferòmetres en els que tenen lloc fenòmens no-clàssics generats per la presència d'elements nolineals, com amplificadors òptics paramètrics i cristalls de conversió paramètrica descendent. OCT és una tècnica interferomètrica no-invasiva per obtenir imatges 3D amb alta resolució axial d'una gran varietat de mostres. La primera contribució original d'aquesta tesi és un esquema experimental que combina dues idees: un esquema d'OCT basat en un interferòmetre nolineal. En aquesta nova proposta, la reflectivitat de la mostra es tradueix en una pèrdua de coherència entre dos feixos de llum, que és la quantitat mesurada. A més a més, permet il·luminar la mostra amb una longitud d'ona diferent a la mesurada. D'aquesta manera, podem guanyar penetració a la mostra mentre utilitzem llum a la longitud d'ona òptima per la detecció. En aquest sentit, presentem i implementem dues configuracions experimentals diferents. La primera es tracta d'un interferòmetre nolineal basat en el concepte de coherència induïda, o també anomenat interferòmetre de Mandel, que opera en el règim de baix guany de conversió paramètrica descendent. Els resultats presentats constitueixen una prova del principi de funcionament, que poden suposar noves aplicacions per OCT, però no estan enfocats a substituir els sistemes convencionals d'OCT. El segon esquema d'OCT que proposem millora algunes de les limitacions de l'anterior proposta. Es tracta d'un SU(1,1), o interferòmetre de Yurke, que opera en el règim d'alt guany de conversió paramètrica descendent. A més d'aprofitar les característiques beneficioses d'aquests nous esquemes, també permet obtenir valors de potència i resolució axial comparables als dels sistemes d'OCT tradicionals. La segona contribució original d'aquesta tesi està relacionada amb aspectes fonamentals que estan rere els interferòmetres nolineals. Presentem dos experiments que estudien dos conceptes importants que expliquen la idea de coherència induïda: la distingibilitat quàntica i l'amplificació paramètrica (o emissió estimulada). En el primer experiment proposem una nova mesura de la distingibilitat quàntica i derivem una relació entre la distingibilitat i la coherència. En el segon experiment, contribuïm al debat vigent sobre el rol de la distingibilitat quàntica i l'emissió estimulada per explicar el concepte de la coherència induïda. Finalment, presentem les bases teòriques d'un nou esquema per obtenir informació espacial d'una mostra utilitzant un interferòmetre nolineal, basat en la projecció dels fotons en uns modes espacials de llum concrets. Anomenen aquesta nova proposta espectroscòpia espacial, i no requereix un escaneig físic-mecànic de la mostra. Demostrem la viabilitat de la tècnica amb un exemple senzill. Aquesta última contribució constitueix una proposta de futur a realitzar amb els interferòmetres nolineals, evidenciant la seva gran versatilitat i potencials aplicaciones en noves àrees.
Abstract Nonlinear interferometry with entangled photons allows for characterizing a sample without detecting the photons interacting with it. This method enables highly sensitive optical sensing in the wavelength regions where efficient detectors are still under development. Recently, nonlinear interferometry has been applied to interferometric measurement techniques with broadband light sources, such as Fourier‐transform infrared spectroscopy and infrared optical coherence tomography. However, they have been demonstrated with photon pairs produced through spontaneous parametric down‐conversion (SPDC) at a low parametric gain, where the average number of photons per mode is much smaller than one. The regime of high‐gain SPDC offers several important advantages, such as the amplification of light after its interaction with the sample and a large number of photons per mode at the interferometer output. This work presents broadband spectroscopy and high‐resolution optical coherence tomography with undetected photons generated via high‐gain SPDC in an aperiodically poled lithium niobate crystal. To prove the principle, reflective Fourier‐transform near‐infrared spectroscopy with a spectral bandwidth of 17 THz and optical coherence tomography with an axial resolution of 11 µm are demonstrated.
Over the past decade, several schemes for imaging and sensing based on nonlinear interferometers have been proposed and demonstrated experimentally. These interferometers exhibit two main advantages. First, they enable probing a sample at a chosen wavelength while detecting light at a different wavelength with high efficiency (bicolor quantum imaging and sensing with undetected light). Second, they can show quantum-enhanced sensitivities below the shot-noise limit, potentially reaching Heisenberg-limited precision in parameter estimation. Here, we compare three quantum-imaging configurations using only easily accessible intensity-based measurements for phase estimation: a Yurke-type SU(1,1) interferometer, a Mandel-type induced-coherence interferometer, and a hybrid scheme that continuously interpolates between them. While an ideal Yurke interferometer can exhibit Heisenberg scaling, this advantage is known to be fragile under realistic detection constraints and in the presence of loss. We demonstrate that differential intensity detection in the Mandel interferometer provides the highest and most robust phase sensitivity among the considered schemes, reaching but not surpassing the shot-noise limit, even in the presence of loss. Intensity measurements in a Yurke-type configuration can achieve genuine sub-shot-noise sensitivity under balanced losses and moderate gain; however, their performance degrades in realistic high-gain regimes. Consequently, in this regime, the Mandel configuration with differential detection outperforms the Yurke-type setup and constitutes the most robust approach for phase estimation.