Publication: Low noise THz detection via optical parametric upconversion
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2020-12
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2021-01-22
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Abstract
Recientemente el interés en plataformas que convierten de manera coherente fotones
de RF al dominio óptico ha aumentado de manera significativa. Esto ha sido
motivado fundamentalmente por el auge de las tecnologías cuánticas, ya que enlaces
fotónicos —operando a temperatura ambiente—serían capaces de transferir estados
cuánticos provenientes de circuitos de microondas superconductores que deben
ser enfriados a temperaturas criogénicas. Debido a esta aplicación tecnológica
prometedora, distintas líneas de investigación actualmente persiguen aumentar
la eficiencia de dichos conversores ascendentes. Sin embargo, los conversores
también pueden ser utilizados como intermediarios para detectar indirectamente
señales de microondas, onda milimétrica y terahercios (THz) en el dominio óptico,
con fotodetectores comerciales que no requieren enfriamiento. Esto permitiría
desarrollar receptores de microondas/THz operando a temperatura ambiente y con
sensibilidad sin precedentes, siendo esto sumamente útil para aplicaciones como
radio astronomía, observación de la tierra, obtención de imágenes y espectroscopía.
En esta tesis se realiza un análisis teórico de los conversores ascendentes desde
radiofrecuencia o THz, hasta frecuencias ópticas, basados en modulación electroóptica
(EOM). El trabajo se centra esencialmente en arquitecturas resonantes,
especialmente en aquellas que utilizan resonadores de modos whispering-gallery
(WGM). Utilizando un enfoque electrodinámico clásico, se desarrolla un modelo
teórico que describe procesos no lineales en guías de ondas arbitrarias hechas de
materiales electroópticos. Con este modelo, posteriormente se estudian resonadores
WGM, considerándolos guías de ondas en lazo en lugar de osciladores armónicos
como se hace en la literatura. El motivo es que un modelo de guía de ondas en
lazo permite tomar en cuenta las no uniformidades de los modos a lo largo del
perímetro de los resonadores, a diferencia del modelo de osciladores armónicos que asume que la energía electromagnética está distribuida equitativamente en
la cavidad. Los resultados obtenidos demuestran que, a diferencia del modelo de
oscilador armónico, el modelo de guía en lazo propuesto no subestima la eficiencia
de conversión cuando el resonador está fuertemente sobreacoplado. De igual forma,
impone un límite mínimo a la temperatura de ruido térmico de un modo enfriado
radiativamente mediante un sobreacoplamiento fuerte, a diferencia del modelo del
oscilador armónico que predice cero ruido en un modo infinitamente sobreacoplado.
Cuando los factores de calidad (Q) intrínseco y de acoplo son altos, ambos modelos
coinciden, excepto para interacciones no lineales excepcionalmente fuertes. Si bien
este caso se daría con niveles de potencia laser y de solapamiento modal que son
muy altos para las demostraciones experimentales reportadas en la literatura hasta
el momento, futuras arquitecturas propuestas podrían necesitar ser analizadas con
el modelo de guías en lazo propuesto en este trabajo.
Por último, se realiza un estudio unificado del ruido total presente en radiómetros
de onda milimétrica y THz basados en conversores ascendentes. Los resultados
muestran que el ruido es fundamentalmente diferente si la etapa de detección óptica
post conversión ascendente es coherente (homodina o heterodina) o incoherente
(conteo de fotones). A diferencia de los mezcladores (conversión descendente)
o amplificadores de bajo ruido (LNA), un conversor ascendente 100% eficiente
no introduce ruido además del ruido térmico que se acopla por su temperatura
física. Esto permite utilizar el conversor ascendente como una interfaz para de
manera indirecta contar fotones de THz en el dominio óptico. Ya que el conteo
de fotones es un proceso incoherente, no está sometido al límite cuántico estándar
de los radiómetros que utilizan receptores coherentes (mezcladores o LNAs). Por
tanto, es físicamente posible evitar el límite cuántico estándar haciendo conteo
de fotones en el dominio óptico tras la conversión ascendente. El sistema de
conversión ascendente está sujeto al límite cuántico sólo cuando se realiza detección
óptica coherente (homodina o heterodina). Las predicciones teóricas muestran
que conversores ascendentes trabajando a temperatura ambiente con eficiencias
fotónicas de 1% o más, serían suficientes para lograr un receptor de THz con
sensibilidad significativamente mayor a la de mezcladores y LNAs del estado del
arte, incluso operando en condiciones criogénicas. Cabe mencionar que dicho
requisito de eficiencia es mucho menos estricto que los necesarios para desarrollar
enlaces fotónicos de estados cuánticos prácticos. En este trabajo se propone un
diseño de conversor ascendente resonante en láminas delgadas de niobato de litio,
cuya eficiencia fotónica teórica se predice en 1% por cada mW de potencia láser.
Recently, the interest in platforms that coherently convert photons from the RF to the optical domain has increased. This has been mainly motivated by the advent of quantum information technologies, because room-temperature photonic links could be used to transfer quantum states between cryogenic superconducting microwave circuits over long distances. While several approaches are under investigation to this end, there is a second potential application of efficient photonic upconverters that is sometimes overlooked. Indirect detection of weak signals at microwave, millimeter-wave, and THz frequencies could be carried out with optical detectors by using a room-temperature upconverter as an intermediary. The advantage is the high sensitivity provided by off-the-shelf photodetectors with no need for cryogenics. Thus, room-temperature microwave/THz receivers with unprecedented sensitivity could be designed. This fact provides upconverters a great potential for bridging the so-called THz gap, being millimeter and submillimeter-wave radio astronomy, earth observation, imaging, and spectroscopy among the direct beneficiaries. In this dissertation, we theoretically analyze RF-to-optical upconversion via electro-optic modulation (EOM) in nonlinear crystals. We emphasize resonant architectures, especially those using whispering-gallery-mode (WGM) resonators. Using a classical approach, we develop a theoretical framework to describe nonlinear interactions in generic waveguides. This framework is later used to study nonlinear mixing in WGM resonators, modeling them as loop-waveguides instead of harmonic oscillators as it is done in the literature. The reason is the harmonic oscillator model might result too simplistic because it presumes the energy is evenly distributed throughout the cavity. The loop-waveguide model allows us to account for the possible non-uniformities of the WGMs along the resonator’s perimeter, caused by exceptionally strong coupling or nonlinear interaction strength. Our results show that both models match when intrinsic and coupling quality (Q) factors are sufficiently large, but for highly overcoupled systems, however, the harmonic oscillator model underestimates the upconversion efficiency. We also found a similar behavior regarding the amount of thermal noise coupled to the upconverter due to its physical temperature: the harmonic oscillator model underestimates the thermal population in a strongly overcoupled WGM. Indeed, the loop-waveguide model imposes a theoretical minimum for the input-referred thermal noise temperature of an infinitely overcoupled resonator, in contrast to the harmonic oscillator model that allows for arbitrary thermal noise reduction. Furthermore, in the high-Q limit, both models agree except for exceptionally strong nonlinear interactions, requiring levels of pump power and modal overlap that are far from the experimental demonstrations reported so far. The waveguide model might be relevant, however, for recent proposals of ultra-efficient upconverter architectures. We propose a resonant upconverter design on a thin-film lithium niobate platform that is analyzed theoretically, predicting photon conversion efficiencies on the order of 1% per milliWatt of pump power. Finally, total noise calculations in a WGM upconverter followed by both incoherent and coherent optical detection stages are presented in a unified manner. It is found that homodyne, heterodyne, and direct optical detection schemes after upconversion lead to fundamentally different noise performance for radiometry. Contrary to a mixer (downconverter) or low-noise amplifier (LNA), there is no fundamental noise added by a 100% efficient sum-frequency-generation (SFG) upconverter except for the coupled thermal noise. Therefore, the upconverter can serve as an interface to indirectly count the incoming THz photons in the optical domain, which is not subject to the conventional quantum limit for coherent receivers. Hence, under certain realistic conditions, the quantum limit in a THz radiometer can be circumvented via upconversion followed by incoherent optical detection. The conventional quantum limit bounds the upconversion system sensitivity only when coherent optical detection is done afterward (homodyne or heterodyne). Predicted noise levels of upconversion-based THz receivers show that photon conversion efficiencies on the order of 1% or higher would significantly improve state-of-the-art room-temperature and cryogenic low noise amplifiers and mixers in the sub-millimeter-wave/THz bands. It is worth mentioning that such efficiency requirements are much less strict for ultra-low noise radiometers than for practical quantum-state links.
Recently, the interest in platforms that coherently convert photons from the RF to the optical domain has increased. This has been mainly motivated by the advent of quantum information technologies, because room-temperature photonic links could be used to transfer quantum states between cryogenic superconducting microwave circuits over long distances. While several approaches are under investigation to this end, there is a second potential application of efficient photonic upconverters that is sometimes overlooked. Indirect detection of weak signals at microwave, millimeter-wave, and THz frequencies could be carried out with optical detectors by using a room-temperature upconverter as an intermediary. The advantage is the high sensitivity provided by off-the-shelf photodetectors with no need for cryogenics. Thus, room-temperature microwave/THz receivers with unprecedented sensitivity could be designed. This fact provides upconverters a great potential for bridging the so-called THz gap, being millimeter and submillimeter-wave radio astronomy, earth observation, imaging, and spectroscopy among the direct beneficiaries. In this dissertation, we theoretically analyze RF-to-optical upconversion via electro-optic modulation (EOM) in nonlinear crystals. We emphasize resonant architectures, especially those using whispering-gallery-mode (WGM) resonators. Using a classical approach, we develop a theoretical framework to describe nonlinear interactions in generic waveguides. This framework is later used to study nonlinear mixing in WGM resonators, modeling them as loop-waveguides instead of harmonic oscillators as it is done in the literature. The reason is the harmonic oscillator model might result too simplistic because it presumes the energy is evenly distributed throughout the cavity. The loop-waveguide model allows us to account for the possible non-uniformities of the WGMs along the resonator’s perimeter, caused by exceptionally strong coupling or nonlinear interaction strength. Our results show that both models match when intrinsic and coupling quality (Q) factors are sufficiently large, but for highly overcoupled systems, however, the harmonic oscillator model underestimates the upconversion efficiency. We also found a similar behavior regarding the amount of thermal noise coupled to the upconverter due to its physical temperature: the harmonic oscillator model underestimates the thermal population in a strongly overcoupled WGM. Indeed, the loop-waveguide model imposes a theoretical minimum for the input-referred thermal noise temperature of an infinitely overcoupled resonator, in contrast to the harmonic oscillator model that allows for arbitrary thermal noise reduction. Furthermore, in the high-Q limit, both models agree except for exceptionally strong nonlinear interactions, requiring levels of pump power and modal overlap that are far from the experimental demonstrations reported so far. The waveguide model might be relevant, however, for recent proposals of ultra-efficient upconverter architectures. We propose a resonant upconverter design on a thin-film lithium niobate platform that is analyzed theoretically, predicting photon conversion efficiencies on the order of 1% per milliWatt of pump power. Finally, total noise calculations in a WGM upconverter followed by both incoherent and coherent optical detection stages are presented in a unified manner. It is found that homodyne, heterodyne, and direct optical detection schemes after upconversion lead to fundamentally different noise performance for radiometry. Contrary to a mixer (downconverter) or low-noise amplifier (LNA), there is no fundamental noise added by a 100% efficient sum-frequency-generation (SFG) upconverter except for the coupled thermal noise. Therefore, the upconverter can serve as an interface to indirectly count the incoming THz photons in the optical domain, which is not subject to the conventional quantum limit for coherent receivers. Hence, under certain realistic conditions, the quantum limit in a THz radiometer can be circumvented via upconversion followed by incoherent optical detection. The conventional quantum limit bounds the upconversion system sensitivity only when coherent optical detection is done afterward (homodyne or heterodyne). Predicted noise levels of upconversion-based THz receivers show that photon conversion efficiencies on the order of 1% or higher would significantly improve state-of-the-art room-temperature and cryogenic low noise amplifiers and mixers in the sub-millimeter-wave/THz bands. It is worth mentioning that such efficiency requirements are much less strict for ultra-low noise radiometers than for practical quantum-state links.
Description
Mención Internacional en el título de doctor
Keywords
Nonlinear optics, Whispering-Gallery-Mode, WGM, Upconversion, Radiometry, Nonlinear parametric processes, Photonics, Quantum noise, Thermal noise