Abstract-Sensitivity limits of millimeter-wave photonic radiometers based on efficient electro-optic upconverters

Gabriel Santamaría Botello, Florian Sedlmeir, Alfredo Rueda, Kerlos Atia Abdalmalak, Elliott R. Brown, Gerd Leuchs, Sascha Preu, Daniel Segovia-Vargas, Dmitry V. Strekalov, Luis Enrique García Muñoz,  Harald G. L. Schwefel,

Fig. 1. Coupling of millimeter-wave and optical radiation to the WGM resonator. The microwave coupling region (enclosed by the dashed curve) is considered small and lossless. Input and output modes in the waveguide and the resonator are defined with normalized field amplitudes 𝑎𝑖(𝜈)$a_i(\nu )$ such that their squared norm equals the power spectral density of the mode. The coupling system can be viewed as a four-port network whose scattering matrix is characterized by the waveguide’s reflection coefficient 𝑟=𝑎2/𝑎1$r=a_2/a_1$ and waveguide-resonator transmission coefficient 𝑡=𝑎3/𝑎1$t=a_3/a_1$. The cavity’s reflection coefficient 𝑟′=𝑎3/𝑎4$r^{\prime }=a_3/a_4$ and cavity-waveguide transmission coefficient 𝑡′=𝑎2/𝑎4$t^{\prime }=a_2/a_4$ have the same magnitudes as 𝑟$r$ and 𝑡$t$, respectively, but a different phase in general

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-5-10-1210

Conventional ultra-high sensitivity detectors in the millimeter-wave range are usually cooled as their own thermal noise at room temperature would mask the weak received radiation. The need for cryogenic systems increases the cost and complexity of the instruments, hindering the development of, among others, airborne and space applications. In this work, the nonlinear parametric upconversion of millimeter-wave radiation to the optical domain inside high-quality (𝑄$Q$) lithium niobate whispering-gallery mode (WGM) resonators is proposed for ultra-low noise detection. We experimentally demonstrate coherent upconversion of millimeter-wave signals to a 1550 nm telecom carrier, with a photon conversion efficiency surpassing the state-of-the-art by 2 orders of magnitude. Moreover, a theoretical model shows that the thermal equilibrium of counterpropagating WGMs is broken by overcoupling the millimeter-wave WGM, effectively cooling the upconverted mode and allowing ultra-low noise detection. By theoretically estimating the sensitivity of a correlation radiometer based on the presented scheme, it is found that room-temperature radiometers with better sensitivity than state-of-the-art high-electron-mobility transistor (HEMT)-based radiometers can be designed. This detection paradigm can be used to develop room-temperature instrumentation for radio astronomy, earth observation, planetary missions, and imaging systems.

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