Characterization of the NEP of Mid-Infrared Upconversion Detectors

We present a scheme to estimate the noise equivalent power (NEP) of the frequency upconversion detectors (UCDs), detecting mid-infrared (MIR) light. The NEP of the UCD is a combined contribution of NEPs from the upconversion process and from the photodetector, used for detecting the upconverted signal. The 2–5- $\mu \text{m}$ MIR range is particularly investigated in this letter using a bulk periodically poled lithium niobate-based CW-intracavity UCD. We measured the NEP of UCD as 20 fW/ $\sqrt {\text {Hz}}$ at the MIR wavelength of $3.39~\mu \text{m}$ . Here, we showed that the limiting factor is not the noise from the upconversion process (estimated NEP is 2.3 fW/ $\sqrt {\text {Hz}}$ at $3.39~\mu \text{m}$ ) but from the electrical noise in the photodetector itself. We also compared the performance of our UCD with the previously published results and with market available direct MIR detectors. In addition, we measured the optical noise of the UCD over its working spectral range (2.9– $3.6~\mu \text{m}$ ) and compared with the numerical simulation.