This article provides an extensive survey of nonlinear parametric upconversion infrared (IR) imaging, starting from its origin to date. Upconversion imaging is a successful innovative technique for IR imaging in terms of sensitivity, speed, and noise performance. In this approach, the IR image is frequency upconverted to form a visible/near-IR image through parametric three-wave mixing followed by detection using a silicon-based detector or camera. In 1968 (50 years back), J. E. Midwinter first demonstrated upconversion imaging from shortwave-IR (1.6 μm) to visible (484 nm) wavelength using a bulk lithium niobate crystal. This technique quickly gained interest, and several other groups demonstrated upconversion imaging further into the mid-and far-IR with significantly improved quantum efficiency. Although a few excellent reviews on upconversion imaging were published in the early 1970's, the rapid progress in recent years merits an updated comprehensive review. The topic includes linear imaging, nonlinear optics, and laser science, and has shown diverse applications. The scope of this article is to provide an in-depth knowledge of upconversion imaging theory. An overview of different phase matching conditions for the parametric process and the sensitivity of the upconversion detection system are discussed. Furthermore, different design considerations and optimization schemes are outlined for application-specific upconversion imaging. The article comprises a historical perspective of the technique, its most recent technological advances, specific outstanding issues, and some cutting-edge applications of upconversion in IR imaging.

Mid-infrared (MIR) imaging has emerged as a valuable tool to investigate biological samples, such as tissue histological sections and cell cultures, by providing non-destructive chemical specificity without recourse to labels. While feasibility studies have shown the capabilities of MIR imaging approaches to address key biological and clinical questions, these techniques are still far from being deployable by non-expert users. In this review, we discuss the current state of the art of MIR technologies and give an overview on technical innovations and developments with the potential to make MIR imaging systems more readily available to a larger community. The most promising developments over the last few years are discussed here. They include improvements in MIR light sources with the availability of quantum cascade lasers and supercontinuum IR sources as well as the recently developed upconversion scheme to improve the detection of MIR radiation. These technical advances can substantially speed up data acquisition of multispectral or hyperspectral datasets thus providing the end user with vast amounts of data when imaging whole tissue areas of many mm2. Therefore, effective data analysis is of tremendous importance, and progress in method development is discussed with respect to the specific biomedical context.

We demonstrate a robust, compact, portable and efficient upconversion detector (UCD) for a differential absorption lidar (DIAL) system designed for range-resolved methane (CH) atmospheric sensing. The UCD is built on an intracavity pump system that mixes a 1064 nm pump laser with the lidar backscatter signal at 1646 nm in a 25-mm long periodically poled lithium niobate crystal. The upconverted signal at 646 nm is detected by a photomultiplier tube (PMT). The UCD with a noise equivalent power around 127 fW/Hz outperforms a conventional InGaAs based avalanche photodetector when both are used for DIAL measurements. Using the UCD, CH DIAL measurements have been performed yielding differential absorption optical depths with relative errors of less than 11% at ranges between 3 km and 9 km.

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We demonstrate efficient upconversion of modulated infrared (IR) signals over a wide bandwidth (up to frequencies in excess of 1 GHz) via cavity-enhanced sum-frequency generation (SFG) in a periodically poled LiNbO. Intensity modulated IR signal is produced by combining beams from two 1547 nm narrow-linewidth lasers in a fiber coupler while tuning their wavelength difference down to 10 pm or less. The SFG crystal is placed inside an Nd:YVO ring cavity that provides 1064 nm circulating pump powers of up to 150 W in unidirectional operation. Measured Fabry-Pérot spectrum at 1064 nm confirms the enhanced spectral stability from multiple to single longitudinal mode pumping condition. We describe analytically and demonstrate experimentally the deleterious effects of using a multimode pump to the high-bandwidth RF spectrum of the 630 nm SFG output. Offering enhanced sensitivity without the need for cooling, the GHz-bandwidth upconverter can readily be extended to the mid-IR (2 - 5 μm) as an alternative to cooled low-bandgap semiconductor detectors for applications such as high-speed free-space optical communications.