Terahertz (THz) noise sources are crucial tools for measuring the noise figure and evaluating the system performance of high-frequency THz devices and systems. They find widespread applications in radar systems, THz imaging, noise figure measurement, and radiometric calibration. Conventional noise sources are based on thermal radiation or solid-state electronic devices, and thus suffer from significant limitations in frequency coverage, excess noise ratio (ENR) and spectral flatness, limiting their applicability in advanced broadband THz systems. Fortunately, the radio-frequency (RF) photonic technology provides a promising alternative. This photonic approach can effectively overcome the inherent bandwidth bottleneck confronted by electronic methods, and show distinct advantages in frequency extension, ENR enhancement, and spectral flatness. This makes the photonic techniques be a research hot in recent years.

We generally review the recent progresses in the field of THz noise source research. Traditional thermal radiation noise sources are limited by low output noise power and thus can not reach a high ENR. As a result, they can only be used as reference standards for calibrating other kind of noise sources. Solid-state electronic noise sources often process high ENR, but limited by the electronic bottleneck, their bandwidth is difficult to be enhanced to the THz domain. Different with them, the RF photonic technologies convert optical noise into electrical noise through photo-mixers such as high-speed photodetectors (PDs), so that they deliver some key advantages simultaneously such as wide frequency range, high ENR, and excellent spectral flatness. In view of this, we emphatically introduce the fundamental principles for THz noise generation, and some typical experimental schemes based on incoherent light sources such as amplified spontaneous noise and laser chaos are reviewed. At the same time, we point that, limited by the high-speed PDs including PIN-PDs and UTC-PDs, only 390 GHz operation bandwidth is implemented using this RF photonic method. Exploring photoconductive antennas to construct new generation of ultrafast photo-mixers may be a promising solution, due to the ultralow carrier lifetime of photoconductive materials.

Focusing on technical requirements and implementation pathways for THz noise sources, this paper systematically summarizes the developmental status of noise generation technologies based on thermal radiation, solid-state electronic and RF photonic technique. Compared with the traditional noise generation scheme, the incoherent optical mixing method based on RF photonic technologies demonstrates significant advantages in operating frequency range, spectral control and output power. However, RF photonic technologies still face several challenges in achieving higher performance and broader application. First, as the core components of THz noise sources, high-speed PDs are limited by material properties and high-frequency coupling and packaging technologies, making it difficult to further enhance the operational frequency of the noise sources. Second, the size of THz noise sources based on RF photonic technologies is significantly larger than solid-state noise sources. This severely restricts applications with strict volume and weight requirements, such as satellite payloads. Furthermore, current applications of terahertz noise sources are primarily focused on measuring the noise figure of millimeter-wave and terahertz devices. To realize broader adoption and functionality, it is imperative to explore their potential in diversified application domains such as non-destructive evaluation, noise radar, and security imaging.