oa Editorial [Hot Topic: Recent Progress on the Frontiers of Optical Signal Processing (Guest Editor: Y. Ding)]

INTRODUCTION Optical signal processing is a quite wide area with the combination of optoelectronic technologies and signal processing technologies. It deals with the various optical signal operations including optical logical operation [1], imaging [2, 3], encoding and decoding [4], data transmission and exchange [5], encryption [6], etc. Owing to the advantages of high-speed and high-capacity signal processing ability, wide spatial imaging scales, and unique terahertz (THz) interactions with biological systems etc, optical signal processing has been widely used in a range of applications, in particular, optical fiber communications [7-9], biomedicical imaging [10-12], sensing [13], security screening [14, 15], vision chips [16, 17], information security [18] etc. Photonic computer might be one of the most potential applications of this field in the future. Lately, far infrared (IR), or THz wave imaging technologies have grown up due to the ever-increasing demand for the security screening as well as medical diagnosis. THz wave imaging technologies have many excellent advantages over those imaging technologies using other electromagnetic spectrum. The THz wave can penetrate a wide range of materials which are opaque to visible or near IR light or produce only low quality images in X-ray region. It has advantages of low damage to the biological tissues and high image resolution. Currently high-power, pulsed THz radiation sources are already commercialized and used in research and industry, however, the need for more efficient and compact THz signal emission still leads to the continuous development of novel quantum-dot based THz devices, which will be introduced in this issue. Although optical holograms have been used in authentication and security applications for several decades, the impact of optical systems on the information security systems becomes much more impressive nowadays, e.g., in the area of optical data transmission. Benefits of optical systems over other traditional electronic systems emphasize that optical processing provides the maximum number of degrees of freedom and that optical encryption is ideal for information storing in holographic media. Various novel schemes have been proposed in the last decade. In this issue, we review and discuss the recent progress on the patents and research articles on the frontiers of optical signal processing. We mainly focus on the latest development of far IR THz imaging technologies, quantum-dot based THz radiation sources and optical information security systems. This special issue aims to bring the researchers insight and prospect of the relevant issues concerning optical engineering, mathematics, imaging as well as optoelectronic devices....