# Editorial Board Members' Collection Series: Nonlinear Photonics
## 2 An Overview of the Articles in This Special Issue
Stimulated Raman scattering (SRS) microscopy is a high-speed imaging modality based on intrinsic molecular vibrations, producing chemical maps in living systems. Such capability, allowing for direct visualization without the perturbation of biological processes, has enabled a plethora of biological and medical applications. In reference [12], after introducing the basic theory and competitive effects of SRS, some crucial features for SRS microscopy implementations, such as noise, spectral bandwidth, speed, chemical sensitivity, spatial resolution, and quantum enhancement, are discussed. Finally, some SRS applications in biological and medical imaging are described. Even if certainly not exhaustive, the review aims to offer a broad overview, providing guidance for newcomers and hinting at a more detailed investigation to interested researchers in this rapidly growing field. Free-space optical communication has seen a surge in interest, both for terrestrial and last-mile solutions, as well as optical satellite communication. Current efforts have focused on utilizing the short-wave infrared (SWIR) atmospheric transmission window, containing the 1530-1565 nm C-band used in telecommunications. However, SWIR communication suffers significantly from scintillation, scattering, and weather effects. The 3-5 \(\upmu\)m mid-infrared (MWIR) atmospheric transmission window should offer improved signal-to-noise ratio and resistance to these adverse effects and could lead to higher link availability and reliability. In reference [13], a diode-pumped solid-state (DPSS) laser, used for intracavity pump-enhanced difference frequency generation (DFG) to create a 3.5-micron laser, is reported. Using a 50 mm long periodically poled lithium niobate (PPLN) crystal inside the cavity of an Nd:YVO\({}_{4}\) solid-state laser at 1064 nm with 4.5 W pump power at 808 nm, and a 310 mW C-band signal at 1529 nm, up to 31 mW of mid-infrared output power at 3499 nm is obtained. The cavity requires no active stabilization and/or locking, and the entire cavity is \(<\)8 cm in length. The obtained output power corresponds to a black-box efficiency of 2.20% W\({}^{-1}\), which is the highest value reported to date for continuous-wave DFG based on a bulk nonlinear optical crystal with no active stabilization. Potential future applications in free-space optical communication are also discussed. Traditional microwave signal processors relying on electronic devices exhibit significant loss and strong crosstalk, which make them suffer from limited operation bandwidths. To overcome this restriction, microwave photonic (MWP) signal processors that perform signal processing functions based on MWP technologies have found wide applications in telecommunication and radar systems. For MWP signal processors implemented by the transversal filter systems, a large number of taps, or the wavelength channels provided by multi-wavelength optical sources, are required to improve their performance. Compared to other multi-wavelength optical sources, optical microcombs can provide significantly increased numbers of wavelength channels by using compact micro-scale resonators, which is critical for improving the processing accuracy of MWP transversal signal processors. Although a range of signal processing functions have been realized, they only used Gaussian input waveforms for demonstrations, while the ability to handle various input signal waveforms is essential for practical applications. In reference [14], the capability of microcomb-based MWP signal processors for dealing with various input signal waveforms is experimentally demonstrated. The processing accuracy of different input waveforms, including Gaussian, triangle, parabolic, super Gaussian, and nearly square waveforms, is quantified. These results provide guidance for microcomb-based MWP signal processors when processing microwave signals of various waveforms.