## 2 History and Basic Principles of Supercontinuum Lasers  
Supercontinuum (SC) lasers are broadband, coherent light sources that are capable of combining the wide spectral range of conventional white lamps (including UV, visible and IR light) with the optical performances of single-mode lasers, in particular their high output intensity, the capability of fast, pulsed emission, and the fine collimation of the beam down to diffraction limit. Such characteristics make this type of photonic technology highly versatile for a number of applications, in particular in biomedical optics and biophotonics, thanks also to recent advancement towards making SC lasers more compact and affordable [14]. The physical process of optical SC generation was discovered and studied in the 60 s and 70 s with milestones publications by [PERSON] and [PERSON] [15; 16]. It is based on the propagation of extreme non-linear effects from a narrow-bandwidth pulsed laser beam in order to generate a broadening of the overall spectrum of the emitted light. These non-linear effects typically include self-phase modulation, four-wave mixing, Raman scattering, and solitons dynamics, among others [14; 17]. The propagation of these effects is performed by dispersing the pulsed laser beam through a non-linear material. These can range from bulk materials, such as simple water cells, silicas, or gases (as in the earliest attempts of SC generation), to more advanced crystal-structured optical fibres [17; 18]. Early SC lasers using bulk materials for non-linear propagation were limited in the efficiency of coupling the input laser light with these propagating material (coupling efficiencies stood at below 10%) and thus they were able to only achieve ranges of about 200-300 nm, with pulses duration in the order of tens of nanoseconds. Further developments in SC generation efficiency and coupling using optic fibres lead to enlarging the range up to 1-2 um and increasing the pulse duration to the range of picoseconds (for commercial, compact lasers), or even femtoseconds (for research-grade, benchtop lasers) [18]. A critical turning point for modern SC lasers to achieve such high performances was the invention and application of photonic crystal fibres (PCF), a particular type of microstructured optic fibre (Figure 1). PCFs, invented in 1996 [19], are currently the most used non-linear propagating materials for SC generation [17; 18]. As the name suggests, this class of optic fibres is based on recreating the optical properties of crystalline structures, in particular their photonic bandgap effect, by combining various high and low refractive indexes in their inner structure. Such features can be achieved in different manners, which typically define the given category of PCFs [20]: (1) hollow-core PCFs present a central hole in their structures, filled with air; (2) hole PCFs are composed of a matrix of several air-filled holes in their cross-sections that is interspersed with the core material; (3) solid-core PCFs involves a solid core surrounded by a matrix of air-filled holes. PCFs offer high coupling efficiencies and low power losses, thus allowing ultra-broadband, high-brightness SC generation at cost-effective access [17; 21]. SC lasers based on PCFs used in biomedical optics applications can typically generate broad spectrum spanning from 350 to 2400 nm, generally showing a central sharp peak which is a residual of the original shape of the pulsed beam. An example of this is shown in Figure 1. Most commercial SC lasers show performances similar to pulsed laser diodes, with integrated output power up to tens of watts and the capability of generating pulses in the order of picoseconds and repetition rates of tens of MHz [22]. Finally, we can report that in the recent year, more companies are able to provide ready-to-use SC solution.