## 3 The Use of Supercontinuum Lasers in Biomedical Diffuse Optics
### Novel Instrument Development
[PERSON] and collaborators used a commercial SCL combined with a ICCD camera to perform functional brain imaging with 2 wavelengths at a frequency of 3 Hz [67]. This system was an upgrade of a previously developed instrument based on pulsed Ti:Sapphire laser [68]. This source was tuneable but the switching time between wavelength was too slow to enable dual-wavelength functional imaging. Therefore, the authors report that the use of SCL unlocked this possibility with their detection scheme. Similarly, [PERSON] and collaborators described a TD-NIRS system developed to measure the human brain tissue physiology [69; 70]. This system was based on commercial SCL for the emission and on an ICCD camera coupled with an imaging spectrometer in order to perform the spectral decomposition. This system allowed to retrieve the typical hemodynamic response, and preliminary results on the detection of oxCCO were also performed. The ability to retrieve oxCCO is exiting for brain applications, as it has been shown that this contrast can be more specific than the oxygenation, and that it can be an early biomarker of neonatal encephalopathy for example [7]. This possibility has been explored by the UCL team by developing a 4-channel 16 wavelengths TD-NIRS system in order to monitor both the oxygenation and oxCCO signal [24; 71]. This system was based on time multiplexing, with a TCSPC detection scheme, but the fast-switching capabilities of the SCL coupled with AOTF allowed the acquisition of all the 16 wavelengths in less than 2 s. This system has also been used to evaluate the long-term reproducibility of the evaluation of cerebral tissue saturation in healthy adults with good reproducibility [72]. This system can be seen in Figure 2, and in an example of data in Figure 3.  
Another approach that has been used using SCL sources is the development of non-contact systems. This can present several advantages to avoid issues attributed to sensor-tissue contact like skin compression and to obtain a reduction in measurement preparation time. A team at the Physikalisch-Technische Bundesanstalt (PTB) has developed a non-contact TD-NIRS system based on a commercial SCL and fast-gated SPAD to explore the human brain physiology [73; 74; 75; 76]. At the time, the authors reported that the use of SCL permitted a shorter pulse width (\(<\)100 ps), an achievable output power considerably larger than picosecond diode lasers, and the possibility to quickly switch between two different wavelengths (i.e., on the \(\upmu\)s to ms time scale). Moreover, the use of the SPAD in that system enables to reject the burst of early photons, which can improve the spatial resolution of the system at very short detector distances [77]. This process had been previously demonstrated by the POLIMI team that had develop a single channel TD-NIRS system based on SCL and a fast gated SPAD detector [78]. The POLIMI team has also worked on the modelling of the parameters of such detection schemes, in order to optimise their design [79]. The non-contact scheme is also very useful for pre-clinical applications. Indeed, several systems, based on SCL for the source, aiming at collecting physiological information of small animals, have been proposed. In the early 2000s, a system based on a home-made SC generation associated with an imaging spectrometer coupled with a streak camera was developed and used to monitor the cerebral hemodynamic of songbirds [80; 81]. More recently, the UCL team have developed a system aiming at investigating both the cerebral hemodynamic and metabolic response (via oxCCO) of the exposed cortex of rodents [25]. In this work, a commercial SCL was used in combination with a dispersive prism, for the emission, and a sCMOS camera, as a detector, to record 11 images (11 wavelengths) of the exposed cortex in less than 0.3 s/wavelength. Here the recording was not time resolved, but the power of the laser and the ability to quickly switch between the wavelength had dictated the choice of this source.