## 3 The Use of Supercontinuum Lasers in Biomedical Diffuse Optics
### Methodology
Such systems based on an integrating sphere and several light sources, including a commercial SCL, have been proposed to spectrally characterise and calibration optical systems [105]. More recently, a digital phantom has been developed to mimic TD-NIRS signal acquisition, and the authors used various sources and detectors in order to test this approach, including a SCL [106]. One other important component to optically characterize is the optode holders used to secure the optical fibres to the patient. In a recent study, [PERSON] and colleagues used a system previously developed to evaluate the optical properties of fruit [107] (based on a commercial SCL and a filter wheel to select 14 spectral band in the range 550 to 940 nm) to spectrally characterised poly lactic acid (PLA), a material used for3D printing [108]. As 3D printing is more and more common in order to produce probe holders, it is important to optically characterise the material used in order to avoid any unwanted effects on the measurement provoked by light interaction with the material. In that study, the authors concluded that the different material tested were not optically the same, and that characterisation was important before their use. The development of all these phantoms has provided a good framework for the developers to compare their systems and methods in order to provide robust clinical information. Regarding the instrumentation, a lot of effort are currently being made in order to improve the detection scheme of TD-NIRS. To that end, SCL have been a source of choice to test new detectors like the promising fast silicon photomultiplier (SiPM) which drastically improve light harvesting compared to the old PMTs [109, 110, 111]. SCL based systems have also been used to characterise novel bio-compatible fibres that can be implanted in the patient with potential use in the monitoring of the evolution of the physiology after a surgical intervention or in photodynamic therapy (PDT) [112, 113]. Regarding the methodology, several new data processing techniques have been explored and tested in conjunction with SCL. Indeed, system based on SCL can collect multidimensional data that can be coupled together in order to improve the accuracy of the data, reduce the noise or explore new contrast. One of the first work regarding this matter dates from 2006, where [PERSON] and colleagues, showed that one could increase the robustness of the determination of the chromophore's concentrations in tissues (hemoglobin, water, lipid) by using a spectral constraint during the fitting procedure [114]. This showed the strength of using the spectral dimension coupled with the DTOF, which is accessible when using SCL. This method has also been used to monitor absorption changes in a layered diffusive medium [115], which is relevant in the monitoring of brain activation for example. Similar approaches were explored in the same group in order to accurately monitor the spectral changes in absorption coefficients of turbid media [116]. One can also use the spatial dimension in order to enhance the amount of information recorded, by recording data from multiple source-detector distances. This approach has been recently tested and it has been shown that coupling the spatial dimension with the DTOF could increase the accuracy of the estimation of the optical properties [117, 118, 119]. Another interesting use of the spatial dimension is the self-calibrating method for TD-NIRS, which uses the spatial dimension in order to avoid the measurement of the IRF, that is needed to remove the effect of the instrument from the response of the tissue [120]. The characterisation of the IRF necessitates an extra step that can be a burden in a clinical environment. Therefore, this method could facilitate the use of TD-NIRS in the clinic. Finally, we can also report that an approach combining the spatial and spectral dimension proved to also improve the accuracy of the results of CW-NIRS.