## 3 The Use of Supercontinuum Lasers in Biomedical Diffuse Optics
### Novel Instrument Development
The system was however not based on a supercontinuum laser generation but on two tuneable lasers for the source, and the detection scheme was based on time-correlated single-photon counting (TCSPC), enabling light detection down to the single photon, which drastically improves the sensitivity of the system. This increased sensitivity is of great advantage to reduce the acquisition time and improve the light collection in the region where water absorption is high (900-1050). However, handling two lasers drastically increases the complexity and bulkiness of the system. Nevertheless, with that system, that team was able to record the optical properties of absorption and scattering of various tissues invivo [34, 35, 36, 37]. The availability of commercial SCL sources gave rise to a new version of that system, where the 2 lasers were replaced by a SCL source coupled with a prism for the spectral selection [38]. This new source allowed to make the system more compact and transportable making it more suitable for the clinic. Moreover, the detector was also replaced (previously photomultiplier (PMT)) by a Single-Photon Avalanche Diode (SPAD) which enable to increase the spectral coverage, allowing a measurement between 600 and 1000 nm. This system could be used to characterise the optical and physiological properties of the tissue of the female breast. Another version of that system was developed, combining the SCL with a Ti-Sapphire laser to expand the spectral coverage to 1100 nm [39]. The SCL was used to cover the 600-900 nm and the Ti-Sapphire was used between 900 and 1100 nm. Indeed, even though the SCL could cover the entire considered wavelength band, the spectral resolution provided by the couple SCL/dispersion prism was too low to provide accurate results, as the source bandwidth increased with wavelength from 3 nm at 600 nm to 10 nm at 900 nm, and to approximately 15 nm at 1100 nm with that solution. This instrument has indeed been used to demonstrate that the broadening of the spectral bandwidth effect could impair the results, as the spectral region within the bandpass that exhibits the lowest absorption will dominate, leading to significant underestimations of the absorption and distortions in the spectral shape [40]. The spectral quality of the acquired data being extremely important, having the narrowest bandwidth is a great advantage. To do so, using a SCL is thus a true advantage as it provides a high power across the entire spectra. However, the method used to filter the white light is very important. As it has been seen above, several techniques can be used, either filtering on the detection side with a spectrometer, or on the source side with a dispersive prism or using an acousto-optic tuneable filter (AOTF), as we will see in systems described below. The POLIMI team has compared the wavelength selection from a SCL with an AOTF and a prism. They have  
Figure 4: Summary of the literature review results. **(a)** Graph showing the number of publications using SCL in biomedical diffuse optics since the first study in 1993. **(b)** Summary of the main focus of the papers reviewed. shown that one had to be careful when using AOTF, as they can produce side lobes in the selected bandpass, which can drastically impair the retrieval of the optical properties [41]. Nevertheless, the extension of the spectral coverage offered by this newly developed system was of particular interest, as several components, like lipid and collagen, have a distinct signature above 1000 nm. The LLC group also reported a similar system, based on a commercial SCL coupled with 2 AOTF and a detection scheme based on TCSPC with a coverage between 650 and 1400 nm [42]. The POLIMII group has then extended this work by focusing on the spectral window between 900 and 1700 nm, by using an InGaAs/InP SPAD as a detector [43; 44; 45].