## 4 Applications
### Optical Spectroscopy beyond 1000 nm  
All the applications presented in the previous paragraphs rely on the use of silicon detectors, thus the wavelength range in which they are used is limited to about 1000 nm. However, in the region from 1100 to 1700 nm, some compounds such as collagen and lipids have spectral features which are of the utmost importance for clinical purposes (e.g., breast cancer risk assessment is correlated with lipid, water and collagen concentration [156]). To this extent, InGaAs/InP SPAD detectors represent a good solution thanks to their high photon detection efficiency in that range.  
The exploitation of these kind of detectors allows one to explore this new spectral region, thus opening up new applications. However, as anticipated in Section 3.2, the main disadvantage of the InGaAs/InP is represented by the high level of background (due to both dark count rate and after pulsing). For this reason, in addition to a moderate detector cooling [157], a TG approach is needed to decrease the contribution of the noise, thus avoiding the detector saturation. In this case, due to the different aim of the gating (suppressing the noise and not rejecting the early arriving photons), there is no need for a sharp rising edge.  
To the best of our knowledge, in diffuse optics a TG InGaAs detector was exploited only in two different papers. In the first one by [PERSON] et al. [158], a phantom validation was presented together with a broadband characterization of a sample of pig fat, while in the latter one (by [PERSON] et al. [159]) the optical characterization of collagen till 1700 nm was shown.  
In both cases, the InGaAs/InP SPAD employed was cooled down to 230 K and gated [160]. The resulting photon detection efficiency was good (i.e., maximum value: 42% at 1100 nm while it is still 15% at 1600 nm). In Ref. [158] it was demonstrated that the system was linear in the recovery of the absorption coefficient, with a range where linearity is preserved depending on the fitting procedure. If the diffusion approximation to the radiative transfer equation was used, a good linearity was maintained till \(\mu_{a}=2.1\) cm\({}^{-1}\) (with a displacement from the linear trend lower than 27%). On  
Figure 16: Relative blood flow index (BFI) during an arterial cuff occlusion recovered from the hardware correlator for both non-gated acquisition and gated one (figure generated with data taken from Ref. [155]).  
the other hand, using a Monte Carlo fitting, the range of linearity reached a value of \(\mu_{a}=2.6\) cm\({}^{-1}\) (displacement from the linear trend <3%). In both cases, no saturation of the recovered absorption value was reported. For what concerns the scattering, the authors proved that using Monte Carlo fitting, also the retrieval of the scattering coefficient was linear till the lower value of 4.5 cm\({}^{-1}\), where the relative error reached 18%.  
For what concerns the recovery of the optical spectra, in Ref [158] the authors were able to measure a lipid sample in the range from 1100 to 1700 nm, retrieving a similar trend to the one published by [PERSON] et al. [161]. Few years later, the same group published the characterization of collagen from 500 to 1700 nm [159]. In this case their setup was based on two different detectors, namely a SiPM module [162] for the range 500-1100 nm and an InGaAs/InP SPAD in the 1100-1700 nm range. The authors were able to retrieve the absorption spectrum of collagen over the whole 1100-1700 nm range. Additionally, thanks to the knowledge of the collagen spectrum, they were able to recover the main constituents of an absorption spectrum recorded in-vivo on a bone prominence location.