## 3 Instrumentation/Technology for Broadband TD-DOS
As sources, two tunable mode-locked lasers were used, namely, a synchronously pumped dye laser in the 610-700 nm range and an actively mode-locked Ti:Sapphire laser in the 700-1050 nm range. For detection, a microchannel plate photomultiplier (MCP) with S1 surface (R1564U, Hamamatsu, Japan) was used, followed by a chain for TCSPC (SPC130, Becker & Hickl, Germany). The key complexity of this system resided in the handling of the laser sources. Both of them were tunable lasers whose spectral scanning required delicate alignment to provide optimal lasing over the entire range but also automatic control--operated via mechanical actuators--for rotating the intracavity Lyd's filters. In addition, the Ti:Sapphire laser demanded automatic switching of the output mirror to avoid spurious laser emission as well as a fine adjustment of the cavity length to compensate for chromatic dispersion. A replica of the instrument response function (IRF) was implemented to compensate for variations in pulse shape and detector response during the wavelength tuning and to provide real-time feedback for automatic optimization of the cavity length. This reference IRF was achieved by splitting a small portion of the laser that was directly fed to the MCP. Despite the inherent complexity of the system, the fully automated operation was achieved, permitting the first acquisition of in vivo absorption and scattering spectra of various biological tissues by time domain diffuse optics over a wide spectral range (600-1000 nm) [61]. Surely, the adoption of TCSPC permitted to reach high sensitivity and dynamic range, and cover the critical region of high water absorption (above 900 nm), where signal attenuation is dramatically increased as compared to the 700-800 nm range. Figure 4: Broadband time domain system based on supercontinuum generation and parallel streak-camera based detection developed by Lund University [60]. The introduction of compact commercial supercontinuum sources based on fiber amplified lasers coupled to photonic crystal fibers [62; 63; 64] and of rugged silicon photomultipliers [65] usable in diffuse optics [43; 66] made it possible to realize movable rack-based systems suitable for use in clinical settings, as shown in Figure 6[23; 67]. Automatic wavelength selection of the supercontinuum source (SC450, Fianium, UK) was achieved by dispersing the light using a rotating prism and selecting a narrow band with a slit. The system was mounted on a cart and permitted non-invasive clinical studies on the application of broadband diffuse optics to bone tissues [29] and thyroid [68]. The implementation of a TCSPC detection scheme--granting single-photon sensitivity, but operated in a parallel configuration--required the use of multianode detectors and multichannel TCSPC boards. A realization is shown in Figure 7[69]. The source was a supercontinuum generated in a photonic crystal fiber (NL-2.4-800, Blaze Photonics, UK) by a self-locking mode-locked Ti:Sapphire oscillator (Tissa 50, CDP Systems Corp., Russian Federation). Diffusely re-emitted light was dispersed by an imaging spectrometer (SP-2150, Acton Research, USA) and detected by a 16-channel multianode PMT (R5900U-01-L16, Hamamatsu, Japan) connected to a TCSPC board (SPC-630, Becker & Hickl, Germany). Routing bits, associated with the PMT channels hit by the impinging photons were processed by the TCSPC router electronics (PML-16, Becker & Hickl, Germany) to separate signals arising from different wavelengths. This configuration enabled the fully parallel acquisition and permitted to track dynamic variations in absorption and scattering spectra following hemodynamic changes. The key constrains were a limited spectral range (520-850 nm) imposed by the available multichannel PMTs as well as reduced dynamics due to the need to accommodate all wavelengths with different signal attenuation in the same acquisition. Figure 5: Broadband time domain system based on sequential tuning of mode-locked lasers and time-correlated single photon counting developed by Politecnico di Milano [22].