In transmission mode, the photons are forward scattered, as opposed to reflection mode, where they are backscattered. The reflection mode technique requires a very sensitive receiver, especially when the absorption coefficient is high in the homogenous media. As a result, a transmission mode technique is adopted here because there is a higher signal-to-noise ratio for a similar receiver separation. The overall measurement setup with custom-designed optical Tx and Rx is depicted in Fig. 2. An automatic network analyzer (Anritsu MS4623B; Test Equipment Connection, Lake Mary, Florida) was employed to measure the forward IP and IL of modulated photons, where the rf source used for modulating optical Tx is compared to a very sensitive received rf. The customized optical Tx used the rf switch from Hittite (HMC245QS16) to drive three high-powered vertical cavity surface-emitting lasers (VCSELs) (at 670, 795, and 850 nm). A printed board was designed and fabricated using a commercial substrate (FR4) to accommodate all surface mount technology (SMT) components. Details of this optical Tx was discussed in 16, and it includes a tri-wavelength source from Vixar (Plymouth, MN; Module V3WLM) with pin designations for three wavelengths: 680, 795, and 850 nm. High-powered lasers are employed with each VCSEL’s threshold currents: 8.5 mA for the 680 nm, 9.5 mA for the 795 nm, and 2.8 mA for the 850 nm. Even though in other publications,26,27 we have described a custom-designed optical Rx with a lower-operating voltage, better light collection efficiency, and better sensitivity than the available commercial avalanche photodiode (APD); however, throughout this set of measurements, we have used optical Rx based on APD module C5658 (Hamamatsu Phonics, Tokyo, Japan) along with a built-in trans-impedance amplifier (TIA). This commercial module provides high-sensitivity optical measurements with a spectral response in the range of 400 to 1000 nm. The built-in TIA provides 34 dB of power amplification gain up to 1 GHz. The overall optical sensitivity is at 800 nm, with a compression dynamic range of about 70 dB for all three NIR wavelengths of interest. We previously showed in 16 the full capability of our built-in system. More specifically, it demonstrated a novel extraction procedure using the analytical DE method for homogenous phantom resembling white brain matter for the fNIR three wavelengths of interest.