Signal Enhancement by Fiber-Dispersion in Sub-GHz Frequency Domain Biophotonic Diagnosis Systems

Pulsed lasers can be utilized to generate intensity-modulated light at the fundamental frequency of the laser repetition rate as well as higher order modulation (HOM) frequencies in the recent study. We also demonstrated the feasibility of using pulse lasers and fiber-wavelength-convertors as a light source in multicolor frequency-domain (FD) biophotonic systems. In this paper, we present a novel approach to enhancing the intensity of signals below 1 GHz in HOM-based FD systems simply by employing the fiber dispersion effect. The energy spectrum of HOMs can be redistributed to enhance the intensity of HOMs within the bandwidth of the photo-detector without the need for an external (optical or electrical) amplifier. We also demonstrate that the length of dispersive fiber can be optimized via numerical calculations, the results of which are in good agreement with experimental measurements. Finally, we demonstrate a frequency-domain-photon-migration system employing the fiber dispersion effect to enhance sub-GHz signals at a wavelength of 1.03 μm in order to recover the optical properties of turbid samples. Our measurements results demonstrate the superiority of the proposed FD source over the conventional directly modulated FD light source. Furthermore, the simple configuration of the proposed scheme makes it applicable to a wide range of FD biophotonic systems.