As the summary, carrier relaxation dynamics in non-doped BSO and Cr doped BSO is studied using the PIA change by nanosecond pulse excitation using a frequency doubled Nd:YAG laser. The time-resolved PIA decay in non-doped BSO relaxes in a few hundred milli second whereas for the Cr doped BSO, this happens in time of the order of hundred seconds. The Cr defects in BSO increase the density of the shallow and deep trap levels that increase relatively the life time of the charge carriers compared to non-doped BSO. The microoptical mechanism for the PIA decay is related to the carrier tunneling between localized state of each Cr ions than carrier trapping. From the optical absorption spectra measurements, the Cr doped BSO has shifted the intrinsic edge to the longer wavelength and gave rise to new absorption bands in the visible and NIR ranges.