We present optical fiber measurements for cell state determination via simultaneous observation of both electrodes of a lithium-ion-battery. The integration of fiber optics is done using commercial electrodes set up in pouch configuration. This system extends the method, shown previously for the graphite anode. Lithium iron phosphate cathodes suffer from poor optical observability due to the addition of carbon black as conductive agent. Hence, carbon black is locally replaced by an electrochromic marker additive. Transmissions through integrated fibers were monitored during battery cycling and results show that the cell state of charge directly influences both transmission signals. Optical data can be utilized as a parameter independent of electrical measurements to estimate the state of charge.