Troubleshooting Frequency Dropout Issues in Instrument with Photodetector

Hello, I am working with a instrument where we frequently get the problem of frequency drop out, i.e. suddenly frequency signal goes missing i.e. goes to zero, this cause the loss in data as shown in the below image. Yellow is received frequency signal from Photodetector and Purple is the demodulated data (our desire output). When frequency goes zero the purple line is constant so data is lost.
Can you help me find a solution for this problem!