Using the Yokogawa2400 spectrum analyzer I measured the correlation between the signals coming from the two photodiodes:

- PM100D (incident power 85mW)       DC 470mV, analog out
- DET10N (incident power 15microW) DC 104mW (load resistance 6kOhm) 

The signals don't look coherent. It could be that they are really not coherent, or that the spikes at high frequencies invalidate the spectrum measurement.