Photodiode in photovoltaic mode, overcoming saturation
Because Si junction has 0.6 V knee and is in parallel to the current source represented by photons reaching the PN junction. So if the voltage drop on the 50 ohm resistor caused by current generated by photons is greater than 0.6 V the parallel diode begins to conduct and seems that saturation is reached. When you place a 1 ohm resistor the maximum illumination can be 50 times greater before 0.6 V is reached.