Some of my student and I are designing a Physics experiment involving a high power LED, a square wave from function generator (50 ohm output impedance) and a digital oscilloscope (1 MOhm imput impedance). The output of the function generator has two BNC cables. One goes into the oscilloscope channel and then other goes into the LED. The LED is coupled with a light sensor (ThorLabs DET36A/M and the output is connected to the another channel on the oscilloscope. Both these channels are plotted on the same channel.
We gave a square tooth wave of amplitude 0.5 V with a off-set of 1.5 V (so it fluctuates between (1.5-2 V). I was expecting that the light pulse from the oscilloscope would follow the same pattern as the square wave (of course with a smaller amplitude). But instead we got the waveform in the attached picture.
The confusing part is the decaying peak even when the voltage is 1.5 V. This trend is similar even if I set the minimum voltage to zero in the cycle. If I extend the duty cycle of the function generator I get the picture attached above.
I am unable to understand this pattern like a RC circuit. I understand some capacitor is involved somewhere, very possibly in the photosensor but I am not able to understand what is going on physically and how to fix this.
There is a known impedance mismatch between the oscilloscope and the function generator.