N-Channel Mosfet to switch a -48V to -54V Power Source
Here is a circuit that would do the task



Replace the pulse source with the opto output; i.e. the optotransistor is connected across ground to the junction of R2 and R3.

R3 is present so that the dark current from the optocoupler does not turn on Q1. 

An advantage to using a zener is you get the same gate drive regardless of the value of the source being tested(-48V to -54V).

I would probably add a resistor across the zener to provide a voltage divider just in case it goes pop for some reason to provide some gate protection so that the maximum Vgs is not exceeded.

Note that I used a 2N2222 because it was handy; I would look to size the part appropriately for a final circuit.

You should probably heat sink the FET you want to use as it has 0.1 ohm on resistance (at 30A, that's 90W).