Not sure why you're fiddling with an ancient NE555.
Those chips need a 15volt supply to reach proper specs.
At 5volt VCC they can only sink (LOW) ~5mA and sourcing (HIGH) drops to <2volt with some current.
What is the input impedance of this "laser". BNC indicates a possible 50 ohm termination resistor.
An Arduino can generate those pulses cleanly, but you might have to look at a mosfet driver chip to boost the Arduino signal to the voltage/current/impedance you need.
How long is that 'transmission line' (coax cable) to the laser.
The wrong cable and termination will kill your rise/fall times.
Leo..