In that case I would go with the opto isolator suggestion in reply #16. There is no need to have a 7805 voltage regulator and it takes the -28V grounding out of the question.
First test that your Laser and arduino work together without the input voltage. If that works then use the -28V to drive the LED side of the opto through a sutiable resistor. Have the transistor side connected between the arduino input pin and ground and enable the internal pull up resistors.