Laser firing units (LFUs) provide fiber optic guided infrared radiation pulses to safely and reliably initiate all types of explosive and pyrotechnic output devices—deflagration, detonation, ignition, pressure and gas generators. They also provide microwatt level built-in-test signals and detect the return from PSEMC laser initiated ordnance devices. The units are compact and lightweight, and have small power demand. Ordnance devices can be initiated tens of meters from the LFU, using strong, lightweight fiber optic cables. Typical initiation times are less than 1 millisecond. All parts of the system are highly immune to electrostatic discharge and electromagnetic radiation. PacSci EMC LFUs use fiber optic-coupled high-power laser diodes, one per initiated device, fully enclosed in a faraday cage. Electric power is transmitted into the faraday cage using patented Radio Frequency Attenuating Couplers (RFACs). Signals are transmitted into and out of the faraday cage using only optocouplers. Firing energy is transmitted to initiated devices via silica optical fibers. As a result, the laser diodes and their switching electronics are extremely well isolated from external electric and magnetic fields. Built in test (BIT) is implemented by sending a very low power signal (less than 1% of the no-fire power of the ordnance devices) to the attached devices and detecting a partial reflection of that power returned over a separate fiber. A positive BIT indication is a certain evidence of the integrity of the firing fiber and connection to the ordnance device.