Pulse duty cycle effects on CO₂ laser radiation coupling with a reflective alloy

The effect on radiation coupling of varying the laser pulse duty cycle without changes in the peak power was investigated for a range of average powers by using a Rofin-Sinar RS3000 RF excited CO₂ laser operating in three different pulse modes: pulse width modulated (PWM), gated and mixed. The conventional mode used for the RS3000, PWM, was found to not couple at low average power levels which were easily coupled using both gated and mixed pulse modes. A coupling map is provided and speculation made concerning the physical explanation. Plasma behaviour was monitored using a photodiode. In particular, plasma ignition behavior was recorded in an attempt to correlate coupling behavior with initial plasma formation. Only a modest correlation was found.