DC-excited vs. RF-excited CO2 lases

laser tubes used in medical CO2 lasers use DC or RF excitation to energize the laser gas.  The main difference in performance is related to the fact that systems with DC-excited tubes typically enable the user to adjust both power level and pulse width, whereas RF-excited tubes, such as that used in the Ultrapulse, only allow the user to vary pulse width. The power level for  RF-excited tubes is usually set by adjusting the duty cycle at a fixed power level resulting in an adjustment to the average power during the pulse.

Also DC-excited lasers can provide enhanced pulses (superpulse), up to 10x power in the leading edge of the pulse, whereas RF-excited lasers generally cannot. This is due to the small gas cavities used in RF-excited tubes. Because of the very small discharge volume and the metal and ceramic construction of RF excited tubes the discharge runs very efficiently and at maximum sustained power levels for several milliseconds.

DC-excited tubes have much larger gas volumes for the same power levels. The discharge is not as effectively cooled so after a few hundred microseconds drops to a steady state power level. In the first 100 microseconds or so the laser tube can sustain discharge powers many times the steady state power, the level being determined by the discharge volume.   When short pulses are required, such as in fractional treatments, the DC-excited laser can have  a substantial cost and performance advantage, at the cost of lower wall plug efficiency. however the running cost of all these lasers is very low. 

In summary, for fractional applications the DC-excited laser typically has thee significant advantages over RF excited laser; 1) ability to easily adjust pulse energy independent of pulse width, 2) the ability to deliver an enhanced pulse, and 3) much be greater value to cost ratio. 

 

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