ANALYSIS: CO2
LASERS
DC-excited glass laser tubes versus RF-excited, metal slab laser tubes
Dr Jason Lee, of Luxinar, reviews competing CO2 laser technologies in terms of processing capability, relative lifetime and cost
In most applications, the processing capability of a laser is determined by its average power, pulsing capability, beam quality and wavelength. Each of these parameters is determined by the engineering of a specific laser design. In particular, in the realm of CO2
power, pulsing capabilities and beam quality vary greatly between direct current (DC)- excited glass laser tubes and radio frequency (RF)-excited metal slab lasers.
Average power The power scalability of DC-excited glass laser tubes is limited by the discharge excitation and related geometry. A light-producing discharge is created by applying a large DC voltage between a
“The long service life and consistent performance of an RF-excited laser represent a sound investment”
26 LASER SYSTEMS EUROPE SUMMER 2022 lasers, the average
cathode and an anode, each located at opposing ends of the discharge. A high electric field of ~20kV/m is required to initiate the discharge and ~13kV/m to sustain it. Handling of these high voltages safely is not trivial and can add unexpected, unseen complexity. The output power of the laser is increased by increasing the length, with typical discharge lengths between 0.5m to 2m to produce output powers between 20W and 160W. The long discharge lengths and high switching voltages limits the power scaling of a single glass tube. Polarisation combining of two glass tubes is possible to increase the maximum output power to ~300W, but this results in crossed polarisation at the output. Some optical arrangements include polarisation-sensitive components, such as beam splitters, acousto-optic modulators (AOMs) and optical isolation, which will not function correctly without
linear polarisation. Therefore, cross-polarisation can limit the applications that combined glass laser tubes can undertake. RF-excited slab lasers do not
suffer the same limitations. RF excitation enables the creation of gas discharges over large areas. Unstable resonators are naturally matched to the geometry of the discharge and so can usefully extract light from the whole discharge. So the combination of RF excitation, slab-like electrodes and unstable resonators result in a laser output that is scaled relative to the area of the discharge, not its length, producing a far more compact device. In fact, single laser tube devices are commercially available with operating output powers between 20W to 1,000W when sealed.
Pulsing capability The average power of both DC-excited glass laser tubes and RF-excited metal slab laser tubes are varied by pulsing the laser. The pulsing capability of a glass laser tube is limited by the difficulty of switching high voltages, typically >10kV, and the low gas pressure required for consistent striking of the discharge. The low discharge pressures result in long rise and fall times, producing modest pulse repetition frequencies before successive pulses overlap and the output no longer consists of distinct pulses. Pulsing of an RF power supply within an RF-excited laser is far more straightforward. The switching voltages are much lower and the small gap between the slab-like electrodes allows much higher pressures within
Luxinar’s OEM series of CO2
lasers employ the RF-excited slab laser design @LASERSYSTEMSMAG |
WWW.LASERSYSTEMSEUROPE.COM
Luxinar
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