ANALYSIS: CO2
the discharge. Therefore, the rise and fall times of the pulses are much shorter, allowing a much higher pulse repetition frequency before successive pulses begin to overlap. Ultimately, the RF-excited
laser is able to operate at pulse repetition frequencies from single shot to hundreds of kilohertz. The shorter rise and fall times of the RF-excited laser result in much less peripheral damage and heat-affected zone, outside of the intended processed area; therefore, combined with the excellent pulsing characteristics of the power supply, a much wider range of high-speed printing, perforating and drilling applications are possible.
Beam quality RF-excited slab lasers ensure a high-quality, near-Gaussian distribution by utilising the combination of waveguiding between the slab electrodes and beam correction. This
is extremely important for applications requiring high- quality, low kerf widths and low heat-affected zones where multimode profiles create features and damage outside the required processing area. DC-excited lasers have limited mode selection largely to ensure the maximum output power. Typically, this results in lower mode and machining quality than RF-excited lasers.
Lifetime DC-excited glass lasers have lifetimes ultimately limited by ‘cathode poisoning’. This is the process of positively charged molecules generated in the gas discharge reacting with the negatively charged cathode, limiting the generation of electrons and, subsequently, the power transferred to the discharge. This causes a drop in the output power before, eventually, emission ceases. RF-excited discharges do not suffer from this effect.
Although improvements in glass tube lifetime have been enabled by the introduction of catalysts, quoted lifetimes for DC-excited glass laser tubes are still, at best, only half of those for RF-excited lasers. In fact, anecdotally, it is apparent that often the lifetime of an RF-excited laser is several times longer than that of the DC-excited glass laser. When glass laser tubes are replaced, often they cannot be regassed or refurbished and so are simply destroyed. However, RF-excited lasers can be regassed and refurbished more than once, significantly reducing the waste created.
Cost
A simple comparison of the initial cost of a DC-excited glass laser tube and RF-excited laser shows that the glass laser tube would be cheaper. However, this is often an over- simplification. The increased capability of the RF-excited laser
LASERS
and significantly longer lifetime ensures that the RF-excited laser is not only the most cost- effective choice, but the only choice for many applications.
Conclusion DC-excited glass tube laser sources have their place, generally in applications where processing speed, precision and quality requirements are relatively low. However, the more robust RF-excited laser technology is the choice for more demanding applications, particularly where the laser is required to operate at high speeds and in harsh environments, and where high- quality, repeatable results are essential. In these applications, the long service life and consistent performance of an RF-excited laser represent a sound investment. l
Dr Jason Lee is technology and innovation director at Luxinar
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