DC and RF Laser Tubes - a Technical Comparison

Learn about the different types of CO2 laser tubes

If you're planning to purchase a CO2 laser, you may want to take a few minutes to educate yourself on the differences between CO2 lasers operated by direct current (DC) and those operated by radio frequency (RF), and ask which type will be installed in your new system. Both types have their own specific advantages and disadvantages (and significant price differences). Below is an overview of the differences between the two types-- including strengths and weaknesses of each and guidance on how to select the right type for your needs.

What is a DC laser?

DC laser sources are typically contained within a glass laser tube. In the glass tube of the DC laser, there is a mixture of different gases: Nitrogen (by far the highest proportion of the gas mixture), carbon dioxide (as the laser-active medium), helium (to effectively dissipate the heat loss), and oftentimes hydrogen and xenon. By applying a high voltage to the electrodes in the tube, current begins to flow. The electrodes must be designed as feedthroughs through the glass body. These feedthroughs are weak points in the laser, and as temperatures fluctuate, the impermeability of the laser tube is difficult to ensure. This is because metals expand more than glass when the temperature rises. Glass and metal thus exert increasing pressure at higher temperatures due to thermal expansion. The sealing is therefore subject to higher forces and may show a leak during cooling (after many thermal load cycles).

The flowing current leads to a gas discharge or to excitation of the nitrogen. This is always the first step (with RF CO2 lasers too) in generating the laser beam. Each CO2 laser also has an end mirror and an output coupler mirror and is constructed as a resonator.

With the energy transfer in the operation of a CO2 laser, a molecule consists of atoms that are connected by springs. CO2 is a triatomic molecule - meaning it consists of three atoms. In this case, it consists of two oxygen atoms and one carbon atom. A nitrogen molecule consists of two nitrogen atoms. The first step is the application of the high voltage to the electrodes (DC lasers) OR the control of the antennas (RF lasers). All processes afterward are identical for DC and RF.

Advantages of a DC laser

DC laser machines are water-cooled, which provides a couple of advantages. Firstly, water cooling enables an extremely quiet operation of the devices. Water cooling also achieves a constant temperature control of the laser - which significantly prolongs the service life. The average life expectancy of a DC laser tube is around two years, which is relativiely short compared to the RF lasers. One advantage of DC systems are the low up-front investment costs. It has a lower beam quality compared to RF lasers, but it it still capable of cutting of textiles, leather and films. One drawback to consider is that if it's used below 20% of the nominal power, it cannot be reliably controlled or emitted with DC devices. In this area, an RF laser of the current generation has the clear advantage, enabling high-linearity power of approximately 2% - 100%.

What is an RF laser?

RF lasers are supplied with energy from power radiated by antennas. As the excitation frequency is in the range of the radio frequencies (typically between 86 MHz and 48 MHz). The radio frequency provides excitation of the nitrogen molecules (vibration). RF lasers can be operated at much higher repetition rates (measured by the number of pulses delivered in one second) than DC lasers. This enables fast engraving/marking, where each contrasting pixel requires a single laser pulse. If the laser resonator is designed as aluminum-oxide-ceramic, this provides the advantage that this material has a negligible characteristic impedance for the excitation frequency. The resonator can therefore be pumped with low loss. As the antennas are outside the resonator, there are no feedthroughs in the resonator that are problematic for gas impermeability.

Advantages of an RF laser

RF laser sources can operate at high repetition rates and have low power drift in operation. In addition, they have higher beam quality.

For example, if paper needs to be marked, the power is set to a low value. The average service life of RF laser tubes is expected to be around six years. After that, an exchange for a new or upgraded laser source is required. With ceramic tubes, contamination of the gas mixture by air flowing in from the outside is rarely or never the case. Often the full laser power can be achieved again by adjusting the power electronics (readjusting the frequency for the antenna driver) or by replacing a defective board (typically a power amplifier defect).

Which laser source is better - DC or RF?

It depends on what your requirements are. DC lasers are clearly the cheapest option if your main concern is up-front cost. However, they need to be replaced after only two years, whereas RF laser sources can be used for almost three times as long. (So in the end, the costs between the two may not be as different as they appear.)

The reason for the striking differences in durability is the material used in the laser tubes. DC systems use comparatively brittle, breakable glass, whereas RF systems use a much more robust metal and ceramic construction.

In addition to acquisition costs, the future purpose plays an important role. If materials with a rather low precision requirement are often to be cut, a laser with a DC system is recommended. These systems have proven to be effective for the textile industry in low-wage countries because a larger kerf (due to the lower beam quality) is irrelevant. The short service life and unpredictable machine downtime make DC systems unsuitable for automated production lines.

Practical consequence of precision blanking wood:

high beam quality of the RF laser (left) vs. low beam quality and thus larger kerf and more smoke (right), due to the present (and frequently encountered) ellipticity of the DC laser beam, it is also noticeable that the vertical kerf is significantly larger than the horizontal one (right)

RF Lasers: Here you can get a reproducible output of around 2% - 100% power.  For the engraving of filigree elements in the medical or cosmetics industry, this is indispensable. Here, the laser power can be precisely adapted to the particular application and optimally controlled, especially in the low power range. In this way, the most precise engravings can be produced, or unwanted body hair can be easily and painlessly removed.

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