CO2 laser tubes are the most common type of laser tube used in laser cutting and engraving machines. They work by exciting carbon dioxide molecules with a high-voltage electric current (for DC excited) or with radio frequency (for RF excited... think microwave oven), which causes the molecules to emit photons. These photons are then collimated into a laser beam that can be used to cut or engrave materials.

There are two main types of CO2 laser tubes: DC excited and RF excited. DC excited laser tubes are the most common type of laser tube. They are relatively inexpensive and easy to maintain. However, they have a shorter lifespan than RF excited laser tubes and are not as well suited for high-speed cutting or engraving.

RF excited laser tubes are more expensive than DC excited laser tubes, but they have a longer lifespan and are better suited for high-speed cutting and engraving. They also produce a higher quality laser beam with a narrower kerf.

DC excited CO2 Laser Tubes

The carbon dioxide laser (CO2 laser) was one of the earliest gas lasers to be developed. It was invented by Kumar Patel of Bell Labs in 1964, and is still one of the most useful. Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available. They are also quite efficient: the ratio of output power to pump power can be as large as 20%. The CO2 laser produces a beam of infrared light with the principal wavelength bands centering on 9.6 and 10.6 micrometers (μm). 

From Wikipedia: https://en.wikipedia.org/wiki/Carbon_dioxide_laser

The Thunder Laser Nova series CO2 laser gantry engravers employ a high-quality (SPT and Reci) Chinese glass tube. Every tube is inspected and tested to ensure that it conforms to our stringent specifications. One of those specifications is that every Thunder Laser must produce at least 10% over the rated tube power. These tubes are water-cooled and DC excited. This means that a laser power supply (LPSU) of 30kVDC could be capable of delivering currents of well over 50mA.  The high-voltage distribution will be with a thick red wire. Read all warnings and refer to the manual for guidance.

Let's identify some parts of the tube for reference:

The anode (positive terminal) is the one on the back side of the tube with the big red wire we mentioned above. The cathode (negative terminal) is on the front near the output lens (output coupler, OC). 

Direct Current excitation in DSP controlled lasers is pulse-width modulated. The LPSU does not excite the tube with a constant output, but rather pulses rapidly. This is how the power is varied. When the power remains constant, even when vectoring, the trigger still pulses. This is very similar to how a 110VAC incandescent light actually cycles on and off, 60 times every second (60Hz). DC excited tubes cannot pulse as fast as RF excited tubes. The larger the tube, the more this behavior manifests. ThunderLaser offers RF machines as well, which we will cover in a separate article.

Striking Voltage

The striking voltage (ionization threshold) is the point where the power modulation is sufficient enough to maintain steady ionization in the pumping chamber. This will output a uniform beam signature and density. Longer, higher-powered tubes will have a higher threshold. Once you exceed the threshold there is still a range just above it where the stability may very. This has been identified in our 100w machines and is evidenced in the appearance of the arc pattern at the anode. Here is more on that:

Banding and Plasma Arc Issues and Striking Voltage Banding and Plasma Arc Issues and Striking Voltage

Tube Degradation

These glass CO2 tubes have a finite life, which varies with the brand and quality. The lifespan of the Thunder Laser tube is reported to be 2000 to 4000 hours under normal operation. The primary reason for tube degeneration is the metal vapor deposition that occurs within the pumping chamber as well as the degeneration of the gasses inside the tube. 

When the tube fires, an arc (and resulting plasma discharge) is produced between the anode and cathode. These electrodes are usually tantalum. The arcing vaporizes the metal, which gets deposited on the optics within the pumping chamber. Over time, this degrades the performance of this tube. This is a normal condition. The 2 biggest killers of co2 glass tubes are overheating and overdriving. Firing the tube regularly does help regenerate or help balance the gasses to a small extent. It is not recommended to keep a 'spare' tube as they also have a shelf life. The inventory is perishable and rotated carefully to ensure the 'freshest' tube. That is not to say that a tube can't sit shelved or idle for 6 months or a year, you just don't want it dormant for extended periods.

Power Supply Response

We use high-quality LPSUs and we calibrate them to the tube so you won't overdrive the tube on a Thunder Laser. The delay in time from the moment the LPSU is triggered to the time it actually fires is less than 1 millisecond, which is fast for laser engravers. However, even the best electronic equipment is not 100% efficient, so this time to fire is actually quite slow, as far as electronics go.

Tube Discharge Timing

When the LPSU fires, there is a bit more delay before the beam is actually formed. Again, this delay is fast as far as lasers go, but still has some effect.

RF excited CO2 Laser Tubes

Which type of CO2 laser tube is right for you?

The best type of CO2 laser tube for you will depend on your specific needs and budget. If you are looking for an inexpensive laser tube that is easy to maintain and suitable for a wide range of materials, then a DC excited laser tube may be a good option for you. However, if you need a laser tube with a longer lifespan and better suited for high-speed cutting or engraving, then an RF excited laser tube may be a better choice.

Ultimately, the best way to decide which type of CO2 laser tube is right for you is to talk to a Thunder Laser Team Member. They can help you assess your needs and recommend the best type of laser tube for your application.