The importance of lasing gas purity

How to minimize impurities


February 14, 2002


Here's some food for thought on lasing gases: How are they created? What are their potential impurities? Which impurities and how much of them are of concern? What lasing gases should be used? How do you protect yoiur high-quality lasing gases from contamination? Giving these items some attention could save you some trouble down the road.

Industrial laser powers have increased dramatically over the past few years. This has made maintaining a high-purity atmosphere within the laser resonator more important than ever.

Certain gaseous impurities can cause significant damage to the resonator. How pure is pure enough, and how can you know for sure that a high-purity gas does not contain damaging impurities? To most laser users, this is critical information, as it can offer cost savings and productivity enhancements or, if ignored, can result in costly repair bills and lost production.

How Are Lasing Gases Created?

Air Separation. A few components found in lasing gases are manufactured directly from the air we breathe. Although the process of air separation may be complicated, the concept is simple.

Each gas has its own unique characteristics that allow separation. Of course, water boils at 100 degrees Celsius, rapidly changing the liquid to a vapor. The same process can happen in reverse with atmospheric gases, except at extremely cold temperatures.

For example, under atmospheric pressure, gaseous nitrogen becomes a liquid at -196 degrees C, argon at -186 degrees C, and oxygen at -183 degrees C. These characteristics allow the air separation process to take place.

Figure 1:
Impurities can be expected during the production of gases.

Manufactured Gas Products. Other gases, such as CO2, carbon monoxide, and hydrogen, can be created in large quantities by using chemical reactions and recovering them as byproducts. Once recovered, they are purified to various grades.

Natural Resource Gas Products. Helium is a natural resource typically extracted from helium-enriched natural gas. This is key to understanding the impurities that can exist in these gases that typically are not produced through conventional air separation.What Are the Potential Impurities in Lasing Gas?

Most lasing gases for industrial lasers are combinations of helium, nitrogen, and CO2. Various laser manufacturers also add small amounts of carbon monoxide, hydrogen, oxygen, and xenon for beam improvement. Figure 1shows the impurities that can be expected during the production of each gas.

Impurities can be introduced when filling high-pressure cylinders if rigorous procedures are not followed. These impurities typically are nitrogen, oxygen, and water.

Figure 2:
Moisture destabilizes the beam, absorbs into the coating of some optics, and decreases power output.

Which Impurities Should Concern You?

This may be surprising, but not all impurities are of concern in lasing gases. Three basic reasons that certain impurities are of no consequence are as follows:

1. The impurity is already a component of the mixture. For instance, there is no reason to worry about the nitrogen impurity in helium because nitrogen already exists in the lasing gas mixture.

2. As soon as a laser is activated, a series of reactions begin to occur within the resonator, dissociating the CO2molecules into carbon monoxide and oxygen. This creates many thousands of parts per million (PPM) of carbon monoxide and oxygen within the resonator—many more than can even be present in the source product to begin with.

3. Rare gas impurities such as argon have little to no effect on the laser resonator unless they are present in quantities larger than a few percent. Although argon is the most abundant of the rare gases, it represents less than 1 percent of the atmosphere; thus, more than a few PPM of argon or another rare gas impurity such as neon, krypton, or xenon would be nearly impossible.

Therefore, the main impurities that can cause problems in the laser resonator are moisture and total hydrocarbons. Moisture destabilizes the beam, absorbs into the coating of some optics, and decreases power output (see Figure 2). Hydrocarbons reduce the gain of the laser, thereby limiting its ability to amplify power (see Figure 3).

Figure 3:
Hydrocarbons reduce the gain of the laser, thereby limiting its ability to amplify power.

What Level of Impurity Causes Damage?

Because it is impossible to eliminate impurities from lasing gases, it is not cost-effective for the laser user to purchase ultrahigh-purity gases. The final mixture of lasing gases should be controlled to less than 5 PPM of moisture and less than 5 PPM of total hydrocarbons.

Research shows that even a gas with 99.995 percent purity could contain as many as 50 PPM of damaging impurities. If unchecked, this could present operational problems in the laser. Therefore, lasing gas purchases should be based on the manufacturer’s conformance to limited impurities.

What Level of Each Impurity Is Acceptable?

Depending on the laser type, the resonator will require either premixed lasing gas or individual pure gases. The impurity control should range from 1 to 5 PPM for each impurity, depending on the premixed gas and the type of laser using it. For laser types that mix their own gases, it is important to examine what the end product will be once they enter the resonator.

For instance, consider the following hypothetical example. A laser is supplied individual pure lasing gases and mixes them at certain percentages (see Figure 4).

This example shows how much each component affects the final impurity level of the mixture. Actual impurity levels found in each gas typically are below the specified limit, but it is important to verify the certification to which each pure lasing gas is manufactured.Laser users who are concerned about maintaining their lasers to a high standard should purchase lasing gases specifically designed for the laser being used. The lasing gas should come with a conformance guarantee assuring that the percentages of each gas will be within tolerances specified for the particular mix.

Figure 4:
Each gas component affects the final impurity level of the mixture.

How Does the Cylinder Affect Gas Purity?

Cylinder cleanliness is critical to gas purity. Even though the source product may be free of impurities, the cylinder may have been contaminated previously, resulting in a mixture that is out-of-tolerance. Many times cylinder cleanliness is the difference between industrial-quality gases and specialty gases.

It is important that the cylinders provided by the manufacturer be dedicated to high-purity service. In addition to gaseous impurities, particulate larger than 10 microns in diameter can cause significant cumulative damage to the surface of the optics within the resonator.

How Can I Protect the Lasing Gas From Contamination?

While it is important to purchase high-quality laser gases, it is just as important to protect that gas from source to point of use. Using gas distribution equipment specifically designed for laser use will prevent contamination of the laser gas from atmospheric permeation through the hoses, improper materials used in regulation equipment, or atmospheric contamination during cylinder changeout. All of these can be prevented with the use of proper equipment.

For more information, contact Jeremy Barr, program manager, welding gases, or Brad Dewhirst, business manager, industrial cylinder gases, with Air Liquide America Corporation, 2700 Post Oak Blvd., Houston, TX 77056, phone 800-820-2522, fax 877-715-4799, e-mail, Web site Air Liquide is a global provider of industrial and medical gases and related services.

Jeremy Barr

Contributing Writer

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