Image credit: Stanford Research Systems | Laser Innovations | USHO

Nitrogen lasers are gas lasers which use molecular nitrogen as their laser medium.

Basic Information

Construction and Operation

Like all lasers, nitrogen lasers typically consist of three basic parts: an energy source (or pump), a laser medium (also known as a gain medium), and an optical resonator.

• As in most gas lasers, the energy source is an electrical discharge provided by a power supply.
• The gain medium is some concentration of N₂ molecules. The laser medium is typically either pure nitrogen, nitrogen-helium mixture, or simply air.
• Unlike many lasers, nitrogen lasers can operate without an optical resonator (the series of mirrors and windows used to amplify and direct emitted radiation). This is due to the fact that stimulation of nitrogen atoms results in amplified spontaneous emission (SAE) — also known as superluminescent light — by achieving population inversion within the gain medium. (Population inversion refers to a state where more atoms exist in an excited state than in a low energy state.) Nitrogen lasers may still include a single reflective mirror at the back of the laser to ensure correct output.

Nitrogen lasers use simpler and cheaper building materials when compared to more powerful gas lasers such as excimer and CO₂ lasers. Because the gas medium is relatively benign, the gas is often contained within a simple plastic (acrylic glass) chamber. The image below shows a typical home-built (non-commercial) nitrogen laser reflecting the simplicity of the parts involved. In this design, the spark gap at right provides the electrical discharge which is transmitted through the plates and into the gas chamber. The chamber itself is made of Plexiglas, while the sole optical components are a microscope slide and covers positioned at the output point.

Image credit: Nu Energy

Nitrogen lasers are exclusively ultraviolet (UV) devices which predominantly emit at 337.1 nm. Their operation consists solely of extremely short, powerful pulses; for more about pulsing and laser power, see the Laser Power section below.

Levels and Population Inversion

As stated above, nitrogen lasers achieve superluminescent emission by using population inversion. In order to accomplish this, they use three different energy levels (and are therefore classified as three-level lasers). It is important to note that only two of the energy levels are specifically important, as the third level represents the unexcited ground state.

A nitrogen laser's upper energy level is directly pumped by a high voltage electrical discharge. This pumping is powerful enough to cause population inversion and stimulates the nitrogen atoms to immediately emit at 337.1 nm; the atoms remain at this level for approximately 20 ns. As the energy level drops the atoms fall into the second level and remain there for much longer (approximately 10 microseconds) than the first level. The effect of this time difference is that the atoms rapidly cease emission, resulting in a fast pulse of radiation. When considering this effect, it is clear that the pulse length is directly determined by the lifetime of the upper level, which in turn is dependent upon the gas medium's pressure: the higher the pressure, the shorter the lifetime of the level (and pulse).

The graph below shows the three energy levels (as well as a fourth "meta-stable" level) of a typical nitrogen laser. The purple line represents the lasing action, which is incited by the pumping of the upper level and terminated at the second level.

Image credit: World of Lasers

Gas Pressure and TEA Lasers

Nitrogen lasers are generally constructed using one of two designs. The first is a "traditional" low-pressure design involving a vacuum pump; this one results in pulses 5-10 ns in length and is most suitable for pumping other lasers.

The second design is a higher-pressure one known as a TEA (transverse electrical discharge at atmospheric pressure) nitrogen laser. (It is important to note that, while TEA lasers are frequently nitrogen types, TEA designs for excimer and carbon dioxide lasers are common as well.) Nitrogen TEA lasers use plain "open" air at atmospheric pressure as their lasing medium, as air is 78% nitrogen. While TEA lasers require no vacuum pump to operate, they require a much faster electrical discharge to achieve effective pulses when compared to lower pressure designs. The home-built laser diagram above represents a TEA laser.

Applications

Compared to other gas lasers, nitrogen lasers are used in a relatively narrow range of applications. Because of their simple construction and inexpensive components, they are highly valued by beginning laser hobbyists. Practical nitrogen laser uses include:

• Nondestructive testing (NDT)
• Measurement of rapid processes such as time of flight (TOF), due to fast pulsing
• Pumping of dye lasers
• UV spectroscopy
• Fluorescence measurement

Laser Power

Because nitrogen lasers produce pulsed output, manufacturers specify their power as average power. Average power is calculated by multiplying the energy within each pulse (in joules) by the number of pulses per minute. The nature of average power makes it difficult to discern a nitrogen laser's actual power, since a laser with an average power of 50 W may output five 10 joule pulses per minute or 100 500 millijoule pulses per minute. To compensate, manufacturers often include specification values for individual pulse energy and repetition rate as well as average power. Nitrogen lasers in particular are capable of very powerful pulses (several megawatts per pulse).

Safety

Safety is an important topic when discussing laser use. A split-second direct exposure to a 200 mW laser emitting 100 yards away can cause permanent eye damage. Pulsed UV radiation from nitrogen lasers is capable of causing eye damage if viewed directly and can also cause skin injuries.

A laser warning sign.

Image credit: Keller Studio

In the United States, the Center for Devices and Radiological Health (CDRH), a division of the Food and Drug Administration (FDA), provides a laser safety classification scheme based on six product classes. Lasers are also classified using the international IEC 60825 standard. The table below describes US domestic and international classes for laser safety. Nitrogen and air lasers typically fall into Class 3B and require UV-blocking eye protection to safely operate.

Laser safety classes. Image credit: Erchonia

References

IHS - IEC 60825 (Laser safety)

Niagara College - The TEA Nitrogen Gas Laser

Sam's Laser FAQ - Homebuilt N₂ Lasers

Wikipedia - Population inversion