Low Power CO₂ Lasers Emphasize Power Stability and Wavelength Tunability

New applications call for portability, stability, and tunability

The CO₂ laser is one of the most dominant light sources in commercial laser applications.  Although it has mainly been used in the industrial realm for cutting, welding and marking, the CO₂ laser capabilities go beyond its ability to deliver thermal energy.   Its unique wavelength (9-11μm) , spectral purity (less than 100 kHz linewidth), coherence (km or more), wavelength tunability, and mode quality set it apart from alternatives and make it very attractive for numerious applications such as trace gas analysis or free-space communications.  Even in thermal applications, such as welding of optical fibers, there are cases when power requirements are not very high but mobility and precision are vital. 

Portability is a key enabling factor for CO₂ laser applications in remote trace gas analysis. The dense distribution of molecular radiation lines from the CO₂ laser within the infrared spectroscopy fingerprint region, makes them very useful in selectively exciting specific molecules. This unique feature provides identification of minute amounts of the targeted gas molecule among a strong background of ambient air, offering sensitivity in parts per billion.  These applications take advantage of the CO₂ laser’s capability to tune from one wavelength to another while maintaining a high degree of both power and frequency stability. 

CO₂ lasers do not have to be bulky

CO₂ lasers have a reputation of being bulky due to their historic implementation as high-power, large-flowing gas light sources for metal processing. More recently, trends in telecommunications seek long-wave and medium-wave infrared sources with low lasing thresholds for precision fabrication of sensitive parts. Within the CO₂ laser development community, the main efforts have always been focused on increasing power in a comfortable volume through folding, array, area scaling, etc. From this prospective, a short-cavity, low-power laser has been considered impractical due to power instability issues. 

With our focus on addressing the needs of the test and measurement market, along with the demands of researchers who need low power, we have developed new technologies that allow CO₂ lasers to have a very low lasing threshold for the RF driver input.  In our experiments, we’ve achieved lasing with as little as 4-watts RF input. Some of our products have a threshold of just 10 watts RF driving power. 

In the picture below (Figure 1) is a complete battery-powered CO₂ laser.  It can deliver output power of 400mW CW over one hour, non-stop, with a single battery charge. Weighing less than 3 pounds including the battery and RF driver, this is the first truly portable CO₂ laser in the marketplace. It is suitable for applications when the laser needs to be carried from place to place.

Stability in both power and wavelength

CO₂ lasers with short cavities typically suffer from severe instability, because there is less overlap between the cavity longitudinal modes and the CO₂ molecular gain distribution. Our laser designs minimize the impact of this and achieve a high degree of stability in both power and frequency.  Lasers enabled with technologies for power and line stability, such as Temperature Stability (-S) and Line Tracker (-T), have achieved long-term stability of better than ±2%, as shown in the following Power vs. Time plot.

Wavelength Tuning with Grating, and without Grating

From the time that tunable CO₂ lasers became commercially viable in the 1980’s, it has been standard practice to use intra-cavity gratings as the dispersion element for line-selection.  However, this approach not only increases complexity but also reduces the laser system’s efficiency because the reflecting gratings have very low efficiency, typically lower than 94%.  For a short-cavity laser, the cavity itself is a Fabry-Perot etalon, which can be used as the dispersion element. In this case, the longitudinal modes are widely separated in the frequency domain.  Strong coupling among the energy levels that participate in CO₂ molecular radiation makes it possible to oscillate only the strongest line, while suppressing all others. The competitiveness of a laser line to dominate this intra-cavity selection mechanism is determined not only by its molecular emission strength, but also by its overlap with a cavity mode.  If there is no cavity mode falling around the center of a strong emission line, then no amplification oscillation will take place. While the frequencies of the CO₂ lines are determined by the structure of the CO₂ molecules, and therefore are constants once a molecular system is selected, the frequencies of cavity modes are determined by the length of the cavity through the following relationship:

where N is an integer from zero to infinity, c is the speed of light, L is the length of the cavity, and Δν is the frequency spacing between adjacent modes.  From this relationship, one can see that a short cavity (small L) results in a widely separated cavity mode pattern (large Δν).  This makes it possible that weaker lines will have a chance to overlap with a cavity mode while stronger lines do not. This is illustrated in the following plots for two laser lines: 10P(20) at 10.59µm and 10P(14) at 10.53µm.

These plots illustrate that small changes to the cavity length, typically less than 1 µm at a time, result in a different longitudinal mode having a higher gain value for a different laser line.  Line selection is therefore achieved if the cavity length can be precisely controlled.  For CO₂ lasers without cavity control, the unwanted line change is called “line hop”, often induced via thermal expansion introducing random drift in cavity length. This has always been a problem for CO₂ laser applications, but with precision, low-cost controls, Access Laser has turned this curse into a blessing for users of CO₂ lasers. 

For long cavities, the modes are close packed.  Therefore, there is always one cavity mode close to the center of the strongest line, making it dominant at all times regardless of the drift in cavity. Figure 5 illustrates this phenomenon.

Although grating-less tuning has the above-mentioned advantages in power and cost, it does not offer as many tunable lines as lasers with a grating.  Typically, a short, grating-less tuned laser can yield between 12 to 17 lines while a grating-tuned laser can have more than 40.