Gas Lasers: CO2 lasers - progressing from a varied past to an application-specific future



Invented by C. Kumar N. Patel in 1964 at Bell Laboratories, carbon-dioxide (CO2) lasers are an ancient technology by photonics industry standards. But despite a long history, CO2 laser technology has survived and thrived because of its unique combination of wavelength, power, and spectral purity.
Because many natural and synthetic materials have strong characteristic absorptions in the 9–12 ?m spectral range spanned by CO2 lasers, there are numerous opportunities in materials processing and spectral analysis. These wavelengths are also contained in an important window for atmospheric transmission, ideal for many sensing and ranging applications



A typical CO2 laser consists of a volume of electric discharge with a gas mixture that includes CO2 molecules. Because the energy levels of molecular vibration and rotation are close together, photons emitted as a result of transitions between these levels are low in energy and long in wavelength compared to visible and near-infrared (NIR) light.
Carbon-dioxide lasers can provide power levels from milliwatts to tens of kilowatts, making them equally suited to instrumentation or brute-force cutting. And because CO2 lasers have very high spectral purity, with <1 kHz of radiated linewidth without a power tradeoff, conversion efficiencies of 10% are possible. These traits allow CO2 lasers to tackle emerging applications in materials processing, light detection and ranging (lidar), thermal vision assistance, and targeted therapeutic medical applications.
In the decades since their invention, hundreds of thousands of CO2 lasers have been used in medicine, manufacturing, and scientific research-from printing a four-digit code on water bottles in a high-speed manufacturing line in China to welding component parts for Mercedes-Benz automobiles in Germany. Even today, with fiber lasers gaining popularity in similar applications and new frontiers being forged by quantum-cascade lasers (QCLs), CO2 lasers will remain one of the most widely deployed lasers on the market if they can progress into application-specific territory.







Challenging the competition
Despite these long-time advantages, CO2 lasers are being challenged on several fronts. Fiber lasers and QCLs have migrated into some of the applications formerly dominated by CO2 lasers.
For industrial applications, high-power fiber lasers can provide higher efficiency and better absorption by metal at a cheaper or comparable cost. However, CO2 lasers remain the only way to process many non-metal materials that do not absorb the NIR wavelengths provided by fiber lasers.
Quantum-cascade lasers are also compact in size and can produce wavelengths spanning 2–12 ?m, making them a great tool for spectroscopy. However, many sensing and spectrum-sensitive industrial and medical applications in the long-wave infrared (LWIR) region from 8 to 12 ?m require a combination of high power, spectral purity, excellent coherence, and stable spatial mode that only CO2 lasers offer.
In addition to technological challenges, price erosion—primarily from China's ever-expanding laser industry—has pushed prices lower and lower. Standard CO2 lasers are becoming a pure commodity, with barriers to entry and margins dropping rapidly. Just three years ago, Chinese companies were purchasing U.S.-made 30 W CO2 lasers for $4500. Now, Chinese laser manufacturers have entered the market with their own CO2 lasers for $2000.
These factors signal an end to the "dollar-per-watt" era, wherein companies produced lasers with a specified average power, the capability to perform a myriad of tasks, and a price proportional to the watts delivered. Using this strategy, highly successful companies like Synrad, Coherent, and Rofin created families of lasers ranging from a few watts to tens of kilowatts, birthing an industry where CO2 lasers were used in plastic manufacturing plants, dentists' offices, and cell phone assembly lines.
Although the ability of CO2 lasers to compete as a one-size-fits-all solution is coming to an end, we are meeting the challenges posed by the emerging world of new materials and increasingly demanding industrial and scientific processes with a new application-specific lasers paradigm that requires a much deeper technical understanding of the true value proposition of a laser, and a different approach to how CO2 lasers are manufactured and marketed.
On the manufacturing side, this new paradigm leverages the wide-ranging specifications of CO2 lasers to closely match specific customer requirements. On the marketing side, it shifts away from average power and dollars-per-watt as the dominant value proposition towards customer-specific solutions where pulse shaping, peak power, wavelength specificity, and operational stability match specific material and applications needs.





Processing new materials
As manufacturing continues its global shift towards full process automation, lasers are changing the value proposition for traditional processes such as knife cutting and punch pressing of materials in industries like packaging. Companies such as Amazon are driving a change from rigid packaging—set sizes of traditional cardboard boxes—to flexible packaging constructed from new plastic materials and created onsite to exactly fit the needs of the particular shipment.
Flexible packaging is lighter in weight and safely biodegradable in a landfill, reducing the fixed costs and environmental impact of shipping and packaging—key future performance metrics for Amazon. This shift is enabled by the fact that lasers never go dull and deliver repeatability and accuracy well beyond the capabilities of mechanical systems.
New types of multilayer plastics can have a very strong absorption peak at one particular line in the CO2 spectrum, or even require a laser that can shift wavelengths on the fly as it goes from layer to layer in the cutting process. Replacing a large-form knife-cutting system would require an array of lasers capable of creating ablation with almost no heat-affected zone, while tracking a web moving hundreds of feet per second. This application-specific laser needs high accuracy at a single wavelength in the spectrum, with potentially very high peak-power-to-average-power ratios of 100 to 1 or even higher, and very fast (>10 kHz) pulses.
Traditionally, such peak powers and fast pulses require Q-switching or other external modulation techniques that are simply too expensive to scale for cost-effective mass deployment in industries such as packaging. However, we are developing CO2 lasers that achieve kilowatt peak powers without external modulation in a small form factor to meet these demands. Indeed, the newest trends in manufacturing, medicine, and materials science are forcing the historical dollar-per-watt value proposition of the last half-century to evolve into a customer-centric, application-specific era for the CO2 laser.