50 Years of the Laser Industry

Paul Sechrist, Coherent Inc.

The relentless innovation that has always characterized the laser industry is now driven and firmly controlled by the needs of applications.

In the technology’s infancy, the main goals were the discovery of new laser materials and the development of different operating regimes. Then came a period, particularly in scientific lasers, where achieving benchmark performance (e.g., shortest pulse width or highest peak power) was the main objective. Next came a slow migration of lasers into industrial/OEM applications and, later, the telecom boom, where the focus became fiber related and brought us a new material – doped ytterbium fiber – and a renewed emphasis on pump (diode) technology.

Today, innovation is driven primarily by the need to satisfy the requirements of specific applications. For example, in solar cell and flat panel display production, development is focused on delivering better results from laser-enabled processes (e.g., increased throughput, lower cost, higher yield). Laser innovation for microscopy applications is targeted at faster imaging and higher spatial resolution. Lower cost of ownership, critical in many industrial applications, is driving developments to improve the reliability and efficiency of existing laser technology. This article will illustrate these trends by examining a few key laser technologies and the applications they enable.

Multiwatt visible lasers

Perhaps no single area epitomizes the evolution of our industry like the CW multiwatt visible laser. The first generation of this technology was the argon-ion laser, which could produce output at several visible and UV wavelengths, the most intense being 488 and 514.5 nm. When introduced in the late 1960s, these lasers generated tremendous interest and market demand. At first, this was as much about how the laser worked as about how it could be used. This focus characterizes the first decade of the laser industry. Later uses include holography, inspection and trabeculoplasty, a treatment for glaucoma that was the world’s first medical laser application. And in scientific applications, these lasers were used as pump sources for dye lasers, which were the next very diverse technological development.

Unfortunately, ion lasers were inefficient, bulky and much less reliable than the lasers of today. Their optics required frequent tweaking, and water cooling was essential for multiwatt operation. The early plasma tubes had a lifetime of a year, making them expensive consumables. Some of these limitations were mitigated by very creative engineering and a switch to ceramic discharge tubes, but these lasers remained user-unfriendly (by today’s standards), and few true OEM applications developed. In contrast, their lower-power, lower-cost, air-cooled cousins became the workhorses for a generation of biomedical instruments and disc mastering machines.

Multiwatt, CW, visible technology was revolutionized by the introduction of diode-pumped solid-state (DPSS) lasers. Unlike ion lasers, these DPSS lasers could emit only one wavelength, 1064 nm, which was then intracavity doubled to produce 532 nm. But this single-wavelength functionality was considered a small price to pay for the massive gain in efficiency, longer lifetime, lower cost of ownership, a ten times reduction in size and the fact that these lasers could be factory-sealed with no subsequent use tweaking required. Needless to say, applications for these solid-state lasers expanded overnight. And existing scientific applications, such as pumping for both CW Ti:sapphire and ultrafast laser systems, switched from ion to DPSS. Other important uses for these lasers included forensics, inspection and holography.

Despite the advantages of DPSS, commercial and scientific applications continued to demand even lower cost of ownership and greater flexibility and reliability, but without any sacrifice in output characteristics. A third generation of visible lasers based on optically pumped semiconductor laser (OPSL) technology arose to meet this need. Here a laser diode pumps a semiconductor chip rather than a doped crystal. This eliminates the thermal lensing issues that plagued DPSS lasers. OPSL also enabled automated “pick and place” manufacturing methods and economies of scale. And today this technology can be tailored to output specific wavelengths over a near-IR range that, when intracavity doubled, provides a selection of visible wavelengths.

Figure 1. Both yellow and blue OPSL lasers are used in high-impact light shows. Photo courtesy of Lightline Lasertechnik, Osnabrück, Germany.

The result is that OPSL technology exceeds the best of ion and DPSS characteristics: It is wavelength-flexible and power-scalable, yet inherently compact and efficient. As a result, we are now at a point where laser output can be defined by the application, rather than vice versa. So while these lasers are available at legacy wavelengths, such as 488 and 514 nm, they are also offered at completely new wavelengths to optimize targeted applications. A standout example is the 577-nm yellow laser that is matched to oxyhemoglobin absorption. This wavelength provides superior results in the photocoagulation used to treat wet-form macular degeneration. This has also turned out to be a useful and popular wavelength for use in laser light shows (Figure 1).

Most recently, multiwatt OPSLs in the red have become available. These wavelengths were developed to provide brighter images and a wider color gamut in entertainment applications as well as to support faster DNA sequencers, which now use a combination of 488-, 532- and 639-nm laser wavelengths (Figure 2).

Figure 2. Fast-automated DNA sequencing relies on laser-excited fluorescence. The use of multiple laser wavelengths is critical, as instrument builders close in on the holy grail of “the thousand-dollar genome,” a price point predicted to cause massive clinical diagnostic market uptake. Courtesy of Coherent Inc.

The diode laser revolution

Many of the advances in laser technology over the past two decades have been enabled through the use of diode lasers as pump light sources. However, just as with other laser types, the diode laser itself started out as a low-power lab novelty with limited reliability. But the need for compact, long-lived sources in data storage and telecommunications drove these devices to higher output powers, narrow linewidths and increased reliability.

Producing even higher power diode lasers and arrays with increased brightness remained a challenge. The principal problems were heat load management and facet failure, since a large amount of optical power is channeled through a facet measuring, at most, tens of microns. In addition, many applications needed a system design that could withstand repeated on/off cycling.

The laser industry met this need by incremental improvements in power, lifetime and brightness. Key technical innovations were the introduction of aluminum-free active region devices to address facet lifetime and indium soldering to address the mounting/cooling interface issues. Market success followed suit, with applications such as optically pumping solid-state lasers in tasks such as chemistry, holography, materials science, biology and industrial micromachining, and materials processing using the direct diode output in tasks such as welding, cladding and hardening. Today, few would argue with the fact that laser diodes (individually or aggregated) are the single most important technology in the laser industry (Figure 3).

Figure 3. Laser diodes and diode arrays are key building blocks of both materials processing systems and other lasers. They are the most mature of all lasers in terms of volume manufacturing. This is reflected in their closer resemblance to integrated electronic components than to other laser technology. Courtesy of Coherent Inc.

CO₂ lasers

The solid-state laser revolution has not been completely universal in its reach. When it comes to industrial applications, the CO₂ gas laser (first commercialized by Coherent in 1966) has long reigned as the lowest-cost-per-watt champion. No other technology can come close to the high power or low cost in the mid-IR. Initially, CO₂ lasers were flowing gas type, with the CO₂ lasing in a long plasma discharge tube. These lasers were eventually scaled up to multiple kilowatts for brute force applications such as cutting steel for automobiles and drilling holes for the aerospace industry.

Materials processing engineers soon realized that the mid-IR was a perfect wavelength for processing a host of organic materials in applications such as cutting and perforating paper, as well as cutting plastics, leathers, metal foils and carbon fiber laminates used in automotive interiors. In addition, emerging applications such as via drilling in the electronics industry needed lower initial and operating costs, flexibility in power and, in some cases, optimization of wavelengths and a quantum leap in miniaturization and operational simplicity compared with the massive water-cooled flowing gas lasers with their fast pumps and expensive bottles of laser gas. These changes to CO₂ lasers ultimately enabled the transition from conventional mechanical processes (drills in the case of microvias) to enhanced laser-based processes. Many applications also needed pulsed operation with fast rise times to avoid charring and damaging the cut edges in these relatively delicate materials.

Figure 4. Sealed carbon dioxide lasers are now widely used for engraving and marking of a wide range of organic materials, with possibly no application more eye-catching than the production of images, cutouts and fake 3-D effects on high-end denim jeans. Courtesy of Coherent Inc.

At low power levels of less than 100 W, this need was met with the development of sealed waveguide lasers using radio-frequency excitation. The low cost and simplicity of these compact lasers continue to feed a healthy and diverse materials processing market, headlined by marking and engraving (Figure 4) as well as surgical and aesthetic applications. At higher powers – i.e., up to a kilowatt – the goals of compact simplicity and high reliability were met with the introduction of sealed slab discharge lasers (Figure 5), where the power scales as the area of the electrodes rather than the length of a waveguide.

These novel lasers enabled a host of new materials processing applications, most notably using robotic and flatbed mounting, in organics, thin metals and particularly in mixed (laminated) materials for electronics and automotive interiors. The sealed CO₂ laser thus became a very mature tool used in many industries and applications. As a result, size, reliability and cost of ownership drive today’s decision making in applications ranging from microvia creation (think mobile phones) to flat panel display manufacturing and other high-volume cutting and drilling applications.

Figure 5. Carbon dioxide lasers have undergone tremendous innovation over the years. The biggest step forward was arguably the development of compact sealed lasers at powers of up to a kilowatt (foreground, Coherent Diamond 500W series) in a platform that bears little resemblance to their large, cumbersome and less reliable forebears (background, Coherent Everlase 500W series, circa 1980). Courtesy of Coherent Inc.

Fiber lasers

Fiber lasers represent another example of innovation where a completely new laser type is pioneered for one application but later modified or upgraded to address the needs of others. Born from the telecom boom, these lasers initially saw life as amplifiers for ultralong-haul communications networks.

These fiber amplifiers used laser diodes to pump light into a doped optical fiber. When married with fiber Bragg gratings, they emitted laser light at about 1.1 μm. First produced as a low-power laboratory novelty, they underwent a rapid acceleration during the telecom bubble. However, when that imploded, the specialty manufacturers of these telecom novelties began to look for other potential applications. It became clear that if fiber lasers could be scaled to higher powers, they could become viable and successful sources for certain materials processing tasks at kilowatt levels.

The first generation of high-power fiber lasers relied on a large number of laser diodes for pumping and on long lengths of doped fiber for distributing the gain and thus minimizing thermal strain effects. This approach has enabled powers ranging from tens of watts up to tens of kilowatts with operating modes from CW and modulated, to even some low-power mode-locked (picosecond/femtosecond) models. The power and optical characteristics of these fiber lasers have enabled them to be quite successful in certain materials processing segments – marking, micromachining and cutting thin materials. Specifically, lower-power (e.g., 20- to 200-W) CW fiber lasers are used to engrave metals and coated metals.

The other sweet spot for fiber lasers has proved to be in the 1- to 2-kW window, where, depending on the material and the application specifics, they share the limelight with CO₂ lasers. Their 1-μm wavelength makes them particularly attractive for cutting metals since their main competitor – the CO₂ laser – is not an ideal wavelength match. But for cutting nonmetals and mixed materials, the lower cost of ownership of sealed CO₂ lasers means this is a very competitive area for the two laser technologies.

Fiber lasers remain a relatively new commercial deployment, and it remains to be seen what the future holds. But reliability and cost of ownership as well as serviceability will play a role. By applying more effective aggregation of diodes and reducing the total part count, a next-generation fiber architecture would enable more applications. In addition, there is a strong trend to produce more specialized fiber laser products, highly optimized for target applications, reflecting the growing maturity of fiber lasers for use in materials processing, scientific research and medical procedures.

The laser industry has always been powered by technology and innovation. In the early years, these were pursued as an end in and of themselves. Applications were almost an afterthought, mirroring the original skepticism that the laser was indeed a solution looking for a problem. However, over the past two decades, this has completely changed. With an explosion of diverse applications, the laser has become arguably the key tool in ushering in the age of the photon. So while technology and innovation are still the watchwords of our industry, these efforts are now focused on developing products demanded by, and defined by, the applications themselves.

Meet the author

Paul Sechrist is senior vice president of Coherent Inc.; e-mail: paul.sechrist@coherent.com.