Same Innovation, Different Motivation
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:
[email protected].