SOLID-STATE LASERS
The Next Generationo
of Analytical Instruments
by Paul Ginouves
SUMMARY
Innovations in solid-state lasers are producing more reliable, compact
instruments for biological analysis.
A
dvances in laser-based analytical instrumentation have finally
begun to catch up with advances
in microprocessor technology and consumer electronic devices. Although faster,
smaller and more-ergonomic devices permeate our personal lives, until recently
users of laser-based analytical tools toiled
with instruments that always seemed to be
one step behind the times. The prevailing laser technology was a limiting factor
in the packaging, ease of use, functionality and reliability of many analytical instruments.
Whether the choice was an air-cooled
argon-ion, HeNe or HeCd laser, or a water-
cooled argon-ion, Kr or mixed-gas laser,
each brought significant drawbacks to analytical instrument design. Some applications required several widely spaced
wavelengths and, therefore, multiple
lasers, giving rise to instruments with enormous footprints. In addition, poor reliability and low electrical efficiency resulted
in analytical instruments that were expensive to operate because of regular
maintenance downtime and three-phase
electrical requirements. In some cases,
they exhibited high heat dissipation that
required either forced-air or water cooling.
In the case of air-cooled lasers, cooling
fans introduced vibration problems and
noise, while water cooling meant bulky
chillers and higher operating costs. Finally,
all these lasers were delicate because they
incorporated plasma tubes that were neither durable nor portable enough for field
instruments.
Solid-state promise
When solid-state lasers were developed
nearly a decade ago, their size, efficiency
and performance promised to usher in a
new era of analytical instrument design.
However, these devices — including
diodes, diode-pumped solid-state and
now optically pumped semiconductors
— have come into their own only during
the past few years.
Upon their introduction, the cost per
milliwatt of green (532 nm) solid-state
lasers was higher than the analytical instrument market would accept. Moreover,
because 532 nm was outside existing par-
TABLE
APPLICATION
WAVELENGTH EMPLOYED (nm)
Raman Spectroscopy
488, 532, 785
Confocal Microscopy
UV, 405, 457, 488, 514, 635, 638
Environmental Monitoring/Bioagent Detection
UV, 488, 532
Flow Cytometry
UV, 457 or 460, 488, 514, 635, 638
DNA Sequencing
488, 514, 532
High-Throughput Screening
488, 514, 532, 635, 638
Reprinted from the January 2003 issue of Biophotonics International © Laurin Publishing Co. Inc.
Figure 1. New solid-state lasers, approximately the size of a computer mouse,
have enabled the creation of the first generation of high-end desktop flow
cytometers. The footprint of this DakoCytomation CyAn flow cytometer is
1 3 1 ft (30 3 30 cm).
adigms for assay protocols and reagent
systems, significant research was required
for successful integration. And most green
lasers suffered from what was commonly referred to as “the green problem”;
namely, unpredictable mode hopping and
output amplitude spikes. Other problems
included changes in performance as ambient temperatures varied and amplitude
noise in low-power, low-cost units.
Advances in technology have produced
many green solid-state lasers that minimize or correct these problems, however.
Packaging posed the main challenge to
incorporating diode lasers into instrumentation. Instrument manufacturers
were accustomed to lasers that emitted
collimated, circular beams and that contained integrated drive electronics and
cooling systems. Also, parts were required
to transform a highly divergent and elliptical diode laser beam into a usable coherent light source. Consequently, diode
lasers made inroads into the analytical
instrument market only after all the features that instrument manufacturers
needed were fully integrated into diode
laser packages. These included beam cir-
Figure 2. New high-end desktop flow
cytometers such as the DakoCytomation
CyAn are compact and efficient.
cularity, light regulation, collimation optics, thermoelectric cooling and integrated
drive electronics.
Solid-state lasers emitting at 488 nm
took a path different from other solidstate lasers. Attempts to create blue-green
lasers using directly doubled diode lasers
and up-conversion fiber lasers have yet to
produce a robust, commercially available
solution because of numerous technical
and material issues. Commercially available 488-nm lasers employ optically
pumped semiconductors. Although this is
only the first generation, these blue lasers
meet or exceed end-user expectations,
owing in part to the experience gained in
the development of green and red solidstate lasers.
After these growing pains, solid-state
lasers are positioned with the right wavelengths (405, 488, 532 and 638 nm), sufficient output power, ergonomic user interfaces and demonstrably superior
longevity to enable instrument manufacturers to capitalize on the inherent advantages of solid-state technology. Not all
red, green and blue solid-state lasers
are created equal, however. The spectrum
of commercially available solid-state lasers
now addresses the technical and economic
needs of nearly every customer.
New areas to explore
These lasers have allowed instrument
manufacturers to develop new markets
and manufacturing approaches as well as
to explore the areas of portability, remote
monitoring and battery operation. Their
impact has been felt on the functionality
and design of modern analytical instruments used for a wide variety of applications, including DNA sequencing, drug
discovery, flow cytometry, confocal microscopy and, most recently, biological
threat detection (see Table).
Genomics and proteomics
chemical properties of cells
in a variety of research,
DNA sequencing and drug
clinical and field applicadiscovery are two areas in which
tions. Depending on the
solid-state lasers are gaining
application, the devices
momentum. As genomics and
employ laser-induced light
proteomics start to play an everscatter and fluorescence to
increasing role in the developmeasure parameters such
ment of new pharmaceuticals,
as DNA content or cell surthis promises to be an area that
face antigens.
will see numerous new laserFor decades, flow cybased instrument paradigms.
tometer manufacturers
The process of ascertainused water- and air-cooled
ing the exact order of the nuargon-ion lasers in their incleotides in DNA, known as
struments because they
DNA sequencing, has traditionwere the only lasers with
ally used air-cooled argon-ion
the requisite combination
lasers. Instrument manufacturof wavelength and output
ers still employ them because of
power. Now manufacturthe convenience of multiline exers have the option to use
citation from a single package.
newly developed solidHowever, several prominent sestate lasers emitting at 488,
quencer manufacturers now use
532 or 638 nm.
488- and 532-nm solid-state
DakoCytomation, which
lasers.
manufactures high-end
One major barrier to broader
flow cytometers for biouse of solid-state lasers in this
medical research, produces
market is the instrument manuinstruments that can perfacturers’ need to support the
form multicolor simultalegacy of their reagent systems;
neous analysis of numerfor example, because many of
ous single-cell parameters
the reagents used in DNA seat throughput rates exquencing were developed for
ceeding 50,000 events per
488 and 514 nm, those wavesecond. In an effort to satlengths are now the de facto standards for the industry. Therefore, Figure 3. Small, rugged and energy-efficient solid-state lasers with isfy seemingly insatiable
it is difficult for manufacturers low heat dissipation have led to the development of self-contained customer demands for
higher performance from
to switch to 532 nm because the flow cytometers for remote field monitoring. The Cytobuoy flow
smaller instruments, the
enormous body of experimen- cytometer in this buoy measures the composition and population
company turned to 488-nm
tal data developed at 488 and of phytoplankton and other aquatic organisms in surface waters
solid-state laser technology.
514 nm would be impossible to of the oceans. Courtesy of Oceanor.
As a result, a 200-mW
reproduce and compare at 532
sapphire laser made by Coherent Inc. is innm. Given the obvious long-term advanenables a significant reduction in instrucorporated into the DakoCytomation
tages of solid-state technology in instrument size, allowing the lab to accommoMoFlo high-speed sorter, and a 20-mW
mentation, this roadblock eventually will
date more machines. Moreover, the highmodel is used in the CyAn high-perforfall by the wayside, leading to even more
reliability and low-maintenance characmance analyzer. John Sharpe, director of
sophisticated and sensitive reagent systeristics of solid-state lasers minimize
research and development at Dakotems based on slightly different waveinstrument downtime, thereby increasing
Cytomation, said, “The laser has given us
lengths.
productivity. Further, many solid-state
the laser horsepower we need. It has fanIn contrast to the DNA sequencing inlasers produce less noise than argon-ion
tastic pointing and power stability, comdustry, drug discovery system manufaclasers, which improves the signal-to-noise
pared to argon lasers, and is only a fraction
turers embraced solid-state laser techratio and ultimately increases instrument
of the size. So our new CyAn products pronology very quickly. In drug discovery apresolution.
vide better performance in a smaller packplications, accuracy and throughput are
age” (Figures 1 and 2). The footprint
mission-critical. Consequently, mainteFlow cytometry
of CyAn is just 1 3 1 ft (30 3 30
nance downtime must be minimal and
Solid-state lasers also have had an imcm), which is considerably smaller than
instrument performance consistent, and
pact on flow cytometry, a common techthe 3 3 3-ft footprint of the previously
the more instruments that can be used in
nique for counting and classifying indiavailable argon-based flow cytometers.
a single laboratory, the better.
vidual cells in suspension. Flow cytomeA similar set of requirements led
The compactness of solid-state lasers
ters can rapidly examine physical and
Figure 4. Compact solid-state laser
modules with consistent and reliable
output beam quality make the
manufacture of analytical instruments,
such as the LaserCyte System, more
efficient. Courtesy of Idexx Laboratories.
Cytobuoy BV of Nieuwerbrug, the Netherlands, to employ solid-state lasers in the
design of their miniature flow cytometers
for aquatic research applications. The instruments can measure the composition
and population of phytoplankton and
other aquatic organisms from inside a
buoy or onboard a ship or submarine
(Figure 3). Because phytoplankton possess several naturally occurring fluorescent pigments that are excited at wavelengths between 430 and 690 nm, these
instruments use compact 488-, 532- or
638-nm solid-state lasers.
In the past, marine researchers had to
transport and install their large laboratory flow cytometers on ships
to perform realtime field studies.
In contrast, Cytobuoy sought to create a compact autonomous instrument that could
function unattended on potentially heavy seas
and in remote ge- Figure 5. Solid-state lasers allowed this two-channel image of mouse fibroblast cells to be taken with a
ographic locations Leica confocal microscope system. The DNA in the left image was DAPI-stained and excited at 405 nm. The
while transmitting middle image shows cytoskeleton that was CY3-stained and excited at 543 nm. The right image shows an
data via telemetry. overlay of the two channels.
Consequently, the
laser requirements for this application
autonomous instruments of this type
able, easy to service and inexpensive.
were demanding.
available for remote field monitoring. We
Consequently, the company chose a modThey needed an energy-efficient laser
have very strict requirements on size, enular design that included a custom diode
that was small enough to fit in a buoy
ergy efficiency and low-heat dissipation.
laser module.
and that gave off very little heat. Because
So, solid-state lasers are really the only
Because this instrument relies on light
the instrument had to operate on battery
choice for our application.”
scatter rather than on fluorescence, it uses
power and/or solar cells, energy could not
Idexx Laboratories Inc. in Westbrook,
a 10-mW, 638-nm diode laser module.
be spared for power-hungry laser cooling
Maine, a manufacturer of diagnostic, deWorking in partnership, Idexx and Cosystems. Third, the laser needed exquistection and information products for the
herent designed a solid-state-sensor modite beam pointing and power stability to
animal health industry, was looking to
ule that combines a diode laser with
operate in remote locations without resupply veterinarians with a flow cytomebeam-conditioning optics, custom drive
quiring regular maintenance and, finally,
ter that could make possible diagnosis
electronics and a light-scatter detection
it had to be rugged enough to withstand
during the pet visit by providing hemasystem (Figure 4).
heavy seas.
tology information within minutes rather
To achieve and maintain a positional
George Dubelaar, CEO of the company,
than the days required when samples are
accuracy of ±1 µm for the laser focal spot
explained, “Before the development of
sent to external labs. To work well in a
relative to the flow sample stream core,
solid-state lasers, there were no compact
clinic, the system had to be small, relithe flow cells were redesigned and incor-
porated into the sensor module as well. In
fact, rather than assemble the module
from a laser component, a flow-cell component and a detector component, those
are mated and delivered as a single module that can be assembled in the instrument without additional adjustment.
Also, the modules are now interchangeable and field-replaceable without
the need for laser-to-flow-cell realignment.
The result is a system called the LaserCyte,
a compact and affordable flow cytometer for in-clinic testing of blood samples
by veterinarians.
Confocal microscopy
Developments in solid-state laser technology have had an impact on confocal
fluorescence microscopy as well. This technique provides high-quality sectional views
even from relatively thick specimens.
However, the only reliable UV source that
is suitable for confocal microscopy is 1 m
long, has a large external power supply,
dissipates more than 5 kW of heat and is
expensive to purchase and to operate.
Manufacturers of confocal microscopes
have heard the same customer demands
as manufacturers of other analytical instruments: smaller, more economical and
easier-to-use instruments. Leica Micro-
systems Heidelberg GmbH of Mannheim,
Germany, which makes confocal microscopes, discovered that, for some applications, near-UV solid-state lasers offer
the combination of small size, ease of use,
no cooling water and low operating cost
to meet these demands.
The company has developed a microscope system that uses a 405-nm solidstate laser in place of a traditional UV
source. As Heinrich Ulrich, manager of
development optics, explained, “Although
405 nm has lower excitation for some UV
dyes, it comfortably replaces UV for DAPI
and Hoechst dyes and is an ideal excitation wavelength for CFP (excitation range:
400 to 430 nm). Also, a new photoactivated GFP has its absorbance maximum
around 400 nm and is, therefore, perfect
for the 405-nm laser.” To compensate for
the focus shift between 405 nm and the
visible range, Leica has designed correction
optics that result in perfect Z-axis overlay
and bright images (Figure 5).
The beam characteristics required for
confocal microscopy are achieved using
a single-mode fiber coupling. The 25-mW,
405-nm solid-state laser used by Leica is
packaged in a 218-mm-long standard
cylindrical HeNe laser tube. This configuration is standard for most instrument
manufacturers and facilitates the use of
solid-state lasers in many analytical instruments.
Furthermore, according to Ulrich, “The
405-nm laser is one-third to one-quarter
the cost of a UV laser system and is much
easier to operate.” Consequently, customers benefit not only from a less expensive system, but also from being able
to spend more time on their experiments
and less time on maintenance.
Solid-state lasers are now positioned
with the right combination of wavelengths, output power, reliability and
beam characteristics to allow analytical
instrument manufacturers to capitalize
on their inherent advantages. In many applications, solid-state lasers are the only
solution. Their combination of small size,
energy efficiency, low heat dissipation,
low operating cost, ease of use and robustness has led to the development of a
new generation of field and desktop analytical instruments as well as of more efficient manufacturing techniques and easier field-servicing of instruments.
G
Meet the author
Paul Ginouves is director of marketing for
Instrument and Medical Markets, Coherent
Inc., in Santa Clara, Calif.
5100 Patrick Henry Drive, Santa Clara, CA 95054
Tel: 1-800-527-3786 Fax: 1-408-764-4983
Email: tech.sales@CoherentInc.com Web: www.CoherentInc.com