Laser power and energy
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August 2017 www.laserfocusworld.com Laser Focus World 48
VISIBLE SEMICONDUCTOR LASERS continued
module for cytometry called Cell X introduced in 2017 takes the
standardized product concept to its conclusion by integrating
up to four lasers inside the module. This integration also allows
cost reduction through consolidation of hardware and electronics, for instance, by using a single laser controller board, common power, and I/O connector.
In multiwavelength flow cytometry, the total number of cell
types discernable is maximized by correlating many different
signals from mul-
tiple excitation (or
light scatter) sourc-
es. To do this, the
instrument needs to
know whether flu-
ed at, say, 550 nm
is because of exci-
tation at 405 or 488
nm. The most com-
mon way to achieve
this is to arrange the
focused laser beams
as a staggered se-
quence of elliptical
foci aligned with the flow stream, as shown in Fig. 1. The use
of a trigger signal and time-delay gating allows for the correla-
tion of sequentially generated signals from a single object into
a single data packet. Thus, signal discrimination as a function
of both emission wavelength selection and position in the flow
stream is achieved.
While providing for these common elements of flow-cy-tometry instruments, the OPSL module provides flexibility
to customize laser beam parameters for specific instrument
designs (see Fig. 3). Most importantly, the output optics for
the four beams are all independently adjustable by the OEM
integrator to optimize focus and to generate different sequences and spacings. The separation can be varied from zero (
co-aligned) to ±250 µm—in addition, each of the four lasers is
independently addressable and controlled through a standard USB connection.
John Abbott is director of LMC Sales, Dan Callen is a product manager, and Matthias Schulze is director of marketing, all at Coherent, Santa
Clara, CA; e-mail: firstname.lastname@example.org; www.coherent.com.
OPSLs and laser diodes
Two complementary solid-state laser technologies service
most of the life sciences market: the optically pumped semiconductor laser (OPSL) and the laser diode. Consequently, a
key milestone was the development of smart, plug-and-play
compact lasers having identical packaging, irrespective of
their internal technology.
In an OPSL, a quantum-well-type indium gallium arsenide
(InGaAs) chip is optically pumped by one or more laser diodes. By changing the size and stoichiometry of the quantum
wells, and using harmonic conversion of the resulting output,
the laser wavelength can be tailored over a wide range from
355 nm throughout the visible and into the infrared. In addition to wavelength scaling, the technology is power-scalable
and power-adjustable because there is no thermal lensing as
in crystal-based lasers. The large-area gain chip also naturally
supports a TEM00 circular output beam.
Once limited to the red and blue, laser diodes are now available at several visible wavelengths. In general, they offer lower power than OPSLs, but conversely, their cost is lower and
they can be directly modulated at even higher speeds of up to
hundreds of megahertz. However, their highly divergent and
asymmetric output beam means that several optics are required to create a collimated near-circular output beam.
The optimum choice of technology for a specific application depends on the wavelength and power level, as well as
the required modulation speed and beam quality. Packaging
both types in an identical plug-and-play format enables the
optimum choice at each wavelength in every application,
as well the capability to select the right wavelength for the
problem at hand.
FIGURE 3. With a bare laser, the beam
quality and pointing are key specifications.
But with a module, adjustable focus
parameters of several elliptical spots are
instead specified. This example is for a four-wavelength CellX module rated at 15 m W at
each laser wavelength.