Optically Pumped Semiconductor Lasers
Dr. Karen Ness
Optically pumped semiconductor lasers offer new solutions in telecommunications and could
replace gas lasers in other markets.
The past few years have
seen a resurgence of interest in optically pumped — rather than electrically
pumped — semiconductor lasers. Optically pumped semiconductor lasers show
significant promise as efficient, compact, reliable sources, offering power levels
of up to several watts and good spatial mode quality at a range of wavelengths.
Such performance, combined with developments in fabrication and device integration
technologies, will lead to a class of versatile devices that could become the
lasers of choice for a variety of applications in market sectors ranging from telecommunications
to biomedical diagnostics.
The growing importance of these lasers as high-performance
devices is due to their advantages in power scaling, spectral and temporal control,
ease of design and processing, wavelength versatility and suitability for pumping
with commercial pump diodes. They combine the design flexibility of a semiconductor-engineered
gain region with extended — if compact — physical laser cavities that
facilitate mode-locking or frequency conversion, thus extending the laser’s
Just emerging are tunable optically
pumped semiconductor lasers, which provide a further degree of flexibility in performance.
Tunability is enabled by microelectromechanical systems (MEMS) technology. Arguably,
no other laser class offers engineers such design freedom, allowing them to tailor
performance to specific application requirements.
Most development activity has focused
on emission wavelengths around 1000 nm, with demonstrated continuous-wave (CW) power
in a circularly symmetric TEM00 mode now exceeding 1.5 W. These sources also have
achieved stable single-frequency operation (unusually, from a linear cavity design)
and more than 200 mW of average power in passively mode-locked picosecond operation.
In materials technology, the gallium
arsenide family provides the basis for devices operating over significant portions
of the visible and near-IR regions of the spectrum. GaAs-substrate-based epitaxial
technology — using active regions including GaInP, GaAs, InGaAs and GaInNAs
— in principle allows devices to be fabricated for any specific wavelength
from 670 to 1500 nm. Effective pumping can be provided by commercially available
pump diodes that emit at 650 to 670, 808 and 980 nm.
Intracavity doubling/tripling can generate
output wavelengths down to the UV. The Coherent Sapphire laser based on this technology,
with an output of 20 mW at 488 nm in a package slightly bigger than a computer mouse,
is now available.
Based on the lasers’ flexibility,
their applications are many — from specific, niche market lasers to larger,
more “mass market” areas, often replacing less efficient or less reliable
devices. In existing markets, they can challenge gas lasers in printing, biomedicine
and scientific applications. As their performance reaches higher power levels, they
could challenge crystalline host solid-state lasers in some industrial applications.
They also could impact biological instrumentation, graphic arts, display, and inspection
of silicon wafers and masks.
Telecommunications is one of the most
exciting application areas for optically pumped semiconductor lasers, where —
despite recent market downturns — the hunger for new technologies likely to
underpin the recovery remains almost undiminished. Here the lasers can be viewed
as a disruptive rather than a replacement technology, offering the potential for
new solutions demanded by innovative network architectures. In recent years, the
GaInNAs family (“dilute” nitrides) has been recognized for its potential
for temperature-insensitive devices at wavelengths that are useful for telecommunications.
The move toward tunable lasers in the
telecommunications industry also could involve the use of optically pumped lasers.
It is estimated that 80 percent of all CW lasers will be tunable by the end of 2002,
with the US market predicted to grow from about $5 million in 2000 to $1.2 billion
by 2004. Some companies are already addressing this market by offering 1.55-μm
MEMS-tunable optically pumped semiconductor sources.
These lasers could well become the
lasers of choice for many markets, both current and yet to come.
Meet the author
Karen Ness is chief executive of the Institute
of Photonics at the University of Strathclyde in Glasgow, UK.
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