Quantum-Dot Lasers’ Performance Makes Great Gains
Tom Tumolillo Jr.
plateau of quantum-well technology has opened the door for its use in lowering costs
and improving the performance of telecom and datacom applications.
The evolution of laser diode design has proved critical to the advancement
of low-cost, high-performance transmitters for telecom and datacom networks, especially
in light of the downturn in the optical communications market. With increased demand
for lower-cost laser sources, companies are finding it impossible to meet these
goals with current quantum-well technology, which has reached a performance plateau.
By comparison, quantum-dot technology offers a
significant reduction in threshold current density (Figure 1). Other benefits include
improved temperature performance, reduced chirp that enables 10-Gb/s direct modulation,
increased lasing slope efficiency and less susceptibility to optical feedback. This
allows isolator-free operation, an important step in reducing system costs. Reduced
transmission laser costs translate directly to lower costs per bit for carrier network
Figure 1. Besides a reduction in
threshold current density, the evolution in laser diode design from double hetero
to quantum-well and then quantum-dot technology also presents a corresponding improvement
in the price/performance ratio (inset).
The current 1310- and 1550-nm quantum-dot
laser diode technology is built upon existing quantum-well design. The quantum dots,
formed in wafer deposition through molecular beam epitaxy, are semiconductor nanostructures
that act as artificial atoms by confining electrons and holes in three dimensions.
Working with either GaAs or InP substrates, the process forms initial deposition
layers that are lattice-matched (or coherently strained) to the substrate, with
deposition of the quantum-dot layer forming the active region. The InAs dots result
from differences in the strain layers that cause self-assembly, also known as the
Stransky-Krastanov process. The core material structure technology is called dots-in-a-well,
or DWELL technology.
Completion of the laser structure involves
depositing material layers lattice-matched to the substrate. Other than the quantum-dot
layer, preceding and subsequent material layers are really no different from existing
quantum-well laser structures. However, it is these tiny nanostructures that lead
the device to a new standard of semiconductor laser performance (Figure 2).
Figure 2. The purer signal of quantum-dot technology compared with quantum-well designs
will enable isolator-free operation and potentially eliminate up to 25 percent of
the component cost.
Fueling the recent commercialization
of quantum-dot systems are properties such as high temperature stability, a purer
signal than conventional quantum-well devices and a broad gain spectrum. End users
can expect further performance gains resulting from ongoing quantum-dot research
by universities, consortiums and industry.
For example, the 1310-nm quantum-dot
semiconductor optical amplifier has demonstrated an 18-dB gain, 9-dBm saturated
output power, an 8-dB noise figure and a 10-ps gain recovery time. These results
show the potential for operation free of pattern effects at high bit rates under
saturation, and commercially viable gain and output power in a device that is roughly
10 times shorter in length than previous quantum-dot semiconductor optical amplifiers.
A broad gain spectrum, temperature insensitivity and potential for an inherently
lower noise figure will enable dot semiconductor optical amplifiers to challenge
erbium-doped fiber amplifiers.
Theoretical work out of the Technical
University of Denmark in Lyngby also shows the potential for quantum-dot semiconductor
optical amplifiers to provide performance equivalent to that of erbium-doped fiber
amplifiers, but at dramatically lower cost. Based on previous gain bandwidth data
from the University of New Mexico in Albuquerque, the researchers predict that amplification
could occur over a 200-nm range. They also want to take advantage of the ultralow
linewidth of quantum-dot lasers — down to 100 kHz — for spectroscopic
applications with wavelengths as long as 2 μm on InP substrates, as well as
to investigate how dot density and multiple stacks of dots impact signal purity
Further research efforts at the University
of New Mexico are focusing on the development of 1310-nm quantum-dot vertical-cavity
surface-emitting lasers for feedback insensitivity and high temperature stability.
When it comes time to commercialize these devices, component designers should expect
tests for reliability and lifetime to show that performance is equal to that of
quantum-well devices. The mortality rate of Zia Laser’s product is comparable
with that of existing InP and GaAs quantum-well systems, and engineers believe that
quantum-dot technology should have no inherent reason for premature failure.
Anticipated innovations include direct-modulation
10-Gb/s 1310-nm uncooled distributed feedback performance with, eventually, isolator-free
operation. Zia expects 10-Gb/s direct modulation to be the overwhelming solution
for advanced optical components for at least the next decade based primarily on
cost, performance and technological feasibility.
It is also important to consider that
adoption of higher-speed networks takes many years to fully develop and implement.
Along with OC-192 bit rates, uncooled performance is critical for lowering the cost
of components. Because of the higher temperature-insensitivity properties of quantum-dot
systems compared with quantum-well lasers, end users can expect to see future quantum-dot
lasers reaching 100 °C performance.
Meet the author
Tom Tumolillo Jr. is director of product management
at Zia Laser Inc. in Albuquerque, N.M.
- optical communications
- The transmission and reception of information by optical devices and sensors.
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