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Development of 850 nm 10 Gbps
TOSA/ROSA for Optical Data
Link Applications
VCSEL Achieves High-Speed Modulation over a Wide Temperature Range |
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VCSEL |
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High-speed modulation operation over
a temperature range of −10 to 90°C |
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High reliability |
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As the performance of digital equipment
continues to improve, the amount of information
we have to deal with is increasing radically.
In information transmission, which up to now
has been provided by metal wires, we are
finally seeing an increasing use of light and
optics for high-capacity transmission. Sony has
been pushing forward with the development
of the vertical-cavity surface-emitting laser
(VCSEL), which will be the key device for
such optical communication. The features of
the 850 nm VCSEL Sony has now developed
include the achievement of high-speed
modulation operation at 10 Gbps over the
wide temperature range of −10 to 90°C so that
high-speed optical communication systems can
be used in a variety of environments. In addition
to excellent high-frequency characteristics,
this device also achieves superb reliability
and Sony plans to push forward with entry
into new optical communications markets.
Furthermore, the range of VCSEL applications
does not only consist of LAN, SAN, and other
optical communication networks, but we also
expect that in the near future this device will be used in optical interconnection technology that
forms connections within and between digital
equipment. High-capacity/high-speed optical
transmission technologies that use such optical
interconnections may be used in next generation
TVs and PCs. Sony is now working to establish
VCSEL reliability in ways that include achieving
higher speeds, VCSEL arrays, and resistance
to environmental influences in the expectation
that VCSEL devices will be deployed in these
and other applications.
In this article, we introduce the device
characteristics, the high-speed modulation
performance, and the reliability of the 850 nm
band optical communication 10 Gbps TOSA/
ROSA with a main focus on the VCSEL, which
is the key device for this area.
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 850 nm VCSEL |
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| The edge-emitting laser, in which the laser beam
is emitted from the edge of the chip, is the most
commonly used device in optical disc pickups
and similar applications. In contrast, in the
VCSEL that Sony has now developed, the laser
beam is emitted from the surface of the chip.
(See figure 1.) While the crystal cleavage plane
is used as the laser cavity mirror in edge-emitting
lasers, in the VCSEL, the laser cavity mirror
(DBR) is built into the device in the vertical
direction relative to the crystal. (See figure
2.) A GaAs quantum well structure is used as
the light-emitting material (active layer) and
an AlGaAs multilayer film with a differing
composition is used as the mirror material that
forms the resonator. This device structure allows
the light emitting area into which current is
injected to be made smaller. As a result, the
threshold value in the current/optical output
characteristics becomes less than 1 mA, thus
making low-power drive possible. (See figure 3.) The far field pattern (FFP) of the VCSEL
emitted laser beam is nearly a perfect circle,
as opposed to that of the edge-emitting laser,
which is an oval. This is a feature that makes
the VCSEL effective for use in optical coupling.
Furthermore, the VCSEL has several features
that are not available with edge-emitting lasers,
such as it being possible to create a 2D array
of VCSEL devices, and it being possible to
evaluate the characteristics of each device with
the devices still in the wafer state. |
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| High-Speed Modulation
over a Wide Temperature Range |
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The 2.5 Gbps data transfer rate has been the
norm up to now in communication systems
using 850 nm band VCSEL devices. Along with
the recent advances in the information society,
however, data transfer speeds are poised to
switch to the high speed of 10 Gbps. Sony has
now developed 10 Gbps TOSA (transmitter
optical sub-assembly) and ROSA (receiver
optical sub-assembly) products that include
10 Gbps optical devices (VCSEL and PD)
as optical components for use in high-speed
optical communication transceivers. (See
photograph 1.)
Recently, however, lower power and a wider
temperature range have come to be required
in optical communication transceivers due
to the high density mounting and support
for further space savings that are associated
with the increasing performance of optical
communication systems.
This Sony VCSEL has a threshold current
under 1 mA, can operate with a bias current
under 10 mA, and is thus a low power device.
Furthermore, we expect that the operating
temperature range will become even wider
in the future.
Sony has led the industry by developing an
850 nm band VCSEL that is capable of 10
Gbps high-speed modulation over the wide
temperature range of −10 to 90°C. To expand
the temperature range, Sony optimized the
device layer structure, doping conditions, and
the conditions in each fabrication process and
thus stabilized the modulation characteristics
with respect to changes in temperature. As a
result, this device achieves a mask margin (the
margin with respect to the standard aperture)
of over 20% for all operating temperatures in
the optical waveform for a modulation speed
of 10.3125 Gbps (PRBS: 231 − 1, ER = 6 dB).
(See figure 4.)
Furthermore, the international standards for
optical communication stipulate the resonant
spectral width during high-speed modulation,
and the Sony-developed VCSEL device holds
the RMS spectral width to under 0.40 nm over
the whole temperature range. In the 10 Gbps
TOSA that uses this VCSEL, it is possible to
detect, at all times, the optical output from the
VCSEL with a built-in photodiode. This TOSA
can be operated in auto power control mode over
the −10 to 90°C temperature range by using this
optical output monitor and the VCSEL stabilized
high-frequency characteristics.
Sony, at the same time, has also developed a new
photodiode (PD) as the photodetector device.
(See photograph 2.) The PD must also have the
same excellent high frequency characteristics
as the VCSEL, and it is important to reduce
device capacitance as well. Sony succeeded in
holding the device capacitance to under 0.2 pF
by reducing the residual carrier density in the crystal, which is the PD's optical absorption
region. Due to this capacitance reduction, Sony
was able to achieve the excellent frequency
response characteristics of f−3dB = 19 GHz in
a GaAs PIN photodiode with a 70 μm diameter
aperture. (See figure 5.) The 10 Gbps ROSA
that uses this PD achieves the excellent
characteristics of a −14 dBm minimum receiver
sensitivity. (See figure 6.) |
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Figure 1 : Device Structure
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 High Reliability |
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See
all articles with figures and tables.  |
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Vol.59 |
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