First of All, the Development
of Optical Devices |
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. |
Leaving for work in the morning, while hurriedly
getting ready I suddenly thought that
it would be great to be able to watch high-definition
video on the Shinkansen (bullet
train). With toast in one hand, a video file
could be downloaded instantly from a high-capacity
storage device to a mobile device.
Without feeling any stress...
If optical interconnections were applied to
digital equipment and home networks, this
sort of scene would become a daily reality.
The key device which will become the core
technology for optical interconnection is the
vertical-cavity surface-emitting laser, a device
called the VCSEL. (See figure 1.) Compared
to the earlier edge-emitting laser, the
active region that emits light in the VCSEL
is smaller, and the VCSEL can achieve laser
oscillation at lower current levels. Furthermore,
the VCSEL is optimal for switching
on and off at high speeds. Researchers had
previously considered using this device for
optical interconnection. |
|
| Rapid Modulation and Stable
Operation Achieved by Unique
Technologies |
| . |
. |
The VCSEL device that Sony has now developed
independently features a wavelength
of 850 nm and supports high-speed operation
at up to 10 Gbps per channel. (See figure
2.)
• Improving the quality of the optical
modulation properties
Since the VCSEL has even more layers in its
structure than the edge-emitting laser, in general,
this would result in the disadvantage of
it having a higher electrical resistance.
The goals of this development project were
the reduction of this electrical resistance and
the parasitic capacitance and the improvement
of the transmission characteristics by optimizing
each of the device parameters. The aperture
ratio for the optical modulation properties
(eye pattern) due at the high-speed operation
of 10 Gbps showed the excellent characteristics
of a waveform margin of 15% or
higher for the IEEE standard (10GBASE-SR)
at all temperatures in the operating temperature
range from -10°C to +85°C. (See figure
3.)
• Assuring high reliability at high
temperatures and high humidity
Another point of this development effort was
that in addition to high-speed operation, the
device should also be highly reliable. We
analyzed the characteristic degradation modes
of the VCSEL device and worked on optimizing
the device fabrication conditions. We
verified that this device can provide stable
operation for over 5000 hours at the overstressed
state of 85°C and 10 mA. (See figure
4.) |
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| High Environmental Resistance
even in the Bare Chip |
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. |
Previously, it was common for the light emitting
and receiving devices used for communication
to be mounted in hermetically sealed
packages. When considering the practical
application of VCSEL devices, however, in
addition to resulting in larger module sizes,
this type of mounting has the problems that
it has limited expandability for the number
of channels and high mounting costs.
At Sony, we assumed that these devices
would be used in the bare chip form and carried
out environmental testing. In an environmental
storage test at a temperature of
85°C and a humidity of 85%, these devices
achieved an environmental withstand ability
of a change in optical output of less than 0.3
dB even after storage for 2000 hours. Thus
we showed that these devices can be expected
to provide significant improvements in both
mounting costs and expandability. |
|
| Optical Modules Designed
for Practical Use |
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. |
If we were simply to provide light emitting
and receiving devices that required different
handling from other electronic components
to end product developers, it would be difficult
for them to take full advantage of these
devices. Therefore we developed, along with the devices themselves, optical modules that
integrate four channels of 10 Gbps VCSEL
(or photodetector (PD) array) with a 4-channel
optical fiber array. (See figure 5.)
A point that becomes an issue in optical module
design is how to lead the laser beam output
in the vertical direction from the VCSEL
to the horizontally oriented optical fibers.
The key point for practical application is how
reliably the beam can be transmitted in a
structure that is at the same time as simple
as possible. The optical module developed
in this effort provides 45-degree mirrors and
optical fiber guide grooves in an optical
waveguide using polymer materials to clear
this hurdle. (See figure 6.)
The laser beam output from the VCSEL device
is reflected by a 45-degree mirror and
the core section with a high index of refraction
acts as a condenser. The laser beam is
propagated through this core section and
coupled to the optical fiber in the fiber guide
groove. |
|
| Low Cost Batch
Integrated Mounting |
| . |
. |
In general, active alignment mounting is used
for the positioning operation when optical
fiber, optical waveguides, and light emitting
and receiving devices are combined in a
module. In this technique, the light emitting
and receiving devices are actually driven and
the strength of (or loss in) the optical signal
measured. The relative positions of the devices
are adjusted (positioning and centering)
one at a time while performing that measurement.
Although this technique allows
precise positioning, it is not possible to avoid
increased complexity in the mounting procedure
and rising costs.
In contrast, passive alignment mounting is a
technique in which image recognition technology
is used and positioning is performed
with just the mechanical precision of the
equipment. In the optical module developed
in this effort, the addition of 45-degree mirrors
and fiber guide grooves to the optical
polymer waveguide made it possible to use
batch integrated mounting based on passive
alignment. Positioning can be performed simply
by surface mounting the 45-degree mirror
and inserting and bonding the optical fiber
in the fiber guide groove in the
waveguide. The optical insertion loss at this
point is held to about 1.7 to 3.2 dB.
When we investigated the optical coupling
tolerance between the VCSEL and the optical
waveguide, we verified that if the positional
displacement was within a ±15 μm
range, the optical coupling loss could be held
to under 1 dB. (See figure 7.) The value ±15
μm is much wider than the tolerance (around
±1 μm) in ordinary optical communication
devices, and can be fully met by ordinary
mounting equipment. That is, this approach
is also appropriate from the standpoint of effective
use of existing manufacturing infrastructure. |
|
| This Approach Also
Supports the Expansion of
the Number of Channels |
| . |
. |
Figure 8 shows our prototype optical
waveguide module. By integration with the
drive ICs for the light emitting and receiving
devices this module can directly input and
output signals by photoelectric conversion of
10 Gbps electrical signals input or output
through the electrical connector at the end of
the package.
Although in this effort, we created an optical
module based on a four-channel array of light
emitting and receiving devices, this structure
can easily support expansion of the transmission
and reception functions or the number
of channels (from 1 to 12 channels) with different
combinations of devices.
Note that the optical waveguide module developed
in this effort also exhibited the excellent
stability characteristics of an output
variation of less than ±0.6 dB, both in a 2000-hour storage test at a temperature of 85°C and
a humidity of 85%, and in a -40°C/+85°C heat
shock test. |
|
|
Towards Use in
Consumer Equipment |
| . |
. |
There is now a strong prospect for the practical
application of this 10 Gbps × 4 channel
optical waveguide module. The following are,
however, required before use in consumer
equipment, which is our end goal, will be
possible.
• Flexible support for a wide range of expansion
needs
• Further improvements in environmental resistance
characteristics
• Market awareness and the creation of actual
results
• Further cost reductions
As a first step towards that goal, we are investigating
the development of a 10 Gbps
TOSA/ROSA for optical communication for
release in 2007. We plan to proceed by accumulating
results in fields such as these and
continue with preparation for use in optical
interconnection for digital home appliances
and home networks. |
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 |
 The time for "Optics" to make its appearance has finally arrived.
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See
all articles with figures and tables.  |
|
Vol.47 |
Sony Corporation Global Headquarters
