Micro-electro-mechanical
systems (MEMS) are microscopic devices, particularly those with moving
parts. The MEMS technology merges at
the nano-scale into nano-electro-mechanical systems (NEMS) and nano-technology.
In this
article, we report on component-level MEMS-based Variable Optical Attenuators
(VOA), which allow for the automated, fine-tune control of the attenuation
level in an optical system. Commonly, MEMS-VOAs incorporate an extremely stable
MEMS mirror to reflect the light that is collected and collimated (lenses) from
the input fiber to aim the light to the output fiber.
MEMS-based
Variable Optical Attenuators are often used in optical communication systems
where the optical signal is too strong and needs to be reduced, in which the
attenuation, also called transmission loss, helps with the long-distance
transmission of digital signals.
Attenuators are commonly used in fiber optic communications, either to
test power level margins by temporarily adding a calibrated amount of signal
loss, or installed permanently to properly match transmitter and receiver
levels.
Fiber optic
switches and variable optical attenuators (VOA) are critically important
functions, especially in optical fiber networks for the telecommunication and
data communication industries. In October 2016, we are seeing service providers
taking an opportunity of their 100G technology implementations, as well as
test/development of 400 Gigabit Ethernet line rate traffic, to upgrade their
optical network infrastructure, and thereby driving an increase in market
demand for advanced photonic switches and VOAs.
The company
Xtera was one of the first optical networking equipment suppliers to embrace
100G coherent technology for increasing the channel rate carried by optical
wavelengths in backbone networks. The
company’s flex-rate interface card combines high transmission performance with
multiple levels of flexibility in optical networking, delivering 100G, 200G,
300G or 400G coherent channels in order to offer the level of capacity and
reach performances required by network operators’ needs and applications, from
regional terrestrial networks to long-haul repeatered submarine cable systems.
Optical
attenuators can take a number of different forms and are typically classified
as fixed or variable optical attenuator (VOA). Fixed attenuators can be
segmented into either build out style or incorporated into a fiber optic patch
cord. The build out variety is a small (~ 1.25 inch long) attenuator with a
male fiber optic connector interface on one end and a female interface
connector on the opposite end. The build out style is typically fabricated with
either air gap attenuation or doped fiber attenuation.
An optical attenuator can use a segment of attenuating fiber interposed
in the optical path. Using a solution doping technique to introduce transition
or rare earth elements into the fiber’s core produces the attenuating fiber.
The dopant reduces the transmission of the fiber. The degree of attenuation
depends upon the material used as the dopant, the dopant level, and the length
of the attenuation segment. In a specific embodiment, an optical attenuator is
provided having a first and second signal carrying optical fibers and an
attenuating fiber segment, each of which has a core, a cladding substantially
coaxial with the core, and a substantially planar end face. The attenuating
fiber segment is fusion spliced between the first and second signal carrying
optical fibers. In a second embodiment a portion of the cladding of the
attenuating fiber is chemically etched.
The variable optical attenuators (VOA) is a basic building block for
several optical systems such as wavelength division multiplexed (WDM) transmission
systems, optical beam formers, fiber-optic adaptive controls, and other
applications.
Built-in
variable optical attenuators may be either manually or electrically
controlled. A manual device is useful
for one-time set up of a system, and is a near equivalent to a fixed
attenuator, and may be referred to as an "adjustable
attenuator". In contrast, an
electrically controlled attenuator can provide adaptive power optimization.
The
advantages of an electrically controlled device include speed of response and
avoiding degradation of the transmitted signal. Dynamic range is usually quite restricted, and power feedback may
mean that long-term stability is a relatively minor issue. Speed of response is
a particularly major issue in dynamically re-configurable systems, where a
delay of one millionth of a second can result in the loss of large amounts of
transmitted data.
Variable attenuators are ideal for
simulating cable loss for research and development (laboratory) testing of
optical communication link power limits or reducing power in the links where
receivers are in the process of being overloaded. Fixed in-line (cable assembly/jumper) attenuators can distinguish
the color band coding process to simplify the specification identification of the
optical communication link components during field installation, stocking, or
maintenance operations. Variable
optical attenuators enable adjustment capabilities, so the injected loss may be
simply reduced as specific components degrade and increase their own
attenuation over a few years.
According to ElectroniCast, MEMS-based VOAs
are the leading and dominate technology for accomplishing electronically and
automatically controlled optical attenuation; therefore we are following the
activities of about twenty (20) vendors active in this product-line (see a
couple of examples below).
The company DiCon Fiberoptics, Incorporated
has a patented core MEMS mirror technology is based on a single-crystalline
silicon construction that does not deform, fatigue or wear out over time. The
mirror (the octagonal area in the middle of the chip) is tilted by applying a
voltage to the electrostatic comb-finger actuators that extend to either side
of the mirror. The moving parts of the structure, including the mirror and half
of the comb-fingers in the actuators, are connected to the fixed parts of the
structure via silicon torsion beams. Thus, the moving parts of the structure
are affectively suspended in space, and never come into physical contact with
the fixed parts of the structure. The mirror tilts over a continuous range of
motion, with a highly repeatable tilt angle as a function of the applied
voltage.
OZ Optics Ltd. offers MEMS-based variable
optical attenuator (VOA) in miniature packages, available either as single
units or as arrays of attenuators and offered with either Single-mode or
Polarization Maintaining (PM) fibers.
In a January (2015) research paper,
researchers from Nanyang Technological University (Singapore), Institute of
Microelectronics, A*STAR (Singapore), and the National Key Laboratory of
Science and Technology on Micro/Nano Fabrication Institute of Microelectronics,
Peking University (China), reported on a Nanoelectromechanical System (NEMS)
Variable Optical Attenuator (VOA) driven by the optical gradient force. The VOA
is realized via a waveguide based directional coupler. The researchers stated
that the NEMS VOAs have the merits of small dimension, low power consumption
and good capability for all optical integration, making it a good candidate for
future applications in silicon photonics circuit and optical communication
devices.
For over 20-years, we have been tracking
the worldwide use of component-level fiber optic attenuators in
communication. In the market forecast, when counting (quantifying)
variable optical attenuator array modules and integrated modules, which may
have more than one component-level attenuator, each component-level attenuator
is counted separately. For example:
with an integrated value-added module, we count only the complete
(component-level) fiber optic attenuator as well as cost-adjusting for the
optics, optical fiber alignments, and optical fiber and associated packaging,
and other required materials.
ElectroniCast Web-site: www.electronicast.com
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