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42 Optics & Photonics News
■
March 2003
Tunable
MEMS
Devices
For Optical Networks
Jill D. Berger and Doug Anthon
Early deployment of tunable lasers and filters
in optical networks that use dense wavelength division
multiplexing has been driven by the inventory advantages of
universal line cards and multiwavelength sparing. But the
greatest benefits can be achieved by using tunable devices
for dynamic wavelength provisioning in reconfigurable
transparent optical networks. A prerequisite for the
realization of dynamic optical networks is the availability
of tunable devices that meet the rigorous performance
requirements of fixed-wavelength transceivers—at
comparable manufacturing costs. Widely tunable
external-cavity diode lasers and diffraction-grating
filters based on silicon microelectromechanical
actuators meet these criteria.
Widely tunable external-cavity diode laser, based on a silicon MEMS actuator, for optical
network applications.
March 2003
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Optics & Photonics News
1047-6938/03/03/0042/8-$0015.00 © Optical Society of America
43
TUNABLE MEMS DEVICES
S
ignificant advances in optical technology, particularly the development of optical amplifiers and
dense wavelength division multiplexing
(DWDM), have transformed the design
of metro, regional, long-haul, and undersea telecommunications networks. Over
the course of the past decade, the number
of channels that can be carried on a single optical fiber has risen from one to
more than 100, while data rates per channel have increased from 622 Mbits/s to
10 Gbits/s. Single-channel systems that
once required optical-electrical-optical
(OEO) regeneration at 40-km intervals
have been supplanted by optically amplified DWDM systems with regeneration
intervals of up to 3000 km. These
advances tend to drive networks away
from electronically switched “opaque”
architectures, with frequent OEO conversions, and towards transparent, optically
switched designs.
Rapid technological progress,
combined with the readily available capital in the late 1990s, generated an abundance of bandwidth that has yet to be
fully employed. Although the marginal
cost of providing bandwidth is rapidly
falling, carriers are still having difficulty
achieving profitability, in part because of
the low prices being paid for data services. For carriers seeking to maximize
revenue and minimize both capital
investments and operating expenses,
dynamic provisioning may be one solution. Opaque networks contain a number
of costly redundancies that can be eliminated with appropriate network design.
For example, it has been estimated that
an agile transparent network, one that
permits nonterminating channels to pass
through nodes in optical form, can eliminate more than half the OEO conversions
found in an opaque network.1
End-user bandwidth demand changes
dynamically, because of both traffic
growth and traffic churn. In fixed-wavelength networks, traffic churn results in
stranded, unusable bandwidth.
Provisioning time frames of up to several
months—without benefit of preprovisioning to meet projected future traffic
demands—make it difficult to respond to
traffic growth. And when forecasting
inaccuracies occur, the result is equipment that is both deployed and operat-
44 Optics & Photonics News
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March 2003
(a)
D
A
Tunable Receiver
Tunable Transmitter
B
1
TX1
Fiber
1 ... N
2
TX2
C
Optical
Switch
N
2
RX2
DEMUX
MUX
TXN
1 R
X1
Optical Add/Drop
Multiplexer
Optical
Amplifier
Terminal
Terminal
N:1
N
RXN
1:N
M:1
Wavelength-Insensitive
Power Multiplexing
1:M
Wavelength-Insensitive
Power Demultiplexing
Figure 1. (a) Optical network based on widely tunC
(b)
able lasers and filters. At node A, signals from an
array of tunable transmitters, each composed of a
D
A
B
tunable laser and modulator capable of transmitting
E
any one of 100 channels, are combined by a waveG
length-insensitive power combiner and transmitted
through an amplified fiber link with transparent
Transparent Node F
nodes B and C. Signals arriving at B and C may be
locally terminated, or passed through to other nodes. Locally added or passed-through
signals can be dynamically switched to any other transparent node. At node D, the signals
are demultiplexed by a wavelength-insensitive power splitter and detected by an array of
tunable receivers, each composed of a tunable filter and photoreceiver. (b) Dynamic traffic
routing in an agile network can be accomplished by changing the tunable transmitter/
receiver wavelength or the optical switch configuration. Traffic traveling from A to C is
rerouted from A to D by changing the transmitter wavelength at A. Traffic travelling from
G to D is rerouted from G to E by changing the optical switch configuration at F.
ing, but not carrying revenue-generating
traffic. In agile optical networks, such as
the system depicted in Fig. 1, bandwidth
can be remotely provisioned in only minutes instead of weeks or months, and subsequently reprovisioned as revenuegenerating traffic demands change. With
appropriate network management, frequency contention problems can be minimized and optical routing becomes an
efficient tool for dynamically reconfiguring networks.2
Tunable transmitters based on tunable
lasers are analogous to fixed-wavelength
transmitters. But in a transparent optical
network, tunable filters can play several
different roles. Single-channel tunable
bandpass filters are used in amplified
transmitters for the suppression of amplified spontaneous emission (ASE) and in
receivers as adaptive prefilters for noise
reduction. Tunable filters can reduce the
costs of optical performance monitoring
by allowing one monitor to select
between multiple channels. More generally, tunable filters can be used in the
reconfigurable optical add-drop multiplexers (ROADM) that are the central
switching element of transparent optical
nodes. A ROADM can be constructed
entirely from single channel tunable filters using a broadcast-and-select architecture. In this case, the incoming signal
is amplified and split into N independent
outputs that are divided into groups of
pass-through channels, branched channels and received channels. Although the
number of channels of each type is determined by the hardware, the use of tunable filters allows for control of the
network configuration by making it possible to modify decisions over which signal is routed where.
Because most optical networks are still
opaque, with a set of point-to-point links
connected through a mesh of electrical
switches, full advantage cannot be taken
of the benefits offered by tunability.
TUNABLE MEMS DEVICES
Nonetheless, tunability offers a solution
to the increasingly difficult problem of
inventory stock and sparing for DWDM
transmitters and receivers used in opaque
networks capable of carrying more than
100 wavelengths per fiber. In case of
hardware failure, a complete inventory of
fixed-wavelength transmitters and
receivers must be carried at numerous
locations throughout the country so that
spares can be deployed into the network.
Widely tunable lasers and filters offer a
cost-effective solution to this problem by
allowing the creation of universal line
cards that can be dynamically tuned to
operate at any one of 100 wavelengths.
Tunable transceivers have to meet or
exceed the rigorous telecom performance
demands made on fixed-wavelength
transceivers—and at comparable costs.
Today, DWDM channel spacing, which
has already migrated from 100 GHz to
50 GHz, is moving toward 25 GHz. Longhaul systems are now being designed to
take advantage of both the C-band
(~1530-1565 nm) and the L-band
(~1570-1610 nm) with 100 channels in
each band. Long-distance, transparent
transmission over distances greater than
3000 km requires tight specifications on
laser parameters (linewidth, frequency
stability and output power) and on filter
parameters (pass bandwidth, loss and
dispersion). Metro systems are now being
designed with 32 wavelengths, a reach of
several hundred kilometers, and data
rates of 2.5 Gbits/s per channel. Although
metro systems use very low-cost directly
modulated lasers, they can still benefit
from tunable lasers and filters that reduce
inventory costs and enable agile optical
networking to handle unstable demand
patterns.3 Because network reconfigurations are infrequent on a 1-s timescale,
acceptable network performance can be
achieved by use of lasers and filters with
tuning times in the tens of milliseconds
range, within the requirements of
SONET restoration. These tuning times
are difficult for thermally tuned devices,
but are achievable using MEMS devices.
Telecom components must also meet
Telcordia reliability criteria and be capable of operating in a variety of environmental conditions.
The widely tunable laser technologies
available for telecommunications appli-
cations include: MEMS tunable externalcavity diode lasers (MEMS-ECL)4,5;
distributed Bragg reflectors (DBR)6-8;
temperature-tuned distributed-feedback
lasers (DFB)9,10; and MEMS tunable vertical-cavity surface-emitting lasers
(VCSELs).11 The technologies employed
for widely tunable filters include MEMS
diffraction-grating filters12 and fiberBragg-grating,13 thin-film-dielectric14,15
and fiber-Fabry-Perot filters.16 Each of
these technologies has advantages and
disadvantages. Yet the fact that multiple
vendors are bringing a variety of components to market has enabled the deployment of tunable devices to begin.
MEMS-based external-cavity diode
lasers and diffraction-grating filters offer
optical performance and environmental
stability rivaling that of high-performance
fixed-wavelength DFB lasers and fiberBragg-grating or thin-film dielectric filters. These devices support full C- or
L-band tuning at 25, 50 or 100 GHz channel spacings, with accurate wavelength
control over the required environmental
conditions. MEMS external-cavity diode
lasers offer high power, high side mode
suppression, narrow linewidth and low
relative intensity noise (RIN); for use in
cost-sensitive metro networks, they can
also be directly modulated at 2.5 Gbits/s.
MEMS diffraction-grating filters offer
narrow bandwidths for high channel isolation, as well as low insertion loss, low
polarization dependent loss (PDL) and
low dispersion. The manufacturing costs
of MEMS-based agile optical components
are on par with the scalable cost structures of their fixed-wavelength counterparts, so that direct replacement of
fixed-wavelength devices can be justified
purely on the basis of reduced inventory
and sparing. This means that dynamic
optical networking with photonic layer
intelligence comes almost for free.
MEMS tunable
external-cavity diode lasers
The tunable laser shown in Fig. 2 is a
Littman/Metcalf external-cavity diode
laser17,18 tuned by a silicon MEMS actuator.4,5 The gain medium is based on a
high-power, 1.55-m InGaAsP/InP multiple-quantum-well laser diode.
Reflections at the intracavity facet of the
laser diode are suppressed by use of a low
reflectance (< 2 10-3) antireflective
coating in combination with an angled
facet; this makes possible an effective
facet reflectance of less than 10-4. The low
intracavity facet reflectance enabled by an
angled facet is highly stable over the lifetime of the device. The laser output beam
is collimated at the opposite diode facet,
which serves as the output coupler of the
resonator. Light emerging from the intracavity diode facet is collimated by a
diffraction-limited, micro-optical lens
Glossary
Agile optical network—An optical network in which data traffic can be
dynamically routed by changing either the tunable transmitter or receiver
wavelength or the optical switch configuration.
Dynamic wavelength provisioning/routing—Remote provisioning or routing
of traffic by changing the tunable transmitter or receiver wavelength in an
agile optical network.
Multiwavelength sparing—Use of a single tunable transceiver as a spare to
replace many different fixed-wavelength transceivers in case of hardware failure.
Opaque network—Optical links connected through a mesh of electrical switches,
where the optical signals are detected, converted to electrical, and retransmitted
as optical (OEO).
Provisioning—Deployment/allocation of bandwidth into an area of the network.
Traffic churn—Changing bandwidth demand patterns in a telecom network.
Transparent network—Optical links connected through optically transparent
nodes without OEO conversion.
Universal line card—Hardware with transceivers that can be remotely controlled
to operate at any one of many optical telecommunications wavelengths.
March 2003
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Optics & Photonics News
45
TUNABLE MEMS DEVICES
and then diffracted at grazing inciof a new class of devices for
Lenses
telecommunications systems.21
dence from a high-efficiency, freePM
Fiber
Many of the large arrays of optical
space diffraction grating. The first
Diffraction
Pigtail
Grating
Laser
switches have been built by use of
diffracted order travels to an external
Wavelength
Diode
Locker
surface micromachining, the techmirror mounted out-of-plane on a
Silicon
nique whereby sensors and actuarotary silicon microactuator.
Mirror
MEMS
tors are made from patterning thin
Wavelength tuning is achieved by
Actuator
films, such as polysilicon, on the
applying a voltage to the microactuShutter/
surface of a silicon substrate and
ator, which rotates the mirror to
Beamsplitter VOA
sacrificially etching another mateallow a specific diffracted wavelength
Isolator
rial, such as silicon dioxide, to free
to couple back into the laser diode.
the mechanical structure. Most
The actual wavelength of the laser
accelerometers for deploying air
output is determined by the gain
bags are now made in this way.
Figure 2. Schematic drawing of the packaged MEMSbandwidth of the diode, the grating
Actuators made by use of this techECL-WLL,
showing
a
scanning
electron
microscope
dispersion and the external-cavity
(SEM)
image
of
the
MEMS-ECL
on
the
left
and
the
isonique are poorly suited for translatmode structure. The laser-diode gain
lator, WLL, shutter/VOA and fiber coupling on the right.
ing or rotating relatively large,
bandwidth is greater than 80 nm,
externally fabricated elements—
while the external-cavity mode spacsuch as mirrors or lenses—because
ing is only 0.16 nm. For this reason, a
their
actuator force is low. For these purlarge number of external-cavity modes
hops by allowing the cavity phase to
poses,
higher force actuators made by
are supported by the diode-gain medium,
change by several wavelengths across the
deep-reactive-ion
etching (DRIE) of a
although the narrow spectral passband of
full tuning range. In this case, adding an
silicon
substrate
are
preferred.19,20
the diffraction-grating filter introduces
independent cavity-length control actuaAlthough thermal actuation and magmore than 1 dB loss at the adjacent side
tor can optimize performance at any
netic actuation are used for some devices,
modes. This spectral filter works in comchannel by adjusting the laser frequency
electrostatic actuation is still preferred.
bination with gain-saturation mechawith respect to the grating-filter passDevices based on electrostatic actuators
nisms to suppress the adjacent side modes
band. The actuator voltage determines the
can be accurately positioned and easily
and preferentially select a single externallaser frequency with an open-loop accudriven with up to 150 V at very low
cavity mode. Design criteria, including a
racy of approximately 10 GHz, and the
current.
spectrally narrow grating-filter passband,
frequency is then stabilized to ± 1.25 GHz
The actuator fabrication process is rela high external-cavity feedback level, low
by use of the error signal from a waveatively simple, requiring only five masks
intracavity diode-facet reflectance and a
length locker (WLL) etalon in a servo that
(see Fig. 3). First, alignment marks are
well-confined single-spatial-mode diode
adjusts the mirror position.
formed on the front and back surface of
waveguide are all key to ensuring stable,
The collimated output beam from the
an oxidized carrier wafer. Shallow cavities
single-mode laser operation.18
ECL, with optical power of up to 70 mW,
are then plasma etched into the silicon on
The laser is tuned by applying voltage
passes through an isolator and is coupled
the front surface. These cavities will
to the comb elements of the MEMS actuinto a polarization-maintaining (PM)
define what portions of the actuators will
ator to produce an electrostatic force that
fiber pigtail with 65% coupling efficiency.
be free to move and which will be
rotates the mirror about its virtual
A beamsplitter reflects 5% of the light
attached to the carrier. A second wafer is
pivot.19,20 The angular range of the actuainto an integrated wavelength locker. The
fusion bonded to this front surface and
tor determines the tuning range; a variety
MEMS shutter is used to block the output
then ground and polished to the desired
of 150-V actuators, with ranges of up to
for dark tuning; with suitable external
actuator device thickness, in this case
± 2.8 degrees, have been used to tune over
feedback, it can also be used as a voltage85 m. The alignment marks from the
a wavelength range of up to 42 nm. The
controlled attenuator (VOA). The
bottom surface are then transferred to
mirror’s geometrical pivot can be
MEMS-ECL-WLL is assembled on a
the new top surface to allow alignment
designed to adjust the cavity length
ceramic substrate, bonded to a thermowith the now enclosed cavities in the
because the laser tunes in a way that
electric cooler (TEC), and packaged, as
device. After oxidation, contact-hole etch,
maintains constant cavity phase at all
shown in Fig. 2, in a hermetic, 18-pin butand metallization deposition and patwavelengths across the tuning range.
terfly package. The result is a compact,
terning, the DRIE etch is performed
The laser wavelength determined by the
environmentally robust, high-perforthrough the thickness of the top wafer
diffraction angle then scans synmance tunable laser that is suitable for
until it intersects the cavity. Those porchronously with the grating-filter passnetwork applications.
tions of the device that are above the cavband, and the laser tunes continuously
ities are then suspended, typically by
MEMS design and fabrication
without mode hops. The pivot point can
narrow flexural elements, to parts of the
also be designed to provide for a limited
device still fusion bonded to the carrier
The availability of high-performance
continuous tuning range between mode
wafer. Electrical connections to the movMEMS actuators has led to the creation
46 Optics & Photonics News
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March 2003
TUNABLE MEMS DEVICES
ing elements are made through the
conductive silicon flexures. Electrical
insulation is provided by a combination of the oxide layer between the
moving or fixed parts of the device
and the carrier wafer, and by etched
trenches that can surround device
features.
Tunable laser assembly
(a)
peak-to-peak intensity modulation,
for example, produces a linewidth of
200 MHz. The linewidth can also be
reduced to near the instantaneous
value by use of a current servo to
lock the laser to an external etalon.
The MEMS-ECL can be directly
modulated with low chirp at data
rates of up to 2.7 Gbits/s by use of
an appropriate low-capacitance laser
diode. Direct modulation at up to
2.7 Gbits/s provides a cost-effective
tunable transmitter solution for
metro applications. The laser can be
tuned to another channel and locked
within 15 ms. Figure 5 shows a sideby-side comparison of the MEMSECL and a high-performance,
fixed-wavelength DFB in an externally modulated, 40-Gbits/s, 500-km
amplified link. The MEMS-ECL and
DFB exhibit nearly identical biterror ratio (BER) performance.
Similar results have been obtained at
2.5 and 10 Gbits/s, and with amplified link lengths up to 3200 km.
The network tunable laser modules
Etch shallow cavity in
carrier wafer to create
available today all use the same funfuture movable areas.
damental package structure, comFusion bond device
bining a tunable laser, free-space
wafer to carrier, polish to
isolator and wavelength locker on a
final thickness.
TEC in a butterfly package with a
Oxidize; open contact
PM fiber pigtail and drive electronholes; deposit and patics. Some tunable laser modules also
tern pad metal.
incorporate a semiconductor optical
DRIE etch through
amplifier (SOA) to reach 20-mW
(b)
device wafer.
output power.6 Tunable laser transFigure 3. (a) Scanning electron microscope (SEM)
mitters combine a tunable laser
image and (b) cross-sectional view of the fabrication
module with an external Machprocess for the DRIE-MEMS actuators used to tune
Zhender or an integrated electroabthe lasers and filters.
sorption (EA) modulator for
10-Gbits/s data rates, and use either
an EA modulator or direct modulation of
lasers currently being used in metro and
Environmental
the laser diode current for metro applicalong-haul systems. The MEMS-ECL
performance and reliability
tions at 2.5 Gbits/s.
tunes over 42 nm in the C or L band,
The MEMS-ECL takes advantage of
with fiber-coupled output powers of up
Thermal and vibrational sensitivities are
dramatically reduced cost and complexity
to 40 mW. Figure 4 shows the laser power
important issues in a telecom tunable
at the laser-diode level compared to other
variation and frequency deviation for a
laser. In the MEMS-ECL, they are
tunable laser technologies. The MEMSlaser locked sequentially to 100 L-band
addressed by a combination of thermoECL is based on a simple, high-power
channels spaced by 50 GHz. Similar
mechanical design and servo control.
Fabry-Perot laser diode that is readily
performance is obtained with channel
Mounting the ECL and WLL components
available from a variety of vendors. The
spacings of 25, 50 or 100 GHz. The
on a TEC eliminates the majority of therother cornerstone of the MEMS-ECL is
MEMS-ECL has a side-mode suppression
mal issues. Figure 4 shows how TEC therthe silicon MEMS actuator. With only
ratio (SMSR) of more than 55 dB, a RIN
mal control of the ECL and WLL, in
five mask levels, standard silicon processof less than -155 dB/Hz from 10 MHz to
combination with active servo control of
ing, hundreds of devices per wafer and
22 GHz, a polarization extinction ratio
the MEMS mirror position and cavity
high yield, these actuators can be fabribetter than 20 dB and a spontaneous
phase, stabilizes channel-locking perforcated at low cost. For this reason, the
emission background of less than
mance. The laser power stability (± 0.1
challenge of MEMS-ECL fabrication
-50 dBc/nm. Due to its longer cavity
dB) and frequency stability (± 0.5 GHz)
resides not at the chip level but at the
length, the MEMS-ECL has a narrower
are shown as the laser is randomly locked
assembly level, where high-level precision
linewidth than a DFB, with a 3-dB
1500 times between 100 channels while
automation is essential. The laser asseminstantaneous linewidth of 125 kHz
the case temperature varies from -10 to
bly process relies on automated robotic
inferred from phase-noise spectral den70° C. Accelerated aging tests at 70° C
vision systems and optical alignment syssity measurements. Submegahertz 1/f
case temperature also show excellent
tems for accurate component placement
and actuator-motion-induced phase
power and frequency stability while the
and attachment, resulting in high
noise produces a time-averaged effective
laser is locked to a single channel for over
throughput and yield, which translates
linewidth of 2 MHz, measured by a 25-s
2000 hours. Coupling to external shock
into low manufacturing costs.
delayed homodyne measurement. As
and vibration is minimized using a zerowith a DFB, this linewidth can be broadmoment balanced MEMS actuator. The
Tunable laser performance
ened for suppression of stimulated
use of active servos to position the tuning
Brillouin scattering (SBS) by adding a
mirror allows the device to operate under
The optical performance of the MEMSsinusoidal dither to the laser current. A
rigorous shock and vibration conditions
ECL matches or exceeds that of the fixed200-kHz current dither, giving a 3%
far exceeding the combined European
wavelength distributed feedback (DFB)
March 2003
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Optics & Photonics News
47
TUNABLE MEMS DEVICES
-4
(a)
(c)
13.1
-5
MEMS-ECL
DFB
13.0
-6
12.9
Sequential locking
12.8 to 100 channels
1.0
Frequency
Deviation (GHz)
(b)
Random locking
to 100 channels
(d)
(e)
Locked to single
channel at 70°C
(f)
0.5
BER
Power (dBm)
13.2
-7
-8
-9
-10
0.0
-11
-36
-0.5
-34
-32
-30
-28
-26
Received Optical Power (dBm)
-1.0
187 188 189 190 191 -10 0 10 20 30 40 50 60 70 0
Laser Frequency
(THz)
Case Temperature
(C)
500 1000 1500 2000
Time
(hours)
Figure 4. Laser power (upper curves a-c) and frequency (lower curves d-f) of the MEMS-ECL
are stabilized by a combination of thermomechanical design and servo control. Power
deviation of less than +/- 0.1 dB and frequency deviation of less than +/- 0.5 GHz are
shown in (a,d) versus laser frequency as the laser is tuned and locked sequentially across
100 channels; in (b,e) versus case temperature from -10 to 70° C as the laser performs
1500 random channel switches; and in (c,f) over a 2000-h accelerated aging test at 70° C
case temperature with the laser locked to a single channel.
Telecommunications Standards Institute
(ETSI) and Telcordia specifications for
earthquake, office and equipment rack
environments, with a maximum frequency deviation of less than 2.5 GHz.5
Reliability of the MEMS-ECL has been
proven through over 150,000 device
hours in Telcordia testing.
MEMS tunable
diffraction-grating filter
The tunable MEMS-diffraction grating
filter shown in Fig. 6 is analogous to the
MEMS-ECL tunable laser, but without
the gain medium. The tunable filter is a
miniature monochromator based on a
free-space Littrow diffraction grating, a
beam expander and a silicon MEMS
actuator. Optical devices using MEMS
actuators work best with relatively lowmass moving parts, such as silicon mirrors, to optimize speed and range of
motion. While traditional Littrow
monochromators tune by grating rotation, the MEMS-grating filter—as in the
case of the tunable laser—tunes by
reflecting light from a rotating silicon
micromirror to change the beam incidence angle at the grating. The optics are
designed to maintain a constant filter
bandwidth as the mirror rotates across
the tuning range. High-efficiency diffraction gratings, optimized for a single
48 Optics & Photonics News
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March 2003
polarization state, have inherently high
polarization-dependent loss (PDL). The
insertion loss and PDL of the MEMSgrating filter are minimized using a
copackaged micro-optical circulator to
impart light of a single polarization state
on the grating.
In the tunable filter schematic of
Fig. 6, the collimated beam from a singlemode input fiber passes through a polarization recovery module (PRM), reflects
from a mirror mounted on a MEMS
actuator and diffracts from a 95% efficiency, Littrow-mounted diffraction grating. The PRM is a micro-optical
circulator consisting of a polarizing
beamsplitter, Faraday rotator and two
half-wave plates. The PRM minimizes
polarization-dependent loss (PDL) and
polarization-mode dispersion (PMD) in
the filter by converting the randomly
polarized input light into p-polarized
light incident at the grating, and then circulating the return light to the output
fiber. The Gaussian output fiber mode is
the wavelength-selective aperture, resulting in a Gaussian filter passband set by
coupling loss versus lateral beam shift at
the output fiber. The filter center wavelength is selected by the diffracted wavelength that returns to the center of the
output fiber. The filter bandwidth is governed by the grating period and the pro-
Figure 5. BER versus received optical power
and eye diagram for single-channel transmission at 40 Gbits/s in an amplified 500-km link.
The MEMS-ECL is compared to a fixed-wavelength DFB. [Courtesy Lucent Technologies].
jected beam size, as determined by the
grating incident angle and the prism
magnification. Different bandwidths are
obtained by changing prisms, and the
angular dependence of the prism magnification is such that the beam size
remains constant at the grating even as
the incident angle changes with tuning. A
constant filter bandwidth is maintained
across the entire tuning range. The filter
center wavelength is tuned over 40 nm by
applying ± 140 V to the comb elements
of the MEMS actuator to produce an
electrostatic force that rotates the mirror
± 1.8 degrees about its center pivot.22
Similar to the tunable laser, the microoptical filter assembly and MEMS actuator are assembled on a ceramic substrate
and packaged in a hermetic, 18-pin butterfly package with dual single-mode
fiber pigtails. The tunable filter package
design can be extended to create a tunable receiver module in which the output
beam is coupled into an integrated spatial
filter and detector, mounted on the same
ceramic platform.
Tunable filter performance
The MEMS-grating filter offers several
performance advantages compared to
other widely tunable filter technologies.
The filter supports full C or L band tuning and narrow bandwidths for high
Output Fiber
Polarization
Recovery
Module
Diffraction Grating
PDL (dB)
Beam Expanders
(b)
Polarization Recovery
Module (PRM)
or
uat
Act
MS
E
M
Silicon Mirror
MEMS Actuator
Ceramic Substrate
Figure 6. (a) Center wavelength of diffractiongrating filter selected by mirror angle, which
determines the wavelength of diffracted light
coupled to output fiber. (b) Micro-optics and
silicon MEMS actuator are assembled on a
common substrate mounted in a hermetically
sealed butterfly package.
channel isolation at 25, 50, or 100 GHz
channel spacing with a wide range of
bandwidths achievable using the same
optical components. The filter has low
insertion loss and PDL, low dispersion
(enabling use at 10-Gbits/s data rates)
and accurate wavelength control.
Figure 7(a) shows the Gaussian filter
passband at a single C-band channel,
with 32 GHz -3 dB bandwidth, 76 GHz
-20 dB bandwidth and 1.3-dB insertion
loss. The measured 50 GHz adjacent
channel isolation is 30 dB, and the nonadjacent channel isolation is greater than
40 dB. Figure 7(b) shows the PDL of less
than 0.2 dB within ± 20 GHz of the filter
center frequency. Dispersion in the grating filter is governed by the free-space
optical path difference between input
polarization states (PMD) and wavelengths (CD). The grating tunable filter
has PMD of less than 0.2 ps, chromatic
dispersion (CD) of less than ± 20 ps/nm,
and chromatic dispersion ripple of less
than 10 ps/nm on all channels across the
40-nm tuning range.
Figure 7(c) shows the filter tuning
over 100 C-band channels spaced by
50 GHz. Figure 7(d) shows a constant
32 GHz -3dB filter bandwidth across the
40-nm tuning range. With appropriate
selection of the prism beam expansion,
-3 dB filter bandwidths ranging from 16
(a)
31.6 GHz
(-3 dB BW)
75.6 GHz (-20 dB BW)
(b)
40 GHz
(PDL < 0.2 dB)
193.76
193.76
193.76 193.76
193.76
Frequency (THz)
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20
38
36
34
32
30
28
26
(c)
Insertion Loss (dB)
Rotating
Mirror
Input Fiber
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20
-22
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-3 db BW (GHz)
Diffraction
Grating
Insertion Loss (dB)
(a)
Insertion Loss (dB)
TUNABLE MEMS DEVICES
(d)
192
193
194
195
196
Frequency (THz)
Figure 7. (a) MEMS-diffraction-grating filter passband centered at 193.8 THz, with 32 GHz
-3 dB bandwidth, 76 GHz -20 dB bandwidth and 1.3 dB insertion loss. A Gaussian fit to the
data is indicated by the dashed line. (b) PDL of less than 0.2 dB within ± 20 GHz of the
filter center frequency. (c) Tuning the filter to 100, 50 GHz ITU channels in the C band.
(d) The 32-GHz, -3-dB-filter bandwidth is constant across the 40-nm tuning range.
to 64 GHz can be achieved to support 25,
50 and 100 GHz channel spacings using
the same components. The Gaussian filter bandwidth can be similarly optimized
to enable optically matched filtering for
enhanced optical signal-to-noise ratio
(OSNR)23 under various modulation formats and data rates, provided the filter
center wavelength is accurately controlled. The actuator voltage determines
the filter center wavelength, which can be
stabilized to within ± 5 GHz by use of the
error signal from a position sensor in a
servo that adjusts mirror angle.
High accuracy wavelength tracking, to
within ± 1.25 GHz of the incoming signal
wavelength, is possible by dithering the
filter mirror position to maximize transmitted output power. Adaptive wavelength tracking to the incoming signal, in
combination with a matched Gaussian
filter passband as narrow as 16 GHz,
enables improved system performance.23
The filter can be tuned to another channel and wavelength-stabilized in less
than 15 ms.
As tunable devices approach cost parity
with fixed-wavelength devices, the economic benefits of reduced inventory and
sparing alone are sufficient to justify the
replacement of fixed components with
tunables in metro and long-haul networks. This conversion, which started
with deployment of narrowly tunable
DFBs, is expanding to widely tunable
devices that enable universal line cards
capable of dynamically tuning to any one
of 100 channels. These are some of the
first steps towards realizing all-optical
networks that promise to increase carrier
revenues and lower operating expenses
by streamlining traffic patterns and
allowing bandwidth to be dynamically
reprovisioned as traffic patterns change.
Conclusion
References
Widely tunable ECLs and diffractiongrating filters based on silicon MEMS
actuators offer the performance required
in DWDM networks at costs comparable
to equivalent fixed-wavelength modules.
Please see OPN Feature Article References,
page 62.
Acknowledgments
The authors gratefully acknowledge the
Iolon development team, John D. Grade,
Hal Jerman and Kevin Y. Yasumura, for
the MEMS material, John McNulty for
the reliability data, and Lucent
Technologies for the BER test data.
Jill D. Berger ([email protected]) and Doug Anthon
([email protected]) are with Iolon, Inc., in San
Jose, California.
March 2003
■
Optics & Photonics News
49
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L.A. Zenteno and D.T. Walton
Tunable MEMS Devices for
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