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Avanex Powers the Mux
by Charlie Burger
They Do It with Mirrors
—Agatha Christie
Dame Agatha had the answer back in 1952 and pasted it on the front of a million
paperbacks. Save for a minor glitch (she got the story wrong), Christie leaves us to
reflect back and forth on the obvious: Avanex’s hugely superior photonic
processing technology. The real story, as you’ll soon discover, begins when a
foreign disrupter encounters another Perot (Fabry). But first, to make sure we’re all
on the same wavelength, let’s get our terms straight.
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The realm of domain
Spatial domain … wavelength domain … frequency domain. Huh? What’s this all
about? Who does what in which? Do we care?
Absolutely.
All multiplexing/demultiplexing technologies including Avanex’s
PowerMux, thin-film filters, fiber Bragg gratings, and arrayed waveguide gratings
align WDM channels to the ITU (International Telecommunications Union)
frequency grid, centering the channels on the preset standardized frequencies at
equal spacing intervals (e.g., 100 GHz or 50 GHz channel spacing). We define the
frequency of light as the number of waveforms or cycles that pass through a point
in one second, measured in Hertz, and the wavelength of light as the distance
between a point on the lightwave and the identical point on the next wave in the
cycle. Since a frequency of light is related to a wavelength of light by the equation
f = c / λ,
where f = frequency, λ = wavelength, and c = speed of light in a vacuum, each of
the ITU periodic WDM frequency-channels corresponds by physical law to a
unique wavelength of light; wavelength and frequency are merely interchangeable
measurements of light. Note, however, that f and λ are not linearly related.
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Channels aligned to an ITU grid that is equally-spaced in frequency have by
definition unequal spacings in wavelength, corresponding to the inverse
relationship.
So … everybody works in the frequency domain? And everybody works in
the wavelength domain? Well, yes, but only because frequency and wavelength are
interchangeable units of measurement. However, if we define frequency domain
strictly as “operating in equally-spaced frequency channels according to the ITU
grid,” and contrast that with a definition of wavelength domain as “operating in
equally-spaced channels in wavelength,” then everyone works in frequency and
nobody works in wavelengths. Here’s why:
It is much easier and more readily accurate to measure and reference light in
frequency than in wavelength.
If the medium in which a light pulse is traveling changes its properties, then its
index of refraction may also change, affecting both the speed of light and the
wavelength. For instance, if the index of refraction increases, light slows down and
its wavelength compresses such that frequency remains unchanged. The reverse is
true when the index of refraction decreases. Mathematically we have
n = c / c(n) and λ(n) = λ / n
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where
n = index of refraction of the medium
c = speed of light in a vacuum
c(n) = speed of light in a medium
λ = wavelength of light in a vacuum
λ(n) = wavelength of light in a medium
Hence f = c(n) / λ(n) = c/n / λ/n = c /λ. In terms of absolute value, we see from the
above that each λ is affected differently by the index of the propagating medium.
Since wavelength depends on solid-state interaction, a λ measurement standard
must include controls on environmental parameters such as temperature and
humidity to ensure that the properties of the unit of space remain constant. Since
the frequency of a light pulse holds steady through thick and thin, the measurement
of frequency boils down to our ability to measure time—highly precise thanks to
atomic clocks.
Now, suppose that your multiplexing technology is wavelength dependent,
meaning that it is highly sensitive to the index of refraction of the propagating
medium?… Keep that thought on the back burner.
Next, let’s clear up the misconception that says spatial domain is simply
another term for wavelength domain. It is not. Every WDM multiplexer maps
individual frequency components of light to the spatial domain, that is, it maps
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light components onto spatially separated output ports. For example, light of
frequency f1 (corresponding to wavelength λ1) from an incoming port might be
directed to output port 1 (a spatial destination). Alternately, the spatial destination
of f1 might have been mapped onto output port 2. So let’s get this right from the
start: wavelength domain refers to light pulses as measured in wavelengths and
spatial domain refers to the mapping of those light pulses in space.
It is true, however, that when it comes to multiplexing technology, the
wavelength (and so by definition frequency) domain is directly related to the
spatial domain. For example, in arrayed waveguide gratings, the spatial separation
and location of input fiber ports and output fiber ports determine the wavelength
(frequency) of each output port. In fiber Bragg gratings, wavelengths separate
based on the spatial density of the gratings. In another example, the frequencies
filtered by thin-film filters depend on the angle of incidence of the light beam on
the filter.
Now, suppose that your multiplexing technology is not spatial domain
sensitive?… Another critical thought to toss into the back pot. We’ll return to it
after a review of the three popular multiplexing methods: arrayed waveguide
gratings, fiber Bragg gratings, and thin-film filters … and, of course, after that all-
important dissection of Avanex’s PowerMux interleaver.
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Guiding lights
An arrayed waveguide grating (AWG) is a direct extension of a Mach-Zehnder
interferometer, so that is where we begin.
A Mach-Zehnder interferometer (MZI) demultiplexes different
wavelengths of light by splitting an input signal into two signals of equal power
and sending one signal down a path (i.e., waveguide) of length L and the other
signal down a longer path of length L + ¨/ before the signals are recoupled. At
that point the signals will have shifted in phase by an amount directly proportional
to the path-length difference ¨L. That is, the phase of the lightwaves traversing the
longer path lags the phase of identical lightwaves traversing the shorter path. So,
for example, the peak of a wave traveling down waveguide L + ¨/ reaches the
recoupling point after its twin brother traveling down waveguide L. When these
phase-shifted copies of the same signal recombine, the ridges and troughs of the
waves add or cancel (i.e., interfere constructively and destructively) depending on
wavelength and propagation angle such that certain wavelengths strengthen in one
direction and so pass onto one output fiber and other wavelengths strengthen in a
second direction and so pass onto a second output fiber.
Another way to look at the process is that light takes a longer time to
traverse the path L + ¨/ than the path L. This delay causes diffraction (changes in
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the propagation direction of light waves) of the interfering light in the output
coupler. Thus the different frequencies (wavelengths) of light leave the coupler at
different angles and the two output fibers are placed in the precise positions to
collect the redirected light.
For a multiplexed input signal of known wavelengths λ(n), a path difference
¨/ can be chosen such that the resulting interference directs alternating
wavelengths onto each output fiber (e.g., λ1, λ3, λ5 pass onto output-1 and λ2, λ4,
λ6 pass onto output-2). That is, in the direction of output-1, λs 1, 3, 5 interfere
precisely in phase and therefore strengthen and λs 2, 4, 6 interfere 180 degrees out
of phase, canceling them out. The opposite occurs in the direction of output-2.
An arrayed waveguide grating (AWG) extends the Mach-Zehnder
interferometer to multiple inputs, outputs, and intervening waveguides (or phased
array of multiple waveguides) called arrayed waveguides. AWGs are typically
fabricated in silica on silicon chips (or all-silicon in the case of Bookham). The
path-length differences between waveguides in the array are chosen so that a
desired phase relation between waves results at the output.
In the case of a demultiplexer, the signal is first split by the input star
coupler into as many parts of equal power as there are waveguides. Each
waveguide has a path length difference of ¨L. That is, the length of waveguide 1 =
L, the length of waveguide 2 = L + ¨L, the length of waveguide 3 = L + 2¨L, and
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so on. This linear variation in path length maps channels that are equally spaced in
frequency. (To map channels in equally-spaced wavelengths, a nonlinear variation
in path length must be chosen.) For a multiplexed input signal of known
frequencies f(n), a path difference ̈/ can be chosen such that the resulting
interference directs f1 onto output-1 (all the other fs are out of phase), f2 onto
output-2, up to f(n) onto output-n. This AWG frequency mapping depends on the
refractive index both of the couplers [n(c)] and of the arrayed waveguides [n(w)],
the grating period d (distance between adjacent waveguides at output), and the
diffractive angle of the output coupler, all in addition to ¨L, such that
f(k) = c k / [n(c) d (sin θ) + n(w) ̈ /@
where k is the diffractive order.
An AWG has a periodic response in frequency (when waveguide lengths
increase linearly)—that is, for a given ¨/� as you go up or down the
electromagnetic spectrum only a finite set of WDM channels equally spaced in
frequency can be demultiplexed (or, alternately, multiplexed) until the process
repeats itself. The frequency range between repeating frequencies is called the free
spectral range (FSR). For instance, given an AWG of certain ¨/ and refractive
index, you might get an FSR that includes f1–f100 corresponding to λ1–λ100. The
process would repeat itself at f101 (corresponding to λ101) such that λ1 and λ101
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would be demultiplexed onto the same output, λ2 and λ102 demultiplexed onto the
same fiber, and so on.
WDM on a fiber diet
Fiber Bragg gratings (FBGs), as the name implies, are in-fiber multiplexers. The
gratings are written into photosensitive fibers by exposing these fibers to
ultraviolet (UV) light which in turn changes the refractive index of the fiber core.
One way to make a photosensitive fiber is to dope a conventional silica fiber with
germanium. We can then write a grating into the fiber by exposing its core to two
interfering UV beams, causing the radiation intensity (and hence the refractive
index) to vary periodically along the length of the fiber.
Like AWGs, indeed, like all gratings, FBG principle of operation involves
interference among multiple optical signals originating from the same source and
undergoing phase shifts. The phase of an electromagnetic wave depends on four
parameters: (1) the angular frequency of the wave, (2) direction of propagation, (3)
a propagation constant which includes the refractive index of the medium and
which is therefore proportional to the speed of light in the medium, and (4) time as
related to parameters 1 and 4. Therefore, for a given medium, a wave of certain
angular frequency and direction of propagation can be split into two waves of
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equal power and phase shifted by sending each wave down a path of same
refractive index but of different length. The waves will take different times to
reach the output coupler and will arrive out of phase. That describes the principle
behind AWGs.
We see from the four phase parameters that we can also achieve phase shifts
by altering the propagating medium. In the case of Bragg gratings, these alterations
take the form of a periodic perturbation, usually a periodic variation of the
refractive index of the fiber. Change the refractive index and you change the
propagation constant and therefore the phase of the lightwave.
Here we are concerned with Bragg gratings written into waveguides. These
gratings reflect wavelengths of light that resonate with the grating period, while
transmitting other wavelengths. Specifically, lightwaves passing through a grating
undergo phase shifts, changing direction (reflecting) based on wavelength. Energy
from a wave traveling toward the grating couples onto a diffracted wave traveling
in the opposite direction at the same wavelength when
λ = 2 n d
where n is the refractive index of the fiber, d is the grating spacing, and λ is the
reflected wavelength. The strength of the reflection depends on the number of lines
in the grating (the more the better), the uniformity of their spacing, and how
strongly they are written. Note that a change in either refractive index or grating
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spacing results in a linear response in wavelength and so by definition a nonlinear
response in frequency, the opposite of the ITU world.
A multiplexed light signal can be demultiplexed by passing the signal
through a set of gratings, each grating reflecting one channel to a circulator and
passing the remaining channels through. The reflected channel can then be coupled
onto an output fiber.
They do it with onion skins
Thin-film filters (TFFs) are based on interferometer technology and take the form
of filters made up of hundreds of layers of dielectric thin-films. (A dielectric is a
nonconductor of direct electric current.) These films or coatings are deposits of
alternating dielectric layers (only a few atoms thick) with alternating high and low
refractive indexes. As with FBGs, this refractive index pattern generates
interference between lightwaves, reflecting some and passing others. The
interference effects depend on the thickness of the coatings, the refractive index of
the coatings, the angle of incidence of the signal, and wavelength.
In a common configuration as a multiplexer, an array of TFFs are placed on
both sides of a substrate. A WDM multiplexed light beam enters the substrate at an
angle and meets the first multilayer dielectric thin-film filter on the opposite side of
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the substrate cavity where one wavelength passes through to an output fiber. All
other wavelengths reflect back (and down) to the opposite side of the substrate (in
a sawtooth fashion) where the beam strikes a second filter which passes one
wavelength through to an output fiber and reflects all other wavelengths. The
process continues until the light has passed all the filters and each wavelength has
been demultiplexed (the number of filters must equal the number of channels).
Reverse the process for multiplexers.
Simon’s cash Cao
Avanex’s fabulous PowerMux is based on the classical, well-understood Fabry-
Perot filter, so that is where we begin our discussion.
A Fabry-Perot filter (F-P) or interferometer is an etalon—two highly
reflective mirrors placed in parallel with reflecting surfaces facing each other. The
input signal enters the etalon cavity perpendicular to one of the mirror surfaces, say
for argument the left mirror. It then traverses the width of the cavity to the right
mirror where part of the beam passes through and part reflects. The percent of total
signal power reflected equals the reflectivity of the mirror, usually 95 percent or
higher to get good isolation per channel. The reflected power returns to the left
mirror where part is again reflected and part passed through. As these reflections
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continue back and forth over the same perpendicular path between the mirrors, the
light fluxes leaving the filter cavity on the right emerge at different points in their
wave cycles, adding in phase for those frequencies which are related to multiples
of 2� the one-way propagation delay across the cavity, represented by
2� �Q�O���F
where
n = refractive index of the cavity
l = cavity length
c = speed of light in a vacuum
Since n = c / c(n), where c(n) is the speed of light in the cavity, we have 2� �O���
c(n), which is the time it takes for light to traverse the etalon cavity.
The power transfer function T(f) of a Fabry-Perot filter—the fraction of the
input light power transmitted as a function of frequency—follows the relationship
T(f) = 1 / (1 + (K sin(2 π�I�2��A���
assuming identical reflectivities and zero absorption loss at each mirror. K is a
constant determined by mirror reflectivity and f = optical frequency. So, T(f) = 1,
its maximum value, for sin(2 π�I�2�� ��� Alternately, T(f) min occurs at sin(2 π�I�2��
= 1. Therefore, passbands, or peaks of power transmitted by the filter, are present
when
2 π�I�2� �N�π
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where k is a positive integer (interference order). Thus, I� �N�����2� �N�F�����Q�O� so
that passbands occur at periodic (evenly spaced) intervals in frequency that are
inversely related to both cavity length and index of refraction of the cavity. One
passband occurs at k = 1, another at k = 2, and so forth.
As touched on under AWGs, the spectral range between two successive
passbands of the filter is called the free spectral range (FSR). Say you have a
multiplexed WDM signal of known equally-spaced frequencies f1, f2, … f9. Now,
assume that you want to filter out all channels except channel f5. In that case you
must set the cavity to a length l that creates a passband at f5 for some integer k and
for which the FSR covers a frequency range of at least 8 channels, placing the
passbands on either side of f5, far to the left and right of f1 and f9 respectively.
For an 8-channel FSR, the passband below f1 would be f(-3) and the
passband above f9 would be f13; i.e., three equally-spaced passbands f(-3), f5, f13
each 8 channels apart and represented by three different interference orders k.
Channels f1 and f9 of our WDM signal are midway between passbands and hence
sufficiently isolated from neighboring passbands, and when mirror reflectivities are
99 percent and higher, f2–f4 and f6–f8 are also significantly isolated from f5 which
would then have a sharp peak much the shape of the Eiffel Tower.
An F-P filter can be tuned on the order of a few milliseconds by changing
the cavity length. Because of the inverse relationship between passband
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frequencies and l, passband spacings tighten as the mirrors are moved further apart.
In our example, to filter out all channels excepting those centered on f1, f5, and f9,
the cavity length must be doubled, which would halve the FSR channel range to 4
channels and result in passbands at f(-3), f1, f5, f9, and f13.
With the PowerMux, Simon Cao has uniquely adapted the dynamically
tunable Fabry-Perot interferometer to a Cinderella telecosmic universe. By
transforming the linear F-B filter into an asymmetric nonlinear interferometer and
thereby introducing Fourier optics, Simon turns a Buick into a Rolls Royce.
In WDM paradise, we approach infinite bandwidth by saturating the
electromagnetic spectrum with data. To close in on the ideal, we not only multiply
channels like rabbits, we pack them together like sardines. Channels that look like
Jack Sprat, lean and well separated, are data starved. Channels that look like Jack’s
wife, nearly rubbing shoulders, are stated with data. Modulating at today’s 1 bit per
Hertz, channels separated at a dense 12.5 GHz (Gigahertz), if well shaped, could
transmit at 10 Gbps (Gigabits per second). Poorly shaped channels could not.
Specifically, Gaussian-shaped channels, spread out across the frequency
spectrum like so many waves in the ocean, overlap at their bellbottoms as they
squeeze together into denser spacings, blocking out bandwidth from below and
leaving less and less free data-carrying space in the overshooting tops. Now, bring
on the shoeboxes. Stand them up vertically. Crowd them together. No overlap, all
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data. Crowd them closer yet. Still no overlap. The ideal case, crisp rectangular
channel shapes allow you to carry the maximum bandwidth per channel. The flat
tops ensure uniform loss and the vertical sidebands (sharp skirts) isolate adjacent
channels. In the real world, we try to approximate this ideal. And Simon does it oh-
so-well.
By varying the index of refraction within the etalon cavity using proprietary
technology which includes waveplates (see U.S. Patent # 6,130,971), Dr. Cao
introduces a phase delay which enables him to add higher orders of frequency
called Fourier transforms. Thus, using Fourier optics, he can approximate
rectangular-shaped channels to a high degree. (The Fourier transform separates a
waveform into sinusoids of different frequency which sum to the original form. To
reconstruct a perfectly rectangular waveform would require the addition of an
infinite number of sinusoids.) Recall that the one-way propagation delay 2� the time
it takes for light to traverse the etalon cavity, can be increased by increasing the
length of the cavity or by increasing the refractive index. An increase in 2 in turn
alters the power transfer function such that the FSR decreases and passband
frequencies become more densely spaced. Multiple reflections of light passing
through different refractive indexes therefore result in multiple fractions of light
transmitted at each frequency, allowing Simon to fashion channel shapes that look
more like rectangles. This filter design has been a winning part of the PM,
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accommodating potentially greater bandwidth per channel and potentially more
closely-packed channels.
Now, if we set the length l of the PM etalon cavity such that the FSR (free
spectral range) is twice the channel spacing of a known WDM signal, the square-
shaped channels marching out the right side of the etalon will center on the
frequencies corresponding to alternating frequencies of the original WDM
channels. Take one more step. Split the incoming light signal into two polarized
beams and pass each through a nonlinear F-P etalon such that alternating odd/even
channels are processed separately and directed to unique output fibers. In a
nutshell, that’s the basic operation of the PowerMux periodic processor, which
translates a set of densely spaced WDM channels to two sets of WDM channels
with twice the spacing (reverse the process for multiplexing). Going back to our F-
P example, PM input channels f1–f9 would output onto two fibers, one carrying
the odd channels (f1, f3, f5, f7, f9) and the other carrying the even channels (f2, f4,
f6, f8).
PM is dynamically adaptive to channel spacing. To change the channel
spacing, you only need change the width of the nonlinear interferometer cavity. By
changing the length from tens of microns to tens of centimeters, the channel
spacing changes from THz to hundreds of MHz.
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Simon chops the suey out of other muxers
Used in 70 percent of WDM systems today, TFFs are most broadly deployed at
coarse channel spacings, 200 GHz and above, at channel counts of 16 or fewer. At
that backend, TFF remains a low-cost technology. However, at 100 GHz spacing
on down, where channel counts increase beyond 16, TFF almost doubles in cost
over AWGs. The difficulty in manufacturing TFFs increases significantly as
channel-count increases because of the precision required for depositing dielectric
layers only a few atoms thick on the surface of a substrate. TFF muxers require not
only a filter per channel, they also require additional dielectric depositions per
filter in order to separate the additional λs. These filters require several hundred
layers of deposition with extremely tight tolerances in large vacuum chambers.
TFFs are spatially sensitive since the channels reflected or passed through
each filter depend on the angle of incidence of the beam and the thickness of
hundreds of dielectric layers in addition to achieving desirable refractive indexes.
These spatial relationships must prove accurate at each filter. TFF is a mature
technology exhibiting good temperature stability, low loss, and excellent channel-
to-channel isolation, as well as broad passbands with flat tops and sharp skirts
achieved much the same way as in Simon’s PM—through multiple reflections of
higher order frequencies. Nevertheless, because of the huge manufacturing and
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packaging challenges, today’s consensus confines TFF muxers to 16-channel
WDM systems and lower.
Oplink and Corning have recently made significant advances in
manufacturing and packaging of TFFs and so may effectively move to 24 channels,
perhaps 100 GHz spacing at the outset, but always a backend product in the
coming lambdasphere.
So the battle lines are now being drawn between advocates of FBGs and
AWGs.
AWGs are manufactured using well-understood semiconductor processes
for mass production and cost control. Employed by Lucent in some of its WDM
systems, AWGs are super spatially sensitive: each channel requires a separate
waveguide grating. Want a thousand-channel system? Then you need a thousand
gratings on a chip, each of the precise length and index of refraction to achieve
accurate channel centers and spacing. Also critical in the manufacturing process:
the index of refraction of the star couplers, the alignment (spacing and direction) of
the grating ends, and the locations of the output fibers. Though the chip layers and
gratings are patterned and etched using variants of standard semiconductor process
techniques, controlling layer thickness, composition, index of refraction, and
defects remains a challenge for devices that are much larger than those produced in
standard IC facilities.
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The sensitivity of AWGs to wavelength (index of refraction) and to the
severe spatial-domain constraints mentioned above results in a temperature-
dependent device. This can be controlled actively with a heating element or
passively with novel chip configurations such as forming a trench in the middle of
a waveguide array and filling it with a material that has a temperature dependence
opposite that of the waveguide material. Such a structure demands precise trench
formation to reduce the phase error and very strict fiber coupling to the AWG chip
to adjust to center-wavelength offset introduced in fabrication.
These spatial and wavelength constraints make AWGs far harder to scale
than the PowerMux which concerns itself with the spacing of several mirrors and
waveplates. Laboratory hero experiments have demonstrated AWG channel-counts
up to 256 and channel spacing down to 12.5 GHz, but the cutting-edge commercial
deployment today is 40 channels. A logical question would be, Why not employ
the Mach-Zehnder approach using two waveguides to separate alternating
channels, giving the same result as the PowerMux periodic processor? Companies
such as Bookham and Kymata do not follow this approach because once you have
to cascade AWGs you loose the advantage of a one-chip function, the Great
Benefit extolled by AWG developers. And you still face the high signal attenuation
or loss inherent in the solid-state approach as well as the cross-talk and inferior
channel shape for crammed lambdas.
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Indeed, Avanex can just as easily multiplex/demultiplex immense arrays of
channels at spacings much less 1 GHz, all at once, by spacing the mirrors
appropriately, rather than follow the interleaver approach. They could beat AWGs
hands-down. But then we lose the advantages of the interleaver approach. By
muxing alternating “odd and even” channels, interleavers require a total of only
three fibers in and out rather than an unwieldy thousands if all channels were
separated or combined at once. For example, a WDM signal with channels spaced
25 GHz apart would be demuxed to two sets of signals at 50 GHz spacing, each
signal with half the number of channels. Each of these two WDM signals could
then in turn be demuxed separately to yield four signals, each spaced at 100 GHz.
This multiplexing architecture ensures flexibility; if a network desires to add
channels, a muxer can be added in front of the 25 GHz interleaver to yield 12.5
GHz—the entire multiplexer module does not need to be torn out and upgraded.
On the opposite end, at channel spacings of 200 GHz and coarser, the traditional
muxing technologies become less expensive and can be employed accordingly.
Philosophically, the AWG concept itself suffers from the challenge of
kidnapping a technology naturally suited to the microcosm and enslaving it in the
world of light. Non-attenuating electrons move around corners with ease and so
call their home solid-state. Photons attenuate, hate corners, and respond best to
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their own natural world of mirrors, crystals, and waveplates which manipulate the
natural properties of light with ease.
For all that, until recently, AWGs seemed to offer the best prospect for
WDM where TFFs leave off. That may be changing. FBGs “are moving up,”
claims Victor Mizrahi, Ciena’s retired chief scientist. Ciena began its life with
FBGs and Mizrahi sees no reason for the company to change course now. In
addition, Nortel apparently has FBG development underway via Qtera. Why?
FBGs exhibit extremely low loss (0.1 dB), high wavelength accuracy (+/- 0.05
nm), uniform response across channels, high crosstalk suppression (40 dB),
channels with flat tops, polarization insensitivity, and couple readily to other
fibers. FBGs can isolate channels better than AWGs, so channel spacing can
potentially be tighter. Alcatel claims to have gotten 160 λs on a fiber using FBGs.
But the jury is still out in the case of FBG v. AWG. Like AWGs, FBGs are
spatially and wavelength sensitive, depending critically on the grating period (line
spacing), including the uniformity of the spacing, the number of lines in the
grating, and the index of refraction of the grating. Since fiber length (and therefore
grating period) varies with temperature, FBGs are heat sensitive. This is controlled
passively by packaging the grating with a material that has a negative thermal
expansion coefficient, keeping the fiber length constant over the range of normal
operating temperatures.
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And, much the same as AWGs requiring a grating per channel, FBGs
require a grating set per channel. However, unlike AWGs which can be packed on
a single chip, in-fiber gratings must be cascaded, resulting in a large form factor
device, a problem for high channel counts, where AWGs are 3 to 4 times cheaper
per channel. The complexity of per-channel cascading would dwarf the cascading
requirements of interleavers while promising none of the benefits.
Researchers at Southampton University in England claim to have developed
“a way of automatically translating requirements for Bragg gratings into patterns of
barriers to be superimposed on the fiber, and then automatically translating the
actual grating.” In English, they’ve demonstrated in the laboratory a better way of
manufacturing FBGs such that they can get longer, finger gratings than anyone
else. Hence they can carry more data per channel and can “squash,” as they put it,
to 25 GHz spacing. For Southampton, 25 GHz is a squash and a squeeze. For
Avanex, 12.5 GHz and tighter is a breeze.
The PowerMux (PM), based on the principle of F-P interferometry,
separates groups of channels in a way that is inherently nonsensitive to the spatial
location of the fiber port. By adjusting the length of the etalon, PM automatically
aligns itself to the ITU frequency grid in a self-organizing fusion. Align one
frequency, and the other equally-spaced frequency channels automatically line up.
As explained, AWGs, TFFs, and FBGs are not only spatially sensitive, requiring
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separate gratings and filters per channel and other spatial alignments, they also
exhibit wavelength dependence because of solid-state optics. With solid-state
devices, especially AWGs, variations in the index of refraction are critical since
wavelength changes with index, each λ responding differently to the same index
adjustment. Free-space devices such as PM are environmentally robust since the
index of refraction of air doesn’t vary enough to meaningfully affect wavelength.
True, in order to attain “square” channel shapes using Fourier transforms,
Simon has added waveplates to his F-P interferometer to make it nonlinear. This
adds spatial constraints, but they pale in comparison to the spatial constraints of
AWGs, TTFs, and FBGs as already discussed. Avanex expertise comes to the fore
in its patented unique design and processing technologies. The physical innards the
PM—the lenses and mirrors—are well-known technologies, leaving Avanex with
numerous, inexpensive sources.
Insensitivity to both the spatial and wavelength domains make PM scalable
to extremely high channel counts. How high? Based on their experimental results,
Avanex has demonstrated that PM nonlinear interferometer technology can resolve
channels down to Hertz separations, potentially opening up 44 trillion connectivity
routes per fiber within the 1310 and 1625 nm spectral regions. While likely never
to be practical for real-life networks at such a fine level of granularity, these results
reveal the filtering precision at Avanex’s disposal.
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The PM NxG (PowerMux Next-Generation), commercially available next
year, promises lower loss (1.2 dB), better PMD, better PDL (.03 dB), half the
footprint, half the number of assembly steps, and half the number of parts
compared to the PM, making it extremely easy to manufacture in high volumes. In
addition to yet more innovations in F-P interferometry, the improvement in the PM
NxG comes with the replacement Avanex’s PowerFilter (a TFF) on the backend
with Holographix’s (bought by Avanex) diffraction gratings. The combined
innovations should drop the cost per λ to $200 immediately and to $100 in two
years compared to $500 per λ for AWGs and FBGs (estimated by Epoch Partners).
At the backend of the muxing process where channel spacing is coarse (200 GHz
and greater), rectangular-shaped channels are not necessary. Exit Fourier optics
and enter holographic gratings on a glass substrate, cheaply mass produced (similar
to the strip on your credit card) and designed to produce interferometric patterns
aligned to the ITU grid.
Blinded by the light
So why all the fuss about AWGs, TFFs, and FBGs? It’s really an attitude thing; a
state of mind. Few industry technologists and analysts understand either the
essentially infinite bandwidth-carrying capacity of fiber or its massive connectivity
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potential. They think in terms of bandwidth conserving, hybrid TDM with
relatively few channels and high clock-speeds—40 Gbps, 100 Gbps, and beyond—
which ultimately limit data transfer due to severe signal distortions in the fiber.
More importantly, connectivity suffers mercilessly. Yet, without connectivity,
network access and hence usability drops dramatically. In an environment in which
the next bit is “free,” connectivity generates carrier revenues.
Present-day commercial “dense” WDM systems generally operate between
16 and 40 channels. So when Mitel Corp. announces that it is developing a device
using standard semiconductor materials that can “cram” as many as 80 channels
onto a beam of light, the bandwidth-challenged listen. Mitel uses a technique
called Echelle gratings instead of AWGs for “better capacity, precision, and
construction.” At the far backend of the muxing process, Mitel may find a place.
But in terms of enabling a WDM micro-rainbow, who cares? Fujitsu has begun
commercial shipment of its 176-channel system based on Avanex’s first generation
PM. Nortel will use PM to get 160 channels. And the lambdasphere has barely
arrived.
Savvy folk look elsewhere
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Avanex’s PM periodic processor is an interleaver, and its true WDM challengers
are competing high-channel-count interleaving technologies from Chorum,
WaveSplitter, and Oplink.
WaveSplitter uses an in-fiber Mach-Zehnder approach. They cascade three
fiber couplers of different coupling ratios which link two pairs of unbalanced fibers
(fibers of different length), thereby interleaving flat-topped channels shaped by
first- and third-order Fourier harmonics. This approach suffers from the spatial
constraint of the temperature sensitivity of the refractive index of fiber and
therefore must be actively temperature compensated much like AWGs. As you
might expect, the denser the channel spacing, the more critical the thermal drift—
bad news for high-end multiplexing. WaveSplitter appears to be substantially
behind Avanex in its product development.
Oplink focuses on breadth and volume production to position itself as an
alternative supplier to JDSU. By contrast, companies such as Avanex and Chorum
attempt to excel in a few specialized parts or technologies. Oplink currently offers
a commercial 50 GHz interleaver and has plans to go to 25 GHz “in the future.”
They incorporate TTFs into the backend to lower the cost of multiplexing as
Avanex does with its current PM. However, recall that Avanex will replace its
prized filters with the even-better Holographix solution in its PM NxG. Oplink
would not reveal its front-end multiplexing technology, but suggested that it has
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now opened the door to two possible routes. So they are still unsettled as to
technology at this late date and also must fend-off a suit by Chorum which claims
Oplink piracy of Chorum polarization technology.
And that brings us to Chorum’s PolarWave interleaver based on their
patented polarization processing and spectrum filtering technologies which
controls light signals by manipulating its polarization state (U.S. Patent #
5,694,233 and # 5,978,116).
Lightwaves are more complex than the simple waveforms described earlier.
In addition to length, frequency, and amplitude, light has the characteristic of
polarization. Lightwaves are oscillating electro-magnetic fields. The electric fields,
or waves, are always perpendicular to the magnetic fields, or waves. Light is
normally a combination of these two polarizations.
Traditionally regarded as a nuisance, the polarization of light is
fundamentally a geometric phenomenon. This has meant bad news for fiber and
other components, unless they could be made perfectly symmetrical. But where
there’s a headache, a beneficial tool almost always lies hidden. Interestingly, some
crystals exhibit noticeable differences in refractive index for vertically and
horizontally polarized light, creating path-length differences for the two
polarization states. Thus, these crystals can split light into two polarized beams of
equal power, at which point they can be manipulated separately and afterward
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recombined. Crystals with refractive indexes that vary vertically and horizontally
are called birefringent. In liquid crystals (LCs), molecules align themselves in
response to an applied electric field thereby changing polarization state. Other
asymmetric materials, called polarization rotators, shift the direction of
polarization as light passes through rather than affecting the amplitudes of the
poles or creating path differences between them. The amount of rotation depends
on the thickness of the material and the strength of the magnetic field.
So why not harness the polarization problem to manipulate light to our
benefit? Answer: problems—slow switching speeds, poor channel shape,
temperature sensitivity, and high signal attenuation. Chorum properly understands
the lambdasphere and so cares not about switching time. Hence, they forged ahead
on the other problems which they have ingeniously solved through proprietary,
highly disciplined process and characterization technology.
With novel arrangements of crystals and polarization rotators, each of
different thickness and varying refractive index profile, Chorum has developed an
impressive interleaver with high-order Fourier transforms for superb channel
shape. Its third-generation interleaver, currently beta testing at a tight 12.5 GHz
channel spacing, is scheduled for commercial rollout in 2001. Chorum has
demonstrated 6,400 channels in the laboratory using the same technology.
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Triumphantly confirming the approaching lambdasphere and Cao’s law,
Chorum speaks of lambdas in abundance, thousands on a fiber as early as mid-
2002, enabled in part by their third-generation interleaver. Far from a threat to
Avanex, Chorum will only spur Avanex to new heights as it helps to enable the
lambdasphere in which both companies will thrive. While the PolarWave
interleaver, like PM, contains orders of magnitude fewer spatial constraints than
the traditional muxing technologies, it requires precise alignments of refractive
index profiles that are likely more difficult to achieve than Avanex’s seemingly
simpler spatial separation of mirrors and waveplates. Chorum laboriously produces
its own birefringent crystals; Avanex’s inexpensive subcomponents are off-the-
shelf. And Avanex sage Simon Cao blazes the path to the lambdasphere in his
vision of a switchless network.