Avanex Powers the Mux - gildertech.comAn arrayed waveguide grating (AWG) is a direct extension of a Mach-Zehnder interferometer, so that is where we begin. A Mach-Zehnder interferometer

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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.


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.


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


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


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.


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


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


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


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


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


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


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


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


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


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


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,


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.


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


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.


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.


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


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.


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


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.


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


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


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


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


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.


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.

Documents

Avanex Powers the Mux - gildertech.comAn arrayed waveguide grating (AWG) is a direct extension of a Mach-Zehnder interferometer, so that is where we begin. A Mach-Zehnder interferometer