CUSTOM PDK
FRAMEWORKWITH IPPROTECTION

ISSUE I 2024

INSIDENews Review, Features
News Analysis, Profiles
Research Review
and much more...

PICMAGAZINE.NET

SUPPORTING AI
WITH OPTICALMODULATORS

To cope with surging internet
traffic, datacentres can be
upgraded with electro-optic
polymer modulators

HIGH-POWER
TRANSMITTERS
FOR NETWORKS

Combine a laser, a modulator andan amplifier in a single device arelaying the foundations for highcapacity access networks

POLARISATIONINDEPENDENTMONOLITHIC PICS

As datacentre infrastructure races
towards 200G per lane, delays
remain significant hurdles inhigh-speed optical receivers

INDUSTRY LEADING WAFER
PROCESSING EQUIPMENT FOR
PHOTONIC INTEGRATED CIRCUITS
Optical and Nanoimprint Lithography
Temporary and Permanent Wafer Bonding
NILPhotonics® Competence Center

EVG®7300
Multifunctional UV Nanoimprint Lithography System

GET IN TOUCH to discuss your process development and manufacturing needs

www.EVGroup.com

VIEWPOINT
By Laura Hiscott, Editor

The forces driving innovation

IT IS OFTEN SAID that “necessity is the mother of
invention,” and it is not difficult to find examplesof this in any environment, least of all thetechnology industry. We are now more than a year on
from reports of ChatGPT reaching one hundred million
subscribers in two months – exceeding the growth rateof any consumer application before it – and it looks likeAI’s data demands may be the necessity that drives not
only the invention of new photonic technologies, but alsoa major scale-up in the use of existing ones.
Photonics as the key to achieving higher data rates isa recurring theme in this issue. POET Technologies, forinstance, distinguishes three ways in which the field is“positioned to be the catalyst for large-scale growthin hardware applications for AI,” while Michael Lebby,
CEO of Lightwave Logic, describes how electro-optic
polymers could serve as faster optical modulators intransmitter PICs, helping datacentres to handle surging
internet traffic.
Meanwhile, GlobalFoundries reports progress in tacklingpolarisation-dependent loss – an important challenge forthe industry to overcome on its way to achieving higher
bandwidths – and Almae explains how a device thatcombines a laser, modulator, and amplifier can pave theway for higher-capacity access networks.
And it is not only the technological solutions themselvesthat are spurred by the need for them; there are ripple
effects throughout the whole surrounding ecosystem.
On page 34, for example, EXFO describes how
increasingly intricate PICs are pushing the boundaries
of high-precision measurement techniques – something
they are striving to address with their testing platforms.
Of course, external requirements are not the only
motivation for technological progress. After all,conventional wisdom also advises us to “build it andthey will come.” Indeed, much of the technology that’s

poised to solve current problems would not be readywaiting if it were not for exploratory research. As thenews pages show, exciting developments are being
made in the lab all the time, from a chip that
does maths with light to a laser printerthat can fabricate rewritable PICs.
Amid these push and pull forces, it’scertainly a time of intense creativity inthe world of integrated photonics.
And with PIC International on thehorizon, it will be fascinating to see
the latest innovations on display there,
whether they are the product oftargeted, application-focusedwork or curiosity-led research.
I look forward to seeing you at
the event in April.

PIC MAGAZINE.NET I ISSUE I 2024 3

CONTENTS

VOL. 28 ISSUE I 2024

The best of both worlds: 14
design freedom andIP protection
VPIphotonics empowers foundries and fablesscompanies by giving them the freedom to customise
PIC designs using process design kits while also
ensuring the security of intellectual property

16 Supporting AI with new opticalmodulators
To cope with surging internet traffic, datacentres canbe upgraded with electro-optic polymer modulators.

22 Lumentum expands Caswell fab,reflecting company-wide plansfor growth
Lumentum is due to complete a 25 percent expansionof its wafer fab at Caswell Science and Technology Parkin April this year
4 ISSUE I 2024 I PIC MAGAZINE.NET

24 Three ways photonics can catalyse
hardware evolution for datacoms
As the computing industry faces ever-growing demands
for higher speeds, photonic technologies can facilitate
the necessary hardware evolution, through hybrid
integration, chip disaggregation, and using 200G per lane

30 Extremely high-power transmittersfor access networks
Sources that combine a laser, a modulator and an
amplifier in a single device are laying the foundations
for high capacity access networks

34 Getting the full PICture: testing
microring resonators with
sub-picometre spectral resolution
Microring resonators are key components in many
PIC technologies, but, as their performance continues
to improve, they require increasingly precise test andmeasurement solutions. EXFO’s testing platforms canhelp to address this need

38 Moving towards polarisationindependent monolithic PICs
As datacentre infrastructure races towards 200G
per lane and beyond, polarisation-dependent loss
and differential group delay remain significant hurdles
in high-speed optical receivers

42 Tuneable lasers: The elusive
trifecta
Integrated tuneable laser assemblies serving in theC- and O- bands excel on three fronts by offering
the unique combination of wide tuning, fastnanosecond switching and a narrow linewidth

06 DustPhotonics raises $24 million infollow-on funding round
07 Synopsys to acquire Ansys
08 New technique enables efficient integrationof III-V with silicon
09 Creating photonic chips with a laser printer
10 Amazec Photonics awarded €1.5 million
for PIC-based medical tech
11 Cisco and Microsoft deliver 800G on
Amitié Transatlantic Cable
12 UK Government announces new fundingfor chips and quantum

11

30
00

EditorLaura Hiscottlaura.hiscott@angelbc.com
Contributing Technical Editor
Richard Stevenson
richard.stevenson@angelbc.com
Sales & Marketing Manager
Shehzad Munshi
shehzad.munshi@angelbc.com
Design & Production Manager
Mitch Gaynor
mitch.gaynor@angelbc.com
Publisher
Jackie Cannon
jackie.cannon@angelbc.com

NEWS

CEO Sukhi Bhadal
CTO Scott Adams

sukhi.bhadal@angelbc.com
scott.adams@angelbc.com+44 (0)2476 718970+44 (0)2476 718970+44 (0)1923 690215+44 (0)1923 690215+44 (0)1923 690214+44 (0)1923 690205

Published byAngel Business Communications Ltd
6 Bow Court, Fletchworth Gate,
Burnsall Road, Coventry CV5 6SP, UK.
T: +44 (0)2476 718 970
E: info@angelbc.com

PIC Magazine is published four times a year on a controlled circulation basis. Non-qualifying individuals can subscribe at: £105.00/e158 pa (UK & Europe), £138.00 pa (air mail), $198 pa (USA). Cover price£4.50. All information herein is believed to be correct at time of going to press. The publisher does not accept responsibility for any errors and omissions. The views expressed in this publication are notnecessarily those of the publisher. Every effort has been made to obtain copyright permission for the material contained in this publication. Angel Business Communications Ltd will be happy to acknowledge
any copyright oversights in a subsequent issue of the publication. Angel Business Communications Ltd © Copyright 2024. All rights reserved. Contents may not be reproduced in whole or part without thewritten consent of the publishers. The paper used within this magazine is produced by chain of custody certified manufacturers, guaranteeing sustainable sourcing.US mailing information: Pic Magazine, ISSN
1096-598X, is published 4 times a year, March, May, August and December by Angel Business Communications Ltd, Unit 6, Bow Court, Fletchworth Gate, Burnsall Rd, Coventry CV5 6SP. UK. The 2024 US
annual subscription price is $198. Airfreight and mailing in the USA by agent named Air Business Ltd, c/o Worldnet Shipping Inc., 156-15, 146th Avenue, 2nd Floor, Jamaica, NY 11434, USA. Periodicals postage paid at Jamaica NY 11431. US Postmaster: Send address changes to Pic Magazine, Air Business Ltd, c/o Worldnet Shipping Inc., 156-15, 146th Avenue, 2nd Floor, Jamaica, NY 11434, USA. Printed by: The
Manson Group. © Copyright 2024. We strive for accuracy in all we publish; readers and contributors are encouraged to contact us if they recognise an error or omission. Once a magazine edition is published[online, in print or both], we do not update previously published articles to align old company names, branding, marketing efforts, taglines, mission statements or other promotional verbiage, images, or logos
to newly created or updated names, images, typographic renderings, logos (or similar) when such references/images were accurately stated, rendered or displayed at the time of the original publication. When
companies change their names or the images/text used to represent the company, we invite organizations to provide Angel Business Communications with a news release detailing their new business objectives and/or other changes that could impact how customers/prospects might recognise the company, contact the organisation, or engage with them for future commercial enterprise.

PIC MAGAZINE.NET I ISSUE I 2024 5

INDUSTRY NEWS

DustPhotonics raises $24 million in follow-on
funding roundThe company says the investment will boost production of its current products for 400G
and 800G applications, as well as accelerate development of new products for 1.6T
applications
DUSTPHOTONICS, a developer ofsilicon photonics technology for
datacentre and AI applications, hasannounced that it has raised an
additional $24 million in its Series B
follow-on funding round. The company
says it will use this new investment toscale production of its Carmel-4 andCarmel-8 products, used for 400G and
800G applications, and to accelerate
development of its next-generation
products, which aim to enable 1.6T
applications.

technology and customer
traction,” said Isabelle AmielAzoulai, founding partner at
Sienna Venture Capital. “We
are excited by the customer
agreements and backlog thatthe DustPhotonics team has
generated with its Carmel-4
and Carmel-8 products, andlook forward to working with
the DustPhotonics team tohelp the company in this next
growth phase.”

DustPhotonics says this latest funding
round was oversubscribed, and was
funded by a combination of new andexisting investors, such as Sienna
Venture Capital, Greenfield Partners,
Atreides Management, and Exor
Ventures.

“Greenfield Partners is excited tocontinue to support DustPhotonics
in this upcoming phase of growth,on the back of strong execution by
the DustPhotonics team across bothproduct innovation and commercial
wins,” said Yuda Doron, managing
partner at Greenfield Partners. “We
continue to be firm believers in theprospect of silicon photonics becoming
a key enabler for datacentres tomeet the increasingly demanding“We looked at innovative technologies
supporting the fast-growing AI compute
market, and DustPhotonics stood out
as a leader in this market due to theirrequirements brought upon by AI, as
demonstrated by the company’s recent
momentum.” Ronnen Lovinger, CEO of
DustPhotonics, commented: “We areseeing a lot of design-win momentum
with our products, and are excited by
the new opportunities ahead in both
AI and cloud service datacentres. We
are grateful to have a strong group ofinvestors who have been supporting us
from the early days, and are delighted
to have new partners join our journeyahead.”

Coherent CEO to step down
COHERENT has announced that the current CEO, Vincent(“Chuck”) Mattera, has informed the board of directors that
he intends to retire as CEO once a successor has been
employed. The company emphasised that this decision was
not due to a dispute or disagreement.
The board of directors has retained an executive search firm,
which will immediately commence a comprehensive search
process, evaluating both internal and external candidates,
to identify a new CEO. A subcommittee of the board has
been formed to oversee this process. Mattera, 68, joined
Coherent 20 years ago, and has served as CEO for the lasteight. He is the company’s third CEO since its founding in 1971
as II-VI Incorporated, and he has also served as chair of its
board of directors since November 2021. During his tenure,
Mattera led the transformation of II-VI into Coherent. “Leading
this 53-year-old company through its multi-decade growthtransformation has been an incredible privilege,” said Mattera.
“I want to acknowledge and deeply thank our employees,

6 ISSUE I 2024 I PIC MAGAZINE.NET

investors, customers, partners,
and especially our leadershipteam, as well as my fellow
board members, for our sharedaccomplishments and for helping
make my years at Coherent so
rewarding and impactful.
“Since the strategic combination of II-VI and Coherent two
years ago, I am most proud of the tremendous progress
we have made to integrate our two organisations, optimise
synergies, and place the company in an advantaged
position for accelerated growth. With Coherent on a clear
path to improved margins and continued profitable growth,
its track record of product leadership, customer intimacy,
operational excellence, and the unstoppable imaginationof our world-class people, I believe that now is the righttime to look toward the next chapter of the company’stransformation.”

INDUSTRY NEWS

Synopsys to acquire Ansys
The move combines the companies’ expertise in electronic design automation, simulation
and analysis
SYNOPSYS and Ansys have enteredinto an agreement under which
Synopsys will acquire Ansys. The
companies say that this step brings
together Synopsys’ pioneering
semiconductor electronic design
automation (EDA) with Ansys’ broad
simulation and analysis portfolio,
creating a leader in silicon to systems
design solutions.

to help maximise the capabilities oftechnology R&D teams across a broad
range of industries. This is the logicalnext step for our successful, sevenyear partnership with Ansys and I
look forward to working closely with
Ajei and the talented Ansys team torealise the benefits of this combinationfor our customers, shareholders andemployees.”“The megatrends of AI, siliconproliferation and software-definedsystems are requiring more computeperformance and efficiency in theface of growing, systemic complexity,”
said Sassine Ghazi, president andCEO of Synopsys. “Bringing together
Synopsys’ industry-leading EDA
solutions with Ansys’ world-classsimulation and analysis capabilities will
enable us to deliver a holistic, powerfuland seamlessly integrated silicon
to systems approach to innovation

Aart de Geus, executive chair andfounder of Synopsys, commented:“Since inception 37 years ago,
Synopsys has been an innovation
pioneer, central to world-changing
semiconductor advances incomputation, networking, and mobility,and now enabling the new era of‘pervasive intelligence’. Joining forces
with Ansys, a company we know wellfrom our long-standing partnership, isthe latest example of how Synopsys
remains at the forefront.

“Our board and management team
carefully evaluated our top strategic
options to lead and win in this fastgrowing new wave of electronics
and system design. The technologybroadening team-up with Ansys is an
ideal, value-enhancing step for ourcompany, our shareholders, and theinnovative customers we serve.”
Ajei Gopal, president and CEO of
Ansys, said: “For more than 50 years,
Ansys has enabled customers todesign, develop and deliver cuttingedge products that are limited only
by imagination. By joining forces with
Synopsys, we will amplify our joint
efforts to drive new levels of customer
innovation. This transformative
combination brings together eachcompany’s highly complementary
capabilities to meet the evolving
needs of today’s engineers and givethem unprecedented insight into theperformance of their products.”

Chip uses light to perform calculations needed for AI
ENGINEERS at the University ofPennsylvania have developed a newchip that uses light waves, rather thanelectricity, to perform the complex
mathematics essential to training
AI. The researchers say that thisdevelopment, reported in a paperin Nature Photonics, could radicallyaccelerate the processing speed ofcomputers while also reducing theirenergy consumption.
The silicon photonic chip applies
the work of Nader Engheta, who is
H. Nedwill Ramsey Professor in the
School of Engineering, whose expertise
includes manipulating materials at thenanoscale to perform mathematicalcomputations using light. In particular,
the chip’s design is the first to combine
Engheta’s research with the silicon
photonic platform, which uses silicon,
the cheap, abundant element used tomass-produce computer chips.
The interaction of light waves withmatter represents one possible
avenue for developing computers thatsupersede the limitations of today’s
chips, which are essentially basedon the same principles as chips fromthe earliest days of the computing
revolution in the 1960s.
In their new paper, Engheta’s group,
together with that of Firooz Aflatouni,
associate professor in electrical andsystems engineering, describe thedevelopment of the new chip. “We
decided to join forces,” says Engheta,
leveraging Aflatouni’s group’s researchon nanoscale silicon devices.
Their goal was to develop a platform
for performing what is known as
vector-matrix multiplication, a coremathematical operation in thedevelopment and function of neuralnetworks, the computer architecturethat powers today’s AI tools. Insteadof using a silicon wafer of uniform

height, explains Engheta, “you makethe silicon thinner, say 150 nm,” but onlyin specific regions. Those variations inheight — without the addition of any
other materials — provide a meansof controlling the propagation of lightthrough the chip, since the variations inheight can be distributed to cause lightto scatter in specific patterns, allowing
the chip to perform mathematical
calculations at the speed of light.
Due to the constraints imposed by thecommercial foundry that produced
the chips, Aflatouni says, this designis already ready for commercial
applications, and could potentially be
adapted for use in graphics processing
units (GPUs), the demand for which
has skyrocketed with the widespreadinterest in developing new AI systems.
“They can adopt the silicon photonics
platform as an add-on,” says Aflatouni,“and then you could speed up training
and classification.”

PIC MAGAZINE.NET I ISSUE I 2024 7

INDUSTRY NEWSNew technique enables efficient integration of
III-V with silicon
The method involves selectively growing III-V materials on silicon-on-insulator laterally,
without thick buffers
RESEARCHERS at the Hong Kong
University of Science and Technology(HKUST) have developed a new
integration technique for efficient
integration of III-V compound
semiconductor devices and silicon.
The scientists say this method paves
the way for photonic integration at low
cost, large volume, and high speed andthroughput, potentially revolutionisingdata communications. The findings
are reported in the journal Laser &Photonics Reviews.
Unlike conventional integrated circuits,
or microchips, that use electrons,
photonic integrated circuits use
photons, or particles of light. Photonic
integration combines light andelectronics to speed up data transfer.
Silicon photonics, in particular, is at theforefront of this revolution as it enables
the creation of high-speed, low-costconnections that can handle massive
amounts of data at once.
While silicon can handle passive optical
functions, it struggles with active tasks,
such as generating light (lasers) or
detecting it (photodetectors) – both
key components for data generation
and readout. This necessitates theintegration of III-V semiconductormaterials (which use elements fromgroups III and V of the periodic table)
onto a silicon substrate for complete
functionality and enhanced efficiency.
However, while III-V semiconductors
do the active tasks well, they do notnaturally work well with silicon. The
team, led by Xue Ying, research
assistant professor, and Lau Kei-May,
research professor from the Division of
Emerging Interdisciplinary Areas (EMIA),
tackled this challenge by finding a way
to make III-V devices work efficiently
with silicon.
They developed a technique called
lateral aspect ratio trapping (LART) – a

8 ISSUE I 2024 I PIC MAGAZINE.NET

novel selective direct epitaxy methodthat can selectively grow III-V materials
on silicon-on-insulator (SOI) in a lateral
direction without the need for thickbuffers.
While no integration methods reported
in the previous literature could solve
the challenge with high coupling
efficiency and high production volume,
their method achieved an in-plane III-V
laser, so that the III-V laser can couple
with silicon in the same plane, making it
efficient.
“Our approach addressed the mismatchof III-V devices and silicon,” said Xue. “It
achieved excellent performance of III-V
devices and made it easy and efficient
to couple III-V with silicon.”
In the past decades, data traffic
has grown exponentially, driven by
emerging technologies such as big
data, cloud applications, and sensors.
The field of integrated circuits, also
known as microelectronics, has enabledthat growth by making electronic
devices smaller and faster thanks to
Moore’s Law, an observation that the
number of transistors on a microchip

doubles about every two years. But
the continued explosion of data traffic
has pushed traditional electronic
devices to their limits. The start of
the Zettabyte Era in 2016 ushered insoaring growth in data generation,
processing, transmission, storage,
and readout. This surge poses criticalchallenges of speed, bandwidth, cost,and power consumption. This is where
photonic integration, in particular silicon
photonics, comes in.
In the next steps, the team plans toshow that III-V lasers integrated withsilicon waveguides can perform well,with a low threshold, high output power,
long lifetime, and the ability to operate
at high temperatures.
Xue added that there are key scientificchallenges to address before the
new technique can be used ineveryday technologies. However,the scientists say it will enable newgeneration communications and various
emerging applications and researchareas, including supercomputers,
artificial intelligence (AI), biomedicine,
automotive applications, and neural andquantum networks.

INDUSTRY NEWS

Creating photonic chips with a laser printerResearchers have developed a new technique for fabricating rewritable PICs using
phase-change materials
AS PICs find applications across a broad
spectrum of near-future technologies,
from LiDAR and biomedical sensors to
6G and quantum computing, accessto fabrication equipment for them is
becoming increasingly important.
However, today’s nanofabrication
facilities cost millions of dollars toconstruct and are well beyond thereach of many colleges, universities,
and research labs. Those who canaccess a nanofabrication facility mustreserve at least a day for the timeconsuming lithographic process.
Furthermore, if there is a design error,
or if the chip doesn’t work properly, it
must be discarded, the design adjusted,
and a new chip fabricated. This often
results in days or even weeks spent in
the cleanroom.
But now, as described in a new paperin Science Advances, a research teamled by the University of Washington
Department of Electrical andComputer Engineering (UW ECE) has
devised a way to bypass expensive
nanofabrication facilities and produce
PICs almost anywhere. The team hasdeveloped an innovative method withwhich these circuits can be written,
erased, and modified by a laser writer
into a thin film of phase-change material
similar to what is used for recordable
CDs and DVDs. With this newprocess, PICs can be constructed andreconfigured in a fraction of the time it
would take at a nanofabrication lab.
The multi-university team was led
by Mo Li, a professor of physics andassociate chair for research at UW
ECE. “Photonics technology is on thehorizon; therefore, we need to train or
educate our students in this field,” says
Li. “But for students to study and havehands-on experience with photonic
circuits, currently, they need access toa multimillion-dollar facility. This newtechnology addresses that problem.
Using our method, photonic circuitsthat previously had to be fabricated in
expensive and hard-to-access facilities
now can be printed and reconfigured
in labs, classrooms, and even garage
workshops, by a speedy, low-cost
device about the size of a conventional
desktop laser printer.”
Researchers can also benefit fromthis advance, which will enable amuch quicker turnaround time for
prototyping and testing out a new
idea before booking valuable timein a nanofabrication facility. And forindustrial applications, a big advantageof this method is reconfigurability.
For example, companies could
possibly use this technology to create
reconfigurable optical connections
in datacentres, especially in systems
that support artificial intelligence andmachine learning, which would reduce
costs and boost efficiencies.
Li’s research team included UW ECE
graduate student Changming Wu, who
is lead author of the paper, and, alongwith Li, came up with the idea for thisnovel technique. UW ECE graduate
student Haoqin Deng also contributed
to the effort. Their work is the latest
result of a six-year line of research that
includes advances in optical computing.
It is also a continuation of a productive
collaboration with professors Ichiro
Takeuchi and Carlos Ríos Ocampo
and their students at the University of
Maryland. The work was funded by
the Office of Naval Research and the
National Science Foundation.
“Being able to write a whole photonic
circuit using only one single step,
without a complicated fabrication
process, is really exciting,” says Wu.
“And the fact that we can make any
modification to any part of the circuit inour own lab and rewrite and redo it isamazing. It’s a matter of minutes versus
a full, day-long process. It’s a huge relief
to be able to finish the whole fabrication

process within a few minutes insteadof what often is several days or even a
week.”The method the team developed has
been proven to work, but it is still an
early-stage concept. However, Li has
filed a provisional patent application,and he is planning to build a desktop
laser writer for PICs. The team’s vision
is of a laser printer that uses a staging
system to move the substrate in amuch more precise manner than in a
traditional desktop printer, and which
can be sold at an affordable price anddistributed widely to research labs
and educational institutions aroundthe world. The researchers are also
looking at reducing optical loss in thephase-change material, to enable theprinting of even more detailed andsophisticated circuits.
Li is already engaging with industry
leaders to promote possible
applications of this new technology inprogrammable PICs and reconfigurable
optical networks. “This technology
can create the photonic circuitry you
want, but it also can be added onto
already-existing electronic circuitry,”
he explains. “And because it isreconfigurable and reusable, it justopens many possibilities for students,
researchers, and industry. What’s most
exciting to me is that we’ll potentiallyhave a huge impact on the field ofphotonics in disseminating this new tooland technology to the broader researchcommunity.”

PIC MAGAZINE.NET I ISSUE I 2024 9

INDUSTRY NEWS

Amazec Photonics awarded€1.5 million for PIC-based medical tech
The seed funding will be used to develop accurate and minimally invasive devices for
cardiovascular monitoring and diagnosis, with clinical trials beginning this year
open the door to many more people
being tested much more regularly.
With this funding round we will beable to build ten prototypes andundertake extensive clinical trials with
the intention of producing and sellingdevices across the EU by 2028.”

AMAZEC PHOTONICS, a company
focused on using integrated photonics
to advance medical technology, has
secured a €1.5 million seed round todevelop its diagnosis devices.
The round was led by PhotonDelta– an accelerator and ecosystem ofphotonic chip technology organisations– with several private investors also
contributing. The funding will be used
to develop devices for clinical trials.
Cardiovascular diseases are the world’s
leading cause of death, accounting for
19 million deaths per year. This is inpart due to the difficulty in diagnosing
conditions, leading to delays intreatment. Amazec says its solution – aneasy-to-use cardiovascular monitoring
tool – is a huge step forward, enablingmuch earlier and much more accurate
diagnosis for minimal costs andcomplexity.
Existing solutions are complex, invasive,and often inaccurate. The mostcommon technique to measure cardiac
output is called thermodilution, which
involves injecting a known volume
of liquid upstream of the heart andthen measuring temperature changes
downstream through specialised
catheters inserted into the patient.
This has several drawbacks, including
an inability to be used reliably
during routine examination, large
variation between measurements,
a lack of sensitivity, and high costs.
Consequently, it often leads to late or
misdiagnosis, severely impacting theoutlook for patients.
According to Amazec, its new solution
uses photonics-based technology tomeasure temperature changes to an
unprecedented precision of 0.0001
degrees Celsius, compared to current
accuracy of 0.01 degrees Celsius.

10 ISSUE I 2024 I PIC MAGAZINE.NET

The monitoring device is external,
meaning there is no need to insert
catheters. The company also says thatmeasurements can be made in realtime, improving reliability comparedwith current methods, which use asingle measurement.
Pim Kat, CEO of Amazec Photonics,
said: “The number of people sufferingfrom cardiovascular diseases has risen
by 93 percent over the past 25 years
and now impacts an estimated 550
million patients worldwide. Many ofthese people will die or suffer poor
health outcomes because the tools wehave to diagnose them simply aren’t
good enough.
“Our solution can make a realdifference because, not only does it
vastly improve the accuracy of testingfor cardiovascular disease, it is alsomuch less invasive and simpler to use.
This will substantially reduce costs andLaurens Weers, CFO of PhotonDelta,
said: “Amazec has leveraged the powerof photonics to create a device thatcan make a profound impact on theworld. Cardiovascular disease is oneof the biggest health challenges weface and better diagnosis can be thekey to saving millions of lives. We’revery proud to be a part of Amazec’s
journey - we believe it has the capacity
to become one of Europe’s mostimportant medtechs and a standard
bearer for a new generation ofphotonics-based technology.”
PhotonDelta’s investment in Amazec
Photonics is the latest step in theorganisation’s goal to create a worldleading photonics industry in the
Netherlands. PhotonDelta aims tohelp build 200 startups, create new
applications for photonic chips anddevelop infrastructure and talent.
Amazec will begin clinical trials of
its device at Catharina Hospital in
Eindhoven this year, with an expansion
to three other hospitals planned in
2025.

“

The number of people suffering from cardiovasculardiseases has risen by 93 percent over the past 25 years
and now impacts an estimated 550 million patientsworldwide. Many of these people will die or suffer
poor health outcomes because the tools we have todiagnose them simply aren’t good enough”

INDUSTRY NEWS

Cisco and Microsoft deliver 800G on Amitié
Transatlantic Cable
The companies say that the trial, which was powered in part by silicon photonics,
outperformed any other industry trial to date with DWDM 800G in a 150 GHz channel
spacing
CISCO has announced that it has
successfully transmitted 800G on the
Amitié transatlantic communications
cable, which runs 6,234 km from
Boston, Massachusetts to Bordeaux,France. The trial was performed with
the Cisco NCS 1014 platform enabled by
Acacia’s Coherent Interconnect Module
8 (CIM 8), which is powered by its Jannu
digital signal processor and advanced
silicon photonics.
The continued growth of cloud andexplosion of AI services is driving
the need for greater subsea network
capacity, which requires advanced
coherent transmission systems that
support higher performance. This trial
was conducted to target improvements
in subsea transmission to provide
increased performance and capacity.
The Amitié submarine cable features
Space Division Multiplexing (SDM)
technology with 16 fibre pairs, morethan traditional subsea cables, withrepeater power shared across thefibre pairs to deliver the highest cablecapacity.
According to Cisco, this real-timefield trial exceeded any industry trial

performance to date with Dense
Wavelength Division Multiplexing(DWDM) 800G in a 150 GHz channel
spacing, equivalent to a spectrum
efficiency of 5.33 bit/s/Hz and amaximum spectral efficiency of
5.6 bit/s/Hz. In addition, 600G wastransmitted over 12,469 km for atrans-atlantic loopback configuration.
The company says this is the first timea 140 Gbaud single carrier signal wasdemonstrated live, and is the longest
distance ever reported at single carrier
600G DWDM transmission on an SDM
cable.
The Amitié cable, a transatlantic
submarine cable connecting theUS, UK, and France, is now part ofMicrosoft’s global network supporting
all Microsoft services.
“In the era of AI, reliable and fast
network connections are more
important than ever,” said Bill Gartner,
senior vice president of opticalsystems and optics at Cisco. “Working
with Microsoft on the Amitié cable todemonstrate the potential for improved
overall network capacity with 800G
at these distances is a significant

milestone for an SDM cable, and we’re
proud to drive the innovations that pave
the way for ever-increasing network
capacity needs.”
Jamie Gaudette, general manager
of cloud network engineering at
Microsoft, added: “The transmissionof 800G over 6,234 km is a milestonethat demonstrates SDM cables candeliver increased capacity overtraditional subsea cables. This field trial
demonstrates what is now a commercial
technology for subsea routes, and we
can improve the network capacity tohelp drive digital transformation forpeople, organisations, and industries
around the world.”

Dedicated webinars for the photonic
integrated circuits (PIC) industry
Using our 30+ years’ experience in B2B vertical technical markets, and as the
publisher of PIC Magazine, we offer effective webinars, ZOOM interview andvirtual events. We help you get your message, to your desired audience, bymarketing to over 15,000 PIC professionals.
In addition to organising and managing your webinar, we can also market yourwebinar to our specialist databases.

Reach Educate Influence£ Brand Awareness£ Lead Generation£ Thought Leadership

Contact: Jackie Cannon
jackie.cannon@angelbc.com+44 (0)1923 690205

PIC MAGAZINE.NET I ISSUE I 2024 11

INDUSTRY I PROCESS DESIGN KITS

The best of both worlds:
Design freedom and IP protection

VPIphotonics empowers foundries and fabless companies by giving them
the freedom to customise PIC designs using process design kits while
also ensuring the security of intellectual property
BY ANDRZEJ POLATYNSKI, STANISLAU SAVITSKI, CHRIS MALONEY,AND ANDRÉ RICHTER FROM VPIPHOTONICS
IN RECENT YEARS, rapid advancement in the
PIC industry has led to the emergence of many
advanced technological platforms for photonic
chips. Those platforms are commercially available
to enterprises and scientific institutions, often
after passing long development cycles. Modern
photonic design automation (PDA) tools provide
access to these technologies via professional
software environments, which support the design ofapplication-specific photonic circuits and systems
with foundry-dedicated building block libraries,
better known as process design kits (PDKs). For
instance, this industry ecosystem allows end users
to create PICs whose spectral range of operation
extends beyond the C-band, enhancing their use
in telecommunications and data transmission
networks, sensor systems, and biomedical
devices.
However, since there is a need to balancecustomisability with intellectual property (IP) security,
end users rarely have complete freedom in how
they use PDKs. Photonic foundries that open theirtechnology to third-party users specify stringent
constraints describing their fabrication processes
and design rules. Typically, only limited building
block geometry and performance data is shared,
as these details represent critical IP belonging to afoundry, collected after many years of development.
Foundries limit the information accessible to end
 Figure 1.
Customisationof the PDK
building
blocks (BBs)based on theexample of animbalanced 2x2
multi-mode
interferometer(MMI).
12 ISSUE I 2024 I PIC MAGAZINE.NET

users to safeguard their proprietary knowledge,
meaning that end users can often use only a basic
subset of the platform’s capabilities. They are usuallyalso restricted to utilising predefined and optimisedbuilding blocks, providing only a few parametrised
attributes to design their PIC.
To enable the broadest range of applications, andfulfil the PIC industry’s needs, PIC designers willneed access to more flexible PDKs, but there are
two main challenges to overcome. First, there needs
to be a way to customise existing building blocks
without breaching the foundry’s IP. Second, end
users need to be able to create new custom building
blocks, either based on device-level simulations orexperimental data. Solving these issues would allow
the designer to supplement existing PDK libraries
with new building blocks, opening a wide range of
new applications.
This is where VPIphotonics comes in. The
company’s solution is to provide a framework forgenerating custom PDKs, allowing designers toenrich existing commercial PDKs and to create
new PDKs for other PIC platforms. This frameworkoffers its users complete freedom to customise an
existing PDK, allowing them to add complementarybuilding blocks and enabling fabless companies todevelop their own internal PDKs while protecting
their IP.

INDUSTRY I PROCESS DESIGN KITS

Custom PDK generation

The process of generating a custom PDK is
depicted in Figure 2, where the starting point is ademonstration library, here called PDK DemoFab.
The user can specify a custom PDK name to create
a new library, which includes all the necessary
features of a standard PDK, including design
automation macros, custom user interface toolbars,
building block templates, and their links to layout
descriptions. This process can be further customised
to adapt the template models to the designer’s
unique requirements.
Moreover, users can add modules from commerciallyavailable PDKs. All added foundry components are
directly encrypted to protect the foundry IP, and theuser can automatically encrypt their custom building
blocks to protect their own IP when shared as partof a custom PDK. Usage of the foundry components
typically requires a confidentiality agreement withthe fab and the allocation of a software license
dedicated to the foundry PDK. Customisedbuilding blocks are licensed under the custom PDK
framework and can be encrypted directly by theuser. Thus, sharing those components does not
require additional licenses and can be done directlyby the user who developed the custom PDK.
An essential feature of professional circuit design
tools is the capability of simulating fabrication
tolerances. Therefore, just as with standard foundry
PDKs, tolerances can be included in custom PDK
libraries by adding the performance distribution
directly to the description of a building block. The
design process and challenges are identical incustom and commercial foundry PDKs and aredescribed in detail in previous literature [1].

analysis of their design. Thanks to the layout-aware
simulation approach, the layout of the designed
chip can be directly verified and visualised fromthe schematic perspective at any time. Moreover,the designer can utilise “elastic” components [2],ensuring that the waveguides used to connectbuilding blocks will be automatically routed to fulfil
the design requirements.

 Figure 2.
Generation ofa custom PDK in
VPIcomponent
Maker Photonic
Circuits.

A typical simulation PDK must include building
blocks verified by the foundry with properinterfacing between the simulation model and thedevice layout. The requirements are the samefor a custom PDK, but now the PDK designer
is responsible for the functional building block
instead of the foundry. To ensure that the custom
module performs correctly, designers must relyon their knowledge from previous foundry runs
or device-level simulations. Characteristics fromthese sources can be included in the PDK library,allowing for circuit and system-level designs. Theinterface between the layout and simulation toolmeans that, with the press of a single button, thelayout can be automatically exported from thesimulation schematic. The designer can definea custom layout description compatible with afoundry’s layer definition. Within the custom PDK
framework available in VPIcomponentMaker
Photonic Circuits, the interface includes templates
for PDK module generation with example layout
descriptions in several layout tools, as shown in

Design workflow, implementation,and interfaces

The workflow for using a custom PDK is equivalent
to that of a standard PDK, utilising a layout-aware
schematic-driven design methodology, as shownin Figure 3. The designer can use custom building
blocks and standard PDK components to createa circuit design that can be simulated and further
optimised. The user can also perform a tolerance Figure 3. Block description of the layout-aware schematic-driven design
methodology and its implementation with VPIcomponentMaker Photonic
Circuits [2].
PIC MAGAZINE.NET I ISSUE I 2024 13

INDUSTRY I PROCESS DESIGN KITS
 Figure 4.
Demonstrationof theautomated
export fromthe simulation
schematic tovarious layout
tools.

Figure 4. Furthermore, dedicated macros can help
the designer build custom PDK building blocks
automatically by loading data files in a standardised
format or via the interface with a device-level
simulation solution called VPIdeviceDesigner.

Many platforms, one solution

Another advantage of the custom PDK is that it isinherently platform agnostic; users can easily adapt
it to any material, operational wavelength, andmanufacturing process. Within the current stateof the art, custom PDKs have been demonstrated
for various platforms, including indium phosphide,lithium niobate, polymer, silicon, silicon nitride,
and germanium on silicon. Several examples
are shown in Figure 5. Achieving both flexibility
for PIC designers and IP security presents aninherent challenge for the end users of PDKs, but
VPIphotonics’ custom PDK framework provides aflexible solution for foundries and fabless companies
alike. Enabling the customisation of foundry building
blocks and the development of customised PDK
libraries, this solution offers all the features of a Figure 5. Demonstrationof custom PDKs dedicatedto various platforms
developed under European
and regional R&D projects,
including polymer, silicon
nitride (PolyChrome project,polychrome-berlin.de), lithium
niobate (ELENA project,www.project-elena.eu), andgermanium on silicon (MIRPIC
project, www.mirpic.eu).
Various waveguide colours
determine different modules’
cross-sections.
14 ISSUE I 2024 I PIC MAGAZINE.NET

typical foundry PDK, including IP protection, while
being easy to share and giving designers the
freedom to add and adapt building blocks to their
specific needs.

FURTHER READING / REFERENCE [1] André Richter, et al., “Schematic-driven
PIC design process considering
manufacturing tolerances,” Proc. SPIE 11690,
Smart Photonic and Optoelectronic
Integrated Circuits XXIII, 1169004 (5 March
2021); https://doi.org/10.1117/12.2577209
 [2] S. Mingaleev, et al., “Rapid virtual
prototyping of complex photonic integrated
circuits using layout-aware schematic-driven
design methodology,” Proc. SPIE 10107, Smart
Photonic and Optoelectronic Integrated
Circuits XIX, 1010708 (20 February 2017);
https://doi.org/10.1117/12.2252001

TECHNOLOGY I ELECTRO-OPTIC POLYMERSSupporting AI with new optical modulators
To cope with surging internet traffic, datacentres can be upgraded withelectro-optic polymer modulators. This technology requires no changes
to the rest of the infrastructure and has performance headroom tosupport continual upgrades for decades.
BY MICHAEL LEBBY, CHIEF EXECUTIVE OFFICER, LIGHTWAVE LOGIC
THERE HAS long been talk of AI’s transformative
potential, and the last year has made it clearer
than ever that the AI revolution has begun. The
technology is already reshaping many aspects ofour daily lives as end users. But behind the scenes,
AI is also impacting the industries and infrastructurethat play a role in providing access to it. And
photonics is no exception.
To understand how this trend might affect photonic
integrated circuits (PICs), we must first consider
how AI relates to the internet and optical networks.
Although we would all like to see optical computing,and optical computational processing, this holygrail has yet to become widely commercialised likesemiconductor microprocessing units (MPUs) orgraphical processing units (GPUs).
For this reason, the industry currently uses
photonics to send the information from MPUs andGPUs from source to destination, with fibre optic
cables that form the architecture of the internet andoptical network.
At the highest level, electronics does thecomputational processing, and is expected
16 ISSUE I 2024 I PIC MAGAZINE.NET

to continue doing so in the near future, while
photonics helps convey huge amounts of generated
information optically.
The photonic components that make up thecommunication are laser diodes, modulators,
and photodetectors. These components are now
becoming integrated into PICs, typically with one
PIC transmitting data, and another receiving it. PICs
are found in transceiver modules, which, in turn, arefound in the switches and router equipment thatmake up a datacentre.
As surging AI usage demands higher data rates,
PICs are therefore one piece of the puzzle in
upgrading datacentres to meet that demand. As well
as looking at the challenges posed by AI, this article
describes how electro-optic polymers are driving
new hybrid technologies for increased
PIC performance.

Skyrocketing data rates

It is widely expected that AI will substantially
increase the bandwidth required to operate
the internet at the speed users have come to
expect. This is because AI users are increasingly

TECHNOLOGY I ELECTRO-OPTIC POLYMERS
experimenting with innovative ways to drive new
applications which require large amounts data to besynchronously exchanged between switches andusers.
In the 1980s, it would typically take tens of minutes
to download a 10 Mb file, but this can be done
today in less than a second. Videos were virtually
impossible to download and send around backthen, but today we share short video clips, evenmovies, with friends and family regularly. Thanks tothese advances, images and videos have become
the largest generators of internet traffic. But it now
seems likely that AI will surpass them. Whereas
it took Netflix 3.5 years to get to one million
subscribers, it took ChatGPT just five days to reachthe same milestone.
Additionally, over the past 60 years, computing
power in high computational processing systems
has skyrocketed. Its growth initially doubled every
3-5 years [1], and, since about 2020, it has increasedby over an order of magnitude, to a doubling ofcomputational power every 3-4 months in termsof petaflops [2]. Naturally, this puts strain on theinfrastructure supporting it.
All parts of the network need to cope with increased
quantities of information, from the fibres that opticalsignals travel down to the places performing opticalrouting and switching – which typically happens
in datacentres. The financial cost for internet andoptical network operators will be substantial. Further,
it is not only the increased performance of sending
information through the optical network that needs
attention, but also ensuring power consumption is
kept at reasonable levels, as data rates increase.

Datacentres and the internet

As AI becomes increasingly integrated within ourdaily activities, we are already seeing datacentres
being upgraded today in a fashion that the industry
has not seen before. This feels reminiscent of theinternet bubble of 2000.

as the main vehicle. Datacentres were looking at
4-channel 100G and 8-channel 50G as key solutions
for aggregation into 400G. Today, this has allchanged; forecasts for 400G are flat if not declining,
and the focus is on 800G.
Unfortunately, the industry is not ready for 4 lanes at
200G, so initial prototypes demonstrated at majorindustry trade shows in 2023 are implementing
800G as best they can: using 8 channels with 100G
lanes. However, the move towards 200G lanes is
accelerating, pulled along by the excitement aroundAI, and by datacentre operators’ motivation to
upgrade equipment as quickly as possible. Simply
creating a 4-channel 200G-lane optical transceiver
is not easy. Many in the industry are now realising Datacentres
need toupgrade tomeet theunabatingdemand forhigher data
rates.

As we are in a growth stage, we don’t yet know
if the trend is bubble-like. However, it is worth
noting that the markets for the internet bubble
collapsed largely due to poor growth and business.
What is different today is that we are seeing
datacentre companies already investing to update
their equipment with solid, committed capital
expenditure. This effect looks to be stronger than in2000 and could drive robust growth for electronic
computational processing chips and photonic chips
over the next decade.
What is interesting from a photonics standpoint isthat datacentre operators have pretty much ignored
the photonics node of 400G and are focusing on
800G and 1.6T (or 1600G). Just two years ago, every
market analyst covering datacentres, and more
specifically optical transceivers, was forecastinghuge growth in 400G with optical transceivers

 High EO S21 RF bandwidth measurement of an electro-optic polymer
modulator with approximate comparisons of other materials.
PIC MAGAZINE.NET I ISSUE I 2024 17

TECHNOLOGY I ELECTRO-OPTIC POLYMERS
industry is addressing this via linear pluggable optics(LPO) to reduce power consumption. Meanwhile, thephotonics industry is turning to innovative opticalmodulator technology for PICs.
One promising solution is electro-optic polymers,
which offer a hybrid solution, as the material isorganic and can be spun or dropped onto an
existing PIC platform made of silicon or anothermaterial. The polymers are physically positioned
in front of lasers that are also part of the PIC.
Excitingly, electro-optic polymers significantly
outperform semiconductor technologies being usedin the internet today. With optical 3 dB bandwidths
exceeding 100 GHz, and that have been measured
to over 250 GHz, electro-optic polymers are well
positioned to enable 800G, 1.6T, 3.2T and evenhigher speeds over the next decade.
 4-channel
200G PAM41310 nm PIC
design using
electro-optic
polymers.

that the photonics need to upgrade also, and 20-30
GHz optical 3 dB bandwidth modulators for thetransmitter PIC are too slow.
In general, optical modulators are components thatswitch and modulate light, and there are millions ofthem embedded in the internet. However, the higher
data rates necessitated by increasing AI usage
are pushing incumbent semiconductor solutions
to their limits. This has led the industry to look forfaster optical modulators that have 3 dB optical
bandwidths double or triple the bandwidths oftoday’s solutions (silicon, indium phosphide, lithium
niobate).
Further, optical transceivers need to rein in theirpower consumption. While the majority of power
is consumed by the electronics, there are still
significant savings to be made through more
efficient use of PICs. Lastly, another major factor
for optical transceivers is size or footprint; astransceiver sizes are shrinking, smaller-footprint
photonics is key.
These requirements of higher speed, lower power,
and smaller size create big challenges for both theelectronics and photonics industries. The electronics

During fabrication, the polymers arealigned through brief application of a highvoltage, enabling ultra-fast modulation
at ultra-low power. Both the polymer
materials and the silicon PICs they areincorporated into perform stably andreliably, meaning they are well positioned
to displace current semiconductor
technologies
18 ISSUE I 2024 I PIC MAGAZINE.NET

Additionally, with drive voltages in the sub 2 V
that range down to the 0.5 V level, this technology
minimises power consumption effectively. And
finally, with electro-optic modulator device structures
such as the slot, footprint sizes are extremely
tiny, and can therefore work with many different
form factors for optical transceivers, within bothpluggable design as well as co-packaged, on-board
optics design.

Lightwave Logic’s electro-optic
polymers

Lightwave Logic [3] has been pursuing thispotential of electro-optic polymers to replace
existing modulators, using its patented Perkinamine
molecular compounds – state-of-the-art organic
materials that can be used to create the polymers.
The company starts with its proprietary organic
chromophores, which are a key ingredient ofpolymers, and deposits them onto a silicon chip to
add an optical modulator function.
During fabrication, the polymers are aligned throughbrief application of a high voltage, enabling ultra-fast
modulation at ultra-low power. Both the polymer
materials and the silicon PICs they are incorporated
into perform stably and reliably, meaning they are
well positioned to displace current semiconductor
technologies.
The silicon-based chips used are about a few
millimetres on each side, and they act as theengine of a fibre optic transceiver, which is a corecomponent of switches and routers in datacentres.
One of the advantages of using polymer modulators
can be understood through the analogy of
automotive vehicles; using these modulators is akinto upgrading a car simply by replacing its engine
with a better one, while keeping the rest of thestructure the same. Similarly, polymer modulators
can improve the fibre optic modules, while leaving
other parts of the datacentre infrastructure as theyare.
Thanks to velocity-phase matching of the electricalsignal and the optical beam, electro-optic polymers

TECHNOLOGY I ELECTRO-OPTIC POLYMERS
have inherently high performance, and, crucially, thepotential for this to increase even further in latergeneration products. Technology with this performanceheadroom is essential to support the continual upgradingthat the internet and optical networks need. Conversely,competing technologies – both those that are incumbent
and those competing for new business – may not work
well beyond the maturation of the current generation oftechnology.
One way to visualise this performance potential is toconsider the same baseline of 3 dB optical bandwidth ineach modulator. Over the past 10 years, semiconductor
modulators have generally been achieving around 20-30
GHz, but recent enhancements to both silicon and indium
phosphide designs have raised their performance to 4050 GHz, occasionally approaching 60 GHz. In general,to achieve 100G (or 100 Gbaud NRZ) and 200G (or 100
Gbaud PAM4) encoding, a 70 GHz 3 dB optical bandwidthis required.
Today, many datacentre operators are seeking
technologies that can achieve 200G per lane. Since
polymer modulators can reach 70 GHz, and even 150
GHz – about double current lane rates – they could pave
the way for 1.6T with 4 lanes at 400G. Moreover, whenenhanced with plasmonic designs, modulator devices
using Lightwave Logic’s electro-optic polymer material
have exhibited 3 dB bandwidths exceeding 250 GHz.

Get the full

PIC
ture

Now with

50x

Better Spectral
Resolution

Polymer modulators for AI

It is the potential performance of new technologies
in photonics that will enable datacentre operators tonavigate the demands of AI. Further, a new technology
platform that can turbo-boost existing transceivers withoutchanging the network architecture and infrastructure is a
low-cost approach to speed up the internet, keep power
consumption at bay, and keep the footprint and size thesame.
The potential for Lightwave Logic’s polymer technology
to make datacentre operators more competitive with
their equipment upgrades has never been higher. As AI
changes our lives, we face grand challenges ahead, but
we have the motivation to utilise our toolkits to meet thosechallenges. I’m sure this will involve developing numerous
new technologies, but electro-optic polymers are certainly
an excellent candidate to move the needle forward for allof us.

T200S / T500S –ContinuouslyTunableLasers

CTP10 –
Component
Testing Platform

Learn more on EXFO.com/CTP10FURTHER READING / REFERENCE [1] OpenAI, The Economist, Data Center World,“Computing power used in training AI systems”
 [2] https://datacenters.lbl.gov/sites/default/files/
Masanet_et_al_Science_2020.full_.pdf
 [3] https://www.lightwavelogic.com
PIC MAGAZINE.NET I ISSUE I 2024 19

INDUSTRY I FAB EXPANSION Caswell Science andTechnology Park was originallybuilt in the countryside to avoid
aerial bombing during WWII.
However, today, this locationmeans there is plenty of spacefor it to grow.

Lumentum expands Caswell fab, reflecting
company-wide plans for growth
Lumentum is due to complete a 25 percent expansion of its wafer fab at Caswell
Science and Technology Park in April this year. Editor, Laura Hiscott visited the facility
to learn how these activities fit into the company’s wider strategy for growth.
IIN THE HEART of the UK countryside, surroundedby fields and more than half an hour’s drive fromthe nearest train station, is a large campus wherearound 400 people gather to work each day.
Caswell Science and Technology Park certainly
seems far from the hustle and bustle of a city, but its
remote location belies the integral role it plays in themodern world.
The site is a 3-inch indium phosphide fab belonging
to the global photonics company Lumentum, and it
manufactures photonic chips for data transmission
over optical fibres – essential components
underpinning the internet and our digital lives.
Caswell’s rural setting is a historical legacy – it was
originally developed during the Second World War,when its founder was encouraged to build it far
away from London and the risk of air raids. Today,
however, being surrounded by empty fields offers anentirely different advantage: there is plenty of space
20 ISSUE I 2024 I PIC MAGAZINE.NET

to grow. And Lumentum is taking full advantage ofthat potential.
After gaining Caswell as part of its acquisition of
optical manufacturing company Oclaro in 2018,
Lumentum initially leased the land it was situated on.
But in September 2023, the company bought theland, and is now investing in significant upgrades.
Chief among these is a major expansion of thecleanroom, which is due to be completed thisspring, increasing its size and production capacity
by 25 percent.
These actions affirm the facility’s value to Lumentum,
and its key role in the company’s plans moving
forward. “Lumentum’s investment in equipment,
infrastructure, and people at Caswell will be key tothe company’s continued success,” says Sal Pavone,vice president of operations. “We are committed
to make this the centre of excellence for the
manufacture of photonic products.

INDUSTRY I FAB EXPANSION
Many of our key products are manufactured at
Caswell and are dependent on the leading-edge
technology that is developed here on site.”
Indeed, Lumentum’s 800G ZR+ and 400G ZR+
0 dBm transceivers, and 130+ GBaud components
and Smart TROSAs, which were on display at ECOC
2023 in Glasgow last year, all contain chips from
Caswell. They include, for example, InP modulator
chips that incorporate multiple Mach-Zehnder
modulators and on-board optical amplifiers, aswell as on-chip photodetectors for monitoring andcontrol functions.
Paul Johnson, senior director of product line
management for cloud and networking, points outthat these chips offer several key advantages of InP
over other PIC platforms, enabling differentiatedperformance for cloud and networking. “Theyhave very high-speed capability beyond 100 GHz
electro-optic bandwidth, optical light generation
and amplification, and ease of integration of thesecapabilities,” he says. “We can combine the bestof laser and modulator or receiver performance
without compromise, enabling optical fibre datatransmission at 800G per fibre or higher, a 1,000
times greater capacity than the typical fibre-to-thehome connection.”

Global network

Caswell is one of eight manufacturing facilities
that Lumentum operates across three continents,
with many more non-fab sites around the world.
This global presence means that the company
can be close to all its various customers. Whiledifferent locations may have a different focus, they
all perform several functions, and employees atdifferent sites work together towards the company’s
overall goals.
An important advantage of having the same
products manufactured at multiple sites is theredundancy and the associated supply chain
security that this distributed structure offers.
Additionally, forming teams of people across thedifferent locations, rather than limiting them to
employees at the same site, widens the possibilities
for finding the optimal combination of skills
and perspectives needed for a particular
project.
The chips made at Caswell
are generally transferred to
Lumentum’s other facilities for
packaging into final products.
However, in addition to
producing photonic chips, thesite also plays an active rolein developing the company’s
future-generation products.
For this reason, it has a smallerscale assembly line to enable
rapid development of prototypes.
There are R&D teams based locallyworking on improving chip designs,

process integration and optical sub-assemblies(OSA). Caswell also has R&D teams researching
the integration of chips into larger components anddeveloping the test infrastructure. “The need forinnovation to move to higher baud rates doesn’tstop,” comments Lloyd Langley, vice president of
R&D. “I remember introducing products with lasers
operating at 10G many years ago. Now we haveproducts operating at 130 Gbaud, and advanced
technology work aimed at 260 Gbaud for 1.6T
per laser applications. These higher rates areoperating with RF signals exceeding 100 GHz. In
addition, other aspects such as minimising power
consumption are an important consideration.” Caswellmanufacturesindiumphosphide
opticalmodulator
chips, which
can be foundin many ofLumentum’s
products.

Company-wide growth

Besides affirming Lumentum’s investment in theregion, the expansions underway at Caswell also
reflect wider growth across the company. Last year,
for example, it announced that it had increased
manufacturing operations and R&D capabilities at
its site in Slovenia, which focuses on custom opticalfibres found in many Lumentum products.
Furthermore, the company has made multiple
acquisitions in recent years, cementing its position
as a major player in the industry, which isitself growing. Among its most notableacquisitions are two manufacturersof optical modules for datacentre
interconnects: NeoPhotonics
in 2022, and Cloud Light in
October last year.
As Lumentum continues
to grow, in terms of both
its manufacturing andresearch capacities, it has
every reason to believe that
there will be demand for asmuch volume and as many
product updates as it can deliver.
After all, our modern lifestyles are
hungry for higher data rates; streaming
PIC MAGAZINE.NET I ISSUE I 2024 21

INDUSTRY I FAB EXPANSION
address that demand. As a combined company, we
believe we can bring advantages such as first-tomarket high-speed transceivers customised for thehyperscale market, utilising in-house manufacturing
for lowest cost at scale, with the security of supply
that the market requires.” At ECOC
2023 in Octoberlast year,
Lumentumintroduced
a 130+
GBaud Smart
TROSA, which
incorporates
components
manufactured
at Caswell.

services and the rise of cloud computing werealready fuelling an unwavering rise in internet traffic,
even before the explosion of widely available AI andmachine learning applications in the past few years.
As computational workload increases continue tooutstrip Moore’s Law, network bandwidth will needto expand by as much as an order of magnitude tohandle all that information, and using optics can help
to achieve that.

As the industry sets its sights on securing
bandwidths of 800G, and moving towards 1.6T,
Caswell and Lumentum are striving to stayahead of the game in providing the supporting
infrastructure. “Our process and design teams workat the cutting edge of compound semiconductor
wafer manufacture and performance of PICs,” says
Langley. “We have design experts in semiconductormaterials, optical waveguides, very high frequency
electromagnetics, test, and reliability.”“AI is driving a need for scalable interconnect
solutions within datacentres. Intra-datacentre
transceivers, operating at 400G to 1.6T, will grow to
be a $10 billion market by 2028,” explains Johnson.
“The addition of Cloud Light to Lumentum helps us

Yet, as the company and its technologies evolve,
one thing looks set to remain the same. As Pavone
puts it: “Caswell has been producing leading edge
technology in the UK and will continue to do foryears to come.”

ESG Commitments
AS WELL as developing its facilities and technology,
Lumentum is evolving several other aspects of its operations
too. In a time of greater emphasis on corporations’environmental and social responsibilities, the company has
been redoubling its commitments to sustainability and to its
employees.
Caswell is one of several Lumentum sites running on 100
percent renewable electricity, with the latest three locations
to make this transition being Navanakorn, Thailand, andtwo sites in the US: San Jose, California and Dallas, Texas.
Overall, 61 percent of electricity used by the company’s
operations for last year was from renewable sources.
“Lumentum remains a leader in reducing its carbon
footprint and continues to invest in green programmes andequipment as part of our corporate responsibility agenda,”says Pavone.
Going beyond the initial impact of devices during the
manufacturing stage, Lumentum has also begun to conduct
energy life cycle assessments of its products, accounting
for aspects including, manufacturing, transport, and usage.
The assessments carried out so far have highlighted that thelargest proportion of emissions associated with a productoccurs in the use phase, motivating Lumentum to continue
improving the energy efficiency of the products themselves,as well as the facilities that manufacture them.
And finally, the company is also helping its employees
to reduce their individual carbon footprints by facilitating
more environmentally friendly choices. Several EV chargers
have been installed at Caswell, for instance, and staff in the

22 ISSUE I 2024 I PIC MAGAZINE.NET

UK can also lease an EV and participate in a car-poolingprogramme. As for the social aspect of “ESG,” Lumentum
has been taking steps to invest in its people as much as its
facilities. The company has an active Diversity, Inclusion &Belonging (DIB) Council with a presence at sites across theworld. Nadege Rebuffie, OSA design manager, is a memberof this council, and says that its initiatives are strongly driven
and supported by the leadership team, including CEO andpresident, Alan Lowe, who joined the coalition CEO Action
for Diversity and Inclusion in 2022.
“As a global company, Lumentum values creating an
inclusive environment for all employees. Each employee’s
unique experiences enhance our competitive edge,” says
Rebuffie. “Our DIB programme aims to enhance diversity
across our workforce, with progress regularly reviewed.”
To this end, one of the main activities of the DIB Council isestablishing and supporting Employee Resource Groups(ERGs), which create spaces for staff facing specificchallenges to come together and be heard and supported.
For example, one ERG with a presence at Caswell is agroup called Women@Lumentum, which was establishedto promote professional development and careeradvancement for women.
Two other ERGs that are active at Caswell are Working
Parents and Caregivers@Lumentum, which supports staff
juggling caregiving responsibilities with their work, and
Next-Gen Luminaries@Lumentum, which helps early-career
employees settle in through regular social activities, tutorials
on soft skills, and access to a “Survival Guide” for their firstdays at the company.

avophotonics.com

Celebrating 20 years at the forefront of the photonics
industry, Avo accelerates your journey from concept
to market. We pride ourselves on our unique ability
to offer comprehensive solutions encompassing
innovative design and high-quality manufacturing.
Discover how Avo can enable you to win.
Join us at the Photonics West conference this January.

Innovative Design
and Precision
Manufacturing
Services in the USA
L I G H T. P R E C I S E LY.

TECHNOLOGY I AUGMENTED SILICON

Three ways photonics can catalyse
hardware evolution for datacoms
As the computing industry faces evergrowing demands for higher speeds, photonic
technologies can facilitate the necessary
hardware evolution, through hybrid integration,
chip disaggregation, and enabling 200G per lane.
BY SURESH VENKATESAN, CHIEF EXECUTIVEOFFICER, POET TECHNOLOGIESOF THE MANY THINGS artificial intelligence has
upended, the microchips industry may be the sector
that has seen the most disruption. AI applications
are gluttonous for speed and power, and, as thecomputing industry has raced to meet the demand,it has become apparent that existing solutions will
not suffice. There is a growing sense of urgency
across the industry as it becomes increasingly
clear that optical interconnects enable large-scale
AI compute and other data-intensive workloads tooperate at bandwidths, energy efficiencies, andlatencies that are unachievable through today’s
electrical-based interconnect technology.
Sixty-plus years after engineers commercialised thefirst monolithic integrated circuit chip, the explosionof AI and the insatiable demand of hyperscale
datacentres for more bandwidth have offered a
remarkable opportunity for chipmakers to move
24 ISSUE I 2024 I PIC MAGAZINE.NET

beyond traditional solutions in a dramatic way. Whilematerials such as indium phosphide and gallium
arsenide have been used in commercial applications
for decades, the growth of material-agnostic opticalplatforms has presented the chance to transition
from “silicon-only” to “silicon-plus” designs.
Whereas the absolute need for silicon will neverwane, the slowing of Moore’s law has opened thedoor to “augmented silicon” technologies.
These approaches aim to enhance silicon’s
innate capabilities and advantages through theincorporation of new materials (germanium,
indium phosphide), new architectures (chiplet andhybrid system partitioning), and 3D packaging andassembly. That’s the reality many manufacturers
and industry observers have recognised so far thisdecade.
One of the ascendent technologies is photonics,
with many companies currently in the midst ofidentifying optical computing solutions that will
drive the next generation of device manufacturing.
The following are three key areas where
photonics — which was once a boutique niche — ispositioned to be the catalyst for large-scale growthin hardware applications for AI and hyperscale
datacentres.

Hybrid Integration

With a hybrid-integration approach to waferscale chip design, developers can assure thoseTECHNOLOGY I AUGMENTED SILICON
manufacturers who have built their entire operations
around silicon that the coming wave of productdesign will have several familiar pieces. In this way,hybrid integration allows for a seamless transition to
future paradigms.
At POET Technologies, our “semiconductorisation ofphotonics” approach is a form of hybrid integration.
That term is meant to underscore the intention
behind our designs: to make it as straightforward
as possible for the semiconductor industry tomove to photonics-first solutions that are material
agnostic. Our elegant design eliminates dozens ofparts because we rely on passively attached opticalcomponents to move data, leveraging the broad
semiconductor industry’s investments in advanced
packaging technology.
Based on a “silicon for photonics” interposer
platform, POET’s products offer manufacturers
a critical piece of the puzzle of achieving highintegration, especially for 1.6T, 3.2T, and beyond.
With its “silicon for photonics” hybrid integration
approach, POET is focused on addressing theshortcomings of more conventional silicon photonics
solutions. To this end, POET’s technology includes
features like passive alignments, low-loss multilayer waveguides, and integrated optical passives
like multiplexers and de-multiplexers that have theflexibility and fungibility to address a broad range ofmarket requirements.
Such flexibility and integration are crucial, because
serial data communication channels have not beenable to keep pace with growing bandwidth. The
number of communications lanes increases as data
rates increase. Those manufacturers who rely on
conventional discrete assembly are challenged
to economically deploy products that performconsistently within eight-channel architecture– which is needed for 800G speeds – and areincapable of accommodating 16-channel lanes –which is necessary for 1.6T and 3.2T, the speeds ofthe future.

 The POET Technologies O-Band LightBar, shown with lasers
assembled, is developed to help AI manufacturers bring next-generation
products to the market.

an optical transmission link using that configuration
at OFC 2023. “The demo not only validated
the feasibility of 200G per lane optical links fordatacentre networking, but also reassured theecosystem that 1.6T optical modules can costeffectively be deployed to address the growing
bandwidth demands in datacentres supporting AI
applications and workloads,” wrote the company’s
master engineer Khanh Lam.
A key component needed to achieve 200G perlane is an externally modulated laser (EML). Already,more than 90% of 800G modules in production
today use EMLs, which present the primary pathto 200G per lane solutions. The performance of
EMLs and the industry trust they have attained
are reasons why POET has retooled its optical The POET
Technologies
400G Infinity
Transmit
Chipletdemonstrates
the flexibility of
the company’s
opticalinterposer
platform.

POET achieves 16-lane capability by using 3D
assembly techniques and stacked non-interacting
waveguides. It’s a simplified process that drives data
communications at unprecedented speeds.

200G per lane

Moving data simultaneously through more lanes is
one way that photonics brings superior capability
to the market. Another is its ability to increase thespeed that can go through each of those lanes.
Conventional speeds are currently 100G per lane
but photonics can operate at 200G per lane, busting
the bottlenecks that cause latency and high power
consumption. At 200G per lane, AI and hyperscale
datacentres and cloud networks can surge towards
new capacity limits.
Among the giants touting the merits of 200G perlane solutions is Broadcom, which demonstrated
PIC MAGAZINE.NET I ISSUE I 2024 25

TECHNOLOGY I AUGMENTED SILICON
dominated semiconductor design for decades
are approaching their physical limitations. The
necessity for faster data transmission has gone
beyond what these chips can capably deliver. As
a result, disaggregation is going to push the needfor photonic connectivity. It’s already happening
at Google, whose AI servers are exploiting
disaggregation to connect to networks with 800G
transceivers — making the search-engine leader the
first company to commit to using that speed for its
datacentres.

 The POET Technologies 800G solution includes 2xFR4 receive opticalengines with a trans-impedance amplifier and photodiodes.

interposer platform to include both EML and DML(directly modulated laser) solutions.

Disaggregation“Chip disaggregation, or chiplets, offers analternative to the traditional monolithic SoC scaling
approach. Aggregating multiple chiplets to performthe function of a single monolithic IC de-risks
the overall system by reducing complexity andincreasing yields,” writes chip producer Rambus.
With the conventional processor-centric computing
architecture and copper interconnects, thechips based on 3 nm technology that have

Not every discussion is aheated debate...

26 ISSUE I 2024 I PIC MAGAZINE.NET

Products such as the POET Infinity chiplet, andthe family of engines built from them, provide ananswer for pluggable transceiver and module
designs. The Infinity, a 400G transmitter chiplet,can be configured in a daisy-chain architecture
to reach speeds of 800G (2x400G chiplets), 1.6T
(4x400G), and beyond. As with all products built
from the POET Optical Interposer, the Infinity chiplet
includes a monolithically integrated multiplexer,
and eliminates wire bonds and active alignments ofcomponents.
Photonics-first innovations such as those being
introduced to the market by POET and othercompanies promise to pave a new path forward in the
AI era. The industry is in the middle of an importantstage as it develops new processes to power
computing performance. The platforms and productsthat are adopted now are likely to be the de facto
choices for chipmakers for decades to come.

 Based around a hot topic for yourcompany, this 60-minute recorded,
moderated zoom roundtable would
be a platform for debate and discussion
 Moderated by an editor, Laura Hiscott,this can include 3 speakers
 Questions prepared and shared inadvanceCost: €5995
Contact: Jackie Cannon
jackie.cannon@angelbc.com

TECHNOLOGY I COMMUNICATIONSExtremely high-power transmitters foraccess networks
Sources that combine a laser, a modulator and an amplifier in a single
device are laying the foundations for high capacity access networks
BY HÉLÈNE DEBRÉGEAS FROM ALMAE TECHNOLOGIESDUE TO THE PHENOMENAL rise of YouTube and
TikTok, many are watching more content on-line. To
accommodate this trend, there’s been a tremendous
rise in internet traffic, with this growth showing
no sign of abating. Consequently, capacity has
to increase throughout the fibre-optic network,from the long-haul links that span many thousand
kilometres to the shorter connections, such as
the optical access networks providing the ‘last
mile’ between internet service providers and theircustomers.
Given this state of affairs, it’s not surprising that
optical access networks are continuing to be rolled
out at great pace. And as well as this additional
deployment, throughput is on the up – initially,transmission speeds were 2.5 G, recently they
climbed to 10 G, and sometimes data rates are now
as high as 25 G and even 50 G.
Unfortunately, increasing the capacity of opticalaccess networks is more complex than it appears
28 ISSUE I 2024 I PIC MAGAZINE.NET

at first glance. While sources with higher opticalmodulation rates are essential, they are not the only
factor that matters.
In addition, there’s a need for a high opticalpower, because access networks – also known
as passive optical networks (PONs) – have no
optical amplification, setting them apart from otherlong-distance networks. This lack of amplificationmeans that the losses resulting from propagationin the fibre and splitting among multiple end users
ultimately limit both the network’s geographicalcoverage and the number of its end users.
To optimise the capacity of PON networks, they must
include highly sensitive photodiodes that detect
the low optical power they receive, and transmittersdelivering the highest possible output power. For
current applications, such as 10G-PON, 25G-PON
and 50G-PON, the modulated power in the fibre
needs to be around 4 mW. However, to increase
capacity, new access networks are demanding a

TECHNOLOGY I COMMUNICATIONS
modulated power of up to 10 mW in the fibre, witheven higher values sure to follow.
For upstream transmissions – that is, transmissions
from the end user to the network – O-band
wavelengths are used. This spectral domain, whichis around 1.3 µm, benefits from an absence ofchromatic dispersion in the fibre. A lack of distortionof the optical signal results, enabling implementationof powerful yet low-cost transmitters on the user’s
premises. In this case, the most suitable source isthe directly modulated laser: by simply modulating
the electric current injected into the laser,
modulation of the emitted power follows.
It’s a very different story for downstream
transmission, which involves the transfer of datafrom the network to the end users. Wavelengths
used for this task vary, ranging from 1342 nm for
50G-PON to 1577 nm for 10G-PON. However, in allcases, there is significant chromatic dispersion in thefibre.
One troublesome consequence of this dispersion
is that the quality of the signal that’s emitted
by directly modulated lasers is insufficient fortransmission with an acceptable degree of errors.
Due to this, externally modulated lasers (EMLs)
are deployed for downstream transmission inaccess networks. This class of laser comprises two
sections: a laser, powered by an electrical current,
that emits a constant optical signal; and an electroabsorption modulator section, which is transparent
at zero voltage, but absorbs light under a negative
voltage. Through application of a modulated
voltage, typically between 0 V and -2 V to themodulator section, successions of bits 0 and 1 canbe transmitted at high data rates.
The downside of the EML is that it is more expensive
to manufacture than the directly modulated laser.
However, the additional cost is acceptable, because
the component is only deployed in core network
terminals and not on the user’s premises. The EML
has a high reputation, thanks to its capability totransmit a much higher quality signal: there is astrong power contrast between the 0s and 1s; andthere is high spectral purity, ensuring the sourceis resistant to chromatic dispersion. Following
continuous improvements to EMLs for accessnetworks, the modulated power in the fibre now
reaches the order of 4 mW, meeting today’s needs.
However, while the power provided by the EML isadequate for access network coverage, it is still alimiting factor. What’s more, this weakness is only
going to become more of an issue, as a further
power increase is a key differentiator in nextgeneration high-capacity access networks.
A radical approach is needed to address thisconcern, because EMLs are getting close to
the glass ceiling for their output power. Even if
the power of the laser section is increased, it’s
not possible to maintain the performance of themodulator. That’s because there is an intrinsic
issue, associated with absorption of photons
that create electrical carriers in the modulator.
As the injected power increases to a very highvalue, carrier evacuation cannot keep up, so thecarriers accumulate and screen the voltage that’s
applied to the modulator. Making matters worse,the photogenerated electric current createsan opposing electric voltage in the modulator
supply circuit. Both these effects contribute to a
distortion of the signal emitted by the EML, andimpose modulation voltages that are too high for
implementation in the system.

Amplifying concerns

Offering a way forward that allows the realisation ofmodulated powers of 10 mW or so is the addition ofa third section to the EML: a semiconductor opticalamplifier (SOA). With this refinement, injection of anelectric current into the SOA ensures optical gain inthis section through a multiplication of the photons
that are passing through it. In theory, the SOA simply
boosts the power emitted by the EML.
There are several advantages that come from thisapproach. First, the EML no longer needs to emit asmuch power, since there is now optical gain from Figure 1.
An illustrationof a Passive
Optical accessNetwork(PON).

PIC MAGAZINE.NET I ISSUE I 2024 29

TECHNOLOGY I COMMUNICATIONS Figure 2.
Top view
image of ahigh-powertransmitter(EML-SOA),
including laser– modulator– amplifier– output
passive tilted
waveguides.

the SOA that is located after the modulator section.
Thanks to this refinement, the problems associatedwith modulator saturation and the loss of modulation
efficiency are eliminated. What’s more, as the SOA
section is very similar to the laser section, it can be
manufactured using the same processing steps. The
only noteworthy change is that the surface area ofthe chip increases. Finally, the SOA section improves
the quality of the optical signal through the additionof so-called negative chirp, increasing the capability
to combat chromatic dispersion in the fibre.
However, it is far from simple to enjoy all thesebenefits. That’s why the EML-SOA is only starting to
be offered as a practical option for the transmitter.
Note that great care is needed when taking thisapproach, because there is the threat that the SOA
can severely degrade the performance of the EML.
One of the causes of this is that the SOA suffersfrom saturation. When too much optical power
is injected, the density of photons propagating
and multiplying reaches a value that leads to theconsumption of all injected electrical carriers. At
this point, the SOA fails to provide amplification, acondition described as gain saturation. This is highly
problematic, as when the modulated optical signal
injected into the SOA goes from a bit 0 to a bit 1,
the injected power suddenly increases and the gain
saturates in a few hundred nanoseconds. Due tothis, the power of bit 1 gradually decreases, resulting
in very high noise levels for bit 1s, an issue thatprevents data transmission.
 Figure 3.
(Left) Output
power
producedby thesemiconductor
opticalamplifier asa function of
input power.
(Right) 25 G
eye diagramwith mask
margin.
30 ISSUE I 2024 I PIC MAGAZINE.NET

A second concern is that the EML is very sensitive
to optical feedback at the output facet. While mostmodulated light leaves the chip, a small portion isreflected by the facet and excites the laser section,
which begins oscillating at a resonance frequency ofa few gigahertz. This oscillation totally disrupts theemitted signal, preventing the correct transmissionof the information bits. To limit this effect on an EML,an anti-reflection coating of around 0.01 percent isapplied to the output facet.
However, with the EML-SOA, optical feedback is
amplified by around a factor of ten as light traverses
back and forth along the SOA. Due to this, there’s
the need for a 0.001 percent facet anti-reflection
coating, which is not industrially feasible.
Finally, the SOA section requires an additional
electrical current. However, this is restricted,
because there’s a need to limit the power
consumption of the transmission modules. It canbe assumed that the total for the laser and the SOA
should be in the same range as the current for justan EML laser.

Addressing the challenges

At Almae Technologies of Marcoussis, France, wehave tackled all these concerns while developing
an EML-SOA for emerging high capacity accessnetworks. Our device, suitable for 10G-PON and
50G-PON, targets a very high modulated power inthe fibre of around 10 mW. To realise this level of
performance we have adapted the design, so it isTECHNOLOGY I COMMUNICATIONS
capable of a high-quality modulated signal and a low
power consumption.
Our design excels by incorporating a fourth opticalwaveguiding output section, comprising a bendleading to a 7° output tilt, and a section withouta waveguide where light diffracts. Thanks to thisarchitecture, the optical mode is very wide and at
an angle of 7° when it encounters the 0.01 percent
anti-reflection coated output facet. This ensures thatthe small portion of reflected light is strongly spreadout spatially, forming an angle of 14° with the opticalguide. It is a configuration that ensures 99 percent of
the reflected light is radiated, while just 1 percent isrecoupled into the optical waveguide, where it could
possibly disturb the laser section. The combinationof the tilt and the unguided zone ensures that
despite the SOA section, the device remains
compatible with a standard 0.01 percent facet antireflection coating, while avoiding laser oscillation.
When designing our EML-SOA, we did not focus on
optimising the laser section by placing our emphasis
on a high power and low threshold current, but
directed our attention to the SOA section. For
this part of our device, we placed a premium on
realising the highest possible SOA saturation
output power. To achieve this, we selected a stackof semiconductor layers that reduced propagation
losses, and ensured a less-confined optical mode
to avoid too high a photon density. These choices
enabled us to more than double the saturation
output power of the SOA.
With these optimisations, our EML-SOA operates atrecord output powers, without any degradation in
the quality of the modulated signal. For deployment
in 50G-PON networks, our device provides 4 mW
modulated in the fibre, for an EML with a laser
current of 110 mA; and 10 mW modulated in the fibre,
from an EML-SOA with only 135 mA of total laser
and SOA current. And for the 10G-PON, we go from
3.5 mW modulated in the fibre with an EML having
a laser current of 110 mA to 7 mW modulated in
the fibre with an EML-SOA having a total laser and
SOA current of just 150 mA. If it seems that there
are too many numbers to digest, just note that wehave more than doubled the modulated output
power for an increase of only 20-30 percent power
consumption.
Our EML-SOA results are extremely importantfor next-generation access networks because inaddition to increasing the modulation rate, they
open the door to very large capacities, in terms ofboth geographical coverage and the number ofusers. Our devices are currently in pre-production
phase, with market launch slated for later this year.
It is possible that our technology may also have arole to play in other networks. It is unlikely to be
deployed in long-haul networks, as in this casetransmission is in the C-band, centred around1.55 µm, where signals can be amplified by erbium-

doped fibre amplifiers. But our EML-SOAs mightserve in networks involving O-band transmissions
around 1.3 µm, as today they do not have any formof in-line optical amplification. The O-band is used
for interconnections between datacentres (400 G
Ethernet and beyond), with data transmission
supported by several very high-speed 100 G EMLs.
Here the transmission distance is limited by the
optical power budget to around 40 km, and theintroduction of our high-power EML-SOAs could
increase that distance by several tens of kilometres.
So it’s clear that our device has plenty of promise.

Almae Technologies: A French
integrated device manufacturerof InP photonic devices
FOUNDED IN 2016, Almae Technologies is a spin-off from
III-V Lab, which is a joint research laboratory between Nokia
Bell Labs, Thales R&D, and CEA-Leti. The aim of Almae is tobuild an industrial supply chain for producing and marketing InP
components coming from advanced research and development.
Today, Almae has 40 employees, all experts in the field. The
company – a well-known Integrated Device Manufacturer (IDM) foradvanced InP based photonic chips in Europe – is located southof Paris. From this location, equipped with a 1,700 m² clean room,
the company undertakes the design, development, and end-toend manufacturing of InP wafers, including the epitaxial growth ofmaterials, testing and prototyping.
Almae’s debut component is a 10 Externally Modulated Laser(EML) for transmission up to 80 km that covers Dense Wavelength
Division Multiplexing (DWDM) channels within the C-band, whichspans 1530 nm to 1565 nm. This laser, now produced in volume on
an in-house production line, is qualified
by external customers for field applications.
The team at Almae engages in a great deal of innovation, enabling
development of many new products for emerging markets. All
these new components, which are based on optimised andspecific designs, use exactly the same technology as the 10 G
EML. This means that these novel components can take advantageof the maturity and efficiency of the production line for designfocused development, to ensure rapid qualification and time-tomarket.
New products in Almae’s market launch phase include 100 G
EMLs for datacentre interconnection, 1577 nm EMLs for 10G-PON,
and high-power O-band or C-band lasers as sources for silicon
photonics modulators, lidar and sensing. EML-SOAs for very highpower 10G-PON and 50G-PON (10 mW modulated in the fibre) arein the pre-production phase, with a market launch scheduled for2024. Another part of Almae’s technology involves the integrationof laser components or gain sections into silicon platforms using
low-cost passive alignment processes.

PIC MAGAZINE.NET I ISSUE I 2024 31

INDUSTRY I TEST + MEASUREMENT

Getting the full PICture:

Testing microring resonators with sub-picometre
spectral resolution
Microring resonators are key components in many PIC technologies,
but, as their performance continues to improve, they require increasingly
precise test and measurement solutions. EXFO’s testing platforms canhelp to address this need.
BY SOPHIE LANGE, BUSINESS DEVELOPMENT ENGINEER AT EXFO
AS THE FIELD of integrated photonics continues
to advance, microring resonators have become akey enabler of this technology. The interferometricdevices create very sharp spectral features, and canbe used as ultra-sensitive detectors or as wavelength
filtering devices in PIC-based low-linewidth tuneable
lasers. They could also be used as compact and ultralow voltage optical modulators, potentially proving to
be a game-changer in the increasingly energy-hungry
telecom and datacom industry.
As the name implies, a microring resonator consistsof a ring-shaped waveguide placed close enough toa linear waveguide (or multiple linear waveguides)
for some light to couple between them, creatinga highly efficient optical resonator. The quality

factor, or Q-factor, is a figure of merit of a device’s
interferometric efficiency (the higher the Q-factor,
the more efficient the device) and is directly linked
to its spectral performance. With the developmentof low-loss structures and materials, interferometers
based on integrated photonic ring resonators canachieve Q-factors of several million.
However, characterising devices like this isbecoming more and more challenging. The
spectral measurement of the Q-factor, among otherperformance parameters, needs to be tested withhigh accuracy and precision, but also within a shorttimespan, since a single integrated photonics wafer
can count several thousands of these components.
This article details the key challenges associatedwith measuring these ring resonator structures andprovides insights into how to achieve fast, accurate,and highly resolved spectral characterisation of
optical microring resonators. A measurementof a high Q-factor resonator, provided courtesyof CEA-Leti, demonstrates a path to efficientcharacterisation of these key components of PICs.

 Figure 1.
A ringresonator
consists of aring-shaped
waveguideclose to a linear
waveguide.
Picturecourtesy of
CEA-Leti.
32 ISSUE I 2024 I PIC MAGAZINE.NET

Spectral characteristics of microring
resonators

The base pattern of a ring resonator’s transmission
spectrum consists of a Lorentzian-shaped troughcorresponding to a resonance of the ring cavity.
This pattern is then repeated at an interval that
corresponds to the free spectral range of theresonator. These high-contrast features depend
strongly on the ring’s physical and optical properties
and on effective interaction with the nearby linear
waveguide(s).

INDUSTRY I TEST + MEASUREMENT Figure 2. Example of spectral characterisation instruments for the swept laser method, consisting of the tuneable laser (T200S/
T500S from EXFO) and a bank of power meters integrated into a component testing platform (CTP10 from EXFO). (DUT represents
the device under test.)

In recent years, ring resonators with a Q-factor ofseveral million have been demonstrated and readily
used as modulators, filters, or sensors. Such Q-factor
values roughly translate into a spectral resonancebandwidth of less than 1 picometre with optical peak
depths of several decibels. When characterisingsuch devices, the spectral position of the peak,spacing between peaks, and their height and width
provide critical parametric information that candetermine whether the cavity can be used for theintended application.
This, however, poses a difficult challenge whenit comes to acquiring such a spectrum. Reliable
readings of both wavelength and optical power
information are needed while passing through these
extremely high-contrast features. As a result, thetest instrument requires femtometre-scale spectral
resolution and needs to deal with power fluctuationof 10 dB per picometre.
The best technique to perform a fast, reliable,
and high-resolution spectral measurement of ring
resonators is to use a continuously tuneable laser
together with one or more fast detectors. The laser
needs to cover the relevant operating wavelength
range while offering low linewidth and good
spectral quality to avoid measurement artefacts.
The detection part of the test system needs tohave excellent linearity and enough dynamic range
so that the full spectral contrast of the device canbe captured in a single measurement without any
distortion or discontinuity.
Additionally, the overall system needs excellent
wavelength accuracy and repeatability as well as
high spectral resolution to collect all the data points
at the correct wavelength with femtometre-scaleprecision within seconds. Finally, polarisation control

is often required to ensure coupling of the signal
into the correct transmission mode.

Efficient and fast spectral testing withfemtometre resolution
Figure 2 shows a spectral characterisation solution
where the component testing platform takes
control of the laser and monitors the wavelength,
power in, and power out of the device under
test synchronously. Such systems have been
demonstrated to also capture return loss andpolarisation-dependent loss measurements and toprovide high-quality spectral performance required
for ring resonator testing.
Figure 3 shows a measurement performed on aring resonator device with a Q-factor of 6 million,
provided by CEA-Leti. The graph demonstrates theaccuracy level achieved with the high-resolution
mode of the test solution used in this case. Thanks
to an “on-the-fly” optical wavelength detection,
the laser wavelength is precisely determined ateach tuning step, avoiding any measurement

 Figure 3.
Spectral
responseof a highQ-factor ring
resonator using
resolutions of
1 picometre(red) and 20
femtometres(blue)
measured with
the CTP10component
testing
platform.
Devicecourtesy of
CEA-Leti.

PIC MAGAZINE.NET I ISSUE I 2024 33

INDUSTRY I TEST + MEASUREMENT Figure 4.
Example of amode splitting
spectral event(on the samering resonator
as in Figure 3)
at a spectral
resolution of 20
femtometresmeasured with
the CTP10component
testing
platform.
Devicecourtesy of
CEA-Leti.

Ring resonators and integrated
photonics as a whole arepushing the boundaries ofspectral testing, as well as
photonics measurement
techniques more generally.

inaccuracies due to factors such as laser speed
variation. Together with the fast, high-dynamicrange photodetectors, the Lorentzian shape ofthe resonance is spectrally resolved without any
distortions or artefacts.
A system like this also provides highly repeatablemeasurements, ideal for characterising theresonance position versus other ring resonatorparameters such as the applied voltage for phaseshift or thermal control. As a matter of fact, thesystem’s femtometre-scale acquisition allows theresolution of complex behaviour such as a mode

splitting effect – a spectral event observed when
defects in the ring create counter-propagating
modes, resulting in two resonant peaks instead ofone, as shown in Figure 4.
Ring resonators and integrated photonics as a whole
are pushing the boundaries of spectral testing, as
well as photonics measurement techniques moregenerally. Ever-increasing Q-factors require higher
and higher spectral precision and resolution, andminimised testing time. This is a challenging but
exciting time for test and measurement engineers
and researchers alike.

SAVE THE DATE

I6 – I7 APRIL 2024
Sheraton Brussels Airport HotelTo find out more about our sponsor and speaker opportunities,

contact us today on: +44 (0) 2476 718970
or email:
info@csinternational.net
info@picinternational.net
info@pe-international.net

info@angel-tech.net
34 ISSUE I 2024 I PIC MAGAZINE.NET

Register TodayA Hybrid Conference — In-Person and Virtual Presentations
Technical Conference: 24 – 28 March 2024
Exhibition: 26 – 28 March 2024
San Diego, California, USA

Plenary Speakers

Anita Döhler

Chief Executive Officer,
NGMN Alliance

OFCConference.org

Inder Monga

Executive Director, ESnet, andScientific Networking Division
Director, Berkeley Lab

David Richardson
Partner Researcher,
Microsoft

TECHNOLOGY I OPTICAL RECEIVERS

Moving towards polarisation-independentmonolithic PICs
As datacentres race towards 200G per lane, polarisation-dependent loss anddifferential group delay remain significant hurdles in high-speed optical receivers.
GlobalFoundries researchers have made significant progress in addressing thesechallenges.
BY YUSHENG BIAN, MASSIMO SORBARA, WON SUK LEE, SUJITH CHANDRAN,
TAKAKO HIROKAWA, ABDELSALAM ABOKETAF, KEVIN DEZFULIAN, RYANSPORER, KEN GIEWONT AND TED LETAVIC, GLOBALFOUNDRIESTO COPE with ever-growing bandwidth demands,
datacentre communication applications aretransitioning to 200G PAM-4 transmission per optical
wavelength (200G per lane), doubling the symbol
rate to 106.25 GBaud compared with 53.125 GBaud
found in 100G-per-lane systems [1, 2]. However, in
200G-per-lane systems, managing the significant
impact of polarisation-dependent loss (PDL) anddifferential group delay (DGD) on optical receivers

is increasingly challenging, particularly in opticalinterconnection schemes deploying monolithic
silicon photonic technology.
To facilitate seamless integration between photonic
and CMOS components with an optimised process
flow (Figure 1), the waveguide layers incorporated
into the monolithic silicon photonic platform are
typically thinner than those in standalone photonic

 Figure 1. GlobalFoundries FotonixTM platform monolithically integrates advanced photonic devices and CMOS components on
the same SOI substrate [3].
36 ISSUE I 2024 I PIC MAGAZINE.NETTECHNOLOGY I OPTICAL RECEIVERS

technologies [3-10]. However, this thinning can resultin non-negligible birefringence and PDL, affecting
both waveguides and other functional devices.
As most optical transmitters are optimised fortransverse electric (TE) operation – as opposed totransverse magnetic (TM) operation – PDL becomespronounced in optical receivers due to the rotatingstate-of-polarisation in the fibre optic cable. In highspeed data communication systems, it is criticalto minimise the net optical path loss and PDL to
optimise the system’s overall signal-to-noise ratio(SNR) performance.
Alongside PDL, significant DGDs between TE andTM signals arise from birefringence and polarisation
mode dispersion in waveguides and other building
blocks within the monolithic optical receiver [11].
These DGDs typically surpass 50 percent of asymbol interval in 200G-per-lane transmission,
resulting in self-induced inter-symbol interference(ISI) out of the photodetector (PD) output current.
This leads to further closure on the eye diagram andshifting of the optimal sampling of the received
PAM-4 signal. This article presents recent
advancements in enhancing the functionality ofcritical monolithic building blocks deployed in an
optical receiver front-end, with a primary focus on
reducing insertion losses (ILs), net PDL as well as
DGD compensation. The performance enhancements
are achieved by optimising both the design andprocesses to improve the balance between the
TE and TM paths in a receiver. Combining all
improvements, we effectively demonstrate a nearlypolarisation-independent receiver circuit with a
PDL of only 0.38 dB and negligible DGD, utilisingmonolithic silicon photonic components.

Reducing IL and PDL for monolithic
silicon photonic components

 Figure 2.
(a) Schematic
illustration ofsilicon nitride
edge coupler
in a monolithic
optical receiver.
(b)-(c)Statistical yield
distributions of
the TE and TM
ILs at 1310 nm.

To explore the optical performance of eachcomponent and associated IL reductions, we begin
by focusing on the silicon nitride edge coupler (EC)using V-groove technology [12]. This component
serves as the broadband high-power optical input/
output (I/O) for transceivers and PICs leveraging
edge coupling schemes (Figure 2a). Design andprocess optimisations, such as adjustments in the
crystallographic etch module and the back-endof-line cladding removal, ensure precise V-groove
dimensions for optimal coupling with the singlemode fibre and protection of the silicon nitride spot-

 Figure 3. Evolution of IL and PDL reduction in silicon-silicon nitride coupler. (a), (b), (c) silicon-silicon nitride vertical coupler. (d),(e), (f) silicon-silicon nitride diagonal coupler. (g), (h), (i) silicon-silicon nitride diagonal coupler featuring an enhanced transition
design for integration with standard silicon waveguide. All statistical measurement data was acquired at a wavelength of 1310 nm.
PIC MAGAZINE.NET I ISSUE I 2024 37

TECHNOLOGY I OPTICAL RECEIVERS

 Figure 4. Progression of IL and PDL reduction in PSR. (a), (b), (c) PSR. (d), (e), (f) PSR with enhanced process implementation.
(g), (h), (i) Enhanced PSR resulting from both process improvement and advanced transition design. All statistical measurement
data was acquired at 1310 nm.

size-converter beneath the dielectric cladding forminimal mode conversion loss.
Our refined process flow yields low IL for both
TE and TM modes (-0.6 dB and -0.8 dB) withminimal standard deviation of 0.23 dB and 0.33 dB
respectively for the two corresponding polarisations(Figure 2b and 2c). This results in a minimal PDL of
0.2 dB at 1310 nm, representing approximately 0.3
dB improvement when compared to the initial PDL
value prior to the process optimisation.
The next component to consider is the silicon-silicon
nitride coupler. Since the monolithic silicon photonic
platform integrates both silicon and silicon nitride
waveguides [13-14], it requires a compact transition
design for efficient, broadband and polarisationindependent light transfer between these layers.
We have thoroughly explored design options within
the platform to continually enhance the coupler
performance.
The evolution of the silicon-silicon nitride coupler,
as illustrated in Figure 3, involves transitioning froma vertical coupler design (Figure 3a) to a diagonal
coupling scheme (Figure 3d), accompanied by an
improved transition design outside the coupler
region (Figure 3g). The initial design (Figure 3a)features an ultra-compact footprint and low TE IL
Component(Figure 3b) but significantly higher TM IL (Figure 3c),
resulting in a pronounced PDL up to 0.77 dB. By
introducing a lateral offset in the silicon nitride layer(Figure 3d), we can achieve a diagonally coupledsilicon-silicon nitride transition, which significantly
reduces TM IL by 0.32 dB, while maintaining low
TE IL (Figure 3e and 3f). Advanced transition
design within the non-tapered silicon region(Figure 3g) further reduces the TM IL to 0.11 dB,
and the TE IL to 0.049 dB, as shown in Figure 3hand 3i, resulting in an impressively low PDL of
0.06 dB.
Finally, Figure 4 illustrates ongoing efforts toenhance the performance of the polarisation splitterrotator (PSR). This component employs an innovative
adiabatic design concept to manipulate TE and TM
polarisations, leveraging a hybrid silicon-silicon
nitride structure coupled to a silicon waveguideto achieve polarisation splitting and rotation [15].
The initial device (Figure 4a) shows a notable loss
imbalance between the two polarisations, with TM IL
substantially higher than TE IL, resulting in > 0.5 dB
PDL (Figure 4b and 4c).
Through continued process optimisations, wehave succeeded in reducing the TM IL by 0.25 dB
while maintaining low TE IL, as evident in Figure
4e and 4f, resulting in a PDL reduction of > 0.2 dB.

TE IL (dB or dB/cm)

TM IL (dB or dB/cm)

PDL (dB or dB/cm)

DGD (ps or ps/mm)

SiN EC

0.6

0.8

0.2

0.05

Si-SiN coupler

0.049

0.11

0.061

0.37

PSR

0.34

0.42

0.08

5.2

Si WG

1.0

0.9

-0.1

5.3

 Table 1. IL and DGDs for critical silicon photonic building blocks in a monolithic optical receiver.
38 ISSUE I 2024 I PIC MAGAZINE.NETTECHNOLOGY I OPTICAL RECEIVERS

 Figure 5. (a) Optical receiver circuits consisting of silicon nitride edge coupler, silicon-silicon nitride coupler, PSR and dual-input PD.
(b)-(c) TE IL, TM IL, PDL of each component and the total net IL and PDL of the receiver circuit. The loss associated with waveguiderouting is neglected in the calculation due to minimal PDL and short waveguide length. (d)-(f) Through-band performance from thesilicon nitride edge coupler to the PSR.

Leveraging the advanced transition design originallydeveloped for the silicon-silicon nitride coupler,we further reduce the PSR TM IL by an additional
approximately 0.2 dB (Figure 4i). This achievement
has yielded a record low PDL of < 0.1 dB at 1310 nm,
surpassing previous PSR demonstration utilising a
poly-silicon waveguide layer within the monolithic
silicon photonic platform [16].

PDL reduction and DGD compensation
for a monolithic optical receiver

The essential building blocks discussed, along withrouting waveguides and photodetectors, form thecore framework of a typical optical receiver. Utilising
the representative receiver architecture shown in
Figure 5a, we explore the cumulative improvementin IL and PDL. Figure 5b and 5c summarise the IL
and PDL for each component before and after theenhancement. Remarkably, substantial reductions in
TM ILs have been achieved in both the silicon-silicon
nitride coupler and the PSR, without compromising
the TE IL, consistent with the data presented in thehistograms in Figure 3 and Figure 4.
This notable enhancement has led to a record-high
1.52 dB reduction in overall TM IL, resulting in a

total PDL reduction exceeding 1.45 dB and a net
PDL of 0.38 dB. Moreover, due to the broadband
nature of the improved design, both the IL and PDL
reductions apply across a wider wavelength range(Figure 5d and 5e). Importantly, these performance
improvements are achieved through design andprocess optimisations without impacting the stackup or integrity of the monolithic silicon photonic
platform.
Turning to DGD compensation, Table 1 presents
the nominal ILs and DGDs of critical passive
components utilised in the monolithic receivercircuit. As passive components are connected in
the receiver towards the photodetector, DGDs
accumulate. If the net DGD at the photodetector
occupies a significant portion of the PAM-4 symbolinterval, it will introduce substantial ISI, necessitatingcompensation by the receiver’s digital signal
processor (DSP).
To demonstrate the impact of DGD on the receiver
performance, Figure 6 illustrates the degradationof a PAM-4 eye diagram resulting from the DGD
introduced at the photodetector output, represented
as a fraction of the symbol interval (T).
PIC MAGAZINE.NET I ISSUE I 2024 39

TECHNOLOGY I OPTICAL RECEIVERS

 Figure 6.
Impact of
DGD and eye
diagram at
PD output
as a fractionof symbol
interval.

In this scenario, we assume equal amplitudes of
the TE and TM components. The degradation in
the received optical eye is observed as the DGD(Δτ) varies from 0xT to 1.0xT. When the added DGD
exceeds half of the symbol interval, the received
eye diagram becomes closed, which significantly
complicates the equalisation and detection of thePAM-4 symbols in the receiver’s DSP.
To mitigate cumulative DGD and further minimise the
PDL, we propose employing a waveguide delay andvariable optical attenuators (VOAs) on both outputsof the PSR before feeding signals into a dual-port
photodetector, as illustrated in Figure 7. For 200G
PAM-4, with a symbol period of T = 9.4 picoseconds,
the accumulated DGD from the silicon nitride edge
coupler to the PSR output, including the routing
waveguide, is 8.96 picoseconds, occupying 95%
of T. Therefore, introducing a waveguide length DL
in the TM-to-TE path of the PSR provides adequate
delay to compensate for the DGD.
With this additional delay, the net DGD added in thereceiver at the photodetector is nearly zero. The
VOAs can be incorporated to further minimise the
PDL. By compensating for DGD and PDL optically,
the received PAM-4 signal will have zero added
ISI, and the eye diagram will be restored for 200G
transmission. Any remaining differential delay at the
PD (for example, less than 10 percent of the symbolinterval) can be compensated for by the adaptive
equaliser in the receiver.

Summary

In this article, we have highlighted our efforts inminimising losses in critical passive silicon photonic
building blocks essential for optical receivers.
Collective improvements in PDL, achieved throughenhancements in edge couplers, silicon-silicon

nitride couplers and PSRs, result in a substantial 1.45
dB reduction in system-level PDL. This translates toa mere 0.38 dB imbalance in TE-TM IL for a typical
optical receiver circuit, with similar PDLs observed
across a broader wavelength range in the O-band.
Furthermore, we have outlined strategies for
mitigating DGD in the receiver, resulting in minimal
ISI and restored eye diagrams for 200G applications.
The approaches and techniques presented inthis article pave the way for the development ofpolarisation-insensitive PICs for diverse applications
beyond datacentres.

AKNOWLEDGEMENTS
 The authors would like to thank:Thomas Houghton, Yarong Lin, Karen Nummy,
Jae Kyu Cho, Arpan Dasgupta,
Zahidur Chowdhury, Farid Barakat, Jason Kim,
Michelle Zhang, Shenghua Song, Helen Wong,
Chris Ritchie, Michal Rakowski, Hanyi Ding,
Qidi Liu, Ming Gong, Francis Afzal,
Salman Mosleh, Fahimeh Banihashemian,
Norman Robson, Teck Jung Tang,
Frederick G Anderson, Brian Popielarski,
Petar Ivanov Todorov Mohamed Gheith,
Ian Stobert, Mini Modh Ghosa,
Apoorva Vakil, Kate McLean, Ryan Gallagher,
Ranjani Sirdeshmukh, Yangyang Liu,
Janet Tinkler, George Gifford, Vikas Gupta,
Anthony Yu, Isabelle Ferain, Ed Kaste,
Gregg Bartlett and the rest of the
GlobalFoundries team for their development
engineering support for FotonixTM technologies.

 Figure 7. Monolithic optical receiver structure for DGD and PDL compensation.
40 ISSUE I 2024 I PIC MAGAZINE.NETTECHNOLOGY I OPTICAL RECEIVERS

FURTHER READING / REFERENCE [1] IEEE Standard for Ethernet - Amendment 11: PhysicalLayers and Management Parameters for 100 Gb/s and
400 Gb/s Operation over Single-Mode Fiber at 100 Gb/s
per Wavelength, IEEE SA - IEEE 802.3cu-2021

 [10] Y. Bian et al., “300 mm monolithic CMOS silicon
photonics foundry technology,” Conference on Lasers
and Electro-Optics, ATu3H.1, 2024.

 [2] IEEE 802.3dj, IEEE P802.3dj 200 Gb/s, 400 Gb/s,
800 Gb/s, and 1.6 Tb/s Ethernet Task Force (ieee802.org)

 [11] Y. Bian et al., “Polarization mode dispersion in
CMOS-integrated monolithic SiPh components:
simulations and experiments,” OFC, Th2A.24, 2023.

 [3] Y. Bian et al., “3D Integrated Laser Attach Technology
on a 300-mm Monolithic CMOS Silicon Photonics
Platform,” IEEE J. Sel. Topics Quantum Electron., Vol. 29,
no. 3, pp. 8200519, 2023.

 [12] Y. Bian et al., “Monolithically integrated self-aligned
SiN edge coupler with <0.6/0.8 dB TE/TM insertion loss,<-39 dB back reflection and >520 mW high-power
handling capability,” OFC, M3C.3, 2023.

 [4] V. Stojanović et al., “Monolithic silicon-photonic
platforms in state-of- the-art CMOS SOI processes,”
Opt. Express, vol. 26, no. 10, pp. 13106–13121, 2018.

 [13] Y. Bian et al., “Monolithically integrated silicon nitride
platform,” OFC, Th1A.46, 2021.

 [5] K. Giewont et al., “300-mm monolithic silicon
photonics foundry technology,” IEEE J. Sel. Topics
Quantum Electron., Vol. 25, no. 5, Sep. 2019, Art. no.
8200611.
 [6] M. Rakowski et al., “45 nm CMOS silicon photonics
monolithic technology (45CLO) for next-generation, lowpower and high speed optical interconnects,” OFC, T3H.3,
2020.

 [14] S. Chandran et al., “High Performance Silicon
Nitride Passive Optical Components on Monolithic
Silicon Photonics Platform,” OFC, Th3H.4, 2024.
 [15] W. Lee et al., “Low-loss and broadband adiabatic
polarization splitter rotator on a CMOS-integrated silicon
photonics platform,” OFC, M1J.5, 2024.
 [16] W. D. Sacher et al., “An O-band polarization splitterrotator in a CMOS-integrated silicon photonics platform,”
Frontiers in Optics, pp. paper FTu2D. 2, 2016.

 [7] Y. Bian et al., “Towards low-loss monolithic silicon andnitride photonic building blocks in state-of-the-art
300 mm CMOS foundry,” Frontiers in Optics, pp. paper
FW5D. 2, 2020.
 [8] T. Letavic et. al., “Monolithic Silicon Photonics,”
BCICTS, 2a.1, 2023.
 [9] T. Hirokawa, et al., “Latest Progress and Challenges in
300mm Monolithic Silicon Photonics Manufacturing,”
OFC, Th3H.2, 2024.

Expertise: Moderators, Markets,
30 Years + Pedigree
Reach: Specialist vertical
databases

Specialists with 30 year+ pedigree and in-depth
knowledge in overlapping sectors

Branding: Message delivery tohigh level influencers via various
in house established magazines,
websites, events and social media

For more information contact:
Jackie Cannon T: 01923 690205 E: jackie@angelwebinar.co.uk W: www.angelwebinar.co.uk
T: +44 (0)2476 718 970 E: info@angelbc.com W: www.angelbc.com
PIC MAGAZINE.NET I ISSUE I 2024 41

TECHNOLOGY I INTEGRATED PHOTONICS A tuneable laser chip on a custom
test PCB - the device has a footprint
smaller than 1 mm by 2 mm.

Tuneable lasers:
The elusive trifecta

Integrated tuneable laser assemblies serving in the C- and O- bands
excel on three fronts by offering the unique combination of wide tuning,
fast-nanosecond switching and a narrow linewidth
BY CAOLÁN MURPHY, SHANE DUGGAN, GAURAV JAIN,
CRISTIAN VARGAS, AND DESI GUTIERREZ FROM PILOT PHOTONICSTUNEABLE LASERS are now established as a key
enabling technology in a wide range of applications,spanning various fields from optical communications
to sensing.
In telecommunication infrastructure, particularly inthe long-haul and metro networks, widely tuneable
lasers with a low linewidth are indispensable
components. When incorporated in coherent
transceivers, their low linewidth enables increased
data rates by supporting advanced modulation
formats within dense wavelength-divisionmultiplexing systems. The wide wavelength
tuneability is a valued asset, simplifying network
design and management by enabling resources
to be dynamically assigned and optimised while
reducing inventory requirements.
42 ISSUE I 2024 I PIC MAGAZINE.NET

Meanwhile, in optical fibre sensing
applications, such as fibre Bragg
grating sensing, fast-swept
tuneable lasers are used tointerrogate sensors with
Bragg gratings inscribed into
optical fibres. These gratings
reflect specific wavelengthsof light, based on the strain andthe temperature applied to thefibre, to create a distinctive spectral
signature. When lasers offer fastswitching and are widely tuneable, thisenables precise and fast wavelength
sweeps across the fibre Bragg grating
sensor’s bandwidth to ensure real-time,
accurate readings of strain or temperature.

TECHNOLOGY I INTEGRATED PHOTONICS
Fast tuneable lasers have also played a key rolein the development of many optical switchingarchitectures, involving rapid adjustments to thelaser wavelength to quickly switch or route opticaltraffic without having to convert signals to theelectronic form.
There is no doubt that the versatility of the tuneable
laser has enabled widespread applications.
However, today’s commercially available widely
tuneable lasers still fall short in some areas. In
particular, they struggle to serve in applications
requiring phase-sensitivity and very fast-switching/sweeping.
For instance, for frequency-modulated continuouswave lidar, a cutting-edge technology inautonomous vehicles and remote sensing, they
are not adept at providing the low linewidth andultra-fast sweeping capabilities needed to precisely
measure distances and velocities. Additionally, insystems that employ coherent optical time-domainreflectometry and optical frequency-domainreflectometry, rapid, phase-sensitive tuning is criticalfor accurate detection of disturbances or variations
along the optical fibre, or for other devices under
test. If optical packet or optical burst systems are tore-emerge, they will have to align with the demandsof today’s high-speed data transmission, which
requires not only very fast wavelength switching,but the ability to handle phase-sensitive modulation
formats.
Unfortunately, today’s widely tuneable
semiconductor lasers are not great allrounders.
Typically, they offer either a narrow linewidth or fasttuning – but not both. An example is the sampledgrating distributed Bragg reflector laser, whichis capable of tuning across tens of nanometresusing the Vernier tuning effect, thanks to a currentinjection-based tuning mechanism that enables
swift optical switching. However, this comes at theexpense of increased phase noise and broader
linewidths, rendering this design unsuitable for the
aforementioned applications.
The alternative is to make use of an equivalent
thermally tuned laser. This class of laser is capableof realising similar tuning ranges, while boasting
low linewidths. However, fast electronic tuning
is sacrificed. Similarly, SiN ring-resonator-based
devices major in ultra-low linewidths via thermal
tuning, but dramatically reduce switching speeds,
also limiting usage in these applications.
The elusive combination of a broad tuning range,
narrow linewidth and either rapid switching or
sweeping, has been a long-standing industrial
challenge. Breaking through this impasse to fulfil
the elusive trifecta is our team at Pilot Photonics of
Dublin, Ireland. Working with the InP foundry Smart
Photonics, we are trailblazing lasers that blend all
three desirable properties.

 Figure 1. Tuneable laser form factor offerings: Photonic Integrated
Circuit (PIC), the laser cavity consists of a gain section providing optical
gain in either the C or O bands, two ring sections that act as tuneable
mode selectors through the Vernier effect, and a common phase
section to allow finer tuning of the laser cavity mode; 14-pin butterfly
package; OEM benchtop unit; nano-integrated tuneable laser assembly
package.
Available in both the C-band and O-band, ourground-breaking lasers unlock new possibilities
across a spectrum of applications, from lidar
to optical coherence tomography, dense
wavelength division multiplexing, coherent opticalcommunication, distributed fibre sensing, gassensing, sensor interrogation, fibre optic testing, andoptical switching. That’s a vast and varied range ofapplications.
Currently available in a 14-pin butterfly package, or
instrument formfactor, our nano-integrated tuneable
laser assembly that’s targeting high-volumecommunication applications will be released later
this year (see Figure 1). Each package features anintegrated feedback control, based on wavelengthmeter and wavelength-locker systems. Incorporated
in the wavelength meter are a linear filter and aphotodetector, employed to estimate the current
lasing wavelength, and a wavelength locker thatconsists of an etalon and photodetector – this allows
the monitoring of the wavelength on a 50 GHz or
200 GHz grid. Combining the two feedback systems
ensures closed-loop control of the device during
operation.

 The threedesirablecharacteristics
for tuneable
lasers, as wellas the typicaldesign rules
required toachieve them.
Few designs
are able toachieve the
combination ofwide tunability,
fast switchingand narrow
linewidth.

PIC MAGAZINE.NET I ISSUE I 2024 43

TECHNOLOGY I INTEGRATED PHOTONICS
 Figure 2. Example of LightCurrent (L-I) and Current-Voltage(I-V) curves. The C-band device
has a threshold current of
30 mA, while the O-band device
has a higher threshold of
50 mA. However, a higher slope
efficiency in the O-band device
results in a higher optical power
at a nominal current of 100 mA.
The O-band device is expected
to operate at a higher voltage
due to the wider bandgap
required to generate light in the
O-band.

Monolithic dual ring architecture

Our lasers are fabricated on a monolithic InP chiputilising an active-passive integration scheme.
The design employs a looped cavity with a gain
section that provides optical gain, and two ringresonators that act as transmission filters forwavelength selectivity through Vernier tuning (seeFigure 1 for details). The ring-resonators, which areelectro-optically tuned through reverse-bias voltage
application, have remarkably low dark currents– they are in the low milliampere range. These
low-power, low-current passive tuning sections are
crucial to realising optimum device performance.
Low dark currents minimise linewidth broadening
and device power consumption, while low heatdissipation in the tuning sections eradicatestroublesome slow thermal transients. Thanks tothese strengths, we realise nanosecond-levelswitching speeds.
Wide-band tuning is achieved with the Verniereffect. The desired emission wavelength is selected

through the overlap of the transmission filters of
two ring resonators, each with a slightly different
free spectral range. These filters produce a seriesof discrete lasing modes when one ring is tuned
relative to the other. A long phase tuning sectionin the large outer loop provides fine tuning of thecavity mode within the ring filter envelope.
One of the benefits of the Vernier mechanism is thatit opens up an avenue for ‘step-and-sweep’ styletuning, important for many frequency-modulated
continuous-wave lidar systems. The idea behind
step-and-sweep is as follows: tune the rings relative
to each other to produce a series of discontinuous
hops between ring modes (step), before ramping thephase section with a defined signal to produce
a continuous change in frequency (sweep) withineach of these modes.
To increase the output power, we integrate asemiconductor optical amplifier. This boosts theoutput to 30 mW (+14 dBm), with further increases

 Figure 3. Mode maps generated by sweeping voltages to the two ring tuners (with current into the gain
section and voltage to the phase section held fixed). The wavelength map on the left shows a series of
discrete regions, in which wavelength is relatively fixed (the modes). A map of the side-mode suppression
ratio (SMSR) on the right indicates regions where only a single lasing mode is dominating. Regions of low
SMSR (<30 dB) indicate a transition between modes (mode hops).
44 ISSUE I 2024 I PIC MAGAZINE.NETTECHNOLOGY I INTEGRATED PHOTONICS
 Figure 4.
A set ofindividual
spectrataken acrossthe opticalO-band. Eachline represents
a discretetuning modeof the laser.

expected. The addition of this amplifier also enables
output power stabilisation, thanks to the possibility
to readjust the output independently from the cavity.
We have recorded the characteristics of our original
C-band designs and our more recent designs in the
O-band. By plotting light-current-voltage curves, wehave determined the likes of lasing threshold, slope
efficiency and power consumption. Our C-band
devices, which do not include a semiconductor
optical amplifier, produce an optical output powerof 0.5-1.0 mW. O-band variants have a higher lasing
threshold but compensate with higher opticalpowers. Without the addition of the amplifier, theoutput power is 1.0-2.5 mW (see Figure 2). During
normal operation, the devices typically consume
130 mW, excluding the power drawn by the thermoelectric cooler.
For the spectral characterisation of our devices,
we use ‘mode-maps’. These two-dimensional maps
represent the tuning space of the laser – in otherwords, the lasing wavelengths produced by every
combination of voltages supplied to the two rings
on a two-dimensional grid (see Figure 3 for moredetails). These wavelength maps feature a series of
discrete regions (modes), in which the wavelength
varies continuously. Crossing a boundary between
the modes results in a wavelength discontinuity,
known as a mode-hop.

Using this method, we have recorded switching
speeds consistently below 10 ns, and as short as
3.6 ns (see Figure 5 for more details). That’s up to
three orders of magnitude faster than typical lasers
based on thermal tuning.
Having established that our laser is fast, thenext crucial question is this: what is its noisecharacteristics, and in particular its linewidth?
To answer this, we have used a delayed selfheterodyne technique to measure the linewidth,
obtaining values from 400 kHz down to 100 kHz inboth C- and O-band devices (see Figure 6). While
this is still broader than most external-cavity-type

Another important quantity to record on these maps
is the side-mode suppression ratio. The value forthis ratio offers an insight into the competing modesin the optical spectrum – when this ratio is low, that’s
below 30 dB, a mode hop is about to occur.
These maps can also be used to determine suitable
operating points for the laser under step-and-sweep
operation, and to identify optical switching pairs.
By sweeping the phase section, the tuning space
for these maps expands into the third dimension,
providing a more comprehensive characterisation.
However, such maps are much more difficult tovisualise.
We have used mode maps to determine the tuning
range for our lasers. It’s nearly 30 nm in the O-band
and over 35 nm in the C-band.
Once a suitable set of operating points are extracted
from the mode maps, it is possible to measure theoptical switching speed (see Figure 4). To validate
the nano-second switching speeds expectedfrom the device, we use a simple frequency
discrimination technique. By supplying one ormore tuners with a low-frequency square-wave
signal, with a suitable amplitude to produce a hop
between two modes, we produce a repetitive opticalswitching signal. Following photodetection, thisproduces an electrical signal of the same frequency
as the input signal, with the rise/fall time being equal
to the optical switching time.

 Figure 5. A switching speed measurement taken using a frequency
discriminator. The square-wave voltage applied to one of the rings (a)produces two complimentary optical signals (b), (c) – one for when the
optical filter is centred on each of the two switching modes. The switchingtime is equal to the fall time of the optical signal, measured to be 3.6 ns.
PIC MAGAZINE.NET I ISSUE I 2024 45

TECHNOLOGY I INTEGRATED PHOTONICS
lasers, it is a significant improvement over fasttuning sampled-grating distributed Bragg reflector
lasers, which would be expected to have linewidths
in the megahertz range. The level of noise
indicates that our devices are suitable for coherent
transmission applications. With our collaborators in
Dublin City University, we have proven this, realising
480 Gbit/s 16-QAM transmission over 25 km of
optical fibre. This result demonstrates the capabilityof our lasers for serving in optical access, metro, anddata-centre networks.

Future impact

According to the Tunable Laser Market Research
Report, published by Market Research Community,
the tuneable laser market is projected to grow by
8.8 percent per year from now to 2030, by whichtime it will climb to $23.2 billion. Helping to fuel thisgrowth will be improvements to the technology,
such as those just described, that will enable new
applications, enhance existing ones and have farreaching impacts on healthcare, telecommunications,transportation and many other sectors.
There is no doubt that producing tuneable lasersthat combine wide tunability with a low linewidth and

FURTHER READING / REFERENCE
 S. Andreou et al. “Electro-Optic Tuning of a MonolithicallyIntegrated Widely Tuneable InP Laser with Free-Running andStabilized Operation,” J. Light. Technol. 38 1887 (2020)
 G. Jain et al. “Widely tunable C-band laser and module with fasttuning and narrow linewidth,” Proc. SPIE 12424 Integrated Optics:
Devices, Materials, and Technologies XXVII, 124241M (2023)
 C. Murphy et al. “Widely Tunable O-band Laser with Narrow
Linewidth, Fast Switching Speed and an Integrated SOA”, ECOC
2023, Th.B.4.2.
 M. Troncoso-Costas et al. “Experimental demonstration of
480 Gbit/s coherent transmission using a nanosecond switchingtuneable laser”, Opt. Commun. 554 130164 (2024)

 Figure 6. Spectrum fitted to a Voigt profile.
Lorentzian linewidths as low as 100 kHz have beenrecorded.
either ultra-fast switching or sweeping is an important
step forward. In the optical networking field, it
enables new datacentre switching architectures thatpromise to trim power consumption by keeping as
much traffic in the optical domain as possible, andonly converting to the electronic domain the trafficthat is necessary.
The evolution of telecom protocols like 5G and theanticipated 6G necessitate ever-faster laser switching
speeds, wide tunability, and low latency – all are
required to ensure the seamless communication
that’s essential for our connected future. Moreover,the impact of lasers that excel on all these threefronts extends to advancements in metrology
techniques, benefiting applications such as lidar,
healthcare diagnostics and industrial sensing, wherephase sensitivity and speed are critical.
Key to the potential of this unique laser is its
monolithic integration. Thanks to this, it can be
manufactured at wafer scale using today’s InP
processing techniques, ensuring a cost profile thatis compatible with the very high-volume applications
envisaged today and into the future.

Dedicated webinars for the photonic
integrated circuits (PIC) industry
Using our 30+ years’ experience in B2B vertical technical markets, and as the
publisher of PIC Magazine, we offer effective webinars, ZOOM interview andvirtual events. We help you get your message, to your desired audience, bymarketing to over 15,000 PIC professionals.
In addition to organising and managing your webinar, we can also market yourwebinar to our specialist databases.

Reach Educate Influence£ Brand Awareness£ Lead Generation£ Thought Leadership46 ISSUE I 2024 I PIC MAGAZINE.NET

Contact: Jackie Cannon
jackie.cannon@angelbc.com+44 (0)1923 690205