PROJECT FINAL REPORT
Project acronym: CRITICAL
Project title: Component Requirements and Integrated Transceiver Investigation for
Coherent Access Links
Project duration: 3 years 1 month from July 2011 to July 2014
Coordinator (organisation): Oclaro Technology Ltd
Scientific representative of the coordinator (name and title): Dr Mike Wale
E-mail: mike.wale@oclaro.com
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Final publishable summary report
Executive summary
The CRITICAL project aimed to develop coherent transceivers and system concepts for ultrahigh bandwidth access systems. Partners for the 37 month project comprised Oclaro
(Coordinator, UK), Deutsche Telekom AG (DE), NPL (UK), Coriant (DE), PMC Sierra (IL) and
University College London (UK).
New indium phosphide (InP) based lasers were designed and developed using AlQ materials
systems to enhance performance. Laser cavity and gratings were redesigned and devices
manufactured, with the laser chips packaged and characterised. Excellent results were
obtained.
Ambitious designs for the optical line terminal (OLT) and optical network unit (ONU) were
addressed using functional building blocks for lasers, modulators and detectors. Combining all
building blocks proved a challenge to complete within the project timescales. However,
significant knowledge and understanding of the InP processing needed to realise the fully
integrated design was gained, and a key output of the project.
Key elements of the real time ONU demonstrator were successfully implemented together
with optimised equalisation enabling significant power reduction. Differential carrier phase
recovery for the quadrature phase shift keying (QPSK) coherent system was also been
investigated using the new laser designs. The differential algorithm was demonstrated to be
robust and reliable for wide-range wavelength tunable lasers.
Prospects for exploitation are good, with short term opportunities in wireless backhaul
networks. Further development work will be necessary to meet the requirements for smaller,
low cost components for the access market.
Summary of project context and objectives
The PIANO+ CRITICAL project addressed technology for the next generation of access
systems, capable of delivering symmetric data rates up to 10Gbit/s, over long spans to a large
number of users. The approach exploited recent developments in digital coherent system
concepts and advanced modulation formats. Coherent access technologies are based on a
combination of spectrally efficient modulation, coherent detection and digital signal processing
(DSP):
• Coherent detection maximises efficacy of DSP and permits the use of spectrally efficient
modulation, allowing an extremely high channel density.
• Coherent detection provides higher sensitivity to received signals which allows for longer
reach and higher split ratios
• DSP simplifies the receiver, provides filtering and compensates for transmission
impairments, increasing the obtainable transmission length.
The objective of the project was to specify, design, build and test a proof of concept system
(technology demonstrator), embodying all of the new technology, which will allow the concept
and associated technology to be assessed and the technical and economic issues to be
investigated.
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Description of the main S&T results/foregrounds
The target architecture and system requirements were defined and the feasibility of such a
system has been shown in a demonstrator setup.
Key to the systems was a high performance monolithic tunable laser. New Indium Phosphide
based lasers were designed by Oclaro using the AlQ material system to enhance behaviour
and performance at higher temperature. The laser cavity and gratings were redesigned to
improve laser linewidth according to the system requirements in WP2 and broadly in line with
other commercially deployed coherent optical communications systems. These new designs
were manufactured in the wafer fab and the laser chips packaged and supplied to NPL for
characterisation and validation of expected performance improvements. Linewidths of less
than 500kHz were obtained with further improvements demonstrated in the latter stages of
project, meeting system needs.
The Forward Error Correction requirements were defined in conjunction with systems
requirements by Coriant, together with UCL and PMCS. Key parameters such as error
correction overhead, signal/coding gain and power consumption were all been defined and
specified. The coding algorithm, written in C was generated and performance verified with
detailed simulations.
Oclaro had very ambitious plans at the start of the project to integrate many optical functional
building blocks onto a single photonic integrated circuit (PIC). This did indeed prove a
significant challenge and full integration was not possible in the project timescales. Oclaro
had several designs on the existing “n+” InP platform, however integrating lasers, modulators
and detectors on this processing platform was not feasible. Therefore, a new platform on
semi-insulating (SI) substrates was further developed with the expectation to integrate the
required functionality. The complexity of defining a semiconductor processing scheme on this
SI platform proved significant and we were unable to complete this task within the project
timescales. However, Oclaro have been able to demonstrate the design and definition of the
individual building blocks needed to eventually construct a complex optical device with high
levels of functionality.
Key elements of the real time ONU demonstrator successfully implemented, in particular
equalisation and carrier recovery with 3 Gb/s throughput. Research revealed power
consumption of equalisation could be reduced by 80% by employing a sign-sign update
scheme. Differential carrier phase recovery for the QPSK coherent system was also
investigated based on the measured phase noise of the integrated tunable laser. The
differential algorithm was demonstrated to be robust and reliable with the digital supermode
(DS) distributed Bragg reflector (DBR) laser across different ITU channels.
A resolution of 5 bits in the analogue-to-digital converter (ADC) has been demonstrated to be
sufficient for the differential carrier phase recovery implemented on the field-programmable
gate array (FPGA).
Access to installed fibre infrastructure for a full technology demonstrator was not possible
within the project timescale, so a lab based configuration and demonstrator was completed.
Arbitrary waveform generators were set up with modulators providing multi carrier signals
with QPSK modulation, and direct detection and demodulation of signals allowing spectral
evaluation.
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Potential impact (including the socio-economic impact and the wider societal
implications of the project so far) and the main dissemination activities and
exploitation of results
Prospects for exploitation are good, with short term opportunities in wireless backhaul
networks. Further development work will be necessary to meet the requirements for smaller,
low cost components for the access market.
The project resulted in several important technological advances which will be exploited
directly by the project partners and have a wider impact through advancing the science and
technology enabling future ultra-high capacity communications systems. The design and
development of very narrow linewidth, low phase noise laser sources that are efficient and do
not need cooling is key to those systems, as are the signal coding algorithms developed here.
While coherent access networks had been an exotic future topic when the project started,
they are now seriously under evaluation from several groups and have found acceptance in
the scientific community as a worthwhile topic for development following the inexorable
demand for bandwidth from consumers and society as a whole.
Results have been disseminated widely via publications in leading international journals and
proceedings (six to date) plus presentations at a range of workshops and conferences.
Project public website and relevant contact details.
www.pianoplus.eu/critical
mike.wale@oclaro.com
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