Funding type: COFUND Marie Slodowska Curie Action
This project is under the Marie Skłodowska-Curie Actions (MSCA) program. There are no nationality conditions but the candidates must fulfill the MSCA mobility conditions, which means that she/he must not have stayed more than 1 year in France during the last 3 years immediately before the call deadline (31/05/2019)

Expected start date: 01/10/2019

Contacts:
Dr. Andy Boes, RMIT
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Pr. Arnan Mitchell, RMIT
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Ass. Prof. Christelle Monat
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websites:

• https://www.rmit.edu.au/about/our-locations-and-facilities/facilities/research-facilities/micronano-research-facility
• http://inl.cnrs.fr/en/

Collaborations/External partners: Thales TRT

Domain and scientific context:

It is well established that photons are suitable for communication and electrons for computing. Yet the border between computing and communication has become somehow blurred when considering emerging research domain such as optical interconnects [1] and photonic neuromorphic computing [2]. In many contexts it has been realized that photonics can help computation by providing an efficient interface for short-distance communication or for performing simple operations but in a massively parallel fashion. Here we consider the problem of dealing with high-speed signals in the optical domain needing to be digitized with very high accuracy. This PhD project will address the most critical operation here, namely extremely accurate sampling with minimal timing error [3].
Keywords: all-optical signal processing, lithium niobate, parametric conversion, photonic integrated circuits

Objectives:

We consider here a photonic integrated circuit converting a high-speed optical signal into a sequence of optical pulses, representing the samples extracted with the most accurate timing. Sampling is controlled by the pulses generated by a mode-locked laser [4], which is actually among the most accurate timing references available and, therefore, suits the need perfectly [5]. In contrast with existing implementations [6], here we use a nonlinear photonic platform based on Lithium Niobate. This allows dealing with signals in the optical domain, which can therefore be generated remotely. Optical samples are then converted into electric signals and digitized. As accurate analog to digital converters (ADC) are limited in speed, the photonic integrated circuit will be used to time-demultiplex the high-speed flow of data and feed multiple ADCs in sequence. We target an aggregate sampling rate of 20Gs/s with noise and error low enough to allow encoding with 10 representative bits.

Scientific challenges:

High-resolution digitization of high-speed signals is ultimately limited by the accuracy at which the signal is sampled. This enforces strict tolerances on the timing of signals propagating in the photonic circuits. In the optical domain, however, issues such as latency are less severe than in electronics and propagation velocity in passive waveguide is well controlled.

On the other hand, accurate sampling requires the optical gating to provide a very large extinction ratio, as well as filtering and demultiplexing requires very large rejection. Finally, sampling requires a nonlinear operation based on sum frequency generation, which need to be efficient enough.  

Expected original contributions:

All-optical sampling in an integrated photonic circuit is a recent and challenging topic. No demonstration has been reported of time-interleaved all-optical sampling. Moreover, an integrated lithium niobate on insulator (LNOI) [7] platform has just become available and this is the first time it will used for this purpose.  
The performances of the periodically poled LNOI waveguides on a silicon platform will be enhanced in order to maximize efficiency and simplify the topology of the all-optical sampling circuit. This will lead to advanced poling configuration and novel design improving the non-linear interaction.

The non-linear functions will be for the first time combined on the same platform, with low losses photonics circuits, for filtering and signal distribution and synchronization.

Research program and methodology:

Program:

Year1: Mostly at INL & Thales.

• Conceive architecture of the all-optical sampling circuits and define requirements for its functional parts (Thales).
• Design and modelling of the different functional elements of the photonic circuit in particular the non-linear waveguide (INL).
• Modelling of the behaviour of the full system and estimation of the expected performances (Thales).
• Implementation of the sampling concept implementing discrete components including existing PPLN waveguides.

Year 2: Mainly at RMIT.

Fabrication & characterization of individual functional building blocks

• Design and simulation of LNOI waveguides and components of the PDK required for full photonic circuit
• Fabrication of periodically poled waveguides in LNOI and other building blocks required for the full photonic circuit
• Characterisation and optical testing of PPLN waveguides for optical sampling (first SHG experiments, then SFG experiments with CW lasers)
• Preliminary design and layout of the full photonic circuit

Year 3: Mainly at INL and Thales.

• Revised architecture and fine tuning of the design.
• Tape out and coordination of fabrication of the full circuit through remote collaboration with RMIT
• Optical sampling experiments on chip (Single channel experiment, Time inter-leaved sampling)

Methodology:

The design of the chip architecture is based on the knowledge of the building blocks. Common commercial electromagnetic simulation software tool such as Fimmwave, Lumerical, Comsol as well as in-house developed tools will be used for modelling while layout will be generated based on the process design kit (PDK) provided by RMIT using an industry standard design framework (IPKISS)

The fabrication of the optical circuits in LNOI will be done in the clean-room facilities at RMIT Micro Nano Research Facility - MNRF.

Optical characterization will consist in dispersion and transmission measurements for instance using Optical Coherence Tomography [8].
The non-linear frequency conversion will be performed in the continuous and pulse regime using CW tunable lasers and pulsed mode-locked sources in the Telecom range.

Sampling experiments will aim at demonstrating the fidelity and accuracy of the operation by retrieving the signal and comparing it to a well-known input. To this extend, actively mode-locked source driven by RF frequency generator. Multi-channel electronic analog to digital converters (ADC) will be cascaded to the optical chip for the digitizing the optically sampled signal. 

feasibility/resources/equipment:

Thales:

• Fully equipped laboratory for optical signal processing on integrated platform (70GHz oscilloscope, 10GHz real time oscilloscope, actively and passively mode-locked lasers, tunable sources, low-noise amplifiers, RF sources, optical amplifiers, fast detectors, …).
• Computing cluster for modelling and available software
• OCT setup for dispersion, reflexion and transmission measurements. Infrared and visible cameras for chip diagnostics.

RMIT:

The project requires a wide-range of skills in photonic and electro-optic device concept, circuit design, fabrication and characterization, nonlinear optics, optical frequency combs and optical communications. Prof Mitchell and his team have nearly 20 years of experience in the design, fabrication and testing of lithium niobate devices, including periodically poled nonlinear structures and high-speed optical modulators. Prof Mitchell is Director of the $40M RMIT Micro Nano Research Facility (MNRF) which has almost all of the tools required to fabricate the proposed devices and a substantial integrated photonics laboratory for characterization and testing of the components. As a Distinguished Professor, Mitchell is entirely research focussed enabling contribute to this project without competing commitments.

• Optics lab: full equipped laboratory for optical signal processing on integrated platform (65GHz oscilloscope, actively and passively mode-locked lasers, tunable sources, low-noise amplifiers, RF sources, optical amplifiers, fast detectors, …), stages for stable fibre coupling to the optical chips, full suite of software tools for design and simulation of integrated photonic components and circuits.
• Micro Nano Research Facility (MNRF): Full suite of equipment for fabricating of optical waveguides (spin coating, lithography, wet etching, reactive ion etching (RIE), plasma enhanced chemical vapour deposition (PECVD), sputtering, e-beam evaporation, atomic layer deposition (ALD), annealing and oxidation furnaces, scanning electron microscopes, atomic force microscopes, …)

Scientific supervision:

• Description of the supervision committee :

• Integration inside the laboratories (percentage of working time inside these laboratories) : 67% at INL (including 10% Thales), 33% at RMIT

PhD funding: Co-Fund Marie Sladowska Curie Action (MSCA) ECL/RMIT (ECLAUsion program)

Profile of the candidate: 

The required skills for the intern will be a good knowledge and a solid background in the field of solid-state physics, optics, nonlinear optics, and semiconductor devices. S/he should work towards his/her Masters/honours or Engineering degree in a field appropriate to one of these areas. An experience in photonics, clean-room fabrication, material deposition or optical modeling and characterization will be strongly appreciated.

Objectives for the valorization of the research work:

The results obtained will be published in peer-reviewed journals with a high impact factor and presented at international conferences in the field (CLEO-US/Europe, SPIE photonics Europe, NLO...). No patent filing or confidentiality constraints are envisaged, but this could change (in consultation with our collaborators).

Skills that will be developed during the PhD:

The student will develop the complete range of "nanophotonics / nanotechnology" skills, from device design (simulation and design of optical microcomponents using different modelling techniques such as FDTD-Finite difference time domain, BPM-Beam propagation method) and their integration into a circuit using an industry standard design framework, the manufacture of these components in clean room environments (deposition, electron beam lithography, chemical etching and dry etching), their characterization (within a complete optical bench - wafer coupling, non-linear characterization, pulsed laser, parametric processes - that the student will have contributed to design and deploy) and data processing.

The highly collaborative and international environment of the project will require the student to develop, in addition to technical and scientific skills, communication, teamwork and project management skills.

Professional opportunities after the PhD:

This conceptual, scientific and technological work of demonstrating the digitization of high-speed signals in the optical domain with very high accuracy is of extreme importance for optical communication and computation technology – a rapidly growing field of research and industrial applications. The future prospects for the student, at the end of his/her thesis, are therefore extremely rich, with the possibility of pursuing an academic career in a prestigious photonics laboratory or entering an industrial field that will be eager for the skills developed by the student.

Bibliographic references about the PhD topic:

1. A. Shacham, K. Bergman and L. P. Carloni, "Photonic Networks-on-Chip for Future Generations of Chip Multiprocessors," in IEEE Transactions on Computers, vol. 57, no. 9, pp. 1246-1260, Sept. 2008.
2. Shen, Yichen, et al. "Deep learning with coherent nanophotonic circuits." Nature Photonics 11.7 (2017): 441.
3. Valley, George C. "Photonic analog-to-digital converters." Optics express 15.5 (2007): 1955-1982.
4. Keller, Ursula. "Recent developments in compact ultrafast lasers." Nature 424.6950 (2003): 831.
5. Kim, Tae Keun, et al. "Sub-100-as timing jitter optical pulse trains from mode-locked Er-fiber lasers." Optics letters 36.22 (2011): 4443-4445.
6. Khilo, Anatol, et al. "Photonic ADC: overcoming the bottleneck of electronic jitter." Optics Express 20.4 (2012): 4454-4469.
7. A. Boes, B. Corcoran, L. Chang, J. Bowers, A. Mitchell, “Status and Potential of Lithium Niobate on Insulator (LNOI) for Photonic Integrated Circuits, "  Laser & Photonics Reviews 2018, 12, 1700256. https://doi.org/10.1002/lpor.201700256
8. Combrié, Sylvain, et al. "Comb of high‐Q Resonances in a Compact Photonic Cavity." Laser & Photonics Reviews 11.6 (2017): 1700099.