Linking fundamental to applied photonics
All-optical and nonlinear signal processing
Using nonlinear optics to perform all-optical signal processing
The nonlinear optical processing group explores all-optical signal processing methods to enable high bitrate optical fibre communications through terabaud super-channels and coherent modulations formats, in collaboration with the Australian National University (ANU), Technical University of Denmark (DTU), AIST Japan and University of York.
In this project optical processing is performed using processes such as four-wave mixing (FWM) and cross-phase modulation (XPM). To maximize system integration, the research group uses on-chip highly-non-linear waveguides, made in media such as chalcogenide glass or slow-light in silicon photonic crystals. The research group also leverages the power of reconfigurable optical processors based on liquid crystal on semiconductor (LCOS) technology.
Current key activities
The research group aims at replacing operations usually performed by Digital Signal Processing (DSP) with all-optical devices with a view to reduce latency and power consumption in optical data transmission. Both linear and non-linear pulse shaping devices are studied.
Operations demonstrated include:
- linear impairments (dispersion, higher orders of dispersion, differential group delay) monitoring and real-time cancellation of their fluctuations;
- demultiplexing of high baudrate optical time division multiplexed signals;
- all-optical matrix product between wavelength division multiplexed signals;
- compensation of linear crosstalk based on dot-product implementation;
- generation and comparison of hash codes for data integrity check upon propagation through a transmission link.
The research group harnesses the ultra-fast Kerr nonlinear response in specially-tailored nonlinear optical waveguides to undo the deleterious impairments suffered by high bit rate optical signals in long distance transmission in optical fibre, stemming from the intensity dependent change in the fibre refractive index known as the Kerr effect. The project focuses on negating the nonlinear phase distortion associated with the Kerr effect in the transmission link by the means of all-optical signal processing via nonlinear optics to conjugate the phase of the signal field during propagating through the optical waveguide. This approach is being explored to improve the transmission performance for higher bit rate signals produced by the emerging complex multi-level amplitude/phase modulation formats combined with coherent detection, and both wavelength division and polarization state multiplexing. The innovative harnessing of nonlinear optics to overcome the limits on the maximum signal transmission distance and bit rate capacity can have an important role in next generation optical fibre communication systems in enabling the challenging ever increasing bandwidth demand to be met.
Mode Division Multiplexing (MDM) differentiates between independent channels propagating in a fibre based on their spatial properties. Using computer-controlled holograms implemented on spatial light modulators (SLM), channels are converted to different spatial patterns as they are coupled into the fibre and a corresponding hologram is used at the receiver to recover the channels.
Mid-Infrared photonics
Developing novel mid-IR sources on a compact chip-scale platform
The mid-infrared wavelength range (3-50 microns) is an extremely useful range of the electromagnetic spectrum. In this range, many molecules exhibit strong, fundamental ro-vibrational absorption. This fact has led to a drive to develop molecular sensing platforms using mid-IR light. One major roadblock to achieving such devices, however, is the current state of broadband sources in the mid-IR. One of the main goals of this mid-IR project is to lead the way in developing these novel sources of light on a compact chip-scale platform. This platform will then be used to demonstrate the usefulness of these devices for sensing applications such as breath analysis, airport security screening, food quality monitoring, and livestock health monitoring.
The mid-IR photonics research group is investigating a number of platforms for developing broadband, mid-IR light on a chip using nonlinear waveguides, in collaboration with industry partners and the Australian National University.
The project focuses on the nonlinear propagation characteristics of pulses and the spectral expansion through various processes.
The research group is also pursuing broadband light generation using a complimentary platform: tapered optical fibres based on highly nonlinear glasses such as Arsenic-Sulphide and Arsenic-Selenide. Using the optical fibre tapering facility at the University of Sydney, the research group can build devices with extreme nonlinearity that exhibit similar effects to those observed on chip-waveguide architectures.
Single photon generation for large-scale photonic quantum systems
Systems utilising single and entangled photons show considerable promise for the implementation of quantum technologies, including quantum key distribution and quantum computing. A major obstacle lies in the lack of an on-demand single photon source. Using photonic integration and active multiplexing, generation rates can be improved by several orders of magnitude, this forming the basis of a crucial tool for the development of quantum photonics.
Recent demonstrations have clearly indicated the advantages of quantum computing over other methods, in particular applications such as Boson sampling. For more general applications, large-scale quantum photonics requires large numbers of indistinguishable single photons. Current techniques generate photons with low-probability, and experiments requiring multi-photon interactions occur with exponentially decreasing probability.
In this project, scalability is addressed using a complementary metal oxide semiconductor (CMOS) compatible process. A silicon nanowire (250*450nm) is used as a nonlinear single photon source. Due to high levels of nonlinearity and engineered dispersion, time correlated single photon pairs are randomly generated via a stochastic, spontaneous four wave mixing (SFWM) process. CMOS implementation offers the potential of including massive numbers of these sources on a single chip, making it scalable.
The other intrinsic limitation investigated in this project is generation efficiency. Crucially, photons within the pair are time correlated, allowing the detection of one photon to herald the existence of its partner. Electronics are then used to multiplex the heralded photon and increase its probability of occurring at a point in time. This technique is known as active multiplexing and has the potential to increase efficiency by a large factor.
In 2016, in work published in Nature Communications, Professor Benjamin J. Eggleton’s group and Professor Philip H. W. Leong’s group gave the first demonstration of active temporal multiplexing and observed nearly 100% enhancement. Indistinguishability of the multiplexed photons was verified via Hong-Ou-Mandel quantum interference with 91% visibility. Based on this work, we then collaborated with Professor Terry Rudolph from Imperial College London and we experimentally demonstrated the relative temporal multiplexing that can further enhance the generation efficiency of N single photons without increasing complexity. In the proof-of-principle demonstration, 90% enhancement and 88% visibility indicate the significance of this scheme in large-scale photonic quantum computing.
This project is mainly founded by an ARC Centre of Excellence, Professor Eggleton’s ARC Laureate Fellowship, and partially funded by Huawei Technologies Co. Ltd. In the future, more components will be integrated.
Benjamin Eggleton
