Periodic structures with a sub-wavelength pitch have been known since Hertz conducted his first experiments on the polarization of electromagnetic waves. While the use of these structures in waveguide optics was proposed in the 1990s, it has been with the more recent developments of silicon photonics and high-precision lithography techniques that sub-wavelength structures have found widespread application in the field of pho- tonics. This review first provides an introduction to the physics of sub-wavelength structures. An overview of the applications of sub-wavelength structures is then given including: anti-reflective coatings, polarization rotators, high-efficiency fiber-chip cou- plers, spectrometers, high-reflectivity mirrors, athermal waveg- uides, multimode interference couplers, and dispersion engi- neered, ultra-broadband waveguide couplers among others. Particular attention is paid to providing insight into the design strategies for these devices. The concluding remarks provide an outlook on the future development of sub-wavelength structures and their impact in photonics.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.201400083

Waveguide sub-wavelength structures: a review of principles and applications

Part I. Silicon Photonics - Introduction: 1. Fabless Silicon Photonics: 1.1 Introduction 1.2 Silicon photonics - the next fabless semiconductor industry 1.3 Applications 1.4 Technical challenges and the state of the art 1.5 Opportunities 2. Modelling and Design Approaches: 2.1 Optical Waveguide Mode Solver 2.2 Wave Propagation 2.3 Optoelectronic models 2.4 Microwave Modelling 2.5 Thermal Modelling 2.6 Photonic Circuit Modelling 2.7 Physical Layout 2.8 Software Tools Integration Part II. Silicon Photonics - Passive Components: 3. Optical Materials and Waveguides: 3.1 Silicon-on-Insulator 3.2 Waveguides 3.3 Bent waveguides 3.4 Code Listings 3.5 Problems 4. Fundamental Building Blocks: 4.1 Directional couplers 4.2 Y-Branch 4.3 Mach-Zehnder Interferometer 4.4 Ring resonators 4.5 Waveguide Bragg Grating Filters 4.6 Code Listings 4.7 Problems 5. Optical I/O: 5.1 The challenge of optical coupling to silicon photonic chips 5.2 Grating Coupler 5.3 Edge Coupler 5.4 Polarization 5.5 Code Listings 5.6 Problems Part III. Silicon Photonics - Active Components: 6. Modulators: 6.1 Plasma Dispersion E 6.2 PN Junction Phase Shifter 6.3 Micro-ring Modulators 6.4 Forward-biased PIN Junction 6.5 Active Tuning 6.6 Thermo-Optic Switch 6.7 Code Listings 6.8 Problems 7. Detectors: 7.1 Performance Parameters 7.2 Fabrication 7.3 Types of detectors 7.4 Design Considerations 7.5 Detector modelling 7.5.2 Electronic Simulations 7.6 Code Listings 7.7 Problems 8. Lasers: 8.1 External Lasers 8.2 Laser Modelling 8.3 Co-Packaging 8.4 Hybrid Silicon Lasers 8.5 Monolithic Lasers 8.6 Alternative Light Sources 8.7 Problems Part IV. Silicon Photonics - System Design: 9. Photonic Circuit Modelling: 9.1 Need for photonic circuit modelling 9.2 Components for System Design 9.3 Compact Models 9.4 Directional Coupler - Compact Model 9.5 Ring Modulator - Circuit Model 9.6 Grating Coupler - S Parameters 9.7 Code Listings 10. Tools and Techniques: 10.1 Process Design Kit (PDK) 10.2 Mask Layout 11. Fabrication: 11.1 Fabrication Non-Uniformity 11.2 Problems 12. Testing and Packaging: 12.1 Electrical and Optical Interfacing 12.2 Automated Optical Probe Stations 12.3 Design for Test 13. Silicon Photonic System Example: 13.1 Wavelength Division Multiplexed Transmitter.

Silicon Photonics Design: From Devices to Systems

Silicon photonics has experienced phenomenal transformations over the last decade. In this paper, we present some of the notable advances in silicon-based passive and active optical interconnect components, and highlight some of our key contributions. Light is also cast on few other parallel technologies that are working in tandem with silicon-based structures, and providing unique functions not achievable with any single system acting alone. With an increasing utilization of CMOS foundries for silicon photonics fabrication, a viable path for realizing extremely low-cost integrated optoelectronics has been paved. These advances are expected to benefit several application domains in the years to come, including communication networks, sensing, and nonlinear systems.

Recent advances in silicon-based passive and active optical interconnects

We design and demonstrate broadband directional couplers that use asymmetric-waveguide based phase control sections, on the silicon-on-insulator platform. Broadband directional couplers with various power splitting ratios, including 10%/90%, 20%/80%, 30%/70%, 40%/60% and 50%/50%, were realized for both transverse electric (TE) and transverse magnetic (TM) modes. Some of the devices exhitbit bandwidths in excess of 100 nm, and all in excess of 75 nm. The footprints of the TE mode couplers are 32 μm ×1.3 μm, or less, and those of the TM mode couplers are 13 μm ×1.3 μm, or less.

Broadband silicon photonic directional coupler using asymmetric-waveguide based phase control.

Quantum key distribution provides an efficient means to exchange information in an unconditionally secure way. Historically, quantum key distribution protocols have been based on binary signal formats, such as two polarization states, and the transmitted information efficiency of the quantum key is intrinsically limited to 1 bit/photon. Here we propose and experimentally demonstrate, for the first time, a high-dimensional quantum key distribution protocol based on space division multiplexing in multicore fiber using silicon photonic integrated lightwave circuits. We successfully realized three mutually unbiased bases in a four-dimensional Hilbert space, and achieved low and stable quantum bit error rate well below both the coherent attack and individual attack limits. Compared to previous demonstrations, the use of a multicore fiber in our protocol provides a much more efficient way to create high-dimensional quantum states, and enables breaking the information efficiency limit of traditional quantum key distribution protocols. In addition, the silicon photonic circuits used in our work integrate variable optical attenuators, highly efficient multicore fiber couplers, and Mach-Zehnder interferometers, enabling manipulating high-dimensional quantum states in a compact and stable manner. Our demonstration paves the way to utilize state-of-the-art multicore fibers for noise tolerance high-dimensional quantum key distribution, and boost silicon photonics for high information efficiency quantum communications. Quantum key distribution (QKD) enables ultimate secure communication guaranteed by quantum mechanics. Most of QKD systems are based on binary encoding utilizing bulky, discrete, and expensive devices. Consequently, a large scale deployment of this technology has not been achieved. A solution may be by photonic integration, which provides excellent performances and are particularly suitable for manipulation of quantum states. The Center for Silicon Photonics for Optical Communication (SPOC) led by Prof. Leif Katsuo Oxenlowe at the Technical University of Denmark demonstrated an integrated solution for manipulation of new high-dimensional quantum states using spatial degrees of freedom (the cores of a multicore fiber). We achieved the first silicon chip-to-chip decoy-state high-dimensional QKD, which is suitable for longer transmission distance with higher secret key rate, better resilience to noise, and higher information efficiency.

/pdf/high-dimensional-quantum-key-distribution-based-on-multicore-3wwvpjodl8.pdf

High-dimensional quantum key distribution based on multicore fiber using silicon photonic integrated circuits

We propose a novel ultracompact (5 μm) hybrid plasmonic polarization rotator operating at telecommunication wavelength for integrated silicon photonic circuits. The polarization mode of a silicon waveguide is rotated with >14 dB polarization extinction ratio and low total insertion losses of 2.1 dB.

/pdf/compact-hybrid-plasmonic-polarization-rotator-4c4atqxnvc.pdf

Compact hybrid plasmonic polarization rotator

We experimentally demonstrate an ultracompact (3.7 μm long) hybrid plasmonic polarization rotator operating around 1.55 μm for integrated silicon photonics circuits. The TM polarization of a silicon waveguide is rotated to the TE polarization with insertion losses as low as 1.5 dB and polarization extinction ratios larger than 13.5 dB.

Experimental demonstration of an integrated hybrid plasmonic polarization rotator

We propose a novel broadband coupler for silicon photonics using a hybrid plasmonic waveguide section. The hybrid plasmonic waveguide is used to create an asymmetric section in the middle of a silicon nanowire waveguide coupler to introduce a phase delay to allow for a 3-dB power coupling ratio over a 150 nm bandwidth around 1.55 µm. The device is very compact (<8.5 µm) and has a low insertion loss (<0.15 dB).

/pdf/compact-low-loss-and-broadband-hybrid-plasmonic-directional-1sv354ce6i.pdf

Compact low loss and broadband hybrid plasmonic directional coupler

We fabricated and measured a compact 3 dB hybrid plasmonic directional coupler for silicon photonics integrated circuits with a length of 212 μm The coupler has a 50∶50 coupling ratio over a spectral bandwidth of more than 100 nm around a wavelength of 155 μm and has an insertion loss of less than 1 dB

Measurement of a compact colorless 3 dB hybrid plasmonic directional coupler

We have experimentally demonstrated the ability to couple an arbitrary polarization state from a fiber to the TE-mode of a single waveguide in an integrated silicon photonics circuit with an extinction ratio larger than 31 dB, measured between the output ports of the integrated photonic circuit. To achieve this we combined a 2D- grating coupler and a Mach-Zehnder Interferometer (MZI). After accounting for setup and coupling losses, for a 1 mW input into the 2D coupler, we obtain an average output power of 0.98 mW at the desired waveguide, with less than 1.2 dB variation across all input polarization states. The experiments were performed at a wavelength of 1.55 μm.

J. Niklas Caspers

Papers

Compact hybrid plasmonic polarization rotator

Experimental demonstration of an integrated hybrid plasmonic polarization rotator

Compact low loss and broadband hybrid plasmonic directional coupler

Measurement of a compact colorless 3 dB hybrid plasmonic directional coupler

Active polarization independent coupling to silicon photonics circuit