Diffraction characteristics of general dielectric planar (slab) gratings and surface-relief (corrugated) gratings are reviewed. Applications to laser-beam deflection, guidance, modulation, coupling, filtering, wavefront reconstruction, and distributed feedback in the fields of acoustooptics, integrated optics, holography, and spectral analysis are discussed. An exact formulation of the grating diffraction problem without approximations (rigorous coupled-wave theory developed by the authors) is presented. The method of solution is in terms of state variables and this is presented in detail. Then, using a series of fundamental assumptions, this rigorous theory is shown to reduce to the various existing approximate theories in the appropriate limits. The effects of these fundamental assumptions in the approximate theories are quantified and discussed.

Analysis and applications of optical diffraction by gratings

Silicon photonics research can be dated back to the 1980s. However, the previous decade has witnessed an explosive growth in the field. Silicon photonics is a disruptive technology that is poised to revolutionize a number of application areas, for example, data centers, high-performance computing and sensing. The key driving force behind silicon photonics is the ability to use CMOS-like fabrication resulting in high-volume production at low cost. This is a key enabling factor for bringing photonics to a range of technology areas where the costs of implementation using traditional photonic elements such as those used for the telecommunications industry would be prohibitive. Silicon does however have a number of shortcomings as a photonic material. In its basic form it is not an ideal material in which to produce light sources, optical modulators or photodetectors for example. A wealth of research effort from both academia and industry in recent years has fueled the demonstration of multiple solutions to these and other problems, and as time progresses new approaches are increasingly being conceived. It is clear that silicon photonics has a bright future. However, with a growing number of approaches available, what will the silicon photonic integrated circuit of the future look like? This roadmap on silicon photonics delves into the different technology and application areas of the field giving an insight into the state-of-the-art as well as current and future challenges faced by researchers worldwide. Contributions authored by experts from both industry and academia provide an overview and outlook for the silicon waveguide platform, optical sources, optical modulators, photodetectors, integration approaches, packaging, applications of silicon photonics and approaches required to satisfy applications at mid-infrared wavelengths. Advances in science and technology required to meet challenges faced by the field in each of these areas are also addressed together with predictions of where the field is destined to reach.

https://iopscience.iop.org/article/10.1088/2040-8978/18/7/073003/pdf

Roadmap on silicon photonics

Silicon-based large-scale photonic integrated circuits are becoming important, due to the need for higher complexity and lower cost for optical transmitters, receivers and optical buffers. In this paper, passive technologies for large-scale photonic integrated circuits are described, including polarization handling, light non-reciprocity and loss reduction. The design rule for polarization beam splitters based on asymmetrical directional couplers is summarized and several novel designs for ultra-short polarization beam splitters are reviewed. A novel concept for realizing a polarization splitter–rotator is presented with a very simple fabrication process. Realization of silicon-based light non-reciprocity devices (e.g., optical isolator), which is very important for transmitters to avoid sensitivity to reflections, is also demonstrated with the help of magneto-optical material by the bonding technology. Low-loss waveguides are another important technology for large-scale photonic integrated circuits. Ultra-low loss optical waveguides are achieved by designing a Si3N4 core with a very high aspect ratio. The loss is reduced further to <0.1 dB m−1 with an improved fabrication process incorporating a high-quality thermal oxide upper cladding by means of wafer bonding. With the developed ultra-low loss Si3N4 optical waveguides, some devices are also demonstrated, including ultra-high-Q ring resonators, low-loss arrayed-waveguide grating (de)multiplexers, and high-extinction-ratio polarizers. Newly developed photonic components could allow large-scale photonic integrated circuits to be built on silicon substrates. Daoxin Dai from Zhejiang University, China, alongside co-workers from the University of California, USA, have proposed several new optical technologies for use in photonic integrated circuits, which substitute or work alongside electrical circuits in optical devices. The researchers have designed new ultrashort polarization-handling devices that split high-intensity beams of light, and a ring optical isolator that reduces reflections. The team have also created a new waveguide based on silicon nitride that can guide optical waves with a minimal loss of energy. These new technologies will allow scientists to construct higher performance, more compact optical devices.

/pdf/passive-technologies-for-future-large-scale-photonic-4doamz9zqi.pdf

Passive technologies for future large-scale photonic integrated circuits on silicon: polarization handling, light non-reciprocity and loss reduction

A novel concept for an ultracompact polarization splitter-rotator is proposed by utilizing a structure combining an adiabatic taper and an asymmetrical directional coupler. The adiabatic taper structure is singlemode at the input end while it becomes multimode at the other end. When light propagates along the adiabatic taper structure, the TM fundamental mode launched at the narrow end is efficiently (close to 100%) converted to the first higher-order TE mode at the wide end because of the mode coupling between them. By using an asymmetrical directional coupler that has two adjacent waveguides with different core widths, the first higher-order TE mode is then coupled to the TE fundamental mode of the adjacent narrow waveguide. On the other hand, the input TE polarization does not change when it goes through the adiabatic taper structure. In the region of the asymmetrical directional coupler, the TE fundamental mode in the wide waveguide is not coupled to the adjacent narrow waveguide because of phase mismatch. In this way, TE- and TM- polarized light are separated while the TM fundamental mode is also converted into the TE fundamental mode. A design example of the proposed polarization splitter-rotator is given by using silicon-on-insulator nanowires and the total length of the device is less than 100μm. Furthermore, only a one-mask process is needed for the fabrication process, which is compatible with the standard fabrication for the regular photonic integrated circuits based on SOI nanowires.

/pdf/novel-concept-for-ultracompact-polarization-splitter-rotator-nl8wj0ny4k.pdf

Novel concept for ultracompact polarization splitter-rotator based on silicon nanowires

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

A simple and general set of design rules for slanted-angle polarization-rotating waveguides is presented in this paper. These design rules are employed to construct single-mode SOI polarization rotators that offer significant advantages in conversion efficiency, optical loss, fabrication tolerance, spectral response and spatial dimensions relative to III-V components.

Design rules for slanted-angle polarization rotators

We describe, model and demonstrate a tunable micro-ring resonator integrated monolithically with a photodiode in a silicon waveguide device. The photodiode is made sensitive to wavelengths at and around 1550nm via the introduction of lattice damage through selective ion implantation. The ring resonator enhances detector responsivity in a 60 μm long waveguide photodiode such that it is 0.14 A/W at −10Vbias with less than 0.2 nA leakage current. The device is tunable such that resonance (and thus detection) can be achieved at any wavelength from 1510 – 1600 nm. We also demonstrate use of the device as a digital switch with integrated power monitoring, 20 dB extinction, and no optical power tapped from the output path to the photodiode. A theoretical description suggests that for a critically coupled resonator where the round trip loss is dominated by the excess defects used to mediate detection, the maximum responsivity is independent of device length. This leads to the possibility of extremely small detector geometries in silicon photonics with no requirement for the use of III-V materials or germanium.

Silicon photonic resonator-enhanced defect-mediated photodiode for sub-bandgap detection

A new compound, tris(dimethylamino)silane was used as an organosilicon source for the deposition of silicon oxynitride thin films. The depositions were carried out at low substrate temperatures (<150 °C) in an electron cyclotron resonance plasma enhanced chemical vapor deposition reactor. Films with compositions varying from Si3N4 to SiO2 were deposited on silicon substrates by varying the N2/O2 flow ratio to the plasma chamber. In situ ellipsometry measurements of the film optical index were well correlated with film composition. Auger electron spectroscopy showed that only low levels of carbon (<3 at. %) were present, while Fourier transform infrared spectroscopy showed low levels of bonded hydrogen. The deposition rate of high quality Si3N4 was as high as 220 A/min.

Electron cyclotron resonance chemical vapor deposition of silicon oxynitrides using tris(dimethylamino)silane

We describe the fabrication and characterization of a silicon waveguide resonant photodetector compatible with the optical-to-electrical conversion of wavelengths at, or around, 1550 nm. Sub-band responsivity is provided through the introduction of defects via inert self-implantation and subsequent annealing. The detector is located within a 20- m radius silicon microring resonator. An 18-dB resonant enhancement in absorption coefficient and 12-dB enhancement in photocurrent were measured, leading to a resonant responsivity of approximately 39 mA/W at 20-V reverse bias.

Defect-Enhanced Silicon-on-Insulator Waveguide Resonant Photodetector With High Sensitivity at 1.55 $\mu$ m

Recent attention has been attracted by photo-detectors integrated onto silicon-on-insulator (SOI) waveguides that exploit the enhanced sensitivity to subbandgap wavelengths resulting from absorption via point defects introduced by ion implantation. In this paper, we present the first model to describe the carrier generation process of such detectors, based upon modified Shockley-Read-Hall generation/recombination, and, thus, determine the influence of the device design on detection efficiency. We further describe how the model may be incorporated into commercial software, which then simulates the performance of previously reported devices by assuming a single midgap defect level (with properties commensurate with the single negatively charged divacancy). We describe the ability of the model to highlight the major limitations to responsivity, and thus suggest improvements which diminish the impact of such limitations.

Paul E. Jessop

Papers

Design rules for slanted-angle polarization rotators

Silicon photonic resonator-enhanced defect-mediated photodiode for sub-bandgap detection

Electron cyclotron resonance chemical vapor deposition of silicon oxynitrides using tris(dimethylamino)silane

Defect-Enhanced Silicon-on-Insulator Waveguide Resonant Photodetector With High Sensitivity at 1.55 $\mu$ m

Modeling Defect Enhanced Detection at 1550 nm in Integrated Silicon Waveguide Photodetectors