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We study the design of ring core fibers (RCFs) supporting orbital angular momentum (OAM) modes for mode division multiplexing (MDM) transmission systems. We develop target criteria to optimize fiber designs using a particle swarm optimization (PSO) algorithm under fabrication constraints. Effective index separation, Δneff, and polarization purity of each OAM mode are known to determine modal crosstalk levels. To reduce the complexity of multiple-input multiple-output (MIMO) processing required to compensate for modal crosstalk, we define an objective function based on these quantities. Our design analysis focuses on four different concepts of step-index RCF leading to different modal and structural characteristics. The optimum design for each concept is derived using the PSO algorithm. We investigate the impact of hollow-core and/or higher-order radial modes on Δneff and polarization purity. Design strategies for increasing Δneff and polarization purity are discussed in light of robustness to fabrication errors. We finally discuss the scalability and potential limitations of this design.

We experimentally demonstrate a Bragg-grating-based modulator operating in reflection with low loss and without a circulator by using mode conversion and an asymmetric Y-branch. With a compact footprint of 240μm, we demonstrate OOK up to 30 Gbit/s with an estimated switching energy of 150 fJ/bit.

We propose a novel design of a silicon photonic modulator that has a high modulation efficiency and that is tolerant to temperature variations. A series of phase-shifted Bragg gratings are placed in each arm of a Mach-Zehnder interferometer in order to provide enhanced phase modulation. The slow light effect in these ultra-compact coupled resonators improves phase modulation efficiency compared to conventional silicon phase shifters. These Bragg grating cavities are designed such that the optical bandwidth is increased compared to other coupled resonators such as micro-rings. This improved bandwidth reduces the temperature sensitivity of the devices. We present in detail how to optimize these modulators considering properties such as modulation efficiency (Vπ×L), optical modulation amplitude (OMA), and optical bandwidth (λBW); the latter property determining the operating temperature range (T). As examples, we present two designs that meet different target specifications for short-reach or long-haul applications. We further provide a model, based on coupled mode theory, to investigate the dynamic response of the proposed modulators. A large signal analysis is performed using finite difference time domain (FDTD) in order to simulate on/off keying (OOK) modulation and eye diagrams up to 110 Gb/s.

Structural slow light is the dispersion engineering process by which the group velocity of light can be drastically reduced in a periodic waveguide structure. Enabling large group delay and enhancing the light-matter interaction on a subwave- length scale, on-chip slow light is of great interest in a vast array of fields such as non-linear optic, sensing, laser physics, telecommunication and computing. In this work, we experimen- tally demonstrate, for the first time, slow light in subwavelength grating waveguides on the silicon-on-insulator platform. We present a comprehensive numerical study in analytical modelling and 3D FDTD. Multiple waveguides variations were fabricated using an electron-beam lithography process. Figures of merit such as group index, bandwidth and loss-per-delay are examined in both theory and experiment. A maximum measured group index of 47.74 with a loss-per-delay of 103.37 dB/ns has been achieved near the wavelength of 1550 nm. A broad bandwidth of 8.82 nm was measured, in which the group index remains larger than 10. We also show that the region of slow light operation can be shifted over a large wavelength span by controlling a single design parameter.

Cladding-pumped optical fiber amplifiers are of increased interest in the context of space-division multiplexing but are known to suffer from low power efficiency. In this context, ytterbium (Yb) co-doping can be an attractive solution to improve the performance of erbium (Er) doped fiber amplifiers. We present a detailed direct comparison between Er/Yb-co-doping and Er-doping using numerical simulations validated by experimental results. Two double-cladding fibers, one doped with Er only and the other one co-doped with Er and Yb, were designed, fabricated and characterized. Using the experimentally extracted parameters, we simulate multi-core fiber amplifiers and investigate the interest of Er/Yb-co-doping. We calculate the minimum gain of the amplifiers over a 35-nm spectral window considering various scenarios.

We propose a weakly-coupled few-mode fiber requiring only 2×2 MIMO equalizer blocks, which makes it compatible with standard coherent receivers with polarization diversity. The fiber has a highly-elliptical core, surrounded by a depressed index trench in the cladding, and supports five spatial modes with twofold polarization degeneracy (ten channels). The fiber is designed to mitigate inter-modal crosstalk since the effective index difference between spatial modes is larger than ∼1×10−3 over the C-band. Through numerical simulations, we report on bending loss and other modal characteristics such as effective area and chromatic dispersion. Finally, we briefly discuss the scalability of the design.

Slow-light is experimentally demonstrated in subwavelength grating waveguides integrated on a silicon photonic chip. At the band-edge, a group index up to 30 is measured. We show that the band-edge wavelength varies linearly with the subwavelength grating period and can be shifted by thermal tuning.

We propose a weakly-coupled few-mode fiber requiring only 2×2 MIMO equalizer blocks, which makes it compatible with standard coherent receivers with polarization diversity. The fiber has a highly-elliptical core, surrounded by a depressed index trench in the cladding, and supports five spatial modes with twofold polarization degeneracy (ten channels). The fiber is designed to mitigate inter-modal crosstalk since the effective index difference between spatial modes is larger than ∼1×10−3 over the C-band. Through numerical simulations, we report on bending loss and other modal characteristics such as effective area and chromatic dispersion. Finally, we briefly discuss the scalability of the design.

Space-division multiplexing (SDM), whereby multiple spatial channels in multimode1 and multicore2 optical fibres are used to increase the total transmission capacity per fibre, is being investigated to avert a data capacity crunch3,4 and reduce the cost per transmitted bit. With the number of channels employed in SDM transmission experiments continuing to rise, there is a requirement for integrated SDM components that are scalable. Here, we demonstrate a cladding-pumped SDM erbium-doped fibre amplifier (EDFA) that consists of six uncoupled multimode erbium-doped cores. Each core supports three spatial modes, which enables the EDFA to amplify a total of 18 spatial channels (six cores × three modes) simultaneously with a single pump diode and a complexity similar to a single-mode EDFA. The amplifier delivers >20 dBm total output power per core and <7 dB noise figure over the C-band. This cladding-pumped EDFA enables combined space-division and wavelength-division multiplexed transmission over multiple multimode fibre spans.

Future super-computer interconnect systems and data centers request ultrahigh data rate links at low cost and power consumption, for which transmitters with high-level of integration and spectral efficient formats are key components. We report 60 Gb/s pulse-amplitude modulation (PAM-4) of an optical signal using a dual-microring silicon photonics circuit, making a low power, DAC-less PAM modulator. The power consumption is evaluated below 100 fJ/bit, including thermal adjustments. To the best of our knowledge, these results feature the lowest reported power consumption for PAM signaling in a DAC-less scheme for data rate beyond 40 Gb/s

The performance of on-chip gas sensors using absorption spectroscopy are currently limited by the small overlap and reduced interaction length between the light and the analyte. Here, the use of slow-light in subwavelength grating (SWG) waveguide integrated on a silicon photonic chip is proposed to improve methane sensing by tunable diode laser absorption spectroscopy in the near infrared. Such SWG waveguide increases the interaction by two means. First, close to the photonic bandgap edge, a SWG waveguide no longer acts as a metamaterial with a homogeneous index, but rather as a 1D photonic crystal in which slow-light effect enhances the light-analyte interaction. Second, the subwavelength segmentation of the waveguide increases the modal overlap with the air. These two enhancement mechanisms results in a six-fold improvement of the interaction with respect to strip waveguides. In this paper, we discuss how to engineer the group index of SWG waveguides to exploit slow-light effect for the first time. Design guidelines for minimizing propagation loss and disorder effect are discussed considering limitations of typical fabrication processes. SWG waveguides could improve the sensitivity and the limit of detection of on-chip trace-gas sensors that provide a compact, fabrication tolerant, inexpensive, and selective sensing technology.

In this work, we demonstrate the measurement of the Brillouin gain spectra of vector modes in a few-mode fiber for the first time using a simple heterodyne detection technique. A tunable long period fiber grating is used to selectively excite the vector modes supported by the few-mode fiber. Further, we demonstrate the non-destructive measurement of the absolute effective refractive indices (n eff ) of vector modes with ~10−4 accuracy based on the acquired Brillouin frequency shifts of the modes. The proposed technique represents a new tool for probing and controlling vector modes as well as modes carrying orbital angular momentum in optical fibers with potential applications in advanced optical communications and multi-parameter fiber-optic sensing.

In this paper, we report on a novel sensor for the contactless monitoring of the respiration rate, made from multi-material fibers arranged in the form of spiral antenna (2.45 GHz central frequency). High flexibility of the used composite metal-glass-polymer fibers permits their integration into a cotton t-shirt without compromising comfort or restricting movement of the user. At the same time, change of the antenna geometry, due to the chest expansion and the displacement of the air volume in the lungs, is found to cause a significant shift of the antenna operational frequency, thus allowing respiration detection. In contrast with many current solutions, respiration is detected without attachment of the electrodes of any kind to the user’s body, neither direct contact of the fiber with the skin is required. Respiration patterns for two male volunteers were recorded with the help of a sensor prototype integrated into standard cotton t-shirt in sitting, standing, and lying scenarios. The typical measured frequency shift for the deep and shallow breathing was found to be in the range 120–200 MHz and 10–15 MHz, respectively. The same spiral fiber antenna is also shown to be suitable for short-range wireless communication, thus allowing respiration data transmission, for example, via the Bluetooth protocol, to mobile handheld devices.

We present the design and characterization of a cladding-pumped amplifier with erbium doping located in an annular region near the core. This erbium-doped fiber is proposed to reduce gain saturation, leading to smaller gain compression when compared to uniform core doping. Through numerical simulations, we first compare the performance of three fibers with different erbium doping profiles in the core or the cladding. When the doped fibers are operated at the optimum length, results show that the smaller overlap of the signal mode field with the annular erbium doping region leads to higher gain and lower saturation of the amplifier. A single-core erbium-doped fiber with an annular doping and a D-shaped cladding was fabricated. Measurements demonstrate less than 4 dB of gain compression over the C-band for input power ranging from −40 dBm to 3 dBm. Small gain compression EDFAs are of interest for applications that require input channel reconfiguration. Higher gain and saturation output power are also key issues in cladding-pumped multi-core amplifiers.

An asymmetric sidewall grating allows to operate a Bragg modulator in reflection without circulator and with less than 1.5 dB on-chip loss. An asymmetric Y-branch directs the incident TE0 mode to the grating, while the reflected TE1 mode is guided to the drop port.

Ultrahigh-speed optical interconnects are essential to future cloud computing. Further increase in optical transmission speed has been hindered by power consumption and limited bandwidth resources, for which integrated optical transceivers using advanced modulation formats, such as pulse-amplitude modulation (PAM), are a promising solution. We report 80 Gb/s PAM operation of a silicon microring modulator (MRM) with an ultralow power consumption below 7 fJ/bit. We also report the first demonstration of PAM-8 modulation of MRMs in the Gb/s order, achieving error-free capability at 45 Gb/s, using 1 fJ/bit. To the best of our knowledge, these results feature the lowest power consumption, per transmitted bit, ever demonstrated at such high data rates. We further demonstrate PAM data transmission up to 64 Gb/s over 5 km. Simultaneous achievement of ultrafast modulation and ultralow power consumption is a critical step toward next-generation optical interconnects.

We propose an analytical time-domain model for microring and microdisk modulators, which considers both their electrical and optical properties. Theory of the dynamics of microring/microdisk is discussed, and general solutions to the transfer matrix representation are presented. Both static and dynamic predictions from the model are compared to measurement results to demonstrate the accuracy of our model. Static predictions and measurements are presented for power and phase responses, whereas dynamic predictions and measurements are presented for small-signal and large-signal operations. The model verifies that the chirping and modulation bandwidth of the modulators depend on the detuning state. Finally, the accuracy and scalability of several techniques employed in the model are discussed.