Coherent O-band WDM transmission of DP-16QAM over a 50-km BDFA-amplified link.

We present wavelength-division multiplexed coherent transmission in an O-band amplified link enabled by bismuth-doped fiber amplifiers (BDFAs). Transmission of 4 × 25 GBd DP-16QAM (4 × 200 Gb/s) is demonstrated over a single span of 50-km length, occupying a bandwidth of 4.7 THz across the wavelengths 1323 nm to 1351 nm.

Experimental investigation of BDFA-based O-band direct-detection transmission using an optical recirculating loop.

We implemented a bismuth-doped fiber amplifier (BDFA) based optical recirculating loop to investigate the performance of amplified O-band transmission over appreciable distances. Both single-wavelength and wavelength-division multiplexed (WDM) transmission were studied, with a variety of direct-detection modulation formats. We report on (a) transmission over lengths of up to 550 km in a single-channel 50-Gb/s system operating at wavelengths ranging from 1325 nm to 1350 nm, and (b) rate-reach products up to 57.6 Tb/s-km (after accounting for the forward error correction redundancy) in a 3-channel system.

Quantitative study of birefringence effects in fiber-based orthogonal-pump FWM systems.

Optical fibers have unwanted residual birefringence due to imperfections in fabrication processes and environmental conditions. This birefringence will randomize the state of polarization of propagating signals and may harm the performance of four-wave mixing based processing devices. Here, we present a quantitative study of the effects of birefringence in orthogonal-pump four-wave mixing systems, and identify different regions of operation of the optical fiber, mainly determined by the relative magnitude between the physical length L and beat length Lb. This finding clarifies the characteristics of the complex interplay between birefringence and four-wave mixing and advises appropriate fiber length selection for minimized polarization dependent gain.

Experimental demonstration of single-span 100-km O-band 4×50-Gb/s CWDM direct-detection transmission.

We report on what is to the best of our knowledge the longest 50-Gb/s/λ O-band wavelength-division multiplexed (WDM) transmission. A pair of in-house built bismuth-doped fiber amplifiers (BDFAs) and the use of Kramers-Kronig detection-assisted single-sideband transmission are adopted to overcome the fiber loss and chromatic dispersion, respectively, in a reach-extended O-band coarse WDM (CWDM) system with a channel spacing of ∼10 nm. Through experiments on an amplified 4×50-Gb/s/λ direct-detection system based on booster and pre-amp BDFAs, we show the superior performance of single-sideband transmission in terms of both optical signal-to-noise ratio sensitivity and uniformity in performance amongst CWDM channels relative to double-sideband transmission after both 75-km and 100-km lengths of single-mode fiber. As a result, up to 100-km reach with comparable performance at all 50-Gb/s channels was achieved without the need for in-line optical amplification.

Highly efficient dual-level grating couplers for silicon nitride photonics.

We propose and numerically demonstrate a versatile strategy that allows designing highly efficient dual-level grating couplers in different silicon nitride-based photonic platforms. The proposed technique, which can generally be applied to an arbitrary silicon nitride film thickness, is based on the simultaneous optimization of two grating coupler levels to obtain high directionality and grating-fibre mode matching at the same time. This is achieved thanks to the use of two different linear apodizations, with opposite signs, applied to the two grating levels, whose design parameters are determined by using a particle swarm optimization method. Numerical simulations were carried out considering different silicon nitride platforms with 150, 300, 400 and 500 nm thicknesses and initially employing silicon as the material for the top level grating coupler. The use of Si-rich silicon nitride with a refractive index in the range 2.7-3.3 for the top layer material enabled to obtain similar performance (coupling efficiency exceeding - 0.45 dB for the 400 nm thick silicon nitride platform) with relaxed fabrication tolerances. To the best of our knowledge, these numerical results represent the best performance ever reported in the literature for silicon nitride grating couplers without the use of any back-reflector.

Polarization Control in Integrated Silicon Waveguides Using Semiconductor Nanowires.

In this work, we show the design of a silicon photonic-based polarization converting device based on the integration of semiconduction InP nanowires on the silicon photonic platform. We present a comprehensive numerical analysis showing that full polarization conversion (from quasi-TE modes to quasi-TM modes, and vice versa) can be achieved in devices exhibiting small footprints (total device lengths below 20 µm) with minimal power loss (<2 dB). The approach described in this work can pave the way to the realization of complex and re-configurable photonic processors based on the manipulation of the state of polarization of guided light beams.

A Review of Capabilities and Scope for Hybrid Integration Offered by Silicon-Nitride-Based Photonic Integrated Circuits.

In this review we present some of the recent advances in the field of silicon nitride photonic integrated circuits. The review focuses on the material deposition techniques currently available, illustrating the capabilities of each technique. The review then expands on the functionalisation of the platform to achieve nonlinear processing, optical modulation, nonvolatile optical memories and integration with III-V materials to obtain lasing or gain capabilities.

Demonstration of >1Tbit/s WDM OWC with wavelength-transparent beam tracking-and-steering capability.

Beam tracking-and-steering is crucial for the operation of high-speed, narrow beam, optical wireless communication (OWC) systems. Using a system based on two sets of low-cost cameras for continuous beam tracking and a set of mirrors for steering, we demonstrate here a high-capacity (>1Tbit/s) ten-channel wavelength-division multiplexed (WDM) OWC system based on discrete multitone transmission. The results, which are achieved over a 3.5-m perpendicular distance and across a lateral coverage up to 1.8 m, constitute to the best of our knowledge, the highest aggregate OWC capacity at this coverage.

Numerical and experimental study on the impact of chromatic dispersion on O-band direct-detection transmission.

The recent emergence of efficient O-band amplification technologies has enabled the consideration of O-band transmission beyond short reach. Despite the O-band being a low chromatic dispersion (CD) window, the impact of CD will become increasingly significant when extending the reach of direct-detection (DD) systems. In this work, we first numerically investigate the 3-dB bandwidth of single-mode fibers (SMF) and the CD-restricted transmission reach in intensity-modulation DD systems, confirming the significant difference between low- and high-dispersion O-band wavelengths. We then carry out experimental transmission studies over SMF for distances of up to 70 km at two different wavelengths, the low-dispersion 1320 nm and the more dispersive 1360 nm, enabled by the use of an O-band bismuth-doped fiber amplifier as a preamplifier at the receiver. We compare three 50-Gb/s optical DD formats, namely, Nyquist on-off keying (OOK), Nyquist 4-ary pulse amplitude modulation (PAM4) and Kramers-Kronig detection-assisted single-sideband quadrature phase shift keying (KK-QPSK) half-cycle subcarrier modulation. Our results show that at both wavelengths, OOK and QPSK exhibit better bit error rate performance than PAM4. When transmitting over 70-km of SMF at the less dispersive wavelength of 1320 nm, 50-Gb/s OOK modulation offers more than 1.5-dB optical power sensitivity improvement at the photodiode (PD) compared to 50-Gb/s QPSK. Conversely, at 1360 nm, the required optical power to the PD can be reduced by more than 3 dB by using QPSK instead of OOK.

Experimental characterization of an o-band bismuth-doped fiber amplifier.

The recent emergence of bismuth-doped fiber amplifiers (BDFAs) offers the potential to transmit high-speed WDM signals over long distances in the O-band spectral region, thereby greatly enhancing the scope of systems utilizing these wavelengths. In this paper, we present a comprehensive experimental study on several basic characteristics of an O-band BDFA based on a phosphosilicate optical fiber, including the frequency-dependent noise figure, gain tilt (static and dynamic), transient response, and polarization dependent gain. We discuss our findings and their implications on the use of BDFA technology in high bit-rate multichannel systems.

Supercontinuum generation in tantalum pentoxide waveguides for pump wavelengths in the 900 nm to 1500 nm spectral region.

We characterize the spectral broadening performance in silica clad and unclad Tantalum pentoxide (Ta2O5) waveguides as a function of the input pulse central wavelength and polarization, sweeping over a wavelength range from 900 nm to 1500 nm, with an average incident power of 110 mW. The waveguides are 0.7 µm high and between 2.2 and 3.2 µm wide, and the SiO2 top cladding layer is 2 µm thick. We model the dispersion of the higher order spatial modes, and use numerical simulations based on the generalized nonlinear Schrödinger equation to analyze the nonlinear behaviour of the spatial modes within the waveguides as well as the dispersive effects observed in the experiments. We achieve octave spanning supercontinuum with an average power of 175 mW incident on the waveguide at 1000 nm pump wavelength.

Co-design of a differential transimpedance amplifier and balanced photodetector for a sub-pJ/bit silicon photonics receiver.

This paper presents the design and implementation of a fully differential optical receiver, which is aimed for short reach intensity modulation and direct detection (IMDD) transceiver links. A Si-Ge balanced photodetector (PD) has been co-designed and packaged with a novel differential transimpedance amplifier (TIA). The TIA design is realized with a standard 28 nm CMOS process and operates with a standard digital supply (1V). Without using any equalization or DSP techniques, the proposed receiver can operate up to 54 Gb/s with a BER less than the KP4 limit (2.2×10-4) under an optical modulation amplitude (OMA) of -8.6 dBm, while the power efficiency has been optimized to 0.55 pJ/bit (0.98 pJ/bit if output buffer is included).

Selective wavelength conversion in a few-mode fiber.

We experimentally demonstrate a means to selectively enhance wavelength conversion of WDM channels on a 100 GHz grid exploiting nonlinear effects between the spatial modes of a few mode fiber. The selectivity of parametric gain is obtained by dispersion design of the fiber such that the inverse group velocity curves of the participating modes are parallel and their dispersion is suitably large. We describe both theoretically and experimentally the observed dependence of the idler gain profile on pump mode (quasi) degeneracy.

Si-rich Si nitride waveguides for optical transmissions and toward wavelength conversion around 2 µm.

We show that subwavelength Si-rich nitride waveguides efficiently sustain high-speed transmissions at 2 µm. We report the transmission of a 10 Gbit/s signal over 3.5 cm with negligible power penalty. Parametric conversion in the pulsed pump regime is also demonstrated using the same waveguide structure with an efficiency as high as -18 dB.

Frequency comb generation in a silicon ring resonator modulator.

We report on the generation of an optical comb of highly uniform in power frequency lines (variation less than 0.7 dB) using a silicon ring resonator modulator. A characterization involving the measurement of the complex transfer function of the ring is presented and five frequency tones with a 10-GHz spacing are produced using a dual-frequency electrical input at 10 and 20 GHz. A comb shape comparison is conducted for different modulator bias voltages, indicating optimum operation at a small forward-bias voltage. A time-domain measurement confirmed that the comb signal was highly coherent, forming 20.3-ps-long pulses.

Fibre-optic metadevice for all-optical signal modulation based on coherent absorption.

Recently, coherent control of the optical response of thin films in standing waves has attracted considerable attention, ranging from applications in excitation-selective spectroscopy and nonlinear optics to all-optical image processing. Here, we show that integration of metamaterial and optical fibre technologies allows the use of coherently controlled absorption in a fully fiberized and packaged switching metadevice. With this metadevice, which controls light with light in a nanoscale plasmonic metamaterial film on an optical fibre tip, we provide proof-of-principle demonstrations of logical functions XOR, NOT and AND that are performed within a coherent fibre network at wavelengths between 1530 and 1565 nm. The metadevice has been tested at up to 40 gigabits per second and sub-milliwatt power levels. Since coherent absorption can operate at the single-photon level and with 100 THz bandwidth, we argue that the demonstrated all-optical switch concept has potential applications in coherent and quantum information networks.

High-efficiency grating-couplers: demonstration of a new design strategy.

We present a simple and practical strategy that allows to design high-efficiency grating couplers. The technique is based on the simultaneous apodization of two structural parameters: the grating period and the fill-factor, along with the optimization of the grating coupler etching depth. Considering a 260 nm Si-thick Silicon-on-insulator platform, we numerically demonstrated a coupling efficiency of -0.8 dB (83%), well matching the experimental value of -0.9 dB (81%). Thanks to the optimized design, these results represent the best performance ever reported in the literature for SOI structures without the use of any back-reflector.

Experimental comparison of direct detection Nyquist SSB transmission based on silicon dual-drive and IQ Mach-Zehnder modulators with electrical packaging.

We have designed and fabricated a silicon photonic in-phase-quadrature (IQ) modulator based on a nested dual-drive Mach-Zehnder structure incorporating electrical packaging. We have assessed its use for generating Nyquist-shaped single sideband (SSB) signals by operating it either as an IQ Mach-Zehnder modulator (IQ-MZM) or using just a single branch of the dual-drive Mach-Zehnder modulator (DD-MZM). The impact of electrical packaging on the modulator bandwidth is also analyzed. We demonstrate 40 Gb/s (10Gbaud) 16-ary quadrature amplitude modulation (16-QAM) Nyquist-shaped SSB transmission over 160 km standard single mode fiber (SSMF). Without using any chromatic dispersion compensation, the bit error rates (BERs) of 5.4 × 10-4 and 9.0 × 10-5 were measured for the DD-MZM and IQ-MZM, respectively, far below the 7% hard-decision forward error correction threshold. The performance difference between IQ-MZM and DD-MZM is most likely due to the non-ideal electrical packaging. Our work is the first experimental comparison between silicon IQ-MZM and silicon DD-MZM in generating SSB signals. We also demonstrate 50 Gb/s (12.5Gbaud) 16-QAM Nyquist-shaped SSB transmission over 320 km SSMF with a BER of 2.7 × 10-3. Both the silicon IQ-MZM and the DD-MZM show potential for optical transmission at metro scale and for data center interconnection.

Si-rich Silicon Nitride for Nonlinear Signal Processing Applications.

Nonlinear silicon photonic devices have attracted considerable attention thanks to their ability to show large third-order nonlinear effects at moderate power levels allowing for all-optical signal processing functionalities in miniaturized components. Although significant efforts have been made and many nonlinear optical functions have already been demonstrated in this platform, the performance of nonlinear silicon photonic devices remains fundamentally limited at the telecom wavelength region due to the two photon absorption (TPA) and related effects. In this work, we propose an alternative CMOS-compatible platform, based on silicon-rich silicon nitride that can overcome this limitation. By carefully selecting the material deposition parameters, we show that both of the device linear and nonlinear properties can be tuned in order to exhibit the desired behaviour at the selected wavelength region. A rigorous and systematic fabrication and characterization campaign of different material compositions is presented, enabling us to demonstrate TPA-free CMOS-compatible waveguides with low linear loss (~1.5 dB/cm) and enhanced Kerr nonlinear response (Re{γ} = 16 Wm-1). Thanks to these properties, our nonlinear waveguides are able to produce a π nonlinear phase shift, paving the way for the development of practical devices for future optical communication applications.

Minimizing inter-channel cross-phase modulation with optical phase conjugation in asymmetric fibre links.

Using analytic and numerical modelling of fibre transmission systems that employ optical phase conjugation (OPC), we show inter-channel cross-phase modulation depends on the integrated square error between nonlinear profiles before and after OPC and that arranging amplifiers and tuning power levels is crucial to minimizing noise. We derive modulation transparent formulas for phase noise and optimal power settings. Examples are shown for 16 and 64 quadrature amplitude modulation.