dB Extinction Ratio Research Articles

Silicon-based dual-mode polarization beam splitter for hybrid mode/polarization-division-multiplexed systems

This paper presents a compact dual-mode polarization beam splitter (PBS) based on silicon on insulator platform. The proposed PBS separates two lowest-order transverse magnetic (TM) modes from two lowest-order transverse electric (TE) ones by employing an optimized tapered directional coupler. To the best of our knowledge, this is the first PBS with the ability of separating two sets of modes with different polarizations, simultaneously. Such a component is an essential block in the hybrid mode/polarization-multiplexing systems. The three-dimensional finite-difference time-domain (FDTD) simulation results confirm that the designed PBS has a low loss of 1.6 dB and high extinction ratio of more than 10 dB over a broad wavelength range of 90 nm. Also, the presented PBS tolerates ±15 nm change in the coupling gap, ±30 nm waveguide width deflection, and more than ±100 nm coupling length variation with an insertion loss lower than 1 dB and extinction ratio higher than 10 dB.

Thermo-optic properties of silicon-rich silicon nitride for on-chip applications.

We demonstrate the thermo-optic properties of silicon-rich silicon nitride (SRN) films deposited using plasma-enhanced chemical vapor deposition (PECVD). Shifts in the spectral response of Mach-Zehnder interferometers (MZIs) as a function of temperature were used to characterize the thermo-optic coefficients of silicon nitride films with varying silicon contents. A clear relation is demonstrated between the silicon content and the exhibited thermo-optic coefficient in silicon nitride films, with the highest achievable coefficient being as high as (1.65±0.08) ×10-4 K-1. Furthermore, we realize an SRN multi-mode interferometer (MMI) based thermo-optic switch with over 20 dB extinction ratio and total power consumption for two-port switching of 50 mW.

Electro-Absorption Optical Modulator Based on Graphene-Buried Polymer Waveguides

By exploiting the electro-absorption characteristic of graphene, we proposed and theoretically demonstrated an electro-absorption optical modulator (EOM) based on the graphene-buried polymer waveguides. By burying the graphene layers inside the polymer waveguide, acquiring a strong graphene-light interaction when the graphene layers buried in the center of core, which bring about an important change of the optical mode and effective mode index in the waveguide. The width of the graphene capacitor was also optimized to enhance the bandwidth of the device. Calculations show that the proposed EOM can realize 29 dB extinction ratio, 7 dB modulation depth and 42 GHz bandwidth with an 800 μm-long active region. The required switching voltage form its minimum to maximum absorption state is 1.6 V and 1.55 pJ/bit energy consumption can be achieved. The proposed modulator can remedy the lack of high-speed optical modulator on the passive polymer waveguide and have a potential application for polymer-based photonic integrated circuits.

High speed graphene-silicon electro-absorption modulators for the O-band and C-band

In the past few years, graphene has drawn interest for applications in optoelectronic devices. Due to its extraordinary properties, i.e. wide optical bandwidth, tunable absorption, high carrier mobility, and CMOS compatibility, it is a candidate to improve current state-of-the-art high-speed optoelectronic devices, such as modulators. In this work, we present a model that describes the DC and high-speed behaviour of single-layer graphene-oxide-silicon electro-absorption modulators (EAM). We compare the theoretical analysis with experimental results, and we find that p-doped graphene combined with p-doped silicon enables high-speed operation at low DC bias. Using this configuration, we demonstrate 75 μm long TM EAMs operating in the O-band and in the C-band. The O-band EAM exhibits 3.1 dB extinction ratio (ER) and 16.0 GHz 3 dB bandwidth at 1 V DC bias. With the C-band EAM we achieve 6.5 dB ER and 14.2 GHz 3 dB bandwidth at 0 V DC bias. Open eye diagrams up to 50 Gbit s−1 are measured using 2.5 Vpp and −0.5 V DC bias at a wavelength of 1560 nm.

Design of Hybrid Plasmonic Multi-Quantum-Well Electro-Reflective Modulators Towards <100 fJ/bit Photonic Links

Realization of on-board and inter-chip optical interconnects requires a photonic data link with power consumption well below their electrical counterparts (i.e., 50 Gb/s requires 2–4 pJ/bit/channel. External reverse-biased modulators could drastically reduce this power consumption. Here we design ultralow power GaAs/AlGaAs multi quantum well electro-reflective modulators operating at 1 V for facile integration with polymer “optical bridges”, utilizing coupled quantum confined Stark effect between adjacent quantum wells and optical coupling to hybrid surface plasmon-slab modes for significantly enhanced extinction ratio and spectral bandwidth. Distinctive from conventional electro-optical or electro-absorption modulators, this new design synergistically leverages ultra-large changes in both refractive index (|Δn|∼0.05) and absorption coefficient (Δα∼104 cm−1), achieving 35-50 dB extinction ratio at 1 V reverse bias with a low insertion loss of 1–3 dB, an incident angle tolerance of ∼5°, and a spectral bandwidth of 7–10 nm. The modulator power consumption is ∼1.9 fJ/bit without the need of thermal tuning, and the RC-limited bandwidth well exceeds 100 GHz. This new modulator enables high bandwidth and ultralow power optical interconnect networks at >100 Gb/s/channel and <100 fJ/bit/channel compatible with ever-scaling CMOS technologies.

Design of silicon waveguide polarization beam splitter based on reversed Δβ directional coupler

We report a directional coupler type polarization beam splitter using silicon waveguides designed for 1550–1600 nm wavelengths. The device uses the coupling strength difference between the TE and TM modes. We use a reversed Δβ structure to achieve a wide operational wavelength range. The gap between the waveguides needs to be wide enough to attain a sufficient extinction ratio for the TE mode. Simulation results show that an over 100 nm operational 20 dB extinction ratio wavelength range can be attained at 1520–1620 nm wavelengths by this simple structure. The fabrication width tolerance exceeds 10 nm which is enough for foundry processes.

Broadband graphene/hexagonal boron nitride modulators based on a Si3N4 waveguide

For exploring a strong electro-optic effect from a graphene/hexagonal boron nitride (hBN) structure, here a comprehensive study for investigating the integrated broadband graphene/hBN modulators has been presented. Two representative configurations with a S i 3 N 4 waveguide were compared, focusing on optimizing the hBN thickness, waveguide width and height, and graphene size to balance their performances. It was found that 200 µm long devices with 65 nm hBN thickness allow for ∼ 80 G H z modulation bandwidth, 35.4 dB extinction ratio, and over 700 nm operation spectral range. Compared to the ridge S i 3 N 4 waveguide, the buried S i 3 N 4 waveguide was more suitable as an optical waveguide for the graphene/hBN modulator to obtain a large modulation depth of ∼ 0.17 d B µ m and a large operation spectral range from 750 to ∼ 1640 n m .

Time-resolved frequency chirp control of semiconductor lasers using digital signal processing

The theoretical possibility of precisely controlling the time-resolved chirp in a directly modulated semiconductor laser using a single drive digital-to-analog converter (DAC) controlled via a novel digital signal processing (DSP) algorithm is investigated. A semi-analytical expression for the requisite drive current is obtained through algebraic back-calculation of the large signal laser rate equations, in tandem with a first-order numerical approximation of the optical power and chirp functional relationship. A 25-GHz peak-to-peak frequency modulation (FM) at a bit-rate of 28-Gb/s is demonstrated at a 6 dB extinction ratio, while completely suppressing the transient chirp contribution. Furthermore, a bipolar frequency chirp signal resulting in a differential phase shift keying (DPSK) modulation format at a bit-rate of 28-Gb/s was achieved with an error vector magnitude (EVM) below 1%. A 5-bit look-up table (LUT) was found to be adequate for capturing the requisite DSP drive current needed to fully suppress the transient chirp as demonstrated by preliminary optical link simulations at 28-Gb/s.

Ultra compact Bragg grating devices with broadband selectivity.

Current silicon waveguide Bragg gratings typically introduce perturbation to the optical mode in the form of modulation of the waveguide width or cladding. However, since such a perturbation approach is limited to weak perturbations to avoid intolerable scattering loss and higher-order modal coupling, it is difficult to produce ultra-wide stopbands. In this Letter, we report an ultra-compact Bragg grating device with strong perturbations by etching nanoholes in the waveguide core to enable an ultra-large stopband with apodization achieved by proper location of the nanoholes. With this approach, a 15 µm long device can generate a stopband as wide as 110 nm that covers the entire ${\rm C} + {\rm L}$C+L band with a 40 dB extinction ratio and over a 10 dB sidelobe suppression ratio (SSR). Similar structures can be further optimized to achieve higher SSR of $ \gt {17}\;{\rm dB}$>17dB for a stopband of about 80 nm.

Reconfigurable nonlinear nonreciprocal transmission in a silicon photonic integrated circuit

We present a programmable silicon photonic integrated circuit (PIC) that can be configured to show nonlinear nonreciprocal transmission at high optical input power. Nonreciprocal transmission in PICs is of fundamental importance in various fields. Despite diverse approaches to generate nonreciprocal transmission, the research on efficient control of this effect is still scarce. The silicon PIC presented here has programmable linear and nonlinear behavior using integrated phase shifters. In the nonlinear regime (high optical power), the device can be configured to be either reciprocal or nonreciprocal between opposite propagation directions with over 30 dB extinction ratio and only 1.5 dB insertion loss. More importantly, the high/low transmission direction can be dynamically reconfigured. Furthermore, nonreciprocal transmission based on nonlinearities usually requires the optical field in both propagation directions to be high, in order to induce a large extinction ratio. For our circuit, only the forward-propagating light needs to have high power to enjoy low-loss transmission while the backward propagating light will always suffer a high rejection. Besides this nonreciprocal behavior, the circuit also offers the ability for all-optical functions, such as switching, optical compute gates, or optical flip-flops, thanks to its unique controllable nonlinear behavior. This work can trigger new research efforts in nonreciprocal photonics circuits.

Fully-Integrated Heterogeneous DML Transmitters for High-Performance Computing

Optical connectivity, which has been widely deployed in today's datacenters and high-performance computing (HPC) systems, is a disruptive technological revolution to the IT industry in the new Millennium. In our journey to debut an Exascale supercomputer, a completely new computing concept, called memory-driven computing, was innovated recently. This new computing architecture brings challenges and opportunities for novel optical interconnect solutions. Here, we first discuss our strategy to develop appropriate optical link solutions for different data traffic scenarios in memory-driven HPCs. Then, we present detailed review on recent work to demonstrate fully photonics-electronics-integrated single- and multi-wavelength directly modulated laser (DML) transmitters on silicon for the first time. Compact heterogeneous microring lasers and laser arrays were fabricated as photonic engines to work with a customized complementary metal-oxide semiconductor (CMOS) driver circuit. Microring lasers based on conventional quantum well and new quantum dot lasing medium were compared in the experiment. Thermal shunt and MOS capacitor structures were integrated into the lasers for effective thermal management and ultra low-energy tuning. It enables a controllable dense wavelength division multiplexing (DWDM) link architecture in an HPC environment. An equivalent microring laser circuit model was constructed to allow photonics-electronics co-simulation. Equalization functionality in the CMOS driver circuit proved to be critical to achieve up to 14 Gb/s direct modulation with 6 dB extinction ratio. Finally, the on-going and future work is discussed towards more robust, higher speed, and more energy efficient DML transmitters.

Low-voltage MEMS optical phase modulators and switches on a indium phosphide membrane on silicon

In this paper, an optical switch based on a microelectromechanical phase modulator is presented. Phase tuning is achieved by tuning the vertical gap between two vertically coupled waveguides through the application of a reverse bias on a p-i-n junction. An effective refractive index tuning Δneff of 0.03 and a phase shift of more than 3π rad at telecom wavelengths are measured with an on-chip Mach–Zehnder interferometer (MZI), with a phase-tuning length of only 140 μm. With a bias voltage of 5.1 V, a half-wave-voltage-length product (Vπ L) of 5.6 × 10−3 V·cm is achieved. Furthermore, optical crossbar switching in a MZI is demonstrated with a 15 dB extinction ratio using an actuation voltage of only 4.2 V. Our work provides a solution to on-chip, low-voltage phase modulation and optical switching. The switch is fabricated on an indium-phosphide membrane on a silicon substrate, which enables the integration with active components (e.g., amplifiers, lasers, and detectors) on a single chip.

Design, optimization, and performance evaluation of GSST clad low-loss non-volatile switches.

In this paper, we investigate the performance of a self-sustained ON-OFF switch incorporating a new phase change material ${\text{Ge}_2}{\text{Sb}_2}{\text{Se}_4}{\text{Te}_1}$Ge2Sb2Se4Te1 (GSST) with a silicon rib waveguide at the telecommunication wavelength 1.55 µm. A full-vectorial $\textbf{H}$H-field finite-element method is used to find the effective index and modal loss of the quasi-TE modes in the GSST-Si waveguide. Both the electro-refraction and electro-absorption type design are studied, and the effects of GSST thickness and Si slab thickness on the device performance are presented. These results suggest that a GSST-Si rib waveguide with a 500 nm wide Si core, 60-90 nm Si slab thickness, and 40-60 nm GSST layer could be a realistic switch design that can yield a very compact, 5 µm long device with only 0.135 dB total insertion loss and more than 20 dB extinction ratio.

Infrared frequency comb generation and spectroscopy with suspended silicon nanophotonic waveguides

Nanophotonic waveguides with sub-wavelength mode confinement and engineered dispersion profiles are an excellent platform for application-tailored nonlinear optical interactions at low pulse energies. Here, we present fully air clad suspended-silicon waveguides for infrared frequency comb generation with optical bandwidth limited only by the silicon transparency. The achieved spectra are lithographically tailored to span 2.1 octaves in the mid-infrared (2.0-8.5 um or 1170--5000 cm-1) when pumped at 3.10 um with 100 pJ pulses. Novel fork-shaped couplers provide efficient input coupling with only 1.5 dB loss. The coherence, brightness, and the stability of the generated light are highlighted in a dual frequency comb setup in which individual comb-lines are resolved with 30 dB extinction ratio and 100 MHz spacing in the wavelength range of 4.8-8.5 um (2100-1170 cm-1). These sources are used for broadband gas- and liquid-phase dual-comb spectroscopy with 100 MHz comb-line resolution. We achieve a peak spectral signal-to-noise ratio of 10 Hz^0.5 across a simultaneous bandwidth containing 112,200 comb-lines. These results provide a pathway to further integration with the developing high repetition rate frequency comb lasers for compact sensors with applications in chip-based chemical analysis and spectroscopy.

High fabrication tolerance and broadband silicon polarization beam splitter by point-symmetric cascaded fast quasiadiabatic couplers

We propose a high fabrication tolerance and broadband silicon polarization beam splitter (PBS) by cascading point-symmetric 3-dB couplers designed using fast quasiadiabatic dynamics (FAQUAD). The FAQUAD strategy only requires access to a single control parameter, and it can shorten the conventional adiabatic designs, maintaining the broadband characteristic and robustness to fabrication errors. The 3-dB FAQUAD coupler for the TM0 mode is made invisible to the TE0 mode due to a large difference in their adiabaticity parameters. Moreover, the system of cascaded point-symmetric devices further enhances the bandwidth and fabrication tolerance of the FAQUAD PBS. The total length of the FAQUAD PBS is 89.4 µm. We find that the FAQUAD PBS exhibits > 10 dB extinction ratio (ER) for the input TM0 mode and the input TE0 mode over a bandwidth of 240 nm and 260 nm, with excess losses below 0.4 dB and 0.021 dB, respectively. The PBS also exhibits excellent fabrication tolerance, showing an ER > 24 dB and an excess loss 16.8 dB and an excess loss 15.3 dB and excess loss 15.9 dB and excess loss < 0.015 dB for the TE0 mode are observed.

High-performance InP-based Mach-Zehnder polarization beam splitter with a 19 dB extinction ratio across C-band.

A high-performance InP-based polarization beam splitter (PBS) using a symmetrical Mach-Zehnder interferometer is experimentally demonstrated. The waveguides are aligned along the [011] direction, which results in a small reverse bias required and easy adjustment to realize the PBS. The experimental results indicate that the polarization extinction ratio (PER) is over 19dB in the wavelength range from 1525 to 1570nm with one arm injected with a 4.32mA current and the other arm reversed biased at 5.14V simultaneously. The PER is measured to remain above 18dB in the same wavelength range even when the width varies by ±200 nm.

CMOS-compatible hybrid bi-metallic TE/TM-pass polarizers based on ITO and ZrN.

Transverse-magnetic (TM) and transverse-electric (TE) pass polarizers based on a silicon-on-insulator platform are studied and analyzed. The proposed structures are CMOS-compatible based on indium tin oxide and zirconium nitride as alternative plasmonic materials. The bi-metallic combination of the plasmonic materials exhibit large coupling between one of the modes (TE or TM) in the silicon core and the surface plasmon mode, while the other mode can propagate with low losses. The numerical simulations for the TE-pass polarizer predict 32.7dB extinction ratio (ER) and 0.13dB insertion loss (IL) at a compact device length of 1.5μm. Additionally, the TM-pass polarizer has 31.5dB ER and 0.17dB IL at a device length of 2μm at an operating wavelength of 1.55μm.

Coupling-enhanced dual ITO layer electro-absorption modulator in silicon photonics

Abstract Electro-optic signal modulation provides a key functionality in modern technology and information networks. Photonic integration has not only enabled miniaturizing photonic components, but also provided performance improvements due to co-design addressing both electrical and optical device rules. The millimeter to centimeter footprint of many foundry-ready electro-optic modulators, however, limits density scaling of on-chip photonic systems. To address these limitations, here we experimentally demonstrate a coupling-enhanced electro-absorption modulator by heterogeneously integrating a novel dual-gated indium-tin-oxide phase-shifting tunable absorber placed at a silicon directional coupler region. This concept allows utilizing the normally parasitic Kramers-Kronig relations here in an synergistic way resulting in a strong modulation depth to insertion loss ratio of about 1. Our experimental modulator shows a 2 dB extinction ratio for a just 4 μm short device at 4 V bias. Since no optical resonances are deployed, this device shows spectrally broadband operation as demonstrated here across the entire C-band. In conclusion, we demonstrate a modulator utilizing strong index change from both real and imaginary parts of active material enabling compact and high-performing modulators using semiconductor near-foundry materials.

High-Performance Silicon 2 × 2 Thermo-Optic Switch for the 2-$\mu$m Wavelength Band

The 2- $\mu$ m spectral window is emerging as a promising candidate for the next generation communication. We present a 2 × 2 Mach–Zehnder interferometric thermo-optic switch at 2- $\mu$ m waveband. The device is fabricated on a 220 nm thick silicon-on-insulator wafer with standard complementary metal oxide semiconductor (CMOS) process. We demonstrate an over 30 dB extinction ratio under the power consumption of 32.3 mW, with an average switching time of ∼15 μs. This proof of principle optical switch paves a way toward full silicon-based chip-scale interconnects in the 2- $\mu$ m waveband.

Demonstration of Q-factor enhancement in a mode splitting-based microdisk-coupled asymmetric Mach–Zehnder interferometer

We demonstrate significant enhancement of an effective Q-factor in a silica planar waveguide-based asymmetric Mach–Zehnder interferometer (AMZI) coupled with a microdisk. To enhance the Q-factor, which is required to more effectively measure the signal changes occurring during biochemical events, the structure utilizes resonance mode splitting. This phenomenon is observed through power cancellation at the same amplitude and a 180° phase difference of each path in the AMZI. In this study, we experimentally showed the effective Q-factor enhancement by comparing resonance characteristics of a fabricated microdisk, AMZI, and the proposed structure at equal fabrication conditions. We measured an effective Q-factor of 9.15 × 104 with a 22 dB extinction ratio.