High-speed All-optical Signal Processing Research Articles

High-speed and high-contrast two-channel all-optical modulator based on solution-processed CdSe/ZnS quantum dots

Recently, all-optical modulators are potentially the most promising candidate to achieve high-bit rate modulation in high-speed all-optical communication technologies and signal processing. In this study, a two-channel all-optical modulator based on a solution-processed quantum dot structure is introduced for two sizes of quantum dots to operate at two wavelengths of MIR spectra (3 µm and 5 µm). To perform numerical and theoretical analysis and evaluate the optical behavior of the proposed all-optical modulator, the coupled rate and propagation equations have been solved by considering homogeneous and inhomogeneous broadening effects. The modulation depth at the 50 GHz frequency and 3 mW probe power is attained, about 94% for channel-1 with the wavelength of 559 nm at 300 Wcm−2 pump power density as well as approximately 83.5% for channel-2 with the wavelength of 619 nm at 500 Wcm−2 pump power density. The introduced two-channel all-optical modulator can operate simultaneously at two wavelengths during the modulation process in which information could be transmitted through both signals from the control light. This approach can present the practical device as a high-contrast and high-speed two-channel all-optical modulator with a high modulation depth in numerous applications such as thermal imaging in night vision cameras, wavelength de-multiplexing, signal processing, free-space communication.

Nanosecond-scale all-optical Ti3C2T x -MXene modulator

Inspired by recent advancements of graphene-based ultrafast photonic devices, Ti3C2T x -MXene, as a graphene-like two-dimensional layer-structure material, has gained extensive interest in nonlinear optical applications, especially in all-optical intensity modulation. Herein, we successfully fabricated an all-optical modulator based on Ti3C2T x -Mxene/polyvinyl-alcohol film with nanosecond-scale response time, modulation speed of 100 kHz, and modulation depth of 10%. Furthermore, the variation of fall edges of modulated signal pulse caused by thermo-optic effect under different pump power was also observed. Considering the ease of fabrication, low cost, and ease of integration, the proposed modulator may open the door for high-speed all-optical communications and signal processing.

Enhanced optical nonlinearity in a silicon–organic hybrid slot waveguide for all-optical signal processing

Silicon photonic integrated devices used for nonlinear optical signal processing play a key role in ultrafast switching, computing, and modern optical communications. However, current devices suffer from limited operation speeds and low conversion efficiencies due to the intrinsically low nonlinear index of silicon. In this paper, we experimentally demonstrate enhanced optical nonlinearity in a silicon–organic hybrid slot waveguide consisting of an ultranarrow slot waveguide coated with a highly nonlinear organic material. The fabricated slot area is as narrow as 45 nm, which is, to the best of our knowledge, the narrowest slot width that has been experimentally reported in silicon slot waveguides. The nonlinear coefficient of the proposed device with a length of 3 mm is measured to be up to 1.43 × 10 6 W − 1 km − 1 . Based on the nanostructure design, the conversion efficiencies of degenerate four-wave mixing showed enhancements of more than 12 dB and 5 dB compared to those measured for an identical device without the organic material and a silicon strip waveguide, respectively. As a proof of concept, all-optical canonical logic units based on the prepared device with two inputs at 40 Gb/s are analyzed. The obtained logic results showed clear temporal waveforms and wide-open eye diagrams with error-free performance, illustrating that our device has great potential for use in high-speed all-optical signal processing and high-performance computing in the nodes and terminals of optical networks.

All-optical high-speed modulation of THz transmission through silicon core optical fibers.

High speed optical modulation of THz radiation is of interest for information processing and communications applications. In this paper infrared femtosecond pulses are used to generate free carriers that reduce the THz transmission of silicon based waveguides over a broad spectral range. Up to 96% modulation is observed from 0.5 to 7 THz in an optical fiber with a 210 µm diameter gold-doped silicon core. The observed carrier recombination time of 2.0 ± 0.2 ns makes this material suitable for high speed all-optical signal processing. These results show both enhanced modulation depth and reduced carrier lifetime when compared to the performance of a high resistivity float zone silicon rectangular guide with comparable cross sectional area.

Error-free data transmission through fast broadband all-optical modulation in graphene–silicon optoelectronics

Ultrafast third order nonlinearity in silicon photonics is promising for all-optical signal processing beyond 100 Gbit/s. However, the coexistence of slower dynamical responses, such as mesoscopic Drude free-carrier plasma or thermal nonlinearities, degrades high-speed signal integrity. Here, we introduce atomic-layer graphene onto the silicon nanophotonic platform to leverage its unique nondegenerate two-photon absorption and ultrafast carrier dynamics, demonstrating broadband and fast bitrate transmission through cross-absorption modulation. Multichannel error-free optical transmission across telecommunication C- and L-bands is demonstrated with 10−12 bit-error rates at a data transmission rate of 1 Gbps. The bandwidth of the hybrid silicon–graphene structure is demonstrated at least up to 25 GHz. The broadband cross-absorption modulation over 640 nm in graphene–silicon optoelectronics provides a promising scalable material platform for high-speed all-optical signal processing.

Investigation of optical bistability in core-shell nanoparticles consisting of nonlocal core and magnetoplasmonic shell

We study the interaction of an electromagnetic field with a core-shell nanostructure and propose a nonlocal effective medium theory for a core-shell structure made of a nonlocal core and a magneto-optical shell cylinder embedded in the host medium. In addition to the appearance of optical bistability and tristability in this system, it is found that the threshold and window of bistability are strongly dependent on the nonlocality and magneto-optical properties of the core-shell nanoparticle, the size of the nanoparticle, and the incident wavelength. By fine modification of these parameters, it is possible to achieve maximal enhancement of optical nonlinearity from the proposed system. Our study offers a theoretical direction to further structure design and optimization and also applications in the high speed all-optical signal processing.

Bistable scattering in graphene-coated dielectric nanowires.

In nonlinear plasmonics, the switching threshold of optical bistability is limited by the weak nonlinear responses from the conventional Kerr dielectric media. Considering the giant nonlinear susceptibility of graphene, here we develop a nonlinear scattering model under the mean field approximation and study the bistable scattering in graphene-coated dielectric nanowires based on the semi-analytical solutions. We find that the switching intensities of bistable scattering can be smaller than 1 MW cm-2 at the working frequency. To further decrease the switching intensities, we show that the most important factor that restricts the bistable scattering is the relaxation time of graphene. Our work not only reveals some general characteristics of graphene-based bistable scattering, but also provides a guidance to further applications of optical bistability in the high speed all-optical signal processing.

Internal Asymmetric Plasmonic Slot Waveguide for Third Harmonic Generation with Large Fabrication Tolerance

We propose an internal asymmetric plasmonic slot waveguide (IAPSW) containing two different materials in the slot region for third harmonic generation (THG) from 2.25 μm. In the IAPSW, the required phase matching condition is satisfied between the 0th-order mode at the fundamental frequency and 1st-order mode at the third harmonic frequency. By choosing an appropriate slot geometry and materials, the third harmonic electric field distribution can be engineered to significantly enhance the nonlinear overlap coefficient for THG. According to our simulation, a conversion efficiency up to 0.67 % with 1 W pump power is predicted within a ∼10 μm IAPSW. Additionally, the waveguide shows large fabrication tolerance in terms of geometry parameters. The proposed waveguide can find potential applications for high-speed all-optical signal processing.

Nonlinear Polarization Rotation Characteristic Phenomenon in a Bulk Semiconductor Optical Amplifier

The phenomena of polarization rotation induced by self-modulation in semiconductor optical amplifier (SOA) are analyzed theoretically. The relationship between polarization parameters and gain as well as phase is obtained by the correlation parameter of ellipse polarization and SOA nonlinearity polarization rotation theory. The experiment employs polarizer drive by walking electromotor and power meter, the light power of 360 degree is measured. The transformation law of output polarization power components is found for obvious polarization rotation in the selected coordinate axes based on connection of polarization state in difference axes. Using this law make the manipulation easily on getting ideal polarization state. It can offer a fine method to realize all-optical switch and other logic elements in experiment. This work is of great significance for the applications of SOA nonlinear polarization rotation at high-speed all-optical signal processing and all-optical logic gate.

Quantum dot semiconductor optical amplifiers: optical pumping versus electricalpumping

A comprehensive comparison between the characteristics of conventional electrical pumpingand the new optical pumping (OP) schemes in quantum dot semiconductor opticalamplifiers (QD-SOAs) has been done and has proved that the optical pumping scheme cansignificantly improve the dynamical properties of QD-SOAs. Analytical solutions have beenpresented to justify the optical properties in both pumping methods in CW operationmode. Also, pulse-mode operation has been investigated through numerically solved rateequations. Besides obviating the deficiencies of electrical pumping, optically pumped SOAs(OP-SOAs) can be attractive alternatives for high-speed all-optical signal processing.

All-optical Flip-flop Operation Based on Polarization Bistability of Conventional-type 1.55-㎛ Wavelength Single-mode VCSELs

We report, for the first time to our knowledge, observation of polarization bistability from 1.55-µm wavelength single-mode VCSELs of a conventional cylinder-shape under control of their driving current, and demonstration of all-optical flip-flop (AOFF) operations based on the bistability with optical set and reset pulse injection at a 50 MHz switching frequency. The injection pulse energy was less than 14 fJ. The average on-off contrast ratio of the flip-flopped signals was about 7 dB. These properties of the VCSELs will be potentially useful for future high-speed all-optical signal processing applications.

Semiconductor Microcavity: An Intrinsic Optical Transistor

Since its first observation, many of the phenomena studied in lasers, such as fluctuations, regenerative pulsations, and optical turbulence, can be observed in passive bistable systems, often under better controlled conditions. Their applications are based on dispersive and temperature switching using OB properties, suggesting possibilities in highspeed all-optical signal processing. Thus we investigate the use of semiconductor microcavities with a GaAs single quantum well of 100Aå with Al0.3Ga0.7As barrier, sandwiched between two DBR mirrors (AlAs/Al0.2Ga0.8As). We describe an anomalous and controllable optical bistable regime where we identify one and two crossing between the upward and downward hysteresis curve. The temporal behavior of the device is discussed were we present a purpose to develop an intrinsic optical transistor.

Single crystal growth of MgO-doped near-stoichiometric lithium niobate crystals and fabrication of Ti:PPLN devices

Quasi-phase matching (QPM) frequency conversion devices have been studied for applications of high-speed all-optical signal processing due to their ultrafast nonlinear optical response and wide conversion bandwidth. Among several ferroelectric crystals used, MgO-doped near-stoichiometric LiNbO 3 (MgSLN) crystals have advantages including low dopant content, low coercive field and high damage threshold. In this work, 2 in boules of MgSLN crystals were grown from Li-rich melts by the melt supplying technique. Then Ti-diffusion waveguides were fabricated on Z-cut plate of as-grown crystals. Finally, periodically poled optical gratings with period of 18 μm were made on the plate by high-voltage electric-field poling process. The compositional uniformity of as-grown crystals was detected by optical spectroscopy and the quality of Ti:PPLN devices was observed by optical microscopy, respectively. The effects of several key parameters including domain structure, compositional uniformity and polish process on the quality of Ti:PPLN devices were discussed. The way to improve the quality of Ti:PPLN devices was proposed.

Applications of Highly-Nonlinear Chalcogenide Glass Devices Tailored for High-Speed All-Optical Signal Processing

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Ultrahigh nonlinear tapered fiber and planar rib Chalcogenide waveguides have been developed to enable high-speed all-optical signal processing in compact, low-loss optical devices through the use of four-wave mixing (FWM) and cross-phase modulation (XPM) via the ultra fast Kerr effect. Tapering a commercial <formula formulatype="inline"><tex> $\hbox{As}_{2}\hbox{Se}_{3}$</tex> </formula> fiber is shown to reduce its effective core area and enhance the Kerr nonlinearity thereby enabling XPM wavelength conversion of a 40 Gb/s signal in a shorter 16-cm length device that allows a broader wavelength tuning range due to its smaller net chromatic dispersion. Progress toward photonic chip-scale devices is shown by fabricating <formula formulatype="inline"> <tex>$\hbox{As}_{2}\hbox{S}_{3}$</tex></formula> planar rib waveguides exhibiting nonlinearity up to <formula formulatype="inline"><tex>$2080\, {\rm W}^{-1}\cdot \hbox{km}^{-1}$</tex></formula> and losses as low as 0.05 dB/cm. The material's high refractive index, ensuring more robust confinement of the optical mode, permits a more compact serpentine-shaped rib waveguide of 22.5 cm length on a 7-cm-size chip, which is successfully applied to broadband wavelength conversion of 40–80 Gb/s signals by XPM. A shorter 5-cm length planar waveguide proves most effective for all-optical time-division demultiplexing of a 160 Gb/s signal by FWM and analysis shows its length is near optimum for maximizing FWM in consideration of its dispersion and loss. </para>

Integrated Small-Sized Semiconductor Ring Laser With Novel Retro-Reflector Cavity

We have successfully designed and fabricated 1.55-mum semiconductor ring lasers with novel parabolic mirror retro-reflector cavities. The parabolic mirror is designed by means of ray-tracing and finite-difference time-domain to optimize its shape and position. The fabricated devices, with a range of sizes down to an equivalent circular ring radius of 16 mum, operate in continuous-wave mode at room temperature. Threshold current of 22 mA and output power up to 140 muW have been achieved for the smallest device. These devices are suitable for high-speed all-optical signal processing and digital photonic functions.

Proposal for All-Optical Switchable or/xor Logic Gates Using Sum-Frequency Generation

All-optical logic gate based on parametric processes in periodically poled lithium niobate (PPLN) waveguides is a promising technique in future high-speed all-optical signal processing. A simple realization of switchable or/xor logic gates at 40 Gb/s is proposed and numerically demonstrated using sum-frequency generation in a PPLN waveguide. By appropriately adjusting the input signal power and choosing the waveguide length, or and xor logic gates can be obtained. The operation performance is simulated, including eye diagrams and Q-factor. The input signal powers and waveguide length are optimized, providing a theoretical basis for achieving the optimal performance for the switchable or/xor logic gates

Investigation on inverted and non-inverted wavelength conversion based on transient cross phase modulation of semiconductor optical amplifier

All-optical wavelength conversion based on transient cross phase modulation of semiconductor optical amplifier (SOA) is a promising approach to achieve high-speed all-optical signal processing. We experimentally demonstrate inverted and non-inverted wavelength conversion (WC) based on single SOA and an optical bandpass filter with 0.4 nm bandwidth. When the central wavelength of the following filter is blue shifted by 0.25 nm or red shifted by 0.05 nm with respect to the wavelength of probe signal, the WC is inverted. When the filter detuning is blue shifted 0.29 nm or red shifted 0.25 nm, the WC is non-inverted. The polarity evolution from inverted WC to non-inverted WC is also presented with numerical simulation. The obtained results are in good agreement with the experiment.

Precise operation-frequency control of monolithic mode-locked laser diodes for high-speed optical communication and all-optical signal processing

We describe a method for pulse-repetition-frequency tuning of mode-locked laser diodes (MLLDs) monolithically integrated with a distributed Bragg reflector (DBR). The pulse-repetition frequency, i.e., the cavity-roundtrip time, is tuned through loss-induced change in the effective length of the DBR. The frequency-tuning range as large as the chip-to-chip frequency deviation caused by cavity-length fabrication variation of 10 μm has been confirmed experimentally, and the MLLDs operate at SDH (synchronous digital hierarchy) frequencies of 9.953, 19.906 and 39.813 GHz. Synchronization with an external system-clock through the hybrid mode-locking operation reduces the timing jitter of the optical pulses to less than 0.3 ps. As an optical pulse source for optical communication, error-free 20-Gbps transmission over 3000 km has been demonstrated, confirming that the MLLD properties satisfy the requirements for use in real systems. The novel application of MLLDs to all-optical clock extraction, one of the essential functions required in all-optical signal processing, has been demonstrated at the 40-GHz SDH frequency.