All-optical Signal Processing Research Articles

Wideband Microwave Phase Noise Analyzer Based on All-Optical Microwave Signal Processing

An approach for wideband phase noise measurement of microwave signal sources is proposed based on all-optical microwave signal processing. In the proposed scheme, an optical carrier is sequentially modulated by the signal under test (SUT) in a phase modulator and a dual-polarization modulator to generate two +1st-order sidebands with orthogonal polarizations. A time delay is introduced between two sidebands by a span of low-loss optical fiber. Combined with a polarization controller and a polarizer, photonic microwave downconversion with a desired phase shift and time delay can be realized after photodetection, and the phase noise of the SUT can be calculated thereafter. Since all the microwave signal processing functions in the conventional photonic-delay-line-based phase noise measurement system are implemented in the optical domain, the proposed scheme has a large operational bandwidth and a high measurement sensitivity. In the experimental demonstration, accurate phase noise measurement of SUTs is achieved in a frequency range of 10-35 GHz, and a phase noise floor as low as −132.16 dBc/Hz at 10 kHz is obtained.

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.

Channel Arrangement Design in Lumped Amplified WDM Transmission over NZ-DSF Link with Nonlinearity Mitigation Using Optical Phase Conjugation

Optical phase conjugation (OPC) is an all-optical signal processing technique for mitigating fiber nonlinearity and is promising for building cost-efficient fiber networks with few optic-electric-optic conversions and long amplification spacing. In lumped amplified systems, OPC has a little nonlinearity mitigation efficiency for nonlinear distortion induced by cross-phase modulation (XPM) due to the asymmetry of power and chromatic dispersion (CD) maps during propagation in transmission fiber. In addition, the walk-off of XPM-induced noise becomes small due to the CD compensation effect of OPC, so the deterministic nonlinear distortion increases. Therefore, lumped amplified transmission systems with OPC are more sensitive to channel spacing than conventional systems. In this paper, we show the channel spacing dependence of NZ-DSF transmission using amplification repeater with OPC. Numerical simulations show comprehensive characteristics between channel spacing and CD in a 100-Gbps/λ WDM signal. An experimental verification using periodically poled LiNbO3-based OPC is also performed. These results suggest that channel spacing design is more important in OPC-assisted systems than in conventional dispersion-unmanaged systems.

A compact realization of Feynman Reversible and NOR logic gate using Plasmonic waveguide based MZI for all-optical signal processing

The reversible logic function becomes a potential solution in the regime of optical switching and computing. The one-to-one mapping between inputs and outputs in a reversible gate is a key advantage, that prevents information loss. The reversible logic gates have shown potential applications in low-power complementary metal-oxide semiconductors and in quantum computing. Therefore, to utilize these benefits of reversible functions, we have proposed an all-optical compact circuit to realize the functioning of Feynman reversible and NOR logic gates. The proposed all-optical circuit is modeled within the footprints of 87μm by using a plasmonic metal–insulator–metal (MIM) waveguide-based Mach–Zehnder interferometer (MZI). The numerical investigation of the MZI is done by evaluating the power imbalance and extinction ratio. The results show that the optical signal propagating in a single MZI with an extinction ratio of 15.1 dB for the power difference of 0.001 W/μm (∼0.1 dB) and 0.0012 W/μm (∼0.79 dB) for logic ‘0’ and logic ‘1’, respectively. The logic ‘0’ and ‘1’ is represented by low (0.028 W/μm) and high (0.038 W/μm) power signals. At logic ‘0’ the optical signal propagates in linear arms of MZI with ‘π’ phase variation, which causes constructive interference (CI) and thrives the signal at the cross-port. For logic ‘1’ the optical signal experiences ‘0’ phase variation in linear arms of MZI, and due to destructive interference reaches its through-port. The investigated results of the proposed circuit are attained by using the finite difference time domain (FDTD) method.

AlGaAs Nonlinear Integrated Photonics.

Practical applications implementing integrated photonic circuits can benefit from nonlinear optical functionalities such as wavelength conversion, all-optical signal processing, and frequency-comb generation, among others. Numerous nonlinear waveguide platforms have been explored for these roles; the group of materials capable of combining both passive and active functionalities monolithically on the same chip is III–V semiconductors. AlGaAs is the most studied III–V nonlinear waveguide platform to date; it exhibits both second- and third-order optical nonlinearity and can be used for a wide range of integrated nonlinear photonic devices. In this review, we conduct an extensive overview of various AlGaAs nonlinear waveguide platforms and geometries, their nonlinear optical performances, as well as the measured values and wavelength dependencies of their effective nonlinear coefficients. Furthermore, we highlight the state-of-the-art achievements in the field, among which are efficient tunable wavelength converters, on-chip frequency-comb generation, and ultra-broadband on-chip supercontinuum generation. Moreover, we overview the applications in development where AlGaAs nonlinear functional devices aspire to be the game-changers. Among such applications, there is all-optical signal processing in optical communication networks and integrated quantum photonic circuits.

Brillouin dynamic grating erasure technique for fast all-optical signal processing.

Brillouin dynamic grating (BDG) is an attractive storage unit for all-optical signal storage and processing. However, the processing speed of the traditional "write-read" scheme is severely limited by the inter-process interference (IPI) due to the residual BDG. Here, we propose an all-optical "write-read-erase" scheme to avoid the IPI effect, which can effectively eliminate the residual BDG through an erase pulse. In a numerical simulation, for multi-processes to store a 7 × 7-bits Simplex code, each time, the residual BDGs from the former process are erased for the proposed scheme, and the power fluctuation of the retrieved waveform is suppressed within ±10%. In a preliminary experiment, residual BDG erase efficiencies up to 88.5% can be achieved by introducing erase pulses to neglect the IPI effect on the retrieved waveform. Without the IPI effect, all-optical signal processing will availably be speeded up, especially for short on-chip integrated circuits.

Quantum Dot Reflective Semiconductor Optical Amplifiers: Optical Pumping Compared with Electrical Pumping.

A comprehensive study has been conducted on quantum dot reflective semiconductor optical amplifiers (QD-RSOAs) with optical pumps (OPs). Moreover, few studies have been completed on OP-based QD-RSOAs. A comparison is made between them and QD-RSOAs with electrical pumps (EPs) in this study. It is shown that the dynamical properties of the device can significantly develop in the optical pumping version. The optical properties are studied for both methods. Moreover, by solving the coupled differential rate and signal propagation equations, the operation of the device in the pulse mode is investigated. Finally, it is proven that OP QD-RSOAs can perform significantly better in applications such as fast all-optical signal processing and wavelength division multiplexing in passive optical networks.

Phase-matched second-harmonic generation in core-shell nanowire hyperbolic metamaterial

Second-harmonic generation is well-known nonlinear frequency conversion technique, which can be applied in nonlinear optical characterization of materials and all-optical signal processing. This phenomenon requires phase-matching to maximize the conversion efficiency of generated second-harmonic field. But the phase-matching condition is difficult to achieve because of dispersion of naturally existing materials. To overcome this limitation, we propose an innovative phase-matching technique, which is called hyperbolic phase-matching, that can be possibly achieved by managing dispersion of a hyperbolic metamaterial. Here, the hyperbolic metamaterial is made of two-dimensional periodic arrays of core-shell nanowires, which have aluminium gallium arsenide as a core and gold as a shell, immersed in anodic aluminium oxide matrix. We have demonstrated phase-matched conditions for two different non-collinear second-harmonic interacting configurations in the metamaterial, which can be created by tuning incident angle of pump field to optimal values. Finally, conversion efficiencies of transmitted and reflected second-harmonic pulses as a function of incident angle and input pulse intensity were examined. The maximum conversion efficiencies are obtained at optimal incident angle and largest pumping intensity.

Reconfigurable all-optical signal processing node architecture with data-symbol and encryption-key superposition function

This paper proposes a reconfigurable all-optical signal processing node architecture with selective all-optical signal processing in one simple configuration without optical-electrical-optical (O/E/O) conversion at the heterogeneous network connection node or repeater node. Proof-of concept experiments of encryption-key data superposition for binary phase shift keying and quadrature phase shift keying show the nonlinear optical effect generation within 1.5-dB and 3.5-dB characteristic degradation. Self-phase modulation and stimulated Brillouin scattering caused by increasing the highly nonlinear fiber input of pump light is the primary cause of signal degradation in the experiment.

Unbiased Plasmonic-Assisted Integrated Graphene Photodetectors.

Photonic integrated circuits (PICs) for next-generation optical communication interconnects and all-optical signal processing require efficient (∼A/W) and fast (≥25 Gbs–1) light detection at low (<pJbit–1) power consumption, in devices compatible with Si processing, so that the monolithic integration of electro-optical materials and electronics can be achieved consistently at the wafer scale. Graphene-based photodetectors can meet these criteria, thanks to their broadband absorption, ultra-high mobility, ultra-fast electron interactions, and strong photothermoelectric effect. High responsivities (∼ 1 A/W), however, have only been demonstrated in biased configurations, which introduce dark current, noise, and power consumption, while unbiased schemes, with low noise and zero consumption, have remained in the ∼ 0.1 A/W regime. Here, we consider the unbiased asymmetric configuration and show that optimized plasmonic enhanced devices can reach for both transverse-electric and transverse-magnetic modes (at λ = 1550 nm), ∼A/W responsivity, and ∼ 100 GHz operation speed at zero power consumption. We validate the model and material parameters by simulating experimental devices and derive analytical expressions for the responsivity. Our comprehensive modeling paves the way for efficient, fast, and versatile optical detection in PICs with zero power consumption.

Intensity and phase encoding for realization of integrated Pauli X, Y and Z gates using 2D photonic crystal

Nano-scale all-optical logic gates systems are desirable for the all-optical computing and signal processing applications. All-optical Quantum computing offers excellent computational abilities and lossless operations. The Pauli X, Y and Z gates are vital quantum logic gates for all-optical systems. In this paper, the integrated all-optical Pauli X, Y, Z gate is realized using a square lattice 2D photonic crystal of silicon rods. The phase encoding and intensity encoding techniques have been used simultaneously for the integrated operation. Pauli X, Y, Z logic gate truth table has been verified at the operating wavelength of 1.55 µm.The system offers a linear operation, compact size, low power requirement, and high contrast ratio of 10.4 dB. The footprint of the proposed structure is about 70µm x 70µm and demonstrates a fast response time of 2 ps, bit rate of 0.25 Tbps, which makes it suitable for photonic integrated circuits (PICs).

Nonlinear all-optical modulator based on non-Hermitian PT symmetry

All-optical modulators with ultrahigh speed are in high demand due to the rapid development of optical interconnection and computation. However, due to weak photon–photon interaction, the advancement of all-optical modulators is consequently hampered by the large footprint and high power consumption. In this work, the enhanced sensitivity around an exceptional point (EP) from parity-time (PT) symmetry theory is initiatively introduced into a nonlinear all-optical modulator design. Further, a non-Hermitian all-optical modulator based on PT symmetry is proposed, which utilizes the large Kerr nonlinearity from indium tin oxide (ITO) in its epsilon-near-zero (ENZ) region. The whole system is expected to operate around EP, giving rise to the advantages of nanoscale integration and large modulation depth. This presented modulator with high efficiency and high-speed all-optical control can be commendably extended to the design methodology of various nanostructures and further prompt the development of all-optical signal processing.

Theoretical investigation of a multi-functional logic device based on plasmonic Mach-Zehnder interferometer waveguides

Optical logic device is an essential part of the all-optical signal processing system. A multi-functional logic device is proposed in this paper, it consists of plasmonic Mach–Zehnder Interferometer (MZI) waveguides and a couple of identical hexagonal resonators with swiveling ellipses embedded in them. The input boolean value is defined by the rotation angle of rotatable ellipses. The output boolean values are distinguished by monitoring the transmissions of the output port and setting the appropriate thresholds. The finite-difference time-domain (FDTD) method is used to simulate the proposed structure. Ultimately, four logical functions: NOR, AND, OR and NOT could be accomplished simultaneously. Furthermore, the contrast ratios of the four logic gates are obtained by calculating the structure simulated results. Compared with the same type of logic devices, the proposed logic device has better performance in contrast ratio. Therefore, the structure has excellent application prospects in optical communications and photonic integrated devices.

All-optical microwave waveform transformation based on photonic temporal processors.

An all-photonic approach of microwave waveforms generation and transformation is proposed and experimentally demonstrated. From the perspective of envelope function operation in time domain, an initial triangular waveform is transformed into square waveform and sawtooth (or reversed-sawtooth) waveform via two types of differentiators, respectively. In addition, by using a SOA as a multiplier, both bright and dark parabolic pulses are achieved, which are further transformed into sawtooth (or reversed-sawtooth) waveform by taking the first derivative operation. The feasibility of the system is verified by theoretical analysis and simulation. In experiment, all of the expected results are successfully demonstrated and agree with the theoretical analysis well. This scheme provides a novel access to implement all-optical microwave waveforms generation, transformation, signal processing and computing.

All-optical light manipulation based on graphene-embedded side-polished fiber.

We present a study of all-optical light manipulation arising in a graphene-embedded side-polished fiber (SPF) with a Norland Optical Adhesives (NOA)-coated structure. With the help of the Pauli blocking effect, such an all-fiber device serves to manage the loss of transverse-electric-polarized light when the control light and the signal light are polarized along the direction parallel to the graphene surface. The insertion loss of this device can be effectively reduced with the NOA coating. An enhanced interaction between the graphene and the propagated light can be achieved via the strong evanescent field of the SPF and longer interaction length. This results in effective all-optical manipulation of light with a modulation depth of 10.4 dB (or modulation efficiency of ∼91%) and a modulation slope of ∼1.3, where the required control power is only about 14 dBm. The device has broadband operation wavelength. The insertion loss for both the signal light and the control light are only about 0.6 dB. The experimental results are well-fitting with the simulation study. Such an all-fiber device has the potential for all-optical signal processing.

Optimization of quantum light sources and four-wave mixing based on a reconfigurable silicon ring resonator.

Being a key component on a photonic chip, the microring usually specializes in a certain nonlinear optical process and can not simultaneously meet different working conditions for different processes. Here, we theoretically and experimentally investigate a reconfigurable silicon microring resonator to act as a optimization strategy for both classical four-wave mixing and quantum light sources. Experimental results show that the four-wave mixing efficiency with continuous wave and pulsed pump can be both optimized to a high value well matching numerical analysis. A variety of quantum light sources - including the heralded single-photon source, two-photon source and multi-photon source - are demonstrated to present a high performance and their key parameters including the pair generation rates (PGR), the heralding efficiency (HE) and the coincidence-to-accidental ratio (CAR) are controllable and optimizable. Such tunable nonlinear converter is immune to fabrication variations and can be popularized to other nonlinear optical materials, providing a simple and compact post-fabrication trimming strategy for on-chip all-optical signal processing and photonic quantum technologies.

Low-power all-optical switch based on a graphene-buried polymer waveguide Mach-Zehnder interferometer.

We propose a low-power all-optical switch based on the structure of a graphene-buried balanced Mach-Zehnder interferometer (MZI), where the signal light is switched between the two output ports of the MZI by the heat generated from graphene's absorption of the pump light. We use orthogonal polarizations for the pump and the signal light to maximize pump absorption and minimize graphene-induced signal loss. Our experimental device fabricated with polymer waveguides buried with 5-mm long graphene shows a pump absorption of 10.6 dB (at 980 nm) and a graphene-induced signal loss of 1.1 dB (at 1550 nm) and can switch the signal light with a pump power of 6.0 mW at an extinction ratio of 36 dB. The actual pump power absorbed by graphene for activating switching is estimated to be 2.2 mW. The rise and fall times of the switch are 1.0 and 2.7 ms, respectively. The switching characteristics are weakly sensitive to ambient temperature variations. Our device can be butt-coupled to single-mode fibers and could find applications in fiber-based and on-chip all-optical signal processing.

All-optical ultrafast differentiation and integration for femtosecond pulse shaping based on programmable differential group delay

We propose and demonstrate a novel scheme of multi-functional all-optical ultrafast signal processor for femtosecond pulse shaping based on programmable differential group delay (DGD), which is capable of performing both all-optical differentiation and integration simultaneously, and can easily switch two functionalities by simply tuning the orientation of a polarizer. The differentiation waveforms and integration windows length can be adjusted by programming the DGD time. Utilizing numerical simulation we obtain the differential pulses and their autocorrelation of ~400-fs Gaussian signal for DGD of 0.5 ps and 4 ps, while achieve its integral pulses and their autocorrelation for DGD of 0.5 ps, 1.5 ps, and 3.5 ps. We further experimentally demonstrate the all-optical differentiation and all-optical integration of ~400-fs Gaussian signals whose results agree well with the theoretical prediction and simulation result. Using autocorrelation method, we observe in detail the differential and integral processing of femtosecond pulses at various DGD for the first time according to our knowledge. Our scheme can work over the whole communication band with variable operation wavelength, and is competitive in ultrafast pulse shaping for quantum control due to highly reconfigurable and flexible features.

Enhanced four-wave mixing in borophene-microfiber waveguides at telecom C-band.

All-optical wavelength conversion technology based on two-dimensional (2D) materials has lately received keen interest. As a new 2D material, borophene displays acceptable photoelectric properties. We demonstrate the all-optical wavelength conversion through four-wave mixing (FWM) in borophene-microfiber hybrid waveguides. Borophene is deposited at the thinnest part of the tapered fiber and enhanced FWM occurs in this photonic device. By optimizing the effect of nonlinear polarization, wavelength tuning, and power variation, the conversion efficiency increases to -19.1dB, corresponding to 3 dB conversion bandwidth in a range of 7.1 nm. In addition, this photonic device is employed to achieve all-optical wavelength conversion of 10 Gb/s non-return-to-zero digital sequence. The signal quality of converted light such as optical signal-to-noise ratio, bit-error-rate, and eye diagram are investigated, which indicates that the proposed wavelength converter has high conversion efficiency and remarkable stability. This study shows that the borophene-microfiber waveguide has potential application prospects in all-optical signal processing.

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.