Optical Signal Processing Devices Research Articles

Phonon-induced anomalous gauge potential for photonic isolation in frequency space

Photonic gauge potentials are crucial for manipulating charge-neutral photons like their counterpart electrons in the electromagnetic field, allowing the analogous Aharonov–Bohm effect in photonics and paving the way for critical applications such as photonic isolation. Normally, a gauge potential exhibits phase inversion along two opposite propagation paths. Here we experimentally demonstrate phonon-induced anomalous gauge potentials with noninverted gauge phases in a spatial-frequency space, where near-phase-matched nonlinear Brillouin scatterings enable such unique direction-dependent gauge phases. Based on this scheme, we construct photonic isolators in the frequency domain permitting nonreciprocal propagation of light along the frequency axis, where coherent phase control in the photonic isolator allows switching completely the directionality through an Aharonov–Bohm interferometer. Moreover, similar coherent controlled unidirectional frequency conversions are also illustrated. These results may offer a unique platform for a compact, integrated solution to implement synthetic-dimension devices for on-chip optical signal processing.

All‐fibre phase shifter based on tapered fibre coated with MoWS 2 ‐rGO

In this work, an all-fibre phase shifter using tapered fibre coated with molybdenum tungsten disulfide - reduced graphene oxide (MoWS2-rGO) is demonstrated. The large evanescent field of tapered fibre allows MoWS2-rGO to absorb injected light from a 1400-nm pump. It subsequently modifies the refractive index of both an MoWS2-rGO solution and tapered fibre because of the thermo-optic effect. A maximum wavelength shift of 2.489 nm is observed and the equivalent phase shift is 0.91π. The efficiency of the phase shifter is calculated at 0.0175 nm/mW. The proposed phase shifter is beneficial for a variety of applications, specifically for future optical beam-forming networks and all optical signal processing devices.

A density functional theory (DFT) study of the doping effect on 4-[2-(2-N, N-dihydroxy amino thiophene) vinyl] benzenamine

In the present work, density functional theory was used at the B3LYP, CAM-B3LYP and B3PW91 levels with the 6–311 + + G (d, p) basis set, in other to study the structural, optoelectronic, nonlinear optical, piezoelectric and thermodynamic properties of 4-[2-(2-N, N-dihydroxy amino thiophene) vinyl] benzenamine (DATVB) molecule doped with chlorine and fluorine respectively. The results obtained show that doping significantly improves the properties of this material. The materials obtained after doping 4-[2-(2-(5-(bis (chlorooxy) amino) thiophene) vinyl] benzenamine (BCATVB) and 4-[2-(2-(5-(bis (fluorooxy) amino) thiophene) vinyl] benzenamine (BFATVB) are good semiconductors because their gap energy is range between 1 and 1.5 eV which is less than 3 ev. Their first order hyperpolarizabilities ({beta }_{mol}) are very high compared to para-nitroaniline (p-NA) known as reference molecule for nonlinear optical properties. The obtained results predisposes them as good nonlinear optical materials with applications in photonics, optical signal processing devices, dynamic imaging, data storage and computer devices. The piezoelectric and pyroelectric coefficients of the fluorinated molecule are higher than those of the reference organic molecule polyvinylidene fluoride. This led us to the conclusion that, these molecules can also be used for applications in piezoelectricity.

Dynamic Hybrid Metasurfaces

Efficient hybrid plasmonic-photonic metasurfaces that simultaneously take advantage of the potential of both pure metallic and all-dielectric nanoantennas are identified as an emerging technology in flat optics. Nevertheless, postfabrication tunable hybrid metasurfaces are still elusive. Here, we present a reconfigurable hybrid metasurface platform by incorporating the phase-change material Ge2Sb2Te5 (GST) into metal-dielectric meta-atoms for active and nonvolatile tuning of properties of light. We systematically design a reduced-dimension meta-atom, which selectively controls the hybrid plasmonic-photonic resonances of the metasurface via the dynamic change of optical constants of GST without compromising the scattering efficiency. As a proof-of-concept, we experimentally demonstrate two tunable metasurfaces that control the amplitude (with relative modulation depth as high as ≈80%) or phase (with tunability >230°) of incident light promising for high-contrast optical switching and efficient anomalous to specular beam deflection, respectively. Our findings further substantiate dynamic hybrid metasurfaces as compelling candidates for next-generation reprogrammable meta-optics.

Integrated Photonic Platform for Rare-Earth Ions in Thin Film Lithium Niobate.

Rare-earth ion ensembles doped in single crystals are a promising materials system with widespread applications in optical signal processing, lasing, and quantum information processing. Incorporating rare-earth ions into integrated photonic devices could enable compact lasers and modulators, as well as on-chip optical quantum memories for classical and quantum optical applications. To this end, a thin film single crystalline wafer structure that is compatible with planar fabrication of integrated photonic devices would be highly desirable. However, incorporating rare-earth ions into a thin film form-factor while preserving their optical properties has proven challenging. We demonstrate an integrated photonic platform for rare-earth ions doped in a single crystalline thin film lithium niobate on insulator. The thin film is composed of lithium niobate doped with Tm3+. The ions in the thin film exhibit optical lifetimes identical to those measured in bulk crystals. We show narrow spectral holes in a thin film waveguide that require up to 2 orders of magnitude lower power to generate than previously reported bulk waveguides. Our results pave the way for scalable on-chip lasers, optical signal processing devices, and integrated optical quantum memories.

Apodization of Silicon Integrated Bragg Gratings Through Periodic Phase Modulation

The sinusoidal phase modulation apodization technique, owing to its high precision and resolution and very low phase noise, shows great promise for spectral tailoring of silicon integrated Bragg grating (IBG) devices for optical telecommunications and signal processing. Here, we extend this promising sinusoidal phase modulation technique by showing that phase modulation apodization of a silicon IBG can actually be accomplished based on any periodic function. This paper also shows a dependence of the apodization characteristic, physical grating structure, and actual apodized grating performance on the periodic function used. Then, we propose a general implementation process of the periodic phase modulation apodization to achieve a desired response on a silicon IBG, and study the limiting factors of the apodization performance, design tradeoffs and optimization, and grating robustness against fabrication constraints for different periodic phase functions, using a computational lithography model together with a structure-aware grating emulator. Finally, the extended periodic phase modulation apodization technique is validated by demonstrating a series of differently designed phase-modulated silicon IBGs, including Gaussian-apodized gratings, single- and multi-channel flat-top filters, and flat-top dispersion-compensating filters, using different periodic phase functions. The work offers an additional degree of freedom for the design and optimization of phase-modulated gratings, and has significant implications for practical implementation of the phase modulation apodization for spectral engineering of silicon Bragg grating devices.

Optomechanically Induced Transparency at Exceptional Points

Exceptional-point (EP) optics allows for innovative devices, such as a sensor based on a whispering-gallery-mode (WGM) microresonator. Meanwhile, WGM-enabled optomechanics has led to important advances, including optomechanically induced transparency (OMIT). The authors show that unconventional EP features can be observed in OMIT: Tuning the relative angle between two external nanoparticles coupled to the same microresonator causes EPs to emerge periodically, strongly modifying both the transmission rate and group delay of the signal, for a slow-light-to-fast-light switch. This approach is a means to engineer optomechanical EP devices for optical communication and signal processing.

Optical waveguide on the basis of a layered magnetoactive metamaterial

Guiding properties of an optical waveguide based on a layered metamaterial formed by layers of iron-yttrium garnet and aluminum-yttrium garnet, magnetized to saturation are considered. Dispersion equations for the waveguide TE and TM modes in the structure were obtained using the approximation of an effective medium and solving the boundary problem (electromagnetic method), and also by means of a 4×4 matrix method. Dispersion spectra of the propagation constant dependence on the cyclic frequency ω(β) for different types of structures are found, localization coefficients of the orthogonal modes in the guiding layer are also derived and their comparative analysis is carried out. The advantages of the approximate electromagnetic method are shown in comparison with the exact 4x4 matrix method. The results of the work can find an application in developing new integrated optical devices for optical signal processing based on layered metamaterials.

Low Loss Electro-Optic Polymer Based Fast Adaptive Phase Shifters Realized in Silicon Nitride and Oxynitride Waveguide Technology

We present a comprehensive study on how to design and fabricate low loss electro-optic phase shifters based on an electro-optic polymer and the silicon nitride and silicon oxynitride waveguide material systems. The loss mechanisms of phase shifters with an electro-optic (EO) polymer cladding are analyzed in detail and design solutions to achieve lowest losses are presented. In order to verify the low loss design a proof of concept prototype phase shifter was fabricated, which exhibits an attenuation of 0.8 dB/cm at 1550 nm and an electro-optic efficiency factor of 27%. Furthermore, the potential of this class of phase shifters is evaluated in numerical simulations, from which the optimal design parameters and achievable figures of merit were derived. The presented phase shifter design has its potential for application in fast adaptive multi stage devices for optical signal processing.

Microscopic optical buffering in a harmonic potential.

In the early days of quantum mechanics, Schrödinger noticed that oscillations of a wave packet in a one-dimensional harmonic potential well are periodic and, in contrast to those in anharmonic potential wells, do not experience distortion over time. This original idea did not find applications up to now since an exact one-dimensional harmonic resonator does not exist in nature and has not been created artificially. However, an optical pulse propagating in a bottle microresonator (a dielectric cylinder with a nanoscale-high bump of the effective radius) can exactly imitate a quantum wave packet in the harmonic potential. Here, we propose a tuneable microresonator that can trap an optical pulse completely, hold it as long as the material losses permit, and release it without distortion. This result suggests the solution of the long standing problem of creating a microscopic optical buffer, the key element of the future optical signal processing devices.

Heterogeneously integrated silicon photonics for the mid-infrared and spectroscopic sensing.

Besides being the foundational material for microelectronics, crystalline silicon has long been used for the production of infrared lenses and mirrors. More recently, silicon has become the key material to achieve large-scale integration of photonic devices for on-chip optical interconnect and signal processing. For optics, silicon has significant advantages: it offers a very high refractive index and is highly transparent in the spectral range from 1.2 to 8 μm. To fully exploit silicon’s superior performance in a remarkably broad range and to enable new optoelectronic functionalities, here we describe a general method to integrate silicon photonic devices on arbitrary foreign substrates. In particular, we apply the technique to integrate silicon microring resonators on mid-infrared compatible substrates for operation in the mid-infrared. These high-performance mid-infrared optical resonators are utilized to demonstrate, for the first time, on-chip cavity-enhanced mid-infrared spectroscopic analysis of organic chemicals with a limit of detection of less than 0.1 ng.

Ultra compact slow light photonic crystal directional coupler design with elliptical rods

We propose a novel design for a photonic crystal waveguide coupler, which uses elliptical shaped rods with refractive index equal to 3.4 arranged in square lattice. We investigate the performance of slow light in this photonic crystal coupler. By providing a slow-light region using elliptical cells, a coupling length of just 2.58μm is achieved at optical wavelength. This is extremely small coupling length then previously reported. The proposed structure would offer significant potential for novel compact high-speed optical signal processing devices.

Nonlinear Optical Signal Processing in Optical Packet Switching Systems

This paper discusses nonlinear optical signal processing employed in optical packet switching systems. Nonlinear optical signal processing provides optical label (header) recognition, optical switching, wavelength conversion, and time buffering with typically higher capacity, lower latency, and lower power consumption than electronic counterparts. In order to provide diverse signal processing functions, large-scale integration of nonlinear optical signal processing devices is essential. We discuss possible future directions in optical packet switching involving nonlinear optical signal processing of optical packets with advanced data and label modulation formats.

Active and Passive Devices Based on Silicon-on-Insulator Waveguides

Silicon on Insulator (SOI) waveguides are attracting considerable attention in recent years, and the devices based on these optical waveguides are important to realize many modern optical signal processing systems. The main driving forces behind these devices are the technological improvements in exploiting the non-linear optical phenomenon and their compatibility with CMOS devices. Although all optical signal processing devices have been investigated intensively, the chip-scale solution provided by the silicon photonics is the most promising. Silicon photonic devices are finding their applications in telecommunication transmission systems and biomedical instruments and in the new generation of high performance computing systems. Recent progress in patents all over the globe, along with important technological developments in this field are reviewed.

Optical–optical isomerization mechanism for the third-order optical nonlinearity in side-chain azobenzene copolymer

The third-order optical nonlinear properties of a side-chain azobenzene copolymer have been studied using the Z-scan technique at 532 nm with 35 ps pulses. The mechanism has been described in detail via an optical–optical isomerization. The two-photon absorption coefficient β was measured to be 234 cm/GW and the nonlinear refraction coefficient γ was measured to be −104×10 −4 cm 2/GW at the on-axis peak intensities I 0 of 0.5623 GW/cm 2. It is found that β and γ decreased significantly with increasing I 0, and the faster optical responses of β and γ should prove valuable in optical signal processing devices.

Free-Space Optical Communications with Channel Shortening Filter and Viterbi Equalizer

This paper is about signal processing techniques for airborne free-space optical (FSO) wireless communications through clouds. FSO channel is known to be accompanied by multi-scattering, which causes severe intersymbol interference. Unlike traditional FSO systems relying on optical signal processing devices, this research paper investigates the feasibility of digital signal processing schemes applied to FSO. By having a channel shortening equalizer (CSE or TEQ for time equalization) at the transmitter and a Viterbi equalizer at the receiver, reliable communication at low-to-mid level optical thickness values is shown to be feasible. We also propose how existing TEQ algorithms can be modified to fit into TEQ-in-transmitter configurations. Performance is evaluated in BER, and compared with conventional equalizations.

Bragg diffraction in thin 2D refractive index modulated semiconductor samples

A very simple model has been developed to describe the diffractive properties of a crossed grating structure of the refractive index formed by a thin transmission grating recorded in a Bragg reflector. When the Bragg condition of the transmission grating coincides with the band edge of the reflection grating seen as a one-dimensional photonic crystal, the diffraction efficiency and wavelength selectivity of the transmission grating are highly enhanced and a Bragg diffraction regime can be obtained, even in very thin samples. The model can be used to design micrometric very efficient new diffracting devices for optical signal processing.

Time resolved nonlinear spectroscopy at the band edge of 1D photonic crystals

Large refractive index changes have been measured at the band edge frequency of 1D photonic crystals (PCs). Results concerning both thin and thick samples of high and low refractive index contrasts, respectively, are presented. The very large value of the refractive index changes obtained at moderate pump powers thanks to the strong enhancement of the local intensity inside the PC opens the way to very small volume devices for optical signal processing. However, time resolved experiments demonstrate the photo-generation of high free carrier densities through two- or even three-photon absorptions, which are shown to be also strongly enhanced at the band edge of the PC. This drawback may most probably be circumvented by using lower pump intensities in PCs showing narrower resonances.

Higher nonlinear indices, Raman gain coefficients, and bandwidths in the TeO2–ZnO–Nb2O5–MoO3 quaternary glass system

A quaternary glass system, TeO2–ZnO–Nb2O5–MoO3, with higher linear and nonlinear indices, third order optical susceptibility (χ3), and Raman gain coefficient and gain bandwidth is reported. The χ3 of these glasses are ∼80 times higher than the silica glass and 1.6 times higher than pure TeO2 glass; Raman gain coefficient is 40 times higher than the silica glass. Amplification bandwidth of one of the glasses is 1.7 times larger than the silica glass. The present glass has a glass transition temperature 80°C lower than the previously reported tellurite showing large Raman amplification bandwidth. These features make the TeO2–ZnO–Nb2O5–MoO3 glass a potential material for photonics devices such as fiber Raman amplifiers and/or nonlinear optical signal processing devices.

Resonant coupling between bulk waves and guided modes in a dielectric slab with a thick holographic grating

What we believe to be a new type of resonant coupling of an incident bulk wave into guided modes of a slab with a thick holographic grating is shown to occur in the presence of strong frequency detunings of the Bragg condition. This happens through the reflection of the strongly noneigen +1 diffracted order with the slab-grating boundaries, the resultant reflected waves forming a guided slab mode. Rigorous coupled-wave analysis is used for the numerical analysis of the predicted resonant effects. Possible applications include enhanced options for the design of multiplexing and demultiplexing systems, optical signal-processing devices, optical sensors, and measurement techniques.