Optical Signal Processing Applications Research Articles

Effects of disorder on optical and electron spin linewidths in Er3+,Sc3+:Y2SiO5

The material Er3+:Y2SiO5 co-doped with Sc3+ is investigated for applications in optical quantum storage and signal processing. Replacing 1% of the Y3+ in the crystal with Sc3+ introduces static strain into the lattice that increases the inhomogeneous linewidth of the Er3+ optical transition at 1.536 μm to 25 GHz, a 50-fold increase compared to Er3+:Y2SiO5 samples without Sc3+ co-doping. Electron paramagnetic resonance spectroscopy shows that electron spin linewidths are also strongly increased, confirming the previously proposed mechanism for decoherence suppression by using disorder to inhibit resonant spin-spin interactions. Analysis of the spin line broadening as a function of magnetic field orientation indicates the presence of contributions that cannot be modeled by a simple change in the electronic g tensor. Optical homogeneous linewidths of less than 2 kHz are observed for a weak magnetic field of 0.1 T and also for fields greater than 2 T with the field oriented near the D2 crystal axis and at a temperature of 1.7 K. These results suggest that this material can be useful for high-bandwidth classical and quantum information processing in the telecom C-band.

Extinction ratio and resonant wavelength tuning using three dimensions of silica microresonators

In this paper, a multidimensional tuning method of the silica microcapillary resonator (MCR) is proposed and demonstrated whereby the extinction ratio (ER) as well as the resonant wavelength can be individually controlled. An ER tuning range of up to 17 dB and a maximum tuning sensitivity of 0.3 dB/μm are realized due to the tapered profile of the silica optical microfiber (MF) when the MF is adjusted along its axial direction. Compared to direct tuning of the coupling gap, this method could lower the requirement for the resolution of displacement stage to micrometers. When the MF is adjusted along the axial direction of the silica microcapillary, a resonance shift of 3.06 nm and maximum tuning sensitivity of 0.01 nm/μm are achieved. This method avoids the use of an applied external field to control the silica microresonators. Moreover, when air is replaced by ethanol and water in the core of the silica microcapillary, a maximum resonance shift of 5.22 nm is also achieved to further enlarge the resonance tuning range. Finally, a microbubble resonator with a higher Q factor is also fabricated to achieve an ER tuning range of 8.5 dB. Our method fully takes advantage of the unique structure of the MCR to separately and easily tune its key parameters, and may broaden its applications in optical signal processing and sensing.

Effect of alpha and Gaussian refractive index profiles on the design of highly nonlinear optical fibre for efficient nonlinear optical signal processing

We propose an index profiled, highly nonlinear ultra-flattened dispersion fibre (HN-UFF) with appreciable values of fibre parameters such as dispersion, dispersion slope, effective area, nonlinearity, bending loss and splice loss. The designed fibre has normal zero flattened dispersion over S, C, L, U bands and extends up to . The maximum dispersion variation observed for this fibre is as low as over the optical fibre transmission spectrum. This fibre also has two zero dispersion wavelengths at 1.487 and and the respective dispersion slopes are 0.02476 and . The fibre has a very low ITU-T cutoff wavelength of and a virtuous nonlinear coefficient of . The wide spectrum of zero flattened dispersion and a good nonlinear coefficient make the designed fibre very promising for different nonlinear optical signal processing applications.

Observation of second-harmonic generation in silicon nitride waveguides through bulk nonlinearities.

We present experimental results on the observation of a bulk second-order nonlinear susceptibility, derived from both free-space and integrated measurements, in silicon nitride. Phase-matching is achieved through dispersion engineering of the waveguide cross-section, independently revealing multiple components of the nonlinear susceptibility, namely χ(2) yyy = 0.14 ± 0.08 pm/V and χ(2) xxy = 0.30 ± 0.18 pm/V. Additionally, we show how the second-harmonic signal may be tuned through the application of bias voltages across silicon nitride. The material properties measured here are anticipated to allow for the realization of new nanophotonic devices in CMOS-compatible silicon nitride waveguides, adding to their viability for telecommunication, data communication, and optical signal processing applications.

Control of dispersion in fiber coupled resonator-induced transparency structure**Project supported by the National Natural Science Foundation of China (Grant Nos. 61307076 and 61275066), the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (Grant No. 2012BAF14B11), and the Postdoctoral Scientific Research Developmental Fund of Heilongjiang Province, China (Grant No. LBH-Q14042).

Induced transparency phenomena and strong dispersion can be produced in a coupled resonator induced transparency (CRIT) structure. In this paper, we investigate the influences of structure parameters, such as amplitude reflection coefficient and loss, on transmission spectrum and dispersion of CRIT structure, and further study the control of dispersion in the structure. The results show that in the CRIT structure, adjusting the loss of resonators is an effective method of controlling dispersion and producing simultaneous normal and abnormal dispersion. When we choose approximate amplitude reflection coefficients of the two couplers, the decrease of transmittance due to loss could be effectively made up. In the experiment, we achieve the control of dispersion and simultaneous strong normal and abnormal dispersion in the CRIT structure comprised of fiber. The results indicate the CRIT structure has potential applications in optical signal processing and optical communication.

Theoretical analysis of optical mode deflection in lithium niobate waveguide with serrated array electrodes

In this paper, we establish a lithium niobate (LN) thin film planar waveguide and an annealed proton exchange (APE) waveguide model with serrated array electrodes configuration to deflect and reshape the optical mode. The refractive index profile of the waveguide can be electro-optically manipulated by applying an external voltage into the serrated array electrodes. Through the simulation results, it is observed that different refractive index manipulation of LN waveguide can make the optical beam have different deflection and reshaping. The comparison between the thin film and APE bulk planar waveguide is investigated. When the beam deflection is modulated via the high-speed electro-optic effect of LN, the temporal and spatial beam smoothing can be achieved for the application of high power laser system and other optical signal processing.

Phase-controlled two-wave mixing in a moving grating

Two-wave mixing is fundamental to applications in optical signal processing, holographic information, and imaging science. Conventionally, it takes place in a nonlinear medium where self-induced optical grating can effectively couple two optical waves. However, it demands high input power to trigger nonlinearities and is complicated by the phase shift of the optical grating for the given medium. Here, we separate these physical processes by studying near-degenerated two-wave mixing in a moving grating induced independently by an external acoustic optical modulator. Two-wave mixing is observed under Bragg’s phase matching condition, which is greatly affected by phase shifting. Moreover, we show that two-wave mixing here is unidirectional due to the propagation of acoustic waves, which may offer new avenues for applications in optical information processing.

Stimulated Brillouin scattering gain bandwidth reduction and applications in microwave photonics and optical signal processing

Stimulated Brillouin scattering (SBS) is one of the most dominant nonlinear effects in standard single-mode fibers and its unique spectral characteristics, especially the narrow bandwidth, enable many different applications. Most of the applications would benefit from a narrower bandwidth. Different methods for the bandwidth reduction of SBS in optical fibers are presented and discussed. A bandwidth reduction down to 17% of the natural gain can be achieved by the superposition of the gain with two losses or the utilization of a multistage system. Furthermore, applications in the field of microwave photonics and optical signal processing like high-resolution spectroscopy of communication signals, the storage of optical data packets as well as the processing of frequency combs including generation of millimeter waves and ideal sinc-shaped Nyquist pulses are presented.

Line Degeneracy and Strong Spin-Orbit Coupling of Light with Bulk Bianisotropic Metamaterials.

Propagation of light in a medium is dictated by equifrequency surfaces (EFSs), which play a similar role as Fermi surfaces for electrons in crystals. Engineering the equifrequency surface of light through structuring a photonic medium enables superior control over light propagation that goes beyond natural materials. In this Letter, we show that a bulk metamaterial with a suitably designed bianisotropy can exhibit line degeneracy in its EFSs that consist of two ellipsoids of opposite helicity states intersecting with each other. Very interestingly, light propagating along the direction of the line degeneracy experiences strong spin-dependent photon deflection, or optical spin Hall effect, which may lead to applications in optical signal processing and spin-optical manipulations. We provide a realistic metamaterial design to show that the required bianisotropy can be readily obtained.

Multiple-Input Multiple-Output Optical Integrator Design by Non-Interacting Control via Dynamic Extension

Semiconductor optical amplifiers (SOAs) have recently attracted much attention as the potential building blocks for active photonic circuits. They are being used in many applications of optical signal processing in addition to amplification of the optical signal. Their use as wavelength converters and optical logic gates has been demonstrated [1,2]. In this paper we propose an SOA based design of a multiple-input multiple-output integrator for optical signals. The design is based on non-interacting state feedback control through dynamic extension. If a nonlinear dynamical system takes the form of Equation (1) where, z is in Rn , f, g and h are n-dimensional smooth vector fields in the neighborhood of z0 in Rn , and it also has a well-defined vector relative degree in the neighborhood of z0, it is well known in the nonlinear control literature that the system can be transformed into a chain of integrators by state feedback [3]. Moreover, the effect of such feedback will result in the outputs which are completely decoupled from each other, with each output yi being affected only by the corresponding input, ui, hence it is named non-interacting control. This control method is commonly used in nonlinear multiple-input multiple-output systems. From the physical governing equations of SOA, the state space model for the device was developed by Knutze et. al, in [4] in the form of Equation (2), where z is the charge carrier density and RA, RB and RC are, respectively, linear, bimolecular and Auger recombination coefficients; I is the electric current, q is the charge of an electron; and V and L are, respectively, the volume and the length of SOA. For each optical channel: ui is the input and, Gi(x)=aiγi(z-ztr,i) is the gain function; ai is the differential gain, γi is the confinement factor; ztr,i is the transparency carrier density; ωc,i is the carrier frequency; and αi is the waveguide loss. The signals yi are the optical outputs. Equation (2) does not take the form of (1) because it has the inputs directly appearing in the outputs. Thus, non-interacting control method cannot be applied directly. Therefore, the dynamics of the system are extended by renaming z, u1 and u2 as z1, z2 and z3, respectively, and the external inputs: v1 and v2are used to control the dynamics of these extended states. Now the state space model becomes Equation (3). Since Equation (3) takes the form of Equation (1), and it was verified that the system has a well-defined vector relative degree 1 in a large range of operating points, the non-interacting feedback laws are derived and the state feedback is applied. After applying feedback, the closed loop device is simulated in Matlab Simulink and it was observed that each optical input signal ui(t) is integrated and its integral yi(t) appears at the output without inter-modal modulation. Simulation results for a pair of typical inputs are shown in Figure 1.

Light-induced polariton clusters and their application in optical signal processing

The properties of coupled long-lived states of the electromagnetic field + matter system that are induced by a collision of two counter-propagating coherent pulses in a dense resonant media are discussed. It is demonstrated that the system possesses properties of a mirrorless polariton nanocavity with a high Q-factor.

Magnetic and nonlinear optical properties of BaTiO3 nanoparticles

In our earlier studies the BaTiO3 samples were processed at higher temperatures like 1000oC and explained the observed magnetism in it. It is found that the charge transfer effects are playing crucial role in explaining the observed ferromagnetism in it. In the present work the samples were processed at lower temperatures like 650oC-800oC. The carrier densities in these particles were estimated to be ∼ 1019-1020/cm3 range. The band gap is in the range of 2.53eV to 3.2eV. It is observed that magnetization increased with band gap narrowing. The higher band gap narrowed particles exhibited increased magnetization with a higher carrier density of 1.23×1020/cm3 near to the Mott critical density. This hint the exchange interactions between the carriers play a dominant role in deciding the magnetic properties of these particles. The increase in charge carrier density in this undoped BaTiO3 is because of oxygen defects only. The oxygen vacancy will introduce electrons in the system and hence more charge carriers means more oxygen defects in the system and increases the exchange interactions between Ti3+, Ti4+, hence high magnetic moment. The coercivity is increased from 23 nm to 31 nm and then decreased again for higher particle size of 54 nm. These particles do not show photoluminescence property and hence it hints the absence of uniformly distributed distorted [TiO5]-[TiO6] clusters formation and charge transfer between them. Whereas these charge transfer effects are vital in explaining the observed magnetism in high temperature processed samples. Thus the variation of magnetic properties like magnetization, coercivity with band gap narrowing, particle size and charge carrier density reveals the super paramagnetic nature of BaTiO3 nanoparticles. The nonlinear optical coefficients extracted from Z-scan studies suggest that these are potential candidates for optical imaging and signal processing applications.

Switchable multi-wavelength Tm-doped mode-locked fiber laser.

We propose and demonstrate for the first time a switchable tri-wavelength Tm-doped ultra-fast fiber laser based on nonlinear polarization evolution (NPE) technique. The NPE effect induces wavelength-dependent loss in the cavity that changes the homogeneous broadening of the effective gain to become inhomogeneous. This inhomogeneous effective gain spectral profile enables the multi-wavelength mode locking. Binary control of three bits can be realized by controlling the polarization in the compact fiber ring cavity. Such switchable laser has potential applications in optical signal processing and communication.

Periodic and aperiodic pulse generation using optically injected DFB laser

Generation of optical pulse trains using semiconductor lasers has attracted significant attention due to their applications in optical communications and signal processing. A pulse generator based on the nonlinear dynamics of an optically injected distributed feedback laser is presented. The proposed system does not require electrical excitation and can be configured to operate in a periodic or aperiodic pulsation regime by controlling the injection conditions. Experiments show periodic pulsation with a repetition rate of 1.034 GHz and aperiodic pulsation with an average repetition rate of 5.618 GHz.

Plasmonic propagation and spectral splitting in nanostructured metal wires

Due to the coupling of photons with the electrons at a metal-dielectric interface, surface plasmons (SPs) can achieve extreflely small wavelengths and highly localized electromagnetic fields. Hence, plasmonics with subwavelength characteristics can break the diffraction limit of light, and thus has aroused great interest for decades. The SP-inspired reflearch, in the application respect, includes extraordinary optical transmission, surface enhanced Raman spectroscopy, sub-wavelength imaging, electromagnetic induced transparency, perfect absorbers, polarization switches, etc.; and in the fundamental respect, includes plasmon-mediated light-matter interaction, such as plasmonic lasing, plasmon-exciton strong coupling, etc.#br#Recently a series of studies has been performed to push the dimensions of plasmonic devices into deep subwavelength by using nanowires. The chemically synthesized metallic nanowires have good plasmonic properties such as low damping. The reported silver nanowire structures show great potential as plasmonic devices for communication and computation. Now we develop the nanostructured metal wires for plasmonic splitters based on the following considerations. One is that we introduce cascade nano-gratings on a metallic nanowire, enabling a single nanowire to act as a spectral splitting device at subwavelength; and the other is that we use silicon as a substrate for the metallic nanowire, making the plasmonic nanowire device compatible with silicon based technologies.#br#In this paper, we continue and develop our previous work on position-sensitive spectral splitting with a plasmonic nanowire on silicon chip (see Scientific Reports (2013) 3 3095). The three parts are organized as follows. In the first part, we derive analytically the dispersion relation of the SPs in a suspended silver nanowire based on Maxwell equations. In the second part, we placed a silver nanowire in the silicon substrate, and use the finite-element method (FEM) to obtain the dispersion relation of the SPs for the practical applications. The calculations show that the SP mode can be confined better in this system, howbeit with larger loss. Starting from the dispersion relation, we then calculate the mode area, the propagation length and the effective index of the SP modes, with respect to the nanowire dimension and the substrate materials. It is shown that a thinner nanowire has smaller mode area and a higher-index substrate induces larger loss. We also perform the finite-difference time-domain (FDTD) simulation to investigate the electromagnetic field distribution in this system. We find that the SP mode is mainly confined around the top surface of the nanowire, and in the crescent gap between the nanowire and the substrate. In the third part, we demonstrate both experimentally and theoretically that the silver nanowire with two cascaded gratings can act as a spectral splitter for sorting/demultiplexing photons at different spacial locations. The geometry of the grating is optimized by rigorous coupled wave analysis (RCWA) calculation. The carefully designed gratings allow the SPs with the frequencies in the plasmonic band and prohibit the SPs with the frequencies in the plasmonics bandgap. Those prohibited SPs areflemitted out through a single groove in front of each grating. Both the detected images and the measured optical spectra demonstrate that the SPs with different colors can be emitted at different grooves along a single nanowire. Thus the structured metal nanowire shows potential applications in position-sensitive spectral splitting and optical signal processing on a nanoscale, and provides a unique approach to integrating nanophotonics with microelectronics.

Polarization-insensitive and broad-angle gradient metasurface with high-efficiency anomalous reflection

Polarization-insensitive metasurfaces are of great value in practical applications. In this paper, we present a polarization-insensitive reflective phase-gradient metasurface operating in optical communication band which has almost the same electromagnetic (EM) responses for both x-and y-polarized incident waves with high-efficiency anomalous reflection.The reflective metasurface employs a typical metal (Au)-insulator (SiO2)-metal (Au) structure, in which the top metal layer consists of periodic arrays of isotropic cross-shaped gold antennas, i.e. unit cells. The supercell of the metasurface is composed of five unit cells with their dimensions different from each other. The normally incident waves are reflected by the metal-grounded plane, but the reflection phases of both x-and y-polarized waves are controlled by changing the dimensions of their unit cells. Based on the finite-difference time-domain simulations, we investigate the polarization-dependent EM responses of this metasurface under the illumination of linearly polarized incident plane waves. Selecting carefully five cross-shaped gold antennas in different dimensions, we obtain polarization-insensitive metasurface with high-performance anomalous reflection in optical communication band.First, in order to investigate the polarization sensitivity of the proposed metasurface, we study the EM responses for x-and y-polarized incident waves, since arbitrary linearly-polarized EM waves can be separated into two orthogonally-polarized components. We find that the two orthogonally-polarized incident EM waves have almost the same phase and amplitude response with the phase nearly linearly changing from 0 to 2up within a supercell, hence a constant gradient of phase discontinuity is introduced and anomalous reflection will occur. We further analyze the reflected electric-field patterns and the far-field intensity distributions, from which we find that the reflected beams exhibit low-distortion wavefronts and the scattered light is predominantly reflected into the anomalous mode. As a consequence, high-efficiency anomalous reflection is realized, with a 70% reflectivity at the operating wavelength of 1480 nm. Furthermore, we look into the incident-angle dependence of the proposed metasurface, and find that the designed metasurface can exhibit polarization insensitivity within a broad incident angle ranging from -30 to 0.In conclusion, we propose a broad-angle polarization-insensitive reflective gradient metasurface with high-efficiency anomalous reflection, which has potential applications in optical communications, signal processing, displaying, imaging and other fields.

Nonlinear switching dynamics in a photonic-crystal nanocavity

We report the experimental observation of nonlinear switching dynamics in an InP photonic crystal nanocavity. Usually, the regime of relatively small cavity perturbations is explored, where the signal transmitted through the cavity follows the temporal variation of the cavity resonance. When the cavity is perturbed by strong pulses, we observe several nonlinear effects, i.e., saturation of the switching contrast, broadening of the switching window, and even initial reduction of the transmission. The effects are analyzed by comparison with nonlinear coupled mode theory and explained in terms of large dynamical variations of the cavity resonance in combination with nonlinear losses. The results provide insight into the nonlinear optical processes that govern the dynamics of nanocavities and are important for applications in optical signal processing, where one wants to optimize the switching contrast.

Integrated Photonic Circuits in Gallium Nitride and Aluminum Nitride

Integrated optics is a promising optical platform both for its enabling role in optical interconnects and applications in on-chip optical signal processing. In this paper, we discuss the use of group III-nitride (GaN, AlN) as a new material system for integrated photonics compatible with silicon substrates. Exploiting their inherent second-order nonlinearity we demonstrate and second, third harmonic generation in GaN nanophotonic circuits and high-speed electro-optic modulation in AlN nanophotonic circuits.

All-Optical Signal Processing

Optical signal processing brings together various fields of optics and signal processing - namely, nonlinear devices and processes, analog and digital signals, and advanced data modulation formats - to achieve high-speed signal processing functions that can potentially operate at the line rate of fiber optic communications. Information can be encoded in amplitude, phase, wavelength, polarization and spatial features of an optical wave to achieve high-capacity transmission. We revisit advances in the key enabling technologies that led to recent research in optical signal processing for digital signals that are encoded in one or more of these dimensions. Various optical nonlinearities and chromatic dispersion have been shown to enable key sub-system applications such as wavelength conversion, multicasting, multiplexing, demultiplexing, and tunable optical delays. We review recent advances in high-speed optical signal processing applications in the areas of equalization, regeneration, flexible signal generation, and optical control information (optical logic and correlation).

Compact suspended silicon microring resonators with ultrahigh quality

We propose and demonstrate compact suspended silicon microring resonators with ultra-high optical quality. We achieve an intrinsic quality factor of 9.2 × 10(5) for the resonator with a radius of 9 μm. The high optical quality factor, high optical confinement together with the suspended structure of our device enable great potential for broad applications in biosensing, quantum photonics, nonlinear photonics, cavity optomechanics, and optical signal processing.