Bacteriorhodopsin (bR) is a promising biomaterial for several applications. Optical excitation of bR at an electrode-electrolyte interface generates differential photocurrents while an incident light is turned on and off. This unique functional response is similar to that seen in retinal neurons. The bR-based bipolar photosensor consists of the bR dip-coated thin films patterned on two ITO plates and the electrolyte solution. This bipolar photocell will function as a biomimetic photoreceptor cell. The bipolar structure, due to the photocurrent being generated in alignment with the cathodic direction, makes the excitatory and inhibitory regions possible. This scheme shows our bipolar cell can act as a basic unit of edge detection and forms the artificial visual receptive field.
We observed a propagation of surface plasmon polaritons (SPPs) along bent Au-wires on a thin SiO2-coat-InP substrate with the bending radius R from 20 to 1000 μm and evaluated the bending-part attenuation coefficients, i.e., the bending losses of propagating SPPs along them with R from 20 to 400 μm by measuring the transmittance of SPPs. We discussed about the SPP propagation and the applicability to an optical electronic device and circuit of SPPs.
A diffusion-current theory based on the superposition theorem of constant-current sources is applied to analyzing nerve
impulses transmitting through arrayed neurons laterally coupled via excitatory and inhibitory lateral connection paths.
Nonlinear dynamic responses to density-modulated impulses are simulated for simple point stimulus. Responses to
spatially distributed stimulus are also simulated to show that the arrayed neurons not only enhance contours of objects to
be detected but also exhibit some illusions.
This paper describes a new approach to realize a bidirectional repeater suitable for sensor networks with absolute time synchronization, and discuss propagation delay measurements of an experimental bidirectional repeatered system. A time-shared quasi-bidirectional linear repeater, that enables to the direction of signal flow to be changed. It uses a unidirectional EDFA, an optical router module composed of magneto-optic components and a delay fiber. The repeater has realized identical propagation delay for both directions of signal flows and attained multipath interference free amplification of optical packet signals for absolute time synchronization and sensing. A 50.078 km bidirectional repeatered line varied its propagation delays by 1.2 ns for both directions of signal flow with laboratory temperature change. Nevertheless, the experimental setup has not detected the propagation delay difference between both directions of signal flows that exceeded +/- 30 ps. This means that highly accurate absolute time synchronization will be feasible using bidirectional transmission system equipped with the time-shared quasi-bidirectional linear repeaters.
Reconfigurable wideband photonic networks based on dense wavelength division multiplexing (WDM) are promising for versatile ubiquitous services. In such networks, wavelength-selectable optical switches will be needed for WDM-based routing, which is a primary function of the networks. A critical requirement is a wavelength selection time of 20 ms or less; otherwise, data packets will be lost. Various filters, including acousto-optic filters or Fabry-Perot etalon filters, are candidates for such switches, but all are inadequate because of high driving power or low accuracy in selecting wavelengths. We propose and demonstrate a wavelength-selectable switch that consists of micromechanically movable in/out filters. A series of thirty-two in/out elements with highly wavelength-managed dielectric filter units, which have flat-top spectral responses according to ITU-T grids, are densely packaged into a small space of 45 x24 x11 mm using miniaturized voice-coil motors (VCMs). By accurately arranging the filter elements along a collimating optical beam between fibers, we achieve small total insertion losses of less than 2.5 dB for all elements. By optimizing the VCM torque, we also achieve a wavelength-selection time of 10 ms (The minimum is 5 ms). We also achieve good wavelength reproducibility with an error of less than 0.1 nm, which was confirmed by a repetition test. These results show that the proposed switches are suitable for practical use.
Surface-plasmon polaritons (SPPs) existing at metal-dielectric interfaces with exponentially damped vertical intensity profiles have attracted much interest for their capability of confining light energy into a small space beyond the diffraction limit. Theoretical considerations have been given to understandings of wave behaviors based on Maxwell's equations taking into account metals as dielectric materials with negative permittivity. However, since this approach assumes metals as homogeneous media, it is difficult to perform detailed analysis on propagation mechanisms in three-dimensional micro and nano structures. Here we propose a theoretical method based on the dynamics of electric dipoles formed by local displacement of free electrons in metals to describe SPP waves. In this method, the Poisson equation is used to describe actual movement of electrons in metals interacting with the electromagnetic field. Based on this method, we have revealed fundamental properties including electron density distribution functions in the area close to metal surfaces, SPP waves are then modeled by reconstructing microscopic features of such novel electromagnetic waves in a given material systems. After this simple verification, we make the best use of this method to explain SPP propagation at ultra-thin metal films or along narrow metal wires.
Silicon-based electronics has matured and holds a dominant position in critical technologies for computing systems. Advances in micro-miniaturization techniques enable us to fabricate nanometric devices with novel functions based on mesoscopic physics, and we expect that such devices will innovate on existing systems. Optics has also made tremendous progress since the first laser to generate quasi-coherent light was developed. Lasers are now widely used in basic science and in practical applications such as information processing and communications systems. Many studies have demonstrated novel functions in logics based on not only non-linear effects of media but also quantum-optic effects in nanometer-scaled structures. However, how to overcome the diffraction limit remains an unsolved fundamental problem how to break down the diffraction limit. Here, we report some ideas for nanophotonics and present a future picture of computing systems.
Chaotic fluctuation of light, which is being intrinsically different from deterministic chaos in lasers, arises from quantum-optic stochastic processes, and it therefore cannot be artificially replicated. When the fluctuation is correlative, however, it will be of more use in practical applications such as cryptographic communications. Throughout various experiments, it was found that a double-ring laser having a common semiconductor gain medium with strong saturation characteristics can produce a stable light beam consisting of negatively correlative dual-color components. Although each component decomposed by chromatic beam splitting is chaotic, their combination regenerates a stable light beam. This means that the photon-number states can be controlled by using an optical processing scheme for a correlative dual-color chaotic beam. How such a beam is generated is explained by a simple numerical simulation using a finite Markov chain model that assumes strong short-term intensity correlation between the components. A possible cryptosystem is presented based on the controllability of the photon-number state.
We propose a metamorphic network system, based on autonomously controlled wavelength division multiplexed devices. We will address the issue of how to assure and manage wavelength accuracy and controllability over the whole network to implement practical optical path routed networks. Our solution uses smart devices with wavelength calibration tables to construct systems that can be self-controlled according to their own stored physical layer information. The devices are interactive and share renewable information and are therefore useful for realizing self-reconfigurable (metamorphic) network systems. To verify the proposed architecture, we performed practical examinations using subsystems with wavelength-managed 'disk filters' in a field trial (Chitose trial).
This paper describes surface plasmon polariton (SPP) modes that show extraordinary propagation behaviors along metal stripes with finite width, including multiple energy condensations. Phenomenological and quantitative investigations are carried out on these behaviors. Images of SPP modes at output edges for wide stripes exhibit multiple spots that spanned the lateral side of the stripes. The spot number is monotonously decreased as the stripe length increases. Besides, the input versus output power ratio shows a stepwise change as changing the propagation distance. We present a model of SPP modes to explain these behaviors.
An autonomic network control scheme is presentd for wavelength-division multiplexing based photonic networks. An aboslute wavelength control scheme using a disk-shaped wavelength tunable optical bandpass filter is presented. The wavelength reproducibility of 0.02 nm for the entire tuning range at various temperatures is promising for the automonic wavelength management. To verify the control scheme, an optical add/drop multiplexing subsystem is constructed using optical switches utilizing oil latchign interferential tension effect and wavelength-tunable devices. Its detailed operating performances are experimentally evaluated. The switching speed of less than 20 ms and extinction ratios of less than 45 dB of the switching element are allowable in practical systems. Error-free transmission performance at 10 Gbps with negligible coherent cross talk at an intersection of the switch is obtained. The optical carriers generated by a wavelength-tunable ring laser usign a disk filter, are allowable for data transmissions. These results show that the autonomic entwork contorl scheme is feasible.
Novel network architecture and key device technology are described for next-generation photonic networks enabling high-performance data communications. To accomplish full-mesh links for efficient data transportaion, time-shared wavelength-division multiplexing is the most promising under the limitation imposed on the total wavelength number available at network nodes. Optical add/drop multipelxing (OADM) using wavelngth-tunable devices is essential for temporal data link fomraiotn. Wavelength managemetn based on absolute wavelength calibraiotn is a key to OADM operations. A simple wavelength dscriminating device using a disk-shaped tunable optical bandpass filter under the synchro-scanned operation is useful for managing the laser wavelengths. High-speed data transmissions of greater than 40 Gbps necessary for efficient operation of the networks are also described. A key is photonic downconversion which enables phase deteciton for optical data streams at above the electrical limitation of around 50 GHz. This technique is applied not only to a phase-locked loop for synchronizing mode-locked pulses to an electrical signal in the much lower frequency range of around 10 GHz, but to timing extraction from 100-Gbps data streams.
The optical micromachines controlling the light in spatial and wavelength domains are based on the micro- optomechatronics which handles small objects in micrometers and covers the fields from monolithic fabrication and control techniques. Their advantageous features include quick response, high mechanical stability, and low driving power because of the small inertia effects. A wide variety of functions such as wavelength tuning and optical path switching has been realized. This paper describes typical applications corresponding to these functions and their possible implementations: optical manipulation for small optical components based on momentum transfer of photons, micro lenses fabricated monolithically on a substrate for optical integration circuits, tunable filters with moving mirrors driven by the electrostatic force fro pulse shaping, and optical switches based on thermo capillary for cross connect or add/drop multiplexing operations in network systems. These applications are useful for next-generation photonic reconfigurable networks.
Photonic devices with ultra-wide and precise controllability for lightwaves are essential for constructing flexible optical networks to serve versatile multimedia applications. However, conventional monolithically fabricated photonic devices suffer from their controllability being limited by the physical characteristics. Micro-optomechatronics based on precise positional control of optical elements is a promising method of meeting the above requirements. This paper presents typical examples, which include repetition- rate tunable optical pulse sources with a micro mechanically controllable cavity length and synchro-scanned tunable disk- shaped optical fiber modules. The operations of these modules were demonstrated to confirm the validity of micro- optomechatronics as the ultimate lightwave control scheme, which will be useful for future optical telecommunications systems.
Optical heads wherein a laser diode (LD) is optically coupled through a small aperture to a recording medium acting as an external mirror are described. A flying head and a fiber- coupled head are presented, and their high-sensitivity, wavelength-free detection characteristics are demonstrated. A micro-lens head is also investigated. Interference undulations of the coupled-cavity laser with various spatial frequencies due to variations in the external-cavity length are completely explained. Anti-reflection (AR) coatings on the LD facet with reflectivities of better than -40 dB greatly suppress these undulations. To achieve practical heads, fundamental fabrication techniques including AR coating and monolithic integration on laser wafers are briefly mentioned.
KEYWORDS: Head, Reflectivity, Signal to noise ratio, Semiconductor lasers, Near field optics, Reliability, Optical storage, Servomechanisms, Optical discs, Waveguides
A feasibility study is shown for an optical disk drive using an optically switched laser (OSL) flying head. This head consists of an AlN slider and 1.3-micrometers wavelength InGaAsP laser diode monolithically integrated with a photodiode. The laser diode has a taper-ridged waveguide on the top of the facet facing the recording medium. The disk consists of an SiN dielectric protective layer, an SbTe Ge recording medium, and 1.5-micrometers -track pitch sampled servo format pits on the glass substrate. The lensless near-field flying head gives excellent read/write performance and head/medium reliability: a high-quality readout SNR of 35 dB, stable write operation with 16 mW of laser power, and a lifetime of 4.4 X 108 passes under experimental room conditions.
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