Modelling of movement of the lamprey: control of oscillators.

This article discusses the control methods of the central pattern generator (CPG). First a control model of the CPG is presented using 2 oscillators, and we suggest that phasic modulation to the CPG by means of phasic information is effective for controlling the phase difference between oscillators. Next, two models for controlling the CPG of a lamprey are proposed. One model describes a control system from the brain stem, in which the reticulospinal neurons control the CPG by receiving feedback signals and sending control signals to the neck region of the CPG. The other is a model for learning an localized control system to generate a desired motor pattern. By means of these models, a role of the efference copy is suggested.

Direct observation of second-harmonic generation from crystalline particles in Ge-doped SiO2 glass films.

The authors report what is, to our best knowledge, the first observation of second-harmonic generation (SHG) directly from the crystalline particles in Ge-doped SiO2 (Ge:SiO2) glass films. Ge:SiO2 glass films with approximately 5 microm thickness were fabricated by chemical vapor-phase deposition. X-ray diffraction (XRD) peaks at around 2theta=22 degrees in thermally crystallized Ge:SiO2 films were observed, and obtained XRD patterns are exactly the same as those in ultraviolet-laser-poled Ge:SiO2 glasses. Using SHG microscopic technique with a Nd:YAG laser, it has been successfully found that SH emitting with 532 nm wavelength is observed directly from the crystalline particles induced in the crystallized films.

A learning model for oscillatory networks.

A learning model for coupled oscillators is proposed. The proposed learning rule takes a simple form by which the intrinsic frequencies of the component oscillators and the coupling strength between them are changed according to the effects of the input signals on the dynamics of the oscillator. In the learning mode, each component oscillator receives a teacher signal of desired phase and frequency, and a desired parameter set for generating the desired pattern is acquired by the proposed learning rule. It is known that the basic locomotor patterns of many living bodies are generated by coupled neural oscillators. The proposed learning rule could be a learning model used by such neural systems to acquire an adequate parameter set for generating a desired locomotor pattern.

Evaluation of trajectory planning models for arm-reaching movements based on energy cost.

Computational studies have suggested that many characteristics of reaching trajectories in a horizontal plane can be effectively predicted by certain models, including, the minimum end point variance model and minimum torque change model. It has also been reported that these characteristics appear to differ from those obtained by the minimum energy cost model that has been reported to explain the characteristics of locomotor patterns. Do these results imply that the human nervous system uses different strategies to resolve the redundancy problem for different tasks? In order to reexamine the optimality of reaching trajectories from a viewpoint of energy cost, we considered the corrective submovements to compensate for positional error due to signal-dependent noise in motor commands and computed the expected value of the total energy costs required to reach a target by repetition of submovements planned by each of the following models: the minimum energy cost model, minimum end point variance model, and minimum torque change model. The results revealed that when the noise is large, the total energy cost required by the minimum end point variance model and the minimum torque change model can be lower than that required by the minimum energy cost model which assumes minimizing energy cost under noise-free condition. This result indicates that the minimization of the expected value of the energy cost would be an important factor in determining the reaching trajectories.

Surface-relief gratings with high spatial frequency fabricated using direct glass imprinting process.

Surface-relief gratings with high spatial frequencies were first fabricated using a direct imprinting process with a glassy carbon mold at the softening temperature of phosphate glass. A grating with maximum height of 730 nm and 500 nm period was formed on the glass surface by the pressing at the softening temperature of glass under constant pressure of 0.4 kN/cm(2). Phase retardation of 0.1 lambda was observed between TE-polarized and TM-polarized light at 600 nm wavelength.

Legged insects select the optimal locomotor pattern based on the energetic cost.

The gait transition in legged animals has attracted many researchers, and its relation to metabolic cost and mechanical work has been discussed in recent decades. We assumed that the energetic cost during locomotion is given by the sum of positive mechanical work and the heat energy loss that is proportional to the square of joint torque and examined the optimal locomotor pattern based on the energetic cost in a simple dynamical model of a hexapod by computer simulations. The obtained results well agree with characteristics in the locomotor patterns in legged animals; for example, the leg protraction time, step length, and the metabolic cost of transport are almost constant for many velocities, the leg cycling period decreases with velocity, and the energetic cost of locomotion induced by carrying loads linearly increases with mass loaded. This correspondence of the results of calculation to experimental results suggest that the heat energy loss for torque generation is proportional to the square of the torque during locomotion, and that the locomotor pattern in legged animals is highly optimized based on the energetic cost.

Learning model for coupled neural oscillators.

Neurophysiological experiments have shown that many motor commands in living systems are generated by coupled neural oscillators. To coordinate the oscillators and achieve a desired phase relation with desired frequency, the intrinsic frequencies of component oscillators and coupling strengths between them must be chosen appropriately. In this paper we propose learning models for coupled neural oscillators to acquire the desired intrinsic frequencies and coupling weights based on the instruction of the desired phase pattern or an evaluation function. The abilities of the learning rules were examined by computer simulations including adaptive control of the hopping height of a hopping robot. The proposed learning rule takes a simple form like a Hebbian rule. Studies on such learning models for neural oscillators will aid in the understanding of the learning mechanism of motor commands in living bodies.

Bragg gratings printed upon thin glass films by excimer laser irradiation and selective chemical etching.

Photon-induced property changes of sputter-deposited GeO(2)-SiO(2) thin glass films were investigated. Irradiation with ArF laser pulses induced the changes in refractive index of -10% and volume of +30% in the film without ablation. A Bragg grating with a positive sinusoid wave pattern was printed upon the film by irradiation with ArF excimer laser pulses through a phase mask. The irradiated area could be quickly etched by a HF solution. The ratio of etching rate of irradiated area to unirradiated area was higher than 30. A Bragg grating with a surface relief pattern was successfully formed on the film by irradiation with excimer laser pulses followed by chemical etching. Diffraction efficiency of the gratings increased by 25 times with the etching.

High photosensitivity and nanometer-scale phase separation in GeO(2)-SiO(2) glass thin films.

Amorphous xGeO(2)-(1-x)SiO(2) thin films exhibit large negative index changes (4-8%) in the high GeO(2) region (x>~0.25) on irradiation with ArF laser pulses. The sign of the index change is opposite the low GeO(2) region X<0.25, and the magnitude of the index change is larger by an order of magnitude than that reported so far. Cross-sectional transmission electron microscope observation has revealed that nanometer-scale phase separation is induced in these highly photosensitive glasses by irradiation with ArF excimer laser light pulses or electron beams. This is a first finding of microphase separation in SiO(2)-GeO(2) glasses by irradiation and provides an essential constraint on the modeling of photonic effects induced by irradiation in these glasses.

Diffraction gratings of photosensitive ZrO2 gel films fabricated with the two-ultraviolet-beam interference method.

Photosensitive ZrO(2) gel films were patterned with a two-beam interference method by use of a 325-nm-wavelength He-Cd laser for the first time to our knowledge. The ZrO(2) gel films were prepared from Zr(O-n-C(4)H(9))(4) chemically modified with benzoylacetone. We fabricated uniform gratings with a 0.5-microm period on Si or SiO(2) substrates by etching the gel films in ethyl alcohol after UV irradiation. A maximum diffraction efficiency of 28% was attained with the grating fabricated on Si substrate under a Littrow mounting condition by use of a 633-nm-wavelength He-Ne laser. Blazed gratings could also be fabricated.

Chalcogenide glass fiber with a core-cladding structure.

GeAsSeTe, GeSeTeTl, and GeSeTe glass fibers with a core-cladding structure were prepared by one of two methods: crucible drawing or preform drawing. The diameter of the core was 340 microm and that of the cladding was 440 microm. The transmission loss spectra of the fibers were measured by the cut-back method. The crucible drawing method was preferable to reduce the loss due to structural imperfections at the interface of the core and cladding. The lowest loss of 1.7 dB/m was achieved at 10.6 microm (the wavelength of the CO(2) laser) in the fibers with a GeSeTe core and a GeAsSeTe cladding. Transmission of CO(2) laser power was examined using a 1.5-m long fiber and an output power of 2 W (2.2 kW/cm(2) in power density) was obtained.

Bending sensitivity of in-series long-period fiber gratings.

The interference fringes formed in a long-period fiber grating pair are sensitive to the loss in the cladding mode, which can be induced by bending or by coating on the fiber. Assuming that there is no loss and that both gratings are identical, the interference fringes are shown to have 100% contrast when each grating has 50% transmissivity. By contrast, if the cladding mode is absorbed or scattered the fringe contrast is shown to be degraded. The spectral behaviors of a long-period fiber grating pair with loss is analyzed by the coupled-mode theory and is compared with experimental results.

Dependence of fringe spacing on the grating separation in a long-period fiber grating pair.

The spectral spacing of the interference fringes formed by a pair of long-period fiber gratings was investigated. The variation of the fringe spacing was measured while the separation between the gratings was changed from 22 to 500 mm. When the grating separation was much longer than the length of the individual grating, the inverse of the fringe spacing became linearly proportional to the grating separation and to the differential effective group index of the fiber. In the third stop band of the grating pair, made along a dispersion-shifted fiber centered at 1.55 microm, the differential effective group index was calculated to be approximately 6.4 x 10(-3), which is approximately twice the differential effective index of the fiber. The discrepancy between the two indices was observed to decrease with the band order, a phenomenon that is explained by the first-order dispersion of the fiber. The measured interference fringes were not regularly spaced in the frequency domain, but regular spacing is required in wavelength-division multiplexing communication systems. Analysis of the second-order dispersion of the fiber and the grating-induced nonlinear phase shift within grating regions as the factors that induce chirping on the fringe spacing is presented.

Preparation of Bragg gratings in sputter-deposited GeO(2)-SiO(2) glasses by excimer-laser irradiation.

GeO(2) - SiO(2) thin glass films were prepared by the rfsputtering deposition method. Changes in the refractive index of -3% and in the volume of +18% were induced in the film by irradiation with excimer-laser pulses. Bragg gratings a (periodic surface-relief pattern with a sinusoidal wave) have been written in the film by irradiation through a phase mask.

In situ observation of photoinduced refractive-index changes in filaments formed in glasses by femtosecond laser pulses.

We investigated the relationship between the formation of filaments and local refractive-index changes induced by femtosecond laser pulses in silica glass. In situ observation revealed that the location of a filament coincided with that of the refractive-index change. Observation also showed that the region of refractive-index change was elongated toward the upstream direction of the laser pulses with increasing exposure time. The region of refractive-index change was several hundred micrometers long, and its diameter was smaller than 2 mum. The refractive-index change was confirmed by two of three different methods to be as large as 0.8 x 10(-2).

Antireflection microstructures fabricated upon fluorine-doped SiO(2) films.

Two-dimensional periodic structures were fabricated upon a fluorine-doped SiO(2) film in which the fluorine content changed gradually in the direction of film thickness. The films were deposited by plasma-enhanced chemical-vapor deposition. The film was periodically patterned into a 1-mum period and an ~1-mum -groove depth by inductive coupled plasma reactive-ion etching followed by chemical etching in a diluted HF solution. A surface reflectance of 0.7% was attained at 1.85-mum wavelength, a value that is one fifth as large as the 3.5% Fresnel reflection of a SiO(2) substrate with a flat surface.

Three-dimensional hole drilling of silica glass from the rear surface with femtosecond laser pulses.

By moving silica glass in a preprogrammed structure, we directly produced three-dimensional holes with femtosecond laser pulses in single step. When distilled water was introduced into a hole drilled from the rear surface of the glass, the effects of blocking and redeposition of ablated material were greatly reduced and the aspect ratio of the depth of the hole was increased. Straight holes of 4-mu;m diameter were more than 200 microm deep. Three-dimensional channels can be micromachined inside transparent materials by use of this method, as we have demonstrated by drilling a square-wave-shaped hole inside silica glass.

Optical seizing and merging of voids in silica glass with infrared femtosecond laser pulses.

We demonstrate that one can seize and translate voids formed by IR femtosecond laser pulses inside silica glass and can also cause two voids to merge into one. We also present clear evidence of a void and its surrounding region by showing scanning electron microscope images of cleaved voids, which we produced by cleaving through the glass along a plane that included a thin laser-ablated line on the surface of the glass and the voids formed inside.