Interferometric method for preventing mode-hopping in tunable external-cavity semiconductor lasers

A problem in frequency-tuning of external-cavity lasers is mode-hopping between neighbouring external-cavity modes. We demonstrate a new interferometric method for monitoring mode-hoping and an automatic control circuit for a 1.3 ?m grating external-cavity laser that maintains single-mode operation when the lasing frequency is tuned.     

Wide-span optical frequency comb generator for accurate opticalfrequency difference measurement

An optical frequency comb (OFC) generator was realized for accurate optical frequency difference measurement of 1.5 μm wavelength semiconductor lasers by using a high frequency LiNbO₃ electrooptic phase modulator which was installed in a Fabry-Perot cavity. It was confirmed that the span of the OFC was wider than 4 THz. By using semiconductor lasers whose spectrum linewidths were narrowed to 1 kHz and a sensitive optical balanced-mixer-receiver for measuring beat signal between the sideband of the comb and the laser, we demonstrated a frequency difference measurement up to 0.5 THz with a signal-to-noise ratio higher than 61 dB, and a heterodyne optical phase locking with a heterodyne frequency of 0.5 THz in which the residual phase error variance was less than 0.01 rad². The maximum measurable frequency difference, which was defined as the sideband frequency with the signal-to-noise ratio of 0 dB, was estimated to be 4 THz     

Modulation characteristic of waveguide-type optical frequency combgenerator

We report the modulation characteristics of a waveguide-type optical frequency comb generator (WG-OFCG) with the advantages of compactness, high modulation index and low driving power. The characteristics of the optical frequency comb (OFC) generated when the modulation index exceeds 2π are discussed. The power of the modulation sidebands was measured by the optical heterodyne method with an external-cavity laser as a local oscillator. The dependence of the modulation index of the WG-OFCG on the modulation frequency up to 40 GHz was measured. The generation span of the OFC at the modulation frequency detuned from an integer multiple of the free spectral range (FSR) is discussed     

Thermal response in thermal energy storage material around heat transfer tubes: effect of additives on heat transfer rates

The effects of carbon-fiber chips and carbon brushes as additives on the thermal conductivity enhancement of phase change materials (PCMs) using in latent heat thermal energy storage are investigated experimentally and numerically by considering the wall effect of the additives. The carbon-fiber chips are effective for improving the heat transfer rate in PCMs. However, the thermal resistance near the heat transfer surface is higher than that for the carbon brushes. As a result, the overall heat transfer rate for the fiber chips is lower than that for the carbon brushes. Consequently, the carbon brushes are superior to the fiber chips for the thermal conductivity enhancement under the present experimental conditions. The carbon brushes are moreover applied to the packed beds of particles to overcome their low thermal conductivity in chemical heat pump/storage. The carbon brushes essentially improve the heat transfer characteristics in the packed beds, though the thermal resistance is observed because the particles obstruct contact between the fibers and the heat transfer surfaces.     

Limit of optical-frequency comb generation due to materialdispersion

The limit of optical frequency comb (OFC) generation (i.e., the limit of frequency difference measurement) due to the material dispersion in the EO crystal is experimentally studied. By using a modified monolithic OFC generator, we observed the OFC spectrum, and confirmed that the envelope of the OFC around 780 nm extended to a span as wide as 16 nm (or 7.6 THz) reaching the limit of the OFC generation. We also proposed a method of stabilizing the Fabry-Perot cavity for the monolithic OFC generator     

Study on ketalization reaction of poly(vinyl alcohol) by ketones. IV. Reaction between poly(vinyl alcohol) and aliphatic ketones and behavior of polyvinylketal in water

Ketalization reaction of poly(vinyl alcohol) (PVA) by aliphatic ketones, dimethylsulfoxide (DMSO) as solvent, under the presence of acidic catalyst, in homogeneous system was carried out and the synthesis of polyvinylketals were successfully performed. The equilibrium constant at 40°C is ca. 0.07 in the case of methyl n-propyl ketone (nPK) and methyl n-butyl ketone (nBK), but is ca. 0.05 in the case of methyl i-propyl ketone (iPK) and ca. 0.01 in the case of methyl i-butyl ketone (iBK) and methyl t-butyl ketone (tBK), respectively. Moreover, the ketalization degree of polyvinylketal by iBK and tBK reached to only ca. 35 mol % as the maximum. It seems that these were due to steric hindrance of bulky side chain of ketones. But the heat of reaction is 7.5 kcal/mol in all aliphatic ketones, it seems to proceed the same ketalization reaction mechanism. Films prepared from the polyvinylketals were soaked in water and degree of swelling, solubility, and hydrolysis of films were measured. The reaction of film with water, in acidic side, at first the film swells, and then, as the deketalization reaction proceeds, the film dissolves in water. The dissolution time is controlled by the kind of ketones, ketalization degree, and pH of water which reveals that deketalization reaction proceeds proportional to proton concentration. It is more difficult to dissolve highly ketalized polyvinylketals obtained by propyl ketones and butyl ketones than that by acetone. The hydrolysis of polyvinylketal film proceeds in the order as follows: acetone > MEK > nPK > iPK ≒ nBK > iBK > tBK. This phenomenon seems to be affected by hydrophobicity of the film surface which depends upon the kind of the original ketones.     

Stochastic Resonance in Recurrent Neural Network with Hopfield-Type Memory

Stochastic resonance (SR) is known as a phenomenon in which the presence of noise helps a nonlinear system in amplifying a weak (under barrier) signal. In this paper, we investigate how SR behavior can be observed in practical autoassociative neural networks with the Hopfield-type memory under the stochastic dynamics. We focus on SR responses in two systems which consist of three and 156 neurons. These cases are considered as effective double-well and multi-well models. It is demonstrated that the neural network can enhance weak subthreshold signals composed of the stored pattern trains and have higher coherence abilities between stimulus and response.     

Importance of multiple-phonon interactions in molecular dissociation and nanofabrication using optical near fields

A quasi-particle (exciton-phonon polariton) model, as a simple model of an optical near-field probe, is proposed to investigate an unresolved problem in photochemical processes, i.e., why a vapor molecule can be dissociated by an incident photon with less energy than the dissociation energy only if, not a propagating far field, but an optical near field is used, and what is the mechanism leading to the photon flux dependence of the deposition rates. Incident photon energy and intensity dependences of Zn deposition rates are analyzed, and good agreement between the theoretical and experimental results is obtained. It suggests that the probe system plays an important role in vibrational transitions as well as electronic transitions in photodissociation processes, and that the couplings between the optical near field and molecular vibrations are enhanced to permit a nonresonant photodissociation inherent in the optical near field.     

Enhanced catalytic activity of nanoshell carbon co-doped with boron and nitrogen in the oxygen reduction reaction

The use of carbon cathode catalysts in polymer electrolyte fuel cells instead of the current platinum catalysts is attracting increasing attention. We claim that two factors are important for enhancing the activity of carbon cathode catalysts in the oxygen reduction reaction (ORR): the formation of a nanoshell structure and co-doping with boron and nitrogen. Herein, we investigate the preparation and characterization of active ORR carbon catalysts that combine the above factors. Boron and nitrogen (BN)-doped nanoshell-containing carbon (BN-NSCC) was prepared by carbonizing a mixture of poly(furfuryl alcohol), cobalt phthalocyanine, melamine, and a trifluoroborane–methanol complex at 1000 °C. Transmission electron microscopy and X-ray photoelectron spectroscopy revealed the formation of nanoshell structures with distorted graphitic layers and the introduction of boron and nitrogen atoms, respectively. The ORR activity was evaluated in oxygen-saturated 0.5 mol dm^?3 H₂SO₄ using Koutecky–Levich analysis. The BN-NSCC showed an eight to ten times higher ORR activity than undoped NSCC, with an increased number of electrons participating in the reaction. Tafel analysis revealed a change in the rate-determining step caused by BN-doping. Thus, the combination of a nanoshell structure and co-doping with boron and nitrogen was found to improve the ORR activity of carbon catalysts.