Low-loss Region Research Articles

Raman amplification of 1.50-µm laser diode light in a low fiber loss region

Raman amplification utilizing the second Stokes line of 1.32-μm Nd:YAG laser light in a single-mode silica fiber was observed. Raman gains of 24 dB have been obtained for 1.50-μm laser diode light in the low-loss region. In addition, the signal-to-noise characteristics of Raman amplification using the second Stokes line was investigated. As a result, it became clear that the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> ratio was inferior to that in 1st Stokes line by about one order of magnitude.

Monitoring method for axis alignment of single-mode optical fiber and splice-loss estimation

A new method for core-axis alignment and precise splice-loss estimation for single-mode optical fibers is presented. By using a differential interference contrast microscope, we can achieve core-axis alignment with an offset below 0.3 microm, which results in a butt-joint loss increase of 0.04 dB compared with that obtained by alignment using a conventional optical monitoring method. Furthermore, splice-loss estimation with a precision of 0.05 dB is attained for a low-loss region without using an index-matching liquid.

分析電子顕微鏡 ＥＥＬＳを中心として

The principle of electron energy loss spectroscopy (EELS) is discussed based on the dielectric function of solid specimen. Electron energy loss spectra in low (<50eV) and high energy loss regions (>50eV) are analysed for graphite and explained by its electronic band structure. Limitations of the spatial resolution and the detectable mass concentration by EELS in TEM are discussed from the point of view of local chemical analysis. The mapping techniques and images formed by characteristic energy losses of each element contained within the specimen are shown for some examples. In addition to EELS in TEM, other examples of EELS in SEM are shown for surface analysis of bulk specimen.

280 Mbit/s single-mode fibre transmission with DFB laser diode emitting at 1.53 μm

By using a distributed feedback buried heterostructure InGaAsP/InP laser diode emitting at 1.53 μm, 21.7 km single-mode fibre transmission at 280 Mbit/s was successfully carried out without any noticeable degradation In a waveform and an error rate. This result indicates that a long-haul optical telecommunication system with a repeater span of over 70 km is possible in the fibre low-loss region of 1.5 μm to 1.6 μm wavelength.

Twin-stripe injection laser with leaky-mode coupling

A new twin-stripe leaky-mode (TLM) injection laser has been demonstrated. The structure consists of two nominally 2-μm-wide stripes on 24-μm centers in a buried-heterostructure configuration (produced by mesa-etching a three-layer double heterostructure and burying with an AlGaAs layer). By adjusting the mole fraction of Al in the burying layer, a negative-index step is produced at the lateral stripe boundary. Thus a substantial fraction of laser power leaks into the low-loss waveguide region between the stripes, and strong stripe coupling is observed. The far field consists of two collimated (divergence ∼1°) beams characteristic of leaky-mode operation that are stable to pulsed drive current as high as 6Ith. Linear output characteristics are observed to powers greater than 150 mW.

Low Energy Loss Structure of Small Aluminum Particles

Usually, configuration effects are ignored in microanalytical work. That is, the energy loss mechanisms in small volumes are expected to be similar to those in large volumes. However, it is shown elsewhere in these proceedings1 that this assumption is not valid for bulk plasmons in small Al spheres. In fact, ‘configurational’ energy loss effects can dominate the low energy loss region when probing small volumes. These effects include: surface plasmons, bulk plasmon spatial quantization, Schottky barriers, and any other mechanisms that rely on the close proximity of surfaces or interfaces. We have examined the energy loss structure for ˜200Å Al particles (formed as explained in Ref. 1) with the VG Microscopes, Ltd. HB5 scanning transmission electron microscope at 100KeV. The particles consist of single crystal Al cores covered by ˜40Å of Al2O3, and possibly further covered by 5-15Å of carbon contamination. We obtained electron energy loss spectra (EELS), and inelastic scattering images of the particles to correlate the observed EELS structure with the spatial structure and to make a tentative identification of the losses. The spectra were recorded with a probe angular diameter of 8mR (1.4Å−1) and an energy loss resolution of 0.7eV. The images were recorded with an energy loss resolution of 2eV.

Energy loss spectroscopy (ELS) on the Si-Au system

We report on the energy loss spectroscopy obtained on the clean cleaved Si(111) covered with different gold thickness. The results clearly indicate that in the Si-Au system the interface is characterized by a Si rich phase with a well defined electronic transition at 7.5 eV energy loss. Increasing Au thickness the main changes in the spectrum arise in the low energy loss region and a gold-like behaviour is observed for a gold coverage of 60 monolayers. The effects of annealing at 350°C are reported.

Wavelength dispersion characteristics of single-mode fibers in low-loss region

Wavelength dispersion characteristics of single-mode silica fibers in a very low-loss region, including zero dispersion wavelengths, are studied in detail using a "difference method." Wavelength dispersion of single-mode fibers is compared for fibers with same material dispersion but with different waveguide structure. Material dispersion is evaluated by extracting waveguide dispersion from experimental results. Effects of waveguide dispersion on the zero dispersion wavelength of the single-mode fiber in a longer wavelength region are clearly analyzed. Single-mode fiber design consideration is given from the wavelength dispersion point of view.

Efficient LiNdP_4O_12 lasers pumped with a laser diode

Efficient wavelength conversions into low-dispersion and low-loss regions of optical fibers are reported. (AIP)

Tailoring zero chromatic dispersion into the 1.5–1.6 μm low-loss spectral region of single-mode fibres

We have designed and fabricated single-mode silica fibres that have zero chromatic dispersion within the 1.5-1.6 μm low-loss spectral region by controlling their waveguide dispersion and dopant-dependent material dispersion. These fibres have small core diameters and large core-cladding refractive-index differences, n = 0.018, corresponding to a 13 mole % GeO2 dopant concentration in the core. The zero-dispersion wavelength was shifted from -0 = 1.375 = to =0 = 1.54 = by decreasing the fibre core diameter from 7 μm to 4.8 μm. This ability to tailor the minimum dispersion wavelength to the ultra-low-loss region near 1.55 μm is important for designing single-mode fibres for very long distance and very high capacity optical transmission.

Single-Mode Pulse Dispersion in Optical Waveguides

The limitations of a widely used method for analyzing pulse distortion in a single-mode waveguiding structure are derived. The results are applied to propagation in optical waveguides, and for cases where material dispersion is dominated by a broad resonance line, pulse attenuation is found to be much more serious than the broadening of the pulse. In extremely low-loss regions, however, other effects may cause the reverse to be true.

Modes of Stable and Unstable Optical Resonators

A method is developed for the approximate determination of the normal modes of stable and unstable optical resonators and the associated resonant frequencies and power losses. The method is based on replacing the finite integration limits in the integral equation for the normal modes by infinite limits and, subsequently, finding a differential equation whose solutions coincide with or approximate the solutions of this integral equation. When the end reflectors of the resonator are conical surfaces, a differential equation is found which corresponds exactly to the integral equation with infinite limits. Moreover, the equivalent differential equation is found to be of the same form as the wave equation for a monochromatic transverse electric wave propagating in an inhomogeneous medium of infinite extent with the inhomogeneity being transverse to the direction of propagation, showing the correspondence between the modes of a homogeneously filled conical resonator and the eigenmodes of an infinite inhomogeneous mdium. For the stable, low loss (convergent) region the solutions of the differential equation are readily found. For the unstable, high loss (divergent) region the solutions are found by using the principle of analytic continuation. The specific example of parabolic end reflectors is treated in more detail, and solutions for the eigenvalues and eigenfunctions are given for the case of infinite strip and circular geometries.

Energieverlustspektren der Alkalihalogenide und der Metalle Cu, Ag und Au und Vergleich mit optischen Messungen

Energy loss spectra of 50 keV electrons have been measured in transmission of the alkali halides and of Cu, Ag, and Au at zero scattering angle. These spectra are compared with the quantity −Im(1/e) determined from optical measurements. The general agreement is good, especially in the low energy loss region. In the case that the foil thicknesses could be measured, the absolute intensity of −Im(1/e) is found to be equal or up to three times larger than from optical data. In addition to the sharp low energy peaks (excitons), characteristic sharp peaks are found at higher energies: 60 eV for the Li-halides, 33 eV for the Na-halides, and 20 eV, 17 eV, and 13 eV for the K-, Rb-, and Cs-halides respectively. These peaks are very little shifted by the different anions in the crystal lattice and may be due to excitons caused by alkali band transitions. We have further examined several peaks in the loss spectra of LiF, KCl, KBr and the Na-halides: 1. The angular dependence of the loss intensity is in agreement with theory. 2. The dispersion of the peaks is much smaller than for plasma losses. 3. The temperature dependence of the peak energy has been found quite different for different peaks.

Effects of nuclear radiations on structure and molecular motions in some crystalline stereoregular polymers

AbstractYoung's modulus and the mechanical damping factor have been determined between −180 and +280°C. (at a frequency of several kilocycles), in samples of isotactic polypropylene, isotactic polystyrene, and trans‐1,4‐polybutadiene, subjected to pile irradiation (γ‐rays and neutrons) at γ‐doses from 90 to 4000 Mrad. In isotactic polypropylene no important structural changes are produced by the irradiation, except for a partial destruction of crystallinity. The samples receiving high radiation doses exhibit a low temperature loss region, which is attributed to the formation of a certain number of branches. Isotactic polystyrene shows very slight modifications of the dynamic mechanical properties at room temperature. At low temperature an increase of intensity of the δ relaxation phenomenon (probably due to oscillations of phenyl rings) with increasing radiation dose is observed. Important structural modifications produced by the radiation, destruction of crystallinity accompanied by crosslinking, which transform the material into a crosslinked rubber, are observed in trans‐1,4‐polybutadiene. Unlike conventional (sulfur) vulcanization, crosslinking by radiation does not cause a marked shift of the glass transition point. A secondary low‐temperature relaxation effect, not existing in the unirradiated material, appears in the mechanical loss curves of the irradiated samples; it is attributed to the formation of CH2sequences in the main chains through saturation of CC bonds. The mechanical spectrum of irradiated polybutadiene is very similar to those shown by crosslinked ethylene–butadiene copolymers.

A Scalar Resonator Theory for Optical Frequencies

A scalar resonator theory for optical or near-optical frequencies is developed. For an arbitrary resonator system a general integral equation is derived which represents a solution of Helmholtz's equation between the reflectors and satisfies Maxwell's equations along the surface of the reflectors. To meet the consequences of the boundary conditions an eigenmode is defined as an energy distribution which when launched from an arbitrary mathematical surface between the reflectors reproduces itself within an amplitude factor after a number of bounces given by the periodicity of the system under consideration. The eigenvalues of the integral equation determine the diffraction losses and the resonance spectrum. The mode patterns between the reflectors can be deduced from the corresponding eigenfunction. The general theory is discussed in connection with resonator systems with spherical reflectors. This reveals a physical picture of the low-loss and high-loss regions as the reflector spacing is varied. With the example of the Fabry-Perot interferometer it is demonstrated that the present resonator theory agrees with geometrical optics as the Fresnel number becomes very large. The scalar formulation to be presented accounts for the phenomenon of diffraction and is consistent with the microwave and the antenna theories. The general theory when applied to a periodic sequence of thin lenses suggests that it may equally well be the basis of a theory for beam waveguides.

Generalized Confocal Resonator Theory

The theory of the confocal resonator is extended to include the effect of unequal aperture size and unequal radii of curvature of the two reflectors. The latter is equivalent to a periodic sequence of lenses with unequal focal lengths. This treatment is in Cartesian coordinates as previously used. In an appendix the modes and resonant formulas are written in cylindrical coordinates. The effect of unequal aperture size of the two reflectors is shown to produce mode patterns of unequal size on the two reflectors of a confocal resonator. The previous computations for diffraction losses are found to be applicable. Generalization of the theory to the case of reflectors of unequal curvature shows the existence of low-loss regions and high-loss regions as the reflector spacing is varied. One of the high diffraction loss regions occurs when the reflector spacing is between the two unequal radii of curvature. Such a region is interpretable in terms of instabilities in a periodic sequence of lenses of unequal focal length. An estimate of diffraction losses is obtained for the low-loss regions. The presence of a high diffraction loss region or unstable region should be of importance in the design of resonators or of a periodic sequence of lenses.

Barrier-layer dielectrics

Barrier-layer dielectrics are bodies exhibiting an anomalously high permittivity as a result of continuous thin layers of low-loss dielectric enclosing regions of high conductivity. This inhomogeneity is brought about in ceramic dielectrics containing titanium dioxide by a reduction process followed by partial reoxidation, the reduced ceramic behaving as an n-type semiconductor. According to the porosity, barrier layers are formed either between the ceramic grains or at the ceramic surface.Dielectrics have been developed with permittivities between 105 and 106, loss tangents below 0.05, and very low temperature coefficients of permittivity between −40°C and +150°C. A limitation on the use of such dielectrics is that they are non-ohmic, the resistance falling steeply with increasing voltage and limiting the working potential to a few volts.The dielectrics are shown to be electrically equivalent to two resistance-shunted capacitors connected in series, corresponding to the stoichiometric and reduced portions of the ceramic. An unexpected feature is that the reduced ceramic behaves in itself as a dielectric with anomalously high permittivity.Properties of barrier-layer dielectrics based on a wide range of materials are given, together with preparatory techniques. Data are also presented on two types of commercial barrier-layer capacitors.