Corrugation Phase Research Articles

Thermal performance analysis of triple casing latent heat system based on sinusoidal function type corrugation

The natural convection disappears and the melting rate of phase change material (PCM) decreases in the microgravity environment of spacecraft operation. For the characteristics of low thermal conductivity of phase change materials, a triple casing based on sinusoidal function type corrugated structure as latent heat storage system (TTES) is proposed to improve the melting performance of the latent heat storage system. In this paper, the study model is simplified to a 2D model using commercial CFD software to numerically simulate the phase change heat transfer process, and the heat transfer characteristics of the sinusoidal function type corrugated triple casing and the effect of Reynolds number on the melting process are investigated. The results show that: the joint action mechanism of sinusoidal corrugation structure and phase change material local heat transfer can significantly shorten the heat charging time; reasonable change of corrugation period and crest height can shorten the heat charging time and increase the heat storage; comprehensive analysis of heat charging time, heat storage and average heat flux, through single-factor optimization to obtain a sinusoidal pipe with corrugation period between 100 mm and 150mm and crest height around 15 mm on The comprehensive performance of flow heat exchange has the best effect.

Динамика широкоапертурных гетеролазеров со сбоем фазы гофрировки брэгговской структуры

Using the coupled-wave method supplemented by the quasi-optical approximation, we study the dynamics of wide-aperture distributed-feedback (DFB) heterolasers with planar resonators having a step of corrugation phase. In configurations where the width of the Bragg structure is several times smaller or equal to the width of the active zone, the diffraction transverse losses provide mode selection with respect to the transverse index and, simultaneously, suppress the excitation of the longitudinal modes with frequencies above the bandgap. Stable single-mode operation of the DFB-heterolaser is possible in the wide range of positions and values of the corrugation phase step, which determine the level and ratio of the values of power radiated from the opposite ends of the DFB heterolaser. Conditions are found that ensure almost unidirectional output of radiation.

Mode splitting effect in FEMs with oversized Bragg resonators

Splitting of the fundamental mode in an oversized Bragg resonator with a step of the corrugation phase, which operates over the feedback loop involving the waveguide waves of different transverse structures, was found to be the result of mutual influence of the neighboring zones of the Bragg scattering. Theoretical description of this effect was developed within the framework of the advanced (four-wave) coupled-wave approach. It is shown that mode splitting reduces the selective properties, restricts the output power, and decreases the stability of the narrow-band operating regime in the free-electron maser (FEM) oscillators based on such resonators. The results of the theoretical analysis were confirmed by 3D simulations and “cold” microwave tests. Experimental data on Bragg resonators with different parameters in a 30-GHz FEM are presented. The possibility of reducing the mode splitting by profiling the corrugation parameters is shown. The use of the mode splitting effect for the output power enhancement by passive compression of the double-frequency pulse generated in the FEM with such a resonator is discussed.

Rayleigh wave dispersion in an irregular sandy Earth's crust over orthotropic mantle

The present paper presents a study of Rayleigh wave propagation in a sandy crustal layer of the Earth lying over an orthotropic mantle with irregular boundary surfaces under the influence of initial stress, gravity field and rigid boundary plane. An attempt has been made to study the dynamics of the individual medium and derive the displacement of the wave for both the media separately. Suitable boundary conditions have been imposed to derive the dispersion equation in closed form. In general, the obtained dispersion equation is complex, which on separating into real and imaginary parts, leads to two different dispersion equations, of which we are neglecting the imaginary part being damping in nature. The real part contains complete information regarding corrugation parameter, wave number, phase velocity, sandy parameter, initial stress and gravity field. The results for irregular boundaries have been obtained in three cases and presented using graphs. It has been found that the initial stress and sandy parameter have a significant effect and remarkable bearing on the phase velocity of Rayleigh surface wave. Corrugation parameters also have a dominant impact on the propagation of Rayleigh wave. This study may be of great help in the field of industrial engineering, submarine structures, civil engineering, aerospace, chemical pipe and metallurgy as well as geophysics.

Peculiarities of the Mode Spectrum in Free-Electron Masers Based on Oversized Bragg Resonators with a Corrugation Phase Step

We study the operating mode splitting caused by interaction of the neighboring Bragg scattering zones in an oversized Bragg resonator with a corrugation phase step, which is operated at the coupled forward and backward waveguide modes with different transverse structures. This effect is described within the framework of the coupled-wave approach using an advanced four-wave model. It is shown that this effect deteriorates the selective properties of the resonator and, finally, restricts the output power and reduces stability of the narrow-band operating regime in the free-electron masers (FEMs) based on such resonators. The results of the theoretical analysis were corroborated by 3D simulations and “cold” electrodynamic tests. Experimental studies of 30-GHz FEMs with the Bragg resonators having different corrugation depths demonstrated the onset of both narrow-band single-mode and multifrequency multimode oscillation regimes in such resonators. The possibility of power enhancement by using passive compression of the FEM output pulse in a double-frequency oscillation regime is discussed.

Evaluation of the imaging properties of Microwave Imaging Reflectometry

Microwave Imaging Reflectometry (MIR) has been developed for unambiguous measurement of electron density fluctuations in fusion plasmas. The loss of phase information limiting the use of conventional reflectometry can be minimized by a large aperture imaging optics and an array of detectors in the MIR embodiment. The evaluation of the optical system is critical for precise reconstruction of the fluctuations. The optical systems of the prototype TEXTOR MIR [2] and newly-designed KSTAR MIR [5]systems have been tested with a corrugated target simulating density fluctuations at the cut-off surface. The reconstructed phase from the MIR system has been compared to the directly measured phase of corrugations taking into account the rotational speed of the target. The effects of optical aberrations and interference between lenses on the phase reconstruction have been investigated by the 2D amplitude measurement of the reflected waves and the diffraction-based optical simulations. (CODE V) A preliminary design of the KSTAR MIR optics has been suggested which can minimize the aberration and interference effects.

Comparitive Study of Coaxial Bragg Resonators

A coaxial Bragg resonator can be performed in four kinds of structures: it is constructed by inserting a regular waveguide between two selective Bragg reflectors (being called the upstream reflector and downstream reflector, respectively), or it consists of only two Bragg reflectors without regular waveguide section between them, where connection of these two reflectors is smooth with phase difference Δϕ = 0 or is step-changed with corrugation phase difference Δϕ = +π or −π, respectively. The present paper is devoted to a comparative study for the response of reflectivity and quality factor on frequency in these four kinds of coaxial Bragg resonators, including the influences of the location and corrugation-phase difference of the step-changed connection of the two reflectors. Results show that a coaxial Bragg resonator possesses most attractive properties when the corrugation-phase difference is Δϕ = +π at the point of connection and the upstream reflector is longer than the downstream reflector.

Possible ways of improvement of FEM oscillator with Bragg resonator

Two methods of efficiency enhancement of free-electron maser (FEM) oscillator with the Bragg resonator having a shift of corrugation phase were studied. The first one is a “starting mode” regime using an additional high-frequency mode located in the vicinity of the main mode. The Bragg resonator having a shift of corrugation phase seems to represent the most attractive oscillator configuration. Another possibility is a special variation of corrugation depth of Bragg reflectors (mirror profiling) providing the possibility of optimizing the longitudinal distribution of the RF field amplitude inside the interaction space and depressing of parasitic side modes of the resonator. Optimization of RF power extraction from FEM, based on Talbot effect, has also been considered. Perspectives of future FEM development are discussed including schemes of pulse power compression and submillimeter wave generation.

Possible ways of improvement of FEM oscillator with Bragg resonator

Two methods of efficiency enhancement of free-electron maser (FEM) oscillator with the Bragg resonator having a shift of corrugation phase were studied. The first one is a “starting mode” regime using an additional high-frequency mode located in the vicinity of the main mode. The Bragg resonator having a shift of corrugation phase seems to represent the most attractive oscillator configuration. Another possibility is a special variation of corrugation depth of Bragg reflectors (mirror profiling) providing the possibility of optimizing the longitudinal distribution of the RF field amplitude inside the interaction space and depressing of parasitic side modes of the resonator. Optimization of RF power extraction from FEM, based on Talbot effect, has also been considered. Perspectives of future FEM development are discussed including schemes of pulse power compression and submillimeter wave generation.

Experiments on precise frequency tuning in the FEM oscillator with the Bragg resonator

It is known that precise frequency tuning of a FEM oscillator with a Bragg resonator could be performed by changing the corrugation phase shift between the reflectors. This report presents new experimental results obtained using a tunable band-pass waveguide filter as well as a heterodyne technique to measure the spectrum. The 30 GHz FEM oscillator was tuned over a frequency range of 6% with a precision of about 0.2%. In all regimes the radiation spectrum width did not exceed 0.1%. The dependence of the operating frequency on the value of the corrugation phase shift is in good agreement with previous simulation results.

Corrugation of roads

We present a one dimensional model for the development of corrugations in roads subjected to compressive forces from a flux of cars. The cars are modeled as damped harmonic oscillators translating with constant horizontal velocity across the surface, and the road surface is subject to diffusive relaxation. We derive dimensionless coupled equations of motion for the positions of the cars and the road surface H( x, t), which contain two phenomenological variables: an effective diffusion constant Δ( H) that characterizes the relaxation of the road surface, and a function a( H) that characterizes the plasticity or erodibility of the road bed. Linear stability analysis shows that corrugations grow if the speed of the cars exceeds a critical value, which decreases if the flux of cars is increased. Modifying the model to enforce the simple fact that the normal force exerted by the road can never be negative seems to lead to restabilized, quasi-steady road shapes, in which the corrugation amplitude and phase velocity remain fixed.

Precise frequency tuning in the millimeter-wave FEL-oscillator with the Bragg resonator

A FEL-oscillator with the traditional Bragg resonator consisting of the waveguide with corrugated ends (mirrors) allows to obtain single-mode generation with high efficiency. However, the output radiation frequency cannot be fixed more precisely than the mirror's reflection band, typically at 2–3%. There exists another scheme of the Bragg resonator without the smooth tube between mirrors. Depending on the corrugation phase shift between the mirrors this resonator has a single high-quality mode inside the reflection band or two modes near the ends of the reflection band. Variation of the corrugation phase shift from 0 to 2π tunes the mode frequency from higher to lower limit of the reflection band. Bragg resonators of this type were designed both for high-efficiency FEL-oscillator and for precise radiation frequency meter. Experimental investigation of the tunable FEL-oscillator is now in progress.

High-efficiency single-mode free-electron maser oscillator based on a bragg resonator with step of phase of corrugation

A new type of high-selective Bragg resonator having a step of corrugation inside the interaction region was used as a microwave system for a free-electron maser (FEM). Using a LINAC LIU-3000 (1 MeV/200 A/200 ns) to drive the FEM oscillator, a single-mode single-frequency operation was achieved at a frequency of 30.74 GHz with an output power of about 50 MW, which corresponded to a record efficiency of 35% for a millimeter wavelength FEM.

Effects of structural parameters on the external optical feedback sensitivity in DFB semiconductor lasers

External optical feedback sensitivity in distributed feedback (DFB) semiconductor lasers is analyzed with special attention to phase-shifted and complex-coupled lasers. The effects of various structural parameters such as coupling strength, facet reflectivity, and corrugation phase angle on external optical feedback sensitivity are studied. The /spl lambda//4 phase-shifted index-coupled DFB laser exhibits low external optical feedback sensitivity for large index-coupling coefficient and high facet reflectivity. Pure gain-coupled DFB lasers perform better than the phase-shiftless uniform index-coupled DFB lasers but worse than /spl lambda//4 phase-shifted index-coupled lasers with high coupling strengths. External optical feedback sensitivity of complex-coupled lasers depends significantly on the index-to-gain coupling ratio, the phase between the index and gain gratings, and the total coupling.

Fluctuations of the laser characteristics and the effect of the index-coupling component in the gain-coupled DFB laser

The normalized oscillation frequency /spl delta/L and threshold gain /spl alpha/L of gain-coupled distributed feedback (DFB) lasers with an AR/AR (symmetric) structure vary significantly with a variation in the corrugation phase at the facets when the facet reflectivity |r| is large, even though the spatial-hole-burning-corrected yield is 100%. For example, the standard deviation for the /spl delta/L, /spl sigma/(/spl delta/L), increases from 0.018 to 0.40 when |r| is increased from 0.03 to 0.30. The magnitude of /spl sigma/(/spl delta/L) and /spl sigma/(/spl alpha/L) are small, however, for the gain-coupled, index-coupled, and complex-coupled DFB lasers when the |r| is kept small. For the HR/AR (asymmetric) structure, these two values are more than an order of magnitude larger than those for the AR/AR structure, except for the /spl sigma/(/spl alpha/L) for the index-coupled laser, even though yields of each of these lasers are relatively large. In the AR/AR structure, the contamination by the index-coupling and gain-coupling components always degrades the device characteristics because of the difference in the phase shifter and when the amount of contamination is small this degradation is particularly severe for the index-coupled laser. Several properties of the index-coupled laser with the HR/AR structure can be improved, however, by introducing the gain-coupling component.

Tapered corrugation for improving characteristics of DFB and DBR lasers

In DFB and DBR lasers, phase of corrugation at the reflective end and possible change of reflectivity at the facets may alter the conditions of oscillation. The purpose of tapering at the grating is to reduce these effects. In addition, with dissimilar tapering, we can increase mode selectivity of such lasers. In this article we consider the reflection coefficient of tapered DBR as a function of end facet reflection coefficient, and deviation from the Bragg wavelength. It is shown that tapering improves the operation of DFB and DBR lasers. © 1992 John Wiley & Sons, Inc.

Stability of the longitudinal mode in lambda /4-shifted InGaAsP/InP DFB lasers

Factors causing the mode stability of lambda /4-shifted distributed-feedback (DFB) lasers, spatial hole burning along the laser axis and residual reflectivity of the facets of laser cavity, are studied. It is experimentally and theoretically confirmed that the spatial hole burning increases the corrugation phase shift from lambda /4 ( pi ) to 3 lambda /8 (1.5 pi ) as the driving current increases in lasers whose coupling constant and cavity-length product kL is 2.0. This hole-burning effect requires the reflectivity of facets to be reduced below 0.2% to get a high reproducibility of lasers oscillating at a stable single mode up to high output powers. >

On the inversion of the atomic beam scattering data in the Eikonal approximation

The inversion method of Celli, Garcia and Rieder requires some a-priori knowledge about the phases of the dominant scattering amplitudes. A perturbational method will be introduced here which is evaluated in the Eikonal approximation, but can in principle be extended to the Helmholtz equation. It is based on the observation that the expansion coefficients P G ( n) of the scattering intensities with respect to k i, the incident wave vector, are functions of the corrugation amplitudes and phase differences and fulfill certain sum rules. If these coefficients are determined by several measurements with varying scattering parameters, then the lowest P G ( n) for n = 2,3.4 are sufficient for finding the essentially unique surface profile function. As an example, simulated surface domain structure scattering data will be inverted.

Temperature Range for DFB Mode Oscillation in 1.5 µm InGaAsP/InP DFB Lasers

The temperature range for distributed feedback (DFB) mode oscillations in 1.5-µm InGaAsP/InP DFB lasers was investigated. The range was determined by the relation between the net gain spectra and the loss difference between the Fabry-Perot and DFB modes. The temperature dependence of the net-gain spectra of 1.5-µm InGaAsP/InP Fabry-Perot (FP) lasers were measured. The loss difference between FP and DFB modes was calculated while taking into account the corrugation phase at facets, the facet reflectivity, and the coupling coefficient. From these results, it is confirmed that DFB lasers with two cleaved facets and a κL value of 2(κ: coupling coefficient, L: cavity length) can oscillate in the DFB mode in a temperature range of at least 100 degrees.

Asymmetry in Output Power of InGaAsP/InP DFB Lasers

The output powers from the two ends of a DFB region were compared. The power ratios and the external differential quantum efficiencies of the lowest-threshold modes were calculated in detail for an asymmetric end structure, one end being a cleaved facet and the other end being non-reflective. Power ratios of 2–5, depending on the corrugation phase at the cleaved facet, were predicted for KL=2–3. Such large ratios were actually estimated experimentally in InGaAsP/InP DFB lasers with a window region at one end. A larger output from a window region may be useful if the window region length is restricted, to about 10 µm or less in this case. The asymmetric output power in an FP-type DFB laser was also demonstrated.