Coherent Power Research Articles

An X-band phase-locked relativistic backward wave oscillator

For the purpose of coherent high power microwave combining at high frequency band, an X-band phase-locked relativistic backward wave oscillator is presented and investigated. The phase-locking of the oscillator is accomplished by modulation of the electron beam before it reaches the oscillator. To produce a bunched beam with an acceptable injected RF power requirement, an overmoded input cavity is employed to provide initial density modulation. And a buncher cavity is introduced to further increase the modulation depth. When the beam enters the oscillator, the modulation depth is enough to lock the frequency and phase of the output microwave generated by the oscillator. Particle-in-cell simulation shows that an input power of 90 kW is sufficient to lock the frequency and phase of 1.5 GW output microwave with locking bandwidth of 60 MHz.

Stable optical vortices in nonlinear multicore fibers

The multicore fiber (MCF) is a physical system of high practical importance. In addition to standard exploitation, MCFs may support discrete vortices that carry orbital angular momentum suitable for spatial-division multiplexing in high-capacity fiber-optic communication systems. These discrete vortices may also be attractive for high-power laser applications. We present the conditions of existence, stability, and coherent propagation of such optical vortices for two practical MCF designs. Through optimization, we found stable discrete vortices that were capable of transferring high coherent power through the MCF.

Combustion Noise Analysis for Combustion and Fuels Diagnosis of a Compression Ignition Diesel Engine Operating with Biodiesels

In this article, the combustion noise of a compression ignition diesel engine operating with biodiesels has been investigated experimentally. It aims to explore an effective method for combustion process monitoring and fuel quality evaluation through analyzing the characteristics of the engine combustion noise. The experiments were conducted on a four-cylinder, four-stroke, direct injection and turbocharged diesel engine fueled with biodiesels (B50 and B100) and normal pure diesel, and operating under different loads and speeds. The signals of cylinder head vibration, engine noise, and in-cylinder pressure were measured during the tests. A coherent power spectrum analysis method was used to investigate the vibration and noise signals that related to the combustion process. The results showed that the noise components at the frequency band of 2–3 kHz are closely related to the combustion process. Subsequently, the Wigner–Ville distribution is employed to present the energy distribution of engine noise in the time-frequency domain. Then a band-pass filter based on fractional Fourier transform is developed to extract the main component of the combustion noise for feature extraction. The results show that the sound pressure levels of the extracted combustion noise of the test diesel engine fueled with biodiesels are higher than that fueled with diesel. This is also identical to the variation of in-cylinder pressure. The results demonstrate that the features of the extracted combustion noise can indicate the combustion characteristics and provide useful information for monitoring the combustion process and evaluating the fuel quality of diesel engines.

Modulation-format-independent OSNR monitoring insensitive to cascaded filtering effects by low-cost coherent receptions and RF power measurements.

Optical signal-to-noise ratio (OSNR) monitoring is indispensable for ensuring robust and flexible optical networks that provide failure diagnosis, dynamic lightpath provisioning and modulation format adaptation. We propose and experimentally demonstrate a low-cost, modulation-format-independent OSNR monitoring scheme utilizing reduced-complexity coherent receptions, electrical filtering and radio frequency (RF) power measurements. By measuring the RF power of the coherently received baseband signals at three different frequency components, the proposed OSNR monitor is also insensitive to spectral narrowing induced by cascaded wavelength selective switches (WSSs). We experimentally demonstrate accurate data-format-transparent and filtering-effect-insensitive OSNR monitoring for 25-Gbaud dual-polarization (DP-) transmissions with QPSK, 16-QAM and 64-QAM signals over various distances with different amount of filtering effects by cascaded WSSs. We further characterize the influence of different system parameters, such as the bandwidth of the electrical low-pass filter, the laser frequency offset and laser linewidth on the accuracy of the proposed OSNR monitor. The robustness of the proposed OSNR monitoring scheme to fiber nonlinearities, calibration parameter mismatches and variations of WSS parameters are also investigated.

A new compact self-coherent high power microwave source based on dual beams

In this paper, a compact self-coherent high power microwave source based on dual beams is presented. It consists of a two-cavity triaxial klystron amplifier (TKA) (noted as the outer sub-source below) and a multiwave Cerenkov generators (noted as the inner sub-source) inserted in the TKA's inner conductor. These two sub-sources share a common cathode and the magnetic field. The injected signals to the outer sub-source are leakage microwaves from the inner sub-source through the anode-cathode gap (A-K gap). Particle-in-cell simulation shows that when the diode voltage is 687 kV and the axial magnetic field is 0.8 T, two microwaves with power of 1.02 GW and 2.65 GW and the same frequency of 9.72 GHz are generated in the inner and the outer sub-source, respectively; the corresponding power efficiencies are 24% and 31%. Two sub-sources reach the phase locking at 23 ns with a phase difference fluctuation within ±3°. The fast and stable phase locking in the voltage ranging from 665 kV to 709 kV further suggests that the proposed source is promising for coherent power combination and to export a higher power of combined microwaves.

The Euro: A Chartalist Critique

:The aim of this article is to develop a Chartalist critique of the prevailing economic theories that have informed the original design of the Eurosystem. In order to understand the structural dynamics of the current crisis, it is necessary to examine the longstanding internal contradictions that the system has inherited from its inception under the Maastricht Treaty of 1992 and the neoliberal strategy, which has governed its evolution. In its bare essentials, the euro lacks the backing of a coherent sovereign power. More specifically, the article argues that this national/supranational dichotomy prevents a more unified response to the current debt crises engulfing the peripheral countries of the Eurozone.

Simulation and experimental study of phase characteristics in high-gain relativistic klystron amplifier

In order to realize the combination of the coherent high power microwave, an S-band high gain relativistic klystron amplifier with high phase stability is presented and studied theoretically and experimentally. The phase characteristics of the output microwave are explored in particle-in-cell code and experiment. The experimental results accord well with the simulation results, which indicates that the parasitic oscillation excited by retrograde electrons is the main cause of phase variation in the development of high-gain relativistic klystron with high phase stability. When the input RF power is 10 kW, a microwave pulse with about ± 10° in relative phase difference fluctuation between output and input is obtained in experiment. And the corresponding locking duration is about 90 ns. Accordingly, the expectation of power combination efficiency of 10 high-gain relativistic klystron amplifiers can be achieved to be 99% with continuous uniform distribution of random relative phase difference.

Field of a short dipole above a dielectric half‐space with rough interface

The accurate prediction of radiowave coverage at high frequency-ultra high frequency over irregular terrain features is of importance in the design and development of low-cost and low-power communication systems. This study studies the problem of electromagnetic wave propagation, excited by a short dipole, above a dielectric ground plane with an arbitrary dielectric profile and an irregular interface. This investigation is a natural extension of the classical Sommerfeld problem with the exception of the random surface irregularities at the interface between the two dielectric media. It is assumed that the interface profile height variations are small compared with the wavelength. First, the bistatic scattering of a plane wave illuminating the rough surface is solved using a perturbation solution of an integral equation for the induced polarisation current. Analytical expressions for the coherent field and incoherent scattered power at an arbitrary observation point are obtained. Then, the solutions for the coherent and incoherent scattered power generated by a small dipole of arbitrary orientation and position are derived by expanding the field of the dipole in terms of a continuous spectrum of plane waves and using superposition. The effects of the rough interface on the surface waves and on depolarisation are studied.

An S-band high gain relativistic klystron amplifier with high phase stability

For the purpose of coherent high power microwave combining, an S-band high gain relativistic klystron amplifier with high phase stability is presented and studied. By the aid of 3D particle-in-cell code and circuit simulation software, the mechanism of parasitic oscillation in the device is investigated. And the RF lossy material is adopted in the simulation and experiment to suppress the oscillation. The experimental results show that with an input RF power of 10 kW, a microwave pulse with power of 1.8 GW is generated with a gain of 52.6 dB. And the relative phase difference fluctuation between output microwave and input RF signal is less than ±10° in 90 ns.

Coordinated radar observations of plasma wave characteristics in the auroral F region

Abstract. Properties of decameter-scale plasma waves in the auroral F region are investigated using coordinated observations of plasma wave characteristics with the Kodiak HF coherent radar (KOD) and Poker Flat Incoherent Scatter Radar (PFISR) systems in the Alaskan sector. We analyze one event on 14 November 2012 that occurred during the first PFISR Ion-Neutral Observations in the Thermosphere (PINOT) campaign when exceptionally good F region backscatter data at 1 s resolution were collected by KOD over the wide range of locations also monitored by PFISR. In particular, both radar systems were observing continuously along the same magnetic meridian, which allowed for a detailed comparison between the line-of-sight (l-o-s) velocity data sets. It is shown that l-o-s velocity correlation for data points strictly matched in time (within 1 s) depends strongly on the number of ionospheric echoes detected by KOD in a given post-integration interval or, equivalently, on the KOD echo occurrence in that interval. The l-o-s velocity correlations reach 0.7–0.9 for echo occurrences exceeding 70%, while also showing considerable correlations of 0.5–0.6 for occurrences as low as 10%. Using the same approach of strictly matching the KOD and PFISR data points, factors controlling coherent echo power are investigated, focusing on the electric field and electron density dependencies. It is demonstrated that the signal-to-noise ratio (SNR) of F region echoes increases nearly monotonically with an increasing electric field strength as well as with an increasing electron density, except at large density values, where SNR drops significantly. The electric field control can be understood in terms of the growth rate of the gradient-drift waves being proportional to the convection drift speed under conditions of fast-changing convection flows, while the density effect may involve over-refraction at large density values and radar backscatter power proportionality to the perturbation density.

Photonic crystal-based flat lens integrated on a Bragg mirror for high-Q external cavity low noise laser

We demonstrate a high reflectivity (> 99%), low-loss (< 0.1%) and aberrations-free (2% of λ rms phase fluctuations) concave Bragg mirror (20mm radius of curvature) integrating a photonic crystal with engineered spherical phase and amplitude transfer functions, based on a III-V semiconductors flat photonics technology. This mirror design is of high interest for highly coherent high power stable external cavity semiconductor lasers, exhibiting very low noise. We design the photonic crystal for operation in the pass band. The approach incorporates spatial, spectral (filter bandwidth= 5nm) and polarization filtering capabilities. Thanks to the mirror, a compact single mode TEM(00) 2mm-long air gap high finesse (cold cavity Q-factor 10(6) - 10(7)) stable laser cavity is demonstrated with a GaAs-based quantum-wells 1/2-VCSEL gain structure at 1μm. Excellent laser performances are obtained in single frequency operation: low threshold density of 2kW/cm(2) with high differential efficiency (21%). And high spatial, temporal and polarization coherence: TEM(00) beam close to diffraction limit, linear light polarization (> 60dB), Side Mode Suppression Ratio > 46dB, relative intensity noise at quantum limit (< -150dB) in 1MHz-84GHz radio frequency range, and a theoretical linewidth fundamental limit at 10 Hz (Q-factor ∼ 3.10(13)).

Development of Terahertz Radiation Sources using Compact Linear Accelerator LEENA

Tunable and high-power terahertz radiation sources using relativistic electron beams from a compact linear accelerator LEENA are under development. The synchrotron radiation and Smith-Purcell radiation in THz regime were successfully measured. The coherent radiation power from an electron bunch shorter than 1ps is estimated theoretically to be about 107 larger than the incoherent one. Recent experimental results on the generation of the THz radiation and the numerical results on the high-power coherent radiation from a short electron bunch are reported.

Coherent-Radiation Spectroscopy of Few-Femtosecond Electron Bunches Using a Middle-Infrared Prism Spectrometer

Modern, high-brightness electron beams such as those from plasma wakefield accelerators and free-electron laser linacs continue the drive to ever-shorter bunch durations. In low-charge operation (~20 pC), bunches shorter than 10 fs are reported at the Linac Coherent Light Source (LCLS). Though suffering from a loss of phase information, spectral diagnostics remain appealing as compact, low-cost bunch duration monitors suitable for deployment in beam dynamics studies and operations instrumentation. Progress in middle-infrared (MIR) imaging has led to the development of a single-shot, MIR prism spectrometer to characterize the corresponding LCLS coherent beam radiation power spectrum for few-femtosecond scale bunch length monitoring. In this Letter, we report on the spectrometer installation as well as the temporal reconstruction of 3 to 60 fs-long LCLS electron bunch profiles using single-shot coherent transition radiation spectra.

10-Gbit/s direct modulation of a TO-56-can packed 600-μm long laser diode with 2% front-facet reflectance

A 600-μm long-cavity laser diode with a front-facet reflectance of 2% is demonstrated as a colorless OC-192 transmitter for the future DWDM-PON, which is packed in a TO-56-can package of only 4-GHz frequency bandwidth but can be over-bandwidth modulated with 10-Gbit/s non-return-to-zero data-stream. The coherent injection-locking successfully suppresses its side-mode intensity and noise floor level, which further improves its modulation throughput at higher frequencies. With increasing the coherent injection-locking power from -12 to -3 dBm, the side-mode suppression ratio significantly increases from 39 to 50 dB, which also suppresses the frequency chirp from -12 to -4 GHz within a temporal range of 150 ps. The dense but weak longitudinal modes (with 0.6-nm spacing) in the long-cavity laser diode suppresses to one single-mode in a 100-GHz wide DWDM channel for carrying the OC-192 data at 9.953 Gbit/s. Such an over-bandwidth modulated laser diode still exhibits an on/off extinction ratio of 6.68 dB and a signal-to-noise ratio of 4.96 dB, which can provide a back-to-back receiving power sensitivity of -12.2 dBm at BER of 10⁻⁹. After 25-km DSF transmission of the OOK data-stream at a bit rate up to 10 Gbit/s, the receiving power sensitivity is -10.1 dBm at a requested BER of 10⁻⁹.

Coherent combination of mutual injection phase-locked fiber lasers with a corner cube reflector

We present a new method of the mutual injection phase-locked coherent combination of fiber lasers based on corner cube reflector (CCR). The theoretic calculation of phase difference and far-field intensity distribution of coherent beams obtained by the CCR has been carried out on this work. The relation between the far-field optical intensity distribution and the CCR rotation angles is analyzed in the case of mutual injection. The coherent combination experiments of mutual injection phase-locked with two and four fiber lasers are performed. The experimental results reveal that the far-field intensity distribution in the two free running fiber lasers is good agreement with the theoretical calculation. In the two internal mutual injection structures, the coherent combination output power goes up as high as 54W, and power combined efficiency of more than 90% and fringe contrast about 90% are obtained. The far-field intensity distribution is the same as the typical coherent combination pattern of two-dimensional fiber laser arrays, the experimental results also show that the coherent combination output with more laser beams and higher power with a good beam quality could be realized by the current scheme.

Revisiting Solar and Heliospheric 1.3-Year Signals during 1970 – 2007

We revisit the 1.3-year (yr) signals observed on the Sun, in the interplanetary space, and in the Earth’s magnetosphere to study the coupling among signals from the three regions for about 40 years (1970 – 2007) covering the three solar cycles 21, 22, and 23. For this, we make dynamic spectra of datasets including three different regions. From this, we estimate the peak frequency around 1.3 yr for each region and the corresponding band power. We found that coherent power only appears during 1987 – 1995 and the coherent behavior is found only in the interplanetary space and Earth, not in the Sun. Although the solar surface magnetic field shows significant power around 1.3 yr, their peak frequencies are statistically different from those of the outer regions, which make us dismiss the existence of coherence among the three regions. But it is notable that the peaks in band power corresponding to the 1.3-yr period are clearly simultaneous in the interplanetary space and Earth.

Coherent Power Combining of Chirped-Seed Erbium-Doped Fiber Amplifiers

Using a rapidly-chirped <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(5\times 10^{14}~{\rm Hz}/{\rm s})$</tex> </formula> continuous-wave optoelectronic swept-frequency seed laser and two 3 W erbium-doped fiber amplifiers, we demonstrate simultaneous stimulated Brillouin scattering suppression, coherent power combining, and electronic beam steering, without imposing strict path-length matching requirements. Our platform presents a viable path toward high-power continuous-wave sources.

Powerful terahertz emission from Bi2Sr2CaCu2O8+δ mesa arrays

Stacks of intrinsic Josephson junctions in high-temperature superconductors enable the fabrication of compact sources of coherent terahertz radiation. Here, we demonstrate that multiple stacks patterned on the same Bi2Sr2CaCu2O8+δ crystal can—under optimized conditions—be synchronized to emit high-power THz-radiation. For three synchronized stacks, we achieved 610 μW of continuous-wave coherent radiation power at 0.51 THz. We suggest that synchronization is promoted by THz-waves in the base crystal. We note that synchronization cannot be achieved in all samples. However even in these cases, powers on the 100-μW scale can be generated.

Aeroacoustic measurements on a NACA 0012 applying the Coherent Particle Velocity method

Aeroacoustic measurements on two NACA 0012 airfoil sections with different chord length and sharp trailing edge were conducted at the Laminar Wind Tunnel (LWT) of the University of Stuttgart. The LWT is a closed test section wind tunnel with a very low turbulence level and an acoustically optimized diffusor section allowing for high-quality aerodynamic as well as aeroacoustic measurements. Trailing edge noise measurements were performed using the Coherent Particle Velocity (CPV) method, which is based on a cross-spectral analysis of two hot-wire sensor signals placed on the suction and the pressure side of the airfoil trailing edge, respectively. At high angles of attack, the cross-spectral analysis of the two sensor signals used for the measurement of the trailing edge noise can be prone to a disturbing influence of hydrodynamic fluctuations. Hence, continuous shifts in the phasing of the cross-correlation are observed mainly for low sensor distances to the trailing edge. The quantitative evaluation of the trailing edge noise predominately in the low frequency range is, therefore, considerably disturbed. A new approach is proposed, which allows for the correction of the cross-correlation function based on the averaged single wire auto-spectrum. The results are compared to measurements with increased sensor distance and show good agreement. In the following, trailing edge noise measurements were performed on a NACA 0012 airfoil in a wide range of angles of attack (α = 0°–8°) and free-stream velocities ( $$u_{\infty} = 30{-}70\,{\hbox{m/s}}$$ ). The tripped flow cases exhibit a very good consistency for the scaling of the 1/3 octave spectra based on outer variables. Moreover, a common intersection point of the sound pressure level was observed for trailing edge noise spectra measured at constant free-stream velocity and different angles of attack. In cases without boundary layer tripping, the presence of an acoustic feedback loop was observed and linked to the presence of a laminar separation bubble on the pressure side in the vicinity of the trailing edge. Finally, a comparison of the aeroacoustic measurements based on the CPV method showed reasonably good agreement with published data obtained with both a microphone array and the Coherent Output Power method in open-test section facilities.

High transverse momentum quarkonium production and dissociation in heavy ion collisions

We calculate the yields of quarkonia in heavy ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC) as a function of their transverse momentum. Based upon nonrelativistic quantum chromodynamics, our results include both color-singlet and color-octet contributions and feed-down effects from excited states. In reactions with ultrarelativistic nuclei, we focus on the consistent implementation of dynamically calculated nuclear matter effects, such as coherent power corrections, cold nuclear matter energy loss, and the Cronin effect in the initial state. In the final state, we consider radiative energy loss for the color-octet state and collisional dissociation of quarkonia as they traverse through the QGP. Theoretical results are presented for $J/\ensuremath{\psi}$ and $\ensuremath{\Upsilon}$ and compared to experimental data where applicable. At RHIC, a good description of the high-${p}_{T}$ $J/\ensuremath{\psi}$ modification observed in central $\mathrm{Cu}+\mathrm{Cu}$ and $\mathrm{Au}+\mathrm{Au}$ collisions can be achieved within the model uncertainties. We find that measurements of $J/\ensuremath{\psi}$ yields in proton-nucleus reactions are needed to constrain the magnitude of cold nuclear matter effects. At the LHC, a good description of the experimental data can be achieved only in midcentral and peripheral $\mathrm{Pb}+\mathrm{Pb}$ collisions. The large fivefold suppression of prompt $J/\ensuremath{\psi}$ in the most central nuclear reactions may indicate for the first time possible thermal effects at the level of the quarkonium wave function at large transverse momenta.