Structural modal excitation using travelling impulses—force frequency shifting

A review of existing hardware and methods for vibration testing of large structures is given by Koss and has shown that the size of inertial vibration shakers, to achieve a specific displacement, has to increase, as a structure becomes larger. In previous papers the concept of “force frequency shifting (ffs) for structural excitation”, was introduced to develop a more compact structural vibration exciter than is presently available for low frequencies. An ffs shaker operates at a frequency much greater than the natural frequency of the structure under test but generates a modal force at the lower frequency of the structure. This effect is accomplished by moving a vibrating force back and forth across the structure while the force is applied normally to its surface. For example, the generalized force generated by an ffs shaker at the fundamental structural frequency for a simply supported beam is given by 1.65Pr/l where P is the high frequency out of balance force, r is the throw amplitude and l is the beam length. The term that reduces the efficiency of force transfer from high to low frequencies is “r/l” as, usually, the length of a structure is much greater than the throw of the force. This paper introduces another force frequency shifting approach that allows r/l to be large. This is accomplished by placing force exciters along a structure-spatial array, spaced a distance ΔX apart, and each force exciter is activated for a short period of time to simulate a travelling force traversing the structure forwards and backwards. The “force throw r “can thus be made large. Results of simulations and experiments verify that force frequency shifting can be accomplished using travelling impulses and modal identification can be achieved.     

FLaser frequency stabilization and shifting by using modulation transfer spectroscopy

The stabilizing and shifting of laser frequency are very important for the interaction between the laser and atoms. The modulation transfer spectroscopy for the 87Rb atom with D2 line transition F = 2 →F＇ = 3 is used for stabilizing and shifting the frequency of the external cavity grating feedback diode laser. The resonant phase modulator with electro-optical effect is used to generate frequency sideband to lock the laser frequency. In the locking scheme, circularly polarized pump- and probe-beams are used. By optimizing the temperature of the vapor, the pump- and probe-beam intensity, the laser linewidth of 280 kHz is obtained. Furthermore, the magnetic field generated by a solenoid is added into the system. Therefore the system can achieve the frequency locking at any point in a range of hundreds of megahertz frequency shifting with very low power loss.     

Laser frequency stabilization and shifting by using modulation transfer spectroscopy

The stabilizing and shifting of laser frequency are very important for the interaction between the laser and atoms. The modulation transfer spectroscopy for the87 Rb atom with D2 line transition F = 2 → F = 3 is used for stabilizing and shifting the frequency of the external cavity grating feedback diode laser. The resonant phase modulator with electro–optical effect is used to generate frequency sideband to lock the laser frequency. In the locking scheme, circularly polarized pump- and probe-beams are used. By optimizing the temperature of the vapor, the pump- and probe-beam intensity, the laser linewidth of 280 kHz is obtained. Furthermore, the magnetic field generated by a solenoid is added into the system. Therefore the system can achieve the frequency locking at any point in a range of hundreds of megahertz frequency shifting with very low power loss.     

Flat optical frequency comb generation using polarization modulator and frequency shifter with double recirculating frequency shifting loops

A scheme to generate ultra-flat and stable optical frequency comb (OFC) based on multi-wavelength optical seed source and polarization-modulator-based complementary frequency shifter (PCFS) has been proposed and demonstrated. The OFC generation scheme includes two stages: 5-carrier light source is generated by utilizing a polarization modulator at the first stage, and the light source is then used as the seed light in the PCFS at the second stage. Compared with the previous flat OFC generator based on a recirculation frequency shifter, the proposed scheme can greatly reduce the recirculation times, and such a property makes the accumulated noise of the system enormously decrease. In order to achieve the frequency comb with the best flatness, the optical amplifier gain, filter stop-band attenuation and modulator drive voltage are optimized. Experiment simulations show that high-quality OFC with hundreds of frequency lines and power fluctuation within 2 dB can be achieved without direct-current bias control and wavelength-selective-switch.     

高等离子体密度区域的光子加速现象

本文研究了激光尾场实验中的透射光谱。和标准尾场实验不同的是,我们采用了更高的等离子体密度。实验中仍然观察到了明显的频谱加宽与频谱分裂,这是光子加速的显著标志。为解释该现象,我们做了二维粒子模拟。模拟结果证明在该高密度等离子体中, 光子加速仍然可以发生。     

Laser beam deflection by frequency shifting

A new type of beam scanner has been developed which utilizes frequency shifting of a laser beam followed by deflection of the beam in a dispersive element. Theoretical considerations involve the maximization of the frequency shift and number of resolvable spots. An experimental deflector has been constructed which employs a mode-locked 0.633 μm He-Ne laser, lithium-niobate frequency shifter, and a Fabry-Perot dispersion element. Scans of 15 resolvable spots have been obtained with a theoretical access time of 18.5 ns.     

Laser Doppler velocimeter with optical frequency shifting

A bi-directional laser Doppler velocimeter utilizing optical frequency shifting has been developed. Optical frequency shifting is achieved by using a rotating radial grating. A high diffraction efficiency permits the system to be used in the fringe mode. The zero velocity frequency shift can be varied from 0.1 to 2.5 MHz with a stability better than 0.2%. Important applications are velocity measurements in reversing flows, highly turbulent flows, two-phase flows, and boundary layers. The system described may also be used for vibration analysis. Experimental results are presented.     

Microwave photon counter based on Josephson junctions

We describe a microwave photon counter based on the current-biased Josephson junction. The junction is tuned to absorb single microwave photons from the incident field, after which it tunnels into a classically observable voltage state. Using two such detectors, we have performed a microwave version of the Hanbury Brown-Twiss experiment at 4 GHz and demonstrated a clear signature of photon bunching for a thermal source. The design is readily scalable to tens of parallelized junctions, a configuration that would allow number-resolved counting of microwave photons.     

Single photon level spectrum measurement at fiber communication band using frequency up-conversion technology

We have developed a polarization independent (PI) spectrometer based on frequency up-conversion technology for single photon level spectrum measurement at the fiber communication band. To overcome the polarization dependence of the frequency up-conversion process, we use two periodically poled lithium niobate (PPLN) waveguides with a polarizing beam splitter. We experimentally study the sensitivity and resolution of the PI up-conversion spectrometer. We demonstrate the spectrometer by way of a spectrum measurement of a single photon level signal in the communication band.     

Single photon frequency up-conversion and its applications

Optical frequency up-conversion is a technique, based on sum frequency generation in a non-linear optical medium, in which signal light from one frequency (wavelength) is converted to another frequency. By using this technique, near infrared light can be converted to light in the visible or near visible range and therefore detected by commercially available visible detectors with high efficiency and low noise. The National Institute of Standards and Technology (NIST) has adapted the frequency up-conversion technique to develop highly efficient and sensitive single photon detectors and a spectrometer for use at telecommunication wavelengths. The NIST team used these single photon up-conversion detectors and spectrometer in a variety of pioneering research projects including the implementation of a quantum key distribution system; the demonstration of a detector with a temporal resolution beyond the jitter limitation of commercial single photon detectors; the characterization of an entangled photon pair source, including a direct spectrum measurement for photons generated in spontaneous parametric down-conversion; the characterization of single photons from quantum dots including the measurement of carrier lifetime with escalated high accuracy and the demonstration of the converted quantum dot photons preserving their non-classical features; the observation of 2nd, 3rd and 4th order temporal correlations of near infrared single photons from coherent and pseudo-thermal sources following frequency up-conversion; a study on the time-resolving measurement capability of the detectors using a short pulse pump and; evaluating the modulation of a single photon wave packet for better interfacing of independent sources. In this article, we will present an overview of the frequency up-conversion technique, introduce its applications in quantum information systems and discuss its unique features and prospects for the future.     

Microwave frequency modulation in CW EPR at W-band using a loop-gap resonator

Loop-gap resonator (LGR) technology has been extended to W-band (94GHz). One output of a multiarm Q-band (35GHz) EPR bridge was translated to W-band for sample irradiation by mixing with 59 GHz; similarly, the EPR signal was translated back to Q-band for detection. A cavity resonant in the cylindrical TE011 mode suitable for use with 100 kHz field modulation has also been developed. Results using microwave frequency modulation (FM) at 50 kHz as an alternative to magnetic field modulation are described. FM was accomplished by modulating a varactor coupled to the 59 GHz oscillator. A spin-label study of sensitivity was performed under conditions of overmodulation and gamma2H1(2)T1T2<1. EPR spectra were obtained, both absorption and dispersion, by lock-in detection at the fundamental modulation frequency (50 kHz), and also at the second and third harmonics (100 and 150 kHz). Source noise was deleterious in first harmonic spectra, but was very low in second and third harmonic spectra. First harmonic microwave FM was transferred to microwave modulation at second and third harmonics by the spins, thus satisfying the "transfer of modulation" principle. The loaded Q-value of the LGR with sample was 90 (i.e., a bandwidth between 3 dB points of about 1 GHz), the resonator efficiency parameter was calculated to be 9.3 G at one W incident power, and the frequency deviation was 11.3 MHz p-p, which is equivalent to a field modulation amplitude of 4 G. W-band EPR using an LGR is a favorable configuration for microwave FM experiments.     

Electron, positron, and photon wakefield acceleration: trapping, wake overtaking, and ponderomotive acceleration

The electron, positron, and photon acceleration in the first cycle of a laser-driven wakefield is investigated. Separatrices between different types of the particle motion (trapped, reflected by the wakefield and ponderomotive potential, and transient) are demonstrated. The ponderomotive acceleration of electrons can be largely compensated by the wakefield action, in contrast to positrons and positively charged mesons. The electron bunch energy spectrum is analyzed. The maximum upshift of an electromagnetic wave frequency during reflection from the wakefield is obtained.     

Far-infrared frequency shifting by optically induced gratings in semiconductors

A first approach to frequency shifting of far-infrared radiation by optically induced “moving” refractive index gratings in semiconductors is reported. The factors determining the performance of this frequency shifting technique are inferred using a one-dimensional analytical model. In a proof-of-principle experiment frequency shifting of 119 μm radiation was observed up to 80 MHz using a 300 mW, 488 nm Ar⁺ laser for optical excitation.     

The frequency shifting of laser light by electro-optic techniques

In velocity measurements by the laser Doppler method, a requirement exists for changing the frequency of one light beam with respect to another derived from the same source. This is particularly useful for the determination of the sign of a velocity. The required frequency shifting or single side-band generation can be accomplished by the use of electro-optical effects and a number of possible arrangements are discussed. Trials were made using Kerr cells and electro-optic crystals and single side-band generation demonstrated by a simple Doppler beating experiment. If a low efficiency can be tolerated as in Doppler beating with a reference beam, the production of a pure frequency shifted beam is not difficult up to frequencies of a few megahertz. The differential Doppler or fringe system however requires shifted and non-shifted beams of equal intensity and hence high efficiency conversion is imperative. Radio-frequency driving power requirements may also be a limitation. The theoretical efficiencies of various multi-cell electro-optic arrangements have been calculated as a function of R.F. excitation. The possible efficiency approaches unity as the number of cells is increased.     

High-precision optical phase-locking based on wideband acousto-optical frequency shifting

We present a high-precision optical phase-locking based on wideband acousto-optical frequency shifting.Increasing the modulating bandwidth stabilizes the loop at a high loop gain,thus improving phase correction capability.An optical phase-locked loop with a wide control bandwidth is constructed.The closed-loop residual phase error is only 0.26°or smaller than λ/1000.The loop exhibits excellent correction capability for high-frequency noises.The correctable frequency range reaches 35 kHz when the noise amplitude is ±λ/2,and becomes even wider for smaller noise amplitudes.     

Enhancing a tone by shifting its frequency or intensity

When a test sound consisting of pure tones with equal intensities is preceded by a precursor sound identical to the test sound except for a reduction in the intensity of one tone, an auditory "enhancement" phenomenon occurs: In the test sound, the tone which was previously softer stands out perceptually. Here, enhancement was investigated using inharmonic sounds made up of five pure tones well resolved in the auditory periphery. It was found that enhancement can be elicited not only by increases in intensity but also by shifts in frequency. In both cases, when the precursor and test sounds are separated by a 500-ms delay, inserting a burst of pink noise during the delay has little effect on enhancement. Presenting the precursor and test sounds to opposite ears rather than to the same ear significantly reduces the enhancement resulting from increases in intensity, but not the enhancement resulting from shifts in frequency. This difference suggests that the mechanisms of enhancement are not identical for the two types of change. For frequency shifts, enhancement may be partly based on the existence of automatic "frequency-shift detectors" [Demany and Ramos, J. Acoust. Soc. Am. 117, 833-841 (2005)].     

Quantum cryptography using photon “frequency” states (example of a possible realization)

A quantum cryptosystem is proposed using single-photon states with different frequency spectra as information carriers. A possible experimental implementation of the cryptosystem is discussed. Zh. éksp. Teor. Fiz. 114, 526–537 (August 1998)     

Active vibration control using optimized modified acceleration feedback with Adaptive Line Enhancer for frequency tracking

Modified acceleration feedback (MAF) control, an active vibration control method that uses collocated piezoelectric actuators and accelerometer is developed and its gains optimized using an optimal controller. The control system consists of two main parts: (1) frequency adaptation that uses Adaptive Line Enhancer (ALE) and (2) an optimized controller. Frequency adaptation method tracks the frequency of vibrations using ALE. The obtained frequency is then fed to MAF compensators. This provides a unique feature for MAF, by extending its domain of capabilities from controlling a certain mode of vibrations to any excited mode. The optimized MAF controller can provide optimal sets of gains for a wide range of frequencies, based on the characteristics of the system. The experimental results show that the frequency tracking method works quite well and fast enough to be used in a real-time controller. ALE parameters are numerically and experimentally investigated and tuned for optimized frequency tracking. The results also indicate that the MAF can provide significant vibration reduction using the optimized controller. The control power varies for vibration suppression at different resonance frequencies; however, it is always optimized.     

Plasma acceleration in a wave with varying frequency

The averaged velocity of a test particle and the averaged velocity of a plasma in an electro-magnetic wave packet with varying frequency (e.g., a radiation pulse from pulsar) is derived. The total momentum left by the wave packet in regions of plasma inhomogeneity is found. If the plasma concentration is changing due to ionization, the plasma may be accelerated parallelly or antiparallelly to the direction of the wave packet propagation, which is relevant for a laser induced breakdown in gas.The author thanks R.Klíma for numerous discussions.