Search results for "FREQUENCY"
showing 10 items of 2158 documents
20 GHz arbitrary radio-frequency waveform generator based on incoherent pulse shaping
2008
We demonstrate a new approach of photonically assisted radiofrequency (RF) waveform generation using a spectrally incoherent light source. The system is based on the so-called generalized frequency-to-time mapping operation. In this work, external modulation of the source is done by concatenating two electro-optic Mach-Zehnder modulators properly biased to achieve short pulse gates which allow for broad bandwidth electrical signals. Also, the spectral shaping stage is performed prior to O/E conversion. A detailed theoretical analysis demonstrates that even in this case the frequency-to-time mapping is preserved. The key is that the noise level is greatly reduced because the amplitude filter…
An evaluation procedure for scanning interferometer based wavemeters
2004
Abstract A common approach to wavelength measurement is the travelling interferometer (wavemeter). We present an evaluation procedure that allows to achieve high precision with a compact device. The zero crossings of the interference signal corresponding to the reference laser is used to sample the signal of the probe laser. Electronic noise and fluctuations of the laser frequency cancel due to the large number of sample points. The evaluation procedure has been applied to a wavemeter based on a continuously moving Mach–Zehnder interferometer. A travelling path of 10 cm was sufficient to yield 3σ-accuracy of Δ λ / λ =3.2×10 −9 , demonstrated by comparison with cesium saturation spectroscopy.
A signal processing scheme based on high-frequency electromechanical oscillations in nanostructures
2009
We explore the characteristics of a new signal processing scheme based on the high-frequency electromechanical oscillations of a nanostructure formed by an oscillating metallic nanoparticle connected to the left and right electrodes by soft links. Because this system shows resonant behavior when the frequency of the applied electric potential is close to the characteristic natural frequency of the oscillating nanoparticle, a parallel arrangement of nanostructures with different frequencies can be excited selectively by an external time-dependent electrical signal with the appropriate resonant frequencies. The highly nonlinear system response makes it possible to devise a signal processing s…
Switching dynamics of dark-pulse Kerr frequency comb states in optical microresonators
2021
Dissipative Kerr solitons are localized structures that exist in nonlinear optical cavities. They lead to the formation of microcombs - chip-scale frequency combs that could facilitate precision frequency synthesis and metrology by capitalizing on advances in silicon photonics. Previous demonstrations have mainly focused on anomalous dispersion cavities. Notwithstanding, localized structures also exist in the normal dispersion regime in the form of circulating dark pulses, but their physical dynamics is far from being understood. Here, we explore dark-pulse Kerr combs generated in normal dispersion optical microresonators and report the discovery of reversible switching between coherent dar…
Identification of Bending Modes of Vibration in Rails by a Laser Doppler Vibrometer on a Moving Platform
2020
This paper introduces a method to identify the bending modes of vibration of railway tracks by using a laser Doppler vibrometer (LDV) mounted on a moving platform. Two sets of experiments were conducted at Transportation Technology Center Inc. (TTCI) in Pueblo Colorado, in order to validate the proposed method. First, the bending vibration modes were identified using the signals collected from a rail span (rail section between two consecutive sleepers) by accelerometers under moving car excitation. Then, vibration measurements from rail spans were obtained by using an LDV mounted on the moving railcar. All tests were carried out at four different rail car speeds: 8 km/h (5 mph), 16 km/h (10…
Optical frequency combs for space applications
2016
Optical frequency comb-based high resolution laser spectroscopy has been demonstrated in space under micro-gravity on two sounding rocket based experiments. The comb has been used to simultaneously measure two different atomic transitions.
The Wigner Distribution of Sum-of-Cissoids and Sum-of-Chirps Processes for the Modelling of Stationary and Non-Stationary Mobile Channels
2016
This paper concerns the time-frequency analysis of stationary and non-stationary multipath flat fading channels. For the modelling of stationary multipath fading channels, we use a sum-of-cisoids (SOCi) process, while the non-stationary channel is modelled by a sum-of-chirps (SOCh) process that captures the time-variant Doppler effect caused by speed variations of the mobile station. For the time-frequency analysis, we apply the concept of the Wigner distribution. Closed-form solutions are provided for the Wigner distribution of SOCi and SOCh processes. It is shown that the obtained Wigner distributions can be expressed by the sum of an auto-term representing the true Doppler power spectral…
A Method Based on Amplitude Probability Density Representation for Sounding High Frequency Noise in Ionospheric Channels
2021
High Frequency (HF) communications efficiency require a precise characterization of the ionospheric channel’s noise. We present a rapid and accurate method to sound the HF ionospheric channels that enables tracing of the time-availability of the channel based on imposed electric field strength thresholds. The method makes use of the amplitude probability density implemented in a real-time spectrum analyzer. Sounding of 3, 10 and 20 kHz bandwidth channels in the 4.8 – 8.8 MHz range is exemplified and specific observations are presented.
Nuclear-spin comagnetometer based on a liquid of identical molecules
2018
Atomic comagnetometers are used in searches for anomalous spin-dependent interactions. Magnetic field gradients are one of the major sources of systematic errors in such experiments. Here we describe a comagnetometer based on the nuclear spins within an ensemble of identical molecules. The dependence of the measured spin-precession frequency ratio on the first-order magnetic field gradient is suppressed by over an order of magnitude compared to a comagnetometer based on overlapping ensembles of different molecules. Our single-species comagnetometer is shown to be capable of measuring the hypothetical spin-dependent gravitational energy of nuclei at the $10^{-17}$ eV level, comparable to the…
Instantaneous diffusion effect on spin-echo decay: Experimental investigation by spectral selective excitation
2001
The influence of the instantaneous diffusion process on spin-echo decay of ${E}_{\ensuremath{\gamma}}^{\ensuremath{'}}$ centers in gamma irradiated silica is experimentally probed by spectral selective excitation within their inhomogeneous resonance line. Our results evidence the different effectiveness of this dephasing mechanism on varying the resonance field, manifesting itself by a faster decay of the echo signal when generated by spin packets located in the central part of the spectrum. It is shown that the dependence of the instantaneous diffusion rate on the spectral position of echo-active spins reproduces the shape of the ${E}_{\ensuremath{\gamma}}^{\ensuremath{'}}$ centers resonan…