Doppler Background Research Articles

Wavelet-based saturated absorption line detection for laser frequency locking

Owing to the presence of noise and the Doppler background, accurate saturated absorption (SA) peak automatic identification technology poses a significant challenge for laser frequency tuning and locking. To address this issue, a novel peak identification algorithm for the SA spectrum is proposed. First, a Gaussian filter based on a Gaussian continuous wavelet transform is proposed to mitigate the spectral high-frequency noise. Subsequently, a hybrid method combining a first-order Gaussian continuous wavelet transformation and adaptive threshold judgment was designed for multi-peak boundary segmentation. Finally, we obtained the target peak and its sweeping voltage based on an adaptive nonlinear fitting algorithm, which was almost unaffected by the peak asymmetry caused by the Doppler background.

TCN-based M-ary mobile spread spectrum underwater acoustic communication

A deep learning-based direct sequence spread spectrum (DSSS) underwater acoustic (UWA) communication system was proposed in this study to improve the performance of the conventional system under Doppler effects and low signal-to-noise ratio (SNR). The Temporal Convolutional Network (TCN) model was used as the receiver of the system. Simulation results showed that the TCN-based M-ary DSSS UWA communication system outperformed the conventional system under Doppler effects and complex shallow water acoustic channel. A transfertraining algorithm was used for fine-tuning of the model based on sea real-time data to achieve optimal performance. Sea experimental results showed that the fine-tuned TCN model improved the communication effect under Doppler background in the complex dynamic environment of shallow water.

High-resolution Cs-Rb two-photon spectrometer for directly stabilizing a Ti:sapphire comb laser.

In this paper, we present a simple scheme for efficiently removing the residual Doppler background of a comb laser based two-photon spectrometer to be better than 10-3 background-to-signal ratio. We applied this scheme to stabilize the frequencies of a mode-locked Ti:sapphire laser directly referring to the cesium 6S-8S transition and rubidium 5S-5D transition. We suggest a standard operation procedure (SOP) for the fully direct comb laser stabilization and evaluate the frequency of two spectral lines at a certain temperature, by which we demonstrate an all-atomic-transition-based Ti:sapphire comb laser merely via a 6-cm glass cell.

Absolute frequency measurement of the 6s21S0 → 6s6p3P1F = 3/2 → F' = 5/2 201Hg transition with background-free saturation spectroscopy.

We report the development of a method for eliminating background-induced systematic shifts affecting precise measurements of saturation absorption signals. With this technique, we measured the absolute frequency of the 6s21S0 → 6s6p3P1 transition in 201aHg (F = 3/2 → F' = 5/2) to be 1181541111051(83) kHz. The measurement was referenced with an optical frequency comb synchronized to the frequency of the local representation of the UTC. This specific atomic line is situated on the steep slope of the Doppler background at room temperature, which results in a frequency systematic shift. We determined the dependence of this shift on the properties of both the spectral line and the background of the measured signal.

Experimental study on transit time broadening in rubidium 5S-5D excitation.

In this experimental work, we report our findings about a transition between states of even parity 5S1/2→5D5/2 of Rb atoms under different conditions of transit time broadening. For this purpose, the blue fluorescence emanating from the associated decay 5D5/2→6P3/2→5S1/2 channel is monitored. Based on the relative magnification of the laser beam, which is used in excitation between levels of even parity, the system exhibits two photon absorption (TPA) through blue fluorescence peaks. The TPA profile appears as a Lorentzian peak on a Doppler background. The change in the laser beam diameter directly influences the relative contributions of Lorentzian and Doppler parts. A direct inference drawn from of this study may be utilized in optimizing the extraction of the first derivative spectrum corresponding to 5S→5D transitions.

Development and Analysis of New NLFM Waveform for Achieving Sidelobe Reduction and Narrowing of Main lobe Width

Aimed at narrowing main lobe width and reduced sidelobe values, we developed three new NLFM chirp waveforms. The ambiguity function and the impact of sampling rate and compression ratios of these waveforms are analyzed. Their performance is examined against the doppler effect and background noise. One of the three designed NLFM chirp waveforms is useful in applications requiring side lobes of -50 dB and narrow main lobe width. The new waveform could achieve reduced sidelobes and narrow main lobe width compared to LFM and other NLFM waveforms

Performance Analysis of NLFM Signals with Doppler Effect and Background Noise

This paper focuses on the study of effect of background noise and Doppler Effect on various Nonlinear Frequency Modulation (NLFM) waveforms designed using two stage piece-wise and three stage piece-wise linear and non linear functions. The background noise investigated is Additive White Gaussian Noise (AWGN). Simulations are carried out for different target speeds ranging from 100 to 5000km/hour to study Doppler Effect. The simulations are carried out using Matlab software. Among the waveforms designed, the NLFM function designed using two piece-wise Linear Frequency Modulation (LFM) is observed to be Doppler tolerant and also not affected by noise, as the SNR changes from -20 to 20 dB the peak side-lobe level (PSL) of this signal is around -34.84 dB.

Exploring hyperfine levels of non-Rydberg excited states in a Ξ system using Autler-Townes splitting.

In this experimental work we report our findings about a cascade (Ξ) transition 5S1/2→5P3/2→5D3/2 of both 85,87Rb atoms under different laser detuning combinations. The relative power levels of two individual lasers are adjusted under a counter-propagating configuration so that the system exhibits Autler-Townes splitting (ATS). However, the ATS, which is otherwise difficult to detect in a room-temperature alkali vapor cell offering large Doppler background, is well resolved here by using a combination of modulation transfer and phase-sensitive detection techniques. The results show that the AT components clearly indicate the hyperfine structure of 5D3/2 level for 87Rb isotope. For 85Rb, the resolution of ATS is limited by the relatively closer proximity of 5D3/2 hyperfine components. The results are also verified through blue fluorescence detection by monitoring the 5D3/2→6P3/2→5S1/2 non-degenerate decay arm. The technique is easy to implement and is able to reveal the hyperfine structure of the excited levels. However, the technique is not a good choice when an excited level with dense hyperfine structure is targeted.

Saturated fluorescence spectroscopy measurement apparatus based on metastable Li+ beam with low energy.

The apparatus for fluorescence spectroscopy measurement is developed to determine the fine structure (FS)/hyperfine structure (HFS) splittings of 1s2p 3PJ(J=0,1,2) states of Li+. The instrument is composed of a low energy Li+ ion source and a saturated fluorescence spectroscopic probing system. A low energy Li+ ion source, containing 1.8(7)% of 1s2s 3S1 metastable ions with an energy of ∼500 eV, is obtained by an electron bombardment process. The ion current can stay more than 250 h with the variation of ∼0.3%, and the divergence of ion beam is ∼0.5 mrad. The symmetric profile of Lamb dip signals of 1s2s 3S1--1s2p 3PJ transitions with linewidths of ∼50 MHz are obtained after subtracting Doppler background from the saturated fluorescence signals. A back-and-forth scan method is adopted to determine the FS/HFS splittings of 1s2p 3PJ states. Under these conditions, as a preliminary test, several splittings of 7Li+ are measured. The statistical uncertainties of the FS/HFS splittings are estimated to be less than 50 kHz, and the results are one order of magnitude better than previous results. The apparatus is feasible to precisely determine the splittings of energy levels of alkali and alkaline earth ions.

Doppler distortion correction based on microphone array and matching pursuit algorithm for a wayside train bearing monitoring system

Doppler distortion and background noise can reduce the effectiveness of wayside acoustic train bearing monitoring and fault diagnosis. This paper proposes a method of combining a microphone array and matching pursuit algorithm to overcome these difficulties. First, a dictionary is constructed based on the characteristics and mechanism of a far-field assumption. Then, the angle of arrival of the train bearing is acquired when applying matching pursuit to analyze the acoustic array signals. Finally, after obtaining the resampling time series, the Doppler distortion can be corrected, which is convenient for further diagnostic work. Compared with traditional single-microphone Doppler correction methods, the advantages of the presented array method are its robustness to background noise and its barely requiring pre-measuring parameters. Simulation and experimental study show that the proposed method is effective in performing wayside acoustic bearing fault diagnosis.

Observation and theoretical simulation of electromagnetically induced transparency and enhanced velocity selective optical pumping in cesium vapour in a micrometric thickness optical cell

Observation of electromagnetically induced transparency (EIT) and enhanced velocity selective optical pumping (VSOP) signals in a micrometric cell with cesium is reported. The line shape and non-linear features observed in the case of fluorescence in the direction parallel to the cell windows and the transmission spectra observed along the propagation direction of the probe beam show considerable differences in the spectral profile. A theoretical model based on five level optical Bloch equations is used to simulate the spectra. The Doppler convolution includes all possible orientations of atomic velocities with respect to the laser beam direction. Atoms moving nearly parallel to the windows and perpendicular to the collinear pump and probe beams have much lower Doppler shift and hence produce considerable narrowing of the Doppler background in the fluorescence spectra. The coherence decay rate is also low for such atoms as they do not meet with the cell walls. The simulated curves reproduce the observed sharp EIT peaks and enhanced broad VSOP signals for the closed probe transition in the fluorescence and absorption spectra. The observed effect of detuning of the pump frequency on the non-linear features is also reproduced by the simulation.

Determining the hyperfine structure constants of caesium 8S1/2 state aided by atomic coherence

High-sensitivity spectroscopy of caesium's higher excited 8S1/2 state is obtained by a coherent two-photon transition via an intermediate resonance state. The ladder-type atomic system is driven by two counter-propagating low-power diode lasers, the probe laser being tuned to the transition from the ground state to the intermediate state (6S1/2–6P1/2), and the coupling laser to that between the intermediate and the final state (6P1/2–8S1/2). By locking the probe laser and scanning the coupling laser, the electromagnetically induced transparency (EIT) peaks appear in the probe transmission when the coupling laser resonates with each of the hyperfine levels. Compared with conventional EIT, where the signal-to-noise ratio is limited by the absorptive Doppler background, here these narrow-linewidth peaks have no Doppler background. The peak centres are well determined from theoretical fits to the experimental data. To accurately measure the 8S1/2 hyperfine structure splitting, we developed a simple method to eliminate error arising from the nonlinear frequency scanning by employing an optical waveguide phase modulator and a confocal Fabry–Perot cavity. The hyperfine structure constants of the caesium 8S1/2 state are obtained from hyperfine structure splitting measurements. Systematic effects from the ac-Stark and Zeeman shifts are studied. The measured hyperfine magnetic dipole constant A = (219.08 ± 0.12) MHz is consistent with previous results.

The wavelength tunable 589 nm laser output based on singly resonant sum-frequency generation and the measurement of saturate fluorescence spectrum of sodium atom

A wavelength-tunable laser output at 589 nm with high conversion efficiency based on sum-frequency generation by using the technique of single-wavelength extra-cavity resonance is achieved. The two fundamental wavelengths are 1583 nm and 938 nm and the nonlinear crystal is the period-poled lithium niobate. After the frequency of 1583 nm laser was locked to the cavity mode and the frequency of 938 nm laser was scanned, a 589 nm laser output with power of 4.96 mW and wavelength tuning range of 7 GHz was obtained and the stability of the output power is improved effectively with the help of servo feedback loop technique of acousto-optic-modulator. Finally, based on this laser, the saturated fluorescence spectrum of sodium D2 line in the temperature range of 348—413 K (75—140 ℃) were measured. The Doppler-free structures of D2a, D2b and crossover lines on Doppler background were observed, which can provide reference signals for the frequency locking of 589nm laser.

Double-resonance optical-pumping effect and ladder-type electromagnetically induced transparency signal without Doppler background in cesium atomic vapour cell

In a Doppler-broadened ladder-type cesium atomic system (6S1/2-6P3/2-8S1/2), this paper characterizes electromagnetically induced transparency (EIT) in two different experimental arrangements, and investigates the influence of the double-resonance optical-pumping (DROP) effect on EIT in both arrangements. When the probe laser is weak, DROP is explicitly suppressed. When the probe laser is moderate, population of the intermediate level (6P3/2 F′ = 5) is remarkable, therefore DROP is mixed with EIT. An interesting bimodal spectrum with the broad component due to DROP and the narrow part due to EIT has been clearly observed in cesium 6S1/2 F = 4−6P3/2 F′ = 5−8S1/2 F″ = 4 transitions.

Electromagnetically induced transparency without a Doppler background in a multilevel ladder-type cesium atomic system

We present an investigation of electromagnetically induced transparency (EIT) without Doppler background due to a locked probe laser. The EIT is theoretically studied based on a multilevel ladder-type cesium atomic system 6${S}_{1/2}$--6${P}_{3/2}$--8${S}_{1/2}$ in a room-temperature vapor cell. The experimental results agree with the theoretical calculations. Compared with the traditional EIT spectra with a Doppler profile limiting the spectral resolution for keeping the coupling laser locked and scanning the probe laser, these EIT spectra with the probe laser locked and the coupling laser scanned have a flat background, which seem be of great benefit for applications such as the measurement of hyperfine intervals between excited states, the study of highly excited Rydberg states, laser-frequency stabilization, etc.

Embolus Detection and Differentiation Using Multifrequency Transcranial Doppler

To the Editor: In our articles in Stroke in August 2002,1,2 we presented our findings using the first multifrequency transcranial Doppler (TCD) to detect and differentiate cerebral emboli. Since this time, we have …

The Advanced Gamma Ray Tracking Array AGATA

New accelerator facilities for radioactive–ion beams will enter into operation in the next few years, providing the opportunity to explore unknown territories of the nuclear landscape. The foreseen harsh experimental conditions require the construction of a new generation of γ–ray detector arrays based on the emerging technique of γ–ray tracking. The “Advanced GAmma Tracking Array” (AGATA), proposed in Europe, will be built out of 120 or 180 highly segmented Ge crystals operated in position sensitive mode by means of digital data techniques and pulse shape analysis of the segment signals. AGATA will be capable of measuring γ radiation in a large energy range (from ∼ 10 keV to ∼ 10 MeV), with the largest possible photopeak efficiency (25% at M γ = 30 ) and with good spectral response. The very good Doppler correction and background rejection capability of this γ–ray tracking array will allow to perform γ–ray spectroscopy experiments using fragmentation beams with sources moving at velocities up to β ∼ 0.5 .

The advanced gamma ray tracking array AGATA

New accelerator facilities for radioactive-ion beams and high-intensity stable beams will start operation in a few years. Although these beams will provide interesting opportunities for exploring unknown territories of the nuclear landscape, the experimental conditions will be very challenging and, indeed, the nuclear structure community has realized that a new generation of powerful arrays for gamma-ray spectroscopy has to be built in order to cope with them. As a result of years of experience with Compton suppressed germanium arrays and of intensive R&D work targeted to extend their limits, it is now clear that the next 4<FONT FACE=Symbol>p g</FONT>-ray spectrometers will be built fully from germanium detectors and will be based on the technique of gamma-ray tracking. The "Advanced GAmma Tracking Array" (AGATA), proposed in Europe, will be an instrument of major importance for nuclear structure studies at the very limits of nuclear stability. It will be built out of 120/180 highly segmented Ge crystals operated in position sensitive mode by means of digital data techniques and pulse shape analysis of the segment signals. AGATA will be able to measure gamma radiation in a large energy range (from ~ 10 keV to ~ 10 MeV), with the largest possible photopeak effi ciency (25 % at Mgamma = 30) and with a good spectral response. In particular, its very good Doppler correction and background rejection capabilities will allow to perform "standard" gamma-ray spectroscopy experiments using fragmentation beams with sources moving at velocities up to beta ~ 0.5.

Saturated absorption spectroscopy of Xe using a GaAs semiconductor laser

Doppler-free absorption spectrum from 5P 56S to 5P 56P transitions of Xe in a hollow cathode discharge was measured in the wavelength region from 810 to 910 nm. The saturated dip signals detected by a frequency modulated probe beam were doubly AM-modulated by a pump beam chopped using a conventional mechanical chopper. The Doppler background was greatly reduced by detection using a second lock in amplifier synchronous to the chopping signal. The isotope shifts of even mass 136−128Xe in natural abundance were resolved for the five transitions from the metastable states of J=2 and 0, while in the transitions from the J=1 states were partially resolved. The nuclear magnetic and quadrupole hyperfine coupling constants of 131Xe and 129Xe determined are listed with those reported previously. The specific mass shifts and the field shifts were estimated using the relation given by King's diagram and compared with those obtained from the changes in the mean square-radii of the nuclear charge distribution.

Laser spectroscopy of calcium in hollow-cathode discharges

We investigated the use of hollow-cathode discharges for high-resolution and high-sensitivity spectroscopy, using atomic calcium. Spectra with sub-Doppler resolution of Cai transitions at 423 (resonant), 610, 612, 616, 645, 657 (intercombination), and 672 nm were obtained by optogalvanic saturation spectroscopy in lamps filled with argon (0.6 and 2.5 Torr) and krypton (0.6 Torr). A Doppler background that is due to velocity-changing collisions, which may severely limit the resolution, can be greatly reduced by the choice of buffer gas. Sub-Doppler linewidths comparable with those achieved in atomic beams have been obtained, making a properly chosen hollow-cathode lamp a convenient tool for high-resolution spectroscopic experiments, providing wavelength references for laser frequency tuning. The sensitivity of optogalvanic detection and the excitation of most electronic levels by the discharge make these lamps attractive also for investigating weak and excited level transitions with the use of a simple experimental setup.