Laser Intensity Fluctuations Research Articles

Pendular trapping conditions for ultracold polar molecules enforced by external electric fields.

We theoretically investigate trapping conditions for ultracold polar molecules in optical lattices, when external magnetic and electric fields are simultaneously applied. Our results are based on an accurate electronic-structure calculation of the polar 23Na40K polar molecule in its absolute ground state combined with a calculation of its rovibrational-hyperfine motion. We find that an electric field strength of 5.26(15) kV/cm and an angle of 54.7° between this field and the polarization of the optical laser lead to a trapping design for 23Na40K molecules where decoherences due laser-intensity fluctuations and fluctuations in the direction of its polarization are kept to a minimum. One standard deviation systematic and statistical uncertainties are given in parenthesis. Under such conditions pairs of hyperfine-rotational states of v = 0 molecules, used to induce tunable dipole-dipole interactions between them, experience ultrastable, matching trapping forces.

Absolute noninvasive measurement of CO2 mole fraction emitted by E. coli and S. aureus using calibration-free 2f WMS applied to a 2004 nm VCSEL.

We report the first demonstration, to the best of our knowledge, of accurate real-time noninvasive measurement of the absolute cumulative mole fraction of metabolic carbon dioxide emitted by Escherichia coli and Staphylococcus aureus over a period of several hours of their life cycles using a recently developed calibration-free wavelength modulation spectroscopy technique. A 1mW vertical-cavity surface-emitting laser is used to interrogate a single rotational vibrational absorption line of carbon dioxide at 2003.5nm. The measurements are immune to laser intensity fluctuations and variable optical coupling that is inevitable in such free-space coupled experiments that run over 10-18h. The cumulative carbon dioxide mole fraction follows the characteristic modified Gompertz model that is typical of bacterial growth in batch cultures. The characteristic growth parameters are extracted from this curve. The technique can be readily extended to study multiple volatile organic compounds that bacteria are known to emit.

Simultaneous Acquisition of the Polarized and Depolarized Raman Signal with a Single Detector.

Polarization-resolved Raman spectroscopy provides much more information than its conventional counterpart. However, it usually either requires a complicated setup with two spectrographs and detectors or two measurements must be performed sequentially. This study presents a simple and straightforward approach to recording both polarization components simultaneously with a single spectrograph and detector. The vertically and a horizontally polarized laser beam exiting a Wollaston prism are focused into the sample with a small spatial separation. The scattered light from both beams is imaged onto the slit of an imaging spectrograph as two spatially separated signals, i.e., the polarized and the depolarized Raman signal. Eventually, both spectra are acquired on a single CCD chip simultaneously. Experimental data of ethanol and dimethyl sulfoxide are shown as proof-of-concept. The new method has a number of advantages, for example, laser intensity fluctuations and the polarization dependence of the diffraction grating do not play a role. The proposed approach will be useful for an improved structural analysis and it will be the enabling technology for temporally resolved enantioselective Raman (esR) spectroscopy.

Intensity fluctuation of a gain-switched semiconductor laser for quantum key distribution systems.

Security certification of quantum key distribution (QKD) systems under practical conditions is necessary for social deployment. This article focused on the transmitter, and, in particular, investigated the intensity fluctuation of the optical pulses emitted by a gain-switched semiconductor laser used in QKD systems implementing decoy-BB84 protocol. A large intensity fluctuation was observed for low excitation, showing strong negative correlation between the adjacent pulses, which would affect the final key rate. The fluctuation decreased and the correlation disappeared as excitation increased. Simulation with rate equations successfully reproduced the experimental results and revealed that the large fluctuation originates from an intrinsic instability of gain-switched lasers driven periodically at a rate comparable to the inverse of carrier lifetime, as in GHz-clock QKD systems. Methods for further reduction of the intensity fluctuation were also discussed.

Theoretical research of influence of laser intensity fluctuation on imaging quality degradation of coherent field

The laser coherent field imaging system emits multiple beams of laser from earth to space, and laser scans remote space target by passing through turbulence atmosphere. Multi-beam laser intensity fluctuation caused by atmosphere turbulence is a key factor affecting high-resolution imaging quality of the coherent field imaging system. Aiming at solving the problem of imaging quality degradation caused by laser intensity fluctuation error, we discuss the mechanism of laser intensity fluctuation error influencing the imaging quality of laser coherent field high-resolution imaging system. The theoretical model about the relationship between laser intensity fluctuation factor and imaging quality is proposed for the first time. Firstly, the laser echo field signal error induced by laser intensity amplitude fluctuation factor is deduced according to laser transmitting atmosphere theory. Then adopting multi-beam phase closure arithmetic, the phase closure coefficient error is derived from the laser intensity fluctuation factor and laser echo field signal. The mechanism of disturbed laser echo signal influencing phase closure coefficient is investigated in detail. In the following, based on reconstructed spectrum theory, the model of imaging frequency spectrum error propagation, caused by laser intensity fluctuation factor, is proposed. Finally, we reveal the mechanism of laser intensity amplitude fluctuation factor influencing reconstructed imaging frequency spectrum and imaging quality. The correctness and validity of the theoretical model are verified in simulation experiment. In the three-beam laser coherent field imaging simulation experiment, the imaging quality is evaluated by the Strehl ratio of the image. Experimental result shows that the Strehl ratio is only related to the light intensity fluctuation of one of the three beams of laser, and the greater the fluctuation of laser intensity, the more serious the degradation of imaging quality is. The research draws the conclusion that the reconstructed imaging frequency spectrum and image quality are mainly affected by the laser intensity fluctuation of the second beam in the three-beam phase closure algorithm, regardless of other two laser intensity fluctuations. Thus, in order to restrain the degradation of imaging quality caused by laser intensity fluctuation, we only need to keep stable the laser intensity of the second beam but not all of the laser beams. In this paper, we reveal the mechanism of laser intensity fluctuation affecting high-resolution imaging quality in the three-beam laser coherent field imaging system. The research provides a theoretical basis for analyzing imaging quality degradation from the laser intensity fluctuation caused by atmospheric turbulence, and reasonably assigning the light intensity stability of multi-beam laser emitter to improve the imaging quality in laser coherent field imaging system.

Efficient cooling of quantized vibrations using a four-level configuration

Cooling vibrational degrees of freedom down to ground states is essential to observation of quantum properties of systems with mechanical vibration. We propose two cooling schemes employing four internal levels of the systems, which achieve the ground-state cooling in an efficient fashion by completely deleting the carrier and first-order blue-sideband transitions. The schemes, based on the quantum interference and Stark-shift gates, are robust to fluctuation of laser intensity and frequency. The feasibility of the schemes is justified using current laboratory technology. In practice, our proposal readily applies to an nano-diamond nitrogen-vacancy center levitated in an optic trap or attached to a cantilever.

Nuclear-state population transfer by a train of coincident pulses

Population transfer of three-level $\mathrm{\ensuremath{\Lambda}}$-like nuclei interacting with x-ray free electron lasers (XFEL) via a train of coincident pulses has been investigated theoretically. This study uses copropagating beams in which the frequency of the pump laser is different from that of the Stokes laser. We calculate the required laser intensities for each step which satisfy the condition of coincident-pulse technique in different nuclei. By employing the master equation and considering the effect of spontaneous emission, we show that complete nuclear population transfer occurs by choosing the appropriate number of pulse pairs. It is shown that the effect of laser intensity fluctuation is suppressed by increasing the number of pulse pairs. In this scheme, after each step, the populations of system are in stable states and the time delays between the neighboring pulses do not affect the transmission efficiency.

Preparation of Entangled States through Hilbert Space Engineering.

We apply laser fields to trapped atomic ions to constrain the quantum dynamics from a simultaneously applied global microwave field to an initial product state and a target entangled state. This approach comes under what has become known in the literature as "quantum Zeno dynamics" and we use it to prepare entangled states of two and three ions. With two trapped ^{9}Be^{+} ions, we obtain Bell state fidelities up to 0.990_{-5}^{+2}; with three ions, a W-state fidelity of 0.910_{-7}^{+4} is obtained. Compared to other methods of producing entanglement in trapped ions, this procedure can be relatively insensitive to certain imperfections such as fluctuations in laser intensity.

Numerical simulations of recent proton acceleration experiments with sub-100 TW laser systems

Recent experiments carried out at the Italian National Research Center, National Optics Institute Department in Pisa, are showing interesting results regarding maximum proton energies achievable with sub-100 TW laser systems. While laser systems are being continuously upgraded in laboratories around the world, at the same time a new trend on stabilizing and making ion acceleration results reproducible is growing in importance. Almost all applications require a beam with fixed performance, so that the energy spectrum and the total charge exhibit moderate shot to shot variations. This result is surely far from being achieved, but many paths are being explored in order to reach it. Some of the reasons for this variability come from fluctuations in laser intensity and focusing, due to optics instability. Other variation sources come from small differences in the target structure. The target structure can vary substantially, when it is impacted by the main pulse, due to the prepulse duration and intensity, the shape of the main pulse and the total energy deposited. In order to qualitatively describe the prepulse effect, we will present a two dimensional parametric scan of its relevant parameters. A single case is also analyzed with a full three dimensional simulation, obtaining reasonable agreement between the numerical and the experimental energy spectrum.

Thin laser beam wandering and intensity fluctuations method for evapotranspiration measurement

We compare in this study two simple optical setups to measure the atmospheric turbulence characterized by the refractive index structure parameter Cn2. The corresponding heat flux values sensed by the laser beam propagation are calculated leading to the plant evapotranspiration. The results are discussed and compared to measurements obtained with a well-known and calibrated eddy-covariant instrument. A fine analysis gives a good insight of the accuracy of the optical devices proposed here to measure the crop evapotranspiration. Additional evapotranspiration values calculated with meteorological sensor data and the use of different models are also compared in parallel.

激光强度起伏及相位噪声对光学双频探测的影响

激光强度起伏及相位噪声对光学双频探测的影响

Wideband fluorescence-based thermometry by neural network recognition: Photothermal application with 10 ns time resolution

Neural network recognition of features of the fluorescence spectrum of a thermosensitive probe is exploited in order to achieve fluorescence-based thermometry with an accuracy of 200 mK with 100 MHz bandwidth, and with high robustness against fluctuations of the probe laser intensity used. The concept is implemented on a rhodamine B dyed mixture of copper chloride and glycerol, and the temperature dependent fluorescence is investigated in the temperature range between 234 K and 311 K. The spatial dependence of the calibrated amplitude and phase of photothermally induced temperature oscillations along the axis of the excitation laser are determined at different modulation frequencies. The spatial and frequency dependence of the extracted temperature signals is well fitted by a 1D multi-layer thermal diffusion model. In a time domain implementation of the approach, the gradual temperature rise due to the accumulation of the DC component of the heat flux supplied by repetitive laser pulses as well the immediate transient temperature evolution after each single pulse is extracted from acquired temporal sequences of fluorescence spectra induced by a CW green laser. A stroboscopic implementation of fluorescence thermometry, using a pulsed fluorescence evoking probe laser, is shown to achieve remote detection of temperature changes with a time resolution of 10 ns.

Subpicotesla Diamond Magnetometry

Diamond defect centers are promising solid state magnetometers. Single centers allow for high spatial resolution field imaging but are limited in their magnetic field sensitivity to around 10 nT/Hz^(1/2) at room-temperature. Using defect center ensembles sensitivity can be scaled as N^(1/2) when N is the number of defects. In the present work we use an ensemble of 1e11 defect centers for sensing. By carefully eliminating all noise sources like laser intensity fluctuations, microwave amplitude and phase noise we achieve a photon shot noise limited field sensitivity of 0.9 pT/Hz^(1/2) at room-temperature with an effective sensor volume of 8.5e-4 mm^3. The smallest field we measured with our device is 100 fT. While this denotes the best diamond magnetometer sensitivity so far, further improvements using decoupling sequences and material optimization could lead to fT/Hz^(1/2) sensitivity.

18pPSA-84 強度バランスSagnac干渉計光熱偏向分光法

18pPSA-84 強度バランスSagnac干渉計光熱偏向分光法

Contribution of multiple electron trajectories to high-harmonic generation in the few-cycle regime

We use a few-cycle, carrier-envelope-phase (CEP) stabilized laser system to generate high-harmonic emission in argon, neon, and carbon dioxide. The high-harmonic spectra consist of discrete harmonic orders whose positions shift as a function of the CEP. Near the cutoff harmonic, the peaks are separated by two photon orders, and can correspond to either even or odd harmonics of the driving laser frequency, depending on the value of the CEP. In the plateau region, harmonic orders are separated by only one photon order. We develop a simple model which predicts the observed behavior. We use the observed dependence of the harmonic peaks as a function of CEP as a method to measure the statistical CEP fluctuations of the laser system. The measured rms fluctuation of 0.17 radians agrees with optical measurements. The high-harmonic approach to measuring CEP stability has the advantage that it is less sensitive to laser intensity fluctuations than are optical methods.

Self-calibration methodology by normalized intensity for wavelength modulation spectroscopy measurement

A methodology of self-calibration for concentration measurement based on wavelength modulation absorption spectroscopy has been developed by normalized fixed intensity. Experimental results show that the simple self-calibration method not only effectively improves the calibration accuracy, but also greatly improves stabilization and reliability as compared with the popular method called self-calibration by 1f normalized. In addition, the proposed system do not need any additional equipment when comparing with the use of traditional wavelength modulation absorption spectrometer. The standard deviation of a concentration measurement with different optical intensity aimed on the detector has been found to be below 1.0%. The dependence of the concentration assessment on the laser intensity fluctuation has also been investigated, which shows that the method of self-calibration could be applied in the field, specially where the dust is easily splattered on the windows of the detector.

Two-wavelength Raman imaging for non-intrusive monitoring of transient temperature in microfluidic devices

The present study proposes a non-intrusive visualization technique based on two-wavelength Raman imaging for in-situ monitoring of the unsteady temperature field in microfluidic systems. The measurement principle relies on the contrasting temperature dependencies of hydrogen-bonded and non-hydrogen-bonded OH stretching modes of the water Raman band, whose intensities were simultaneously captured by two cameras equipped with corresponding bandpass filters. The temperature distributions were then determined from the intensity ratio of the simultaneously-obtained Raman images, which enables compensation for temporal fluctuation and spatial inhomogeneity of the excitation laser intensity. A calibration experiment exhibited a linear relationship between the temperature and the intensity ratio in the range 293–343 K and least-regression analysis gave an uncertainty of 1.43 K at 95% confidence level. By applying the calibration data, time series temperature distributions were quantitatively visualized in a Y-shaped milli-channel at a spatial resolution of 6.0 × 6.0 µm2 with an acquisition time of 16.5 s. The measurement result clearly exhibited the temporal evolution of the temperature field and was compared with the values obtained by thermocouples. This paper therefore demonstrates the viability of employing the two-wavelength Raman imaging technique for temperature measurements in microfluidic devices.

Relative Phase Noise-Induced Phase Error and System Impairment in Pump Depletion/Nondepletion Regime

Although the pump relative intensity noise transfer phenomenon in traditional Raman amplified amplitude-shift keying fiber communication systems has been investigated intensively, the stochastic intensity fluctuation of Raman pump laser will interplay with fiber nonlinearity and leads to additional phase noise of signal in the multilevel modulated coherent optical communication system. Such phase noise is defined as relative phase noise (RPN). In this paper, we present comprehensive analysis regarding the characteristics and impairments of RPN through both theoretical derivation and Monte Carlo simulation. We derive the analytical expressions of phase error variance for M-ary PSK and 16QAM modulation formats in consideration of RPN using decision-aided maximum-likelihood phase estimation algorithm. The analysis can be used to predict RPN induced impairment theoretically. Bit error rate (BER) performance is evaluated and compared for QPSK, 8PSK, 16PSK, and 16QAM modulation formats, and the result shows that in the context of RPN 16QAM signal outperforms 16PSK which has the same spectral efficiency. Finally, we extend the analytical result obtained in pump nondepletion regime to depletion regime via numerical analysis. It is observed by comparison that Q-factor penalty will increase in depletion regime.

Nonlinear microscopy looks beneath the surface of historic artwork

For the first time, it’s possible to create depth-resolved images of a painting without damaging it.

Relative phase noise estimation and mitigation in Raman amplified coherent optical communication system

The interplay between the stochastic intensity fluctuation of Raman pump laser and cross-phase modulation (XPM) effect in transmission optical fiber leads to additional phase noise, namely, relative phase noise (RPN) of signal in multi-level modulated coherent optical communication system. Both theoretical analysis and quantitative simulation have been performed to investigate the characteristics and impact of RPN. Being low-pass in nature, RPN is different from XPM induced phase noise in PSK/OOK hybrid system, and has not been considered yet. The noise power of RPN can accumulate incoherently along transmission links. With a proper signal model, we study the impact of RPN to the coherent optical communication system through Monte Carlo simulation. RPN will cause more cycle slips in Viterbi-and-Viterbi (V-V) phase estimation (PE), and the quantitative analysis of cycle slip probability is carried out. When using sliding window V-V without any optimization, the Q factor penalty of RPN on DQPSK signal can be as large as around 5 dB in strong RPN condition. However, it can be reduced by over 3 dB when using an optimal block size or optimal averaging weights.