The return flux from a sodium laser guide star suffers, at large angles between the geomagnetic field and the laser beam, from the reduction in optical pumping due to spin-precession of sodium atoms. This detrimental effect can be mitigated by modulating the circular polarization of a continuous-wave laser beam in resonance with the Larmor frequency of sodium atoms in the mesosphere. We present an investigation based on numerical modeling to evaluate the brightness enhancement of a laser guide star with polarization modulation of a continuous-wave laser beam at different observatories.
In this work we discuss a mechanism for generation of a coherent source of light from the mesosphere as a new concept of directional laser guide star. In contrast to the near-isotropic spontaneous emission, nonlinear processes in atomic vapors like amplified spontaneous emission can yield highly directional emission in the forward and backward directions. Along with directional emission, excited sodium atoms also radiate at different wavelength creating a polychromatic laser guide star (PLGS). If feasible, a directional PLGS would provide a net gain in the return flux of several orders of magnitude compared to traditional LGS schemes, making possible laser-guided tip/tilt-correction in adaptive optic systems.
Recent developments in magnetic field sensing with negatively charged nitrogen-vacancy centers (NV) in diamond employ magnetic-field (MF) dependent features in the photoluminescence (PL) and eliminate the need for microwaves (MW). Here, we study two approaches towards improving the magnetometric sensitivity using the ground-state level anti-crossing (GSLAC) feature of the NV center at a background MF of 102.4 mT. Following the first approach, we investigate the feature parameters for precise alignment in a dilute diamond sample; the second approach extends the sensing protocol into absorption via detection of the GSLAC in the diamond transmission of a 1042nm laser beam. This leads to an increase of GSLAC contrast and results in a magnetometer with a sensitivity of 0.45 nT/√Hz and a photon shot-noise limited sensitivity of 12.2 pT/√Hz.
COSMA: Coherent Optics Sensors for Medical Application is an European Marie Curie Project running from 2012 to March 2016, with the participation of 10 teams from Armenia, Bulgaria, India, Israel, Italy, Poland, Russia, UK, USA. The main objective was to focus theoretical and experimental research on biomagnetism phenomena, with the specific aim to develop all-optical sensors dedicated to their detection and suitable for applications in clinical diagnostics. The paper presents some of the most recent results obtained during the exchange visits of the involved scientists, after an introduction about the phenomenon which is the pillar of this kind of research and of many other new fields in laser spectroscopy, atomic physics, and quantum optics: the dark resonance.
We report on the comparison between observations and simulations of a completed 12-month field observation campaign at Observatorio del Teide, Tenerife, using ESO's transportable 20 watt CW Wendelstein laser guide star system. This mission has provided sodium photon return flux measurements of unprecedented detail regarding variation of laser power, polarization and sodium D2b repumping. The Raman fiber laser and projector technology are very similar to that employed in the 4LGSF/AOF laser facility, recently installed and commissioned at the VLT in Paranal. The simulations are based on the open source LGSBloch density matrix simulation package and we find good overall agreement with experimental data.
Recent numerical simulations and experiments on sodium Laser Guide Star (LGS) have shown that a continuous wave (CW) laser with circular polarization and re-pumping should maximize the fluorescent photon return flux to the wavefront sensor for adaptive optics applications. The orientation and strength of the geomagnetic field in the sodium layer also play an important role affecting the LGS return ux. Field measurements of the LGS return flux show agreement with the CW LGS model, however, fluctuations in the sodium column abundance and geomagnetic field intensity, as well as atmospheric turbulence, induce experimental uncertainties. We describe a laboratory experiment to measure the photon return flux from a sodium vapor cell illuminated with a 589 nm CW laser beam, designed to approximately emulate a LGS under controlled conditions. Return flux measurements are carried out controlling polarization, power density, re-pumping, laser linewidth, and magnetic field intensity and orientation. Comparison with the numerical CW simulation package Atomic Density Matrix are presented and discussed.
In this work, we optimize a dynamical decoupling (DD) protocol to improve the spin coherence properties of a dense ensemble of nitrogen-vacancy (NV) centers in diamond. Using liquid nitrogen-based cooling and DD microwave pulses, we increase the transverse coherence time T2 from ∼ 0.7 ms up to ∼ 30 ms. We extend previous work of single-axis (Carr-Purcell-Meiboom-Gill) DD towards the preservation of arbitrary spin states. After performing a detailed analysis of pulse and detuning errors, we compare the performance of various DD protocols. We identify that the concatenated XY8 pulse sequences serves as the optimal control scheme for preserving an arbitrary spin state. Finally, we use the concatenated sequences to demonstrate an immediate improvement of the AC magnetic sensitivity up to a factor of two at 250 kHz. For future work, similar protocols may be used to increase coherence times up to NV-NV interaction time scales, a major step toward the creation of quantum collective NV spin states.
We present a novel technique of efficient optical pumping of open, high-angular-momentum systems. The method combines two well-established approaches of population manipulation (conventional optical pumping and coherent population transfer), offering the ability to achieve higher population of a sublevel with the highest or lowest quantum number m (the “end state”) than obtainable with either of the techniques. To accomplish this task, we propose to use coherent-population-transfer technique (e.g., adiabatic fast passage) to arrange the system in such a way that spontaneously emitted photon (conventional optical pumping) carries away more entropy than in conventional schemes. This enables reduction of a number of spontaneous decays Nsd required to pump the system with the total angular momentum J from Nsd = J decays in the conventional scheme to Nsd ≲ log2(2J) decays in the proposed scheme. Since each spontaneous-emission event is potentially burdened with a loss of population (population is transferred to a dark state), this enables increasing population accumulated in the “end state”, which is important for many applications.
We investigate a four-wave-mixing process in an N interaction scheme in Rb vapor placed inside a low-finesse ring cavity. We observe strong amplification and generation of a probe signal, circulating in the cavity, in the presence of two strong optical pump fields. We study the variations in probe field gain and dispersion as functions of experimental parameters with an eye on the potential application of such a system for enhanced rotation measurements. Density-matrix calculations are performed to model the system and are shown to provide good qualitative agreement with the experiment.
We investigate the propagation of a weak probe laser field in a medium of warm Rb atoms, controlled with two strong resonant pump fields tuned to the D1 and D2 optical transitions to form an N-scheme arrangement. We have shown theoretically that four-wave mixing has a profound effect on the probe-field group velocity and absorption, allowing the probe-field propagation to be continuously tuned from superluminal to slow-light regimes with amplification. We have also identified the experimental conditions for observation of such tunable slow-to-fast light regime (continuously through the point of zero group index) with positive probe-field gain, and demonstrated that the spectral range corresponding to the zero group index can be tuned by controlling the power of one of the pump laser.
Almost all sodium laser guide star (LGS) systems in the world are based on pulsed lasers. We review the relevant
sodium physics and compare different laser pulse formats. Selected formats are discussed on the basis of numerical
simulation results. One of the key findings is that the brightness of most existing LGS facilities could be boosted at, as
we argue, reasonable expense. Recommendations are presented to enhance the LGS return flux and to design future LGS
lasers, including those suitable for spot tracking in the mesosphere to mitigate the spot elongation problem.
A prototype magnetometer for anti-submarine warfare applications is being developed based on nonlinear magneto-optical
rotation (NMOR) in atomic vapors. NMOR is an atomic spectroscopy technique that exploits coherences among magnetic
sublevels of atoms such as cesium or rubidium to measure magnetic fields with high precision. NMOR uses stroboscopic
optical pumping via frequency or amplitude modulation of a linearly polarized laser beam to create the alignment. An
anti-relaxation coating on the walls of the atomic vapor cell can result in a long lifetime of 1 s or more for the coherence and
enables precise measurement of the precession frequency. With proper feedback, the magnetometer can self-oscillate,
resulting in accurate tracking and fast time response.
The NMOR magnetic resonance spectrum of 87Rb has been measured as a function of heading in Earth's field. Optical pumping of alignment within the F=2 hyperfine manifold generates three resonances separated by the nonlinear Zeeman
splitting. The spectra show a high degree of symmetry, consisting of a central peak and two side peaks of nearly equal
intensity. As the heading changes, the ratio of the central peak to the average of the two side peaks changes. The amplitudes
of the side peaks remain nearly equal. An analysis of the forced oscillation spectra indicates that, away from dead zones,
heading error in self-oscillating mode should be less than 1 nT. A broader background is also observed in the spectra. While
this background can be removed when fitting resonance spectra, understanding it will be important to achieving the small
heading error in self-oscillating mode that is implied by the spectral measurements.
Progress in miniaturizing the magnetometer is also reported. The new design is less than 10 cm across and includes fiber
coupling of light to and from the magnetometer head. Initial tests show that the prototype has achieved a narrow spectral
width and a strong polarization rotation signal.
KEYWORDS: Diamond, Magnetic sensors, Luminescence, Infrared detectors, Infrared radiation, Fourier transforms, Photonics, Quantum computing, Current controlled current source, Medium wave
We demonstrate magnetometry by detection of the spin state of high-density nitrogen-vacancy (NV) ensembles in diamond using optical absorption at 1042 nm. With this technique, measurement contrast and collection efficiency can approach unity, leading to an increase in magnetic sensitivity compared to the more common method of collecting red fluorescence. Working at 75 K with a sensor with effective volume 50x50x300 μm3, we project photon shot-noise limited sensitivity of 5 pT in one second of acquisition and bandwidth from DC to a few MHz. Operation in a gradiometer configuration yields a noise floor of 7 nTrms at ~110 Hz in one second of acquisition. We also present measurements of the zero-field splitting parameters as a function of temperature, a calibration which is essential for ultra-sensitive magnetometry at low frequencies.
We extend previous sodium LGS models by integrating the return flux across the mesosphere, taking into account
variable mesospheric gas density, temperature, and local sodium density. This method allows us to produce accurate
predictions of the actual return flux on the ground, relevant for determining the performance of adaptive-opticsassisted
instruments. We find that the flux distribution across the sky depends strongly on geographic location and
laser parameters. Almost independent of location, future sodium LGS will be about three times brighter at zenith
than at the observing horizon.
A self-oscillating magnetometer based on nonlinear magneto-optical rotation using amplitude-modulated pump light and unmodulated probe light (AM-NMOR) in 87Rb has been constructed and tested towards a goal of airborne detection of magnetic anomalies. In AM-NMOR, stroboscopic optical pumping via amplitude modulation of the pump beam creates alignment of the ground electronic state of the rubidium atoms. The Larmor precession causes an ac rotation of the
polarization of a separate probe beam; the polarization rotation frequency provides a measure of the magnetic field. An anti-relaxation coating on the walls of the atomic vapor cell results in a long lifetime of 56 ms for the alignment, which enables precise measurement of the precession frequency. Light is delivered to the magnetometer by polarization-maintaining optical fibers. Tests of the sensitivity include directly measuring the beat frequency between the magnetometer and a commercial instrument and measurements of Earth's field under magnetically quiet conditions, indicating a sensitivity of at least 5 pT/νHz. Rotating the sensor indicates a heading error of less than 1 nT, limited in part by residual magnetism of the sensor.
We demonstrate a magnetometric technique based on nonlinear magneto-optical rotation using amplitude modulated
light. The magnetometers can be operated in either open-loop (typical nonlinear magneto-optical rotation with
amplitude-modulated light) or closed-loop (self-oscillating) modes. The latter mode is particularly well suited for
conditions where the magnetic field is changing by large amounts over a relatively short timescale.
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