Counterpropagating Pump Beams Research Articles

Optical magnetic combination method for suppressing the Rb polarization-induced magnetic gradient in Rb-Xe NMR co-magnetometers.

The Rb polarization-induced magnetic field gradient affects the Xe nuclear spin relaxation properties and degrades the long-term stability of the NMR co-magnetometers. This paper proposes a combination suppressing scheme, which uses the second-order magnetic field gradient coils to compensate for the Rb polarization-induced magnetic gradient under counter-propagating pump beams. Based on the theoretical simulation, we find that the spatial distribution of the Rb polarization-induced magnetic gradient and the magnetic field distribution generated by gradient coils are complementary. The experimental results indicate that the compensation effect is 10% higher under the counter-propagating pump beams scheme compared with the compensation effect under the conventional single beam. Besides, due to a more uniform spatial distribution of electronic spin polarization, the Xe nuclear spin polarizability is improved and the signal-to-noise ratio (SNR) of NMR co-magnetometers could be further enhanced. The study provides an ingenious method for suppressing magnetic gradient in the optically polarized Rb-Xe ensemble, which is expected to improve the performance of the atomic spin co-magnetometers.

Theoretical calculation of saturated absorption spectroscopy of atomic ytterbium beam

Saturated absorption spectroscopy is a technique of measuring the transmission of a probe beam induced by a counter-propagating pump beam, which can achieve a nearly Doppler-free spectral resolution. In this study, the saturated absorption spectra of individual isotopes of ytterbium atoms are calculated using rate equations for various polarizations of the probe and pump beams under an assumption that the distribution of velocities for atoms within the atomic beam is a Maxwell-Boltzmann distribution. The fourth-order Runge-Kutta method is used for calculation of the rate equations, then the absorption coefficient in the vicinity of the atomic transitions is calculated, and the transmission of the probe beam is obtained by Beer-Lambert law. We compare the calculations with an experimental result from the literature and obt ain a good agreement.

Polarization enhanced tunable Doppler-free dichroic lock technique for laser frequency locking

We present a technique, called the polarization enhanced tunable Doppler-free dichroic lock (PET-DFDL) technique, for frequency stabilization of a laser system. This technique is based on the magnetically induced circular dichroism for a pair of circularly polarized probe beams overlapped by oppositely circularly polarized counterpropagating pump beams. The amplitude and slope of the signal generated in the PET-DFDL technique at cooling transition F = 2 → F ′ = 3 in 87 R b atom are nearly 4 times higher than those of signal generated from conventional Doppler-free dichroic lock (DFDL). The proposed PET-DFDL technique can also provide tunable lock-position. The frequency stability of a laser system locked with the PET-DFDL technique has been improved by factor of 2 as compared to frequency stability obtained by the DFDL technique. The dependence of PET-DFDL signal on pump beam power and magnetic field for the cooling transition of 87 R b atom is also investigated.

Improvement of spin polarization spatial uniformity in optically pumped atomic magnetometers based on counter-propagating pump beams and atomic diffusion

In an optically pumped alkali vapor cell with a high density of atoms, the attenuation of the pump light generates a spatially non-uniform distribution of the electronic spin polarization of alkali atoms, which is detrimental to biomagnetism applications of magnetometers as well as the hyperpolarization of noble gas atoms. Therefore, in this study, we propose a new scheme to generate a nearly uniform, unsaturated spin polarization region based on counter-propagating pump beams and atomic diffusion. A finite element method-based simulation is used to demonstrate the three-dimensional distribution of the spin polarization in a spherical cell. The effects of cell temperature and pump light power on the homogeneity of the spin polarization are studied. The distribution of spin polarization near the center of the cell is experimentally measured and a 1 cm uniform spin polarization region is achieved in the center of the cell. The uniformity of spin polarization in the center region of the cell increased by 50% compared with single beam pumping. The advantage of our proposed scheme is that it can generate an unsaturated uniform region of spin polarization in the center of a cell using a single species of alkali atoms.

In-situ monitoring by thermal lens microscopy of a photocatalytic reduction process of hexavalent chromium

In this work, we describe the application of a micro-spatial thermal lens spectroscopy setup (thermal lens microscope, TLM) with coaxial counter-propagating pump and probe laser beams and an integrated passive optical Fabry-Perot to quantify the Cr-VI concentration in water during a photocatalytic reaction in-situ. A series of test samples was analyzed using the 1,5 diphenil carbazide colorimetric method. A calibration curve was obtained by plotting of the TLM signal as a function of the concentration of Cr(VI) in a range between 0 and 10 μg/L (1 μg/L = 1 ppb, part per billion), with a detection limit of 53 ng/L (1 ng/L = 1 ppt, part per trillion). A solution of 10 μg/L Cr(VI) in distillated water was placed into a cell in contact with an iron-incorporated titanium dioxide film, which was previously grown onto a 1 mm thick glass microscope slide by the sol-gel dip-coating technique. The TLM signal was registered as a function of the photocatalysis time measured from the beginning of the process, radiating the film with UV-violet light. The Cr(VI) concentration was determined with the calibration curve and after the first 50 minutes a reduction of 95 % of Cr(VI) was observed, being the chemical reaction kinetic described by a potential time decreasing function.

Trace detection and photothermal spectral characterization by a tuneable thermal lens spectrometer with white-light excitation

In the thermal lens experimental set-up we replaced the commonly employed pump laser by a halogen lamp, combined with an interference filter, providing a tuneable, nearly monochromatic pump source over the range of wavelengths 430–710 nm. Counter-propagating pump and probe beams are used and a 1 mm path-length sample cell together with the interference filter makes an optical cavity, providing amplification of the thermal lens signal, which leads to enhancement of the measurement sensitivity, and enables detection of absorbances on the order of 5 × 10−6. Amplified thermal lens signal allows us to replace the typical lock-in amplifier and digital oscilloscope with a silicon photodetector, Arduino, and a personal computer, offering the possibility for a compact, robust and portable device, useful for in-field absorption measurements in low concentration or weakly absorbing species. The use of a white light source for optical pumping, an interference filter for wavelength selection and direct diagnostic of the thermal lens signal increase the versatility of the instrument and simplifies substantially the experimental setup. Determination of Fe(II) concentrations at parts per billion levels was performed by the described white-light thermal lens spectrophotometer and the absorption spectrum for 50 μgL–1 Fe(II)-1,10-phenanthroline was well reproduced with an average measurement precision of 4%. The obtained limits of detection and quantitation of Fe(II) determination at 510 nm are 3 µgL−1 and 11 µgL−1, respectively. The calibration curve was linear in the concentration range of LOQ-500 µgL−1 with reproducibility between 2% and 6%, confirming that this instrument provides good spectrometric capabilities such as high sensitivity, tuneability and good reproducibility. In addition, the versatility of the instrument was demonstrated by recording the photothermal spectrum of gold nanostructured material and determination of excitation wavelength with most efficient optical to thermal energy conversion, which differs considerably (cca 100 nm) from the absorption maximum of the investigated sample.

Tunable and polarization-controlled high-contrast bright and dark coherent resonances in potassium.

We demonstrate high-contrast electromagnetically induced absorption (EIA) bright resonances on the D1 line of K39 with characteristics comparable to those of the electromagnetically induced transparency (EIT) dark resonances observed in the same conditions. EIA is produced by the interaction of a weak probe beam with the atomic ground state driven in a degenerate coherent superposition by either a co- or counter-propagating pump beam. We have obtained an order of magnitude increase of the EIA's contrast with respect to previous similar experiments, performed with other alkalis, without compromising its linewidth. Furthermore, we show that the magneto-optic resonances can be continuously tuned from EIT to EIA by changing the relative handedness of circular polarizations of pump and probe beams, or depending on whether they co- or counter-propagate. This opens new perspectives in the use of EIA in a broad range of physical domains and in a large wealth of potential applications in optics and photonics.

High sensitivity thermal lens microscopy: Cr-VI trace detection in water

In this work, a low detection limit for hexavalent chromium in water of parts per trillions (21ng/L) was achieved using a micro-spatial thermal lens spectroscopy setup with coaxial counter-propagating pump and probe laser beams and an integrated passive optical Fabry-Perot resonator, aided with a well-established diphenyl carbazide colorimetric method. Cr-VI concentrations in the range of μg/L, i.e. well-below the toxicity thresholds in humans and animals (26 and 190mg/L respectively) and below those delimited by international regulations for drink water (~0.05–0.5mg/L), have been obtained by measurements in bottled and tap water samples. The developed thermal lens microscope is also capable to detect Cr-VI directly in potassium dichromate solutions using pump beam wavelengths within the very low optical absorption region in the visible part of the spectrum, i.e., without the use of any colorimetric method.

Two-photon-absorption line strengths for nitric oxide: Comparison of theory and sub-Doppler, laser-induced fluorescence measurements.

We discuss the results of high-resolution, sub-Doppler two-photon-absorption laser-induced fluorescence (TPALIF) spectroscopy of nitric oxide at low pressure and room temperature. The measurements were performed using the single-longitudinal mode output of a diode-laser-seeded optical parametric generator (OPG) system with a measured frequency bandwidth of 220 MHz. The measurements were performed using a counter-propagating pump beam geometry, resulting in sub-Doppler TPALIF spectra of NO for various rotational transitions in the (0,0) vibrational band of the A2Σ+ - X2Π electronic transition. The experimental results are compared with the results of a perturbative treatment of the rotational line strengths for the 20 different rotational branches of the X2Π(v″ = 0) → A2Σ+(v' = 0) two-photon absorption band. In the derivation of the expressions for the two-photon transition absorption strength, the closure relation is used for rotational states in the intermediate levels of the two-photon transition in analogy with the Placzek treatment of Raman transitions. The theoretical treatment of the effect of angular momentum coupling on the two-photon rotational line strengths features the use of irreducible spherical tensors and 3j symbols. The final results are expressed in terms of the Hund's case (a) coupling coefficients aJ and bJ for the X2Π(v″ = 0) rotational level wavefunctions, which are intermediate between Hund's case (a) and case (b). Considerable physical insight is provided by this final form of the equations for the rotational line strengths. Corrections to the two-photon absorption rotational line strength for higher order effects such as centrifugal stretching can be included in a straightforward fashion in the analysis by incorporating higher order terms in these coupling coefficients aJ and bJ, although these corrections are essentially negligible for J < 50. The theoretical calculations of relative line intensities are in good agreement both with our experiment and with published experimental results. In addition, the calculated line shapes and relative intensities for closely spaced main branch and satellite transitions are in excellent agreement with our experimental measurements.

(Invited) Photonic Chip for Laser Stabilization to a Rubidium Atomic Vapor

— We report on recent progress on stabilization of an optical laser to a photonic chip based MEMS fabricated rubidium vapor cell. The interrogation light is delivered to the cell from the edge of the photonic chip via a fiber-pigtailed SiN waveguide. The light is directed into the vapor cell from the waveguide using a diffractive grating coupler. We will discuss the stability and environmental sensitivity of a laser locked to the photonic chip device. Additionally, observation of sub-Doppler features in the atomic absorption spectrum have been observed using a counter propagating pump beam generated from a partial reflector placed above the photonic chip. The development of high precision, ultraminiature, quantum based measurement devices has the potential to transform measurement science, enabling precision metrology outside the laboratory [1]. To this end, we are developing a photonic chip based platform for precision optical spectroscopy of alkali vapors. A laser stabilized to an optical transition in an alkali vapor can provide an accurate wavelength standard. The photonic chip is based on a silicon nitride/silicon dioxide on silicon structure. Fig. 1 shows an optical image and schematic of the photonic chip. Rubidium is introduced into the evacuated MEMS cell via optical heating of an alkali metal dispenser pill located inside the MEMS cell [2]. Waveguides direct probe light from the edge of the photonic chip to the Rb vapor cell. Grating output couplers direct the probe light out of the waveguide up through the Rb vapor cell and onto a silicon photodetector placed on top of the chip. The lower right panel of Fig. 1 shows an absorption spectrum of the Rb D1 line at 795nm taken using the photonic cell. We will report on the stability of a laser locked to this transition using the photonic chip. Retro-reflection of the probe beam by a partial reflector allows for the observation of sub-Doppler features. These sub-Doppler features allow for higher stability laser stabilization. Additionally, we will compare the performance of photonic chips with grating couplers that provide different output laser beam diameters. Figure 1. Top Left: Image of photonic chip. The chip contains two wells of dimension 1mm x 1mm. The left well contains a Rb alkali metal dispenser. The right well contains the photonic structures. Upper Right: Close up of photonic waveguides and grating output couplers. Lower Left: Cross section of the photonic chip describing the planar structure. PD: photodetector. Lower Right: Rubidium D1 line absorption spectrum at 795 nm. The photonic cell is heated to a temperature of 90 °C. Acknowledgment This work is a contribution of NIST and is not subject to copyright.

Bismuth-doped fibre amplifier operating between 1600 and 1800 nm

We report the first bismuth-doped fibre amplifier operating between 1600 and , which utilises bidirectional pumping (co-propagating and counter-propagating pump beams) by laser diodes at a wavelength of . The largest gain coefficient of the amplifier is , at a wavelength of . It has a noise figure of gain bandwidth of and gain efficiency of .

Highly efficient counter-propagation-beamsnarrow-band ultraviolet frequency conversion in a quantum gas

We show that highly efficient ultraviolet frequency up conversion can be established in a single-component quantum gas in the counter-propagating weak pump beam geometry where no frequency up conversion can occur in a normal gas. We also show that all light-wave mixing and scattering processes in quantum gases originating from elementary excitations characterized by efficient collective atomic recoil motion are stimulated Raman/hyper-Raman in nature.

All-optical delay of images by backward four-wave mixing in metal-nanoparticle composites

We theoretically study a novel method for all-optical delay of images based on backward four-wave mixing in composites containing metal nanoparticles. In this approach a delayed phase conjugate probe pulse is generated by the interaction of two counter-propagating pump beams and a non-collinear shaped probe pulse in the nanocomposite.The fractional delay and the reflectivity for the phase-conjugate signal pulses are studied as function of the input pump intensity. It is shown that this scheme can be used for delayed imaging combined with the elimination of optical diffraction. The advantages of this method include minituarized design, tunable wavelength range up to the telecommunication range and wide bandwidth.

Polarization spectroscopy and magnetically-induced dichroism of the potassium D_2 lines

We study modulation-free methods for producing sub-Doppler, dispersive line shapes for laser stabilization near the potassium D2 transitions at 767 nm. Polarization spectroscopy is performed and a comparison is made between the use of a mirror or beam splitter for aligning the counter-propagating pump and probe beams. Conventional magnetically-induced dichroism is found to suffer from a small dispersion and large background offset. We therefore introduce a modified scheme, using two spatially separated pump-probe beam pairs. Finally we compare our results to methods using phase modulation and heterodyne detection.

Hamiltonian tools for the analysis of optical polarization control

The study of the polarization dynamics of two counterpropagating beams in optical fibers has recently been the subject of a growing renewed interest, from both the theoretical and experimental points of view. This system exhibits a phenomenon of polarization attraction, which can be used to achieve a complete polarization of an initially unpolarized signal beam, almost without any loss of energy. Along the same way, an arbitrary polarization state of the signal beam can be controlled and converted into any other desired state of polarization, by adjusting the polarization state of the counterpropagating pump beam. These properties have been demonstrated in various different types of optical fibers, i.e., isotropic fibers, spun fibers, and telecommunication optical fibers. This article is aimed at providing a rather complete understanding of this phenomenon of polarization attraction on the basis of new mathematical techniques recently developed for the study of Hamiltonian singularities. In particular, we show the essential role that play the peculiar topological properties of singular tori in the process of polarization attraction. We provide here a pedagogical introduction to this geometric approach of Hamiltonian singularities and give a unified description of the polarization attraction phenomenon in various types of optical fiber systems.

Light-by-Light Polarization Control of 10-Gb/s RZ and NRZ Telecommunication Signals

Controlling the state of polarization of a light beam propagating in a standard single-mode fiber by means of a lossless and instantaneous interaction is a fundamental effect of great interest for telecommunication applications and all-optical signal processing. In this paper, we experimentally demonstrate light-by-light polarization control via a nonlinear interaction occurring in single-mode optical fiber between a signal wave and a counterpropagating control pump beam. We observe a polarization attraction and stabilization of a 10-Gb/s optical telecommunication signal around 1550 nm for either return to zero or nonreturn to zero modulation format. These experimental results confirm yet another fascinating possibility to all-optical control the light properties in optical fiber.

Amplification of light in a plasma by stimulated ion acoustic waves driven by multiple crossing pump beams

Experiments demonstrate the amplification of 351 nm laser light in a hot dense plasma similar to those in inertial confinement fusion ignition experiments. A seed beam interacts with one or two counter-propagating pump beams, each with an intensity of 1.2×10(15) W/cm2 at 351 nm, crossing the seed at 24.8° at the position where the flow is Mach 1, allowing resonant stimulation of ion acoustic waves. Results show that the energy and power transferred to the seed are increased with two pumps beyond the level that occurs with a single pump, demonstrating that, under conditions similar to ignition experiments where each beam has a low gain exponent, the total scatter produced by the multiple beams can be significantly larger than that of the individual beams. It is further demonstrated that the amplification is greatly reduced when the pump polarization is orthogonal to the seed, as expected from models of stimulated scatter.

Magnetoassisted pump–probe spectroscopy of cesium atoms

We studied the spectral behavior of a probe transmission in the presence of a counterpropagating pump beam for various laser polarizations and magnetic fields with and without the assistance of an additional repopulation laser beam. These parametric studies enabled us to distinctively extricate the influence of saturation, optical pumping, and resonant light pressure. The enhanced probe absorption resulting from the pump-beam-induced Zeeman substate dressing was maximized when the change in the resonance frequency was suitably compensated by the change in the Doppler shift arising from resonant light pressure. The possibility of tuning the laser frequency locked to saturation absorption spectroscopy was investigated and implemented to a laser cooling and trapping setup.

Theory of lossless polarization attraction in telecommunication fibers

In this work, polarization attraction is meant to be the conservative nonlinear effect that transforms any arbitrary input state of polarization (SOP) of an intense optical signal beam fed to a nonlinear medium into approximately one and the same SOP at the output, provided that the medium is driven by a relatively stronger counterpropagating pump beam. Essentially, the combination of the nonlinear medium and the pump beam serves as a lossless polarizer for the signal beam. The degree of polarization of the outcoming signal beam can be close to 100% (90% in our present simulations). With an eye toward the development of such lossless polarizers for fiber optics applications, we theoretically study the polarization attraction effect in the optical fibers that are used in telecommunication links; i.e., randomly birefringent fibers. A generic model for the fiber-based lossless polarizers is derived, and a statistical scheme for the quantification of their performance is proposed.

Influence of the volume speckle on fiber specklegram sensors based on four-wave mixing in photorefractive materials

In this work, the dependence on the speckle size in the performance of a micro displacement sensor based on fiber specklegrams stored in a photorefractive BSO (Bi 12SiO 20) crystal is experimentally demonstrated. In our experimental setup, a plastic optical fiber (POF) was used to generate a subjective speckle pattern which was recorded in the crystal by using a four-wave mixing arrangement in transmission geometry. The speckle size was controlled by modifying the diameter of a pupil aperture adjacent to a lens producing the image of the speckle. The signal speckle beam was mixed into the crystal with two counter propagating pump beams to generate a fourth beam which is proportional to the conjugate of the original speckle beam. Real time fringe patterns were obtained at the output of the system by producing micro displacements of the fiber output end. Increases of the phase conjugation reflectivity and the visibility of the fringe patterns were appreciated when the speckle length was increased by decreasing the pupil aperture diameter. This behavior allowed recovering the autocorrelation functions of fringe patterns associated to micro displacements that initially led to decorrelation, and therefore, to improve the dynamic range of the metrological system. Until the best of our knowledge this is the first report about the influence of the speckle size on fiber specklegrams sensors recorded on photorefractive materials by four-wave mixing.