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We propose and investigate a method for controlling the spectrum of the vertical-cavity surface-emitting laser by simultaneous modulation of the injection current at single and doubled frequencies. We experimentally demonstrate the ability to control the power asymmetry of the first-order sidebands and to suppress the carrier by the proposed method. These possibilities are beneficial to improve frequency stability of atomic clocks based on the effect of coherent population trapping.

We have found a fine structure in the frequency dependence of modulation efficiency of a diode laser (DL) with a strong selective optical feedback. The FM response of an extended cavity diode laser (ECDL) to the microwave current modulation has two peaks whose frequencies are close to the intermode frequency of the laser longitudinal modes. The frequency splitting of the peaks depends on the power of laser radiation and on the difference between the frequency of operating ECDL mode and the frequency of the closest DL mode. We have observed similar splitting in AM noise spectra with the same type of dependencies on power and aforementioned frequency detuning. The peculiarity of the AM noise spectrum has been observed and explained previously by nonlinear interaction of fields in the active region of a DL that results in the frequency splitting of the modes. This dynamic splitting of the laser cavity modes explains the specific features of the modulation characteristics.

A new external cavity diode laser (ECDL) structure with an interference filter (IF) as a selective element is proposed. Key units of the developed ECDL, providing high mechanical stability, and hence, reproducible device characteristics, are the units of selective element rotation and radiation output from the laser cavity. Such ECDL is proposed to be used in onboard electro-optical systems.

The study of the previously proposed method for improving the parameters of the coherent population trapping resonance on the 87Rb D1 line using two counterpropagating bichromatic fields with orthogonal circular polarizations is continued. The method consists in elimination of trap states existing in the conventional σ+ measurement scheme and limiting the resonance amplitude. The resonance parameters are studied in a wide range of pump radiation intensities and in various configurations implementing the σ+-σ− scheme.

We theoretically and experimentally demonstrate that the light shift of the coherent population trapping resonance frequency depends on the buffer gases pressure. The light shift suppression becomes impossible when a certain value of the buffer gases pressure is exceeded. We estimate the minimal dimensions of an atomic cell at which the zero light shift and the lowest ground-state relaxation rate can be achieved simultaneously. A new technique of the light shift cancellation by means of a feedback utilizing the RF power modulation is proposed.

A method of dynamic continuous-wave spectroscopy of coherent population trapping (CPT) resonances using phase modulation of the jump type is developed. The time evolution of the spectroscopic signal is investigated. A method for the formation of an error signal for frequency stabilization is proposed. We show that our approach has a reduced sensitivity to the lineshape asymmetry of the CPT resonance. The experimental results are in good qualitative agreement with theoretical predictions based on a mathematical model of a three-level $\Lambda$ system in a bichromatic field. This method can be used in atomic frequency standards (including chip-scale atomic clocks)., Comment: 10 pages, 15 figures

We demonstrate the possibility of suppressing the linear response of the coherent population trapping resonance frequency to variations in the external magnetic field. The suppression can be achieved at a specific value of the clock magnetic field B due to the competition between the frequency pulling by neighboring resonances and the quadratic Zeeman shift. The effect is possible only with certain combinations of the optical field polarization and the direction of the magnetic field. An incorrect field configuration can significantly increase the magnetic-induced frequency shifts.

Magnetic resonance at the Fg = 1 \( \rightleftarrows \)Fe = 1 transition of the D1 line in 87Rb has been studied with pumping and detection by linearly polarized radiation and detection at the double frequency of the radiofrequency field. The intervals of allowed values of the static and alternating magnetic fields in which magnetic resonance has a single maximum have been found. The structure appearing beyond these intervals has been explained. It has been shown that the quadratic Zeeman shift is responsible for the three-peak structure of resonance; the radiofrequency shift results in the appearance of additional extrema in resonance, which can be used to determine the relaxation constant Γ2. The possibility of application in magnetometry has been discussed.

Motivated by recent developments in atomic frequency standards employing the effect of coherent population trapping (CPT), we propose a theoretical framework for describing the CPT resonances. Our extended model includes several realistic experimental factors not accounted for in the standard approaches, namely, the FM-based technique for generating the error signal, skewed laser spectrum, etc. We provide a simple yet non-trivial analytical solution revealing the magnitude and structure of the light shift. The performance of the model is checked against numerical simulations, the agreement is good to perfect.

Modulation characteristics of absorption at the resonance D 1 line of cesium atoms in a cell with antirelaxation coating are studied. It is experimentally shown that the minimum of the response to harmonic modulation in a certain frequency band coincides with Maxwellian velocity distribution maximum, despite the line profile distortion observed during linear frequency scanning [1]. The effect is theoretically explained.

We propose and demonstrate a simple technique for identifying the central Ramsey fringe of pulsed coherent population trapping resonance. An auxiliary optical field is applied during the free evolution time. It suppresses the nearby fringes but does not change the amplitude of central fringe practically, which marks it out clearly. The theory based on the density matrix equations for the Λ-system configuration of levels and Ramsey interrogation that takes into account the auxiliary optical field is presented and compared to the experiment with 87 Rb atoms. We also propose a technique for improving the middle- and long-term stability of compact atomic clocks.

We study the peculiarities of the absorption spectra in D1,2-lines of Cs, caused by optical pumping in cells with antirelaxation coating. In these cells the internal state of the atom, which arose under optical pumping by a monochromatic laser field, is preserved with a high probability in a collision with the wall. As a result, the optical pumping action extends to the entire volume of the cell and to all the velocities of the atoms. This leads to the speed-dependent scanning distortions of the absorption line profile. The detected features should be considered when using laserpumped quantum magnetometers with antirelaxation-coated cells.

A brief review is made of the investigations conducted at the Laboratory of Frequency Standards at the P N Lebedev Physical Institute (FIAN) Division of Quantum Radiophysics along an important line of inquiry into the development of quantum frequency standards?the elaboration of a subminiature clock based on the coherent population trapping effect in alkali metal atoms with semiconductor laser pumping (frequency stability of 10?11?10?12 for an averaging time of ~ 104 s) and compact master oscillators/optical clocks with a frequency stability of 10?14?10?15 (for an averaging time of 1 s) based on He?Ne/CH4 and fiber femtosecond lasers.

The saturated absorption resonance in an atomic gas in the field of counterpropagating light waves with an arbitrary elliptical polarization is investigated. A conclusion about the peculiarities of the resonance shape is drawn from a general analysis of the symmetry of the problem. In particular, a new effect has been found: the polarization parameters of the light waves can lead to an asymmetry and shift of the resonance. These conclusions are corroborated by an approximate analytical solution for the F g = 1 → F e = 2 transition and by numerical calculations. Apart from their fundamental importance, the results obtained can have important applications in metrology (frequency and time standards).

Short-cavity diode lasers (SCDLs) emitting at the 894-nm D1 line of caesium are developed. Low threshold currents and power consumption will make it possible to use these lasers in chip-size atomic clocks (CSACs) and magnetometers. The SCDL parameters are comparable with the parameters of surface-emitting lasers.

The resonance of the intersection of sublevels in a probe laser field resonant to the cyclic transition corresponding to the D287Rb line has been investigated in a zero magnetic field. The strong effect of an additional laser field acting on the adjacent transition has been revealed. In a cell without the buffer gas and antirelaxation coating, the amplitude of the probe-field absorption resonance can increase by more than an order of magnitude in the presence of a counterpropagating wave. The effect is observed at the laser frequency tuned to the cross resonance, when the counterpropagating waves simultaneously act on moving atoms at the cyclic and open transitions with the common lower level. The theoretical analysis of the effect of the additional field on the electromagnetically induced absorption resonance is in agreement with the experimental data.

A transmission peak has been experimentally observed in the center of the signal of double radio-optical resonance in rubidium atomic vapors at the hyperfine splitting frequency of the ground state. It has been shown that this structure appears due to the Doppler detuning of the optical field and the corresponding optical shift of the microwave field. An analytical expression has been obtained for the shape of this resonance and the effect has been qualitatively explained.

We propose and study, theoretically and experimentally, a new scheme of excitation of a coherent population trapping resonance for the D1 line of alkali atoms with nuclear spin I = 3/2 by bichromatic linearly polarized light (lin‖lin field) under the conditions of spectral resolution of the excited state. The unique properties of this scheme result in a high contrast of dark resonance for the D1 line of 87Rb.

Doppler-free absorption spectra of resonance laser fields are studied upon two-stage excitation of cesium atoms according to the schemes 6S{sub 1/2{yields}}6P{sub 3/2{yields}}6D{sub 5/2} and 6S{sub 1/2{yields}}6P{sub 3/2{yields}}8S{sub 1/2}. The obtained experimental results are in qualitative agreement with the theory. In the case of weak absorption saturation, the width of resonances is mainly determined by two-photon transitions. The efficiencies of the two variants of two-stage excitation of the 7P{sub 3/2} level are compared. The possibility of fabrication of a gas laser operating on the 455-nm 7P{sub 3/2{yields}}6S{sub 1/2} transition with the optical depopulation of the lower operating level by an additional laser is discussed. (active media)

The amplitude of Λ resonance in alkali atoms is limited by perturbing cycling transitions in the case of D2 line or by existence of additional trapping states in the case of D1 line. We propose to eliminate these extra trapping states by using two counter-propagating bichromatic fields of orthogonal circular polarizations. The experiment is in accordance with the theoretical proposal. The result refers to small-size cells and is important for applications in miniaturized atomic clocks.

A new method providing a significant increase in the amplitude and contrast of dark resonances is proposed. The method is based on the use of the σ+−σ− configuration of polarized counterpropagating waves, D1-line excitation in alkali metal atoms, and small-sized cells. Qualitative considerations of the scheme are confirmed by the results of numerical calculations. A variant of a standing wave with homogeneous circular polarization is also discussed.

A system is developed which allows one to stabilise the diode laser frequency at any point in the vicinity of the cyclic D2-line transition in Rb in the interval from +40 to -150 MHz and to switch the laser frequency within this interval for ~1 ms. A method is proposed and realised for increasing the contrast of the reference sub-Doppler resonance observed in circularly polarised fields. The ultimate contrast of the resonance is estimated. This system can be used to study the anomalous light pressure force acting on atoms in an optical molasses. A magnetooptical trap for Rb atoms is described.

A method for fabricating atomic vapor cells with physical dimensions of order 1 mm3 or below is described. Cells with integrated lenses are made from hollow-core glass fiber, and fused shut at either end using highly localized heating from a CO2 laser beam. Such cells, subsequently loaded with alkali atoms and a buffer gas, could form the basis for future generations of compact frequency references or magnetometers.

A new scheme of the external cavity of a diode laser is described. The scheme provides emission of two closely spaced (Δλ < 10-3 nm) orthogonally polarised modes, which can be tuned within the gain line, with the mode interval continuously changed.

The refractive index of coherently driven rubidium vapour is experimentally investigated in a three-level cascade con® guration using a selective reection technique. The maximum measured resonant change in the refractive index is ¢n ' 0:1. The selective reection is accompanied by a four-wave-mixing process that can reach π90% eA ciency.

We demonstrate a scanning fiber-optic probe for magnetic-field imaging where nitrogen-vacancy (NV) centers are coupled to an optical fiber integrated with a two-wire microwave transmission line. The electron spin of NV centers in a diamond microcrystal attached to the tip of the fiber probe is manipulated by a frequency-modulated microwave field and is initialized by laser radiation transmitted through the optical tract of the fiber probe. The two-dimensional profile of the magnetic field is imaged with a high speed and high sensitivity using the photoluminescence spin-readout return from NV centers, captured and delivered by the same optical fiber.

We demonstrate fiber-optic magnetometry using a random ensemble of nitrogen-vacancy (NV) centers in nanodiamond coupled to a tapered optical fiber, which provides a waveguide delivery of optical fields for the initialization, polarization, and readout of the electron spin in NV centers.

The electron spin of nitrogen--vacancy (NV) centers in diamond offers a solid-state quantum bit and enables high-precision magnetic-field sensing on the nanoscale. Implementation of these approaches in a fiber format would offer unique opportunities for a broad range of technologies ranging from quantum information to neuroscience and bioimaging. Here, we demonstrate an ultracompact fiber-optic probe where a diamond microcrystal with a well-defined orientation of spin quantization NV axes is attached to the fiber tip, allowing the electron spins of NV centers to be manipulated, polarized, and read out through a fiber-optic waveguide integrated with a two-wire microwave transmission line. The microwave field transmitted through this line is used to manipulate the orientation of electron spins in NV centers through the electron-spin resonance tuned by an external magnetic field. The electron spin is then optically initialized and read out, with the initializing laser radiation and the photoluminescence spin-readout return from NV centers delivered by the same optical fiber.

A model for stabilisation of a laser frequency is considered which takes into account the spectral splitting of the mode and the dependence of its Q factor on coupling with the laser. It is shown that the stabilisation regime is optimal when the coupling coefficient with a whispering-gallery mode is greater than the critical coefficient at which the mode is split. The presence of the amplitude — phase correlation in the active region of the laser determines the maximum value of the stabilisation parameter caused by the dynamic restriction of the feedback level. The single-mode lasing disappears in the case of a strong feedback. It is shown that the appropriate choice of the feedback-phase level suppresses the dynamic instability, which provides in principle a strong feedback with a high-Q external resonator.

A new modification of external optical feedback OFB was used to narrow the line of a diode laser DL . A 'whispering . gallery' WG mode of a high- Q microsphere was excited by means of frustrated total internal reflection while the feedback for optical locking of the laser was provided by the intracavity Rayleigh backscattering. The ; 600 MHz beatnote of the two laser diodes optically locked to a pair of orthogonally polarized modes of the same microresonator had the indicated spectral width of 20 kHz, and the stability of 2 = 10 y6 over averaging time of 10 s. The feasibility of miniature sub-kHz-line width

An analysis is made of the self-excitation condition for a laser in which the active medium is a 'quantum dot' and a whispering-gallery mode of a dielectric microsphere acts as the cavity. It is pointed out that the interaction of a quantum dot with an ensemble of degenerate or near-degenerate modes increases the laser excitation coefficient. A calculation is made of the dependence of the effective volume of a whispering-gallery mode on its index. It is shown that the field maximum of an E-type whispering-gallery mode lies on the surface of a sphere, whereas for an H-type mode this maximum is shifted along the radius into the sphere. Calculations suggest that it should be possible to construct a microlaser even on the basis of a single quantum dot.

We propose a non-standard spectroscopic technique that uses a feedback control of the input probe field parameters to significantly increase the contrast and quality factor of the atomic resonances. In particular, to apply this technique for the dark resonances we sustain the fluorescence intensity at a fixed constant level while taking the spectra process. Our method, unlike the conventional spectroscopy, does not require an optically dense medium. Theoretical analysis has been experimentally confirmed in spectroscopy of atomic rubidium vapor in which a considerable increase (one-two order) of the resonance amplitude and a 3-fold decrease of the width have been observed in optically thin medium. As a result, the quality factor of the dark resonance is increased by two orders of magnitude and its contrast reaches a record level of 260%. Different schemes, including magneto-optical Hanle spectroscopy and Doppler-free spectroscopy have also showed a performance enhancement by using the proposed technique., 8 pages, 9 figures, 1 table

An experimental investigation was made of the influence of the length of an external cavity on the threshold current and on the normal-mode spectrum of gain-guiding diode lasers. The wavefront curvature was taken into account. An analogy between a laser with an external resonator and one with a curved facet was confirmed. This was used to propose a method for simulating the influence of the wavefront curvature on the degree of feedback. A numerical calculation was used to show why the use of an external cavity in which the wavefront is 'conjugated' leads to the greatest reduction in the threshold current in such lasers.

We develop a generalized principle of electromagnetically induced transparency (EIT) vector magnetometry based on high-contrast EIT resonances and the symmetry of atom-light interaction in the linearly polarized bichromatic fields. Operation of such vector magnetometer on the ${D}_{1}$ line of $^{87}\mathrm{Rb}$ has been demonstrated. The proposed compass-magnetometer has an increased immunity to shifts produced by quadratic Zeeman and ac-Stark effects, as well as by atom-buffer gas and atom-atom collisions. In our proof-of-principle experiment the detected angular sensitivity to magnetic field orientation is ${10}^{\ensuremath{-}3}$ deg/Hz${}^{1/2}$, which is limited by laser intensity fluctuations, light polarization quality, and magnitude of the magnetic field.

We present a joint theoretical and experimental characterization of the coherent population trapping (CPT) resonance excited on the D1 line of 87Rb atoms by bichromatic linearly polarized laser light. We observe high-contrast transmission resonances (up to 25%), which makes this excitation scheme promising for miniature all-optical atomic clock applications. We also demonstrate cancellation of the first-order light shift by proper choice of the frequencies and relative intensities of the two laser field components. Our theoretical predictions are in good agreement with the experimental results., 8 pages, 7 figures

The resonance of saturated absorption in counterpropagating light fields is experimentally and theoretically studied. We focus on two cases: parallel and linearly polarized waves, driving an open dipole transition and the general case of elliptically polarized waves, driving closed dipole transition. The former reveals a new Doppler-free resonance as a peak within the saturated-absorption dip. The latter case reveals a new polarization effect, causing a shift and asymmetry of the saturated-absorption resonance. The results obtained can be found useful in metrology.

The resonance of saturated absorption in counterpropagating light fields is experimentally and theoretically studied. We focus on two cases: parallel and linearly polarized waves, driving an open dipole transition and the general case of elliptically polarized waves, driving closed dipole transition. The former reveals a new Doppler-free resonance as a peak within the saturated-absorption dip. The latter case reveals a new polarization effect, causing a shift and asymmetry of the saturated-absorption resonance. The results obtained can be found useful in metrology.

Electromagnetically-induced absorption resonances in Hanle-configuration are experimentally and theoretically studied in the case of counterpropagating waves. The presence of a backward wave provides a significant increase of the amplitude. The results may be useful for magnetometry.

Electromagnetically induced absorption (EIA) in Hanle configuration versus ellipticity of an optical running wave is investigated. All theoretical results are confirmed experimentally quite well. It is shown that magneto-optical resonance parameters (amplitude, width and amplitude-to-width ratio) depend on the wave polarization substantially. For example, the resonance amplitude in the case of elliptic polarization of the light wave can be the order more than for linear polarization.

Electromagnetically induced absorption (EIA) in Hanle configuration versus ellipticity of an optical running wave is investigated. All theoretical results are confirmed experimentally quite well. It is shown that magneto-optical resonance parameters (amplitude, width and amplitude-to-width ratio) depend on the wave polarization substantially. For example, the rsonance amplitude in the case of elliptic polarization of th elight wave can be the order more than for linear polarization.

We propose a simple and effective way of creating pure dark superposition states. The generation of pure states is carried out by using bichromatic radiation with controllable polarization ellipticity. We derived analytic formulas for polarization elipticity to obtain pure dark states of different Zeeman sublevels of alkali atoms. Experimentally we accumulated ~60% of the atoms in the 0-0 dark state of the D1 line of Rb87, Comment: 9 pages, 3 figures

We report on the fabrication of millimeter-sized vapor cells and their performance in atomic clocks based on coherent population trapping (CPT). We discuss two fabrication techniques. The first one is based on hollow-core pyrex fibers, fused with a CO/sub 2/ laser or micro-torch, and the second one involves anodic bonding of micro-machined silicon wafers to pyrex. Key aspects of the discussion are the performance of the cell in frequency references, the potential for further miniaturization of the cells and the ability to manufacture them on a large scale with reproducible performance.

Fiber-optic probes coupled with nitrogen-vacancy (NV) centers in diamond and integrated with a microwave transmission line are shown to enable fiber-format optical thermometry. Temperature measurements with an accuracy of 0.02 K are performed by combining this NV-diamond fiber thermometer with a properly optimized differential lock-in detection technique.

We describe several novel experimental effects resulting from optical coherences in multilevel atoms driven with coherent laser fields. Four configurations were explored using Rb vapor cells: lambda dark-line resonances, single beam double-lambda oscillations, and cascade two-photon transitions in co- and counter-propagating geometries. Experiments were performed with high spectral-resolution using low-power diode lasers sources and optically thick cells. Two of the most striking effects we observed are: (1) a self-oscillation that occurs at 3.0 GHz on the 85Rb hyperfine frequency when a single laser field pumps the atoms, and (2) a coherent blue-beam emission that we observe in the co-propagating cascade two-photon case. These phenomena lead to complex lineshapes, and the effects sometimes dominate the well-known 3-level coherence effects. Additional higher-order mixing or cascaded interferences are apparent in all four experimental configurations. Many of these can be viewed as multi-wave mixing enhance...

The effect of intracavity Electromagnetically Induced Transparency on the properties of optical resonators and active laser devices is discussed theoretically. A pronounced frequency pulling and cavity linewidth narrowing are predicted. The effect can be used to substantially reduce classical and quantum phase noise of the beat-note of optical oscillators. Fundamental limits of this stabilization mechanism are discussed as well as its potential application to high-resolution spectroscopy.

We present a proof-of-principle experiment demonstrating a resonant enhancement of the index of refraction accompanied by vanishing absorption in a cell containing a coherently prepared Rb vapor. The results are in good agreement with detailed theoretical predictions.

Laser oscillation without population inversion is demonstrated experimentally in a V-type atomic configuration within the ${D}_{1}\mathrm{and}{D}_{2}$ lines of Rb vapor. It is shown that the effect is due to the atomic interference. The experimental results, as first predicted by careful theoretical analysis, are in a good agreement with detailed calculations.

A Joint session of the P N Lebedev Physical Institute Research Council of the Academy of Sciences and the Scientific session of the Physical Sciences Division of the Russian Academy of Sciences and the United Physical Society of the Russian Federation was convened on 6 April 2009 to mark the 75th anniversary of the P N Lebedev Physical Institute of the Academy of Sciences (FIAN). The following papers were read to the session: (1) Mesyats G A (P N Lebedev Physical Institute RAS) "75 years of the P N Lebedev Physical Institute RAS which traces back its origin to the Physics Room of Peter the Great's Kunstkamera created 225 years ago"; (2) Bolotovskii B M (Division of Theoretical Physics, P N Lebedev Physical Institute RAS) "Vavilov–Cherenkov radiation"; (3) Kopaev Yu V (Division of Solid State Physics, P N Lebedev Physical Institute RAS) "Toroidal states in crystals"; (4) Kardashev N S (Astro-Cosmic Center of the P N Lebedev Physical Institute RAS) "Science program of the Radioastron mission"; (5) Zatsepin G T (Nuclear Research Institute RAS), Roganova T M (M V Lomonosov Moscow State University, D V Skobeltsyn Nuclear Physics Research Institute) "Cosmic ray studies"; (6) Masalov A V (Optics Division, P N Lebedev Physical Institute RAS) "Spectroscopy: from atoms to cosmic objects"; (7) Gubin M A, Velichansky V L (Division of Quantum Radiophysics, P N Lebedev Physical Institute RAS) "Small-size optical frequency standards"; (8) Dagkesamanskii R D (Radioastronomical Observatory in Pushchino of the Astro-Cosmic Center of the P N Lebedev Physical Institute RAS) "Prospects for radioastronomical research in Pushchino". Articles written on the basis of these talks are printed in this special issue of Uspekhi Fizicheskikh Nauk (Usp. Fiz. Nauk 179 (11) 1145 – 1256 (2009) [English version: Phys. Usp. 52 (11) (2009] journal. • P N Lebedev Physical Institute RAS: past, present, and future, G A Mesyats Physics-Uspekhi, 2009, Volume 52, Number 11, Pages 1084–1097 • Vavilov–Cherenkov radiation: its discovery and application, B M Bolotovskii Physics-Uspekhi, 2009, Volume 52, Number 11, Pages 1099–1110 • Toroidal ordering in crystals, Yu V Kopaev Physics-Uspekhi, 2009, Volume 52, Number 11, Pages 1111–1125 • Radioastron: a radio telescope many times the size of Earth. Research program, N S Kardashev Physics-Uspekhi, 2009, Volume 52, Number 11, Pages 1127–1137 • Cosmic ray investigations, G T Zatsepin, T M Roganova Physics-Uspekhi, 2009, Volume 52, Number 11, Pages 1139–1146 • Spectroscopy: from atoms to cosmic objects, A V Masalov Physics-Uspekhi, 2009, Volume 52, Number 11, Pages 1147–1152 • Laser frequency standards at the P N Lebedev Physical Institute, V L Velichansky, M A Gubin Physics-Uspekhi, 2009, Volume 52, Number 11, Pages 1153–1158 • The Pushchino Radio Astronomy Observatory of the P N Lebedev Physical Institute Astro Space Center: yesterday, today, and tomorrow, R D Dagkesamanskii Physics-Uspekhi, 2009, Volume 52, Number 11, Pages 1159–1167