Doppler-free Resonances Research Articles

Frequency-stable and ultranarrow-linewidth semiconductor laser locked directly to an atomic-cesium transition

We describe a new method for semiconductor laser FM noise reduction. A Doppler-free Faraday resonance in Cs vapor provided optical feedback, and FM sideband saturation spectroscopy in a second Cs cell provided electronic feedback. The combined optical and electronic feedback allowed us to reduce the low-frequency FM noise power by more than 6 orders of magnitude, which resulted in a sub-100-Hz-linewidth semiconductor laser locked directly to an atomic transition frequency.

ドップラーフリー光・光二重共鳴分光法

ドップラーフリー光・光二重共鳴分光法

Optical pumping effects in high resolution laser spectroscopy

A theoretical analysis of the relative intensities of the Doppler-free signals in saturation spectroscopy, velocity selective optical pumping (VSOP) spectroscopy, polarization spectroscopy and a mixing of saturation and polarization spectroscopies is presented using the approximation of a single cycle of optical pumping in a four-level system. An optical pumping process in the Zeeman sublevels is taken into account owing to the important role of spontaneous emission even if the sublevels are degenerate.Arbitrary polarization combinations are considered for pump and probe beams. The similarities and differences between three kinds of spectroscopy are made clear. Both circular and linear optical anisotropies induced by the pump beam are considered in this theoretical calculation. The magnitude and sign of Doppler-free resonances in each spectroscopy are quite different owing to the polarizations of the pump and probe beams. The four-level system is important as a three-level approximation. The results obtained from the four-level optical pumping theory are in good agreement with experimental reports.

Unmodulated external-cavity diode laser stabilised on caesium D2 line

Frequencies of two external-cavity diode lasers were locked to Doppler-free resonances on the caesium D/sub 2/ line using the Zeeman modulation technique. In this technique the laser frequency is unmodulated. The Allan variance of beat note between two identical lasers was measured. Frequency stabilities of better than 10/sup -12/ were observed for averaging times from 10 to 300 s.

Measurement of fast frequency fluctuations: Allan variance of a grating-stabilized diode laser

Fast frequency fluctuations of a 780 nm grating-stabilized diode laser are directly observed by the measurement of the Allan variance. Time intervals as short as 10 ns and up to 1 s are realized by means of two independent time interval counters. Frequency fluctuations with the laser locked to a Doppler-free resonance transition of atomic rubidium are examined in detail.

A modified commercial Ti:sapphire laser with 4 kHz rms linewidth

We have modified a commercial Ti:sapphire laser to allow optical phase stabilization to an extremely stable semiconductor laser, which in turn is locked to a Doppler-free resonance in a cesium vapor cell. For time scales from 10 μs up to several hours the combined system has a rms linewidth of 4 kHz with respect to the cesium resonance. The system allows the resolution of extremely narrow resonances in a cloud of trapped atoms.

Quasienergy spectrum of a two-level atom in the field of high-intensity modulated radiation

An asymptotic method is proposed for the determination of quasienergy levels of an atom in the field of modulated optical radiation. A comparison of the asymptotic and exact solutions is used to show that the range of validity of the new method is considerably wider than those of the known asymptotic methods. An analysis is made of the shifts of the quasienergy levels of an atom under the influence of bichromatic radiation and of a periodic pulse sequence. A modulation method is proposed for the attainment of Doppler-free resonances in gases.

Line shape of resonances recorded in selective reflection: influence of an antireflection coating

The theory of selective reflection from the boundary of rarefied atomic vapor and a transparent dielectric medium covered by an antireflection coating is developed. The line shape of the selective reflection signal is shown to depend strongly on the refractive indices of the materials of the antireflection coating as well as on its thickness. Conditions are derived under which the line shape depends only on the imaginary part of the admittance of the vapor at the boundary. As an application, Doppler-free resonances, recorded in selective reflection are discussed.

Pump-probe spectroscopy in potassium using an AlGaAs laser and the second-harmonic generation of an InGaAsP laser for frequency stabilization and linking

Pump-probe spectroscopy in /sup 41/K by using a grating-feedback 0.77-/spl mu/m AlGaAs laser and the second-harmonic wave of a 1.54-/spl mu/m InGaAsP laser for providing a frequency reference in the 1.5 /spl mu/m region was performed. The pump laser frequency was locked to the Doppler-free resonance with frequency fluctuations less than 50 kHz estimated from the error signal. The Doppler-free absorption spectra in both co- and counter-propagating schemes were obtained, and the feasibility of simultaneous frequency stabilization and linking for two diode lasers with a frequency difference as large as 200 THz was demonstrated. >

Linewidth reduction and frequency stabilization of a semiconductor laser with a combination of FM sideband locking and optical feedback

We describe a novel method for semiconductor laser noise reduction that uses a combination of optical and electronic feedback. A Doppler-free Faraday resonance in Cs vapor provided both optical feedback and discrimination for an electronic feedback scheme incorporating FM sideband spectroscopy. The introduction of electronic feedback further reduced the low-frequency fluctuation noise power by more than 2 orders of magnitude, resulting in a linewidth of 1.4 kHz.

High-resolution spectroscopy and frequency measurement of the midinfrared 5d/sup 2/D/sub 3/2/-5d/sup 2/D/sub 5/2/ transition of a single laser-cooled barium ion

The narrow 12.5- mu m, 5d/sup 2/D/sub 3/2/-5d/sup 2/D/sub 5/2/ fine-structure transition in a laser-cooled, single Ba/sup +/ atom was probed with a tunable sideband from a frequency-stable /sup 15/NH/sub 3/ laser operating on the sP(8, 6) transition. Single-ion Doppler-free resonances with negligible time dilation shift are observed without any detected motional sidebands for each Zeeman component. Linewidths of 11 kHz are observed for the components, which are currently limited by the probe laser frequency excursions. A measurement of the D-D transition center frequency relative to the sP(8, 6) saturated absorption is performed. A preliminary measurement of the sP(8, 6) line center frequency gives the line center for the /sup 138/Ba/sup +/ isotope transition as 24012.048.5+or-0.2 MHz. >

Nonlinear Doppler-free spectroscopy of the 6 1S 0-6 3P 1 intercombination transition in barium

Doppler-free saturated absorption resonances on an intercombination transition of Ba (λ=791 nm) have been observed with a highly coherent laser diode. A width of 820±50 kHz and a contrast of 12% are obtained. The isotopic shift for 138Ba and 136Ba has been found to be 109±2 MHz. The properties of the intercombination lines in Ba, Ca and resonant transitions of alkali metals as a reference for the frequency stabilization of the laser diode have been compared.

Atomic spectroscopy with squeezed light for sensitivity beyond the vacuum-state limit

A frequency tunable source of squeezed light has been developed which is suitable for a variety of spectroscopic applications. In initial experiments continuous tunability over a range of 2 GHz has been achieved with a directly observed nonclassical noise reduction of 6 dB relative to the vacuum-state limit in a balanced homodyne detector. A process of light-induced absorption in the nonlinear crystal has been identified as the principal loss mechanism which prevents the observation of yet larger degrees of squeezing. Although our source is potentially broadly tunable over the range of wavelengths from 840 to 970 nm, the current research centers on the performance at 852 nm for spectroscopy of the D 2 line of atomic cesium. For frequency-modulated (FM) saturation spectroscopy in a vapor cell, an improvement of 3.1 dB in sensitivity relative to the usual quantum limit is demonstrated for the detection of Doppler-free resonances. When corrected for the thermal noise of the detector, the enhancement in signal-to-noise ratio brought by the squeezed field is 3.8 dB relative to the shot-noise limit set by the vacuum fluctuations of the probe field.

Spectroscopy with squeezed light.

A frequency-tunable source of squeezed light is employed for spectroscopic measurements of atomic cesium. Relative to the usual quantum limit associated with vacuum-state fluctuations, enhanced sensitivity is demonstrated for the detection of Doppler-free resonances in saturation spectroscopy, with an improvement of 3.1 dB directly observed and 3.8 dB inferred after correcting for detector thermal noise.

Self-quenching of the semiconductor laser linewidth below the Schawlow–Townes limit by using optical feedback

We demonstrate theoretically and experimentally self-quenching of the fundamental semiconductor laser frequency fluctuations to a level that is orders of magnitude below the Schawlow-Townes limit for a solitary laser. It is shown that the main operative mechanism is the combined action of a frequency-dependent internal loss and amplitude-to-phase coupling. The internal frequency-dependent loss is introduced by means of spectrally narrow external optical feedback, which provides a strong frequency-dependent dispersion. Linewidth reduction by a factor of 2 x 10(3) is demonstrated by using a narrow Doppler-free Faraday resonance in Cs vapor.

Intensity and concentration dependence of Doppler-free resonance in selective reflection

Influence of laser radiation intensity on the amplitude and width of the Doppler-free resonance of selective reflection in the vicinity of the 87Rb D 1 line has been investigated. The shift of the selective reflection resonance with respect to the frequency of unperturbed atomic transition is found to be red and have the value of δ = (2.6±0.7) MHz. It does not vary within the accuracy limit with concentration in the range of 2 × 10 13-3 × 10 14 cm -3 (which corresponds to the selfbroadening variation from 2 to 30 MHz) and with the 2–120 mW/cm 2 intensity change. An explanation of the absence of the concentration depended shift and the dependence of reflection resonance amplitude on intensity is proposed. It is based on the consideration of the optical pumping in the hyperfine levels of the ground state.

Second-order doppler-free spectroscopy

The observation of second-order Doppler-free optical resonances with a width of 50 Hz are reported for the first time. It was achieved due to the use of optical selection of cold particles from an absorbing gas. The experiments have been carried out by using a new laser spectrometer, supposed to obtain the saturated absorption resonances with a relative width 10−13–10−14. The results of experimental and theoretical studies of second-order Doppler-free effect influence on the shape of nonlinear optical resonances in transit-time conditions are considered.

Influence of a magnetic field on Doppler-free resonance absorption due to optical alignment of the ground atomic state

Influence of a magnetic field on Doppler-free resonance absorption due to optical alignment of the ground atomic state

Power broadening of saturation absorption resonance on the D 2 line of rubidium

Doppler-free resonances in saturated absorption on the D 2 line 85Rb are investigated by using a tunable external-cavity-diode laser. The linewidths of the most prominent crossover resonances are investigated as a function of the saturating beam intensity. In spite of the fact that many levels and different mechanisms of optical pumping are involved in the saturation, the dependences are found to fit well the simple formula, obtained for a two-level system with a phenomenologically defined saturation parameter.

Doppler-free resonances of the second-order Raman scattering

An analysis is made of the possibility of observation of Raman scattering resonances free of the influence of the Doppler effect. Such Doppler-free resonances result from the excitation of a standing wave of Raman vibrations in a gas by two standing optical waves differing in frequency by half the Raman frequency. Complete compensation of the Doppler shifts occurs as a result of simultaneous interaction of atomic particles with two pairs of counterpropagating waves. Scattering of traveling pump waves by a standing wave of Raman vibrations in the fifth order with respect to the field gives rise to resonances in the radiation at the Stokes and anti-Stokes frequencies. Coherent scattering is observed in the presence of dispersion. A resonance in the number of particles excited to the upper level of a Raman transition appears in the eighth order in the field. The expressions obtained are applied to the specific example of neon.