Doppler-free spectroscopy of mercury at 253.7 nm using a high-power, frequency-quadrupled, optically pumped external-cavity semiconductor laser

We have developed a stable, high-power, single-frequency optically pumped external-cavity semiconductor laser system and generate up to 125 mW of power at 253.7 nm using successive frequency doubling stages. We demonstrate precision scanning and control of the laser frequency in the UV to be used for cooling and trapping of mercury atoms. With active frequency stabilization, a linewidth of <60 kHz is measured in the IR. Doppler-free spectroscopy and stabilization to the 6(1)S(0)-6(3)P(1) mercury transition at 253.7 nm is demonstrated. To our knowledge, this is the first demonstration of Doppler-free spectroscopy in the deep UV based on a frequency-quadrupled, high-power (>1 W) optically pumped semiconductor laser system. The results demonstrate the utility of these devices for precision spectroscopy at deep-UV wavelengths.     

Simple laser frequency locking based on Doppler-free magnetically induced dichroism

We present a simple and reliable optical setup for active stabilization of a diode laser. The method combines Doppler-free spectroscopy and magnetically induced dichroism in atomic vapor. The long- and short-term stability is achieved using a simplified optical and laser setup. We apply the setup to stabilize an external-cavity diode laser to the D2 atomic resonance in potassium vapor. With the laser locked we attain long-term stabilization to the atomic line with the reduction of laser frequency jitter down to 0.5 MHz on a 50 ms timescale.     

Doppler-free frequency-modulation spectroscopy of atomic erbium in a hollow-cathode discharge cell

The erbium atomic system is a promising candidate for an atomic Bose–Einstein condensate of atoms with a non-vanishing orbital angular momentum (L≠0) of the electronic ground state. In this paper we report on the frequency stabilization of a blue external cavity diode laser system on the 400.91 nm laser cooling transition of atomic erbium. Doppler-free saturation spectroscopy is applied within a hollow-cathode discharge tube to the corresponding electronic transition of several of the erbium isotopes. Using the technique of frequency-modulation spectroscopy, a zero-crossing error signal is produced to lock the diode laser frequency on the atomic erbium resonance. The latter is taken as a reference laser to which a second main laser system, used for laser cooling of atomic erbium, is frequency stabilized.     

A laser system for the spectroscopy of highly charged bismuth ions

We present and characterize a laser system for the spectroscopy on highly charged ²⁰⁹Bi⁸²⁺ ions at a wavelength of 243.87?nm. For absolute frequency stabilization, the laser system is locked to a near-infra-red laser stabilized to a rubidium transition line using a transfer cavity based locking scheme. Tuning of the output frequency with high precision is achieved via a tunable rf offset lock. A?sample-and-hold technique gives an extended tuning range of several THz in the UV. This scheme is universally applicable to the stabilization of laser systems at wavelengths not directly accessible to atomic or molecular resonances. We determine the frequency accuracy of the laser system using Doppler-free absorption spectroscopy of Te₂ vapor at 488?nm. Scaled to the target wavelength of 244 nm, we achieve a frequency uncertainty of σ _{244 nm}=6.14?MHz (one standard deviation) over six days of operation.     

Buffer-gas-assisted polarization spectroscopy of 6Li

We report on the demonstration of Doppler-free polarization spectroscopy of the D2 line of (6)Li atoms. Counterintuitively, the presence of an Ar buffer gas, in a certain pressure range, causes a drastic enhancement of the polarization rotation signal. The observed dependence of the signal amplitude on the Ar buffer pressure and the pump laser power is reproduced by calculations based on simple rate equations. We performed stable laser frequency locking using a dispersion signal obtained by polarization spectroscopy for laser cooling of (6)Li atoms.     

Laser frequency stabilization for atom cooling and magnetic-field compression of the trap

We report a frequency stabilization technique of a diode laser in a Doppler-free atomic transition used for obtaining the magneto-optical trapping of Rb atoms. This technique, based on side locking to an atomic transition using a servo controller, is very simple and can be implemented straightforwardly to lock the laser at the red detuned frequency position required for laser cooling experiments. The number of trapped atoms and the temperature of the cold cloud have been determined. The effect of trapping the magnetic field on the cloud radius has also been analyzed.     

Optimization of frequency modulation transfer spectroscopy on the calcium 41S0 to 41P1 transition

We present experimental results of frequency modulation transfer spectroscopy in a vapor of neutral atomic calcium. The observed line shapes agree well with the theoretical model. We use numerical calculations in order to improve the signal shape such that its magnitude and its slope at the zero-crossing is maximized. When optimized this way, the frequency modulation transfer signal can be used for the sensitive optical detection of rare species or isotopes, Doppler-free frequency measurements or as a sensitive error signal for laser frequency stabilization. PACS 42.62.Fi; 32.70.Jz; 39.30.+w     

Magnetic field modulation spectroscopy of rubidium atoms

The magnetically modulated saturation absorption profile is studied for a wide range of external DC magnetic field. The salient features of Doppler-free signal generated by laser frequency modulation and atomic energy level modulation are compared. The DC offset of the signal profile is found to be unstable as the external DC magnetic field is changed. The technical difficulty of tuning laser frequency under locked condition over a large frequency span is discussed along with possible solutions.     

Laser frequency stabilization and large detuning by Doppler-free dichroic lock technique: Application to atom cooling

We present results of a study of frequency stabilization of a diode laser (λ = 780 nm) using the Doppler-free dichroic lock (DFDL) technique and its use for laser cooling of atoms. Quantitative measurements of frequency stability were performed and the Allan variance was found to be 6.9 × 10¹¹ for an averaging time of 10 s. The frequency-stabilized diode laser was used to obtain the trapping beams for a magneto-optic trap (MOT) for Rb atoms. Using the DFDL technique, the laser frequency could be locked over a wide range and this enabled measurement of detuning dependence of the number and temperature of cold atoms using a relatively simple experimental set-up.     

A Narrow Line Width Tunable Diode Laser System

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Laser frequency stabilization using a balanced bi-polarimeter

We report on a Doppler-free polarization spectroscopy based technique of laser frequency stabilization using a balanced bi-polarimeter set-up. Two linearly polarized weak laser beams are used to probe birefringence induced by two oppositely circularly polarized strong pump beams in a vapour cell. Subtraction of balanced polarimeter signals obtained from the two probe beams results in a background-free dispersion-like reference signal without frequency modulation. The dispersion-like signal corresponding to the closed transition 5 ² S _1/2 (F=2) →5 ² P _3/2 (F^′=3) of ⁸⁷Rb was used for frequency locking of a diode laser. The frequency fluctuations and the drift were measured to be less than 0.25 MHz and 0.02 MHz, respectively, for an observation period of more than 10 hours. PACS 42.62.Fi; 42.55.Px; 32.30.-r     

Two-photon spectroscopy in a fast atomic beam

We suggest the use of a fast atomic beam to obtain resonance enhancement in Doppler-free two-photon spectroscopy. The method requires only one laser frequency thus avoiding the complications due to a residual Doppler shift.     

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.     

碘吸收激光稳频中调制信号的理论计算

激光频率标准是将激光频率锁定于原子或分子的超精细能级间的跃迁频率,从而获得高精密度的激光频率输出。在激光稳频研究工作中,通常将碘分子的吸收谱线采用导数谱、FM光谱和调制转移光谱MTS(ModulationTransferSpectroscopy)进行频率的精密控制。为更好地消除吸收谱线的本底噪声,目前最好的稳频方案是采用调制转移光谱法。它具有高灵敏度、高分辨率和无多普勒背景等特点。从物理学的原理出发,分析了激光对介质的极化机理,导出介质对激光的吸收和色散作用的数学模型,用信号处理的技术从理论上推导了MTS谱线线型,分析了光谱线型的特性,讨论了提高激光频率稳定度的各种情况。     

Ne absorption tube in an alternating magnetic field inside a laser cavity as a frequency standard

A neon absorption tube placed in an alternating magnetic field is used as a frequency standard for the frequency stabilization of a He-Ne laser. This absorption tube, which is situated within the laser cavity, permits a very precise frequency adjustment (<10^-12) due to the formation of the inverse Lamb dip. An excellent beam coherence is also obtained because this method of stabilization does not superimpose any frequency modulation on the oscillation.     

Rb saturation spectroscopy with an optically narrowed FM diode laser

A fast-frequency modulated (FM) diode laser has been optically narrowed using the technique of resonant optical feedback, to provide linewidths in each FM mode of ≈ 200 kHz peak-to-peak. With a drive frequency of 50 MHz and modulation index of 0.2, the FM laser has been used for the first time to obtain Doppler-free FM spectra of the Rb D₁ line at 795 nm. The potential use of this system for laser frequency stabilisation is discussed.     

Doppler-free optoacoustic spectroscopy

The first observation of Doppler-free optoacoustic spectroscopy is reported. As a first example the P (193) line of the 11-0 band of the B←X transition of ¹²⁷I₂ is used. The output of cw single mode dye laser is split into two equal intensity beams chopped at frequencies ω₁ and ω₂. The nonlinear compoment of the optoacoustic signal at the frequency (ω₁ + ω₂) is detected and Doppler-free resolution is obtained. Comparing the Doppler-free optoacoustic and fluorescence spectra of iodine measured under similar conditions, good agreement is found. Since optoacoustic and fluorescence methods complement each other, this opens up new possibilities for weakly or nonfluorescing molecules.     

Combination of doppler-free polarization spectroscopy and magnetic rotation for the example of IBr

We describe a spectroscopic method which combines for the first time Doppler-free laser polarization spectroscopy with the magnetic rotation technique. This is achieved by the application of a modulating weak magnetic field to the overlap region of modulated pump and probe beam. By the double modulation, molecular levels with effectiveg-factors as low as 0.05 Gm_B can easily be detected and distinguished from diamagnetic levels. This is demonstrated for the molecule IBr where theB'0⁺ state is perturbed by a repulsive = 1 state which leads to increasingg-factors with increasing vibrational levels. With the normal polarization spectroscopy, several of these levels are not detectable within the manifold of overlapping lines from I₂ and Br₂ which are always present in this chemical system. The new method is well suited for characterizing perturbed molecular levels.Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday     

Flexible control of semiconductor laser with frequency tunable modulation transfer spectroscopy

We introduce a new method of simultaneously implementing frequency stabilization and frequency shift for semiconductor lasers. We name this method the frequency tunable modulation transfer spectroscopy(FTMTS). To realize a stable output of 780 nm semiconductor laser, an FTMTS optical heterodyne frequency stabilization system is constructed. Before entering into the frequency stabilization system, the probe laser passes through an acousto-optical modulator(AOM) twice in advance to achieve tunable frequency while keeping the light path stable. According to the experimental results, the frequency changes from 120 MHz to 190 MHz after the double-pass AOM, and the intensity of laser entering into the system is greatly changed, but there is almost no change in the error signal of the FTMTS spectrum. Using this signal to lock the laser frequency, we can ensure that the frequency of the laser changes with the amount of AOM shift. Therefore,the magneto-optical trap(MOT)-molasses process can be implemented smoothly.