Stabilization Of Laser Power And Frequency In Ultracold Dipolar Atom Experiment

In recent years,ultracold atomic physics has greatly promoted the progress of quantum simulation,quantum information and precision measurement,so it has gradually become the focus of physics research.In contrast to conventional ultracold alkali atomic gases,lanthanide atoms(e.g,erbium(Er),thulium(Tm),dysprosium(Dy))have exceptionally large magnetic dipole moments due to non-S electronic ground states.Therefore,lanthanides not only have short-range isotropic contact interactions,but also have long-range anisotropic dipole-dipole interactions(DDI).As a result,lanthanides have important scientific significance in the many-body dipolar system.Erbium with rich energy level structure and different abundance of isotopes,including bosons and fermions,offers convenience for studying novel physical phenomena of dipolar quantum gas.Therefore,erbium is an ideal candidate for studying dipolar gases.In an ultracold atomic experiment,it is necessary to load an atom cloud from a magneto-optical trap(MOT)into the far-off resonance optical dipole trap(ODT)for evaporative cooling to obtain a quantum gas.The intensity fluctuation of the dipole far-detuning capture laser will lead to the change of the trap depth,resulting in the heating of atoms and consequently shortening of the capture lifetime,which will affect the final atomic number and temperature.Because of that our ultracold erbium atom system is very sensitive to the intensity noise of low frequency,an experimental system of laser power stability is established in this paper by external servo feedback of radiofrequency power of acousto-optic modulator.In addition,by analyzing and comparing the spectrum of residual power noise of the homemade PI circuit and the commercial SIM960 PID controller,the homemade circuit has obvious advantages in suppressing the low-frequency intensity noise.In the ultracold erbium atom experiment,due to the narrow linewidth of the 583-nm transition(natural linewidth ? = 2? × 190 k Hz),it has a low Doppler cooling temperature limit(4.6?K),so the 583-nm laser is widely used as the capture light in the magneto-optical trap.In order to maintain the stabilization during running the experiment,the 583-nm laser with narrow linewidth and high stability is one of the key technologies in the laser cooling and trapping erbium atom experimental system.The Pound-Drever-Hall(PDH)technique is one of the most powerful active laser stabilization techniques to narrow the linewidth and suppress the short-term frequency fluctuation,where the laser frequency is locked to the resonance of a reference cavity.However,due to the internal stress of the cavity material and temperature fluctuation,the reference cavity inevitably has long-term drift.In order to solve this problem,we employ a two-stage locking system by combining the PDH technique together with the modulation transfer spectrum(MTS)of iodine molecule.Specifically,the583-nm laser is first pre-stabilized by the reference cavity via the Pound-Drever-Hall(PDH)technique to suppress short-term fluctuation and narrow laser linewidth,while the MTS of iodine molecule serves as the secondary feedback to overcome the inevitable long-term drift of the optical reference cavity.To evaluate the stability of the two-stage locking laser,the stabilized laser beats with an optical frequency comb.During up to 4 hours of monitoring,the long-term drift of laser frequency stabilized only cavity is 205 k Hz.By contrast,the maximum fluctuation is within ±12k Hz in the case of two-stage laser stabilization,which meets the requirements of long-term stable operation of ultracold erbium experiment.The laser power and frequency stabilization schemes in this paper are universally used in the ultracold experiment.Moreover,it may provide a strategy to build up other possible dipolar experimental systems with lanthanides such as europium(Eu)and thulium(Tm).