Backward Resonant Scattering Enhancing Mechanism Of Sodium Atom Assembly

Large ground-based telescopes are not only big eyes for investigating universe mystery,but also pre-warning monitors to prevent collisions between space objects,however their resolution is limited by atmospheric turbulence.To compensate wavefront distortion induced by atmospheric turbulence,scientists are developing adaptive optics?AO?that uses sodium laser guide stars?LGS?as artificial beacons.The brightness of LGS is critical for precise and fast wavefront detecting in AO systems.The insufficient brightness of LGS is still limiting the current AO systems,while the next generation of AO systems require unprecedented LGS brightness.In this paper,basing on the most current laser technology,spin polarization mechanisms of sodium atom assembly and the excitation to higher atomic levels are studied to pursue brighter LGSs.1.To improve LGS brightness,it is important to research photon-atom interaction mechanisms under the unique condition of mesospheric layer.In this paper,atoms in LGS are treated as atom assembly that has special evolution character in both state and velocity space.Bloch equations are re-deduced here,electric-dipole transition,interaction of the magnetic moment with the external magnetic field,spin-exchange collisions,spontaneous emission,transit damping,recoil and velocity-changing collisions are included.Cusp kernels,which are more similar to real kernels,are adopted to deal with velocity-changing collisions.Basing on Bloch equations,we introduce a parallelly computing model,which is quite suitable for the computations of the temporal evolution of sodium atom assembly in both state and velocity space.2.Dual-line optical repumping is not only an efficient approach to prevent down-pumping,but also the precondition of spin polarization in sodium atoms.Basing on a single frequency laser and a sodium cell,the optimum repumping offset frequency and repumping power fraction are measured,which are 1712 MHz and 14%respectively.It means that the(3S_1/2,F=1)to(3P_3/2,F'=2)transition can provide an efficient channel to repump atoms from(3S_1/2,F=1)state back to(3S_1/2,F=2)state.The experimental data agrees well with the numerical predictions and on-sky test,which demonstrates that cell based lab-bench is valuable for LGS mechanism study.The repumping efficiency dependence on light intensity and spin damping rate is analyzed.The difference of repumping effect between different reports is well explained.Using the return flux enhancement factor due to repumping,a novel method for remotely measuring the spin-damping time of mesospheric sodium is proposed.3.In the optimization of LGS,it is important to analyze the atomic distribution in both state and velocity space simulaneously.Spin polarization induced by circularly optical pumping is beneficial for LGS efficiency improvement,however,geomagnetic field inducing Larmor precession will depolarize the atoms.Though it is advisable to improve light intensity to enhancing spin polarization,recoil and saturation are harmful at the same time.All these factors should be considered in LGS efficiency optimization.We found for the continue wave?cw?LGS laser with power of 20-50 W,single frequency to tens MHz bandwidth is prefered,for the long pulsed LGS laser that can provide an order of magnitude higher light intensity,the optimum bandwidth depends on both laser format and light spot.Considering the Doppler width of sodium atom,the current cw or long pulsed lasers can only make use of a small fraction of atoms,the LGS brightness can be boosted if more powerful lasers with spectra optimized are built.4.It might be also effective to improve the LGS brightness by exciting the atoms to states higher than 3P_3/2 state.We proposed to create 589 nm+819 nm LGS though 3S_1/2?3P_3/2?3D_5/2 closed cycling transitions.Basing on two single frequency lasers at the wavelength of 589 nm and 819 nm respectively,the experiment of double resonant scattering in cascade level system is carried out.The absorption spectrum of 819 nm and relative brightness of 589 nm and 819 nm fluorescence are measured,which can provide valuable data about 589 nm+819 nm LGS before on-sky test,including the requirements of the stability of laser frequency,linewidth and light intensities.Using Bloch equations,the 589 nm+819 nm LGS are numerically calculated.We found that,compared with 589nm LGS,589 nm+819 nm LGS can get more than 40%photons at 819 nm with 589nm photons nearly unchanged,that less than 3%photons lost,and the overall brightness is enhanced by a factor of more than 1.37.The 819 nm return flux might reach 8-17×10⁶ph/s/m²,7.5-6.5 magnitude,where the each average power of 589 nm and 819 nm lasers is 100 W.This new concept and brighter LGS could also provide a most practical way to realize near-infrared LGS.