Some of the adaptive optics systems contemplated to ameliorate the effects of the atmospheric turbulence utilize guide stars. Guide stars are artificial beacons formed in the atmosphere by the interaction of a laser with matter in the atmosphere. Sodium guide stars are formed as a result of laser interaction with the sodium layer in the upper atmosphere. A small rf FEL is one of the candidates considered as a potential source capable of creating a sodium beacon. The pulse format of such a laser consists of trains of short optical pulses. The duration of an individual pulse is typically shorter than that of relevant relaxation times in the medium. The separation of the pulses may be longer or of the order of the relevant atomic memory time scale. To properly describe interactions of the atmospheric sodium layer with a laser characterized by such a pulse format one must utilize the optical Bloch equations (OBE). The OBE have been solved numerically using a simplified two-level model, and the resulting sodium layer response has been characterized. Additionally, an analytic solution has been obtained for the case in which both the population and coherence relaxation times are equal. The model is capable of providing useful insights.
Some of the adaptive optics systems contemplated to ameliorate the effects of the atmospheric turbulence utilize guide stars. Guide stars are artificial beacons formed in the atmosphere by the interaction of a laser with matter in the atmosphere. Sodium guide stars are formed as a result of laser interaction with the sodium layer in the upper atmosphere. A small rf FEL is one of the candidates considered as a potential source capable of creating a sodium beacon. The pulse format of such a laser consists of trains of short optical pulses. The duration of an individual pulse is typically shorter than that of relevant relaxation times in the medium. The separation of the pulses may be longer or of the order of the relevant atomic memory time scale. To properly describe interactions of the atmospheric sodium layer with a laser characterized by such a pulse format one must utilize the optical Bloch equations (OBE). The OBE have been solved numerically using a simplified two-level model, and the resulting sodium layer response has been characterized. Additionally, an analytic solution has been obtained for the case in which both the population and coherence relaxation times are equal. The model is capable of providing useful insights.
line by nanosecond time scale pulsed lasers is described. By numerically integrating transition rates in the sodium hyperfine structure, the return flux per sodium atom is predicted as a function of laser power. The simulation should be useful for studies of mesospheric sodium and adaptive optics. Applications include the estimation of sodium column density from lidar return flux, and of laser guide star brightness for different pulsed laser formats. The simulation assumes that the pulse repetition frequency is sufficiently low (smaller than a few kilohertz) that atomic collisions restore local thermodynamic equilibrium between pulses. It is also assumed that the pulse length is short compared to the Larmor precession time scale. The numerical results are well-approximated by a simple analytic model for a three-level atom. The number of emitted photons is found to be primarily dependent on the product of the length of the laser pulse and the energy density.
Adaptive Optics (AO) based on artificial beacons is the key to achieve high resolution images from large ground-based telescopes. Long pulsed lasers are preferable to create sodium laser guide stars (LGS) as they allow for Rayleigh blanking. However, these lasers may increase the effective light intensity irradiated at the sodium layer, which may lead to transition saturation, and then decline the normalized return flux efficiency. The return flux might be boosted by optical repumping, which could make full use of the advantages of optical pumping without trapping the atoms to the F=1 ground state. In this paper, we study the optical repumping effect by using a small scale long pulsed sodium laser developed in Technical Institute of Physics and Chemistry (TIPC), Chinese Academy of Sciences, whose pulse format may be pretty suitable for large telescopes. An electro-optic phase modulator is used to produce 1.713 GHz sidebands from the D2a center wavelength with the fraction of 20%. As for a vacuum sodium cell at the temperature of 40°C, when the effective laser intensity increases from 4.53×102 W/m2 to 6.99×105 W/ m2, resonant fluorescence with and without repumping is measured. The result illustrates that the resonant scattering brightness with repumping can be as over 3 times as without it when the light intensity changes between 4.53×102 W/m2 to 5 ×104 W/ m2. The saturated phenomenon is also observed. This gives direct evidence that repumping could improve the performance of sodium laser guide stars based on TIPC long pulsed lasers. To our knowledge, this is the first experimental demonstration of the repumping effect with the TIPC type long pulsed laser in laboratory.
Sodium laser guide star (LGS) becomes an essential part in modern astronomical adaptive optics system, especially for the next generation extremely large ground based telescope. The LGS technology has experienced the developmental stages as natural guide star, Rayleigh LGS, sodium LGS and constellation of LGS. The sky coverage is still limited in that the LGS cannot not be used to detect the tip/tilt aberrations. While the polychromatic laser guide star (PLGS) is one of the most effective ways to enlarge the sky coverage to 100%. Previous simulation models are insufficient for the accurate calculation of the return flux, especially for the simulation model of PLGS which is generated by one-photon excitation of mesospheric sodium atoms. The simulation model based on Bloch equations proposed in this paper can be used to compute the return flux of one-photon excited PLGS precisely. Doppler broadening, beam atom exchanging, collisions and recoil are taken into account in the model. The return flux is validated by the return efficiency. The simulation results indicate that with one-photon excitation of sodium atoms, a return efficiency of 330 nm is minimum compared with those of other wavelengths; the saturation power density will decrease with recoil increasing and increase with collision rate increasing; an optimal line-width exists up to maximum the photon return efficiency. In the best case, when the power density is 10 W/m2 at the sodium layer, the maximum return efficiency at 330 nm is 0.907 photons/s/sr/atom/(W/m2) with an optimal laser line-width of 18 MHz.
Summary form only given. Adaptive optics systems, guided by artificial sodium beacons have reached fundamental limits, because of (i) the low densities of atoms in the mesosphere; (ii) the low level of excitation dictated by the requirement to avoid saturation, and (iii) the monochromatic return which makes it impossible to correct for differential tilt (the largest error-about 90%-in image formation). Different attempts are being made to taylor laser pulse profiles to more efficiently access the Doppler groups of the atomic lines, while avoiding saturation (e.g. only 6.4 mW/cm/sup 2/ for the D2 line), and to assist in the correction for differential tilt by adding Rayleigh or natural beacons. Both problems severely limit the further advancement in this field due to the increased complication and cost. We have presented an alternative, which circumvents these problems by using coherent excitation of sodium with short pulses. The saturation limitation is avoided in this approach in which each pulse totally inverts (3s /spl rarr/ 4s) the interaction volume. Subsequent cascade resonance fluorescence at two wavelengths allows for correction of the tilt.
Sodium laser guide star (LGS) is the key for the success of modern adaptive optics (AO) supported large ground based telescopes, however, for many field applications, Sodium LGS’s brightness is still a limited factor. Large amounts of theoretical efforts have been paid to optimize Sodium LGS exciting parameters, that is, to fully discover potential of harsh environment surrounding mesospheric extreme thin sodium atoms under resonant excitation, whether quantum or Monte Carlo based. But till to now, only limited proposals are demonstrated with on-sky test due to the high cost and engineering complexities. To bridge the gap between theoretical modeling and on-sky test, we built a magnetic field controllable sodium cell based lab-bench, which includes a small scale sum-frequency single mode 589nm laser, with added amplitude, polarization, and phase modulators. We could perform quantitative resonant fluorescence study under single, multi-frequency, side-band optical re-pumping exciting with different polarization, also we could perform optical field modulation to study Larmor precession which is considered as one of devils of Sodium LGS, and we have the ability to generate beams contain orbital angular moment. Our preliminary sodium cell based optical re-pumping experiments have shown excellent consistence with Bloch equation predicted results, other experimental results will also be presented in the report, and these results will give a direct support that sodium cell based lab-bench study could help a Sodium LGS scientists a lot before their on-sky test.
We describe a two-level model for the mesospheric sodium resonance fluorescence excited by a train of pulses. For pulse durations that are short compared with the 16-ns natural lifetime, population-rate equations are inadequate and must be replaced by density-matrix (Bloch) equations. We briefly contrast these two approaches and discuss several issues associated with pulse-train excitation. Analytical approximations to averages over atomic velocities and the transverse spatial profile of the laser are described. Estimates of return photon numbers for trains of pulses, ranging in duration from tens of picoseconds to half a nanosecond, are made and these estimates are compared with those of more conventional long-pulse lidar schemes. Roughly similar photon numbers are predicted for both long and short pulses whenever their average intensities are comparable. For average intensities, less than ~ 10 W/cm(2), trains of asymptotically equal to 0.5-ns pulses yield greater photon returns than trains of asymptotically equal to 30-ps pulses. For larger intensities the reverse can be true.