Adaptive optics (AO) system can be used to detect and compensate the aberrational wavefront of beam in real time. The compensative ability will be affected by the detecting precision of wavefront. Noise is one of the most important factors that affect the detecting precision of Hartmann-Shack (HS) sensor. Noise can induce the errors of centroid detected by HS sensor, consequently influences the wavefront reconstruction. Based on the characteristic of Charge Coupled Devices (CCD), the model that simulates the detecting precision of wavefront affected by noise is built. Based on the factual application scene of artificial beacon, for typical HS sensor, the effects of noise on the detecting precision of wavefront are simulated, and the relations of wavefront beam spot in CCD, and the number of pixel used to calculate spot centroid are analyzed. Several results that can be used in engineering application are obtained.
In this paper we adopt a full density matrix treatment to deal with the interaction between long-pulse,
circularly-polarized light and the sodium atoms. The time evolution curve of every 24 sub-state's population probability
can be obtained by solving the Bloch equation of twenty-four hyperfine levels. We find that during the long pulse
time(more than 100 ns), the final transition is only between the sub-level 3S1/2 (2,2) and 3P3/2 (3,3).What is more,
the probability of the excited sub-state has become steady. Then we start the comparison between the 2-level model and
the full density matrix method to show the rationality of rate equation and 2-level model for the long pulse,
circularly-polarized light. We prove the rationality upon adopting 2-level model to deal with interaction between
long-pulse, circularly-polarized light and the sodium atoms and thus bring the convenience to solve the interaction.
Key words: Sodium beacon, long pulse, circularly polarized light, 2-level atoms, adaptive optics
The forecasts of the optical turbulence in the marine surface layer are made in different seasons based on the numerical
products of the numerical weather prediction model. It is found that the seasonal variation of the surface optical
turbulence is not prominent over the oceans between 30°S and 30°N, but much larger over the higher latitude oceans
with weaker surface optical turbulence in the summer hemisphere and stronger surface optical turbulence in the winter
hemisphere. The surface optical turbulence strength in the height 10m above the sea level is greater than 10-15m-2/3 for
10.6μm, but less than 10-15m-2/3 for 0.55μm over the most parts of the oceans around the world. The horizontal patterns
of the forecasted surface optical turbulence strength are similar to each other based on the same time products
respectively from the two different numerical weather prediction models, but the horizontal pattern of the forecasted
surface optical turbulence is much sharper with the higher model horizontal resolution.
The beam spot will break up into some cracks when laser propagates through the turbulent atmosphere. The
characteristics of the cracked beam spot are studied statistically for different apertures and beam qualities, and with
different turbulent strengths. It is shown that, with the degeneration of the beam quality and the turbulence being
stronger, the total size of the beam spots and the numbers of the fragments will increase; meanwhile the scale of the
fragments will keep invariable almost. Four special situations are analyzed and the physical hypostasis of Fourier
transformation is discussed to understand these results.
On the base of the detailed numerical computations by 4-D wave optics code, the scaling laws for estimating the thermal blooming effects on target of the atmospheric propagation of high energy laser, for both horizontal and slant paths are given.
In laser beam propagation through the atmosphere under condition where strong scintillation is present, real zeros can appear in the beacon field. On these zero points the phases are undetermined, which are called branch points. The occurrence of branch points causes the conventional least squares phase estimation to fail.
The scintillation index is defined of wave front sensor. The relations between branch point number and index versus Rytov variance are simulated numerically. The number of branch point increases with Rytov variance and the index increases and saturates with Rytov variance. The branch point is not only related to Rytov variance but also Fresnel number. Under the given Rytov variance, the larger Fresnel has the more branch point number. The corrected Strehl ratios are compared with least square method and extended least square method for various turbulent conditions.
Scintillation is increased when laser propagates long distance near horizontally in atmospherics, which limits the ability of conventional adaptive optical system. A theoretical analysis is presented based on extend Huygens-Fresnel theory for Gaussian beam profile. Numerical simulations based on wave optics computer code are given for different atmospheric condition. For given Rytov variance different turbulence strength and propagating distance are considered. Various receiving and projecting apertures are also considered. Rytov variance and propagation Fresnel have more effect on Strehl ratio. The expressions of Strehl ratio versus Rytov variance are obtained from weak to strong scintillation. For given Rytov variance, Strehl ratio with Fresnel number is studied. Large aperture has benefit for correction. The results show that Strehl ratio increases with Fresnel number and saturates to the limit of ideal phase-only correction for given Rytov variance.
The key step for full field correction in adaptive optics with two deformable mirrors is to match the amplitude profile of the main laser with the same of beacon laser on deformable mirror 2. Amplitude and phase corrections for turbulence are studied based on Y-G wave front retrieval algorithms. The simulation result show that amplitude and phase corrections can improve Strehl ratio by a factor of 1.27 to 2.50 compared with phase-only correction.
Laser propagating through atmosphere is distorted by thermal blooming and turbulence. Thermal blooming in atmospheric laser propagation is studied using Fast Fourier Transform (FFT). Variation of steady state thermal blooming with laser power and propagation distance are shown and power threshold is also calculated. Time-dependent thermal blooming effects with and without wind are investigated. Comparisons between simulated and experimental results reach good agreements.