Boris V. Fortes

Adaptive Beaming and Imaging in the Turbulent Atmosphere

Adaptive Beaming and Imaging in the Turbulent Atmosphere (SPIE Press Monograph Vol. PM109) By Vladimir P. Lukin, Boris V. Fortes Due to the wide application of adaptive optical systems, an understanding of optical wave propagation in randomly inhomogeneous media has become essential, and several numerical models of individual AOS components and of efficient correction algorithms have been developed. This monograph contains detailed descriptions of the mathematical experiments that were designed and carried out during more than a decades worth of research.

Phase-correction of turbulent distortions of an optical wave propagating under conditions of strong intensity fluctuations.

Phase correction of a plane wave and a spatiolimited beam propagating through a turbulent layer of atmosphere were considered. The required adaptive corrector element size and the system bandwidth were found by numerical simulation. These requirements were determined to be the same as for a weak-intensity scintillation approximation. The size of the required segmented mirror element was found to be equal to Fried length r0, whereas the tolerable time lag was r0/V, where V is the wind velocity. However, the local slope sensors then became impractical, as did tip-tilt correction over the corrector subapertures.

Modeling of the image observed through a turbulent atmosphere

Computer simulations of a wavefront distorted by atmospheric turbulence are considered focusing on large scale and dynamic simulations. Attention is also given to wavefront sensor and wavefront corrector simulations. Computer modeling of adaptive optical systems takes into account an optical wave field in the plane of receiving aperture, a wavefront distortion sensor, a wavefront distortion corrector, and quantum fluctuation of the optical wave intensity.

Phase correction of an image turbulence broadening under conditions of strong intensity fluctuations

Phase correction of a plane wave, propagating through a turbulent layer, is considered. The required adaptive corrector element size and the system bandwidth were found by numerical simulation. These requirements were determined to be the same as for weak intensity scintillation approximation. The size of the required segmented mirror element was found to be equal to Fried length r0 while the tolerable time lag was r0/V, where V is the wind velocity. However, the local slope sensors become therewith impractical as well as the tip-tilt correction over the corrector subapertures.

Estimation of turbulent and thermal blooming degradation and required characterization of adaptive system

A laser beam propagating in the atmosphere are influenced simultaneously by thermal blooming and turbulence that results in aberrations of a focal spot. Character of thermal blooming depends not only on atmospheric conditions but also on beam power and velocity of a target. Turbulent aberrations prevail at large sped of scanning, from this point of view they are more important. In the previous papers it was pointed out that thermal aberrations of laser beams decrease sharply as the object velocity increases. If Mach number is greater than unity and laser power is not too great, the influence of thermal blooming is negligible. Effectiveness of phase correction for thermal blooming increases at increase of scanning velocity, because in this case the thermal lens is placed near the transmitting aperture. On the contrary, effectiveness of correction for turbulent aberrations decreases when the object velocity increases. Turbulent aberrations do not depend on sped of angular scanning but in the case of a moving object the requirements to the adaptive system bandwidth are higher than that for a motionless target. For beams with power about 500 kW and wavelength (lambda) equals 1.315 micrometers on the upper atmosphere paths turbulent aberrations are always stronger compared to thermal blooming even for motionless targets. So if the correction for turbulent aberrations is possible, the correction for thermal blooming is possible too.

Potential capabilities of adaptive-optical systems in the atmosphere

Thermal-blooming correction can reduce high-power laser beam distortions in the atmosphere. We use computer simulations to estimate the efficiency of different phase and amplitude-phase correction algorithms over horizontal and vertical atmospheric paths.

Partial correction for turbulent distortions in telescopes.

To provide complete compensation for turbulent distortions in the visible range at aperture dimensions typical for modern telescopes (6-10 m), one needs to develop adaptive systems with hundreds of control channels. More simple adaptive systems that provide complete compensation in the infrared range can give an essential advantage in angular resolution in the visible range too. In this case the image brightness characterized by the Strehl ratio remains much less than that in the diffraction-limited case, i.e., the system provides only partial compensation. We present the results of numerical calculations of the partially corrected point-spread function and discuss possible approaches to composing the adaptive system configuration.

Effective outer scale of turbulence for imaging through the atmosphere

We discuss here some possibilities of introducing the distortions of an optical wave phase, propagating along vertical atmospheric path, as an integral characteristic, describing the turbulence along the path. Several models of the turbulence outer scale profile have been analyzed as well as the structural characteristic of the atmospheric refractive index fluctuations in order to find the value of the efficient outer scale. The error in the Strehl ratio determination was estimated. This ratio was computed using the efficient outer scale and compared to its value computed by the model profile of the outer scale.

Adaptive forming beams and image simulation through the atmosphere

The problem of compensation for atmospheric distortions of a wavefront has been studied for a sufficiently long time. The first papers on this subject were published in the mid 1960s. At that time, however, the engineering base gave no way for designing the efficient devices for compensating for atmospheric distortions. In recent years much progress has been made in developing wavefront sensors and correctors and then fitting the optical facilities operating under atmospheric distortions with these devices. In this connection there has been an increased interest in theoretical works concerning the optical design and configuration of the wavefront corrections.

Four-dimensional computer dynamic model of an atmospheric optical system

A computer code for simulation of high-power beams thermal blooming in the turbulent atmosphere and imaging in a ground- based telescope is described. This code also allows one to simulate the components of adaptive systems, such as the Hartmann-Shack wavefront sensor and various flexible and segmented mirrors. Our software can be used for estimation of beams and images parameters in the atmosphere and for the investigations of adaptive optical system efficiency.