Measuring Low Velocity Turbulence Field With Hartmann And The Extend Of Small Aperture Beam Technique

Aero-optics concerns with the optical aberrations which the aberrating flowfield is of relatively short propagation length in the near field of the optical system. The term "near field" in the present context refers to optical path very near the exit aperture of an outgoing collimated beam or at the receiving aperture for an imaging system, the optical path length is on the order of beam aperture. Optical wave front aberrations are typically measured in terms of the relative difference in optical path length across the aperture, the reductions in the system performance from these aberrations are described in the terms of the Strehl Rate. Along with the development and application of the high energy laser, airborne imaging system and optical guiding technology, people have made deeper researches on the interaction law between beam and complex flow field. Aero-optical flowfields are usually associated with propagation through turbulent shear and boundary layers. A wave-front sensor is demanded for these researches with the temporal resolution from 2 to 100 kHz. The small aperture beam technique has been developed to increase the temporal resolution by Fitzgerald et al. to increase the temporal response. The wavefront aberrations were measured real time and wavefront reconstructions were made by the 37-subaperture Hartmann wavefront sensor, at the same time, the temporal and spatial slope variation of subaperture measured was analysed with correlation function, the convection velocity and scale of large eddy were presented.The small aperture technique bases on the theory of Hartmann wavefront sensor, transforms beam spatial deviation into slope of locale optical wavefront, carries out the real time measurement of optical wavefront aberrations. SABT can gain the time series of optical aberrations on the relative large aperture, advances the temporal and spatial information of optical wavefront aberration. For the sensor with fixed temporal sample frequency, We advanced the extend of SABT in theory. In the extend of SABT, spatial resolution of wavefront sensor is enhanced, high temporal spatial resolution OPD field is constructed, with dense beam sampling, imposing frozen-flow hypothesis. Both SABT and the extend of SABT advanced by us base on frozen-flow hypothesis, so the approximate satisfied degree of the hypothesis must be considered, when the validity of using the relation between the small aperture jitter information and optical wavefront aberrations is discussed. The two technique are validity and finity, they optimize the wavefront sensor and flow field.