Correction of Atmospheric Effects on Laser Beams Propagating through Strong Turbulence

Atmospheric effects limit the performance of any electro-optical (EO) system. Tasks such as delivery of directed energy and laser communications are significantly affected by turbulence and refraction. A correction of atmospheric effects on the propagation of light can be done by adaptive optics (AO). Nevertheless, challenging scenarios like strong turbulence near the ground lead to high failure rates of the traditional AO systems. Unconventional wavefront sensors and sensing strategies are developed at Fraunhofer IOSB to provide alternatives for measuring the wavefront deformation of a laser beam and to improve the performance of energy delivery and laser communications applications even in strong turbulence and/or horizontal- or slant-path propagation

Modal vs. zonal wavefront-sensorless adaptive optics for free-space laser communications

Atmospheric effects limit the performance of any electro-optical system. Tasks such as laser communication and delivery of directed energy are significantly affected by turbulence and refraction. A correction of atmospheric effects on the propagation of light can be done by adaptive optics (AO). Unconventional wavefront sensing strategies are being developed to provide alternatives for measuring the wavefront deformation of a laser beam propagating through strong turbulence and/or along a horizontal-path. In this paper we present results from two "wavefrontsensorless" approaches: stochastic parallel gradient descent (SPGD) and its modal version (MSPGD)

Optimization of wavefront-sensorless adaptive optics for horizontal laser beam propagation in a realistic turbulence environment

Unconventional wavefront sensing strategies are being developed to provide alternatives for measuring the wavefront deformation of a laser beam propagating through strong turbulence and/or along a horizontal path. In this paper we present a modified wavefront-sensorless (WFS) adaptive optical (AO) system where the well-known stochastic parallel gradient descent (SPGD) algorithm is extended with a-priori knowledge of the spatial and temporal statistics related to atmospheric turbulence. Here, a modal implementation of the correction algorithm allows us to exploit modal wavefront decomposition to decrease SPGD optimization complexity. We also propose an implementation of a modal decomposition based on Karhunen-Lo´eve polynomials instead of the common Zernike polynomials. Appropriate calibration of the deformable mirror is also presented. Performance evaluation of this modified wavefront-sensorless AO system is carried out in a realistic simulated turbulence environment and the results are compared against the traditional, zonal SPGD algorithms

Challenges for Wavefront Sensing and Correction beyond Large Astronomical Installations and Vision Science

The use of adaptive optics, wavefront sensing and wavefront correction beyond the large astronomical telescopes or vision science behoves careful analysis of the media being corrected for, and identification of non-traditional systems and devices

Ultimate turbulence experiment: simultaneous measurements of C n

We have performed a series of experiments in order to simultaneously validate several devices and methods for measurement of the path-averaged refractive index structure constant ( \u1d436\u1d45b 2). The experiments were carried out along a horizontal urban path near the ground. Measuring turbulence in this layer is particularly important because of the prospect of using adaptive optics for free-space optical communications in an urban environment. On one hand, several commercial sensors were used: SLS20, a laser scintillometer from Scintec AG, BLS900, a largeaperture scintillometer, also from Scintec, and a 3D sonic anemometer from Thies GmbH. On the other hand, we measured turbulence strength with new approaches and devices developed in-house. Firstly, an LED array combined with a high-speed camera allowed for measurement of \u1d436\u1d45b 2 from raw- and differential image motion, and secondly a two-part system comprising a laser source, a Shack-Hartmann sensor and a PSF camera recoded turbulent modulation transfer functions, Zernike variances and angle-of-arrival structure functions, yielding three independent estimates of \u1d436\u1d45b 2. We compare the measured values yielded simultaneously by commercial and in-house developed devices and show very good agreement between \u1d436\u1d45b 2 values for all the methods. Limitations of each experimental method are also discussed