James K. Gruetzner

Amplitude and phase beam characterization using a two-dimensional wavefront sensor

We have developed a two-dimensional Shack-Hartman wavefront sensor that uses binary optic lenslet arrays to directly measure the wavefront slope (phase gradient) and amplitude of the laser beam. This sensor uses an array of lenslets that dissects the beam into a number of samples. The focal spot location of each of these lenslets (measured by a CCD camera) is related to the incoming wavefront slope over the lenslet. By integrating these measurements over the laser aperture, the wavefront or phase distribution can be determined. Since the power focused by each lenslet is also easily determined, this allows a complete measurement of the intensity and phase distribution of the laser beam. Furthermore, all the information is obtained in a single measurement. Knowing the complete scalar field of the beam allows the detailed prediction of the actual beams characteristics along its propagation path. In particular, the space-beamwidth product, M2, can be obtained in a single measurement. The intensity and phase information can be used in concert with information about other elements in the optical train to predict the beam size, shape, phase and other characteristics anywhere in the optical train. We present preliminary measurements of an Ar+ laser beam and associated M2 calculations.

Propagation of self-focusing laser pulses in atmosphere: experiment versus numerical simulation

Numerical simulations of self-focusing laser pulses in atmosphere obtained via the slowly evolving wave approximation are compared with experimental results. The model includes group velocity and third order dispersion as well as instantaneous Kerr and delayed Raman nonlinearities. The magnitude of n/sub 2/ is inferred.

Use of beam parameters in optical component testing

We are investigating the use of a Shack-Hartmann wavefront sensor for measuring optical component quality during manufacture and testing. In a variety of fields, an optical component is designed to pass an optical signal with minimal distortion. Quality control during the manufacturing and production process is a significant concern. Changes in beam parameters, such as RMS wavefront deviation, or the beam quality parameter M2, have been considered as indications of optical component quality. These characteristics can often be quickly determined using relatively simple algorithms and system layouts. A laboratory system has been prepared to investigate the use of a wavefront sensor to measure the quality of an optical component. The instrument provides a simultaneous measure of changes in M2 and induced RMS wavefront error. The results of the investigation are presented.

Specialized wavefront sensors for adaptive optics

The performance of an adaptive optical system is strongly dependent upon correctly measuring the wavefront of the arriving light. The most common wavefront measurement techniques used to date are the shearing interferometer and the Shack-Hartmann sensor. Shack-Hartmann sensors rely on the use of lenslet arrays to sample the aperture appropriately. These have traditionally been constructed using MLM or step and repeat technology, and more recently with binary optics technology. Diffractive optics fabrication methodology can be used to remove some of the limitations of the previous technologies and can allow for low-cost production of sophisticated elements. We have investigated several different specialized wavefront sensor configurations using both Shack-Hartmann and shearing interferometer principles. We have taken advantage of the arbitrary nature of these elements to match pupil shapes of detector and telescope aperture and to introduce magnification between the lenslet array and the detector. We have fabricated elements that facilitate matching the sampling to the current atmospheric conditions. The sensors were designed using a far-field diffraction model and a photolithography layout program. They were fabricated using photolithography and RIE etching. Several different designs are presented with some experimental results from a small-scale adaptive optics brass-board.

Multisegment coherent beam combining

Scaling laser systems to large sizes for power beaming and other applications can sometimes be simplified by combining a number of smaller lasers. However, to fully utilize this scaling, coherent beam combination is necessary. This requires measuring and controlling each beams pointing and phase relative to adjacent beams using an adaptive optical system. We have built a sub-scale brass-board to evaluate various methods for beam-combining. It includes a segmented adaptive optic and several different specialized wavefront sensors that are fabricated using diffractive optics methods. We have evaluated a number of different phasing algorithms, including hierarchical and matrix methods, and have demonstrated phasing of several elements. The system is currently extended to a large number of segments to evaluate various scaling methodologies.

Optical and control modeling for adaptive beam-combining experiments

The development of modeling algorithms for adaptive optics systems is important for evaluating both performance and design parameters prior to system construction. Two of the most critical subsystems to be modeled are the binary optic design and the adaptive control system. Since these two are intimately related, it is beneficial to model them simultaneously. Optic modeling techniques have some significant limitations. Diffraction effects directly limit the utility of geometrical ray-tracing models, and transform techniques such as the fast Fourier transform can be both cumbersome and memory intensive. We have developed a hybrid system incorporating elements of both ray-tracing and Fourier transform techniques. In this paper we present an analytical model of wavefront propagation through a binary optic lens system developed and implemented at Sandia National Laboratories. This model is unique in that it solves the transfer function for each portion of a diffractive optic analytically. The overall performance is obtained by a linear superposition of each result. The model has been successfully used in the design of a wide range of binary optics, including an adaptive optic for a beam combining system consisting of an array of rectangular mirrors, each controllable in tip/tilt and piston. Wavefront sensing and the control models for a beam combining system have been integrated and used to predict overall systems performance. Applicability of the model for design purposes is demonstrated with several lens designs through a comparison of model predictions with actual adaptive optics results.

Multitiered wavefront sensor using binary optics

Wavefront sensors have been used to make measurements in fluid-dynamics and for closed loop control of adaptive optics. In most common Shack-Hartmann wavefront sensors, the light is broken up into series of rectangular or hexagonal apertures that divide the light into a series of focal spots. The position of these focal spots is used to determine the wavefront slopes over each subaperture. Using binary optics technology, we have developed a hierarchical or fractal wavefront sensor that divides the subapertures up on a more optimal fashion. We have demonstrated this concept for up to four tiers and developed the wavefront reconstruction methods for both segmented adaptive optics and continuous wavefront measurement.