Gary C. Loos

Laser diode coherence length variation for balancing fiber optic interferometers

A visible light (675-nm) laser diode is used as a variable coherence length source to allow the rapid adjustment to equal optical pathlengths of a fiber interferometer. The diodes spectral width, measured for decreasing drive current, changes from less than 0.2 to 16 nm. The corresponding change in coherence length is from 2.3 mm to 28 μm, bridging a significant portion of the gap between a gas laser and a broadband source. The fiber interferometers air paths are first adjusted with the laser at high drive current, then fine tuned as the current is reduced below threshold. An experimentally observed interferogram confirms the theoretical prediction for fringe visibility at a current near threshold.

Phase diversity as an on-line wavefront sensor: experimental results

We present the preliminary results of a laboratory experiment using phase diversity as a wavefront sensor. Computer simulations of this experiment were also performed. The phase diversity algorithm used the ordinary finite-difference method to solve the transport equation of intensity and phase. This method of phase diversity retrieves the phase directly and may prove to be useful for low light level applications and for extended objects. This entertains the possibility of using phase diversity as an on-line wavefront sensor for adaptive optics.

The Magdalena Ridge Observatory: a look ahead

The Magdalena Ridge Observatory project has received first- year funding to complete planning and environmental work. The observatory will have three 2.4-meter telescopes that can be used individually for conventional single-telescope projects or linked to do interferometry. The layout of the observatory will allow fixed east-west baselines as long as 75 meters and may include one telescope that can be moved north-south 100 meters or more to improve coverage in the u- v plane.

Concurrent computation of Zernike coefficients used in a phase diversity algorithm for optical aberration correction

This paper describes a method to compute the optical transfer function, in terms of Zernike polynomials, one coefficient at a time using a neural network and gradiant decent. Neural networks, which are a class of self-tutored non-linear transfer functions, are shown to be appropriate for this problem as a closed form solution does not exist. A neural network provides an approximation to the optical transfer function computed from examples using gradient descent methods. Orthogonality of the Zernike polynomials allow image wavefront aberrations to be described as an ortho-normal set of coefficients. Atmospheric and system distortion of astronomical observations can introduce an unknown phase error with the observed image. This phase distortion can be described by a set of coefficients of the Zernike polynomials. This orthogonality is shown to contribute to the simplicity of the neural network method of computation. Two paradigms are used to determine the coefficient description of the wave front error to provide to a compensation system. The first uses a phase diverse image as input to a feedforward backpropagation network for generation of a single coefficient. The second method requires the transfer function to be computed in the Fourier domain. Architecture requirements are investigated and reported together with saliency determination of each input the the network to optimize computation and system requirements.

Use of electro-optical devices for optical path-length (OPL) compensation

We present the results of some laboratory experiments of the use of electro-optical (EO) devices to control the optical path length (OPL) of an interferometric array. One of the most important problems in interferometric beam combination is the control of the path length; this is coupled with the need for partial wavefront compensation in order to increase the sensitivity of the interferometer. Traditional approaches to such problems are often very expensive and sometimes impractical. For this reason we started an effort, both theoretically and experimentally, in order to investigate if less costly and more effective techniques can be applied. In our experiments we used single-cell LCDs in order to eliminate piston terms in a two- aperture interferometer. We used phase diversity techniques for extracting the phase information. Although the experimental results are still partial we believe that there is enough evidence that such devices can be used for the OPL control and partial wavefront compensation. Further testing is needed in order to assess the real capabilities of commercially available LCDs and the need, if any, of customization.

Wavefront control using a 64x64-pixel liquid crystal array

Results of wavefront control and a simple example of wavefront correction using a 64 X 64 pixel nematic liquid crystal array are presented.

Adaptive optics performance considerations for the Magdalena Ridge Observatory

The Magdalena Ridge Observatory now being planned for a site within the Magdalena Mountains near Socorro, NM will have an optical/near infrared interferometric imaging array as its primary observing instrument. We are presently evaluating the use of array telescope apertures ranging from 1.2 to 2.0 m in diameter for this application. Telescopes in this size range are many times the size of Frieds coherence parameter r0 anywhere within the .6 to 2.2 μm wavelength range of interest and to be useful for interferometry will require the use of adaptive optics (AO) techniques to restore and maintain the spatial coherence of the telescope pupils. We review some of the practical limitations related to the use of AO systems on interferometer telescopes and discuss the enhanced interferometric performance that might thereby be attained.

Magdalena Ridge Observatory optical interferometer: a status report

The Magdalena Ridge Observatory (MRO) project is presently funded to design and build a facility including an optical/infrared imaging interferometer composed of up to 10 1.5 meter class telescopes and a single conventional 2.4 meter class telescope. The interferometer array will be arranged in a “Y” configuration and the use of movable telescopes will allow its reconfiguration from a very compact array with baselines up to tens of meters to a true long baseline configuration with baselines up to 400 meters. We plan to introduce adaptive optics systems on the array telescopes.

Synthetic images from simulations of long-baseline 2D optical interferometers

We present computer simulations of variable baseline 2D imaging optical interferometers operating at visible and infrared wavelengths. Sparse apertures of fewer than 10 receivers, baselines up to 400 m and aperture size from 1 to 2 m are considered. SNR limitations pose significant problems with dilute apertures observing faint sources; we explore various ways to address this problem. We simulate pupil plane visibility measurements under perfect conditions and in the presence of wavefront aberrations due to atmospheric distortion. A variety of ideal sources are studied, including stellar photospheres with features and artificial satellites. Earth rotation aperture synthesis over extended periods improves spatial frequency coverage of astronomical sources, while observation at multiple wavelengths improves coverage for geosynchronous satellites. We introduce a hybrid technique for performing a fast analytic pixel transform, similar to the fast discrete Fourier transform, which allows complex sources to be represented in pixel form but which admits the full floating point accuracy of analytic calculation. We study image deconvolution techniques to enhance the final image. An algorithm is presented for improvement of images formed from pupil plane interferometric data. Values are added to the frequency domain with the dominant constraint being an image taken with a small filled aperture instrument. A deconvolution technique using the entropy function is developed to enhance reconstruction of the truth image. Examples of image improvement obtained by the algorithm are presented.

Low-light-level adaptive optics for a two-telescope interferometer scheme

We study the use of a cross-correlation algorithm as a sub-aperture slope estimator in a low light-level adaptive optics scheme for a two-telescope interferometer. Using a computer simulation of a Hartmann-Shack wavefront sensor to compare the algorithm against a conventional intensity-centroid estimator, we find that the cross-correlation algorithm marginally reduces the mean-square slope-measurement error under shot-noise limited conditions and significantly reduces the error in the presence of additive noise. We compare the performance of the two estimators in a two-telescope interferometer scheme by introducing wavefront sensor noise with appropriate variance in a full adaptive optics simulation, calculating measures-of-merit appropriate to interferometric imaging such as fringe visibility and coherent energy.