Adaptive-additive algorithm

In the studies of Fourier optics, sound synthesis, stellar interferometry, optical tweezers, and diffractive optical elements (DOEs) it is often important to know the spatial frequency phase of an observed wave source. In order to reconstruct this phase the Adaptive-Additive Algorithm (or AA algorithm), which derives from a group of adaptive (input-output) algorithms, can be used. The AA algorithm is an iterative algorithm that utilizes the Fourier Transform to calculate an unknown part of a propagating wave, normally the spatial frequency phase (k space). This can be done when given the phase’s known counterparts, usually an observed amplitude (position space) and an assumed starting amplitude (k space). To find the correct phase the algorithm uses error conversion, or the error between the desired and the theoretical intensities.

## Contents

* 1 The algorithm
* 1.1 History
* 1.2 Algorithm
* 1.3 Example
* 1.3.1 Limits
* 2 See also
* 3 References
* 4 External links

## The algorithm

### History

The adaptive-additive algorithm was originally created to reconstruct the spatial frequency phase of light intensity in the study of stellar interferometry. Since then, the AA algorithm has been adapted to work in the fields of Fourier Optics by Soifer and Dr. Hill, soft matter and optical tweezers by Dr. Grier, and sound synthesis by Röbel.

### Algorithm

1. Define input amplitude and random phase
2. Forward Fourier Transform
3. Separate transformed amplitude and phase
4. Compare transformed amplitude/intensity to desired output amplitude/intensity
5. Check convergence conditions
6. Mix transformed amplitude with desired output amplitude and combine with transformed phase
7. Inverse Fourier Transform
8. Separate new amplitude and new phase
9. Combine new phase with original input amplitude
10. Loop back to Forward Fourier Transform

### Example

For the problem of reconstructing the spatial frequency phase (k-space) for a desired intensity in the image plane (x-space). Assume the amplitude and the starting phase of the wave in k-space is A0 and ϕnk respectively. Fourier transform the wave in k-space to x space.

A0eiϕnk→FFTAnfeiϕnf

Then compare the transformed intensity Inf with the desired intensity I0f, where

Inf=(Anf)2,

ε=(Inf)2−(I0)2.

Check ε against the convergence requirements. If the requirements are not met then mix the transformed amplitude Anf with desired amplitude Af.

A¯nf=[aAf+(1−a)Anf],

where a is mixing ratio and

Af=I0.

Note that a is a percentage, defined on the interval 0 ≤ a ≤ 1.

Combine mixed amplitude with the x-space phase and inverse Fourier transform.

A¯feiϕnf→iFFTA¯nkeiϕnk.

Separate A¯nk and ϕnk and combine A0 with ϕnk. Increase loop by one n->n+1 and repeat.

#### Limits

* If a=1 then the AA algorithm becomes the Gerchberg–Saxton algorithm.
* If a=0 then A¯nk=A0.

## See also

* Gerchberg–Saxton algorithm
* Fourier optics
* Holography
* Interferometry
* Sound Synthesis

## References

* Dufresne, Eric; Grier, David G; Spalding (December 2000), "Computer-Generated Holographic Optical Tweezer Arrays", Review of Scientific Instruments 72 (3): 1810, doi:10.1063/1.1344176, Bibcode: 2001RScI...72.1810D .
* Grier, David G (October 10, 2000), Adaptive-Additive Algorithm, http://www.physics.nyu.edu/~dg86/cgh2b/node6.html .
* Röbel, Axel (2006), "Adaptive Additive Modeling With Continuous Parameter Trajectories", IEEE Transactions on Audio, Speech, and Language Processing 14 (4): 1440–1453, doi:10.1109/TSA.2005.858529 .
* Röbel, Axel, Adaptive-Additive Synthesis of Sound, ICMC 1999
* Soifer, V. Kotlyar; Doskolovich, L. (1997), Iterative Methods for Diffractive Optical Elements Computation, Bristol, PA: Taylor & Francis, ISBN 978-0-7484-0634-0

## External links

* David Grier's Lab Presentation on optical tweezers and fabrication of AA algorithm.
* Adaptive Additive Synthesis for Non Stationary Sound Dr. Axel Röbel.
* Hill Labs University of Maryland College Park.]

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