Development and application of a ray-based model of light propagation through a spherical acousto-optic lens.

Geoffrey J Evans,R Angus Silver,K M Naga Srinivas Nadella,Paul A Kirkby,Bóris Marin

doi:10.1364/oe.23.023493

Abstract

A spherical acousto-optic lens (AOL) consists of four acousto-optic deflectors (AODs) that can rapidly and precisely control the focal position of an optical beam in 3D space. Development and application of AOLs has increased the speed at which 3D random access point measurements can be performed with a two-photon microscope. This has been particularly useful for measuring brain activity with fluorescent reporter dyes because neuronal signalling is rapid and sparsely distributed in 3D space. However, a theoretical description of light propagation through AOLs has lagged behind their development, resulting in only a handful of simplified principles to guide AOL design and optimization. To address this we have developed a ray-based computer model of an AOL incorporating acousto-optic diffraction and refraction by anisotropic media. We extended an existing model of a single AOD with constant drive frequency to model a spherical AOL: four AODs in series driven with linear chirps. AOL model predictions of the relationship between optical transmission efficiency and acoustic drive frequency including second order diffraction effects closely matched experimental measurements from a 3D two-photon AOL microscope. Moreover, exploration of different AOL drive configurations identified a new simple rule for maximizing the field of view of our compact AOL design. By providing a theoretical basis for understanding optical transmission through spherical AOLs, our open source model is likely to be useful for comparing and improving different AOL designs, as well as identifying the acoustic drive configurations that provide the best transmission performance over the 3D focal region.

Highlights

Spherical acousto-optic lenses (AOLs) are dynamic diffractive devices that can focus and steer an optical beam with high speed and precision
We have developed an experimentally constrained ray-based model of light propagation through a spherical AOL that incorporates refraction and acousto-optic diffraction, and is able to handle chirped acoustic frequency drives for each of the four acousto-optic deflectors (AODs)
The close match between AOL model predictions and experimental results from a 3D two-photon AOL microscope (AOLM) confirm that the model reproduces the main properties of light propagation through the AOL over a wide range of acoustic centre frequencies and linear frequency chirps

Summary

Introduction

Spherical acousto-optic lenses (AOLs) are dynamic diffractive devices that can focus and steer an optical beam with high speed and precision. The advantages of an AOL over a galvanometer-based system for laser scanning are the ability to shift the focus axially as well as laterally and high speed, inertia-free jumping between 3D spatial positions. This makes AOLs well-suited for imaging rapid neuronal signalling distributed in 3D space using activity-dependent fluorescent indicators. Three spherical AOL designs, based on two orthogonal cylindrical AOLs, have been developed: Two use relay optics between AODs resulting in a long optical path length [5, 8]. An advantage of the compact design is it can be added to an existing twophoton microscope with relative ease

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