This script produces aberration-correction holograms for use with a digital spatial light modulator (SLM), for the purpose of achieving uniform focal conditions independent of focal depth during femtosecond laser irradiation within transparent samples. Supports irradiation through an additional window layer, as in the case of a sample within a high temperature stage.
Python 2.x, matplotlib, numpy, and scipy.
Experimental parameters are specified in the inputs.py script, in the manner illustrated by the included example. Parameters are grouped into several classes and passed as instances to create_bitmaps().
A successful run will calculate the aberration correction patterns and place them in a folder called 'bitmaps'. If a distortion correction file is specified, an additional set of patterns will be calculated which include both aberration and distortion corrections.
The distortion correction is used to compensate for phase distortion due to the SLM surface not being perfectly flat. An example 'distortion.bmp' pattern is included, but in practice each SLM module should have its own pattern provided by the manufacturer to be used here. The distortion- corrected patterns should be used for actual experiments, while the pre- distorted patterns are useful for aligning the SLM module.
Output filenames are formatted as
window{thickness}_depth{focal depth}_z{focal displacement}
Focal depth here refers to the target depth of the focus below the sample surface. Focal displacement refers to the stage displacement required after focusing on the surface in order to achieve the intended focal depth after the refraction and aberration correction is accounted for. Values are given in microns. For example:
window1000_depth500_z263
This hologram would be used when there is a 1 mm window to focus through, and a 500 micron focal depth after correction is desired. The sample should be positioned by focusing on the sample surface, then raising the stage by 263 microns, in order to achieve a real focal depth of (approximately) 500 microns.
The algorithm presented by Itoh et al. (http://dx.doi.org/10.1364/OE.17.014367) was generalized to account for multiple refracting layers, in order to apply aberration correction while irradiating a sample inside a high-temperature stage through a silica window. Details of the derivation and experimental implementation are provided in
Adam Stone et al. "Multi-layer aberration correction for depth-independent
3D crystal growth in glass by femtosecond laser heating." JOSA B, Vol. 30,
Issue 5, pp. 1234-1240 (2013). <http://dx.doi.org/10.1364/JOSAB.30.001234>
An example aberration-correction pattern with slit beamshaping before applying distortion correction:
After applying an additional distortion correction:
