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. 2009 Dec 31;14(6):064033. doi: 10.1117/1.3275468

Figure 2.

Figure 2

Compensation of spectral dispersion. (a) Principles of uncompensated (top row) and grating-compensated (bottom row) spectral dispersion. Spectral dispersion elongates the collimated beam along the axis of deflection. A diffraction grating whose groove frequency is equivalent to the center frequency of the AOD recollimates the center beams and reduces the dispersion of the outer beams. Note: angles are exaggerated for the purposes of this schematic. (b) Diagonal scan patterns in fluorescent material. The uncompensated spots are elongated along the scan axis by 5 to 9×, while the compensated spots are ∼1× near the center of the scan pattern. (c) Variation of spot elongation S as a function of AOD frequency. As suggested by Eq. 9, S increases with increasing AOD frequency. With the addition of a diffraction grating tuned to the center frequency, the S is minimized in the middle of the scan field and is greatly improved at the edges of the scan pattern. (d) Images of 0.5-μm fluorescent beads in water at 5× zoom factor. Without compensation, the spherical beads appear lengthened along the vector sum of the two deflection axes (i.e., along the 45 deg) as a result of the elongated focal spot. With the grating inserted to recollimate the beams, the beads appear qualitatively circular. The zoom-ins were taken near the center of the scan range, so the observed elongation does not reflect the maximal spectral dispersion at the extrema of the scan pattern. Lines indicate examples of manually selected cross sections taken approximately parallel (solid black) and orthogonal (dotted red) to the axis of dispersion. (e) Average intensity of parallel (par) and orthogonal (orth) cross sections without compensation (uncomp, n=11) and with compensation (comp, n=10), respectively. The orthogonal axis FWHM is similar in both cases (FWHMuncomp,orth=0.53 μm; FWHMcomp,orth=0.52 μm), whereas the parallel axes are quite dissimilar (FWHMuncomp,par=1.67 μm; FWHMcomp,par=0.89 μm). The fact that the parallel and orthogonal axes are similar when compensated (i.e., grating is inserted) indicates that the PSF has been nearly restored to its circular shape. (Color online only.)