FTu3A.32.pdf
FiO/LS Technical Digest © OSA 2012
Confocal Signal-based Wavefront Sensing Md. Atikur Rahman Jewel and Brian Vohnsen Advanced Optical Imaging Group, School of Physics, University College Dublin, Dublin 4, Ireland Author e-mail address:
[email protected]
Abstract: A confocal signal-based wavefront sensor has been developed and compared to a commercial Hartmann-Shack wavefront sensor. Its main limitation is speed but it may be used with high-power applications where camera-based sensors are problematic. OCIS codes: (010.7350) Wave-front sensing; (220.1000) Aberration compensation; (220.1080) Active or adaptive optics
1. Introduction Aberrations reflect deviations of a wavefront from that of a diffraction-limited system with typically planar or spherical wavefronts. System aberrations may be largely eliminated by the proper choice of optics, but also sampleinduced aberrations degrade the system performance in terms of resolution and signal-to-noise ratio [1-3]. Therefore, aberration-free optical systems are desirable. For wavefront correction at high laser powers a tiny fraction of the light may be directed towards a camera-based sensor such as the Hartmann-Shack (HS) [4]. However, this only partially compensates the aberrations by not including the sample. Sample-embedded fluorescent beads may be used as guide stars [5] although ultimately it is desirable to optimize the wavefront by only monitoring the signal strength or a related image parameter. The aim of this work is to explore the feasibility of signal-based wavefront sensing for use with high-power laser applications and compare its performance with that of a commercial Hartmann-Shack wavefront sensor. 2. Experimental realization A commercial adaptive optics (AO) kit (Thorlabs) that contains a deformable mirror (DM) and a HS has been combined with a spatially-filtered HeNe laser (wavelength 633nm). A confocal 25µm pinhole in combination with a 60mm focal length aspheric lens has been used to focus the light onto a power meter PM100D (ThorLabs) mounted in parallel to the HS. With the optical system properly aligned negligible amounts of aberrations are present and the light is focused through the pinhole to provide a maximum of signal power corresponding to a plane wavefront on the HS. With aberrations present, less light is focused trough the confocal pinhole and the quality of the wavefront can be read from the HS. To calibrate the system, different Zernike mode aberrations were introduced on the DM via custom made LabView software. The voltage array of the 140 DM actuators was controlled by the following equation:
V app [V maxV0 ] V0
(1)
In Eq. (1) Vapp is the applied voltage array for a given Zernike mode, Vmax is the maximum voltage array allowed onto the actuators of that mode, and V0 is a voltage array corresponding to 50% bias stroke of the DM. The parameter α sets the strength of the applied aberration mode. Due to the non-local influence function of the DM we do not expect a direct relationship between the applied voltages and the mode, but a stepwise closed-loop correction may provide sufficient accuracy to optimize the wavefront based on signal readings only. Voltage arrays were introduced Vapp at α = 1 and α = 0.5 for Tilt ( ) and ( ), Defocus ( ), Astigmatism ( ) and ( ), Coma ( ) and ( ), Trefoil ( ) and ( ), and Spherical aberration ( ). RMS measurements were noted for the different modes using the HS for different values of α. This calibration is needed prior to closed-loop correction of any wavefront based on the confocal signal only. 3. Results and discussion Fig.1 shows a matrix representation, A, of the measured RMS wavefront modes (and coupling between terms) when different Zernike polynomials were generated with the DM through the voltage vector, Vapp. As expected, maximum correspondence was found for the selected mode on the DM and the resulting reading on the HS. However, Coma ( ) and ( ) and Spherical ( ) were less singular. As the diagonal element of the matrix, A shows the maximum RMS value, its inverse A-1 for α = 1 can be used to calculate the vector α required to correct any given
FTu3A.32.pdf
FiO/LS Technical Digest © OSA 2012
wavefront aberration. Ideally, these matrices would be diagonal but coupling among actuators of the DM also induce other terms than those applied sequentially.
(a)
(b)
(c)
Fig.1. Matrix representation of the RMS wavefront values (in m) at α = 1 (a) and at α = 0.5 (b) obtained from the HS. The horizontal axis shows the applied Zernike modes whereas the vertical axis shows the measured Zernike modes (a, b). The resemblance of the two matrices confirms an approximate linear dependence on the scaling parameter in Eq. (1). The graph of measured signals power versus different α is shown in (c) where different colors are representing different aberrations and the maximum power is achieved at α = 0 corresponding to a near diffractionlimited performance.
=
(2)
In Eq. (2) α is the vector scaling the Zernike terms required to correct the aberrations contained in the Zernike vector R. That the square control matrix, A, and its inverse, A-1, can correct any given aberration has been confirmed by correction in closed-loop based on the power signal readings. The system will be implemented into a multi-photon microscope setup to improve the quality of the focus and thus the nonlinearly-generated signal. 4. Acknowledgements This research has been funded by Science Foundation Ireland (grants: 07/SK/B1239a and 08/IN.1/B2053) 5. References [1] X. Tao, B. Fernandez, O. Azucena, M. Fu, D. Garcia, Y. Zuo, D. C. Chen, and J. Kubby; “Adaptive optics confocal microscopy using direct wavefront sensing,” Opt. Lett. 36, 1062-1064 (2011) [2] M. J. Booth, D. Debarre and A. Jesacher; OPN Optics and Photonics News, Page 22, January (2012) [3] M.J. Booth, M. A. A. Neil, R. Juskaitis, and T. Wilson, “Adaptive aberration correction in a Confocal microscope,” PNAS 99, 5788-5792 (2002) [4] J. M. Bueno, B. Vohnsen, L. Roso, and P. Artal, “Temporal wavefront stability of an ultrafast high-power laser beam,” Appl. Opt. 48, 770 777 (2009) [5] P. Vermeulen, E. Muro, T. Pons, V. Loriette, and A. Fragola, “Adaptive optics for fluorescence wide-field microscopy using spectrally independent guide star and markers,” J. Biomed Opt. 16 (7), 076019 (2011)
