Biomedical subjects

Martin J Booth

Publications and source records attributed to Martin J Booth.

5 recordsLinked to original sources

PubMed2007

Wavefront sensorless adaptive optics for large aberrations.

In some adaptive optics systems the aberration is determined not by using a wavefront sensor but by sequential optimization of the adaptive correction element. Efficient schemes for the control of such systems are essential if they are to be effective. A scheme is introduced that permits the efficient measurement of large amplitude wavefront aberrations that are represented by an appropriate series of modes. This scheme uses an optimization metric based on the root-mean-square spot radius (or focal spot second moment) and an aberration expansion using polynomials suited to the representation of lateral aberrations. Experimental correction of N aberration modes is demonstrated with a minimum of N+1 photodetector measurements. The geometrical optics basis means that the scheme can be extended to arbitrarily large aberrations.

Journal Article↗

PubMed2003

Dynamic axial-position control of a laser-trapped particle by wave-front modification.

The axial position of a laser-trapped particle has been controlled by modification of the wave front by means of a membrane deformable mirror. The mirror gives wave-front modulation in terms of Zernike polynomials. By modulation of the Zernike defocus term we can modulate the particle position under conditions of laser trapping. A polystyrene particle of 1-microm diameter was moved along the optical axis direction for a distance of 2370 nm in minimum steps of 55.4 nm. We also demonstrated particle oscillation along the optical axis by changing the focal position in a sinusoidal manner. From the frequency dependency of the amplitude of particle oscillation we determined the spring constant as 91.7 nN/m.

Journal Article↗

PubMed2002

Adaptive aberration correction in a confocal microscope.

The main advantage of confocal microscopes over their conventional counterparts is their ability to optically "section" thick specimens; the thin image slices thus obtained can be used to reconstruct three-dimensional images, a capability which is particularly useful in biological applications. However, it is well known that the resolution and optical sectioning ability can be severely degraded by system or specimen-induced aberrations. The use of high aperture lenses further exacerbates the problem. Moreover, aberrations can considerably reduce the number of photons that reach the detector, leading to lower contrast. It is rather unfortunate, therefore, that in practical microscopy, aberration-free confocal imaging is rarely achieved. Adaptive optics systems, which have been used widely to correct aberrations in astronomy, offer a solution here but also present new challenges. The optical system and the source of aberrations in a confocal microscope are considerably different and require a novel approach to wavefront sensing. This method, based upon direct measurement of Zernike aberration modes, also exhibits an axial selectivity similar to that of a confocal microscope. We demonstrate an adaptive confocal fluorescence microscope incorporating this modal sensor together with a deformable membrane mirror for aberration correction. Aberration corrected images of biological specimens show considerable improvement in contrast and apparent restoration of axial resolution.

Fourier Analysis↗

PubMed2002

Active aberration correction for the writing of three-dimensional optical memory devices.

We describe an active optical system that both measures and corrects the aberrations introduced when writing three-dimensional bit-oriented optical memory by a two-photon absorption process. The system uses a ferroelectric liquid-crystal spatial light modulator (FLCSLM) configured as an arbitrary wave-front generator that is reconfigurable at speeds as great as 2.5 kHz. A method of aberration measurement by the FLCSLM wave-front generator is described. The same device is also used to correct the induced aberrations by preshaping the wave fronts with the conjugate phase aberration as well as to scan the focal spot in three dimensions. Experimental results show the correction of both on- and off-axis aberrations, allowing the writing of data at depths as great as 1 mm inside a LiNbO3 crystal.

Journal Article↗

PubMed2002

New modal wave-front sensor: application to adaptive confocal fluorescence microscopy and two-photon excitation fluorescence microscopy.

Confocal and multiphoton microscopes are particularly sensitive to specimen- or system-induced aberrations, which result in decreased resolution and signal-to-noise ratio. The inclusion of an adaptive optics correction system could help overcome this limitation and restore diffraction-limited performance, but such a system requires a suitable method of wave-front measurement. By extending the concept of a modal wave-front sensor previously described by Neil et al. [J. Opt. Soc. Am. A 17, 1098-1107 (2000)], we present a new sensor capable of measuring directly the Zernike aberration modes introduced by a specimen. This modal sensor is particularly suited to applications in three-dimensional microscopy because of its inherent axial selectivity; only those wave fronts originating in the focal region contribute to the measured signal. Four wave-front sensor configurations are presented and their input response is characterized. Sensitivity matrices and axial responses are presented.

Microscopy, Confocal↗