Mark A. A. Neil

New modal wave-front sensor: a theoretical analysis

We present a new design of a modal wave-front sensor capable of measuring directly the Zernike components of an aberrated wave front. The sensor shows good linearity for small aberration amplitudes and is particularly suitable for integration in a closed-loop adaptive system. We introduce a sensitivity matrix and show that it is sparse, and we derive conditions specifying which elements are necessarily zero. The sensor may be temporally or spatially multiplexed, the former using a reconfigurable optical element, the latter using a numerically optimized binary optical element. Different optimization schemes are discussed, and their performance is compared.

Confocal microscopy by aperture correlation

Most confocal microscopes do not produce images in real time with nonlaser light sources. The tandem scanning confocal microscope does produce such images but, because the pinhole apertures of the Nipkov disk must be placed far apart to reduce cross talk between neighboring pinholes, only 1% or less of the light available for imaging is used. We show that, by using aperture correlation techniques and relaxing the requirement to obtain a pure confocal image directly, one can obtain real-time confocal images with a dramatically increased (25% or even 50%) light budget.

Method for the generation of arbitrary complex vector wave fronts.

We describe an extremely versatile method that permits the accurate generation of arbitrary complex vector wave fields. We implement the scheme using a reconfigurable binary optical element that also permits additional fine tuning, such as aberration correction, to be performed. As examples we demonstrate the generation of both azimuthally and radially polarized beams.

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.

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.

Programmable multiple-level phase modulation that uses ferroelectric liquid-crystal spatial light modulators

We present a novel method of producing arbitrarily valued binary phase-only modulation from a commercially available ferroelectric liquid-crystal spatial light modulator that is used in conjunction with simple polarization components. By cascading of such stages, modulators with four and eight equally spaced phase levels are constructed with 128 × 128 pixels. Near-diffraction-limited performance, when stopped down to 64 × 64 pixels, is reported in producing simple diffraction patterns and when used to generate asymmetric spot arrays in the Fourier plane of a lens.

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.

Programmble diffractive optics with a ferroelectric liquid crystal SLM in a binary phase-only mode

We report on the performance and application of a commercially available 128 X 128 pixel, electrically addressed ferroelectric liquid crystal spatial light modulator (FLCSLM). In combination with appropriate polarization optics the FLCSLM can be configured as a 2-level phase only modulator. The performance of this `programmable phase transformer (PPT) is demonstrated by using it in conjunction with a positive lens to produce near diffraction limited, reconfigurable, arrays of spots in the Fourier plane of the lens. Near diffraction limited performance is also shown for the PPT with pixels configured to act as a Fresnel zone plate. Finally, the pixel configuration for spot array generation is combined with that for zone plate to produce spot arrays without the aid of a lens. Improved performance with 2-dimensional over 1-dimensional optimization procedures is also demonstrated.

Point spread functions with extended depth of focus

The use of an annular pupil plane filter may be used to increase the depth of focus of an objective lens without significant deterioration of the lateral resolution. However this approach is very inefficient since most of the illumination light is blocked by the annular filter. We describe a method which uses a diffractive optical element to increase significantly the depth of focus but with dramatically increased light efficiency.

Characterizing high-quality microscope objectives: a new approach

We present a novel technique for testing of the high quality microscope objective lenses. The characterization of the lens is achieved by using a point light source approximated by a 40 nm colloidal gold bead scatterer and simultaneously measuring the field distribution in the pupil plane (pupil function) and light intensity in the image plane (point spread function). Aberrations introduced by the lens are then expanded into Zernike polynomials. The proposed technique is particularly suited for measuring apodization and vignetting effects and allows for easy measurements of the off-axis aberrations.