Shack-Hartmann wavefront sensing using interferometric focusing of light onto guide-stars.

Optical microscopy provides noninvasive imaging of biological tissues at subcellular level. The optical aberrations induced by the inhomogeneous refractive index of biological samples limits the resolution and can decrease the penetration depth. To compensate refractive aberrations, adaptive optics with Shack-Hartmann wavefront sensing has been used in microscopes. Wavefront measurement requires light from a guide-star inside of the sample. The scattering effect limits the intensity of the guide-star, hence reducing the signal to noise ratio of the wavefront measurement. In this paper, we demonstrate the use of interferometric focusing of excitation light onto a guide-star embedded deeply in tissue to increase its fluorescent intensity, thus overcoming the excitation signal loss caused by scattering. With interferometric focusing, we more than doubled the signal to noise ratio of the laser guide-star through scattering tissue as well as potentially extend the imaging depth through using AO microscopy.

Adaptive optical two-photon microscopy using autofluorescent guide stars.

We demonstrate a fast, direct wavefront-sensing method for dynamic in vivo adaptive optical two-photon microscopy. By using a Shack-Hartmann wavefront sensor and open-loop control, the system provides high-speed wavefront measurement and correction. To measure the wavefront in the middle of a Drosophila embryo at early stages, autofluorescence from endogenous fluorophores in the yolk were used as reference guide stars. The method was tested through live imaging of a Drosophila embryo. The aberration in the middle of the embryo was measured directly for the first time. After correction, the contrast and signal intensity of the structure in the middle of the embryo was improved.

Live imaging using adaptive optics with fluorescent protein guide-stars.

Spatially and temporally dependent optical aberrations induced by the inhomogeneous refractive index of live samples limit the resolution of live dynamic imaging. We introduce an adaptive optical microscope with a direct wavefront sensing method using a Shack-Hartmann wavefront sensor and fluorescent protein guide-stars for live imaging. The results of imaging Drosophila embryos demonstrate its ability to correct aberrations and achieve near diffraction limited images of medial sections of large Drosophila embryos. GFP-polo labeled centrosomes can be observed clearly after correction but cannot be observed before correction. Four dimensional time lapse images are achieved with the correction of dynamic aberrations. These studies also demonstrate that the GFP-tagged centrosome proteins, Polo and Cnn, serve as excellent biological guide-stars for adaptive optics based microscopy.

Adaptive optics microscopy with direct wavefront sensing using fluorescent protein guide stars.

We introduce a direct wavefront sensing method using structures labeled with fluorescent proteins in tissues as guide stars. An adaptive optics confocal microscope using this method is demonstrated for imaging of mouse brain tissue. A dendrite and a cell body of a neuron labeled with yellow fluorescent protein are tested as guide stars without injection of other fluorescent labels. Photobleaching effects are also analyzed. The results shows increased image contrast and 3× improvement in the signal intensity for fixed mouse tissues at depths of 70 µm.

Adaptive optics confocal microscopy using direct wavefront sensing.

Optical aberrations due to the inhomogeneous refractive index of tissue degrade the resolution and brightness of images in deep-tissue imaging. We introduce a confocal fluorescence microscope with adaptive optics, which can correct aberrations based on direct wavefront measurements using a Shack-Hartmann wavefront sensor with a fluorescent bead used as a point source reference beacon. The results show a 4.3× improvement in the Strehl ratio and a 240% improvement in the signal intensity for fixed mouse tissues at depths of up to 100 µm.

Adaptive optics wide-field microscopy using direct wavefront sensing.

We report a technique for measuring and correcting the wavefront aberrations introduced by a biological sample using a Shack-Hartmann wavefront sensor, a fluorescent reference source, and a deformable mirror. The reference source and sample fluorescence are at different wavelengths to separate wavefront measurement and sample imaging. The measurement and correction at one wavelength improves the resolving power at a different wavelength, enabling the structure of the sample to be resolved.

Wavefront aberration measurements and corrections through thick tissue using fluorescent microsphere reference beacons.

We present a new method to directly measure and correct the aberrations introduced when imaging through thick biological tissue. A Shack-Hartmann wavefront sensor is used to directly measure the wavefront error induced by a Drosophila embryo. The wavefront measurements are taken by seeding the embryo with fluorescent microspheres used as "artificial guide-stars." The wavefront error is corrected in ten millisecond steps by applying the inverse to the wavefront error on a micro-electro-mechanical deformable mirror in the image path of the microscope. The results show that this new approach is capable of improving the Strehl ratio by 2 times on average and as high as 10 times when imaging through 100 microm of tissue. The results also show that the isoplanatic half-width is approximately 19 microm resulting in a corrected field of view 38 microm in diameter around the guide-star.

Breadboard testing of a phase-conjugate engine with an interferometric wave-front sensor and a microelectromechanical systems-based spatial light modulator.

Laboratory breadboard results of a high-speed adaptive-optics system are presented. The wave-front sensor for the adaptive-optics system is based on a quadrature interferometer, which directly measures the turbulence-induced phase aberrations. The spatial light modulator used in the phase-conjugate engine was a microelectromechanical systems-based piston-only correction device with 1024 actuators. Laboratory experiments were conducted with this system utilizing Kolmogorov phase screens to simulate atmospheric phase distortions. The adaptive-optics system achieved correction speeds in excess of 800 Hz and Strehl ratios greater than 0.5 with the Kolmogorov phase screens.

High-speed horizontal-path atmospheric turbulence correction with a large-actuator-number microelectromechanical system spatial light modulator in an interferometric phase-conjugation engine.

Results of atmospheric propagation for a high-speed, large-actuator-number adaptive optics system are presented. The system uses a microelectromechanical system- (MEMS-) based spatial light modulator correction device with 1024 actuators. Tests over a 1.35-km path achieved correction speeds in excess of 800 Hz and Strehl ratios close to 0.5. The wave-front sensor was based on a quadrature interferometer that directly measures phase. This technique does not require global wave-front reconstruction, making it relatively insensitive to scintillation and phase residues. The results demonstrate the potential of large-actuator-number MEMS-based spatial light modulators to replace conventional deformable mirrors.