Guy Indebetouw

Optical Vortices and Their Propagation

Abstract Free space propagation of an array of optical vortices nested in a smooth (Gaussian) beam is studied in the paraxial regime. It is found that when the vortices have all the same charge, their relative positions, as well as their positions within the host beam are invariant upon propagation. The array simply expands or contracts with the host beam and rotates rigidly. Vortices of opposite charges, in contrast, attract each other. Pairs can collide and annihilate. As an illustration, numerical simulation is used to compare the propagation of a pair of vortices of equal charges with that of a pair of opposite charges.

Nondiffracting optical fields: some remarks on their analysis and synthesis

Recently Durnin pointed out the existence of nondiffracting beams that propagate in free space with their energy confined around their axis but that experience no spread or divergence [ J. Opt. Soc. Am. A4, 651 ( 1987)]. This Communication provides an alternative way of interpreting these results. The approach is based on the McCutchen theorem from which the necessary conditions for a nondiffracting field are derived. This description may be useful in suggesting several different means of synthesizing such fields and in providing a convenient way of estimating their practical limitations. Some examples, inspired by known techniques studied for depth of field enhancement, are briefly mentioned.

Imaging through scattering media with depth resolution by use of low-coherence gating in spatiotemporal digital holography

We demonstrate optical gating through scattering media based on low-coherence spatiotemporal digital holography. The method combines the advantages of low-coherence gating, both temporal and spatial, with the advantages of methods using heterodyning and phase-sensitive detection. Spatiotemporal data are captured on a CCD detector in a single exposure, without mechanical scans, and processed digitally. Examples of reconstructions and sectioning of objects hidden behind a ground-glass diffuser are shown.

Three-dimensional holographic fluorescence microscope

Most commonly used methods for three-dimensional (3D) fluorescence microscopy make use of sectioning techniques that require that the object be physically scanned in a series of two-dimensional (2D) sections along the z axis. The main drawback in these approaches is the need for these sequential 2D scans. An alternative approach to fluorescence imaging in three dimensions has been developed that is based on optical scanning holography. This novel approach requires only a 2D scan to record 3D information. Holograms of 15-microm fluorescent latex beads with longitinal separation of ~2 mm have been recorded and reconstructed. To our knowledge, this is the first time holograms of fluorescent specimens have been recorded by an optical holographic technique.

Imaging with Fresnel zone pupil masks: extended depth of field

Fresnel zone plates are used as pupil apertures to extend the depth of field of an incoherent imaging system. The defocused optical transfer function is evaluated approximately, and the results are confirmed by experiments and by subjective evaluations of the images of a test chart. With a pupil having four zones, for example, the resolution limit at four depths of field out of focus is improved by a factor of 10.

Twin-image elimination experiments for three-dimensional images in optical scanning holography

Twin-image elimination in the context of optical scanning holography has recently been proposed. The proposed technique involves simultaneously acquiring sine and cosine Fresnel holograms. A complex hologram is then formed by complex addition of the holograms, and twin-image rejection is predicted by computer simulations. An experimental verification of the technique by optical acquisition of the two holograms and subsequent reconstruction of the complex hologram digitally is reported. Three-dimensional image reconstruction without twin-image noise is demonstrated.

Imaging properties of scanning holographic microscopy.

Scanning heterodyne holography is an alternative way of capturing three-dimensional information on a scattering or fluorescent object. We analyze the properties of the images obtained by this novel imaging process. We describe the possibility of varying the coherence of the system from a process linear in amplitude to a process linear in intensity by changing the detection mode. We illustrate numerically the properties of the three-dimensional point-spread function of the system and compare it with that of a conventional imaging system with equal numerical aperture. We describe how it is possible, by an appropriate choice of the reconstruction algorithm, to obtain an ideal transfer function equal to unity up to the cutoff frequency, even in the presence of aberrations. Some practical implementation issues are also discussed.

Scanning holographic microscopy of three-dimensional fluorescent specimens

We demonstrate experimentally the three-dimensional reconstructions of fluorescent biological specimens using scanning holographic microscopy. Three-dimensional reconstructions with transverse resolution below about 1 microm of transmission and fluorescence emission images are presented and analyzed. The limitations of the method are discussed.

Synthesis of Polychromatic Light Sources with Arbitrary Degrees of Coherence: Some Experiments

Abstract A simple optical system for synthesizing two-dimensional planar sources having a wide variety of one-dimensional spectral degrees of coherence is analysed. A pupil-mask acting on the dispersed image of a polychromatic incoherent slit is used to control the spectral degree of coherence of the source. Examples of sources exhibiting interesting spectral shifts and modulations in the far field are shown.

Periodic and chaotic spatiotemporal states in a phase-conjugate resonator using a photorefractive BaTiO 3 phase-conjugate mirror

The dynamics of an externally pumped phase-conjugate resonator that uses four-wave mixing in BaTiO3 is investigated experimentally. The emergence of spatiotemporal instabilities as the degree of transverse confinement is varied by changing the Fresnel number is described in detail. Local intensity time series show that relaxing the transverse confinement leads the system from a stationary state to periodic, then to quasi-periodic, motions and finally to chaotic behavior. In some regions of parameter space two frequencies are identified in the power spectra of the time series, indicating a route to chaos following the Ruelle–Takens–Newhouse scheme [ Commun. Math. Phys.64, 35– 40 ( 1978)]. Wave-front topological defects are identified by interferometry. As the system’s confinement is varied, the phase-defect density and the spatial correlation index are found to follow similar trends, indicating that the observed spatiotemporal dynamics may indeed be an example of defect-mediated turbulence.