Gilad Barach

Manipulating the spatial coherence of a laser source.

An efficient method for controlling the spatial coherence has previously been demonstrated in a modified degenerate cavity laser. There, the degree of spatial coherence was controlled by changing the size of a circular aperture mask placed inside the cavity. In this paper, we extend the method and perform general manipulation of the spatial coherence properties of the laser, by resorting to more sophisticated intra-cavity masks. As predicted from the Van Cittert Zernike theorem, the spatial coherence is shown to depend on the geometry of the masks. This is demonstrated with different mask geometries: a variable slit which enables independent control of spatial coherence properties in one coordinate axis without affecting those in the other; a double aperture, an annular ring and a circular aperture array which generate spatial coherence functional forms of cosine, Bessel and comb, respectively.

Phase locking of even and odd number of lasers on a ring geometry: effects of topological-charge

The effects of topological charge on phase locking an array of coupled lasers are presented. This is done with even and odd number of lasers arranged on a ring geometry. With an even number of lasers the topological-charge effect is negligible, whereas with an odd number of lasers the topological-charge effect is clearly detected. Experimental and calculated results show how the topological charge effects degrade the quality of the phase locking, and how they can be removed. Our results shed further light on the frustration and also the quality of phase locking of coupled laser arrays.

Rapid and efficient formation of propagation invariant shaped laser beams

A rapid and efficient all-optical method for forming propagation invariant shaped beams by exploiting the optical feedback of a laser cavity is presented. The method is based on the modified degenerate cavity laser (MDCL), which is a highly incoherent cavity laser. The MDCL has a very large number of degrees of freedom (320,000 modes in our system) that can be coupled and controlled, and allows direct access to both the real space and Fourier space of the laser beam. By inserting amplitude masks into the cavity, constraints can be imposed on the laser in order to obtain minimal loss solutions that would optimally lead to a superposition of Bessel-Gauss beams forming a desired shaped beam. The resulting beam maintains its transverse intensity distribution for relatively long propagation distances.

Teaching an old laser new tricks: Solving the inverse scattering problem rapidly

Inverse scattering problems, namely reconstructing the structures of objects from their scattered intensity distributions occur in many fields of science and technology[1], such as tomographic imaging, seismology, single shot X-ray scattering[2] and imaging. Solving the inverse scattering problem where all the phase information is lost, is generally very difficult. The difficulty is alleviated by resorting to some a priori knowledge such as the boundaries within which the object lies (compact support), sparsity or other spatial features. Then it is possible to reconstruct the object using iterative algorithms, such as the well-known Gerchberg-Saxton algorithm. Unfortunately, the algorithms are time consuming and do not always converge to the right solution even with advanced computational resources.

Efficient in-phase locking of coupled lasers

In-phase locked array of lasers, where all have common frequencies and phases, can serve as single powerful laser with the high beam quality of an individual laser. Talbot and Fourier diffractions are commonly used for strong coupling between the lasers in the array. When used separately, each has some disadvantages. Talbot diffraction can lead to efficient out-of-phase locking, but requires additional diffractive elements to achieve stable in-phase locking. Fourier diffraction can directly lead to in-phase locking, but with low efficiency, high alignment sensitivity and possible deleterious damage to elements.

Digital degenerate cavity laser

In the past, we investigated degenerate cavity lasers (DCL) which allows manipulation of both near-field and far-field properties of the output beam. The DCL was comprised of a gain medium, two lenses in a 4f telescope configuration, an output coupler at one end and a back mirror at the other end. With the DCL we investigated topological defects in arrays of coupled lasers[1], simulation of classical spins arrays in a frustrated geometry[2], beam focusing after scattering media[3], and lasers with controllable coherence functions for speckles reduction[4]. In these investigations, the DCL usually included metallic masks of holes and filters that had to be specifically designed and fabricated for each application.

Lasing with propagation invariant shaped beams

Control of the propagation properties of complex beams is desired for many applications. Here we present a novel method to generate propagation invariant shaped beams. Our method is based on a modified degenerate cavity (MDC) [1], [2], which has a huge number of degrees of freedom (300, 000 modes in our system), that can be coupled and controlled. Specifically, the MDC allows direct access to both the x-space and k-space components of the laser beam. Accordingly, placing two amplitude masks, one in x-space and one in k-space, enables control of the output beam. Varying the geometric properties of the mask in x-space changes the shape of the output beam, and varying the geometric properties of the mask in k-space breaks the degeneracy between modes and forms spatial correlations (partial spatial coherence) in the output beam [1].

Rapid manipulation of the spatial coherence

Efficient method for manipulating the spatial coherence of a laser is presented. Different mutual intensity coherence functions, such as cosine or Bessel functions, are obtained, and number of modes is controlled in 1D and 2D.