Spatial-coherence modulation for optical interconnections

Abstract

The spatial coherence of a laser beam depends on the number and the relative weights of the spatial modes supported by the laser waveguide. By electro-optic modulation of the cavity geometry, the spatial-coherence function can be modulated between zero and one at predictable locations across the beam and thus carry information. A simple integrated-optic interferometer is used to decode the signal. Spatial coherence can be modulated independently of the beam intensity and can be used as another level of multiplexing in addition to amplitude modulation, wavelength-division modulation, etc. One can implement a free-space optical interconnection scheme by carrying data on the intensity and address information on the spatial coherence.

The Optics of Spatial Coherence Wavelets

“…In fact, as predicted by the classic Van Cittert-Zernike theorem (Born and Wolf, 1993), the spatial coherence state of the optical field emitted by a primary source at a plane in the Fraunhofer domain can be modulated by changing the shape of the source. Electro-optic modulation (Anderson and Pelz, 1995) and optical resonator tuning (Qiang and Ligang, 2000) inside the cavity of laser sources have also been reported. It is also known that the spatial coherence of a laser can be reduced by interposing a rotating ground glass in its propagation (Shirai and Wolf, 2001), which is widely used in techniques that require control of the spatial coherence of the illumination, as in laser beam characterization (Castañ eda et al, 2007a), beam modulation (Ostrovsky and Hernandez-Garcia, 2005), and analysis of the spatial coherence moiré (Castañ eda et al, 2007b), among others.…”

The application of laser-induced multi-scale surface texturing

“…15 Spatial coherence is typically responsible for the speckle patterns produced by lasers and is related to the size of the source. Loss of spatial coherence occurs through volume interaction and can be limited or prevented by using high-quality optics in a clean environment.…”

“…For multi-mode systems, a loss of spatial coherence occurs both as the number of modes increases and as the contribution from the secondary modes is increased. 15 Finally, spatial coherence increases with distance from the source (as the source becomes more point-like) and can be improved by using a spatial fi lter consisting of a condensing lens and an aperture.…”

“…Perfect coherence would occur if light were emitted at a single frequency, essentially a delta function, but since no source is able to produce such pure light, the light produced in narrow bands is said to be quasi-monochromatic. 15 Maintaining temporal coherence only really becomes a problem when the optical path difference (OPD) of the two interfering beams becomes large (greater than the coherence length). The coherence length is related to the coherence time by Equation 1, where Δx c is the coherence length, c is the wave speed, and Δt c is the coherence time.…”

Spatial coherence modulation using plane waves generated with a digital micromirror device

“…Modulating the spatial coherence of light beams provides another level of control that could be applied in optical communications, as a multiplexing method, or in imaging systems for speckle reduction, apodization, and spatial frequency bandwidth enhancement [2]- [7]. Thus, several methods to modulate the spatial coherence have been developed using random phase distortion through different kinds of diffusers or stochastic phase masks on a liquid crystal spatial light modulator [3], [8]- [10].…”