SPATIAL COHERENCE

We examine how to exploit the properties of the spatial coherence of laser to measure the spatial modes of a laser or fiber, and as a possible communication channel.

You recall Young's double slit experiment, in which the fringes produces by two different parts of a given beam interfering tell how coherent these two points on the beam are.

This is a cumbersome experiment, however. If you want to plot the spatial coherence as a function of position across the beam, you have to keep putting in different sets of slits. Good luck aligning them all correctly every time! So we do this experiment differently; we use a twin-fiber interferometer.

This way you can move the fibers around anywhere you want within the beam, and compare any two points. Turns out if you know the the spatial coherence between any two points on a beam, you can find out the spatial modes. Consider the horizontal mode structure. If we take every point on a single cross-section, and compare it with every other point, we can generate a matrix of spatial coherence data. If, then, you solve the matrix, (get this!) the eigenvectors are the modes, and the eigenvalues are the modal weights! This is important because you don't need to know anything about the mode structure in advance. All previous methods required you to assume some particular set of modes, say, Hermite-Gaussian, but semiconductor lasers just arearen't that well-behaved. There are some papers about this on my publications page, but this is the most recent.

We also explored a way to modulate the spatial coherence of beam on purpose. This could, in principle, be used as an additional form of multiplexing after you've used up all the others, or a secret way to send data on a beam that looks like it's doing something else. you'd have to know to look for this! There are a couple of papers on that idea here.

Research Page

Home

 

Last revised 8/10/2003