Synthesizing a Cylindrical Vector Beam with a Mach-Zehnder Interferometer
Josh Lieber, Oyster Bay High School, John Noe, Laser Teaching Center,
Stony Brook University
A cylindrical vector beam (CVB) is a beam of monochromatic light whose
linear polarization varies symmetrically with respect to the beam's axis
of propagation. The two basic CVB's are either radially or azimuthally
polarized. The many interesting applications of CVB's include enhanced
laser cutting and drilling, high-resolution microscopy, and particle
acceleration.
CVB's can be generated by precisely combining the two orthogonal first-order
Hermite-Gauss laser modes (denoted HG1,0 and HG0,1) while also giving them
orthogonal linear polarizations. The first-order HG modes resemble two spots
of light an equal and opposite distance from the center of the beam; they
differ by being aligned along either a vertical or horizontal line. A correctly
synthesized CVB will look like a bagel. When the bagel is analyzed with a
linear polarizer it turns back into an HG mode which rotates as the polarizer
is turned.
In this project, a radial CVB was generated within a Mach-Zehnder
interferometer, a device which first divides and then recombines a beam using
two mirrors and two beamsplitters in a rectangular configuration. One HG mode
was obtained from an open-cavity HeNe laser, and the second was generated by
rotating the beam in one arm of the interferometer 90 degrees with a Dove
prism. Similarly, the two orthogonal polarization states were obtained by
re-orienting the initial polarization direction using polarizers in each arm
of the interferometer.
We were successful in creating a radial CVB that passed the test described
above, although the recreated HG patterns were somewhat irregular in shape.
Very precise adjustments were needed to make the re-combined beams perfectly
collinear and coaxial, and to have the correct phase relationship. In the
future we hope to create a mathematical model of the synthesis process that
will help guide the challenging alignment procedure.