  The use of femtosecond lasers to introduce controlled stress states has
recently been demonstrated in silica glass. We use this technique, in
combination with chemical etching, to generate and control stress-induced
birefringence over a well-defined region of interest, demonstrating
direct-write wave plates with precisely tailored retardance levels. This
tailoring enables the fabrication of laser-written polarization optics that can
be tuned to any wavelength for which silica is transparent and with a clear
aperture free of any laser modifications. Using this approach, we achieve
sufficient retardance to act as a quarter-wave plate. The stress distribution
within the clear aperture is analyzed and modeled, providing a generic template
that can be used as a set of design rules for laser-machined polarization
devices.
