Tunable Phase Masks 103 slightly linear-tapered thickness. Applying strain to the slave mask introduced a precise chirp. Complex PDMS aperiodic phase masks can thus be implemented with a uniform grating silica or PDMS mask. A PDMS phase mask is useful for writing into planar waveguide structures, as the contact of the phase mask does not pose a threat of damage to either the planar circuit or the phase mask. The soft mask easily attaches to the surface of the planar circuit, allowing several gratings to be written using one exposure. 3.2.1 Fabrication of Long-Period Gratings Long-period gratings (LPGs) can be fabricated by using a number of tech- niques. These include the exposure to an amplitude mask with the appropriate period using UV radiation as shown in Fig. 3.41 [142], or simply with a linear amplitude mask [147], through heating with a laser [148], an electric arc [149], or a CO 2 laser [150]. Because heating is localized to 10 s of microns with a focused laser beam, the use of the CO 2 laser also allows ultrashort devices such as microcouplers only 200-mm long, long-period gratings, and transversely excited whispering gallery mode ring resonator devices to be fabricated in preta- pered fibers [150]. Many of these systems have been used for LPGs with periods of tens to hundreds of microns. However, LPGs with gratings of greater than a