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This paper presents planar long period grating (LPG) devices based on a periodic thickness variation in the waveguide
core, fabricated by etching into the lower cladding layer prior to definition of the waveguide layer. This periodic
geometric change results in a stable grating structure and a permanent refractive index modulation of 10-4 or higher,
which is comparable to the index modulation in Ge-doped silica material induced by photo irradiation techniques widely
used in fiber grating fabrication. This grating produces a strong resonance at a particular wavelength in the transmission
spectrum, enabling a range of applications from wavelength filtering to signal distribution in communication networks.
In this work, a polymer and silica hybrid architecture has been implemented in order to achieve wavelength tunability.
Using a thermally oxidized silicon layer as a lower cladding, a Ge-doped silica ridge is patterned using conventional
photolithography and reactive ion etching to form the waveguide core, which is then covered with a low index
fluorinated polymer cladding. While the silica waveguides offer a lower propagation loss and an easy processability, the
top polymer allows the device to be thermally tuned over a wide wavelength range by exploiting the opposite thermo-optic
coefficient between fluorinated polymer and silica, and the high sensitivity of the underlying LPG to the refractive
index of the cladding layer. Strong rejection bands have been demonstrated in the C+L band, in good agreement with
theoretical calculations. Corrugated structures have been defined across an extended area under multiple waveguides
resulting in coupling of light from the fundamental mode into cladding modes and back into the neighboring waveguides
located far from the evanescent coupling distance. This kind of coupler can facilitate devices that require extraction and
control of a particular waveguide mode for applications such as multiple channel signal distribution and temporal pulse
shaping. Implementation of LPGs for these applications will be discussed.
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