Control of planar optical waveguides using photosensitive liquid crystal cladding layers

 

Integrated photonic active and passive devices are topics of great interest for many applications in telecommunications, computing, and sensing. By impressing a Bragg grating onto an optical waveguide or fiber, its optical transmission properties can be modified to perform such functions as filtering for wavelength division multiplexing, coupling, and laser tuning. Current Bragg waveguide devices are fabricated with only a fixed or slightly tunable periodicity, however tunability would be highly useful and desirable. We are working on methods that use a photosensitive cladding layer to allow us to write or erase tunable Bragg waveguide devices1. The usual problems with scattering from the photosensitive LC polymer are reduced by using a passive, high transmission inorganic core.

The waveguides have a 0.5 µm thick silicon oxynitride core of refractive index 1.89 grown over a 3 µm subcladding layer on a doped silicon wafer. The polymer top cladding has both nematic and azoic groups to achieve its photosensitive properties. We write the Bragg grating onto the top cladding using the interference pattern from two crossed green laser beams. A single circularly polarized beam can erase the grating. Figure 1 shows the experimental setup and Figure 2 shows a spectral response curve of the waveguide with an impressed Bragg grating. It can be seen that the waveguide is effective as a filter for 1440 nm light.

Figure 1. Converging green circularly polarized laser beams interfere, causing the top cladding polymer to orient alternately parallel and perpendicular to the waveguide propagation direction, with a periodicity and angle that matches the Bragg condition.


Figure 2. The waveguide was rotated around the plane of incidence of the green beams to change the Bragg condition and collect the waveguide transmission data.

 

References

[1] Y. Wang, A. Klittnick, N.A. Clark, and P. Keller, Applied Physics Letters 93, 143506 (2008).

Text and images contributed by Art Klittnick.