Optical polymers have attracted much attention for the implementation of integrated optical systems because they have many desirable features such as good processibility, excellent adhesion to a number of substrates, and flexibility in material selection. Especially, different types of polymers can be integrated within the same optical circuit to perform the specific functions.
In this thesis, we have proposed and demonstrated a band-rejection filter based on the grating-assisted forward coupling between an electro-optic (EO) polymer waveguide and a passive polymer waveguide. The passive polymer waveguide is used as a low-loss optical path and the EO polymer waveguide is used for dropping the optical signal at a given wavelength. The grating is etched on top of the EO polymer waveguide, which satisfies the phase-matching condition for the power exchange between the two asynchronous waveguides. The proposed device is designed by using the coupled mode theory and the beam propagation method combined with the effective index method. The polymer layers are formed by spin-coating and curing. The waveguides and the grating are fabricated by the reactive ion etching in oxygen.
First, to implement the proposed filter, we have studied the fabrication process for integrating the corrugated EO polymer waveguide and the passive polymer wavguide. Using the grating with a period of 63.4㎛ we have obtained significant power exchange between the two waveguides. The grating-interaction length is 11 mm. The measured FWHM (Full Width at Half Maximum) bandwidth is 4.3 nm with the maximum attenuation of 13.3 dB at 1533.1 nm. The proposed filter can be used as a band selection filter if the output of the EO polymer waveguide is monitored.
Second, we have presented a convenient post-fabrication tuning method that uses photobleaching to adjust the center wavelength of the proposed filter. Photobleaching reduces the effective index of the EO polymer waveguide, which makes it possible to adjust the center wavelength. We can easily realize the filters with a desired center wavelength by exposing ultraviolet light on them with a proper energy even after the device fabrication. With a photobleaching energy of $180 J/cm^{2}$, the center wavelength is decreased by 25 nm without extra loss. Finally, we have investigated changes of the filter characteristics induced by poling and demonstrated a wavelength tuning by the poling-induced EO effect. The center wavelength shifts as great as 10Å have been achieved with ±500 V. We may improve the tuning voltage and the poling efficiency by adopting a cladding layer with low resistivity and an EO polymer with a high EO coefficient. Theoretical calculation shows that the tuning voltage for the center wavelength shift of 100 Å will be reduced to less than 30 V, if we reduce the device thickness to 10 mm and employ APC/CLD, a polymer with an EO coefficient of 55 pm/V at 1550 nm, as the EO polymer core layer.
The fabricated polymeric filters may be used as input-output couplers, wavelength-band filters, and noise filters for optical amplifiers in the optical communication systems and networks. Their features include low coupling loss to fibers, simple fabrication process, and potential for high-speed tuning by the poling-induced EO effect. The involved fabrication techniques will be useful for the implementation of integrated optical circuits with diverse functions composed of active and passive polymer waveguides.
