Polymer waveguide notch filters using long-period gratings

Abstract

Polymer waveguide notch filters using long-period gratings are proposed and investigated theoretically and experimentally. They are integrated-optical filters analogous to long-period fiber gratings. They give more freedom in choosing waveguide materials and structures than long-period fiber gratings. Hence, desired filter characteristics may be achieved by choosing them appropriately. The notch filter consists of a channel-type core, a cladding surrounding the core, and buffer layers sandwiching the cladding. For the notch filters, long-period gratings are formed in two ways: corrugated long-period grating (CLPG) and thermo-optic long-period grating (TOLPG). The former is permanently made on the surface of the core. The latter is temporarily induced by periodic heaters formed on the upper buffer layer. TOLPG means both the periodic heaters and the thermo-optic index change induced by them. By the CLPG or the TOLPG, the phase-matching condition is satisfied such that the core mode in the channel waveguide is coupled to a cladding mode mainly confined in the multimode slab region. An analysis method for the proposed filters is described. The coupled-mode theory is applied to the notch filters approximated by the effective index method. For the TOLPG, the heat transfer theory is used. Several analytic expressions necessary to calculate the characteristics of the notch filters are derived. Based on the analysis method, the notch filters are designed and analyzed. The design procedure consisting of three steps is described. Through the analysis of the designed filters, it is shown that the notch filters have desired characteristics. To demonstrate the feasibilities of the proposed notch filters, commercially available thermo-curable polymers are used. The notch filters are fabricated by using conventional processes. The notch filter using the CLPG is demonstrated experimentally. The grating period is 51.4㎛ and the length of the region perturbed by the CLPG is 24 mm...