Page 340 - Introduction to Information Optics

6.3. Thin-Film Waveguide Couplers •p. 5

Fig. 6.24. SEM picture of the grating on 1608D polyimide.

mental photograph in Fig. 6.25(b). The grating is designed to surface-normally
couple the laser beam into the 9120D polyimide waveguide with an operating
wavelength at 1.3 /mi. The coupling into the planar waveguide with unidirec-
tional propagation can be clearly observed with a measured efficiency of 5%.
The coupling efficiency is relatively low because the difference between the
refractive index of the guiding and cladding layers of the waveguide is very
small ( — 0.015) in the case of 9120D polyimide. This experimental result is
consistent with theoretical calculation based on the coupled-mode theory, as
shown in the curve in Fig. 6.18. Furthermore, the calculated results indicate

Surface-normal input light
Guiding mode
(a)

Fig. 6.25. (a) The schematic of coupling of a surface-normal input light into a waveguide using the
tilted grating, (b) The experimental photograph of coupling a surface-normal input laser into the
9120D polyimide waveguide.