Page 344 - Introduction to Information Optics

6.3. Thin-Film Waveguide Couplers

Substrate lOiim
Fig. 6.28. THE SEM micrograph of the cross section of the 45° microcoupler.

6.3.2.2. Output Profile of Micromirror Coupler
The output profiles from one of the 45° microcouplers are shown in Fig.
6.30(a), (b), and (c). These figures correspond to z — 100 fan, z = 1 ram and
z = 5 mm, respectively, where z is the distance from upper surface of the
coupler to the point of observation. The output coupling efficiency of this 45°
microcoupler is nearly 100%. The half width at half maximum (HWHM) of
the output profile at the microcoupler is about 60 /mi, which is comparable
with the active region of a silicon-based photodetector having a bandwidth of
6 GHz [22]. If the photodetectors are mounted close to the microcouplers,
most of the light can reach the photodetectors and thus the coupler-to-detector
coupling efficiency can be very high.
The output profile from the 45° microcoupler can be determined using
diffraction theory. According to Fresnel approximation [23,24], the near field
distribution U(x, y) is given by:

exjp(.//cz) A
JZA 2z

Fig. 6.29. A planar polyimide waveguide with waveguide mirror.