The architecture of WDM passive optical networks could be simplified by replacing the transmitter lasers at the subscriber terminal with optical modulators. A fraction of the light transmitted from the central office could be detected at the subscriber's site and the remainder is remodulated with upstream data and return to the central office. Thus, this network does not require WDM sources at the subscriber terminal, obviating the need for their wavelength control. These modulators could operate at relatively low speed, but should be inexpensive and polarization-insensitive. We propose a thermo-optic modulator which could satisfy these conditions.
A ZnS Fabry-Perot etalon filter was constructed by multilayer dielectric coatings on a 100-㎛ thick $SiO_2$ substrate. The spacer was 100 quaterwave layers of ZnS and placed between the two mirrors made of five pairs of $ZnS/ThF_4$ quaterwave layers. The free-spectral range and passband were measured to be 30 nm and 0.55 nm, respectively. This filter could be tuned 0.064 nm/℃ with temperature. A thin nichrome layer attached on the top of the filter was used to apply heat. The resulting modulation bandwidth was about 500 Hz. This slow response is due to the extremely low thermal conductivity of the epoxy layer used to attach this filter to the copper heat sink. A theoretical model predict that the modulation bandwidth could be improved to several MHz by eliminating the $SiO_2$ substrate and replacing the epoxy layer with a material which has better thermal conductivity.
