Realization Of Fabry-Perot Filters For Wavelength Demultiplexing

Filters intended for demultiplexing radiations whose wavelengths differ from some 20 or 30 nm must exhibit high performances. Fabry Perot filters with high finesse and contrast are generally used: single Fabry Perot filters fulfilling these conditions are made with a large number of quarterwave layers. The contrast can be improved by using a system constituted of several single Fabry Perot filters in series (D H W, T H W). To carry out the production of such filters with very strict manufacturing tolerances, a direct optical monitoring method has to be used. In this case, following up the evolution of the optical properties of the stack during its growth allows an automatic error compensation to be obtained. We have developed an apparatus allowing a continuous observation of the spectral profile of the filter during its formation, in a range of some ten nanometers centered on the peak transmission of the filter. For the centering wavelength of the filter, calculation and evolution of the two par-tial derivatives ∂T/∂e (with respect to the time) and II1/X (with respect to the wavelength) can then be made. With some precise examples, a discussion about the sensitivity of the different methods together with the choice of the best fitted one for the stack monitoring is presented.