An etch tunable antireflection coating for the controlled elimination of Fabry-Perot oscillations in the optical spectra of transverse modulator structures

A technique is described whereby an Si/sub x/N/sub y/ coating of a noncritical thickness in excess of 1/4 ( lambda /n/sub c/) is deposited and, using a process of repeated wet etching and measurement of the optical transmission spectrum, the thickness of the film is precisely adjusted to obtain an optimized antireflection coating. Although this technique was developed for transverse modulators, it should prove beneficial for optically characterizing any structure susceptible to unwanted Fabry-Perot effects. Peak-to-peak oscillations as large as 20% of the maximum transmitted signal are reduced to zero over a 35-nm range. This effective range can be progressively shifted through the spectra so that spectra completely free of Fabry-Perot oscillations can be obtained.<>