Mechanical stress effects on optical multi-layer dielectric thin film multi-cavity Fabry-Perot interference films

The Dense Wavelength Division Multiplexing (DWDM) technology is the key enabler for the next generation of high capacity telecommunication networks. Among the available platforms to realize DWDM, thin film filters (TFF) are key due to their reliable performance and flexible applications. Based on the Fabry-Perot Etalon mechanism, it consists of multi-layer thin film coatings on a glass substrate with alternating high-index and low index materials. During the filter deposition process, compressive intrinsic stress is built up by the ion beam assisted bombardment coating method: and the temperature difference at coating and operation also introduces thermal stress in the films. The existence of internal stress inevitably changes the optical performance of the TFF through its effects on the film layer thicknesses and the refractive indices. Not much attention has previously been paid to the stress effects on the TFF optical performance because up to now the manufacturing tolerance could overcome the variations. However, as the increased capacity requires more optical channels, the much narrower channel spacing has diminished the tolerance and the stress has to be more tightly controlled.; This study is concerned with analyzing the internal stress distributions of a TFF of 50 GHz spacing in the substrate and thin film layers and relating it to the filter's optical characteristics. Based upon Townsend's theory of multi-layer structures, and the observations that the substrate is free of clamping during the coating process and the bending is gradually built-up, we introduce a linear approximation for the intrinsic strain using the initial length method that is dependent on the relative position of each layer along the thickness direction. The linear coefficient a is obtained through radius of curvature measurements. The analytical results for the center wavelength drift resulting from the grinding process, the trend of the thermal drift, the reflection spectrum isolation and chromatic dispersion, and the transmission spectrum polarization mode dispersion (PMD), are in fairly good agreements with the experimental data and empirical statistics, indicating validity of the theory.; Through this model, the effects of the process parameters such as the substrate type and its coefficient of thermal expansion (CTE), the final substrate thickness, the coating temperature, and coating temperature rate of increase, on the optical performance of the transmission spectrum center wavelength, transmission passband ripple and isolation, the reflection isolation, the chromatic dispersion and PMD of both paths are analyzed. With the assistance of Davis's theory, the relationship between the deposition control parameters, such as the ion beam current, ion beam voltage, and deposition rate, and the intrinsic stress is qualitatively evaluated.; The intrinsic stress determines the radius of curvature and thus has a major effect on the optical performance. The simulations show the effects of substrate thickness on the filter center wavelength thermal drift coefficient and thus correct the prior misunderstanding in industry that this parameter depends only on the substrate CTE.