| Summary: | 博士 === 國立中央大學 === 光電科學研究所 === 94 === One of the important applications of sub-nanometer narrow band-pass filters is for dense wavelength division multiplexing, DWDM, in fiber optic communication systems. To fabricate a sub-nanometer narrow bandpass filter is a very tough coating technology. Particularly, uniformity is one of the most important issues in the fabrication of narrowband pass filters for DWDM.
The basic design of narrow bandpass filter is constructed on the Fabry-Perot interferometer. We have derived the theory of all-dielectric narrow bandpass filter for DWDM by multiple-beam interferometry. The dependence of the uniformity on the design of filters has been derived and analyzed theoretically. Essentially, the uniformity of multicavity DWDM filters is dependent on the spacer design of each cavity. First of all, using a same spacer layer in each cavity can insure the spectra are the same and independent on the tooling factors of materials. Besides, by using high order and good tooling factor of material or low index material to be a spacer will achieve the improvement of the uniformity.
Excepting the improvement of designing a DWDM filter with large tolerance of layer thickness to obtain the good uniformity, the tooling factors of the materials also play the important role. We provide a technology of ion etching effect to improve the uniformity of the tooling factors. Based on the analyses, the uniformity of filters has indeed been improved not only by the tooling factor of each material deposited by E-gun, but also by the etching profile of the ion source. The etching profile could be controlled by adjusting of the working parameters of the ion source, such as the ion-beam voltage, ion-beam current, and acceleration voltage. The parameters have to be controlled to an optimum condition to accommodate the deposition rates of the two coating materials.
The ion etching effect provides a method to modify and improve the tooling factors. However, it is a problem to measure the film thickness with very high precision. To verify the tooling factor to be less than 1/10,000, the accuracy of the thickness has to be at the order of sub-atom. We provide an analyzed method of shaping tooling factor (STF) to improve and enlarge the useful coating area. Instead of deposited a single layer, a single cavity of narrow bandpass filter was deposited and measured the central wavelengths to analyze the distribution of thickness. Using the method of STF the precision of tooling factors can be as good as less than 10-5.
Finally, we applied the technologies shown above including the spacer design rules, ion-etching technique, and shaping tooling factor to achieve the best result. Before any improvement, the useful coating area is limited at the monitoring point. By applying the spacer design rules, the useful coating areas cover 4.4% of the 10-cm substrate. Besides, the useful coating area for a single channel is about 185 mm2, 2.3% of the whole substrate. In addition, under the improvements of ion-etching technique and the shaping tooling factor, the uniformity of the DWDM filter was better than +/-0.003% over an area of 50 mm in diameter and +/-0.0006% over that of a 20 mm diameter. The useful coating area for a single channel exceeds 2300mm2, 29% of the whole substrate, which is twelve times of the area if only the design was improved.
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