How is a Thin Film filter fabricated？ （）
A. The glass core is doped with an Erbium in periodic segments of the fiber.
B. Layers of high index ？and low index ？material are deposited on a glass substrate.
C. Thin layers of silica glass are placed onto wafers to produce a prism-like effect.
D. Fiber is etched with an ultraviolet laser that is precisely spaced for specific wavelength filtering.
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