| 摘要 |
An apparatus that features an optical fiber with a core surrounded by a cladding, wherein at least one of the core and the cladding has a reduced thickness along an interval of the fiber to enlarge a modal field diameter of the guided mode in the fiber. A control layer is disposed on an external surface of the cladding along the interval. The control layer exhibits a strong change in its index of refraction as a function of temperature. The apparatus further includes a heating/cooling element and a controller to change the temperature of the control layer to more than two different levels, responsive to more than two different, corresponding attenuation settings for an optical signal to be guided by the fiber. In another embodiment of the invention, a coreless optical fiber is fused to the cladding of the optical fiber at the interval to achieve a flat wavelength response behavior. The control layer is formed at least on the external surface of the optical fiber, and may further be formed on an external surface of the coreless optical fiber.
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