An optical attenuator and method, the optical attenuator having a core which is covered by a cladding material with a section of polymer cladding of an identifiable length. The polymer cladding has an index of refraction that varies with temperature. A thermal device such as a thermoelectric heater or cooler is placed adjacent to the polymer cladding to control the temperature of the cladding. The index of refraction of the polymer cladding is manipulated by changing its temperature by supplying power to the thermal device. The index of refraction of the polymer cladding will range from values below and above the index of refraction of the core material. A light wave transmitted through the core will experience a degree of attenuation due to leakage into the polymer cladding material when the index of refraction of the polymer cladding is equal to or greater than the index of refraction of the core. The light wave may be either attenuated or blocked entirely.