Variable optical attenuator with thermo-optic control

A method for manufacturing an attenuator using an optical fiber having a core surrounded by a cladding, comprising:

• heating and pulling the optical fiber to form a tapered interval in which at least one of the core and the cladding have a reduced thickness;
• and
• forming a control layer on an external surface of the cladding along the tapered interval, the control layer including a material that exhibits a change in its index of refraction as a function of temperature.

A method for attenuating an optical signal, comprising:

• generating an optical signal to be guided by an optical fiber having a core surrounded by a cladding, at least one of the core and the cladding having a reduced thickness along an interval of the fiber to enlarge a modal field diameter of a guided mode in the fiber, a control layer disposed on an external surface of the cladding along the interval and whose index of refraction can change as a function of temperature;
• and
• changing the temperature of the control layer to more than two different levels in response to more than two different attenuation settings.

An apparatus comprising:

• an optical fiber having a core, the core being covered by first and second cladding sections that are spaced in a longitudinal axis along the core by an intermediate section which (1) is in contact with a surface of the core, (2) is capable of enlarging a modal field diameter of a guided mode in the fiber, and (3) has an index of refraction that can change as a function of its temperature so that at certain temperatures, its index of refraction is different than that of the first and second claddings;
• and
• means for setting the temperature of the intermediate section to more than two different levels in response to more than two different attenuation settings for an optical signal guided by the fiber.