A thermally responsive shutter interrupts or transmits light through an optical fiber. A small heater, such as a resistance heater, is positioned centrally under a gap in an optical fiber. At one end of the fiber is an illumination source; the opposite end of the fiber is the output end. The gap between the two segments of the optical fiber is filled by an opaque dye solution comprising a volatile organic or other liquid, and a high opacity dye. In the normal state, the dye solution interrupts the light passing through the glass fiber. Upon activation of the heater, the gap is sufficiently small that a bubble will form between the ends of the glass fibers. This removes substantially all of the dye from the gap between the fibers, and permits light to pass from the light source to the output end of the optical fiber.
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