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Temperature measurement of an optical fiber core is a challenge as there is no way to reach the tiny core in order to probe its temperature using regular methods. By exploiting the birefringence properties of a polarization maintaining (PM) fiber, a highly sensitive temperature sensor can be realized to monitor the variations of temperature in the optical fibers. This temperature sensor is particularly useful to detect very small temperature changes of the gain fiber in radiation-balanced fiber lasers. The sensor performance is demonstrated by measuring the temperature changes of a PM fiber attached to a Peltier cooler with the sensitivity of 0.02 C for 6 cm of PM fiber. Also, the temperature rise in the core of a piece of PM fiber carrying a few mW of a cw laser is measured to be 0.18 C mW with a response time of 125 microseconds. PM fibers are designed in different ways to create different indices of refraction along two orthogonal slow and fast axes. The polarization of a linearly polarized light input to a PM fiber along one of these axes is maintained while propagating in the fiber. Any other input polarization will be periodically modified along the PM fiber. The transmitted polarization state depends on the initial polarization, fiber length, and the birefringence of the fiber which is varied by small temperature changes. In the case of using single-mode gain fiber in radiation-balanced fiber lasers, the temperature detection can be done by attaching the gain fiber to the PM fiber sensor. The possibility of direct fiber core temperature monitoring in laser cooled fibers will be achieved by using polarization maintaining fiber for laser cooling. The same fiber can then be used as temperature sensor of its core using circularly polarized light as probe.
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