A Stable Twin-Core-Fiber-Based Integrated Coupler Fabricated by Thermally Diffused Core Technique

A type of stable integrated couplers based on the twin-core fiber (TCF) is proposed and demonstrated. The integrated coupler is fabricated by heating a piece of TCF with the thermally diffused core technique. During the heating process, the dopant in the cores of the TCF will diffuse to the cladding which leads to expansion of the core mode fields. As a result, the optical power in the two cores can be coupled to each other. Because the external shape of the coupling region of the TCF coupler does not change, such type of couplers have high mechanical strength and stable responses to temperature, strain, and surrounding refractive index changes. The coupling characteristics between the cores of the thermally diffused TCF are theoretically analyzed and experimentally investigated. Results show that the coupler is almost stable when the temperature changed from 30 to 120 °C, the strain changed from 0 με to 1923 με, and the external refractive index ranged from 1.3435 to 1.3676.