It is not straightforward to fabricate low loss fused coupler with polarization maintaining or depressed cladding fibers. Therefore, polished couplers are commonly used with such fibers. However, coupling ratio of a polished coupler strongly depends on the temperature.
In this work, we fabricated directional couplers with polarization maintaining fiber (PMF) and depressed cladding fibers using the combination of polishing and fusion process. The fabricated couplers showed insertion loss between 10% and 30%. For the best case, the loss was less than 3%. The insertion loss mainly come from the imperfect cleaning of the polished surfaces, inhomogeneous heating and microbending in the fusion process. Some fraction of the loss can be reduced in the pulling process. The pulling length needed to the control coupling ratio is very short compared to fused couplers. A coupler fabricated with standard communication fiber showed a low temperature dependence of coupling ratio with the coefficient of 0.2%℃. For PMF couplers, the coefficients were 0.15 and 0.31%℃ for fast axis and slow axis, respectively. Polarization isolation was 21dB and 19dB for throughput port and coupled port, respectively.
We have measured the bending effect of the fused-polished coupler. For in-plane bending, the coupling ratio was antisymmetrically changed in respose to bending angle and the coefficient was about 0.43 percent per degree. This antisymmetric change in the coupling ratio results from the initial phase mismatching that is introduced by different strain applied to the two fibers in the process of atraching the fibers to the polishing jig. Theoretical fitting of the experimental results shows the strain difference of 0.07 percent. For out of plane bending case, coupling ratio did not show much change, as expected.
