Quick fabrication method of a thermally expanded core in polarization-maintaining fibers using CO2 laser and fiber rotation

Maximizing the transmission of high-peak-power ultrashort pulses through joints between polarization-maintaining optical fibers with different mode field diameters is crucial for improving the efficiency of all-fiber laser systems. Here, we propose a method of fabricating a thermally expanded core by using a CO${}_2$ laser as a heating source that does not require a priori splicing of fibers. Unlike standard methods utilizing continuous heating of the fiber, usually on the time scale of minutes, we present the pulsed approach, which can reduce the duration of the whole process to below 30 s. Via fiber rotation, we tailor our method to polarization-maintaining optical fibers. Furthermore, we apply the thermally expanded core method to manufacturing mode field adapters between commercially available polarization-maintaining optical fibers. Exemplary splices reveal an increase in transmission of 30 %, enabling insertion losses lower than 0.4 dB. The presented methods of fabricating a thermally expanded core and mode field adapters show high potential for large-volume production, especially for high-power applications, as the splices can withstand peak powers as high as 50 kW.