Temperature-Insensitive Mode Converters With CO2-Laser Written Long-Period Fiber Gratings

We fabricate mode converters by directly writing long-period gratings in a two-mode fiber with a CO₂ laser. These mode converters allow the fundamental LP₀₁ mode to be converted to any of the four higher order cylindrical vector modes or the LP₁₁ modes. The transmission characteristics of these devices are insensitive to temperature variations and the mode conversion efficiency is insensitive to the polarization state of the input light. One of our typical gratings, which contains 15 grating periods, can provide a conversion efficiency >99% over a bandwidth of 34.0 nm in the C-band. These robust mode converters could find applications in mode-division-multiplexing systems and other applications that require cylindrical vector modes.