Broadband optical fiber mode converters

Abstract

Mode Division Multiplexing (MDM) has been more and more widely used in the field of telecommunication. The application of MDM in communication requires converting the mode of light from LP01 mode to higher order modes. Our project aimed to widen the bandwidth of conversion from light of LP01 mode to LP11 mode to cover as much of C band (1530nm - 1565nm) and L band (1565nm - 1625nm) as possible by the means of Long Period Fiber Grating (LPFG). The grating was fabricated in a computer-controlled CO2 laser machine on a two-mode step index fiber. My design work consisted of simulation and experiment parts. The simulation was conducted based on transfer matrix method through programming with MATLAB, serving for finding suitable parameters, such as the length of grating, the number of sections the grating contains and coupling coefficient, for experiments. Length apodization with phase shifts was applied into the new profiles I designed. Experiments done through CO2 laser and mode field verification were used to fabricate the grating and to validate whether the light has been converted into LP11 mode. By observing the -20dB, -10dB bandwidth of the transmission spectrum of the fundamental mode, we could specify the characteristics and performance of the mode converter. The results showed that some of the profiles I designed widened -10dB bandwidth of the conversion from LP01 mode to LP11 mode with relatively longer grating length, compared to the simulation results of uniform grating with similar length, although it did not work so well on -20dB bandwidth.