Broadband optical fiber mode converters

Abstract

Objective of this project is to fabricate a broadband optical fiber mode converter which couples light from fundamental core mode LP01 to forward higher order core mode LP11 for applications in Mode-division multiplexing (MDM) system. Mode converter is fabricated by introducing long-period fiber grating (LPFG) on a two-mode step index fiber. Input light excites LP01 mode and energy is coupled into LP11 mode at the output port near the resonance wavelength. Fiber grating is written by point-by-point CO2 laser writing method where grating profile is programmed by computer. Bandwidth enhancement of LP01 transmission is achieved by introduction of chirped grating. -10 dB and -20 dB bandwidth in LP01 transmission spectrum are considered. Grating parameters affecting transmission bandwidth are pitch, grating length, coupling coefficient and chirp parameter characterizing pitch variations in chirped grating. Previous researches have already be conducted to explore the effect total grating length may have on transmission bandwidth in uniform grating, so this project will focus on the bandwidth broadening effect of different chirp parameters and coupling coefficients for 20-period LPFG. Simulation and experiments are conducted to find out optimum settings for chirp parameter and coupling coefficient for widest bandwidth compared with uniform grating. Two types of chirped gratings are discussed: one is positive linearly chirped grating where pitch increases linearly period by period, the other one is step-changed period chirped grating where the whole grating can be regarded as a series of cascaded uniform gratings and pitch increases linearly between adjacent uniform sections. According to simulation and experiment results, positive linearly chirped grating can achieve better performance. Optimum bandwidth is achieved when pitch difference between adjacent periods is 0.005 mm where -10 dB bandwidth is 239.82 nm, around 4.8 times of that in uniform grating; -20 dB bandwidth is 29.28 nm, around 2.8 times of that in uniform grating.