Broadband spatial mode conversion by chirped fiber bending

We solve the coupled-mode equations for linearly chirped codirectional mode couplers and use the results to develop design rules for broadband couplers. By comparing a chirped coupler with a uniform coupler having the same effective coupling length we find that high coupling efficiency can be obtained over a substantially wider bandwidth. We design a chirped device to have a 99% bandwidth of 47 nm at 1090 nm, which is 5.8 times larger than for the uniform coupler. Experimentally the bandwidth was improved by a factor of 5.4.