We present an approach to modify emission spectra of amplifiers by local density of states engineering with structured waveguides. More specifically, an inverse design algorithm is adopted to obtain the geometric parameters of the proposed structure that meets the local density of states target. The finite-difference time-domain numerical results indicate that the 3 dB bandwidth and the figure of merit on flatness in the structure we propose grow by over 22% and 32%, respectively, compared with the values of the t…