Using loss to flat the delay lines of finite coupled-resonator optical waveguides

We derive the expressions of transfer function and group delay of finite coupled-resonator optical waveguides (CROWs), using the method of coupling of modes in time, and investigate the influences of loss on the delay line. The results show that loss can eliminate the ripples and sharp peaks in the delay line. A Lorentzian response of the delay line gradually appears with the increase of loss, and the bandwidth is proportional to the amplitude decay rate. Combining with coupling coefficients loss can produce the delay line with a wide flat top where the group velocity dispersion is almost zero. We also experimentally demonstrate this in a fiber CROW which can be utilized for rotation sensing and Mach-Zehnder interferometers. The results can be used to design delay lines for the applications of CROWs.