This paper presents an accurate theoretical model for the study of concatenation of optical multiplexers/demultiplexers (MUXs/DMUXs) in transparent multiwavelength optical networks. The model is based on a semianalytical technique for the evaluation of the error probability of the network topology. The error-probability evaluation takes into account arbitrary pulse shapes, arbitrary optical MUX/DMUX, and electronic low-pass filter transfer functions, and non-Gaussian photocurrent statistics at the output of the direct-detection receiver. To illustrate the model, the cascadability of arrayed waveguide grating (AWG) routers in a transparent network element chain is studied. The performance of the actual network is compared to the performance of a reference network with ideal optical MUXs/DMUXs. The optical power penalty at an error probability of 10 9 is calculated as a function of the number of cascaded AWG routers, the bandwidth of AWG routers, and the laser carrier frequency offset from the channel's nominal frequency. Index Terms-Error analysis, optical filters, optical receivers. I. INTRODUCTION I N TRANSPARENT multiwavelength optical networks, each lightwave signal may be optically multiplexed/demultiplexed several times during propagation from its source to its destination [1]. Optical multiplexers/demultiplexers (MUXs/DMUXs) exhibit nonideal amplitude and phase transfer functions within the optical signal band. That is, their amplitude transfer functions might present passband curvature, tilt, and ripple. In addition, their phase transfer functions might not vary linearly with frequency. These impairments are enhanced when a large number of these devices are cascaded together. Consequently, optical MUX/DMUX concatenation