To provide a wavelength multiplexing device and the like that can effectively identify whether impairment occurs at a location before or after a separation and switching module, a wavelength multiplexing device (100) is connected to one or more optical fiber line systems and to one or more optical transceiver systems and is disposed between the optical fiber lines and optical transceivers so as to input and output optical signals, and is provided with the following: first optical switches (12a-12c) that output an optical signal input from an optical line to an optical transceiver; second optical switches (12d-12f) that output an optical signal input from an optical transceiver to an optical fiber line; and a local optical loopback circuit (13) that feeds back and outputs an optical signal input from an optical transceiver (21-23) to that optical transceiver.

In some embodiments, techniques for creating a design for a physical device are provided. A computing system receives an initial design of the physical device. Performance of the physical device is simulated using the initial design. A performance loss value is determined for the physical device based on the simulated performance at a target wavelength and one or more delta wavelengths. The performance loss value is backpropagated to determine a gradient corresponding to an influence of changes in the initial design on the total performance loss value. The initial design of the physical device is revised based at least in part on the gradient.