The significant growth in bandwidth demand has resulted in the utilization of fiber infrastructure reaching its limits. To fulfill this requirement, optical add-drop multiplexers (OADMs) were introduced in metro/access networks, serving as the simplest elements to introduce wavelength management capabilities by enabling the selective add and drop of optical channels. Since the OADM is based on low-loss, low-cost passive devices and does not need any power supply, a reliable, cost-effective and scalable network can be achieved with its help.
As a key component for DWDM and UW-WDM (ultra wide wavelength division multiplexing) optical networks, OADM is used for selectively dropping and inserting optical signals into a transparent DWDM network. We know that the main function of an optical multiplexer is to couple two or more wavelengths into the same fiber. If a demultiplexer is placed and properly aligned back-to-back with a multiplexer, it is clear that in the area between them, two individual wavelengths exist. This makes it possible for OADM to remove or insert individual wavelengths.
An OADM generally consists of three parts: an optical multiplexer and demultiplexer, a method of reconfiguring the paths between the optical demultiplexer and the optical multiplexer, as well as a set of ports for adding and dropping signals. The multiplexer is used to couple two or more wavelengths into the same fiber. Then the reconfiguration can be achieved by a fiber patch panel or by optical switches that direct the wavelengths to the optical multiplexer or to drop ports. The demultiplexer separates the multiple wavelengths into a fiber and directs them to many fibers.
Thin-film filter (TFF) ：for OADM configuration with TFF, an arbitrary signal wavelength is branched/dropped from wavelength-multiplexed signals via a narrow band-pass filter (BPF), whereby only the desired signal wavelength being transmitted while others reflected. Meanwhile, an arbitrary signal wavelength can be inserted/added into wavelength-multiplexed signals via a narrow BPF, whereby the desired signal wavelength being transmitted is combined with the reflected signal wavelengths.
