of the waveguides as shown in Figure 4.46. In the figure, an example of filter working is also indicated, in particular the response to two signals entering from the same port. An adjustable delay element controls the optical path length in one of the waveguides, resulting in a phase difference between the two signals when they are recombined. In order to widen the bandpass, sometimes the MZI branches are not straight, as also shown in the figure. Wavelengths for which the phase difference is 180 are filtered out. By constructing a chain of these elements, a single desired optical wavelength can be selected [75]. While the MZI chain promises to be a low-cost device because it can be fabricated on semiconductor material [76], its tuning control is complex, requiring a delay element set- ting in each stage to be dependent on the settings in previous stages of the chain. Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices Delay MZ bandpass control function 1490 nm port 1490 nm 1550 nm Bandpass setting FIGURE 4.46 Block scheme of a wide band MZI filter. 4.4.2.3 Microrings Filters Optical microring resonators (OMRs) are versatile elements for implementing filtering function in integrated photonic circuits [77]. A microring is generally constituted by a ring waveguide resonator with two tangent waveguides that, in the nearer point, passes at a very small distance from the ring. This distance, which is a critical parameter determining the coupling efficiency between the resonator and the input/output waveguides, is called characteristic gap. Each ring, with its input and output ports, constitutes a single-pole add/drop periodic fil- ter: resonant frequencies are sent from the input to the so-called drop port, while nonreso- nant frequencies are sent to the so-called line port. This behavior is represented in Figure 4.47. The single-pole filter has a Lorentzian bandpass characteristic and can be used as build- ing blocks for multipole add-drop filters (see Figure 4.47). The bandwidth of the