A successful strategy since the first cavemen earned their tribe’s dinner by separating a single woolly mammoth from the herd has been “divide and conquer.” In modern times, it has morphed into a technique to design algorithms for solving complex mathematical problems. And researchers at MIT in Cambridge, Mass., have invoked the same fundamental concept to design and demonstrate optical filters that they believe surpass the performance of all previous, similar devices.In a wavelength division multiplexed (WDM) telecom system, multiple wavelengths carry information on a single optical fiber. At various points along the fiber, individual wavelength channels are pulled off and others are added into the spectral slots formerly occupied by the dropped channels. The device that performs this function is called an add/drop multiplexer, and it must meet stringent requirements: The channels dropped, added and passed through the multiplexer must all experience minimal loss, and there must be minimal crosstalk between them.That is where the MIT divide-and-conquer strategy comes into play. Rather than attempt to accomplish all the functions in a single step (or in a series of coherently coupled steps), the scientists designed a series of independent, incoherent filters to separate the wavelengths (Figure 1). The task of achieving the desired characteristics of the through channel is substantially separated from the same problem for the drop and add channels because each problem is addressed by a different subset of filters.Although all three filters in the top row operate on the through channels, the first filter is the only one concerned with the drop channel, and the last, the only one concerned with the add channel. Therefore, the first and last filter can be independently designed to fit the drop and add requirements, respectively, and then the middle three-ring filter, which is concerned with the through channel only, can be designed to provide the desired through channel performance when combined with the already set first and last stages. The ultimate limitation to this scheme is that each filter adds an unavoidable amount of loss and dispersion near resonance, so the total number of filters is limited. Figure 1. By incoherently cascading microring filters, the researchers separated the tasks of tailoring the spectral characteristics of the through, add and drop channels.The individual filters in Figure 1 comprise three microring resonators, which are resonant only with light whose wavelength fits an integral number of times around their circumference. When the rings are resonant, they extract power from the incoming waveguide and transfer it, ring to ring, to the drop waveguide, and vice versa, for the add waveguide. In other words, only the wavelength that is resonant with the microrings will emerge from the drop waveguide, and all other wavelengths will emerge from the through waveguide. Figure 2. A scanning-electron micrograph shows a three-stage add/drop multiplexer fabricated in silicon-rich Si3N4. Experimentally, the researchers fabricated one-, two- and three-stage multiplexers by direct-write scanning-electron-beam lithography (Figure 2). They believe that the optical performance of their three-stage filter exceeded that of any similar microring filters. Its 40-GHz-wide, flattop through channel provided an extinction ratio greater than 50 dB with only 2 dB of loss, and the drop channel provided 30 dB of adjacent-channel rejection (Figure 3).Figure 3. The three-stage multiplexer had better than 50-dB extinction in the through port for the 40-GHz-wide passband. Excess loss seen by the dropped channel was a mere ~2 dB because of traversing the ring resonators.They also think that such high-extinction filters might be used in such applications as fluorescence spectroscopy and quantum key distribution.Optics Letters, Sept. 1, 2006, pp. 2571-2573.