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Optoelectronics & Communications

Nested architecture comprises fewer elemental switches

A novel multimode interference ring switching system has advantages over conventional architectures, including fewer waveguide crossovers.
13 January 2010, SPIE Newsroom. DOI: 10.1117/2.1201001.002545

The Internet is a complex network of optical interconnections between sources and receivers where signals are amplified, relayed, and filtered along transmission paths. Cross-connect switching is an important function in such networks where signals are moved between different channel paths. At large network nodes in multiplexing systems, for example, high volumes of optical signals are added or dropped using optical switches established with efficient architectures. Power consumption, waveguide loss, cost-effectiveness, and device real estate are some of the issues constantly being engineered and improved as communication capacity or the number of user connections continues to grow. They also are important steps toward realizing future ultra-compact integrated switching architectures.

In earlier days, conventional N-input port to N-output port (N×N) switching architectures, such as the Crossbar, Clos,1 and Benešs,2,3 have been standards for non-blocking, multi-port electrical switching systems. The same function can also be realized by a network of optical fibers interconnected by a number of 2×2 or M×N elemental switches. As the density of photonic devices increases and the physical size of switching elements scales down, it is inefficient and cumbersome to connect a large network of optical fibers. Integrated optics4 can compact all switches and waveguide interconnects onto a single chip to support spatial switching with most architectures.

While the Crossbar, Clos, and similar architectures offer non-blocking features (input signals are able to connect to any available output signals), the number of elemental switching devices is large, which results in higher costs and more power consumption. Of concern for integration, these switching architectures have large amounts of waveguide cross-over connections that increase the scattering loss of optical signals. The Beneš architecture is one of the most efficient in terms of reducing the number of elemental switches, and its path loss difference is zero so all outputs have equal power levels. This is not the case for the Crossbar and Clos architectures, where path loss differences are large. However, in an integrated Beneš system, the number of cross-over interconnections also remains large.

It is beneficial to use an architecture for integrated optics with fewer cross-over connections, a smaller constant path loss difference, the ability for multi-directional access, more reductions to the number of elemental switches, and additional functions.

We have proposed a novel photonic switching architecture with nested rings using multimode interference (MMI)5 or Mach-Zehnder interferometer (MZI) coupler-based switches. Called a multimode interference ring switch (MIRS), it is shown with the 4×4 arrangement in Figure 1. Progressively, an 8×8 arrangement can be realized by nesting the 4×4 arrangement in an additional MIRS ring (see Figure 2). Conventional N×N switching systems composed of 2×2 or similar elemental switches are cascaded in the direction of signal propagation. By contrast, the MIRS system extends in multiple directions and is beneficial for use in densely integrated switching networks. Also, optical signals can be routed in both directions of MIRS ring waveguides, allowing for connection to nearby or distant ports, which is difficult to realize in conventional switching architectures.

Figure 1. A multimode interference ring switch with 1-4 input ports, 5-8 output ports, and elemental switch labels A-H.

Figure 2. The 8×8 MIRS with the nested 4×4 core switch. There are three elemental switches in each arm extension and a total of 4 outer MIRS rings. The 1-4 are input ports, 5-8 output ports, and the elemental switch labels are A-H for the 4x4 arrangement. Si is an intermediate switching element, Sr a ring switching element, and Sa an access switching element.

To reduce power consumption and cost, this new switching architecture can be modified to correspond to a reduced version of the conventional rearrangeble non-blocking Beneš switch with fewer elemental switches. Another important advantage is an input signal feedback function, which may be required for supervisory and/or connectivity in photonic networks. The ability to extend input and output ports in multiple directions for ultra-compact integrated photonic grid systems is enhanced further by constant low-path difference losses and the availability for waveguide cross-over reductions.

In general, as the number of MIRS ports increases, the number of MIRS ring waveguides increases to surround the nested 4×4 MIRS. The MIRS has fewer elemental switches for non-blocking, resulting in a lower number of switches than for the Beneš architecture. This advantage is illustrated in Figure 3, where the number of elemental switches is compared between common architectures and the reduced MIRS. A comparison between the path loss differences of the reduced MIRS and common architectures6 is shown in Figure 4. Similar to the Beneš architecture, as the number of ports increases, path loss difference is constant for the reduced MIRS. In addition, the MIRS allows certain inputs to be fed back to other inputs to protect the system or provide privacy. Since the MIRS can use multimode interference couplers for cross-over connects and cross-over angles are generally small, connections can be dramatically reduced compared to the Beneš architecture.

Figure 3. Comparison of the number of switching elements between the Beneš and other common architectures highlights the advantages of the reduced MIRS approach.

Figure 4. Comparison of largest path loss difference between various switching architectures.

With improvements to elemental switch compactness, power consumption, and propagation loss, the MIRS can be integrated to greatly enhance functional efficiency. Compared to other switches, this architecture offers reduced numbers of switching components and allows for a constant maximum path loss difference. The reduction of path difference loss, number of waveguide crossovers, power consumption,7 and number of switching elements will allow the MIRS to be applied to future densely integrated switching and routing systems. The MIRS also incorporates input feedback and the capability to establish grid topologies for dense integration of cascaded components. In addition to the demonstration of a fabricated MIRS device, our future work focuses on further reducing the number of N×N switching elements using modified ring topologies. Fewer elemental switches or lower power consumption, cost, propagation loss, and complexity due to switch crossovers are some of the important steps toward the realization of future ultra-compact integrated N×N MIRS switching architectures.

Nan Xie, Katsuyuki Utaka
Waseda University
Tokyo, Japan