Directional-coupler (DC)-based switching systems can switch signals at the rate of several terabits per second. Such switches can also transmit signals with multiple wavelengths simultaneously. Despite these advantages, DCs suffer from an intrinsic crosstalk problem that must be overcome in building a robust switching system. In this paper, the principles of constructing strictly nonblocking DC-based photonic switching systems under various crosstalk constraints are explored. We demonstrate how crosstalk adds a new dimension to the theory of switching systems. We find the sufficient nonblocking condition for photonic networks under crosstalk constraints and demonstrate that some well-known nonblocking networks can tolerate a stricter crosstalk constraint while retaining their hardware complexity. The theory developed in the paper can guide us in making the design tradeoff between the level of crosstalk and the amount of hardware.

Directional couplers are electro-optical switching devices that are capable of switching multiple wavelength signals. Once the state of a coupler is set up, optical signals can pass through the coupler with a rate of tera bits per second. A directional coupler-based photonic switching system however, suffers from the intrinsic crosstalk problem. This shortcoming has been the most limiting factor in building a large switching network of this kind. It has been shown that by using more hardware (couplers) we can reduce the number of crosstalk-generating couplers along the path of a connection in a Banyan-type switching system. We explore the wavelength approach to achieve this task. We show that if there are two wavelengths available, the number of couplers suffering the first-order crosstalk can be reduced to zero while the total number of couplers will remain the same as a network without any crosstalk constraint.

Photonic switching systems based on directional coupler switching elements can switch multiple-wavelength signals at the speed of tera-bits per second. Crosstalk is an intrinsic shortcoming of the directional couplers which must be overcome in the construction of an efficient switching system. Therefore, it adds a new dimension to the design of the networks based on directional couplers. We explore the principles of constructing strictly nonblocking directional coupler-based photonic switching systems under various crosstalk constraints. We show that a lower level of crosstalk can be achieved at a cost of additional hardware. We also introduce a rule that limits the routing of the new connections based on the state of the switching network. Such a rule can significantly reduce the undesired crosstalk and the hardware complexity of the strictly nonblocking network at the same time. The resulting network is therefore categorized as a wide-sense nonblocking network.

Directional couplers are capable of switching signals with multiple wavelengths. They are the ideal technology for building light-path switches to configure wavelength division multiplexing (WDM) transmission facilities. This technology, however, has a major constraint: crosstalk. Design of a strictly nonblocking directional coupler-based photonic switching system under a specified crosstalk constraint has previously been studied. The results indicate that a lower level of crosstalk can be achieved at a cost of some extra amount of hardware. In this paper, we show that if a simple rule is followed in setting up the connections, both the amount of hardware for nonblocking networks and the connection setup time can be greatly reduced. The resulting networks thus belong to the class of wide-sense nonblocking networks.