To date, most communication networks use optical fiber as a point-to-point provider with electro-optical transmitters and receivers providing the conversion of optical signals to electrical signals at important points, e.g. switches, in the network. However, because the net throughput is limited by the electronics, such architectures do not effectively utilize the available bandwidth of the optical fibers used for transmission.
Wavelength-division multiplexing (WDM), as applied to optical communications, impresses several data signals upon respective optical carriers of different wavelengths. The optical signals are combined upon a single optical fiber at its transmitting end. At the receiving end of the optical fiber, the optical carriers are optically demultiplexed into beams each carrying a single data signal, and conventional optical detectors are dedicated to each of these beams. Although this simple WDM architecture increases the fiber throughput by the factor of the number of optical carriers, the nodes of the network become complex and expensive because of the need to convert optical signals to electrical signals at various points in the network. As such, there exists a need for low-cost, highly efficient all-optical switches.
Various prior art concepts have been developed for all-optical WDM networks in which the nodes of the networks switch different ones of the WDM wavelength channels in different directions without ever converting the optical signals to electronic form. For example, early implementations of transparent all-optical networks include wavelength-division multiplexing (WDM) switches which can selectively switch the wavelength-multiplexed optical signals in different directions dependent upon their wavelengths. The signals are not converted to electrical form at the switches but remain in optical form throughout. In such transparent all-optical networks, access nodes are interconnected through the WDM switches, the destination access node of a signal leaving an originating access node being determined by the wavelength of its optical carrier. The WDM switches are reconfigurable within times of the order of seconds and remain in a set configuration for minutes or even days. The reconfigurability allows the optical wavelengths to be reallocated to connecting different pairs of the access nodes. Such a switching of WDM signals is referred to as space switching even though the signals are switched in different directions with the directions being determined by the wavelength.
It should be noted however, that the number of discrete wavelength channels is limited. For example, a network of sufficient size requires amplification, which at the present time depends upon erbium-doped fiber amplifiers having an amplification bandwidth of about 40 nm. With a realistic optical channel spacing of about 4 nm in a moderately simple architecture, such a bandwidth can accommodate only about 10 optical channels. Hence, wavelength reuse will be required, but the wavelength reuse in such networks is limited. Such limited reuse is insufficient for a network intended to connect large numbers of users in a public network. More extensive wavelength reuse is required. Of perhaps greater importance, on a larger scale, one or more of the access nodes, may simultaneously be connected to another transparent all-optical network with additional access nodes attached to the second network. This architecture allows the overall network to be scaled to very large sizes, however, the scalability requires that an access node connecting the two networks be able to translate the wavelength for the optical carrier of the data signal being transferred between the two all-optical networks to a wavelength dictated by the second network. That is, large WDM networks will require wavelength translation of a signal at many points in the network.
Several switches have been proposed for wavelength translation of a signal in a WDM system. For example, an all-optical switch previously proposed for wavelength translation of a signal is a four-wave optical mixer. Four-wave mixing, however, suffers several disadvantages over difference frequency generation. For a single pump signal, the pump frequency ωp is between the two optical carrier frequencies ω1, ω2 and the tails of the pump signal, which needs to be large for a third-order non-linear conversion, overlap the optical carrier frequencies. Furthermore, four-wave mixing, being more complex, generates more cross terms, which can interfere with the optical carrier signals. As a result, it is more suitable for converting a single wavelength and is difficult to apply to bulk conversion, that is, the simultaneous conversion of multiple wavelengths.
Yet another all-optical switch previously proposed is a single-pump parametric wavelength cross-connect. However, such a parametric wavelength cross-connect is limited to the conversion of a single wavelength for each non-linear optical element provided.