As modern societies demand faster Internet and Ethernet service, video on demand, and more advanced data services, the need for additional channels ("slices" of the electromagnetic spectrum) that can carry greater data-loads is rapidly increasing. Optical fiber-based communication is currently the most promising approach for satisfying this growing demand for bandwidth. To increase data-carrying capacity, many channels, centered around different wavelengths, are multiplexed into the same fiber. This technology, called dense wavelength division multiplexing (DWDM), still does not fully utilize the theoretical transmission bandwidth. In practice, various signal degradation mechanisms, such as chromatic dispersion (CD), polarization-mode dispersion (PMD), polarization-dependent loss/gain (PDL/G) and wavelength-dependent loss/gain (WDL/G) limit both the number of channels and the data-transmission rate in each. Such degradation occurs because different components of the light propagate through the system with different transmission characteristics. CD arises, for example, because different frequency components travel with different group velocities; this leads to a broadening of the light pulses (which contain many neighboring frequencies) and limits the overall signal transmission rate.
Engineers are beginning to use dynamic optical equalizers (DOE) to help mitigate these degradation effects. When a DOE is added to an optical transmission link, it can be adjusted to emulate the inverse transfer-function (a description of the system-induced distortion) of the link, thus canceling it out (equalizing it). The link's transmission characteristics are thus greatly improved. There are many possible configurations for DOE's. Dr. Avishay Eyal of the TAU Faculty of Engineering has been investigating a DOE approach based on a lattice of several variable-interference filters which, with relatively minor design modifications, can be used to equalize (correct) all four types of distortion. Many different DOE systems have been tried, including all-fiber, microoptics and planar lightwave circuit systems.
One interesting approach uses a series (cascade) of Mach-Zehnder interferometers, used as filters (Figure 1). By controlling the optical phase-difference between the arms of the interferometers, one can custom-tailor the desired response. If only one input and one output port of the device are used, it can equalize such scalar properties of the optical system as CD or WDL/G. If both input and both output ports are used, two-dimensional polarization characteristics of the system, such as PMD or PDL, can be mitigated.
To achieve optimal real-time equalization, the device needs to be initialized and then dynamically controlled. This requires efficient synthesis and control algorithms. Similar synthesis and control problems have been studied extensively in the framework of finite impulse-response filter design, and Dr. Eyal has successfully adapted tools and techniques from that field to design optimal equalizers for both PMD and WDL/G. In PMD equalization the goal is to minimize the differential group delay (DGD) between the fastest and slowest polarization components for each wavelength in the transmission band.
In a series of computer simulations, his new and quite novel approach provided excellent broadband compensation for PMD. A comparison of the residual (uncompensated) DGD before and after equalization by 0, 5, or 8 cascaded interferometric filters ("stages" in Figure 2) revealed the dramatic reduction in distortion achievable by the new TAU algorithm. The same figure also shows simulation results for a non-optimized equalizer. The improvement in DOE performance due to Dr. Eyal's optimization procedure is clearly visible.
