Special Report: Next Generation Internet Applications

Optical Dispersion Compensation Longer wavelengths tend to propagate faster than shorter wavelengths. As optical signals travel through long fiber runs, a significant time offset proportional to the distance develops between the different wavelengths. Unfortunately, since all real optical pulses contain a small range of wavelengths around the intended wavelength, different components of the optical pulse travel at slightly different speeds. When this occurs, the optical pulse widens, possibly overlapping with other optical pulses and corrupting the optical signal. Known as "dispersion," this optical side effect limits the maximum transmission rate of the fiber by forcing an increase in the optical pulse separation. Figure 9 depicts the effects of optical dispersion on transmission rate.

Signals in long fiber runs encounter several forms of dispersion. Chromatic dispersion is caused by the variation in the propagation speed through a fiber as a function of wavelength. Modal dispersion occurs in multi-mode fiber and results from the different distances, or modes, traveled by the optical signals. Polarization Mode Dispersion (PMD) occurs when the orthogonal optical pulses in a fiber arrive at different times due to physical imperfections in the fiber core.

Dispersion effects must be compensated for to extend the bandwidth and range of optical fibers. One technique for dispersion compensation is dispersion-shifted fiber that compensates for the chromatic dispersion by canceling its effect with waveguides. Another compensation technique is the inclusion of opposing dispersive elements within the optical chain to strategically cancel the effects of dispersion. Other methods being developed include dispersion-flattened fiber and dispersion-optimized fiber.

2.2.3 Wavelength Conversion Wavelength conversion is a critical optical networking component that will allow routing to be achieved entirely within the optical domain. This technology will complement optical switching, by providing additional functionality within high-speed core networks. All-optical wavelength conversion reduces the wavelength contention issue in photonic switching. Optical switching is wavelength dependent; therefore, only one information stream can be transmitted on any given wavelength throughout the network segment at a given time. Wavelength conversion provides a method for translating from one wavelength to another, allowing the optical switch to complete the operations without blocking either source. Having this performed completely in the optical domain provides a speed benefit and minimizes corruption losses due to the elimination of the OEO conversion process.

Figure 10 provides an illustration of the contention issue faced by photonic switches. Input Signal A: Wavelength A1 Input Signal B: Wavelength A1 Input Signal C: Wavelength A1 Input Signal A: Wavelength A1 Input Signal B: Wavelength A1 Input Signal C: Wavelength A1 Output Signal A: Wavelength A1 Output Signals B and C Blocked Output Signal A: Wavelength A1 Output Signal B: Wavelength A2 Output Signal C: Wavelength A3

Currently, wavelength conversion is achieved via an OEO conversion process, which adversely affects transmission speed due to conversion. Laser modulation is time consuming and hinders a system's ability to be rapidly reconfigured in the event of optical channel corruption. Work is underway to move the wavelength conversion process to the optical domain in an effort to simplify network design and management. Research for all-optical wavelength conversion focuses on advanced semiconductor design for exploiting substrate nonlinearities and optical properties. Methods include cross-gain modulation, four-wave mixing, and cross-phase modulation.[4]

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