1. Field of the Invention
The present invention relates generally to dispersion compensation, and particularly to an integrated system for providing chromatic dispersion compensation and polarization mode dispersion compensation.
2. Technical Background
Chromatic dispersion (CD) occurs because each wavelength of light travels through a given medium, such as an optical fiber, at a different speed. Since the various wavelengths of light have different velocities, a given wavelength of light will arrive at a receiver disposed at the end of a transmission fiber before another wavelength of light will arrive at that receiver. The time delay between different wavelengths of light leads to pulse broadening. Chromatic dispersion is obtained in an optical fiber by measuring fiber group delays in the time domain. Chromatic dispersion is a relatively stable phenomenon. FIG. 1 is a chart showing chromatic dispersion wavelength dependence. As shown, the main contribution is refractive index dispersion. CD wavelength dependence can be used to predict CD effects for different wavelength channels. Thus, passive chromatic dispersion compensation is relatively easy to perform.
FIG. 2 is a chart showing chromatic dispersion penalty curves for some ideal receiver at transmission rates of 10 G/s and 40 Gb/s. The curves shown in FIG. 2 correspond to a given bit error rate (BER=10−13). Chromatic dispersion can be in the range of 300-500 psec in a 10 Gb/s system before incurring a 1 dB power penalty. In a 40 Gb/s system, the range decreases to 18-25 psec.
FIG. 3 is a chart showing sensitivity characteristics for chromatic dispersion for different receivers. Typically, chromatic dispersion compensation is adjusted to the optimized operating region of each receiver, as shown in FIG. 3.
Polarization is a critical parameter in optical communications. The fundamental mode of a single mode optical fiber is the solution to the wave equation that satisfies the boundary conditions at the core-cladding interface. Although this appears to be counter-intuitive, there are two solutions to the wave equation that correspond to the fundamental mode. The fiber is deemed to be a single mode fiber because both solutions have the same propagation constant. The two solutions are referred to as the polarization modes. These polarization components are mutually orthogonal. The state of polarization refers to the distribution of light energy between the two polarization modes. In practice, since the cross-sectional area of a fiber is not perfectly circular, the two polarization modes have slightly different propagation constants that give rise to pulse spreading. One polarization mode is referred to as the “fast-mode,” and the other polarization mode is known as the “slow-mode.” The fast mode and the slow mode mix as they travel down the fiber, becoming indistinguishable. The resulting difference in propagation time between polarization modes is known as the differential group delay (DGD).
FIG. 4 is a chart showing the wavelength dependency of polarization mode dispersion. A comparison of FIG. 1 and FIG. 4 reveals that PMD wavelength dependency is much more complicated than CD wavelength dependency. CD can be time variant as a result of changes with temperature or stress, but typically, the time variance of CD is not particularly strong. PMD, on the other hand, is very time variant, and thus, compensation should track with time. PMD describes the statistical broadening of optical pulses within an optical fiber caused by polarization effects. This broadening effect, similar to pulse broadening from chromatic dispersion, ultimately prevents the correct detection of the waveform at the receiver. DGD is usually described statistically using Maxwell's distribution. As discussed above, PMD is unlike chromatic dispersion because DGD fluctuates with time, wavelength, environmental conditions and with other parameters. The statistical behavior of PMD makes passive PMD compensation ineffective.
PMD is the major limiting factor for high bit-rate transmissions. FIG. 5 shows approximate PMD power penalty curves for 10 Gb/s and 40 Gb/s optical transmission speeds. These curves are approximate because a real curve depends on various transmitter and receiver properties. A PMD power penalty of 1 dB corresponds to an instantaneous DGD value of 0.4 times the bit period. For a 10 Gb/s system this translates to approximately 40 psec. For a 40 Gb/s system it translates to about 5 psec, which is significantly lower.
Currently, chromatic dispersion and PMD are compensated for separately. Optical transmission link power or signal-to-noise ration (SNR) budgeting typically takes into account CD and PMD caused penalties separately. However, a more accurate chromatic dispersion compensation allows for a higher PMD penalty on the link and vice-versa. What is needed is an integrated approach to chromatic dispersion and PMD compensation. What is needed is an integrated dispersion compensation method that is performed to optimize a specific receiver's performance. Further, the optimization should be specific to a given transmitter-receiver combination.