1. Field of the Invention
The invention is related to the field of communications, and in particular, to a communication system for processing received optical signals that carry user information.
2. Description of the Prior Art
Transmission technologies such as multiplexing are used to increase the bandwidth provided by optical fibers. One transmission technology called time division multiplexing (TDM) interleaves multiple channels or signals into one optical signal. Another transmission technology called wavelength division multiplexing (WDM) transmits multiple signals at different wavelengths, where low data rates are used on each wavelength. Commercial WDM systems are limited by channel spacing of about 50 GHz due to selectivity of optical filters and limitations in the wavelength stability of semiconductor lasers.
Another transmission technology called sub-carrier multiplexing (SCM) multiplexes multiple signals in the radio frequency (RF) domain and transmits on a single spectral carrier. In an SCM system, the receiver generates a composite electrical signal from the heterdyne beating between the carrier and the sub-carrier. SCM is advantageous because microwave devices are more mature than optical devices. Some of these transmission technologies are combined such as WDM/SCM optical systems to provide even greater transmission capacity.
Optical system that utilize these transmission technologies experience signal degradation such as non-linear crosstalk, chromatic dispersion, and polarization mode dispersion (PMD) especially with higher data rates and long distance transmissions. Optical systems with data rates of 10 Gb/s and higher require precise dispersion compensation and careful link engineering.
PMD results from birefringent properties in the optical fiber, where the material in the optical fiber displays two different indices of refraction due to asymmetries in the optical fiber. PMD varies in time with ambient temperature, fiber movement, and mechanical stress on the fibers. Thus, compensating for PMD can be difficult because of the time varying nature and randomness of PMD. PMD is measured like a vector quantity, where a differential group delay (DGD) is the magnitude of the vector and the principal state of polarization (PSP) are the direction. There are two PSPs associated with PMD. The two PSPs propagate at slightly different velocities with the distribution of signal power varying with time. The generally acceptable limit for DGD is about 15% of the bit time for the non-return-to-zero (NRZ) modulation format.
FIG. 1 depicts a PMD compensation system 100 in the prior art. The transmitter 102 transmits an optical signal to the polarization controller 106 via the optic fiber 104. The polarization controller 106 aligns polarization of the optical signal based on feedback links 126 and 128 from the compensation algorithm 124. The polarization beam splitter 110 then splits the optical signal by the two PSPs of the optical fiber into link 114 and the optical delay line 112. The optical delay line 112 is tunable and introduces a DGD that is equal to the instantaneous DGD from the PMD. After the two split optical signals are combined, the photodetector 116 converts the optical signal into a corresponding electrical signal. The bandpass filter 118, the square law detector 120, and the low pass filter 122 then modify the electrical signal. The compensation algorithm 124 then performs an algorithm to determine how much DGD to introduce into the optical signal and how to control the polarization controller 106.
One problem with the PMD compensation system 100 is the optical delay line 112. The optical delay line 112 uses a mechanical means to cover a sufficient DGD range. For example, an optical system with a 200 ps DGD requires a 60 mm dynamic range of physical delay for the tunable optical delay line, which is accomplished by motorized machines. One limitation of this tunable optical delay line with a motorized machine is the speed of tuning, which is inversely proportional to the tuning range. The optical delay line is slow and also bulky. Another problem is the long term reliability of mechanical moving parts of the optical delay line, which perform continuous adjustment in an operational optical system.
In another prior solution, two photodetectors convert the optical signals from the splitter into two electrical signals. An RF delay line delays one of the electrical signals similar to the optical delay line in FIG. 1 except this RF delay line is in the electrical domain. Unfortunately, this prior solution also experiences the same problems of the optical delay line such as reliability with the RF delay line.
In an optical SCM system, the data rate carried by each RF carrier is relatively low compared to a conventional TDM system. Therefore, PMD-induced signal distortion is not considered significant because the width of the data pulse is long. However, PMD-induced signal fading in SCM systems depends on the frequency of the sub-carrier, which is typically higher than the data rate.