Optical communication system and apparatus for compensation or emulation of PMD effects
An apparatus for compensation or emulation of polarization mode dispersion (PMD) effects occurring in an optical signal traveling over an optical fiber based link comprises controllable birefringence sections and devices for polarization conversion. Each section comprises birefringence optical members or parts thereof, and each section is followed or preceded by one of the controllable devices for polarization conversion. The birefringence sections are at least three in number, and at least one of these has a propagation delay different from that of the other sections. The apparatus comprises a detection device for PMD output and control of the controllable devices for feedback adjustment.
1. An apparatus for compensating or emulating polarization mode dispersion (PMD) effects occurring in an optical signal traveling over an optical fiber based link, comprising: birefringence sections, controllable polarization transformation devices, each birefringence section including birefringence optical members or parts thereof, each birefringence section being followed or preceded by one of the controllable polarization transformation devices, the birefringence sections being at least three in number, at least one of the birefringence sections having a propagation delay time different from that of the other birefringence sections, and two of the birefringence sections having propagation delay times of around T c and αT c respectively, and the remaining birefringence sections having a total propagation delay time of around (1−α) T c with T c such that
DGD
_
4
≤
T
c
≤
2
DGD
_
where {overscore (DGD)} is an average differential group propagation time of the PMD to be compensated or emulated and with 0<α<1.
2. The apparatus in accordance with claim 1 , characterized in that each birefringence section has a propagation delay time different from that of the other birefringence sections.
3. The apparatus in accordance with claim 1 , characterized in that the birefringence sections are arranged in series by decreasing order of the propagation delay time in a direction of propagation of the signals over the birefringence sections.
4. The apparatus in accordance with claim 1 , characterized in that 0.5<α<0.7.
5. The apparatus in accordance with claim 1 , characterized in that the devices are at least one polarization controller and two polarization rotators.
6. The apparatus in accordance with claim 5 , characterized in that a first of the birefringence sections is preceded by the polarization controller, and a second of the sections is preceded by one of the two rotators, and a third of the birefringence sections is preceded by the other of the two rotators.
7. The apparatus in accordance with claim 6 , characterized in that a fourth of the birefringence sections is preceded by another polarization controller.
8. The apparatus in accordance with claim 7 , characterized in that the third and the fourth birefringence sections have propagation delay times of β(1−α) T c and (1−β) (1−α) T c respectively with 0<α<1, and with 0<α<1.
9. The apparatus in accordance with claim 8 , characterized in that 0.5<α<0.7.
10. The apparatus in accordance with claim 8 , characterized in that β is between 0.5 and 0.95.
11. The apparatus in accordance with claim 10 , characterized in that β is around 0.9.
12. The apparatus in accordance with claim 1 , characterized in that each birefringence section has a propagation delay time such that there is at least one condition of adjustment of the devices for which DGD·β=ε on all frequencies within a bandwidth on which the apparatus is made to operate, with DGD equal to a differential delay time assumed by the apparatus, and with ε expressed in radians such that 0≦ε≦2.5 and 0<β1.
13. The apparatus in accordance with claim 12 , characterized in that ε is zero.
14. The apparatus in accordance with claim 7 , and comprising a detection and control device for detecting output PMD and controlling the controllable polarization transformation devices, the detection and control device receiving at an input a signal output from the apparatus, and sending feedback adjustment control signals for the controllable polarization transformation devices to keep the detected output PMD below a predetermined amount.
15. The apparatus in accordance with claim 14 , characterized in that the detection and control device privileges adjustment of the other polarization controller which precedes the fourth birefringence section in such a manner as to send control signals to change only the adjustment of the other polarization controller when the detected output PMD increases more than the predetermined amount by an amount falling within adjustment range of the other polarization controller and of the fourth birefringence section.
16. An optical communication system using an apparatus for compensating or emulating polarization mode dispersion (PMD) effects occurring in an optical signal traveling over an optical fiber based link, the apparatus comprising: birefringence sections, controllable polarization transformation devices, each birefringence section including bi refringence optical members or parts thereof, each birefringence section being followed or preceded by one of the controllable polarization transformation devices, the birefringence sections being at least three in number, at least one of the birefringence sections having a propagation delay time different from that of the other birefringence sections, and two of the birefringence sections having propagation delay times of around T c and αT c respectively, and the remaining birefringence sections having a total propagation delay time of around (1−α) T c with T c such that
DGD
_
4
≤
T
c
≤
2
DGD
_
where {overscore (DGD)} is an average differential group propagation time of the PMD to be compensated or emulated and with 0<α<1.