Optical waveguides – Polarization without modulation
Reexamination Certificate
2002-04-05
2004-05-11
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Polarization without modulation
C385S024000, C385S029000, C359S490020
Reexamination Certificate
active
06735351
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is based on a priority application DE 101 21 025.6 which is hereby incorporated by reference.
The invention relates to a method and a device for adjusting the polarization of an incoming optical signal to a defined polarization state using modulated-up data, with a polarization divider, which processes the incoming optical signal such that two optical signals with orthogonal polarization states are present at its output, with a series-connected first phase setter in the signal path of at least one of the two output signals from the polarization divider, with a first optical coupler, which distributes the light of the two output signals respectively in roughly equal parts to two outputs, with a second phase setter in the signal path of at least one of the two output signals from the first optical coupler, and with a series-connected second optical coupler, an actuating signal being supplied to the first and second phase setter respectively in a feedback loop, said actuating signal being obtained from a quality signal derived from the processed optical signal.
A method of this kind is known for example from the conference papers of T. Ozeki and T. Kudo, “Adaptive equalization of polarization-mode dispersion”, Techn. Dig. OFC'93, W19, 1993, pp. 143-144 or by L. Möller, “Broadband PMD Compensation in WDM Systems”, proc. ECOC 2000, Sep. 3-7, Munich, Germany, 2000, P 1.15.
Electromagnetic waves in the frequency range of visible light are frequently used today to transmit signals, in particular data signals. In this case, as known from high-frequency engineering in the radio wave range, a data signal is modulated onto a carrier signal. On the optical transmission path, which normally comprises mirrors, optical fibres and other dispersive elements, distortions of the optical signal transmitted occur that can lead to a corruption of or fault in the data transmitted. Such distortions derive for example from chromatic dispersion or also from polarization mode dispersion (=PMD).
In systems with 40G channel rate transmission over connection lengths of several hundred kilometers, for example, operators of optical communications networks are now using very many optical fibres with such a high PMD that dispersion compensation with the features described at the beginning is essential. This is normally realized by a dispersion compensator installed on the receiver side in the transmission network.
Thus in the article by L. Möller quoted at the beginning, a PMD compensator is described for example that can compensate at least partly for distortions on the receiver side owing to PMD effects.
Hitherto only effects of a first and at best low orders can be processed using these known compensators.
Simple arrangements such as described for example in
F. Heismann et al., “Automatic compensation of first-order polarization mode dispersion in a 10 Gb/s transmission system”, proc. ECOC'98, WdC11, 1998, and in F. Roy et al., “A simple dynamic polarization mode dispersion compensator”, Techn. Dig. OFC/IOOC'99, 1999, TuS4, only compensate for first-order PMD. Thus fibre PMD can be compensated for a maximum of roughly 35% of the bit duration. This value (35 ps for 10 Gb/s) will be too small in particular for 40 Gb/s systems, as it signifies a PMD of only 8.8 ps.
If this PMD limit value is exceeded, then distortions of a so-called higher order occur. Initial equalizer arrangements for even higher PMD orders are described for example in the article by L. Möller quoted above. On the one hand, it is evident that as the orders increase, the outlay on signal processing increases sharply, and on the other hand, sufficiently swift adaptation of such an arrangement with many free parameters is not guaranteed.
The methods defined above and related devices can be used as circuits for adjusting the polarization even of non-PMD-distorted optical signals, which therefore arrive only with one defined polarization state. A disadvantage of the solutions known from the prior art and described above here is that only a single quality signal is used in each case to optimize several manipulated variables. To adjust the polarization, a sweep method with a maximum search is used in each case here, which on the one hand is unwieldy and on the other hand works imprecisely, as an optical maximum is normally difficult to locate exactly. The known methods are also very time-consuming and require an expensive logic circuit on the apparatus side for control or adjustment purposes.
By comparison, the object of the present invention is to develop a method and a device of the type described at the beginning with the simplest possible means to the effect that only a single related quality signal is used for each manipulated variable, that instead of a maximum search, optimization can be effected by a search for a minimum signal or a balance, that the quality signals used are bipolar and thus carry information regarding the signal direction, that the corresponding adjustment method can be executed more quickly and simply without complicated sweeping (“dithering”) of the manipulated variables and that a control logic that is as inexpensive as possible to implement can be used for the feedback.
This object is achieved according to the invention in a manner that is as surprisingly simple as it is effective in that a subtraction element is provided, to which the two parallel optical signals are supplied from a point of their signal paths after the first optical coupler, a bipolar quality signal being obtained in the subtraction element from the difference of the two signals supplied, and that a control element is connected downstream of the subtraction element, which control element generates an actuating signal to one of the phase setters that adjusts the phase setter such that the amount of the quality signal is minimized and in particular adjusted to zero.
Due to the simultaneous and bipolar adaptation of all manipulated variables of the optical processor part of the device according to the invention, it is possible on the one hand to react to changing PMD distortion by orders of magnitude more quickly (in 10 is instead of in 10 ms). The invention also facilitates the adjustment of equalizers of PMD of higher orders (higher number of manipulated variables) if multiple feedbacks are used. The technical structure of the device according to the invention is very compact and can be executed in an integrated optical manner, so that it is suitable for DWDM systems, which require a dedicated PMD compensator for each wavelength. An embodiment of the device according to the invention in which the control element is executed as an integrator, which integrates the quality signal supplied timewise and forwards it as an actuating signal to the phase setter, is particularly preferred. The device according to the invention can thus be executed in a particularly simple and compact technical manner.
The advantages of the present invention can be utilized in one embodiment also for a device with a further phase setter if a second subtraction element with a series-connected second control element is provided that supplies actuating signals for the second phase setter.
A development of this embodiment in which the signals supplied to the first and second subtraction element are tapped at the same points in the signal paths of the parallel optical signals between the first and the second optical coupler or between the second phase setter and the second optical coupler is particularly compact.
An alternative embodiment provides that signal tapping for the two input signals into the first subtraction element takes place from a point in the signal path of both parallel optical signals following the first optical coupler and before the second phase setter or a point between the first and the second optical coupler and signal tapping for the two input signals into the second subtraction element takes place from a point after the second phase setter and before the second optical coupler or a point between
Alcatel
Pak Sung
Sughrue & Mion, PLLC
LandOfFree
Fast-adapting PMD compensator does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Fast-adapting PMD compensator, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fast-adapting PMD compensator will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3240499