Apparatus and method for polarization mode dispersion emulation and compensation

• US 6,538,787 B1
• Filed: 09/24/1999
• Issued: 03/25/2003
• Est. Priority Date: 09/24/1999
• Status: Expired due to Term

• Current Assignee: WSOU Investments, LLC (WSOU Holdings, LLC)
• Original Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Inventors: Kogelnik, Herwig, Moeller, Lothar
• Primary Examiner(s): TONG, NINA C
• Application Number:
  US09/404,892
• Time in Patent Office: 1,278 Days
• Field of Search: 359/161, 359/158, 359/177, 359/183, 359/156, 359/249, 359/140, 385/37, 385/11, 385/27, 385/28, 372/25, 372/27, 372/29.01, 372/30
• US Class Current: 398/158
• CPC Class Codes: G02B 6/272 comprising polarisation means for beam splitting and combining
• CPC Class Codes: G02B 6/274 based on light guide birefringence, e.g. due to coupling between light guides G02B6/105, G02B6/024 take precedence; mechanically induced birefringence G02F1/0134
• CPC Class Codes: G02B 6/278 Controlling polarisation mode dispersion [PMD], e.g. PMD compensation or emulation PMD minimised transmission systems H04B10/2569
• CPC Class Codes: G02B 6/2861 using fibre optic delay lines and optical elements associated with them, e.g. for use in signal processing, e.g. filtering delay lines in general H03H, H01P9/00; optical computing devices G06E
• CPC Class Codes: G02B 6/29352 in a light guide
• CPC Class Codes: G02B 6/29395 configurable, e.g. tunable or reconfigurable switching G02B6/35
• CPC Class Codes: H04B 10/2569 due to polarisation mode dispersion [PMD]

Abstract

A polarization mode dispersion (PMD) emulator may include one or more modular “cells” that emulate PMD. Each of the cells may include optical delay and phase modulation components. The optical delay and/or the phase modulation components may be adjusted to account for differences in PMD and two or more of the cells may be combined to further adjust the overall PMD of the apparatus. Similarly, a PMD compensator may include one or more modular “cells” that compensate for PMD, with each of the cells including optical delay and phase modulation components. The optical delay and/or the phase modulation components may be adjusted to compensate for various PMD values and two or more of the cells may be combined to further adjust the overall PMD compensation of the apparatus. The PMD compensator apparatus may be used to compensate for PMD in wideband applications, such as wavelength division multiplexed (WDM) systems.

Claims

1. Apparatus for compensating the polarization mode dispersion (PMD) of an optical signal imparted by a device, said device characterized by a transfer function U, said apparatus comprising:

a first mode mapper responsive to the reception of a randomly polarized optical signal by dividing the signal into orthogonally polarized component signals and routing each of the component signals into a separate path, with one of the separate paths configured to rotate the polarization state of one of the component signals until it is parallel to the polarization of the other component signal;

a lossless two by two filter network connected to receive the component signals from the mode mapper, the filter network being characterized by a transfer function U<sup>−

1</sup> that is the inverse of the transfer function of the device which imparts polarization mode dispersion to the optical signal; and

a second mode mapper responsive to the reception of a the orthogonally polarized component signals by combining the signals into an optical signal characterized by substantially less PMD than the optical signal received at the first mode mapper.

2. Apparatus of claim 1 wherein said first mode mapper comprises:

a polarization beam splitter configured to split an incoming, randomly polarized optical signal into separate orthogonally polarized component signals; and

first and second optical paths connected to receive the orthogonally polarized component signals from the polarization beam splitter, one of the optical paths including a polarization rotator configured to rotate the polarization of the component signal travelling in the optical path to an orientation that is parallel with that of the orthogonal component signal within the other optical path.

3. The apparatus of claim 2 wherein the polarization rotator is an optical fiber that is rotated through 90°

along its length.

4. The apparatus of claim 2 wherein each of the first and second optical paths comprises a polarization maintaining optical fiber (PMF).

5. The apparatus of claim 1 wherein the filter network comprises at least two filter cells connected in series.

6. The apparatus of claim 5 wherein each cell within the filter network comprises a Mach-Zehnder Interferometer (MZI).

7. The apparatus of claim 6 wherein each MZI comprises two optical paths, with one optical path providing a first delay relative to the other optical path.

8. The apparatus of claim 7 wherein each MZI further comprises a phase modulator that produces an adjustable delay, of lesser magnitude than the first delay, within one optical path relative to the other optical path.

9. The apparatus of claim 8 wherein the filter cells are connected in series and include one or more Mach-Zehnder switches connected to select among geometrical scaled delay lines.

10. The apparatus of claim 8 wherein the filter network exhibits a transfer function that is an artificial inverse jones matrix (JM) created by PMD function interleaving.

11. The apparatus of claim 10 wherein the artificial inverse JM is constructed with a period of N(Bhc)−

Bsi, where N is an integer, Bhc is the channel spacing of the WDM system, and Bsi is the bandwidth of the signal for which PMD is being compensated.

12. Apparatus of claim 1 further comprising:

a monitoring network connected to receive a portion of at least one of the signals within the second mode mapper and to produce a monitoring output signal; and

a modulating input connected to the filter network to respectively modulate the phases of first and second orthogonally polarized signals within the filter network by +φ

+ sin(ω

τ

) and −

φ

−

sin(ω

τ

).

13. Apparatus of claim 12 further comprising a feedback circuit connected to control the modulation of the phases of the first and second orthogonally polarized signals in response to variation in the monitoring output signal.

14. Apparatus of claim 1 wherein said first mode mapper is implemented as an inegrated optical device.

15. Apparatus of claim 1 wherein one or more component elements of said lossless two by two filter network is implemented as an integrated optical device.

16. Apparatus of claim 1 wherein one or more elements of said second mode mapper is implemented as an integrated optical device.

17. A polarization mode dispersion emulator comprising:

a first optical network exhibiting the transfer matrix $R^{+1}<br><br>\left(\mathrm{\omega <br><br>}\right)=\left[\begin{array}{cc}\cos <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)& +\sin <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)\\ -\sin <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)& \cos <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)\end{array}\right]$

a second optical network connected to receive the output of the first optical network and exhibiting the transfer matrix $D<br><br>\left(\mathrm{\omega <br><br>}\right)=\left[\begin{array}{cc}d<br><br>\left(\mathrm{\omega <br><br>}\right)& 0\\ 0& d^{*}<br><br>\left(\mathrm{\omega <br><br>}\right)\end{array}\right];<br><br>$

and

a third optical network connected to receive the output of the second optical network and exhibiting the transfer matrix $R^{-1}<br><br>\left(\mathrm{\omega <br><br>}\right)=\left[\begin{array}{cc}\cos <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)& -\sin <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)\\ +\sin <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)& \cos <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)\end{array}\right]$

 

where;

d(ω

)=exp(j(Δ

τ

₀ω

+(1/2)Δ

τ

₁ω

²)/2).

18. A method of compensating for polarization mode dispersion (PMD) imparted to an optical signal by a device characterized by a transfer function U, said method comprising the steps of:

(A) in a first mode mapper, mapping the polarization modes of a randomly polarized optical signal by dividing the signal into orthogonally polarized component signals and routing each of the component signals into a separate path, with one of the separate paths configured to rotate the polarization state of one of the component signals until it is parallel to the polarization of the other component signal;

(B) passing the mode-mapped component signals from the first mode mapper through a lossless two by two filter network connected to receive the component signals from the first mode mapper, the filter network being characterized by a transfer function U<sup>−

1</sup> that is the inverse of the transfer function of the device which imparts polarization mode dispersion to the optical signal; and

(C) in a second mode mapper, mapping the polarization modes of the filtered component optical signals received from the filter network by rotating one of the filtered component signals to a polarization state orthogonal to that of the other filtered component signal and combining the component signals in a polarization beam combiner.

19. The method of claim 18 wherein step (A) comprises the steps of:

(A1) splitting the randomly polarized optical signal in a polarization beam splitter into separate orthogonally polarized component signals

(A2) coupling the orthogonally polarized component signals from the polarization beam splitter into separate optical paths; and

(A3) rotating the polarization of one of the component signals in a polarization rotator to a polarization that is parallel to that of the component signal within the other optical path.

20. The method of claim 19 wherein the component signal is rotated in step (A3) by a polarization maintaining optical fiber that is rotated through 90°

along its length.

21. The method of claim 18 wherein the filter network of step (B) comprises a network of cells, each of which comprises a Mach-Zehnder Interferometer (MZI).

22. The method of claim 21 wherein step (B) comprises the step of:

(B1) passing each component signal through a separate path of a MZI, with one optical path providing a first delay relative to the other optical path.

23. The method of claim 22 wherein step (B) comprises the step of:

(B2) adjusting a delay, of lesser magnitude than the first delay, within one optical path relative to the other optical path.

24. The method of claim 23 wherein step (B) comprises the step of:

(B3) switching the component signals through geometrical scaled delay lines, using Mach-Zehnder switches to select among geometrical scaled delay lines.

25. The method of claim 23 wherein step (B) further comprises the step of:

(B4) interleaving a wavelength division multiplexed signal to produce a filter network characterized by an artificial inverse jones matrix (JM).

26. The method of claim 25 wherein step (B4) comprises the step of:

(B5) constructing the artificial inverse JM with a period of N(Bhc)−

Bsi, where N is an integer, Bhc is the channel spacing of the WDM system, and Bsi is the bandwidth of the signal for which PMD is being compensated.

27. The method of claim 18 further comprising the step of:

(D) tapping a portion of at least one of the signals within the second mode mapper to produce a monitoring output signal; and

(E) respectively modulating the phases of first and second orthogonally polarized signals within the filter network by +φ

+ sin(ω

_mτ

) and −

φ

−

sin(ω

_mτ

), where ω

_m is a monitoring signal frequency.

28. The method of claim 27 further comprising the step of:

(F) feeding back the monitoring output signal to control the modulation of the phases of the first and second orthogonally polarized signals.

29. A method of emulating polarization mode dispersion comprising the steps of:

(A) rotating an optical signal in an optical network characterized by a frequency dependent rotatation matrix $R^{+1}<br><br>\left(\mathrm{\omega <br><br>}\right)=\left[\begin{array}{cc}\cos <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)& +\sin <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)\\ -\sin <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)& \cos <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)\end{array}\right]$

(B) delaying the principle states of polarization of the signal in an optical network characterized by a frequency dependent rotation matrix $D<br><br>\left(\mathrm{\omega <br><br>}\right)=\left[\begin{array}{cc}d<br><br>\left(\mathrm{\omega <br><br>}\right)& 0\\ 0& d^{*}<br><br>\left(\mathrm{\omega <br><br>}\right)\end{array}\right];<br><br>$

and

(C) rotating an optical signal in an optical network characterized by a frequency dependent rotatation matrix $R^{-1}<br><br>\left(\mathrm{\omega <br><br>}\right)=\left[\begin{array}{cc}\cos <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)& -\sin <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)\\ +\sin <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)& \cos <br><br>\left(k<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\omega <br><br>}\right)\end{array}\right]$

 

where;

d(ω

)=exp(j(Δ

τ

₀ω

+(1/2)Δ

τ

₁ω

²)/2).