Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-07-22
2003-02-04
Mullen, Thomas (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C385S037000
Reexamination Certificate
active
06515778
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to compensating for polarisation mode dispersion, particularly across the signal spectrum of an optical signal of an optical communications system, and to an optical element for providing a variable differential delay primarily but not exclusively for use in such polarisation mode dispersion compensation.
BACKGROUND TO THE INVENTION
Existing communications systems typically rely, for transmission over long distances, upon the use of nominally single mode optical fibres which carry optical signals and provide transmission of signal data at rates of 10 Gb/sec or more over distances of the order of 100 kilometers or greater. Although such fibres are nominally single mode, propagation of optical signals is generally characterised in such fibres by two orthogonally polarised HE
11
modes for which slightly different group velocities exist in the presence of birefringence.
An optical fibre may be regarded as a concatenation of birefringent elements through which the optical signal propagates. As the different modes of the signal will propagate through the fibre at different velocities due to the differing refractive indices experienced by the modes, the modes will become temporally separated.
For a given span of optical fibre, the difference in transmission time for these two modes (often referred to as “fast” and “slow” modes to indicate the difference in final propagation velocities) is termed polarisation mode dispersion (PMD).
FIG. 1
shows a schematic representation of this effect, with both the fast and slow modes
10
,
12
of the optical signal being launched into the optical fibre at time t. Both modes propagate along the fibre, with the faster mode
10
arriving at the end of the fibre span at time T and the slower mode
12
arriving at time T+dt, where dt is the difference in transit time between the signals due to polarisation mode dispersion.
FIG. 2
shows a part of a typical optical communication system that might include a polarisation mode dispersion compensator. An optical fibre (
1
) provides a transmission path for propagation.
It is known from U.S. Pat. No. 5,473,457 to analyse a received optical signal in a manner which permits the received optical signal to be separated into fast and slow mode components, the fast mode component then being subject to a compensating delay by means of transmission of both components through a polarisation maintaining optical fibre of pre-determined length and high polarisation dispersion to provide a differential delay, albeit a fixed differential delay.
FIG. 2
shows a part of a typical optical communication system that might include a polarisation mode dispersion compensator. An optical fibre (
1
) provides a transmission path for propagation of an optical signal from a polarised light emitting transmitter (
2
) to a receiver (
3
). This transmission path includes an erbium doped optical fibre amplifier (
4
), and, adjacent the receiver (
3
), a PMD compensator (
5
).
The optical fibre (
1
) is a nominally circular symmetric single mode fibre extending over a substantial distance. Over a distance of this length the departures from perfect circular symmetry of that fibre, for example as a result of bending strain, are liable to be of sufficient magnitude for the fibre to function as a concatenation of birefringent elements of random relative orientation, as described above. Consequently, such optical fibres require compensation for polarisation mode dispersion. Moreover, the birefringence of the fibres will change due to such effects as heating and cooling of the fibre, and changes in strains imparted to the fibre (particularly in overhead optical cables and cables affected by maintenance crews or other human intervention). Obviously, such systems require a PMD compensator that is variable.
It is known from WO97/50185 to compensate for PMD by splitting the received optical signal at the receiver into two polarisation states and to apply switched delays of different length to the separated components, thereby providing a variable delay, albeit a delay that is not continuously and smoothly variable, and that also requires a relatively complex optical switching configuration.
The inventor of the present invention has previously disclosed in U.S. Pat. No. 4,953,939 the use of an optical fibre chirped Bragg grating reflector in combination with a directional coupler to introduce a delay which is wavelength dependent because the periodicity of the Bragg grating varies with position along the fibre, so that different wavelengths are reflected from different positions along the fibre. In pending U.S. patent application Ser. No. 09/135,967 filed Aug. 18, 1998 (granted as U.S. Pat. No. 6,271,952), which is incorporated herein by reference, the present inventor provides an optical element for providing a variable differential delay and an associated method for polarisation mode dispersion compensation by separating the optical signal into fast and slow mode components and utilising a chirped Bragg reflector to provide a variable delay.
All of the above techniques relate to first order polarisation mode dispersion, as indicated in FIG.
1
. Each technique requires the use of polarisation state controllers, to align with and separate the different polarisation states, prior to providing a differential delay to the separate states. It would be advantageous to provide a PMD compensator that does not require such a polarisation state controller.
GB2184252 discloses how the use of squeezer elements applying stress to an optical fibre may be used to change the birefringence of the fibre to produce an optical state-of-polarisation modulator.
The differential group delay is dependent upon the wavelength of the optical signal. The mechanism by which the PMD value of a fibre is seen to vary across the signal spectrum is often referred to as higher order PMD. As the differential group delay (DGD) due to PMD is not a single value across the signal bandwidth, accurate PMD compensation cannot simply be achieved by introducing a single value of differential delay in opposition to the differential group delay induced by the PMD of the fibre (as is described in the above techniques). For low values of PMD, the dispersion does indeed approximate to a single value across the signal bandwidth, thus permitting the use of such simple compensator designs as described above. However, for larger PMD values there exists a need to provide an improved method of providing a continuously variable optical delay and for compensating for polarisation mode dispersion in optical fibres across the signal spectrum.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides an apparatus for compensating for polarisation mode dispersion, the apparatus comprising a chirped Bragg reflector extending longitudinally along an optical waveguide, said waveguide being susceptible to stress birefringence; and at least one tuning means located at a position along the length of said waveguide, said tuning means being operable, in use, to apply a stress to said waveguide so as to alter the magnitude and the orientation of the birefringence of said waveguide for compensation of the polarisation mode dispersion of the optical signal.
It is well known that with low chirp fibre gratings, quite low levels of birefringence, can cause significant PMD. This invention exploits this fact and provides a means of compensating for differing PMD. This approach has the advantage over PMD compensation techniques described in the prior art, in so far as it does not require a separate polarisation state controller to align with the different states prior to applying different delays to the orthogonal states.
Preferably, the apparatus comprises at least two of said tuning means, each tuning means being located at a different position along the length of said waveguide. Advantageously, such an apparatus may be used to affect the birefringence of the waveguide so as to compensate for the polarisation mode dispersion caused by the birefringence of
Epworth Richard Edward
Watley Daniel A
Lee Mann Smith McWilliams Sweeney & Ohlson
Mullen Thomas
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