Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-08-18
2001-08-07
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06271952
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to compensating for polarisation mode dispersion in 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 10 Gb/sec or more over distances of the order of 100 km. Although such fibres are nominally single mode, propagation of optical signals is generally characterised in such fibres by two orthogonally polarised HE
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modes for which slightly different group velocities exist in the presence of birefringence.
For a given span of optical fibre, the difference in transmission time for these modes is termed polarisation mode dispersion.
For the given span of optical fibre, it is possible to define a pair or orthogonal principal polarisation states such that an optical pulse launched into the fibre in only one of the principal polarisation states will be received at the other end of the fibre without polarisation mode dispersion being evident, the principal polarisation states therefore representing the fast and slow axis modes of propagation. In practical systems however, it is difficult to control the launch state to always correspond to one of the principal polarisation states so that an optical signal typically comprises the sum of fast and slow mode components.
Environmental factors affecting the optical fibre produce variation over time in the birefringence effects causing polarisation mode dispersion and the resulting dispersion is observed to vary relatively slowly for fibres in buried cables and more quickly for fibres contained in overhead cables.
It is known from U.S. Pat. No. 5473457, Ono, to analyze a received optical signal in a manner which permits the principal states of polarisation to be determined and the received pulse 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 predetermined length and high polarisation dispersion to provide a differential delay. This technique however has the disadvantage of making available only a fixed amount of compensation and therefore does not allow variable compensation of polarisation mode dispersion suitable for a practical communications system. A further disadvantage is that a delay element providing optical delay by transmission via a fibre will typically require a relatively long length of fibre in the range 10 to 100 meters.
It is known from WO 97/50185 to compensate for polarisation mode dispersion 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. A disadvantage of this system is that the delay is not continuously and smoothly variable and also requires a relatively complex optical switching configuration.
The inventor of the present invention has previously disclosed in U.S. Pat. No. 4953939 the use of a 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. The inventor has also disclosed in U.S. Pat. No. 5602949 a technique for manufacturing a suitable chirped fibre by producing strain in a non-chirped fibre for this purpose. GB-A-2316761 discloses a method of directly writing a chirped grating into a fibre for the purpose of compensating chromatic dispersion.
There remains a need to provide an improved method of providing a continuously variable optical delay and for compensating for polarisation mode dispersion in optical fibres.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and apparatus of compensating for polarisation mode dispersion in a manner which adapts continuously to environmental changes affecting birefringence of the optical fibre.
It is a further object of the present invention to provide an improved optical element for providing a variable delay in an optical signal.
According to the present invention there is disclosed apparatus for compensating for polarisation mode dispersion in an optical signal transmitted via an optical waveguide of an optical communications system, the apparatus comprising;
separating means operable to separate the optical signal into first and second components having a dispersion delay therebetween;
delay means operable to delay the first and second components by respective delays differing by a delay increment;
recombining means operable to recombine the delayed first and second components to form an output signal; and
control means operable to control the delay increment such that in the output signal the dispersion delay is substantially compensated by the delay increment;
wherein the delay means comprises at least one delay line having a chirped Bragg reflector extending longitudinally of the delay line, wherein said respective delay results from a time taken for said respective component to traverse the delay line to and from a reflection position at which Bragg reflection occurs, and wherein the control means is operable to variably regulate at least one parameter determining the conditions under which Bragg reflection occurs to variably define the reflection position relative to the Bragg reflector.
Preferably the parameter controlled by the control means is a characteristic of spatial periodicity as a function of position of a chirped refractive index grating formed in an optical medium of the Bragg reflector.
Conveniently the control means produces strain in the optical medium of the Bragg reflector, thereby providing a shift in the position of the Bragg reflection which is sensitive to a control signal to an actuator producing the strain.
Conveniently the Bragg reflector is formed as a refractive index grating in an optical fibre and the control means may comprise an electromechanical actuator deforming the fibre or a temperature controller allowing dimensions of the fibre to be regulated by thermal effects.
The optical fibre may comprise a sampled grating having a series of pass bands giving corresponding positions of Bragg reflection for different wavelengths of a wavelength division multiplexed optical signal.
REFERENCES:
patent: 5715265 (1998-02-01), Epworth
patent: 5822100 (1998-10-01), Robinson et al.
Takahashi, J., et al., “Automatic Compensation Technique for Timewise Fluctuating Polarisation Mode Disperson in in-line Amplifier Systems,” Electronics Letters, Feb. 17th, 1994, vol. 30, No. 4, pp. 348-349.
Yoshimura, et al., “Polarization Mode Disperson Equalization,” Fifth Optoelectronics Conference (Dec. 1994) Technical Digest, Jul. 1994, Makuhari Messe, pp. 258-259.
Lee Mann Smith McWilliams Sweeney & Ohlson
Negash Kinfe-Michael
Nortel Networks Limited
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