Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2000-08-21
2002-10-15
Spyrou, Cassandra (Department: 2872)
Optical waveguides
With optical coupler
Plural
C359S199200, C359S341100, C359S337200
Reexamination Certificate
active
06466712
ABSTRACT:
TECHNICAL FIELD
The present invention relates to optical fiber transmission links and, more particularly, to the such links as used in WDM optical transmission systems.
BACKGROUND OF THE INVENTION
Stimulated Raman scattering (SRS) is one of the major nonlinear optical processes that can impair the performance of optical fiber transmission links. SRS causes energy exchange between light components of different frequencies propagating in a medium such as an optical fiber.
One impact of SRS on wavelength-division-multiplexed (WDM) transmission links is known as SRS crosstalk, in which energy is transferred from at least one channel of higher frequency to at least one channel of lower frequency. In principle, SRS crosstalk can be eliminated via spectral inversion techniques.
Another impact of SRS on both single and multiple channel transmission links is referred to as channel depletion. Channel depletion occurs because any “optical noise” present in the transmission link will be amplified by the channels via SRS. The origins of the optical noise may be side modes of the channels or the optical noise created by spontaneous Raman scattering of the channels. The amount of noise amplification depends on the frequency shift of the optical noise away from the channels and on the channel powers. In fused silica fibers, the maximum noise amplification occurs roughly 13 THz away from the channels. For example, a channel with a wavelength of 1550 nm will amplify optical noise preferentially around ~1660 nm. In a transmission link with high aggregate channel power (such as in a massive WDM link), the noise amplification can be large enough to cause significant channel depletion. Thus, a need remains in the art for overcoming the problems associated with optical noise in optical transmission inks.
SUMMARY OF THE INVENTION
The need remaining in the prior art is addressed by the present invention, which relates to optical transmission links and, more particularly, to the inclusion of one or more high pass optical filters in the transmission link to reduce the channel depletion due to stimulated Raman scattering (SRS).
In accordance with the present invention, a high pass optical filter is included in the optical transmission link, where the filter exhibits a cut-off frequency substantially immediately below the channel frequency, with low loss at this frequency and high loss at the unwanted optical noise frequencies. In arrangements utilizing multiple input channels, the high pass filter is configured to exhibit a cut-off frequency immediately below the lowest channel frequency. The resultant high loss at Raman noise frequencies will lead to reduced interactions of the optical noise with the channels, thus permitting greater channel power to reach the end of the optical fiber. Thus, the channel depletion due to SRS is reduced.
In one embodiment, the high pass filter may comprise a discrete element, such as a fused silica coupler, a dielectric stack, or long period Bragg gratings. Multiple filters may be utilized in order to reduce noise interaction along the length of the fiber.
In an alternative embodiment, the high pass filter may be of the “distributed” type, achieved by inserting absorbing ions into the core of the transmission fiber, or placing an absorbing layer around the core of the fiber.
REFERENCES:
patent: 5107360 (1992-04-01), Huber
patent: 5131069 (1992-07-01), Hall
patent: 5777793 (1998-07-01), Little
patent: 6094296 (2000-07-01), Kosaka
patent: 6181465 (2001-01-01), Grubb
patent: 6327062 (2001-12-01), King
Christodoulides Demetrios Nicolau
Delavaux Jean-Marc Pierre
McIntosh Christopher Michael
Toulouse Jean
Boutsikaris Leo
Koba Esq. Wendy W.
Lucent Technologies - Inc.
Spyrou Cassandra
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