Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1999-09-23
2001-03-20
Ngo, Hung N. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S011000, C385S033000
Reexamination Certificate
active
06205270
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fiber optic networks, and more particularly to fiber optic dense wavelength division multiplexers.
BACKGROUND OF THE INVENTION
Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Multiple signals and/or wavelengths may be transmitted simultaneously over the same optical fiber. For purposes of this discussion, a signal is a single stream of logical information carried by an optical fiber and a compound signal is the totality of all such simultaneously transmitted signals. If each signal carried in a particular direction along an optical fiber corresponds to a separate wavelength, then the network is a wavelength multiplexed system.
A crucial feature of a wavelength-multiplexed fiber optic network is the separation of a compound optical signal into its component single-wavelength signals, or “channels,” typically by a dense wavelength division multiplexer. This separation must occur for the exchange of wavelengths between signals on communications “loops” within networks to occur. As the demand for information carrying capacity over existing fiber networks continues to grow, the performance constraints on optical network components continue to become more severe.
One potential difficulty with optical channel separators in wavelength multiplexed systems with dense channel packing is imperfect optical separation or add-drop performance as a result of imperfections in optical components. Such imperfections would include unwanted reflections of light at nominally transparent components, unwanted transmissions of light through nominally reflective components, and light scattering from surface roughness of nominally flat surfaces. All such imperfections lead to stray light that can propagate in random and uncontrollable directions. The presence of this stray light can cause signals to propagate along undesired and incorrect pathways, thereby causing imperfect isolation of one set of signals from another. Such imperfect channel separation can cause signals to branch to incorrect portions of the network and can ultimately lead to contamination, for instance, of a signal comprising a particular wavelength channel with spurious incorrectly routed signals carried on the same channel.
As an example,
FIG. 1
shows a schematic representation of optical channels as they are transmitted through a dense optical channel separator. A three-port channel separator
101
initially separates an initial optical input signal
103
, comprised of a plurality of optical channels, into two sub-signals, a first sub-signal
104
comprised primarily of “odd” channels and a second sub-signal
105
comprised primarily of “even” channels, respectively. Histograms attached to each signal and sub-signal represent, hypothetically, the intensities of the channels comprising each. If the channel separator
101
is perfect then the sub-signal
104
is comprised only of odd channels and the sub-signal
105
is comprised only of even channels. However, since, in general, the separator
101
is not perfect, there is some “leakage” of even channels into sub-signal
104
and of some odd channels into sub-signal
105
. Depending upon the level of isolation between sets of channels required by the wavelength multiplexed system, these spurious channel signals could provide unacceptable cross talk if, for instance, new sets of even and odd channels, respectively, are subsequently added to the sub-signals
104
and
105
, as illustrated in FIG.
1
.
Accordingly, there exists a need for a mechanism for minimizing channel leakage in a dense wavelength division system. The mechanism should be easy to manufacture and be cost effective. The present invention addresses such a need.
SUMMARY OF THE INVENTION
The present invention provides a dense wavelength division multiplexer for separating an optical signal into optical channels which minimizes channel leakage. The multiplexer includes a mechanism for inputting an optical signal, the optical signal having a plurality of optical channels; at least one channel separator optically coupled to the inputting mechanism; at least one dense optical channel comb filter optically coupled to the at least one channel separator; and a mechanism for outputting the separated plurality of optical channels along a plurality of optical paths, optically coupled to the at least one dense optical channel separator. In the preferred embodiment, the dense optical channel comb filter includes a lens optically coupled to the input mechanism disposed at a distance equal to a focal length of the lens; a birefringent wedge optically coupled to the lens at a side opposite to the inputting means; and a non-linear interferometer optically coupled to the birefringent wedge at a side opposite to the lens. The comb filter of the present invention provides an improved efficiency of separation of channels when used to supplement an optical channel separator. The present invention also provides a higher tolerance to drifts than is provided by conventional optical filters due to the increase in the widths of the pass bands. The materials required to manufacture and implement the dense optical channel comb filter in accordance with the present invention are readily available. The present invention thus does not require special or expensive materials or processes. It is thus cost effective.
REFERENCES:
patent: 5170290 (1992-12-01), Land et al.
patent: 5225930 (1993-07-01), Land et al.
patent: 5699378 (1997-12-01), Lealman et al.
patent: 6023542 (2000-02-01), Pan et al.
Avanex Corporation
Ngo Hung N.
Sawyer Law Group LLP
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