Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
2001-12-20
2004-07-06
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling function
C385S024000, C398S159000
Reexamination Certificate
active
06760513
ABSTRACT:
The present invention is directed to the field of dispersion compensation as applied to co-transmitted optical signals having different wavelengths, in optical communications networks.
In optical communications networks set up in known methods heretofore, one installed, almost exclusively, standard single-mode fibers having an attenuation of about 0.4 dB/km and a dispersion minimum at 1310 nm.
To an increasing degree, the wavelength range of around 1550 nm is used for optical communications. This is due to the lower attenuation of approximately 0.2 dB/km, the increasing use of wavelength division-multiplex transmission, and the availability of a virtually perfected optical-fiber light amplifier, the EDFA (erbium doped fiber amplifier), which can be used to amplify several channels simultaneously within a broad range of around 1550 nm.
One deficiency of the above approach is that the transmission bandwidth and the amplifier distances are limited by the high dispersion of standard single-mode fibers of about 17 ps
m×km at 1550 nm. Therefore, for longer transmission routes and bandwidths in the Gb/s range, it is necessary to install dispersion-compensating elements.
It is generally known to use dispersion-compensating fibers (DCF) which exhibit a high negative dispersion. −100 ps
m×km is given as a typical value for the dispersion of a DCF. Accordingly, 17 km of DCF are needed to compensate for the dispersion of a 100 km long standard single-mode fiber. The compensation fibers are wound onto spools, which must be at least 10 cm in diameter to avoid loss of curvature. There are several drawbacks associated with using a dispersion-compensating fiber DCF for dispersion compensation:
A substantial length of a relatively expensive, special fiber is needed.
The fiber spool has large dimensions. This can lead to problems in line repeater stations or in cable jointing chambers, particularly when working with multi-core optical cables.
A supplementary attenuation is added. Due to their special core structure, dispersion-compensating fibers exhibit an attenuation of about 0.5 dB/km, i.e., a fiber length of 17 km yields an attenuation of about 9dB.
The above described properties and possible applications of dispersion-compensating fibers DCF, as well as the wavelength division-multiplex transmission and optical-fiber light amplifiers are described in detail in “Optische Telekommunikationssysteme” by H. Hultzsch, Damm Publishers, Gelsenkirchen (1996) p. 123 and pp. 296-298.
Another dispersion compensation method is based on the use of optical fiber gratings (see likewise in “Optische Telekommunikationssysteme” by H. Hultzsch, Damm Publishers, Gelsenkirchen (1996) pp. 152-153). However, optical fiber gratings of about one meter length are required to compensate for dispersion over broad wavelength ranges, e.g., the EDFA range of 1530 nm-1570 nm. The manufacturing of very long optical fiber gratings having the necessary tolerances with respect to the grating constants and the requisite long-term stability is expensive and is still in the development stage.
The technical objective of the present invention is directed to an economical approach that requires little overall space to compensate for the dispersion of co-transmitted optical signals having different wavelengths &lgr;.
The achievement of the objective in accordance with the present invention is based on the use of photonic crystals Photonic crystals are periodic arrays of dielectric materials having high and low dielectric constants, alternately disposed as one-, two-, or three-dimensional gratings having periods of &lgr;/3 and rod or cubic diameters of &lgr;/6. See J. D. Joannopoulus et al.: Photonic Crystals: Molding the Flow of Light, ISBN 0-691-03744-2 (1995).
In accordance with the present invention, co-transmitted optical signals of different wavelengths which, after propagating through a line section, exhibit dispersion-induced transit-time differences, are coupled via an optical fiber input E into an arrangement configured as a network, which is made up of photonic crystals K
1
through Kn positioned one after another on an optical waveguide
2
. Photonic crystals K
1
through Kn are, therefore, optically connected to one another. Photonic crystals K
1
through Kn are formed in such a way that they reflect or divert signals of a specific wavelength and allow signals having other wavelengths to pass through, unattenuated. For example, the formation of first photonic crystal K
1
is such that it exclusively reflects the signals of a first wavelength. Optical signals of other wavelengths pass through photonic crystal K
1
, unattenuated, and are coupled into the downstream photonic crystal K
2
. Of those signals transmitted through first photonic crystal K
1
, the subsequent, second photonic crystal K
2
reflects, in turn, only those signals having a second wavelength. It likewise permits the signals having other wavelengths to pass through. In accordance with this principle, the signal continues to be passed on by a photonic crystal to a further photonic crystal until the signals of all wavelengths have been reflected by photonic crystals K
1
through Kn assigned to them.
Since the positive dispersion value of the signals coupled into the optical fiber input is known for the individual wavelengths, path lengths, which are afflicted by negative dispersion in the array made up of successively ordered photonic crystals K
1
through Kn, are defined in accordance with the individual wavelengths, are dimensionally designed to alter or completely cancel the dispersion differences of the signals of the individual wavelengths. Before the signal of a defined wavelength is reflected in one of the successively ordered photonic crystals K
1
-Kn, it has already traveled a path length up to the element reflecting the defined wavelength in the photonic crystal and acted upon by such a negative dispersion This path length is defined by the distance between optical fiber input E and the reflecting mirror in photonic crystal K
1
through Kn in question.
The dispersion-compensated signals of different wavelengths reflected by the photonic crystals are again coupled into a shared optical fiber output A to be retransmitted by a suitable module, such as an optical circulator
1
.
The method of the present invention shall now be explained in greater detail on the basis of five exemplary embodiments.
Assuming the case where optical signals transmitted with different wavelengths &lgr;
i
, e.g., three wavelengths &lgr;
i
, &lgr;
i+1
, &lgr;
i+2
, exhibit dispersion-induced transit-time differences after propagating through a line section, the specific embodiments are especially directed to once again compensating for these transit-time differences. However, these approaches also include the option of setting a predistortion including defined transit-time differences for the signals of the individual wavelengths, for example for the wavelengths &lgr;
1
, &lgr;
i+1
, &lgr;
i+2
.
FIG. 1
depicts an arrangement for compensating for dispersion, where the optical signals afflicted by transit-time differences are coupled via a shared optical fiber input E into an optical circulator
1
. The optical signals afflicted by transit-time differences are coupled by optical circulator
1
into a module made up of photonic crystals KS
1
through KSn which are disposed one after another as selective reflection filters on a waveguide
2
.
In this context, each of photonic crystals KS
1
through KSn is tuned to only reflect the signals having a specific wavelength of those signals coupled in via optical circulator
1
, but to allow the signals of the other wavelengths to pass through. It, is the actual transit-time difference of the signals of the particular wavelength that determines which photonic crystal KS
1
through KSn is designed as a reflection filter for which wavelength. The greater the transit-time difference is, the longer the optical path must also be that the signal needs to travel until complete dispersion compensation i
Heitmann Walter
Koops Hans W. P.
Deutsche Telekom AG
Healy Brian
Wood Kevin S.
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