Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-01-25
2002-10-08
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06462844
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a wavelength-division multiplexing transmission system which wavelength-division-multiplexes optical signals. at a plurality of wavelengths and transmits the same, a method for designing a loss difference compensator for optical devices used in the wavelength-division multiplexing transmission system, and a method for configuring the wavelength-division multiplexing transmission system.
(2) Description of the Related Art
In order to realize a future multimedia network, there is, in these years, a demand for an optical communication system by which a larger capacity communication is feasible. A lot of researches have. been conducted on a time-division multiplexing (TDM) system, an optical time-division multiplexing (OTDM) system in the optical domain, a wavelength-division multiplexing (WDM) system and the like, as multiplexing (transmission) systems enabling very large capacity communications.
Among them, the WDM transmission system is expected as a means to realize a optical network which can flexibly multiplex different types of services by crossconnecting or branching and inserting at the optical level using a wide gain band of an erbium doped fiber amplifier (EDFA).
In the WDM transmission system (wavelength-division multiplexing transmission system), optical repeaters each having an optical amplifier such as an EDFA or the like are, in general, connected in plural stages according to a transmission power (a transmittable distance) of a used optical transmission line, whereby a long-distance transmission of a WDM signal is possible.
However, an optical amplifier such as an EDFA or the like has an amplification (gain) characteristic dependent on wavelength, as well known. For this reason, even if a WDM signal having a flat power at each wavelength is transmitted, there are generated differences in loss of power of the wavelengths (gain tilt) of the received WDM signal at a receiving end.
In a system using a single mode fiber (SMF) whose zero-dispersion characteristic is 1.3 &mgr;m (micrometer) in a transmission line to WDM-transmit a lightwave centered at a 1.55 &mgr;m band, for example, an optical signal at a shorter wavelength has a smaller power at the receiving end, since the optical amplifier has a characteristic that the longer the wavelength, the larger the gain in the used wavelength band is [for example, 1530 to 1570 nm (nanometer)] (tilt upward toward the right). In the worst case, it is impossible to receive the optical signal at a shorter wavelength.
For the purpose of equalizing (compensating) differences in loss of the wavelengths generated in the WDM signal due to the gain characteristic of the optical amplifier, there have been proposed various techniques.
A technique disclosed in Japanese Patent Laid-Open Publication No. 9-436-47, for example, flattens the gain characteristic of the optical amplifier (EDFA) using a bending loss caused by a bending of an optical fiber. Namely, taking advantage of a fact that when an optical fiber is bent, the longer the wavelength, the larger the bending loss is, an optical fiber having a transmission characteristic approximately reverse to the gain characteristic of the optical amplifier is added to flatten the gain characteristic of the optical amplifier.
However, this technique can yield only an optical filter having a linear transmission characteristic since the technique utilizes a bend of an optical fiber. The gain characteristic of an actual optical amplifier is not linear so that such an optical filter cannot effectively flatten the gain characteristic of the optical amplifier, of course.
There is also proposed a technique in which a Fabry-Perot etalon filter is used as an optical filter being able to accomplish a non-linear transmission characteristic to flatten the gain characteristic of the optical amplifier more effectively.
Meanwhile, an optical fiber (SMF) used in the present WDM transmission system has absorption losses inherent to silica glass that is a material of the optical fiber such as ultraviolet loss, infrared loss and the like, and scattering losses such as Rayleigh scattering and the like (refer to FIG.
26
). A transmission loss characteristic
100
is generated dependent on, mainly, Rayleigh scattering and infrared absorption among these losses.
FIG. 27
shows a 1.55 &mgr;m band shown in
FIG. 26
when enlarged. As seen from
FIG. 27
, a loss difference (a tilt downward to the right) are generated in a used wavelength band
101
.
For this, even if a WDM signal having a flat power at each wavelength is inputted to an optical fiber, a difference is generated in the receiving level of the channel (a wavelength) at a receiving end as shown in
FIG. 28
, for example, due to the wavelength-dependent transmission loss characteristic of the optical fiber, leading to a variation in the transmission characteristic (S/N ratio) of each channel.
The above phenomenon is not a serious issue in the case where a used wavelength band is narrow (a WDM signal at one to several wavelengths, for example) and a transmission distance is short [several tens km (kilometer), for example] since the phenomenon can be avoided by setting the used wavelength band
101
shown in
FIG. 27
in a portion where the transmission loss characteristic
100
is almost flat. However, in the case where the used wavelength band
101
is wide, a tilt inevitably exists in the transmission loss characteristic
100
so that a difference in loss is generated according to a transmission distance, leading to a very serious issue.
Particularly, a recent demand is to transmit more wavelengths (16 wavelengths or more, for example) for a longer distance (several hundreds kilometers, for example). For this, a loss difference generated due to the transmission loss characteristic
100
of the optical fiber is not negligible.
In the case where an SMF is used to perform optical transmission (WDM transmission) with a lightwave in a 1.55 &mgr;m band, there is often used a technique in which a dispersion compensation fiber (DCF) is used to cancel dispersion generated in a WDM signal due to a dispersion characteristic of the SMF. However, since the DCF is an optical fiber whose basic material is silica glass similarly to the SMF, the DCF has a transmission loss characteristic similar to that of the SMF.
When the DCF is used, a wavelength-dependent loss difference similar to that of the SMF is generated in not only the SMF but also the DCF so that a larger difference is generated in S/N ratio of each channel, as compared with a case of only the SMF, due to the transmission loss characteristic of both of the SMF and the DCF.
In the case where a used wavelength band
101
for a WDM signal is 1530 to 1570 nm, for example, a tilt (a loss difference) of 0.4 dB is generated when the WDM signal is transmitted for 80 km if a loss difference in the used wavelength band of the SMF is 0.005 dB/km. Moreover, if the DCF is used to compensate dispersion degradation in the SMF, a DCF with −1000 to −1200 pb
m is required for 80 km of the SMF. This corresponds to about 10 km of a fiber length, generating a loss:difference of about 1 dB.
When the SMF and DFC are connected in several stages (3 or 4 stages, for example) to linearly repeat, a loss difference of about 5 to 6 db in total is generated by the SMFs and the DCFs. As a result, an S/N ratio of each channel is largely degraded. In the worst case, it is impossible to receive a wavelength (a channel) on the shorter wavelength's side.
When the SMF or the DCF is used in a WDM transmission system, it is important to compensate the above loss difference, in consideration of a wavelength-dependent transmission loss characteristic of the SMF or the DCF itself. Particularly, when 16 or more lightwaves are wavelength-division-multiplexed and a WDM signal whose used wavelength band is 12 nm or more is transmitted, the above compensation is very important since a tilt in the transmission loss characteristic appe
Kai Yutaka
Onaka Hiroshi
Fujitsu Limited
Negash Kinfe-Michael
Staas & Halsey , LLP
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