Optical communications – Diagnostic testing – Determination of communication parameter
Reexamination Certificate
2000-10-31
2003-11-25
Negash, Kinfe-Michael (Department: 2633)
Optical communications
Diagnostic testing
Determination of communication parameter
C398S025000, C398S034000
Reexamination Certificate
active
06654561
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for measuring an optical signal-to-noise ratio of a wavelength-division multiplexing optical signal where optical signals are multiplexed in a high density, in a method and. apparatus for measuring an optical signal-to-noise ratio of wavelength-division multiplexing optical signals. The present invention further relates to a pre-emphasis method, an optical communication system, a measuring circuit, and a controlling apparatus each utilizing this optical signal-to-noise ratio measuring method.
Recently, an optical communication system with an ultra-long distance and large capacity has been demanded in order to constitute a multi-media network. To achieve this. object, a wavelength-division multiplexing (WDM) system has been researched and developed since it has advantages in effectively utilizing optical fibers characteristics of broadband and large capacity.
It is particularly a recent trend to narrow wavelength gaps between optical signals that are wavelength-multiplexed, in order to satisfy the requirement for a greater capacity. A method of measuring an optical signal-to-noise ratio of WDM optical signals at high-density has been needed in order to maintain signal quality even when such a WDM optical signal is transmitted.
2. Description of the Related Art.
When optical signals are transmitted in a long distance, in general, optical signals are amplified by an optical repeater station provided with an optical amplifier between an optical signal sending station and an optical signal receiving station to compensate for a transfer loss of an optical transmission line. When amplified, the optical signal is superposed with amplified spontaneous emission (ASE) generated by the optical amplifier. This ASE is cumulatively superposed every time when the optical amplifier amplifies the optical signal. The cumulative ASE is a noise to the optical signal. To keep predetermined signal quality, therefore, the optical SNR of the optical signal has to be measured.
An optical SNR measuring method according to the prior art will be explained with reference to FIG.
20
.
FIG. 20
shows the case where a WDM optical signal is amplified by several optical amplifiers and is superposed with ASE. The WDM optical signal contains four-wave optical signals of channels
1
to
4
that are wavelength-multiplied. Each optical signal is arranged with a sufficient wavelength space at its part of a low optical level lest they overlap with one another as shown in FIG.
20
. Hereinafter, the term “channel” will be abbreviated as “ch.”
In such a WDM optical signal, an optical level at a substantial intermediate wavelength (indicated by X in
FIG. 20
) between adjacent channels corresponds to the optical level of ASE. Therefore, the optical SNR at a channel can be measured by the steps of measuring the spectrum of the WDM optical signal by using a spectrum analyzer, and calculating a ratio of the optical level at a wavelength at which the channel is disposed (indicated by black circle &Circlesolid; in
FIG. 20
) to the optical level at an intermediate wavelength (indicated by X in
FIG. 20
) between the channel and a channel adjacent to this channel from the measurement result.
A level diagram of an optical communication system is generally designed for light at a predetermined wavelength. When the WDM optical signal is transmitted in such an optical communication system, each channel of the WDM optical signal is amplified at a different gain due to nonuniform gain wavelength characteristics as well as gain saturation characteristics in the optical amplifier inside the optical repeater station. Further, each channel loses optical power at a different loss due to nonuniform loss wave characteristics of an optical transmission line. Inconsequence, the more the level diagram in the optical signal whose wavelength deviates from the predetermined wavelength, the more it deviates from the designed level diagram. This causes a greater bit error ratio in the channel having low optical power than the allowable one in the optical communication system.
Therefore, the WDM optical signal is pre-emphasized so that the bit error ratio becomes lower than the allowable bit error ratio.
Next, a conventional pre-emphasis method will be explained with reference to FIG.
21
.
Referring to
FIG. 21
, the WDM optical signal generated by an optical sender (OS)
911
inside an optical sending station
901
is amplified by a plurality of optical repeater stations
903
disposed in the optical transmission line
902
so as to compensate for the loss of the optical transmission line
902
and the loss by the optical repeater stations
903
, is then transmitted to the optical receiving station
904
and is processed. The loss in the optical repeater stations
903
results from optical components in each station such as a dispersion compensating fiber (DC).
A pre-emphasis control circuit
912
inside the optical sending station
902
pre-emphasizes the WDM optical signal when the optical sending station
901
sends the WDM optical signal to the optical transmission line
902
.
The pre-emphasis control circuit
912
pre-emphasizes the WDM optical signal by regulating the optical level of the optical signal generated by the optical sender
911
and corresponding to each channel. The optical SNR of each optical signal is measured by an optical SNR measuring circuit
922
disposed inside the optical-receiving station
904
, such as a spectrum analyzer, and is transmitted through a line
931
.
Here, the pre-emphasis control circuit
912
regulates the optical level of each optical signal in the following way, for example.
As a first step, the pre-emphasis control circuit
912
calculates an average value of the optical SNR for all the channels in the optical receiving station
904
.
As a second step, it calculates the difference of this average value from the optical SNR of the channel ch.
1
in the optical receiving station
904
.
As a third step, the optical sending station
901
increases or decreases the optical level sufficient to compensate for this difference from the channel ch.
1
and regulates the optical level of the channel ch.
1
.
As a fourth step, the process of the second and third steps is applied to each optical signal corresponding to each channel of the WDM optical signal.
The optical sending station
901
regulates the optical level of each optical signal in the way described above so that the optical SNR of each channel in the optical receiving station
904
becomes equal to one another.
When the optical level of each optical signal is regulated in this way, the level diagram is regulated for each optical signal. Consequently, when pre-emphasis is applied, the optical SNR of each optical signal in the WDM optical signal can be optimized.
Japanese Unexamined Patent Application Publication Nos. 06-069891, 09-261205, and 11-103287 disclose pre-emphasis in an optical communication system between two terminal stations.
Besides the optical communication system for sending and receiving the WDM optical signals between two terminal stations, an optical communication system having ADM functions of selectively passing only optical signals having a specific wavelength among wavelength-multiplexing optical signals inside a station interposed in an optical transmission line, dropping the optical signals having other wavelengths in this station, or adding separate optical signals and sending them to other stations has been required in recent years. It is a recent trend that the ADM function is realized without converting the WDM optical signal from the optical signal to the electric signal but by utilizing an optical add/drop multiplexer (OADM) for dropping and adding the optical signals as they are.
When a channel is dropped/added, OADM removes the channel to be dropped by using an optical filter, for example, and then adds the optical signal that is to be added, into this channel. In this case, the optical filter removes not on
Murakami Makoto
Onaka Hiroshi
Sakamoto Takeshi
Sekiya Motoyoshi
Terahara Takafumi
Fujitsu Limited
Negash Kinfe-Michael
Staas & Halsey , LLP
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