Optics: measuring and testing – By dispersed light spectroscopy
Reexamination Certificate
2001-05-08
2004-03-16
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
C398S079000
Reexamination Certificate
active
06707547
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of analyzing wavelength-division multiplexed signal light. More particularly, it relates to a method and an apparatus for analyzing wavelength-division multiplexed signal light which feature the capability of determining reference wavelengths automatically.
2. Description of the Related Art
In optical communications where information signals are carried on intensity-modulated light, the use of wavelength-division multiplexing (WDM) is rapidly increasing today. In WDM, light output from n light sources of different wavelengths (hence, frequencies) is multiplexed so that optical signals having different wavelengths (hence, frequencies) are treated as separate channels.
FIG. 1
shows an exemplary optical spectrum of wavelength-division multiplexed signal light. The wavelength (hence, frequency) of each channel of this light is customarily of any value that is selected from among reference wavelengths &lgr;C
1
, &lgr;C
2
, . . . &lgr;Cm spaced at equal intervals as specified by the ITU-T (International Telecommunication Union-Telecommunication sector) standards. In
FIG. 1
, &lgr;An is the reference wavelength of channel n as selected from among those standard reference wavelengths. The optical intensities of the respective channels are represented by P
1
, P
2
, . . . Pn and it is usually considered ideal that they have no variations but are constant.
FIG. 2
shows an exemplary optical spectrum of actual wavelength-division multiplexed signal light having n channels. In the optical spectrum shown in
FIG. 2
, the central wavelength &lgr;Bn of each channel contains an error &Dgr;&lgr;n with respect to the reference wavelength &lgr;An specified by the ITU-T standards and the optical intensity Pn of each channel also contains an error &Dgr;Pn. In optical communications using wavelength-division multiplexed signal light, interference will occur if the errors &Dgr;&lgr;n and &Dgr;Pn are significant in each channel.
Therefore, in the actual use of wavelength-division multiplexed signal light in optical communications, the errors &Dgr;&lgr;n and &Dgr;Pn in the optical spectrum of the light are usually subjected to preliminary analysis, typically using an optical spectrum analyzer that measures intensity vs. wavelength characteristics by means of a spectrometer having in its interior a diffraction grating or some other device to separate the incident light into its spectral components.
FIG. 3
illustrates how the spectrum waveform of a single channel n in the wavelength-division multiplexed signal light is analyzed. To analyze the central wavelength &lgr;Bn and the peak power Pn, the spectrum waveform's peak level Pn is determined within the range of ±&Dgr;&lgr;
R
from the reference wavelength &lgr;An of the channel of interest; then, a line indicating Pn minus a threshold level TH (=Pn−TH) is drawn parallel to the horizontal axis of the graph in
FIG. 3
; since the line for Pn−TH crosses the spectrum waveform at two points, the wavelength of the midpoint between the two crossing points is determined as the central wavelength &lgr;Bn of the channel of interest; the difference between the reference wavelength &lgr;An and the central wavelength &lgr;Bn is taken to determine the wavelength error &Dgr;&lgr;n.
In order to analyze optical spectra by the above-described method, the reference wavelength &lgr;An must preliminarily be known for each channel and the value of &lgr;An for each channel must be registered in a memory before analysis starts.
FIG. 4
shows an exemplary optical spectrum of 4-channel wavelength-division multiplexed signal light. In the illustrated case, four of the reference wavelengths &lgr;Cm as determined by the ITU-T standards are allotted as the reference wavelengths &lgr;An of the four channels and they are &lgr;C
1
, &lgr;C
3
, &lgr;C
5
and &lgr;C
7
corresponding to &lgr;A
1
, &lgr;A
2
, &lgr;A
3
and &lgr;A
4
, respectively, which must be registered in the memory before actual analysis of the spectrum starts.
FIG. 5
shows another exemplary optical spectrum of 4-channel wavelength-division multiplexed signal light. In the illustrated case, four of the reference wavelengths &lgr;Cm as determined by the ITU-T standards are allotted as the reference wavelengths &lgr;An of the four channels and they are &lgr;C
1
, &lgr;C
2
, &lgr;C
5
and &lgr;C
6
corresponding to &lgr;A
1
, &lgr;A
2
, &lgr;A
3
and &lgr;A
4
, respectively, which must be registered in the memory before actual analysis of the spectrum starts.
Thus, prior to analyzing optical spectra, the reference wavelength &lgr;An of each channel in the spectrum to be analyzed need be set in accordance with the channel wavelength &lgr;Cm of the spectrum.
There may be a case where analysis of an optical spectrum comprising one combination of channels, say, the spectrum shown in
FIG. 4
, is followed by analysis of another optical spectrum comprising a different combination of channels, say, the spectrum shown in FIG.
5
. In a case like this, the reference channel &lgr;An of each channel must be reset in accordance with the channel wavelengths &lgr;Cm of the spectrum under analysis.
The above-described prior art method may be used to analyze n-channeled wavelength-division multiplexed signal light in accordance with the flowsheet shown in FIG.
6
.
First, reference wavelengths &lgr;A
1
, &lgr;A
2
, . . . &lgr;An corresponding to the respective channels of the wavelength-division multiplexed signal light to be analyzed are input and stored in a memory (step A
1
).
Then, the optical spectrum of the wavelength-division multiplexed signal light is measured with an optical spectrum analyzer or any other suitable apparatus and the measured spectrum waveform data is stored in the memory (step A
2
).
Then, on the basis of the measured waveform data, the spectrum waveform's peak level Pn is determined within the range of ±&Dgr;&lgr;
R
from the reference wavelength &lgr;An of each channel; a line indicating Pn minus a threshold level TH (=Pn−TH) is drawn parallel to the horizontal axis of the spectrum graph; since the line for Pn−TH crosses the spectrum waveform at two points, the wavelength of the midpoint between the two crossing points is determined as the central wavelength &lgr;Bn of each channel (step A
3
).
Subsequently, the difference between the reference wavelength &lgr;An and the central wavelength &lgr;Bn is taken to determine the wavelength error &Dgr;&lgr;n (step A
4
).
Then, the central wavelength &lgr;Bn and the wavelength error &Dgr;&lgr;n are output to a display or some other device as the results of analysis (step A
5
).
If the analysis is not to be repeated, the process ends but if the user wants another analysis, the process continues (step A
6
).
If the wavelength-division multiplexed signal light to be analyzed varies in channel wavelength &lgr;An, the sequence returns to the step of inputting another set of reference wavelengths &lgr;A
1
, &lgr;A
2
, . . . &lgr;An corresponding to the respective channels of the wavelength-division multiplexed signal light (step A
1
); if there is no such change, the sequence returns to step A
2
.
As will be understood from the foregoing description, the prior art method of analyzing wavelength-division multiplexed signal light has the disadvantage of time-consuming input step since the reference wavelengths &lgr;An corresponding to the respective channels must be input one by one before analysis starts.
The same disadvantage occurs if the combination of channels in the optical spectrum to be analyzed is different from the previous one and this is because the reference wavelengths &lgr;An that correspond to the respective channels in the optical spectrum under current analysis have to be reset one by one.
SUMMARY OF THE INVENTION
An object of the present invention is to ensure that the data about the reference wavelengths &lgr;An which are necessary to analyze the optical spectrum of wavelength-division multiple
Ando Electric Co. Ltd.
Smith Zandra V.
Stock Gordan J.
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