Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2000-07-26
2002-08-13
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06433864
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an apparatus for monitoring optical signal-to-noise ratio of optical signals in wavelength-division-multiplexing (WDM) optical transmission system, and more particularly, to an apparatus for monitoring optical signal-to-noise ratio of each optical signal by measuring noise signal occurring when an optical signal is detected through de-multiplexing optical signals at each channel in wavelength-division-multiplexing (WDM) optical transmission system.
BACKGROUND OF THE INVENTION
Wavelength-division-multiplexing (WDM) optical transmission system is a system that transmits several transmission lasers with different wavelengths from each other by multiplexing them in an optical fiber.
By using the system, there is an advantage to significantly increase transmission capacities per optical fiber, even though each laser operates with relatively low transmission rate.
It is necessary to use an optical fiber amplifier for amplifying optical signals so as to increase transmission range in these systems. However, due to amplified spontaneous emission (ASE) light, occurring when the optical fiber amplifier amplifies optical signals, optical signal-to-noise ratios of the optical signals are deteriorated and therefore, that causes performance degradation of total system.
That is to say, as optical signal-to-noise ratio of an optical signal is directly related to the performance of system, the performance of wavelength-division-multiplexing (WDM) optical transmission system can be measured by monitoring optical signal-to-noise ratio. Also, more effective maintenance of a system is achieved by comprehending precise performance of the system.
Especially, in the case of all optical transmission networks, which is expanded wavelength-division-multiplexing (WDM) optical transmission system, due to different optical signal-to-noise ratio of an optical signal at each channel, the monitoring of optical signal-to-noise ratio for each optical signal is indispensable.
Conventional method for monitoring optical signal-to-noise ratio of an optical signal was using optical spectrum analyzer with rotating diffraction grating.
Even though these optical spectrum analyzers have advantages of wide measurement range and high accuracy, there is disadvantage of additional installation cost in wavelength-division-multiplexing (WDM) optical transmission system caused by high volume and high cost.
Several methods for monitoring wavelength-division-multiplexing (WDM) optical signal-to-noise ratio of an optical signal, while complementing the disadvantage, have been proposed.
First, there is one technique of “Signal Monitoring Apparatus for Wavelength-division-multiplexed Optical Communication” [U.S. Pat. No. 5,796,479], which was issued for a patent by Dennis Derickson and Roger Lee Jungerman and registered.
This method separates wavelength-division-multiplexing (WDM) optical signals, which are incident upon via an optical fiber by using diffraction grating, at each wavelength and then monitors optical signal-to-noise ratio of the optical signal by using photo diode array.
However, this method has problems of low accuracy in measurement and instability of optical spatial alignment on account of spatial distance between the optical fiber and the diffraction grating.
Next, another technique was described in a paper entitled “A High-Performance Optical Spectrum Monitor with High-Speed Measuring Time for WDM Optical Networks” written by K. Otsuka, Y. Sampei, Y. Tachikawa, N. Fukushima, and T. Chikama, in “97 European Conference on Optical Communication, pp. 147-150, 1997”. As this method also used a diffraction grating and photo diode array, there were problems of instability in optical spatial alignment and low accuracy in measurement.
Next, another technique was described in a paper entitled “High Resolution Fiber Grating Optical Network Monitor” written by Chris Koeppen, Jefferson L. Wagner, Thomas A. Strasser, and John KeMarco, in “National Fiber Optic Engineers Conference '98, Sep. 14-17, 1998”. This method used a blazed bragg grating and photo diode array.
However, this method had problems of not having precise measurement result of optical signal-to-noise ratio of an optical signal unless spatial alignment between the blazed bragg grating and photo diode array is stable.
Besides, there is method for monitoring optical signal-to-noise ratio of an optical signal by using Fabry-Perot Filter.
However, these methods for monitoring optical signal-to-noise ratio of an optical signal are useful only when optical signal-to-noise ratios are nearly same and the characteristic of amplified spontaneous emission (ASE) light, occurring from optical amplifier, is flat. Particularly, in the case of all optical transmission network, which is expanded wavelength-division-multiplexing (WDM) optical transmission system, as each channel is added/dropped by an optical add-drop multiplexer, transmission range at each channel is different from each other.
Therefore, the intensities of amplified spontaneous emission (ASE) lights, which are occurring from optical amplifier, are different from each other and as a result, optical signal-to-noise ratio at each channel is different.
FIG. 1
shows optical spectrums in wavelength-division-multiplexing (WDM) optical transmission network, after passing several optical add-drop multiplexers. Referring to
FIG. 1
, it is known that the spectrums of optical noises, occurring from optical amplifier, does not have flat characteristics by arrayed waveguide grating within optical add-drop multiplexer and optical signal-to-noise ratio at each channel is different from each other.
Moreover, as the optical signal is distorted by optical filter such as array waveguide grating, core device of optical add-drop multiplexer, these conventional methods have problem of impossibility in the measurement of optical signal-to-noise ratio.
FIG.
2
A and
FIG. 2B
show optical spectrums measured by optical spectrum analyzer with resolution of 0.05 nm after passing an optical signal with 25 dB of optical signal-to-noise ratio through wavelength-division-multiplexing optical add-drop multiplexer.
In the
FIG. 2A
is capable of monitoring optical signal-to-noise ratio when passing through optical add-drop multiplexer with passband of 1.1 nm. However,
FIG. 2B
is difficult to monitor optical signal-to-noise ratio when passing through optical add-drop multiplexer with passband of 0.3 nm.
Due to the low resolution, the measurement of optical signal-to-noise ratio is impossible in the case of
FIG. 2B
, which is impossible to be measured by optical spectrum analyzer with high resolution. Therefore, new method for monitoring optical signal-to-noise ratio, which is also applicable to wavelength-division-multiplexing (WDM) optical transmission network, is indispensable.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an apparatus for monitoring optical signal-to-noise ratio in wavelength-division-multiplexing optical transmission system, in which maintenance and management of system can be effectively performed by de-multiplexing the optical signal of each channel in wavelength-division-multiplexing optical transmission system, applying the signal to optical detector, and then monitoring optical signal-to-noise ratio from the quantity of noise occurring when the applied optical signals are detected by the optical detector.
To achieve the object according to the present invention, the present invention applicable to optical add-drop multiplexer, in which wavelength-division-multiplexed optical signals are split at each channel, provides an apparatus for monitoring optical signal-to-noise ratio in wavelength-division multiplexing optical transmission system, comprising: an optical splitting means for splitting optical signals applied from an external; an optical power measuring means for measuring intensities of optical signals out of a portion of said split optical signals; a noise measuring means for measuring intensities of noises occurring
Chung Yun Chur
Park Keun Joo
Shin Seung Kyun
Akin Gump Strauss Hauer & Feld L.L.P.
Font Frank G.
Korea Advanced Institute of Science and Technology
Nguyen Tu T
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