Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
2000-09-29
2004-05-11
Negash, Kinfe-Michael (Department: 2633)
Optical communications
Multiplex
Wavelength division or frequency division
C398S094000, C398S196000, C398S197000, C372S032000, C372S031000
Reexamination Certificate
active
06735395
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to wavelength division multiplexing (WDM) multichannel communication systems propagating WDM optical signal encoded with a large amount of data via an optical fiber. More particularly, the present invention relates to WDM communication systems utilizing a plurality of WDM optical sources having stabilized wavelengths and light intensity characteristics.
BACKGROUND OF THE INVENTION
The performance of multichannel WDM optical communication systems requires the integrity of signals produced by WDM optical sources. For contemporary WDM optical communication systems the wavelength and power of each WDM optical source have to remain stable for at least 10 years. Since aging of components and environmental variation cause the wavelength and power drifts, an active stabilization of wavelength and power of each WDM source has to be provided.
A conventional method of control and stabilization of the optical power of each channel of WDM optical transmission system is disclosed, for example by Onaka et al (the U.S. Pat. No. 5,894,362).
FIG. 1
of accompanying drawings is a diagram illustrating Onaka et al teaching. WDM multichannel communication system
100
comprises a plurality of diode lasers
101
connected to a corresponding plurality of data modulators
102
. A WDM multiplexer
103
combines optical outputs of the data modulators
102
. A small portion of the multiplexed optical power is tapped by coupler
104
and launched through a transmission line
105
to a wavelength selective device
106
. The wavelength selective device may be a spectrum analyzer having a resolution higher than the WDM channels spacing. The spectrum of the sample signal is then analyzed by a microprocessor
107
. The output signals of the microprocessor is used to control output power of the diode lasers
101
.
Theoretically this method could also be used for the wavelength stabilization of the high performance WDM communication systems. However it is very difficult to implement it in practice since to stabilize the center wavelength of each laser to the accuracy of the order of 0.01 nm, the spectrum analyzer must have very high spectral resolution.
A conventional method of wavelength stabilization is illustrated by diagram shown in FIG.
2
. Each optical source (usually a semiconductor diode laser)
201
of a WDM communication system generates an optical signal. A small portion of the output from each optical source
201
is diverted by an external tap coupler
202
and directed to a wavelength reference device
203
, commonly referred to as “wavelength locker”. The error signal generated by the wavelength locker is then fed back to a laser temperature control unit
204
(as described in U.S. Pat. No. 5,798,859; No. 5,825,792; and No. 6,005,995.) The laser temperature control unit
204
shifts the laser wavelength to the reference wavelength. Some commercially available diode lasers have built-in wavelength lockers (U.S. Pat. No. 5,825,792). More frequently, however, external wavelength lockers are used for wavelength stabilization (U.S. Pat. Nos. 5,798,859 and 6,005,995).
Customarily each WDM source has its own wavelength locker. As the number of WDM channels increases, the number of required wavelength lockers increases proportionally. The cost of an external wavelength locker is comparable or even higher than the cost of a WDM source. In addition, the spectrum analyzer used for channel power monitoring is extremely expensive. As a result, the stabilization devices represent a significant portion of the total cost of the multichannel WDM system.
SUMMARY OF THE INVENTION
The present invention provides a cost effective, integrated solution for the wavelength and power stabilization of all channels in a WDM communication system, where a low frequency, small depth modulation is superimposed on the light intensity of each individual WDM channel before the corresponding channel is multiplexed by the WDM muliplexer. After WDM channels are multiplexed, modulations having a specific frequency for each channel, serve as identifiers for the corresponding channels. By applying a Fourier transform on total light intensity of the combined optical signal, the characteristic frequency components, and related information, of each channel is extracted.
According to the present invention a WDM communication system for propagating a plurality of optical signals produced by a corresponding plurality of WDM optical sources via an optical fiber comprises a transmission system for generating and transmitting optical signals. The optical signals are modulated with distinguishing low frequency electrical signals having small modulation depth and used as identifiers for each WDM optical source. The modulated optical signals are mixed by the WDM multiplexer. To detect a small portion (about 1%) of WDM optical signal and obtain first and second electrical signals carrying information on light intensity and wavelength of each WDM optical source, a detection system is coupled to the transmission system. A control system is inserted between the transmission system and the detection system for analyzing the first and second electrical signals by Fourier transform and obtaining information on wavelength and light intensity for each WDM optical source. Wavelength and light intensity are adjusted accordingly.
According to one aspect of the present invention the transmission system comprises a plurality of data modulators that are connected respectively to the plurality of WDM optical sources for modulating the optical signal in each WDM channel by an electrical signal of low frequency and small depth of modulation. A low frequency generator is coupled to each data modulator for generating a distinguishing electrical signal of low frequency and small modulation depth. Each data modulator comprises high frequency input for transmitting the optical signal of the WDM optical source and low frequency input for applying the distinguishing electrical signal. A plurality of variable optical attenuators is coupled to a respective plurality of data modulators for setting a predetermined optical power for each WDM optical source. A WDM multiplexer combines output signals of the variable optical attenuators into a WDM optical signal. About 1% of WDM optical signal is diverted by an external tap coupler. A splitter divides a diverted portion of WDM optical signal into first and second sample signals for directing them into respective first and second transmission paths. A first detector is placed within the first transmission path for detecting and converting this first sample signal into a first electrical signal. A wavelength locker and a second detector are placed within the second transmission path for providing wavelength selectivity of the second sample signal and converting it into a second electrical signal. A microprocessor is coupled to the output of the first and second detectors for analyzing first and second electrical signals by transforming them into Fourier components corresponding respectively to intensity and wavelengths of the WDM optical sources. Feedback connectors are coupled between the microprocessor and WDM optical sources for providing digital feedback on wavelength characteristics of WDM optical sources, and between the microprocessor and variable optical attenuators for providing digital feedback on light intensity of WDM optical sources.
According to another aspect of the present invention, a plurality of LiNbO
3
modulators are utilized as data modulators. A bias control unit coupled to each LiNbO
3
modulator generates a distinguishing electrical signal of low frequency and small modulation depth. Each LiNbO
3
modulator comprises high frequency input for transmitting the optical signal of the WDM optical source and low frequency input for applying the distinguishing electrical signal of low frequency and small modulation depth.
According to yet another aspect of the present invention, directly modulated diode lasers are utilized as both WDM optical sources and data mo
Andrews & Kurth LLP
Bush Gary L.
FutureWei Technologies, Inc.
Negash Kinfe-Michael
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