Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-06-30
2001-08-07
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06271944
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of optical communication systems. More particularly, the present invention provides a method and apparatus for controlling and stabilizing laser wavelengths in a dense wavelength division multiplexer transmission system.
BACKGROUND OF THE INVENTION
Over the past few years, the use of fiber optic networks in communication systems has increased dramatically. Such fiber optic networks are commonly employed, for example, in long distance telecommunication systems, cable television systems, and Internet cable systems. In the future, the use of fiber optic networks will become even more prevalent as a preferred medium for transferring information as the marketplace for wide-bandwidth services matures. For instance, such services may include video-on-demand, interactive television and games, image networking, and video conferencing.
As the demand for fiber optic networks increases, the development of new supporting technologies and the refinement of existing technologies is required for the implementation of the above-identified services. One key for any such fiber optic network is the ability to multiplex and demultiplex optical signals. One preferred optical device for performing such functions is a wavelength division multiplexer (WDM).
A WDM is a device with multiple optical paths, each of which exhibits a particular wavelength passband. Each passband permits passage of one or more particular wavelengths (i.e., a Achannel@) along the respective optical path, to the substantial exclusion of others. Thus, the WDM can be used to a divide multichannel optical signal into specific wavelength channels, or to combine various channels on respective optical paths into one multichannel optical signal on one optical path.
Three basic classes of WDMs are commonly used, and are classified as coarse, intermediate, and dense. Coarse WDMs are configured for dividing and combining two channels that are spaced relatively far apart, e.g., a 1310/1550 nanometers (nm) WDM used to separate wavelength channels with a 100 nm bandwidth centered around 1310 nm and 1550 nm. Intermediate WDMs are configured for dividing and combining two to three channels that are spaced closer than those of the course WDMs, e.g., a 1540/1560 nm WDM used to space two channels approximately 20 nm apart in the 1550 nm wavelength band. Lastly, and subject of the present invention, dense WDMs (also referred to as DWDMs) are configured for dividing and combining four or more channels that are very closely spaced, e.g., 32 channels having a spacing of less than 1.0 nm.
DWDM transmitters in closely spaced DWDM transmission systems require accurate wavelength setting and stabilization. In many cases, to ensure system reliability, active wavelength monitoring and stabilization techniques are used to independently stabilize each transmitter in the DWDM array. Unfortunately, previously available wavelength monitoring and stabilization techniques are often complex, expensive, difficult to implement, and have limited effectiveness.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for controlling and stabilizing laser wavelengths in a dense wavelength division multiplexer transmission system.
The lasers in an optical communication system are each modulated in a known manner by a data signal. In addition, the present invention further modulates each laser using a plurality of test signals each having a predetermined frequency. At the optical receiver, a frequency analyzer examines the frequency test signals for distortions and/or changes in amplitude. Any distortions and/or changes in amplitude of the frequency test signals indicate a change in the wavelength of a corresponding laser. A control signal may be returned to the laser controller of each laser to regulate the laser wavelength. Alternately, a fault signal may be generated indicating that the wavelength of a laser has changed, drifted, etc., more an acceptable amount.
Generally, the present invention provides an optical communication system comprising: a plurality of optical sources each having a distinct wavelength; a system for modulating each of the optical sources with a data signal and a plurality of frequency test signals, thereby producing a plurality of optical signals; a wavelength division multiplexer (WDM) for receiving the plurality of optical signals and for outputting a multiplexed optical signal; a wavelength division demultiplexer (WDD) for receiving and separating the multiplexed optical signal into the plurality of optical signals; and a system for analyzing the frequency test signals in each optical signal to indicate a change in the wavelength of the corresponding optical source.
The present invention additionally provides an optical communication method comprising the steps of: providing a plurality of optical sources each having a distinct wavelength; modulating each of the optical sources with a data signal and a plurality of frequency test signals to produce a plurality of optical signals; converting the plurality of optical signals into a multiplexed optical signal; separating the multiplexed optical signal into the plurality of optical signals; and analyzing the frequency test signals in each optical signal to indicate a change in the wavelength of the corresponding optical source.
The present invention further provides a wavelength stabilization system comprising: a plurality of optical sources each having a distinct wavelength; a system for modulating each of the optical sources with a data signal and a plurality of frequency test signals to produce a plurality of optical signals; a system for analyzing the frequency test signals in each optical signal to indicate a change in the wavelength of the corresponding optical source, and for generating a plurality of control signals each corresponding to one of the optical signals; and a controller for regulating the wavelength of each of the optical sources based on the corresponding control signal generated by the analyzing system.
The present invention also provides a method for wavelength stabilization comprising the steps of: providing a plurality of optical sources each having a distinct wavelength; modulating each of the optical sources with a data signal and a plurality of frequency test signals to produce a plurality of optical signals; analyzing the frequency test signals in each optical signal to indicate a change in the wavelength of the corresponding optical source, and generating a control signal in response thereto; and regulating the wavelength of each of the optical sources based on the corresponding control signal generated by the analyzing system.
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Mutalik Venkatesh G.
Schemmann Marcel F. C.
Bello Agustin
Pascal Leslie
Philips Electronics North America Corp.
Piotrowski Daniel J.
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