Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/501—Structural aspects
  • H04B10/506—Multiwavelength transmitters
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/572—Wavelength control
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems

Definitions

• the present invention is in the field of optical transmission 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.
• fiber optic networks are commonly employed, for example, in long distance telecommunication systems, cable television systems, and Internet cable systems.
• 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.
• such services may include video-on-demand, interactive television and games, image networking, and video conferencing.
• WDM wavelength division multiplexer
• 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 "channel") along the respective optical path, to the substantial exclusion of others.
• 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.
• dense WDMs also referred to as DWDMs
• DWDMs dense WDMs
• dense WDMs 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.
• active wavelength monitoring and stabilization techniques are used to independently stabilize each transmitter in the DWDM array.
• previously available wavelength monitoring and stabilization techniques are often complex, expensive, difficult to implement, and have limited effectiveness.
• the present invention provides a system for providing feedback control information in an optical transmission system that comprises a dense wavelength division multiplexer.
• the present invention generally includes an optical transmission system comprising: a plurality of optical sources each having a distinct wavelength; a first control device for modulating each of the optical sources with a data signal, thereby producing a plurality of optical signals; a second control device for inserting a unique signature into each of the plurality of optical signals; a wavelength division multiplexer (WDM) for receiving the plurality of optical signals and for outputting a multiplexed optical signal; a Febri Perot (FP) cavity that receives the multiplexed optical signal; and a wavelength decoder that examines an output of the FP cavity and identifies each of the multiplexed optical signals based upon the inserted signature.
• the unique signature inserted into each of the plurality of optical signals may comprise a predetermined frequency modulation that allows a respective optical signal to be readily identified, examined, and stabilized.
• the invention also comprises a method for controlling optical signals in a transmission system utilizing a wave division multiplexer, comprising the steps of: inserting a unique signature into each of the optical signals; multiplexing the optical signals with the DWDM to create a multiplexed signal; inputting the multiplexed signal into a Febri Perot
• FP FP cavity
• analyzing an output of the FP cavity to identify each optical signal based upon its unique signature analyzing an output of the FP cavity to identify each optical signal based upon its unique signature
• providing feedback control signals to a controller to control the optical signals based upon the analyzing step It is therefore an advantage of the present invention to provide a single element (e.g., FP cavity) with a known transmission function versus wavelength to stabilize more than one transmitter.
• the element e.g., FP cavity
• Fig. 1 is a block diagram depicting an optical transmission system in accordance with a preferred embodiment of the present invention
• Fig. 2. depicts FP cavity transmission coefficients as a function of wavelength in accordance with a preferred embodiment of the present invention
• Fig. 3 depicts an FP cavity transmission coefficient in accordance with a preferred embodiment of the present invention
• Fig. 4 depicts the relationship between wavelength misalignment and the intensity of the output of an FP cavity in accordance with a preferred embodiment of the present invention.
• Fig. 1 illustrates a dense wavelength division multiplexer (DWDM) optical transmission system 10 incorporating a wavelength stabilization system in accordance with a preferred embodiment of the present invention.
• DWDM dense wavelength division multiplexer
• the optical transmission system 10 includes a DWDM grid or array comprising a plurality of transmitters (e.g., lasers) 12 ⁇ , 12 2 , .... 12 context, each having a predefined wavelength or channel, ⁇ ls ⁇ 2 , ..., ⁇ utilizat.
• the output of each of the transmitters 12 ⁇ , 12 , ..., 12 n is modulated in a manner known in the art by an electrical data signal Si, S 2 , ..., S n , respectively, under control of a first control device 14j, 14 2 , ..., 14 n .
• each of the transmitters 12 1 ⁇ 12 2 , ..., 12 n is additionally modulated by a predetermined "signature" Mi, M , ..., M resort, respectively, under control of a second control device 16 ⁇ , 16 2 , ..., 16 n . That is, a disturbance is added to the output of each of the transmitters 12 ⁇ , 12 2 , ..., 12 n.
• the signatures Mi, M 2 , ..., M n are used to separate and identify optical information corresponding to each individual transmitter 12 ls 12 2 , .... 12 n , respectively, from the output of a Febri Perot (FP) cavity 30.
• FP Febri Perot
• the present invention intentionally adds a small magnitude disturbance, e.g., a distinct, identifiable signature M, to the output of each transmitter.
• a small magnitude disturbance e.g., a distinct, identifiable signature M
• the output of each transmitter 12 ⁇ , 12 , ..., 12 n is wavelength modulated a predetermined amount by the signatures Mj, M 2 , ..., M n , (e.g., 1Hz, 2Hz, 3 Hz, etc.).
• the magnitude of the signatures Mi, M 2 , ..., M ⁇ is chosen to be small enough to not adversely affect the performance of the optical transmission system 10.
• a third control device 18], 18 , ..., 18 n provides a means by which the output of each transmitter 12 ⁇ , 12 2 , ..., 12 n , respectively, can be modified based upon feedback information provided by a wavelength decoder 20.
• the feedback information can be used for any purpose, including to stabilize the output of the transmitters 12 ⁇ , 12 2 , ..., 12 n , to perform error detection, etc.
• the control devices 14, 16 and 18 can be implemented with a microcontroller, software, the combination of both hardware and software, or any other known means.
• the modulated optical signals ST, S 2 ', ... S n ', produced by the transmitters 12 ⁇ , 12 2 , ..., 12 n> are then directed into an optical multiplexer 22.
• the modulated optical signals Si', S ', ..., S n ', are combined by the optical multiplexer 22 into a transmission signal S t in a manner known in the art.
• the transmission signal S t is transmitted along an optical guide 24, e.g., a fiber optic cable, where it is received and demultiplexed at a receiving section (not shown) of the optical transmission system 10.
• the optical transmission system 10 of the present invention includes a feedback loop comprising the FP cavity 30, a photodetector system 26, and the wavelength decoder 20.
• the FP cavity 30 receives the transmission signal S t through a splitter 28 that couples a small fraction of the light to the FP cavity 30.
• the spacing of the peaks of the FP cavity 30 is chosen to be equal to the wavelength spacing of the transmitters 12 ⁇ , 12 2 , ..., 12 n , of the DWDM grid.
• the FP cavity 30 simultaneously transmits individual optical signals at a ratio depending on their momentary wavelength.
• the output of the FP cavity 30 corresponds to the combination or summation of the optical outputs of all of the transmitters 12 ⁇ , 12 2 , ..., 12 n .
• FP cavities are known in the art and will not be described in detail herein. While this preferred embodiment contemplates utilizing a FP cavity 30, any system that incorporates a wavelength dependent transmission function could be used, e.g., distributed feedback (DFB).
• DFB distributed feedback
• an FP cavity transmission coefficient as a function of wavelength is depicted in graphical form.
• the FP cavity 30 selectively passes through discrete bands of the transmission signal S t .
• the bands are spaced equally to the wavelength spacing of the transmitters 12 ⁇ , 12 2 , ..., 12 n .
• Discrete wavelengths at the center of these bands have the highest transmission coefficients.
• the FP cavity 30 offers an inexpensive method to match up an array of transmitter wavelengths against the series of transmission peaks available.
• a system must be provided to identify and separate the optical components corresponding to each individual transmitter 12 ⁇ , 12 2 , ..., 12 n , from the optical output of the FP cavity 30. In accordance with the present invention, this is achieved through the use of the signatures Mi, M , ..., M n .
• the FP cavity 30 is very sensitive to wavelength. Thus, it is possible to observe the modulation of the transmitters 12 ⁇ , 12 2 , ..., 12 n , caused by the addition of the signatures Mi, M , ..., M n.
• the wavelength decoder 20 can tune into each of the signatures Mi, M 2 , ..., M n , using the photodetector system 26, which preferably comprises a single photodetector, to separate the optical component of each individual transmitter 12 ⁇ , 12 2 , ..., 12 n , from the combined optical output of the FP cavity 30.
• the separated optical components can then be analyzed to detect a change in the wavelength of a corresponding transmitter 12 ⁇ , 12 , ..., 12 n , and to generate feedback information, e.g., for control and stabilization of the transmitters 12 ⁇ , 12 2 , ..., 12 lake.
• the feedback for each transmitter 12 ⁇ , 12 2 , ..., 12 n is provided to the third control device 18 ⁇ , 18 2 , ..., 18 n , of the transmitters 12 ⁇ , 12 , ..., 12 n , respectively.
• the wavelength decoder 20 can tune into the optical output of a specific one of the transmitters 12 ⁇ , 12 , ..., 12 n , based on the signatures Mi, M 2 , ..., M Practical, using the photodetector system 26.
• the wavelength decoder 20 can be implemented by a microcontroller device, software application, or any combination of hardware and software capable of recognizing and distinguishing among the signatures.
• the photodetector system 26 which preferably comprises a single photodiode or the like, converts the optical output of the selected transmitter 12 into an electrical signal that is supplied to the wavelength decoder 20.
• the wavelength decoder 24 and the control devices 14, 16, and 18 are shown as separate devices, it is understood that any of these devices could be combined into a single control device, or divided or combined with other devices without departing from the scope of the present invention.
• An example of an FP cavity 30 transmission coefficient for a transmitter 12 N having a modulated wavelength ⁇ (t) centered about ⁇ N is illustrated in FIG. 3. If the wavelength of the transmitter 12 N is misaligned by an amount ⁇ the intensity transmitted by the FP cavity 30 will vary accordingly.
• the amount of correction to be applied to a specific transmitter 12 can be determined by the waveform decoder 20 based on the intensity of the output of the FP cavity 30 compared to its expected value.
• the sign of the misalignment ⁇ i.e., a positive or negative misalignment, determines the sign of the transmitted signal I(t).
• a graph is depicted that illustrates transmitted intensity 1(f) as a function of wavelength.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Optical Communication System (AREA)
• Semiconductor Lasers (AREA)

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Citations (4)

• 2000
  • 2000-06-20 AT AT00942104T patent/ATE359676T1/de not_active IP Right Cessation
  • 2000-06-27 EP EP00942140A patent/EP1108296A1/de not_active Withdrawn
  • 2000-06-27 WO PCT/EP2000/005958 patent/WO2001003338A1/en not_active Application Discontinuation
  • 2000-06-27 JP JP2001508083A patent/JP2003503939A/ja active Pending

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