Abstract:
A method and apparatus for controlling and stabilizing laser wavelengths in a dense wavelength division multiplexer (DWDM) transmission system. The lasers in the transmission system are each modulated in a known manner by a data signal. In addition, the lasers are each modulated by a plurality of test signals each having a predetermined frequency. At the optical receiver, a frequency analyzer examiners 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. A fault signal may be generated indicating that the wavelength of a laser has changed, drifted, etc., more an acceptable amount.

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention will best be understood from a detailed description of the invention and a preferred embodiment thereof selected for the purposes of illustration and shown in the accompanying drawings in which: 
     FIG. 1 illustrates an optical communication system including a wavelength stabilization system in accordance with a preferred embodiment of the present invention; and 
     FIG. 2 illustrates a plurality of frequency test signals for a laser. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The features and advantages of the present invention are illustrated in detail in the accompanying drawings, wherein like reference numerals refer to like elements throughout the drawings. Although the drawings are intended to illustrate the present invention, the drawings are not necessarily drawn to scale. 
     Referring now to the figures, FIG. 1 illustrates a dense wavelength division multiplexer (DWDM) optical communication system  10  incorporating a wavelength stabilization system in accordance with a preferred embodiment of the present invention. Generally, the optical communication system  10  includes a transmission section  12 , a receiving section  14 , and a feedback control loop  16 . Although the wavelength stabilization system of the present invention is described in conjunction with a DWDM, in which wavelength control and stabilization is important, it should be apparent to those skilled in the art that the wavelength stabilization system of the present invention may also be used to control and stabilize laser wavelengths in any type of optical communication system. 
     The transmission section  12  includes a plurality of lasers  18   1 ,  18   2 , . . . ,  18   n , each having a predefined wavelength or channel, λ 1 , λ 2 , . . . , λ n . Each of the lasers  18   1 ,  18   2 , . . . ,  18   n , is modulated in a manner known in the art by an electrical data signal S 1 , S 2 , . . . , S n , respectively. In addition, each of the lasers  18   1 ,  18   2 , . . . ,  18   n  is modulated by a plurality of test signals f each having a predetermined frequency. In the optical communication system  10  illustrated in FIG. 1, each of the lasers  18   1 ,  18   2 , . . . ,  18   n  is modulated by a pair of test signals f. Additional test signals f may also be used to modulate the lasers  18   1 ,  18   2 , . . . ,  18   n , without departing from the intended scope of the present invention. As shown in detail in FIG. 2, the frequency test signals f are chosen to be out of the main signal bands of the data signals S 1 , S 2 , . . . , S n  to prevent interference with the data signals. As illustrated in FIG. 1, the laser  18   1  is modulated by data signal S 1  and first and second frequency test signals f 1  and f 2 . Similarly, the lasers  18   2 ,  18   3 , . . . ,  18   n , are modulated by the data and frequency test signals S 2 , f 3  and f 4 , S 3 , f 5  and f 6 , . . . , respectively. 
     The modulated optical signals S 1 ′, S 2 ′, . . . S n ′ produced by the lasers  18   1 ,  18   2 , . . . ,  18   n  are then directed into an optical multiplexer  20 . The modulated optical signals S 1 ′, S 2 ′, . . . , S n ′ are combined by the optical multiplexer  20  into a transmission signal S t  in a manner known in the art. The transmission signal S t  is transmitted along an optical guide  22 , e.g., a fiber optic cable, where it is received and demultiplexed at the receiving section  14  of the optical communication system  10 . The transmission signal S t  is demultiplexed by an optical demultiplexer  24  in a known manner and is separated into the modulated optical signals S 1 , S 2 ′, . . . , S n ′. 
     In the receiver section, each individual modulated optical signal S 1 ′, S 2 ′, . . . , S n ′ is directed into a respective receiver node  26   1 ,  26   2 , . . . ,  26   n . The receiver node  26   1 , is illustrated in detail in FIG.  1 . The remaining receiver nodes  26   2 ,  26   3 , . . . ,  26   n  include similar components. 
     Receiver node  26   1  includes an optical receiver  28   1  for extracting the data signal S 1  and the frequency test signals f 1  and f 2  from the optical signal S 1 ′ in a known manner. In addition, the receiver node  26   1  includes a frequency analyzer  30   1  for examining the frequency test signals f 1  and f 2  for distortion and/or changes in amplitude. 
     Distortion of the frequency test signals f 1  and f 2  is preferably determined by analyzing any relative frequency changes between f 1  and f 2 . For example, distortion of the frequency test signals f 1  and f 2 , which indicates a change in the wavelength λ 1  of the laser  18   1 , may be determined according to Δ(f 1 +f 2 ) or Δ(f 1 −f 2 ). Alternately, or in addition, distortion of the frequency test signals f 1  and f 2  may be determined by examining changes in amplitude of one or both of the signals. Other techniques may also be used to determine distortion of the frequency test signals f 1  and f 2 . Distortion of the frequency test signals associated with the remaining lasers  18   2 ,  18   3 , . . . ,  18   n , may be determined in a similar manner. 
     Information generated by the frequency analyzer  30   1  may be transmitted back to the transmission section  12  via the feedback control loop  16  to control and stabilize the laser  18   1 . Alternately, the output of the frequency analyzer  30   1  may be fed into a fault detection/alarm system  32 . The fault detection/alarm system  32  may be used to detect faults (e.g., excessive wavelength drift), log such faults, generate alarms, etc., in response to output of any of the frequency analyzers  30   1 ,  30   2 , . . . ,  30   n . Further, part or all of the signal and/or distortion/amplitude analysis may be performed at the transmission section  12  by the controllers of the lasers  18   1 ,  18   2 , . . . ,  18   n  or other control system. In this case, the frequency test signals f 1  and f 2  are transmitted back (see below) to the transmission section  12  for analysis. 
     If the information provided by the frequency analyzer  30   1  is to be transmitted back to the transmission section  12 , the receiver node  26   1  is provided with a return laser  34   1 . Each of the remaining receiver nodes  26   2 ,  26   3 , . . . ,  26   n , is also provided with a corresponding return laser  34   2 ,  34   3 , . . . ,  34   n . The return lasers  34   1 ,  34   2 , . . . ,  34   n  each have a predefined wavelength or channel, λ r1 , λ r2 , . . . λ rn , respectively. Each of the return lasers  34   1 ,  34   2 , . . .  34   n  is modulated in a manner known in the art by an electrical data signal S r1 , S r2 , . . . , S rn , respectively. In addition, or alternately, each of the return lasers  34   1 ,  34   2 , . . . ,  34   n  is modulated by a control signal C 1 , C 2 , . . . , C n , respectively, that contains information regarding changes in the wavelengths λ 1 , λ 2 , . . , λ n  of the lasers  18   1 ,  18   2 , . . . ,  18   n . 
     The modulated optical signals S r1 ′, S r2 ′, . . . , S rn ′ produced by the return lasers  34   1 ,  34   2 , . . . ,  34   n  are then directed into an optical or electrical and optical multiplexer  36 . The modulated optical signals S r1 ′, S r2 ′, . . . , S rn ′ are combined by the optical multiplexer  36  into a transmission signal S rt  in a manner known in the art. The transmission signal S rt  is transmitted along an optical guide  38 , e.g., a fiber optic cable, where it is received and demultiplexed at the transmission section  12  of the optical communication system  10 . The transmission signal S rt  is demultiplexed by an optical demultiplexer  40  in a known manner and is separated into the modulated optical signals S r1 ′, S r2 , . . . , S rn ′. 
     As illustrated in detail in FIG. 1, the control signal C 1  is extracted from the modulated optical signal S r1 ′ by an optical receiver  441 . The control signal C 1  is subsequently fed into the controller  46   1  of the laser  18   1  to control and stabilize the wavelength of the laser. Similarly, the control signals C 2 , C 3 , . . . , C 1  are fed into the corresponding controllers  46   2 ,  46   3 , . . . ,  46   n  of the remaining lasers  18   2 ,  18   3 , . . . ,  18   n . 
     The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.