LASER WAVELENGTH CONTROL IN AN OPTICAL TRANSMISSION SYSTEM

Abstract: An optical transmission system having a dense wavelength division multiplexer (DWDM). A disturbance is inserted into the output of each wavelength transmitter. A feedback control system for stabilizing the wavelength of each transmitter employs a method for ananalyzing the output of a device having wavelength dependent transmission for signatures of the disturbances.

Description: Laser wavelength control in an optical transmission system 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. Over the past few years, the use of fiber optic networks in transmission 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 "channel") 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. 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) cavity; analyzing an output of the FP cavity to identify each optical signal based upon its unique signature; and 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. It is therefore a further advantage of the present invention to provide a system for monitoring the output of the element (e.g., FP cavity) when it is excited with the combination of all wavelengths (i.e., after multiplexing), separating out information corresponding to each individual transmitter, and providing feedback control information to each individual transmitter. This is accomplished by modulating the wavelength of each transmitter individually with a known function. The invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which: 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; and 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. 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 transmission system 10 incorporating a wavelength stabilization system in accordance with a preferred embodiment of the present invention. 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 transmission system. The optical transmission system 10 includes a DWDM grid or array comprising a plurality of transmitters (e.g., lasers) 12ι, 12 2 , .... 12„, each having a predefined wavelength or channel, λ ls λ 2 , ..., λ„. The output of each of the transmitters 12ι, 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 . The output of each of the transmitters 12 1} 12 2 , ..., 12 n , is additionally modulated by a predetermined "signature" Mi, M , ..., M„, respectively, under control of a second control device 16ι, 16 2 , ..., 16 n . That is, a disturbance is added to the output of each of the transmitters 12ι, 12 2 , ..., 12 n. Advantageously, as will be presented in greater detail below, 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. Thus, instead of maintaining the output of each transmitter 12 l5 12 2 , ..., 12„, as stable as possible, as is commonly attempted in known optical transmission systems, the present invention intentionally adds a small magnitude disturbance, e.g., a distinct, identifiable signature M, to the output of each transmitter. In the preferred embodiment of the present invention, the output of each transmitter 12ι, 12 , ..., 12 n , is wavelength modulated a predetermined amount by the signatures Mj, M 2 , ..., M n , (e.g., 1Hz, 2Hz, 3 Hz, etc.). Alternatively, random modulation or programmable modulations schemes could also be used. Preferably, the magnitude of the signatures Mi, M 2 , ..., M π , 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 12ι, 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 12ι, 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 12ι, 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. To stabilize the wavelengths of the transmitters 12 ls 12 2 , ..., 12 n , 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. Preferably, the spacing of the peaks of the FP cavity 30 is chosen to be equal to the wavelength spacing of the transmitters 12ι, 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. In the present optical transmission system 10, the output of the FP cavity 30 corresponds to the combination or summation of the optical outputs of all of the transmitters 12ι, 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). Referring now to Fig. 2, an FP cavity transmission coefficient as a function of wavelength is depicted in graphical form. As shown, the FP cavity 30 selectively passes through discrete bands of the transmission signal S t . In this embodiment, the bands are spaced equally to the wavelength spacing of the transmitters 12ι, 12 2 , ..., 12 n . Discrete wavelengths at the center of these bands have the highest transmission coefficients. Thus, the FP cavity 30 offers an inexpensive method to match up an array of transmitter wavelengths against the series of transmission peaks available. To provide stabilization feedback to the transmitters 12j, 12 2 , ..., 12„ , however, a system must be provided to identify and separate the optical components corresponding to each individual transmitter 12ι, 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 12ι, 12 2 , ..., 12 n , caused by the addition of the signatures Mi, M , ..., M n. Advantageously, in the present invention, 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 12ι, 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 12ι, 12 , ..., 12 n , and to generate feedback information, e.g., for control and stabilization of the transmitters 12ι, 12 2 , ..., 12„. The feedback for each transmitter 12ι, 12 2 , ..., 12 n , is provided to the third control device 18ι, 18 2 , ..., 18 n , of the transmitters 12ι, 12 , ..., 12 n , respectively. The wavelength decoder 20 can tune into the optical output of a specific one of the transmitters 12ι, 12 , ..., 12 n , based on the signatures Mi, M 2 , ..., M„, 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. Although 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 λ(t) centered about λ N is illustrated in FIG. 3. If the wavelength of the transmitter 12 N is misaligned by an amount δλ the intensity transmitted by the FP cavity 30 will vary accordingly. Therefore, 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 δλ, i.e., a positive or negative misalignment, determines the sign of the transmitted signal I(t). Referring now to Fig. 4, a graph is depicted that illustrates transmitted intensity 1(f) as a function of wavelength. Thus, by examining the transmitted intensity of each signal, means are provided for determining the amount of misalignment, if any, of each transmitter 12 1} 12 2 , ..., 12„. 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.

Claims: CLAIMS: 1. An optical transmission system comprising: a plurality of optical sources (12) each having a distinct wavelength; a first control device (14) for modulating each of the optical sources with a data signal, thereby producing a plurality of optical signals; a second control device (16) for inserting a unique signature into each of the plurality of optical signals; a wavelength division multiplexer (WDM) (22) for receiving the plurality of optical signals and for outputting a multiplexed optical signal; a Febri Perot (FP) cavity (30) that receives the multiplexed optical signal; and a wavelength decoder (20) that examines an output of the FP cavity and identifies each of the multiplexed optical signals based upon the inserted signature. 2. The optical transmission system of claim 1 , further comprising a photodetector system (26) for converting an output of the FP cavity to an electrical signal. 3. The optical transmission system of claim 1, further comprising a third control device (18) for modifying the wavelength of each optical source based on an output of the wavelength decoder. 4. The optical transmission system of claim 3, wherein the third control device includes a stabilization mechanism. 5. The optical transmission system of claim 3, wherein the third control device includes a system for identifying a fault status in any of the optical sources. 6. The optical transmission system of claim 1 , wherein each unique signature comprises a predetermined modulation. 7. An optical transmission system utilizing a wave division multiplexer (WDM) that receives a plurality of optical signals at different wavelengths and outputs a multiplexed signal suitable for transmission, the optical transmission system having a feedback control loop for controlling the plurality of optical signals as the signals are inputted into the WDM, the transmission system comprising: a first control device (Cl) for inserting a unique signature into each of the plurality of optical signals; an element having a known transmission function versus wavelength that receives the multiplexed signal from the WDM; a mechanism (26) for converting an output of the element into electrical signals; a wavelength decoder (20) for analyzing the electrical signals and for identifying each of the plurality of optical signals based upon the unique signatures, and for creating feedback control signals for each of the optical signals; and a second control device (16) for controlling each of the plurality of optical signals based upon the feedback control signals received from the wavelength decoder. 8. The optical transmission system of claim 7, wherein the element is a Febri Perot (FP) cavity. 9. The optical transmission system of claim 7, wherein each unique signature comprises a predetermined modulation. 10. The optical transmission system of claim 7, wherein the feedback control signals comprise fault detection information or stabilization information. 11. An optical transmission system having a dense wave division multiplexer (DWDM), comprising: a first control device for adding a unique signature to each of a plurality of optical signals being inputted into the DWDM; an optical element for receiving a multiplexed signal outputted from the DWDM, and for outputting a plurality of decomposed optical signals; a device for analyzing the decomposed optical signals and for identifying each of the plurality of optical signals based upon the unique signatures; and a device for providing control information regarding each of the plurality of optical signals. 12. The optical transmission system of claim 11 , wherein the optical element is a Febri Perot (FP) cavity. 13. A method for controlling optical signals in a transmission system utilizing a dense wave division multiplexer (DWDM), 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) cavity; analyzing an output of the FP cavity to identify each optical signal based upon its unique signature; and providing feedback control signals to a controller to control the optical signals based upon the analyzing step. 14. The method of claim 13, wherein the step of inserting a signature comprises the step of modulating each optical signal at a predetermined frequency.

Inventors: MUTALIK VENKATESH G, SCHEMMANN MARCEL F C, MUTALIK, VENKATESH, G., SCHEMMANN, MARCEL, F., C.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H04B10
IPC codes: H01S5, H04B10, H04J14