Abstract:
A bidirectional optical link has a plurality of wavelengths to carry information in first and second differing transmission directions to optimize system performance. First and second sets of wavelengths of the plurality of wavelengths are determined wherein the wavelengths of the first set alternate with the wavelengths of the second set. Transmitting is performed in the first transmission direction by way of the first set of wavelengths and in the second transmission direction by way of the second set of wavelengths whereby the transmission directions of adjacent wavelengths of the plurality of wavelengths differ. The wavelengths of the plurality of wavelengths are wavelength division multiplexed within the optical link and the wavelengths transmitted in the same direction are multiplexed with each other. The wavelengths of the plurality of wavelengths can be substantially evenly spaced apart from each other or adjacent wavelengths transmitted in different directions may overlap. The wavelengths of the first set of wavelengths and the wavelengths of the second set of wavelengths are transmitted by way of a single optical fiber. The single optical fiber is provided with a bidirectional amplifier. Selected wavelengths of the plurality of wavelengths can be moved during the transmitting of the plurality of wavelengths in order to avoid interference.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of optical communication systems and, in particular, to the field of bidirectional optical communication links within such systems wherein a single optical communication link transmits a plurality of carrier wavelengths. 
     BACKGROUND OF INVENTION 
     Optical networks having a plurality of optical transmission lines permit high bandwidth data communications. In optical data networks high speed data is modulated on light waves that are transmitted through the optical links of the data network. Optical transmission links or lines of this type can be used in telephone systems and various other types of data communication systems. Further bandwidth improvement can be achieved in an optical network by modulating different electrical data signals on distinct light wave carriers wherein each light wave carrier has a different wavelength. This technique is known as wavelength division multiplexing (WDM). Optical systems using WDM therefore require a plurality of optical transmitters and optical receivers operating at different light frequencies. 
     When several light wave carriers operate within the same optical fiber, as in the case of WDM, unwanted interference signals can sometimes be formed in the available optical band of the fiber. For example, second harmonic distortion can produce sums of differences of the traveling wavelengths that can tend to lie outside the band of interest. However, third harmonic distortion can be within the band of interest and therefore have an interfering effect on the carrier spectrum. Interference of modulated signals with each other in this manner is referred to as crosstalk. 
     Several techniques are known in the prior art for minimizing the crosstalk between modulated signals within optical fibers. For example, it is known to select the transmission frequencies of modulated signals within an optical fiber such that the possible interference frequencies of the modulated signals do not fall within the transmission bands of other modulated signals within the fiber. Additionally, it is known to restrict the modulation bandwidths of an optical fiber in order to minimize crosstalk. Furthermore, it is known to provide guard bands between adjacent wavelengths of an optical fiber. However, each of these techniques is hindered by the limited selectivity and stability of the optical filters required for implementation. 
     Therefore, it is desirable to provide an improved system and method for optimizing system performance within optical fibers transmitting modulated wavelengths that overcomes the drawbacks of the prior art methods. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention is a system and method for communicating information within a bidirectional optical link having a plurality of wavelengths that transmits information in a first direction receiving information and from a second differing direction. First and second sets of wavelengths of the plurality of sequential wavelengths are determined wherein the wavelengths of the first set alternate with the wavelengths of the second set. The first set of wavelengths is transmitted in a first direction and the second set of wavelengths is transmitted in a second direction such that the propagation directions of adjacent wavelengths differ from each other. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a schematic representation of a prior art unidirectional wavelength division multiplexing channel plan for optical communication; 
     FIG. 2 shows a schematic representation of a prior art bidirectional wavelength division multiplexing channel plan for optical communication; and 
     FIG. 3 shows a schematic representation of the bidirectional wavelength division multiplexing channel plan of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, there is shown prior art unidirectional channel plan  10 . Unidirectional channel plan  10  is a 16-wavelength channel plan wherein the first eight wavelengths of the channel plan are used for transmission in one direction and the second eight wavelengths are used for transmission in the opposite direction. It is known in the art of optical communication systems to provide 2-, 4-, and 8-wavelength channel plans as well as 16-wavelength channel plans such as unidirectional channel plan  10 . 
     Unidirectional channel plan  10  operates upon both 8-wavelength optical communication system  12  and 8-wavelength optical communication system  16 . Optical communication systems  12 ,  16  are separate communication systems that can each transmit eight modulated wavelengths in one of two opposite directions and can be combined to form channel plan  10 . 
     Within 8-wavelength optical communication system  12  of channel plan  10  sequential optical wavelengths  14   a-h  are applied to individual optical transmitters  18   a-h  to provide transmit wavelengths  14   a-h  for transmission in the same direction through an optical link. Each transmit wavelength  14   a-h  of communication system  12  has a unique wavelength. In one embodiment of communication system  12  the sequential wavelengths of transmit wavelengths  14   a-p  can be separated from each other by approximately 1.57 nanometers. For example, transmit wavelength  14   a  can be 1530.30 nanometers, transmit wavelength  14   b  can be 1531.90 nanometers, and transmit wavelength  14   c  can be 1533.47 nanometers, etc. 
     Transmit wavelengths  14   a-h  are applied by optical transmitters  18   a-h  to WDM multiplexer  20  where they are WDM multiplexed. The multiplexed signal at the output of WDM multiplexer  20  is amplified by amplifier  24  and transmitted by way of optical fiber  25 . The signal transmitted by way of optical fiber  25  within  8 -wavelength optical communication system  12  is received and amplified by amplifier  27 . The amplified signal at the output of amplifier  27  is applied to WDM demultiplexer  26  where it is demultiplexed. 
     WDM demultiplexer  26  separates the amplified signal of amplifier  27  into eight demultiplexed signals. Each of the eight demultiplexed signals from WDM demultiplexer  26  is applied to an individual optical receiver  30   a-h . Optical receivers  30   a-h  receive the demultiplexed signals and provide received wavelengths  14   a-h . The wavelengths of the eight received wavelengths  14   a-h  provided at optical receivers  30   a-h  correspond to the wavelengths of the eight transmit wavelengths  14   a-h  at the opposite end of channel plan  10 . 
     Within 8-wavelength optical communication system  16  of channel plan  10 , sequential optical wavelengths  14   i-p  are applied to individual optical transmitters  58   a-h  to provide transmit wavelengths  14   i-p  for transmission in the direction opposite to the direction of transmit wavelengths  14   a-h . Transmit wavelengths  14   i-p  have unique wavelengths that can be separated from each other by approximately 1.57 nanometers. Optical transmitters  58   a-h  apply transmit wavelengths  14   i-p  to WDM multiplexer  54  where they are WDM multiplexed. The multiplexed signal at the output of WDM multiplexer  54  is amplified by amplifier  52  and transmitted by way of optical fiber  50 . 
     The signal transmitted by way of optical fiber  50  within optical communication system  16  is received and amplified by amplifier  48 . The amplified signal at the output of amplifier  48  is applied to WDM demultiplexer  44  for WDM demultiplexing. Each of the demultiplexed signals from demultiplexer  44  is applied to an individual optical receiver  40   a-h . Optical receivers  40   a-h  provide received wavelengths  14   i-p . The eight received wavelengths  14   i-p  at the outputs of optical receivers  40   a-h  correspond to the eight transmit wavelengths  14   i-p  at the opposite end of unidirectional channel plan  10 . 
     It will be understood by those skilled in the art that prior art unidirectional channel plans such as channel plan  10  can provide satisfactory results with respect to transmitting a plurality of optical signals through an optical link. However, it will also be understood that the required use of a second unidirectional communication system in order to obtain bidirectional communication results in substantial inefficiency in channel plans such as channel plan  10 . 
     Referring now to FIG. 2, there is shown prior art bidirectional channel plan  100  having sequential optical wavelengths  114   a-p . Bidirectional channel plan  100  is thus a 16-wavelength channel plan. The sequential wavelengths of optical wavelengths  114   a-p  within bidirectional channel plan  100  are unique. Adjacent sequential wavelengths  114   a-p  can be separated from each other by approximately 1.57 nanometers in one possible embodiment of channel plan  100 . 
     Within 16-wavelength bidirectional channel plan  100  the first eight sequential optical wavelengths  114   a-h  are applied to optical transmitters  118   a-h  to provide transmit wavelengths  114   a-h  for transmission in the same direction as each other. Transmit wavelengths  114   a-h  are applied by optical transmitters  118   a-h  to WDM multiplexer/demultiplexer  120  where they are WDM multiplexed. The multiplexed signal at the output of WDM multiplexer/demultiplexer  120  is amplified by bidirectional amplifier  124 . The amplified output signal of bidirectional amplifier  124  is transmitted by way of optical fiber  1 within channel plan  100 . 
     The output signal of amplifier  124  transmitted by way of optical fiber  125  is received and amplified by bidirectional amplifier  127 . The amplified output of bidirectional amplifier  127  is applied to WDM multiplexer/demultiplexer  126  to be demultiplexed. WDM multiplexer/demultiplexer  126  separates the amplified signal into eight demultiplexed signals. Each of the eight demultiplexed signals from WDM multiplexer/demultiplexer  126  is applied to an individual optical receiver  130   a-h . Optical receivers  130   a-h  receive the demultiplexed signals and provide received wavelengths  114   a-h . The eight received wavelengths  114   a-h  at the output of optical receivers  130   a-h  correspond to the eight transmit wavelengths  114   a-h  applied to optical transmitters  118   a-h  at the opposite end of channel plan  100 . 
     Also within bidirectional channel plan  100 , the next eight sequential optical wavelengths  114   i-p  are applied to individual optical transmitters  158   a-h . Optical transmitters  158   a-h  thus provide transmit wavelengths  114   i-p  for transmission through channel plan  100  in the direction opposite to the direction of transmit wavelengths  114   a-h . The outputs of optical transmitters  158   a-h  are applied to WDM multiplexer/demultiplexer  126  where they are WDM multiplexed. The multiplexed signal at the output of WDM multiplexer/demultiplexer  126  is received and amplified by bidirectional amplifier  127 . The amplified signal at the output of bidirectional amplifier  127  is transmitted by way of optical fiber  125 . 
     The signal from bidirectional amplifier  127  is received and amplified by bidirectional amplifier  124 . The amplified signal at the output of bidirectional amplifier  124  is applied to WDM multiplexer/demultiplexer  120  for WDM demultiplexing into eight demultiplexed signals. Each of the eight demultiplexed signals from WDM multiplexer/demultiplexer  120  is applied to an individual optical receiver  140   a-h . Optical receivers  140   a-h  provide received wavelengths  114   i-p . The eight received wavelengths  114   i-p  at the outputs of optical receivers  140   a-h  correspond to the eight transmit wavelengths  114   i-p.    
     Both prior art channel plans  10 ,  100  are subject to interference between the various modulated optical wavelengths transmitted during the communication process. For example, both prior art channel plans  10 ,  100  can have nonlinear crosstalk and four wave mixing because of the narrow spacing between the wavelengths. Furthermore, in channel plans  10 ,  100  the combination of the two wavelengths (2λ−λ) in the same transmission direction can cross over between channels and interfere with another wavelength in an adjacent channel with a wavelength traveling in the same direction. 
     Referring now to FIG. 3, there is shown bidirectional channel plan  200  of the present invention. Bidirectional channel plan  200  has sixteen optical wavelengths  214   a-p  and is thus a 16-wavelength channel plan. It is adapted to provide transmission of eight wavelengths in each of two differing directions. While the system and method of the present invention is thus described with respect to a 16-wavelength channel plan for illustrative purposes, it will be understood that the present invention can be advantageously applied to channel plans of any size. Also, in this embodiment the channels have sequential optical wavelengths, however, the channel wavelengths need not be sequential, e.g., there may be a gap in wavelengths between channels. Alternately, if capacity is an issue, adjacent channels may have overlapping wavelengths. 
     The optimization in system performance within bidirectional channel plan  200  of the present invention is accomplished by alternating the transmission directions of adjacent optical wavelengths  214   a-p  available within the optical link. For example, in the case wherein optical wavelengths  214   a,b,c  are sequential, optical wavelength  214   a  can be used as a transmit wavelength at one end of the optical link. At the same end of the optical link, optical wavelength  214   b  can be received by an optical detector. Optical wavelengths  214   a,b , adjacent to each other, are transmitted in different directions. This helps to reduce unwanted interference within bidirectional channel plan  200 . 
     The next optical wavelength in the sequence, optical wavelength  214   c , can then be used as the next transmit wavelength at the same end of the optical link. The previously allocated wavelength that is adjacent to optical wavelength  214   c  is optical wavelength  214   b . Since adjacent optical wavelength  214   b  is a received wavelength at this end of channel plan  200  while transmit wavelength  214   c  is a transmit wavelength, crosstalk between optical wavelengths  214   b,c  is also reduced. 
     If the optical wavelengths  214   a-p  within channel plan  200  are sequential and do not overlap, as in the embodiment in FIG. 3, the distance between any two wavelengths  214   a-p  transmitted in the same direction is never less than twice the distance between two adjacent wavelengths  214   a-p . For example, the distance between the first two transmit wavelengths  214   a,c  in the example of channel plan  200  is approximately three nanometers rather than 1.57 nanometers. This makes the likelihood of cross phase modulation between transmit wavelengths  214   a,c  substantially lower. Furthermore, the combination of the two transmit wavelengths  214   a,c , 1530.33 and 1533.47 nanometers, does not fall within the transmission channel range of channel plan  200 . 
     In another embodiment, the optical wavelengths  214   a-p  within channel plan  200  may overlap. For example, each optical wavelength  214   a-p  may be positioned only 1 nanometer apart while having a bandwidth of 1.57 nanometers. Optical signal  214   a  has a 1.57 nanometer bandwidth and is transmitted at 1530.33 nanometer; optical signal  214   b  has a 1.57 nanometer bandwidth and is transmitted at 1531.33 nanometer; optical signal  214   c  has a 1.57 nanometer bandwidth and is transmitted at 1532.33 nanometer, etc. As a result, Two wavelengths transmitted in the same direction, such as  214   a ,  214   c , are separated by 0.44 nanometer while adjacent wavelengths  214   a ,  214   b  overlap by 0.56 nanometer. This emodiment increases the optical capacity at the expense of increasing possibility of interference. 
     Referring to FIG. 3, the system of the present invention is now described in further detail. If minimizing interference is the main priority, then the optical wavelengths  214   a-p  may be positioned further apart. For example, if the bandwidth of the optical signals is 1.57 nanometers, the optical wavelengths may be positioned 2 nanometers apart at 1530.33 nanometers, 1532.33 nanometers, 1534.33 nanometers, etc. 
     At one end of 16-wavelength bidirectional channel plan  200 , a set of optical wavelengths  214   a,c,e,g,i,k,m,o  is applied to optical transmitters  218   a-h  for transmission in the same direction as each other. The outputs of optical transmitters  218   a-h  are applied to WDM multiplexer/demultiplexer  220  where they are WDM multiplexed. The resulting multiplexed signal is amplified by bidirectional amplifier  224  and transmitted through channel plan  200  by way of optical fiber  225 . 
     The multiplexed signal transmitted by way of optical fiber  225  is amplified by bidirectional amplifier  227  and applied to WDM multiplexer/demultiplexer  226 . WDM multiplexer/demultiplexer  226  separates the amplified signal into eight demultiplexed signals that are applied to individual optical receivers  230   a-h . Optical receivers  230   a-h  provide received wavelengths  214   a,c,e,g,i,k,m,o . The eight received signals at the output of optical receiver  230   a-h  correspond to the eight transmit signals of optical transmitters  218   a-h.    
     Also within bidirectional channel plan  200 , a second set of optical wavelengths  214   b,d,f,h,j,l,n,p  is applied to optical transmitters  258   a-h  for transmission in the direction opposite to the direction of transmit wavelengths  214   a,c,e,g,i,k,m,o . The outputs of optical transmitters  258   a-h  are applied to WDM multiplexer/demultiplexer  226  where they are WDM multiplexed. The multiplexed signal is amplified by bidirectional amplifier  227  and transmitted by way of optical fiber  225 . 
     The signal from bidirectional amplifier  227  is received and amplified by bidirectional amplifier  224 . The amplified signal at the output of bidirectional amplifier  224  is applied to WDM multiplexer/demultiplexer  220  for WDM demultiplexing. The demultiplexed signals from WDM multiplexer/demultiplexer  220  are applied to optical receivers  240   a-h . Optical receivers  240   a-h provide received wavelengths  214   b,d,f,h,j,l,n,p . The eight signals at the outputs of optical receivers  240   a-h  correspond to the eight transmit wavelengths  214   b,d,f,h,j,l,n,p.    
     The system and method of the present invention can be advantageously combined with many other communication methods. For example, it can be combined with communication methods wherein the channels of an optical link are moved during transmission of a plurality of predetermined wavelengths therethrough in order to reduce harmonic interference. This method is taught in U.S. Pat. No. 5,600,467, entitled, “Method And Apparatus For Reducing Harmonic Interference On Multiplexed Optical Communication Lines,” issued to John A. Fee, on Feb. 4, 1997, which is incorporated by reference herein. 
     In the method taught by Fee the content of the spectrum of the wavelengths within an optical link is examined using a wave analyzer or a spectrum analyzer  260  in order to derive a representation of the optical activity within the link. The measured spectrum is compared with the desired spectrum of the predetermined wavelengths of the link. If an unwanted interfering wavelength is detected within the spectrum a detect signal is provided. When the detect signal is determined to be present the locations of the predetermined wavelengths within the link can be recalculated in order to prevent any of the predetermined wavelengths from coinciding with the unwanted wavelength. It will be understood that the predetermined wavelengths of the analyzed spectrum in the method taught by Fee can carry signals transmitted in alternating directions in accordance with the system and method of the present invention. 
     While the present invention has been described in terms of the preferred embodiments, for example, for specific numbers of optical carriers at specific wavelengths, it should be evident to those skilled in the art that variations of the preferred embodiments can be practiced without departing from the scope of the invention. The invention should only be restricted as defined in the appended claims.