Abstract:
One embodiment of the present invention includes a plurality of first optical elements and a second optical element optically coupled to one another in cascaded fashion. Each first optical element has a group delay response characterized by a first period such that only one group delay peak occurs within a first channel. By contrast, the second optical element has a group delay response characterized by a second period, which is less than the first period, such that more than one group delay peak occurs within the first channel. The preferred embodiment uses cascaded GT etalons to provide the desired group delay responses. One advantage of the present invention is that the passband of the dispersion compensator is increased relative to prior art designs without increasing insertion losses. Alternatively, the same passband common in prior art designs may be achieved with fewer GT etalons, thereby reducing insertion losses.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to the field of fiber optic networks and more specifically to a tunable, single-channel dispersion compensator for high-speed optical systems.  
         [0003]     2. Description of the Related Art  
         [0004]     Fiber optic communication systems use wavelength division multiplexing (WDM) to transfer large amounts of data at high speeds. In order to use WDM, channels are specified within a wavelength band. For example, it is generally accepted that the C-band begins at 1530 nm and extends to 1565 nm, and each channel in the C-band is approximately 0.8 nm wide and corresponds to a channel spacing of 100 GHz. The International Telecommunications Union (ITU) sets the standards for channel spacing, channel width and other communication band parameters for this and other optical communication bands.  
         [0005]     Lasers that transmit data on optical media, such as optical fibers, provide a narrow spectrum of light (i.e., a light pulse) that includes many wavelengths. Chromatic dispersion is a variation in the velocity of this light according to wavelength. Among other things, this variation in velocity causes the light pulses of an optical signal to broaden as they travel through the optical media. This phenomenon, known as “pulse spreading,” can cause increased bit error rates if the light pulses spread to a point where they begin to overlap with one another.  
         [0006]     Chromatic dispersion is particularly problematic in high-speed optical systems because the light pulses associated with higher bandwidths have broader wavelength spectra, resulting in relatively more pulse spreading, and the light pulses typically are narrower and transmitted closer together. The combination of these factors creates a system more susceptible to light pulse overlap and increased bit error rates.  
         [0007]     As is also known, chromatic dispersion is the rate of change of the group delay response of the light pulses of an optical signal as a function of wavelength. Thus, one approach to compensating for chromatic dispersion involves passing the optical signal through a dispersion compensator that exhibits a rate of change of the group delay response as a function of wavelength opposite to that caused by the optical medium.  
         [0008]     For example, U.S. Pat. No. 6,724,482 presents a dispersion compensator that includes a series of cascaded Gires-Tournois interferometers (i.e., GT etalons). Each GT etalon in the dispersion compensator has an individual group delay response. The group delay response of the dispersion compensator (hereinafter referred to as the “aggregate group delay response”) is the summation of the individual group delay responses of each of the cascaded GT etalons. The disclosed dispersion compensator is designed such that the aggregate group delay response across a channel has a rate of change as a function of wavelength opposite to that caused the optical medium, thereby compensating for the chromatic dispersion within a single channel of a multi-channel WDM communication system.  
         [0009]     In addition, a GT etalon has a periodic group delay response that repeats as a function of wavelength. The free-spectral-range (FSR) is a device parameter of a GT etalon that determines, among other things, the period of the group delay response. Based on these principles, U.S. Pat. No. 6,724,482 also teaches that by designing each GT etalon to have an FSR that aligns with the ITU&#39;s channel spacing scheme, the disclosed dispersion compensator can provide dispersion compensation across several channels simultaneously. For example, in a multi-channel WDM communication system having a 100 GHz channel spacing, if the dispersion compensator includes only GT etalons having an FSR of 100 GHz, then the dispersion compensator will provide the same aggregate group delay response, and therefore dispersion compensation across each channel of the system.  
         [0010]     One drawback of this type of dispersion compensator is that the passband is limited by the relatively high insertion losses associated with each GT etalon stage. More specifically, in order to extend the passband of the dispersion compensator, more GT etalons must be used. However, the additional GT etalons increase the total insertion losses across the dispersion compensator, which is undesirable. Thus, to keep the total insertion losses at an acceptable level, only a limited number of GT etalons can be used in the dispersion compensator. Limiting the number of GT etalons, however, results in a passband that is not optimized for high-speed optical systems, such as 40 Gb/s per channel optical systems.  
         [0011]     As the foregoing illustrates, what is needed in the art is a dispersion compensator with an increased passband for high-speed optical system applications that does not introduce increased insertion losses.  
       SUMMARY OF THE INVENTION  
       [0012]     A tunable, single-channel dispersion compensator, according to one embodiment of the present invention, includes a plurality of first optical elements and a second optical element optically coupled to one another in cascaded fashion. Each first optical element has a group delay response characterized by a first period such that only one group delay peak occurs within a first channel. By contrast, the second optical element has a group delay response characterized by a second period, which is less than the first period, such that more than one group delay peak occurs within the first channel.  
         [0013]     One advantage of the disclosed dispersion compensator is that the passband of the dispersion compensator is increased relative to prior art designs without increasing insertion losses. Alternatively, the same passband common in prior art designs may be achieved with fewer optical elements, thereby reducing insertion losses.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0015]      FIG. 1  is a conceptual diagram illustrating a dispersion compensator, according to one embodiment of the invention;  
         [0016]      FIG. 2  is a graphical diagram illustrating a group delay response of a GT etalon having an FSR of about 50 GHz, according to one embodiment of the invention;  
         [0017]      FIG. 3  is a graphical diagram illustrating the aggregate group delay response of the dispersion compensator of  FIG. 1 , according to one embodiment of the invention;  
         [0018]      FIG. 4  is a graphical diagram illustrating the aggregate group delay response of the dispersion compensator of  FIG. 1  across three channels, according to one embodiment of the invention; and  
         [0019]      FIG. 5  is a graphical diagram illustrating the aggregate group delay response of the dispersion compensator of  FIG. 1  across three channels after tuning, according to one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]      FIG. 1  is a conceptual diagram illustrating a dispersion compensator  100 , according to one embodiment of the invention. The dispersion compensator  100  is realized by sequentially cascading a plurality of GT etalons and, as shown, includes GT etalons  101 ,  102 ,  103 ,  104 ,  105 ,  106 ,  107 ,  108 ,  109 ,  110 ,  111  and  112 . In alternative embodiments, the dispersion compensator  100  may include more or less than 12 GT etalons. Each GT etalon has an associated group delay response. As is well-known, the aggregate group delay response of the dispersion compensator  100  is the summation of the individual group delay responses of the GT etalons  101 ,  102 ,  103 ,  104 ,  105 ,  106 ,  107 ,  108 ,  109 ,  110 ,  111  and  112 .  
         [0021]     As will be described in further detail below in  FIGS. 2 and 3 , the passband of the dispersion compensator  100  can be increased without increasing insertion losses by including one or more GT etalons in the dispersion compensator  100  that contribute more than one group delay peak within the channel for which dispersion compensation is being provided.  
         [0022]      FIG. 2  is a graphical diagram illustrating a group delay response  202  of a GT etalon having an FSR of about 50 GHz, according to one embodiment of the invention. As shown, a channel  200  in the C-band is approximately 0.8 nm wide. Importantly, the group delay response  202  of the GT etalon is twice periodic in a channel of this size and therefore contributes two group delay peaks within the channel  200 . As described in further detail below in  FIG. 3 , dispersion compensator  100  may be configured to include at least one GT etalon with a group delay response that is twice periodic within the channel  200  and, thus, contributes two group delay peaks to the aggregate group delay response of the dispersion compensator  100 . Among other things, such a configuration increases the passband of the dispersion compensator  100 , without increasing insertion losses.  
         [0023]      FIG. 3  is a graphical diagram illustrating an aggregate group delay response  320  of the dispersion compensator  100  of  FIG. 1 , according to one embodiment of the invention. As shown, the dispersion compensator  100  includes both GT etalons having an FSR of approximately 100 GHz and GT etalons having an FSR of about 50 GHz. The GT etalons  101 ,  102 ,  103 ,  104 ,  105 ,  106 ,  107 ,  108 ,  109  and  110  have FSRs of approximately 100 GHz and therefore contribute one group delay peak within the channel  200 , as shown by group delay response curves  301 ,  302 ,  303 ,  304 ,  305 ,  306 ,  307 ,  308 ,  309  and  310 , respectively. By contrast, GT etalons  111  and  112  have FSRs of about 50 GHz and therefore contribute two group delay peaks within the channel  200 , as shown by group delay response curves  311  and  312 , respectively. Again, the aggregate group delay response  320  of the dispersion compensator  100  is the summation of the individual group delay responses provided by each GT etalon. Thus, the GT etalons  101 ,  102 ,  103 ,  104 ,  105 ,  106 ,  107 ,  108 ,  109 ,  110 ,  111  and  112  provide fourteen group delay peaks within the channel, as compared to twelve group delay peaks that are provided when the GT etalons have an FSR of approximately 100 GHz. The two extra group delay peaks provided by GT etalons  111  and  112  advantageously increase the passband of the dispersion compensator  100  relative to that of prior art designs. For example, experiments have shown that a dispersion compensator including twelve GT etalons that contribute twelve group delay peaks to the aggregate group delay response has a passband of 0.58 nm. In contrast, a dispersion compensator including twelve GT etalons that contribute fourteen group delay peaks to the aggregate group delay response has a passband of 0.62 nm, an increase of 0.04 nm.  
         [0024]     One advantage of the disclosed dispersion compensator design is that the passband is increased without using more GT etalons. Since the number of GT etalons does not increase, the insertion losses are not increased. Conversely, the dispersion compensator design of  FIGS. 2 and 3  may use fewer GT etalons to achieve the same aggregate group delay response of a dispersion compensator that includes only GT etalons having a common FSR. Thus, in such an implementation, the same passband can be achieved using fewer GT etalons, resulting in lower insertion losses.  
         [0025]     One consequence of the disclosed design is that if the dispersion compensator  100  includes GT etalons with FSRs that do not align with the ITU channel spacing scheme, then the aggregate group delay response of the dispersion compensator  100 , and, hence, the dispersion compensation, is not repeated across each channel in the C-band. For example, in a system with 100 GHz channel spacing, a GT etalon with an FSR smaller than 100 GHz has a group delay response with group delay peaks having varying relative positions within the different channels across the system. Because the relative positions of these group delay peaks change from channel to channel, the summation of the group delay responses of the individual GT etalons of the dispersion compensator  100  also changes from channel to channel. As a result, the dispersion compensator  100  may provide the desired dispersion compensation to only one channel at a time. This phenomenon is described in further detail below in  FIG. 4 .  
         [0026]      FIG. 4  is a graphical diagram illustrating the aggregate group delay response of the dispersion compensator  100  of  FIG. 1  across three channels, according to one embodiment of the invention. As shown, a first channel  401 , a second channel  403  and a third channel  405  represent channels in the beginning of the C-band. A curve  402  represents the aggregate group delay response of the dispersion compensator  100  across the first channel  401 , a curve  404  represents the aggregate group delay response of the dispersion compensator  100  across the second channel  403 , and a curve  406  represents the aggregate group delay response of the dispersion compensator  100  across the third channel  405 . Comparing the aggregate group delay response curves  402 ,  403  and  404  to aggregate group delay response curve  320  of  FIG. 3  shows that the dispersion compensator  100  provides the desired aggregate group delay response to the first channel  401 , but not to the second channel  403  or the third channel  405 . Thus, as previously described herein, the dispersion compensator  100  provides the desired dispersion compensation only to the first channel  401 .  
         [0027]     Although the dispersion compensator  100  may provide the desired dispersion compensation to only one channel at a time, the dispersion compensator  100  is well-suited for use in high-speed optical systems where only one channel is used at any given time, such as 40 Gb/s optical systems. As described in further detail below in  FIG. 5 , the dispersion compensator  100  may be advantageously tuned on a channel-by-channel basis to provide the desired dispersion compensation to any channel in a high-speed optical system.  
         [0028]      FIG. 5  is a graphical diagram illustrating the aggregate group delay response of the dispersion compensator  100  of  FIG. 1  across three channels after tuning, according to one embodiment of the invention. As shown, a first channel  501 , a second channel  503  and a third channel  505  represent channels in the middle of the C-band. A curve  502  represents the aggregate group delay response of the dispersion compensator  100  across the first channel  501 , a curve  504  represents the aggregate group delay response of the dispersion compensator  100  across the second channel  503 , and curve  506  represents the aggregate group delay response of the dispersion compensator  100  across the third channel  505 . Again, comparing the aggregate group delay response curves  502 ,  504  and  506  to aggregate group delay response curve  320  of  FIG. 3  shows that the dispersion compensator  100  provides the desired aggregate group delay response only to the second channel  503 , but not to the first channel  501  and the third channel  505 . Further, a comparison of  FIGS. 4 and 5  shows that the dispersion compensator  100  has been tuned to produce the desired dispersion compensation across the channel  503 , whereas, the dispersion compensator  100  initially provided the desired dispersion compensation across the channel  402 .  
         [0029]     Such tuning may be achieved by modifying some or all of the individual group delay responses of the GT etalons  101 ,  102 ,  103 ,  104 ,  105 ,  106 ,  107 ,  108 ,  109 ,  110 ,  111 , and  112  on an as-needed basis. As is well-known, the FSR (and, thus, the group delay response) of a GT etalon may be modified by varying the temperature of the GT etalon. Thus, by altering the temperatures of the GT etalons in the dispersion compensator  100  in an appropriate fashion, the individual group delay responses of those GT etalons may be modified to produce the aggregate group delay response curve  504  across the second channel  503  of  FIG. 5  from the aggregate group delay response curve  402  across the first channel  401  of  FIG. 4 . Temperature control is only one means by which the group delay response of a GT etalon may be modified. Persons skilled in the art will recognize that, in alternative embodiments, the individual group delay responses of the GT etalons may be modified by other means such as air pressure modulation, piezo tuning, or the like.  
         [0030]     In addition to the foregoing, although the dispersion compensator  100  is described in  FIGS. 2 and 3  as including GT etalons with an FSR of about 50 GHz, alternative embodiments may include any GT etalon with a periodic group delay response that contributes more than one group delay peak within the channel for which dispersion compensation is being provided. Further, in other embodiments, the GT etalons may be replaced with waveguides, Bragg gratings or other optical devices that are mathematically equivalent to a GT etalon and display the described periodic group delay responses. Finally, the dispersion compensator  100  may be tuned to provide dispersion compensation to any channel. Therefore, in yet other alternative embodiments, the dispersion compensator  100  may be configured to operate on other optical communication bands, such as the L-band and the S-band, that have 100 GHz channel spacing or otherwise.  
         [0031]     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.