Abstract:
An optical asynchronous chirp device having an input port and an output port, comprising first and second optical phase modulators optically connected in series between the input port and the output port of the device, a local oscillator connected to the first and second phase modulators, and a phase shifter connected between the local oscillator and one of the phase modulators, and systems and methods related thereto.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application No. 60/492,520, filed Aug. 5, 2003, which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     It is known in the field of optical communications to utilize “chirp” to compensate for effects such as non-linearities. Chirping causes a signal&#39;s spectrum to spread and, therefore, reduces the power density. As a result, non-linear effects are also reduced. Chirping can also be used, for example, to compensate for dispersion caused when optical signals travel through optical fiber, as well as to compensate for other effects. 
     On prior art method is to use a fixed chirp which is inherent in a Mach-Zehnder-based (“MZ”-based) intensity modulator. Another prior art method is to use a separate phase modulator (“PM”) to implement chirp through phase modulation of the signal. The prior art, however, requires the PM to be synchronous. This is accomplished by driving the PM with the same clock source as drives the data modulator(s). 
       FIG. 1  illustrates one embodiment of a prior art implementation of a chirped wavelength division multiplexed optical communications transmitter  10 . In that embodiment, a laser  12  and several modulators  14  are used to produce an optical signal for one of the wavelength division multiplexed channels. In this particular embodiment, the laser  12  produces an optical carrier, one modulator is used to modulate data onto an optical carrier in non-return to zero (“NRZ”) format, another modulator is used to amplitude modulate (“AM”) the NRZ format signal to produce a return to zero (“RZ”) format, and a third modulator is used to phase modulate (“PM”) the RZ format signal. The phase modulator introduces the chirp to the signal. 
     Each of the modulators  14  in the illustrated prior art embodiment are driven by a common or master clock signal generated by a master clock or oscillator  16  so that the operation of each of the modulators  14  is precisely synchronized. The synchronization of the modulators  14  is necessary in the prior art because the degree of chirp introduced by the phase modulator is a function of the relative phase between the data and the clock signals. This dependence on phase can be seen more clearly with reference to data points “ 1  mod” in  FIG. 6 , which is described in more detail hereinbelow. As a result, if the phase modulators of the prior art are not synchronized with the other modulators, the extent of the chirp cannot be controlled and the performance of the system will suffer. U.S. Pat. No. 5,526,162, issued to Bergano, illustrates the prior art approach in which a master clock is used to precisely synchronize various modulators in an optical transmission system. 
     For example, if a local oscillator is used to drive the phase modulator, it may be unsynchronized, resulting in the chirp of the optical signal to vary over time. This can cause significant detrimental effects because the variation in chirp can vary significantly. 
     The use of synchronous chirp has certain disadvantages. For example, an optical transmitter which is initially designed to operate without chirp, cannot be easily or inexpensively upgraded to introduce chirp. That is because it is difficult and expensive to upgrade a produce with a synchronous phase modulator. As a result, using synchronous chirp according to the prior art limits the ability to modify products in light of new or changing business environments. 
     Therefore, there is a need for systems, apparatuses, and methods which allow for signals to be chirped without the requirements and limitations of the prior art. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to systems, apparatuses, and methods for producing and utilizing chirped signals and, more particularly, for systems, apparatuses, and methods for producing and utilizing asynchronous chirped signals. 
     In one embodiment of the present invention, chirped optical signals are produced using an asynchronous chirp module. The chirp module may utilize a local oscillator, two or more optical phase modulators, and an electrical phase shifter. The local clock drives the phase modulators such that the two driving signals are ninety (90) degrees out of phase due to the phase shifter. As a result, the dependence of the phase modulation produced by the chirp module will be modified. The particular manner in which it is modified will depend on factors such as the characteristics of the phase modulators and the amount of phase shift introduced. 
     The chirp module may be utilized, for example, in a transmitter and may be used with or without wavelength division multiplexing. In wavelength division multiplexed applications, the chirp module may be upstream or downstream of the wavelength division multiplexer. The present invention may be embodied as a discrete module which may be added to a pre-existing apparatus, or it may be integrated into an apparatus without any modularity. 
     Unlike synchronous chirp designs, the asynchronous nature of the present invention allows it to be more easily used to upgrade devices which were not originally designed to utilize chirp. For example, the present invention may be used to upgrade pre-existing communications systems and apparatuses from unchirped systems and apparatuses to chirped systems and apparatuses. This is particularly valuable in undersea systems in which the submerged portions are not easily modified or upgraded, although it is also valuable for terrestrial systems. The present invention is also particularly useful for new system builds which must utilize existing equipment. 
     Those and other embodiments and advantages of the present invention will be described in more detail hereinbelow. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIG. 1  illustrates one embodiment of a prior art implementation of a chirped wavelength division multiplexed optical communications transmitter; 
         FIGS. 2 and 3  illustrate several embodiments of optical communications transmitters according to the present invention; 
         FIG. 4  is a graph illustrating performance of one embodiment of the present invention at different power/channel settings; 
         FIG. 5  is a graph illustrating synchronous chirped, asynchronous chirped, and non-chirped performance; 
         FIG. 6  is a graph illustrating synchronous chirped and asychronous chirped performance through 360 degrees of phase angle; and 
         FIGS. 7 and 8  illustrate embodiments of systems in which the present invention may be implemented. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in terms of optical, wavelength division multiplexed communications systems, although the present invention has broader applicability. For example, the present invention may be used without wavelength division multiplexing. Furthermore, the particular types of signal generation and modulation techniques and formats may also be varied according to the teachings of the present invention. 
       FIG. 2  illustrates one embodiment of a wavelength division multiplexed optical communications transmitter  20  according to the present invention. In that embodiment, chirped optical signals are produced using an asynchronous chirp module  22 . The chirp module  22  in the illustrated embodiment utilizes a local oscillator  24 , two phase modulators  26 , and a ninety degree phase shifter  28 . The local oscillator  24  drives the two phase modulators  26  and the phase shifter  28  between the local oscillator  24  and one of the phase modulators  26  causes the phase modulators  26  to produce different relative phase modulations of the optical signals. As a result, the dependence of the phase modulation produced by the chirp module  22  will be modified. The particular manner in which it is modified will depend on factors such as the characteristics of the phase modulators  26  and the amount of phase shift introduced. The combination of optical phase modulators  26  and a phase shifter  28  according to the present invention allows for better control of phase modulation or chirp over a range of relative phase angles such as, for example, to provide a relatively constant chirp over a wide range of relative phase angles. 
     Although the phase shifter  28  is illustrated as a ninety degree phase shifter, it may provide more or less than ninety degrees of shift. For example, the particular embodiment of the phase modulators  26  may dictate a different phase shift in order to achieve desired performance. Furthermore, the particular application may require some other phase shift in order to achieve the desired results. For example, the phase shifter  28  may produce forty-five degrees of shift, sixty degrees of shift, or some other value. Furthermore, the phase shifter  28  may be variable so that, for example, it may be tuned or adjusted for better performance, such as to compensate for performance changes over time. In other embodiments, the phase shifter  28  may be varied more regularly, such as to improve averaging of, or otherwise control, the chirp over the range of relative phase angles. In other embodiments, more than one phase shifter  28  may be used. For example, two or more phase shifters  28  may be used, along with additional phase modulators  26 , to further refine the dependence of chirp on the relative phase angle of the data and clock signals. 
     The local oscillator  24  may operate at the same frequency as the master oscillator  30  used to drive the other modulators, or it may operate at a different frequency. One advantage of using a local oscillator  24  having a different frequency is that phase dependent variations in phase modulation will be constantly changing, thereby providing a better averaging effect which is desirable for some applications. In other embodiments, more than one local oscillator  24  with different frequencies may be used to provide signals which sweep across each other. 
     The chirp module  22  is illustrated as including one local oscillator  24 , a ninety degree phase shifter  28 , and two phase modulators  26 . However, many variations of the chirp module  22  are possible. For example, the chirp module  22  may contain more than two phase modulators  26  and more than one phase shifters. Furthermore, the phase shifter may be more or less than ninety degrees. Also, the phase modulator  26  utilizing the phase shifter  28  is illustrated as being the second modulator, although it may instead be used as the first modulator. Furthermore, one or more parts may be variable, such as to allow for better control. The drivers  30 , for example, may be fixed or variable or, in some embodiments, may be eliminated. 
     In the illustrated embodiment, the chirp module  22  is polarization sensitive and is illustrated downstream of the wavelength division multiplexer (“WDM”)  34 . As a result, polarization maintaining fiber and devices are used between the optical sources, such as lasers  36 , and the PM modulators  26 . The use of polarization maintaining fiber and devices is sometimes undesirable and expensive, particularly in a WDM  34 , and it will sometimes be desirable to locate the chirp module  22  upstream of the WDM  34  to reduce the use of polarization maintaining fiber and devices. Furthermore, a dispersion compensation unit (“DCU”)  38  is provided after the PM modulators  26  in this embodiment, because the DCU  38  disturbs the polarity of the data signals. In other embodiments, such as those which can PM modulate with polarization insensitive modulators  26 , or with DCUs  38  which do not disturb the polarity of the data signal, different configurations may be used. As illustrated in  FIG. 2 , the chirp module  22  has an input port connected to the output of the wavelength division multiplexer  34  and an output port connected to the dispersion compensation unit  38 . As described below, the chirp module  22  may be connected in other configurations and the input and output ports may be connected to other devices. 
       FIG. 3  illustrates another embodiment of an optical communications transmitters  20  according to the present invention. In this embodiment, the chirp module  22  is located upstream of the WDM  34  and can eliminate the need to utilize polarization maintaining fiber and devices through the WDM  34 , although more chirp modules  22  are used. This embodiment also illustrates a DCU  38  in the WDM  34 . 
       FIG. 4  is a graph illustrating performance of one embodiment of the present invention at different power/channel settings. This graph was produced with simulated data over a 12,000 kilometer system utilizing carrier-suppressed RZ format optical signals according to the present invention. The graph illustrates the present invention producing predictable performance over a range of launch powers. 
       FIG. 5  is a graph illustrating synchronous chirped, asynchronous chirped, and non-chirped performance over various signal launch powers. This graph was produced with simulated data over a 12,000 kilometer system utilizing carrier-suppressed RZ format optical signals according to the present invention. The present invention produced performance comparable to that of a comparable synchronous chirped system, and produced significantly better performance than an unchirped system. 
       FIG. 6  is a graph illustrating one example of synchronous chirped and asychronous chirped performance through 360 degrees of relative phase angle between the data and driver signals. The prior art synchronous chirped curve (identified as “ 1  mod”) exhibits significant peak to peak variation. As a result, without synchronization, the prior art will produce significant performance variations which can be undesirable. In contrast, the asynchronous chirped curve (identified “ 2  mod”) exhibits less peak to peak variation, thereby providing for more predictable phase modulation without the need for synchronization. 
     The present invention also includes systems utilizing the teachings of the present invention. For example, a system may transmit data in chirped form by including one or more transmitters  20  which produce asynchronously chirped signals according to the present invention.  FIG. 7  illustrates one embodiment of an optical communications system  40  which includes optical paths  42  forming links  43  and connecting nodes and network elements  44 , which may include, for example, transmitters  20 , receivers  46 , switches  48 , add/drop multiplexers  50 , amplifiers  52 , interfacial devices  54 , multiplexers/combiners  34 , and demultiplexers/distributors  58 , as well as filters, dispersion compensating and shifting devices, monitors, couplers, splitters, and other devices. One embodiment of one node  44  is illustrated in  FIG. 7 , although the nodes  44  can have many other variations and embodiments. 
     Advantages of the present invention can be realized with many system  40  configurations and architectures, such as an all optical network, one or more point to point links, one or more rings, a mesh, other architectures, or combinations of architectures. The system  40  illustrated in  FIG. 7  is a multi-dimensional network, which can be implemented, for example, as an all optical mesh network, as a collection of point to point links, or as a combination of architectures. The system  40  can employ various signal formats, and can also convert between formats. The system  40  can also include more or less features than those illustrated herein, such as by including or deleting a network management system (“NMS”)  60  and changing the number, location, content, configuration, and connection of nodes  44 . 
       FIG. 8  illustrates another embodiment of the system  10  including a link  43  of four nodes and network elements  44 . That link  43  can be, for example, all or part of a point to point system, or it may be part of a multi-dimensional, mesh, or other system. One or more of the nodes or network elements  44  can be connected directly to the network management system  60  (not shown). If the link  43  is part of a larger system, then as few as none of the nodes or network elements  44  can be connected to the network management system  60  and all of the nodes and network elements  44  can still be indirectly connected to the NMS  60  via another node or network element  44  in the larger system  40 . 
     Many variations and modifications can be made to described embodiments of the invention without departing from the scope of the invention. For example, transmitters are illustrated as utilizing data modulators and amplitude modulators, while the present invention may be utilized with a single modulator, with more than two modulators, with different modulators, or with no external modulators at all. Other variations, modifications, and combinations are taught and suggested by the present invention, and it is intended that the foregoing specification and the following claims cover such variations, modifications, and combinations.