Asynchronous chirped systems, apparatuses, and methods

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.

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'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. 1illustrates one embodiment of a prior art implementation of a chirped wavelength division multiplexed optical communications transmitter10. In that embodiment, a laser12and several modulators14are used to produce an optical signal for one of the wavelength division multiplexed channels. In this particular embodiment, the laser12produces 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 modulators14in the illustrated prior art embodiment are driven by a common or master clock signal generated by a master clock or oscillator16so that the operation of each of the modulators14is precisely synchronized. The synchronization of the modulators14is 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 “1mod” inFIG. 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.

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. 2illustrates one embodiment of a wavelength division multiplexed optical communications transmitter20according to the present invention. In that embodiment, chirped optical signals are produced using an asynchronous chirp module22. The chirp module22in the illustrated embodiment utilizes a local oscillator24, two phase modulators26, and a ninety degree phase shifter28. The local oscillator24drives the two phase modulators26and the phase shifter28between the local oscillator24and one of the phase modulators26causes the phase modulators26to produce different relative phase modulations of the optical signals. As a result, the dependence of the phase modulation produced by the chirp module22will be modified. The particular manner in which it is modified will depend on factors such as the characteristics of the phase modulators26and the amount of phase shift introduced. The combination of optical phase modulators26and a phase shifter28according 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 shifter28is 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 modulators26may 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 shifter28may produce forty-five degrees of shift, sixty degrees of shift, or some other value. Furthermore, the phase shifter28may 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 shifter28may 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 shifter28may be used. For example, two or more phase shifters28may be used, along with additional phase modulators26, to further refine the dependence of chirp on the relative phase angle of the data and clock signals.

The local oscillator24may operate at the same frequency as the master oscillator30used to drive the other modulators, or it may operate at a different frequency. One advantage of using a local oscillator24having 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 oscillator24with different frequencies may be used to provide signals which sweep across each other.

The chirp module22is illustrated as including one local oscillator24, a ninety degree phase shifter28, and two phase modulators26. However, many variations of the chirp module22are possible. For example, the chirp module22may contain more than two phase modulators26and more than one phase shifters. Furthermore, the phase shifter may be more or less than ninety degrees. Also, the phase modulator26utilizing the phase shifter28is 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 drivers30, for example, may be fixed or variable or, in some embodiments, may be eliminated.

In the illustrated embodiment, the chirp module22is 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 lasers36, and the PM modulators26. The use of polarization maintaining fiber and devices is sometimes undesirable and expensive, particularly in a WDM34, and it will sometimes be desirable to locate the chirp module22upstream of the WDM34to reduce the use of polarization maintaining fiber and devices. Furthermore, a dispersion compensation unit (“DCU”)38is provided after the PM modulators26in this embodiment, because the DCU38disturbs the polarity of the data signals. In other embodiments, such as those which can PM modulate with polarization insensitive modulators26, or with DCUs38which do not disturb the polarity of the data signal, different configurations may be used. As illustrated inFIG. 2, the chirp module22has an input port connected to the output of the wavelength division multiplexer34and an output port connected to the dispersion compensation unit38. As described below, the chirp module22may be connected in other configurations and the input and output ports may be connected to other devices.

FIG. 3illustrates another embodiment of an optical communications transmitters20according to the present invention. In this embodiment, the chirp module22is located upstream of the WDM34and can eliminate the need to utilize polarization maintaining fiber and devices through the WDM34, although more chirp modules22are used. This embodiment also illustrates a DCU38in the WDM34.

FIG. 4is 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. 5is 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. 6is 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 “1mod”) 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 “2mod”) 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 transmitters20which produce asynchronously chirped signals according to the present invention.FIG. 7illustrates one embodiment of an optical communications system40which includes optical paths42forming links43and connecting nodes and network elements44, which may include, for example, transmitters20, receivers46, switches48, add/drop multiplexers50, amplifiers52, interfacial devices54, multiplexers/combiners34, and demultiplexers/distributors58, as well as filters, dispersion compensating and shifting devices, monitors, couplers, splitters, and other devices. One embodiment of one node44is illustrated inFIG. 7, although the nodes44can have many other variations and embodiments.

Advantages of the present invention can be realized with many system40configurations 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 system40illustrated inFIG. 7is 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 system40can employ various signal formats, and can also convert between formats. The system40can also include more or less features than those illustrated herein, such as by including or deleting a network management system (“NMS”)60and changing the number, location, content, configuration, and connection of nodes44.

FIG. 8illustrates another embodiment of the system10including a link43of four nodes and network elements44. That link43can 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 elements44can be connected directly to the network management system60(not shown). If the link43is part of a larger system, then as few as none of the nodes or network elements44can be connected to the network management system60and all of the nodes and network elements44can still be indirectly connected to the NMS60via another node or network element44in the larger system40.

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.