Polarization mode dispersion mitigation of multiple optical communication channels

An optical transmitter for an optical communication system is provided. Included in the transmitter is a first optical delay element configured to generate a second optical signal from a first optical signal. A second optical delay element is configured to generate a fourth optical signal from a second optical signal. An optical multiplexer is configured to combine the third and fourth optical signals to produce a fifth optical signal. Also included is an optical modulator configured to alter a pulse width of the fifth optical signal to generate a sixth optical signal. An optical delay controller is configured to control the first optical delay element and the second optical delay element based on the sixth optical signal.

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MICROFICHE APPENDIX

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BACKGROUND OF THE INVENTION

1. Field of the Invention

Aspects of the invention relate generally to optical communication systems, and more particularly to mitigation of polarization mode dispersion (PMD) of multiple optical communication channels.

2. Description of the Prior Art

Optical communication systems typically employ optical fibers for carrying optical communication signals over significant distances. These optical signals typically take the form of a series of light pulses carrying encoded voice information or digital data. As shown in the simplified diagram ofFIG. 1, an optical communication system1typically includes an optical transmitter2and an optical receiver4coupled by way of an optical fiber6. Using such a system1, the optical transmitter2transforms an electrical communication signal8into an optical communication signal10, which is sent to the optical receiver4over the optical fiber6. The optical receiver4then converts the optical communication signal10back to a received electrical signal12.

Like other forms of communication, the optical communication signal10of the optical communication system1is subject to various forms of noise or distortion, thus possibly reducing the fidelity of the optical signal10after passing along an optical fiber6. One particular form of distortion of optical signals10is polarization mode dispersion (PMD). Oftentimes, due to manufacturing processes, mechanical stresses, and the like, the cross-sectional shape of the optical fiber6may become asymmetric. As a result, such a fiber6exhibits asymmetric light propagation characteristics that allow light propagating in a first plane of polarization to propagate more quickly than light propagating in a second plane of polarization perpendicular to the first. These two planes are normally referred to as the principal states of polarization (PSP), and the resulting time delay between the two PSPs is often referred to as differential group delay (DGD). As a result, pulses from the optical transmitter2at one end of an optical fiber6tend to disperse, or spread in time, by the moment they arrive at the optical receiver4at the opposing end of the fiber6. Thus, each optical pulse may appear as two separate, but closely situated, optical pulses to the receiver4. Also, adjacent optical pulses may begin to merge. In either case, the “eyes” of an eye pattern produced by the optical pulses tend to shrink or close completely, typically resulting in data corruption or loss at the receiving end.

The deleterious effects of PMD worsen as data rate increases. While PMD has proved to be somewhat problematic at data rates of 10 gigabits-per-second (10 Gbps, or 10 G), PMD has proven to be a major barrier to implementing newer 40 G systems being developed to increase optical communication bandwidth and capacity.

Some methods for mitigating PMD at an optical receiver4have been devised. Normally, such methods involve employing a compensator that separates the received optical signal into its two PSPs, typically by way of a polarization beam splitter. The compensator then delays the faster of the two PSPs by the exhibited DGD via a feedback controller to essentially negate the effects of the PMD at the receiver4.

Unfortunately, the efficacy of PMD compensators is restricted in optical communication systems employing wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM). In such systems, multiple communication channels are carried over a single optical fiber by dividing the available bandwidth into several relatively narrow bandwidth or frequency ranges, with each range carrying a single communication channel.FIG. 2illustrates in a simplified manner a typical WDM optical communication system100having a WDM optical transmitter110, a WDM optical receiver120, and an optical fiber130. The optical transmitter110includes single-channel optical transmitters112, each for translating an electrical communication signal111into an associated WDM communication channel signal113for a particular WDM channel. A WDM multiplexer114is used to combine the various WDM channel signals113into a combined optical communication signal140to be transferred over the fiber130. Similarly, the optical receiver120includes a WDM demultiplexer124to separate the various received WDM channel signals123comprising the combined optical signal140, and multiple single-channel receivers122for translating each received WDM optical channel signal123into a corresponding received electrical signal121carrying the communicated information.

As no PMD compensation is provided in the optical communication system100, PMD exhibited by the fiber130is likely to adversely affect the quality of the received WDM optical signals123being processed by the optical receiver120. In addition, the magnitude of the delaying effects of PMD is known to be wavelength-dependent. Thus, simultaneous PMD mitigation of all WDM channel signals113, as embodied in the combined optical signal140, by way of a single PMD compensator is normally ineffective. As a result, multiple compensators are typically required for PMD mitigation of the optical signal140, thereby increasing the cost and complexity of the optical communication system100.

SUMMARY OF THE INVENTION

One embodiment of the invention, described below, provides an optical transmitter for an optical communication system. The transmitter includes a first optical delay element configured to generate a third optical signal from a first optical signal. Similarly, a second optical delay element is configured to generate a fourth optical signal from a second optical signal. The transmitter also includes an optical multiplexer configured to combine the third and fourth optical signals to produce a fifth optical signal. An optical modulator is employed to alter a pulse width of the fifth optical signal, thus generating a sixth optical signal. In addition, the transmitter employs an optical delay controller configured to control the first optical delay element and the second optical delay element based on the sixth optical signal.

Another embodiment of the invention provides a method of transmission in an optical communication system. At least one of a first optical signal and a second optical signal is delayed in time, resulting in a third optical signal and a fourth optical signal. The third and fourth optical signals are combined to produce a fifth optical signal. A pulse width of the fifth optical signal is altered to produce a sixth optical signal. The delaying of at least one of the first and second optical signals is controlled by way of the sixth optical signal.

Additional embodiments and advantages of the present invention will be realized by those skilled in the art upon perusal of the following detailed description, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3illustrates an optical transmitter200according to one embodiment of the invention. The optical transmitter200includes a first optical delay element220aconfigured to generate a third optical signal215afrom a first optical signal213a. Similarly, a second optical delay element220bis configured to generate a fourth optical signal215bfrom a second optical signal213b. An optical multiplexer230is configured to combine the third optical signal215aand the fourth optical signal215bto produce a fifth optical signal217. An optical modulator240is configured to alter a pulse width of the fifth optical signal to generate a sixth optical signal219. An optical delay controller250is configured to control the first optical delay element220aand the second optical delay element220bbased on the sixth optical signal219.

FIG. 4depicts a method300of transmission in an optical communication system. At least one of a first optical signal and a second optical signal is delayed, resulting in a third optical signal and a fourth optical signal (operation302). The third and fourth optical signals are then combined to produce a fifth optical signal (operation304). A pulse width of the fifth optical signal is altered to produce a sixth optical signal (operation306). The delaying operation of at least one of the first and second optical signals is controlled by way of the sixth optical signal (operation308).

In one particular embodiment of the invention, as depicted inFIG. 5, a PMD-mitigated WDM optical communication system400includes a PMD-mitigating optical transmitter410coupled with an optical receiver420by way of an optical fiber430. The optical transmitter410receives an electrical communication signal411for each of n WDM channels provided by the communication system400. Each electrical signal411is converted into a corresponding WDM optical channel signal413by way of a single-channel optical transmitter412. Each WDM optical channel signal413is potentially delayed by way of a dedicated optical variable delay element470, resulting in a delayed optical channel signal472for each WDM channel. The delayed optical channel signals472are then multiplexed into a combined WDM optical signal474by way of a WDM multiplexer414. The combined WDM optical signal474is processed by an optical signal modulator450, which in turn produces a modulated WDM optical signal476, which is carried to the WDM optical receiver420over the optical fiber430. An optical delay controller460employs the modulated WDM optical signal476to control each of the optical delay elements470.

In one embodiment, an optical tap455couples the optical modulator450with the optical delay controller460and the optical fiber430. The optical tap455is employed to divert a portion of the power of the modulated WDM optical signal476to the optical delay controller260, while allowing the majority of the power of the modulated WDM optical signal476to propagate down the optical fiber430to the optical receiver420.

Upon receiving the modulated WDM optical signal476, the optical receiver420utilizes a WDM demultiplexer424, which separates the modulated WDM optical signal476into n received WDM optical channel signals423. Each of the received WDM optical channel signals423is then converted to a received electrical signal421by way of a single-channel optical receiver422.

In one embodiment, the various optical signals of the WDM optical communication system400employ a series of light pulses to convey information from the optical transmitter410to the optical receiver420. In one particular example, the optical signals employ a return-to-zero (RZ) encoding scheme, whereby the light pulses associated with a particular WDM channel exhibit generally the same pulse width.

Other elements possibly employed by the WDM optical communication system400have been omitted for the sake of brevity and clarity. For example, an add/drop multiplexer, which allows the adding or dropping of WDM channels to or from the optical fiber430, may be employed in other embodiments.

In one embodiment, the optical delay controller460controls the variable delay of each of the optical delay elements470so that pulses of each of the delayed WDM optical channel signals472are aligned with each other in time. Thus, the optical delay elements470, in conjunction with the optical delay controller460, compensate for any phase differences between the WDM optical channel signals413, thus synchronizing the delayed WDM optical channel signals472with each other. The operation of the optical delay elements470is shown graphically inFIG. 6.

In one particular implementation, the optical delay controller460employs spectrum analyzer461configured to determine the power of each of the individual WDM optical channel signals embodied in the modulated WDM optical signal476, at various points in time (operation310). The optical delay controller460thus adjusts the delay of each of the delayed WDM optical channel signals472so that the maximum power of each of the individual optical channel signals of the modulated WDM optical signal476is synchronized (operation312).

In another embodiment, the optical delay controller460uses optical demultiplexer462to separate the modulated WDM optical signal476into its component WDM optical channel signals. One or more power monitors463may then be employed to determine the power of each of the component WDM optical channel signals at various points in time to determine the relative phase differences (operation310). Using this information, the optical delay controller460may then control the optical delay elements470so that the component optical channel signals are synchronized (operation312). Such synchronization often aids in the optical receiver420sampling or clocking the optical pulses at or near their peak power, thus potentially reducing any misinterpretation of the data carried via the optical pulses.

After the delayed WDM optical channel signals472are combined into a combined WDM optical signal474, the optical signal modulator450adjusts the width of the pulses to reduce the effects of PMD, thus producing the modulated optical signal476. In one embodiment, the optical signal modulator450adjusts the pulse widths for each WDM optical channel signal embedded within the modulated WDM optical signal476so that the half-height bandwidth (HHBW) of the component optical channel signals is between approximately 45 percent and 55 percent. In other words, the width of each signal pulse at half of the height of the pulse is about 45 percent to 55 percent of a bit period of the optical channel signals.FIG. 7graphically portrays a comparison between optical signal pulses exhibiting pulse widths of 100 percent, 75 percent, and 50 percent of HHBW.

Referring again to the optical delay controller460ofFIG. 5, the width of the optical pulses of the modulated WDM optical signal476produced by the optical modulator450may be checked or verified by the optical delay controller460. Thus, the controller460may inform the modulator450of any necessary corrections to yield the desired pulse width.

Reduction of the width of the optical pulses transmitted over the optical fiber430typically mitigates the effects of PMD by providing an optimum optical pulse width which allows proper interpretation of the modulated WDM optical signal476in spite of PMD effects induced by the fiber430. For example, relatively wide optical pulse widths tend to result in adjacent optical pulses merging at an optical receiver420. Conversely, relatively narrow pulse widths tend to result in a single pulse being split into two separate pulses being detected at the optical receiver420. However, employing a pulse width at half-height on the order of 45 percent to 55 percent of a bit period tends to reduce the occurrence of either phenomenon. Thus, the need for PDM mitigation at the optical receiver420of the optical communication system400is greatly reduced or eliminated.

In one embodiment, the modulator450employs a clock signal482provided by a clock generator480, as shown inFIG. 5. The clock signal482exhibits the same frequency as the communication signals provided in the combined WDM optical signal476, and is employed by the modulator450to adjust the pulse widths of the incoming combined optical signal474to generate the modulated optical signal476.

One advantage of the particular optical transmitter410displayed inFIG. 5is that only a single modulator450is required to implement the narrow optical pulses for each of the component optical signals embedded in the modulated WDM optical signal476. Modification of the pulse widths of each embedded channel signals is desirable, as PMD induced by the optical fiber430affects each channel differently, since the magnitude of the resulting DGD is wavelength-dependent, as mentioned above. While multiple optical delay elements470are implemented in the transmitter410, each delay element470is typically much less expensive when compared to the cost of the modulator450. Thus, the optical transmitter410ofFIG. 5employs a cost-effective PMD mitigation system.

While several embodiments of the invention have been discussed herein, other embodiments encompassed within the scope of the invention are possible. For example, while the specific embodiments discussed herein specifically involve WDM signals, other optical communication systems, such as those employing DWDM signals, may also benefit from various aspects of the present invention as set forth above. Also, aspects of one embodiment may be combined with aspects of other embodiments disclosed above to produce additional embodiments not heretofore discussed. Thus, while the present invention has been described in the context of specific embodiments, such descriptions are provided for illustration and not limitation. Accordingly, the proper scope of the present invention is delimited only by the following claims.