The present invention relates generally to the optical transmission and optical networking fields. More specifically, the present invention relates to the bi-directional application of a dispersion compensating module (DCM) in a regional system. Advantageously, the systems and methods of the present invention have the potential to reduce dispersion compensation costs in optical transmission systems by nearly 50%.
Cost reduction in optical transmission systems using various dispersion compensation techniques has been the subject of a number of recent studies. These dispersion compensation techniques include: dispersion compensating fiber (DCF) optimization, the use of etalons, the use of fiber Bragg gratings, the use of planar light-wave circuits (PLCs), and electrical dispersion compensation (EDC) by signal pre-distortion, in addition to the use of non-zero dispersion shifted fiber (NZDSF). NZDSF is manufactured with a more perfectly circular fiber core and a more complex refractive index profile than conventional single-mode fiber, resulting in less dispersion than conventional single-mode fiber. Disadvantageously, NZDSF addresses only polarization mode dispersion (PMD), described in greater detail herein below, and exacerbates slope mismatch dispersion, also described in greater detail herein below.
DCF optimization involves placing spools of DCF at predetermined intervals along a network—approximately 15 km of DCF for approximately every 80 km of network fiber, for example. These spools of DCF are typically stacked on top of telecommunications racks. Disadvantageously, DCF optimization addresses only chromatic dispersion (CD), described in greater detail herein below, and is typically set up to accurately correct CD on a center wavelength of multiple wavelengths carried on a fiber. Thus, dispersion accumulates at the other wavelengths and creates a problem at the edge of a band of wavelength channels.
The use of etalons involves using a Fabry-Perot interferometer arranged with two flat reflecting surfaces that are aligned to be parallel, and either a transparent plate (such that reflections from both of the flat reflecting surfaces are exploited) or an air gap in between the two flat reflecting surfaces. The etalon acts as an optical resonator or cavity, optionally with controllable resonant frequency, providing CD.
The use of fiber Bragg gratings involves using multiple short lengths of fiber that each reflect a particular wavelength. Fiber Bragg gratings incorporate periodically spaced zones in a fiber core that each have a predetermined refractive index that is slightly higher than the fiber core, for example. This structure selectively reflects a predetermined range of wavelengths, while selectively transmitting other wavelengths. Fiber Bragg gratings are each typically between about 1 mm and about 25 mm long, and are formed by selectively exposing a fiber to ultraviolet (UV) light. Advantageously, fiber Bragg gratings have relatively low insertion loss when inserted into a network, as a given light wave is not routed outside of the fiber.
The use of PLCs involves using PLC chips incorporating Mach-Zehnder interferometry, for example, to compensate for CD and the like. Advantageously, these devices have relatively low insertion loss when inserted into a network, are quickly tunable, and are relatively simple to operate.
EDC by signal pre-distortion involves pre-distorting the amplitude and phase waveforms of a transmitted signal in order to achieve dispersion compensation. Advantageously, these techniques eliminate the need for bulky and expensive optical dispersion compensation components.
DCMs incorporate DCF optimization, the use of etalons, the use of fiber Bragg gratings, the use of PLCs, and/or a variety of other dispersion compensation techniques. These devices are placed in front of receivers in a network and make continual signal adjustments based on information derived from the analysis of a sample of an optical pulse as it travels through the DCM. The degree to which the optical pulse is corrected is based on its state, as read by a detector associated with the DCM. Advantageously, DCMs are either remotely or adaptively tunable, have a relatively small form factor, and are relatively inexpensive and simple to replace.
DCF optimization remains the most stable and reliable field technique. Thus, what are needed are improved systems and methods using DCMs that incorporate DCF optimization, as well as the use of etalons, the use of fiber Bragg gratings, and/or a variety of other dispersion compensation techniques. These improved systems and methods are provided by the present invention.
In various exemplary embodiments, the present invention provides for the bi-directional application of a DCM in a regional system. Advantageously, the systems and methods of the present invention have the potential to reduce dispersion compensation costs in optical transmission systems by nearly 50%.
In one exemplary embodiment of the present invention, a system for the bidirectional application of a dispersion compensating module in a regional system includes a dispersion compensating module configured to receive a first optical signal traveling along a first path and a second optical signal traveling along a second path, wherein the dispersion compensating module provides dispersion compensation to the first optical signal and the second optical signal. The system also includes a first circulator in optical communication with the dispersion compensating module and the first path, wherein the first circulator delivers the first optical signal to the dispersion compensating module. The system further includes a second circulator in optical communication with the dispersion compensating module and the second path, wherein the second circulator delivers the second optical signal to the dispersion compensating module. The first circulator is further in optical communication with the second path, receives the second optical signal from the dispersion compensating module, and transmits the second optical signal along the second path subsequent to dispersion compensation. The second circulator is further in optical communication with the first path, receives the first optical signal from the dispersion compensating module, and transmits the first optical signal along the first path subsequent to dispersion compensation.
In another exemplary embodiment of the present invention, a method for the bi-directional application of a dispersion compensating module in a regional system includes providing a dispersion compensating module configured to receive a first optical signal traveling along a first path and a second optical signal traveling along a second path, wherein the dispersion compensating module provides dispersion compensation to the first optical signal and the second optical signal. The method also includes providing a first circulator in optical communication with the dispersion compensating module and the first path, wherein the first circulator delivers the first optical signal to the dispersion compensating module. The method further includes providing a second circulator in optical communication with the dispersion compensating module and the second path, wherein the second circulator delivers the second optical signal to the dispersion compensating module. The first circulator is further in optical communication with the second path, receives the second optical signal from the dispersion compensating module, and transmits the second optical signal along the second path subsequent to dispersion compensation. The second circulator is further in optical communication with the first path, receives the first optical signal from the dispersion compensating module, and transmits the first optical signal along the first path subsequent to dispersion compensation.
In a further exemplary embodiment of the present invention, a dispersion compensating system includes an optical device configured to receive an optical signal traveling in a first direction, provide first dispersion compensation to the optical signal, receive the optical signal traveling in a second direction, and provide second dispersion compensation to the optical signal. The system also includes a mirror for changing the direction of travel of the optical signal from the first direction to the second direction. The system further includes a circulator in optical communication with a first path and a second path, wherein the circulator receives the optical signal traveling in the first direction from the first path and delivers the optical signal traveling in the first direction to the optical device. The circulator further receives the optical signal traveling in the second direction from the optical device and transmits the optical signal traveling in the second direction along the second path.
In a still further exemplary embodiment of the present invention, a dispersion compensating method includes providing an optical device configured to receive an optical signal traveling in a first direction, provide first dispersion compensation to the optical signal, receive the optical signal traveling in a second direction, and provide second dispersion compensation to the optical signal. The method also includes providing a mirror for changing the direction of travel of the optical signal from the first direction to the second direction. The method further includes providing a circulator in optical communication with a first path and a second path, wherein the circulator receives the optical signal traveling in the first direction from the first path and delivers the optical signal traveling in the first direction to the optical device. The circulator further receives the optical signal traveling in the second direction from the optical device and transmits the optical signal traveling in the second direction along the second path.
The present invention is illustrated and described herein with reference to the various drawings, in which like reference numbers denote like system components and/or method steps, as appropriate, and in which:
CD is based on the principal that different colored pulses of light, with different wavelengths, travel at different speeds, even within the same mode, and is the sum of material dispersion and waveguide dispersion. Material dispersion is caused by the variation in the refractive index of the glass of a fiber as a function of the optical frequency. Waveguide dispersion is caused by the distribution of light between the core of a fiber and the cladding of a fiber, especially with regard to a single-mode fiber. CD concerns are compounded in today's high-speed transmission optical networks.
Slope mismatch dispersion is a subset of CD, and occurs in single-mode fiber because dispersion varies with wavelength. Thus, dispersion builds up, especially at the extremes of a band of wavelength channels. Slope mismatch dispersion compensation typically requires slope matching or tunable dispersion compensation at a receiver.
PMD results as light travels down a single-mode fiber in two inherent polarization modes. When the core of a fiber is asymmetric, the light traveling along one polarization mode travels faster or slower than the light traveling along the other polarization mode, resulting in a pulse overlapping with others, or distorting the pulse to such a degree that it is undetectable by a receiver. Again, PMD concerns are compounded in today's high-speed transmission optical networks. Further, PMD varies dynamically with temperature changes, infinitesimal asymmetries in the fiber core, etc., and therefore requires adaptively tunable dispersion compensation.
Referring to
The main concerns related to bi-directional DCM application are BRS and SBS. BRS is the backward scattering of light by particles that are smaller than the wavelength of the light. SBS is the stimulated scattering of light particles that occurs when light in a medium interacts with density variations and changes its path. These density variations can be associated with acoustic modes, such as phonons, or temperature gradients. As illustrated in
Referring to
The BRS and SBS generated by the DCM 16 in the recirculation loop 30 are illustrated in
Q=[(1/Q02)+C·MPI]−1/2 (1)
where Q0 is unimpaired Q by the equivalent MPI and C is a constant related to the eye closure factor. C=1.28 is used for all OSNR values.
The bi-directional application of a DCM 16 (
Referring to
Referring to
It should be noted that
Although the present invention has been illustrated and described herein with reference to preferred embodiments and specific examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples can perform similar functions and/or achieve like results. All such equivalent embodiments and examples are within the spirit and scope of the present invention and are intended to be covered by the following claims.
The present non-provisional patent application claims the benefit of priority of U.S. Provisional Patent Application No. 60/875,449 (Wenxin ZHENG, Harshad SARDESAI, and Jean Luc ARCHAMBAULT), filed on Apr. 28, 2005, and entitled “BI-DIRECTIONAL APPLICATION OF A DISPERSION COMPENSATING MODULE IN A REGIONAL SYSTEM,” which is incorporated in-full by reference herein.