1. Field of the Invention
This invention relates to dispersion compensation in optical networks. More specifically, the invention relates to the use of uniform fiber Bragg gratings to provide dispersion compensation. The wavelength of light for which the dispersion compensation is provided by a grating may be dynamically controlled.
2. Description of the Problem
Dispersion compensation is a well-known technique for increasing the usable bandwidth of long-haul fiber systems that suffer from limitations caused by pulse broadening due to chromatic dispersion. Without dispersion compensation, a pulse, upon propagation through a dispersive system, will accumulate a phase chirp, leading to a broadening of pulses in the time-domain. The broadening of pulses in a data string then produces overlap of adjacent pulses and resulting detection errors at the receiver. An example of such broadening is shown in FIGS. 1 and 2, where eye diagrams of a non-dispersed, and a dispersion-broadened data string are shown. An eye diagram is a synchronous measurement of the received signal, where signals from multiple bit intervals are plotted over one another. Undistorted transmission leads to xe2x80x9copen eyes,xe2x80x9d where the binary zero and one signals are clearly separated during the measurement interval as in FIG. 1, while dispersion-induced broadening leads to xe2x80x9cclosed eyesxe2x80x9d as in FIG. 2. U.S. Pat. No. 5,861,970 issued Jan. 19, 1999 provides a good discussion of dispersion in optical networks and is incorporated herein by reference.
Dispersion within an optical network can be measured in a number of ways. U.S. Pat. Nos. 5,717,510, issued Feb. 10, 1998, and 5,815,294, issued Sep. 29, 1998, address the monitoring of specific signal characteristics, and are incorporated herein by reference. The monitoring systems discussed in these patents, as well as some other known monitoring systems, require synchronous detection of data with subsequent processing. Such systems provide accurate results, but require large amounts of storage and processing hardware, making them complex and expensive.
Two methods for dispersion compensation are currently used in practical systems. In the first method, dispersion compensators, formed by long lengths of optical fiber (with dispersion properties that cancel the dispersion characteristics of the transmission channel), are placed at strategic points in the network to keep the dispersion within limits, and at the receiver to reduce accumulated dispersion. At the receiver, careful adjustment of fiber lengths is necessary to match these characteristics at a specific wavelength for a specific system. Adjustment is made by switching long lengths of fiber into or out of the compensator. The long lengths necessary for this component introduce signal attenuation, so an optical amplifier is typically needed to offset this loss.
The second currently employed method uses linearly chirped fiber Bragg gratings. Here, the period of a fiber grating is varied linearly with distance, with a mean period that approximately satisfies the Bragg condition for strong reflection at the optical wavelength of interest. The linearly-varying period imparts chromatic dispersion on the reflected pulse which can be used to compensate for accumulated dispersion in long-haul signal propagation. These components typically operate over a narrow bandwidth, and the amount of dispersion compensation, which depends on the grating length, chirp parameter, and index modulation depth, cannot be adjusted easily. Various aspects of the fabrication and use of chirped fiber Bragg gratings are described in U.S. Pat. No.: 5,420,948, issued May 30, 1995; U.S, Pat. No. 5,602,949, issued Feb. 11, 1997; and U.S. Pat. No. 5,718,738, issued Feb. 17, 1998; all of which are incorporated herein by reference.
An alternative technique is the use of a uniform fiber Bragg grating in transmission. It is known that a pulse propagating at a wavelength just outside the reflection band of a grating structure undergoes significant chromatic dispersion. This effect was observed in the paper, F. Ouellette, Optics Letters, vol. 29, no. 32, pp. 4826-4829, 1990. Use of this technique for dispersion compensation has been suggested in the articles: B. Eggleton, et al., Electronics Letters, vol. 32, p.1610, 1996; N. Litchinitser, et al, Journal of Lightwave Technology, vol. 15, pp. 1323-1328, 1997; and N. Litchinitser, et al., Journal of Lightwave Technology, vol.15, pp.1303-1313. These papers demonstrated the effectiveness of this technique for a single, fixed wavelength in a particular system geometry. Examples of the power transmission and dispersion versus wavelength for a uniform grating are shown in FIGS. 3 and 4. At wavelengths slightly longer than the long-wavelength reflection band edge of the grating, negative dispersion is produced, while at wavelengths below the short-wavelength reflection band edge, positive dispersion is provided. For large dispersion magnitudes (greater than 1000 ps/nm), a large index modulation (greater than 0.001) and a long grating (greater than 10 cm) are typically required. In addition, proper apodization (i.e., smoothing of the profile at the grating edges) is necessary to remove sidelobe structure present in a uniform grating. The graphs in FIGS. 3 and 4 are for a grating of 10 cm in length with a refractive index of 0.0018 and super-Gaussian apodization. It should be emphasized that dispersion compensation by chirped gratings is achieved through reflection, whereas dispersion compensation by uniform gratings is achieved through transmission, two very different mechanisms that exhibit markedly different physics.
A related technology is that of strain-tuned fiber gratings. It is known that the resonant wavelength of an individual fiber grating may be tuned by either tensile strain (i.e., stretching) or compressive strain. Several techniques have been proposed for producing tensile strain, including thermal and piezo-electrical mechanisms. U.S. Pat. Nos. 5,007,705, issued Apr. 16, 1991 and 5,469,520, issued Nov. 21, 1995 describe tensile and compressive strain tuning, respectively, and are incorporated herein by reference. Strain tuning has been applied to a uniform grating used for filtering and to a chirped grating used for dispersion compensation.
Present dispersion compensation methods, as described above, have several shortfalls. Dispersion compensators formed by long lengths of compensating fiber are cumbersome and their properties can only be changed in discrete steps since change is accomplished by switching lengths of fiber in and out of the compensator. A chirped fiber Bragg grating has a narrow bandwidth, and even if strain tuned, is only adjustable over a small range. Additionally, a chirped grating typically requires a length on the order of meters for full compensation. A uniform fiber Bragg grating only provides dispersion when the incident wavelength is just outside a reflection band edge, and therefore, it is difficult to manufacture such a grating precisely enough to provide the required compensation, and difficult to maintain the dispersive characteristic, even with strain tuning. What is needed is a way to provide a reliable, fully adjustable (tunable), broadband dispersion compensator with a wide dynamic range. Additionally, such a compensator could be enhanced through the development of a monitoring and control system that monitors dispersion asynchronously and controls dispersion compensating elements using relatively inexpensive hardware.
This invention solves the above-described problem by providing a fully tunable dispersion compensator with a wide dynamic range in dispersion. The dispersion compensator according to the invention is based on tuned uniform fiber Bragg grating technology. By small adjustments of grating period, dispersion magnitude may be varied from zero to values of 2000 ps/nm or higher for a given wavelength, depending on grating length. The gratings employed in this invention require lengths of 5-20 cm for full compensation. Additionally, a control signal input is provided for a transducer that is coupled to a grating, so that precise control, including feedback control, can be used to tune the gratings to maintain the appropriate characteristics. A single compensation element can be used to provide dispersion compensation at a single frequency. Or, multiple elements can be linked together in series or parallel to provide a multiple wavelength or broadband optical dispersion compensator. A highly efficient, low-cost, dispersion monitoring and control system for the dispersion compensators of the invention is also disclosed.
When we use the terms xe2x80x9ctunedxe2x80x9d or xe2x80x9ctuningxe2x80x9d or words like xe2x80x9cadjustablexe2x80x9d with respect to a uniform fiber Bragg grating, we refer to the act or process of adjusting the grating periodicity to provide optimum dispersion compensation. Our use of these words is independent of whether this is done by a manual or automatic process and independent of whether it is done one time, prior to a grating being put into use, or done continuously while the grating is in operation. When we refer to the grating as being xe2x80x9cdynamically tunedxe2x80x9d or xe2x80x9cdynamically adjustedxe2x80x9d we are referring specifically to the act or process of adjusting the grating while it is in operation to maintain the appropriate characteristics, whether this is done manually or through the use of a control system using feedback principles.
In order to describe our invention, we describe, in this disclosure, multiple example embodiments. According to one embodiment, an adjustable, optical dispersion-compensating element is provided for compensation at a single frequency. The element includes a uniform fiber Bragg grating and a transducer coupled to the uniform fiber Bragg grating to deform the uniform fiber Bragg grating in response to a control signal. Dispersion is provided for a dynamically selectable wavelength of light being transmitted through the optical path by transmission just outside a reflection band edge of the uniform fiber Bragg grating. Compensation by this element can be for negative or positive group velocity dispersion by selecting the transmission point to be just outside either the short or long wavelength end of the reflection band.
According to other embodiments of the invention, multiple dispersion compensating elements as described above can be connected together and coupled to optical fiber connectors and other components to create multiple-wavelength or broadband optical dispersion compensators. In one type of compensator, a plurality of cascaded uniform fiber Bragg gratings are connected between input and output connectors, each uniform fiber Bragg grating for compensating for dispersion at a specific wavelength. Light at other wavelengths is well outside the reflection band of a specific grating, and is transmitted through the system, to be compensated for by a different grating.
In another type of compensator an optical circulator is provide which has connections for input and output optical fibers and for one or more uniform fiber Bragg gratings, each to compensate for dispersion at a specific wavelength. A transducer is coupled to a grating, and the transducer is responsive to a control signal so that the specific wavelength for the uniform fiber Bragg grating can be dynamically changed. If there is only one grating, a reflection element is connected to reflect the light back to the uniform fiber Bragg grating and to the optical circulator. Upon being reflected back through the grating, further dispersion compensation is provided. If there are multiple gratings with transducers, the gratings work in pairs. The first grating of a pair is tuned so that light of the specific wavelength is transmitted through the grating slightly outside the reflection band edge so that compensation is provided. The second uniform fiber Bragg grating of the pair is tuned so that the light is reflected back to the first uniform fiber Bragg grating and to the optical circulator. If the wavelengths are chosen carefully, gratings before the pair simply pass the unselected wavelengths through the system. A final reflection element is needed to reflect the final wavelength back through the system. This can be a final grating or simply a mirror.
Another embodiment of the invention involves connecting the uniform fiber Bragg gratings in parallel. In this case, an N-wavelength optical dispersion compensator is built by connecting N parallel, uniform fiber Bragg gratings between a demultiplexer and the multiplexer, each uniform fiber Bragg grating for compensating for dispersion at a specific wavelength as described earlier. When the demultiplexer receives a broadband signal having multiple wavelengths, the signal is split into individual streams of light pulses, each at a different wavelength. Each signal is passed through the appropriate grating just outside the reflection band edge as before, and the signals are reassembled by an N by 1 multiplexer. The demultiplexer and multiplexer have optical fiber connectors for connection within an optical fiber network. All the embodiments of the invention fundamentally operate the same way. Light waves at one or more selected wavelengths are received from an input. Dispersion compensation is provided at each of the selected wavelengths by passing light of the selected wavelength through a uniform fiber Bragg grating that is tuned so that the selected wavelength is slightly outside a reflection band edge of the grating, and the light is transmitted to an output. Multiple gratings can be connected together, and reflection and/or circulation and retransmission techniques can be used to provide additional compensation. The arrangements of components described provide the means to achieve a fully tunable dispersion compensator with a wide dynamic range in dispersion.
A control system can be connected to control signal inputs for the transducers used in the dispersion compensators described above. A control block is connected to the control signal inputs and receives signals from RF detectors that are connected to an opto-electronic receiver or converter. One RF detector measures spectral power directly, while another measures it after being passed through a filter. The signals from the detectors are used to determine a fractional spectral power transmitted through the filter. The details of the determination depend on the type of filter being used, that is whether the filter is a low-pass, high-pass, or band-pass filter. Measurements can be made in-line via an optical tap, or at a termination, in which case the opto-electronic receiver can be part of the data/clock recovery system for the optical network.
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