TUNABLE GRATING-BASED DISPERSION COMPENSATOR

Abstract: A tunable dispersion compensating device includes a grating element (102) in the form of a bulk or large diameter waveguide, having an outer cladding (104) disposed about an inner core (106). The grating element may be etched, grounded or machined to form a generally "dog bone" shape, wherein the end portions (108) of the grating element has a larger diameter than the center portion (110) disposed therebetween. A chirped grating (112) is written or impressed within the portion of the core disposed in the center portion of the grating element. The center portion is tapered to allow different stresses to be applied along the grating length when the grating element is compressed longitudinally by force F, and thereby vary chirp of the grating to tunable compensate for dispersion.

Description: TUNABLE GRATING-BASED DISPERSION COMPENSATOR Technical Field The present invention relates to dispersion compensators, and more particularly to a tunable dispersion compensator based on a Bragg grating that allows the dispersion characteristics of a Bragg grating element to be tuned. Background Art Fiber Bragg gratings can be used as effective filter elements in a range of applications in optical communications networks. The dispersion characteristics of a communication link are crucial in determining performance and transmission fidelity. Gratings, particularly chirped gratings can be used effectively for dispersion compensation. The ability to tune the dispersion introduced by a grating element is important in providing an adjustable element for performance optimization. As optical networks evolve, there is a trend towards the development of "all-optical" networks, in which communications signal are dynamically routed in the optical domain. One of the issues that affect the ability of network engineers to increase transmission distances & data rates is signal impairments. One of the important parameters of a link is the dispersion of the system, which can create a serious deleterious effect on the signal quality in a high-Gbit optical transmission systems such as 10 Gbits/s and 40 Gbits/s. Chirped Bragg gratings can be used to compensate for dispersion in a communications link. FIG. 1 depicts a known optical transmission system 10. Optical data 12, which is generated by a laser transmitter 14, is transmitted along a fiber link 16, that may or may not contain optical amplifiers and other devices (that may in themselves be dispersive) to a receiver 18. During the transmission along the fiber link 16, the optical data 12 will be corrupted to a certain degree by differential group delay experienced by different wavelength components of a given wavelength signal. This group delay "smears-out" the data 12 producing poor contrast in the received bit stream 20 at the receiver end. The differential group delay in a signal can be compensated for by the use of an appropriately chirped grating 22. As shown, the light (or received bit stream) 20 from the fiber link 16 is coupled though a circulator 24 to a chirped grating 22. The chirped grating 22 has a bandwidth B and length L. The chirping of the grating 22 results in a strong group delay that can be chosen to be equal to, but opposite to that of the fiber link 16. For example, for a grating length of 10 cm, the differential group delay for a wavelength component reflected from the front of the grating compared to one reflected from the rear of the grating is ~ 1000 ps. If the grating has a bandwidth of 1 nm, then it's effective dispersion is 1000 ps/nm. Currently, the grating 22 has to be designed to match a given dispersion induced group delay error in a communications link. Summary of the Invention An object of the present invention is to provide a tunable dispersion compensator having a Bragg grating that allows the dispersion characteristics of a Bragg grating element to be tuned. In accordance with an embodiment of the present invention, an optical waveguide includes an outer cladding disposed about an inner core. A portion of the cladding of the waveguide includes a tapered region. The cladding has a minimum cross-sectional dimension of 0.3 mm. A chiφed grating is written in the inner core of the tapered region of the cladding. In accordance with another embodiment of the present invention, a tunable dispersion compensator includes an optical waveguide having an outer cladding disposed about an inner core. A portion of the cladding has a tapered region. The cladding has a minimum cross-sectional dimension of 0.3 mm. A chirped grating is written in the inner core of the tapered region of the cladding. Brief Description of the Drawings FIG. 1 is a block diagram of an optical transmission system including a Bragg grating-based dispersion compensator that is known in the art; FIG. 2 is a side view of a tunable dispersion compensating device in accordance with the present invention; FIG. 3 is a side view of a grating element of a tunable dispersion compensating device having a chiφed grating written in a first "sense" in accordance with the present invention; FIG. 4 is a graphical representation of the chiφ rate of the grating element of FIG. 3 as the grating is longitudinally compressed; FIG. 5 is a graphical representation of the reflectivity profile of the grating element of FIG. 3 when no compressional load is applied; FIG. 6 is a graphical representation of the reflectivity profile of the grating element of FIG. 3 when a compressional load is applied; FIG. 7 is a side view of a grating element of a tunable dispersion compensating device having a chiφed grating written in a second "sense" in accordance with the present invention; FIG. 8 is a graphical representation of the chiφ rate of the grating element of FIG. 7 as the grating is longitudinally compressed; FIG. 9 is a side view of an alternative embodiment of a grating element in accordance with the present invention; and FIG. 10 is a side view of an alternative embodiment of a grating element in accordance with the present invention. Best Mode for Carrying Out the Invention Referring to FIG. 2, a tunable dispersion compensating device, generally shown as 100, comprises an optical Bragg grating element 102, best shown in FIG. 3. The grating element 102 is a bulk or large diameter waveguide, having an outer cladding 104 disposed about an inner core 106. The grating element may be etched, grounded or machined to form a generally "dog bone" shape, wherein the end portions 108 of the grating element has a larger diameter than the center portion 110 disposed therebetween. A chiφed grating 112 is written or impressed within the portion of the core 106 disposed in the center portion 110 of the grating element 102. The center portion 110 is machined into a tapered form to allow different stresses to be applied along the grating length when the grating element 102 is compressed longitudinally by force F. In the embodiment shown, the cross-sectional area of the center portion 110 of the grating element 102 is gradually reduced from one value to a second value in a linear fashion. In other words, the outer diameter of the center portion tapers from one end to the other as a function of the square rot of the distance such that longitudinal compression of the element results in a linear increase in wavelength tuning of the chiφed grating 112 along its length. This allows the grating chiφ rate 114 a-c to be increased or decreased depending on the 'sense' of the original chiφ and the physical taper in the grating element 102. For example, consider the grating element 102 depicted in FIG. 3 wherein the sense of the chiφed grating 112 results in the Bragg wavelength increasing from left to right as depicted. In other words, the spacing of the gratings at the wide end of the center portion of the grating element is greater than the spacings of the gratings at the narrower end. The physical taper in the grating element 102 results in the longer wavelength having a higher sensitivity to compression load than the shorter wavelengths. Consequently, referring to Fig. 4, as the grating element is compressed longitudinally by force F, the grating chiφ rate 114 a-c decreases. Furthermore, the compressional loading of the grating element results in a reduction of the chiφ bandwidth, as illustrated in FIGs. 5 and 6, and therefore, the dispersion of the grating increases (same group delay is attained over a smaller bandwidth). Specifically, FIG. 5 shows that the chiφ bandwidth 113 of the grating element 102 is relatively wide, when no compressional load is applied. FIG. 6 shows that the chiφed bandwidth 115 of the grating 102 is reduced and the center wavelength 0 is shifted to λ d , when a compressional load is applied. Alternatively, as shown in FIG. 7, the original chiφ may be written into the grating element 102 in the opposite sense, wherein the sense of the. chiφed grating results in the Bragg wavelength increasing from right to left as depicted. In other words, the spacing of the gratings at the wide end of the center portion 112 of the grating element 102 is less than the spacings of the gratings at the narrower end. The physical taper in the grating element results in the shorter wavelengths having a higher sensitivity to compression load than the longer wavelengths. Consequently, referring to FIG. 8, as the grating is compressed longitudinally by force F, the grating chiφ rate 120a-c increases. Conversely to the grating element of FIG. 3, compressional loading of the element results in an increase of the chiφ bandwidth, and therefore, the dispersion of the grating decreases (same group delay is attained over a smaller bandwidth). Consequently, the overall chiφ of the grating 112 of grating element 102 is increased by compressive loading, and thus the dispersion is reduced. Either way, the dispersion of the grating element can be tuned up or down from a present value. Tuning over a 1:3 range or greater should be possible using this approach. As shown in FIG. 6, one limitation of this approach is the compressional tuning also tunes or shifts the overall center wavelength λco of the grating 112. This can cause an issue if the grating bandwidth does not overlap that of the signal at all times and over it's tunable range. This can be compensated by thermal tuning of the grating element, as shown in broken lines at 121 in FIG. 6. Thermal tuning would allow correction of any center wavelength caused by compressional tuning. Thermal tuning could be effected using either heating elements or peltier (TEC) cooler/heater elements 122, as shown in FIG. 2. Referring to FIG. 3, the optical grating element 102 of the tunable dispersion compemsating device 100 is disposed within a compressing device or housing 124. The grating element comprises silica glass (SiO2) having the appropriate dopants, as is known, to allow light to propagate along the inner core, and has an outer diameter (d) of at least 0.3mm. The grating element 102 may be formed by using fiber drawing techniques now know or later developed that provide the resultant desired dimensions for the core and the outer dimensions, similar to that disclosed in co-pending US Patent Application, Serial No. 09/455,868 entitled "Large Diameter Optical Waveguide, Grating, and Laser". The grating element 102 may then be etched, grounded or machined to form the "dogbone" shape. A pair of fibers or "pigtails" 126 may be attached to the ends of the grating element by known techniques, such as epoxy or glass fusion. Alternatively, the optical grating element 102 may be formed by heating, collapsing and fusing a glass capillary tube to a fiber by a laser, filament, flame, etc., as is described in copending US Patent Application, Serial No. 9/455,865, entitled "Tube-Encased Fiber Grating", which is incoφorated herein by reference. Other techniques may be used for collapsing and fusing the tubes to the fiber, such as is discussed in US Patent No. 5,745,626, entitled "Method For And Encapsulation Of An Optical Fiber", to Duck et al., and/or US Patent No. 4,915,467, entitled "Method of Making Fiber Coupler Having Integral Precision Connection Wells", to Berkey, which are incoφorated herein by reference to the extent necessary to understand the present invention, or other techniques. Alternatively, other techniques may be used to fuse the fiber to the tube, such as using a high temperature glass solder, e.g., a silica solder (powder or solid), such that the fiber, the tube and the solder all become fused to each other, or using laser welding/fusing or other fusing techniques. The chiφed Bragg grating 112 is written (embedded or imprinted) into the inner core 106 of the grating element 102. The Bragg grating 112 reflects a portion of the light as indicated by a line 128, and passes the remaining wavelengths of the incident light (within a predetermined wavelength range), as indicated by a line 130. The chiφed grating 112, as is known, is an aperiodic variation in the effective refractive index and/or effective optical absoφtion coefficient of an optical waveguide, such as that described in US Patent No. 4,725,110 and 4,807,950, entitled "Method for Impressing Gratings Within Fiber Optics", to Glenn et al; and US Patent No. 5,388,173, entitled "Method and Apparatus for Forming Aperiodic Gratings in Optical Fibers", to Glenn, which are hereby incoφorated by reference to the extent necessary to understand the present invention. However, any wavelength-tunable grating or reflective element 112 embedded, written, etched, imprinted, or otherwise formed in the inner core 106 may be used if desired. As used herein, the term "grating" means any of such reflective elements. Other materials and dimensions for the optical grating element 102 may be used if desired. For example, the grating element 102 may be made of any glass, e.g., silica, phosphate glass, or other glasses, or made of glass and plastic, or solely plastic. An actuator 132, such as a piezoelectric actuator, axially compresses the grating element 102 within the compression device or housing 124, as indicated by arrows 134. Accordingly, the PZT actuator 132 provides a predetermined amount of force to compress the grating element 102, and thereby tune the grating 112 to a desired center wavelength. In response to a control signal generated by a controller 136 via conductor 138, the PZT actuator 132 is energized to provide the appropriate compression force necessary to tune the grating element to the desired chiφ (i.e., bandwidth and center wavelength of reflectivity profile) of the grating 112. The controller 136 adjusts the expansion and retraction of the actuator 132 in response to a displacement sensor 140 that provides feedback representative of the strain or compression of the grating element 102 to form a non-optical closed-loop control configuration. In other words, light 12 propagating through the network or device is not used to provide feedback for the tuning of the grating 112. In one embodiment, the displacement sensor 140 includes a pair of capacitive elements 142, similar to that disclosed in co-pending US Patent Application, Serial No. 09/519,802 entitled, "Tunable Optical Structure Featuring Feedback Control", filed March 6, 2000, which is incoφorated by reference in its entirety. The controller measures the capacitance between the capacitive plates 144; and provides a sensed signal via conductors 146, indicative of the measured capacitance, to the controller 136. As the grating element 102 is strained, the gap between the parallel capacitive plates 144 will vary, thereby causing the capacitance to change correspondingly. The change in capacitance may be directly or inversely proportional to the change in the chiφ of the grating 112, depending on the sense of the chiφed grating. Although the invention has been described with respect to using a capacitor to measure the gap distance, it should be understood by those skilled in the art that other gap sensing techniques may be used, such as inductive, optical, magnetic, microwave, time-of- flight based gap sensors. Moreover, the scope of the invention is also intended to include measuring or sensing a force applied on or about the compressive element, and feeding it back to control the compression tuning of the optical structure. While the embodiment of the present invention described hereinbefore includes means to provide feedback of the displacement of the grating element 54, one should recognize that the grating units may be accurately and repeatedly compressed/tuned to allow the grating unit to operate in an open loop mode. Alternatively, the grating 112 may be tuned by mechanically stressing (i.e. tension, bending) the grating elements 102. Furthermore, while the grating 112 has been described as being within a grating element 102 in the form of a bulk waveguide, one will appreciate that the grating may be written into a fiber, wherein the grating may be toned thermally or mechanically (compression or tension). FIG. 9 shows another embodiment of a grating element 160 similar to the grating element 102 of Fig. 3. Contrary to grating element 102, the taper of the center portion 110 of grating element 160 is linear and the grating 162 is periodic. Consequently, as the grating elements are longitudinally compressed, the grating becomes variably chiφed and provides a quadratic chiφ in the grating. Fig. 10 shows yet another embodiment of a grating element 170 similar to the grating element 102 in Fig. 3. Contrary to grating element 102, the grating 172 of grating element 170 is periodic such that the grating becomes linearly chiφed as the grating element 170 is longitudinally compressed. The dimensions and geometries for any of the embodiments described herein are merely for illustrative puφoses and, as much, any other dimensions may be used if desired, depending on the application, size, performance, manufacturing requirements, or other factors, in view of the teachings herein. It should be understood that, unless stated otherwise herein, any of the features, characteristics, alternatives or modifications described regarding a particular embodiment herein may also be applied, used, or incoφorated with any other embodiment described herein. Also, the drawings herein are not drawn to scale. Although the invention has been described and illustrated with respect to exemplary embodiments thereof, the foregoing and various other additions and omissions may be made therein without departing from the spirit and scope of the present invention.

Claims: Claims What is claimed is: 1. An optical waveguide comprising: an inner core; and an outer cladding disposed about the inner core, a portion of the cladding including a tapered region, the cladding having a minimum cross-sectional dimension of 0.3 mm; and a chiφed grating written in the inner core of the tapered region of the cladding. 2. The optical waveguide of claim 1, wherein the tapered region varies from a first end to a second end as a function of the square root of the distance therebetween, the first end having a greater cross-section than the second end. 3. The optical waveguide of claim 1, wherein the tapered region varies linearly from a first end to a second end, the first end having a greater cross-section than the second end. 4. The optical waveguide of claim 2, wherein the chiφed grating is written in the inner core such that the Bragg wavelength of the chiφed grating increases from the first end to the second end of the tapered region. 5. A tunable dispersion compensator comprising: an optical waveguide including: an outer cladding disposed about an inner core, a portion of the cladding including a tapered region, the cladding having a minimum cross-sectional dimension of 0.3 mm; and a chiφed grating written in the inner core of the tapered region of the cladding. 6. The tunable dispersion compensator of claim 5, wherein the optical waveguide comprises: an optical fiber, having a reflective element written therein; and a tube, having the optical fiber and the reflective element encased therein along a longitudinal axis of the tube, the tube being fused to at least a portion of the fiber. 7. The tunable dispersion compensator of claim 5, further includes a compressing device for compressing simultaneously and axially the optical waveguide, wherein chiφed grating is disposed along an axial direction of the optical waveguide. 8. The tunable dispersion compensator of claim 5, further includes a straining device for tensioning axially the optical waveguide to tune the chiφed grating, wherein the chiφed grating is disposed along an axial direction of the optical waveguide. 9. The tunable dispersion compensator of claim 7, further includes a thermal element for varying the temperature of the unchiφed grating to tune the unchiφed grating to a selected center wavelength. 10. The tunable dispersion compensator of claim 7, wherein the displacement sensor includes a capacitance sensor coupled to the optical waveguide for measuring the change in the capacitance that depends on the change in the displacement of the optical waveguide. 11. The tunable dispersion compensator of claim 5, wherein the tapered region varies from a first end to a second end as a function of the square root of the distance therebetween, the first end having a greater cross-section than the second end. 12. The tunable dispersion compensator of claim 5, wherein the tapered region varies linearly from a first end to a second end, the first end having a greater cross-section than the second end. 13. The tunable dispersion compensator of claim 11 , wherein the chiφed grating is written in the inner core such that the Bragg wavelength of the chiφed grating increases from the first end to the second end of the tapered region.

Inventors: BAILEY TIMOTHY J, KERSEY ALAN D, PUTNAM MARTIN A, BAILEY, TIMOTHY, J., KERSEY, ALAN, D., PUTNAM, MARTIN, A.
Applicants: CIDRA CORP, CIDRA CORPORATION
Application year: 2001
Main classification: G02B6
IPC codes: G02B6, G02B26