Abstract:
The method and apparatus of the present invention achieves the trimming and, therefore, tuning of fiber optic devices by, in on embodiment, precisely heating a small area of a fiber to allow its elongation when mounted under tension in its package. By pulsing a source of heat in precise amounts, the elongation can be precisely controlled within 1 picometer precision over a tuning range of about 200 picometers. In another embodiment with fibers having core dopants which can be diffused, the optical length of an optical fiber can be trimmed with nanometer precision. By employing a controlled source of localized energy applied to the optical fiber, real time trimming can be achieved in both systems by injecting a broad band source of energy at the input of the device and coupling a spectral analyzer at its output to monitor the frequency characteristic of the optical device during trimming. In a preferred embodiment, the energy source comprised a CO 2  laser having a relatively narrow beam corresponding to the diameter of the optical fiber. Energy from the laser is directed to a small area of the optical fiber in pulses which provide precise control of the trimming process.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the trimming of optical fiber components and particularly to a method and apparatus for achieving the trimming of the optical path length of an optical fiber component. 
     2. Technical Background 
     Optical fiber based devices are widely utilized as components for optical communications due to their relatively low insertion loss and low cost. Foremost of optical fiber components are fiber Bragg gratings (FBG) which are typically made by ultraviolet (UV) wavelength energy exposure. Once an FBG is mounted to a substrate and annealed, it is no longer photosensitive and cannot be further tuned. Thus, it is necessary to empirically predict the final frequency of such a grating which can lead to a significant error and gratings which are not within specifications. Due to the uncertainty of the wavelength shift resulting from the attachment process and annealing, the center wavelength of a package fiber Bragg grating typically has an error of +/−20 picometers from the desired center wavelength. Such a wavelength error combined with a wavelength drift of, for example, distributed feedback lasers, which may be from +/−50 picometers, and the residual temperature dependence of +/−20 picometers imposes a highly stringent requirement on the design of, for example, 50 GHz fiber Bragg gratings. 
     Infused fiber Mach-Zehnder interferometers are also wavelength selective and are used in a variety of communication devices, such as optical switches, filters, wave division multiplexers, demultiplexers, and add/drop filters as examples. In Mach-Zehnder based devices, the optical performance critically depends on the phase difference and/or optical path length difference between two interfering arms. Phase trimming has been attempted utilizing UV exposure to the fibers, however, such fibers must be photosensitive and, once annealed after such UV exposure, the trimming processes cannot be further controlled. Additionally, the maximum amount of trimming utilizing UV exposure is limited to a few wavelengths due to the relatively small refractive index change induced by UV radiation. In some applications, such a trimming process may not be sufficient to achieve the optical path length change necessary. 
     With optical path length sensitive fiber-based devices, therefore, not only is the tuning range a serious limitation by prior techniques, so is the tuning accuracy. There exists a need, therefore, for a system for the tuning of fiber optic devices over a relatively wide band of wavelengths, as well as to a precise wavelength. 
     SUMMARY OF THE INVENTION 
     The method and apparatus of the present invention achieves the tuning of fiber optic devices by, in one embodiment, precisely heating a small area of a fiber adjacent a grating mounted under tension to allow the grating length to change. By pulsing a source of heat in controlled amounts, the optical length of such a grating can be precisely controlled within 1 picometer precision over a tuning range of about 200 picometers. 
     In fibers having dopants which can be diffused, the length of an optical fiber can be trimmed with nanometer precision by thermal diffusion which directly affects the refractive index of the fiber, thereby effectively changing its optical path length. 
     In either embodiment, real time tuning is achieved by injecting a broad band source of energy at the input of the device and coupling a spectral analyzer at its output to monitor the center frequency of the optical device during trimming using a controlled source of localized energy applied to the optical fiber. In a preferred embodiment of the invention, the energy source comprised a laser and particularly a CO 2  laser having a relatively narrow beam corresponding to the diameter of the optical fiber employed in the device. Energy from the laser is directed to a small area of the optical device in pulses which provide precise control of the trimming process. 
     A method of trimming an optical fiber component by directing a source of radiation onto a section of the component for heating the section, coupling a broad band source of signals to an input of the component, coupling an optical analyzer to an output of the component, and monitoring the signal at the output of the component while selectively applying the radiation to the component from the source to achieve a predetermined trimming effect. 
     Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings. 
     It is to be understood that the foregoing description are exemplary of the invention only and are intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provided a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an enlarged schematic side elevational view of a fiber Bragg grating, shown partially packaged; 
     FIG. 2 is a top plan view of the fiber Bragg grating shown in FIG.  1  and the structure employed in practicing the method of the present invention and their relationship to the grating; 
     FIG. 3 is a waveform diagram illustrating the trimming of a fiber Bragg grating, employing the apparatus and method illustrated in FIG. 2; and 
     FIG. 4 is a schematic view of a Mach-Zehnder interferometer employed as an add drop filter, which can be trimmed according to an alternative embodiment of the present invention at locations indicated in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, there is shown a typical fiber Bragg grating assembly  10  which includes an optical fiber  12  on which there is imprinted in a central area  14  spanning a width of approximately 30 millimeters, a grating desired to be tuned to 1559.25 nanometers (nm). The optical fiber  12  is supported on a negative coefficient of expansion substrate  16 , such as β-eucryptite, by a pair of spaced glass frits  18  and  20 . Between the glass frits  18  and  20 , there exists end zones  22  and  24  spanning the grating  14  and each having a length of approximately 10 millimeters such that the overall distance between frits  18  and  20  is approximately 50 millimeters. 
     Fiber  12  is mounted to the substrate  16  under a tension of approximately 10 kpsi, with grating  14  being conventionally formed utilizing an ultraviolet light to change the index of refraction of the core of the fiber  12  in a pattern selected for a wavelength of approximately 1559.2 nanometers as an example. Naturally, other frequency fiber Bragg gratings can be trimmed with the trimming method of the present invention. The grating assembly  10  is then annealed, which eliminates the photosensitivity of the fiber  12 , preventing any further tuning. As a result, the exact frequency of the grating mounted to the substrate  16  can vary significantly. Before the final packaging of the grating shown in FIG. 1, it can be precisely trimmed, resulting in its tuning to an exact frequency utilizing the method and apparatus shown in FIG.  2  and now described. 
     In FIG. 2, a top plan schematic view of the grating assembly  10  is shown. The grating is trimmed for the desired optical path length of the grating area  14  itself by heating an isolated area encircled as area A in FIG. 2 in end section  22  of the grating, although the trimming can take place at either of ends  22  or  24  or both, if desired. It was discovered that the area of fiber  12  adjacent grating  14  can be heated to change it viscosity, which is a function of the width “w” of an impinging laser beam and indirectly proportional to three times the elongation speed of section  22  upon heating according to the following formula:          η                   (     T   ,   z     )       =                  σ                 w       3                 υ                              
     after heating, where “T” is the heated temperature and “z” is the position of the laser beam. 
     After heating for a time “t”, the localized fiber elongation Δ (causing the grating spacing to shorten) is:              Δ              =         σ                 w     3            t     η                   (     T   ,   z     )         .               (equation  1)                                
     Thus, it was discovered that by controlling the local viscosity utilizing an appropriate heat source and exposure time, a small amount of elongation in the order of nanometers can be achieved. Before describing a specific example of the results obtained, a brief description of the equipment and method of FIG. 2 follows. 
     In FIG. 2, real time trimming of the grating  14  can be achieved by providing a broad band source  30  of optical energy to an input  19  of grating assembly  10 , while an optical spectral analyzer  32  is coupled to an output  21  of the grating assembly  10 . Thus, before trimming, the wavelength of the grating can be determined by viewing the display output from the analyzer  32 . Assuming it is desired to trim the grating assembly  14 , tuning it to a desired frequency, the localized heat source is applied to the center of the 10 mm section  22 , that is 5 mm in from frit  18  in the preferred embodiment in the location identified by encircled area A. The energy employed is provided by a CO 2  laser  40  which has a frequency which provides heating of the area A. A conventional CO 2  laser, such as an SYNRAD 48-2W having a power stability within +/−1-percent per hour was employed in the preferred embodiment of the invention. The light beam  42  from CO 2  laser  40  is passed through a control shutter  43 , through a beam splitter  44  onto a focusing lens  46  made of ZnSe having a 1.5 inch focal length, thus positioned approximately 1.5 inches from position A on the fiber Bragg grating. The laser beam  42  passes through the beam splitter  44 , which is employed to coaxially align a visible light beam  45  from a helium neon laser  48  positioned to provide a combined beam  42  and  45  which is visible such that a microscope  50  can be positioned in the area adjacent position A for visibly inspecting the area to which the laser beam  42  is to be directed, assuring its freedom from dust particles or other contaminants and precisely aligning the CO 2  laser onto area A of the optical fiber. The microscope has a magnification of approximately 100× to monitor the heated region A. 
     The focused laser beam has a diameter substantially equivalent to the diameter of the optical fiber and, in the preferred embodiment, 125 μm. By providing the impinging heating light beam  42  from laser  40  laterally, as illustrated in FIG. 2, the substrate  16  is not heated, thereby not interfering with the trimming of the fiber Bragg grating  14 . An example of the trimming of a fiber Bragg grating  10  having a length of 50 mm between the frits and a tension σ equal to 10 kpsi, a laser width “w” of 0.2 mm, trimming of 1.5 picometers in one second requires a viscosity of 13.0 dPa.s. This results in a temperature of 1,200° C. for the viscosity of silica employed for the optical fiber. 
     To achieve a trimming speed of 1 picometer per second, the laser power and focusing condition is first calibrated utilizing a packaged grating with a similar tension and attachment length. The grating is mounted on an XYZ translation stage  60  (shown in phantom in FIG. 2) and positioned utilizing the helium neon laser beam  45  and microscope  50 . Due to the relative shallow depth of the field of the microscope, it effectively registers both lateral and axial positions of the heated region. If necessary for more precise registration, a second microscope orthogonal to the direction of the first microscope and to the laser direction can be employed for precise alignment. Once the reference fiber Bragg grating is aligned, the stage  60  can receive other gratings moved into the same initial position. Final adjustments and inspection of each grating being trimmed is similarly achieved using visible laser beam  45 , microscope  50 , and stage  60 . 
     The laser beam  42  is pulsed using the controlled electro-mechanical shutter  43  which provides pulses of selectable duration and frequency typically from 1 to 30 seconds in duration. The shutter  43  is a commercially available unit and allows laser  40  to remain continuously on for stability. The target tuned wavelength of the grating can be approached in small increments as illustrated by the waveform diagram of FIG.  3 . Thus, for example, a trimming step of 4 picometers is obtained with the laser power adjusted to 0.660 watts and an exposure time of 2.5 seconds. FIG. 3 illustrates five consecutive exposures indicating a relatively linear shift in a gratings tuned wavelength. The wavelength shift is stabilized after about 10 seconds from the exposure to the laser beam  42 . An exposure time of 0.5 seconds results in a wavelength shift of less than 1 pm, which is beyond the resolution of the optical spectral analyzer  32 . The wavelength shift is shown by the following equation:            Δ                   λ   Bragg                      λ   Bragg         =     -     Δ   l                              
     Where “l” is the fiber length between the two frits  18  and  20 . The amount of trimming of the grating and tuning of the Bragg wavelength, therefore, is:          Δ                   λ   Bragg       =       -                  σ                 w       3                 l              (     t     η                   (     T   ,   z     )         )                     λ   Bragg                              
     where Δ is derived from equation 1 supra. 
     As can be seen from the above, a tuning step of 1.5 picometers corresponds to a fiber elongation of 50 nanometers, allowing grating  14  to contract and increase in frequency by as much as 200 picometers. This system, therefore, can be employed for providing precise trimming of, for example, Lucent 50 GHz gratings for center wavelength tuning. 
     The apparatus of FIG. 2 can also be employed for tuning of optical devices which are not mounted under tension and, therefore, the optical length changed not by allowing the relaxation of the optical fiber but instead by the diffusion of a dopant of an optical fiber. This changes the refractive index and, therefore, varies the optical length for precise tuning of, for example, a Mach-Zehnder interferometer as illustrated in FIG.  4 . 
     Referring now to FIG. 4, there is shown a Mach-Zehnder optical device configured as a add-drop filter  60  which can be precisely tuned employing the method of the alternative embodiment of the present invention. The Mach-Zehnder add-drop filter  60  comprises first and second optical fibers  62  and  64 , which are fused in a first coupler  63 , and has branches with gratings  66  and  68 , respectively, formed therein which can be tuned to matching wavelengths λ 1  as described in greater detail below. The fibers continue to a second fused coupler  65  and terminate in an input  69  for fiber  62  and an output  70  for fiber  64 . Fiber  62  has an input  61  which receives, as an example, eight discrete wavelengths λ 1 -λ 8  modulated with signal information. The add-drop filter  60  drops the λ 1  frequency at output terminal  67 , allowing the addition of λ 1 ′ at input  69  to signals λ 2 -λ 8  at output  70 . 
     The Mach-Zehnder device  60  is conventionally fabricated and must be trimmed to tune gratings  66  and  68 . Typically, Mach-Zehnder devices, such as device  60 , have been trimmed utilizing ultraviolet radiation, however, as noted above, such radiation is ineffective once a device is annealed, and such annealing affects the optical path length thereby changing any trimming which may have taken place. Additionally, ultraviolet radiation is not as effective in changing the index of refraction and, therefore, the optical path length of the optical devices. As a result in the past, frequently an isolator was placed before the add port  69  to reduce multi-reflection interference. By using the trimming apparatus illustrated in FIG.  2  and applying heat at a precise location and in a controlled amount, the gratings  66  and  68  of the Mach-Zehnder device  60  can be precisely phase matched. The couplers  63  and  65  can also be trimmed to change their effective coupling length for providing a 50/50 coupler. 
     By applying precise amounts of energy through laser beam  42  in a manner similar to that described above with reference to FIG. 2 to the optical fiber area adjacent either of the interfering gratings  66  and  68  as shown by arrow B, for example, in FIG. 4, the core dopant diffuses toward the cladding, lowering the refractive index and shortening the optical path of one of the interfering arms  66 ,  68 . The energy can be applied to either side of the center of gratings  66  or  68 , depending on which of the legs needs to be optically shortened for phase matching of the two interfering gratings. Trimming can be done in real time by providing a broad band signal at input  61  and monitoring the output at port  70  to determine the absence of energy at the wavelength λ 1  for a filter dropping λ 1 . The amount of energy is slightly greater than discussed with respect to the first embodiment with a G e  doped fiber requiring somewhat larger pulses from laser  40  of about 10 to 30 seconds to raise the temperature of the optical fiber at target area B to about 1600° C. For fibers doped with F 1  or Bo, a lower temperature of about 1400° C. results in the desired change in the refractive index n of the fiber to achieve trimming. Real time trimming is achieved by progressively applying pulses of the laser beam  42  while watching the output of analyzer  32  for the desired maximum rejection of the λ 1  frequency. Alternatively, output  67  can be monitored for the maximum level of λ 1  frequency reflected at the drop port  67 . 
     In addition to phase matching the interfering gratings  66  and  68 , the coupler  63  and decoupler  65  can be adjusted for providing equal splitting of energy by applying the laser energy to one or the other legs, as indicated by arrow C and D, respectively, again to cause the diffusion of the core&#39;s dopant material, lowering the index of refraction “n”. 
     The same diffusion trimming technique can also be employed with unbalanced and lattice filters by phase trimming the amount of unbalance in the optical path length between the two arms of the Mach-Zehnder device, as well as the cross talk and optical switches may be optimized to better than 40 dB with such phase trimming and coupler trimming. 
     In the first embodiment of the invention, the grating spacing is shortened to increase the center frequency of the grating by lengthening the edge connection of the grating to the frit. In the second embodiment, the refractive index is lowered to decrease the optical length of a fiber optic component. With either embodiment, precise tuning of an optical fiber component can be achieved. 
     It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.