Abstract:
The invention provides a structure of tunable optical filter grating. The structure includes a bimetallic strip and a compression spring to set up suitable strain for adjusting the position of the central wavelength. The bimetallic strip can also be specially designed to compensate for any drift in central wavelength due to temperature change so that temperature monitoring and feedback control designs is obsolete. Therefore, the operating wavelength of the optical grating is adjustable yet insensitive to temperature changes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 88119200, filed Nov. 4, 1999. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to an optical fiber grating. More particularly, the present invention relates to an optical fiber grating capable of filtering light wave around a central wavelength with position of the central wavelength being adjustable. 
     2. Description of Related Art 
     Following rapid progress in high-density wavelength division multiplexing techniques for optical transmission, the ability to extract or insert signals at a definite wavelength at a particular node point become increasing important. To increase the variability of transmission capacity, optical designs with wavelength regrouping capability is in great demand. 
     Bragg type of optical grating has an optical core whose refractive index varies cyclically. Cyclically varying refractive index is generated by crossing two ultraviolet light beams so that interference lines are formed within the optical fiber. Alternatively an ultraviolet light beam is shone on a photomask to produce the interference lines in the optical fiber. Due to the photosensitivity of optical fibers, variation in refractive index is achieved. 
     When an incoming light beam transmitted through an optical fiber grating and if one of the wavelengths within the incoming light beam satisfies the Bragg condition, that particular wavelength will be reflected (in other words, same as the central wavelength of the optical grating). In general, the central wavelength reflected from the optical grating is twice the average effective refractive index times the period of refractive change. On the other hand, wavelengths that do not satisfy the Bragg condition will just pass through the optical grating. 
     As temperature around the optical grating changes, corresponding changes in the refractive index will lead to a shift in the central wavelength of reflection. For an optical grating that operates at a wavelength of 1550 nm, the wavelength/temperature coefficient is about 0.012 nm/° C. In other words, for every 1° C. change in temperature, the central wavelength will shift by 0.012 nm. 
     In addition, forces acting on the optical grating change, both the refractive index and the period of fiber grating will be affected leading also to a shift in the central wavelength of reflection. For an optical grating that operates at a wavelength of 1550 nm, the wavelength/strain coefficient is about 1.2 nm/millistrain. 
     Due to possible strain and temperature changes, consideration must be made with regard to the application of the optical grating. For example, in U.S. Pat. No. 5,694,503, the optical grating is fastened onto a ceramic plate that has a negative coefficient of thermal expansion. Furthermore, in U.S. Pat. No. 5,841,920 and World Intellectual Property Organization Patent No. WO9827446, an assembled structure consisting of two components having very different coefficient of thermal expansion are used to produce a negative coefficient of thermal expansion. Both types of inventions are applied to an optical grating working at a fixed wavelength. Strain produced by temperature change in the ceramic plate or the assembled structure is capable of compensating for the drift in central wavelength. In other words, drift in wavelength is minimized when temperature around the optical grating changes, or equivalently, the wavelength/temperature coefficient is reduced. 
     In addition, methods of adjusting the central wavelength of an optical grating are also developed. For example, in U.S. Pat. No. 5,812,711, changes in magnetic strain is used to adjust the operating wavelength of the optical grating. In U.S. Pat. No. 5,469,520, the optical grating is strained to adjust the operating wavelength. On the other hand, in European Patent No. EP0867736, the optical grating is heated to adjust the operating wavelength. 
     As the number of multiplexing channels is increased, channel spacing has been reduced from about 1.6 nm to just under 0.4 nm. Consequently, position of the central wavelength must be very precise and tolerance of the central wavelength has to be lower than ±0.025 nm. If the heating method is used to adjust operating wavelength, power for heating up the grating has to be constantly supplied. If piezoelectric material (for example, lead zirconium titanium oxide PbZrTiO 3  or PZT) is used to adjust operating wavelength, constant power supply is also needed. If the magnetic flux method is used to adjust operating wavelength, although power is needed only when adjustment is required, consideration must still be made regarding the wavelength/temperature coefficient and the thermal expansion coefficient of the magnetic material. Finally, if operating wavelength is adjusted by straining the optical fiber in the axial direction, the resolution of the mechanic has to be in the micrometer range. Therefore, demands in manufacturing precision are exceptionally high. 
     In all the aforementioned methods of adjusting the central wavelength of an optical grating, drift in central wavelength due to a temperature change is still ignored. In other words, even if the operating wavelength has been precisely set, any changes in temperature may still cause some undesirable drifting. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the present invention is to provide an optical fiber grating that uses a bimetallic strip and a compression spring to position the central wavelength of reflection for an incoming light beam. In addition, the grating is capable of offsetting any drifting of the central wavelength due to a temperature change. 
     A second object of this invention is to provide an optical fiber grating whose central wavelength of reflection can be adjusted yet quite insensitive to temperature fluctuation. Hence, there is no need for monitoring surrounding temperature and performing feedback control actions constantly. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an adjustable light-reflecting component. The light-reflecting component includes a waveguide that constrains and transmits an incoming light beam. The light-reflecting component is incorporated with the waveguide for reflecting incoming light signals around a central wavelength of the light-reflecting component. The light-reflecting component is fastened onto one side of a bimetallic strip. Any deformation of the bimetallic strip will cause some strain in the light-reflecting component and lead to a shift in the central wavelength of the reflected light signals. In addition, differences in coefficient of thermal expansion between the two metallic strips of the bimetallic strip will produce bending stress that can almost compensate for any drift in central wavelength when the temperature changes. One end of an elastic prop is attached to one side of the bimetallic strip while the other end is attached to a drawing point. Therefore, the bimetallic strip is bent by a force transmitted through the elastic prop. 
     The invention also provides an adjustable optical fiber grating system. The system includes a bimetallic strip formed by joining the flat surfaces of a first and a second metallic strip respectively. The coefficient of thermal expansion of the first metallic strip is smaller than the second metallic strip. The system also includes an optical fiber that incorporates an optical grating for reflecting incoming light signals around a central wavelength. The optical fiber grating is fastened onto one side of the first metallic strip so that any deformation of the bimetallic strip will cause some strain inside the grating and ultimately will result in a shift in the central wavelength. Furthermore, differences in the coefficient of thermal expansion between the two metallic strips will result in a bending stress in the bimetallic strip capable of compensating almost any drift in central wavelength due to a temperature change. One end of an elastic prop is in contact with one side of the second metallic strip while the other end is attached to a drawing point. Hence, the bimetallic strip is bent by a force transmitted through the elastic prop. 
     The invention also provides a method for adjusting an adjustable light-reflecting component. The method includes the steps of providing a waveguide having a light-reflecting component capable of reflecting incoming light signals around a central wavelength. A bimetallic strip with the light-reflecting component attached to one surface of the bimetallic strip is next provided such that any deformation of the bimetallic strip will cause some strain in the light-reflecting component. Furthermore, deformation in the bimetallic strip due to a temperature change can be used to offset any drift in the central wavelength of reflection due to the temperature fluctuation. Finally, an elastic prop is provided for exerting a force on the side of the bimetallic strip so that the resulting deformation in the bimetallic strip can be used to adjust the central wavelength of reflection for the light-reflecting component. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
     FIG. 1 is a sketch showing an adjustable optical fiber grating system according to a first embodiment of this invention; 
     FIG. 2 is a sketch showing an adjustable optical fiber grating system according to a second embodiment of this invention; 
     FIG. 3 is a graph of central wavelength versus distance traveled by the moving plate of an adjustable optical grating system; and 
     FIG. 4 is a graph of central wavelength versus temperature showing the drift in central wavelength of an adjustable optical grating for three different central wavelength settings. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     FIG. 1 is a sketch showing an adjustable optical fiber grating system according to a first embodiment of this invention. As shown in FIG. 1, the adjustable optical fiber grating system at least includes an optical fiber  10 , an optical grating, a bimetallic strip  30  and a compression spring  40 . 
     The optical fiber (waveguide)  10  is a light conduit that guides and transmits a beam of incoming light. The optical grating (light-reflecting component)  20  is incorporated into the optical fiber  10  so that a portion of the incoming light signals around a central wavelength is reflected while the remaining portion of the incoming light signals is transmitted through. The optical grating  20  is fastened onto a surface of the bimetallic strip  30 . Hence, when the bimetallic strip  30  deforms due to the application of an external force or a change in temperature, the optical grating  20  will similarly be strained. Straining the optical grating  20  will result in a shift in the filtered wavelength by the optical grating  20 . 
     The bimetallic strip  30  carries out two major functions: 
     1. The coefficient of thermal expansion of the first metallic strip  32  of the bimetallic strip  30  is smaller than the second metallic strip  34  of the bimetallic strip  30 . Since the first metallic strip  32  and the second metallic strip  34  are attached side by side to each other, the bimetallic strip  30  will bend downward when the temperature rises to a high temperature. Conversely, the bimetallic strip  30  will bend upward when the temperature drops to a low temperature. The degree of bending is proportional to length of the bimetallic strip  30  and the temperature change while inversely proportional to the thickness of the metallic strips  32  and  34 . Through the differential expansion of each metallic strip with respect to an increase in temperature, shift in the central wavelength of reflection for the optical grating  20  due to a rise in temperature can be automatically compensated. 
     2. Since the central wavelength of reflection for the optical grating  20  shifts when the optical fiber  10  is subjected to tension, a compression spring (an elastic prop)  40  can be installed to provide tension to the optical fiber  10 . A force is applied to the side of the bimetallic strip  30  through the compression spring  40  so that the amount of tension inside the optical grating  20  can be changed. By varying the applied force on the compression spring  40 , the desired central wavelength of reflection can be obtained. 
     A relationship between the bimetallic strip  30  and the compression spring  40  can be derived from structural mechanics. The relationship between strain and temperature can be given by the following formula: 
     
       
         Δε=αΔl+βΔT; 
       
     
     where α=         α   =       8      yLk           L   3        k     +     4        Ebt   3             ,     β   =     -                  8        yaEbt   2             L   3        k     +     4        Ebt   3               ,                          
     Δε: strain on the surface of the bimetallic strip; 
     Δl: drawing distance of the compression spring; 
     ΔT: change in external temperature; 
     y: distance between the outer surface and the neutral surface of the bimetallic strip; 
     a: deflection constant of the bimetallic strip; 
     b: width of the bimetallic strip; 
     t: thickness of the bimetallic strip; 
     L: length of the bimetallic strip; 
     E: the elastic modulus of the bimetallic strip; 
     k: the elastic modulus of the compression spring. 
     According to the above formula, the addition of the compression spring  40  will lower the negative thermal expansion of the bimetallic strip  30 . Hence, when suitable materials arc chosen to form the bimetallic strip  30  and the compression spring  40 , the drift in central wavelength of reflection for the optical grating  20  due to temperature changes can be reduced to a minimum. 
     FIG. 2 is a sketch showing an adjustable optical fiber grating system according to a second embodiment of this invention. As shown in FIG. 2, the adjustable optical fiber grating system includes an optical fiber  110 , an optical grating  120 , a bimetallic strip  130 , a compression spring  140 , a supporting stand  150 , a pair of side springs  160 , a movable plate  170 , a pair of guiding tracks  180 , an outer casing  190  and a micrometer drive  200 . 
     The bimetallic strip  130  is positioned on the supporting stand  150 , and the optical fiber  110  including the optical grating  120  are affixed to the upper surface of the bimetallic strip  130 . When the micrometer drive  200  rotates, distance of separation between the movable plate  170  and the supporting stand is changed. Hence, the degree of bending and the amount of straining on the bimetallic strip  30  through the compression spring  40  are modified. Consequently, the central wavelength of reflection for the optical grating  120  is shifted. 
     FIG. 3 is a graph of central wavelength versus distance traveled by the moving plate of an adjustable optical grating system. As shown in FIG. 3, the slope in the graph is about 0.42 nm/mm when the temperature is kept constant. By forming a structure composed of a bimetallic strip and a compression spring as shown in FIG. 2, a linear relationship between the shift of central wavelength and the moving distance of the moving plate is obtained. In fact, the degree of shifting of the central wavelength can be finely adjusted using the micrometer drive  200 . If the resolution of the micrometer in the micrometer range, ultimate resolution of the wavelength can be as small as 0.42 pm. In this embodiment, the desired shifting range is about 1.2 nm. Since the shifting range is related to the maximum strain in both the bimetallic strip  130  and the compression spring  140 , the desired shifting range can be obtained by proper design and selection of materials. 
     FIG. 4 is a graph of central wavelength versus temperature showing the drift in central wavelength of an adjustable optical grating for three different central wavelength settings. According to the strain versus temperature formula, the addition of the compression spring  140  will lower the negative thermal expansion of the bimetallic strip  130 . However, the overall thermal expansion is linear. Hence, by designing suitable bimetallic strip  130  and compression spring  140 , the optical grating  120  can work within different central wavelengths. In other words, a working central wavelength can be selected such that any variation in central wavelength due to temperature change can be compensated by the bimetallic strip  130 . For example, the wavelength/temperature coefficient can be as small as 0.001 nm/° C. as shown in FIG.  4 . 
     In summary, this invention provides an adjustable optical grating system that utilizes the combination of a bimetallic strip and a compression spring. The system not only provides a range of central wavelengths, but also prevents any drifting in central wavelength due to a change in temperature. Another advantage of this invention is that no constant monitoring and feedback of the central wavelength is necessary. Consequently, no special setups for feedback control are needed and part of the production cost is saved. 
     It will be apparent to those skills in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.