Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 88109893, filed Jun. 14, 1999. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to an optical fiber grating package. More particularly, the present invention relates to a structure and manufacturing method for a passive temperature compensated fiber grating package. 
     2. Description of Related Art 
     Fiber grating can be used as a sensor in a wavelength measuring system. It also applies to wavelength division multiplexing (WDM), dispersion compensation, laser stabilization, gain flattening of optical amplifiers and so on, in a communication system. However, this is a problem in fiber grating applications. The Bragg wavelength of the fiber grating changes with the environmental temperature or external stress. In order to prevent the above conditions, several methods have been developed. For example, in an optical active system, the environmental temperature is dynamically maintained in a stable condition by a temperature control system, so as to maintain a stable central wavelength of the fiber grating. This active method has a drawback in that it consumes power when controlling the temperature. Another choice is to employ a passive system, which structure is less complex than the active system. The optical passive system can be made insensitive to temperature through a thermal compensation mechanism. 
     In the region where the fiber grating is located, the refraction index and the pitch of the fiber grating are inevitably affected by the temperature. The refraction index of the grating is usually very sensitive to temperature and thus hard to control. The control method in the passive system typically makes use of the expansion property of the material, which expansion property varies with the temperature. The central wavelength of the fiber grating therefore can be controlled by temperature. A ceramic material with a negative coefficient of thermal expansion (CTE) proposed by U.S. Pat. No. 5,694,503 is currently in use for compensation of the temperature effect in the passive system. This method has the advantages of small device dimensions and a simple structure, but it has at least one drawback in that the ceramic material is easily broken. Moreover, the negative CTE of ceramic material must be accurately controlled during fabrication. 
     In U.S. Pat. No. 5,042,898, two kinds of material with different but positive CTEs are used. The two materials are affixed together to form two tubes. Two ends of the fiber are respectively affixed to the two tubes of the materials. As temperature increases, the fiber length is loosened by the material with the larger CTE through release of the fiber so as to achieve thermal compensation. The central wavelength of the fiber grating therefore can be kept the same. The drawback of this method is that very high precision is necessary when affixing the fiber onto the two kinds of material. If such precision is not met, the compensation of expansion will fail. 
     Another method, using the plates made of two kinds of materials, is also proposed by patent WO98/27446. The fiber grating is fixed on the plate having a smaller CTE. As temperature increases, the plate becomes concave due to the difference of CTE between the two kinds of material, so that the fiber grating can be compensated. This method also has its drawback. 
     In the patent WO98/27446, quartz is used as the low CTE material, but it is difficult to bend quartz bends. As a result, temperature compensation is difficult to perform. Moreover, if the two plates are not properly fixed together, the thermal stress cannot be properly released. The reliability of the fiber grating package is reduced. In addition, a thin ion plate is used as the higher CTE plate . The temperature compensation of the thin ion plate is adjusted by its width. The device dimension cannot be effectively reduced. 
     In U.S. Pat. No. 5,841,920, a similar principle is applied in which two material elements with different CTEs and different geometric structures are used. By making use of the different CTEs and geometric structures, the material distorts to compensate for the fiber grating. This method still has a drawback in that an external impulse may shift the fiber grating, indirectly influencing the temperature compensation. Moreover, the method cannot allow adjustment of the fiber grating once the fiber grating is adhered, affixed or packaged on the element. This causes a low yield when a filter device is assembled. 
     As WDM transmission technology continues to develop, a precise and stable wavelength for transmission is essential. It becomes an important issue to have a precise and stable wavelength. The typical temperature coefficient of the fiber grating is about 0.01 nm/° C., which is insufficient for the requirement in the WDM transmission. Moreover, as the channel number of the WDM increases, the channel spacing is reduced from 1.6 nm down to 0.4 nm. In the current WDM transmission technology, the wavelength precision is very important. The central wavelength of the fiber grating demands an error within +/−0.025 nm. It is very important to precisely control the central wavelength of the fiber grating during packaging the fiber grating. 
     SUMMARY OF THE INVENTION 
     It is at least an objective of the invention to provide an improved structure for an optical fiber grating package, such that the packaged fiber grating and signals reflected or transmitted from the fiber grating are insensitive to the environmental temperature. The central wavelength of the fiber grating is therefore not affected by the environmental temperature. After packaging, the central wavelength of the fiber grating meets the specifications of the International Telecommunication Union (ITU). In addition, a method for fabricating the structure of the optical grating package is also provided so as to allow the structure to easily meet the specifications of the ITU. Adjustment of the central wavelength can be performed when the structure is on-line, resulting in a high yield of product. 
     The invention provides a structure for an optical fiber grating package. The structure includes a fiber grating that is mounted on a multi-layer metal plate. The fiber grating is formed on a fiber at the desired portion. The ends of the fiber grating are secured to the two ends of the multi-layer metal plate. The multi-layer metal plate includes, for example, a two-layer metal plate and a thinner metal plate on the two-layer metal plate. The two layers of the bimetal plate have different coefficients of thermal expansion (CTE). The thinner metal plate usually is, for example, about 10 times thinner than each of the two layers. The thinner metal plate is adhered to, for example, the one of the two layers with the greater CTE by a contact length, which is adjustable so as to compensate the CTE of bimetal plate. The structure further includes an adjusting plate located on the multi-layer metal plate at the side where the fiber grating is mounted so that the adjusting plate, serving as a pad, can lift the fiber of the fiber grating. The adjusting plate is also located in a position between the grating portion of the fiber grating and one fixed end. As a result, a grating pitch of the fiber grating can be further precisely adjusted by shifting the relative location of the adjusting plate to cause a necessary tension of the fiber. The multi-layer metal plate with the fiber grating is held by a substrate, such as metal substrate. The metal substrate is located in a tube casing, for example, for protection. Tube caps cover both ends of the tube so that the metal substrate is protected by the tube casing. Each cap includes, for example, an aperture to allow the fiber to pass through. 
     The invention also provides a method for fabricating the optical fiber grating package. The method includes providing a bimetal plate, which is planar at a packaging temperature of, for example, about 100° C. This temperature is relatively higher than the normal environmental temperature. A thin metal plate is adhered to the bimetal plate at one side so that a multi-layer metal plate is formed and has a substantially flat structure at the packaging temperature. A desired contact length between the thin metal plate and the bimetal plate is set, according to design requirements for compensating the CTE of the bimetal plate. This structure is now called a multi-layer metal plate. A fiber grating portion of a fiber, whose central wavelength is adjusted to a desired value, is secured to the two ends of the multi-layer metal plate. An adjusting plate is inserted between the fiber and the multi-layer metal plate and is located between the fiber grating portion and one of the two ends. The adjusting plate is used to further adjust the central wavelength of the fiber grating. The multi-layer metal plate with the fiber grating is mounted on a substrate. A portion of the fiber other than the fiber grating portion is also attached to the two ends of the substrate with a reserved additional length, so that the fiber between the multi-layer metal plate and the ends of the substrate is not pulled due to a mechanical effect such as a distortion of the multi-layer metal plate as the environmental temperature changes. The substrate is further inserted in a tube casing, which is then covered with two end caps. Each end cap has an aperture through which the fiber passes. 
     The invention uses a multi-layer metal plate to hold a fiber grating so as to reduce the thermal effect on the fiber grating. An adjusting plate is also used to further adjust the central wavelength of the fiber grating. The central wavelength therefore remains stable as environmental temperature varies. 
     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, 
     FIGS. 1A and 1B are cross-sectional views, schematically illustrating distortion of a bi-metal plate as temperature changes; 
     FIG. 2 is a cross-sectional view, schematically illustrating a multi-layer metal plate used in a passive temperature independent fiber grating package, according to a preferred embodiment of the invention; 
     FIG. 3 is a cross-sectional view, schematically illustrating a fiber grating on the passive temperature independent fiber grating package, according to the preferred embodiment of the invention; 
     FIG.  4 A and FIG. 4B are a top view and a cross-sectional view, schematically illustrating the p tempera e independent fiber grating package, implemented in a tube casing for protection, at a packaging temperature, according to the preferred embodiment of the invention; 
     FIG.  5 A and FIG. 5B are a top view and a cross-sectional view, respectively, schematically illustrating the passive temperature independent fiber grating package, implemented in a tube casing for protection, at a normal working temperature, according to the preferred embodiment of the invention; and 
     FIG. 6 is a plot of a shifted central wavelength of the passive temperature independent fiber grating as the environmental temperature varies in range of −20° C. to 80° C., according to the preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Material can physically linearly expand or contract as its temperature varies. The coefficient of thermal expansion (CTE) is a typical material constant, which can be timed with a temperature difference to determine a dimension difference. The dimension difference can also determine the temperature difference because they are proportional. 
     Metallic material is usually sensitive to the temperature and different metallic materials have different CTEs. If two metal layers of different materials are adhered together as a bimetal plate, the two metal layers have different expansion rates as the temperature varies. As a result, the bimetal plate is distorted as its temperature changes due to the different CTEs. 
     FIGS. 1A and 1B are cross-sectional views, schematically illustrating distortion of a bimetal plate as temperature changes. In FIG. 1A, the bimetal metal plate  104  includes a metal layer  100  with a greater CTE and a metal layer  102  with a smaller CTE. As temperature decreases, the contraction rate of the metal layer  102  is smaller than the contraction rate of the layer  100 . Since the layers  100 ,  102  are adhered together, a shearing stress exists at their interface. The arrows on the layer  102  indicate the directions in which distortion occurs as temperature is reduced. The contraction rate of the layer  100  is sufficiently larger and layer  102  is therefore bent by the interface with layer  100 . As a result, the bimetal plate  104  is distorted into a concave shape. As the temperature of the bimetal plate  104  is raised to a certain level, the distortion disappears, since the layer  100  has larger expansion rate than that of the layer  102 . The bimetal metal plate  104  becomes flat at a higher temperature. These effects are natural physical phenomena. 
     In more detail, the actual expansion rate can be described as curve rate A, which is a shift distance of one end to a tangential line of the middle point on a circular curve as the bimetal plate  104  is circularly distorted at a low temperature. The curve rate A is derived as:                A   =       a   ·   dT   ·     L   2         4      S         ,           (   1   )                                
     where α is a deflection constant in K −1 , including the CTE and other possible linear parameters. dT is the temperature difference, L is the length of the bimetal plate, and S is the thickness of the bimetal plate. The quantity of the curve rate A indicates the tilted distance on one end, while the middle point is treated as fixed point. The deflection constant usually is about 5.5-21 for a commercial product, which amount depends on the different materials used for the two layers  100  and  102 . The curve rate A is linearly proportional to the temperature difference and can be used to determine the temperature. 
     According to the theory of material mechanics, as a material is curved due to a shearing stress, the material suffers a strain. The strain of the material along the x-axis is shown in Eq. 2:                  ɛ   x     =         α   ·   dT     +       σ   x     E       =       α   ·   dT     +     ɛ   B           ,           (   2   )                                
     where ε x  is the total strain along the x-axis, α is the CTE, σ x  is the shearing stress along the x-axis, E is the elastic modulus, and ε B  is surface strain of the bimetal plate  104 . 
     The above thermal effect induced by the temperature difference can be used for thermal compensation in the fiber grating. For the usual product on the market, the deflection constant a is provided. The parameter ε B  can be related to the deflection constant a by Eq. 3:                  ɛ   B     =       2   ·   a   ·   dT   ·   y     S       ,           (   3   )                                
     where y is the distance between a neutral surface and an active surface. Eq. 3 provides information necessary for selection of a desired bimetal plate  104 . 
     The bimetal plate is widely used by industry, so its price is low and its reliability is high. However, the available product is not suitable for use with a fiber grating. The invention therefore chooses a bimetal plate with a CTE greater than the actual need of the fiber grating. Then the invention employs an additional metal plate with a certain CTE and adheres it on one layer of the bimetal plate so as to adjust the compensation rate. The additional metal plate is preferably adhered on the layer with greater CTE. 
     FIG. 2 is a cross-sectional view, schematically illustrating a multi-layer metal plate used in a passive temperature independent fiber grating package, according to a preferred embodiment of the invention. In FIG. 2, a bimetal plate  204  like the one shown in FIG. 1A includes a layer  200  with a higher CTE and a layer  202  with a lower CTE. The thickness of the bimetal plate  204  is about 0.2 mm, and the width is usually about 2 mm so that the plate is a thin strip with a length, for example, of 30 mm. The additional metal layer  206 , also called a third metal layer  206  if layer  200  is called the first metal layer, and layer  202  is called the second layer, is preferably adhered to the first layer  200  having the higher CTE by a contact length L. The third metal layer  206  preferably is thinner than each of the two layers  200 ,  202  of the bimetal plate  204 . The third metal layer  206  preferably includes stainless steel and is about 0.03 mm thick. The third metal layer  206  together with the bimetal plate  204  form a multi-layer metal plate  208  of the passive temperature independent fiber grating package of the invention. 
     The contact length L between the third metal layer  206  and the first metal layer  200  is used to control and monitor a net thermal compensation of the main-body plate  208 . The thermal compensation can be adjusted by optimizing the contact length L. The contact length can be changed by, for example, stripping both ends of the third metal layer  206  away from the first metal layer  200 . The dimensions of the bimetal plate  204  are, for example, the typical dimensions of the available commercial product. In actual application, the dimensions of the bimetal plate  204  can vary with the design, and the third metal layer  206  can also be adhered to the metal layer  202 . 
     After a fiber grating is formed on a desired portion of a fiber, the fiber grating must be fixed on a multi-layer metal plate  208 . A desired shape of the bimetal plate is a concave structure at normal temperatures, where the layer with low CTE serves as the outer layer. The fiber grating is secured on the multi-layer metal plate as follows. Also referring to FIG. 2, at a stable packaging temperature such as 100° C., the bimetal plate  204  is flat. The third metal layer  206  is adhered to the layer  200  with a high CTE with, for example, thermal glue. There is a contact length L. The third metal layer  206  includes, for example, stainless steel with a thickness of about 0.03 mm. The purpose of the third metal layer  206  is to preliminarily adjust the CTE of the bimetal plate  204  to a desired quantity, according to design. The adjustment is necessary because the bimetal plate  204  is a commercial product with a specified CTE. 
     Since the CTE of the third metal layer  206  is usually not much different from that of the bimetal plate  204 , the third metal layer  206  can be firmly glued on the layer  200  and generates less shearing stress at 100° C. The reliability remains at a high level. 
     The portion of fiber having the fiber grating (not shown in FIG. 2 but shown in FIG. 3) is mounted on the metal layer  202  at both ends so that the fiber grating is located on the side with the low CTE. The central wavelength of the fiber grating is adjusted to the desired wavelength that meets the specification of the ITU. 
     Since the normal working temperature is usually within a certain range lower than the packaging temperature, as the temperature decreases to the normal temperature, the multi-layer metal plate  208  becomes convex toward the side having the lower CTE. This is a natural physical effect due to the contraction of the layer  200  with the high CTE. This phenomenon is described in FIG.  1 . 
     FIG. 3 is a cross-sectional view, schematically illustrating a fiber grating on the passive temperature independent fiber grating package, according to the preferred embodiment of the invention. In FIG. 3, the multi-layer metal plate  302  is similar to the one in FIG. 2 but stays at a lower temperature. The top surface of the multi-layer metal plate  302  is on the side of the layer  202  with the lower CTE. Both ends  304  of the grating portion of a fiber  300  are fixed on the multi-layer metal plate  302 . An adjusting metal plate  306  is also used to further adjust the central wavelength of the fiber grating. The adjusting metal plate  306  is inserted between the fiber  300  and the multi-layer metal plate  302  so that the fiber  300  is lifted up. The adjusting metal plate  306  is located between the grating portion and one of the ends  304 . Different locations of the adjusting metal plate  306  can generate different tensions of the fiber. This allows a more precise adjustment of the grating pitch. The central wavelength is therefore more precisely adjusted. 
     At this stage, the fiber grating is mounted on the multi-layer metal plate  302  of the invention. For the practical use, it is still necessary to mount the multi-layer metal plate  302  on a metal substrate, which is further mounted in a protection casing, such as a tube casing. FIG.  4 A and FIG. 4B are a top view and a cross-sectional view, respectively, schematically illustrating the passive temperature independent fiber grating package implemented at a packaging temperature in a tube casing for protection, according to the preferred embodiment of the invention. 
     In FIG.  4 A and FIG. 4B, the multi-layer metal plate  302  with the fiber grating is mounted on a metal substrate  310 , and is put in a tub casing  314 . The fiber grating  308  is indicated by a portion of grating on the fiber  300 . The adjusting plate  306  is inserted, for example, during a packaging period. The multi-layer metal plate  302  is mounted on a substrate  310 , which preferably is metallic. The substrate  310  is further mounted on the tube casing  314 . Both ends of the tube casing  314  are covered by caps  316 . Each cap  316  has an aperture to allow the fiber  300  to enter the tube casing  314  so that the fiber grating is protected by the tube casing  314 . The fiber grating  308  is affixed to the ends  304  on the multi-layer metal plate  302 . The substrate also has two ends  312 , not part of the grating portion, by which the fiber is secured. Since the multi-layer metal plate  302  contracts as temperature is reduced, the fiber length at the portion between the ends  304  and the ends  312  includes a reserved length to make the fiber sufficiently long so as to avoid pulling the fiber  300  due to different temperatures or other mechanical factors. FIGS. 4A and 4B show the fiber grating  308  in the tube casing  314  at the temperature 100° C. The multi-layer metal plate  302  remains flat. 
     As the temperature is reduced from the packaging temperature, the multi-layer metal plate  302  becomes convex as shown in FIG.  5 B. FIG.  5 A and FIG. 5B are a top view and a cross-sectional view, respectively, schematically illustrating the passive temperature independent fiber grating package implemented at a normal working temperature in a tube casing for protection, according to the preferred embodiment of the invention. In FIG. 5B, the convex structure of the multi-layer metal plate  302  is shown. Since the temperature dependence of the Bragg wavelength in the fiber grating  308  is compensated by the multi-layer metal layer  208  (FIG. 2) and the central wavelength is finely tuned by the adjusting metal plate  306 , the central wavelength does not vary away from the desired quantity at the normal temperature. Moreover, the fiber  300  at the portion between the ends  304  and the ends  316  is stretched but not overly stretched or broken due to the reserved length. The passive temperature independent fiber grating package is therefore formed. 
     According to the invention, a stable performance is achieved as shown in FIG.  6 . FIG. 6 is a plot of a shifted central wavelength of the passive temperature independent fiber grating as the environmental temperature varies in range of −20° C.-80° C., according to the preferred embodiment of the invention. In FIG. 6, the data points are the measured shifted wavelength from the set central wavelength of the fiber grating. In the temperature range of about −20° C.-80° C., the shifted wavelength only varies from about 0.35 nm to 0.425 nm. The variation rate dλ/dT is less than 1 pm/° C., while the conventional variation rate is 10 pm/° C. The shifted wavelength is sufficiently stable. For an ideal situation, the shifted wavelength is varies linearly with the temperature. In FIG. 6, the wavelength temperature coefficient at the low temperature of −20° C. is larger due distortion. The absolute quantity can be compensated without problem by a proper adjustment through the third metal layer  206  and the adjusting plate  306 . 
     A bimetal plate is a commercial product with low price and has been widely used in industry. Its working temperature is also about between −20° C. and 80° C. with an acceptable linearity. The two layers of the bimetal plate are firmly adhered together by a conventional discharging joining method. The bimetal plate is reliable so as to assure directly the reliability of the invention. 
     The invention employs the bimetal plate and a third metal layer to firmly form the multi-layer metal plate. The thermal compensation can be easily adjusted by the third metal layer. The fabrication process is simple and has no extra repetitive steps. After the thermal compensation, the central wavelength of the fiber grating can easily meet the requirements of the ITU. 
     The passive temperature independent fiber grating package of the invention includes no ceramic material so it is not easily broken. The fiber grating package is also protected by a casing. The protection is further reinforced. 
     The passive temperature independent fiber grating package does not require highly precise fabrication. The dimension of the fiber grating package is also small. For the aspect of basic thermal compensation, the invention uses two plates with different CTE to achieve thermal compensation. The available bimetal plate is highly reliable. The suitable compensation of CTE is achieved by using the third metal layer, which is thin, to further precisely adjusting the curve rate of the contact surface so that grating pitch is adjusted to the desired quantity. This allows the thickness and the width of the fiber grating package to be effectively reduced. 
     Moreover, the adjustment of the contact length between the third metal layer and the bimetal plate is performed in an on-line manner. The location of the adjusting plate is also adjusted on-line. The yield of the fiber grating is effectively improved. 
     In conclusion, the method of the invention for packaging the fiber grating has several advantages as follows: 
     1. The invention employs a bimetal plate, which is set to be flat at a relatively high temperature. A thin metal layer is adhered to the bimetal plate with a contact length by a thermal glue. 
     2. The contact length is adjusted to achieve a central wavelength variation rate of less than 1 pm/° C. 
     3. The central wavelength of the fiber grating can be adjusted to achieve a precision of about +/−0.025 nm through the distortion of the multi-layer metal plate. As a result, the requirements for WDM communication made by the ITU are satisfied. 
     4. The adjusting plate between the fiber and the multi-layer metal plate is used to further precisely adjust the central wavelength of the fiber grating. 
     5. The fiber on each side secured between the end of the multi-layer metal plate and the end of the substrate has a reserved length at a fabrication temperature, so that the fiber is not overly stretched or broken at normal temperatures. 
     The fiber grating package of the invention also has several advantages as follows: 
     1. The multi-layer metal plate includes a bimetal plate and a thin metal layer so as to effectively compensate the CTE of the intrinsic bimetal plate. 
     2. The multi-layer metal plate is flat at the packaging temperature and is concave at normal temperatures. 
     3. The adjusting plate is inserted between the fiber and the multi-layer metal plate. The central wavelength is further precisely adjusted by adjusting the location of the adjusting plate. 
     It will be apparent to skill 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.

Technology Category: 3