Abstract:
A temperature-compensated optical fiber component for use in high-density WDM optical communication includes an optical fiber which has a Bragg grating serving as a monochromatic filter, an inner package which supports the optical fiber and causes the Bragg grating to have a temperature-compensating capability, and an outer package arranged outside the inner package. A clearance having a heat insulating function is provided between the outer package and the inner package.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to an optical fiber component used in the field of optical communications, and more particularly to a temperature-compensated optical fiber component, suitably used in a WDM (Wavelength Division Multiplexing) communication system.  
           [0003]    2. Description of the Related Art  
           [0004]    Conventionally, a temperature-compensated optical fiber component is known which has an optical fiber grating as part of an optical fiber.  
           [0005]    At the optical fiber grating, the effective refractive index of a core of the optical fiber is periodically changed along the axis of the optical fiber. The optical fiber grating is used for reflecting a light having a relatively narrow wavelength range which is called a Bragg reflection wavelength as a center wavelength. The Bragg reflection wavelength is determined by a Bragg grating period and the effective refractive index of the fiber core. Here, the Bragg reflection wavelength λ, the effective refractive index n, and the Bragg grating period Λ have the relationship expressed by the following equation (1):  
           λ=2 nΛ   (1)  
           [0006]    For this reason, the optical fiber grating is usually referred to as the Bragg grating, and employed in the WDM communication system as a single-wavelength filter excellent in selecting a specific wavelength. However, the Bragg reflection wavelength λ has temperature dependency in respect of both of the effective refractive index n of the fiber core and the Bragg grating period Λ, as shown in the equation (1). Therefore, the temperature-compensated optical fiber component which employs a temperature-compensating package, as means for compensating or suppressing the temperature dependency of the optical fiber grating, is proposed.  
           [0007]    Temperature-compensated optical fiber components have been proposed, for example, in Japanese Unexamined Patent Publication (Kokai) No. Hei 10-96827, and Japanese Unexamined Patent Publication (Kokai) No. 2000-347047. According to them, to enable temperature compensation of the above optical fiber components, a member having a negative coefficient of linear expansion is used. Alternatively, a combination of two kinds of members having different coefficients of linear expansion are used. More specifically, the optical fiber having a tension applied thereto beforehand is fixed or bonded to the above member(s), for example, by an organic adhesive, by which a negative temperature dependency is imparted to the Bragg grating period Λ of the Bragg grating. The negative temperature dependency of the Bragg grating period Λ is canceled by the positive temperature dependency of the effective refractive index n of the fiber core, which enabling the temperature compensation of the optical fiber component. As a result, the temperature dependency of the Bragg reflection wavelength λ is compensated for, whereby a stable monochromatic filter for use in the WDM communication system can be obtained.  
           [0008]    However, the aforementioned temperature-compensating mechanism cannot effectively work if an external force is applied to the optical fiber component to deform a fiber-supporting portion, or the optical fiber component is used under circumstances where it is partially heated, or the predetermined tension applied to the optical fiber is changed with the lapse of time.  
         OBJECT AND SUMMARY OF THE INVENTION  
         [0009]    It is an object of the present invention to provide a temperature-compensated optical fiber component so as to preserve its desired optical quality even if an external force is applied thereto, or its environmental temperature may rapidly change, or there is a heat source in its vicinity, or even after it is used over a long period of time.  
           [0010]    According to the present invention, an optical fiber component comprises an optical fiber, an inner package connected to the optical fiber, the inner package having a portion for processing a light transmitted through the optical fiber, and outputting the processed light, cover means for covering the inner package and permitting the optical fiber to be guided out of the optical fiber component, the cover means including an outer package surrounding an outside of the inner package in a circumferential direction, and heat insulating means for reducing heat transfer between the outer package and the inner package.  
           [0011]    According to this component, the outer package prevents the inner package from being deformed by an external force, such a deformation deteriorating the accuracy of the Bragg grating. The heat insulating means protects rapid heat transfer to the inner package from outside owing to a change of an environmental temperature, and prevents the inner package from having a temperature gradient even though there is a heat source in its vicinity. More specifically, when the processing portion is a Bragg grating formed on the optical fiber, the outer package prevents a change in the tension of the Bragg grating caused by the deformation of the inner package. The heat insulating means can include an air layer or a heat insulating member provided between the inner package and the outer package, which enhancing heat insulation from the outside to preserve the thermal uniformity of the inner package.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein:  
         [0013]    [0013]FIG. 1 is a diagram schematically showing a temperature-compensated optical fiber component, according to an embodiment of the present invention;  
         [0014]    [0014]FIG. 2 is a cross-sectional view showing a temperature-compensating package of FIG. 1;  
         [0015]    [0015]FIG. 3 is a perspective view showing an optical fiber bonding portion;  
         [0016]    [0016]FIG. 4 is a cross-sectional view of the bonding portion FIG. 3;  
         [0017]    [0017]FIG. 5 is a diagram showing a modification of the optical fiber bonding portion;  
         [0018]    [0018]FIG. 6 is a sectional view taken along the line VI-VI of FIG. 1;  
         [0019]    [0019]FIG. 7 is a diagram schematically showing a temperature-compensated optical fiber component according to another modification of the present invention; and  
         [0020]    [0020]FIG. 8 is a diagram schematically showing a case in which a heat insulating member is inserted between an inner package and an outer package. 
     
    
     DETAILED DESCRIPTION  
       [0021]    Embodiments of the invention will now be described with reference to the accompanying figures, wherein like numerals refer to like elements throughout. The terminology used in the description presented herein is not intended to be interpreted in any limited or restrictive manner, simply because it is being utilized in conjunction with a detailed description of certain specific embodiments of the invention. Furthermore, embodiments of the invention may include several novel features, no single one of which is solely responsible for its desirable attributes or which is essential to practicing the invention herein described.  
         [0022]    [0022]FIG. 1 shows a temperature-compensated optical fiber component according to an embodiment of the invention. The optical fiber component includes a temperature-compensating package  14 , and an optical fiber  10  extending through the package  14 . As shown in FIG. 2, a Bragg grating  12  is formed on the optical fiber  10 , where the effective refractive index n is periodically changed longitudinally.  
         [0023]    Further, the package  14  includes a base  16  in the form of a hollow cylinder having an outer diameter of 4.8 mm. The base  16  is formed with a groove  17  which extends along the axis of the base  16  from one end portion to the other end portion thereof, and the optical fiber  10  extends through the groove  17 . A rack  18  is arranged in the groove  17 . More specifically, the groove  17  has one end portion formed deeper than the remaining portion thereof, and the rack  18  is accommodated in the one end portion, with a desired distance between the rack  18  and an opposed step surface  17   a  of the groove  17 . The base  16  is formed of invar 36FN (registered trade mark) having a small coefficient of linear expansion, while the rack  18  is formed of aluminum (Al) having a large coefficient of linear expansion. Further, the base  16  and the rack  18  are fixed to each other at one end of the groove  17  by a bonding portion  20 . The bonding portion  20  includes a projection  18   a  formed on one end portion of the rack  18 , and a recess  16   a  formed in one end portion of the groove  17 . The rack  18  is fixed to the base  16  by fitting the projection  18   a  into the recess  16   a . That is, the rack  18  is fixed to the base  16  only by the projection  18   a , and the other surfaces of the base  16  and the rack  18  are not fixed to each other. Consequently, the base  16  and the rack  18  are substantially in contact with each other and hence they are permitted to thermally expand and contract independently of each other due to a change of environmental temperature.  
         [0024]    Optical fiber bonding projections  22   a  and  22   b  (bonding portions  22 ) are formed at a free end of the rack  18  and the other end portion of the base  16 , respectively. The optical fiber  10  is fixed to the bonding projections  22   a  and  22   b  by using adhesives  23   a  and  23   b . Therefore, the optical fiber  10  is in a state of floating from the bottom of the groove  17  and the rack  18 . The above Bragg grating  12  is positioned between the bonding projections  22   a  and  22   b.    
         [0025]    [0025]FIGS. 3 and 4 show details of the optical fiber bonding portions  22  (bonding projections  22   a  and  22   b ).  
         [0026]    Each of the optical fiber bonding portions  22  has an inserting groove  23  formed in the top thereof. The inserting groove  23  has a U-shaped cross-section. The optical fiber  10  extends through the center of the inserting groove  23 . The groove  23  is covered by a lid  24  which is arranged on the top of the bonding portion  22 .  
         [0027]    An adhesive  25  is filled between the inner walls of the inserting groove  23  and the lid  24 , and the optical fiber  10 . That means the adhesive  25  joins the bonding portion  22  and the optical fiber  10  as well as the bonding portion  22  and the lid  24 .  
         [0028]    As clearly shown in FIG. 4, the adhesive  25  has a thickness defined by a gap between the inner wall surface of the inserting groove  23  and the peripheral surface of the optical fiber  10 . The thickness of the adhesive  25  is uniform along the circumference of the optical fiber  10 , and set to be equal to or smaller than 0.1 mm. More specifically, assuming that the optical fiber  10  has an outer diameter of 0.125 mm, for instance, the inserting groove  23  is configured to have a width W of 0.325 mm or less and a depth of 0.325 mm or less. In addition, the inserting groove  23  has a bottom which has a curvature radius of W/2 or less. If the inserting groove  23  is configured to have the width, depth and bottom of such sizes, and the adhesive  25  is filled around the optical fiber  10  positioned at the center of the inserting groove  23 , the thickness of the adhesive  25  filled between the optical fiber  10  and the inner walls of the inserting groove  23  is set to be equal to or smaller than 0.1 mm.  
         [0029]    Further, preferably, the inserting groove  23  has a length T of at least 2.2 mm or more.  
         [0030]    The inserting groove  23  is not limited to a U-shaped one. As shown in FIG. 5, it may have a semicircular cross-section. In the case of the inserting groove  23  having such a cross-section, the lid  24  may be formed with an arcuate groove  24   c  in the under surface thereof. In this case, both of the inserting groove  23  and the arcuate groove  24   c  have a curvature radius of W/2 or less.  
         [0031]    With such a bonding portion  22 , the optical fiber  10  is positioned on the axis of a circular inserting path defined by the inserting groove  23  and the arcuate groove  24   c , and the inserting path is filled with the adhesive  25 . Thus, the adhesive  25 , filled as described above, can have a thickness of 0.1 mm or less along the whole circumference of the optical fiber  10 .  
         [0032]    When the optical fiber  10  is fixed to the bonding projections  22   a  and  22   b , it is important to apply a predetermined tensile stress to the Bragg grating  12  in advance. More specifically, a tension necessary for canceling the temperature dependency of the Bragg reflection wavelength λ, as aforementioned, within a temperature range (e.g. from −20° C. to 85° C.) of an environment in which the temperature-compensating optical fiber component is used, must be applied to the Bragg grating  12  beforehand.  
         [0033]    In the following, a mechanism of the package  14  which compensates suppresses the temperature dependency of the Bragg reflection wavelength λ will be described in detail.  
         [0034]    Now, assume that the environmental temperature is changed, and the temperatures of the base  16 , the rack  18 , and the Bragg grating  12  are raised by the same degree. In this case, both of the base  16  and the rack  18  are thermally expanded independently, and as a result, the distance between the bonding projections  22   a  and  22   b  becomes shorter owing to a difference of the coefficients of linear expansion between the base  16  and the rack  18 , reducing the tension applied to the Bragg grating  12 . Therefore, the Bragg grating period Λ is shortened to be imparted with a negative temperature dependency. On the other hand, the effective refractive index n of the core of the optical fiber  10  at the Bragg grating  12  has a positive temperature dependency. Consequently, the negative temperature dependency and the positive temperature dependency cancel each other to compensate for the temperature dependency of the Bragg reflection wavelength λ as a whole. As far as the temperature of the package  14  is in the range of the environmental temperature described hereinabove, a change in the Bragg reflection wavelength λ is allowable within limits.  
         [0035]    However, although the package  14  has the above temperature-compensating mechanism, if the tension applied to the optical fiber  10  is changed with the lapse of time, that is, the negative temperature dependency of the Bragg grating period Λ becomes inaccurate, it is impossible for the intended temperature compensation to be executed correctly.  
         [0036]    Therefore, intensive study of causes of the inaccuracy or the change in the tension applied has been done. It is finally found that a faulty fixing or bonding of the optical fiber  10  to the bonding projections  22   a  and  22   b  is one of the causes responsible for the change in the tension of the optical fiber  10  with the lapse of time. More specifically, unless the adhesive  25  has a uniform thickness, curing of the adhesive  25  does not proceed uniformly, thereby making it impossible to accurately give a desired tension to the optical fiber  10 . Further, when the adhesive  25  has an excessively large thickness, the intended or required bonding force thereof is lowered, so that the tension of the optical fiber  10  is liable to be changed with the lapse of time.  
         [0037]    In the case of the present invention, however, as described hereinbefore, the adhesive  25  for bonding the optical fiber  10  and the bonding portion  22  has a uniform thickness between the inner wall of the inserting groove  23  and the optical fiber  10 , and furthermore the thickness is set to be equal to or smaller than 0.1 mm, i.e. very thin. Accordingly, when the optical fiber  10  is fixed to the bonding portions  22  as mentioned above, it is possible to avoid the faulty curing of the adhesive  25  and to hold a desired bonding force of the adhesive  25 , thereby making it possible to accurately preserve the tension of the optical fiber  10  for a long period of time.  
         [0038]    As shown in FIG. 1, the package  14  is surrounded by an outer package  26  with a uniform clearance of 0.10 to 0.25 mm therebetween, for example. The outer package  26  is formed of stainless steel pipe having an inner diameter of 5.1 mm. A silicone resin  28  is locally filled between the base  16  of the package  14  and the package  26 , as a securing member. The silicone resin  28  secures the packages  14  and  26  to each other, and keeps the clearance therebetween uniform, having the intended bonding force.  
         [0039]    More specifically, the resin  28  is filled between one end portion of the base  16  and the outer package  26 , and positioned such that the resin  28  and the groove  17  are at diametrically opposite locations, as shown in FIG. 6. This location of the resin  28  allows the base  16  to have freedom of thermal expansion and contraction along the axis of the optical fiber  10  on one hand, and prevents the resin  28  from intruding into the groove  17  of the base  16  on the other hand. Intrusion of the resin  28  into the groove  17  is not desirable since it rigidly fixes the base  16  and the rack  18  to each other, impairing the temperature-compensating capability of the optical fiber component. The above-mentioned local filling of the resin  28  between the base  16  and the package  26  has an advantage in that it simplifies and facilitates the manufacturing process of the optical fiber component. The length L over which the resin  28  is filled is shown in FIG. 1.  
         [0040]    According to experiments, if the Young&#39;s modulus of the resin  28  is within a range of 0.8 to 2.0 MPa, the resin  28  can keep the uniform clearance between the inner package  14  and the outer package  26 , and furthermore can be used for a good shock absorber of the inner package  14 . On the other hand, if the Young&#39;s modulus of the resin  28  is out of the above range, it is impossible for the resin  28  to effectively absorb a shock to the inner package  14  applied from outside.  
         [0041]    Further, if a combination of materials of the package  26  and the base  16  are properly chosen in view of a difference of the coefficients of thermal expansion between the outer package  26  and the base  16 , the resin  28  can be filled between the lower portion of the base  16  and the package  26  along the full length of the base  16 . When the base  16  is formed of titanium and the outer package  26  is formed of stainless steel, for instance, the coefficients of thermal expansion of the outer package  26  and the base  16  are about 8×10 −6 /° C. and 11×10 −6 /° C. (at an ordinary temperature or therearound), respectively, so that the difference of the coefficients of thermal expansion between outer package  26  and the base  16  is relatively small. Accordingly, the thermal distortion of the resin  28  is suppressed to small amount, even if the resin  28  is filled between the lower portion of the base  16  and the outer package  26  along the full length of the base  16 , or locally filled between the lower portion of the base  16  and the package  26  from two opposite end portions(not shown).  
         [0042]    On the other hand, the invar as a material of the base  16  has a coefficient of thermal expansion of approximately 1×10 −6 /° C. or less, and hence the difference of thermal expansion between the invar and the stainless steel as for the material of the outer package  26  is very large. Therefore, in this case, it is preferable to fill the resin  28  locally in a manner shown in FIG. 1. Of course, there occurs no thermal distortion of the resin  28  as long as the base  16  and the outer package  26  are formed of the same material. If the resin  28  is filled along the full length of the base  16 , it is preferable to keep the constant clearance between the base  16  and the outer package  26 . However, this has a disadvantage in that the quantity of the resin  28  may be increased in consumption.  
         [0043]    Further, in the above embodiment, a silicone resin  30  for securing members is filled in opposite end portions of the outer package  26 . The silicone resin  30  serves the function of securing the package  14  to the outer package  26 . Moreover, rubber caps  32  are attached to the opposite end portions of the outer package  26 , and press the opposite end portions of the inner package  14  via the resin  30 .  
         [0044]    As shown in FIG. 1, there is arranged a laser diode (LD)  33  in the vicinity of one rubber cap  32   a . If the optical fiber  10  is optically connected to the LD  33 , the optical fiber component is liable to be raised by heat from the LD  33 . Therefore, it is preferred that the rubber cap  32   a  has an excellent heat resistance, and at the same time has a small thermal conductivity. Since the thermal conductivity of the rubber is approximately the same as that of the silicone resin, intrusion of heat from the LD  33  into the inner package  14  is effectively reduced so long as the rubber has a thickness of several millimeters or more.  
         [0045]    Next, a method of manufacturing the above temperature-compensated optical fiber component will be described in detail.  
         [0046]    First, as shown in FIG. 2, the optical fiber  10  including the Bragg grating  12  is attached to the package  14 . More specifically, at an ordinary temperature, both portions of the optical fiber  10  on opposite sides of the Bragg grating  12  are fixed to the bonding projections  22   a  and  22   b  of the rack  18  and the base  16 , respectively, by using epoxy adhesives  25 ,  25  of a UV curing type. The optical fiber  10  having the Bragg grating  12  between the above bonding portions  22  of the same, is being stretched. That is, a predetermined tension is provided to the Bragg grating  12 . As the adhesives  25 , a low-melting glass, a metal solder, a thermoplastic organic adhesive, and the like can also be suitably used.  
         [0047]    Second, the inner package  14  is inserted into the outer package  26 . Then, the silicone resin  28  is locally filled between the outer package  26  and the base  16  of the package  14  to keep the above-mentioned clearance. After then, the silicone resin  30  is filled in the opposite end portions of the outer package  26 . After the package  14  has been fixed to the package  26 , the rubber caps  32   a  and  32   b  are attached to the opposite end portions of the outer package  26 , respectively. The rubber caps  32   a  and  32   b  press the opposite end portions of the package  14  via the silicone resin  30 .  
         [0048]    As described above, the optical fiber component according to the present embodiment has a double package construction in which the inner package  14  is surrounded by the outer package  26 , so that even if an external force is applied to the outer package  26 , the force is protected to transmit to the inner package  14 , which makes it possible to avoid deformations of the base  16  and the rack  18  supporting the optical fiber  10 . As a result, the optical fiber  10  can keep the predetermined tension applied to the Bragg grating  12 , thereby making it possible to obtain a desired Bragg reflection wavelength λ constantly.  
         [0049]    The outer package  26  formed of stainless steel pipe has a relatively low thermal conductivity. Moreover, the clearance between the outer package  26  and the inner package  14  has a distance of 0.10 to 0.25 mm, and is filled by an air and the resin  28  both of which having low thermal conductivity. This clearance constructed above can suppress rapid heat transfer into the inner package  14  from outside due to a change of the environmental temperature.  
         [0050]    Heat transmission between the inner package  14  and the outside in the direction orthogonal to the axial direction of the optical fiber  10  may be produced through two heat paths. One of the heat paths includes the air layer and the other includes the resin layer  28 . Since the thermal conductivity of the air differs from that of the resin  28  by an order of magnitude, the quantity of heat transmitting through the resin layer  28  is comparable or smaller to that of the air layer so long as the resin-filling area is sufficiently smaller than the air-filling area.  
         [0051]    According to experiments, when the base  16  was formed of titanium, there was no problem in the temperature-compensating performance of the inner package  14  on condition that a resin-filling area S was equal to or smaller than one-fourth of the whole circumference of the inner package  16 , as viewed along the circumference of the inner package  14  in FIG. 6, and at the same time the resin-filling length L was within a range of 50% of the full length of the base  16 .  
         [0052]    Further, the outer package  26  may include a mirror-finished outer surface  42  and a mirror-finished inner surface  43 . From a microscopic point of view, such surfaces  42  and  43  are very smooth. Therefore they can reflect radiant-heat effectively and can suppress the mutual transfer of the radiant-heat. Similarly, the base  16  as well can have a mirror-finished outer surface  44  to add radiant-heat reflecting properties thereof. Of course, a radiant-heat reflecting film  50  made of a material causing the same effect as those of the mirror-furnished surfaces  42  to  44  may be attached to the surface of the outer package  26  and/or the base  16 . It is possible to deposit the radiant-heat reflecting film  50  on the surfaces of the rubber cap  32   a  and a rubber plug  34 , both arranged in the vicinity of the LD  33 , as indicated by two-dot chain lines in FIGS. 1, 7 and  8 .  
         [0053]    A fiber reinforced plastic (FRP) has a strength comparable to stainless steel, and at the same time has a thermal conductivity lower than that of stainless steel by one or more orders of magnitude. Therefore, as a variation, the outer package  26  may be formed by FRP.  
         [0054]    In the above embodiment, the silicone resin  30  is filled in the opposite end portions of the outer package  26 , so as to hold and secure the inner package  14  at a predetermined axial location within the outer package  26 . But this is not limitative. That is to say, the silicone resin  30  may be dispensed with. As shown in FIG. 7, the rubber plugs  34  may be used in place of the rubber caps  32 , for instance. Two rubber plugs  34  are fitted into the opposite end portions of the outer package  26 , respectively, such that they directly press the opposite end portions of the inner package  14  to hold the same at the predetermined axial location.  
         [0055]    As shown in FIG. 8, a heat insulating effect can also be obtained by inserting a heat insulating member  36 , such as a glass wool, a foamed sheet, or the like, which have approximately the same heat insulating property as that of the air, into the clearance between the inner and outer packages  14  and  26 . In addition, the heat insulating member  36  can prevent any parts of the inner and outer packages  14  and  26  being brought into contact with each other.  
         [0056]    The heat insulating member  36  is not only suitable for absorbing a shock from an external force but also contributes to keeping the clearance constant between the inner and outer packages  14  and  26  without the resin  28 . This makes it easier to manufacture the optical fiber component.  
         [0057]    In the above embodiment, a hybrid construction is employed for the temperature-compensating package  14  using two kinds of members having different coefficients of linear expansion, i.e. the base  16  made of the invar and the rack  18  made of Al are combined together. It is clear that the package  14  may be replaced by a package which has a hybrid construction using other kinds of members having the same characteristics as described above, or by a package using one kind of member having a negative coefficient of linear expansion.  
         [0058]    The temperature-compensated optical fiber grating component according to the present invention, described heretofore, can provide the following advantageous effects:  
         [0059]    According to the present invention, the inner package  14  which enabling the Bragg grating  12  of the optical fiber  10  to have the temperature-compensating capability, is surrounded by the outer package  26 . Therefore, if it happens that an external force is applied to the outer package  26 , the force transmitted to the inner package  14  is reduced by the outer package  26 , which means to keep the predetermined tension of the Bragg grating  12  unchanged. Further, heat from the outside is prevented from directly transmitting to particular part of the inner package  14 , thereby the package  14  can maintain its temperature-compensating mechanism properly. Furthermore, since the optical fiber  10  is fixed to the bonding projections  22   a  and  22   b  by using the adhesive  25  of a thin and uniform thickness, the tension of the optical fiber  10  continues to be stable for a long period of time. Therefore, a desired Bragg reflection wavelength λ with high accuracy can be always obtained by using the Bragg grating  12 .  
         [0060]    In addition, it is clear that the double package construction according to the present invention can be applied to other optical fiber modules, including an optical isolator, an optical circulator, and other optical fiber components.