Abstract:
A package for holding a temperature sensitive optical device. The package includes a substrate, and a first link connecting to the optical device at a first attachment point, and connecting to the substrate at a location remote from the first attachment point. The package also includes a second link connecting to the optical device at a second attachment point that is remote from the first attachment point, the second link connecting to the substrate at a location remote from the second attachment point. The substrate, the first and the second links imposing a strain variation to the optical device in dependence of temperature.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to packaging of optical devices that are temperature sensitive. More specifically, the present invention provides a package and a method for packaging an optical device, such as an optical fiber including a grating.  
       BACKGROUND OF THE INVENTION  
       [0002]     An optical waveguide, such as an optical fiber is formed by a core section conveying the light signal, and a cladding section that surrounds the core to confine the light signal to the core. The light signal remains captive in the core by virtue of the difference between the refractive indexes of the core and the cladding sections and their geometries. In an optical fiber, the core section is cylindrical and the cladding surrounding it is tubular and in contact with the cylindrical core.  
         [0003]     A fiber Bragg grating is an axial periodical change of the effective refractive index (n) that induce harmonic back reflection of the light component at a certain wavelength (λ), called the Bragg wavelength. The Bragg wavelength is related to the period length (Λ) of the effective index change by: 
 
λ=2 nΛ   (1) 
 
         [0004]     Variations of the Bragg wavelength due to the effects of the temperature (T) and strain (ε) can be calculated by the derivative of eq. 1: 
 
Δλ/λ=[(1 /n ) dn/dT +( I /Λ) dΛ/dT]ΔT+[ 1+(1/ n ) dn/dε]Δε   (2) 
 
Δλ/λ=(ζ+α f )Δ T +(1+ p   e )Δε  (3) 
 
 where the thermo-optic coefficient for the fiber material, ζ=[(1/n)dn/dT; the coefficient of thermal expansion (CTE) of the fiber material, α f =(1/Λ) dΛ/dT; and the photo-elastic constant, p e =−(1/n) dn/dε. 
 
         [0005]     This dependence of the Bragg wavelength on temperature and strain is sometimes useful in applications such as temperature and/or stress sensors. But for other applications, such as channel filtering of DWDM communication system, the Bragg wavelength must be kept constant for all environmental conditions. Consequently, various packaging have been developed in the past to maintain Bragg wavelength constant in changing environmental conditions. Active packages using energy to control the temperature and/or the strain of the Bragg grating are relatively expensive, require access to energy source and often need active monitoring of the Bragg wavelength reflection. To overcome this drawback, various passive package designs using the strain dependency of the Bragg wavelength to counter-balance its dependency to temperature have been proposed. These passive athermal packages mostly use a mechanical structure to which the Bragg grating is fixed at a set tension and that would naturally impose to this Bragg grating a strain variation in function of the temperature according to the following equation: 
 
Δε/Δ T =−(ζ+α f )/(1+ p   e )  (4) 
 
 Using eq. 4 to replace Δε in eq. 3, we obtain: 
 
Δλ/λ=(ζ+α f )Δ T +(1 +p   e )[−(ζ+α f )/(1+ p   e )]Δ T= 0  (5) 
 
         [0006]     The first category of those passive athermal package designs is shown in  FIG. 1  and it uses a combination of two materials with different coefficients of thermal expansion (CTE). The material having the lower CTE (α low ) serves as a substrate  50 , while the material having the higher CTE (α high ) serves as a thermal compensator  52 . One end of the Bragg grating  54  is fixed to one end of the thermal compensator  52 ; the Bragg grating and the thermal compensator  52  are fixed in series to the substrate  50  at the other end. As the temperature increases, the thermal expansion of the thermal compensator  52  will push on the Bragg grating  54 , decreasing its strain; inversely, as the temperature decreases, the thermal compensator  52  contracts and increases the strain in the Bragg grating. The strain variations in the Bragg grating can be expressed by the following equation: 
 
Δε/Δ T =( L   subs α low   −L   comp α high )/( L   subs   −L   comp )  (6) 
 
         [0007]     where L subs  is the length of the substrate  50  and L comp  is the length of the thermal compensator  52 . Different athermal package designs using the serial thermal compensator mounting are described in the following US patents and US patent applications:  
                                       Patents or   Date of grant/           applications number   publication   Inventors                   5042898   Aug. 27, 1991   Morey et al.       5844667   Dec. 1, 1998   Maron et al.       5914972   Jun. 22, 1999   Siala et al.       6112553   Sep. 5, 2000   Poignant et al.       6374015   Apr. 16, 2002   Lin       6377727   Apr. 23, 2002   Dariotis et al.       6393181   May 21, 2002   Bulman et al.       2002/0141700   Oct. 3, 2002   Lachance et al.       2002/0146226   Oct. 10, 2002   Davis et al.       2002/0150335   Oct. 17, 2002   Lachance et al.                  
 
         [0008]     This serial configuration arrangement has a number of drawbacks. Firstly, it increases the length of the device, which makes it unsuitable for applications where component footprint and density are important factors. Secondly, since the thermal compensator is located on only one side of the Bragg grating, when the device is placed in a thermal gradient environment, the Bragg grating and its thermal compensator will be at different temperatures inducing an offset in the resulting Bragg wavelength. This could be a major concern when these devices are used in photo-electronic modules where a lot of heat is generated locally at proximity of the devices.  
         [0009]     Another popular configuration is the cantilever design, shown at  FIG. 2 . This approach also uses two materials with different CTEs, one as a substrate  60 , the other for the thermal compensator  62 . Both sides of the Bragg grating  64  are fixed at a set tension on top of the arms of a substrate  60  having an H shape. The thermal compensator  62  is fixed to the substrate arms parallel to the Bragg grating  64 . If the thermal compensator  62  is placed in the upper arms section of the H shaped substrate  60 , its CTE should be lower than the substrate; if it is placed in the lower section, its CTE should be higher than the substrate. Strain variations in the Bragg grating can be defined as follows: 
 
Δε/Δ T=H   Bragg ( L   subs α subs   −L   comp α comp )/( H   comp   L   Bragg )  (7) 
 
         [0010]     where H comp  is the parallel distance between the substrate  60  and the thermal compensator  62 , and H Bragg  is the shortest of the parallel distance between the Bragg grating  64  and either the substrate  60  or the thermal compensator  62 . Different athermal package designs using the cantilever thermal compensator mounting are described in U.S. Pat. Nos. 5,841,920, 6,044,189, 6,144,789, 6,175,674, 6,181,851, 6,295,399, 6,327,405, 6,370,310, 6,396,982 and 6,453,108. The cantilever configuration is also subject to thermal gradient offsets since the thermal compensator is located only at one side of the Bragg grating, but requires less footprint since it is parallel to the Bragg grating, instead of being in series with it. Also, by locating the thermal compensator close to the Bragg grating, the temperature gradient offsetting effect can by diminished, but not nullified.  
                                       Patents or   Date of grant/           applications number   publication   Inventors                   5841920   Nov. 24, 1998   Lemaire et al.       6044189   Mar. 28, 2000   Miller       6144789   Nov. 7, 2000   Engelberth et al.       6175674   Jan. 16, 2001   Lin       6181851   Jan. 30, 2001   Pan et al.       6295399   Sep. 25, 2001   Engelberth       6327405   Dec. 4, 2001   Leyva et al.       6370310   Apr. 9, 2002   Jin et al.       6396982   May 28, 2002   Lin       6453108   Sep. 17, 2002   Sirkis                  
 
         [0011]     By using a substrate with negative thermal expansion, the need for a thermal compensator can be eliminated. Moreover, by fixing the fiber directly on that substrate reduces the parallel distance between the thermal compensation element and Bragg grating, reducing, but not nullifying, offsetting effects induced by a temperature gradient. That is the approach discussed in the following US patents and US patent applications numbers:  
                                       Patents or   Date of grant/           applications number   publication   Inventors                   5694503   Dec. 2, 1997   Fleming et al.       6087280   Jul. 11, 2000   Beall et al.       6187700   Feb. 13, 2001   Merkel       6209352   Apr. 3, 2001   Beall et al.       6233382   May 15, 2001   Olson et al.       6240225   May 29, 2001   Prohaska       6258743   Jul. 10, 2001   Fleming et al.       6317528   Nov. 13, 2001   Gadkaree et al.       6362118   Mar. 26, 2002   Beall et al.       6377729   Apr. 23, 2002   Merkel       6400884   Jun. 4, 2002   Matano et al.       6403511   Jun. 11, 2002   Fleming et al.       6477299   Nov. 5, 2002   Beall et al.       6477309   Nov. 5, 2002   So       6490394   Dec. 3, 2002   Beall et al.       2001/0021292   Sep. 13, 2001   Merkel       2001/0031692   Oct. 18, 2001   Fleming et al.       2002/0146230   Oct. 10, 2002   So                  
 
         [0012]     Chemical composition and fabrication process of those substrates is critical to obtain the exactly matching negative CTE to compensate for thermal effects on the Bragg wavelength. In addition, those formulations must ensure repeatability, reproducibility and stability for all the operational conditions encountered by the devices. New formulations have to be developed for each change in fiber composition (chemical or geometrical) or Bragg grating exposure processes since these parameters will slightly change the Bragg wavelength dependency on temperature and strain. In this respect, U.S. Pat. No. 6,240,225 (Prohaska., May 29, 2001) proposes the use of an anisotropic negative substrate, where by changing the angle on which the Bragg grating is fixed to the substrate, the negative CTE can be adjusted for each type of fiber and grating.  
         [0013]     In contrast to the serial and cantilever configurations discussed earlier which allow the possibility of fine tuning the Bragg wavelength after the fixing points are stabilized, the negative CTE substrate approach does not permit to readjust the Bragg wavelength after curing and stabilization of the fiber anchoring points. Since those processes induce strain variation on the Bragg grating, they must be predictable and repeatable during the pre-tensioning for fiber fixation to insure acceptable yield.  
         [0014]     U.S. Pat. No. 6,148,128 (Jin et al., Nov. 14, 2000) and 6108470 (Jin et al., Aug. 22, 2000) disclose yet a different athermal package design, using a negative thermal expansion substrate and a fine Bragg wavelength adjusting mechanism for post-assembly corrective tuning. These tuning mechanisms use programmable, latchable magnets to control gap distances between magnets due to magnetic force fields. In addition to increasing the cost of the package, the use of programmable, latchable magnets may cause some long-term reliability concerns resulting from changes of the magnetic properties of these magnets, as well as, changes of their equilibrium positions in the magnetic field as a result of mechanical shocks and vibrations. These patents, as well as U.S. Pat. No. 6,101,301 (Engelberth et al., Aug. 8, 2000) and 6243527 (Dawson-Elli, Jun. 5, 2001), also disclose another athermal configuration similar to the serial design, that is shown in  FIG. 3 . A thermal compensator  70  is parallel to the Bragg grating  72  and a low thermal expansion extension  74  is used to join the pushing end of the thermal compensator  70  to the pulled end of the Bragg grating  72 . In this parallel configuration the thermal compensator  70  should have a higher CTE than the substrate  76 ; and since there is only one thermal compensator  70  on one side of the Bragg grating  72 , it is sensitive to offsets due to a thermal gradient.  
         [0015]     Tubular thermal compensator configurations covering the Bragg grating have also been proposed to avoid offsetting effects of temperature gradient. The most popular approach is to use a matching negative CTE coating material discussed in the following US patents/applications:  
                                       Patents or   Date of grant/           applications number   publication   Inventors                   4923278   May 8, 1990   Kashyap et al.       6067392   May 23, 2000   Wakami et al.       6233386   May 15, 2001   Pack et al.       6466716   Oct. 15, 2002   Olge       2002/0090174   Jul. 11, 2002   Girardon et al.                  
 
         [0016]     This approach presents the same drawbacks as the negative thermal expansion substrate, repeatability, reliability and absence of fine-tuning mechanisms. U.S. Pat. No. 6,449,293 (Pedersen et al., Sep. 10, 2002) and US patent application # 2002/01811908 (Pedersen et al., Dec. 5, 2002) propose to hold the Bragg grating between two negative matching thermal expansion substrates; an approach similar to negative CTE coating and which present the same major benefits and drawbacks. U.S. Pat. No. 6,147,341 (Lemaire et al., Nov. 14, 2000) discusses a tubular version of the cantilever configuration and U.S. Pat. No. 6,453,092 (Trentelman, Sep. 17, 2002) proposes a tubular version of the parallel configuration. U.S. Pat. No. 6,449,402 (Bettman et al., Sep. 10, 2002) also proposes a tubular version of the parallel configuration, as well as an axially symmetrical flat version of the parallel configuration. Both versions counteracts offsetting effects of thermal gradients without any mechanism for post-packaging fine-tuning, but the flat version is easier to assemble. Tubular versions usually require the Bragg grating to be threaded into the package, which limits the potential for automation of assembly, the length of the Bragg grating and makes difficult to produce multiple gratings in series along the same fiber. Another drawback of the tubular configuration is that since the grating is completely covered by the thermal compensator, it is impossible to deliver energy to it without affecting the thermal compensator; so, they are not compatible with grating writing on a pre-packaged fiber, nor to annealing or exposition tuning of the Bragg grating in a stabilized athermal package.  
         [0017]     Another related art publication that is of interest to the present subject is U.S. Pat. No. 5,719,974 (Kashyap, Feb. 17, 1998) that proposes to cut open a window over an optical fiber laser diode&#39;s pigtail to enable laser writing of a Bragg grating on the fiber. U.S. Pat. No. 6,349,165 (Lock, Feb. 19, 2002) also proposes the use of two opposite windows cut on a tubular package to enable laser exposure of a pre-packaged fiber. The use of a matching negative CTE material to make the tube covering the Bragg grating, in which the two opposite windows are cut enables the athermalisation of the Bragg grating. Since the package is made of matching negative CTE material, it has the same drawbacks as all packages of that type, namely finding a formulation compatible with long term environmental stability and manufacturing repeatability. In addition, this package does not allow for post assembly tension fine tuning, nor for thermal compensation adjustment; and the tubular configuration requires fiber treading, which limits the automation potential and the use in serial athermal Bragg gratings applications. The energy beam must be strictly confined inside the windows apertures, otherwise the negative CTE material will absorb energy and contract, and so interfere with the grating formation; also, no method is provided to counteract the energy scattered by the fiber to reach and be absorbed by the negative CTE tube. U.S. Pat. No. 6,236,782 (Kewitsch et al., May 22, 2001) presents an athermal package for couplers comprising a Bragg grating in the waist region. The inert structure is designed to enable energy exposition of pre-packaged couplers, but no method is proposed to block scattering energy. The athennalisation is performed by a serially mounted thermal compensator, so it is subject to temperature gradient offsets. In addition, the requirement to tread the long structure is complex, rendering assembly automation difficult. Bragg wavelength and thermal compensation can be fine tuned after the grating is formed but not independently from one another.  
       SUMMARY OF THE INVENTION  
       [0018]     In a first broad aspect, the invention provides a package for holding a temperature sensitive optical device. The package includes a substrate, and a first link connecting to the optical device at a first attachment point, and connecting to the substrate at a location remote from the first attachment point. The package also includes a second link connecting to the optical device at a second attachment point that is remote from the first attachment point, the second link connecting to the substrate at a location remote from the second attachment point. The substrate, the first and the second links imposing a strain variation to the optical device in dependence of temperature.  
         [0019]     In a second broad aspect, the invention provides a package for holding a temperature sensitive optical device. The package has a substrate receiving the optical device and a thermally compensating component mounted to the optical device and to the substrate. The substrate has a window to allow the optical device to be exposed to optical energy electromagnetic radiation to change optical properties of the optical device.  
         [0020]     In a third broad aspect, the invention provides a method for manufacturing a packaged optical component. The method comprises placing an optical component in a substrate, making a connection between a thermally compensating link and the optical component and stabilizing the connection. The method also includes exposing the optical component to optical energy electromagnetic radiation subsequent to the stabilizing to change an optical property of the optical component and affixing the thermally compensating component to the substrate subsequent to the exposing.  
         [0021]     In a fourth broad aspect, the invention provides a method for manufacturing a packaged optical component. The method includes providing an optical component mounted on a substrate and a thermally compensating component link connected to the optical component. The method also includes exposing the optical component to optical energy electromagnetic radiation to change an optical property of the optical component and shielding the thermally compensating component from optical electromagnetic radiation scattered by the optical component during the exposing.  
         [0022]     In a fifth broad aspect, the invention provides a method for manufacturing a packaged optical component. The method includes providing an optical component mounted on a substrate and a thermally compensating component link connected to the optical component. The method also includes exposing the optical component to optical energy electromagnetic radiation to change an optical property of the optical component. The thermally compensating component is located relative the optical component such that optical electromagnetic radiation scattered by the optical component during the exposing is precluded from causing the thermally compensating component to induce a strain in the optical component. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     A detailed description of examples of implementation of the present invention is provided hereinbelow with reference to the following drawings, in which:  
         [0024]      FIG. 1  shows an athermal serial configuration package, according to the prior art;  
         [0025]      FIG. 2  shows an athermal cantilever configuration package, according to the prior art;  
         [0026]      FIG. 3  shows an athermal parallel configuration package, according to the prior art;  
         [0027]      FIG. 4  is a view at a conceptual level of an optical device package according to a non-limiting example of the present invention;  
         [0028]      FIG. 5  is a detailed front elevational view of an optical device package according to a specific but non-limiting example of implementation of the present invention.  
         [0029]      FIG. 6  is a side elevational view of the package shown at  FIG. 5 ; and  
         [0030]      FIG. 7  is a cross-sectional view taken along lines  7 - 7  of  FIG. 6 . 
     
    
       [0031]     In the drawings, embodiments of the invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for purposes of illustration and as an aid to understanding, and are not intended to be a definition of the limits of the invention.  
       DETAILED DESCRIPTION  
       [0032]      FIG. 4  shows at  80  a new configuration for athermal packaging which is capable of reversing the natural positive thermal expansion of the outside frame  82 , into a negative thermal expansion for the fiber section containing the fiber grating. More specifically, the outside frame  82  defines an internal space for receiving a fiber  84  on which a grating such as a Bragg grating can be formed. The fiber  84  is attached to the expanding outside frame  82  by two links  86  and  88  made of material having a lower CTE than the CTE of the outside frame  82 . Each link  86 , 88  attaches to the fiber  84  at a location past the point of attachment of the other link  86 , 88  to the outside frame  82 .  
         [0000]     The strain variations with temperature in the arrangement of  FIG. 4  is given by: 
 
Δε/Δ T =( L   1 α link88   +L   2 α link86   −L   3 α frame82 )/ L   4   (8) 
 
 where L 1 , L 2 , L 3 , L 4  are respectively: 
        1. the axial distance between the junction points  88   a  and  88   b  of the link  88 ,     2. the axial distance between the junction points  86   a  and  86   b  of the link  86 ,     3. the axial distance between the junction points  86   b  and  88   b  on the outside frame  82 ,     4. and the axial distance between the junction points  86   a  and  88   a  on the fiber  84 . 
 
 α link88 , α link86 , α frame82  are respectively the coefficients of thermal expansion (CTE) of the material of the link  88 , of the link  86  and of the outside frame  82 . The selection of materials and of the junction points positions are done so eq. 8 satisfies eq. 4, rendering the Bragg wavelength substantially insensitive to temperature variations: 
 
( L   1 α link88   +L   2 α link86   −L   3 α frame82 )/ L   4 =−(ζ+α f )/(1+ p   e )  (9) 
       
 
         [0037]     As can be seen in eq. 9, this new configuration allows for seven parameters to adjust the package to fit the desired strain/temperature relationship, of which the four relative positions of the junction points. These four degrees of liberty on the position of the junction points enable independent fine tuning and adjustment of the strain level in the packaged section of the fiber, for Bragg wavelength, and strain slope as a function of the temperature, for fine tuning of the athermalisation; even if the two junction points to the glass fiber are already made and stabilized. Junction points to the glass fiber may be made by adhesive, such as glass solders and epoxies that OK require high temperature exposition, inducing stresses on the structures that would need relaxation, and/or curing completion; so these junctions should be stabilized. When this stabilization process is made on a strain fiber already mounted on the package, some fiber strain variations may occur. However, if the junction points to the fiber are stabilized before the assembly of the package, the strain level variations in the fiber grating due to curing completion or induced stresses relaxation are significantly reduced. This new configuration allows effecting the stabilization of the junction points to the fiber before mounting it to the package. Further more, since this configuration is compatible with writing the grating on a pre-packaged fiber, the adhesion processes to the fiber does not have to take into account its effects on the fiber grating since that grating can be effected after the adhesion is completed. Once the fiber is joined to the links in a stable manner, the links can be joined to the substrate. The junction points between the links and the substrate can be selected to obtain the desired strain level and strain thermal slope in the fiber grating. The stability of the junctions will depend on the materials, adhesive and process selection. Since the geometry of the structure can be arranged to adjust the thermal compensation for the fiber grating, material selection of the links and substrate can be made as a function of junction&#39;s stability, price, ease of processing etc.  
         [0038]     The novel package configuration also allows optical frequency electromagnetic radiation such as a laser beam, to be delivered on the fiber without affecting the components of the package, enabling grating writing and annealing on a pre-packaged fiber. Since there is no functional limitations on the radial gaps between the fiber and the links, designs can be made to allow insertion of energy absorbing shields in those gaps so energy scattered by the fiber during expositions do not reach the components of the package. The hollow package also allows for putting a thin non-adhesive and/or flexible coating on the fiber section with bare glass to protect it from the environment and from potential scratching. This concept of grating writing on a pre-packaged fiber could be extended to grating writing on fiber mounted on other structures, such as modules, waist region of couplers, inner waveguides of Mach-Zehnder structures and planar waveguides.  
         [0039]     A more detailed example of an athermal package  10  according to the invention is shown in  FIG. 5 ,  FIG. 6  and  FIG. 7 . The substrate  12  has two identical parts that can slide against one another. Joined to each of these parts, links  14  of lower CTE are joined to the opposite side of the fiber grating  20  to transmit the thermal expansion of the outside substrate  12  to the fiber grating  20 , and transform it to a thermal contraction. The two links  14  and the fiber grating  20  are located in a common imaginary plane. OK  
         [0040]     When the fiber grating  20  is a Bragg grating characterized by a certain wavelength and when in use it is desired to maintain this wavelength constant over a certain temperature range in which the package  10  is expected to operate, the CTE of the links  14  and the CTE of the substrate  12  are selected such that changes in temperature will induce strain in the optical fiber grating  20  that will compensate for the temperature induced wavelength change. As a result, the wavelength of the Bragg grating will remain stable over the temperature range of interest.  
         [0041]     To permit a better adhesion of the links  14  to the fiber grating  20 , a glass insert  18  is affixed into the fiber end&#39;s part of the link  14 . This way the glass fiber  16  will be fixed to a glass surface having similar physical and chemical properties. Since, in this example, the junction between links  14  and fiber  16  is made on glass to glass and the fiber grating  20  could be written after the fiber  16  and package  10  are assembled, the optical fiber  16  is stripped of all coating from one junction point to the other. One eyelet  22  is placed at the bottom front part of each half of the outside substrate  12 . These eyelets  22 , in combination with spring O-rings, could be uses to provide for loose anchoring points to enable the athermal structure to float inside a mechanical protective sealed box.  
         [0042]     With reference to  FIG. 6 , the optical fiber  16  passes loosely into a groove  24  in the front part of the substrate  12 . The beginning of the stripped region is fixed to the glass insert  18  at the end of one of the links  14 . The other end of the link  14  is soldered to the substrate  12  at a position further than the axial center of the fiber grating  20 . The junction between the link  14  and the substrate  12  is made by a soldering line in a C shape using a separable solder connection, with the top and bottom part of the C axially parallel to the fiber  16 . Two grooves are made on the substrate  12  to ease up solder addition or removing on the top and bottom parts of the C shaped soldering line to enable fine adjustment of the anchoring position of the link  14  to the substrate  12 . Four outside eyelets are positioned near the corner of the outside substrate  12  that will serve to anchor the pre-packaged fiber to a fiber grating writing and annealing station.  
         [0043]     With reference to  FIG. 7 , the C shaped front panel  26  of the substrate  12  enables the optical fiber  20  to loosely enter the package without any obstruction or risk of scrapping during its utilization. A separable solder connection is used to fix the two sliding halves of the substrate  12  together. The separable solder connection allows to move the halves of the substrate  12  with relation to one another such as to perform fine adjustments before soldering them permanently in their final position.  
         [0044]     This example of athermal package according to the invention has a planar symmetry along the fiber axis and toward the grating axial center, so its response in a thermal gradient will match a linearized temperature at the center of the grating. The use of separable solder connection for junction points on the outside frame enables readjustments of both strain level tension in the fiber, by repositioning junction points between the two halves of the frame  12 , and strain thermal slope, by repositioning junction points between the two links and the outside frame. The planar design and the low number of components ease up the assembly automation and the axially serial usage on a continuous fiber length.  
         [0045]     A new assembly process for the athermal package according to the present example includes the following steps. For example, the fiber can be stripped and affixed to the glass inserts of the links as a first step. The second step is to anneal these junction points to stabilize them. The third step is to position and solder each link to its respective half of the substrate. The fourth step is to slide the two halves of the substrate along each other to obtain the desired strain level, and then permanently solder them together. The pre-packaged fiber is then stabilized since the fiber junction points have been annealed and ready for fiber grating writing and annealing. Grating writing and annealing can be done by exposition of part of the stripped fiber section to optical frequency electromagnetic radiation, such as a laser energy beam. The exposition is done through a window in the substrate. In the example shown in the drawings the two halves of the substrate  12  define two opposite openings that form windows that can be used for grating writing. The four eyelets near the corners of the side of the frame can be used to screw the pre-packaged fiber on a positioning plate on a laser exposition alignment set-up, where the grating can be written and stabilized, which constitutes the fifth and final step.  
         [0046]     Optionally, thin energy absorbing shields can be inserted between the fiber and the links on the alignment set-up during laser exposition to protect the package against energy scattered by the fiber. Such energy absorbing shields can be in the form of opaque material that will prevent or at least limit scattered energy from reaching the links  14  and the substrate  12  and induce mechanical distortions in the fiber during the exposition as a result of the expansion of the links  14  and substrate  12 . Advantageously, the window on the substrate through which the fiber exposition is made should be made big enough to allow the shields to be inserted through it and removed through it as well. Also a thin coating can be deposited on the bare glass section of the fiber to environmentally protect it, as long as the coating does not strain the grating either by non-adhesion between the coating and the glass or large elasticity of the coating. A possible alternative to the energy absorbing shields is to design the package such that the links  14  and the substrate are located sufficiently far from the fiber such that energy scattered from the fiber will not affect them sufficiently to cause mechanical distortions in the fiber.  
         [0047]     Although various embodiments have been illustrated, this was for the purpose of describing, but not limiting, the invention. Various modifications will become apparent to those skilled in the art and are within the scope of this invention, which is defined more particularly by the attached claims.