Stress-controlled packaging scheme for optical fiber-based optical devices

Abstract

The invention relates to a packaging scheme for an optical fiber-based optical device, comprising a substrate for supporting the optical fiber-based optical device, having a set of individual adhesives for adhering the device to the substrate. A separate bond is disposed along the length of the device so as to reduce the likelihood that the optical fiber moves after encapsulation even if there is a change in ambient temperature during the lifetime of the encapsulated optical device.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 316,662, filed on March 4, 2022, which is incorporated herein by reference. Technical Field

[0003] Disclosed herein is a packaging scheme particularly configured for use with fiber-based optical devices and, more particularly, to a packaging scheme that includes the use of a series of individual splices formed along the length of the device to reduce the likelihood of post-packaging fiber movement even in the presence of changes in ambient temperature over the life of the packaged optical device. Background Art

[0004] In many applications, fiber-based optical devices need to withstand an operating temperature range of -40°C to +85°C (for example). Over such a wide temperature range, any significant mismatch in the coefficient of thermal expansion (CTE) between the packaging material encapsulating the device and the fiber device itself can introduce stresses into the device. The stresses can result in the introduction of unwanted bends along the fiber device, which can cause compromises in the optical signal quality and / or mechanical integrity of the packaged device.

[0005] In the field of optics, the presence of bends along an optical fiber span that exceed a defined radius of curvature can result in outcoupling of a portion of the propagating signal (i.e., bend-induced losses). Depending on the type and application of the fiber optic device, unwanted bends may also introduce mode coupling between different light modes supported by the bent fiber. From a mechanical point of view, the stress changes experienced by a fiber optic device under loads (such as those associated with temperature-related shrinkage relative to the packaging of the fiber optic device) are most troublesome in areas that have already experienced mechanical weakening due to the manufacturing process. For example, in the manufacture / formation of an optical device, one or more thermal treatments may need to be performed, which may result in various areas with different mechanical properties.

[0006] In the formation of a tapered fiber bundle, a defined portion of the collected optical fibers is subjected to an elevated glass softening temperature in order to fuse the individual optical fibers together. At the same time, the collected optical fibers are subjected to longitudinal forces that gradually taper the fused collection of optical fibers (forming a fused cone). As a result of this thermal processing, the manufactured TFB will have a mechanically weakened zone, including the interface between the individual optical fibers and the fused cone. In particular, this weakened zone can be referred to as a "hot / cold interface", where the "hot zone" corresponds to the portion of the device that has been subjected to thermal processing (e.g., fusion), and the "cold zone" corresponds to the unprocessed element (e.g., the original collection of optical fibers).

[0007] In addition to thermal processing leading to the formation of weakened regions, mechanical processing (such as stripping the outer layer from the optical fiber), environmental processing (such as handling individual optical fibers during device assembly), etc., may introduce mechanically weakened regions in fiber-based optical devices.

[0008] It is well known that glass corrosion may occur in any mechanically weakened area of ​​the optic if precautions are not taken. Corrosion may be due to the presence of moisture as part of the environmental conditions when the fusion bonding process begins (for example). It is well known that certain specific heat treatments used for fusion bonding also produce moisture as a by-product.

[0009] Glass corrosion affects the fiber device structure in the weakened area, making the structure susceptible to cracks that propagate along the length (and along the width) of the fiber device. The CTE mismatch between the packaging material and the fiber glass may introduce mechanical stresses (due to the generated thermal stresses). These various results of glass corrosion remain problematic during the lifetime of the packaged optical device and may lead to damage to the optical device at some point in the future.

[0010] Any of these resulting mechanically weakened regions may be further compromised when the final fiber optic device is packaged in a material having a relatively high CTE, where various materials that otherwise exhibit desirable properties (such as high thermal conductivity) are present.

[0011] As fiber-based optical devices are increasingly used in conditions that subject the packaged device to extreme temperatures (as well as other environmental conditions), the choice of materials used for the package and the design details of encapsulating the device into the package become more important. In practice, both mechanical failure and optical damage rely on changes in the radius of curvature of the packaged fiber-optic device, which in this case may be caused by temperature-based changes (particularly shrinkage) in the physical dimensions of the package relative to the fiber-based optical device. For this reason, once the fiber-based optical device is packaged, it is desirable to minimize the potential for fiber movement despite changes in stress on the fiber related to differences in the CTE between the fiber and the packaging material. Summary of the invention

[0012] The present disclosure solves the above-mentioned problems and is directed to a packaging scheme particularly configured for use with fiber-based optical devices (such as tapered fiber bundles (TFBs)). More specifically, it is directed to a packaging scheme that includes the use of a series of individual splices formed along the length of the device to reduce the possibility of fiber movement after packaging, even in the presence of changes in ambient temperature throughout the life of the packaged optical device.

[0013] The present invention provides a packaging device for optical fiber devices, which is based on the selection of a suitable material (or combination of materials) for use as a mounting substrate for the optical fiber device, combined with the use of a series of separate splices to fix the optical fiber device to the mounting substrate, so as to reduce the possibility of optical fiber movement within the packaged device when it is exposed to extreme environmental conditions (e.g., the cryogenic limit of a defined operating temperature range) after the packaged device is exposed. For the purpose of discussion, a "suitable material" can be a material that exhibits relatively high thermal conductivity, and its CTE is a secondary consideration.

[0014] In one exemplary embodiment, a packaging device is provided in which selected individual splices can be positioned so that a mechanically weakened region identified along the optical device is supported between a pair of splices. In other words, the position of the splices is selected so that the mechanically weakened portion of the optical fiber device remains compressed for an expected predefined load range during the life of the optical fiber device.

[0015] In many cases, the predefined load range is a function of the extremes of the operating temperature range of the packaged fiber optic device, because there is usually a large difference in CTE between the packaging material and the glass fiber, the packaging material shrinks relative to the glass fiber at low temperatures and exerts stress on the fiber optic device.

[0016] In another embodiment, the packaging device can be configured such that the length of unsupported optical fiber between individual splices is not subjected to bending beyond a radius of curvature at which bend-induced coupling losses and / or mode coupling of the propagating signal occur (even in the presence of compressive loads caused by temperature extremes).

[0017] Certain bonding members may be formed to extend to cover the entire length of a feature of a fiber optic device (eg, a taper feature of a TFB); in other embodiments, only a portion of a selected feature may be covered by the bonding member.

[0018] It should be understood that while the following discussion focuses primarily on the formation of fiber-based optical devices in the form of a photosynthetic combiner that includes a TFB that exhibits at least one mechanically weakened area due to its manufacturing process, the same principles apply to the packaging of any fiber-based optical device that may experience a large amount of unwanted bending due to mechanical, thermal and / or environmental factors. Fiber-based optical devices can range from simple optical splices to complex assemblies based on multiple optical fibers and are used in various types of applications, including communication systems, sensors, medical devices, etc.

[0019] Various embodiments of the packages disclosed herein may include the use of one or more specific CTE matched substrate inlays located at identified mechanically weak locations or near fiber spans where bending beyond a known radius of curvature would compromise the quality of the propagating optical signal, while the remainder of the substrate is formed of a material having preferred heat transfer properties.

[0020] One exemplary embodiment of the present disclosure may take the form of a packaging device for a fiber-based optical device, the packaging device comprising a substrate for supporting the fiber-based optical device (the substrate being formed of a material that can undergo shrinkage relative to the fiber-based optical device at cryogenic extremes) and a plurality of splices for securing the fiber-based optical device to the substrate. The plurality of splices are disposed along the length of the fiber-based optical device such that an unsupported portion of the optical fiber between a pair of splices is restrained from either bending beyond a defined radius of curvature or rupturing in a mechanically weakened region during operation of the fiber-based optical device within a defined temperature range.

[0021] Other and further features and embodiments will become apparent during the following discussion and by reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Referring now to the drawings, wherein like numerals represent like parts throughout the several views:

[0023] Figure 1 is a cross-sectional view of a photosynthetic beam combiner including a tapered fiber bundle (TFB);

[0024] Figure 2 is used for Figure 1 An isometric view of a typical prior art packaging arrangement for a photosynthetic beam combiner is shown;

[0025] FIG3 is a cross-sectional side view of the prior art packaging device shown in FIG2;

[0026] Figure 4 comprising graphs showing the onset of two different buckling modes based on an Euler critical load analysis, graph (a) being associated with a first (fundamental) buckling mode and graph (b) being associated with a subsequent (secondary) buckling mode;

[0027] Figure 5 Contains information showing Figure 4 A graph showing the data as the CTE of a package used to support an optical fiber device is varied;

[0028] Figure 6 The buckling load factor (BLF) of a mechanically weakened hot / cold interface region in a simple 2×1 photosynthesizer is plotted as a function of the unsupported length L.

[0029] Figure 7A simplified diagram including strategic positioning of adapters to support the mechanically weakened hot / cold interface regions found within the TFB portion of a photosynthetic beam combiner; and

[0030] Figure 8 Contains curves showing how BLF varies with unsupported fiber span length for various tapered fiber bundle configurations. DETAILED DESCRIPTION

[0031] Figure 1 is a cross-sectional view of a photosynthetic combiner 10 including a tapered fiber bundle (TFB) 12 and an output fiber 13. For purposes of the present invention, the tapered fiber bundle 12 is described and characterized by its thermal processing history, which directly affects its ability to maintain stability after packaging. Likewise, it should be understood that while the following discussion focuses primarily on the formation of a photosynthetic combiner including a TFB, the same principles apply to the packaging of fiber optic devices, which may include mechanically weakened portions that are created during manufacturing by any type of machining, thermal processing, product handling environment, etc. Furthermore, in addition to designing specific packaging configurations to avoid failure / damage of mechanically weakened areas, the packaging schemes of the present invention can also be used to develop device packages that minimize the likelihood that an unsupported length of the packaged optical fiber will be bent beyond a radius of curvature that could introduce optical damage.

[0032] like Figure 1 As shown, TFB 12 is formed of a plurality of N individual optical fibers 14, which are stripped of their outer jacket layers and assembled into a closely packed assembly configuration 14C. The closely packed assembly 14C is then subjected to multiple heat treatments to soften the glass and fuse the individual optical fibers together, while also applying a pulling force to gradually reduce the diameter of the fused bundle. Figure 1 Also shown is a taper fusion region 16 which terminates in a taper waist 16W which is spliced ​​or otherwise coupled to the output optical fiber 13.

[0033] Of particular importance to the present disclosure is the formation of a hot / cold interface region 18 in the region between the tapered fusion region 16 (which has been subjected to thermal processing in the form of one or more heat treatments at the glass softening temperature) and the closely packed optical fiber assembly 14C (which has not been further processed). During the fusion process, the glass component of the interface region 18 is mechanically weakened, primarily due to moisture corrosion of the glass caused by the presence of moisture associated with the ambient conditions (or possibly the heat source used for the fusion process, such as an H2-O2 flame torch). Glass corrosion weakens the structure of the interface region 18 by forming cracks along the fiber bundle surface, which may then propagate in the longitudinal (or radial) direction of the tapered fiber bundle 12 in the presence of tensile stresses. Ultimately, glass corrosion may lead to glass failure by mechanical fracture. Within the interface region 18, corrosion is typically greatest along the weakening plane WP, which is the first location to be inspected after mechanical failure is identified. In actual practice, the fiber strength is typically reduced to 20 kpsi (from a nominal value of about 100 kpsi) or even as low as 10 kpsi in the weakened region.

[0034] Figure 2 is used for Figure 1 FIG. 2 and FIG. 3 show an isometric view of a typical prior art packaging device for a photosynthetic combiner 10, and FIG. 3 shows a cross-sectional side view of the prior art packaging device. As shown in FIGS. 2 and 3, the photosynthetic combiner 10 is located on a substrate 30, which is formed of a material having a thermal conductivity value sufficient to provide a heat dissipation function. Aluminum is a typical choice for the substrate 30 because of its relatively high thermal conductivity (approximately 200 W / mK), but other materials may also be used, particularly in applications where heat dissipation requirements can be relaxed.

[0035] 2, the substrate 30 may be formed to include a recessed area 31 to support the various portions of the optical combiner 10. In this prior art configuration, the optical combiner 10 is held in place within the recess 31 on the substrate 30 by a pair of adhesive members 32, 34 located at opposite ends of the substrate 30. The adhesive member 32 is shown attached to a portion of the output optical fiber 13 at a first end 33 of the substrate 30, and the adhesive member 34 is shown as being arranged to hold the closely spaced optical fibers 14C in place at a second end 35 of the substrate 30.

[0036] As described above, the tapered fiber bundle 12 (particularly the mechanically weakened interface region 18) may experience tensile stresses due to the CTE mismatch between the material selected for the substrate 30 and the glass material of the photosynthetic combiner 10. In many applications, the material selected as the packaging material is selected based on having a high thermal conductivity value (i.e., excellent heat dissipation capabilities); the CTE mismatch is a secondary consideration. The specific nature of the thermal stress is based on whether the ambient temperature in which the photosynthetic combiner is packaged is above or below a "strain reference temperature," which is typically room temperature (associated with zero strain). In the event that the ambient temperature is above the strain reference temperature, the substrate 30 formed of the high CTE material will expand relative to the photosynthetic combiner 10, thereby applying tensile stress to the packaged optical devices. In the event that the ambient temperature is below the strain reference temperature, the high CTE material will contract, thereby applying compressive stress to the optical fiber devices. The latter situation leads to the type of optical signal damage and mechanical failure that is addressed and overcome by the packaging solution of the present disclosure.

[0037] The detailed characteristics of the structural changes of optical fiber devices in the presence of compressive stress can be understood using the Euler boundary conditions obtained for a "slender compression rod". The Euler critical load is defined as the compressive load at which the slender rod will suddenly bend or buckle:

[0038]

[0039] Where E is the Young's modulus of the column material (here, the specific glass composition of the optical fiber device), I is the minimum area moment of inertia of the column cross section, L is defined as the unsupported length of the column, and K is the effective length factor of the column. As shown in the relationship, the critical load (P) at which the column may suddenly buckle cr ) is inversely proportional to the square of the length of the unsupported column.

[0040] The effective length factor K is related to whether the ends of the column can rotate or translate. For situations where the ends cannot rotate or translate, such as in the present case where epoxy joints (e.g., 32, 34) hold the opposing ends fixed, the K factor is defined to have a value of 0.5 (thus being half the length L over which the critical load may occur, L eff =0.5L). Figure 4 It shows the length L between the two end points. eff=KL, each specific buckling mode having its own critical load value. (a) shows the first (fundamental) buckling mode, and (b) shows the higher second buckling mode. The maximum displacement (dt) is also shown in both figures. Referring to the basic buckling mode shown in (a), this type of buckling may be allowable for packaging optical devices if the surface tensile stress generated in the buckling is less than the predefined maximum allowable tensile force. The radius of curvature associated with the first mode shown in (a) is smaller than the radius of curvature associated with the second mode shown in (b), where the radius of curvature is an important factor when designing a splice arrangement to address optical coupling issues (such as the possibility of minimizing bend-induced losses when packaged optical fiber devices are exposed to cryogenic extremes).

[0041] Applying the Euler critical load analysis to a conventional package as shown in FIGS. 2 and 3 , the unsupported length L includes all of the individual components of the optical combiner 10; namely, the closely packed fiber assembly 14C, the tapered fiber bundle 12, and the output fiber 13. The length L of a commercial optical combiner may be in the range of about 25-100 mm, depending on the physical parameters of the combiner (including the number of individual fibers being combined). Since the critical load is inversely proportional to the square of the unsupported fiber span length, this relatively long span L results in a P cr This means that even a small shrinkage in package size may generate loads greater than the critical value. When using conventional packages with relatively high CTE values, P cr Values ​​may limit the temperature range over which the device can be used without experiencing buckling and related mechanical problems (including damage), or optical signal integrity problems (such as the occurrence of bend-induced losses or unwanted mode coupling). As is well known in the art, bending of an optical fiber changes its effective refractive index value, resulting in the likelihood that a certain amount of bending will produce an effective index change that supports coupling to another (unwanted) mode. Outcoupling of a propagating signal is a known consequence of bending an optical fiber beyond a certain radius of curvature, and depends on the specific fiber design.

[0042] Figure 5Contains two graphs showing the variation in unsupported fiber span length (d1) and the associated variation in the peak fiber lateral displacement (dt) for various CTE values. Graph (a) shows the variation in the length dl required for the onset of buckling for both mode 1 and mode 2 as the CTE of the encapsulation material increases. As shown, for both buckling modes, an increase in the CTE of the encapsulation material results in a significant decrease in the acceptable unsupported fiber span length. These results also show that for relatively low CTE values, the unsupported length associated with the onset of buckling for mode 2 is approximately twice the unsupported length associated with the onset of buckling for mode 1, with the difference between the two lengths decreasing as the encapsulation CTE is greater than about 1.0e-5. As shown in graph (b), for CTE values ​​greater than about 1.0e-5, the maximum displacement (dt) for both buckling modes varies relatively the same as a function of CTE. Unfortunately, there are few, if any, material choices for encapsulating fiber-based optical devices that exhibit CTEs in this lower range while also providing the desired high levels of thermal conductivity.

[0043] These difficulties of the prior art are addressed by the present invention, which is directed to configurations of packaging arrangements for optical fiber devices utilizing strategically placed splices (which may or may not be free to rotate and translate). In particular, the individual splices are arranged so that the unsupported length of the device (i.e., the length between two splice locations) does not exceed a value that may induce bending / buckling of the optical fiber when the packaged device is exposed to cryogenic extremes. Since the CTE of a given packaging material is typically greater than the CTE of the optical fiber, the shrinkage of the packaging material at cryogenic temperatures introduces loads to the device, and thus there is a need to control this applied load with respect to a known critical load value.

[0044] For the special case of evaluating the placement of the splice around a mechanically weakened region of an optical fiber, the goal is to keep the region in compression. By controlling the placement of the splice in the region so that only a relatively short span of the optical fiber device is unsupported, the mechanically weakened region can withstand much higher loads before any bending occurs (bending is defined as a buckling that includes the mechanically weakened device region and the situation where the maximum surface stress exceeds the device strength while the bend is maintained to less than a known radius of curvature) compared to the prior art arrangements of Figures 2 and 3. In order to maintain acceptable optical transmission characteristics, the length of the unsupported fiber needs to be such that any bend is less than a known radius of curvature at which bending losses are likely to occur. Reference Figure 4 , it is clear that the radius of curvature is related to the buckling mode being excited.

[0045] As described below, the packaging configuration of the present invention takes into account the above critical load P CR and applied load P ap Both. In this case, the applied load P apDefined as the compressive load applied to a packaged optical fiber device by the contraction / shrinkage of the substrate 30 relative to the optical fiber device at the low end of the device's defined operating temperature range. cr / P ap The ratio of P to BLF can be defined as a “buckling load factor” (BLF), where values ​​of BLF greater than 1 may be preferred to minimize the chance of buckling occurring in a particular mode under a particular end condition (i.e., P cr >P ap ). As described below, the larger the BLF value, the more likely it is that an unsupported, mechanically weakened portion of the optical fiber component will remain in compression when exposed to extreme operating conditions (e.g., the cryogenic boundary of a defined operating temperature range).

[0046] Thus, in accordance with the teachings of the present invention, the sites of the splices are selected and spatially positioned so that each known mechanically weakened region along the fiber optic device (e.g., the interface region 18 of the optical combiner 10) is located between a pair of splices. Thus, the length L of the "unsupported post / fiber" (e.g., the interface region 18) is determined so that the unsupported fiber remains in compression (or, in the worst case, experiences a small surface tensile stress) even when the supporting package substrate shrinks to a minimum dimension associated with the lowest operating temperature allowed for the application of the packaged fiber optic device.

[0047] A simple optical combiner can take the form of a 2×1 combiner, where a pair of input fibers are fused together and tapered to splice to an output fiber. For example, the original outer diameter of each input fiber may be 125 μm, and the original outer diameter of the output fiber may also be 125 μm. Figure 6 The buckling load factor (BLF) of the mechanically weakened hot / cold interface region of this simple 2×1 beam combiner is plotted as a function of the unsupported length L. The results show that when the unsupported length L is greater than 3 mm, the first mode of buckling (e.g. Figure 4 Buckling in the second mode may occur (as shown in Figure (a)). The higher second mode of buckling (Figure (b)) may not occur until the unsupported length L exceeds 5 mm. For buckling in the second mode, the maximum surface tensile stress can be an order of magnitude higher than the stress associated with the fundamental mode. Similarly, the optical degradation caused by the second buckling mode may be significantly higher than that caused by the first mode. Because the maximum surface tensile stress (and the optical degradation discussed above) depends on the radius of curvature of the bend, buckling in the fundamental mode may be allowable, while buckling in the second or third modes may result in catastrophic failure.

[0048] Figure 7A simplified diagram of the strategic positioning of adhesives used to support the mechanically weakened hot / cold interface region 18 of the TFB 12 with the photosynthesizer 10 is included. For purposes of discussion and clarity, the adhesives are shown as "open" elements so that the underlying portions of the photosynthesizer 10 are visible. Figure 7 (a) shows an exemplary positioning of a pair of splices 50, 52 relative to the interface region 18 of the TFB 12 (particularly about the weakening plane WP within the region 18). Sometimes referred to as "inner splices" (relative to the location of the outer splices 32, 34), splices 50 and 52 are positioned so that the unsupported fiber length L is relatively short and the interface region 18 is maintained in compression at the lowest temperature limit associated with the operation of the optical combiner 10. Therefore, this short length is able to withstand relatively high applied loads (i.e., exhibit a high BLF) before buckling. Some examples of calculated values ​​are discussed below. Figure 7 The device of (a) is also shown to include joints 32, 34 for further supporting the physical attachment of the optical combiner 10 to the metal substrate 30. For example, a preferred embodiment of the device of the optical combiner 10 includes a plurality of four joints (joints 32, 50, 52 and 34, respectively, as shown) to keep all parts of the assembly in a straight line and in compression at low operating temperatures, thereby avoiding mode coupling of propagating signals at these temperatures. In some optical combiner designs, it is desirable to separate the desired fundamental mode and all other higher order modes (primarily LP 11 The mode coupling between the modes) is maintained at a value of less than 15db; this is just an example.

[0049] Another factor that may be considered in determining a splice configuration is the physical length of the splice attachment that is positioned to surround a mechanically weakened area of ​​the fiber optic component. Figure 7 (b) shows an embodiment of the bonding element length factor. In this particular embodiment, the first bonding element 50T is shown as having a length B greater than B. T In this particular example of the optical combiner 10, the length B may be selected to T , so that the adhesive member 50T completely covers the taper area 16 (with Figure 7 The configuration shown in (a) is different, in that the adhesive 50 covers only a portion of the tapered region 16).

[0050] like Figure 7 The exemplary configuration of the present invention shown may be referred to as a "four-point" joint of the optical combiner 10, which supports three adjacent sections of the combiner 10 on the heat-dissipating metal substrate 30. Figure 7As shown, the first segment I corresponds to the output optical fiber 13 and is supported between the first terminal splice 32 and the first inner splice 50. The second segment II corresponds to the hot / cold interface region 18 of the TFB 12 and is supported between the inner splices 50 and 52. The third segment III corresponds to the "cold" region of the optical combiner 10 (i.e., the closely packed collection of individual optical fibers 14 supported between the second inner splice 52 and the second terminal splice 34). The properties of these three segments will be described below, particularly with regard to the improvement in BLF when the interface region 18 is supported between the inner splices 50, 52.

[0051] like Figure 7 As shown in (a), in an example related to the placement and size of the joint, the metal substrate 30 is made of aluminum, and the minimum operating temperature of the optical combiner 10 is defined as -40°C, resulting in an applied load P caused by the shrinkage of aluminum. ap = -37N. Segment I is defined as length L Ⅰ is 22 mm, the same length as segment III (L Ⅲ =L Ⅰ ). The length of the interface section II (including the weakening plane WP) is L Ⅱ is 5 mm. Using the above relationship, according to L Ⅱ =5mm, calculate P of segment II cr = -532 N. For these values, and assuming that the endpoints are free to rotate, the BLF (i.e., P cr / P ap ) is 14.48, far exceeding the required condition BLF>1 to avoid buckling, thereby ensuring that the interface region 18 will remain in compression (ie, not buckle) at this extreme ambient temperature value.

[0052] In contrast, P cr It is calculated to be only -4.3N, with an associated BLF of 0.11 (the aforementioned P ap The value of P is -37N). The results of segment III are similar. cr =-31.5 N, and the BLF is 0.85. Since both segments I and III have load factors less than 1, in some cases these two segments may experience buckling at the cryogenic extreme, and therefore it may be desirable to avoid buckling of segments I and III; that is, to minimize the likelihood of buckling along any portion of the optic, such as minimizing the likelihood of optical damage in terms of bend-induced losses or unwanted mode coupling.

[0053] As described above, some prior art solutions for addressing buckling of optical fiber devices at low packaging temperatures are related to using a substrate metal material that exhibits a CTE similar to that of the glass material of the optical fiber device. For example, a nickel-iron alloy known as Invar has a CTE of 1.5e-6 / C, which is relatively close to the CTE of glass (about 0.50e-6 / C), which minimizes substrate dimensional changes with temperature (e.g., minimizes its shrinkage relative to the optical fiber at the lowest temperature values ​​in the operating range). However, Invar does not exhibit the degree of thermal conductivity that is typically required for applications that package optical fiber devices (i.e., its thermal conductivity is 12 W / mK, while aluminum has a thermal conductivity of 220 W / mK). One approach might be to utilize a hybrid substrate that utilizes a reduced CTE material (such as Invar) in the packaging area that supports the weakened area, and utilizes aluminum (or another material with similarly high CTE and high thermal conductivity) in the remainder of the packaging substrate. In another approach, an inlay of a material having a reduced CTE may be disposed within a recessed region of the aluminum substrate, positioning the inlay within the weakened region most affected by compressive stresses (e.g., interface region 18, such as Figure 7 (a) and (b) are shown in section II). In this case, the allowable length between the bonding members can be determined based on the CTE of the material to which the device is bonded and the desired maximum surface stress allowed within the desired temperature range.

[0054] Continuing to describe the specific experimental results based on the parameters defined above, in Section II, Invar alloy is added to P ap At -40℃ (reduced value), it was increased to -2.2N. Adding Invar alloy increased the P in Section II cr The values ​​(lower magnitude) increased to a value of -228 N. Thus, for the region of the joint that allows rotation and buckling into the fundamental mode, the BLF of Section II of Invar under load is about 103, a significant improvement over the value of about 14 associated with the all-aluminum substrate.

[0055] The length of the mechanically weakened zone supported between a pair of joined parts also significantly affects the BLF. In the above example, the interface area 18 is defined as the weakened zone (including the weakened plane WP). For the above example (and reference to Figure 6 ), the segment length L Ⅱ Defined as 5mm. Figure 8 The buckling load factor versus length L for various tapered fiber bundles (from 3×1 to 19×1 configurations, all based on the combination of individual fibers with an outer diameter of 125 μm) is included. Ⅱ The figure shows the BLF of a 7×1 fiber configuration with an outer diameter of 250μm as L Ⅱ Changes in the graph. Figure 7An expanded region of the above plot is also shown, showing the L at a larger BLF scale as the BLF approaches the unity threshold. Ⅱ value.

[0056] It should be noted that the scope of the present invention is not limited to the above preferred embodiments, and those skilled in the art may make various modifications and changes without departing from the creative concept and scope of the present invention as defined in the appended claims.

Claims

1. A packaging device for an optical device based on an optical fiber, comprising: a substrate for supporting the fiber-based optical device, the substrate being formed of a material that undergoes contraction relative to the fiber-based optical device at cryogenic extremes; and a plurality of splices for securing the fiber-based optical device to the substrate, the plurality of splices being arranged along the length of the fiber-based optical device such that an unsupported portion of the fiber-based optical device having a length L between a pair of splices is restrained from any of the following: (1) bending beyond a radius of curvature at which bend-induced damage occurs; and (2) fracture occurs in a mechanically weakened region along the unsupported portion during operation of the fiber-based optical device within a defined temperature range.

2. The packaged device according to claim 1, wherein the defined length L is defined as the critical buckling load P cr The function of And E is the Young's modulus of the glass material of the fiber-based optical device, I is the minimum area moment of inertia of the cross-section of the fiber-based optical device, and K is the column effective length factor equal to 0.5 for the unsupported length set between a pair of fixed points.

3. The packaged device of claim 2, wherein a buckling load factor (BLF) is used to determine the defined length L, the BLF being defined as P cr With maximum load P ap Ratio, maximum load P ap Applied as a compressive load on the unsupported portion of the fiber-based optic.

4. The packaging device according to claim 3, wherein the maximum load P ap Related to the maximum shrinkage of the substrate at the lowest temperature value within the operating temperature range. The packaged device of claim 4 , wherein the defined length L is selected such that BLF is greater than 1. 6 . The packaged device of claim 1 , wherein the substrate comprises aluminum.

7. The packaged device of claim 1, wherein the fiber-based optical device operates in a temperature range of -40°C to +85°C. 8 . The packaging device according to claim 1 , wherein the plurality of bonding members further comprises a pair of terminal bonding members located at opposite ends of the metal substrate.

9. The packaged device of claim 1, wherein the fiber-based optical device includes a mechanically weakened region within the unsupported portion.

10. The packaged device of claim 9, wherein the defined length L is selected such that the mechanically weakened region remains in compression below a temperature associated with zero stress on the packaged fiber-based optical device within an operating range of the packaged fiber-based optical device. 11 . The packaged device of claim 10 , wherein the substrate comprises a mixture of at least two different materials having two different CTE values, a first material having a lower CTE value being disposed proximate to the mechanically weakened region. 12 . The packaged device of claim 11 , wherein the first material having a relatively low CTE value is in the form of an inlay disposed in a recessed region of the substrate adjacent to the mechanically weakened region.

13. The packaged device of claim 11, wherein the mixture of at least two different materials comprises aluminum and Invar.

14. The packaging device of claim 19, wherein the fiber-based optical device comprises a tapered fiber bundle having a mechanically weakened region portion at an interface between an input portion comprising a plurality of closely spaced optical fibers and an output portion comprising a fusion-spliced, tapered connection of the plurality of optical fibers.

15. The packaged device of claim 1, wherein the plurality of splices are spaced apart by an amount such that each unsupported portion of the fiber-based optical device exhibits a radius of curvature that is unable to support mode coupling and bend-induced losses at the cryogenic limit of a defined temperature range.

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