Stress-controlled packaging technology for fiber-based optical devices.

Abstract

A packaging structure for a fiber-based optical device includes a substrate for supporting the fiber-based optical device, and a set of individual bonds are used to secure the device to the substrate, the individual bonds being positioned along the length of the device to reduce the likelihood of fiber migration after packaging, even in the presence of ambient temperature changes over the lifetime of the packaged optical device.

Description

[Technical field]

[0001] [Reference to Related Applications] This application claims the benefit of U.S. Provisional Application No. 63 / 316,662 (filed March 4, 2022), which is incorporated herein by reference. Disclosed herein is a packaging technique particularly configured for use with fiber-based optical devices, and more particularly, relates to a packaging technique that includes the use of a series of individual bonds formed along the length of the device to reduce the likelihood of fiber migration after packaging, even in the presence of ambient temperature changes over the lifetime of the packaged optical device. [Background technology]

[0002] In many applications, fiber-based optical devices are required to withstand an operating temperature range of (for example) -40°C to 85°C. Over this wide temperature range, the significant mismatch in the coefficient of thermal expansion (CTE) between the packaging material housing the device and the fiber optic device itself introduces stresses into the device. The stresses can result in undesirable bending along the fiber optic device, which can lead to compromise of the optical signal quality and / or mechanical integrity of the packaged configuration.

[0003] In the optical domain, the appearance of bends along an optical fiber span that exceed a defined radius of curvature can result in outcoupling (i.e., bend-induced loss) of a portion of the propagating signal. Depending on the type and application of the optical fiber device, undesired bends can also result in mode coupling between different optical modes that may be supported by the bent fiber. From a mechanical perspective, changes in the stresses experienced by an optical fiber device under load (e.g., loads associated with temperature-related shrinkage of packaging for an optical fiber device) are most problematic along regions that have already experienced mechanical weakening as a result of the manufacturing process. For example, the need to perform one or more heat treatments during the manufacture / formation of an optical device can result in the creation of various regions of different mechanical properties.

[0004] In forming a Tapered Fiber Bundle, a defined segment of bundled fibers must be subjected to a high glass softening temperature that fuses the individual fibers together. At the same time, the bundled fibers are subjected to a longitudinal force that tapers the fused assembly of fibers (forming the fused taper). As a result of this heat treatment process, the fabricated TFB will have a mechanically weakened region that encompasses the interface between the individual fibers and the fused taper. Notably, this weakened region is referred to as the "hot / cold interface," with the "hot region" corresponding to the portion of the device that is subjected to the heat treatment (e.g., fusion) and the "cold region" corresponding to the untreated elements (such as the initial assembly of optical fibers).

[0005] In addition to thermal processes that cause weakened regions to form, mechanical processes (such as peeling of the outer layer from the fiber), environmental processes (such as handling of individual fibers during device assembly), etc. can introduce mechanically weakened regions into fiber-based optical devices.

[0006] It is known that glass corrosion can occur within any mechanically weakened area of ​​an optical device if preventative measures are not taken. Corrosion can be due to (for example) the presence of moisture as part of the ambient conditions when the fusing process is initiated. Some specific heat processes used in fusing are also known to produce moisture as a by-product.

[0007] Glass corrosion affects the fiber optic device structure in the weakened regions, leaving the structure susceptible to crack propagation along the length (and across the width) of the fiber optic device. CTE mismatch between the packaging material and the glass of the optical fiber can result in mechanical stresses (due to the thermal stresses generated). These various consequences of glass corrosion remain a problem during the life of the packaged optical device and may result in failure of the optical device at some point in the future.

[0008] Any of these created mechanically weakened regions can be further compromised when the final optical fiber device is packaged in a material having a relatively high CTE, as there are a variety of materials that would otherwise exhibit desirable properties such as high thermal conductivity.

[0009] Fiber-based optical devices are increasingly used in conditions that expose the packaged device to extreme temperatures (as well as other environmental conditions), making the selection of materials for use in the packaging, as well as the design details for encapsulating the device within the package, more critical. Indeed, both mechanical and optical failures depend on changes in the radius of curvature of the packaged optical fiber device, which can be caused by temperature-based changes (especially shrinkage) in the physical size of the packaging relative to the fiber-based optical device. To that end, it is desirable to minimize the possibility of fiber migration after the fiber-based optical device is packaged, regardless of changes in stress on the fiber associated with differences in CTE between the fiber and the packaging material. Summary of the Invention

[0010] The above-mentioned problems are addressed by the present disclosure, which relates to a packaging scheme specifically configured for use with fiber-based optical devices such as tapered fiber bundles (TFBs), and more particularly to a packaging scheme that includes the use of a series of individual bonds formed along the length of the device in a manner that reduces the likelihood of fiber migration after packaging, even in the presence of ambient temperature changes over the life of the packaged device. A packaging configuration is proposed for optical fiber devices based on the selection of a material (or combination of materials) suitable for use as a mounting substrate for the optical fiber device, in combination with the use of a series of individual bonds to attach the optical fiber device to the mounting substrate in a manner that reduces the likelihood of fiber migration within the packaged device when subsequently exposed to extreme ambient conditions (e.g., cryogenic temperatures in a defined operating temperature range). For purposes of discussion, a "suitable material" may be one that exhibits a relatively high thermal conductivity, with its CTE a secondary consideration property.

[0011] In an exemplary embodiment, a packaging configuration is proposed in which selected individual bonds may be positioned such that mechanically weakened regions identified along the optical device are supported between a pair of bonds. In other words, bond locations are selected such that the mechanically weakened regions of the fiber optic device remain in compression over a predetermined range of loads expected over the life of the fiber optic device.

[0012] In many cases, the predetermined load range is a function of the extremes of the operating temperature range of the packaged fiber optic device because there is typically a substantial difference in CTE between the packaging material and the glass fiber, and the packaging material will shrink against the glass fiber at low temperatures, imparting stress to the fiber optic device.

[0013] In another embodiment, the packaging configuration can be configured such that the length of unsupported fiber between individual bonds is not bent beyond a radius of curvature where bend-induced coupling losses and / or mode coupling of propagating signals occur (even in the presence of compressive loads occurring at extreme temperatures). Certain bonds may be formed to extend over the entire length of a particular feature of the optical fiber device (e.g., a fused taper feature of a TFB), and in other embodiments, only a portion of a selected feature may be covered by the adhesive bond.

[0014] While the following discussion focuses primarily on the formation of a fiber-based optical device in the form of an optical combiner that includes a TFB that exhibits at least one mechanically weakened region due to its fabrication process, it should be understood that the same principles apply to the packaging of any fiber-based optical device that may experience significant undesirable bending as a result of mechanical, thermal and / or environmental factors. Fiber-based optical devices range from simple optical splices to complex multiple fiber-based assemblies and may be used in various types of applications including communication systems, sensors, medical devices, and the like.

[0015] Various embodiments of the disclosed packages may incorporate the use of one or more specific CTE-matched substrate inserts that are positioned in close proximity to identified mechanically weakened locations or fiber spans where bending beyond a known radius of curvature would impair the quality of the propagating optical signal, while the remainder of the substrate is formed of a material with favorable heat transfer properties.

[0016] An exemplary embodiment of the present disclosure can take the form of a packaging configuration for fiber-based optics comprising a substrate for supporting a fiber-based optical device (the substrate being formed of a material that undergoes contraction relative to the fiber-based optical device at cryogenic temperatures) and a plurality of bonds for securing the fiber-based optical device to the substrate. The plurality of bonds are disposed along a length of the fiber-based optical device such that an unsupported region of the optical fiber between a pair of bonds is restricted from either bending beyond a defined radius of curvature or breaking at a mechanically weakened region during operation of the fiber-based optical device over a defined temperature range. Other and further features and embodiments will become apparent during the course of the following discussion and by reference to the accompanying drawings.

[0017] Reference will now be made to the drawings, in which like reference numerals represent like parts in the several views. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a cross-sectional view of an optical combiner including a tapered fiber bundle (TFB). [Diagram 2] FIG. 2 is an isometric view of a typical prior art packaging configuration for the optical combiner shown in FIG. [Diagram 3] FIG. 3 is a cutaway side view of the prior art packaging device shown in FIG. [Figure 4] FIG. 4 illustrates the onset of two different buckling modes based on Euler's critical load analysis, with FIG. 4(a) relating to the first (fundamental) buckling mode and FIG. 4(b) relating to the subsequent (secondary) buckling mode. [Diagram 5] FIG. 5 is a graph showing the change in the data shown in FIG. 4 as a function of the change in the CTE of the package used to support the fiber optic device. [Figure 6]FIG. 6 plots the buckling load factor (BLF) as a function of unsupported length L for a mechanically weakened hot / cold interface region in a simple 2×1 optical combiner. [Figure 7] FIG. 7 includes a simplified diagram of the strategic positioning of bonds to support a mechanically weakened hot / cold interface region such as that found in the TFB portion of an optical combiner. [Figure 8] FIG. 8 contains plots of the BLF as a function of unsupported fiber span length for a variety of different tapered fiber bundle configurations. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] FIG. 1 is a cross-sectional view of an optical combiner 10 including a tapered fiber bundle (TFB) 12 and an output optical fiber 13. For purposes of the present invention, the characteristics of the tapered fiber bundle 12, including its thermal processing history, directly affect its ability to maintain stability after packaging. Also, while the following discussion focuses primarily on the formation of an optical combiner including a TFB, it should be understood that the same principles apply to the packaging of optical fiber devices that may include mechanically weakened regions created during manufacturing by any type of mechanical process, thermal process, product handling environment, etc. In addition, beyond designing a particular package configuration to avoid failure / breakage at mechanically weakened regions, the packaging scheme of the present invention may be used to develop device packages that minimize the likelihood of unsupported lengths of packaged fiber bending beyond a radius of curvature that may result in optical failure.

[0020] The TFB 12 is shown in FIG. 1 as being formed from a plurality of N individual fibers 14, which are stripped of their outer jacket layers and assembled into a tightly packed assembly configuration 14C. The tightly packed assembly 14C is then subjected to multiple heat treatments to soften the glass and fuse the individual fibers together, while simultaneously applying a pull-out force to taper the diameter of the fused bundle. A tapered fused region 16 is also shown in FIG. 1, which terminates in a tapered waist 16W that is spliced ​​or otherwise coupled to the output fiber 13.

[0021] Of particular importance to the present disclosure, the hot / cold interface region 18 is formed in the region between the tapered fused region 16 (which has been subjected to a thermal process in the form of one or more heat treatments at the glass softening temperature) and the tightly bundled fiber assembly 14C (which has not been further processed). The glass composition of the interface region 18 is mechanically weakened during the fusion process primarily due to moisture corrosion of the glass driven by the presence of moisture associated with the ambient conditions (or possibly the heat source used for the fusion process, e.g., H2-O2 flame torch). The glass corrosion weakens the structure of the interface region 18 by forming cracks along the bundle's surface, which may then propagate along the longitudinal (or radial) direction of the tapered fiber bundle 12 in the presence of tensile stresses. Ultimately, the glass corrosion may result in glass failure due to mechanical fracture. The corrosion is typically greatest along the weakened plane WP in the interface region 18, which is the location of first inspection upon recognition of a mechanical failure. In practical situations, it is common to reduce the fiber strength in the weakened region to 20 kpsi (from a nominal value of about 10 kpsi), or even as low as 10 kpsi.

[0022] Figure 2 is an isometric view of a typical prior art packaging configuration for the optical combiner 10 of Figure 1, and a cutaway side view of the prior art packaging configuration shown in Figure 3. As shown in Figures 2 and 3, the optical combiner 10 is positioned on a substrate 30 formed of a material having a thermal conductivity value sufficient to provide a heat sink function. Aluminum is a typical choice for the substrate 30 because aluminum has a relatively high thermal conductivity (approximately 200 W / mK), although other materials may be used, especially in applications where the heat sink requirements can be relaxed.

[0023] As best shown in Figure 2, the substrate 30 may be formed to include recessed areas 31 to support various regions of the optical combiner 10. In this prior art configuration, the optical combiner 10 is held in place within the recessed areas 31 on the substrate 30 by a pair of adhesive bonds 32, 34 disposed at opposite end terminations of the substrate 30. Bonds 32 are shown at a first end termination 33 of the substrate 30 as being affixed to a portion of the output fibers 13, and bonds 34 are shown at a second end termination 35 of the substrate 30 as being positioned to hold the tightly bundled fibers 14C in place.

[0024] As discussed above, the tapered fiber bundle 12 (particularly the mechanically weakened interface region 18) may be subject to tensile stress as a result of the CTE mismatch between the material selected for the substrate 30 and the glass material of the optical combiner 10. In many applications, materials selected for use as tapered packaging materials are chosen based on having high thermal conductivity values ​​(i.e., good heat sinking capabilities), with the CTE mismatch being a secondary consideration. The specific nature of the thermal stress is a function of whether the ambient temperature of the packaged optical combiner is above or below a "strain reference temperature," which is typically room temperature (and is associated with zero strain). In situations where the ambient temperature is above the strain reference temperature, the substrate 30, which is formed from a high CTE material, will expand relative to the optical combiner 10, thus applying a tensile stress to the packaged optical device. Below the strain reference temperature, the high CTE material will contract, thus applying a compressive stress to the optical fiber device. It is the latter situation, which leads to types of optical signal impairments and mechanical failures, that is addressed and overcome by the disclosed packaging solution.

[0025] Detailed characteristics of the structural changes of optical fiber devices in the presence of compressive stress can be understood by using Euler boundary conditions derived for an "elongated member." In particular, Euler's critical load is defined as the compressive load at which an elongated column suddenly bends or buckles.

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[0026] The effective length factor K relates to whether the ends of the column can rotate or translate. For conditions in which the ends cannot rotate or translate, such as the present case where epoxy joints (e.g., 32, 34) hold the opposing ends in place, the K factor is 0.5 (hence, anything that halves the length L at which the critical load can occur is equal to 0.5). eff = 0.5L. FIG. 4 shows the relationship between the length L of a fixed point between two endpoints. eff FIG. 4(a) shows the onset of two different buckling modes for a column of φ=KL, with each particular buckling mode having its own critical load value. FIG. 4(a) shows the first (fundamental) buckling mode, and the second, higher buckling mode is shown in FIG. 4(b). The maximum displacement (dt) is also shown in both figures. With reference to the fundamental buckling mode shown in FIG. 4(a), it is possible that this type of buckling may be tolerable for a packaged optical device if the surface tensile stresses generated at buckling are less than a predetermined maximum allowable tension. The radius of curvature associated with the first mode shown in FIG. 4(a) is smaller than that of mode 2 shown in FIG. 4(b), and the radius of curvature is an important factor when designing coupling arrangements to address optical coupling issues, such as minimizing the possibility of bend-induced losses when packaged optical fiber devices are exposed to extremely low temperatures.

[0027] Applying Euler's critical load analysis to the conventional packaging arrangement shown in Figures 2 and 3, the length L of the unsupported column is approximately 100 mm long, including all of the individual components of the optical combiner 10, i.e., densely packed fiber assembly 14C, tapered fiber bundle 12, and output fiber 13. Commercially available optical combiners may have lengths L in the range of about 25-100 mm, depending on the physical parameters of the combiner, including the number of individual fibers bundled together. Because the critical load is inversely proportional to the square of the length of the unsupported fiber span, this relatively long span L is approximately 100 mm long, which is approximately 100 mm long. crThis results in a small value of P, which means that even a small shrinkage in package size can generate a load larger than the critical value. When using conventional packages with relatively high CTE values, this P cr The value may limit the temperature range over which the device may be used without bending and associated mechanical problems (including breakage), or optical signal integrity problems such as the onset of bend-induced losses or undesired mode coupling. As is well understood in the art, bending an optical fiber changes its effective refractive index value, which creates the possibility that a certain amount of bending will result in a change in the effective refractive index that aids in coupling to another (undesired) mode. Outcoupling of propagating signals is a known consequence of fiber bending beyond a certain radius of curvature and is a function of the particular fiber design.

[0028] FIG. 5 includes two plots showing the change in unsupported fiber span length (dl) and the associated change in peak transverse fiber displacement (dt) for a variety of different CTE values. Plot (a) shows the change in the length dl required for the onset of buckling for mode 1 and mode 2 as the CTE of the packaging material increases. As shown, an increase in the CTE of the packaging material results in a significant decrease in the allowable length of the unsupported fiber span for both of these buckling modes. The results also show that for relatively low CTE values, the unsupported length associated with the onset of buckling for mode 2 is approximately twice that of mode 1, and the gap between the two lengths becomes smaller as the CTE of the package increases above approximately 1.0e-5. As shown in plot (b), the change in maximum displacement (dt) as a function of CTE is relatively the same for both buckling modes for CTE values ​​greater than approximately 1.0e-5. Unfortunately, there are few (if any) material options for packaging fiber-based optical devices that exhibit this lower range of CTE while also providing the required high levels of thermal conductivity.

[0029] These difficulties of the prior art are addressed by the present invention, which relates to the construction of a packaging configuration for fiber optic devices that utilizes strategically placed bonds (which may or may not be free to rotate and translate). In particular, the individual bonds are positioned such that the unsupported length of the device (i.e., the length between two bond sites) does not exceed a value that can induce bending / buckling of the fiber when the packaged device is exposed to cryogenic temperatures. Since the CTE of a given packaging material is typically greater than the CTE of the optical fiber, contraction of the packaging material at low temperatures will result in a load on the device, and it is desirable to control this applied load against a known critical load value. For the specific case of evaluating bond placement around a mechanically weakened region of the fiber, the goal is to maintain that region in compression. By controlling the placement of bonds in this region, the mechanically weakened region, where only a relatively short span of the fiber optic device is unsupported, can withstand a much larger load compared to the prior art configurations of Figures 2 and 3 before any bending occurs (bending defined as including buckling of the mechanically weakened device region, as well as situations where the bend remains below a known radius of curvature but the maximum surface stress exceeds the strength of the device). With respect to maintaining acceptable optical transmission properties, the length of unsupported fiber must be such that any bends remain below a known radius of curvature where bending losses may occur. With reference to Figure 4, it is clear that the radius of curvature is related to the buckling mode that is excited.

[0030] As will be explained below, the packaging configuration of the present invention is cr and the applied load P ap In this case, the applied load P ap is defined as the compressive load applied to the packaged fiber optic device due to the contraction / shrinkage of the substrate 30 relative to the fiber optic device at the lower end of the device's defined operating temperature range. cr / P apThe ratio of P to BLF can be defined as the "buckling load factor" (BLF), and values ​​of BLF greater than 1 may be preferred to minimize the likelihood of buckling for a particular mode at a particular end condition (i.e., P Cr >P ap As discussed below, the larger the value of the BLF, the more likely it is that unsupported, mechanically weakened regions of a fiber optic device will remain in compression when exposed to extreme changes in operating conditions (e.g., the cold boundary of a defined operating temperature range).

[0031] Thus, in accordance with the teachings of the present invention, sites for bonding are selected and spatially positioned such that each known area of ​​mechanical weakness along the fiber optic device (e.g., interface region 18 of optical combiner 10) is located between pairs of bonds. As a result, the length L of the "unsupported column / fiber" (e.g., interface region 18) is determined such that the unsupported fiber remains compressed (or in the worst case experiences slight surface tensile stress) even when the supporting packaging substrate shrinks to its minimum size associated with the minimum operating temperature allowed for the application of the packaged fiber optic device.

[0032] A simple optical combiner takes the form of a 2×1 combiner, where a pair of input fibers are fused together and tapered to be spliced ​​to an output fiber. For example, each of the input fibers may have an original outer diameter of 125 μm, and the output fiber may also have an outer diameter of 125 μm. Figure 6 plots the buckling load factor (BLF) as a function of unsupported length L for the mechanically weakened hot / cold interface region in this simple 2×1 combiner. Results show that buckling in the first mode (shown as Figure 4(a)) can occur for unsupported lengths L greater than 3 mm. Buckling in the second, higher mode (Figure 4(b)) may not appear until the unsupported length L exceeds 5 mm. The maximum surface tensile stress for buckling in the second mode can be an order of magnitude higher than that associated with the fundamental mode. Similarly, the optical degradation caused by the second buckling mode can be significantly higher than for the first mode. Since the maximum surface tensile stress (and the optical degradation mentioned above) depends on the radius of curvature at the bend, buckling in the fundamental mode may be tolerable, but buckling in the secondary or tertiary modes may cause catastrophic failure.

[0033] FIG. 7 is a simplified diagram of strategic positioning of adhesive bonds to support the mechanically weakened hot / cold interface region 18 with the TFB 12 of the optical combiner 10. For purposes of explanation and clarity, the bonds are shown as "open" elements, and therefore the area underneath the optical combiner 10 is shown. FIG. 7(a) shows an exemplary positioning of a pair of bonds 50, 52 relative to the interface region 18 of the TFB 12 (specifically surrounding the weakened surface WP in region 18). The bonds 50 and 52, sometimes referred to as "internal bonds" (with respect to the location of the external bonds 32, 34), are positioned such that the unsupported fiber length L is relatively short and the interface region 18 remains in compression at the extremely low temperatures associated with the operation of the optical combiner 10. As a result, this short length can withstand a relatively high applied load before buckling (i.e., exhibit a high BLF). Some example calculations are described below. The arrangement of FIG. 7(a) also illustrates the inclusion of couplings 32, 34, which are used to further support the physical attachment of the optical combiner 10 to the metal substrate 30. For example, a preferred embodiment of the optical combiner 10 configuration includes a plurality of four couplings (shown here as couplings 32, 50, 52, and 34, in order) to keep all segments of the assembly straight and under compression at low operating temperatures, thus avoiding mode coupling of the propagating signals at these temperatures. In some optical combiner designs, the desired fundamental mode and all other higher order modes (mainly the LP II It is desirable to keep the mode coupling between the optical fiber and the optical waveguide (modes) to less than 15db, but this is by way of example only.

[0034] Another factor that may be considered in determining the bond configuration is the physical length of the bond attachment that is disposed around the mechanically weakened region of the fiber optic device. The diagram in FIG. 7(b) shows an embodiment that illustrates this bond length factor. In this particular embodiment, the first adhesive bond 50T has a length B that is greater than B. T In this particular example of optical combiner 10, the length B Tmay be selected such that adhesive bond 50T completely covers tapered region 16 (as opposed to the configuration shown in FIG. 7(a) in which adhesive bond 50T covers only a portion of tapered region 16).

[0035] An exemplary configuration of the present invention as shown in FIG. 7 may be referred to as a "four-point" coupling of the optical combiner 10, supporting three adjacent segments of the combiner 10 on a heat sink metal substrate 30. A first segment I is shown in FIG. 7 as corresponding to the output fiber 13 and is supported between the first termination coupling 32 and the first internal coupling 50. A second segment II corresponds to the hot / cold interface region 18 of the TFB 12 and is supported between the internal couplings 50 and 52. A third segment III corresponds to the "cold" region of the optical combiner 10, i.e., a closely packed collection of individual optical fibers 14 is supported between the second internal coupling 52 and the second termination coupling 34. Properties of these three segments are described below, particularly showing the improvement in the BLF of the interface region 18 when supported between the internal couplings 50, 52.

[0036] In one example of the coupling arrangement and size shown in FIG. 7(a), the metal substrate 30 is formed of aluminum, the minimum operating temperature of the optical combiner 10 is defined as −40° C., and the applied load P ap =-37N. Segment I has a length L of 22 mm. I and segments III have the same length (L III =L I ). The interface segment II (including the weakened surface WP) has a length L of 5 mm. II Using the above relationship, P of segment II cr , L II = 5 mm, it is calculated to be -532 N. For these values, assuming the ends are free to rotate, the BLF (i.e., P cr / P ap) is calculated to be 14.48, well beyond the desired condition of BLF>1 to avoid buckling, thus ensuring that the interface region 18 remains in compression (i.e., does not buckle) at this extreme ambient temperature value.

[0037] In contrast, P in segment I cr was calculated to be only −4.3 N, with an associated BLF of 0.11 (P ap (Recall that P has a value of -37N.) The results for segment III are similar, with P cr The load factor was -31.5 N and the BLF was 0.85. Since the load factors of both segments I and III are less than 1, these two segments may potentially buckle at cryogenic temperatures, and it may be desirable to avoid buckling of segments I and III as well, i.e., to minimize the possibility of buckling along any portion of the optical device, e.g., to minimize the possibility of optical failure in terms of bending induced losses or undesired mode coupling. As mentioned above, some prior art solutions to address buckling of fiber optic devices at low packaging temperatures involve the use of substrate metal materials that exhibit a similar CTE to the glass materials of the fiber optic device. For example, a nickel-iron alloy known as Invar has a CTE similar to that of glass (approximately 0.50e -6 / C) is relatively close to 1.5e -6Invar has a CTE of 0.1 / C, thus minimizing the change in size of the substrate as a function of temperature (e.g., minimizing its shrinkage relative to the optical fiber for the lowest temperature values ​​in the operating range). However, Invar does not exhibit the thermal conductivity typically required for packaged fiber optic device applications (i.e., thermal conductivity is 12 W / mK compared to 220 W / mK for aluminum). One approach may be to utilize a hybrid substrate utilizing materials in the low CTE regions of the package (e.g., Invar) and aluminum (or another material with a similarly high CTE and high thermal conductivity) that support the weakened regions relative to the remainder of the package substrate. In another approach, an inset of material with a reduced CTE may be placed within a recess in the aluminum substrate, with the inset located in the weakened region most affected by compressive stresses (e.g., interface region 18, segment II, as shown in Figures 7(a) and (b)). In this case, the allowable length between bonds may be determined based on the CTE of the materials to which the devices are bonded and the desired maximum surface stress that is allowed over the desired temperature range.

[0038] Continuing with the description of specific experimental results based on the parameters defined above, the addition of Invar in segment II increases (decreases in magnitude) Pap at -40°C to -2.2N. The addition of Invar reduces P in segment II. cr value (lower magnitude) to a value of -228 N. As a result, the BLF for the Invar loaded segment II is about 103 for the bonded region that allows rotation and buckling to the fundamental mode, a significant improvement over the value of about 14 associated with the all-aluminum substrate. The BLF is obviously also affected by the length of the mechanically weakened region supported between the pair of bonds. In the above example, the interface region 18 is defined as the weakened region (including the weakened plane WP). For the above example (and with reference to FIG. 6), the segment length L IIwas defined as 5 mm. Figure 8 shows the length L for a variety of different tapered fiber bundles (ranging from a 3 × 1 configuration to a 19 × 1 configuration, all based on a bundle of individual fibers with an outer diameter of 125 μm). II As shown, the plot of BLF vs. L for a 7 × 1 configuration of fiber with an outer diameter of 250 μm is included. II A magnified region of the plot is also shown in Figure 7, where the BLF approaches the unity threshold, L II Values ​​are given on the larger BLF scale.

[0039] It should be noted that the scope of the present invention is not limited to the above-described preferred embodiments, and various modifications and changes may be made by those skilled in the art without departing from the concept and scope of the present invention as defined by the appended claims.

Claims

1. In a packaging structure for a fiber-based optical device, a substrate for supporting the fiber-based optical device, the substrate being formed from a material that contracts relative to the fiber-based optical device at cryogenic temperatures; a plurality of bonds for securing the fiber-based optical device to the substrate, the bonds being positioned along a length of the fiber-based optical device such that an unsupported region of the fiber-based optical device, a length L between a pair of bonds, is restricted from either: (1) bending beyond a radius of curvature at which bend-induced failure occurs; and (2) fracture at a mechanically weakened region along the unsupported region, during operation of the fiber-based optical device over a defined temperature range; A packaging structure comprising:

2. The length L defined above is the critical buckling load P ｃｒ As a function of [0010] is defined by 2. The packaging structure of claim 1, wherein E is the Young's modulus of a glass material of the fiber-based optical device, I is the minimum area moment of inertia of a cross section of the fiber-based optical device, and K is an effective length factor of a column equal to 0.5 for an unsupported length disposed between a pair of fixed points.

3. A buckling load factor (BLF) is used in determining the defined length L, and the BLF is defined as the maximum load P that can be applied as a compressive load to the unsupported region of the fiber-based optical device. ａｐ P for ｃｒ 3. The packaging structure according to claim 2, characterized in that the ratio is defined as:

4. The maximum load P ａｐ 4. The packaging structure of claim 3, wherein: t is related to the maximum shrinkage of the substrate at a minimum temperature value within the operating temperature range.

5. The packaging structure of claim 4 , wherein the defined length L is selected such that the BLF is greater than 1.

6. 2. The packaging structure of claim 1, wherein the substrate comprises aluminum.

7. 10. The packaging structure of claim 1, wherein the fiber-based optical device operates over a temperature range of -40°C to 85°C.

8. The packaging structure of claim 1 , wherein the plurality of coupling portions further comprises a pair of terminal coupling portions located at both ends of the metal substrate.

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

10. 10. The packaging structure of claim 9, wherein the defined length L is selected such that the mechanically weakened region remains compressed over an operating range of the packaged fiber-based optical device below a temperature associated with zero stress on the packaged fiber-based optical device.

11. 11. The packaging structure 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 adjacent the mechanically weakened region.

12. 12. The packaging structure of claim 11, wherein the first material having the lower CTE value has the form of an insert disposed in a recessed region of the substrate adjacent to the mechanically weakened region.

13. The packaging structure of claim 11 , wherein the mixture of at least two different materials includes aluminum and invar.

14. 10. The packaging structure of claim 1, wherein the fiber-based optical device comprises a tapered fiber bundle having a mechanically weakened region at an interface between an input region comprising a plurality of closely packed optical fibers and an output region comprising a fused tapered splice of a plurality of optical fibers.

15. 2. The packaging structure of claim 1, wherein the plurality of couplings are spaced apart by an amount such that each of the unsupported regions of the fiber-based optical device exhibits a radius of curvature that cannot support mode coupling and bend-induced losses at cryogenic temperatures in the defined temperature range.

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