JP2866090B2 - Manufacturing method of single mode fiber optic coupler - Google Patents

Description

The present invention relates to an optical fiber coupler and a method for manufacturing the same.

Certain fiber optic systems require that at least a portion of the light propagating in the optical fiber be coupled to one or more output fibers. The present invention relates to such fiber optic couplers and in particular to a cost effective and reproducible method for making such couplers.

In multi-core devices, it is known that coupling occurs between two cores that are closely spaced from each other. Coupling efficiency increases with decreasing core gap and, in the case of a single mode core, with decreasing core diameter. Many couplers based on these principles have been developed. Such couplers are capable of low loss operation, and they typically exhibit excess loss of less than about 1 dB.

Multi-mode and single-mode couplers position multiple fibers in a juxtaposed relationship along their appropriate lengths, and fuse the claddings of the fibers together to secure the fibers and increase the spacing between cores. It is formed by reducing. U.S. Patent No. 442621
Coupling can be improved by stretching and rotating the fibers along their fused length, as taught in No. 5, but rotating the fiber is useful for certain purposes. Is disadvantageous. Also, U.S. Pat.
As taught in 781, portions of the cladding may be removed by etching or grinding. These couplers must be provided with a hermetic coating, since the coupling area is fragile and exposed to the atmosphere. These methods are labor intensive and therefore expensive, lack long-term integrity, and do not always result in couplers having predetermined desired coupling characteristics. These drawbacks are:
Certain single-mode couplers whose coupling core portions should be in parallel with each other to match the propagation constants, and those single-mode couplers that must have optical properties such as polarization retention. This is particularly noticeable when creating a mode coupler.

While most couplers are made by applying heat directly to the fiber to be spliced, US Pat.
It teaches how to insert a fiber into a capillary that can overlap the end. The capillary is made of glass having a lower refractive index than the fiber cladding material. Heat is applied to the capillary near the portion where the fibers overlap, and the capillary is elongated until its diameter is approximately equal to the diameter of the original fiber. The original core of the stretched portion is so small that it disappears, their elongated diameter is only about 1/100 of the original diameter, and the cladding of the original fiber is Become the core. Such long and thin couplers are very cumbersome and fragile. In addition, this type of coupler is lossy because it replaces the core where the original cladding has disappeared. fiber·
In the region of the coupler, which tapers from a dimension small enough for the core to vanish to a perfect dimension, the amount of power returned from the cladding to the core is insufficient. Moreover, it is difficult to keep the cores straight and parallel to each other when the fiber is inserted into a tube that is drawn unless a specific step for positioning the fiber is performed. Such a non-linear coupler core may reduce the coupling efficiency in a single mode coupler.

Japanese Patent Publication No. 60-140208 discloses a coupler formed by pre-twisting a pair of fibers, inserting them into a quartz tube, and heat stretching the central portion of the tube to reduce its diameter. Has been disclosed. A resin is applied to the end of the tube to seal the fiber. This coupler has the following disadvantages. Upon collapse of the tube against the fiber, the capillaries are not evacuated and thus the fibers are not held in tension. Thus, the fibers meander in the tube, which prevents a predetermined connection from being obtained if the tube is stretched by a predetermined length. This also makes it difficult to reduce coupler loss. The fibers are pre-twisted to obtain contact between the fibers over a length sufficient to provide sufficient coupling. Such couplers cannot maintain the polarization of the input optical signal, and it is difficult to create a wavelength division multiplex coupler with twisted fiber.

Accordingly, one object of the present invention is to provide a method that overcomes the above-mentioned problems of the prior art. Another object of the present invention is to provide a method of making an optical coupler that can withstand environmental harms such as temperature changes and mechanical effects, yet still provide reliable and predictable energy transfer between adjacent fibers. To provide. Yet another object is to provide a low cost and efficient fiber optic coupler that can easily connect fibers in the field such as a shop. Another object is to provide an automated method for making optical couplers whose optical properties closely match predetermined values. Yet another object is to provide a mechanically stiff and inexpensive optical coupler capable of providing reliable and predictable energy transfer between adjacent fibers.

According to the method of the present invention, a glass tube is provided having first and second ends, an intermediate region, and a longitudinal hole extending from the first end to the second end. At least two suitably made glass optical fibers, each having a core and a cladding, are disposed within the longitudinal bore of the tube in a manner extending in both directions from each end of the tube. If the glass fibers have a coating, a portion of the coating between the ends is removed and an uncoated portion of the glass fiber is placed in the longitudinal hole. These fibers are held in tension to create tension.

The interior of the structure thus formed is cleaned by applying a vacuum to one end of the structure and flowing a suitable fluid, such as air or a cleaning solution, through the holes.
The intermediate region of the structure thus formed is heated to squeeze it around the fiber and stretched to a predetermined diameter. Crushing the central portion of the tube may be facilitated by generating a lower pressure in the hole than at the outer surface of the tube prior to the step of heating the tube.

The fact that the fiber is in tension during the crushing process is the primary reason for the low loss of couplers made in accordance with the method of the present invention. Each fiber is secured to a first end portion of the tube, a portion of the fiber extending from the hole is tensioned at the second end of the tube to tension the fiber, and each fiber is attached to the second end of the tube. The anchoring keeps the fiber taut during the crushing process. An adhesive is applied to the fibers to mate them with the first and second tubes.
The fiber can be secured to the end of the tube by sealing to the end of the tube. Applying an adhesive to at least one of the ends of the tube comprises applying an adhesive to a portion of the tube less than the entire perimeter area, leaving an opening between the holes and the adhesive; By making it accessible to its end. Applying the adhesive to the fiber in such a way that the holes are not blocked by the adhesive is facilitated by using a capillary with enlarged and tapered holes at the ends.

Although low loss couplers have been made by squeezing the tube against the fiber and stretching or stretching the middle region of the tube in a single heating operation, it is advantageous to perform these steps separately. More control can be provided for each step if the tubes are cooled before heating for the drawing operation. The central portion of the crushed and solid intermediate region can be stretched, thereby maintaining the stretched portion of the optical fiber completely enclosed within the matrix glass of the tube. This improved hermeticity is beneficial because it prevents the drawn portion of the fiber from being adversely affected by water or the like, a factor that can alter the optical properties of the coupler in a negative direction.

Access to the holes for cleaning or degassing purposes is possible by placing a hollow filament within the end of the tube that is continuous with the two glass fibers before applying the adhesive. After the step of crushing and stretching the middle region of the tube, the stretched hollow filaments are removed and the resulting holes or holes are sealed.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Referring to FIG. 1, there is shown a hollow cylindrical glass tube 10 having a longitudinal hole or hole 12 formed along a longitudinal axis. Tube 10 may comprise a capillary, which may be formed as described in detail below. The tapered holes 14 and 16 form a funnel-shaped entrance to the longitudinal hole 12 at the end faces 18 and 20, respectively. These tapered holes facilitate insertion of the fiber into hole 12 because their maximum cross-sectional dimension is less than 400 μm.

The softening point temperature of the tube 10 must be lower than that of the fiber to be inserted into it. Suitable tube compositions, SiO ₂ and 0.1 doped with 1-25 wt% B ₂ O ₃
A SiO ₂ doped with about 2.5 wt% fluorine. Preferred compositions are Borokeriketo glass made of SiO ₂ doped with 8-10 wt% B ₂ O _3. B ₂ O ₃ and F are advantageous because they lower the softening point temperature of SiO ₂ and also lower its refractive index.

Referring to FIG. 2, a pair of optical fibers 22 and 24, each having a core, a cladding, and a protective coating, extend through the longitudinal bore 12, each fiber having a length sufficient to connect with the tube 10. It extended from each end of the tube, and a length of 1 m was found to be sufficient. A portion of the coating between the ends of each fiber is removed a distance slightly less than the length of the hole 12. The fibers are wiped to remove residual material. An uncoated portion of the fiber is located between end faces 18 and 20 of tube 10. Preferably, the uncoated portions of fibers 22 and 24 are centrally located in bore 12 in the longitudinal direction. For convenience of illustration, the fibers 22 and 24 are shown as thin lines inside the hole 12 and bold lines from inside the end of the hole to outside the tube 10. The thin lines indicate the uncoated portions of those fibers, and the thick lines indicate the coated portions. FIG. 2a is an enlarged sectional view of one end of FIG. 2 showing the coatings 23 and 25 on the optical fibers 22 and 24.

The fibers may be arranged parallel to one another in the bore 12 or may be twisted by more than 180 ° as shown in FIG. Twisting the fibers is a known technique used to hold the fibers in contact with each other during the process of fusing the fibers. If a vacuum is used when squeezing tube 10, twisting the fiber is not important, as the vacuum helps to keep the tube squashed and the fiber in longitudinal contact with each other. In some couplers, such as WDM couplers and polarization maintaining couplers, the fibers must be kept untwisted and kept parallel to each other.

The assembly of fiber 10 and the fiber extended therein is preferably subjected to a final cleaning step before crushing tube 10 and fusing the stripped fiber portions together. This cleaning step is important because small particles of coating material and other contaminants can remain on the uncoated portion of the fiber after the fiber is inserted into the tube 10. This cleaning step may comprise flowing a cleaning fluid into the aperture 12 and over the stripped portions of the fibers 22,24. The cleaning fluid may be a liquid such as a 30% ammonia solution or a gas such as air. Further, it is preferred that the fiber be held in tension during the tube crushing step. Various techniques can be used to perform the steps of cleaning the holes and tensioning the fiber. The following examples describe preferred techniques among them.

In a first embodiment, a pair of optical fibers 22 and 24, each having a core, cladding and protective coating, are suitably prepared by removing the portion of the coating between their ends. The uncoated portion is wiped with a lintless cloth to remove residual material. Fibers 22 and 24
Is fed through the longitudinal bore 12 and extends from each end of the tube 10 by a suitable length for connection purposes. The uncoated portions of these fibers are located between the end faces 18 and 20 of the tube 10 as shown in FIG.

For some couplers, the fibers 22 and 24 are about 180 ° in the longitudinal bore 12 as shown in FIG.
Only can be twisted. A hollow glass filament 26 is inserted into the end of the tube 10 and extends into the hole 12 a short distance. The fibers 22, 24 and the filament 26 are secured to the end of the tube 10 by applying a predetermined amount of adhesive 30 in and around the tapered hole 14. This process is repeated by inserting a second hollow glass filament 28 into longitudinal bore 12 and applying a predetermined amount of adhesive 32 into and around bore 16. When the adhesive 32 is cured, a slight tension is applied to the fibers 22, 24. Adhesives 30, 32 can be made of any adhesive material, such as cement, but are preferably ultraviolet curable epoxies.

The structure thus formed is mounted on a suitable mounting device or holder 34 such as a chinner clamp. Hollow filament 28 may be connected to a suitable vacuum source (not shown) as indicated by arrow 36. Alternatively, a tube connected to a vacuum source may be positioned around the end of the capillary tube 10 such that the hollow filament and fibers 22, 24 extend into the evacuated tube. When the hollow filament 26 is inserted into the liquid cleaning fluid, the fluid is drawn through the longitudinal hole 12 by the vacuum applied to the hollow filament 28, thereby causing the interior of and within the longitudinal hole 12 to enter. The placed fibers 22, 24 and the hollow filaments 26, 28 are cleaned.

If a liquid cleaning fluid is used, the intermediate region 38 of the structure thus formed will evaporate the liquid and dry the structure as shown in FIG. Heated by a suitable heat source. This heating step is not necessary if a gas is used as the cleaning fluid.

According to one embodiment of the present invention, the tube 10 is heated, crushed against the fibers 22, 24, and then the intermediate region of the tube 10 is heated and drawn to form a type of coupler. Bring the cores of the fibers closer together. This is achieved by heating the intermediate region 38 to the softening point of the borosilicate glass tube 10 by a heating source 40 such as an oxygen / hydrogen burner or a gas / oxygen burner. The heating source 40 may be stationary or may move through the intermediate region 38 in a direction toward the vacuum source 36, as indicated by the arrow 41 in FIG. Applying a vacuum source to both hollow filaments 26, 28 is an optional feature of the tube crushing process, in which case the direction of burner travel does not matter. The step of subjecting the intermediate region to a heating source comprises, as shown in FIG.
Acts to crush around 22,24. FIG. 6 shows the intermediate region 38 of the tube 10 crushed around the fibers 22, 24, along the lines 6-6 in FIG. The portion called the intermediate region 38 is a solid region, and it is preferable that there are no air lines or bubbles in this region.

The structure thus formed is removed from the holder 34 and mounted on a precision glassworking lathe indicated by the members 42, 44 in FIG. Solid middle area 38
Is exposed to a flame from an oxygen-hydrogen burner until a portion of it is heated to its softening point. When the entire intermediate region is stretched, the end portion of the optical coupling region of the fiber can be exposed to the hole. Stretching only the central portion of the crushed intermediate region ensures that the bonding region of the fiber is embedded in the matrix glass of the capillary. The flame is removed and the softened portion of the intermediate region 38 is drawn and stretched under the action of the glassworking lathe to reduce its diameter as shown at region 48 in FIG. The diameter of the stretched region 48 will vary as a function of the various fibers and operational parameters. Reduced diameter of region 48 and intermediate region
38 ratio to the initial diameter (draw down ratio)
Is determined by the optical properties of the particular device being created. Such draw ratios include the ratio of the signal split between the fibers, the refractive index difference between the tube and the cladding of the fiber,
It is a function of the outer diameter of the fiber cladding, the diameter of the fiber core, the operating wavelength of the signal, the cutoff wavelength, the allowable excess loss, and the like. The preferred range of draw ratio is from about 1/2 to 1/20, but couplers having draw ratios outside this range can be made.

As shown in FIG.
The portion of the tube 10 held by is secured and the portion of the tube 10 held by the lathing member 44 is moved in the direction of arrow 50 to form an elongated region 48.
In practice, such pulling or stretching takes about 1/2 second. In another stretching technique, the laminating member 42 is moved in the same direction as the moving direction of the member 44 or in a direction opposite to the moving direction of the member 44.
It is done to move.

The assembly does not need to be rotated if the extension of the intermediate region 38 is heated by a ring burner that heats uniformly around it. The stretching method will be otherwise the same. In embodiments using a ring burner, the step of crushing the tube 10 against the fibers 22 and 24 and the step of forming the drawing region 48 can be performed in the same apparatus. If the crushing step and the stretching step are performed in the same apparatus, the tube 10 is preferably cooled before being heated again for the stretching step. By separating the two steps in time in this way, better process control and thus better reproducibility is obtained. further,
Tube 10 may be arranged in any orientation, including vertical and horizontal, during the crushing and / or stretching steps.

After drawing, the exposed ends of the hollow filaments 26 and 28 are removed by separating them from the surfaces of the adhesives 30, 32, and their open ends are filled with a predetermined amount of adhesive as described above. Sealed by 54, 56. The structure thus obtained constitutes the fiber optic coupler 52 shown in FIG. The coupler may be further provided with a package (not shown) for increasing rigidity as required. The coupler 52 connects the optical fiber signal to the optical fiber 24.
And vice versa.

According to the above embodiment, the crushing step and the stretching step are performed separately. Advantageously, if the tube is cooled prior to heating for stretching, more control is added to each step. The central portion of the crushed solid intermediate region is stretched so that the stretched portion of the optical fiber can be completely confined within the matrix glass of the tube. This improved hermeticity is advantageous because it prevents the stretched portion of the fiber from being adversely affected by moisture and the like, which can change the optical properties of the coupler in the wrong direction.

Other embodiments in which the step of crushing the tube against the fiber and the step of stretching the middle region of the tube are performed in a single heating operation also produced low loss couplers. According to this modified embodiment, the fiber is inserted into the capillary and is held taut by being glued to the end of the capillary such that there is an opening to access the hole. This structure is mounted on a precision glass working lathe as described above. A flame is applied to a small portion of the intermediate region until the softening point of the material is reached, and the heated portion is stretched. For some bonding, the degree of stretching of the tube is greater in this embodiment than in the previous embodiment where the tube squeezing step and the intermediate area stretching step are performed separately. Finally, an adhesive is applied to the end of the device to close the opening to the hole.

The disadvantage of this embodiment is that the hermeticity is reduced and the production reproducibility is adversely affected, i.e. stretching to a predetermined length does not always produce the desired engagement characteristics. However, this embodiment has several advantages over other methods. This method is simpler because it can be performed without a vacuum and does not require a separate tube crushing step. This method produced a low loss device with a measured loss of the device as low as 0.05 dB at 1300 μm.

In yet another embodiment, a hollow fiber is used at only one end of the tube 10. Such an embodiment is:
Coupler except that the internal cleaning process described above is impractical.
It is the same as that forming 52.

In the embodiment of FIGS. 8 to 10, the hollow fiber has been completely removed and elements similar to those of FIG. 2 are indicated by the same reference numerals with dashes.

The initial steps required to form the embodiment of FIG. 8 are the same as those used to form the embodiment of FIG. Filaments similar to filaments 28, which may be hollow or solid, have holes 1 as shown in FIG.
2 'and tapered hole 1
It is inserted through 6 '. After an uncoated portion of the fibers 22 ', 24' is centered in the hole 12 ', a predetermined amount of adhesive 58 is applied in and around the tapered hole 16'. As shown in this figure, the adhesive advantageously extends into hole 12 'a distance sufficient to contact the cladding of fibers 22', 24 '. When the adhesive 58 begins to harden, holes 59 through which the adhesive 58 cannot flow and cannot be closed by removing excess filaments.
Viscosity is high enough to remain. A hole similar to hole 59 may be formed in the adhesive located at the opposite end of tube 10 '. A vacuum mounting tube may be placed around the end face 20 'of the tube 10' as described above to clean and / or evacuate the hole 12 '.

In the embodiment of FIG. 9, the adhesive 61 completely seals the tapered hole 16 '. A radial hole 62 near the end 20 'of the tube 10' is a hole 1 for cleaning and / or degassing purposes.
Allows access to 2 '. Hole 12 'is tube 1
By attaching an annular vacuum fitting 64 to 0 ', the air is evacuated through a hole 62, which opens into an annular slot 63. Arrow V indicates that a vacuum source is connected to fixture 64. A radial hole similar to hole 62 is provided at the opposite end of tube 10 'for the purpose of supplying cleaning fluid or gas to hole 12' and / or evacuating hole 12 '. Can be formed.

In order to always produce a coupler with predetermined optical properties in the coupler manufacturing process, all steps, including inserting the fiber into the capillary, must be performed uniformly for each coupler to be produced. No. A preferred fiber insertion method for improving process uniformity, together with the method of forming the embodiment of FIGS. 10 and 11, will now be described. It is advantageous to use a fiber insertion station that meets the following criteria: The mechanism for holding the fiber must be properly aligned, since the fiber is preferably kept twisted and straight. Means are provided for holding the fiber in a slight tension during the adhesive application step to prevent the fiber from sagging or sagging during subsequent processing steps, particularly during the step of crushing the capillary against the fiber. There must be. If any one or both fibers become slack, the resulting device may exhibit excessive losses. The area around the station must be free of excess dust and other particulates that are drawn into the capillary and accumulate therein. Because
This is because seeds can be generated from such particulate matter during the crushing step and the redrawing step. Excessive attenuation that can result from such a seed renders the coupler unusable.

A suitable fiber insertion station is shown in FIG. 12, which comprises aligned blocks 67, 74, 76, 79, 82 and 83. Rubber surface clamps 70, 71 can hold the optical fiber against block 67. Similar clamps 84, 85 are associated with block 83. A spring-biased clamp abutting the block can be released from contact with the block by depressing a handle coupled thereto. The block 74 is aligned with the grooves 80, 81 of the block 82 and is spaced apart from the groove 72,
Has 73. A single groove 75 in the surface of the block 76
Are aligned with similar grooves in block 79. The illustrated groove may be U-shaped and have just enough width to slide and accommodate one or more fibers disposed therein.

A length of coated optical fiber 22 ', 24' is cut from the fiber reel. Each end of these fibers 22 ', 24' is secured by clamps 70, 71, respectively. These fibers are generally wiped with a lintless cloth moistened with a suitable cleaning solution such as ethyl alcohol.

Capillary tube 10 'is selected, and its aperture is preferably just large enough to receive the coated portion of the optical fiber. Such a relationship between the coated fiber and the hole prevents the fiber end from twisting within the tube. As shown in FIG. 11, a hole cross-section such as a diamond, square, etc., facilitates proper alignment of the fibers within the tube. The hole diameter must not be so small that it is difficult to pass the fiber through it. This is because such a small size may cause the coating to be applied to the inside of the tube. Such a swabbed area of the tube can cause the coupler to have a seed that degrades its performance. To make the tube easier to move along the fiber,
A small amount of ethyl alcohol can be injected into the tube. It acts as a temporary lubricant and evaporates immediately. The capillary is passed over the fiber and moved about the position shown adjacent block 76. The fiber is pulled slightly to have some tension,
And their ends are held by clamps 84,85. A mechanical stripping tool is used to remove a portion of the coating from each fiber at a location between tube 10 'and block 79. The length of the stripped portion of the fiber is slightly less than the length of the hole in the capillary, and the coating can be extended at both ends of the hole, thereby properly positioning the fiber within the cross section of the hole. The lengths of the stripped areas must be approximately equal and must be adjacent to each other.

Using a moistened lintless cloth, the two fibers are grasped and wiped evenly at the left end of tube 10 ', where the cloth is moved across the bare area and away from the tube. This step removes debris from the stripping step, leaving a clean surface in bare areas of the fiber.
The fibers are then placed in grooves 75 and 78, which hold the fibers in a straight and adjacent relationship to each other. Clamp 84 is released and fiber
After 22 is nervous again, it is tightened again. In this case, the fiber 24 'is likewise re-tensioned.

The capillary is moved toward block 79 and is centered over the bare area shown in FIG. On the sides of the fibers 22 'and 24', a small amount of adhesive 87 is applied to adhere the fibers to one side of the tapered hole 16 ', where the adhesive 87 and the tapered hole 16' are applied. ', An opening 88 is provided to allow access to the longitudinal bore 12'. An adhesive drop 89 is similarly applied between the fiber and its tapered hole 14 ', leaving an access opening 90 between it and the tapered hole 14'. Tube tapered, depending on capillary hole size
Without 14 ', 16', it would be difficult or even impossible to glue the fiber to the end of the tube without closing the hole. The openings 88, 89 allow fluid to flow through the holes 12 'during the last wash and allow the holes 12' to be evacuated when the tube 10 'is crushed. If the adhesive is a UV curable resin, a UV light source 86 irradiates the first applied drop of epoxy to cure the first drop before the second drop is applied to the remaining end. After the second drop has been applied, the UV source 86
Is moved as indicated by the arrow and illuminates the second drop.

The pigtail of the fiber extending from the end of the tube 10 'may be color coded. In that case,
The fiber in the capillary is visually checked for internal twist. A twist of 180 ° or more is visible to the naked eye. Also,
The laser beam may be incident on the end of the fiber 22 ′ protruding from the clamp 84. If no twist is present, light will come out of the end of the fiber protruding from the clamp 70. An orientation mark may be placed on the top surface of the tube 10 'so that the fiber can be oriented in the same manner with respect to the drawing device for each coupler to be made, so that each coupler preform has uniform processing conditions.

A preferred apparatus for performing the tube crushing and stretching steps is shown in FIG. The chucks 92, 93 used to secure the coupler preform to the device are mounted on a motor controlled stage, which is preferably controlled by a computer. The numbers 92 and 93 also indicate those stages. Symmetry is an important requirement for the squash and draw steps, so the chucks 92, 93 have a negative effect on equipment loss and are dependent on the bidirectional nature of the coupler, i.e., which end of the fiber is selected as the input port. However, the coupler output must be aligned to prevent the coupler from having an offset, which can adversely affect the characteristic that the output characteristic is substantially uniform. The bidirectionality of the coupler is also improved by centering the burner along the coupler preform so as to heat the preform uniformly. A burner designed symmetrically, such as a ring burner 94, is suitable for uniformly heating the middle region of the capillary. A heat shield 95 protects the device located above the burner.

The coupler preform 91 shown in FIG. 10 is inserted through the ring burner 94 with the alignment mark directed in a predetermined direction. The preform is clamped on a stretching chuck, and vacuum attachments 96, 101 are mounted on its ends.
The vacuum attachment 96, shown in cross-section in FIG. 10, comprises a short, somewhat rigid rubber tube having a vacuum line 97 extending in the radial direction. One end of a thin rubber tube 98 is attached to the end of the vacuum attachment opposite the preform 91, and the other end of the tube extends between the clamp jaws 99.
Similarly, the upper vacuum attachment 101 has a line 102, a tube 10
3 and clamp jaws 104. Fibers 22 ', 24' extend from tubes 98, 103.

A vacuum is applied to the lower portion of the coupler preform 91 for a time sufficient to clean the hole 12 'by the clamping jaws 99 on the tube 98. Clamp jaw
By keeping 104 open, the upper line is vented to atmosphere during this time. This “air cleaning” pulls out the debris accumulated during the fiber insertion step from the hole 12 ′. In that case, jaws 104 are clamped against tube 103 to apply a vacuum to the upper portion of preform 91.

The capillary squeezing process requires heating the coupler preform with a flame from the ring burner 94 for a short period of time, typically on the order of 25 seconds, to raise the temperature of the middle region of the tube to the softening temperature. . Combined with the action of the differential pressure on the tubes, this causes the matrix glass to be squeezed onto the fibers and the fibers to come into contact with each other. The matrix glass of the tube surrounds the fibers and fills the holes to form a solid structure, eliminating air lines in the coupling area. The holes are preferably evacuated from both ends during the crushing step. The length of the area to be crushed is determined by the flame temperature, which is determined by the flow of glass to the burner, and the duration of the flame.

The central portion of the crushed intermediate region of the tube can be stretched without removing the structure from the device in which the tube is crushed. After the tubes have cooled,
The flame is ignited, and the center of the crushed area is reheated. The duration of the flame for the drawing process depends on the desired coupler properties and is usually between 10 and 20 seconds. Such a short heating time for the stretching step results in a stretched area that is shorter than the crushed area. After the crushed tube is reheated, stages 92, 93
Pulls in the opposite direction until the coupler length is increased by a predetermined amount. Appropriately aligned equipment is used,
And if the process parameters are carefully controlled, all couplers formed by this process will have similar optical properties.

The degree of stretching that must be imparted to the capillary to obtain a given type of coupler depends on one of the crushed coupler preforms.
It is first determined by injecting light energy into two incoming fibers and monitoring its operation. To this end, one of the fiber pigtails is aligned with the light source, and at the other end of the device both pigtails are coupled to a photodetector. A predetermined ratio of the dynamic output power can be used as an interrupt to stop pulling the sample to stages 92,93. After determining the appropriate stretching distance to obtain the desired bonding properties, the device:
The stage can be programmed to be moved by an appropriate extension distance when a coupler having the predetermined characteristics is created.

As shown in U.S. Pat.Nos. 4,932,712, 4,726,664, British Patent Application GB 2183866A and International Publication No.WO 84/04822, output signals are used to control process steps during the fabrication of optical devices. Monitoring has conventionally been performed. Further, a computer is often used in a feedback system that automatically performs such monitoring and control functions. To perform these functions, an appropriately programmed PDP 11-73 microcomputer may be utilized. The timing sequences used in creating a particular type of coupler can be placed in a separate multiple command file that the computer recalls at run time. The crushing and stretching steps required to make such a particular type of coupler are:
It can be run continuously by a computer for each coupler preform to produce a coupler with reproducibility. Process parameters that can be controlled by a computer to ensure coupler reproducibility are heating time and temperature, gas flow rate, and the rate at which the stage pulls and stretches the coupler preform. The reproducibility is also a function of the resolution of the stages 92 and 93.

After the coupler has cooled, the vacuum line is removed from the coupler and a drop of adhesive is applied to each end of the capillary,
It at least partially flows into the longitudinal bore. This forms a hermetic seal and increases the tensile strength of the device. There the coupler can be removed and packaged.

Although the above description has been directed to couplers made with pairs of optical fibers, it will be apparent that the invention is also applicable to couplers made with more than one optical fiber.

The examples below utilized glass capillaries formed in the manner described in US patent application Ser. No. 082679. U.S. Patent No.
Re.28029, 3884550, 4125388 and 428697
Glass particulate material was applied to a cylindrical mandrel, consolidated, stretched, and dried according to the teachings of No. 8. More specifically, the particulate material was deposited on a mandrel to form a cylindrical porous preform. The mandrel is removed, the porous preform is fused and solidified to form a tubular glass body, and the glass body is heated and stretched again to about
The outer diameter was 2.8-3mm. One end of the capillary thus obtained was attached to a pneumatic source and, while the tube was being rotated, a flame was emitted at a predetermined interval into the tube. The air pressure in the tube caused bubbles to form in each area of the tube softened by the flame.
The tube was scored at the center of each bubble and cut at the line of each score to create a capillary with a tapered hole at each end. U.S. Patent Application No. 082679 discloses a method of shrinking a tube over a carbon mandrel having a desired cross section and then burning out the mandrel to form a hole with a non-circular cross section.

Embodiment 1 This embodiment will be described with reference to FIGS. Capillaries 10 (hereinafter referred to as tubes) were formed as described above, which had an outer diameter of about 2.8 mm, a hole diameter of 400 μm, and a length of 5.1 cm. Tube 10 is 8% by weight B ₂ O ₃
Formed from borosilicate glass. 125 outside diameter
2 μm single-mode optical fibers about 2 m each
Cut to length. Each fiber consists of a core, a cladding, and a urethane acrylate resin coating. Approximately 3.8 cm of the center of the fiber using a commercially available mechanical stripper
(11/2 inch) removed the resin coating.

The uncoated portions of those fibers are wiped with a lintless cloth to remove residual material, and the fibers are pulled through longitudinal holes 21 until the uncoated portions are centered. Can be A hollow glass fiber 28 with an outer diameter of about 125 μm is about 0.3 to 0.6 cm (1/8
〜1 / 4 inch) was inserted into the end 20 of the tube. A predetermined amount of Norland UV curable adhesive was placed in the tapered hole 16 around the three fibers and cured by exposure to UV light for about 1 minute. Thus, the optical fiber and the filament 28 were firmly fixed to the end of the tube 10.

The two optical fibers were twisted 180 ° in the hole 12 and a second hollow fiber 26 was inserted into the other end of the hole 12 by about 0.3-0.6 cm. A slight tension was applied to the two fibers and a drop of UV curable adhesive was applied to the fibers in the tapered hole 14. The adhesive was cured as described above. The assembly formed in this way is, when the coupler assembly is mounted, in the intermediate region.
Attached to the modified chiner clamp by cutting off the center and one end of the clamp area so that 38 and one end 18 were exposed. A tube connected to a vacuum source was connected to one end of the capillary, and an optical fiber and a hollow filament were placed in the evacuated tube. In this way, the longitudinal holes 12 have been degassed through the hollow filaments 28. The hollow fiber 26 was inserted into a 30% ammonia solution beaker. The ammonia solution was drawn into the holes 12, thereby cleaning the holes and the outer surface of the optical fiber for about 10 seconds. The hollow fiber 26 was removed from the cleaning solution beaker. After as much liquid as possible has been removed from the hole 12 by the vacuum source, the tube 10
A burner was directed at the tube 10 for about 20 seconds to help dry the interior of the tube.

The intermediate region 38 of the tube 10 was heated by the oxygen / hydrogen burner to the softening point of the borosilicate glass, which crushed the glass around the optical fiber in the longitudinal hole. As the flame was moved through the intermediate region in the direction of the vacuum source and the tubing material was crushed around the optical fiber, the residual material in the longitudinal holes was sucked out by the vacuum. In this way, a solid intermediate area without air lines or bubbles was formed.

The structure formed in this way is modified
Removed from clamp and mounted on precision glassworking lathe. The lathe is a Heathway with a computer controlled pull down or drone down mechanism.
y) It was a glass processing lathe. A flame from an oxygen / hydrogen gas burner is applied to a small portion of the solid intermediate region until the softening point is reached, and a computer-controlled pull-down device stretches the heated portion for a period of about 0.5 seconds. I let it. The diameter of the pulled-down portion was about 0.7 mm.

Thereafter, the hollow filaments 26, 28 were cut and a UV curable adhesive was applied to the end of the device to cover the resulting holes. This assembly was then packaged in a stainless steel tube for strength. The signal loss measured for a coupler made in this way is typically 0.05-0.7 dB at a wavelength of 1300 μm
Was in the range. This resulted in a 50:50 signal split in the fiber with a 1200 μm cutoff wavelength.

Example 2 A capillary 10 of the type described in Example 1 was provided. Two single mode optical fibers of the type described in Example 1 were made according to that example. After the uncoated portion of the fiber had been wiped, it was pulled through the longitudinal hole 12, where the uncoated portion was approximately centered within the hole. A predetermined amount of a no-land UV curable adhesive was carefully applied between the fiber and one side of the tapered hole 16, leaving a small opening in the hole 12. The adhesive was cured by exposure to ultraviolet light for about one minute. Thus, the optical fiber was firmly fixed to the end face of the tube 10. After some tension was applied to the two fibers, a drop of UV-curable adhesive was carefully applied and cured as described above to secure the fibers to the tapered holes 14.

The structure formed in this way was mounted on the precision glass processing lathe of Example 1. A flame from an oxygen-hydrogen gas burner was applied to a small portion of the intermediate region until the softening point was reached, with a computer-controlled pull-down device stretching the heated portion for a period of about 0.6 seconds. The extent of the capillary stretching was about 4 cm.
Was about twice the degree of tube stretching required.
The diameter of the stretched portion was about 0.4 mm. A UV curable adhesive was applied to the end of the device to close the opening of the hole.

The device formed by this method served as a 3dB coupler. That is, a 50:50 signal split occurred.
The measured signal loss for these devices was 1300 μm
It was 0.05 dB.

Embodiment 3 Using the apparatus shown in FIGS. 12 and 13, a single mode
The following steps were performed to create a 3dB coupler.
Again, reference is made to the coupler preforms of FIGS. 10 and 11. Two predetermined lengths 22 'and 24' of coated single mode optical fiber were cut from a reel of fiber.
The optical fiber had a diameter of 125 μm and the diameter of the coated fiber was 160 μm. Each fiber strip was approximately 2 meters long. The ends of these fibers were secured by clamps 70 and 71, and the fibers were wiped with a lintless cloth moistened with ethyl alcohol.

The capillary 10 'had an outer diameter of about 2.8mm and a length of about 4.12cm. The holes in the longitudinal direction are diamond-shaped, and the length of each side is about 31
It was 0 μm. Tube 10 'is formed of borosilicate glass containing 8 wt% B ₂ O _3. The minimum cross-sectional dimension of the diamond-shaped hole was large enough to receive the coated portion of the optical fiber in the manner shown in FIG. A small amount of ethyl alcohol was injected into the capillary and the tube was passed over the fiber and moved to approximately the position shown in FIG. The fiber was slightly taut and the other end was clamped. A length of about 1.25 inches (3.2 cm) of coating is removed from each fiber at a location between tube 77 and block 79. The length of the stripped portion of the fiber was slightly shorter than the length of the capillary bore. Again, the two fibers were wiped with a lintless cloth moistened with ethyl alcohol to remove debris from the coating removal process. These fibers were placed in grooves 75, 78, and were tautened and restrained by clamps 84, 85.

The tube 10 'was centered over the bare area as shown in FIG. 10 and the fiber was secured to the end of the tube using Dymax 911 UV curable adhesive as described above. . A small amount of adhesive 87 was carefully applied to one side of the fibers 22 'and 24' at each end of the tube, ensuring that the openings 88, 90 were present. The adhesive was exposed to a Dymax PC-3 UV light source for 30 seconds at each end of the tube. The fiber pigtail extending from the coupler preform was color coded. In this case, the fiber in the capillary was visually checked for twist. Also, the fiber protruding from the clamp 84
A beam of HeNe laser light was incident on the end of 22 '. Light from the other end of the fiber indicated that there was no partial twist. An alignment mark was placed on the upper surface of the tube 10 '.

The coupler preform 91 was inserted through the ring burner 94. The end of the preform was fixed to the chucks 92 and 93 with the alignment mark facing the operator. Vacuum attachments 96, 101 were attached to the ends of the preform as shown in FIG. Jaw 99 was clamped onto tube 98 to apply a vacuum to the lower portion of the coupler preform with the upper end vented. This "air wash" lasted about 30 seconds. Jaw 104 was clamped against tube 103 to apply a vacuum adapted to stabilize at approximately 12 inches Hg to the upper portion of preform 91.

The ring burner was turned on for about 25 seconds to raise the temperature in the middle region of the tube to the softening temperature of the borosilicate glass. This crushed the tube along the approximately 0.6 cm length of the tube onto the fiber. After the coupler preform had cooled for about 30 seconds, the flame was re-ignited and the crushed area was reduced to about
It was reheated for 16 seconds. Stages 92 and 93 moved in opposite directions to increase the length of the coupler by about 1.1 cm. All process steps performed in the tube crushing and stretching steps were performed under the control of a PDP 11-73 microcomputer.

After the coupler had cooled, the vacuum line was removed from the coupler and a drop of Dymax 304 adhesive was applied to each end of the capillary and exposed to ultraviolet light for 30 seconds. Next, the coupler was removed from the stretching device.

In this way, 3d operating at a given wavelength, such as 1300nm
A B coupler was obtained. The intermediate excess equipment loss was about 0.3 dB, and the lowest measured loss was 0.01 dB.

[Brief description of the drawings]

FIG. 1 is a sectional view of a glass tube suitable for the purpose of the present invention, and FIG. 2 is a first view in which a pair of optical fibers is disposed.
2a is a cross-sectional view of one end of the tube of FIG. 2, FIG. 3 is a cross-sectional view showing another step in the method of the present invention, and FIG. 4 is another cross-sectional view of the method of the present invention. FIG. 5 is a sectional view showing a state in which a glass tube is crushed around a fiber to form a solid intermediate region, and FIG. 6 is a sectional view taken along line 6-6. FIG. 7 is a cross-sectional view of the fiber coupler of the present invention after being stretched and the ends sealed, and FIGS. Sectional view showing another way of making it accessible to
FIG. 11 is a cross-sectional view showing another method of making the tube hole accessible and a method of degassing the tube, FIG. 11 is a cross-sectional view taken along line 11-11 of FIG. FIG. 12 is a schematic view of an apparatus for inserting a fiber into a tube, and FIG. 13 is a schematic view of an apparatus for crushing a tube and extending an intermediate region thereof. In the drawing, 10 is a tube, 12 is a longitudinal hole, 22,
24 is an optical fiber, 23 and 25 are coatings, 26 and 28 are hollow glass filaments, 30 and 32 are adhesives, and 38 is an intermediate region.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02B 6/28

Claims (10)

(57) [Claims] A first and second end and an intermediate region (38).
And a glass tube (1) having a longitudinal hole (12, 12 ') extending from the first end to the second end.
0, 10 ') and at least two glass optical fibers (22, 24; 22', 24 ') each having a core and a cladding, in said longitudinal holes (12, 12'). Arranging the tube (10, 10 ') to extend through an intermediate region (38); heating the intermediate region (38) of the tube (10, 10') A heating step of squashing a tube onto the fibers (22, 24; 22 ', 24'); and a stretching step of stretching the resulting coupler preform (91) to reduce the cross-sectional area of its central portion. A method of manufacturing a single mode fiber optic coupler (52), wherein in the heating step of heating the intermediate region (38) of the tube, the holes (12, 12 ') are evacuated to reduce a pressure difference and a heat difference. Due to the superposition effect, the intermediate region (38) of the tube is Squeezing over the fiber (22, 24; 22 ', 24') and performing the stretching step after the tube is crushed, stretching only the central portion of the middle region of the tube to reduce its diameter A method for manufacturing a single-mode fiber optic coupler, comprising: 2. The method according to claim 1, wherein the thickness of the tube is greater than the diameter of the fiber. 3. Prior to the drawing step, the tubes (10, 10 ') are cooled and an intermediate region (3, 10) of the tubes is cooled.
The method according to claim 1, further comprising a reheating step of reheating at least the central part of (8). 4. The method of claim 3, wherein said reheating step is shorter in time than said heating step. 5. A vacuum attachment (96, 101) mounted on at least one end of said tube (10 ') to evacuate said hole (12'). 3. The method according to 3. 6. A hollow glass filament is inserted into said hole (12) so as to extend from one end of said tube and an adhesive (32) is applied to said fiber (22, 24) and said hollow filament. 4.) The method of claim 1, 2 or 3 wherein the first and second ends of the tube are glued to the first end of the tube and the holes are evacuated through the hollow filament. 7. The method of claim 1, wherein the step of heating comprises radiating a flame to one end of the intermediate region and moving the flame along the intermediate region. The described method. 8. The at least two glass fibers (2)
2, 24; 22 ', 24') coated on itself (23, 25; 23 ', 2
5 ') removing a portion of the coating from a respective portion of the fiber; and removing the uncoated portion of the fiber from the respective portion into the longitudinal bore of the tube. And placing. The method of claim 1, further comprising: 9. The step of disposing each stripped portion of said fiber in said longitudinal bore of said tube comprises the step of placing each of said fibers (22, 24; 22 ', 24'). Securing to said first end of said tube (10, 10 '); and said fiber extending from said bore (12, 12') at said second end of said tube. 9. The method of claim 8, comprising the steps of pulling a portion and securing each of said fibers to said second end of said tube. 10. The ratio of the diameter of the elongated portion of the intermediate region to the initial diameter of the tube is between about 1/2 and 1
A method according to one of claims 1 to 9, which is between / 20.