Patent Publication Number: US-2004047553-A1

Title: Injection device for optical fibre and preparation method

Description:
[0001] The present invention rel+ates to optical fibers.  
       [0002] More specifically, the present invention relates to an optical injection device for an optical fiber, that is to say a device designed to inject an optical signal into a fiber.  
       [0003] The present invention may be applied especially in the production of lasers or optical amplifiers.  
       [0004] The production of a high-power monomode fiber laser or a high-gain fiber amplifier requires high-power pump lasers. These are generally semiconductor diodes. In particular, multimode diodes are known. However, the characteristics of the output beam of the latter do not allow satisfactory optical coupling into a monomode fiber core. Thus, at the present time only a single monomode pump diode can be effectively coupled into the monomode core of a fiber.  
       [0005] Fiber lasers and fiber amplifiers are consequently power-limited, or gain-limited, respectively, by the power of the monomode pump diodes.  
       [0006] To inject light emanating from a pump diode into a fiber, one end of the fiber may in theory be used. Using suitable optics, such as for example a lens system, it is possible to obtain effective optical coupling [Ref. 1]. However, this means that only the other end of the fiber is then available. This type of injection therefore does not allow access to both ends of the fiber. Now, for an optical fiber amplifier, both ends are required. It is therefore not possible to use this type of injection.  
       [0007] Moreover, the power of pump diodes is sometimes insufficient for some laser or amplifier applications. It would therefore be desirable to combine the power of several diodes in order to obtain the required power. However, longitudinal-type injection into the end of a fiber does not allow this type of multiplexing to be easily accomplished.  
       [0008] Other methods of injection such as that described in [Ref. 2] have been proposed for trying to effectively couple pump diodes into fibers having large multimode sections. However, notching the fiber as proposed in that document weakens it. The risks of breakage over time are considerable. This method therefore does not meet the qualifications required for products used in the telecommunications field.  
       [0009] Another method of injection is that proposed by [Ref. 3]. With that method, a fiber bundle is fused together and then drawn in order to achieve the dimensions of the multimode section of an injection fiber. From the ratio of the numerical apertures of the fibers of the bundle to that of the injection fiber and the ratio of the sections of the bundle to the multimode section of the injection fiber it is possible to calculate the optimum configuration for efficient optical coupling. To optimize the coupling, the multimode section of the injection fiber is generally hexagonal or star-shaped. To have good coupling, it is necessary to maintain the geometrical extent of the N input fibers and of the injection fiber. In general, the number of fibers in the bundle is limited to seven. In order to have access to both ends of the fiber in the case of the production of an amplifier, the central fiber of the bundle must be a monomode fiber.  
       [0010] Reference 4 also proposes various systems for injection via the side. However, in practice, the systems proposed in that document, which use a double-clad fiber and a multimode section, having a large numerical aperture, typically 0.4, and an initiating fiber with a rectangular section and similarly a large numerical aperture, are not satisfactory.  
       [0011] The objective of the present invention is to provide an injection device which improves the coupling and the injection into a fiber.  
       [0012] This objective is achieved within the context of the present invention by means of a device comprising:  
       [0013] a main fiber; and  
       [0014] an auxiliary fiber whose beveled end is placed on the side of the main fiber, in which device the auxiliary fiber has a numerical aperture smaller than the numerical aperture of the main fiber.  
       [0015] The present invention also relates to a method of preparing an injection device for an optical fiber, characterized in that it comprises the steps consisting in:  
       [0016] beveling one end of an auxiliary optical fiber; and  
       [0017] placing and fastening this beveled end of the auxiliary fiber on the side of a main fiber having a numerical aperture larger than the numerical aperture of the auxiliary fiber. 
     
    
    
     [0018] Other features, objectives and advantages of the present invention will become apparent on reading the detailed description which follows, and with regard to the appended drawings given by way of nonlimiting examples, in which:  
     [0019]FIG. 1 shows a cross-sectional view of a double-clad fiber used preferably as the main fiber within the context of the present invention;  
     [0020]FIG. 2 shows schematically a multiple injection device according to the present invention;  
     [0021]FIG. 3 shows the optical indices of the various elements making up a multimode core fiber forming the auxiliary fiber and a monomode core fiber forming the main fiber, respectively, used within the context of the present invention;  
     [0022]FIG. 4 shows schematically the guiding of an optical beam in a multimode fiber forming the auxiliary fiber;  
     [0023]FIG. 5 shows schematically the injection carried out within the context of the present invention;  
     [0024]FIG. 6 shows the critical angle for total reflection as a function of the numerical aperture of the fibers used;  
     [0025]FIG. 7 shows schematically the beveled end of an initiating fiber used as the auxiliary fiber within the context of the present invention;  
     [0026]FIG. 8 shows an auxiliary initiating fiber placed on the side of a main fiber, according to the present invention, seen from the side in the case of FIG. 8 a  and seen from above in the case of FIG. 8 b , respectively;  
     [0027]FIG. 9 shows schematically a first method of fastening an auxiliary initiating fiber to a main fiber, according to the present invention;  
     [0028]FIG. 10 shows schematically a second method of fastening an auxiliary initiating fiber to a main fiber, according to the present invention;  
     [0029]FIG. 11 shows schematically a third method of fastening an auxiliary initiating fiber to a main fiber, according to the present invention; and  
     [0030]FIGS. 12 a  and  12   b  illustrate two alternative methods of injection according to the present invention, with two and four diodes respectively. 
    
    
     [0031] As indicated above, the basic structure of the injection device according to the present invention comprises:  
     [0032] a main fiber; and  
     [0033] an auxiliary fiber  200  whose beveled end  210  is placed on the side of the main fiber  100 , the auxiliary fiber  200  having a numerical aperture smaller than the numerical aperture of the main fiber  100 .  
     [0034] Preferably, within the context of the present invention, the main fiber  100  is a double-clad fiber with a monomode core, of the type illustrated in FIG. 1.  
     [0035] A general description of such a fiber  100  may be found in the document [Ref. 5].  
     [0036] The fiber  100  illustrated in the appended FIG. 1 comprises a monomode core  102 , a multimode section  104 , which has at least one flat  105  and surrounds the core  102 , a low-index cladding  106  and an external mechanical cladding  108 . Such a double-clad fiber  100  generally has a monomode core  102  doped with one or more rare earths, which acts as an amplifying medium and as optical guide for the monomode field. The dimensions of the fiber are generally around 4 μm for the diameter of the core 102 and 21×10 3  μm 2  for the multimode section  104 . The index of the low-index cladding  106  is typically 1.35.  
     [0037] However, for some applications, the fiber may have different characteristics. For example, the core may have a diameter greater than 4 μm.  
     [0038] The multimode section  104  advantageously has an index less than that of the core  102 . The low-index cladding  106  advantageously has an index less than that of the multimode section  104 , while the external mechanical cladding  108  has a higher index, greater than that of the core  102 .  
     [0039] As a variant, the main fiber  100  used in the context of the present invention may be a more conventional multimode fiber.  
     [0040] The auxiliary fiber  200  is advantageously a fiber having a multimode core  202 , surrounded by a lower-index optical cladding  204  and an outer mechanical cladding  206 , having an index greater than that of the core  202 .  
     [0041] As illustrated in FIG. 2, several auxiliary fibers may be associated with a main fiber  100 .  
     [0042] The transverse injection proposed within the context of the present invention thus allows efficient optical coupling of one or more pump diodes  300  each placed opposite the free end of a respective auxiliary fiber  200 , in the multimode section  104  of a fiber  100 . The ends of this fiber  100  are therefore available.  
     [0043] The diodes  300  are advantageously high-power multimode pump diodes.  
     [0044] To obtain optimum coupling, it is preferable to have a diode pigtail, that is to say, between each diode  300  and the input of the associated auxiliary fiber  200 , a length of fiber  250  which is matched to the diode and the core size and numerical aperture of which are similar to those of the initiating multimode fiber  200 . Typically, the pigtail  250  is a 100/125 fiber of 0.15 numerical aperture. The pigtail  250  is bonded to the initiating fiber  200  using a standard process.  
     [0045] If the diode  300  is not pigtailed, it is possible to couple the light directly into the initiating fiber  200  by means of a lens system.  
     [0046] The multimode auxiliary initiating fiber  200  has a core  202  of index less than or equal to the index of the multimode section  104  of the main fiber  100  (as may be seen in FIG. 3). The greater the index difference, the less the propagation of the electromagnetic field  4  in the multimode section  104  is disturbed upon crossing the point of junction. In the case of an index difference greater than or equal to the index difference between the multimode section  104  and the low-index cladding  106 , the propagation of the field is not affected upon crossing the point of junction.  
     [0047] The numerical aperture of the initiating fiber  200  must be sufficiently small—typically this numerical aperture NA is 0.15—while remaining compatible with a good optical coupling coefficient with the pump diode  300 .  
     [0048] The main fiber  100  must have the largest possible numerical aperture, typically NA=0.4. The index difference between the multimode section  104  and the low-index cladding  106  must be as large as possible. Preferably, to obtain such numerical apertures, the low-index cladding  106  is a silicone.  
     [0049] The present invention is based in particular on the following considerations.  
     [0050] An analysis of the critical angles of the interface between two media of different indices stresses the importance of the numerical apertures of the main fiber  100  and the auxiliary fiber  200 .  
     [0051] If n co  and n c1  are the index of the core and of the optical cladding, respectively, of a multimode fiber as shown schematically in FIG. 4, the maximum angle of reflection at the core/cladding interface is given by:  
     θ max   =a  sin ( NA/n   co )  (1) with  
       NA ={square root}{square root over ( n   2   co   −n   2   c1 )}  (2)  
     [0052] where NA represents the numerical aperture of the fiber.  
     [0053] Let us consider the case in which the multimode section  104  of the fiber  100  and the core  202  of the initiating fiber  200  have the same optical index. For total reflection, the following equation is obtained between the numerical apertures:  
     θ 1 +θ max1 =π/2  (3) with  
     θ 1   =a  sin ( n   c1 2   /n   co2 )  (4) and  
     θ r =π/2−θ max2 −θ p   (5)  
     [0054] in which:  
     [0055] θ max1  is the maximum angle for total reflection in the double-clad fiber  100  (multimode propagation);  
     [0056] θ max2  is the maximum angle for total reflection in the auxiliary initiating fiber  200 ;  
     [0057] θ r  is the angle of reflection of the most inclined beam emanating from the auxiliary initiating fiber  200  at the core/cladding interface of the fiber  100 ;  
     [0058] θ p  is the polishing angle of the initiating fiber  200 ; and  
     [0059] θ 1  is the critical angle at the core/cladding interface of the fiber  100 .  
     [0060] To have total reflection of the most inclined beam emanating from the auxiliary initiating fiber  200  at the core/cladding interface of the main fiber  100 , the following condition must be respected:  
     θ r ≧θ 1   (6) i.e.  
     π/2−θ max1 −θ p ≧θ 1   (7),  
     [0061] which may be written as:  
     (π/2)− a  sin ( NA   1   /n   co1 )θθ p   ≧a  sin ( n   cl2 /n co2 )= a  sin (1−( NA   2   2   /n   2   co2 )) 1/2   (8) i.e.  
       a  sin ( NA   1   /n   co1 )≧(π/2)−θ p   −a  sin (1−( NA   2   2   /n   2   co2 )) 1/2   (9)  
     [0062]FIG. 6 illustrates the critical angle of total reflection as a function of the indices of the fibers  100  and  200 . FIG. 6 shows three curves corresponding to bevel angles of 6°, 8° and 10° respectively, for the end of the auxiliary fiber  200 . To the left of these curves, there is partial reflection. In contrast, to the right of these curves, there is total reflection.  
     [0063] It is thus apparent from equation (9) and FIG. 6 that it is necessary to minimize the numerical aperture of the initiating fiber  200  and maximum the numerical aperture of the main fiber  100 . The polishing angle must also be as small as possible.  
     [0064] Optical coupling takes place by positioning the auxiliary initiating fiber  200  on the side of the main fiber  100 .  
     [0065] In the assembled state, the two fibers  100  and  200  have their longitudinal axes coplanar.  
     [0066] The auxiliary initiating fiber  200  is polished beforehand at its end  210  with an angle of about 1° to 20° (FIG. 7). The polishing is carried out by a standard process. The tolerance on the polishing angle depends on the tolerance on the numerical apertures of the initiating fiber  200  and the main fiber  100 . The greater the difference in numerical aperture, the greater this angle may be.  
     [0067] For an initiating fiber  200  of 0.15 numerical aperture, and a main fiber  100  of 0.34 numerical aperture, the polishing angle is typically 6°.  
     [0068] The positioning of the initiating fiber  200  on the side of the main multimode fiber  100  must be carried out in a precise manner, as shown schematically in FIG. 8. To ensure this positioning, a few millimeters of the low-index cladding  106  of the main fiber  100  must first be removed.  
     [0069] The auxiliary initiating fiber  200  must then be fastened to the side of the multimode section of the main fiber  100 .  
     [0070] Various manufacturing processes may be used for this purpose.  
     [0071] To have satisfactory coupling, the initiating fiber  200  may be cemented to the side of the main fiber  100  or fusion-bonded with the latter.  
     [0072]FIG. 9 illustrates a first implementation in which the auxiliary initiating fiber  200  is fastened by cementing to the main fiber  100 .  
     [0073] The cement  310  used must have an index lying between that of the core  202  of the initiating fiber  200  and that of the multimode section  104  of the main fiber  100 . A UV-curable epoxy cement compound is suitable for this application. A microdrop is sufficient for the cementing. It is necessary to prevent the cement  310  from extending beyond the interface between the initiating fiber  200  and the main fiber  100 .  
     [0074] Any other component meeting these criteria may be used. The limitation is the resistance to the intense flux from the pump laser and the aging over time. The index of the cement  310  and its transparency must not change. The cement  310  must be able to meet the qualifications imposed by the application in question.  
     [0075] Once the cement  310  has cured, the main fiber  100  must be reclad with the low-index cladding  106  as illustrated in FIG. 9 under the reference  320 .  
     [0076] As mentioned above, the auxiliary initiating fiber  200  may also be fastened to the main fiber  100  by fusion bonding.  
     [0077] This fusion bonding may, for example, be carried out by a microtorch.  
     [0078] The fusion bonding is then preferably carried out by means of the flame of a microtorch of the oxidant/butane type. Other gas mixtures may also be envisioned. The size of the flame is such that the area heated covers the area of contact between the two fibers  100  and  200 . To prevent the initiating fiber  200  deforming during the fusion bonding, it is possible to use a glass with a low Tg, such as B 2 O 3 , to pre-cement the end of the initiating fiber to the injection fiber (as shown schematically in FIG. 10 a ). The temperature of the flame, its position and its composition are critical parameters of the fusion bonding process. To make the heated area uniform, it is possible to make the flame undergo an oscillating longitudinal movement as shown schematically in FIG. 10 b.    
     [0079] In FIG. 10 a , the element made of B 2 O 3  glass has the reference  330 . In FIG. 10 b , the torch has the reference  340 , its flame  350  and the oscillating movement of the torch  340  is shown schematically by the arrow with the reference  360 .  
     [0080] After fusion bonding, the main fiber  100  must be reclad with the low-index cladding  106 .  
     [0081] Another solution consists in fastening the auxiliary fiber  200  by fusion bonding it with a laser, for example using a CO 2  laser.  
     [0082] An alternative to fusion bonding using a flame is in fact the use of a CO 2  power laser. The emission line at 10.6 μm of the CO 2  laser is strongly absorbed by the glass. Such a laser therefore makes it possible to control the area of heating more precisely and is more flexible to use than a flame. The beam  370  is focused by means of a lens  380  designed to have a focal spot of the same size as the interface of the two fibers  100 ,  200  to be cemented (as illustrated in FIG. 11). The temperature gradient between the top of the fiber  100  and the lower face is very large. The advantage is that it is possible to reach a temperature slightly above the T g  on the upper face without reaching this temperature at the center of the fiber  100  or at the lower face. The risk of deforming the fiber is therefore lessened.  
     [0083] After fusion bonding, the fiber  100  must be reclad with the low-index cladding  106 . The use of B 2 O 3  glass is also possible for the reasons described above.  
     [0084] According to yet another preferred embodiment of the present invention, the fastening of the fiber  200  is carried out by combining a flame with a CO 2  laser.  
     [0085] Cementing using a flame is in fact a difficult manufacturing process to control. The temperature at the interface between the two fibers  100 / 200  must be stable and well controlled, slightly higher than the T g  of the core  202  of the initiating fiber  200 . Too high a temperature deforms the fibers and, conversely, too low a temperature does not allow cementing.  
     [0086] The CO 2  laser gives a very localized heating area. The high temperature gradient does not allow homogenization of the stresses in the fiber.  
     [0087] Combining the two processes (flame and CO 2  laser) allows these difficulties to be overcome. The flame heats the fibers  100  and  200  locally to a temperature below the T g . The flame produces a much smaller temperature gradient in the fiber than CO 2 . The amount of heat needed to reach a temperature above the T g  is produced by the CO 2  laser beam. In this case, the flame-regulating parameters are less critical. The temperature at the interface is controlled by adjusting the power of the CO 2  laser.  
     [0088] After fusion bonding, the fiber  100  must again be reclad with the low-index cladding  106 . The use of B 2 O 3  glass is also possible for the reasons described above.  
     [0089] Trials carried out by the Applicant have led to the following coupling coefficients.  
     [0090] The coupling coefficient is defined by the ratio of the power injected into the initiating fiber  200  to the power output by the main fiber  100 .  
                                           Epoxy cement   Flame and/or CO 2         Type of cementing   compound   fusion bonding                  Typical coupling   65%-75%   75%-85%       coefficient                  
 
     [0091] As was indicated above, it is possible to combine several injections in order to multiplex the power of the pump diodes.  
     [0092] The distance between the various injection systems is not critical and may vary from a few centimeters to a few meters. The position along the main fiber  100  of the various injections and their orientations depend on the application. FIGS. 12 a  and  12   b  show a configuration example for two and four pump diodes, respectively.  
     [0093] According to FIG. 12 a , the injections are carried out in opposite directions.  
     [0094]FIG. 12 b  shows a variant comprising two injections in a first direction and two injections in the opposite direction.  
     [0095] It is possible to use other configurations.  
     [0096] A person skilled in the art will understand that the present invention makes it possible in particular to obtain high-power monomode lasers or high-gain amplifiers.  
     [0097] Of course, the present invention is not limited to the particular embodiments that have just been described, rather it extends to all variants in accordance with the spirit of the invention.  
     REFERENCES  
     [0098] 1) “Design of a device for pumping a double-clad fiber with a laser diode bar”, L. A. Zenteno, Applied Optics, Vol. 33, No. 31, 1994.  
     [0099] 2) “High efficiency side-coupling of light into optical fibers using imbedded V-grooves”, D. J. Ripin and L. Goldberg, Elect. Letters, Vol. 31, No. 25, 1995.  
     [0100] 3) U.S. Pat. No. 5,864,644.  
     [0101] 4) U.S. Pat. No. 4,815,079.  
     [0102] 5) U.S. Pat. No. 5,534,558.