Abstract:
Method and apparatus for mounting an optical fiber for coupling to a high power light source, the fiber being secured to its mount with a low refractive index adhesive. The low refractive index adhesive serves to reduce the coupling of light traveling within the fiber to the fiber mount, thereby reducing undesirable, potentially destructive heating in the fiber mount. The adhesive preferably comprises sol gel.

Description:
TECHNICAL FIELD 
   The inventions relate generally to the use of optical fibers for transmission of power, and more particularly to mounting optical fibers for coupling to light sources. 
   BACKGROUND 
   Optical fibers can be used to transmit power. To transmit along a fiber, electrical power is first converted into light with a power conversion device, such as a multimode pump chip. The high power light is directed into the fiber at a fiber tip, and then travels down the fiber to a destination, or is coupled into another fiber. To achieve optimal coupling at the fiber tip, the fiber tip is accurately aligned with the light source, and, once aligned, securely held in place. A typical securing technique involves stripping a 10-20 mm length of the jacket off the fiber at one end, metallizing the exposed glass of the fiber, inserting the fiber through a mounting tube, and securing the fiber to the mounting tube. The mounting tube is then secured to a mounting block. 
   High-powered optical fibers have been secured to a mounting tube with a metallic solder applied to the metallized surface of the fiber. When light traveling inside the fiber reaches the fiber wall, a significant portion of the light is deflected out into the metal. The deflected light is rapidly absorbed since the metallized surface of the fiber, as well as the solder, do not transmit light. Furthermore, the interface between the fiber and the solder contains a complex web of oxides and other dielectric materials that also absorb light. Modem multimode power-carrying optical fibers typically carry a total power of about 10 watts. Since about 10% of the fiber&#39;s power can be coupled to the metal layer and solder, this coupling can result in the deposition of about one watt within a few millimeters around the solder junction. Such energy deposition can cause intense localized heating, which can cause the solder to melt, and thus cause serious damage to the fiber and the surrounding components. 
   In one alternative approach, the metallic solder is replaced with glass solder or with epoxy. However, because these materials have a refractive index that is similar to that of glass or even a little higher, they refract light out of the fiber, also causing power loss. Oxides within the glass solder are efficient light absorbers, and the result can again be significant localized heating with potentially destructive consequences. 
   SUMMARY 
   The described embodiments reduce the coupling of power from a power-carrying fiber to its surroundings, particularly the mounting means. This reduction is achieved by securing the fiber with its polymer cladding stripped off to its mount with an adhesive that has a refractive index lower than that of the outer glass cladding of the fiber. 
   In general, in one aspect, the invention features a method of mounting an optical fiber for coupling to a light source. The method involves providing a portion of the fiber with its polymer cladding stripped off in a mounting tube, applying an adhesive having a refractive index lower than the refractive index of the fiber core and glass cladding to a junction between the fiber glass cladding and the mounting tube to secure the optical fiber to the mounting tube, and hermetically sealing the mounted fiber and the light source within a module housing. 
   Embodiments include one or more of the following aspects. The adhesive has a refractive index of less than 1.5; the adhesive may comprise sol gel, and may be transmissive of visible light; the adhesive further may have a curing time of less than 30 minutes at room temperature. The sol gel comprises 3-mercapptopropyl-trimethoxysilane and methyltrimethoxysilane. The mounting tube is mounted such that the optical fiber is aligned with the light source. The mounting tube may be mounted on a mounting block connected to the module housing. The fiber core and cladding may comprise glass, in addition to a polymer outer cladding. Prior to placing the fiber inside the mounting tube, a tip portion of the fiber glass cladding may be exposed by stripping off the polymer cladding. 
   In general, in another aspect, the invention features an apparatus with a mounted optical fiber. The apparatus includes an optical fiber having a tip portion where the fiber with its polymer cladding stripped off is exposed; a mounting tube surrounding at least a portion of the tip portion of the optical fiber, the fiber being secured to the mounting tube with an adhesive having a refractive index lower than the refractive index of glass; and a light source that is optically coupled to the light source. 

   
     Embodiments include one or more of the following aspects. The fiber core is made of glass. The adhesive has a refractive index of less than 1.5; the adhesive curing time is less than 30 minutes at room temperature; the adhesive may be a sol gel adhesive; and the adhesive may be transmissive of light. The mounting tube is mounted so as to align the fiber with the light source. Other features and advantages will become apparent from the drawings and detailed description. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a high power multi-mode laser pump module assembly. 
       FIG. 2  is an illustration of selected components used to secure a high power optical fiber. 
   

   DETAILED DESCRIPTION 
   Multimode optical fibers typically transmit between one and 10 watts of power as light within an individual fiber, but power transmission may be as high as 100 watts per fiber. The power to be transmitted is normally provided to the fiber as an electrical current traveling along a conducting cable. A high power multimode pump chip converts the incoming electrical energy into optical energy in the form of laser light, which is coupled to an optical fiber. The multimode pump chip, the fiber tip (one end of the fiber), and associated components are all housed within a hermetically sealed module. One source of malfunction in the assemblies that couple electrical signals to optical fibers is leakage of non-coupled energy out of the core of the fiber into the cladding and into other material that is used to adhere the end of the fiber to its mount. Such energy leakage can cause intense localized heating with consequent damage to the fiber and its surrounding material, and can cause the system to fail, potentially catastrophically. 
   Energy is diverted from a glass fiber when a material surrounding the fiber has a refractive index equal to or higher than that of the fiber&#39;s glass, i.e., greater or equal to about 1.4. When glass solder is used to adhere the fiber to a mount, the adhering material has the same or higher refractive index as the fiber, which results in considerable coupling between the light incident on the inner fiber wall and the surrounding material. The degree of coupling may also be affected by the nature of the core-to-glass solder interface, which may have a layer of surface oxides. 
   The coupling of light to the surrounding material can be drastically reduced by using a material having a lower refractive index than that of the glass fiber. In this situation, light traveling along the fiber that impinges on the glass/surround boundary is incident from the higher-refractive index side of the junction. If incident at an angle greater than the critical angle (which is typically the case for light traveling along the fiber), the light is internally reflected back into the fiber. Low refractive index surrounding material thus reduces the coupling between the fiber and its surround. 
   The described embodiment uses a sol gel adhesive, referred to herein as sol gel 1612, to secure the optical fiber to the mounting tube. Sol gel 1612 is a colloidal suspension of silicon dioxide that is gelled to form a solid. It comprises 3-Mercapptopropyl-Trimethoxysilane (MPTMOS), Methyltrimethoxysilane (MTMOS), and Ceramabind 644-A Colloidal Alumina Aqueous Solution. 
   The following is a typical mixing procedure and sequence.
         1. Weigh 2.5 grams MTMOS into Trace-clean, 2 nd  glass bottle using pipette.   2. Weigh 2.5 grams 644A to same bottle using second pipette.   3. Add 5 drops MPTMOS using third pipette.   4. Screw on lid.   5. Shake by hand for 5 minutes.   6. Allow to rest for 5 minutes.   7. Add 0.75 grams Acetone.   8. Shake bottle.   9. Label bottle contents, batch and mix date.   10. Use or refrigerate.       

   The sol gel is an effective adhesive, and serves to replace the metallic solder and/or the glass solder or epoxy adhesives used in other systems. The 1612 compound sol gel has a refractive index of 1.38 at a wavelength of 589 nm, significantly below the 1.5 refractive index of glass. 
   Sol gel is also optically transmissive, which means that any light that is coupled into it is not rapidly absorbed and does not cause localized heating. In contrast, the glass used in glass solder contains oxides that are efficient light absorbers that would cause power from coupled light to be deposited close to the contact surface with the fiber core. A further advantage of sol gel is that it is stable at room temperature, having a long shelf life. It also cures relatively rapidly (15 to 30 minutes) at room temperature. This property enables the assembly process to proceed rapidly. It also removes the need for the high temperatures required to melt and apply solder adhesives. This allows the assembly to be fixed while on the assembly station, avoiding possible internal movements within the assembly during removal from the assembly station. The low-temperature cure also beneficially avoids temperatures that could cause other solders in the module to soften or move, which could result in thermal damage to the polymer cladding of the fiber. 
   In another embodiment, the fiber is adhered to the mount with an adhesive having a refractive index of less than 1.5; in another embodiment, the adhesive has a refractive index of less than 1.45; in yet another embodiment, the adhesive has a refractive index of less than 1.4. 
     FIG. 1  is an illustration of a high power, hermetically sealed, multimode pump module that incorporates a sol gel adhesive, and that is used to couple optical power into a fiber. Fiber  102  is mounted inside module  104  with fiber tip  106  aligned with multimode pump chip  108  and its carrier  110 . Carrier  110  is mounted on submount  112 . Fiber  102  is mounted and secured by ferrule  114 , which serves as a mounting tube, and is mounted on mounting block  116 . Fiber  102  is adhered to ferrule  114  using a sol gel (not shown in  FIG. 1 ). The fiber then passes through second ferrule  118  that exits sealed module  104  through package ferrule  120 . First ferrule  114  and second ferrule  118  preferably comprise metals or metal alloys. Package ferrule  120  is soldered to wall  122  of module  104 , and forms a hermetic seal with wall  122 . Fiber  102  is hermetically sealed to second ferrule  118  with a glass solder. The purpose of first ferrule  114  is to secure fiber  102  in correct alignment with multimode pump chip  108 , while second ferrule  118  surrounds the fiber with a hermetic seal before it exits sealed module  104 . Sealed module  104  includes enclosure  124  containing a gettering material (not shown), which removes impurities from within module  104  through porous housing  126 . 
   Outside sealed module  104 , fiber  102  exits second ferrule  118  and, after a short gap, is covered with acrylate fiber jacket  128 . The acrylate jacket is a covering that is normally supplied with the optical fiber, but here, the jacket has been stripped off to expose the glass core of fiber  102  to a distance of 18±0.5 mm from fiber tip  106 . Second ferrule  118  is secured to jacket  128  with notched tube  130 , providing strain relief for the fiber gap between second ferrule  118  and jacket  128 . The entire assembly from package ferrule  120  to jacket  128  and beyond is covered with protective rubber strain relief boot  132 , preferably comprising flame-retardant rubber. 
     FIG. 2  is an illustration showing the fiber assembly in more detail. The figure shows the portion of fiber  102  from tip  106  extending about 18 mm along the fiber, corresponding to the portion for which fiber  102  has been stripped down to the glass core. At a distance of 1-4 mm, and preferably at about 2 mm, from fiber tip  106 , the fiber enters first ferrule  114 . The fiber is secured to ferrule  114  by sol gel  202 , which serves as a low refractive index, highly transmissive adhesive layer. Sol gel  202  is applied to the first ferrule  114 , and, through capillary action, wicks along fiber  102 , reaching approximately 80-100% along the length of ferrule  114 . Fiber  102  exits from back face  204  of ferrule  114 , and after a short gap of approximately 2 mm, enters second ferrule  118 , where it is secured with a hermetic seal provided by glass solder  206 . Fiber  102  then exits second ferrule  118 , and, after short gap  208 , enters jacket  128 . Strain relief is provided by notched tube  130 , which is secured by epoxy joints  210  to second ferrule  118  at one end and to fiber jacket  128  at the other. 
   Glass solder is used to secure fiber  102  to second ferrule  118  in order to provide a hermetic seal. This joint allows light to leak from the glass cladding into the glass solder adhesive, which causes heating around the hermetic seal. This heating can be tolerated for low power parts in the 10 W range since the heat can be dissipated in the seal vicinity without destabilizing the fiber coupling or compromising the hermetic seal. However, when used with high power modules, such light leakage could cause destructive heating. In such applications it would be desirable to use a low refractive index adhesive, such as sol gel, to provide the required hermetic seal at this joint (as for first ferrule  114 ) without the light leakage and resulting heat dissipation associated with a glass solder joint. 
   Typical applications of optical fibers transmitting high power multimode laser light include ordnance initiation, soldering, photodynamic therapy, and marking. The laser light may also be used to provide pump power to other lasers, such as to diode-pumped solid state lasers, or to fiber lasers. Since the packages are fully hermetic, they can be used in challenging environments, such as underwater or in space. 
   Other embodiments are within the following claims.