Abstract:
An amplifying optical fiber is proposed (particularly for the 1300 nm wavelength range) whereby a high amplification can be achieved with the smallest possible pumping power. Agglomerates comprising a number of complexes are incorporated into the core of the amplifying optical fiber. The complexes are composed of a rare-earth element and a separator substance.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention concerns an amplifying optical fiber, comprising an optical fiber core and an optical fiber sheath, where most of the volume of the optical fiber core is made of a material with good transmission properties, and in which, as amplification material, complexes composed of rare-earth ions sheathed with a separator substance, are statistically distributed. 
     2. Discussion of Related Art 
     Such an amplifying optical fiber and a method for its production is known from my German application DE 44 20 287 and its corresponding U.S. Pat. No. 5,710,852 which is hereby incorporated by reference in its entirety for background. In the first place the known amplifying optical fiber is one in which the amplification of the light takes place in the 1550 nm wavelength range. Such amplifying optical fibers exhibit a favorable relationship between the pumping power and the amplification. For example, if the core of the amplifying optical fiber is doped with erbium, a pumping power of 30 mW can achieve a 40 dB amplification. 
     To obtain the same result with an amplifying optical fiber in the 1330 nm wavelength range, where the core is doped with praseodymium for example, a pumping power of 500 mW is required. 
     SUMMARY OF INVENTION 
     The technical problem, to which the invention is directed, is therefore to create an amplifying optical fiber which can achieve the highest possible amplification with the smallest possible pumping power. This applies basically to all types of amplifying optical fibers, i.e., both fibrous optical waveguides as well as planar optical conductors arranged on a substrate, and for all imaginable wavelength ranges. Since the production of amplifying optical fibers with sufficient amplification in the 1330 nm wavelength range is greatly desired, such a one is proposed whereby a sufficient amplification requires a pumping power that is well under 100 mW. In the amplifying optical fibers to be created, the likelihood, that a portion of the light power generated by the pumping laser, will be absorbed by the phonons of the host lattice, should be eliminated or very sharply reduced. 
     The invention solves this technical problem in that the amplification material is composed of amorphous agglomerates from a number of complexes. By this measure it is achieved that as large a number of rare-earth ions as possible is included in the core of the amplifying optical fiber, so that a given pumping power optimally excites the latter into stimulated emission, and a coherent light amplification is achieved. 
     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a two-dimensional illustration of the complex; 
     FIG. 2 is a schematic illustration of a section of an amplifying optical fiber with agglomerates; 
     FIG. 3 is a schematic illustration of an agglomerate; 
     FIG. 4 is a section of an amplifying optical fiber with a core as illustrated in FIG. 2; 
     FIG. 5 schematically illustrates a device for producing an amplifying optical fiber core which contains agglomerates; 
     FIG. 6 schematically illustrates another device for producing an amplifying optical fiber core which contains agglomerates; 
     FIG. 7 schematically illustrates a device for producing a powder-gas mixture, and 
     FIG. 8 schematically illustrates a device for producing a gas mixture which contains rare-earth ions. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The complex shown in FIG. 1 comprises the rare-earth ion  1  in the center, which is surrounded by a separator substance. The latter has the function of optically isolating the rare-earth ions from each other. If the complex is built into an amplifying optical fiber core, the rare-earth ion is an erbium ion for example. The separator substance was created from aluminum fluoride (AlF 3 ) for example, so that the rare-earth ion  1  is surrounded by aluminum ions  2  and fluorine ions  3 . The separator substance is not restricted to AlF 3  however and could be any other suitable substance such as a lanthanum compound. 
     In accordance with the invention, agglomerates  4  or lumps are now formed from a number of these complexes, for example from several thousand complexes, which are nonuniformly, i.e., statistically distributed in the material of the amplifying optical fiber core  5 , as can be seen in FIG.  2 . Such an agglomerate for example comprises more than a thousand individual molecules and has a diameter of several hundred nanometers or more or more generally, between 10 −9  and 10 −6  m. The material of the core  5  can be the ordinary material used for optical fibers, such as silica (SiO 2 ) for example, whose index of refraction has been adapted by doping with another material, germanium dioxide (GeO 2 ) for example. However other materials can basically also be used as core material. 
     FIG. 3, schematically illustrates an agglomerate  4  and is used to explain the amplifying effect on the light passing through the amplifying optical fiber core, when the rare-earth ions of the complexes in the agglomerate  4  are excited by a pumping laser. The light  6  penetrating the agglomerate  4  is reflected multiple times and can therefore be amplified multiple times when it passes through several complexes. The amplified light  7  can pass successively through several agglomerates  4  in the pumped amplifying optical fiber core  5 , where it is amplified once again. The result is an altogether higher amplification than with the known amplifying optical fibers. The rare-earth ions are composed of elements which correspond to the range of wavelengths to be amplified, such as erbium, neodymium or praseodymium. 
     If the core material and the agglomerates are amorphous, the value of the wavelength width of the amplified light is optimal. 
     FIG. 4 shows a sectional view of an amplifying optical fiber which has a core  5  with agglomerates  4  according to FIG.  2 . The core  5  is surrounded by the optical fiber sheath or cladding  8  which, as is usually the case with optical fibers, is composed of a material with an index of refraction which is lower than that of the core  5 . In the case of the refraction indexes of the transmission material and the amplification material, they can be different as well, e.g., a difference on the order of about 40×10 −3 . 
     The amplifying optical fiber according to FIG. 4 can be a filamentary or a planar optical fiber. In a filamentary optical fiber, the sheath  8  concentrically surrounds the core  5 . In a planar optical fiber, the sheath  8  is composed of planar layers on both sides of the strip-shaped core  5  for example. 
     FIG. 5 illustrates the configuration of a device that is able to produce an optical fiber core with agglomerates. It contains a known oxyhydrogen burner whereby layers of glass or porous glass layers can be produced in accordance with the flame hydrolysis method. 
     Such devices are known from U.S. Pat. No. 4,224,046 which is hereby incorporated by reference in its entirety for background. They comprise several concentrically arranged tubes, where the material from which the glass layer will be produced, is supplied to the burner, in which the tube  9  is located in the center. As a rule, oxygen is supplied through tube  10  and hydrogen through tube  11 . Tube  12  conducts an inert gas, for example argon, which has several functions, for example to shield the stream of burner gases on the outside, and to cool it. 
     In the device that can be seen in FIG. 5, the tube  9  is supplied from the two tubes  13  and  14 . Tube  13  for example supplies the raw materials for the core glass in gaseous form, for example silicon tetrachloride and germanium tetrachloride in conjunction with argon. Tube  14  supplies the raw materials for the production of the agglomerates. 
     There are two possibilities of supplying the agglomerate materials. In one instance the materials from which the complexes are created, i.e., the rare-earth material and the separator material, are supplied in gaseous form in conjunction with a gas such as argon. However the complexes or the agglomerates can also be supplied to the burner as a fine powder in a gas stream, which is also composed of argon. 
     The device illustrated in FIG. 6 differs from the device in FIG. 5 in that the tubes  13  and  14  are routed in a coaxial manner to the oxyhydrogen burner. The components of the device, which are identical to the components of the device in FIG. 5, have the same reference numerals. 
     FIG. 7 illustrates a device which is suitable for producing a fluid from a fine powder (d&lt;0.2 μm), and the fluid can be supplied to the oxyhydrogen burner in conjunction with a gas stream. The device comprises a container  15  in which the powder material is located on a gas-permeable bottom  16 . The underside of the container  15  is equipped with a tube connection  17  through which a gas is supplied. This gas permeates the powder layer  18  in the container  16  and is exhausted through the gas outlets  19 . A tube  20 , which ends in the tube  21 , dips into the powder layer  18  and conducts the gas stream, whereby the powder-gas mixture is supplied to the oxyhydrogen burner. 
     A negative pressure exists in the narrow section  22  of tube  21 , which causes parts of the fluid in the container  15  to be drawn into the gas stream in tube  21  where they are transported. 
     FIG. 8 illustrates a device whereby the raw materials of the agglomerates can be produced in gaseous form, so that they can be glass-encapsulated together with the raw materials of the optical fiber core during its production. This device comprises a pressure-resistant container  23  which is equipped with a heater  24 . The top of the container  23  has a very small opening  25  which ends in a tube  20 . The tube  20  in turn ends in the tube  21 . Tubes  20  and  21  correspond to those of the device in FIG.  7 . The raw materials for the production of the complexes or the agglomerates are placed on the bottom of container  23 . The device in FIG. 8 is a so-called Knudsen cell, which is also described in DE-A-42 09 004. 
     The gaseous materials for the complexes or the agglomerates are produced in the following manner: the materials in the container  23  are for example erbium chloride and aluminum chloride. When the container  23  is heated to a temperature of about 800° C. for example, the easily volatile aluminum produces a high pressure. The gas  27 , with the erbium chloride/aluminum chloride composition, is located above the mixture of solid materials  26  and can escape through the very small opening  25  which has a diameter of about 100 to 200 μm. Depending on the size of the opening  25 , agglomerates of a hundred or more individual molecules are created. The agglomerates are carried along by the gas stream in the tube and are transported to the vitrification area of the core glass material, where they are incorporated into the core glass. 
     Although the invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.