Temperature-insensitive optical fibers
BACKGROUND OF THE INVENTION 
     This invention pertains generally to optical fibers and more particularly to optical fibers with reduced temperature sensitivity. 
     An optical fiber is generally a composite construction comprising a light-propagating core, at least one outer layer, often referred to as cladding, one of which has an index of refraction lower than that of the core, and usually one or two protective coatings. If two or more outer layers are used, the last layer is often referred to as the substrate. The protective coating is usually plastic or metal. For example U.S. Pat. No. 4,213,672 by Aulich et al. discloses a single and a double plastic protective coating. 
     Performance of an optical fiber is improved by either improving the properties of the core or one of the layers or making certain matches of properties between the core and the different layers. Two examples of borosilicate optical fibers having variations in compositions to improve the optical fiber. are given. In U.S. Pat. No. 4,089,586 by French et al., the ratio of SiO 2  and B 2  O 3  in the core and cladding is varied, but is never less than 3:1, to produce a single mode optical transmission. A multi ingredient borosilicate glass having no more than 15 weight percent of B 2  O 3  is disclosed in U.S. Pat. No. 4,265,667 by Ikeda et al. 
     Temperature response of optical materials has been studied and researched for many years, but recently the research has increased significantly. The following are examples of recent work. In Roy M. Waxler and G. W. Cleak &#34;The Effect of Temperature and Pressure on the Refractive Index of Some Oxide Glasses&#34; J. Res. NBS 77A-6, p. 755-63, Nov.-Dec., 1973, the effect of temperature on the index of refraction is noted and the resulting adverse effect on light scattering is noted. However, no solutions to this problem are given. An earlier study on the effect of temperature and photoelastic constants on the index of refraction is given in R. M. Waxler et al. &#34;Optical and Mechanical Properties of Some Neodymium-Doped Laser Glasses&#34; J. Res. NBS 75A:3, p. 163-74, May-June, 1971. A mismatch between a borosilicate elliptical cladding and a pure silica outerjacket is reported in, V. Ramaswamy et al. &#34;Birefringence in Elliptically Clad Borosilicate Single-Mode Fibers&#34; App. Opt. 18:24, p. 4080-4, Dec. 15, 1979, to cause stress-induced strain birefringence. 
     Only until recently has much research been undertaken concerning the temperature sensitivity of the phase of light propagating in an optical fiber. An example of this work is N. Lagakos et al. &#34;Temperature-Induced Optical Phase Shifts in Fibers&#34; App. Optics 20:13, p. 2305-8, July 1, 1981, which discusses maximizing the temperature sensitivity of the fibers. 
     This sensitivity can be an important consideration in optical fiber design. In communication systems, changes in phase can deteriorate the signal in high-bandwidth communications. Temperature-induced phase shifts add to the noise of fiber sensors, e.g. acoustic, magnetic, and gyros. Similar problems exist with fiber-optic temperature sensors in that the sensing fiber should be temperature sensitive, but the optical fiber lead and reference fiber should not be. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of this invention to fabricate optical fibers with reduced thermal sensitivity. 
     A further object of this invention is to minimize the phase shift of light propagating in a composite optical fiber. 
     These and other objects are achieved by a composite optical fiber comprising a light-propagating core having a minimum variation of the index of refraction with temperature at constant density and at least one outer layer capable of restricting the axial expansion of the core rod without interfering with any radial expansion of the core. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An optical fiber according to the present invention comprises a light-propagating core, generally two outer layers and a protective layer. The composition of the different layers is selected and matched in the following manner in order to significantly reduce the sensitivity of the fiber to temperature. 
     The core comprises a material having an optical clarity sufficient to propagate light over the desired length. In communications, this length would be several kilometers. For sensor fibers the length may be only a meter or less. This property, like tensile strength and other processability properties, has values comparable to presently used optical fibers. The critical property for the core material is the parameter ##EQU2## which does not exceed 5.5×10 -6  K -1 , preferably 4.5×10 -6  K -1 , and most preferably 3.5×10 -6  K -1  and in which n represents the refractive index, T represents the temperature, and ρ represents the density of the core material. Values slightly in excess of 5.5×10 -6  K -1  would have some reduction in temperature sensitivity, but fibers with such a value would have little utility as temperature-insensitive fibers. Examples of excellent materials for the core are boron oxide (1.4×10 -6  K -1 ), borosilicate glasses having at least about 20 percent and preferably at least 30 percent of the total glass composition weight of boron oxide, barium borate glass having from 30 to 50 weight percent and preferably 35 to 45 weight percent of barium oxide. Silica phosphate glasses having at least about 30 percent and preferably 40 percent of the total glass composition weight of P 2  O 5 , barium borate-boron oxide  glasses, glasses having at least about 50 weight percent barium borate, boron oxide, or both and at least two from the group consisting of alumina, sodium oxide, and potassium oxidel. A preferred glass comprises 63 to 68 weight percent of SiO 2 , 3 to 4 weight percent of NaO, 16 to 22 weight percent of K 2  O, 3 to 4 weight perccent of Na 2  O, 16 to 20 weight percent of K 2  O, 3 to 4 weight percent of BaO, 1 to 2 weight percent of PbO, 3 to 4 weight percent of Nd 2  O 3 , and 3 to 4 weight percent of Sb 2  O 3 . Many additional glasses can be used. Plexiglass would be an excellent core material with a parameter equalled to about -2.5×10 -6  K -1 , but the port optical clarity of presently available plexiglass prevents any extensive use of this material. The widely used silica at about 6.3×10 -6  K -1  is too high to produce a fiber with any appreciable degree of temperature insensitivity. 
     The cladding material generally has a lower index of refraction in order to reduce signal loss through the surface of the fiber. Processing the fiber is generally improved if the cladding material is the same type of material as the core, but there is no requirement for this similarity. The optical clarity of the material is not important. Except for the Poisson&#39;s ratio, the choice of material is made in accordance with standard considerations. The Poisson&#39;s ratio of the material is about equal (within about 10 percent) of that of the core material. The thickness of the cladding layer is at least about one-half of the radius of the core. If the fiber is to be used as a sensor, the thickness is from one-half to five times the radius of the core. Such a thickness has some attenuation of the signal passing through the fiber, but the short lengths of fiber used in sensors make the loss unimportant. For communications, the thickness should at least be about 15 times the radius of the core. 
     The term substrate comes from the usual way fibers are prepared. A cladding layer is placed on the inside surface of a tube of the type of material as the cladding. Again matching the materials can improve processing, but there is no requirement for this. Then the center is filled with the core material and the composite is processed into fibers. In the usual fiber the substrate provides physical strength but has little effect on the fiber&#39;s optical performance. With the fibers of the present invention, the substrate contributes much to the temperature insensitivity of the fiber by reducing the axial strain in the core without affecting the radial strain of the core. 
     The coefficient of thermal expansion of the substrate layer is not more than about one fourth, preferably not more than one tenth, and most preferably not more than one twenty-fifth of that of the core. The Young&#39;s modulus of the substrate layer is greater than that of the core and preferably about from 20 to 50 percent greater. The Poisson ratio is within 25 percent of that of the core. In order ensure that this layer is large enough to exert a significant force on the axial strain of the core, the thickness should at least be equal to about one and one-half times the radius of the core and preferably at least five times and most preferably ten times greater than the core radius. Examples of suitable substrate material are silica, borosilicate glasses, with no more than 5 weight percent of B 2  O 3 , silica with 4 to 10 percent of total glass composition weight of titanium, silicanitrate glasses with at least about 90 percent of the total glass composition weight of silica, and certain ceramics. As can be seen many materials are suitable so long as the axial thermal expansion of the substrate is much less than that of the core. 
     The protective layer protects the fiber from moisture, chemical attack, and damages from handling the fibers. Since the non-glass material has an extremely large thermal expansion, it is necessary for the material to be elastomeric; so that, the material can deform and thereby minimize the force being exerted on the optical fiber. The Young&#39;s modulus of the material is at least about 1×10 -3  times that of the core, preferably 1×10 -4  times and most preferably 1×10 -6  times that of the core. The Poisson ratio is from 0.48 to 0.52. Preferably the thickness is at least about five times and most preferably at least ten times greater than the radius of the core. Examples of suitable elastomers are cis polyisoprene, polybutadiene, butyl rubber, and silicone rubbers. 
     The fibers are fabricated in the usual manner. Any of the commonly used methods would be suitable. 
     Having described the invention in general the following examples are given by way of illustration and are not meant to limit this disclosure or the claims to follow in any manner. 
     
                                           TABLE I                                 
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             Core-1 Core-2                                                
                        Clad   Substrate                                  
                                    Coating                               
__________________________________________________________________________
Composition  Nd-laser glass                                               
                    Barium                                                
                        95% SiO.sub.2 +                                   
                               SiO.sub.2                                  
                                    Silicone                              
                    Borate                                                
                        5% B.sub.2 O.sub.3                                
                                    Rubber                                
Diameter (μm)                                                          
             4      4   30     90    190                                  
Young&#39;s modulus                                                           
             61.5   65.4                                                  
                        50.76  72.45                                      
                                    0.0035                                
(10.sup.10 dyne/cm.sup.2)                                                 
Posisson&#39;s ratio                                                          
             0.226  0.290                                                 
                        0.219  0.17 0.49947                               
Thermal expansion                                                         
             97     60  17     5.5  1500                                  
coefficient                                                               
(10.sup.-7 /°C.)                                                   
Refactive index n                                                         
             1.52   1.58                                                  
 ##STR1##     0.45   0.44                                                 
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     The composition of the neodymium laser glass (core-1) is given in Waxler et al. J. Res NBS 75A:3, May-June 1971 at p. 173. The glass is identified as glass -D and comprises 66.3 weight percent of SiO 2 , 3.4 weight percent of Na 2  O, 18.2 weight percent of K 2  O, 3.5 weight percent of BaO, 1.8 weight percent of PbO, 3.5 weight percent of Nd 2  O 3 , and 3.2 weight percent of Sb 2  O 3 . 
     The composition of the barium borate glass (Core-2), designated as barium borate E-1583, is given in Waxler et al. J. Res. NBS 77A;6, Nov.-Dec. 1973 at p. 756 and comprises 59 weight percent of B 2  O 3  and 41 weight percent BaO. 
     As the table shows, the fibers are identical except for the core material. The fiber with core-1 has greater temperature insensitivity than the one with core-2, but both are more than four times more insensitive than optical fibers with a silica core and substrate and a hard plastic coating. Additional details concerning the temperature insensitive fibers of this invention are given in N. Lagakos and J. A. Bucaro &#34;Minimizing Temperature Sensitivity of Optical Fibers&#34; App. Optics 20, p. 3276-8, Oct. 1, 1981 which is incorporated herein. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.