Method of making infrared crystalline fiber and product

The present invention is directed to silver halide fibers. In one embodiment, the fiber has been doped with AgI or a metal compound of the formula MY wherein M is selected from Li, Na, K, Rb, Cr, Mg, Ca, St, Ba, Cd or Hg, and Y is selected from Cl, Br, or I. The fiber has large, elongated grain core structure and decreased infrared transmission losses and a more even/cladding interface in cladded embodiments.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of making silver halide 
crystalline fibers capable of transmitting infrared light and to the fiber 
product made by the process. More specifically, it involves such methods 
and products which relate to improved AgCl.sub.x Br.sub.1-x files (where x 
is 0 to 1.0). 
2. Prior Art Statement 
Silver halide fibers consisting of silver chloride (AgCl), silver bromide 
(AgBr), and their solid solutions (AgCl.sub.x Br.sub.1-x) show promising 
properties for the transmission of middle infrared radiation. The fibers 
can either be bare core fibers, with gas functioning as the lower 
refractive index, total reflecting medium or polymer coated fibers or 
core/clad fibers where the cladding contains more AgCl for total internal 
reflection. These are described in U.S. Pat. Nos. 4,253,731; 4,381,141; 
4,504,298 and 4,552,434 and considerable improvements have been reported 
over time. 
Nevertheless, there are some drawbacks in present silver halide crystalline 
fibers. One basic disadvantage of both bare core and stepindex core/clad 
silver halide fibers is their tendency to deteriorate optically and 
mechanically probably due to recrystallization of the grain structure, 
phase separation and silver colloids formation as well as adsorption and 
diffusion of extrinsic impurities. 
Transmission and elasticity have been observed to decrease remarkably when 
present state of the art fibers are bent repeatedly. This is a result of 
defect formation linked to the plastic deformation of the granular 
structure of the fibers. Present core/clad fibers frequently show 
intermeshing between the core and the clad material during their extrusion 
and as a result bad transmission. This defect arises due to inhomogeneous 
deformation during the conventional extrusion process. However also fibers 
produced by alternative manufacturing methods such as multiple 
step-deformation by drawing the fiber through a row of dies as in, for 
example, U.S. Pat. Nos. 4,381,141; 4,504,298 and 4,552,434, show defects 
such as microvoids and microcracks. It is assumed that these scattering 
defects result from the stretching stress applied in this manufacturing 
method. The core/cladding boundary also shows defects if the process of 
successive rolling by grooved rolls is applied as suggested in some of the 
above prior art probably due to the non-symmetric deformation mechanism. 
Furtheron, trace contamination by lubricants or other extrinsic impurities 
is generally observed when multiple deformation methods are employed and 
these methods are more complicated and less productive than the 
conventional extrusion process. 
SUMMARY OF THE INVENTION 
The present invention is directed to a method of making silver halide 
fibers and to the product resulting from that method. In one embodiment, 
the present invention method involves reducing friction during extrusion 
by moving an extrusion die in a direction opposite from flowing fiber and 
against a stationary preform arrangement. In another embodiment, the 
preform is doped with AgI or a metal compound of the formula MY wherein M 
is selected from Li, Na, K, Rb, Cr, Mg, Ca, Sr, Ba, Cd or Hg, and Y is 
selected from Cl, Br and I. The resulting product has large elongated 
grain core structure and decreased infrared transmission losses and a more 
even core/cladding interface in cladded embodiments.

DETAILED DESCRIPTION OF THE INVENTION 
One object of the present invention is to provide a material composition 
and process for producing an infrared light transmitting crystalline fiber 
having good and stable elasticity and a stable, low level of optical 
losses. 
The production method should guarantee a grain structure reacting favorable 
to multiple bending of the fibers. At the same time the method should 
avoid the current shortcomings of core/clad intermeshing and 
microcracks/microvoids as well as surface contamination. 
The present invention achieves these objectives and overcomes the 
deficiencies of conventional techniques by stabilizing the composition of 
the main silver halide components by suitable dopants, by a preextrusion 
treatment of the preform to form a strengthened profile of texture 
elongated along the preform axis, and by maintaining laminar flow 
conditions during the extrusion process by applying the extrusion pressure 
to the prepared preform in a direction opposite to the fiber flow. 
We have found that photodegradation can be remarkably reduced by partially 
replacing halide anions by iodine and/or partially replacing silver 
cations by any alkali metals of by double valent cations. It is assumed 
that the homogenous distribution of the above listed elements in the fiber 
material creates additional barriers for the sliding and recrystallization 
effect in polycrystalline fibers and restricts the silver ion diffusion. 
This self-diffusion of interstitial silver ions determines the formation 
of silver colloids, that absorb the transmitted radiation. 
Metal compounds suitable for doping are MY, type compound wherein M is a 
metal selected from Li, Na, K, Rb, Cr, Mg, Ca, Sr, Ba, Cd, or Hg and Y is 
Cl, Br or I. 
The expected effect of AgI crystallographic structure transition during the 
cooling of this crystal after its growth from the melt was surprisingly 
suppressed by limiting the content of AgI to less then 5% of the AgCl-AgBr 
matrix. 
It was also determined that the concentration of double valent cations (Hg, 
Dc, Pb, etc.) needs to be restricted to less than 0.001% probably due to 
the substantial physical difference between them and the host cations of 
the lattice matrix. 
The positive effect of the doping of the silver halide crystals on the 
transmission stability of the fibers can be observed with any chosen fiber 
structure (for examples, core only with gas as lower index surrounding, 
core with polymer cladding, core/clad fiber with silver halide core and 
lower refractive index silver halide compositions cladding or graded index 
silver halide fiber). 
It was observed that the grain structure in the finished fiber can be 
influenced by pretreating the preform used for the fiber extrusion. If the 
preform is carefully deformed by exerting pressure to its lateral surface 
by preextrusion or by other methods with a low extent of deformation (&lt;2) 
a texture with elongated grains is achieved. The texture is finer at the 
outside of the deformed preform while larger crystalline grains are 
prevailing at the inner core area. We have observed that fibers extruded 
from above described preforms exhibit similar texture and show improved 
transmission and stability. It is assumed that the superfine (0.01 to 0.2 
.mu.m) ellipsoidally shaped crystalline grains in the periferical fiber 
layers deform more elastically and with less defect formation when 
multiple bending of the fiber occurs, than conventional, round shaped 
grains. Furtheron, the essentially longitudinal directed radiation rays 
pass over fewer gain boundaries and defects, which may further explain the 
enhanced transmission properties. 
Referring now to FIG. 1, a conventional prior art extrusion process is 
shown for a stepindex type preform. This involves applying pressure with a 
plunger 1 and heat with heater 3 in high pressure chamber 2 to the 
crystalline preform comprising preform core 4 and cladding preform 5 to 
force the material in the direction of the applied force and to afterwards 
deform it through the orifice of a die 6 to obtain a fiber 7 with the 
desired diameter. 
FIG. 2 shows a cut cross section of fiber 7 from the method shown in FIG. 
1. It is observed, unfortunately, that the resulting fiber 7 shows 
pronounced deformations of the geometry of core 8 and clad 9 and shows 
intermeshing of core 8 and clad 9 materials. This rough core-cladding 
interface frequently causes high scattering losses in the fiber. 
We attributed this shortcoming of the extrusion process to the friction of 
the material on the cylindrical chamber surface and the resulting 
turbulences in the material flow. We have found, however, that by 
inversing the plunger movement (movement of the plunger in a direction 
opposite to the fiber movement) the problem can be essentially overcome 
and the fiber geometry remarkably improved. Thus, FIG. 3 shows one present 
invention method with apparatus 21. 
In FIG. 3, high pressure chamber 22, includes heater 23, core preform 24, 
and cladding preform 25. However, here plunger 29 is below the preforms 
and moves upwardly, i.e. against the flow. Cap 27 holds the preforms in 
place and, as plunger 29 moves die 28 upwardly, fiber 30 is extruded 
downwardly therefrom. 
FIG. 4 shows a cross-sectional cut of fiber 30 from the present invention 
FIG. 3 process, wherein clad 31 and core 32 have a smooth, consistent 
interface with a relatively consistent clad thickness and core shape. 
The main advantage of this method is probably the fixed position of a 
preform in the chamber which eliminates the friction at the lateral 
surface of the preform and therefore also eliminates the release of 
friction energy. The inhomogeneous deformation of complex step-index or 
gradient-index preforms due to friction-caused turbulence is practically 
inhomogeneous. The intermeshing between core and cladding layers is 
remarkably reduced. 
But even with the above described extrusion process, some residual 
irregularities for the starting stage of the extrusion may be observed. 
FIG. 5 shows a further advance. Here the same equipment as shown in FIG. 3 
is used, with like parts identically numbered. However, to reach the 
laminar flow geometry faster, additional cladding material 35 can be 
inserted not only at the side but also below the cylindrical core preform 
36, as shown as base 37. 
Further, by keeping the plunger diameter marginally smaller than the 
preform diameter, surface contamination from the chamber surface or 
preform surface defects are effectively excluded from entering the 
finished fiber. In this way, our claimed extrusion process for silver 
halide fiber production shows additional benefits also for core-only or 
plastic clad core fiber manufacturing. 
FIG. 6 illustrates a cross-section of a preform successively deformed of 
its periphery consisting of a core 74 and a crystalline cladding 75, a 
layer of polymer lubrication 72 and a protective coating 73 of metal or 
polymer. This preform is particularly advantageous for the present 
inventive method. Thus, FIG. 8 illustrates a cross section of a present 
invention system which includes chamber 92, fluid inlet 91, heater 94, die 
95, preform 93 and pressurizing fluid 91, to produce fiber 96. 
The enhanced quality of the core/cladding boundary in step-index fibers and 
the improved symmetry in gradient index fibers can also be realized by 
applying isotropic pressure through gaseous or liquid media. A lubricant, 
which could be used to reduce friction, can also serve as the liquid 
medium. Silicon oil or liquid flouropolymers could be used for this 
purpose. Argon or other inert gases seem particularly suitable for gas 
extrusion purposes. 
The core/clad preform being subjected to extrusion by a plunger moving in a 
direction opposite to the fiber flow or to liquid or gas extrusion may 
also be surrounded by an additional layer of relatively soft metal. This 
metal layer can then be extruded together with the core/clad (or gradient 
index) crystalline preform to form a fiber already protected by an 
external metal sheath against light, stresses or contamination. 
The above described types of essentially friction free extrusion 
substantially suppress lateral surface friction--the main cause of 
macrostructure defects and turbulence intermeshing at the core/cladding 
boundary. 
The present invention is further illustrated by the following examples, but 
is not limited thereby. 
EXAMPLE I 
A crystal of solid solution of silver halide with a 50/50 ratio of chlorine 
and bromine, i.e. Ag Cl .sub.0.5 Br .sub.0.5 was grown with a Hg .sup.++ 
dopant (concentration 0,0003%) by known silver halide crystal growing 
methods and prepared as extrusion billet in cylindrical form (15 mm in 
diameter and 40 mm long). The billet was extruded into a fiber with 1 mm 
diameter by the present invention friction free process as shown in FIG. 3 
and described above, i.e. by moving the plunger of 14 mm in diameter in 
the direction opposite to the fiber flow at about 200.degree. C. The 
optical losses measured in the fiber by the cut-back method at 10.6 .mu.m 
were 0.2 dB/m immediately after the extrusion and 0.5 dB/m 1 year later. 
For comparative purposes a fiber of equal diameter was manufactured at the 
same temperature from a crystal of AgCl .sub.0.5 :AgBr .sub.0.5 solid 
solution by conventional extrusion in FIG. 1 above. Its optical losses 
were measured as 1 dB/m right after the extrusion and 2.4 dB/m 1 year 
later. Both fibers were stored in black loose polymer tubes at controlled 
laboratory conditions to avoid environmental damages. 
EXAMPLE II 
The preform of a structure such as is shown in cross-section in FIG. 6. 
Structure 73 was formed from a AgCl .sub.0.5 AgBr.sub.0.5 core rod 
(diameter 15 mm), and a AgCl .sub.0.7 :AgBr .sub.0.3 cladding tube (15/18 
mm diameter), a polyethylene coating tube (18/19 mm diameter) and a 
OFHC-copper ductile tube (19/20 mm diameter) with a bottom part in a form, 
suitable for laminar extrusion. Two successive extrusions of this preform 
through the raw of conical dies were realized with silicon oil as outer 
lubricant, and the diameters of the orifices were successively decreased 
from 17 mm to 15 mm. A final friction free extrusion using the FIG. 3 
method was realized to obtain 1 mm diameter fiber. The optical losses in 
the fiber were 1.5 dB/m at 10.6 .mu.m. The tensile strength was remarkably 
high (120 MPa). Structure investigations of chemically etched fiber 
samples revealed a texture similar to that shown in FIGS. 7a and 7b, 
showing a fiber 50 with a core 51 and cladding 54. Note that large, 
elongated grains 52 are formed toward the center and smaller elongated 
grains are formed towarad and in the cladding, such as grains 53. 
It was observed that the fiber could withstand significantly more and 
several bend cycles than fibers produced directly from nonpredeformed 
crystals described above. 
EXAMPLE III 
A step-index type fiber was produced by the method illustrated in FIG. 3 
from a predeformed preform (as shown in FIG. 5). The extrusion billed (15 
mm diameter and 40 mm long) consisted of a single crystalline rod of AgCl 
.sub.0.24 :Br.sub.0.02 solid solution (10 mm diameter and 38 mm long) 
being inserted in a hollow cylinder cladding of AgCl .sub.0.5 Br.sub.0.5 
solid solution (inner diameter 10 mm, outer diameter 15 mm, length 38 mm) 
and a disk of cladding material was placed at the bottom (diameter 15 mm 
and 2 mm thickness). The billet was hot extruded (at about 200.degree. 
.sup.C) by friction free extrusion into a fiber of 0.7 mm outside diameter 
and 0.47 mm core diameter. Good quality of the core-cladding boundary was 
observed over the length of about 10 mm. 
Without the insertion of the extra cladding material at the bottom (near 
the die) much smaller yield was observed due to several meters of start up 
loss. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from its spirit and scope.