- INTRODACTION
- VISUALIZATION OF PHASE OBJECTS
- THE RAISED ATTENATION CAUSES
- THE OPTICAL MODULE SPATIALLY DISTRIBUTED IMPERFECTIONS DETECTION
- THE DIRECT-SHADOW METHOD
The spatial distributed imperfections visualization method is additional one to nondestructive techniques for diagnostics and testing of fiber optical line of communication (FOLC), which is allowing to determine the reason of appearance defects. Whether these module imperfections are exploitation or manufacturing. The shadow method of detection of module imperfections allows in a new fashion to cast a glance at the requirements for manufacture of fiber systems on the other aspect. The simplicity and obviousness of the shadow coherent-optical monitoring schemes enables widely to use them for nondestructive monitoring of quality of an industrial production. This method also allows carry out trials in the real season temperature and to prevent emerging a number of imperfections, beforehand showing: under what climatic zones the tested optical modules can be used.
The refractive index of inhomogeneous phase object is function
of three space coordinates 
(fig.
1.) Under the Snell's law the laser light ray passes a point 
on the observed plane, while undeflected beam would come in a point 
.
The laser light ray passed, coming in a point ,
deviates from initial trajectory. Also the light rays passes an optical
path in a heterogeneity, defined from integral 
,
where s - geometric path length of a ray in object.
Thus except visual direct observation in the image plane it is possible to measure the following quantities: ray displacement, deflection ones in a heterogeneity, phase shifts. The detection of phase objects by the direct-shadow method is fulfilled without the optical elements because visualization happens due to focusing properties of the object (the fiber module with water-blocking buffer gel). The optical module parts is illuminated by divergent beam from helium-neon laser (fig. 2.).
The light beams, which are passing through heterogeneity of the water-blocking buffer gel, change the ones direction. It reduces to light intensity redistribution in the image plane (fig. 3.). The formed direct-shadow image depends on absolute value of angular deflection. The variation of the light intensity is proportional to second-order derivative of refractive index
,
where 
- incident light intensity, 
-
variation of light intensity, 
-
refractive index.
Quantitative responses can be received with the help direct-shadow method in the case of neglects to a ray curvature in a heterogeneity and also additional displacement on a path between a heterogeneity and observed plane, These assumptions are not always admitted for examination in the field of sharp variation of refractive index. Therefore, this method seldom applied to quantitative measurements. Usually it use for a qualitative detection of zones with great a heterogeneity index density. The image quality can be rose by use a cylindrical lens in the final cascade of the image formation system. The lens focus length must be in conformity with a diameter of the research module and parameters of a laser beam.
In the 1999-2000 winter the attenuation increasing was
detected at an evaluation of quality of the "Novosibirsk-Khabarovsk" FOLC
on some parts in connective couplings, which connected the dielectric cable
with storm-rope cable. The measurements by the OTDR have shown, that the
attenuation was raised in optical fibers of a dielectric cable, which are
placed above a ground surface and was most subjected to influence of low
temperatures (fig. 4.). It is necessary to mark, the imperfection was not
precisely location. It was a spatially distributed defects for the space
of ten meters. For the first time, the raised attenuation was discovered
in January-1998. In January-2000 similar problems was revealed in a dielectric
cable on six parts in outcome of test-prophylactic measurements. A maximum
value of attention has made about 5,4 dB. There are some peculiarities such
as: the given effect is exhibited at negative temperatures (below - 
)
and on a vertically cable part which is going out to support from a high-voltage
line.
The most probable reason of the increased signal attenuation in a dielectric cable is represents by local non-uniformity origin in hydorfobous compound (gel) of the optical module because of thermal gel polymeric structure collapse a at low temperatures. The gel structure was changed thanks to modification of moisture-matter in the gel layer. But a real gel structure changes is while unknown in exploitation process. This situation is complicated also by difference of gel properties in the module as a thin layer, from properties of a massive gel sample.
Reasons for a modification of a structure has arose from
specific thermal evolution of boundary layers of a hydorphobos compound
in the optical module in a warmer season. The thermal inter seasonal fluctuations
are in a limit from 
to 
, and daily
variations of temperature amount to .
The non-coordination of thermal, adhesive and capillary properties are the
basic reasons for origin of imperfections. Also the losses in the FOLC can
be conditioned by fiber imperfections, which are formed by micro bends due
to a buffer tube compression in the considerable part of module at low temperature.
The fiber micro bends are caused by local variations of a hydorphobos compound
density (gel density), which filling the buffer tube. A losses are increased
in the micro bends. The surplus geometrical fiber length in the module and
inappropriate thermal expansion of the optical module are conditions for
shaping the increased number of micro bends. The extension of the module
happens in a warm season. In the winter the inverse compression process
is difficult because of the increased viscosity of a hydorphobous compound.
Thanks to it fibers in buffer tube are twisted in a spiral spontaneously,
i.e. take place a twist-structure of optical fibers.
The researches of properties and gel composition were carried out. The structural-capillary properties were also taken into account such as temperature gel collapse, variation of moisture-matter and presence availability of capillary-adhesive occurrences in gel layer. The optical configurations for visualization and documenting of non-uniformity in the module were specially developed with the of extreme coherent laser radiation for visible range (fig. 5).
The module was fixed in the scheme (fig. 5). The divergent
laser beam with a high intensity (
)
is driving into fiber end-face. This beam was spread in volume of the optical
module instead of on a single fiber. Diffusion a laser radiation on non-uniformity
are "gas bubbles" and weighed inclusions in the gel
layer. It is necessary to mark, the presence of gas bubbles through the
module with half-transparent buffer tube can be observed visually without
use of the laser.
Gas bubbles by a diameter of the order 
were observed on module part of the order 
.
The diffusion only on non-uniformity with the size of the order 
was observed at 
.
Gas bubbles were observed through an interval 
.
The detection of local non-uniformity makes possible the supposition about
spatially distributed imperfections. These local inclusions can be a consequence
of
a) violation of the requirements at manufacture of optical cables (introduction of an air and weighed inclusions in the gel);
b) structural modifications of gel at exploitation of the optical cable in the climatic conditions of East Siberia and vertical disposition of an optical cable.
Probably, gas bubbles lead to spatially distributed. These
imperfections can lead to great losses in the FOLC. At a vertical disposition
of a cable gas bubbles by gravity and at high temperature (up to 
)
can move in optical module because of diffuse capillary currents of gel
inside the module. Gas bubbles promote forming of micro bends in fibers.
The existence of gas bubbles and weighed inclusions can lead to gradual
attenuation of a signal for the space of all vertical part of a cable.
(fig.
2). The holder allows to move the module along a vertical axes. The enlarged
image of the module on a screen was exceeded with the help of the lense. As
the researched module has a half-transparent buffer tube, it is possible to
visualize the module inside.
) was
well observed (fig. 6(a)). These imperfections can lead to forming of fiber
micro bends because of the gel pressure modification on fiber deformations.
Also, bubbles with a diameter 
are observed. These imperfections give a diffusion image of module with clearly
looked fibers systems (fig. 6 (a, b)). Fibers was stuck by gel in these parts
of module with gas bubbles. Fibers in the module form system of fibers. This
configuration of fibers places on a center of the module, that lead to strain
of fibers along their own axes.
micron. The assigning
glass crumb in gel layer remains vague. Probable, areas of weighed particles
destroy fibers at the friction existence, which is a consequence of diffuse
microcapillary currents in gel layer inside the module by gravity at vertical
disposition. Also, the inside walls of buffer tube can be destroy that lead
to diffusion of water steam through buffer tube increased of a moisture amount
in a gel layer and structural modifications.