Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a method and apparatus for
use in in-line monitoring of the eccentricity of a plastic jacket being
extruded around a drawn optical fiber.
Plastic jackets are used directly as the cladding for
fused silica cores to create very useful large numerical aperture
fibers, and also as an overcoat to glass-clad fibers to reduce
microbending losses and to enhance mechanical protection. It is highly
desirable that the coating be applied concentrically around the fiber
to ensure reliable splicing as well as optimum strength and
transmission behaviour.
Plastic coatings are applied by various methods, and
techniques have been proposed and implemented with varying degrees of
success to aid in their concentric application. In general,
micropositioning and microscopic observations are necessary to align
the fiber at the start of each coating application, and only by
preparing and microscopically examining sections of the fiber after a
production run can the concentricity of the coating be assessed. In
addition to being time consuming and destructive, microscopic
examination may not detect certain problems, such as geometrical
nonuniformities, that can seriously impair the transmission
characteristics of the fiber. More importantly, real-time information
to enable the fabricator to make corrections, evaluate various
applicators, or stop the process completely, is not available as the
coating is being applied.
In 1976 a sensitive, non-destructive, and non-compacting
method to detect the eccentricity of transparent jackets was proposed
by Marcuse and Presby, Applied Optics, September 1977, Volume 16, No. 9.
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The method is based on the locakion of unique fringes in the
backscattered light arising from a beam that is incident at right
angles to the fiber axis. The method is capable of providing real-time
information on coating concentricity as the coating is being applied.
This known method for monitoring fiber eccentricity is
relatively complex, requiring a coherent light source and a relatively
large imaging area if the fringe pattern is to be electronically
converted. The image area cannot be reduced using a lens system since
passage through the lens system alters opkical path lengths and thus
destroys or distorts the fringe pattern. Another disadvan-tage of this
method is that broad maxima in the fringe pattern can often mask the
presence of relatively narrow minima.
A simpler method and apparatus are now proposed which
gives eccentricity data as detailed as the system previously described
and which can also be used to provide information relating to the
jacket compositional quality and the jacket and fiber surface quality.
According to one aspect of the invention~ there is
provided a method of monitoring the core-jacket concentricity of an
optical fiber having a core and a jacket the method comprising:-
directing a beam of light from a light source to the
fiber9 scanning light reflected from the fiber using a detection means
adapted to scan about an axis of the fiber, detecting the inkensity of
the scanned reflected light, and generating an intensity profile of the
detected light as a function of angle of reflection from the fiber
relative to a preset angle of incidence.
Preferably reflected light which diverges from the fiber
is rendered converging by a lens. The light can be scanned by a mirror
oscillatable about an axis in a plane containing the fiber and
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the light source, the mirror being located between the lens and a
primary focussed position of the converging light. The light can be
directed through a slit aperture onto a detector, the slit
aperture/detector combination located between the mirror and a
secondary focussed position of the converging light imaged by the
mirror.
For a concentric core-jacket combination, the intensity
profile is symmetric relative to the preset angle of incidence but any
eccentricity of core position produces asymmetry of the profile.
Particularly for an on-line fiber jacketing process, any detected
eccentricity can then be compensated for in the jacketing apparatus.
According to another aspect of the invention there is
provided apparatus for monitoring the core-jacket concentricity of an
optical fiber having a core and a jacket, the apparatus comprising a
light source for directing light at the fiber, scanning means for
scanning light reflected from the fiber, the scanning means adapted to
scan about a longitudinal axis of the fiber, detection means for
detecting the intensity of scanned reflected light, and means for
generating an intensity profile of the detected light as a function of
angle of reflection from the fiber relative to a preset angle of
incidence.
The scanning means preferably include said converging
lens, and a mirror sited to receive light from the lens. The mirror is
preferably rotatable about an axis parallel to the fiber axis and
within a plane containing the fiber axis and the light source.
The detection means can include a slit aperture also
extending parallel to the axis of the fiber, the slit aperture located
to pass light onto a photodetector located adjacent the slit aperture.
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The mirror can be a facet of a rotatable polygonal
precision scanner. In an alternative embodiment the scanning means and
detection means can be combined as a linear image sensor.
An embodiment of the invention will now be described by
way of example with reference to the accompanying drawings in which:-
Figure 1 is a schematic plan view, not to scale, of
apparatus according to the invention;
Figure 2 is a schematic view, not to scale and from one
side of the apparatus of Figure 1, showing light incident on and
reflected from a fiber;
Figure 3 shows the passage of two exemplary parallel
rays, I and II, into the fiber with reflection intensity envelopes
produced by each ray; and
Figure 4(a) and 4(b) are graphical representations of
reflection intensity profiles obtained using the apparatus of Figure 1.
Referring in detail to Figure 1, a light source 10
directs light at a jacketed optical fiber 12. Light reflected from the
fiber 12 passes through a converging lens 14 and is directed towards a
photodetector 18 after reflection from a mirror 16 forming part of a
precision scanner. The light source 10 is a Spectra-Physics Model 155
He-Ne laser having an output of 0.5mW and producing a 1 mm diameter
beam of uniform wavefront intensity. The jacketed optical fiber 12~ in
comparison, is typically 0.3 mm diameter. The light from source 10 is
reflected from the fiber 12 with an intensity envelope which depends on
the refractive indices of the fiber and jacket materials, on the
dimensions of the fiber and jacket, and on the angle at which the light
is incident on the jacket surface or the fiber-jacket interface.
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Converginy light from the lens 14 is incident on the
mirror 16 which forms one facet of a multi-faceted precision scanner of
of a type obtainable from the Lincoln Laser Company. The scanner is
mounted on a spindle 22 and is rotatably driven about the spindle axis
which extends in a plane containing the fiber axis. Light from the
scanner is directed towards the photodetector 18 through a slit
aperture 20 in a mask 25, the slit aperture extending within a plane
containing the fiber axis. The detector is so positioned that the
converging light from the mirror is not focussed so that the
photodetector 18 receives only one angular component of the light
reflected from the mirror. A relatively long slit aperture can be used
in order to generate a high photodetector current. Alternatively, the
aperture is made relatively short and the scanner made to scan the
fiber image more rapidly. In this way, increased resolution along the
length of the fiber can be achieved, this being important for a rapidly
longitudinally moving fiber. On the other hand, operating in this mode
demands a high intensity source and a high sensitivity detector.
Although not apparent from Figure 1, the source 10 is vertically
displaced from the detecting optics, i.e. mirror 16 and photodetector
18, so that the significant components of the incident and reflected
light are inclined to the fiber axis as shown in Figure 2.
As shown by the exemplary rays I and II of Figure 3, each
ray can be considered as having primary reflection intensity envelopes
as shown. In the case of ray I, a single predominant external
reflection occurs at the jacket surface, whereas in the case of ray II,
external reflections occur both at the jacket surface and at the
interface of the fiber and jacket. Although these are primary or
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external reflections, there will, in fact, be multiple internal
reflections of relatively low intensity within the fiber and jacket
which contribute to the total reflection obtained from the jacketed
fiber for a particular ray. The specific orientation of the mirror 16
corresponds to a particular viewing angle ~ as indicated in Figure 3,
all light radiating from the fiber at the corresponding viewing angle
being directed to the photodetector 18. As shown in Figure 1 at one
angle of the mirror one componen-t of reflected light is received at the
detector 18 and at a different angle another component reflected at a
different angle is incident on the detector.
Because of the difference in refractive indices of the
jacket and the optical fiber, the intensity profile of the received
light in a range~3n t~~n and as a function of mirror orientation is as
shown in Figure 4(A) where ~= 0 corresponds to the angle of incidence.
In fact, using the photodetector arrangement, an electrical analog of
the optical signal is generated. The intensity profile, which
corresponds to the combined reflection from the central core and the
jacket, has a peak at 3c If the core is eccentrically positioned
within the jacket, the profile departs from symmetry as shown in Figure
4(b). Prior calibration using a jacketed fiber having known
core-jacket eccentricity is performed in order to relate the asymmetry
of intensity profile to fiber jacket eccentricity so that the necessary
correction can be calculated.
It will be appreciated that on-line monitoring of
jacketed fiber production can take place using the apparatus and method
described. The apparatus of Figure 1 in fact permits only the
monitoring of eccentricity in a plane perpendicular to the lens optic
axis. In order to monitor any eccentricity perpendicular to that
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plane, the apparatus including the light source 10 can either be
duplicated but located 90 around the fiber or be driven around the
fiber axis as fiber is produced.
Although the invention has been described in terms of
monitoring a silicone jacket layer formed on a doped fused silica
fiber, the method can be used in many other structures in which there
exists a reflective discontinuity at the interface of transparent
materials.
In addition, although the converging lens and precision
scanner combination represents a convenient way of monitoring fiber
reflectivity as a function of image position, other arrangements could
be used to project the reflected image of the fiber onto an imager to
permit the measured intensity to be related to image position. Thus
the scanner can be obviated, and the image focussed directly onto a
linear imager; for example, a Toshiba CCD linear image sensor TCD 101
C. Alternatively an imager with a slit aperture can be positioned
relative close to the fiber and made to oscillate perpendicularly to
the fiber axis.
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