Method and apparatus for detecting defects in an optical fiber coating

The present invention provides an optical detection system for detecting defects in an optical fiber. The system includes a light source for coupling light into the secondary coating of an optical fiber at a preselected angle with respect to the longitudinal axis of the fiber and an optical detector positioned adjacent the fiber at a preselected distance from the point at which the light is coupled into the fiber coating. In accordance with one embodiment, the light is coupled into the fiber coating at a sufficiently shallow angle with respect to the longitudinal axis of the fiber to cause the light to travel through the coating in a direction substantially parallel to the axis of the optical fiber for some distance before exiting the coating. By placing the detector a preselected distance from the coupling point, detection of light reflected from the fiber at the coupling spot is avoided and only light reflected by defects contained in the primary or secondary coating layers, or light reflected by delaminations between the primary coating layer and the fiber, will be detected by the detector.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to a method and apparatus for detecting 
defects in the coating layers of an optical fiber. More particularly, the 
present invention relates to a detection system which can be incorporated 
into an optical fiber manufacturing process for optically detecting air 
bubbles or delaminations occurring in the coating layers of optical fibers 
as they are being drawn. 
BACKGROUND OF THE INVENTION 
The successful implementation of a light wave communication system requires 
high quality light guide fibers having mechanical properties sufficient to 
withstand the stresses to which they are subjected. Each fiber must be 
capable of withstanding over its entire length a maximum stress level to 
which the fiber will be exposed during installation and service. The 
importance of fiber strength becomes apparent when one considers that a 
single fiber failure will result in the loss of several hundreds of 
circuits. 
The failure of light guide fibers in tension is commonly associated with 
surface flaws which cause stress concentrations and lower the tensile 
strength below that of pristine unflawed glass. The size of the flaw 
determines the level of stress concentration and, hence, the failure 
stress. Even micron-sized surface flaws cause stress concentrations which 
significantly reduce the tensile strength of the fibers. 
Long lengths of light guide fibers have considerable potential strength, 
but the strength is realized only if the fiber is protected with a layer 
of a coating material such as, for example, a polymer, soon after it has 
been drawn from a perform. This coating serves to prevent airborne 
particles from impinging upon and adhering to the surface of the drawn 
fiber, which would weaken it or even affect its transmission properties. 
Also, the coating shields the fibers from surface abrasion, which could 
occur as a result of subsequent manufacturing processes and handling 
during installation. The coating also provides protection from corrosive 
environments and spaces the fibers in cable structures. 
Light guide fibers are usually coated during a wet-coating process which 
typically involves drawing the light guide fiber through a reservoir of a 
liquid polymer material and then curing the liquid polymer material to 
harden it by exposing it to curing radiation (e.g., ultraviolet light). 
During the wet coating process, air bubbles may become entrained between 
the fiber and the primary coating layer or within the primary or secondary 
coating layers. Bubbles in the fiber coating may cause several problems. 
Larger bubbles may extend through the coating thereby exposing the fiber 
to the environment and to mechanical effects, such as abrasion. Smaller 
sized bubbles cause losses in transmission. Bubbles may also prevent the 
fiber from being centered within the coating which can cause serious 
transmissions problems due to misalignment of the fiber cores when fibers 
are coupled. 
Several attempts have been made in the prior art to prevent bubbles from 
forming in the fiber coating. In one method, the fiber is advanced through 
an opening in a baffle plate positioned in a reservoir to alleviate or 
substantially eliminate entrapment of air and consequent bubble formation 
in the fiber coating. Bubbles are stripped from the region about the fiber 
due to a hydrodynamic pressure increase in the fluid pressure as the fiber 
passes through the constriction in the fluid path caused by the baffle 
plate. Pressure changes are made by changing the geometry of the 
arrangements such as, for example, the size of the baffle plate opening. 
Another coating technique is disclosed in Ohls, U.S. Pat. No. 4,246,299. A 
fiber is passed through an applicator having a die body that defines a 
small, vertically orientated, longitudinal tapered passage having a 
reservoir disposed about it. A series of radial ports provide fluid 
communication between the reservoir and the passage. Turbulence within the 
coating material, which causes entrapment of air bubbles, is reduced by 
maintaining the level of coating material in the passage. 
C. M. G. Jochem and J. W. C. Van der Ligt, in an article entitled "Cooling 
and Bubble-Free Coating of Optical Fibers at a High Drawing Rate", Journal 
of Lightwave Technology, Vol. LT-4, No.7, July, 1986, disclose a method 
for preventing air bubbles from forming in optical fibers during the wet 
coating process by using force coating. After the fiber is drawn from the 
perform heated by the draw furnace, the fiber is passed through a 
water-cooled tube made of a heat-conducting material such as aluminum. The 
tube is provided with special locks at the top and the bottom to prevent 
air from entering the tube as much as possible. The tube is filled with a 
gas which transfers heat from the fiber to the inner wall of the 
water-cooled tube. The inlet tube of the force-feed coating applicator is 
made of a material that is not wetted by the liquid acrylic. During the 
coating process, coating liquid is automatically fed into the coating 
applicator in such a way that the inlet tube is just filled so that the 
liquid surface is perpendicular to the fiber axis. This prevents a 
downward meniscus from being formed. The force-feed coating applicator 
thus is designed to avoid problems associated with open-cup coating 
applicators wherein air may be drawn into the liquid coating material 
contained in the open-cup coating applicator as the fiber is drawn into 
it. 
Although the techniques discussed in the foregoing may be successful in 
reducing the number of air bubbles contained in the coating layers 
surrounding an optical fiber, these techniques are not capable of 
completely preventing air bubbles from forming in the coating layers. 
Therefore, a need exists for a method of detecting when bubbles have 
formed in the coating layers during the optical fiber manufacturing 
process and for controlling the manufacturing process in accordance with 
the detection of bubbles to prevent or minimize their formation in the 
future. Ideally, such a detection system should be non-contact in order to 
reduce the possibility of abrading the fiber coating. 
It is generally known in the industry to monitor optical fibers as they are 
being drawn during the manufacturing process to determine whether defects 
exist in the optical fibers. However, such techniques generally are 
directed to monitoring the optical fibers themselves, rather than the 
coating layers, to determine whether defects such as cracks or holes exist 
in the optical fibers. 
It is also generally known to remove the defective optical fiber and/or to 
control the manufacturing process in accordance with the defects detected 
to prevent or minimize defects. For example, Bondybey et al, U.S. Pat. No. 
4,021,217, discloses a system for detecting optical fiber defects to 
determine the tensile strength of optical fibers as they are being 
manufactured and for adjusting the drawing conditions of the manufacturing 
process to eliminate, or reduce, the severity or number of defects. The 
apparatus disclosed in Bondybey et al. projects a focused beam of 
monochromatic light onto an optical fiber in a direction perpendicular to 
the axis of the fiber as it is being drawn. A photodetector, such as a 
photomultiplier, is positioned off axis with respect to the direction in 
which the light is projected onto the optical fiber so that it receives 
only scattered light unique to defects contained in the fiber. The output 
of the detector is received by an electrometer strip chart recorder which 
plots a scattering trace corresponding to the light detected. The peaks in 
the scattering trace correspond to defects in the optical fiber. 
Button et al., U.S. Pat. No. 5,185,636, discloses a method for detecting 
defects such as holes in a fiber. The apparatus utilizes a laser for 
projecting a beam of light onto the optical fiber in a direction 
perpendicular to the axial direction of the fiber. Two optical detectors 
are positioned on each side of the optical fiber at an angle with respect 
to the direction of the laser beam being projected onto the optical fiber. 
As a result of the coherence and monochromaticity of the laser beam, 
interference patterns are created in the far field which are detected by 
the optical detectors. Holes contained in the optical fiber result in 
fewer fringes in the interference patterns created in the far field. The 
apparatus analyzes the interference patterns by a variety of techniques to 
determine the presence of a hole in the optical fiber and can also monitor 
the growth of a hole over time. The apparatus can be used in conjunction 
with a control system which controls the drawing of optical fibers. 
Button et al., also discloses that a plurality of light sources must be 
used in order to ensure that light passes through the entire fiber, i.e., 
so that no blind spots exist. This is intended to ensure that light will 
be reflected off of holes contained at any location within the optical 
fiber and thus will be detected by the optical detectors. Spatial 
frequency spectra are generated based on the output of the light detectors 
and the spectra are analyzed to determine whether a hole exists in the 
optical fiber. 
The systems disclosed in Button et al. and Bondybey et al. are not well 
suited for detecting and distinguishing defects in the primary and 
secondary coatings of an optical fiber because those systems are 
configured such that defects contained in the fiber core will be detected 
and no provision is made for distinguishing between defects existing in 
the fiber core and defects existing the fiber coatings. Accordingly, a 
need exists for a non-contact detection system which detects defects 
contained in the primary or secondary coating layers of the optical fiber 
and delaminations between the primary coating layer and the fiber itself. 
A need also exists for a system which detects defects in optical fibers in 
real time as the fibers are being manufactured and which controls the 
manufacturing process to eliminate future defects and to separate 
defective fiber from non-defective fiber. 
SUMMARY OF THE INVENTION 
The present invention provides an optical detection system for detecting 
defects in an optical fiber. The system comprises a light source for 
coupling a beam of light into the coating layer of an optical fiber at a 
coupling spot along the optical fiber. The light source is positioned so 
that the beam of light is coupled into the coating layer at a preselected 
angle with respect to the longitudinal axis of the optical fiber. An 
optical detector is positioned adjacent the optical fiber a predetermined 
distance away from the coupling spot. The coupling spot is between the 
light source and the optical detector. By separating the optical detector 
from the coupling spot by a predetermined distance, detection of light 
reflected at the coupling spot from the outer surface of the coating layer 
is avoided. The optical detector receives at least a portion of the light 
coupled into the coating layer by the light source as the light coupled 
into the coating layer is reflected outward from the optical fiber by 
defects existing in the coating layer. A signal processor is electrically 
coupled to the optical detector for receiving an output signal from the 
optical detector and for processing the output signal to determine whether 
or not one or more defects have been detected. 
In accordance with a first embodiment, the light is coupled into the fiber 
coating at a sufficiently shallow angle with respect to the longitudinal 
axis of the fiber to cause the light to travel through the coating in a 
direction substantially parallel to the axis of the optical fiber for some 
distance before being attenuated by the coating. The angle at which the 
light is projected onto the fiber is selected in accordance with Snell's 
Law, and preferably is in the range of 50.degree. to 30.degree.. When 
defects such as air bubbles or delaminations are present in the coating 
layers, the light will be reflected out of the coating layers by the 
defects and away from the fiber. By placing the detector a preselected 
distance from the coupling point, detection of light reflected from the 
fiber at the coupling spot is avoided and only light reflected by defects 
contained in the primary or secondary coating layers, or delaminations 
between the primary coating layer and the fiber, will be detected by the 
detector. The output of the detector is processed and analyzed to 
determine whether or not defects, such as air bubbles and/or 
delaminations, have been detected. Based on this determination, the 
manufacturing process of the fiber can be controlled to prevent additional 
defects from forming. 
In accordance with the first embodiment of the present invention, a laser 
is used as the light source for projecting light into the fiber coating. 
The laser projects a laser beam into the coating, preferably at an angle 
of approximately 15 with respect to the outer surface of the secondary 
coating. The coupling spot will be located between the laser and the 
optical detector approximately 1 cm from the detector. The laser 
preferably is modulated by a square-wave transistor-transistor logic (TTL) 
signal having a minimum value of 0 volts and a maximum value of +5 volts. 
The laser is modulated at a predetermined rate which is sufficient for the 
speed at which the fiber is being drawn. The TTL signal preferably is 
generated by a processor which receives the output of the optical 
detector. In order to sample the fiber properly, the processor compensates 
for changes in the output of the detector caused by on and off switching 
of the laser. In accordance with one embodiment, this is accomplished by 
taking the difference between the output of the detector when the TTL 
signal is low and the output of the detector when the TTL signal is high 
and averaging the differences over a number of cycles of the TTL signal. 
This value is then analyzed to determine whether defects, such as air 
bubbles or delaminations, have been detected. 
In accordance with a second embodiment of the present invention, the laser 
is modulated with a square wave in the manner described above with respect 
to the first embodiment. However, in accordance with the second 
embodiment, synchronous detection is used to compensate for differences in 
the voltage level output from the detector caused by the on and off 
switching of the laser. Generally, the sampling of the optical detector is 
synchronized with the on and off switching of the laser to compensate for 
differences in the output of the detector resulting from modulation of the 
laser. 
In accordance with another embodiment of the present invention, the light 
is projected onto the optical fiber in a direction approximately 
perpendicular to the longitudinal axis of the optical fiber. Light 
reflected out of the fiber is detected by a high-speed photodetector. The 
output of the photodetector is processed and analyzed, preferably in real 
time, to determine whether defects, such as air bubbles or delaminations, 
have been detected. A processing means determines the type of defect 
detected by the photodetector by analyzing the levels of light detected 
and the period of time over which particular light levels are detected.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a preferred embodiment of the present invention for 
detecting bubbles in the primary and secondary coating layers of an 
optical fiber and delaminations between the primary coating layer and the 
fiber itself. Generally, an optical fiber is comprised of the fiber 1, a 
primary coating layer 11 and a secondary coating layer 13. The present 
invention comprises a light source 10, which preferably is a laser, and an 
optical detector 15, which preferable is a photodiode. In accordance with 
the preferred embodiment of the present invention, a coherent beam of 
substantially monochromatic light 12 is projected by laser 10 at a 
preselected angle 14 onto the secondary coating layer 13 of an optical 
fiber. The location 17 at which the light 12 is coupled into the secondary 
coating layer 13 of the fiber is a preselected distance d from optical 
detector 15, as discussed in more detail below with respect to FIGS. 6A 
and 6B. Preferably, the light source 10 is a 13 mW, 635 nm, diode laser 
sold by Power Technology, Inc. of Ark. The optical detector 15 preferably 
is a silicon photodiode with a built-in amplifier sold by Radio Parts of 
Denmark. 
The angle 14 generally is in the range of approximately 5.degree. to 
approximately 30.degree. and preferably is approximately 15.degree.. The 
angle 14 preferably is sufficiently shallow to cause the light to be 
refracted such that it travels through the primary and secondary coatings 
in a direction substantially parallel to the longitudinal axis of the 
fiber. The manner in which this angle is selected is discussed in detail 
below with respect to FIGS. 6A and 6B. Bubbles contained in the primary 
and secondary coating layers 11 and 13, respectively, and delaminations 
between the primary coating layer 11 and the fiber 1 will reflect light 
into detector 15, which preferably comprises a lens (not shown) for 
focusing the light onto a photodetector (not shown) contained in optical 
detector 15. 
FIG. 2 illustrates a block diagram of the preferred embodiment of the 
bubble detector of the present invention. A processor 16, which may 
comprise a microprocessor and other circuit components, modulates the 
laser 10 with a square wave. In accordance with one embodiment of the 
present invention, the processor 16 generates a TTL signal having a 
maximum value of +5 volts and a minimum value of 0 volts. Processor 16 
receives the output from detector 15 and processes the output to determine 
whether bubbles or delaminations exist. In accordance with a determination 
that bubbles or delaminations exist, the processor 16 can provide 
information to the manufacturing process (not shown) for adjusting 
parameters of the manufacturing process to prevent bubbles or 
delaminations from occurring in the future. 
In accordance with a first embodiment of the present invention, the 
processor 16 averages the output of detector 15 over a predetermined 
number of cycles of the square wave used to modulate laser 10 in order to 
compensate for changes in the output of detector 15 caused by differences 
in the amount of light detected by detector 15 when the laser 10 is off 
and when it is on. 
FIG. 3 illustrates a circuit diagram of an alternative embodiment of the 
present invention for generating the TTL signal for driving laser 10 and 
the trigger signal for providing synchronous sampling of the detector. An 
oscillation circuit 18 produces an oscillating waveform which is output to 
a laser driving circuit 21 and to a clamping circuit 20. The laser driving 
circuit 21 comprises a transistor 19 which is driven into saturation by 
the oscillating waveform to produce a square wave at collector 23 of 
transistor 19. The square wave is provided to laser 10 for driving the 
laser. 
The oscillating waveform produced by oscillation circuit 18 is provided to 
the noninverting input of operational amplifier 23 of clamping circuit 20. 
In addition to operational amplifier 23, the clamping circuit 20 comprises 
a pair of diodes 25 and a potentiometer 24 which cooperate to clamp the 
signal fed back to operational amplifier 23 such that the signal provided 
to phase shift circuit 26 is a smooth sinusoidal signal. Phase shift 
circuit 26 comprises a potentiometer 27 connected to the emitter of 
transistor 28 and connected in parallel with a capacitor 30. The capacitor 
30 is connected to the collector of transistor 28. The phase of the signal 
31 output from phase shift circuit 26 can be altered by adjusting 
potentiometer 27. 
FIG. 4 illustrates the synchronous detection circuitry which receives the 
output signal 31 of phase shift circuit 26. An optical detection circuit 
32 comprises a photodiode 33 which receives light reflected by bubbles and 
delaminations and produces an output voltage signal in response to the 
light detected. The output of photodiode 33 is input to operational 
amplifier 34 which amplifies the signal. The detection circuit 32 is 
coupled by a capacitor 35 to an amplification circuit 37 which comprises a 
potentiometer 38 for adjusting the gain of amplifier 40 to a suitable 
level. The capacitor 35 filters out the dc light detected by photodiode 
33. The output of amplifier 40 is input to synchronous detector circuit 
41, which preferably comprises an AD630 integrated circuit 42 which is 
designed to produce a dc signal proportional to the amount of light 
detected by photodiode 33. The output 31 of phase shift circuit 26 is used 
to switch the AD630 integrated circuit 42 on an off, which ensures that 
the output of photodiode 33 is sampled at the proper time. The output 31 
is provided to amplifier 45 which amplifies the signal to a level suitable 
for switching the AD630 integrated circuit 42 off and on. 
The output signal 50 of the detection circuitry of FIG. 4 is provided to an 
integrator circuit 51 which integrates the signal to produce an output 
voltage across resistor 52. The output signal across resistor 52 may be 
provided to a bar graph display device which displays LED bars to an 
operator which indicate whether bubbles or delaminations are being 
detected and, if so, the frequency of occurrence and/or the size of the 
bubbles or delaminations. The signal output from integrator circuit 51 is 
also provided to a differentiator circuit 55 which takes the derivative of 
the integrated signal and provides an output signal to a nixi-type 
display. The nixi-type display comprises a circle of LEDs, only one of 
which is lit at any particular time. Events which are too short to be 
picked up by the bar graph display will cause the nixi display to advance 
the LED lit by one, thus allowing short events, e.g., the presence of a 
single bubble or a small group of bubbles, to be observed by an operator. 
In response to a determination by the operator that bubbles have been 
detected, the operator can adjust the manufacturing parameters to 
eliminate the bubbles or to prevent bubbles from forming in the future. 
For example, the operator may decrease the coating pressure of the wet 
coating process by regulating a pump which provides coating material to a 
coating die. In accordance with an alternative embodiment of the present 
invention, the output signal 50 of detection circuitry 41 is provided to a 
processor, such as processor 16 shown in FIG. 1, which processes the 
signal and provides control signals to the manufacturing process to 
automatically control parameters of the manufacturing process in real time 
to eliminate bubbles and delaminations. 
When bubbles or delaminations have been detected, or when the number and/or 
size of the bubbles detected has exceeded a preselected threshold, the 
fiber being drawn may be switched from a first take-up roll (not shown) to 
a second take-up roll (not shown) such that the "good" fiber is separated 
from the "bad" fiber. This can be done by an operator or it can be 
accomplished automatically by processor 16 and suitable machinery (not 
shown). The manner in which this may be accomplished will be apparent to 
those skilled in the art. The bubble detector of the present invention 
preferably is mounted on a draw tower at a location along the 
manufacturing line after the primary and secondary coating layers have 
been applied. 
FIGS. 6A and 6B demonstrate the importance of the spatial relationship 
between the coupling spot and the detector. In accordance with this 
embodiment of the present invention, it has been determined that the 
optical detector should be located a distance d ranging from approximately 
1 mm to approximately 10 cm from the coupling point, as shown in FIG. 6A. 
This relationship ensures that light reflected at the coupling point from 
the outer surface of the fiber coating will not be picked up by the 
detector. The angle at which the light is projected onto the secondary 
coating of the optical fiber preferably is within the range of 50.degree. 
to 30.degree., and preferably is approximately 15.degree.. However, this 
angle will depend on the refractive index of the coating material. The 
relationship between the angle of projection and the index of refraction 
of the coating is defined by the well known law of refraction, also known 
as Snell's Law. Snell's Law is stated as: sin.theta..sub.1 
/sin.theta..sub.2 =n.sub.2 /n.sub.1, where .theta..sub.1 is the angle 
between the normal to the surface of the coating and the light ray, 
.theta..sub.2 is the angle between the refracted ray and a line continuing 
from the normal below the surface of the coating (.apprxeq.90.degree.), 
n.sub.1 is the refractive index of air (.apprxeq.1.0), and n.sub.2 is the 
refractive index of the coating layer (.apprxeq.1.5-1.7). Since 
.theta..sub.2, n.sub.1 and n.sub.2 are known, .theta..sub.1, the angle of 
incidence of the light ray being projected onto the fiber, can be easily 
determined. Once .theta..sub.1 is determined, the angle of projection 
(indicated by numeral 14 in FIG. 1) is calculated by subtracting 
.theta..sub.1 from 90.degree.. Once the light is coupled into the coating 
layers, the light will travel within the coating layers until either the 
light is attenuated by the coating layers, or it is reflected out of the 
coating layers by defects therein. 
As shown in FIG. 6A, when bubbles or delaminations are present, a portion 
of the light projected onto the coating layer is reflected by the bubbles 
or delaminations through the fiber coating and out of the coating toward 
the detector. FIG. 6B demonstrates that when bubbles or delaminations are 
not present, light projected onto the coating layer passes through the 
fiber to a beam dump which absorbs the light. The light also passes 
through the coating layers in the axial direction of the fiber, with 
virtually no light being reflected or refracted toward the detector. It 
should be noted that the detector will even detect the defects located on 
the opposite side of the optical fiber from the detector. This is because 
the light from the light source floods the coating layers, sending light 
around the entire circumference of the fiber. 
FIGS. 7A-7C illustrate the relationship between the draw speed of the fiber 
and the formation of bubbles or delaminations. In FIG. 7A, the upper trace 
represents draw speed and the lower trace represents the output of the 
detection circuitry. FIG. 7A demonstrates that most of the bubbles or 
delaminations occur at start up as the draw speed is being increased. The 
output of the detection circuitry is shown to be approximately -1 volt at 
start up, which corresponds to the largest magnitude of the detection 
signal. FIG. 7B shows an expanded view of the voltage trace of FIG. 7A. 
The signal contains noise from dc light and other sources which is easily 
filtered out by the synchronous detection circuit of the present 
invention. FIG. 7C shows the detection of bubbles formed during and after 
start up. In order to prevent or minimize the detection of light from 
other sources, the bubble detector of the present invention may be located 
inside of a housing assembly (not shown) so that only light projected onto 
the fiber at the coupling point and reflected by bubbles or delaminations 
is detected by the detection circuitry. The wavelength of light used may 
also be chosen so as to eliminate unwanted noise. The optical detector of 
course will be compatible with the wavelength of light used. 
FIG. 8 illustrates an alternative embodiment of the present invention for 
detecting air bubbles, delaminations and other defects in an optical 
fiber. A light source 70, preferably a 900 nm narrowband infrared laser, 
projects light onto the optical fiber 75 at a preselected angle with 
respect to the longitudinal axis of the fiber 75. The angle of projection 
is almost perpendicular to the axis of the fiber and preferably is 
approximately 75.degree.. A magnification lens 80 is positioned a 
preselected distance away from the coupling point adjacent a high speed 
photodetector 82. The photodetector 82 preferably is a reverse biased, 
gallium aluminum arsenide infrared photodetector (operating in the 
photoconductive mode). Light reflected by bubbles, delaminations or other 
defects contained in the fiber coating or between the fiber coating and 
the fiber itself is focused by lens 80 through a slit 85 onto 
photodetector 82. 
FIG. 9 illustrates an enlarged top view of slit 85. Slit 85 preferably is 
approximately 50 micrometers in width. The amount of light detected by 
photodetector 82 will depend on the amount of light passing through slit 
85 which, in turn, will depend on the size and type of defect detected. 
Therefore, the size and/or number of air bubbles, delaminations or other 
defects can be determined. Furthermore, the type of defects existing in 
the coating layers or between the primary coating and the fiber can be 
determined as well. In accordance with the present invention, it has been 
determined that the output of the photodetector will be different for air 
bubbles and delaminations. When delaminations are being detected, the 
output of the photodetector will undergo a voltage transition from high at 
the beginning of the delamination to low for the duration of the 
delamination to high at the end of the delamination. In contrast, when air 
bubbles are being detected, the output of the photodetector will be high 
when a bubble is present and low otherwise. Thus, the duration of the 
photodetector output signal as well as the magnitude of the output signal 
can be analyzed to determine the size and type of defect being detected. 
FIG. 10 is a plot of light intensity as a function of time which represents 
the output of photodetector in the time domain relative to a baseline 
voltage. The difference between the output of the photodetector when a 
bubble is detected as opposed to when a delamination is detected can 
readily be seen in the figure. When a bubble is detected, the light 
intensity increases from the low range to a maximum level in the high 
range for the duration of the bubble and then decreases until it returns 
to the baseline voltage. In contrast, when a delamination is detected, the 
light intensity increases from the low range to a maximum value in the 
high range at the leading edge of the delamination and then decreases to a 
level in the mid range which is greater than the baseline voltage but less 
than the maximum light level detected at the leading edge of the 
delamination. At the end of the delamination, the light intensity detected 
by the photodetector once again increases to its maximum level and then 
decreases to the baseline value. These characteristics are used to 
determine whether defects exist and, if so, the types of defects. 
FIGS. 11A and 11B are flow charts illustrating the data acquisition and 
defect determination processes of the present invention in accordance with 
the embodiment of FIG. 8. As stated above, in order to determine the type 
of defect being detected, the fiber speed and the photodetector output as 
a function of time must be taken into account. In block 100, a timer is 
started and the fiber speed is stored in memory. The timer may be, for 
example, a counter which is periodically incremented. Blocks 102-106 
correspond to a routine for establishing a baseline voltage which will be 
compared to the processed output of the photodetector for determining 
relative increases and decreases of the light intensity. Thus, blocks 
102-106 correspond to a calibration routine. 
In block 102, the analog-to-digital converter (ADC) which receives the 
output of the photodetector (see FIG. 12) is sampled. In block 103, the 
sampled output of the ADC is stored in memory. Preferably, the output of 
the ADC is stored in the accumulator of the microprocessor so that the 
data can be processed very quickly. In block 104, the ADC is sampled 
again. In block 105, a determination is made as to whether the sampled 
output of the ADC is in the low range. If the value is in the low range, 
the process proceeds to step 106 where the value is averaged with the 
value stored in the accumulator in block 106 and the average of the values 
is saved in the accumulator in step 103. The averaged value saved in the 
accumulator corresponds to the baseline voltage shown in FIG. 10. 
By averaging the digitized output of the photodetector when the values are 
in the low range, small variations in light intensity, which may be the 
result of noise, are ignored. In block 105, if the value obtained from the 
ADC in block 104 is not in the low range, the value of the ADC and the 
time at which the ADC is sampled are stored in memory in step 108. A 
delamination flag, described in more detail below, is reset at this point 
in the process. When the value of the ADC is not in the low range, the 
system determines that a defect has been detected. In block 109, a 
determination is made as to whether the sampled value is in the high 
range. If so, the process proceeds to step 110 where the "high" flag is 
set. The ADC is then sampled again in step 111 and the process returns to 
step 109 where a determination is again made as to whether the sampled 
value of the ADC is in the high range. The process continues in the loop 
defined by blocks 109-111 until the sampled value of the ADC is no longer 
in the high range. When a determination is made at step 109 that the value 
of the ADC is not in the high range, the process proceeds to step 113 
where a determination is made as to whether the value of the ADC is in the 
mid range. If the value of the ADC is in the mid range, then the system 
determines that a delamination is being detected. The mid range flag is 
then set in step 114 and the ADC is again sampled in step 115. The process 
continues in the loop defined by blocks 113-115 until a determination is 
made in block 113 that the sampled value of the ADC is no longer in the 
mid range. 
Once a determination has been made in block 113 that the value of the ADC 
is no longer in the mid range, the process proceeds to block 116 where a 
determination is made as to whether the value of the ADC is in the high 
range. If the value of the ADC is in the high range, a determination is 
made at block 117 as to whether the high and mid range flags have 
previously been set. If so, the system determines that a delamination has 
been detected and an indication is provided at block 118. A defect 
attribution set may also be stored in memory at this point which 
corresponds to the type of defect detected. If a determination is made at 
block 1 16 that the value of the ADC is not in the high range, the process 
returns to step 105 where the process starts over. If a determination is 
made at block 116 that the value of the ADC is in the high range, and then 
a determination is made at block 117 that the high and mid range flags 
have not previously been set, then the system determines that a bubble, as 
opposed to a delamination, has been detected. The process then returns to 
block 105. 
FIG. 12 is a block diagram of the detection and defect determination 
circuitry of the embodiment shown in FIG. 8 which performs the data 
acquisition and defect determination routines shown in FIGS. 11A and 11B. 
A reverse biased photodetector 130 detects the light reflected by defects 
in the fiber. As stated above, the photodetector preferably is a gallium 
aluminum arsenide infrared detector with a band width of 900 nanometers. 
Preferably, a visible light filter 131 is coupled to the photodetector 130 
for filtering out visible light. The output of photodetector 130 is input 
to a high-bandwidth, high-gain DC-coupled transimpedance amplifier 132. 
The amplifier converts the current signal output from the photodetector 
130 into a voltage signal. The amplified voltage signal output from 
amplifier 132 is input to analog-to-digital converter (ADC) 135, which 
preferably samples the output of amplifier 132 at a rate of 40 MHz. 
The digital signal produced by ADC 135 is provided to processing circuitry 
136 which may comprise, for example, an Intel 80486 microprocessor 
operating at 100 MH. The microprocessor is programmed with software for 
processing the output of ADC 135 to determine the types of defects 
detected by photodetector 130. The software preferably performs the 
routines discussed above with respect to FIGS. 11A and 11B. However, it 
will be apparent to those skilled in the art that software capable of 
accomplishing the goals of the present invention can be written in a 
variety of different ways and that the present invention is not limited to 
routines discussed above. The output of the processing circuitry 136 
preferably is coupled to a flaw annunciator 140 which provides an 
indication that a flaw has been detected and the type of flaw detected. 
The processing circuitry 136 is coupled to a memory device which serves as 
a data logger by storing all of the information relating to the output of 
photodetector 130 including a time domain representation of the output of 
photodetector 130. The processing circuitry 136 must be sufficiently fast 
so that it is capable of analyzing the digital signals from ADC 135 in 
real time. 
It should be noted that although the present invention has been described 
with respect to particular embodiments, the present invention is not 
limited to these embodiments. It should also be noted that modifications 
and alterations to the method and system of the present invention can be 
made without deviating from the spirit and scope of the present invention. 
For example, it will be apparent to those skilled in the art that 
components other than those discussed above can be used to construct the 
system of the present invention. The type of optical detector used can be 
any type suitable for detecting defects in the fiber. For example, a 
photodiode or a charge coupled device (CCD array) may be used to detect 
the defects. A frame grabber may also be used in conjunction with the 
photodetector to capture images of the fiber from which a determination 
will be made as to the existence or nonexistence of defects. It will also 
be also be apparent to those skilled in the art that methods other than 
those discussed above can be used to accomplish the goals of the present 
invention which are within the spirit and scope of the present invention. 
For example, it will be apparent to those skilled in the art that the 
optical fibers can be inspected after the primary coating layer has been 
applied but before the secondary coating has been applied, although the 
optical fibers preferably are inspected after both the primary and 
secondary coating layers have been applied. It should also be noted that 
the number of and/or the arrangement of the light sources and detectors 
can be modified so that optimum lighting and detection conditions are 
obtained.