Method for detecting bubbles and inclusions present in optical fiber preform and apparatus for detecting same

A method and an apparatus for accurately, thoroughly and automatically inspecting an optical fiber preform for the presence of bubbles and/or inclusions are herein provided. The inspection apparatus comprises light source 2 for making light rays incident upon the end face 1a of rod-like optical fiber preform 1 as a subject to be inspected, video camera 7 for photographing the side face of the optical fiber preform 1 and image-processing circuit 11 for discriminating and detecting bubbles and/or inclusions present in the preform 1 through processing of image signals outputted from the video camera 7 and inputted to the circuit 11.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for photoelectrically detecting 
the presence of bubbles and/or inclusions in an optical fiber preform as 
well as an apparatus for detecting these bubbles and/or inclusions. 
Bubbles and inclusions present in an optical fiber become causes of fatal 
defects, since they scatter and/or absorb light rays which transmit 
through the optical fiber to thus attenuate the intensity of the 
transmitting light rays and reduce the strength of the optical fiber. When 
drawing an optical fiber preform into an optical fiber, the optical fiber 
may sometimes be broken due to the presence of these bubbles and 
inclusions. It is thus needed to inspect an optical fiber preform for the 
presence of bubbles and inclusions prior to drawing it into an optical 
fiber in order to eliminate the foregoing problems. An optical fiber 
preform has conventionally been inspected for the presence of bubbles 
and/or inclusions by making light rays incident upon the preform and 
visually observing the preform, with the naked eyes, for the presence of 
bright spots formed by the bubbles and/or inclusions present therein which 
scatter the light rays incident upon the preform. This is one of sensory 
tests and therefore, there are differences between individuals in the 
detection of such bright spots. Moreover, there has been desired for the 
establishment of a method for mechanically inspecting optical fiber 
preforms, for the elimination of the foregoing problem and for the 
elimination or reduction of labor. 
A method for inspecting the influence of bubbles present in a quartz tube 
used as a jacket for an optical fiber on various properties of the 
resulting optical fiber was reported at the Annual Meeting, the 59th year 
of Showa, of Electronics Telecommunication Society of Japan, A National 
Convention of the Branch of Optics Radio Wave. The gist thereof is 
disclosed in the article entitled "Effects of Bubbles Present in Quartz 
Tubes on the Strength of Optical Fibers", the published Resume for the 
Meeting, 2-165, 421. According to this article, the method comprises 
making a laser beam incident upon the end face of a quartz tube while 
rotating the quartz tube and detecting light rays scattered and reflected 
by bubbles present in the tube with a photomultiplier. The quartz tube is 
irradiated with the laser beam along the circumference of the tube as the 
tube is rotated and the quartz tube can be inspected by scanning the 
photomultiplier along the longitudinal direction of the tube. 
However, this method is inconvenient for the detection of bubbles 
throughout the whole length of the quartz tube. This is because, the cross 
sectional area of the laser beam is substantially smaller than the 
thickness of the quartz tube and accordingly, it is impossible to 
irradiate the whole cross section of the tube with the laser beam. When 
inspecting, in particular, a subject having a circular cross section such 
as an optical fiber preform, only a part thereof can be inspected by 
irradiating the cross section thereof with a laser beam. For this reason, 
there has not yet been proposed any method for mechanically detecting 
bubbles and/or inclusions present in an optical fiber preform. 
SUMMARY OF THE INVENTION 
The present invention has been developed for the purpose of solving the 
foregoing problems associated with the conventional techniques for 
inspecting an optical fiber preform for the presence of bubbles and/or 
inclusions and accordingly, an object of the present invention is to 
provide a method for automatically and accurately inspecting an optical 
fiber preform throughout the entire cross section and length thereof for 
the presence of bubbles and/or inclusions as well as an apparatus for 
practicing the method. 
The method for inspecting an optical fiber preform for the presence of 
bubbles and/or inclusions according to the present invention comprises 
making light rays incident upon the whole end face of a rod-like optical 
fiber preform, photographing an image of the side face of the optical 
fiber preform by a video camera and analyzing the image signals of the 
photographed image to detect and discriminate bubbles and/or inclusions. 
According to another aspect of the present invention, the method for 
inspecting an optical fiber preform for the presence of bubbles and/or 
inclusions comprises making light rays incident upon the whole end face of 
a rod-like optical fiber preform, photographing an image of the side face 
of the optical fiber preform by a video camera, differentiating the image 
signals of the photographed image with respect to intensity change in the 
direction of the longitudinal axis of the optical fiber preform and 
comparing the resulting differential value with a predetermined reference 
value to thus detect and discriminate bubbles and/or inclusions which are 
defined to be bright spots whose value obtained through the 
differentiation is not less than the reference value. 
The inspection of the rod-like optical fiber preform may be performed in 
the air or the preform may be inspected while dipping it in a matching 
oil. 
When a rod-like optical fiber preform is inspected, in particular, in the 
air, the detection and discrimination of the presence of bubbles and/or 
inclusions are preferably carried out by differentiating the image signals 
of the photographed image with respect to intensity change in the 
direction of the longitudinal axis of the optical fiber preform and 
comparing the resulting differential value with the predetermined 
reference value to thus determine whether the former is greater than the 
latter and to detect and discriminate bubbles and/or inclusions which are 
defined to be bright spots whose value obtained through the 
differentiation is not less than the reference value. 
In the foregoing inspection method, it is preferred to make light rays 
incident upon the optical fiber preform and photograph the images of the 
preform by the video camera while rotating the optical fiber preform 
around the longitudinal axis of the optical fiber preform. 
According to a further aspect of the present invention, there is provided 
an apparatus for inspecting an optical fiber preform for the presence of 
bubbles and/or inclusions which comprises, as will be seen from FIG. 1 
showing an embodiment of the present invention, light source 2 for making 
light rays incident upon end face 1a of rod-like optical fiber preform 1 
as a subject to be inspected, video camera 7 for taking photographs of 
images of the side face of the preform 1 and image-processing (or 
-analyzing) circuit 11 for detecting and discriminating bubbles and/or 
inclusions on the basis of image signals outputted from the video camera 7 
and inputted thereto. 
The image-processing circuit 11 comprises, as shown in FIG. 2, 
differentiation circuit 15 which differentiates the image signals with 
respect to the intensity change along the direction of the longitudinal 
axis of the optical fiber preform and discrimination circuit 17 for 
comparing the value obtained through the differentiation performed by the 
differentiation circuit 15 with a reference value and this structure 
permits the elimination of any influence, on the inspection, of the light 
rays scattered by the outer surface of the optical fiber preform 1. 
As will be seen from FIG. 3, any influence of the light rays scattered by 
the outer surface of the optical fiber preform 1 may likewise be 
eliminated if the apparatus is designed such that it is provided with 
means 25 for dipping the optical fiber preform 1 as the subject to be 
inspected in a matching oil. 
The inspection apparatus preferably comprises driving means 20 for rotating 
the optical fiber preform 1 around the longitudinal axis of the preform 1 
as shown in FIG. 1. 
In the inspection apparatus shown in FIG. 1, the whole end face 1a of the 
rod-like optical fiber preform 1 is irradiated with light rays emitted 
from the light source 2. The incident light rays are scattered by bubbles 
and/or inclusions present in the preform 1. At this stage, the side face 
of the preform 1 is photographed by the video camera 7 and the images of 
these bubbles and/or inclusions are simultaneously photographed by the 
video camera 7. The image signals are outputted from the video camera 7 
and then inputted to the image-processing circuit 11 which processes the 
image signals to detect and discriminate bubbles and/or inclusions present 
in the preform. If bubbles and/or inclusions are not present in the 
optical fiber preform 1, the light rays incident upon the end 1a transmit 
through the optical fiber preform without causing any scattering and any 
image is not detected by the video camera 7. Thus, it can be concluded 
that the optical fiber preform 1 is substantially free of bubbles and/or 
inclusions. 
When the rod-like optical fiber preform 1 is examined in the air as shown 
in FIG. 1, reflected images formed by the outer surface of the preform 1 
serve as noises during photographing the preform 1 by the video camera 7 
and often make the discrimination or detection of bubbles and/or 
inclusions difficult. However, these bubbles and/or inclusions can 
correctly be discriminated if the image-processing circuit 11 has a 
structure as shown in FIG. 2. More specifically, image signals relating to 
bubbles and/or inclusions present in the optical fiber preform 1 exhibit 
rapid change in the intensity along the longitudinal axis of the preform 
1, while image signals corresponding to the images formed due to the 
reflection of light by the outer surface of the preform 1 do not show any 
significant change in the intensity along the longitudinal axis of the 
preform 1. Therefore, the image signals outputted from the video camera 7 
are differentiated with respect to the intensity change in the direction 
of the longitudinal axis of the preform 1 through the action of the 
differentiation circuit 15 and the value obtained through the 
differentiation operation performed in the differentiation circuit 15 is 
compared with a reference value previously stored in discrimination 
circuit 17 to detect and discriminate bubbles and/or inclusions which are 
defined to be positions whose value obtained through the differentiation 
is not less than the reference value and to thus eliminate any influence 
of the light rays scattered by the outer surface of the optical fiber 
preform 1. 
When a rod-like optical fiber preform 1 is inspected for the presence of 
bubbles and/or inclusions while dipping the preform in a matching oil, 
bright spots formed on the photographs taken by the video camera 7 are 
ascribed to bubbles and/or inclusions present in the preform 1, since the 
outer surface of the preform does not affect these bright spots. 
As has been shown in FIG. 3, any influence of the reflection by the outer 
surface of the optical fiber preform 1 on the detection or discrimination 
of bubbles and/or inclusions can be eliminated by making use of means 25 
for dipping the preform 1 in a matching oil during the inspection and, 
therefore, any image formed through the reflection of light rays on the 
outer surface of the preform 1 is not detected by the video camera 7. 
Thus, bright spots formed on the photographs taken by the video camera 7 
are correctly ascribed to bubbles and/or inclusions present in the preform 
1. 
As has been explained above in detail, the method and apparatus of the 
present invention permit correct, thorough and automatic detection of 
bubbles and/or inclusions present in an optical fiber preform 1. Moreover, 
the method and apparatus permit the elimination of the need for 
macroscopic inspection in which bubbles and/or inclusions are observed 
with the naked eyes which has conventionally been adopted and serve to 
economize the inspection of the optical fiber preforms. Thus, the method 
and apparatus according to the present invention allows the elimination of 
various drawbacks associated with the production of optical fibers, at the 
stage of the preform. For instance, they can prevent any breakage of an 
optical fiber during drawing an optical fiber preform into an optical 
fiber and can prevent the formation of optical fibers having poor 
transmittance.

DETAILED EXPLANATION OF THE INVENTION 
Embodiments of the method and apparatus of the present invention will 
hereinafter be described in more detail. 
FIG. 1 is a perspective view showing an embodiment of the apparatus for 
inspecting an optical fiber preform for the presence of bubbles and/or 
inclusions according to the present invention. The inspection apparatus 
shown in FIG. 1 is assembled on chassis 3 and comprises light source 2 
emitting light rays which are incident upon the whole end face 1a of 
optical fiber preform 1, video camera 7 for taking a photograph of the 
side face of the optical fiber preform 1 and image-processing circuit 11 
for detecting and discriminating bubbles and/or inclusions present in the 
preform 1 through processing of the image signals outputted from the video 
camera 7 and inputted to the circuit 11. The optical fiber preform 1 is 
connected to rotating-driving means 20 which comprises motor 21 and gears 
22 and 23. 
One end of the optical fiber preform 1 is held in chuck 30a fitted to 
rotary holder 31a and the end 1a thereof is exposed and positioned on the 
side of the light source 2. Another end of the preform 1 is held in chuck 
30b fitted to rotary holder 31b. The gear 23 is fitted to a rotary shaft 
of the chuck 30b and engages with the gear 22 connected to the motor 21. 
The light source 2 comprises white lamp 4, aperture mask 5 and condenser 
lense 6, wherein the aperture mask 5 is positioned at the focal point of 
the condenser lense 6. The aperture of the condenser lense 6 has a size 
sufficient for allowing the encompassment of the whole end face 1a of the 
optical fiber preform 1 and the light rays emitted from the white lamp 4 
is uniformly incident upon the whole end face 1a of the optical fiber 
preform 1 in the form of approximately parallel rays. 
The video camera 7 is a CCD (charge coupled device) camera and 
photographing lense 29 is slidably fitted to frame 33, fixed to the 
chassis 3, in such a manner that the lense 29 faces the side face of the 
optical fiber preform 1 and is parallel to the longitudinal direction of 
the preform 1. The photographing range which is encompassed by the 
photographing lense 29 corresponds to the range of the optical fiber 
preform 1 capable of being inspected. If the photographing range does not 
embrace the whole longitudinal length of the optical fiber preform 1, the 
optical fiber preform 1 can be separately inspected, over several times, 
throughout the whole longitudinal length thereof by sliding the video 
camera 7 along the frame 33. In general, the thickness of the optical 
fiber preform 1 is substantially smaller than the length thereof and, 
therefore, the inspection of the whole range in the direction of the 
thickness of the preform can be ensured by a single photographing. 
The video camera 7 is connected to memory 10 for storing the photographed 
image signals, to image-processing circuit 11 for detecting and 
discriminating bubbles and/or inclusions through the processing of the 
image signals invoked from the memory 10, to display device 12 for 
displaying the bubbles and/or inclusions detected by the image-processing 
circuit 11 and to printer 13 for printing out the contents displayed on 
the device 12. 
As will be seen from FIG. 2, the image-processing circuit 11 comprises 
differentiation circuit 15, reference value-establishing circuit 16 and 
comparison-discrimination circuit 17. The differentiation circuit 15 
performs differentiation of image signals with respect to the intensity 
change thereof in the direction of the longitudinal axis of the preform 1 
to thus determine the rate of variation (magnification of the change in 
the intensity of the image signals). The reference value-establishing 
circuit 16 can establish a reference value which is n times the rate of 
variation (wherein n ranges from 2 to 20). The comparison-discrimination 
circuit 17 compares the rate of variation determined by the 
differentiation circuit 15 with the reference rate of variation (threshold 
level) established by the reference value-establishing circuit 16, then 
judges whether the value obtained through the differentiation is greater 
than the threshold level or not to thus discriminate bubbles and/or 
inclusions present in the optical fiber preform 1 whose value obtained 
through the differentiation is not less than the threshold level and 
outputs the coordinates of these bubbles and/or inclusions to the display 
device 12 and the printer 13. 
A sample of an optical fiber preform which has been proved to have 6 
bubbles having a diameter on the order of about 0.1 mm was practically 
inspected by fitting it to the inspection apparatus having the structure 
as shown in FIG. 1. The reference rate of variation in the reference 
value-establishing circuit 16 of the image-processing circuit 11 was set 
to 5 times (i.e., n=5). When the white lamp 4 is lighted, light rays 
emitted from the lamp were uniformly incident upon the end face 1a of the 
optical fiber preform 1. The light rays were scattered by bubbles present 
in the optical fiber preform 1, the resulting optical image was 
photographed by the video camera 7, the image signal outputted from the 
video camera was processed in the image-processing circuit 11 and 
displayed on the display device 12. At this stage, these 6 fine bubbles 
having a diameter on the order of about 0.1 mm were all displayed on the 
device 12 and simultaneously spots were appeared at the positions 
corresponding to the line showing the external shape of the optical fiber 
preform 1. However, these spots displayed at the positions corresponding 
to the line showing the external shape of the preform 1 were disappeared 
and only these 6 bubbles were displayed on the device 12 when the 
reference rate of variation in the reference value-establishing circuit 16 
was reset to 10 times (i.e., n=10). Thus, only the coordinates of these 
bubbles could be confirmed. 
By way of comparison, the following inspection was performed. An apparatus 
used in this inspection was slightly different, from that shown in FIG. 1, 
in the structure of the image-processing circuit 11. More specifically, 
the comparison-discrimination circuit 17 of this image-processing circuit 
11 compares the intensity of an image signal with the established 
reference value of n times the intensity of the signal serving as the 
threshold level. The established reference value was set to 20 times 
(i.e., n=20) and the same sample of an optical fiber preform used in the 
foregoing example was inspected. Strong noises were appeared on the 
display device 12 at the positions corresponding to the line showing the 
external shape of the preform 1 and accordingly, any clear images of the 
fine bubbles could not be displayed on the device 12. At this stage, when 
the reference rate of variation in the reference value-establishing 
circuit 16 was gradually increased and reset to 50 times (i.e., n=50), the 
noises were attenuated to an extent that spots were displayed on the 
device 12 at the positions corresponding to the line showing the external 
shape of the preform 1, but 2 bubbles having smaller diameters out of 
these 6 bubbles were disappeared (only 4 bubbles were displayed and thus 
confirmed). Moreover, when the reference rate of variation was reset to 80 
times (i.e., n=80), the noises corresponding to the line showing the 
external shape of the preform 1 were completely disappeared, but 1 
additional fine bubble having a smaller diameter was also disappeared and 
only 3 out of the 6 bubbles could be confirmed. 
When the motor 21 of the apparatus shown in FIG. 1 is operated, the 
rotational motion thereof is transmitted to the gears 22 and 23 and as a 
result, the optical fiber preform 1 is rotated. If the motor 21 is not 
operated, only the coordinates of bubbles and/or inclusions present along 
the longitudinal direction of the preform 1 can be confirmed since the 
positions of these bubbles and/or inclusions displayed on the display 
device 12 are not changed. On the other hand, when the motor 21 is 
operated, the positions of bubbles and/or inclusions displayed on the 
device 12 cause fluctuation due to the rotational motion of the preform 1 
and accordingly, the precise positions of these bubbles and/or inclusions 
in a particular cross sectional area of the preform 1 can be confirmed. 
FIG. 3 shows another embodiment of the apparatus according to the present 
invention, which is capable of removing noises generated due to the line 
showing the external shape of the optical fiber preform 1. This apparatus 
shown in FIG. 3 has a structure identical to that shown in FIG. 1 except 
that the optical fiber preform 1 is dipped in matching oil 25. In FIG. 3, 
the reference numeral 26 represents a container of the matching oil 25 and 
the reference numeral 27 represents a packing. The matching oil 25 is an 
oil having a refractive index approximately identical to that of the 
optical fiber preform 1. Light rays are incident upon the end face of the 
preform 1 from the direction indicated by an arrow and the side face of 
the preform 1 is photographed by the video camera 7 (not shown in FIG. 3) 
through the photographing lense 29 and the matching oil 25. The line 
showing the external shape of the preform 1 is not detected by the video 
camera 7 due to the presence of the matching oil and the video camera 7 
simply receives images of bright spots formed through scattering of the 
light rays by bubbles and/or inclusions present in the optical fiber 
preform 1. For this reason, the use of the foregoing image-processing 
circuit 11 is not necessary in this embodiment. 
In the foregoing embodiments, white lamp 4 is used as a light source, but 
the use of a source of diffused light such as a halogen lamp and a 
tungsten lamp is effective since light rays emitted from the source can be 
incident upon the whole end face 1a of the optical fiber preform 1.