8/10/2019 Unit 5-VI

1/2

Lecture No: 35 Date: 09/04/2013

Period: V

Topic: Fiber diameter measurements

Fiber diameter measurements:

Outer diameter:On-line diameter measurement systems are used to provide accuracy better than 0.3% at ameasurement rate greater than 100 Hz (i.e. a typical fiber draing velocity is 1 m s!1".

#he most

common on-line measurement techni$ue uses fiber image proection (shado method" and is

illustrated in &igure '.13.

n this method a laser beam is sept at a constant velocity transversely across the fiber and a

measurement is made o) the time interval during hich the fiber intercepts the beam and casts ashado on a photodetector. n the apparatus shon in &igure '.13 the beam )rom a laser

operating at a avelength o) 0.*3+, m is collimated using to lenses ( 1and +". t is then

re/ected o)) to mirrors (1and +" the second o) hich (+" is driven by a galvanometer

hich ma2es it rotate through a small angle at a constant angular velocity be)ore returning to itsoriginal starting position. #here)ore the laser beam hich is )ocused in the plane o) the fiber by a

lens (3" is sept across the fiber by the oscillating mirror and is incident on the photodetector

unless it is bloc2ed by the fiber. #he velocity dsdt o) the fiber shado thus created at thephotodetector is directly proportional to the mirror velocity d4dt )olloing5

6here Iis the distance beteen the mirror and the photodetector.

&urthermore the shado is registered by the photodetector as an electrical pulse o) idth 6ehich is related to the fiber outer diameter doas5

#hus the fiber outer diameter may be $uic2ly determined and recorded on the printer. #he

measurement speed is largely dictated by the inertia o) the mirror rotation and its accuracy by the

rise time o) the shado pulse.

8/10/2019 Unit 5-VI

2/2

Lecture No: 35 Date: 09/04/2013

Period: V

Topic: Fiber diameter measurements

Figure 5.13 The shadow method for the on-line measurement of the fiber outer

diameter.

Core diameter:

#he core diameter )or step inde7 fibers is defined by the step change in the re)ractive inde7

profile at the core8cladding inter)ace. #here)ore the techni$ues employed )or determining the

re)ractive inde7 profile (inter)erometric near-field scanning re)racted ray etc." may be utilizedto measure the core diameter. raded inde7 fibers present a more di)ficult problem as in general

there is a continuous transition beteen the core and the cladding. n this case it is necessary to

define the core as an area ith a re)ractive inde7 above a certain predetermined value i)re)ractive inde7 profile measurements are used to obtain the core diameter. 9ore diameter

measurement is also possible )rom the near-field pattern o) a suitably illuminated (all guided

modes e7cited" fiber. #he measurements may be ta2en using a microscope e$uipped ith amicrometer eyepiece similar to that employed )or o))-line outer diameter measurements.