unit 5-iv

8/10/2019 Unit 5-IV

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Lecture No: 33 Date: 08/04/2013

Period: VII

Topic: Fiber refractive inde pro!"e #ea$ure#ent$

Fiber refractive index profile measurements:

The measurements may be performed using a number of different techniques each of whichexhibit certain advantages and drawbacks. In this section we will discuss some of the more popular methods which may be relatively easily interpreted theoretically, without attempting to

review all the possible techniques which have been developed.

Interferometric methods:

Interference microscopes (e.g. Mach!ehnder, Michelson" have been widely used to determinethe refractive index pro#les of optical #bers. The technique usually involves the preparation of a

thin slice of #ber (slab method" which has both ends accurately polished to obtain square (to the

#ber axes" and optically $at surfaces. The slab is often immersed in an index%matching $uid, and

the assembly is examined with an interference microscope. Two ma&or methods are thenemployed, using either a transmitted light interferometer (Mach!ehnder" or a re$ected light

interferometer (Michelson". In both cases light from the microscope travels normal to the

prepared #ber slice faces (parallel to the #ber axis", and differences in refractive index result indifferent optical path lengths. This situation is illustrated in the case of the Mach!ehnder

interferometer in 'igure .)(a". *hen the phase of the incident light is compared with the phase

of the emerging light, a #eld of parallel interference fringes is observed.

Figure 5.8 (a) The principle of the Mach!ehnder interferometer. (b) The interference fringe pattern

obtained "ith an interference microscope from a graded index fiber

The refractive index difference between a point in the #ber core (e.g. the core axis" and the

cladding can be obtained from the fringe shift q, which corresponds to a number of fringedisplacements. This difference in refractive index +n is given by

*here x is the thickness of the #ber slab and - is the incident optical wavelength.

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#ear$field scanning method:

The near%#eld scanning or transmitted near%#eld method utilies the close resemblance that exists between the near%#eld intensity distribution and the refractive index pro#le, for a #ber with allthe guided modes equally illuminated. It provides a reasonably straightforward and rapid method

for acquiring the refractive index pro#le. *hen a diffuse /ambertian source (e.g. tungsten

#lament lamp or /01" is used to excite all the guided modes then the near%#eld optical power density at a radius r from the core axis 21(r" may be expressed as a fraction of the core axis near%

#eld optical power density 21(3" following

*here n4(3" and n4(r" are the refractive indices at the core axis and at a distance r from the core

axis respectively, n5 is the cladding refractive index and 6(r, " is a correction factor. The

correction factor which is incorporated to compensate for any leaky modes present in the shorttest #ber may be determined analytically.

Figure 5.%: &xperimental setup for the near$field scanning measurement of the

refractive index profile.

7n experimental con#guration is shown in 'igure .8. The output from a /ambertian source isfocused onto the end of the #ber using a microscope ob&ective lens. 7 magni#ed image of the

#ber output end is displayed in the plane of a small active area photodetector (e.g. silicon pin

photodiode". The photodetector which scans the #eld transversely receives ampli#cation fromthe phase%sensitive combination of the optical chopper and lock%in ampli#er. 9ence the pro#le

may be plotted directly on an :; recorder.

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Period: VII


'efracted near$field method:

7 schematic of an experimental setup for the <=' method is shown in 'igure .43. 7 shortlength of #ber is immersed in a cell containing a $uid of slightly higher refractive index. 7 smallspot of light typically emitted from a >?? nm 9e=e laser for best resolution is scanned across

the cross%sectional diameter of the #ber. The measurement technique utilies that light which is

not guided by the #ber but escapes from the core into the cladding. 9owever, light escaping fromthe #ber core partly results from the power leakage from the leaky modes which is an unknown

quantity. The effect of this radiated power reaching the detector is undesirable and therefore it is

blocked using an opaque circular screen, as shown in 'igure .43(a".

Figure 5. 'efracted near$field method for the measurement of refractive index

profile: (a) experimental arrangement* (b) illustration of the ra+ tra,ectories.

The refracted ray tra&ectories are illustrated in 'igure .43 (b" where @A is the angle of incidence

in the #ber core, @ is the angle of refraction in the #ber core and @B constitutes the angle of therefracted inbound rays external to the #ber core. 7ny light leaving the #ber core below a

minimum angle @B min is prevented from reaching the detector by the opaque screen ('igure.43 (a"". Moreover, it may be observed from 'igure .43 (b" that this minimum angle

corresponds to a minimum angle of incidence @A min. 9owever, all light at an angle of incidence@AC @A min must be allowed to reach the detector. To ensure that this process occurs it is advisable

that input apertures are used to limit the convergence angle of the input beam to a suitable

maximum angle @A max corresponding to a refracted angle @B max. In addition, the immersion of the #ber in an index%matching $uid prevents re$ection at the outer cladding boundary. 9ence all

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Period: VII


the refracted light emitted from the #ber at angles over the range @B min to @B max may be

detected.The detected optical power as a function of the radial position of the input beam 2(r" is measuredand a value 2(a" corresponding to the input beam being focused into the cladding is also

obtained. The refractive index pro#le n(r" for the #ber core is then given by

*here n5 is the cladding refractive index. 'urthermore, 0q. can be written as

It is clear that n(r" is proportional to 2(r" and hence the measurement system can be calibrated to

obtain the constants k 4 and k 5. 'or example, a calibration scheme in which the power that passesthe opaque screen is monitored as it is translated along the optical axis provided an early

strategy.

unit 5-iv

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