Cliapter 3 - If you haven't measured it you haven't made it -- Wayne Knox Cliaracterization of dye doped andundoped pofymer optica[ fi6er ... ," ..... .... : ',.'. . .. .. ."" .. .. with dye iJptictilfibers.· Fluorescence out on dye'!ope4pol!meroptica!jibers. stl!dies,.t/ueto micro bends and sensitivity to als« carried out for. smart sensor .
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Cliapter 3
-If you haven't measured it you haven't made it -- Wayne Knox
Figure 3.11 cFigure 3.11 a, b, c): The 'peeling the curve' method applied to In([ )versus z plot at A = 610 nm at different pump powers. The solid linesrepresent the linear fits to the data
Chapter 3
3.5 Homogeneity in fiber diameter and uniformity in
dye concentration
The homogeneity of the fiber diameter as well as dye concentration are
important parameters which has direct effects on the fiber fluorescence. An
inhomogeneous dye doped fiber will have an inhomogeneous fluorescence
distribution which directly affects the overall gain of the fiber. The
homogeneity of the fiber diameter can be characterized by calculating the
correlation length of the fiber as described below. If the fiber is not perfectly
homogeneous in diameter or in dye concentration, the coefficient that
describes the fluorescence source along the coordinate z is
C(Am , A, z) = C(Am , A)S(Z) --------------------- [3.t)where: Am is the wavelength of the beam, C(Am, A) is the fluorescence yield
The deviation of S (z) from unity gives a measure of the inhomogeneity of
the fiber [8, 9]. We can quantify the inhomogeneities by using the
autocorrelation function, which measures the similarities between the
intensity generated at one position in the fiber, Z , and the intensity generated
at a neighboring point, Z + ( , over a length Z of fiber and is defined for a
fixed wavelength Ao as
(I(O)I«()) = ~~n;(~ )JJ(Z)JCZ + ()dz, [3.2)
The form of the autocorrelation function is,
Characterisation ofPOF
N
I8I j 8Ijk
(&(z)8I(z + (») Norm == ..:-j=-ON~~
:L8I/j=O
The autocorrelation function is often modeled as an exponential or Gaussian
function of ( .By fitting the autocorrelation function to one of these models,
one determines the length scale of fluctuations and is called its correlation
length. Mathematically, the correlation length ((c) is the value of ( when
the correlation function is at lIe of its magnitude. The correlation length is
thus related to the homogeneity of the fiber and can be used to characterize
the fiber quality.
Figure 3.12a shows the plot of fluorescence emission as a function of the
pump position with respect to the fiber end from where emitted light is
collected. As is clearly seen in the figure, fluorescence intensity fluctuates at
shorter path lengths. Also at shorter lengths it is observed that there is slight
deviation from the theoretical values in the florescence intensity. The dye
doped fiber which is used for our studies does not have a cladding. Hence a
considerable amount of scattered fluorescence intensity will be emanating
from the sides of the fiber. This is collected by the lens which is used for
coupling light to the monochromator which causes the intensity fluctuations
at shorter lengths. Theoretical fit according to Beer-Lambert's law is also
shown in the figure. To understand the inhomogeneity, autocorrelated
function was evaluated for various space lags (figure 3.12 b). The lie point in
the plot gives the value of space lag as 1.56mm which is the measure of the
homogeneity of the fiber. Larger the correlation length better is the
Figure 3.12 a: Fluorescence emission as a function of the pump position
li;j1:<l JjiSpace lag (mm)
Figure 3.12 b: Autocorrelated function evaluated fof various space lags
Characterisation of POF
Diffraction technique can also be used to test the homogeneity in diameter of
the fiber. Variation in diameter is found to be less than 1% so that the effect
of inhomogeneity in loss mechanism can be neglected.
The uniformity in dye concentration has an important role to play in
amplification . If the dye concentration is not uniform or if there are aggregate
fonnation of the dye, it will quench the fluorescence propagating through the
fiber. This will adversely affect the overall gain of the fiber amplifier. In
order to study the uniformity in dye concentration through out the length of
the fiber a fiber of total length of 20 cm was taken. The fiber was then cut
into four equal lengths of 5 cm each and the fluorescence spectrum was
charted for each of these individual lengths. The graph shown in figure shows
the fluorescence intensity of the four individual fibers cut from the same fiber
sample.
a•a.•a,a.Osa. ,a.a."a,00
000
000
00·
........
..............
88
Chapter 3
From the plots we can see that the four samples show the same fluorescence
behaviour which implies that the dye is uniformly doped in the fiber for a
total length of 20 cm.
3.6 Conclusion
Characterisation of undoped polymer optical fibers was carried out for its
potential application in smart sensing. The sensitivity to temperature and
microbends is found to be high in comparison with glass fibers.
Using the side illumination technique, position-dependent tuning of
fluorescence light emitted from a Rhodamine 60 doped polymer optical fiber
is observed. The fluorescence data collected from the fiber is used to
characterize the loss mechanisms of the fiber. It has been observed that at
longer wavelengths, there is a lowering of attenuation towards larger
distances of propagation in the fiber. The mechanism for such position
dependent loss parameter is attributed to re-absorption and re-emission
process taking place along the length of the fiber. This suggests that
appropriate design of the fiber will lead to a gain on the longer wavelength
side. It is also found that the drawn fiber is having a good correlation length
which confirms its homogeneity. The doping of dye in the fiber was found to
be uniform from the fluorescence studies.
89
Characterisation ofPOF
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Chapter 3
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