Vol.2, No.8, 868-872 (2010) Natural Science
Copyright © 2010 SciRes. OPEN ACCESS
Polymeric microelement on the top of the fiber
formation and optical loss in this element analysis
Maria I. Fokina, Nina O. Sobeshuk, Igor Y. Denisyuk*
Saint-Petersburg State University of Informational Technologies, Mechanics and Optics, Saint-Petersburg, Russia;
*Corresponding Author: denisiuk@mail.ifmo.ru
Received 8 February 2010; revised 15 March 2010; accepted 21 March 2010.
This paper describes the formation technology
of polymeric microlenses self-organized on the
fiber top in the limited reaction volume. The loss
inserted to the optical coupling channel and
their sources was investigated.
Keywords: Self-Focusing; Microlens; Optical Fiber;
Photopolymer; Coupling
Currently one of the important problems of fiber optics
is the coupling of optical fibers with light sources and
the introduction of light into the fiber with minimal
losses. Direct welding of two fibers is optimal only for
fibers with the same diameter and the aperture, while in
other cases losses increasing require the use of focusing
and collimating external systems. Loss of the light at
fiber coupling whit LED laser or diode is too large, be-
cause emitting area, as the laser and the diode, always
larger then diameter of fiber core. One solution to this
problem is the use of focusing microelements on the top
of the fiber.
A numerous effective technologies for microelements
preparation, including microlenses made directly at the
fiber top now are developed. Creating elements immedi-
ately at the fiber top is preferred because provides a self-
alignment (coaxiality of axes) microlens and the fiber
and no need for further manual assembly.
There are several well known methods for optical mi-
croelements preparation on the top of the fiber such as:
laser fusion, or a gas burner (microoptical elements of
various forms and purposes) [1], chemical etching (light
coneconcentrator) [2], ion beam engraving (diffractive
microlensese) [3]. The first two methods have found
wide application, but the disadvantage is the possible
misalignment of the microelement and core, and the de-
fect of a secondthe unavoidable chemical contamina-
tion of the surface. Technology of ion beam engraving
allows to obtain micro-relief of very high quality and
small size, but this method is technically difficult in ex-
pensive [3].
In recent years carried out many researches in the
field of preparation of the polymeric optical microele-
ments on the top of fiber [4-6]. The method is based on
monomer composition polymerization deposited on the
fiber top by laser radiation coming from the fiber. This
technology allows obtaining high-quality microelements,
as well as to control their size and profile [6]. However,
all developments in this, of course, promising area, are at
the stage of laboratory research, and experimental results
depend from many different conditions such as type of
monomer, initiator concentration, polymerization vol-
ume and differ in different works. In this work we
prolong our experiments with microlens in fiber top
self-writing made previously [7-9]. We find that proc-
esses of photopolymerization depend from speed of
oxygen diffusion from monomers surface to polymerized
area and so it depend from monomer volume. In this
work we accomplish self-writing photopolymerization
process in small monomer drop placed in fiber top in
condition of fast oxygen diffusion from it surface. As
oxygen will inhibit acrylate photopolymerization process
This work is devoted to investigation of the polymeric
microelements formation processes at the end of optical
fiber using laser radiation coming from the fiber at con-
ditions of oxygen high speed diffusion to polymerization
area. Figure 1 shows the scheme of "growing" elements.
Into the one end of the fiber 50/125 was introduced a
N2 laser irradiation (λ = 337 nm), on the other, vertically
mounted end, a drop of the liquid monomer composition
was deposited. The height of the drop was controlled
M. I. Fokina et al. / Natural Science 2 (2010) 868-872
Copyright © 2010 SciRes. OPEN ACCESS
Figure 1. Scheme of the formation a polymeric element on the
top of the fiber.
visually over microscope. A mixture of acrylates was
used: 2-carboxyethyl acrylate (50%), Bisphenol A glyc-
erolate (20%), 1,6-hexanediol diacrylate (10%) and
Trimethylolpropane ethoxylate (1 EO/OH) methyl ether
diacrylate (20%). All reagents were purchased from Al-
drich. As the initiator dimetoksifenilatsetofenon (0.1%)
with absorption maximum at 250 nm was used. At 337 nm
its absorption length was few millimeters. The refractive
index of the mixture is –1.48. Exposure ranged from 3
up to 45 seconds (t = 3, 5, 10, 15, 25, 45 sec).
The elements formation process can be divided into
following stages:
1) The process of polymerization doesn’t begin im-
mediately, but from the moment when the absorbed by
monomer energy reaches a certain threshold. Delay ap-
pears because of the fact that the first impulses burn the
oxygen in the monomer, which is the inhibitor of the
process, after that only formation of free radicals from
initiator molecules begin.
2) With a minimum exposure the formation of small,
quasi-lens is observed, Figure 2.
This stage is the most difficult to reproduce because
polymerization process takes place very rapidly and con-
3) Formation of cylindrical/conical shape element
with rounded top, Figure 3.
4) The formation of the narrowing structure is possi-
ble (Figure 4).
5) There is an alignment of profile of the element while
increasing exposure. Experiments have shown that this
form (Figure 5) has a high repeatability (the probability
of almost 100%) when exposure element is sufficient.
After that visible light passing through microelements
Figure 2. Element at the fiber top with the
minimum exposure. Line of 50 μm is shown
in the photo.
Figure 3. Polymeric element at the top
of the fiber, exposure10 sec.
Figure 4. Polymeric element at the top of the
fiber, exposure15 sec.
Figure 5. Polymeric element on the top of the
fiber, exposure45 sec.
M. I. Fokina et al. / Natural Science 2 (2010) 868-872
Copyright © 2010 SciRes. OPEN ACCESS
was investigated. The experimental scheme is shown on
Figure 6.
The dependence of the radiation intensity on the dis-
tance between the fiber end without microelement and a
scanning fiber connected to a photodetector was meas-
ured in the first part of the experiment. In the second
the dependence of intensity on the distance between the
scanning fiber and the fiber with polymeric microele-
ment is on the end. As the scanning fiber 9/125 μm was
used, because most interesting is efficiency of using mi-
croelements in the coupling of fibers of different core
diameters (single mode with multimode).
Figure 7 shows the results of measurements (de-
pendence of the photodetector signal on the distance
between the fibers). Line 1 corresponds to the first part
of the experiment, the line 2the second.
Figure 6. а) Measurement using fiber without microlens, b)
Measurement using the fiber with polymeric microelement
on the end.
Figure 7. Dependence of the photodetector signal from the
distance between the fibers: 1 (full line)fiber without mi-
crolens; 2 (dashed line)polymeric microelement on the end
of the fiber.
Curve 2 has a clearly defined maximum, which con-
firms that the microelement focuses light. However, the
graph shows that even at the maximum signal, conse-
quently the intensity of radiation is lower than when
fibers connected back to back. To clarify the source of
the losses the following experiment was made: radiation
of He-Ne laser was introduced into the fiber with mi-
crolens at its end. Figure 8 shows the obtained result:
element remains dark, but there are significant losses at
the interface polymer-fiber.
The dynamics of growth of self writing waveguide and
formation of polymeric microelements at the end of the
optical fiber was investigated in this work. Micro-photos
made in series during formation of self-writing wave-
guide in the top of the fiber. The main stages of element
formation are shown in Figure 9 (in the leftexposition
time for each photo). One of the features of obtained
samples is that the width of all elements, starting from
the third formation phase, is approximately equal to the
half-width of fiber core. This can be explained in the
following way: after reaching the threshold energy
Figure 8. Passage of light through the fiber with a
polymeric element at the end.
Figure 9. Empirical reticulation rate of the formulation
versus absorbed energy. Eth is the threshold energy
(achieved when the polymerization starts) and n the re-
fraction index of the formulation.
M. I. Fokina et al. / Natural Science 2 (2010) 868-872
Copyright © 2010 SciRes. OPEN ACCESS
the polymerization process takes place continuously and
is accompanied by increasing of the refractive index
Figure 9 [5].
For this composition increasing of the refractive indi-
ces is 0.04 (the monomer composition1.48, the poly-
mer1.52). As a result, we see the effect of self-
focusing of the light in the polymer, i.e. formation of the
waveguide, the core of which is a polymer, and the clad-
dingunpolymerized monomer. It is known that even
with full internal reflection light goes abroad the sub-
stance to the distance equal to half the wavelength, and,
therefore, slow growth element in wide might be ex-
pected. However, this does not happen, because energy
of emergent radiation is insufficient to burn oxygen and
initiate the polymerization.
One of the components of monomer composition
2-carboxyethyl acrylate having a linear structure also
provides directional growth of element from fiber glass
surface. 2-carboxyethyl acrylate contains acid groups
chemically active with quartz, which provide reaction of
monomer with fiber top and beginning of element grown
from fiber top away.
Also it was found that element length will stop in some
moment and the radius of curvature of the drop increases
when increasing exposure. Figure 10 shows photographs
illustrating this effect.
We think that stopping of micro-element grown is a
result of approaching of polymerized area to surface of
monomer drop and, therefore, stopping of photopoly-
merization in result of oxygen inhibition influence.
Our investigation results showed that the fabrication
method requires improving and optimization: significant
losses in the microelement aren’t observed, which con-
firms its waveguide structure, but there are significant
losses at the interface between microelement and optical
fiber. In fact is it the main component of optical losses
and its removing will result on diminishing of optical
losses to appropriate value.
Certainly, optical losses at interface: optical fiber/self
writing waveguide resulting from narrowing of element
in comparison to fiber core (fiber core 50 um; waveguide
25 um). Stepped transformation of waveguide core from
50 to 25 um will result on radiation going away at this
In our opinion narrowing of polymerized area is a re-
sult of photo polymerization reaction inhibition by oxy-
gen diffused from monomer drop volume to exposed
area and slowing down of polymerization in surface of
waveguide. Certainly this effect is negative and can be
more powerful for single mode optical fiber having core
diameter 7 um.
To overcome or reduce effect of waveguide narrowing
according to our opinion are needed to diminish oxygen
diffusion speed in monomer mixture, so are needed to
Photos of element Exposure,
Figure 10. Dependence the radius of curvature of element
from exposure.
change monomer composition to increase its viscosity or
add traps for oxygen.
Now we prepare series of monomers composition
with increased viscosity and with addition of nanoparti-
cles as possible traps for oxygen diffusion. First experi-
ments show improvement of waveguide formation.
Results obtained with these new UV-curable nano-
composite materials will be subject of next publication.
M. I. Fokina et al. / Natural Science 2 (2010) 868-872
Copyright © 2010 SciRes. OPEN ACCESS
In this work the processes of microelement self-writing
by photopolymerization of composition drop deposited
at the top of the optical fiber by laser radiation coming
from the fiber are analyzed. Experimental investigation
of focusing light by obtained microlenses was made, this
study shows possibility of coupling microlens prepara-
tion with focal length of about 400 um.
It was shown that the observed loss of light in the mi-
crolenses formed by this method are determined by dif-
ference of diameters of self-organized light concentrator
and fiber core, the losses directly in the microelement
almost are not.
This work was made under support of Russian Minis-
try of Education grant RNP.
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