Optical Activity Measurements in the Photorefractive Bi 12TiO 20 Single Crystal Fibers

Abstract

In the present work the specific optical activity coefficient of the photorefractive Bi12TiO20 single crystal fibers have been determined. The samples were illuminated by λ=0.6328 μm and λ=0.5145 μm. The proposed experimental technique is simple, fast and accurate. It is based on the automatic electronic phase shift detection in a dynamic polarimetric system. 6.62±0.06 deg/mm was obtained for λ=0.633 μm and 10.0±0.2 deg/mm for λ=0.514 μm.

Full text

July 1998
Ž.
Optical Materials 10 1998 201–205
Optical activity measurements in the photorefractive Bi TiO
12 20
single crystal fibers
R.M. Ribeiro a,), A.B.A. Fiasca a,1, P.A.M. dos Santos a,2, M.R.B. Andreeta b,3,
A.C. Hernandes b,4
aInstituto de Fısica, UniÕersidade Federal Fluminense, AÕ. Gal Milton TaÕares de Souza s rn Gragoata, 24020-005Niteroi, RJ, Brazil
´´
bDepartamento de Fısica e Ciencia dos Materiais, Instituto de Fısica de Sao Carlos, UniÕersidade de Sao Paulo,
´ˆ ´˜ ˜
AÕ. Dr. Carlos Botelho, 1465, Sao Carlos, SP, Brazil
Received 15 September 1997; accepted 24 October 1997
Abstract
In the present work the specific optical activity coefficient of the photorefractive Bi TiO single crystal fibers have
12 20
been determined. The samples were illuminated by
l
s0.6328
m
m and
l
s0.5145
m
m. The proposed experimental
technique is simple, fast and accurate. It is based on the automatic electronic phase shift detection in a dynamic polarimetric
system. 6.62"0.06 degrmm was obtained for
l
s0.633
m
m and 10.0"0.2 degrmm for
l
s0.514
m
m. q1998 Elsevier
Science B.V. All rights reserved.
Keywords: Optical activity; Photorefractive; Single crystal fiber crystal
1. Introduction
To the present day, some published papers can be
found where attention is given to the necessity of
obtaining accurate measurements of optical activity
wx wx
1,2 and Faraday rotation 3,4 . Both phenomena
produce a rotation in the light polarisation plane after
crossing through the length of an optically active or
non-null Verdet constant material, respectively.
)Corresponding author. E-mail: rmr@if.uff.br.
1E-mail: fiasca@if.uff.br.
2E-mail: pams@if.uff.br.
3E-mail: marcello@ifqsc.sc.usp.br.
4E-mail: hernandes@ifqsc.sc.usp.br.
It is well known that natural optical activity plays
an important role in the photorefractive effect in bulk
wx
5 materials, mainly for the para-electric sillenite
family crystals. The quite interesting photorefractive
Ž.
material, the Bi TiO BTO crystal, used in the
12 20
present work in single crystal fiber form belongs to
this important class of materials. Basically, the
anisotropic character of the diffracted light from
dynamic holograms, in this kind of material, is
strongly dependent on the optical activity rotation of
wx
the reading beams 5–9 . This means that all the
optical holographic properties, starting from the beam
coupling inside the sample, like energy transfer,
phase conjugation and anisotropic self diffraction,
are influenced by optical activity. Furthermore, it has
00925-3467r98r$19.00 q1998 Elsevier Science B.V. All rights reserved.
Ž.
PII S0925-3467 97 00164-X

()
R.M. Ribeiro et al.rOptical Materials 10 1998 201–205202
Fig. 1. Classical polarimetric system.
wx Ž
been reported 10 that the optical activity and the
.
induced birefringence influences the optical spatial
soliton propagation in photorefractive waveguides.
The stated questions justify the necessity for accu-
rate optical activity characterisation, not only in pure
crystal samples but also in crystals where the con-
wx
trolled addition of dopants 11 changes the value of
its optical activity coefficients.
In the present work the natural optical activity
measurements of the non-doped photorefractive BTO
single crystal fibers are described. The proposed
experimental technique is simple, fast and accurate.
It is based on the automatic electronic phase shift
detection in a dynamic polarimetric system. The
measurements were made in samples grown by the
Ž.
LHPG laser heated pedestal growth method along
wx
the 011 crystal axis to the laser light wavelengths
632.8 and 514.5 nm.
2. Method
The experimental technique proposed here is an
Ž.
extension of a classical polarimetric system Fig. 1 .
In this set-up the sample to be measured is posi-
tioned between two crossed polarizers and the light
Ž.
is detected after the last one the analyser , which is
rotated until a next null is found to compensate the
optical activity or Faraday rotation angle offset.
The main limitation of this technique is the accu-
racy in the measurements which is strongly depen-
dent on the quality of the extinction in the search for
a null by crossing the polarizers. In addition, another
limitation is the repositioning of the sample each
time to measure the polarization rotation caused by
the optical activity.
In the present case, there are additional difficul-
ties caused by the geometrical characteristics of the
single crystal fibers analysed. These samples are
about 500
m
m in diameter in the entrance phase and
the lengths vary from 1 to 10 mm. The light injec-
tion, alignment to avoid internal reflections and scat-
tering, are critical questions to be considered in the
classical polarimetric system. Therefore, a new po-
larimetric system is proposed here where these limi-
tations are removed to obtain optimal conditions in
single crystal fiber optical activity measurements.
The measurement method of the system illustrated
in Fig. 2 takes into account a time varying plane
polarized light produced through a
l
r2 waveplate
that rotates with a constant angular frequency
v
.
After this, the time varied polarized light beam is
split into two arms that cross two analysers before
detection and a fixed phase difference is automati-
cally stabilised by a digital lock-in amplifier using
Ž.
one arm S as the reference signal. The normalised
Fig. 2. Automatic electronic phase shift detection system.

()
R.M. Ribeiro et al.rOptical Materials 10 1998 201–205 203
Ž.
light beam intensity, It, in the arm without the
F
Ž.
sample F is
cos22
v
ty
a
y
u
Ž.
F
Its.1
Ž. Ž.
F2
cos
a
q
u
Ž.
F
In this expression
a
is the initial polarization
angle at the laser exit,
u
is the polarization angle of
F
the analyser in this arm, referred to as a horizontal
plane. Before the sample is introduced, both arms
have the same light intensity. With the introduction
of the sample in this arm, all mentioned details are
taken into account and a new automatic phase detec-
Ž.
tion takes place. In the arm S where the light
intensity is given by
cos22
v
ty
a
y
u
q
c
Ž.
S
Its.2
Ž. Ž.
S2
cos
a
y
u
y
c
Ž.
S
In this expression
u
is the polarization angle of
S
the analyser and
c
is the phase difference related to
the other arm originating from the optical activity of
the sample. With this method the optical activity
measurement is quickly, easily and more precisely
obtained than the classic polarimetric system. In
Section 3 the experimental set-up and results are
presented for both kinds of systems.
3. Experimental set-up and results
With the objective of obtaining more reliable
results for the optical activity coefficients, measure-
ments were made with both systems: the classical
polarimetric system, shown in Fig. 1, and the pro-
posed automatic phase shift detection system, illus-
trated in Fig. 2.
In Fig. 1 the classical polarimetric system is
Ž
composed of a laser, two polarizers P and P the
12
.
analyser and the detector D. The sample S is intro-
duced between the two crossed polarizers and the
Ž
light intensity is measured by a power meter not
.
shown in Fig. 1 .
Fig. 2 shows a sketch of the experimental system
proposed in the present work. Unlike the classical
polarimetric system, the light polarization is continu-
ously rotated before being split into two arms by a
Fig. 3. Photorefractive Bi TiO single crystal fiber sample.
12 20
glass prism shown as a Y-configuration. In this way,
the introduction of the sample causes a phase mis-
match between the arms and using the light intensity
in D like the reference one, the light intensity
1
detected by D in a digital lock-in amplifier pro-
2
duces an automatic self-adjustment of the phase
value. The analysers P and P are adjusted before
12
the sample S is positioned on the system which
begins without any phase difference. Fig. 3 shows
the single crystal fiber sample arrangement and the
results obtained from both experimental systems are
indicated in Tables 1 and 2.
Table 1
Ž.
Optical activity coefficient
r
degrmm determined by the classi-
cal polarimetric system
Ž.Ž.
Single crystal fiber
r
degrmm
r
degrmm
Ž. Ž.Ž.
length mm 632.8 nm 514.5 nm
1 2.76 6.7"0.4 11.0"0.5
2 4.80 6.5"0.2 —
3 10.00 6.7"0.1 —
Table 2
Ž.
Optical activity coefficient
r
degrmm determined by the auto-
matic electronic phase shift detection system
Ž.Ž.
Single crystal fiber
r
degrmm
r
degrmm
Ž. Ž.Ž.
length mm 632.8 nm 514.5 nm
1 2.76 6.6"0.1 10.0"0.2
2 4.80 6.57"0.05 —
3 10.00 6.69"0.03 —

()
R.M. Ribeiro et al.rOptical Materials 10 1998 201–205204
4. Discussion
In 1970, to our knowledge, the optical activity
and Faraday rotation measurements in several bis-
muth oxides, including BTO bulk samples illumi-
nated by the visible spectrum of light at room tem-
wx
perature, were published for the first time 12 . From
this paper it is only possible to determine that the
optical activity coefficient
r
)6 deg mmy1at 633
nm and that
r
(11 deg mmy1at 515 nm.
wx
In a subsequent work 13 published in 1975, a
more precise result of optical activity measurements
is given as 6.3 deg mmy1for a BTO bulk sample.
wx
Recently 11 , in 1995, more precise results have
been published relating to the optical activity of the
sillenite family of BSO-type crystals for different
Ž.
wavelengths 450 to 1200 nm . The measurements
were made in bulk non-doped Al, P and V and doped
AlqP samples. Up to this point we have some
conclusions about the optical rotatory power of the
bismuth oxide materials:
ØAll these oxides show a strong, negative sense,
optical activity value and among them BTO shows
the smallest value.
ØThe optical activity decreases to the greatest
wavelengths of the visible light spectrum. Doping is
not significant to the optical activity at wavelengths
far from the absorption band edge but is significant
nearest to the absorption band edge. In quantitative
terms, for wavelengths higher than 590 nm, the
change in the optical activity due to dopant addition
in BTO is around 1 to 2.5%.
ØFor the BTO case, using Al, P and V dopants
the optical activity increases in all spectral ranges.
An exception is BTO:Al in the wavelengths
l
)1130
nm where a decrease occurs. But at 480 nm the
effect increases the optical coefficient to 55.7% for
the Al dopant, 55.4% for P and 118.3% for AlqP.
wx
More recently 14 the optical activity measure-
ments using a classical polarimetric system yield
6.3"0.2 deg mmy1at 633 nm and 11.9"0.2 deg
mmy1at 515 nm for BTO bulk samples.
wx
According to Prokofiev and co-workers 15 , be-
sides the obvious importance of the photorefractive
single crystal fiber growth, looking at some techno-
wx
logical applications 16–18 , there is also an in-
creased interest in studying the problems related to
the growth of optical material with small dimensions,
in this case small diameters of the single crystal
fibers. Among these problems we can mention the
behaviour of the solidification, metastable phase for-
mation, the morphology of the growth of the single
crystal fiber and crystal defects.
The single crystal fibers of Bi TiO were pre-
12 20
pared by the LHPG technique, as described in a
wx
previous paper 15 . A 100 W CO laser, operating
2
at 10.6
m
m, was used as a heat source. In order to
minimise the temperature oscillations in the molten
zone, an additional power control was applied, in
which a photodetector signal proportional to the
emitted light from the molten zone was observed
through a monocular microscope. The pulling and
source rates were 0.15 and 0.05 mmrmin, respec-
wx
tively, and the fibers were pulled in the 011 direc-
tion.
The LHPG method allows growth of the single
crystal fibers without contamination and stress and
the source material can be easily doped. For some
specific optical applications the fibers can be grown
in a defined direction. In a two wave mixing experi-
wx
ment, the 011 direction allows a maximal beam
coupling effect in the sillenite family crystals. The
morphologic structure can be verified by X-ray
diffraction and the Bi and Ti concentrations are
determined by microanalysis in an electronic micro-
scope. The X-ray diffraction measurements show
that the crystal fibers have a cubic centred structure
with the same lattice parameter as the bulk shape
samples. The optical activity of a single crystal fiber
wx
of BTO crystal 15 was measured to be 6.3 deg
y1wx
mm in the 011 direction at 632.8 nm which is
the same as the bulk version of the BTO crystal.
In the present work with a classical polarimetric
system we have obtained the specific optical activity
and an error bar with a precision in the order of 0.1
deg mmy1. Using the automatic proposed system the
same values have essentially been obtained but with
a better precision in the order of 0.01 deg mmy1.
Moreover, the measurements made with the pro-
posed automatic system are in good agreement with
those obtained using the classical polarimetric sys-
tem at 632.8 nm. However, in both cases the value
of the optical activity in 632.8 nm is around 6.6 deg
y1Žy1.
mm more accurately 6.62"0.06 deg mm .

()
R.M. Ribeiro et al.rOptical Materials 10 1998 201–205 205
This value is f5% higher than the more accepted
6.3 deg mmy1for the BTO crystal in bulk or in a
single crystal fiber shape.
5. Conclusion
In the present work we have shown measurements
of optical activity in BTO single crystal fibers with a
proposed experimental setup which is a classical
polarimetric system modification. We have obtained
more accurate measurements with this phase polar-
ization sensitive setup. Our system can also be used
in Faraday rotation measurements.
We have obtained optical activity values for the
BTO single crystal fiber that are not so much differ-
ent from those published up until now. However, we
have obtained a more precise approach concerning
the proposed system. We believe that, mainly for
new materials, after all questions pointed out in the
discussion, accurate measurements of optical activity
are needed for both single crystal fiber and the bulk
shape samples.
Acknowledgements
This work had the financial support of the Brazil-
Ž
ian agencies FAPERJ Fundac¸ao de Amparo a
˜`
.
Pesquisa do Estado do Rio de Janeiro , FINEP
Ž.Ž
Financiadora de Estudos e Projetos , CNPq Con-
selho Nacional de Desenvolvimento Cientıfico e
´
.Ž
Tecnologico and CAPES Coordenac¸ao de
´˜
.
Aperfeic¸oamento de Pessoal de Nıvel Superior .
´
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