T
water
oil
(6)
n
n
n
water
oil
T( n ) T( n )
2
1
(
S n
)
(7)
2
n
1
n
194
Microsensors
with respect to the rapidity of commutation of the microsensor, this property rests in the
bandwidth of the photodetector used in the microsensorial system. Nevertheless in
theoretical terms, the rapidity of commutation, t , is shown through Eq. 8.
rise
t
0.9( t t )
rise
2
1 (8)
where, t2, is the instant where it happens the maximum value, and t1, is the instant where the minimum voltage of Vphoout happens. Nevertheless in practical terms, and valid for
optoelectronic devices, it is possible to be calculated, through Eq. 9.
0.35
t
rise
(9)
BW
where BW is the bandwidth of the photodetector.
5. Experimental results and discussions
Like a pair of immiscible liquids, it has been selected to oil and to the water, by the
importance that it has in the economic growth in the world. In this work it was investigated
firstly, the effect of the state of polarization of the electric field to the input of the fiber
microsensor on the voltage induced in the photodetector, Vphoout. The configuration used in
this measurement is shown in Fig. 10.
According to Eq. 2, the voltage that appears in the photodetector, Vphoout, does not have to
depend on the polarization of the incident electric field to the microsensor input. However
in the interface semigota-liquid, reflections of Fresnell take place, where the polarization of
the incident beams is polarimetrically influenced by the characteristics of the same. In order
to verify experimentally, that the voltage that appears in the photodetector, Vphoout, is
independent of the polarization, it was made vary the state of polarization of the field, Ein , by means of the use of a polarization controller of three coils.
When varying the angular position of the coil modifies the angular inclination of state of
linear polarization of the field, Ein , to the input of the microsensor. The dependency of the
voltage that appears in the photodetector, Vphoout, with respect to angle , of polarization at the input of the microsensor, as shown in Fig. 11. This experiment was realized at the
wavelength of 1550 nm, with interface semidrop-water.
It is appraised that the voltage that appears in the photodetector, Vphoout, has small
dependency of the angle of polarization. Probably in the region of the semidrop, the
dependency of the polarization is remarkable, but this is inhibited by the fiber segments, the
input fiber segment, and the output fiber segment, which are standard and non-preservers
of the polarization, whose lengths are of not less than 30 cm. In this experiment, the voltage
that appears in the photodetector, Vphoout, presents fluctuations within the width vicinity ±
0,1 µV, which demonstrates that, Vphoout, is independent of the polarization.
In order to research the corresponding transmittances for each interaction medium: nair,
nwater, y noil, was used an irradiation source that operates in the vicinity of the 1550 nm, for these reason it implemented the gain spectrum of a 10 m segment of erbium doped fiber
(EDFA). This irradiation source is an EDFA arrangement without signal at the input, is
unidirectional, reason why it emits at the output the spectrum gain of the EDFA, as shown
in Fig. 12. EDFA spectrum output is shown in Fig. 13. These spectrums own intensities
Optical Fiber Microsensor of Semidrop
195
different from zero, in the interval from 1500 from 1600 nm. In order to enter this known
spectrum at the input of the fiber microsensor, the output of EDFA was coupled with the
input end of fiber microsensor
Fig. 10. The semidrop is immersed in two immiscible mediums, whose refractive indices are
n1, n2, additionally nair
10
9
8
V
7
age, 6
volt 5
or 4
ect
3
odet 2
oth 1
P
0
0
20
40
60
80
100
120
140
160
180
Polarization rotation,
Fig. 11. Voltage variations of the microsensorial system output
196
Microsensors
Fig. 12. Setup of EDFA, without input signal
180
160
a.
140
u.m, 120
u
100
spectr
80
ain
60
G
40
20
0
1500 1510 1520 1530 1540 1550 1560 1570 1580 1590 1600
Wavelength, nm.
Fig. 13. Irradiation spectrum used to determine the spectral response of optical fiber
microsensor
The experimental scheme used to obtain the output spectra is shown in the Fig. 14. The
output spectra are useful to determine the functions of transmittance of the microsensor
corresponding to each surrounding mediums, according to Eq. 1, for each refractive index
nair, nwater, and noil.
The Fig. 15 shows to the value of transmittance functions for each system semigota-water,
semidrop-air, and semidrop-oil, in the region of the 1500-1600 nm. These functions are
standardized with respect to the transmittance spectrum of the system semidrop-air, which
presents the higher peak of transmission in 1527 nm, and it is different from zero from 1508
nm to 1560 nm. The transmittance spectrum of the system semidrop-water presents a peak
Optical Fiber Microsensor of Semidrop
197
in 1522 nm, and it is different from zero from 1515 nm to 1560 nm. The transmittance
spectrum of the system semidrop-oil is equal to zero from 1500 nm to the 1600 nm. Reason
why to distinguish oil of air, is propitious to use wavelengths in the 1508-1560 nm range,
and to distinguish oil of water, is propitious to use wavelengths in the 1515-1560 nm range.
Nevertheless, to discriminate the water of oil, the spectral region that offers this possibility,
we divided it in two: the one of low contrast, and the one of high contrast. The one of low
contrast begins from 1500 to 1515 nm, and from 1534 nm to 1562 nm, as it is possible to be
appreciated in the Fig. 15.
The spectral region of high contrast begins from 1523 nm to 1534 nm. This procedure
ensures the success of operation of the optical fiber microsensor, and it is attainable for any
other pair of immiscible liquids of industrial or biomedical interest.
Remarkable contrast exists in the values of T(n) for the mediums air, oil and air for the
wavelength of 1550 nm, as is shown in Fig. 16. During the experiment and measuring, the
decays of power, Pout, were measured in percentage of the power compared with the output
power. In other words, the Pout of the system semigota-air represents the 100% of the
microsensor.
When submerging the microsensor towards oil, was observed a reduction to 0,0% of the la
Pout power of the air. Nevertheless, when the microsensor made contact with the water, the
power was restituted to 18,5% with respect to the Pout of the air. These results are in Table 1.
These data show the capacity of discrimination of the microsensor for the wavelength of
1550 nm.
In order to demonstrate the effectiveness of microsensor of fiber for the detection of the
interfaces formed by immiscible liquids, the graph shown in the Fig. 17 was developed. In
this graph the changes of transmittance acquire knowledge that detects the microsensor of
fiber, when realizing an orthogonal sweeping, from the bottom towards the surface, crossing
the interfaces that form in a tank cistern that contains interfaces water-oil, , hwater, oil-air, h oil.
Fig. 14. Experimental setup to obtain output spectrum in the IR region (1550-1600nm)
198
Microsensors
1.0
). 0.9
( , n 0.8
T
0.7
nair
0.6
0.5
n
0.4
water
transmmitance, 0.3
d
n
0.2
oil
alize
0.1
rm
No 0.01500
1520
1540
1560
1580
1600
Wavelength, nm.
Fig. 15. Transmittance spectra for semigota-water systems, semigota-air, and semigota-oil
0.20
).
( , nT 0.15
nair
tance,
0.10
ransmmi
ed t 0.05
noil
aliz
nwater
rm
No 0.001540 1542 1544 1546 1548 1550 1552 1554 1556 1558 1560
Wavelength, nm.
Fig. 16. Spectral region of low contrast to nwater and npil detection
Optical Fiber Microsensor of Semidrop
199
Fig. 17. Transmittance values corresponding to levels of water, oil, and air
It is observed that in these levels, a commutation in the transmittance of the fiber
microsensor happens, that is interpreted like the existence of each of these interfaces that
appear in the axis of h, level. This evaluation was performed for a wavelength of 1550 nm.
Note that the maximum transmittance for semidrop-air system, considered as 100% for the
semidrop-water system is 18,5%, and semidrop-oil system is zero. For this reason, the fiber
microsensor is functional for the detection of interfaces formed between the two immiscible
liquids.
To research the sensitivity of the optical fiber microsensor, referred to Eq. 7, which requires
the information specified in Table 1, and the indices of refraction of water 1,3, oil 1,48, and
air 1,0. Note in Table 2, the fiber microsensor has a high sensitivity to cross the air-oil,
because the change of transmittance is from 1,0 to 0,0, for a small variation of refractive
indices. In the case of oil-water interface, transmittance values are very close, as their
refractive indices, so that the sensitivity of fiber microsensor suffers a significant reduction.
Surrounding medium
T(n) to 1550 nm, (%)
air 100
water 18,5
oil 0,0
Table 1. Percentage decay of the microsensor T(n)
Interfaces
S( n)
water-oil 1,0
oil-air 2,0
Table 2. Optical fiber microsensor sensitivity
With a similar procedure and by using a white radiation source (light emitting diode of high
brightness) and a silicon photodetector, gives the percentage values of T(n) of the system
interacting with the same mediums, see Table 3. Note that the contrast in the detection for
the air, water and oil has improved. The advantage to radiate with white light source is that,
all wavelengths of the interval from 390 to 780 nm have contributed, so photodetector collect
more power.
200
Microsensors
Surrounding medium
T(n) to 390-780 nm, (%)
air 100
water 13
oil 20
Table 3. Percentage decay of the microsensor T(n) for visible region
Like any proximity sensor device, the optical fiber microsensor when passing the interface
of air, oil, and water, indirectly reports the location of the interface, by location or area of
switching their output voltages: H (high) or L (low). Knowledge of minimum time, that the
microsensor can resolve a change of medium, is of importance to the location of the
interfaces of immiscible liquids. The switching speed of the microsensor is strongly
restricted by the bandwidth of the photodetector to the optical output of the microsensor (13
kHz). Under this, to know the response time of the microsensor, it applies the criterion of
rise time, tr, (risetime) with respect to bandwidth of LTI system, which in this case is the
bandwidth, BW, of InGaAs photodetector. The relationship applied in this deduction is for
Eq. 9, so that, tr = 26,92 µm. Finally, the finish of fiber optic microsensor is shown in Fig. 18,
and specifications of optimized optical fiber microsensor are shown in Table 4.
Fig. 18. Physical illustration of the optical fiber microsensor, increased 220 times in a
monitor screen
Optical Fiber Microsensor of Semidrop
201
Elements Characteristics
Optical fibers
8,3/125 m
ILD
1550 0,5 nm
Power range
5 mW – 6,5 mW
Photodetector
Si low power, BW= 1,75 MHz
Max. response time
< 2 s
Dynamic range
Refractive index < 1,9
Sensitivity
- 5,714 x 10-4 V, water-oil
Temperature
0° a 50 °C
Table 4. Specifications of optimized optical fiber microsensor
6. Conclusions
This research has shown that the fiber optical microsensor is possible to manufacture by
electric arc technology. We demonstrated that is possible to improve the response detection
of optical fiber microsensor, for certain immiscible liquids, by proper choice of a specific
wavelength. This wavelength or radiation spectral region should provide a high
discrimination of the two liquids. Speed detection of the microsensor, strongly dependent
on the bandwidth of the photodetector. The method used in this research can be applied in
the detection of other mediums, including other regions of the optical spectrum. Due to
geometrical and physical properties of fiber microsensor, it can be applied to biomedicine,
because it is micrometer-scale, flexible like being guided in an artery or a catheter inserted,
and it is easy to sterilize. As future research work, we have considered using the
microsensor to implement a system of characterization and recognition of organic and
inorganic liquids (alcohols, acids, neutrals, blood, urine, water, etc.) by using spectroscopy
in the infrared and/or visible region. Optical fiber microsensor represents an alternative in
the industrial applications, and mainly in the detection of explosive, corrosive and/or
highly harmful liquids, for the human being.
7. Acknowledgment
We are thankful to the Benemérita Universidad Autónoma de Puebla, and the group of full
professors of the Facultad de Ciencias de la Electrónica, Optoelectronic department, by to
have looked for at the time, the ways and possibilities, to acquire these efficient equipments
of measurement, to have been able to make reality this project.