Determination of the Concentration of LiBr Using an
Optical Fiber Based System and an Interface in LabView
Edgar Eduardo Antúnez-Cerón1, Miguel Ángel Basurto-Pensado1,
and Alberto Ochoa-Zezzati2
1 Centro
de Investigaciones en Ingeniería y Ciencias Aplicadas, UAEM (Mexico)
2 Universidad Autónoma de Ciudad Juárez (Mexico)
Abstract. Heat pumps work with a refrigerant lithium bromide-water pair
(LiBr-H2O), and the efficiency of a heat pump may be adjusted depending on
the concentration of this pair. On the basis of the last, in this work an analysis
of LiBr concentration by Multi-Mode Interference (MMI) in an SMS
(Singlemode-Multimode-Singlemode) structure optical fiber is presented. In
this type of fiber structures the sensing element is a section of Multimode Fiber
(MMF) spliced between two Singlemode Fibers (SMF). Multimodal fiber
section has no cladding, so that its exposed core interacts with various
concentrations of the LiBr-H2O pair, which refractive indices are between 1.423
and 1.472. Different refractive indices of the solutions covering the exposed
multimode fiber core generate variations in the modes coupling inside the
multimode section of the SMS fiber, and therefore, a different response for each
case. The transmitted intensity is directed to an optical detector, then it is
processed by the suitable electronics and finally through a data acquisition
system is deployed on an interface created in LabView as a voltage response.
The results show that for concentrations with a smaller refractive index of
1.440, the voltage response can be correlated using a mathematical equation and
thus can be determined LiBr concentration of the sample
Keywords: Multimode Interference, SMS Optical Fiber, Refractive Index, Data
Acquisition System, LabView Interface.
1
Introduction
The operation of heat pumps depends largely on the cooling mixture or "pair” used in
the absorption system. The best known is the LiBr-H2O solution (lithium bromidewater), in which the water acts as the working fluid (refrigerant) and the LiBr as the
absorbent. Actually this concentration is not measured “in situ” and it is important to
know the value of that concentration to avoid the proximity of crystallization of the
solution to certain operating conditions, as this would severely damage the heat pump
[1]. The techniques used to determine the concentration of LiBr in heat pumps are
two: Diagrams Dühring and Refractometry. An alternative method for determining
the concentration of LiBr solutions is to use sensors which operate on the basis of
some of the optical properties of the solution, such as: the index of refraction (n).
Reason why this paper proposes the use of an optical technique called multimode
interference (MMI) to carry out such measurement of the concentration. The
information obtained from the optical system is processed and then collected by a data
acquisition system and presented in a LabView interface to facilitate the estimation of
LiBr to the user.
2
Principle of Operation
MMI is a useful basis for the implementation of a number of optical waveguide
devices. MMI was investigated and proposed at first for planar waveguides. MMI
based devices implemented in planar waveguides have been developed for optical
signal processing applications [2, 3], and for optical sensing applications [4, 5].
Multimode Interference (MMI)
A useful basis for visualizing and gaining a better understanding of MMI in a
multimode waveguide is the phenomenon of “self-imaging”. Self-imaging can be
defined as a property of multimode waveguides by which an input field profile is
reproduced due to constructive interference to form single or multiple images of the
singlemode input field at periodic intervals along the propagation direction of the
guide [6]. The self-imaging phenomenon in a waveguide due to MMI was studied and
described in [7].
To illustrate self-imaging due to MMI in a multimode waveguide, a structure
consisting of a multimode waveguide placed between input and output singlemode
waveguides is presented in Fig. 1, where the length of the multimode waveguide is L.
An input field profile existing at Z = 0 will be decomposed into the modal distribution
of all possible modes in the multimode waveguide. The field profile at a distance Z =
L can be expressed as a superposition of the modal distribution of all possible modes.
Under certain circumstances, the field at L = Z will be a reproduction or self-image of
the input field at Z = 0 [6].
Fig. 1. Schematic of a multimode waveguide placed between input and output singlemode
waveguides
Fig. 2 shows the simulated field profile within the multimode section and it is clear
that self-imaging of the input field takes place so that at periodic intervals, a single
image of the input field is reproduced. This occurs in Fig. 2 at 2708, 5415, and 8122
μm. Multi-fold images of the input field can also be found, for example, two-fold
images can be found at 1354, 4062, 6770, and 9478 μm. Self-imaging occurs at
specific lengths only for certain wavelengths [6].
Fig. 2. Field profile within the multimode waveguide showing self-imaging of input field [6]
The MMF is the key component required to support several modes (≥3). After the
supported modes are excited by launching a field using the input SMF, the
interference between the modes propagating along the MMF gives rise to the
formation of self-images of the input field along the MMF. Therefore, the length of
the MMF has to be precisely cleaved in order to have a self-image right at the facet of
the output SMF. The MMI effect is well known and the length of the MMF can be
calculated using [8]:
Where
is the beat length:
Here nMMF and DMMF correspond respectively to the refractive index and diameter
of the MMF core, with λ0 as the freespace wavelength. As shown in Eq. (1), selfimages are periodically formed along the MMF. However, due to the nature of the
MMI effect, there is a difference regarding the bandwidth observed for the true
images (every fourth image) and the pseudo-images (any other image) of the input
field. The pseudo-images exhibit a wider bandwidth, which is ideal for measuring
wider ranges of refractive index.
For our purposes a SMS fiber structure was designed to generate a first self-image
of the input profile at 1555 nm as shown in Fig 3. Basically, this is the principle of
operation: the SMS fiber (particularly the MMF section which has no cladding) will
be surrounded by LiBr-H2O solutions at different concentrations which have distinct
refractive indices according to the concentration, thus the transmitted intensity (the
self-image will be shifted from its original lenght of reproduction) will experience a
variation depending on the concentration of the sample.
Fig. 3. SMS fiber structure designed for the generation of a first self-image of the input profile
3
Experimentation
The experimental set-up for the determination of LiBr concentrations via MMI
technique is presented in Fig. 4.
Fig. 4. Experimental set-up for Multi-Mode Interference (MMI)
The source of the system is diode laser (1555 nm) connected to input of the SMS
fiber (the MMF section is placed in a container where the sample will be poured), the
output of the SMS fiber is connected to a photodetector (Thorlabs FGFA04) which
acts as a transducer converting de transmitted light into a voltage signal, then this
signal is directed to the electronics (noise filters and an instrumentation amplifier) in
order to give a better treatment to the voltage signal. Then, the improved signal is
directed to a data adquisitor (National Instruments USB-6259) and finally it is
displayed on an interface created in LabView as a voltage response.
SMS fiber in both MMF and SMF sections have a stepped index profile and the
refractive index of the MMF section (core) is approximately 1,440. This fiber was
designed to generate a self-image at a wavelength of 1555 nm and the MMF section
length is 14.5 mm.
An important part in the development of this project was how to present the
information to the user. To achieve this goal we use an adquisitor data which we send
information to a PC, where previously, a graphical interface was implemented in the
signal processing software LabView.
In the design of the graphical user interface (GUI) we considered necessary carry
out an additional treatment to the signals, that is, perform filtering and signal
averaging via software for the purpose of generating a better response signal. The
GUI designed for this application is showed in Fig. 5.
Fig. 5. Graphical User Interface for the display of the measurements
Interface presented in Figure 5 is constituted basically of the following elements: a
data capture button, the interface receives three voltage signals in response to a
particular test on concentration of the solution LiBr-H2O, the interface thus allows the
user to capture information from each test in the appropriate time. This interface also
has a table in which data is captured from each of the measurements (concentration
value and the three voltage responses) so that the user can watch the previous results.
Additionally it has the option to export the data table to a text file, in order to keep
faithfully the information generated.
4
Results
For the system testing were prepared 13 concentrations of LiBr-H2O solution for
reliable results (ranging from 44.30% to 60.69% of LiBr concentration). The voltage
responses were performed under the following parameters: current in the laser driver
(IDL) to 68.69 mA, real output power of the laser diode (PDL) of 3.5 mW and all
tests were performed at room temperature of 25 ° C.
To observe the optical behavior of the solutions a transmittance test was
performed, by this optical technique each solution was irradiated with light of a white
source, the amount of light transmitted by each of the solutions were processed on a
spectrometer to generate the optical spectra of each solutions in the optical software
(SpectraSuite) on PC. Fig. 6 shows the optical responses of each concentration at
1555 nm.
Fig. 6. Optical responses of different concentrations of LiBr-H2O
Fig. 6 shows that the optical responses of the concentrations of the solution are low
at 1555 nm reason which it is expected a low transmitted intensity for each
concentration. Furthermore, as a result of each of the solutions presents a different
refractive index (n) the transmitted intensity response generated by the system it is
expected to be different too. The results obtained for MMI (voltage response using the
experimental setup of Fig 4) are presented in Figure 7.
Fig. 7. Voltage responses of different concentrations of LiBr-H2O for MMI
The voltage response curve for MMI shown in Fig. 7 does not seem to obey a
particular trend, but if you look closely you can see three behaviors of particular
interest.
The first tendency is given by the concentrations of LiBr-H2O between 44.3% and
50.87% as shown also in Table 1 (shaded green). For these concentrations the voltage
response presents an increasing behavior, although in small increments of 0.821 to
0874 V. These solutions have a refractive index between 1,421 and 1,439,
respectively.
A second performance of the characterization of the voltage response is given by
the solutions which concentrations are between 52.49% and 57.05%. In this range the
response starts to decrease significantly from 0.86 to 0312 V as shown in Fig. 7 as
well as in Table 1. In theory one would expect an increasing behavior for the voltage
response, but obviously a very important factor for the MMI technique is the core
refractive index of the MMF section. This intrinsic parameter of the optical fiber
reports a value of approximately 1,440, and as can be seen from Table 1 this value is
exceeded by the refractive indices of concentrations greater than 50.87% of LiBr. The
above explains the reason why the response is generated with a decreasing tendency
in transmitted intensity. It is also important to mention that the basic principle to
confine and transmit information within the core of a fiber is not being fulfilled: total
internal reflection (TIR), since for this to happen it is necessary that the refractive
index of the core of the fiber is slightly greater than the refractive index of the coating
(normally 1%), condition that for concentrations greater than 50.87% is not met. This
represents a loss of information (scattering of radiation to the outside of the fiber) and
as a consequence provides an attenuation of the transmitted intensity within the SMS
fiber structure.
Table 1. Refractive index, voltage response and concentration of LiBr-H2O solutions for MMI
Refractive
Index (n)
1.421
1.425
1.426
1.439
1.444
1.447
1.4518
1.453
1.457
1.46
1.466
1.47
1.476
Voltage (V)
0.821
0.833
0.836
0.874
0.86
0.838
0.776
0.701
0.542
0.312
0.0746
0.0769
0.0781
Concentration
% LiBr
44.3
45.86
46.24
50.87
52.49
53.41
54.83
55.17
56.27
57.05
58.53
59.43
60.69
Nevertheless, we see a third behavior in the last three concentrations tested
(58.53% to 60.69%), although these responses are in a range of very low voltage
response is interesting to note that as the concentration of the solution increases, the
response in voltage also increases, although once again in very small increments, but
are detectable by the instruments used in the experimental set-up. The voltage
response experienced little growth ranging from 0.0746 to 0.0781 V. This peculiarity
could be explained from a chemical standpoint. It is known; by the properties of the
solution of LiBr-H2O at high concentrations the solution undergoes crystallization.
We assume that these concentrations are high enough to indicate that we are on the
threshold of a new physical state of this cooling mixture: the solid state, a fact which
is believed could generate this behavior.
In addition to the reported results, Fig 8 shows the estimate of the concentration of
LiBr using a linear correlation. In Fig. 8 you can see that for solutions lower than
50.87% (refractive indices less than 1.440) the concentration of LiBr can be
determined by the linear expression (3) with a square factor of R2 = 0.999, from the
above we can conclude that this linear approximation fits the response behavior of the
voltage provided by the optical system and thereby generates reliable results.
In Eq. (3) the variable V represents the voltage response generated by the optical
system.
Fig. 8. Linear fit for concentrations lower than 50.87% according to voltage response
5
Conclusions
We designed an experimental setup based on an optical property of the solutions of
LiBr-H2O: the refractive index (n) for the Multi-Modal Interference (MMI) technique.
Success is achieved with good stability and resolution in the voltage response to the
proposed instrumentation circuits also a LabView interface was designed for
acquiring and supporting data in the measurements. It accomplished the purpose of
determining the concentration of LiBr using an equation which is a function of the
voltage responses (ie, the equivalent of the optical response) generated by each of the
concentrations. The results are considered reliable and repeatable. The square factor
of the proposed equation is R2 = 0.999. MMI technique can not be used to determine
the concentration of LiBr for the whole concentration range, because the refractive
index of sensing section (MMF) of the SMS fiber is exceeded by the different
concentrations of the solution. The concentration range of LiBr proposed in this study
was established under the criteria of actual operation of a heat pump. The refractive
index of the solution plays a decisive role in the performance of the MMI, and the
condition of optimal performance is due to ensure that the refractive index of the
environment that surrounds the MMF fiber (sensing area) does not exceed the
refractive index of core.
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