Operation of a prism optical fiber sensor for liquid level detection is described. ... one acting as receiving fiber) and a glass prism providing the total and partial.
Iranian Physical Journal, 1-2, 61-66 (2007)
A Prism Based Optical Fiber System for Liquid Sensing M. Razani1, H. Golnabi2 1
Physics Department, Islamic Azad University, Karaj Branch, Karaj, Iran Institute of Water and Energy, Sharif University of Technology, Tehran, Iran
2
Abstract Operation of a prism optical fiber sensor for liquid level detection is described. This sensor operates on light intensity modulation, where modulations result from alteration of total internal reflection into partial reflection at the interface. The modulated intensity has been measured by using a source light, a pair of plastic optical fibers (one transmitting and the second one acting as receiving fiber) and a glass prism providing the total and partial reflections. The performance of this sensor is tested with a laser source and the results are reported here. To analyze the operation of such a sensor theoretical formulation for the sensor operation is also given in this study. A comparison is made between the theoretical estimation and the experimental results.
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Introduction Optical fiber sensors have found many applications in recent years [1-3]. Physical and chemical effects have been used to develop a variety of sensors for monitoring displacement [4], pressure [5], high temperature [6] , and other parameters [7-10]. For example, a sensor design using a coated lens optic for displacement measurements was reported in Ref.[11]. In recent years, fiber optic sensors are advanced enough to become a good candidate for smart structure technology [12]. Hence optical fiber sensors have the potential to be used in design and construction of liquid level sensing devices. In any fiber optic probe used for measuring refractive index or liquid level, the transmitted or reflected light in the fiber is a function of the refractive index of the probe and the refractive index of the liquid to be measured. Using this principle researchers have been able to measure the index of refraction of liquids [13]. Optical refractometer using attenuation of cladding modes also have been reported [14]. In this respect evanescent sensors rely on the measurable loss of guidance from an optical fiber as a means of detecting external changes [15,16]. At a liquid-air interface the change of refractive index can be used for measuring the specific gravity of the liquid or measuring chemical parameter such as PH of a solution. A typical design has been the use of fiber guidance with inverted tip to provide internal reflection at the tip of the fiber [17]. Changes in refractive index of the liquid have been used for detection of hydrocarbons in water [18]. One can use these sensors to measure the liquid level [19,20], absorption temperature, PH factor, and temperature. The fiber refractometer technique can be used to measure specific gravity of storage batteries [21]. In a similar way fiber optic sensors have been used for liquid mixture
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composition determination [22, 23]. Using this method they have been able to measure the local concentration of liquid mixture. Liquid level control and measurement, in general, are based on principles such as capacitive, hydrostatic, ultrasonic, radiometric, and electromechanical measurements. Vibrating probes, conductivity switches, and microwave barriers can be used as liquid level switches. Now there are a variety of such devices in market including intrusive and non-intrusive level sensors based on RF capacitance technology, infrared pulse excitation, vertical buoyant level sensors based on contact tuning fork, and non-contact ultrasonic level sensors [24]. Point level switches based on a magnetostrictive device, as well as a capacitance change, have been fabricated for on/off control of liquids. As mentioned optical fiber sensors have a great potential for use as a level sensor or a switch. An optical level switch consists of a source light, an optical dipstick, and optical detector as reported in Ref. [25]. The goal of this study is to provide theoretical and experimental details on the optimum operation of such an optical fiber sensor for liquid level measurements. Experimental Design
The experimental arrangement used to test the constructed sensor is shown in Fig.1. It includes a light source, a transmitting plastic fiber, a fixture containing the glass prism, a receiving plastic fiber, and digital multimeter as a read out module that can be interfaced to a PC. The test fixture shown in Fig. 1 is made of either Plexiglass or Teflon block with dimensions of 50 mm × 80 mm × 16 mm, but for special applications Teflon material is preferred. A pair of optical fibers (step-index multimode) with a core diameter of about 450 µm and overall diameter
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Iranian Physical Journal, 1-2 (2007)
of 1.5 mm is closed coupled as shown in Fig. 1. The glass prism is a 45-90-45 degree type as shown in Fig. 2 with a dimension of 23 mm×44 mm×16 mm. In this design, source light is transmitted by a fiber, and collected from a receiving fiber by means of total or partial internal reflections in a glass prism. The second fiber transmits the reflected light to the photodetector (PD), which is mounted in a proper holding piece. Fig. 2: Optical path-lengths in the coupling 45-90-45 degree prism for the normal incident ray.
Using Fresnel relations the reflected light amplitude from an interface depends upon the refractive index of the two media, angle of incidence, angle of refraction and state of incident light polarization. Considering all interfaces involved the amplitude of the light Tp emerging at glass- air interface( point 4) for the p-polarized light is
A digital multimeter with a precision of 0.1 mV is used for the output voltage measurements. By employing beam-splitting optics between the light source and the fiber one can use a single optical fiber. A 3-dB coupler may be used for the light splitting as indicated by other reports [25]. The photodetector used in this experiment is a planar silicon PIN diode type. Typical characteristics of this detector are: Active area of 1.5 mm2, responsivity of 0.2 A/W at 450 nm, 0.55 A/W at 900 nm and 0.4 A/W at 632.8 nm (He-Ne laser red wavelength). Source light is transmitted via a transmitting fiber to an optical prism and the photodetector measures the reflected light from the prism via the receiving fiber. During the level measurements, when a liquid in a vessel touches the 45-degree faces of the 45-90-45-degree prism, the total internal reflection is disturbed, and the reflected light is modulated. When there is air around the prism most of the light is reflected up in the fiber while there is liquid, a considerable amount of light is lost through the prism into the fluid. In this way the probe acts as a level switch, changing its state at one particular light intensity level.
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Tp = f p 4 f p 3 f p 2 f p1 Ap ,
Fig. 1: Experimental arrangement for a prism-based optical level sensor.
where
(1)
f p1 =
2µ air cosθ1 , µ g cosθ1 + µ air cos r1
(2-a)
f p2 =
µ a cosθ 2 − µ g cosθ 2 t , µ a cosθ 2 + µ g cosθ 2 t
(2-b)
f p3 =
µ a cosθ 3 − µ g cosθ 3t , µ a cosθ 3 + µ g cosθ 3t
(2-c)
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f p4 =
2 µ g cosθ 4
, (2-d) µ air cosθ 4 + µ g cos r4 and Ap is the amplitude of the p-polarized light incident at initial air-glass interface (point1), θ in general is the incident angle at each interface and r indicated the angle of refraction at each interface. µair is the refractive index of air, µg is the refractive index of prism material (glass) and µa is the refractive index of the ambient, which is liquid in this case. Angle θ2t is the refraction angle in the ambient medium at point 2 and θ3t is the refraction angle in the ambient at point 3. In a similar way the amplitude of the s-polarized light, Ts, emergent at the final interface glass-air is given by
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Ts = f s 4 f s 3 f s 2 f s1 As , where 2µ air cosθ1 , f s1 = µ air cosθ1 + µ g cos r1
Theoretical Computations The characteristic of the reported sensor can be compared with the case of two similar collinear fibers coupled through a prism. The beam path in the prism is shown in Fig.2. As shown in Fig. 2, the light path can be considered at four points (numbered as points 1,2,3 and 4 ).
fs2 =
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µ g cosθ 2 − µ a cosθ 2 t , µ g cosθ 2 + µ a cosθ 2 t
(3) (4-a) (4-b)
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A Prism Based Optical … f s3 =
Iranian Physical Journal, 1-2 (2007)
µ g cosθ 3 − µ a cosθ 3t , µ g cosθ 3 + µ a cosθ 3t
Based on the formulas developed in the last section important parameters are computed by using the MATLAB programs and the results are discussed here. Variation of the obliquity factor K with the incident angle is also computed by using the MATLAB program and the result is plotted in Fig.3.
(4-c)
2 µ g cosθ 4
f s4 =
, (4-d) µ g cosθ 4 + µ air cos r4 and As is the amplitude of the s-polarized incident light. For the case of isosceles prism (45-90-45) and normal incident light the incident and reflected angles at point 1 and 4 are zero; and angles at points 2 and 3 are identical (45 degree). According to Eqs. (2) and (4), the relative emergent intensity for the un-polarized light is the average of s and p polarized light that is ⎞ (f ⎟=
⎛ I ⎜⎜ ⎟ ⎝ I TIR ⎠
f f f ) + ( f s 4 f s 3 f s 2 f s1 ) p 4 p 3 p 2 p1 2
(f
0.98
Obliquity Factor
0.96
f p1 ) + ( f s 4 f s 1 )
,
2
(5)
0.88
0.86
⎞ ( f p3 f p 2 ) + K 2 ( f s3 f s 2 ) ⎟⎟ = , 1+ K 2 ⎠ 2
2
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10 15 20 Incident Angle (Degree)
Variation of the obliquity factor K is calculated for assuming the glass refractive index of 1.5. As can be seen in Fig.3, this factor shows a decrease from one to about 0.86 for a variation of 30 degree in the incident angle. Considering the result it is noted that the maximum intensity ratio is obtained for the case of normal incidence and total internal reflection. Any deviation of the parallel beam from the normal causes reduction in the computed ratio. However there is a limit for the obliquity factor K in such a way that below that value there is no reflected light emerging point 4 in the prism. The relative intensity as indicated in Eq.(6) is plotted as a function of the liquid refractive index. The curve in Fig. 4 show the variation for the spolarized and p-polarized light, respectively. In Fig. 4 the value of the refractive index changes from 1.5 to 2.5 for this study. A sharp increase is noted for a change of 1.5 to 2.5 and for the lower index
µ (7) where µ rel = g . For the symmetric case that θ1 µ air = r4, and r1= θ4 then K reduces to ⎛ f f ⎞ ⎛ µ cosθ1 + cos r1 ⎞ ⎟⎟ . K = ⎜ s 4 s1 ⎟ = ⎜⎜ rel ⎜ f f ⎟ ⎝ p 4 p1 ⎠ ⎝ cosθ1 + µ rel cos r1 ⎠
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(6)
⎞ ⎛ ( µ rel cos θ 1 + cos r1 )(cosθ 4 + µ rel cos r4 ) ⎞ ⎟=⎜ ⎟ ⎟ ⎜ (cos θ + µ cos r )( µ cos θ + cos r ) ⎟ 1 rel 1 rel 4 4 ⎠ ⎠ ⎝
0
Fig. 3: Variation of the obliquity factor K as a function of the incident angle.
2
where factor K is determined from ⎛ f f K = ⎜ s 4 s1 ⎜ f f ⎝ p 4 p1
0.92
0.9
where I is the intensity of the reflected light from the final interface and ITLR is the light intensity when total internal reflection takes place at two intermediate interfaces(points 2 and 3). For total internal reflection (TIR) case fp2=fs2=fp3=fs3=1. After some algebraic manipulation Eq. (5) can be written as ⎛ I ⎜⎜ ⎝ I TIR
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2
2
p4
1
(8)
0.12
For normal incidence (θ1 = r1=0) on the first interface of the prism fs4 fs1= fp4 fp1 and results K=1. The parameter K represents the obliquity factor and can have values less than or equal to 1. The parameter K having values other than unity is a measure of departure from normal incidence at the first interface of the prism. For case that K approaches one the denominator of Eq.(6) becomes 2 and such an equation is used for the intensity ratio computations.
Relative Intensity
0.1
0.08
0.06
0.04
0.02
0 1.5
Results
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1.7
1.8
1.9 2 2.1 Refractive Index
2.2
2.3
2.4
2.5
Fig. 4: Intensity ratio for the un-polarized light as a function of liquid index of refraction.
In the first study the variation of the relative intensity and the obliquity factor K are investigated.
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5 Output Signal (V)
value the ratio shows a very small value which could not be plotted in the same figure. The intensity ratio for the un-polarized light is the average value of the s- and p-components, which have equal part in the total intensity. As can be seen in Fig.4, the intensity ratio for the un-polarized light is very small for the lower liquid index of refraction and most of the light is not reflected back to the receiver fiber. For example for a liquid index of 1.5 the ratio is about 1%. Experimentally, the incident beam is not completely parallel and there is a dispersion angle for the incident beam. for the coherence source beam the divergence angle is small in mrad range while for the LED diode and the white lamp there a higher divergence angle. Thus the obliquity factor plays a more important role for the incoherent light sources. Such effect result in the ray of beams with obliquity to also hit the prism and as a result the measured signal is lost due to the described effect. Hence the optimum operation results for the case of normal incidence angle and TIR condition corresponding to K value of one. To test the system for liquid flow two methods can be used. First case requires a regular up flow of the liquid in a vessel when the sensor probe is fixed at a vertical position. The second method is to immerse the sensor probe in a vessel containing the constant level liquid. In this experiment since it was easier to measure the liquid height accurately the second immersion method is used. Up and down movements of the sensor probe is possible by the height variation using a translation stage. In this experiments the height of the liquid touching the prism is about 5-8 mm. In the first test of the device we measured its operation for different liquids. We have measured the output signal for different liquids. These samples are Water, Methanol, NaCl (10% molar) and NaCl (10% molar) dissolved in water. The result of such measurements for these samples are shown in Fig.5 for the case of the dry and wet signals. When the probe is about 4 mm above the liquid surface level the out put signal is about 4V. at this point by touching the surface level as can be seen in Fig.5, the output drops from 4V(dry signal) to about 60 mV(wet signal). For all the cases reported here a total background signal (dark current noise signal plus stray light signal) of about 20 mV is subtracted from the output signal.
Water
4
NaCl10% 3
NaCl20% CH3OH
2 1 0 0
2
4
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8
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Liquid Height (mm)
Fig. 5: Output signals for level sensing of different liquids.
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To check the repeatability of the sensor, in Fig.6 the results for two different runs for the sensor are plotted together and indicated as series1 and series 2. As can be seen in Fig.6, the reported results for this sensor are quite reproducible and the minor difference in the two runs is due to the difference in the immersing speed of the probe in the liquid and mainly the laser light fluctuations. This fluctuation for the He-Ne laser is about (4%) of the full scale output signal.
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Output Signal (V)
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Series1
3.0
Series2
2.5 2.0 1.5 1.0 0.5
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0.0 0
2
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10
Water Height (mm)
Fig. 6: Reproducibility of the results for the sensor.
Another important factor in the operation of the sensor is the hystresis effect. By this effect one can see the difference between upward and downward operation of the reported design. Performance of the probe when approaching the water level and the reverse case when the probe is getting away from the sample surface is shown in Fig.7. As can be seen the output resultd for both cases are very close to each other. This indicates that the reported level sensor has a small hystresis, which is one of the advantages of such a system.
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A Prism Based Optical …
Iranian Physical Journal, 1-2 (2007) Our computations clearly show that the obliquity factor plays important role in the operation of the reported sensor. The observation here and the results given in [26] suggest that for more sensitive experiments coherent light sources provide higher sensitivity and output voltages. The voltage difference by using a diode laser is improved by one order of magnitude while for He-Ne laser is better than two orders of magnitude. For this reason the He-Ne laser provides a higher intensity with respect to the diode laser and both of them show a higher output signal in comparison with the regular light emitting diode. However, as we improve the sensor performance by using a laser, the cost is higher for a laser source sensor and some justification is required for the optimum applications.
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Output Signal (V)
4 up
dow n
3 2 1 0 0
2
4
6
8
10
Liquid Height (mm)
Fig. 7: The hystresis effect of the sensor operation.
The Accuracy of the reported system for water level measurement is about ±1 mm, repeatability of ±0.5 mm, dry output voltage for water sample is 4 V (4000 mV) and the wet signal voltage is about 60 mV. Using a similar technique liquid flow sensing was reported in [26]. However no theoretical details for the operation of such a system was given in that report. In the work reported here both the theoretical and experimental results are described together. In this section we compare our results with the result of the previous experiment [26]. In the previous report, operation of a liquid level sensor was given by using different light sources (LED diode, diode laser and He-Ne laser). In this study a He-Ne laser was used as the light source and a new experiment was arranged using the same technique of the signal detection. In the case of He-Ne laser in Ref.[26] its accuracy was ±1 mm in level detection, with a repeatability of about ±0.5 mm. The difference on the wet and dry output voltages for the He-Ne laser light in [26] was about 3488 mV, while for this experiment is about 3940 mV whereas the other parameters are about the same. Our new experiment shows a notable increase in the sensitivity of the sensor (signal difference). By using high quality components in the experimental setup and a better fiber coupling the difference signal was improved considerably. In construction of such sensors care must be taken in order to maximize the detector output difference between total internal reflection and loss of optical guiding system (dry and wet operations). For the case of using a prism for the power coupling it must have a good transmission at the source light wavelength. In the case of direct optical fiber sensors with no prism the total internal reflection can be accomplished by shaping the fiber end or by providing the optimum radius of the curvature for the fiber. At the end, for any sensor device the described intensity ratio should be minimized for an optimum operation.
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Conclusion
The reported optical fiber sensor based on the measurement of the transmitted light can be used for liquid level sensing, liquid level switches, and for refractive index measurements. The sensor probe can be mounted vertically at top of the measuring tank. The high sensitivity of the reported sensor is one of the advantages of this sensor with the similar ones [26]. The simplicity of the design, in particular, systems using a white lamp, can lead to the fabrication of more cost effective sensors. It is fair to say that with all these advantages, the reported sensor system has a few limitations. Since it is a contact type probe so some liquids can affect the prism contact surface and contaminate it. The Teflon mount can be resistive to most organic and inorganic chemical solvents and compounds. However, the operation of reported sensor is limited to the clean liquids. In spite of these limitations, such sensor still could be a proper device for liquid level sensing and other related measurements. Typical applications of described sensors could be level sensing of water, clean white petroleum, and specific gravity checking of some aqueous solutions.
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Acknowledgments This work was supported in part by the Islamic Azad University and the first author gratefully acknowledges the grant devoted to this research for the experimental work. References [1] Krohn DA. Fiber Optics sensors: Fundamentals and applications. Instrument Society of America, NC , 1998. [2] Bishnu PP, Fundamentals of fiber optics in telecommunication and sensor systems. New Dehli: Wiley Eastern Ltd., 1994.
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