# The Influence of Incident Power on the Magnetic Fluid Sensor Sensitivity Based on Optical Transmission Properties

Authors: Yongqian Li; Hao Zhang; Zhi Yang; Baoyi Yuan; Ze Yuan; Hailong Xue
Journal: Mathematical Problems in Engineering
 (2018)

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## Abstract

Sensitivity is a critical characteristic of sensors, and increasing the sensitivity of the sensor is valuable for measurement study. Incident power has an important influence on the sensitivity of magnetic fluid sensors based on optical transmission properties. Variation in the magnetic field sensitivity at different incident powers was investigated by the measurement of transmitted power through the magnetic fluid sensors. As the magnetic field strength increases, the sensitivity variation of the magnetic fluid film sensor can be divided into four stages: first decreasing sharply, secondly increasing, then decreasing gradually, and finally tending toward a small stable value. The magnitudes of the change in the sensor sensitivity are influenced by the incident power, because the structural pattern of the nano-magnetic particles in the magnetic fluid sensor changes, the Soret effect and the Photonic Hall effect co-define the sensing system. In the weak magnetic field range, when a higher sensitivity is required, it is appropriate to select a larger incident power; however, in a large magnetic field range, when a higher sensitivity is required, a small incident power should be selected. Therefore, the magnetic fluid film sensor exhibits different sensitivity characteristics if different incident power values are chosen. The appropriate incident power can be selected according to the range of the magnetic field to be measured to improve the sensitivity in the magnetic field measurement study.


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## Body

## 1. Introduction

Magnetic fluid, composed of nanoscale magnetic particles, a carrier liquid, and surfactants, is a type of functional material, possessing the fluidity of liquids and the magnetic properties of the colloidal solids. The properties of the magnetic fluid include the magneto-optical effect [1–4], the thermal lens effect [5–7], and the tunable refractive index [8–12]. Many properties of magnetic fluids can be exploited in a wide range of applications for optical devices, fiber optic sensors, and optical communication. In particular, the magnetic fluid sensors based on optical transmission properties have a wide application prospect in the field of online condition monitoring of power equipment.In recent years, many scholars have studied the magnetic fluid sensors, such as magnetic field sensors [13–17], temperature sensors [18, 19], and other kinds of sensors [20, 21]. Sensitivity is an important characteristic of sensors. There are many factors that affect the sensitivity of the sensor, such as sensor type, sensor structure, and sensor material. However, for the magnetic fluid sensor based on the optical transmission properties, the incident power is a key factor. Therefore, this paper studies the influence of the incident power on the sensitivity of the magnetic fluid sensor based on the optical transmission properties.


## 2. Materials and Methods

The sensing principle is primarily based on the variation of the optical transmission properties of magnetic fluid with magnetic field. When the sensor is applied with an external magnetic field, the magnetic field sensitivity of the light transmitted through the magnetic fluid changes because the structural pattern of the nano-magnetic particles in the magnetic fluid sensor changes. Thereupon, the Soret effect and the Photonic Hall effect co-define the sensing system [22–25].We defined the experiment parameters used in the paper as follows.(1) The incident powerP
I is the light power emitted from the Amplified Spontaneous Emission based broadband light source and attenuated to the designed optical power by the optical adjustable attenuator, which is the light power incident to the magnetic fluid sensor.(2) When light is incident from one end of a cylindrical magnetic fluid film sensor, the optical power transmitted from the other end of the film after transmission in the magnetic fluid is transmitted powerP
T.(3) The magnetic fieldM is generated by the magnet and applied to the magnetic fluid sensor so that the sensor is in the designed magnetic field strength.(4) The sensor sensitivityS as the ratio of the transmitted power change Δ
P to the magnetic field change Δ
M in the steady-state operation of the sensor. That is,(1)
S
=
Δ
P
Δ
M
=
P
2
-
P
1
M
2
-
M
1where P
2 and P
1 are the values of transmitted power when the magnetic field strength is M
2 and M
1, respectively. The unit of sensitivity S is mW/Oe, this is, the ratio of the units of the Δ
P and Δ
M.The experimental setup to investigate the sensitivity of the magnetic fluid sensors based on optical transmission properties is shown in Figure1. The light is emitted from the Amplified Spontaneous Emission based broadband light source with output power of 34 mW and the operating wavelength of 1527-1568 nm. The optical adjustable attenuator is adjusted to determine the incident power to the magnetic fluid sensor. An optical power meter is used to measure the transmitted power of the magnetic fluid sensor, and the measurable range and operating wavelength of the optical power meter are from -70 to +10 dBm and from 800 to 1650 nm, respectively. The external magnetic field strength applied to the magnetic fluid sensor is measured by a Gauss meter, the measurable magnetic field strength of the Gauss meter ranges from 0 to 20,000 Oe, and the resolution of the Gauss meter probe is 0.1 Oe. The incident light and the external applied magnetic field are perpendicular and parallel to the surface of the magnetic fluid film sensor, respectively.Figure 1
Experiment setup to investigate the sensitivity of the magnetic fluid sensors based on optical transmission properties.The structure of the magnetic fluid film sensor is shown in Figure2. In the experiment, the volume concentration of the synthetic ester-based Fe3O4 magnetic fluid, APGL17, produced by FerroTec Corporation is 4%, and the mean physical diameter of the magnetic nanoparticles is 10 nm. The saturation magnetization of the magnetic fluid is 110 Gs. The capillary is filled with magnetic fluid. The two fibers are precisely machined by cutting and inserted from both sides of the capillary. And the two junctions of the fibers to the capillary are fixed with glue. The distance between the ends of the two optical fibers is the thickness of the magnetic fluid film, which is selected to be 200 or 400 μm.Figure 2
Structure of the magnetic fluid film sensor.


## 3. Results and Discussions

As can be seen from Figures3 and 4, we confirm that the incident power has an important influence on the sensitivity of the magnetic fluid sensors. Figures 3 and 4 demonstrate that sensitivities as a function of magnetic field at different incident powers are similar for the 200 and 400 μm magnetic fluid sensors. With the enhancement of the magnetic field, the variation in the sensor sensitivity can be divided into four stages: first decreasing sharply, secondly increasing, then decreasing gradually, and finally tending toward a small stable value. Magnetic nanoparticles aggregation is the main cause of the stages changes in sensitivity [26]. With the change of light power incident into magnetic fluid and applied magnetic field, the structural pattern of the nano-magnetic particles in the magnetic fluid sensor changes, so the Soret effect and the Photonic Hall effect work [22–25].Figure 3
Sensitivity of 200μm magnetic fluid sensor as a function of magnetic field at different incident powers.Figure 4
Sensitivity of 400μm magnetic fluid sensor as a function of magnetic field at different incident powers.First of all, the sensor sensitivity decreases sharply for 200 and 400μm sensors. When the incident power is 5 mW, the sensor has a higher sensitivity in the weak magnetic field range; the 200 and 400 μm sensors are 2.5 times and 62 times more sensitive than those of 1 mW, respectively. In Tables 1 and 2, the decreasing proportion and the increasing proportion are the ratio of the final value to the initial value after the decrease or increase of the sensitivity in the magnetic field region, respectively. The sensitivities of the 200 and 400 μm sensors are sharply reduced to 8.4% and 10.8%, respectively, in the magnetic field strength range of 50-150 Oe when the incident power is 5 mW. However, for the incident power is 1 mW, the sensitivity of the 200 and 400 μm sensors decreased to only 48.7% and 42.9% in the magnetic field range of 50-100 Oe, and the sensitivity gradually increased thereafter. So in the weak magnetic field region, the larger the incident power the higher the sensitivity, but the magnetic field range is small. Above all, as described in the particle aggregation theory, when the magnetic field is applied, the magnetic nanoparticles of magnetic fluid sensor change from random motion to aggregating into clusters. Moreover, due to the Soret effect, when the light passes through a magnetic fluid, the magnetic fluid is heated and the temperature rises, forming a temperature gradient, and the magnetic particles diverge. Furthermore, magnetic field-induced nanoparticle aggregation leads to the Photonic Hall effect, which enhances the scattering [24, 25]. These influencing factors make the sensor’s sensitivity decrease rapidly.Table 1
Variation in sensitivity of the 200μm magnetic fluid sensor at different incident power.
Incident power (mW)
Sensitivity decrease
Sensitivity increase
Sensitivity decrease
Sensitivity saturation
magnetic field (Oe), decreasing proportion
magnetic field (Oe), increasing proportion
magnetic field (Oe), decreasing proportion
magnetic field (Oe)
1
50-100, 48.7%
100-250, 177%
250-650, 40.5%
-
3
50-150, 25.2%
150-200, 207%
200-650, 33.3%
-
5
50-150, 8.4%
150-200, 469%
200-550, 0.7%
550-650Table 2
Variation in sensitivity of the 400μm magnetic fluid sensor with different incident power.
Incident power (mW)
Sensitivity decrease
Sensitivity increase
Sensitivity decrease
Sensitivity saturation
magnetic field (Oe), decreasing proportion
magnetic field (Oe), increasing proportion
magnetic field (Oe), decreasing proportion
magnetic field (Oe)
1
50-100, 42.9%
100-200, 546%
200-650, 43.9%
-
3
50-150, 22.2%
150-250, 332%
250-500, 3.1%
500-650
5
50-150, 10.8%
150-250, 418%
250-550, 5.3%
550-650Secondly, the sensitivity of both 200 and 400μm sensors increases sharply. As shown in Tables 1 and 2, the sensitivities are sharply increased to 177% and 546% at the incident power which is 1 mW for both sensors, respectively. In addition, when the incident power is 5 mW, the sensitivities of the 200 and 400 μm sensors are sharply increased to 469% and 418%, respectively. With the enhancement of magnetic field, the nanoparticles form short chains at this stage, and the cross section area of the sensor occupied by the nanoparticles is decreased. Due to the increase of the magnetic field strength, the diffusion caused by the temperature gradient is suppressed; meanwhile, the scattering caused by the Photonic Hall effect increases, but the magnetic short chains formed by the magnetic fluid nanoparticle are the dominant factor. Therefore, the sensitivity of the sensor increases.Then, the sensitivity of the magnetic fluid sensor is slightly reduced. When the incident power is 5 mW, the sensitivities decrease to 3.1% and 5.3% in the magnetic field range of 200-550 Oe and 250-550 Oe for 200 and 400μm sensors. However, when the incident power is 1 mW, the sensitivity of 200 and 400 μm sensors decreases to 40.5% and 43.9% of the maximum in the magnetic field range of 250-650 Oe and 200-650 Oe, respectively. With the increase of magnetic field strength, the short chains of nano-magnetic particles become longer and thicker, which play a decisive role in reducing the sensitivity, and the influences of the Soret effect and the Photonic Hall effect are small. So the sensors have high sensitivity in a larger range of magnetic field at 1 mW.Finally, when the magnetic field reaches 500 or 550 Oe, the arrangement of nanoparticles gradually stabilizes, the sensor sensitivity tends toward a small stable value. The sensor with an incident power of 1 mW still has a certain sensitivity whereas the sensor sensitivity has been reduced to almost near to 0 at an incident optical power of 5 mW. It is clearly seen that a smaller incident power is more tolerant to sensitivity decrease.When the incident light power is large, such as 5 mW, both of the sensors have a high sensitivity in the weak magnetic field range, and the sensitivity rapidly decreases to 0 when the applied magnetic field is strong. In contrast, when the incident power is small, such as 1 mW, the sensitivity of the sensors is relatively stable, maintained at 0.7μW/Oe and 0.1 μW/Oe in the range 50-650 Oe and 150-650 Oe, respectively. Therefore, if the magnetic field to be measured is small, we select the large incident power of 5 mW. On the contrary, if the magnetic field range is large, we choose the small incident power of 1 mW. In general, the sensitivity of the magnetic fluid sensors is related to the incident power. When the sensor of the system is determined, the magnetic field sensitivity of the sensor can be changed by adjusting the incident power; when the measuring range of magnetic field and the sensitivity of the sensor are determined, the appropriate incident power and sensor film thickness can be selected.


## 4. Conclusions

The influence of the incident power on the sensitivity of magnetic fluid sensors based on optical transmission properties has been demonstrated. Based on the measurement of the optical transmission properties of the magnetic fluid, we obtain the sensitivity variation of the sensor with the magnetic field strength at different incident powers. As the magnetic field strength increases, the sensitivity variation of the magnetic fluid film sensor can be divided into four stages: first decreasing sharply, secondly increasing, then decreasing gradually, and finally tending toward a small stable value. This is due to the aggregation effect of magnetic nanoparticles, the Soret effect, and the Photonic Hall effect. The influence of incident power on sensor sensitivity is of great value in the study of magnetic field sensors. In a weak magnetic field such as 50-150 Oe, we choose a large incident power of 5 mW. In a large range of magnetic field such as 200-650 Oe, we choose 1 mW. The results show that the sensor sensitivity is improved effectively by selecting appropriate incident power.



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*Source: 9026071-2018-09-06.xml*