3.1. Experiment Setup
In order to evaluate the spectral characteristics of the twisted tapered POFs, a broadband light source (DH-2000, Ocean Optics) with operating wavelength from 215 to 2000 nm and a spectrometer (NOVA, Ideaoptics) were used. The light was guided into one of the twisted fibers, and the spectra for the twisted region in air, water, and glycerin solution were recorded. The fiber with the input light was called the active fiber, and the other one was called the passive fiber. Twisted tapered POFs with the active fiber diameter of 100 μm and passive fiber diameter of 200 μm were tested. The results are shown in Figure 2
; it was found that the transmission loss (the integration time of the spectrometer was set at 10 ms) was less obvious between the twisted tapered active POF and the non-twisted one, but it decreased when the twisted region was surrounded by water or glycerin solution. For the passive fiber, it was found that there was a very large transmission loss (the integration time of the spectrometer was set at 20 ms) when the twisted region was surrounded by water, but when the medium was changed to glycerin solution (RI of 1.39), the loss became smaller. In order to evaluate the sensing performance of the probe, a laser diode (TLS001-635, Thorlabs) with the wavelength of 635 nm and two photodiodes (S120, Thorlabs) with the responsivity of 0.41 A/W at 635 nm were used.
A schematic diagram of the experiment setup is shown in Figure 3
. As shown in the figure, the twisted tapered POFs were fixed on a substrate. The light source was launched into one of the input ports, and two photodiodes with a resolution of 1 nW were used to detect the light signals at the two output ends. The glycerin–water solutions with different volume concentrations (different RIs of external media) were prepared as the measured liquids. The RIs of the solutions were measured by an Abbe refractometer and described by the relation n
= 1.33 + 0.13C
, where C
is the volume concentration of the glycerin at 20 °C. The glycerin–water solutions were dropped around the twisted region as the external media with different RIs. For each measured liquid RI, the experiment was carried out five times repeatedly, and the averaged values were calculated. After each measurement, the sensing area was washed with deionized water and dried for the next measurement. The experiment was carried out at a room temperature of 20 °C.
3.2. Experiment Results and Discussion
shows the RI sensing performances for the sensors with active fiber diameter of 100 μm, while the passive fibers’ diameters were of 100, 200, and 300 μm, respectively. The twisted region length was 12 mm. The coupling ratio K
, where Ppas
were the average sensor outputs of passive and active fiber, respectively. Error bars are given for all measured data. The sensitivity of the investigated sensor was given as S
× 100%, where ΔK
is the change of the coupling ratio and Δn
is the change of measured liquid RI. As shown in Figure 4
, it was found that the K
displayed a non-monotonic change with the external medium RIs. When the liquid RI was smaller than 1.41, the cladding RI of the POF, the coupling ratio K
increased with the increased nex
, but when the nex
was larger than 1.41, K
decreased. This may be because that when nex
was equal to the fiber cladding, all the cladding modes were no longer guided and transformed to the radiation modes, which led to a decreased K
. This indicates the proposed sensor could detect a liquid RI higher than that of the fiber cladding, and the RI sensing range can be divided into two parts with the RI point of 1.41. It was also found that the sensor had a linear response in the RI ranges of 1.37–1.41 and 1.41–1.44. The sensor had the highest sensitivity when the passive fiber diameter was 200 μm. The sensitivity results from Figure 4
are shown in Table 1
shows the experiment results for the sensors with the active fiber diameter of 200 μm, while the passive fibers’ diameters were 100, 200, and 300 μm, respectively, and the twisted region length was 12 mm. It was found that the sensitivity was the lowest when the passive fiber diameter was 100 μm. The sensitivity results from Figure 5
are shown in Table 2
shows the RI sensing performances for the sensors with the active fiber diameter of 300 μm, passive fiber diameters of 100, 200, and 300 μm, and twisted region length of 12 mm. It was found the sensitivity was relatively low for sensors with passive fiber diameter of 100 μm or 200 μm. The sensitivity results from Figure 6
are shown in Table 3
From the experimental results above, it was found that the sensor had a better sensitivity when the diameter of the active fiber was smaller than that of the passive one, and the situation was the opposite otherwise. This may be because an active fiber with a small diameter would introduce more evanescent field and make more light energy leak out of fiber; on the other hand, a passive fiber with a large diameter has a larger surface area, which may receive more energy. Thus, more light could be modulated when nex changes. However, it was also found that a too-large passive fiber diameter caused the sensitivity to decrease. This may be because of the large cladding thickness for the fiber with a large diameter, which may cause the coupling efficiency to decline.
The influence of the twisted region length on the RI sensing performance was investigated as shown in Figure 7
, where the active and passive fiber diameters of the sensor were 100 and 200 μm. The twisted region lengths were 6, 12, and 18 mm. It was found that the sensitivity increased as the twisted region length increased. The sensitivity results from Figure 7
are shown in Table 4
The temperature dependence for the sensor was also tested, as shown in Figure 8
. The active and passive fiber diameters for the sensor were 100 and 200 μm, respectively, and the twisted region length was 12 mm. To test the influence of temperature, the twisted region was immersed in water with different temperature ranging from 20 to 60 °C. The results showed that the sensitivity to temperature was about 0.00001/°C, which may have been caused by the thermo-optic and thermal expansion effects of the twisted tapered POFs.
We also estimated the RI sensing performance for the probe with a macrobending structure as shown in Figure 9
. The active and passive fiber diameters of the sensor were 100 and 200 μm, respectively. The twisted region length was 12 mm, and the macrobending radius was 3 mm. It was found that K
decreased as the RI increased in the range of 1.33–1.38. This may be because the bending introduced more transmission loss. In the RI range of 1.38–1.44, the variation trend of K
was the same as that of the straight probes. This may be because more light energy was coupled to the passive fiber as the RI increased (less than 1.41).
shows a comparison of the RI sensing performance for the sensor described in this investigation as well as the other POF RI sensors cited in this article. From Table 5
, it is clear that the achieved RI sensitivities of the sensor in this investigation compare favorably with other sensors, and the proposed sensor has a large sensing range. In addition, this proposed sensor also achieved a self-referencing measurement, which can diminish the influence of light source fluctuations.