1. Introduction

Temperature is a very important physical variable for monitoring and control in the manufacturing process of chemicals, polymers & polymer products and drugs, in oil refining, in the food processing industry, and in biomedical applications [1–5]. Moreover, in sensors based on flow, strain, humidity, and pressure, there is an effect of temperature on the measured properties. Hence, it is important to measure temperature accurately so that the effect of temperature can be compensated and decoupled from the actual measured values [4]. Moreover, it is important to accurately sense temperature in structures such as bridges and buildings as changes in temperature affect the expansion or contraction of these structures. Some of the attributes of good temperature sensors include high sensitivity and response times, ability to withstand high temperatures, compact size, and low weight. In temperature sensing applications, it is also important to isolate the effect of temperature, on the optical spectrum of a sensor optical fiber, from other factors such as losses due to connectors and light absorption in the fiber [3].

As remote sensing and large area distributed sensing are important characteristics for some of these applications, it is important to develop temperature sensors on a platform, such as an optical fiber, such that these sensors can be easily incorporated into other materials or structures [3, 6]. Moreover, there is substantial interest in fiber optic temperature sensors, since they are immune to electromagnetic interference and can often be used in chemical environments. This allows them to be used in a wide variety of demanding applications such as the monitoring of temperature in transformers, in the borehole of oil wells as well as in a variety of other industrial applications. A wide variety of approaches have been used to construct fiber optic temperature sensors including the use of fiber Bragg gratings [7] or interferometric sensors [8–11], the measurement of fluorescence lifetimes [12, 13], measurement of optical properties of semiconductors [14–16] and more recently monitoring the shift and broadening of surface plasmon resonance [17]. In addition, the fabrication of in-line optical fiber sensors has also been demonstrated where optical elements formed by graded index and coreless optical fibers can be used to engineer an optical system within the optical fiber [18].

For single crystal semiconductors, the spectral position of the optical absorption edge is dependent on the band gap, which in turn is dependent on the temperature. Known as the Varshni effect [19, 20], this temperature dependence typically follows the form:

Therefore, vanadium oxide compounds were considered as alternative temperature sensitive materials because of their ability to exhibit large changes in optical properties near room temperature, unlike Si and GaAs. Vanadium oxide (VO_x) thin films are useful for a variety of applications ranging from smart windows, temperature sensors and bolometers, storage devices, ultra-fast optical switches, and as catalysts [21–29]. The vanadium oxide material system is complex consisting of many different phases with different valance states of vanadium in the oxide [21–23]. The value of ‘x’ in the different vanadium oxide (VO_x) phases varies between 1 and 2.5 and some of the common vanadium oxide phases are the VO, VO₂, V₂O₃, V₄O₇, and V₂O₅ phases [21–23]. It has to be noted that although most of the phases of vanadium oxides (VO_x) exhibit a phase transition upon heating, the vanadium dioxide (VO₂) phase of VO_x exhibits a first order phase transition between semiconducting and metallic states at ~68° C [21–23]. This phase transition can also be produced by illuminating a vanadium dioxide film with a laser. This semiconductor to metal phase transition leads to a deformation of the lattice structure of vanadium dioxide from monoclinic to tetragonal or rutile structure, wherein the optical constants in the metallic phase are different from those in the semiconductor phase [20–22]. The phase transition also has an effect on the electrical and optical properties of the vanadium oxide film. Upon phase transition, the material transforms from being semiconducting and transparent to being conductive (metallic) and reflecting in the infrared region. Thus, one can employ a red or an infrared light source and monitor the change in the transmission intensity through a vanadium oxide thin film as it undergoes the semiconductor to metal transition. Moreover, one can also monitor the shift in the temperature-dependent band edge in the blue-green region of the visible spectrum to sense changes in temperature. In the case of un-cooled bolometer technology, it has been useful to tailor the composition of the films such that a mixture of phases [23] coexist in order to maximize the changes in electrical conductivity with temperature while minimizing the hysteresis from the phase changes.

The temperature sensors, proposed in this paper, have been fabricated by incorporating thin films of a semiconductor material – with temperature dependent optical properties – such as vanadium oxide (VO_x) on the tips of optical fibers (‘Tip-based’) as well as inside the optical fiber matrix (‘Matrix-based’). These can be employed as accurate temperature sensors based on a change in their transmission spectrum–including a band-edge shift–as the temperature surrounding the sensor region of these optical fibers is varied. Their applicability as efficient and robust temperature sensors is further justified because of the possibility of using them in remote temperature sensing applications. Moreover, incorporation of vanadium oxide films inside an optical fiber matrix allows the formation of robust continuous inline optical fiber sensor structures. In addition, if the VO₂ state of the VO_x compound is realized, these optical fiber sensors also have the ability to form an inline ultrafast optical switch pertaining to the fast optical switching offered by vanadium dioxide.

2. Experimental

There are a wide variety of methods employed for producing vanadium oxide films including sol gel process, hydrothermal process, RF magnetron sputtering, oxidation of vanadium metal deposited by evaporation, and pulsed laser deposition [24–28]. In this paper, the vanadium oxide films (~100 nm thick) were deposited on the tips of optical fibers by employing pulsed laser deposition (PLD) at room temperature. Figure 1(a) shows the pulsed laser deposition of a vanadium oxide film on the fiber tip. Polycrystalline V₂O₃ targets, one inch in diameter, were prepared from V₂O₃ powder (~120 mesh, 99.995% pure) by pressing under 5000 PSI, and sintering in air at 650 °C for 9 hours. The sintered targets were then polished till they obtained a smooth surface. The optical fibers, employed in this work, were F-MLD multimode fibers obtained from Newport Corporation with a 100 μm core and a 140 μm cladding diameter. It must be noted that the normal communications-grade MMFs (e.g., 50/62.5 um core and 125um diameter) could also be employed in the fabrication of such fiber-optic sensors. The ends of the optical fiber were stripped and cleaved using a diamond cleaver. The cleaved fiber sample (mounted on the substrate holder) and the target were loaded in the PLD chamber which was pumped down to a base pressure of ~1.4 x 10⁻⁷ Torr before carrying out the deposition. A capacitance manometer and throttled gate valve were used to maintain oxygenpressure at 5 mTorr during the deposition process. During the deposition, the KrF Excimer laser at 248 nm was operated at ~150 mJ per 10 nanosecond pulse at a frequency of 10 Hz.

 

Fig. 1 (a) Pulsed laser deposition to deposit vanadium oxide on the tip of an optical fiber, (b) Pulsed electron beam deposition (PED) of fused silica on top of an annealed vanadium oxide film so as to protect the film before fusion to another fiber, and (c) Schematic showing the PED process for depositing fused silica on the VO_x film over the optical fiber tip.

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After the pulsed laser deposition, the vanadium oxide-coated optical fibers were removed from the PLD chamber and annealed to obtain polycrystalline films from the ‘as-deposited’ amorphous vanadium oxide (VO_x) films. The annealing of the vanadium oxide films was carried out using controlled, low-intensity plasma arcs employing a Type-36 Sumitomo Electric fusion splicer, as shown in Fig. 2(a). A schematic illustrating the formation of a ‘tip-based’ fiber sensor with PLD-deposited vanadium oxide on its tip is shown in Fig. 3(a).

 

Fig. 2 (a) Application of plasma arcs to the vanadium oxide film deposited on the tip of an optical fiber, and (b) Effect of application of plasma arcs to the vanadium oxide film deposited on the tip of an optical fiber; An increase in the absorption edge was observed upon application of subsequent plasma arcs indicating the increase in crystallinity of the film.

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Fig. 3 (a) Schematic showing the formation of a tip based fiber sensor with vanadium oxide deposited on the tip of the fiber by using pulsed laser deposition, (b) A fiber with vanadium oxide on its tip after annealing of the film by the application of controlled plasma arcs, and (c) Schematic of the setup employed for evaluation of temperature dependence of the spectrum of the optical fiber containing the vanadium oxide film on the tip of the optical fiber.

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Figure 3(b) shows an optical microscope image of the optical fiber with plasma-arc annealed vanadium oxide on its tip. Optical transmission measurements were made on vanadium oxide-coated optical fibers, for both the ‘as-deposited’ and the ‘annealed’ vanadium oxide films, using a SpectraPro-500 spectrograph having a Spectra-drive stepping motor scan controller. The vanadium oxide-coated optical fiber tips and a light collector fiber were aligned inside a micro-coil heater as shown in Fig. 3(c), and the effect of temperature on the transmission spectrum of the vanadium oxide film was evaluated. Similar measurements were carried out on an uncoated silica optical fiber and these served as the reference spectral measurements. Incorporation of vanadium oxide (VO_x) inside an optical fiber involved taking the optical fiber — having an annealed vanadium oxide film on its tip — and depositing a 3-4 μm layer of fused silica on the VO_x film using pulsed electron beam deposition, as shown in Fig. 1(b)-1(c). This silica-coated fiber was fused to an uncoated optical fiber to form a continuous optical fiber structure containing VO_x inside the fiber matrix. A schematic, illustrating the formation of a continuous inline fiber sensor with PLD deposited vanadium oxide inside the optical fiber matrix, is shown in Fig. 4(a).Figure 4(b) shows an optical microscope image of an optical fiber with an annealed vanadium oxide film incorporated inside an optical fiber matrix to form a continuous in-line fiber optic temperature sensor. The temperature dependent transmission properties of the ‘Matrix-based’ fiber sensors – i.e. the optical fibers containing vanadium oxide inside the fiber matrix – was evaluated by placing these fibers in a micro-coil heater as shown in Fig. 4(c).

 

Fig. 4 (a) Schematic showing the formation of an inline fiber sensor with vanadium oxide incorporated inside the optical fiber matrix, (b) A fiber with an annealed vanadium oxide film incorporated inside an optical fiber matrix to form a continuous in-line fiber optic temperature sensor, and (c) Schematic of the setup employed for evaluation of temperature dependence of the spectrum of the optical fiber containing the vanadium oxide film inside the fiber matrix.

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In order to identify the phases of the vanadium oxide (VO_x) films deposited by PLD and annealed thereafter, Raman spectra were taken using an innoRam Raman spectrometer (Model BWS445). Light from a 785 nm laser with a power of 150 mW was incident on the VO_x coated fiber tip, and the backscattered signals were collected by a fiber-optic probe and incident on a TE cooled back-thinned CCD. The spectra for intensity of the Raman signals versus the wavelength shift (cm⁻¹) were thus obtained. However, to clearly observe the Raman peaks, the peak heights were baseline corrected. The spectra were also corrected to remove the etalons.

3. Results and discussion

A novel a annealing process was employed for annealing the vanadium oxide films deposited on the tips of optical fibers. This process involved application of low intensity plasma arcs on the tip of the optical fiber as shown in the Fig. 1. This process can be considered analogous to rapid thermal annealing of thin films. One can observe in Fig. 2(b) that before the applicationof plasma arcs, the vanadium oxide film was amorphous as indicated by the lack of a sharp band edge. On the application of one plasma arc, the band edge slope changes but the film is still amorphous. It can be seen in Fig. 2(b) that on application of 2 or 3 plasma arcs, the film displays a sharper band edge indicating a polycrystalline character of the vanadium oxide film. Moreover, the shape of the transmission spectrum and the position of the band edge indicate that a mixture of several phases of vanadium oxide (VO_x) are present, more specifically V₂O₅ being the dominating phase along with smaller proportions of VO₂.

The annealed vanadium oxide film on the tip of an optical fiber was evaluated for its transmission spectrum as a function of temperature by employing the experimental setup shown in Fig. 3(c) and a white light source. The effect of temperature on the spectrum of the ‘Tip-Based’ fiber sensor is shown in Fig. 5(a). It was observed that there was a substantial decrease in the normalized optical transmission, especially in the red region of the visible spectrum, as the temperature of the micro-coil heater was increased. The indication of large changes of transmission with temperature in the red region of the visible spectrum are also indicative of crystalline vanadium oxide (VO_x) being formed. It can also be observed fromFigure 5(a) that the transmission increased as the temperature of the micro-coil heater was decreased. The spectra in Fig. 5(a) were normalized to the intensity of the room temperature spectrum at ~700 nm wavelength, such that the values in Fig. 5(b) could be obtained from the transmission values in Fig. 5(a) by multiplication with a normalizing factor of 2.63. Figure 5(b)shows the effect of temperature on the optical transmission of the ‘Tip-Based’ fiber sensor evaluated at 561 nm wavelength of input radiation. One can observe a substantial decrease in the intensity of the transmitted light as the temperature surrounding the fiber tip is increased. One can observe in Fig. 6 that as the temperature of the film increases, there is a shift in the band edge towards the right. It can also be seen that lowering the temperature back to room temperature causes the band edge to nearly return to the original position, i.e. the band edge position before the heating experiment was carried out. The shift in absorption edge withtemperature can be explained by the temperature dependence of the bandgap energy. As the absorption coefficient of the vanadium oxide film is dependent on the bandgap energy, the absorption coefficient and therefore the intensity of light transmitted through the oxide film are functions of temperature. Figure 7 shows the temperature-dependent transmission spectrum of a continuous inline fiber containing a vanadium oxide film inside the fiber matrix. This inline fiber was formed by fusing a fiber, containing vanadium oxide on its tip and over-coated with fused silica, to another optical fiber (See Fig. 4). It was observed that there was a substantial shift in the band edge upon heating the film and that the band edge nearly came back to the original position on lowering the film temperature back to room temperature. In the case of the optical fiber containing the vanadium oxide film inside the fiber matrix, it was observed that the transmission does not decrease with temperature in the red region of the spectrum. While VO₂ is expected to have large changes in its transmission spectra with temperature, V₂O₅ is not expected to have such large changes indicating that V₂O₅ is the dominant phase in the vanadium oxide material.

 

Fig. 5 (a) Effect of temperature on Spectrum of an optical fiber containing an annealed Vanadium oxide film on the fiber tip, and (b) Effect of temperature on the optical transmission of an optical fiber, containing an annealed vanadium oxide film on its tip, evaluated at 561 nm wavelength of input radiation. The spectrum in (a) was normalized to the intensity of the room temperature spectrum at ~700 nm wavelength. The relatively weak hysteresis loop that is observed is consistent with VO_x thin films [30–32].

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Fig. 6 Effect of temperature on the normalized spectrum of the optical fiber, containing vanadium oxide film on the tip of the optical fiber, indicating a shift in the band edge upon increase in temperature around the optical fiber. The band edge comes back to nearly the original position upon cooling back to room temperature. The spectrum was normalized to the intensity of the room temperature spectrum at ~700 nm wavelength.

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Fig. 7 Effect of temperature on the spectrum of the optical fiber containing a vanadium oxide film inside the optical fiber matrix, indicating a shift in band edge upon increase in temperature around the optical fiber.

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The typical loss in the proposed temperature sensors is ~12dB/km taking into account the attenuation loss, mis-alignment loss, insertion loss and loss due to the absorption in the surrounding VO_x film. The sensitivity of these temperature sensors can be determined by measuring the amount of red shift − of the band edge (in nanometers) − with temperature. The band-edge shift can be measured at the mid-point (on the y-axis) of the maximum and minimum values of the normalized relative transmission spectra shown in Figs. 6 and 7. The sensitivity of the tip-based and matrix-based sensors was found to be 0.28 nm/°C and 0.19 nm/°C respectively. The temperature sensitivity of the typical Fiber Bragg Grating (FBG) temperature sensors is ~0.01 nm/°C [38], though more recently FBG temperature sensors with a sensitivity of ~0.028 nm/°C have been reported [39]. Hence, the temperature sensitivity of the sensors proposed by us is better than that reported previously for the FBG sensors. Along with good temperature sensitivity, the inline optical fiber sensors proposed in this paper are compact and low-weight and can be easily integrated into films or media whose temperature needs to be measured.

To identify the different phases of vanadium oxide (VO_x) — present in the VO_x film deposited on the tip of an optical fiber and annealed using plasma arcs — Raman spectra were taken from these VO_x films. The etalon-corrected and baseline-corrected Raman spectra of the annealed VO_x films on the tips of the optical fibers, are shown in Fig. 8.It was found that the Raman peaks occur at 489 cm⁻¹, 666 cm⁻¹, 701 cm⁻¹, 810 cm⁻¹, 1048 cm⁻¹, 1174 cm⁻¹, 1353 cm⁻¹, 1434 cm⁻¹, 1553 cm⁻¹ and 2331 cm⁻¹. Of these peaks, the peak at 489 cm⁻¹ corresponds to that of crystalline V₂O₅. It occurs because of the bending vibrations of the bridging V-O-V bonds [24]. The peak at 701 cm⁻¹ is a characteristic Raman peak of crystalline V₂O₅ [33] and corresponds to the doubly coordinated oxygen (V₂-O) stretching mode. The Raman peak at 666 cm⁻¹ is attributed to the V-O-V stretching mode of VO₂ [24]. The Raman peak at 810 cm⁻¹ is thought to be arising because of a very small percentage of V₂O₇^4- in the film which has V-O-V bond vibration at 810 cm⁻¹ [34]. The next peak at 1048 cm⁻¹ is observed because the vanadium species in our case are deposited on the Silica fiber [35, 36]. Therefore, the terminal V = O stretching mode of the terminal oxygen atoms when the vanadium species is bonded to the SiO₂ film, vibrates at 1048 cm⁻¹. The peaks at 1353 cm⁻¹ and 1605 cm⁻¹ are attributed to the presence of residual carbon during the deposition process. On the basis of the characteristic peaks occurring at 489 cm⁻¹ and 701 cm⁻¹, it can be concluded that the dominant compound forming the deposited and annealed film is crystalline V₂O₅. Furthermore, on the basis of the transmission spectra versus wavelength (See Fig. 2 and Fig. 5) curves, it can be seen that the transmission band edge occurs at around ~500 nm which corresponds to that of V₂O₅ [37].

 

Fig. 8 Raman Spectra for the VO_x thin film deposited on the tip of the silica fiber. The peaks at 489 cm⁻¹ and 701 cm⁻¹ correspond to a crystalline phase of V₂O₅.

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Therefore, on the basis of the information collected from the Raman spectra and inferred from the transmission band edge, it can be concluded that the material on the tip of the fiber contains crystalline V₂O₅ in conjunction with small amounts of VO₂. This is similar to the formation of other VO_x materials, for example, those used in uncooled bolometers.

4. Conclusions

Optical fibers containing vanadium oxide films, on their tips and inside the fiber matrix, were employed for sensing temperature. The sensing mechanism was based on monitoring the shift in the transmission spectrum, either a change in intensity at a given wavelength or a band-edge shift, of the optical fiber structures as a function of temperature. Amorphous vanadium oxide films were deposited on the tips of optical fibers using pulsed laser deposition and plasma arc annealing, of these vanadium oxide-coated tips, was carried out to develop crystallinity in the films. These fiber tips, having the polycrystalline vanadium oxide films on their surface, were over-coated with fused silica using pulsed electron beam deposition and then fused to another optical fiber to form a robust in-line fiber optic sensor. It was observed that there was a significant change in the optical transmission spectrum and a band edge shift when temperature of the optical fibers, having vanadium oxide films inside the fiber matrix or on the fiber tip, was increased from room temperature to temperatures above 100 °C and then decreased back to room temperature. Therefore, on the basis of the Raman spectra and the position of the band edge in the transmission spectra, it can be concluded that the VO_x deposited on the fiber optic tips is primarily crystalline V₂O₅, with trace amounts of some other VO_x phases including VO₂.