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We demonstrated temperature sensing of a fiber with nanostructured cladding, which was constructed by titanium dioxide (TiO2) nanoparticles self-assembled onto a side polished optical fiber (SPF). Significantly enhanced interaction between the propagating light and the TiO2 nanoparticles (TN) can be obtained via strong evanescent field of the SPF. The strong light–TN interaction results in temperature sensing with a maximum optical power variation of ~4dB in SPF experimentally for an external environment temperature varying from −7.8°C to 77.6°C. The novel temperature sensing device shows a linear correlation coefficient of better than 99.4%, and a sensitivity of ~0.044 dB/°C. The TN-based all-fiber-optic temperature sensing characteristics was successfully demonstrated, and it is compatible with fiber-optic interconnections and high potential in photonics applications.
© 2014 Optical Society of America
1. Introduction
Nanostructure on optical fiber can be realized by various techniques, such as nanofiber manufactured through flame heated method [1, 2], nanoscaled thin film or nanorod deposited on fiber-optic core, microfiber, and side polished fiber [3–5], nanopatterned metallic material arranged in optical fiber tip [5–7], etc. Light from the fiber-core coupled to nanostructure region enhances the optical interaction between the device and the ambient environment giving rise to a significantly enhanced sensitivity for photonic devices.
In terms of science and technology domains, TiO2 nanomaterials have found a vast range of applications since it is a wide band-gap and environmentally friendly semiconductor, possessing good biocompatibility and stability. Diverse applications in photocatalysis, photoelectric conversion and biosensor were demonstrated [8–11]. Furthermore, because TiO2 can be prepared in various nanostructures, TiO2 nanostructured thin films and arrays possess large surface area and exhibits unique chemical, optical, and electronic properties that are different from those of bulk materials. Various techniques have been devoted to develop and fabricate the TiO2 photoanodes, including sol–gel dip-coating, spray pyrolysis, atomic layer deposition, spin-coating, sputtering, chemical vapor deposition, electrodepostion, and anodic oxidation [8]. This bestows them to construct novel sensing and optoelectronic devices, Such as ultra-sensible fiber-optic based humidity sensor [9], Optical fiber refractometer [10], optical gas sensor [11], supercapacitors [12], integrated TiO2 resonators and modulator [13, 14], etc. Due to the significant thermal effect on the carrier concentration and efficiency of the luminescence, TiO2 nanoparticle is believed as the desirable temperature sensor, thus a practical material in the combined optical fiber devices. Furthermore, TiO2 nanoparticles (with relative larger refractive index than SiO2) deposited onto substrate induced field confined and enhanced in the interface between the substrate and TiO2, which introduces the enhanced sensing function or others potential optoelectronic application.
Side-polished fiber (SPF) can be fabricated by side-polished technique with a part of fiber cladding removed to bring about a polished region, where evanescent light can escape out from the core to the polished surface leading to a strong interaction between evanescent light and the external environment. Taking the advantages of SPF, several kinds of fiber-optic sensors [15–18] and optical devices have been fabricated, such as ultraviolet (UV) power sensor [19–21], fiber-optic integrated power monitor [22], light-commandable polarization controller [23], etc.
Within the different temperature sensors, optical sensors like fiber-optic sensors provide an alternative to traditional electrical sensors for the applications with high accuracy and stability, optimum performance in harsh condition. For instance, the fiber Bragg gratings (FBG) based sensor possesses a sensitivity up to 20pm/°C with a fiber probe tapered to a point [24]. By using long-period gratings the sensitivity can up to 0.6nm/°C [25]. High birefringence fiber-loop mirrors (HiBi-FLM) have higher temperature sensitivity (0.9435nm/°C) [26]. Since several years, PCF-based temperature sensors have been exploited showing high sensitivities (6.6nm/°C) [27]. All these fiber-based sensors can reach high sensitivities but a spectral analyzer is required for determining the wavelength shift and thereafter its sensitivity. Macro-bend single-mode fiber loop employed in a ratiometric power measurement (0.042dB/°C) and other techniques can realized the temperature sensing function via the transmitted power variations [28]. In this paper, a fiber with nanostructured cladding was proposed and constructed with TiO2 nanoparticles (TN) self-assembled onto a side-polished fiber (SPF) and demonstrated its temperature sensing characteristics. Since TiO2 is a high refractive index medium to silica fiber, strong evanescent light of SPF can be coupled into the interface between the nanostructured TN and polishing surface of fiber, which enhances the interaction between light and the surrounding medium and consequently results in a stronger sensitivity to change of external environment. The proposed device can follow the changes in the surrounding temperature through the optical transmitted power variations with a sensitivity of ~0.044 dB/°C, which induces to an effective temperature sensing function.
2. Device fabrication
The TiO2 nanoparticles with diameter from 20 to 60 nm were synthesized by the sol-gel method from tetrabutyl titanate, ethanol, hydrogenperoxide and ammonium hydroxide [29]. Firstly, 2mL butyl titanate was diluted with 6mL ethanol, and then was carefully transferred to the 250mL distilled water to produce Ti(OH)4 precipitation through hydrolysis process. Thereafter, 10mL H2O2 was added with vigorous stirring until the solution became transparent. Its pH value was adjusted to 7 using NH3H2O. After 24 hours aging, flocculation emerged and came to the precipitation, followed by the vacuum distillation for 2 hours to remove the organic residuals. Finally, by utilizing filtration to remove the NH4 +, the solution was poured into a sealed glass container immerged in a heating water bath for 5 hours, TiO2 nanoparticles suspension was successfully prepared for later experiments.
The used SPF was fabricated by wheel side-polishing technique [21, 30]. Its advantage is to make long length of polished region, where is parallel to the core other than to have a tilt angle with the fiber core. Instrument (XS-01-05-001) with resolution of 0.1μm can measure the polishing depth of the side polished area on SPF. Figure 1(a) shows the polishing depth along the fiber, it displays the length of polished upper part of cladding is ~24mm, the length of polished plane above the core is of ~15mm with polished depth of ~58μm. As the diameters of standard single-mode fiber (SMF) and fiber core are 125μm and ~8μm respectively, the residual thickness of cladding is of ~1.5μm, which is also a considerable compromise between light-TN interaction and the excess loss of SPF. Figure 1(b) presents the scanning electron microscope (SEM) image of polished surface of SPF. The roughness of ~0.8μm on the polished surface results in the enhancement of scattering loss.
Fig. 1 Morphological charateristic of SPF. (a) Transverse profile of the SPF; (b) SEM image of top polished plane of the SPF.
During the self-assembled process of TN onto the SPF, a basin on a glass slide was introduced here to encompass liquid TN solution (with ethanol). Firstly, the SPF was fixed by UV glue onto a glass slide, and the polished region surrounded by a 27x7x1 mm3 basin was constructed by a curing adhesive as shown in Fig. 2.
Fig. 2 Three dimensional schematic of basin used for self-assembled TN and configuration of a fixed side polished fiber on a glass slide.
The prepared TN solution was treated by ultrasonication for 20 minutes in order to distribute homogeneously the TN solution to avoid the agglomeration. Then, 20ml of prepared TN solution was instilled into the prepared basin and the ethanol in the solution was evaporated for about 4 hours in ambient temperature. The TN was self-assembled onto the polished surface by spontaneous evaporation from the TN solution.
Figure 3 is a representative SEM images which illustrate the morphology of the SPF with TN self-assembled onto the polished surface of SPF. An enlarged view with higher magnification for the region marked by a dotted line (in Fig. 3(a)) was shown in Fig. 3(b). According to the Fig. 3, the polished surface was covered with TN and the diameter ranges from ~20 - 60 nm. As the diameter of TN is not homogeneous, the self-assembled TN were close-packed forming a compact but disordered nanostructure with random defects, which constitutes of the nanostructured cladding with thickness of ~4.5μm, when the light transmits along to the SPF with nanostructured cladding, evanescent light would strongly be confined the interface between the surface of SPF and the TN cladding leading to enhanced interaction light-matter within it.
Fig. 3 (a) SEM image of polished surface of SPF with TN; (b) enlarged view for the region marked by a dotted line.
3. Experiments
The experimental setup of temperature sensing is mainly composed of a 1550nm DFB laser, a 1x3 optical fiber coupler, a chamber with variable temperature range from −10°C to 80°C and constant humidity, three optical power meters and a computer, as schematically presented in Fig. 4. The output of the DFB laser through the coupler is divided into three beams as light sources of three fiber-optic type samples: a standard single mode fiber (SMF) with coating layer removed; a bare SPF without TN; a SPF coated with TN. Optical power meters connected by a computer were utilized to measure the transmitted optical power from these optical fiber samples. The standard SMF is used not only to monitor the stability of output power of DFB laser source, but also to investigate the temperature response of SMF in the same chamber environment. The three samples were put inside the same chamber with stable humanity of 40% RH during the whole experimental process. The experiments were conducted in a temperature cycle: an increase of temperature from −7.8°C to 77°C, and then decrease from 77°C to −7.8 °C at a step of ~10°C. During the experiment, each temperature step maintains for more than 10 minutes. A thermocouple meter was simultaneously utilized to investigate and record the real temperature of the chamber.
Fig. 4 Schematic of experimental setup for temperature sensing of 3 fiber samples with or without TN. The bottom box shows the side view of the 3 fiber samples.
The experimental results are shown in Fig. 5. Figure 5(a) shows the adjusted temperature steps inside the chamber over time. The optical transmitted powers through three different samples are respectively shown in Fig. 5(b) for the standard SMF with coating layer removed, Fig. 5(c) for the bare SPF and Fig. 5(d) for the SPF with self-assembled TN. According to the Fig. 5(b), a slight optical power variation (~0.04dB) denotes that the optical transmitted power through the SMF remains almost unchanged, signifying that the DFB laser source is stable. The optical transmitted power through the bare SPF without TN is not regular and the maximum variation is of ~0.1dB as shown in Fig. 5 (c). While the maximum variation of optical transmitted power through the SPF with TN can be achieved up to ~4dB as shown in Fig. 5 (d), that exhibits a sensitivity almost 40 times larger than that of the bare SPF from −7.8°C to 77°C. This phenomenon can be mainly explained as following: when the device is exposed in an increasing temperature surrounding, the effective refractive index of nanostructured TN is modulated and increased similarly with the temperature increase. It leads to a corresponding increase of leakage light from the core of SPF. Then it enhances a stronger light coupling and interaction between the SPF and the self-assembled TN with large surface to volume ratio. Finally, the device attains a high sensitivity to temperature changes. Therefore, the optical transmitted power reduces correspondingly as shown in experiment results. Through the Fig. 5 (a) and Fig. 5(d), the changes of optical transmitted power of SPF with TN can follow the change of temperature in the chamber, and thus the SPF with TN can be utilized as an all fiber-optic temperature sensor.
Fig. 5 (a) Variation of temperature in the chamber measured by a thermocouple meter and variation of optical transmitted power through: (b) standard single-mode fiber with coating layer removed, (c) bare SPF; (d) SPF with coated TN film.
Through the analysis of Fig. 5(a) and 5(d) we can evaluate the relation between the optical transmitted power and the sensing temperature as displayed in Fig. 6. The red squares markers and the black circles markers represent the measured optical transmitted powers, where red square markers are represented the increase process in the temperature circle, and black circles are for the decrease process. The red solid line is the linear fitting curve with a linear correlation coefficient of 99.7% for the temperature increasing process. The black dotted line represents the linear fitting curve with a linear correlation coefficient of 99.4% for the temperature decreasing process. From experimental results and analysis, it can be seen that the TN-based optical fiber sensor possesses a good repeatability because red and black curves almost overlap each other. In addition, according to the two fitting curves, the sensitivity of the sensor can also be evaluated to be 0.044dB/°C. Through the Fig. 5(d), the average and standard error of optical transmitted power of the sensor at different temperature would be estimated. The experimentally measured standard error of transmitted power can be assessed with its maximum of 0.014dB, which signifies the optical fiber temperature sensor is largely stable.
Fig. 6 Variation of optical transmitted power in the SPF with coated TN film as a function of the temperature.
4. Conclusions
In summary, by combining the SPF and self-assembled TN, we have successfully fabricated a optical fiber temperature sensor with nanostructured cladding constructed by TN self-assembled onto the SPF. We demonstrated its application as an all fiber-optic temperature sensor with a linear correlation coefficient of better than 99.4% and a sensitivity of 0.044dB/°C, furthermore, the device possesses a good repeatability. Such a TN-based nanostructured cladding optical fiber is low cost, compatible with fiber-optic systems and possesses high potentiality in photonics applications. We believe further optimizations of the geometric configuration for the SPF and improvement of TN self-assemblage can pave the path towards the development of fiber-optic sensors or novel photonic devices.
Acknowledgments
This work was supported by National Nature Science Foundation of China (NSFC) (No. 61405075, 61177075, 11104218, 11004086, 61475066); National Major Special Project of China (22104001); Key Technology R&D Project Of Strategic Emerging Industries Of Guangdong Province, China (2012A032300016; 2012A080302004; 2011A081302002; 2012A08030100); Fundamental Research Funds for the Central Universities, China (No. 21613325; 21613405; 21612437); H. Yu and G. Jing acknowledges SRF for ROCS, SEM.
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