Violet light sensing of a micro fiber (MF) with cladding of titanium dioxide (TiO2) nanoparticles on the tapered region is demonstrated in this paper. The absorption characteristics of TiO2 can be tuned by violet light illumination. Significantly enhanced interaction between the propagating light and the TiO2 nanoparticles can be obtained via strong evanescent field coupling from the MF. Experimental results reveal that the transmitted optical power of the MF increases with an ~3 dB relative variation in broadband (1520-1620 nm) operation when the violet light is illuminated onto the TiO2 with power ranging from 0 mW to 11 mW. The device has a sensitivity of ~0.28 dB/mW (1520 nm), which indicates that the TiO2 holds great potential in photonic applications such as fiber-optic sensors or controlled devices.
© 2016 Optical Society of America
In recent years, the preparation and characterization of titanium dioxide (TiO2)  nanostructures have attracted much interest due to their unique chemical, optical, and electronic properties. TiO2 is a wide band-gap and environmentally friendly semiconductor, possessing good biocompatibility and stability. TiO2 has wide applications in catalysis , solar cells , photo-catalysis , optical devices , sensors , and gene therapy . It has been demonstrated that the physical and chemical properties of TiO2 nanostructures depend strongly on their crystalline structure, morphology and particle size . Thus, TiO2 nanostructures can be used to construct novel sensing and optoelectronic devices, such as optical fiber refractometers , optical gas sensors , supercapacitors , resonators  and modulators . Hence, TiO2 is a promising material for future use in fiber-optic devices with excellent characteristics.
Micro fiber (MF) is a special optical fiber with a diameter of several to over 10 micrometers . MF has received considerable attention due to several striking optical properties including low optical loss, outstanding mechanical flexibility, tight optical confinement and large fractional evanescent fields . And MF shows strong and rapid near-field interaction between the guided light and the surroundings because of the strong evanescent fields. As a combination of fiber optics and nanotechnology, MF has been emerging as a novel platform for exploring fiber-optic technology on the micro or nanoscale, such as Mach-Zehnder interferometers, optical ring resonators, all-optical tunable resonators, compact filters, all-optical switchers, and optical sensors [16–19].
In this work, MF with self-assembled TiO2 nanoparticles (TN) coated onto the tapered region is demonstrated for violet light sensing. The absorption characteristics of MF with TN can be tuned by violet light illumination, which shows potential for optical sensing and other controllable optical devices. And the enhanced interaction between strong evanescent light of the MF and nanostructured TN results in a stronger sensing functionality. The experimental results demonstrate that the change of the transmitted optical power of the device is ~3 dB in broadband ranging from 1520 nm to 1620 nm when the violet power increases from 0 mW to 11 mW. The sensitivity of the proposed device is ~0.28 dB/mW at λ = 1520 nm, which may signify a fiber-optic violet sensor can potentially be realized. This TN-based all-fiber device further expands the potential of fiber devices in sensing technology.
2. Device fabrication
TNs with diameter from 20 to 100 nm are synthesized by the sol-gel method from tetrabutyl titanate, ethanol, hydrogen peroxide and ammonium hydroxide . Firstly, 6 ml ethanol was used to dilute 2 ml butyl titanate, and then the mixed solution was carefully added to 250 ml distilled water to produce Ti(OH)4 precipitation through hydrolysis process. Thereafter, 10 ml H2O2 was added with vigorous stirring until the solution became transparent, and mixed with NH3H2O till pH 7. After 24 hours aging, flocculation emerged and became precipitation. And the organic residuals were removed after 2 hours vacuum distillation. After removing the NH4+ by utilizing filtration, the solution was poured into a sealed glass container immersed in a heating water bath for 5 hours. Finally, a TN suspension was successfully prepared .
As shown in Fig. 1, the Raman spectrum of TN excited by a 514 nm laser (Ar Ion Laser) is measured with LabRAM HR Evolution (HORIBA JY, France). The laser power on TN is ~8 mW, and the diameter of the normal incidence light spot size is ~1 mm. The Raman spectrum of TN is detected by a CCD with a 0.35 cm−1 resolution. The anatase and rutile phases of TN can be identified by Raman spectroscopy based on the Raman spectrum . The anatase phase shows major Raman bands at 399, 515, and 639 cm−1 . The typical Raman bands due to rutile phase appear at 235, 447, and 612 cm−1, respectively . As shown in Fig. 1, the observed peaks at 391, 512, and 634 cm−1 can be attributed to the characteristics of the anatase phase, which indicates that the anatase is the predominant phase structure.
X-ray diffraction (XRD) has the advantage of being a fairly standard laboratory technique which identifies the crystalline structure and phase composition in nanocrysalline materials . Figure 2 shows XRD diffraction patterns of TN prepared via the sol-gel method. The XRD diffraction patterns are measured by D8 ADVANCE (Bruker, Germany) with the Cu bar. The operation voltage and current are 40 kV and 40 mA. And the scan speed is 10 deg/min. As shown in Fig. 2, TiO2 in the anatase phase shows main diffraction peaks at 2θ of 25.28 °, 37.82 °, 48.1 °, 53.9 °, 55.06 °, 62.8 °, and 69.98 °. And the rutile phase shows main diffraction peaks at 2θ of 27.44 °, 36.12 °, 41.28 °, and 56.62 ° . Two polymorphic forms of TN (rutile R and anatase A) are present. As it can be seen, the fraction of anatase largely dominates over rutile, which is consistent with the Raman spectrum.
TiO2 is a wide band-gap and environmentally friendly semiconductor. The absorption spectrum of TiO2 measured by an UV-VIS spectrophotometer (UV-2600, SHIMADZU) is shown in Fig. 3. TiO2 has strong light absorption in the wavelength range from 200 to 400 nm, and relatively weak light absorption in the wavelength range from 600 to 800 nm, resulting in broadband operation TiO2-based devices.
Figure 4 shows the schematic diagram of the basin and configuration of a fixed MF on a glass slide. The MF is manufactured from a single mode optical fiber by using “flame-brushing” technique. A standard single-mode fiber (SMF, with a core diameter of 8 μm and a cladding diameter of 125 μm from Corning Inc.) is heated with a flame and is elongated at a drawing speed of 0.2 mm/s. The taper shape can be determined with great accuracy by precisely controlling the flame movement and the fiber elongation. A appropriate waist of the MF taper is the key for fabricating a good sensor. In order to obtain the appropriate value of the waist, a 1550 nm distributed feedback (DFB) laser as a light source has been used to monitor the transmitted optical power of the MF during the deposition process of TN.
The small variation of transmitted optical power (5.37 dB) shown in Fig. 5(a) indicates that the interaction between TN and the evanescent wave of MF (with a diameter of 25 μm) is insufficient. The variation of transmitted optical power is 68.66 dB in Fig. 5(c) when the diameter of MF is of 7 μm, but the transmitted optical power is less than −70 dBm, indicating the exceeding loss, which means the detected light of optical power meter is insufficient. Thus, the waist of MF taper should not be too large or too small. In our case, the waist is about 14 μm, the variation of transmitted optical power (28 dB) as shown in Fig. 5(b) is just appropriate to reach a compromise between the light interaction (TN and evanescent light of MF) and the loss. Then the tapered segment is immobilized onto a glass slide by a UV curing adhesive with the purpose of improving the mechanical strength of the device. To contain the TiO2 solution, a basin (20 mm × 5 mm × 1 mm) surrounding the tapered segment is constituted by using the UV adhesive (Loctite 352, Henkel Loctite Asia Pacific) as shown in Fig. 4. And the UV adhesive basin is cured by a UV light source (365 nm, USHIO SP7-250DB) for 10 min illumination.
The prepared TN solution is treated by ultrasonication for 20 minutes for the purpose of distributing homogeneously the TN solution to avoid TN agglomeration. Then, 20 ml of prepared TN solution is introduced into the basin, and the ethanol in the solution is evaporated for about 6 hours in ambient surrounding. Thus, the TN is self-assembled onto the waist of the MF taper by spontaneous evaporation from the TN solution.
An SEM image of the MF with TN self-assembled onto the waist is shown in Fig. 6(a). Figure 6(b) shows an enlarged view with higher magnification for the region marked by a dotted line in Fig. 6(a). As can be seen, the diameter of the TN covered on the waist of MF ranges from ~20 to 100 nm. The self-assembled TN are close-packed forming a compact but disordered nanostructure with random defects because of the inhomogeneous diameter of TN. Since TiO2 is a high refractive index medium compared to silica fiber, strong evanescent light from the MF can be coupled into the interface between the nanostructured TN and the waist of the fiber, which enhances the interaction between the light and the surrounding medium and consequently results in a stronger sensitivity to change of the external environment.
The experimental setup is mainly composed of a 1520-1620 nm tunable DFB Laser, a violet light laser (λ = 405 nm) for illuminating onto the TN, the prepared sample (MF with TN) and an optical spectrum analyzer (OSA), as schematically presented in Fig. 7. The light from the tunable DFB laser is coupled into an SMF and then passed through the MF with the TN. The transmission spectrum of the sample is recorded with the OSA (AQ6317C from YOKOGAWA Inc.). The bandwidth of the violet power (LSR405NL from Lasever Inc) is 10 nm, and the diameter of violet light spot size on the tapered region is 5 mm. The unpolarized violet power is changed from 0 mW to 11 mW in the experiments.
The experimental results of the MF without TN are shown in Fig. 8. When the adjustable violet laser power changes from 0 mW to 11mW, the transmitted optical power of the MF remains almost unchanged. This implies the violet light illumination cannot influence the transmitted optical power of the MF without TN.
As shown in Fig. 9, when the adjustable violet laser changes from 0 mW to 11 mW, the transmitted optical powers of the MF with TN responds with a relative variation power of ~3 dB (λ = 1520 nm). In addition, we perform the same measurement in the wavelength ranging from 1520 nm to 1620 nm.
As shown in Fig. 8 and 9, the introduction of the TN onto the tapered MF reduces the transmission of the light by approximately 27 dB. In our opinion, both the scattering and absorption of TN are the causes of transmission loss. When the TN are self-assembled onto the waist of the MF taper, more light will be transported to the TN from MF because TiO2 has high refractive index. The light will then be scattered by TN, which results in the transmission reduction. In addition, the absorption of the TN can also change the transmission of light. The transmission loss will be larger with the increased absorption of the TN. This feature may open up the potential applications in optical controllable devices. As shown in our paper, the violet light with the order of mW can manipulate the absorption of TN and the transmitted optical power of the device. Shortening the length of MF, optimizing the diameter of MF, and improving the quality of TN can reduce the transmission loss.
The results displayed in Fig. 10 indicate that the violet light illumination can change the absorption of TN, which shows that the sensing functionality of violet light in a relative broad band (1520-1620 nm) can be potentially achieved. If the minimum power resolution of the OSA is 0.001 dBm and the device has a sensitivity of ~0.28 dB/mW (λ = 1520 nm), the minimum detectable change in violet power is 0.00357 mW. And the error bars show minor variances of different wavelengths with respect to transmission light. The uncertainty of this measurement produced by the instability of violet light and tunable DFB laser, the subtle change of surrounding humidity and temperature.
The MF with cladding of TN for violet power sensing may be explained as below: When violet power increases, electrons and holes produced by the violet light will occupy energy levels in the conduction and valence bands, which causes saturable absorption [26, 27] in TN. The concentration of excited electrons-holes increases with the increased violet power [25, 28], resulting in a real part reduction of dynamic conductivity. The real part of dynamic conductivity determines the light absorption of TN. Thus, the light absorption is decreased because of the dynamic conductivity reduction of TiO2 following the increase of illuminating violet power . The transmitted loss of the MF with TN is decreased while the violet power is increased. Consequently, the sensing functionality of violet light power can be achieved by the MF with TN.
In summary, a fiber-optic device has been successfully fabricated by combining the MF and self-assembled TN. The absorption characteristics of the device can be tuned via violet light pump over a broad band (1520 nm to 1620 nm). The experimental results demonstrate that the change of the transmitted optical power for the MF with TN is ~3 dB with the violet power increases from 0 mW to 11 mW. Thus, the MF with TN can be used for sensing functionality of violet light with a sensitivity of 0.28 dB/mW at λ = 1520 nm. Such a TN-based all-fiber device is suitable for applications in fiber-optic systems and holds great potential in photonics applications. Further optimizations of the geometric configuration for the MF and quality improvement of the TN deposition can pave the path towards the development of fiber-optic sensors or novel photonic devices.
National Natural Science Foundation of China (61505069, 61177075, 61275046, 61361166006, 61475066, 61405075, 61401176, 61575084); Natural Science Foundation of Guangdong Province (2014A030313377, 2014A030310205, 2015A030306046, 2015A030313320, 2016A030311019, 2016A030313079, 2016A030310098); Science and Technology Projects of Guangdong Province (2012A032300016, 2014B010120002, 2014B010117002, 2015A020213006, 2015B010125007, 2016B010111003, 2016A010101017); Project of Guangdong High Education (2013CXZDA005, 2014KQNCX025, YQ2015018); Science and Technology Projects of Guangzhou (201607010134, 201506010046, 201605030002).
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