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Abstract
Hybrid optical fibers have been widely investigated in different architectures to build integrated fiber photonic devices and achieve various applications. Here we proposed and fabricated hybrid microfiber waveguides with self-growing polymer nanofilms on the surfaces of microfibers triggered by evanescent field of light for the first time. We have demonstrated the polymer nanofilm of ∼50 nm can be grown on the microfiber with length up to 15 mm. In addition, the roughness of nanofilm can be optimized by controlling the triggering laser power and exposure duration, and the total transmission loss of the fabricated hybrid microfiber is less than 2 dB within a wide wavelength range. The hybrid polymer nanofilm microfiber waveguides have been characterized and their relative humidity (RH) responses have also been tested, indicating a potential for RH sensing. Our fabrication method may also be extended to construct the hybrid microfibers with different functional photopolymer materials.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Integrated hybrid fiber devices, which can provide waveguide structure to enhance light-matter interaction and take advantages of functional materials, have played an irreplaceable role in numerous fields, such as gas lasers, harmonic generations, optofluidics, surface plasmon resonance sensors etc. [1–13]. As a useful specialty optical fiber, microfibers have been widely utilized to combine with various materials such as semiconductor, metal oxide and two-dimensional materials etc. for nonlinear optics, low-threshold lasing and sensing applications [14–20], owing to its low optical waveguide loss, strong evanescent field, tight mode field confinement, and more importantly, the capability to seamlessly connect with the normal fibers and systems. In the sensing field, compared to fiber Bragg gratings (FBGs) and fiber Fabry-Perot interferometers which require spectra measurement [21,22], sensors based on microfibers may be implemented by only power measurement, indicating the possibility of simple and cost-effective sensing [23,24]. Since the sensitivity of bare fused silica microfiber is not high to many chemical or biological samples, suitable sensitive materials applied to the bared microfibers are essential to overcome the limitation of fused silica and achieve the desirable performance [20,25,26].
Polymer is a kind of flexible material with diverse structures that can be artificially altered to achieve different functions which is attractive for sensing applications due to its compatibility and sensibility with various molecules [27,28], i.e., some molecules can be easily bounded to the surface or filled into the interstitial gaps of the polymers, resulting in the change of average refractive index [28,29]. Moreover, polymers can also combine with a variety of functional dopants, such as metal oxides, fluorescent dyes and enzymes, to improve their optoelectronic properties or offer abundant choices for sensing schemes [27,30]. In particular, polymer nanowires have been widely used in sensing applications because of their unique geometry with large surface-to-volume ratio [28,31], however, it may be difficult to be efficiently integrated with peripheral optoelectronic device. A useful way is the use of microfiber to connect the polymer nanowires with the conventional optical fibers, thus reducing the coupling loss and increasing the integration efficiency and design flexibility of photonic devices, as shown in Fig. 1(a) [28]. To further improve robustness of integrating polymer materials with microfibers and make full use of the advantages of polymer nanostructures, new strategies are highly desirable. Recently, the photopolymer material with the photopolymerization process triggered by light has been utilized to construct the fiber-tip waveguide, and the self-growing polymer microtips on the end facet of fibers have facilitated many applications such as optical coupling [32], generating Bessel-like beams [33] and relative humidity (RH) sensing [21]. The fiber-based photopolymerization provides a useful way to integrate polymer materials with the fiber waveguides. Although numerous progresses on fiber tip-based polymer structure have been demonstrated, integration of self-growing polymer nanofilm triggered by light on the surface of microfibers has not yet been reported.
Fig. 1. Schematic diagrams of (a) a polymer nanowire with two ends coupled with microfibers [28]; (b) a polymer nanofilm-coated microfiber hybrid waveguide.
In this work, we have proposed an approach to integrating self-growing polymer nanofilms with microfibers, to form hybrid microfiber waveguides triggered by evanescent field of light for the first time. Guided by the evanescent field of microfiber, the light will trigger the polymerization of the liquid photopolymer surrounding the microfiber, and the polymer nanofilm of ∼50 nm can grow on the surface of the microfiber with long length up to 15 mm. In addition, the roughness of nanofilm can be optimized by controlling the laser power and exposure duration. The fabricated hybrid polymer microfiber waveguides have been characterized and their RH responses have been tested. Owing to high sensitivity of polymer to RH and strong evanescent light of the microfiber, the hybrid microfiber waveguide was employed for RH sensing successfully. In a wide RH range between 45% and 96%, the RH sensitivity of the hybrid microfiber was 0.06 dB/%RH which is comparable to the previous polymer nanowire [28], however, our hybrid polymer nanofilm microfiber can be robustly integrated with the traditional fibers. Furthermore, the hybrid microfiber waveguide demonstrated the fast response time of ∼0.45 s in monitoring human breathing. Our hybrid polymer nanofilm-coated microfiber may provide a promising candidate for RH sensing applications. And the unique way to fabricate the nanofilm on the surface of microfibers using the evanescent field triggered self-growing process may also be extended to construct the hybrid microfibers with different functional materials, which can expand the applications of microfiber waveguide in various fields.
2. Fabrication of hybrid polymer nanofilm coated microfibers
The microfiber was fabricated by a flame-brush tapering technique via a home-made taper rig [34,35]. First, the fiber coating was stripped and the bared fiber was cleaned using acetone. Then, utilizing oxy-hydrogen flame to heat the fiber as well as stretching from two ends, the standard single mode fiber (SMF-28, Corning Inc.) was tapered down to the diameter of around 4.0 µm, which is quite stable in this scale and can be well operated [36]. As displayed in Fig. 1(b), as-fabricated microfiber with the gradual transition symmetric tapered structure is easily coupled with the input and output through the SMF pigtails at the both ends. In addition, the optical transmission was measured in real-time during the pulling process. The overall optical insertion loss of as-prepared microfiber was less than 0.1 dB.
The mixed liquid photopolymer, for fabricating self-growing polymer nanofilm, consists of three basic components: pentaerythritol triacrylate (PETA), methyldiethanolamine (MDEA) and eosin Y (Sigma-Aldrich Inc.) [33,37]. PETA, one of multifunctional acrylate monomers, is used for forming the backbone of the polymer network, accounting for 91.5% of the weight; MDEA accounts for 8% and is used as an amine cosynergist; The remaining component is made of eosin Y, which is used as the sensitizer dye [21,38]. As the eosin Y is irradiated with green light, the triplet state of eosin reacts with the amine to initiate the polymerization of the acrylate monomer so that the photopolymer liquid becomes solid [38]. The refractive index of this mixed liquid polymer is 1.48 and the cured polymer is 1.52 [38]. Moreover, the physical and chemical properties of the mixed liquid photopolymer, for example, viscosity, sensitivity and polymerization threshold energy, can be modified by adjusting the proportion of the components [38].
The fabrication of hybrid polymer nanofilm microfiber was then introduced. First, the microfiber was immersed in the mixed liquid photopolymer, and then the green laser at the wavelength of 532 nm, which can trigger the polymerization of the mixed photopolymer, was coupled from the end of the conventional fiber which is the pigtail of the microfiber, as illustrated in Fig. 1(b). Through the evanescent field of the microfiber, the mixed liquid photopolymer exposed in the evanescent field of green laser was polymerized, inducing the liquid polymer surrounding the microfiber to self-grow the polymer nanofilm on the surface of the microfiber, and the unreacted liquid polymer was washed off by the methanol carefully. During the fabrication process, the uniformity and flatness of the polymer nanofilm on the surface of microfiber can be optimized by controlling the laser power and exposure duration. Compared to the polymer nanowire requiring to be precisely supported by the low refractive index platform and coupled to microfibers, as shown in Fig. 1(a), the prepared hybrid microfiber waveguide integrated with self-growing polymer nanofilm can be naturally connected to the normal fiber and fiber systems, which greatly improves the robustness and integration of the system.
3. Characterization of the hybrid microfibers
The self-growing polymer nanofilms on the surface of the microfibers were characterized by a scanning electron microscope (SEM). Figure 2(a) and (b) is the cross section of microfiber integrated with self-growing polymer nanofilm when we broke the hybrid microfiber carefully, we can find that the microfiber was tightly wrapped by the polymer nanofilm with the thickness of ∼50 nm. We noticed that the optical power of the triggering laser can effectively affect the roughness of the self-growing polymer nanofilms on the surface of the microfibers. Figures 2(c) and (d) are the SEM images of hybrid microfiber surfaces when the laser powers are 1.0 µW and 3.2 µW, respectively, with the exposure duration of 60 s. When the laser power is relatively low (1.0 µW), the polymer nanofilm will be uneven with random micro and nano cracks on the surface, as shown in Fig. 2(c), which may be raised by the incomplete polymerization process. Nevertheless, when the laser power increases to 3.2 µW, the polymer nanofilm is smooth and uniformly integrated with the microfiber surface, as shown in Fig. 2(d), indicating a full polymerization process. And a higher triggering laser power can also produce a smooth photopolymer film coating. The close-up image of the hybrid microfiber surface is shown in the insert of Fig. 2(d).
Fig. 2. SEM images of microfibers integrated with self-growing polymer nanofilms: (a, b) cross section; (c, d) surface sections. Two hybrid microfibers with different fabrication conditions: (c) polymer exposed at 1.0 µW, (d) polymer exposed at 3.2 µW. The insert is a close-up image.
Figure 3(a) shows the hybrid polymer nanofiber coated microfiber guided with the green laser. It can be seen that the green laser can be well guided in the polymer nanofilm on the surface of the microfiber, in the waist region of the microfiber (about 15 mm long length). Besides, under the optical microscope, we can observe that the nanofilm coating is uniform in a relative long length up to more than 10 mm when the hybrid microfiber waveguide transmits green laser, as shown in Fig. 3(b).
Fig. 3. (a) Photograph and (b) optical micrograph of hybrid polymer nanofilm coated microfiber waveguide exposed to green laser emission at 532 nm. The white arrows indicate the direction of light propagation.
In order to characterize the transmission properties of the hybrid microfibers with different fabrication parameters, we utilized a supercontinuum source and an optical spectrum analyzer to measure their transmission spectra, as shown in Fig. 4. Under the same polymerization exposure duration of 60 seconds, the overall propagation loss of the hybrid microfiber with the triggering laser power of 1.0 µW (purple curve) is larger than that with the laser power of 3.2 µW (blue curve), For example, the loss introduced by the fabrication process of polymer nanofilm in the blue curve is 1.26 dB and the corresponding loss in the purple curve is 5.6 dB at the wavelength of 1550 nm. The larger transmission loss of the purple curve may be due to the scattering caused by the unevenness of the polymer nanofilm as shown in Fig. 2(c). Under the same laser power of 3.2 µW, we noticed that if the exposure duration is short, e.g., 30 seconds, the polymer nanofilm may not be well grown on the surface of microfiber or easy to be washed off during fabrication process, and we did not observe the nanofilms on the microfiber samples in this case. As shown in the measured red curve in Fig. 4, the transmission spectrum of the hybrid microfiber sample is consistent with that of the bare microfiber (green curve), confirming that there may be no polymer nanofilm on the surface of the microfiber. The propagation loss of the fabricated hybrid microfiber (laser power of 3.2 µW, exposure duration of 60 s) is less than 2 dB in the wavelength range of 1200 - 1700nm except the absorption peak at around 1380 nm wavelength, which is caused by the O-H bonds induced by the fiber tapering process [39].
Fig. 4. The transmission spectra of as-fabricated hybrid microfiber waveguide integrated with polymer nanofilm under different fabrication conditions.
As a comparison, the microfiber with the diameter of around 4 µm was applied with the liquid photopolymer using a normal dip coating method, and then it was directly exposed to the green laser from the outside to cure the polymer film on the surface of the microfiber. The measured transmission spectrum is illustrated in the black curve of Fig. 4, and a significantly increased transmission loss of around 15 dB was noticed. There are two main reasons for the high attenuation of the hybrid fiber with a dip coating technique, one is that the light will couple from the microfiber to the polymer film with the higher refractive index when the coating thickness is not well controlled in the nanoscale. The other is the scattering loss of the polymer material when the coating is not uniform. Therefore, optical microfibers integrated with the self-growing polymer nanofilms can provide an effective way to construct the low-loss hybrid microfiber waveguide.
4. Characterization of RH sensing response
The schematic diagram of RH sensing experiment is shown in Fig. 5. A temperature/humidity chamber (DHTHM-27, Doaho Ltd.) was used to implement the RH sensing experiment. An EDFA light source (ALS-15-B-FA, Amonics Ltd.) covering the wavelength range from 1530 to 1560 nm was employed. A power meter (PM 100D, Thorlabs Inc.) with a power head (S145C) was used for measuring the average optical transmitted power. During the whole experiment, the hybrid microfiber was placed in the chamber, and the temperature in the chamber was set at 25 °C.
Using the humidity/temperature chamber in Fig. 5, RH sensing was repeatedly investigated by measuring the optical transmitted power changes of microfiber samples at different RH levels, and the corresponding results is shown in Fig. 6(a). In a wide RH range from 45% to 96%, the optical transmitted power linearly increased about 3 dB, corresponding to a sensitivity of ∼0.06 dB/%RH. When the RH dropped from 96% to 45%, the variations of optical transmitted power showed a good consistency and reversibility. The experimental data are linearly fitted in ascending and descending processes, as shown with blue and red lines in Fig. 6(a), respectively. The sensitivity of bare microfiber was also investigated under the same experimental environment. As shown in Fig. 6(a), the bare microfiber just has an extremely low RH sensitivity of 0.0001 dB/%RH (dotted lines) in both ascending and descending processes. The experimental results indicate that the sensitivity of the microfiber is effectively increased by the polymer nanofilm on the surface of the microfiber.
Fig. 6. (a) Optical transmitted power measured as a function of RH; and (b) optical transmitted power as a function of temperature at 55% RH.
It is known that water molecules can intrinsically be bound to the surface or diffuse into the matrix of the polymers [28] while RH increases, resulting in the refractive index variation of polymer. In addition, the typical hydrophilic groups containing hydroxyl and carboxyl in the mixed photopolymer is easy to interact with water molecules [40–42]. The refractive index of dry polymer is about 1.52 [38], while that of water is 1.33. Therefore, when the polymer nanofilm absorbs more water molecules, the average refractive index will decrease [21,28,29,43], which will reduce refractive index contrast between the polymer nanofilm and the microfiber, and subsequently strengthen the confinement of the guided light, resulting in the boost of the optical transmitted power [28].
In order to compare the difference between the self-growing polymer nanofilm and the dip-coating polymer film in terms of the response of RH sensing, although the microfiber with the dip-coating polymer film has a much larger transmission loss, we conducted the comparative experiment using the microfiber with the dip-coating polymer film. Under the same experimental environment, the sensitivity of microfiber with the dip-coating polymer film was tested with only ∼0.028 dB/%RH, which is lower than that of the hybrid microfiber with the self-growing polymer nanofilm, as displayed in Fig. 6(a). The thick polymer coating may reduce the variation of the mode confinement with the change of the RH. The experimental results verified that RH sensitivity of the hybrid microfiber with polymer coating could be enhanced effectively by the evanescent field triggering self-growing polymer nanofilm.
In order to investigate the influence of temperature, further experiments were implemented by placing the sensor in the humidity/temperature chamber (Fig. 5). The temperature inside the chamber was raised from 20 °C to 50 °C at a step of 5 °C, and then decreased to 20 °C, while the RH in the chamber was set at 55%. As shown in Fig. 6(b), the sensor is much less sensitive to temperature variation with low sensitivities of 0.0136 dB/°C for heating process and 0.0173 dB/°C for cooling process, respectively. The results indicate that small temperature variation may have little effect on this hybrid microfiber waveguide.
As an important application of humidity sensors, breathing monitoring is widely used to estimate the sensing characteristics especially in response and recovery time [44–46]. Therefore, as shown in Fig. 7(a), the sensing performance of the hybrid nanofilm-coated microfiber was investigated by the same measurement method reported in [46,47]. The typical performance of the sensor was measured via monitoring the variation of optical transmitted power within 13 cycles of human breathing at ambient temperature, as shown in Fig. 7(b). When the breathing was on, the optical power increased imdediately, which is the response process. Vice versa, the optical power returned to the original level quickly when the breathing was off, defined as the recovery process. There was a slight difference in optical power change and response time between each breathing cycle, which may be due to the breathing fluctuations. The variation of optical power during 11th and 12th breathing cycles is enlarged in Fig. 7(c). The response and recovery time were estimated about 0.6 s and 0.45 s, respectively. The experimental results indicate the potential of the sensor to act as a promising candidate for monitoring actual human breathing.
Fig. 7. (a) Schematic diagram for breathing monitoring; (b) variation of optical power in human breathing cycles; and (c) zoom in on areas of 11st and 12nd cycles.
Table 1 summarizes the response time of typical optical fiber waveguides integrated with different functional materials. Compared with other optical fiber waveguides, our hybrid photopolymer nanofilm-coated microfiber waveguide has the characteristics of fast response speed, short recovery time and cost-effective. The results indicate that hybrid photopolymer nanofilm-coated microfiber waveguide is a promising and compact platform for RH sensing applications.
Table 1. Comparison of response time of various fiber waveguides with different functional materials
5. Conclusion
In conclusion, we proposed and fabricated a hybrid microfiber waveguide integrated with an evanescent field-triggered self-growing photopolymer nanofilm, and characterized its properties and tested its performance on RH sensing. Guided by the evanescent field, the light can trigger the polymerization of the liquid photopolymer to self-grow into a nanofilm on the surface of the microfiber by controlling the laser power and exposure duration. Due to high sensitivity of polymer nanofilm to RH and strong evanescent light of the microfiber, the sensitivity of 0.06 dB/%RH was achieved in a wide RH range from 45% to 96%. In addition, the hybrid microfiber waveguide possessed quick response time of ∼0.45 s in monitoring human breathing. The results validated that our hybrid polymer nanofilm-coated microfiber waveguide, with the advantages of flexible and robust all-fiber constructure, may act as a promising candidate for RH sensing applications. Also, our fabrication method can be extended to construct different hybrid photopolymer microfiber waveguides, the dispersion and nonlinearity properties of the hybrid microfiber may be tailored by optimizing the thickness of polymer nanofilms [18,19], and it may be further enriched by modification of the surface polymers and expand the applications of microfiber waveguide in various fields.
Funding
Israel Science Foundation (12161141018); National Natural Science Foundation of China; Shanghai 2021 Science and Technology International Cooperation Project “Program of Action for Science and Technology Innovation” (21530710400); Yiwu Research Institute of Fudan University (20-1-15).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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