Open Access
Add to BibSonomy
Abstract
The microfiber-based optical structures have been attracting increasing research interests in communications and sensing fields. However, the fabrication of forming structures on fragile microfibers requires delicate operations, which limits the developments of their practical applications. In this work, a one-step-tapering technique is proposed to manufacture structures on microfibers. As a demonstration, the fiber preform, consisting of sawtooth shaped solid-air interfaces with designed dimensions, is obtained using a femtosecond laser milling technique. By one-step tapering the preform, periodic bumps are formed, resulting in a bamboo-like microfiber device. The fabricated structure shows spectral characteristics of a long-period grating, of which extinction ratio is up to 18.2 dB around 1553.3 nm. The response to refractive index is measured to be ∼875.02 nm/RIU and the temperature coefficient is ∼5.78 pm/°C. The theoretical analysis shows good agreement with the experimental results. The microfiber-based structure fabricated using the one-step-tapering-preform technique is featured with flexibility of design, reproducibility, and structural stability.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Devices fabricated with optical microfibers have been attracting much research attention for numerous potential applications including optical coupling, filtering and sensing [1–6]. With diameters scaling down, microfiber devices possess characteristics of compactness, low bending loss, manageable wavelength dispersion, and high evanescent field for refractive index sensing applications [7,8]. However, as the fiber scale decreases, e.g., to several or tens of micrometers, forming structures on the microfiber becomes challengeable. Some techniques have been developed to form structures on a microfiber. After tapering a standard fiber to a microfiber, the surface structure can be periodically modulated via CO$_2$ laser irradiation [9], femtosecond laser inscription [10,11], or arc discharge techniques [12]. By functionalizing the microfiber with polymethyl methacrylate (PMMA) jackets, the inscription of structures can be achieved via a frequency-doubled argon laser [13]. The microfiber surface can also be modified by adding extra materials or structures, for example, via point-by-point dip coating with polydimethylsiloxane (PDMS) ellipsoid rings [14] or coiling another microfiber on the pre-tapered microfiber surface [15]. However, on one hand, the process of formatting structures on the microfiber surface requires delicate operations; on the other hand, the fabricated structures may suffer from geometrical instability or low robustness, for example, the extra materials on the microfiber may drop off or deviate from the original positions.
In this work, we propose a one-step-tapering-preform technique to fabricate structures on a microfiber. As a demonstration, using the proposed technique, a microfiber-based long-period grating (mLPG) with bamboo-like structures is manufactured. Using femtosecond (fs) laser inscription, periodic notches are fabricated on the surface of a standard optical fiber, and the formed compact structure is termed as the preform. By drawing the preform assisted by flame brushing technique, silica bumps are formed periodically while the preform scales down to a few or tens of micrometers. The finalized bamboo-like fiber shows spectral characteristics of long-period gratings. Compared to the conventional methods of modulating structures on a microfiber, the one-step-tapering-preform technique relies less on delicate operation, because the inscription process only happens on a standard fiber of which the size is much larger than that of a microfiber. Furthermore, the fabricated microfiber devices possess more stability due to the naturally integrated bumps on the microfiber.
2. Configuration and fabrication
The one-step-tapering-preform technique to fabricate microfiber devices is illustrated in Fig. 1. First, the polymer jacket of commercial single mode fiber (SMF-28, Corning) is removed. The fiber preform is then fabricated via periodic grooving using a femtosecond laser (Fig. 1(a)). To form designed structures, the fiber is placed in a rotational holder fixed on a translational stage with a step precision of 10 nm. A pulsed laser with a wavelength of 800 nm is focused on the uncoated fiber via an objective lens. The duration and repetition rate of the laser pulse are 40 fs and 1 kHz, respectively. When the irradiation power of the laser is set as 5 mW, the scratch width on the fiber is $\sim$2 $\mathrm{\mu}$m on the upper surface of the fiber. During the inscription process, the laser spot scans transversely across the fiber to fabricate a rectangle notch with the designed depth and width. The fiber is then moved longitudinally, so that another notch with the same dimension is fabricated by repeating the transverse scan of the laser spot. Finally, the fiber preform with a set of notches is manufactured.
Fig. 1. Fabrication of bamboo-like fiber structure via the one-step-tapering-preform technique. (a) Fabrication process: i) preparing an uncoated optical fiber; ii) milling periodic notches on the preform by femtosecond laser inscription; iii) heating and pulling the preform to form periodic bumps on the bamboo-like fiber structure. (b) Microscopic images of the side view (left) and top view (right) of the fs-laser-milled fiber preform. (c) Typical SEM images of a fabricated bamboo-like structure.
Subsequently, via a heating-and-tapering process, the diameter of the preform scales down to several tens of micrometers. The two ends of the preform is separately fixed by translational stages. A hydrogen torch is applied to heat and soften the preform. The flame is scanned in a range of 10 cm along the fiber, while the two translational stages symmetrically moved apart at a relative velocity of 0.4 mm/s. When pulling a standard fiber, the final taper diameter is dependent on the initial fiber diameter, pulling time and heater conditions [16–18], and a uniform tapered region can be obtained by controlling the fabrication parameters [19]. In contrast, the initial diameter of the fiber preform is periodically modified by the notches, which introduces nonuniform pulling parameters along the fiber (e.g., stress and temperature). As a result, the diameter of the microfiber shows a periodical variation, leading to a bamboo-like structure (Fig. 1(a)). Due to the heating and pulling technique, the microfiber structure surface is relatively smooth, especially compared to the fs-laser milled mLPGs [10,11], which would be beneficial for decreasing the light transmission loss [18].
As a demonstration, a comb-like fiber preform is obtained by fabricating 15 notches with a depth, width and period of only $\sim$10 $\mathrm{\mu}$m, $\sim$5 $\mathrm{\mu}$m and $\sim$10 $\mathrm{\mu}$m respectively as shown in Fig. 1(b). During the process of tapering the fiber preform, the diameter decreases while the pitch of the notches increases. The size of the thinner region of the preform decreases faster than that of the thicker region does. The SEM images show that the step of the pitch is stretched to be $\sim$497.5 $\mathrm{\mu}$m, which is almost 50 times longer than that of the fiber preform (Fig. 1(c)). Although the notches in the preform only locate on one side of the fiber, the finalized microfiber structure is approaching angularly symmetric due to surface tension during the pulling process. The diameter of the waist between two bumps is about $d=15.2$ $\mathrm{\mu}$m. The raised notch on the preform stretches to a seemly spindle-shaped bump with a length of $L_{\textrm {bump}}\sim$140.4 $\mathrm{\mu}$m and with a largest diameter of $d_{\textrm {bump}}=21.2$ $\mathrm{\mu}$m.
When stretching the fiber preform, both of the length of the pitch and effective refractive indices of the modes change accordingly. The modal coupling could occur when the resonance condition, $\Delta n\cdot \Lambda =\lambda$, is satisfied, where $\Delta n=n_1-n_2$, $n_1$ and $n_2$ represents the effective indices of the fundamental and the high-order modes respectively, and $\lambda$ is the resonance wavelength. The local coupling strength is determined by [12]:
To evaluate the evolution of the transmission spectrum during the one-step-tapering-preform process, the pigtails at two fiber ends are connected to a broadband light source (Golight, 1250–1650 nm) and an optical spectrum analyzer (Yokogawa, AQ6370D) respectively. Figure 2 shows the transmission spectra when the waist diameters are 26.5 $\mathrm{\mu}$m, 26.3 $\mathrm{\mu}$m, 25.8 $\mathrm{\mu}$m, and 25.6 $\mathrm{\mu}$m, respectively. The resonance wavelength blue-shifts from $\sim$1600.33 nm to $\sim$1382.20 nm as the diameter decreases from 26.5 $\mathrm{\mu}$m to 25.6 $\mathrm{\mu}$m. Theoretically, as the fiber preform is stretched, both the length of the pitch $\Lambda$ and the intermodal index $\Delta n$ changes accordingly. By taking a small variation of the fiber size $d$ from the resonance condition, we obtain
Fig. 2. Evolution of the transmission spectrum during the fiber tapering process. The red dashed lines indicate resonance wavelengths corresponding to the microfiber waist with diameters of $\sim$26.5 $\mathrm{\mu}$m, 26.3 $\mathrm{\mu}$m, 25.8 $\mathrm{\mu}$m, and 25.6 $\mathrm{\mu}$m, respectively.
The transmission spectrum of a finalized fiber device is shown in Fig. 3(a), with $d=15.2$ $\mathrm{\mu}$m and $\Lambda =497.5$ $\mathrm{\mu}$m. The diameter of the bump is 21.2 $\mathrm{\mu}$m. The resonance dip occurs at 1553.3 nm, with a large depth of $\sim$18.2 dB. The full-width-half-maximum (FWHM) of the resonance is $\sim$16.6 nm. A simplified model is established to analyze the mode coupling of microfibers with different diameters. In the theoretical model, the fiber diameter $d$ is uniform along the longitudinal direction, and the grating pitch $\Lambda$ increases with the decrease of the fiber diameter, as illustrated in the inset of Fig. 3(b). If the volume of the structure cell is a constant, $\Lambda$ is inversely proportional to the square of $d$. The original standard fiber is assumed to have an index contrast between the core and the cladding of 0.004 and a core-cladding diameter ratio of 8.5/125. The material dispersion of the fused silica fiber [16] is taken into account. Utilizing a full-vector finite-element method [18], the resonance wavelengths, induced by coupling between the fundamental and higher-order modes, are calculated with respect to different diameters, as indicated in Fig. 3(b). The experimental measured resonance ($d=15.2$ $\mathrm{\mu}$m, $\lambda =1553.3$ nm) is also indicated by a point as a comparison in Fig. 3(b). The coupling occurs between the fundamental HE$_{11}$ mode and the LP$_{11}$ group (i.e., TM$_{01}$, HE$_{21}$ and TE$_{01}$). For the resonance wavelength of 1553.3 nm, the calculated $d$ is about 16.45 $\mathrm{\mu}$m, which is slightly higher than the waist diameter of the fiber. This discrepancy comes from the non-uniformity of the bamboo-like structure in practice: the spindle-shaped bumps forming the fringe mLPG show diameter variations along the longitudinal direction.
Fig. 3. (a) Transmission spectrum of the bamboo-like microfiber structure with $d=15.2$ $\mathrm{\mu}$m and $\Lambda =497.5$ $\mathrm{\mu}$m. (b) Resonance wavelength as function of the fiber diameter for the coupling between fundamental mode and different higher-order mode groups. The solid curves are calculated results and the point indicates the experimentally measured value.
3. Refractive index and temperature responses
Variation of external refractive index (RI) produces change on the effective refractive indices of the guiding modes of the bamboo-like grating, leading to a wavelength shift of the resonance dip. To measure the response of the resonance to variations of the external RI, the fabricated device is immersed into aqueous sucrose solution of which the RI is modified by changing the concentration at room temperature ($\sim$26 $^{\circ }$C). When the external RI changes from 1.3308 to 1.3505, the resonance wavelength in the transmitted spectra of the mLPG redshifts from $\sim$1635.87 nm to $\sim$1653.35 nm, as shown in Fig. 4. The resonance wavelength shift has a positive dependence on the external RI change, and the RI coefficient is measured to be $\sim$875.02 nm/RIU (Fig. 4(b)). The sensitivity is commonly larger than that of a long period grating inscribed in the standard single-mode fiber but is relatively smaller than that of a grating in microfibers with thinner diameters [9,11,12]. Theoretically, the wavelength dependency on external index ($S_{RI}$) can be obtained by taking a small variation of RI,
It is clear that the RI sensitivity is determined by the wavelength $\lambda$, the dispersion factor $\gamma$, and the RI-induced variation of the intermodal index $(1/\Delta n)(\partial \Delta n/\partial RI)$. According to the resonance condition, the resonance wavelength can be selected by designing the corrugated period of the perform and the tapered diameter. Both $\gamma$ and $(1/\Delta n)(\partial \Delta n/\partial RI)$ are strongly dependent on the fiber diameter, so is the sensitivity. The magnitude of the sensitivity can be increased by further scaling down the microfiber size [9,11,12]. The intermodal index decreases with an increase of RI and $\gamma <0$ [9,10,20] , suggesting a redshift of the resonance wavelength according to Eq. (3). Utilizing the full-vector finite element method [21] for our theoretical model shown in Fig. 3(b), when $d=15.2$ $\mathrm{\mu}$m, we calculate that $\gamma =-0.884$ and $(1/\partial \Delta n)(\partial \Delta n/\partial RI)=-0.468$, which yields the RI sensitivity of 867.07 nm/RIU around $RI$ = 1.333 and $\lambda = 1635.87$ nm. The calculated result is in good agreement with the measured value. The small discrepancy ($\sim$0.91$\%$) may be attributed to the geometric variation of the realistic bamboo-like structure.
Fig. 4. (a) Several transmission spectra when the device is placed in solutions with different external RI. The arrow indicates the resonance wavelength redshifts with increasing RI. (b) The resonance wavelength as a function of the external RI. The red line indicates the linear fitting result.
The RI variation range is only limited by the spectral range of the light source, since the wavelength beyond 1650 nm corresponds to a distinct drop of light power of the source. Theoretically, if applying a light source and a spectrometer with wavelength ranges broad enough, the measurement range of RI may be limited by the refractive index of the microfiber material, because as the external RI is increased and close to that of the microfiber, more mode energy will extend out of the microfiber region and the grating strength will be decreased similarly as in [12].
The temperature coefficient is investigated by placing the fabricated mLPG in a resistance furnace in air, of which the temperature is controlled by an electric circuit and recorded by a thermometer. The resonance wavelength slightly redshifts from 1553.28 nm to 1553.70 nm when the temperature increases from 26 $^{\circ }$C to 100 $^{\circ }$C with a step of 5 $^{\circ }$C. Figure 5 shows selected transmission spectra when the device is at three different temperatures. Linear fitting to experimental data produces a coefficient of $~$5.78 pm/$^{\circ }$C Fig. 5(b), which is comparable to that reported in [10–12]. Similar to the expression of RI sensitivity, the temperature coefficient can be derived by differentiating the resonance condition of the grating,
Fig. 5. (a) Selected transmission spectra when the device is at different temperatures. The arrow indicates the resonance wavelength redshifts with increasing temperature. (b) The resonance wavelength as a function of temperature. The red line indicates the linear fitting.
4. Conclusion
A one-step-tapering-preform technique is proposed to manufacture microfiber-based devices. The preform is fabricated by milling a standard single mode fiber with a femtosecond laser milling technique. By drawing the fiber preform, a microfiber and the designed structures can be manufactured in one step at the same time. As a demonstration, a structure of bamboo-like grating is demonstrated and fabricated by heating and tapering a comb-like preform. A grating with a minimum diameter of $\sim$15.2 $\mathrm{\mu}$m shows an extinction ratio up to 18.2 dB around 1553.3 nm. The measured RI sensitivity and the temperature coefficient are 875.02 nm/RIU and 5.78 pm/$^{\circ }$C, respectively. The proposed one-step-tapering-preform technique can be applied to manufacture structures featured with high stability and robustness, which has potential applications in tunable filtering, mode converting, and sensing areas.
Funding
National Natural Science Foundation of China (61575083, 61705083, 61805103, U1701268); Natural Science Foundation of Guangdong Province (2019A1515012100); Youth Top-notch Scientific and Technological Innovation Talent of Guangdong Special Support Plan (2019TQ05X136); Fundamental Research Funds for the Central Universities (21618320); The Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2019BT02X105).
Disclosures
The authors declare no conflicts of interest.
References
1. A. M. Vengsarkar, P. J. Lemaire, J. B. Judkins, V. Bhatia, T. Erdogan, and J. E. Sipe, “Long-period fiber gratings as band-rejection filters,” J. Lightwave Technol. 14(1), 58–65 (1996). [CrossRef]
2. A. Othonos, “Fiber Bragg gratings,” Rev. Sci. Instrum. 68(12), 4309–4341 (1997). [CrossRef]
3. X. Shu, L. Zhang, and I. Bennion, “Sensitivity characteristics of long-period fiber gratings,” J. Lightwave Technol. 20(2), 255–266 (2002). [CrossRef]
4. J. K. Bae, S. H. Kim, J. H. Kim, J. Bae, S. B. Lee, and J. M. Jeong, “Spectral shape tunable band-rejection filter using a long-period fiber grating with divided coil heaters,” IEEE Photonics Technol. Lett. 15(3), 407–409 (2003). [CrossRef]
5. Y. J. Rao, Y. P. Wang, Z. L. Ran, and T. Zhu, “Novel fiber-optic sensors based on long-period fiber gratings written by high-frequency CO2 laser pulses,” J. Lightwave Technol. 21(5), 1320–1327 (2003). [CrossRef]
6. Y. Yu, T. H. Xiao, H. L. Guo, and Z. Y. Li, “Sensing of microparticles based on a broadband ultrasmall microcavity in a freely suspended microfiber,” Photonics Res. 5(3), 143–150 (2017). [CrossRef]
7. B.-O. Guan, J. Li, L. Jin, and Y. Ran, “Fiber Bragg gratings in optical microfibers,” Opt. Fiber Technol. 19(6), 793–801 (2013). [CrossRef]
8. J. Y. Lou, Y. P. Wang, and L. M. Tong, “Microfiber optical sensors: A Review,” Sensors 14(4), 5823–5844 (2014). [CrossRef]
9. H. Xuan, W. Jin, and M. Zhang, “CO2 laser induced long period gratings in optical microfibers,” Opt. Express 17(24), 21882–21890 (2009). [CrossRef]
10. H. Xuan, W. Jin, and S. Liu, “Long-period gratings in wavelength-scale microfibers,” Opt. Lett. 35(1), 85–87 (2010). [CrossRef]
11. Y. Zhi, X. Li, Y. Li, J. Li, and B.-O. Guan, “Superstructure microfiber grating characterized by temperature, strain, and refractive index sensing,” Opt. Express 28(6), 8853–8861 (2020). [CrossRef]
12. P. Fan, L.-P. Sun, Z. Yu, J. Li, C. Wu, and B.-O. Guan, “Higher-order diffraction of long-period microfiber gratings realized by arc discharge method,” Opt. Express 24(22), 25380–25388 (2016). [CrossRef]
13. Z. Y. Xu, Y. H. Li, and L. J. Wang, “Long-period grating inscription on polymer functionalized optical microfibers and its applications in optical sensing,” Photonics Res. 4(2), 45–48 (2016). [CrossRef]
14. X. Y. Zhang, Y. S. Yu, C. Chen, C. C. Zhu, R. Yang, Z. J. Liu, J. F. Liang, Q. D. Chen, and H. B. Sun, “Point-by-point dip coated long-period gratings in microfibers,” IEEE Photonics Technol. Lett. 26(24), 2503–2506 (2014). [CrossRef]
15. L.-P. Sun, J. Li, L. Jin, and B.-O. Guan, “Structural microfiber long-period gratings,” Opt. Express 20(16), 18079–18084 (2012). [CrossRef]
16. T. A. Birks and Y. W. Li, “The shape of fiber tapers,” J. Lightwave Technol. 10(4), 432–438 (1992). [CrossRef]
17. S. C. Xue, M. A. van Eijkelenborg, G. W. Barton, and P. Hambley, “Theoretical, numerical, and experimental analysis of optical fiber tapering,” J. Lightwave Technol. 25(5), 1169–1176 (2007). [CrossRef]
18. H. L. Sorensen, E. S. Polzik, and J. Appel, “Heater self-calibration technique for shape prediction of fiber tapers,” J. Lightwave Technol. 32(10), 1886–1891 (2014). [CrossRef]
19. X.-C. Yu, Y. Zhi, S.-J. Tang, B.-B. Li, Q. Gong, C.-W. Qiu, and Y.-F. Xiao, “Optically sizing single atmospheric particulates with a 10-nm resolution using a strong evanescent field,” Light: Sci. Appl. 7(4), 18003 (2018). [CrossRef]
20. Y. Z. Tan, L. P. Sun, L. Jin, J. Li, and B.-O. Guan, “Microfiber Mach-Zehnder interferometer based on long period grating for sensing applications,” Opt. Express 21(1), 154–164 (2013). [CrossRef]
21. M. Koshiba and Y. Tsuji, “Curvilinear hybrid edge/nodal elements with triangular shape for guided-wave problems,” J. Lightwave Technol. 18(5), 737–743 (2000). [CrossRef]
22. S. W. James and R. P. Tatam, “Optical fibre long-period grating sensors: characteristics and application,” Meas. Sci. Technol. 14(5), R49–R61 (2003). [CrossRef]
