Long-period grating inscription on polymer functionalized optical microfibers and its applications in optical sensing Download: 870次
1. INTRODUCTION
Optical microfibers have attracted much attention because of the high fractional evanescent field, wavelength/subwavelength diameters, low loss, easy fabrication, and full integration with fiber systems [13" target="_self" style="display: inline;">–
An LPG can be inscribed on optical microfibers, with the advantage of compactness due to the shorter grating pitches compared with LPGs in conventional optical fibers [13]. Moreover, the high fractional evanescent field in microfiber LPGs (MLPGs) promises possible applications in the sensing of surroundings, such as the refractive index of surrounding liquids [14]. In addition, the diameters of optical microfibers are much smaller than those of single-mode fiber, so microfibers have been adopted for high-precision sensing of external force [15]. All these features provide MLPGs may have application in highly sensitive optical sensing for strain, temperature, and refractive index [13,14,1618" target="_self" style="display: inline;">–
Polymers exhibit many applications in optical sensing because of their sensitivity to ultraviolet [19], alcohol [20], humidity [21], and temperature [16], and to their biocompatibility [22]. Polymers on optical microfibers can be used for the fabrication of functional microstructures, without any damage of silica microfibers, which bestows them with much higher mechanical strength and flexibility. In particular, polymer jackets on microfibers under point-by-point ultraviolet exposure could enable an agile and versatile technique for flexible microstructure fabrication on optical microfibers.
In this work we demonstrate LPG inscription on optical microfibers functionalized with ultraviolet-sensitive polymers. We inscribe 2 mm long MLPGs with a pitch of 80 μm on a 5.4 μm diameter polymethyl methacrylate (PMMA)-coated microfiber via point-by-point ultraviolet exposure. A resonant dip of 15 dB was observed at 1377 nm. We also demonstrate possible applications in optical sensing with MLPGs. The strain sensitivity of the MLPG was measured to be
2. FABRICATION AND CHARACTERIZATION
Microfiber LPGs couple the fundamental mode and high-order modes in optical microfibers. This resonant coupling gives the phase-matching wavelength [23]:
The symbols
In terms of strain sensing, the sensitivity of an MLPG is given by [23] where
The optical microfibers were directly drawn from commercial single-mode fibers on a homemade taper drawing work station via the traveling flame taper-drawing scheme [24]. The diameters of the optical microfibers were measured to be around 5.4 μm and the insertion loss was less than 0.1 dB. To fabricate MLPGs, the optical microfibers were first coated with PMMA jackets via a modified dip-coating method [25]. The thickness of the PMMA jackets was estimated to be 100 nm under SEM photographs (not shown here), and the propagation loss was measured to be less than 0.5 dB/cm. We chose PMMA as the coating material because it can show remarkable refraction index changes and even photoetching under ultraviolet exposure [26,27].
The fabrication and measurement setup for MLPGs is shown in Fig.
Fig. 1. Schematic for the inscription of MLPGs via point-by-point ultraviolet exposure. A supercontinuum source (SCS) is used for illumination and the transmission spectrum is displayed on an optical spectrum analyzer (OSA) in real time.
In optical microfibers, the grating pitches of MLPGs (e.g., less than 100 μm) can be much shorter than the traditional LPGs [23] because of the relatively large differences between the propagation constants of the fundamental mode and high-order modes. The measured transmission spectrum of a 25-period MLPG is shown in Fig.
Fig. 2. (a) Transmission spectrum of a 25-period MLPG. The grating pitch is 80 μm and the diameter of the optical microfiber is 5.4 μm. (b) The calculated grating pitches for different resonant dip wavelengths in optical microfiber with a diameter of 5.4 μm.
3. APPLICATIONS IN OPTICAL SENSING
The resonant wavelengths of MLPGs are sensitive to strain, temperature, and the refractive index of surrounding liquids. We measured the spectral responses to strain and temperature changes for the MLPGs and showed potential applications of MLPGs in optical sensing.
The measured spectral responses of the MLPG to strain are shown in Fig.
Fig. 3. (a) Spectral responses of the MLPG to strain. The applied strain ranged from 0 to 7533 μϵ. (b) The measured resonant dip wavelengths (square scatters) and the linear fitting result (red line). The corresponding applied axial force is shown as the top axis.
In optical microfibers, the diameters are so small that a much higher sensitivity of axial force can be expected in MLPGs than that of LPGs in single-mode fibers, as shown in Eq. (
The spectral responses of the MLPG to temperature changes were measured with a digitally controlled oven. The temperature increased from 20°C to 50°C, and the transmission spectra of the MLPG at different temperatures are shown in Fig.
Fig. 4. Spectral responses of (a) unsealed and (b) sealed MLPGs to temperature change. (c) The measured resonant wavelengths of sealed (square scatters) and unsealed (circle scatters) MLPGs at different temperatures. The linear fitting results are shown as red lines.
However, high temperature sensitivity of MLPGs is desirable for temperature sensing. To this end, we sealed the MLPG inside a PMMA housing that had a high thermal expansion coefficient. Single-mode fibers at the ends of optical microfibers were attached to the PMMA box, and the waist region was suspended in the air and kept straight in the box. The MLPG at the waist followed the expansion and shrinking of the PMMA housing caused by the temperature. In Fig.
4. CONCLUSION
In conclusion, we have successfully inscribed LPGs on polymer functionalized optical microfibers via point-to-point ultraviolet exposure, and possible applications in optical sensing with the MLPGs are also demonstrated. For a 2 mm long MLPG with a pitch of 80 μm, the strain and axial force sensing sensitivity were
5 Acknowledgment
Acknowledgment. This work is supported by National Natural Science Foundation of China (Grant No. 61505096).
Article Outline
Z. Y. Xu, Y. H. Li, L. J. Wang. Long-period grating inscription on polymer functionalized optical microfibers and its applications in optical sensing[J]. Photonics Research, 2016, 4(2): 02000045.