A fiber Bragg grating written in a photosensitive microfiber using KrF excimer laser via a uniform phase mask is demonstrated. We have successfully fabricated two Bragg gratings in microfibers having different diameters. In the reflection spectrum of a microfiber Bragg grating (MFBG), we observed two reflection peaks,which agrees with our numerical simulation results. Compared with the fundamental mode reflection, the higher-order reflection mode is more sensitive to the refractive index (RI) variation of the surrounding fluid due to its larger evanescent field. The measured maximum sensitivity is ~102 nm/RIU (RI unit) at an RI value of 1.378 in an MFBG with a diameter of 6 μm.
©2010 Optical Society of America
In the past few years, optical microfibers/nanofibers have attracted increasing interests owing to their intrinsic advantages, such as large evanescent field, small effective mode field diameter, high nonlinearity, and low-loss interconnection to single-mode fiber (SMF) [1,2]. As a result, microfibers have been used in many different fields including passive photonic devices [3–6], sensing [7–13], lasing [14,15], signal processing , and atom trapping [17,18]. In the field of sensing, microfibers have certain advantage since a considerable fraction of the optical power coupled into the fiber propagates in the evanescent field outside the physical boundary of a microfiber. So far, microfibers have been utilized to sense humidity , ambient refractive index (RI) [8,9], temperature , acceleration , gas concentration , and absorption of particles on the microfiber surface . Nearly all reported sensing schemes adopt microfiber ring resonators, namely microfiber loop, knot, or coil resonators, since the changing of the ambient medium parameters can induce the transmission spectrum shift of a microfiber ring resonator owning to the effective RI variation of the microfiber.
Recently, microfibers inscribed with Bragg gratings using femtosecond laser pulse have also been utilized for RI sensing . In addition, long period gratings in microfibers, fabricated using CO2 lasers  and femtosecond lasers , have also been demonstrated. As far as we know, all these gratings in the above references [19–21] are inscribed using laser pulses with high intensities to cause physical deformation or damage on the microfiber, and they have all been used to sense RI by measuring the magnitude of the transmission spectra shift.
In this paper, we present a microfiber Bragg grating (MFBG) fabricated using a KrF excimer laser in a highly Ge-doped photosensitive microfiber. To the best of our knowledge, there has been no report to date of microfiber Bragg grating whose reflection and transmission characteristics are not due to fiber surface damages but due to the change of refractive index of the microfiber. Two such Bragg gratings with periods of 532.85 nm and 541.15 nm had been successfully fabricated in the photosensitive microfibers with diameters of 6 μm and 6.5 μm, respectively. Two reflection peaks are observed in the reflection spectrum of each MFBG. The reflection peak induced by the higher-order mode is selected to sense ambient RI, because the higher-order mode has a larger evanescent field outside the microfiber and thus it is more sensitive to ambient RI, compared with the fundamental mode reflection. The proposed RI sensor based on the higher-order mode reflection of an MFBG possesses compact size, high sensitivity and low cost, as well as the convenience of sensing head operation.
2. Fabrication and principle of microfiber Bragg grating
The microfibers were fabricated using a flame-heated taper-drawing technique. The flame with a width of about 5 mm was generated by the burning of mixed butane and propane. Through adjusting the flux of butane and propane, the flame intensity can be controlled to ensure a sufficiently high temperature to soften an SMF. The microfibers with different diameters can be easily fabricated by controlling the taper-drawing speed. The fabricated microfibers show high diameter uniformity, excellent surface smoothness, and low loss of less than 0.01 dB/mm.
The SMF used as the preform of the microfiber is a photosensitive fiber, which has a highly Ge-doped (22 mol%) 3.6-μm-diameter core, a 25-μm-diameter inner cladding co-doped with Ge and B to further increase photosensitive area, and a 125-μm-diameter outer cladding. Once the photosensitive fiber is transformed into a microfiber having a diameter of 6 μm, the highly doped Ge (22 mol%) in core may disperse to inner cladding. However, only very little doped Ge could disperse to outer cladding, since the doping concentration is very low (1 mol%) in inner cladding. The diameter of the inner cladding is around 25 μm in a 125-μm-diameter fiber, so that the diameter of the photosensitive region is approximately 1.2 μm in a 6-μm-diameter microfiber. To increase its photosensitivity, the fabricated microfiber is loaded with hydrogen at a temperature of 70 °C and a pressure of 2000 psi for 240 hours. After hydrogen loading, we cleaned the microfiber using ethanol. The insertion loss of the hydrogen loaded microfiber was measured to be around 2 dB more than that of the initial microfiber without hydrogen loading.
In fabrication of MFBGs, we used 248 nm KrF excimer laser with a pulse repetition rate of 10 Hz and fluence of around 500 mJ/cm2 to implement two groups of experiments. In the first experiment, we used a 25 mm long phase mask with a pitch of 1065.7 nm to fabricate a grating in a 6.0 μm diameter microfiber, which we have labeled as MFBG1. Figure 1(a) shows the scanning electron microscope (SEM) image of the MFBG1. In the second experiment, we used a 10 mm long phase mask with a pitch of 1082.3 nm to inscribe a grating in a 6.5 μm diameter microfiber, which we have labeled MFBG2 and shown in Fig. 1(b). The MFBG1 and MFBG2 are both formed after around 6000 pulses of irradiation.
As known, a microfiber is an optical waveguide, with a circular cross-section, an infinite air cladding thickness, and a step-index profile . In the two-layer step-index model for a cylindrical waveguide, we find that a microfiber with a diameter of less than 1.1 μm can only support fundamental mode propagation at a wavelength of around 1.5 μm. In the microfibers with diameters of 6.0 and 6.5 μm in our case, tens of higher-order modes besides the fundamental mode can be supported as well. While introducing a grating in the microfiber, the periodical index perturbations result in couplings amongst the modes. Mode coupling of such small diameter fiber is a function of phase synchronism and the amount of mode field overlaps in the grating region. The phase-matching condition, ensuring a coherent exchange of energy between the modes, is given by 1(c), the intensity patterns of HE11 and HE12 modes have sufficient overlaps in the grating region (marked by the blue circle), but other higher-order modes (TE01, HE21, TM01, EH11 and HE31 modes) have no obvious field distributions in the grating region. Therefore, the coupling between the forward-propagating HE11 mode and the backward-propagating HE11 mode produces a long-wavelength reflection peak. The coupling between the counter propagating HE11 and HE12 modes generates a short-wavelength reflection peak.
3. Experimental results of refractive index sensing
As show in Fig. 2 , the solid line represents the reflection spectrum of the fabricated MFBG1 immersing in air, measured by an optical spectrum analyzer (OSA) with resolution of 0.01 nm. It can be seen that two main reflected peaks occur in the reflection spectrum. As explained previously, the reflected peak generated at long-wavelength is due to the coupling between the counter propagating fundamental HE11 modes, and the other peak at short-wavelength corresponds to the coupling between the counter propagating HE11 and HE12 modes. The effective modal indices of HE11 and HE12 modes in a microfiber with a diameter of 6 µm can be numerically solved. Hence, for a grating with a period of 532.85 nm, the two reflected resonance wavelengths can be calculated from Eq. (1). In simulation, the effective modal indices of HE11 and HE12 modes in MFBG1 are calculated to be 1.4339 and 1.3854. Based on Eq. (1), the calculated fundamental mode and higher order mode resonance wavelengths are 1528.1 nm and 1502.3 nm, respectively, which agree well with the experimental results as shown in Fig. 2. When the MFBG1 is immersed in the isopropyl alcohol (IPA) (n = 1.378), the corresponding reflection spectrum is shown as the red dashed line in Fig. 2. It can be observed that, the two reflection peaks red shift concurrently. The red-shift of the higher-order mode is greater than that of the fundamental mode, since the higher-order mode has larger evanescent field outside the microfiber than the fundamental mode. Figure 3 shows the reflection spectra of MFBG2 immersed in air (blue solid line) and in IPA (red dashed line). We also find that the two resonance wavelengths of MFBG2 agree well with the numerically calculated results which are presented as dash-dot lines. In simulation, the effective modal indices of HE11 and HE12 modes in MFBG2 are calculated to be 1.4351 and 1.3943. From Eq. (1) the calculated fundamental mode and higher order mode resonance wavelengths are 1553.2 nm and 1531.1nm, respectively. It should be noted that, since MFBG2 has a shorter grating length (10 mm) as compared to MFBG1 (25 mm), the reflectivity of the two reflected peaks shown in Fig. 3 is less than that shown in Fig. 2.
The RI sensing is demonstrated by immersing the MFBG1 and MFBG2 into different concentration IPA solutions with RI ranging from 1.334 to 1.378 at room temperature. Figure 4 shows the dependence of the reflection wavelengths shift with the change of ambient RI. It can be found that, the wavelength shift of the higher-order mode reflection is significantly greater than that of the fundamental mode reflection in each MFBG. That is, the higher-order mode reflection has a higher RI sensitivity than the fundamental mode reflection. Moreover, for the selected fundamental or higher-order mode reflection, the smaller diameter microfiber results in a greater wavelength shift. From Fig. 4 we calculated the maximum RI sensitivity at the reflection peak induced by coupling between HE11 and HE12 modes in the 6-μm-diameter microfiber. The maximum RI sensitivity is ~102 nm/RIU (RI unit) at an RI value of 1.378. The stability of the MFBG sensor is determined by the stability of the whole measuring system. In the experiment, no resonance wavelength fluctuation was observed due to the limited 0.01 nm resolution of the OSA. Since the measured maximum sensitivity is ~102 nm/RIU (RI unit) at the RI value of 1.378, the MFBG sensor can sense a refractive index variation of 9.8 × 10−5. As a sensing head, the proposed MFBG sensor has a higher RI sensitivity compared to the existing techniques [24–26]. The etched FBGs [24–26] sense the ambient RI based on the wavelength shift of the fundamental mode resonance, which has a lower sensitivity compared to the higher order mode resonance in the same microfiber. Also, the MFBG with a diameter of 6 µm has a comparable sensitivity in respect to LPGs [27,28]. However, LPG cannot be used as a sensing head since its transmission spectrum is taken into concern.
In conclusion, we have presented a RI sensor based on the higher-order reflection of an MFBG, which is fabricated by using KrF excimer laser irradiation. Two Bragg gratings have been successfully fabricated in the microfibers with different diameters, and each of them has two reflection peaks corresponding to the fundamental mode and higher-order mode. The experimental results are in a good agreement with the numerically calculated results. The measured maximum RI sensitivity is ~102 nm/RIU at the RI value of 1.378 in an MFBG with a diameter of 6 μm, and the RI sensitivity can be further increased by decreasing the diameter of the microfiber. Acting as a sensing head, the RI sensor based on higher-order mode reflection of an MFBG owns compact size, low cost, high sensitivity and great multiplexing capability.
The authors thank Dr. Li Xia and Ping Zhao for their help in experiments, as well as Feng Tu and Jing Li from Yangtze Optical Fibre & Cable Co. Ltd for discussion and providing photosensitive fiber. This work was supported by National Basic Research Program of China (Grant 2006CB302805), National High Technology Research and Development Program of China (Grant No. 2006AA03Z414), National Natural Science Foundation of China (Grant No. 60877056 and No. 60707005).
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