High-Efficiency Ultraviolet Inscription of Bragg Gratings in Microfibers

Transcription

1 High-Efficiency Ultraviolet Inscription of Bragg Gratings in Microfibers Volume 4, Number 1, February 2012 Yang Ran Long Jin Yan-Nan Tan Li-Peng Sun Jie Li Bai-Ou Guan DOI: /JPHOT /$ IEEE

2 High-Efficiency Ultraviolet Inscription of Bragg Gratings in Microfibers Yang Ran, Long Jin, Yan-Nan Tan, Li-Peng Sun, Jie Li, and Bai-Ou Guan Institute of Photonics Technology, Jinan University, Guangzhou , China DOI: /JPHOT /$31.00 Ó2012 IEEE Manuscript received November 2, 2011; revised December 18, 2011; accepted December 23, Date of publication December 30, 2011; date of current version January 24, This work was supported by National Natural Science Foundation of China under Grant , Grant , and Grant and the Fundamental Research Funds for the Central Universities under Grant Corresponding author: B. O. Guan ( Abstract: In this paper, we demonstrate high-efficiency inscription of Bragg gratings in microfiber. Strong Bragg gratings are easily inscribed using a 193-nm excimer laser and phase mask in microfiber drawn from the 62.5/125-m standard multimode fiber (MMF) with no additional photosensitization treatment. The enlarged photosensitive core offered by the MMF provides a sufficient overlap between the fundamental mode and the induced refractive index modulation region, which significantly enhances the grating inscription efficiency. The grating inscription in the microfiber is even more efficient than that in the original MMF because the multibeam interference-induced fringe fuzziness in the normal size fiber does not occur as the fiber diameter down to the scale that is comparable with the Talbot length of the ultraviolet (UV)-writing light. The proposed method is beneficial for sensing and device applications of microfiber Bragg grating (mfbgs) because of the ease of fabrication, high coupling strength, and large wavelength separation between individual resonant dips. Index Terms: Fiber gratings, microoptics, sensors. 1. Introduction Microfiber Bragg grating (mfbg), which is fiber Bragg grating (FBG) fabricated in microfiber, not only offers great flexibility and compactness compared with conventional FBG but enables high sensitivity to external surroundings via evanescent field interaction as well, leading to a promising application as photonic sensors for bio/chemical targets. This application requires an efficient and cost-effective means of grating formation in microfibers. Most recently, a number of different methods have been demonstrated for the fabrication of mfbgs, including femtosecond (fs) laser irradiation [1], focus ion beam (FIB) milling [2] [4], 248-nm and 193-nm laser writing for silica microfibers [5], [6], and 633-nm side-irradiation or 1550-nm internal-irradiation for chalcogenide glass microfibers [7], [8]. High-reflectivity mfbgs have been successfully fabricated by using fs laser irradiation [1] and the FIB milling method [2] [4], where the grating is formed by introducing structural damage or composing microcarved geometry, resulting in a degradation of mechanical strength. The FIB-microcarved mfbgs are tiny in dimension, which has the advantage of device compactness but the disadvantage of incapability of achieving narrow grating reflection bandwidth. It is desirable to photoinscribe index gratings into microfibers using conventional ultraviolet (UV) inscription techniques. However, the difficulty lies in the limited photosensitivity when the fiber diameter is down to several micrometers. To overcome this problem, photosensitive microfiber was fabricated from a specially designed silica fiber with a Ge/B-codoped inner cladding for grating Vol. 4, No. 1, February 2012 Page 181

3 Fig. 1. Microscope images of the fabricated microfibers with different diameter. inscription [5]. However, hydrogen loading is needed to further enhance the photosensitivity for 248-nm laser irradiation, which requires specific care for the micron-scaled fibers. Using a 193-nm laser as laser a source can release the requirement on fiber photosensitivity, and mfbgs have been successfully inscribed in microfibers that are drawn from standard single-mode fiber (SMF) [6]. However, the inscription efficiency is low and writing a strong mfbg is difficult. An alternative approach is to use chalcogenide glass microfiber. Strong mfbgs have been fabricated in As 2 Se 3 chalcogenide microfiber by side-writing with a 633 nm laser [7] or internal writing with a 1550 nm laser [8]. However, the high refractive index (RI) (typically 2.7) of As 2 Se 3 chalcogenide fiber makes it difficult to pigtail with normal silica fiber. In this letter, we report high-efficiency inscription of index gratings in silica microfibers using conventional UV writing technique. Strong mfbgs have been easily inscribed by use of a 193 nm excimer laser in microfibers drawn from a 62.5/125-m standard multimode fiber (MMF) without hydrogen loading or other photosensitization treatment. The grating inscription efficiency is greatly enhanced due to the enlarged photosensitive region over the microfiber cross section, offered by the MMF. The convenient and high-efficiency inscription of mfbg facilitates its applications in sensing and other fields. 2. High-Efficiency Inscription of mfbg The microfibers are drawn from 62.5/125-m MMF (manufactured by Lucent), which are connected with SMFs at both ends in advance, by use of a 2-mm-wide flame as the heat source. The fiber diameter can be determined by controlling the drawing speed. Fig. 1 shows the microscope images of some of the fabricated microfibers with different diameters. The length of the microfiber is 8 cm. The cross-sectional shape of the microfiber is assumed unchanged during the drawing process. A typical loss of 1.5 db is introduced, mainly due to the mode mismatch between the SMF and MMF. Interferometric fringes can be observed in the transmission spectrum as a result of the excitation of higher-order modes at the SMF/MMF interface. The fringes can be minimized as drawing the fiber down to several microns since the higher-order modes can be effectively filtered out at the waist of the taper. FBGs are inscribed in the microfibers by use of a 193-nm ArF excimer laser and a phase mask with a period FBG of nm. The microfiber is placed parallel to the phase mask with a distance of 100 m. The single-pulse energy and repetition rate of the laser are 3 mj and 200 Hz, respectively. A cylindrical lens is used to focus the UV beam onto the microfiber and the energy density can achieve 120 mj/cm 2. Owing to the high efficiency associated with two-photon excitation at 193 nm [9], [10], hydrogen loading or other photosensitization treatment are not needed and mfbgs can be directly inscribed in the silica microfibers. Fig. 2 shows the growth of mfbg in a 7.7-m microfiber. The transmission spectrum of the mfbg is monitored by use of a broadband light source and an optical spectrum analyzer. The length of the mfbgs is 3 mm, which is determined by the laser aperture. The resonant wavelength is nm. Based on the phase-matching condition B ¼ 2n fund FBG, the effective index of the fundamental Vol. 4, No. 1, February 2012 Page 182

4 Fig. 2. Growth of the mfbg in a microfiber with a diameter of 7.7 m. The black curve is the transmission background of the fabricated microfiber from MMF. Fig. 3. Measured changes of Bragg wavelength and coupling strength as a function of UV fluence for (a) the mfbg and (b) the FBG in MMF. mode n fund is , which is a little larger than that of SMF-based microfiber, due to the enlarged index-raised region. A 13-dB mfbg is fabricated within 63 seconds. Fig. 3(a) shows the changes of Bragg wavelength and coupling strength, respectively, as a function of exposure dosage. A red-shift of the reflection peak is observed, which denotes that the index change is positive, and thus, the grating induction is type I. The measured shift by 0.22 nm corresponds to an average index change n dc of 2: , calculated based on the phase-matching condition. According to the coupled mode theory, the reflectivity of an FBG is expressed by [11] R ¼ tanh 2 ðlþ (1) where L is the grating length and is the coupling coefficient, which is defined by ¼ ZZ!! n ac E E dx dy (2) b Ge where n ac is the index modulation, which is considered as uniform over the Ge-doped region, E! represents the transverse electric field of the fundamental mode, and the integral denotes the overlap between the fundamental mode and the photosensitive region. Based on (1), the coupling coefficient is estimated as 2.58 cm 1. The calculated amplitude of index modulation induced in the mfbg is 1: , and thus, the fringe visibility of the induced longitudinal index variation ¼ n ac =n dc is as high as 94%. The grating inscription efficiency has been greatly improved due to the enlarged photosensitive region offered by the MMF. Equation (2) suggests that the inscription efficiency can be improved by increasing the overlap integral. To numerically obtain the overlap integrals, cross-sectional RI profiles of microfibers have been constructed with a finite element method software. The transverse electric fields are then calculated for microfibers with different diameters by use of a mode solver. Fig. 4(a) and Vol. 4, No. 1, February 2012 Page 183

5 Fig. 4. Calculated fundamental mode energy distribution along fiber diameter for microfibers drawn from (a) 62.5/125 MMF and (b) conventional SMF. The diameter for both microfiber is 7.7 m. (c) Calculated overlap integral between the fundamental mode and the photosensitive region for MMF and SMF-based microfiber, as a function of fiber diameter. (b) exhibits the mode energy distribution along the fiber diameter drawn from a standard SMF and the 62.5/125-m MMF, respectively. The two microfibers present almost identical profiles because the mode property is mainly determined by the silica-air waveguide structure. With the calculated electric fields, the overlap integral can be calculated considering the profile of the photosensitive region. Fig. 4(c) shows the calculated overlap integrals for the two microfibers. The MMF-based microfiber apparently presents much higher overlap integrals due to the larger Ge-doped region. Take the diameter D ¼ 7:7-m, for example, the integral for the SMF-based microfiber is only In contrast, the amplitude of the integral is 0.66 for the MMF-based microfiber, which is 38 times higher. Consequently, it is much easier to achieve a high resonant strength in the MMF-based microfiber. Note that the 193-nm UV irradiation can also induce index modulation over pure-silica region, but needs much higher exposure dosage. The inscription process usually takes several tens of minutes. Since the inscription duration is typically 1 min, only grating formation in the Ge-doped region is taken into consideration in this paper. In comparison, Fig. 3(b) shows the evolution of Bragg wavelength and coupling strength for a FBG inscribed in the 125-m MMF under identical exposure conditions. It takes 220 sec to reach the coupling strength of 12.5 db, which is much longer than that needed for the mfbg. The reflection peak experiences a red-shift of 0.68 nm. The fringe visibility is estimated to be 33%, much lower than 94% for the mfbg. In the ideal case, the 1st-order diffraction beams create a uniform index modulation pattern over the fiber core. One may expect a higher efficiency of grating inscription in the original MMF, because of the higher overlap integral, as shown in Fig. 4. However, due to the imperfection of the phase mask, the zeroth and higher-order diffraction beams are produced, which affects the spatial distribution of the interference fringes and results in a complex index modulation pattern. According to the observation in [12], an index modulation containing interleaving layers with -phase offsets is formed when taking the consideration of the second-order diffraction beam. The spacing between the individual layers equals the BTalbot length.[ Although no quantitative analysis based on coupled mode theory or other theories have been presented so far to Vol. 4, No. 1, February 2012 Page 184

6 Fig. 5. Measured transmission and reflection spectra of (a) the mfbg in a 2.9-m microfiber and (b) the FBG fabricated in the 125-m MMF. Fig. 6. (a) Measured responses of mfbgs with different diameters to surrounding refractive index. (b) calculated mode energy distribution at around the silica/liquid interface for microfibers with different diameters when placed in air and liquids with n ¼ 1:33 and n ¼ 1:36, respectively. calculate the reflectivity for a FBG with such index modulation structure, it is believed that the coupling efficiency is significantly reduced as a result of the -phase offset between the adjacent layers. In contrast, for the microfiber in which the diameter of the Ge-doped region is close to the Talbot length, a much simpler index modulation pattern can be formed, which enables a higher efficiency of grating formation. 3. Spectral Characteristics and Response to RI Fig. 5 shows the transmission and reflection spectra of FBGs written in the original MMF and after drawn down to 2.9 m over a wider wavelength range. A large amount of core modes are established in the MMF due to the large core. Because, a number of higher-order mode resonant peaks can be found in the shorter wavelength side of the fundamental mode peak within the 10-nm range. Drawing down the fiber decreases the mode indices and leads to larger index differences between the individual modes. Consequently, the wavelength separation for the mfbg is as large as 45 nm, which is beneficial for sensing and device applications. Vol. 4, No. 1, February 2012 Page 185

7 We have fabricated a series of mfbgs with different diameters by use of this method. The response of mfbgs to the ambient RI is characterized by immersing the gratings into the sucrose solution. Fig. 6(a) shows the measured Bragg wavelength shifts as a function of surrounding RI. The measured RI sensitivities for mfbgs fabricated in the 8.7-m, 3.8-m, and 2.9-m microfibers are 1.67, 20, and 170 nm/riu, respectively, for RIs between 1.33 and The RI sensitivity of mfbgs relies on the evanescent field interaction with the surrounding medium. Fig. 6(b) shows the calculated evanescent field distributions for the three microfibers when the surrounding medium is air ðn ¼ 1:0Þ and liquid (n ¼ 1:33 and n ¼ 1:36), respectively. The wave equation requires a continuous distribution of tangential components (E and E z ), but the radial distribution ðe r Þ does not have to be continuous at the silica-liquid interface. As a result, the energy distribution presents a ripple at the interface as shown in Fig. 6(b). For thinner microfiber, more fractional mode energy is in the form of evanescent field, which results in a higher RI sensitivity. Since the mode property is mainly determined by the silica/air(liquid) structure, the employment of the MMF with a larger indexraised core hardly affect the RI sensitivity, compared with our previous result in [6]. 4. Conclusion In conclusion, a high-efficiency method for the fabrication of mfbgs is demonstrated, by use of MMF-based microfiber. The efficiency has been greatly improved due to the higher overlap between the mode field and the photosensitive region over fiber cross section. The grating formation is even more efficient than that in the original MMF because a simpler index fringe pattern is formed. The proposed method hopefully brings forward the application of the mfbgs as bio/ chemical sensors and other photonic devices due to the ease of fabrication, high coupling strength and the convenience for connection with standard SMFs. References [1] X. Fang, C. R. Liao, and D. N. Wang, BFemtosecond laser fabricated fiber Bragg grating in microfiber for refractive index sensing,[ Opt. Lett., vol. 35, no. 7, pp , Mar [2] Y. X. Liu, C. Meng, A. P. Zhang, Y. Xiao, H. K. Yu, and L. M. Tong, BCompact microfiber Bragg gratings with high-index contrast,[ Opt. Lett., vol. 36, no. 16, pp , Aug [3] M. Ding, M. N. Zervas, and G. Brambilla, BA compact broadband microfiber Bragg grating,[ Opt. Exp., vol. 19, no. 16, pp , Jul [4] K. P. Nayak, F. L. Kien, Y. Kawai, K. Hakuta, K. Nakajima, H. T. Miyazaki, and Y. Sugimoto, BCavity formation on an optical nanofiber using focused ion beam milling technique,[ Opt. Exp., vol. 19, no. 15, pp , Jul [5] Y. Zhang, B. Lin, S. C. Tjin, H. Zhang, G. H. Wang, P. Shum, and X. L. Zhang, BRefractive index sensing based on higher-order mode reflection of a microfiber Bragg grating,[ Opt. Exp., vol. 18, no. 25, pp , Dec [6] Y. Ran, Y. N. Tan, L. P. Sun, S. Gao, J. Li, L. Jin, and B. O. Guan, B193 nm excimer laser inscribed Bragg gratings in microfibers for refractive index sensing,[ Opt. Exp., vol. 19, no. 19, pp , Sep [7] R. Ahmad, M. Rochette, and C. Baker, BFabrication of Bragg gratings in sub-wavelength diameter As 2 Se 3 chalcogenide wires,[ Opt. Lett., vol. 36, no. 15, pp , Jul [8] R. Ahmad and M. Rochette, BPhotosensitivity at 1550 nm and Bragg grating inscription in As 2 Se 3 chalcogenide microwires,[ Appl. Phys. Lett., vol. 99, no. 6, pp , Aug [9] F. Bilodeau, B. Malo, J. Albert, D. C. Johnson, K. O. Hill, Y. Hibino, M. Abe, and M. Kawachi, BPhotosensitization of optical fiber and silica-on-silicon/silica waveguides,[ Opt. Lett., vol. 18, no. 12, pp , Jun [10] J. Albert, B. Malo, K. O. Hill, F. Bilodeau, D. C. Johnson, and S. Theriault, BComparison of one-photon and two-photon effects in the photosensitivity of germanium-doped silica optical fibers exposed to intense ArF excimer laser pulses,[ Appl. Phys. Lett., vol. 67, no. 24, pp , Oct [11] T. Erdogan, BFiber grating spectra,[ J. Lightw. Technol., vol. 15, no. 8, pp , Aug [12] N. M. Dragomir, C. Rollinson, S. A. Wade, A. J. Stevenson, S. F. Collins, G. W. Baxter, P. M. Farrell, and A. Roberts, BNondestructive imaging of a Type I optical fiber Bragg grating,[ Opt. Lett., vol. 28, no. 10, pp , May Vol. 4, No. 1, February 2012 Page 186