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A simple, polymer-microfiber-assisted approach to fabricating the silica microfiber knot resonator
Optics Communications 321 (2014) 157–161
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
A simple, polymer-microfiber-assisted approach to fabricating the silica microfiber knot resonator Yiping Xu a,b, Liyong Ren a,n, Jian Liang a, Chengju Ma a,b, Yingli Wang a, Nana Chen a,b, Enshi Qu a a b
State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, China University of Chinese Academy of Sciences, Beijing 100049, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 November 2013 Received in revised form 13 January 2014 Accepted 30 January 2014 Available online 11 February 2014
With the assistance of the polymer microfiber, we propose a simple approach to fabricating silica-based optical microfiber knot resonators (MKRs) in this paper. A pre-fabricated knot ring made of the polymer microfiber can be readily driven to the silica microfiber owing to the intensive van der Waals and the electrostatic forces between the polymer and the silica microfibers. The fabrication process is introduced in detail. Several kinds of MKRs are fabricated and their optical performances are demonstrated accordingly. & 2014 Elsevier B.V. All rights reserved.
Keywords: Microfiber Silica microfiber Microfiber knot resonator (MKR)
1. Introduction In recent years, miniature photonic devices assembled by microfibers with diameters of micrometer scale have attracted significant interest. Various structures of microfiber resonators, including loop, knot and coil, have been investigated [1–11]. Among them the optical microfiber knot resonator (MKR) [2], due to its high Q factor, tunable free spectral range, high stability and compact structure, has been extensively applied to optical filters [12], optical sensing [13–17], microfiber lasers [18], and nonlinear resonators [19]. Currently, most MKRs are fabricated based on the micromanipulation method proposed by Jiang et al. [2]. That is, the microfiber knot is assembled with the assistance of two tapered-drawing fiber probes under an optical microscope. Obviously, operating under an optical microscope increases the inconvenience of the fabrication. Recently, for the first time Xiao et al. proposed a new method to directly fabricate a MKR from a double-ended tapered fiber [20]. The merit of this fabrication method is that light can be coupled in and out of the resonator directly via integral full-size fiber ports, which benefits the high finesse. However, this method also has some other deficiencies for the fabrication. For example, one has to draw a longer microfiber with a high degree of uniformity. In fact, this is a challenging problem being addressed in several papers [21–23].
n
Corresponding author. E-mail address: [email protected] (L. Ren).
http://dx.doi.org/10.1016/j.optcom.2014.01.077 0030-4018 & 2014 Elsevier B.V. All rights reserved.
In this paper, we demonstrate an approach to fabricating the silica MKR under the assistance of a polymer microfiber. Several kinds of MKRs with different structures, such as those with a longer intertwisted overlap at the contact area, with a two-ring serial connection structure and with a two-ring parallel connection structure, have been fabricated by use of this technique. It is found that this technique is quite simple and is beneficial to fabricating multi-ring MKRs with more complicated structures.
2. Basic principle of fabrication The schematically fabrication process is illustrated in Fig. 1, which can be divided into four steps. (1) Using the high-temperature tapered-drawing technique one fabricates a silica microfiber with a diameter of micrometer scale from the standard single mode fiber (SMF). As a result, there is a tapered-drawing region which connects the silica microfiber to the SMF at each end of the silica microfiber. Cutting one end of the silica microfiber makes it become a freestanding end. As shown in Fig. 1(a), a polymer microfiber with a diameter of tens micrometers, drawn from solvent polymers [24], is tailored to a suitable length. Then let's manually wind a knot ring with a diameter of several millimeters from the above polymer microfiber and adhere one end of the knot ring to the freestanding end of the silica microfiber. (2) The polymer microfiber knot ring is driven to the silica microfiber with the assistance of a tapereddrawing fiber probe until the polymer microfiber is completely drawn out of the knot ring. As shown in Fig. 1(b), in this case, the knot ring is only composed of the silica microfiber. (3) As shown in
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Fig. 1(c), continuously drawing the polymer microfiber thus finely tunes the diameter of the knot ring under an optical microscope. (4) Departing the polymer microfiber from the freestanding end of the silica microfiber and adhering the other silica microfiber produced in the step (1) to it, as shown in Fig. 1(d), one finally fabricates a silica MKR. Note that the zoom-in image in Fig. 1(d) is the intertwisted overlap at the contact area. The key point of this approach is to take advantage of the strong van der Waals and electrostatic forces between the silica microfiber and the polymer microfiber. The van der Waals and
electrostatic forces between two molecules have the following relationship F¼
2 I 1 I 2 α 1 α2 1 ; 3 I1 þ I2 r6 ð4πε0 Þ2
ð1Þ
where r is the interaction distance, ε0 is the permittivity in vacuum. I1 and I2 are the ionization energy, α1 and α2 are the polarizabilities of the two molecules, respectively. It is easy to see that the van der Waals and electrostatic forces are inverse proportional to the sixth power of the interaction distance. And the interaction distance is usually limited less than 0.165 nm [25]. The fabricated microfiber possesses such a high degree of diameter uniformity and surface smoothness [26,27] that, when two microfibers adhere together, most molecules located at the surfaces of them can contact firmly, which produces strong van der Waals and electrostatic forces between them. Moreover, the distance between molecules becomes shorter when the diameters of the microfibers become smaller, resulting in an increase of the van der Waals and electrostatic forces. Therefore, in principle, it is beneficial for us to drive the knot ring from the polymer microfiber to the silica microfiber when the diameters of these two microfibers are as small as possible. However, as far as the manual fabrication of the polymer microfiber knot ring is concerned, a polymer microfiber with a too small diameter inevitably increases the fabrication difficulties of the knot ring. Repeated experiments indicate that for silica microfibers with the diameters of 1–4 μm, it is better to select the diameters of the polymer microfibers to be 10–20 μm. The adhering length between the silica microfiber and the polymer microfiber should be longer than the circumference of the polymer microfiber knot ring.
3. Experimental results and discussions
Fig. 1. (a)–(d) Schematic of the fabrication process.
Experimentally, we fabricated a MKR with this technique. Fig. 2 (a) shows an optical microscope image of the knot resonator, in which the polymer microfiber ring has being driven to the freestanding end of the silica microfiber. As one can see in Fig. 2(a), the
Fig. 2. Optical microscope images of (a) the MKR located in the adhered region between the polymer microfiber and the silica microfiber on an MgF2 substrate, (b) the zoomin knot region in (a), (c) the final fabricated MKR, (d) the zoom-in knot region in (c).
Y. Xu et al. / Optics Communications 321 (2014) 157–161
bold line is the polymer microfiber with a diameter of about 12 μm, while the slim line is the silica microfiber with a diameter of about 1 μm. Fig. 2(b) shows a zoom-in optical microscope image of the knot region in Fig. 2(a). It is easy to see that the polymer microfiber and the silica microfiber are adhered together firmly. This is attributed to the strong van der Waals and electrostatic forces between them. The final knot resonator consisted of the silica microfiber is shown in Fig. 2(c). The diameter of the fabricated MKR is about 409 μm. Fig. 2(d) shows a zoom-in optical microscope image of the knot region in Fig. 2(c). It should be pointed out that the substrate used to place the MKR is an MgF2 crystal. It has a refractive index of 1.37 which is lower than that of the silica. Because of the large index contrast between the silica microfiber and the MgF2 substrate, the light is well confined on the surface of the silica microfiber and guided along it. To see the spectral property of this MKR, a broadband amplified spontaneous emission (ASE) light was launched into the MKR and the spectrum from the output port was collected. As an example, Fig. 3 shows the transmission spectrum in the wavelength range of 1548– 1554 nm, where an extinction ratio of about 5 dB is obtained. The uniformity of the output spectrum is ideal, which indicates that the influence of the polymer microfiber on the quality of the fabricated silica microfiber is slight. The measured full width at half-maximum (FWHM) ΔλFWHM is about 0.21 nm. And the measured free spectral range (FSR) ΔλFSR is about 1.21 nm at the wavelength around 1550 nm, which is in good agreement with the theoretical one ( 1.29 nm) calculated from ΔλFSR ¼
λ2 ; nef f π D
ð2Þ
where neff ¼1.45 is the effective refractive index of the propagating mode and D is the diameter of the MKR. Obtained Q factor (Q¼ λ/ΔλFWHM) and finesse (F¼ΔλFSR/ΔλFWHM) of the resonator are 7381 and 5.76 at the wavelength around 1550 nm, respectively. Comparing with the initially reported Q factor for a similar knot structure [21], the value obtained here is increased by nearly four times, but it is less than the reported value in Refs. [2,20]. The reason is that the diameter uniformity and the surface smoothness of the microfiber we fabricated are not good enough, which produces large propagation and bending losses in the MKR. When a knot ring with a longer intertwisted overlap at the contact area is pre-fabricated from the polymer microfiber, as shown in Fig. 4 (the schematic zoom-in image of the intertwisted overlap), we can
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fabricate a silica microfiber knot resonator with a longer intertwisted overlap at the contact area using this method. In this case, the MKR with a stronger coupling strength might be fabricated readily via this technique. Indeed, as shown in Fig. 5(a), we show an optical microscope image of the fabricated MKR with a longer intertwisted overlap at the contact area. The diameter of the MKR is about 1225 μm with a silica microfiber diameter of about 2 μm. Fig. 5(b) shows an optical microscope image of the zoom-in knot region with an intertwisted overlap length of about 734 μm. The transmission spectrum of this MKR is given in Fig. 5(c). It is seen that the resonator is not ideal enough if we observe the fluctuation of the resonance spectrum. This is mainly because the quality of the microfiber we fabricated is not good enough at present. Besides, it is also related to the unflatness of our ASE source. The measured FWHM is about 0.14 nm with an extinction ratio of about 8 dB. And the measured FSR is about 0.38 nm at the wavelength of around 1550 nm, which is in good agreement with the theoretical value ( 0.42 nm) obtained according to Eq. (2). Obtained Q factor and finesse of the MKR are 11,446 and 2.81, respectively. Comparing with the obtained values in the previous fabricated MKR, the extinction ratio and the Q factor in this MKR are increased considerably, this is due to the increase of the coupling strength at the contact area. Therefore, through increasing the intertwisted overlap length at the contact area, one can obtain a large extinction ratio and a large Q factor in this kind of MKR. Using this method, we can also fabricate MKRs with a multiring connection structure readily. Fig. 6(a) shows an optical microscope image of MKRs with a two-ring serial connection structure. The diameter of the silica microfiber is about 2 μm, and the diameters of the small ring and the big ring are about 680 μm and 723 μm, respectively. A corresponding transmission spectrum is given in Fig. 6(b). The whole transmission spectrum is the overlapped consequence of the two transmission spectra produced by the two knot rings. Measured FSR and FWHM are about 0.43 nm and 0.05 nm, respectively, with an extinction ratio of about 5 dB at the wavelength of around 1550 nm. And obtained Q factor and finesse of the resonators are about 31,000 and 8.6, respectively. Fig. 6(c) shows an optical microscope image of MKRs with a two-ring parallel connection structure. The diameter of the silica microfiber is about 2 μm and the diameters of the small ring and the big ring are about 788 μm and 1605 μm, respectively. A corresponding transmission spectrum is given in Fig. 6(d). The whole transmission spectrum is not simply an overlapped consequence of the two transmission spectra produced by the two knot rings. Measured FSR and FWHM are about 0.14 nm and 0.04 nm, respectively, with an extinction ratio of about 2 dB at the wavelength of around 1550 nm. And obtained Q factor and finesse of the resonators are about 38,750 and 3.5, respectively. It should be pointed out that the narrow transmission peaks in Fig. 6(d) actually realize a series of quasi-electromagnetically induced transparency windows [28], which can be used to fabricate miniature lasers, filters and group delay devices for optical communications as well as for local sensing of physical and chemical parameters of the environment.
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4. Conclusion
-63 -64 1548
1549
1550
1551
1552
1553
1554
Wavelength (nm) Fig. 3. Transmission spectrum of the fabricated MKR.
Fig. 4. Schematic zoom-in image of the intertwisted overlap at the contact area.
In summary, we propose and demonstrate an approach to fabricating the silica MKR under the assistance of the polymer microfiber. The key point is, by using the van der Waals and electrostatic forces between the silica microfiber and the polymer microfiber, the prefabricated knot ring made of the polymer microfiber can be driven to the silica microfiber. Using this technique, we have successfully fabricated several kinds of MKRs with different structures, such as those with a longer intertwisted overlap at the contact area, with a two-ring serial connection structure and with a two-ring parallel
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Fig. 5. Optical microscope images of (a) the fabricated MKR with a longer intertwisted overlap at the contact area, (b) the zoom-in knot region in (a). (c) Transmission spectrum of the fabricated MKR.
Fig. 6. (a) An optical microscope image of MKRs with a two-ring serial connection structure. (b) Corresponding transmission spectrum of (a). (c) An optical microscope image of MKRs with a two-ring parallel connection structure. (d) Corresponding transmission spectrum of (c).
connection structure. At present, though the Q factors obtained in these structures are much higher than the one in a knot structure initially reported [21], they are less than the reported in Refs. [2,20]. This is owing to the fact that the diameter uniformity and the surface smoothness of the microfiber we fabricated are not good enough. We believe that, with the improvement of our fabrication technique, the quality of our MKRs can be further enhanced. In addition, comparing with other fabrication methods, this technique is quite simple and is easy to fabricate much more complicated multi-ring MKRs.
Acknowledgment This work was supported in part by the National Natural Science Foundation of China under Grants 61275149 and 51207159, and the Advanced Programs of Technological Activities for Overseas Scholars.
References [1] M. Sumetsky, Y. Dulashko, J.M. Fini, A. Hale, Appl. Phys. Lett. 86 (2005) 161108. [2] X.S. Jiang, L.M. Tong, G. Vienne, X. Guo, A. Tsao, Q. Yang, D.R. Yang, Appl. Phys. Lett. 88 (2006) 223501. [3] G. Vienne, A. Coillet, P. Grelu, M.E. Amraoui, J.C. Jules, F. Smektala, L.M. Tong, Opt. Express 17 (2009) 6224. [4] Y.H. Chen, Y. Wu, Y.J. Rao, Q. Deng, Y. Gong, Opt. Commun. 283 (2010) 2953. [5] F. Xu, G. Brambilla, IEEE Photonics Technol. Lett. 19 (2007) 1481. [6] M. Sumetsky, J. Lightwave Technol. 26 (2008) 21. [7] F. Xu, G. Brambilla, Opt. Lett. 32 (2007) 2164. [8] Y.M. Jung, G.S. Murugan, G. Brambilla, D.J. Richardson, IEEE Photonics Technol. Lett. 22 (2010) 1638. [9] G. Brambilla, F. Xu, P. Horak, Y. Jung, F. Koizumi, N.P. Sessions, E. Koukharenko, X. Feng, G.S. Murugan, J.S. Wilkinson, D.J. Richardson, Adv. Opt. Photonics 1 (2009) 107. [10] L. Tong, Front. Optoelectron. Chin. 3 (2010) 54. [11] G. Brambilla, J. Opt. 12 (2010) 043001. [12] X.S. Jiang, Y. Chen, G. Vienne, L.M. Tong, Opt. Lett. 32 (2007) 1710. [13] K.S. Lim, S.W. Harun, S.S.A. Damanhuri, A.A. Jasim, C.K. Tio, H. Ahmad, Sens. Actuators, A 167 (2011) 60.
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[14] Y. Wu, Y.J. Rao, Y.H. Chen, Y. Gong, Opt. Express 17 (2009) 18142. [15] Y. Wu, X. Zeng, Y.J. Rao, Y. Gong, C.L. Hou, G.G. Yang, IEEE Photonics Technol. Lett. 21 (2009) 1547. [16] G. Vienne, Y.H. Li, L.M. Tong, IEEE Photonics Technol. Lett. 19 (2007) 1386. [17] G. Brambilla, Opt. Fiber Technol. 16 (2010) 331. [18] X.S. Jiang, Q.H. Song, L Xu, J. Fu, L.M. Tong, Appl. Phys. Lett. 90 (2007) 233501. [19] G. Vienne, Y.H. Li, L.M. Tong, P. Grelu, Opt. Lett. 33 (2008) 1500. [20] L. Xiao, T.A. Birks, Opt. Lett. 36 (2011) 1098. [21] L.M. Tong, R.R. Gattass, J.B. Ashcom, S.L. He, J.Y. Lou, M.Y. Shen, I. Maxwell, E. Mazur, Nature 426 (2003) 816. [22] G. Brambilla, V. Finazzi, D.J. Richardson, Opt. Express 12 (2004) 2258.
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[23] S.G. Leon-Saval, T.A. Birks, W.J. Wadsworth, P.S.J. Russell, Opt. Express 12 (2004) 2864. [24] S.A. Harfenist, S.D. Cambron, E.W. Nelson, S.M. Berry, A.W. Isham, M.M. Crain, K.M. Walsh, R.S. Keynton, R.W. Cohn, Nano Lett. 4 (2004) 1931. [25] J. Israelachvili, Intermolecular and Surface Forces, CA: Academic Press, SanDiego, 1992. [26] M. Sumetsky, Y. Dulashko, J.M. Fini, A. Hale, D.J. Digiovanni, J. Lightwave Technol. 24 (2006) 242. [27] L.M. Tong, L.L. Hu, J.J. Zhang, J.R. Qiu, Q. Yang, J.Y. Lou, Y.H. Shen, J.L. He, Z.Z. Ye, Opt. Express 14 (2006) 82. [28] K. Totsuka, N. Kobayashi, M. Tomita, Phys. Rev. Lett. 98 (2007) 213904.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
A simple, polymer-microfiber-assisted approach to fabricating the silica microfiber knot resonator Yiping Xu a,b, Liyong Ren a,n, Jian Liang a, Chengju Ma a,b, Yingli Wang a, Nana Chen a,b, Enshi Qu a a b
State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, China University of Chinese Academy of Sciences, Beijing 100049, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 November 2013 Received in revised form 13 January 2014 Accepted 30 January 2014 Available online 11 February 2014
With the assistance of the polymer microfiber, we propose a simple approach to fabricating silica-based optical microfiber knot resonators (MKRs) in this paper. A pre-fabricated knot ring made of the polymer microfiber can be readily driven to the silica microfiber owing to the intensive van der Waals and the electrostatic forces between the polymer and the silica microfibers. The fabrication process is introduced in detail. Several kinds of MKRs are fabricated and their optical performances are demonstrated accordingly. & 2014 Elsevier B.V. All rights reserved.
Keywords: Microfiber Silica microfiber Microfiber knot resonator (MKR)
1. Introduction In recent years, miniature photonic devices assembled by microfibers with diameters of micrometer scale have attracted significant interest. Various structures of microfiber resonators, including loop, knot and coil, have been investigated [1–11]. Among them the optical microfiber knot resonator (MKR) [2], due to its high Q factor, tunable free spectral range, high stability and compact structure, has been extensively applied to optical filters [12], optical sensing [13–17], microfiber lasers [18], and nonlinear resonators [19]. Currently, most MKRs are fabricated based on the micromanipulation method proposed by Jiang et al. [2]. That is, the microfiber knot is assembled with the assistance of two tapered-drawing fiber probes under an optical microscope. Obviously, operating under an optical microscope increases the inconvenience of the fabrication. Recently, for the first time Xiao et al. proposed a new method to directly fabricate a MKR from a double-ended tapered fiber [20]. The merit of this fabrication method is that light can be coupled in and out of the resonator directly via integral full-size fiber ports, which benefits the high finesse. However, this method also has some other deficiencies for the fabrication. For example, one has to draw a longer microfiber with a high degree of uniformity. In fact, this is a challenging problem being addressed in several papers [21–23].
n
Corresponding author. E-mail address: [email protected] (L. Ren).
http://dx.doi.org/10.1016/j.optcom.2014.01.077 0030-4018 & 2014 Elsevier B.V. All rights reserved.
In this paper, we demonstrate an approach to fabricating the silica MKR under the assistance of a polymer microfiber. Several kinds of MKRs with different structures, such as those with a longer intertwisted overlap at the contact area, with a two-ring serial connection structure and with a two-ring parallel connection structure, have been fabricated by use of this technique. It is found that this technique is quite simple and is beneficial to fabricating multi-ring MKRs with more complicated structures.
2. Basic principle of fabrication The schematically fabrication process is illustrated in Fig. 1, which can be divided into four steps. (1) Using the high-temperature tapered-drawing technique one fabricates a silica microfiber with a diameter of micrometer scale from the standard single mode fiber (SMF). As a result, there is a tapered-drawing region which connects the silica microfiber to the SMF at each end of the silica microfiber. Cutting one end of the silica microfiber makes it become a freestanding end. As shown in Fig. 1(a), a polymer microfiber with a diameter of tens micrometers, drawn from solvent polymers [24], is tailored to a suitable length. Then let's manually wind a knot ring with a diameter of several millimeters from the above polymer microfiber and adhere one end of the knot ring to the freestanding end of the silica microfiber. (2) The polymer microfiber knot ring is driven to the silica microfiber with the assistance of a tapereddrawing fiber probe until the polymer microfiber is completely drawn out of the knot ring. As shown in Fig. 1(b), in this case, the knot ring is only composed of the silica microfiber. (3) As shown in
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Y. Xu et al. / Optics Communications 321 (2014) 157–161
Fig. 1(c), continuously drawing the polymer microfiber thus finely tunes the diameter of the knot ring under an optical microscope. (4) Departing the polymer microfiber from the freestanding end of the silica microfiber and adhering the other silica microfiber produced in the step (1) to it, as shown in Fig. 1(d), one finally fabricates a silica MKR. Note that the zoom-in image in Fig. 1(d) is the intertwisted overlap at the contact area. The key point of this approach is to take advantage of the strong van der Waals and electrostatic forces between the silica microfiber and the polymer microfiber. The van der Waals and
electrostatic forces between two molecules have the following relationship F¼
2 I 1 I 2 α 1 α2 1 ; 3 I1 þ I2 r6 ð4πε0 Þ2
ð1Þ
where r is the interaction distance, ε0 is the permittivity in vacuum. I1 and I2 are the ionization energy, α1 and α2 are the polarizabilities of the two molecules, respectively. It is easy to see that the van der Waals and electrostatic forces are inverse proportional to the sixth power of the interaction distance. And the interaction distance is usually limited less than 0.165 nm [25]. The fabricated microfiber possesses such a high degree of diameter uniformity and surface smoothness [26,27] that, when two microfibers adhere together, most molecules located at the surfaces of them can contact firmly, which produces strong van der Waals and electrostatic forces between them. Moreover, the distance between molecules becomes shorter when the diameters of the microfibers become smaller, resulting in an increase of the van der Waals and electrostatic forces. Therefore, in principle, it is beneficial for us to drive the knot ring from the polymer microfiber to the silica microfiber when the diameters of these two microfibers are as small as possible. However, as far as the manual fabrication of the polymer microfiber knot ring is concerned, a polymer microfiber with a too small diameter inevitably increases the fabrication difficulties of the knot ring. Repeated experiments indicate that for silica microfibers with the diameters of 1–4 μm, it is better to select the diameters of the polymer microfibers to be 10–20 μm. The adhering length between the silica microfiber and the polymer microfiber should be longer than the circumference of the polymer microfiber knot ring.
3. Experimental results and discussions
Fig. 1. (a)–(d) Schematic of the fabrication process.
Experimentally, we fabricated a MKR with this technique. Fig. 2 (a) shows an optical microscope image of the knot resonator, in which the polymer microfiber ring has being driven to the freestanding end of the silica microfiber. As one can see in Fig. 2(a), the
Fig. 2. Optical microscope images of (a) the MKR located in the adhered region between the polymer microfiber and the silica microfiber on an MgF2 substrate, (b) the zoomin knot region in (a), (c) the final fabricated MKR, (d) the zoom-in knot region in (c).
Y. Xu et al. / Optics Communications 321 (2014) 157–161
bold line is the polymer microfiber with a diameter of about 12 μm, while the slim line is the silica microfiber with a diameter of about 1 μm. Fig. 2(b) shows a zoom-in optical microscope image of the knot region in Fig. 2(a). It is easy to see that the polymer microfiber and the silica microfiber are adhered together firmly. This is attributed to the strong van der Waals and electrostatic forces between them. The final knot resonator consisted of the silica microfiber is shown in Fig. 2(c). The diameter of the fabricated MKR is about 409 μm. Fig. 2(d) shows a zoom-in optical microscope image of the knot region in Fig. 2(c). It should be pointed out that the substrate used to place the MKR is an MgF2 crystal. It has a refractive index of 1.37 which is lower than that of the silica. Because of the large index contrast between the silica microfiber and the MgF2 substrate, the light is well confined on the surface of the silica microfiber and guided along it. To see the spectral property of this MKR, a broadband amplified spontaneous emission (ASE) light was launched into the MKR and the spectrum from the output port was collected. As an example, Fig. 3 shows the transmission spectrum in the wavelength range of 1548– 1554 nm, where an extinction ratio of about 5 dB is obtained. The uniformity of the output spectrum is ideal, which indicates that the influence of the polymer microfiber on the quality of the fabricated silica microfiber is slight. The measured full width at half-maximum (FWHM) ΔλFWHM is about 0.21 nm. And the measured free spectral range (FSR) ΔλFSR is about 1.21 nm at the wavelength around 1550 nm, which is in good agreement with the theoretical one ( 1.29 nm) calculated from ΔλFSR ¼
λ2 ; nef f π D
ð2Þ
where neff ¼1.45 is the effective refractive index of the propagating mode and D is the diameter of the MKR. Obtained Q factor (Q¼ λ/ΔλFWHM) and finesse (F¼ΔλFSR/ΔλFWHM) of the resonator are 7381 and 5.76 at the wavelength around 1550 nm, respectively. Comparing with the initially reported Q factor for a similar knot structure [21], the value obtained here is increased by nearly four times, but it is less than the reported value in Refs. [2,20]. The reason is that the diameter uniformity and the surface smoothness of the microfiber we fabricated are not good enough, which produces large propagation and bending losses in the MKR. When a knot ring with a longer intertwisted overlap at the contact area is pre-fabricated from the polymer microfiber, as shown in Fig. 4 (the schematic zoom-in image of the intertwisted overlap), we can
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Transmission (dBm)
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fabricate a silica microfiber knot resonator with a longer intertwisted overlap at the contact area using this method. In this case, the MKR with a stronger coupling strength might be fabricated readily via this technique. Indeed, as shown in Fig. 5(a), we show an optical microscope image of the fabricated MKR with a longer intertwisted overlap at the contact area. The diameter of the MKR is about 1225 μm with a silica microfiber diameter of about 2 μm. Fig. 5(b) shows an optical microscope image of the zoom-in knot region with an intertwisted overlap length of about 734 μm. The transmission spectrum of this MKR is given in Fig. 5(c). It is seen that the resonator is not ideal enough if we observe the fluctuation of the resonance spectrum. This is mainly because the quality of the microfiber we fabricated is not good enough at present. Besides, it is also related to the unflatness of our ASE source. The measured FWHM is about 0.14 nm with an extinction ratio of about 8 dB. And the measured FSR is about 0.38 nm at the wavelength of around 1550 nm, which is in good agreement with the theoretical value ( 0.42 nm) obtained according to Eq. (2). Obtained Q factor and finesse of the MKR are 11,446 and 2.81, respectively. Comparing with the obtained values in the previous fabricated MKR, the extinction ratio and the Q factor in this MKR are increased considerably, this is due to the increase of the coupling strength at the contact area. Therefore, through increasing the intertwisted overlap length at the contact area, one can obtain a large extinction ratio and a large Q factor in this kind of MKR. Using this method, we can also fabricate MKRs with a multiring connection structure readily. Fig. 6(a) shows an optical microscope image of MKRs with a two-ring serial connection structure. The diameter of the silica microfiber is about 2 μm, and the diameters of the small ring and the big ring are about 680 μm and 723 μm, respectively. A corresponding transmission spectrum is given in Fig. 6(b). The whole transmission spectrum is the overlapped consequence of the two transmission spectra produced by the two knot rings. Measured FSR and FWHM are about 0.43 nm and 0.05 nm, respectively, with an extinction ratio of about 5 dB at the wavelength of around 1550 nm. And obtained Q factor and finesse of the resonators are about 31,000 and 8.6, respectively. Fig. 6(c) shows an optical microscope image of MKRs with a two-ring parallel connection structure. The diameter of the silica microfiber is about 2 μm and the diameters of the small ring and the big ring are about 788 μm and 1605 μm, respectively. A corresponding transmission spectrum is given in Fig. 6(d). The whole transmission spectrum is not simply an overlapped consequence of the two transmission spectra produced by the two knot rings. Measured FSR and FWHM are about 0.14 nm and 0.04 nm, respectively, with an extinction ratio of about 2 dB at the wavelength of around 1550 nm. And obtained Q factor and finesse of the resonators are about 38,750 and 3.5, respectively. It should be pointed out that the narrow transmission peaks in Fig. 6(d) actually realize a series of quasi-electromagnetically induced transparency windows [28], which can be used to fabricate miniature lasers, filters and group delay devices for optical communications as well as for local sensing of physical and chemical parameters of the environment.
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Wavelength (nm) Fig. 3. Transmission spectrum of the fabricated MKR.
Fig. 4. Schematic zoom-in image of the intertwisted overlap at the contact area.
In summary, we propose and demonstrate an approach to fabricating the silica MKR under the assistance of the polymer microfiber. The key point is, by using the van der Waals and electrostatic forces between the silica microfiber and the polymer microfiber, the prefabricated knot ring made of the polymer microfiber can be driven to the silica microfiber. Using this technique, we have successfully fabricated several kinds of MKRs with different structures, such as those with a longer intertwisted overlap at the contact area, with a two-ring serial connection structure and with a two-ring parallel
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Fig. 5. Optical microscope images of (a) the fabricated MKR with a longer intertwisted overlap at the contact area, (b) the zoom-in knot region in (a). (c) Transmission spectrum of the fabricated MKR.
Fig. 6. (a) An optical microscope image of MKRs with a two-ring serial connection structure. (b) Corresponding transmission spectrum of (a). (c) An optical microscope image of MKRs with a two-ring parallel connection structure. (d) Corresponding transmission spectrum of (c).
connection structure. At present, though the Q factors obtained in these structures are much higher than the one in a knot structure initially reported [21], they are less than the reported in Refs. [2,20]. This is owing to the fact that the diameter uniformity and the surface smoothness of the microfiber we fabricated are not good enough. We believe that, with the improvement of our fabrication technique, the quality of our MKRs can be further enhanced. In addition, comparing with other fabrication methods, this technique is quite simple and is easy to fabricate much more complicated multi-ring MKRs.
Acknowledgment This work was supported in part by the National Natural Science Foundation of China under Grants 61275149 and 51207159, and the Advanced Programs of Technological Activities for Overseas Scholars.
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