Novel All-Fiber Band Pass Filter and Multimode-Single-mode Converter for Interconnection Between Multimode Fiber and Single Mode Fiber Network

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1 Invited Paper Novel All-Fiber Band Pass Filter and Multimode-Single-mode Converter for Interconnection Between Multimode Fiber and Single Mode Fiber Network Yong ZHU*, Hao MEI, Xiaoqin LI, Tao ZHU Key Laboratory for Optoelectronic Technology & System (Chongqing University), Education Ministry of China, Chongqing, China ABSTRACT In this paper, a novel all-fiber band pass filter based on a concatenated structure of multimode fiber, single-mode core mode blocker and a single-mode long period fiber grating was reported. It can simultaneously serves as a band pass filter and multimode-single-mode converter for interconnection between multimode fiber and single mode fiber network. The theoretical analysis, designing, fabrication and experiments result were presented. Keywords: All fiber band-pass-filter, Multimode-single-mode converter, Long period fiber grating, Optical communication 1. INTRODUCTION In recent years, high-speed local area networks (LANs) are stunningly growing due to the increasing needs of ultrahigh-speed interconnection between computers, and a couple of research groups achieved the speed record of hundreds gigabits per second and even terabits per second under laboratorial circumstance[1-3]. Multimode fibers are used predominantly in LANs because of their relatively low modal group velocity dispersion and large fiber core size[]. With its explosive expansion in the passed decade, the geographic scale of LANs has reached the distance limitation of multimode fiber, and therefore leaded to a paradox of speed and scale. The regular solution of this problem is to construct a hybrid network with single-mode and multimode fiber. That is to say, a large scale LAN can be divided into many small sub-networks with multimode fiber communication inside, and then interconnected them by single-mode fiber links[4].the practical challenges of real world applications may include (1) wavelength-selective components, which are suitable for CWDM in multimode-to-single-mode fiber network, () multimode-to-single-mode converter with the data rate from gigabit to terabit per second. Fiber Bragg grating (FBG) and long period fiber grating (LPFG) were demonstrated to be the most promising all fiber wavelength-selective components. In the 003, FBG was fabricated on multi-cylindrical shell multimode fiber by Sun et al.[5], and then wavelength-division multiplexing in a conventional 6.5µm diameter multimode fiber using a fiber Bragg grating written on multimode fiber were demonstrated by Gu et al. [6]. However, FGB is not a band-pass filter but a band-reject filter, the needs for a fiber x coupler leads to extra power loss and raise the cost and complexity of the system. Choi et al. [7] reported an all fiber band pass filter based on two concatenated LPFGs, but this method required two LPFG have the same performance; it restricted the possibility of mass production. In addiction, this all fiber band pass filter is only suitable to single-mode fiber systems. The second challenge has also been studied by many groups[8], and there are some commercial model of mode converter. However, these commercial products are base on opto-electronic repeater technique, they are active devices that need power supply, and their speed are comparatively slow but price is high. Unfortunately, to the best knowledge of the authors, none of the current available techniques solve all two aforementioned challenges simultaneously. * yongzhu@cqu.edu.cn, Phone: , Fax: Photonic Fiber and Crystal Devices: Advances in Materials and Innovations in Device Applications II, edited by Shizhuo Yin, Ruyan Guo, Proc. of SPIE Vol. 7056, 70560N, (008) X/08/$18 doi: / SPIE Digital Library -- Subscriber Archive Copy Proc. of SPIE Vol N-1

2 In this paper, we report a novel all-fiber band pass filter (BPF) based on a concatenated structure of multimode fiber, single-mode core mode blocker and a single-mode long period fiber grating. The BPF can be not only used as an effective wavelength selection device, but also served as a multimode-to-single-mode converter, which can deal with those two challenges at the same time.. Principle For an ideal LPFG, the phase-matching condition that describes the center wavelength of the transmission resonance can be expressed as[9] λ = ( n core n υ clad ) Λ (1) υ where Λ is the grating period of the LPFG, ncore and n υ clad are the effective refractive index for fundamental core mode and the νth-order cladding mode respectively. The phase-matching condition shows that, at the resonance wavelength, the LPFG s fundamental core mode couples with cladding modes. For a common LPFG, the fundamental core mode is coupled to cladding modes and present a property of band rejection on the transmission spectrum. Reciprocally, if one block the input end of fiber core and launches light into cladding, cladding modes which meet phase-matching condition will be couple back to the fundamental core mode, and then the LPFG will shows a band pass transmission spectrum. Based on this idea, an all-fiber band pass filter was proposed and illustrated in Figure1. Input Core mode blocker Cladding mode LPFG Core mode Cladding of SMF Core of MMF Output Fig.1 Schematic diagram of band pass filter structured by a combination of multimode fiber, a core mode blocker and single-mode fiber LPFG As shown in Figure 1, the all fiber BPF device includes a multimode fiber, a LPFG wrote on a single mode fiber and a core mode blocker between them. When incident light propagates through the interface between multimode and singlemode fiber, A small fraction of light encounter the core mode blocker and are extinguished, but most parts of light entered the cladding of single mode fiber, because the diameter of multimode fiber is much larger than that of singlemode fiber. At the grating area, the cladding modes that satisfy the phase-matching condition will be efficiently coupled back into the core mode and other light that don t satisfy the phase-matching condition radiate out of the fiber, and thereby formed a transmission type BPF as well as a multimode fiber to single-mode fiber converter. Proc. of SPIE Vol N-

3 Because the core diameter of multimode fiber is 6.5 µm, the LPFG s parameter is critical to make sure: (1) the resonant cladding mode has peaks in core and cladding, () the size of the cladding mode should be smaller than 6.5 µm. The dispersion relation equation for cladding modes is always adopted to solve this kind of problem:[10] Where ζ ζ 0 ζ 0 = ζ 0 () σ σ u u sl ( a ) u ( JK + ) p ( l a) Kql( a) + Jrl( a) ) naa u 1 1 = u u u u u [ J K ] p ( a ) + q ( a ) + r ( a ) σ l l l n a n1 a1 n1 a1 n1 a1 u3 n3 u1 u3 u1 σ1[ u( J K) p l( a) + ql( a) + rl( a)] a n a1 a a1 0 = n3 σσ 1 u1u3 n3 n u JK + p l a Kq l a + Jrl a s l a n n1 aa 1 n1 n1 u ( ) ( ) ( ) ( ) ( ) (3) (4) The following definitions are used in equation σ 1 = ilneff / Z0 (5) σ = ilneffz 0 (6) 1 1 u 1 = (7) u u1 1 1 u 3 = + (8) ω 3 u uj = ( π / λ) ( nj neff )( j = 1,) (9) ω 3 = ( π / λ) ( neff n3 ) (10) Jl ( u1a1) J = u1jl( u1a1) (11) K l ( 3a ) K = ω ω 3K l ( ω 3a ) (1) pl( r) = Jl( ur) Nl( ua1) Jl( ua1) Nl( ur) ql( r) = Jl( ur) Nl ( ua1) Jl ( ua1) Nl( ur) rl( r) Jl ( ur) Nl( ua1) Jl( ua1) Nl ( u = r) s r = J u r N u a J u a N u r l( ) l ( ) l ( 1) l ( 1) l ( ) (13) where l is the azimuthal number, Z 0 is the vacuum impedance. n 1, n, n 3, and n eff represent the refractive index of the core, the cladding, and the surrounding medium (consider as air) and the effective refractive index of the cladding modes in the cladding region respectively. a 1 and a are the radius of the core and the cladding. J and N are the Bessel Proc. of SPIE Vol N-3

4 functions of the first and the second order. Given the fiber parameter of Corning SM-8 fiber, and solving the equation with the software Matlab, the effective refractive index of the νth-order cladding modes are obtained (see Tab. 1). ν n eff β Tab. 1: Effective index and propagation constant of 10 cladding modes of the first order Consequently, the vector components of the electric and magnetic field for the 1 st order cladding modes in the core (r a 1 ) and cladding (a 1 <r a ) can be obtained, and the optical intensity can be then calculated (Figure ). Optical Intensity 3 Radius io m (a) odd order modes (b) even order modes Fig. The optical intensity of the 1 st order odd and even modes Considering the mode size match between multimode fiber core and cladding mode of single mode fiber, Figure just gave the calculated optical intensity distribution with the radius less than 31.5µm. As shown in Figure, low-order even modes contain very little optical intensity in the fiber core while the low-order odd modes have a peak value in the core. That is to say, the coupling between the 1 st order odd modes and the fundamental core mode are expected to be much stronger than that between the 1 st order even modes and the fundamental core mode. Therefore, the 1 st order odd cladding modes were selected as resonant cladding modes. Also shown in Figure, LP 15 has the mode radius less than Proc. of SPIE Vol N-4

5 0µm and was selected to be the working cladding mode of our all fiber BPF. According to Equation 1, to have a LP 15 peak near 1550 nm, the period of the LPFG should be 450µm. 3. Experiment In our experiment, Corning SMF-8E fibers and a high-frequency CO laser were used to fabricate LPFGs. Due to our unique grating-fabricating method, the period perturbation of refractive index occurred asymmetric in the fiber cladding and the most effective coupling happened between LP 01 core mode and 1 st order cladding modes LP 1v [11]. As the grating constant is 450µm, v equals to 5 in our experiment. The transmission spectrum of LPFG is shown in Figure 3, the main dip is at 1530nm and the depth is 0dB. -10 Intensity (dbm) Wavelength (nm) Fig.3 Transmission spectrum of LPFG Fig.4 Photo of core mode blocker A core mode blocker was made by lithography technique (Figure 4). A tunable laser (Photonetics TUNICS-PRI) which has a wavelength from 1500nm to 1600nm was selected as the broadband light source, and an optical spectrum analyzer (Ando 6317B), to acquire the transmission spectrum. The experimental setups are shown in Figure 5. orqpiuq C FILCG V\WT1 FbE( (JJ4J] e1jbu4jjjjjj U1JJ]AL COLG moqc Pu DCJGL Fig.5 Schematic diagram of experimental setup Proc. of SPIE Vol N-5

6 Before we connected the multimode fiber, the core mode blocker and the LPFG on single mode fiber together, another experiment was carried out to check if our idea works. We fused a multimode fiber and a LPFG without core mode blocker together, and if the cladding mode could be coupled back to core mode, Mach-Zehnder interference should be observed. The reason is, the two beams of light has the same frequency but different optical paths, one passed along the fiber core and go through the grating area directly and the other was coupled back to the core from the cladding. The experimental result is shown in figure 6, Mach-Zehnder interference was observed as expected Intensity (dbm) Intensity (dbm) Wavelength (nm) Wavelength (nm) Fig.6 The transmission spectrum of the Mach-Zehnder interference Fig.7 The transmission spectrum of the band-pass filter Whereafter, the multimode fiber was cleaved off and a core mode blocker was deposited on the end surface of the LPFG, then the multimode and the LPFG with the blocker were fused together again. The fusion parameter of the fusion splicer is very critical to ensure the success of the experiment. The blocker is a thin metal film deposited on the surface of the fiber, it could be easily damaged by the fusing arc, therefore the electric current was decrease to a minimal value. As a result, the fusion point is very frangible, and carefully handle is necessary. After finishing these preparations, we got the transmission spectrum of the whole BPF device, which is shown in Figure 7. The over all transmission spectrum of the proposed device has a band-pass filter feature, and there are one main transmission peaks over 100nm range: 1500 to 1600nm. The main peak is at 1531nm, the total insertion loss at 1531nm is 7.16dB, BPF bandwidth is 7nm, and the peak depth is larger than 30dB. This result experimentally demonstrated the validity of the core mode blocker as well as the BPF with a concatenated structure of multimode fiber, a core mode blocker and a single-mode LPFG. In the current experiment, the insertion loss and some spectral deformation are mainly due to: (1) the splice loss at the interface between the multimode and the single-mode fiber, () the spectral mismatch between the cladding modes of the single-mode fiber and the core modes of the multimode fiber, (3) the arc induced damage on the core mode blocker. However the all fiber BPF and the multimode to single-mode converter with only one LPFG was experimentally demonstrated. Proc. of SPIE Vol N-6

7 4. Conclusion In this paper, we have proposed developed a novel all-fiber band pass filter (using only one LPFG), which consists of a multimode fiber, a LPFG based on a single-mode fiber and a core mode blocker between them. It can also work as a multimode fiber to single mode fiber converter interconnecting the LANs with the WANs. The unique capability of simultaneously filtering and mode converting was enabled because this BPF had the following key features: (1) the multimode fiber and the core mode blocker could couple broadband light into the cladding of the single-mode fiber. () The LPFG has wavelength selective capability that can couple the resonance wavelength from the cladding modes to the core mode. (3) The high frequency CO pulsed laser induced LPFG is based on the cladding index variation, which makes it possible to control the index changing amount and the distribution to ensure the proper coupling between the core mode LP01 and a specified cladding mode. Future works should be focused on: (1) To ensure low insertion loss of the device, the mode conversion efficiency from multimode fiber to the specified cladding mode should be significant increased. () A substituted connection method should be studied to replace the arc fusion, in order to avoid the arc damage of core mode blocker, and consequently reduce the spectral deformation the proposed all fiber BPF. Acknowledgements The authors acknowledge financial support of National Natural Science Foundation of China ( )and Natural Science Foundation of Chongqing Municipality(CSTC007BB315). References [1] E. J. Tyler, P. Kourtessis, M. Webster, E. Rochart, T. Quinlan, S. E. M. Dudley, S. D. Walker, R. V. Penty, and I. H. White, Toward terabit-per-second capacities over multimode fiber links using SCM/WDM techniques, J. Lightw. Technol. 1, 337(000) [] X. J. Gu, Waleed Mohamood, Peter W. Smith, Demonstration of all-fiber WDM for multimode fiber local area networks, IEEE Photon. Technol. Lett. 18, (006) [3] L. A. Buckman, B. E. Lemoff, A. J. Schmit, R. P. Tella, and W. Gong, Demonstration of a small-form-factor WWDM transceiver module for 10-Gb/s local area networks, IEEE Photon. Technol. Lett. 14, (00) [4] Stefano Bottacchi, Multi-Gigabit Transmission over Multimode Optical Fibre: Theory and Design Methods for 10GbE Systems, Wiley-IEEE Press, 1 (006) [5] Y. Sun, T. Szkopek, and P. W. E. Smith, Demonstration of narrow-band high-reflectivity Bragg gratings in a novel multimode fiber, Opt. Commun. 3, (003) [6] X. J. Gu, Waleed Mohammed, and Peter W. Smith, Demonstration of All-Fiber WDM for Multimode Fiber Local Area Networks, IEEE Photon. Technol. Lett., 18(1), (006) [7] S. Choi, T. J. Eom, J. W. Yu, B. H. Lee, and K. Oh, Novel All-Fiber Bandpass Filter Based on Hollow Optical Fiber, IEEE Photon. Technol. Lett. 14(1), (00) [8] Yi Yang, A Study on Light Guiding Devices with Longitudinal Modulations, PhD Thesis, Department of Electrical Engineering, The Pennsylvania State University, USA, 1 (005) [9] A. M. Vengsarkar, P. J. Lemaire, J. B. Judkins, V. Bhatia, T. Erdogan, and J. E. Sipe, Long-period fiber grating as band-rejection filters, J. Lightwave Technol. 14, (1996) [10] Turan Erdogan, Cladding-mode resonances in short-and long-period fiber grating filters,j. Opt. Soc. Am. 14, (1997) Proc. of SPIE Vol N-7

8 [11] Y.J. Rao, T. Zhu, Z.L. Ran, Y.P. Wang, J. Jiang, A.Z. Hu, Novel long-period fiber gratings written by highfrequency CO laser pulses and applications in optical fiber communication, Optics Communications, 9 (1), 09-1 (004) Proc. of SPIE Vol N-8